Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.ijhydene.2024.07.061

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

Recently, OH planar laser-induced fluorescence (PLIF) using the broadband, ultrashort femtosecond-duration (fs-duration) and the thermally assisted vibrational transfer in excited state has been investigated in flames. In this present work, we first measured temperature by thermally assisted OH laser-induced fluorescence (TALF) method with a single ultrashort broadband fs laser. In the experiment, the fs excitation of OH at ultraviolet wavelength is followed by fluorescence detection from two different vibrational bands. The ratio of two measured (1–0) and (0–0) band fluorescence is calibrated with calculated temperature using Chemkin PRO PRIMIX. The calibrated results are used in measuring temperature distributions in different laminar flames. It is found that TALF method using the fs laser can detect 2D temperature distribution in the burnt area with high OH fluorescence signal. However, OH chemiluminescence brings inevitable noise at the flame front that the TALF method does not perform well. And because (1–0) band fluorescence is so weak, the noise from the camera sensor and imaging intensifier (I.I.) remains at the measured temperature imaging. In conclusion, quantitative temperature measurement based on OH TALF based on a single broadband, ultrashort fs laser can be applied in laminar flames with high frequency by a simple experiment setup.

DOI： 10.1007/s00340-024-08199-9

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Other Link： https://link.springer.com/article/10.1007/s00340-024-08199-9/fulltext.html

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.proci.2024.105629

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Informa UK Limited  

DOI： 10.1080/00102202.2023.2249220

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Informa UK Limited  

DOI： 10.1080/13647830.2023.2209047

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Various aspects of machine learning (ML) are explored to resolve limitations appearing in current ML-based subgrid scale (SGS) stress modeling. Graph neural network (GNN), applied in the present study, allows flexible and rigorous use of spatial convolution regardless of the proximity to physical boundaries and mesh uniformity. Along with GNN, the proposed feature scaling method relies only on the local quantities and can be applied for a range of flow configurations. A data augmentation method is also proposed to consider the rotational invariant. All these techniques are implemented in the present model, and the model is compared with versions of corresponding ML-based models including a typical multilayer perceptron (MLP) for various flow configurations. The results showed that both GNN and MLP models yield reasonable prediction overall. However, GNN shows superior performance near-wall due to spatial convolution. Although the present method implements the rotational invariant discretely, the augmentation method is found to produce consistent performance for any rotated coordinates. The minimal flow configuration, which can train a model to predict a range of flow configurations, is also explored. It is found that a model trained based on turbulent channel flows alone yields a close level of prediction robustness to the ones trained with multiple flow configurations. The developed GNN model is implemented in OpenFOAM, and large eddy simulation (LES) results are compared with corresponding direct numerical simulation data. With these proposed techniques, ML-based SGS models can be improved in terms of robustness and usability for a range of LES applications.

DOI： 10.1063/5.0134471

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DOI： 10.1016/j.proci.2022.07.020

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DOI： 10.1007/s10494-022-00360-8

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Other Link： https://link.springer.com/article/10.1007/s10494-022-00360-8/fulltext.html

Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.combustflame.2021.111660

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DOI： 10.1016/j.combustflame.2021.111696

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DOI： 10.1016/j.combustflame.2021.01.017

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DOI： 10.1016/j.combustflame.2020.10.025

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DOI： 10.1080/13647830.2021.1976422

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DOI： 10.1063/5.0039850

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DOI： 10.1016/j.proci.2020.05.031

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DOI： 10.1177/1468087420912136

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DOI： 10.1299/jtst.2021jtst0010

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DOI： 10.1016/j.fuel.2020.117216

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DOI： 10.1007/s10494-019-00102-3

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DOI： 10.1016/j.ijhydene.2019.12.175

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DOI： 10.1177/1468087418774710

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DOI： 10.1016/j.ijhydene.2019.06.146

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DOI： 10.4271/2019-01-0042

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DOI： 10.4271/2019-01-2345

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DOI： 10.1016/j.fuel.2018.11.021

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DOI： 10.1177/1468087419853432

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DOI： 10.1016/j.ijhydene.2019.03.233

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DOI： 10.1016/j.ijheatfluidflow.2018.05.007

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DOI： 10.1016/j.combustflame.2018.02.019

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DOI： 10.1080/00102202.2017.1399368

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DOI： 10.1016/j.ijhydene.2018.02.140

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DOI： 10.1103/PhysRevFluids.3.084601

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DOI： 10.1016/j.fuproc.2018.03.039

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DOI： 10.1016/j.fuel.2016.10.096

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DOI： 10.1299/jmsesdm.2017.9.a205

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DOI： 10.1016/j.proci.2016.08.033

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DOI： 10.1016/j.ijhydene.2016.04.006

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DOI： 10.1299/jtst.2016jtst0047

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DOI： 10.1080/00102202.2016.1195820

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DOI： 10.1007/s10494-016-9770-1

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DOI： 10.1080/00102202.2016.1195821

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DOI： 10.1007/s10494-016-9756-z

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DOI： 10.1007/s00348-016-2146-4

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DOI： 10.1016/j.ijhydene.2015.08.049

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DOI： 10.1016/j.proci.2014.07.057

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DOI： 10.1016/j.proci.2014.09.007

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DOI： 10.1016/j.proci.2014.07.059

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DOI： 10.1016/j.proci.2014.05.153

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DOI： 10.1016/j.proci.2014.10.003

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DOI： 10.1016/j.proci.2012.08.005

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DOI： 10.1016/j.proci.2012.06.166

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DOI： 10.1080/14685248.2012.720022

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DOI： 10.1615/ICHMT.2012.ProcSevIntSympTurbHeatTransfPal.2680

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DOI： 10.1016/j.proci.2010.05.026

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DOI： 10.1115/AJK2011-02020

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DOI： 10.1080/14685248.2010.542752

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DOI： 10.1016/j.proci.2010.07.034

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DOI： 10.1016/j.proci.2010.09.002

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DOI： 10.1007/978-90-481-9603-6_45

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DOI： 10.1063/1.3665619

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DOI： 10.1299/jtst.5.266

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Three-dimensional direct numerical simulation (DNS) of methane-air turbulent premixed flame with a realistic reduced kinetic mechanism has conducted to investigate effects of turbulence characteristics on the local heat release rate. Numerical condition of the DNS is classified into thin reaction zones of combustion diagram proposed by Peters. Quite low heat release rate regions corresponding to the local extinction are observed. In the vicinities of these regions, flame elements are strongly stretching into two tangential directions. The local extinction mechanism was clarified by introducing life time of reactive species relative to strong coherent fine scale eddies in turbulence.

DOI： 10.1299/jsmeted.2009.13

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Language：Japanese   Publisher：一般社団法人 日本機械学会  

DOI： 10.1299/jsmemecjsm.2009.9.0_179

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Heat Transfer Society of Japan  

DOI： 10.11368/nhts.2009.0.62.0

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Language：Japanese   Publisher：The Heat Transfer Society of Japan  

DOI： 10.11368/nhts.2009.0.32.0

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Language：English   Publishing type：Research paper (international conference proceedings)  

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

To investigate statistics of the clusters of coherent fine scale eddies, direct numerical simulation databases of homogeneous isotropic turbulence up to Re_λ =344.1 have been analyzed. To measure the length scale of the clusters quantitatively, PDF variance, which is based on deviations of the concentration PDF (probability density function) of the coherent fine scale eddy from that of an uniform field, has been introduced. The concentration measure reveals that the mean distance of the coherent fine scale eddies is Taylor micro scale and the size of the cluster of the strong coherent fine scale eddies is about the integral length scale.

DOI： 10.1299/jsmecmd.2008.21.814

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

The effects of turbulence characteristics on local flame structure and the fractal feature of flame front are investigated by direct numerical simulation (DNS) of hydrogen-air turbulent premixed flames propagating in three-dimensional homogeneous isotropic turbulence at high Reynolds number. Flame peninsulas towards unburned mixture are frequently observed and tend to diminish due to the heat loss to surrounding low temperature unburned mixture. Multi-layer structure in large scales is created by flame folding and connections. Even for hydrogen-air premixed flames, local extinctions occur at the edge of the corrugated flame front. The fractal characteristics of the flame front are also investigated and strong correlation between the inner cutoff in Kolmogorov length unit and the ratio of the diameter of coherent fine scale eddy to the laminar flame thickness is shown.

DOI： 10.1299/jsmecmd.2008.21.818

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DOI： 10.1007/BF03026959

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DOI： 10.1115/FEDSM2007-37234

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

A new SGS combustion model is proposed based on hierarchical structure of turbulent premixed flames. The hierarchical structure is clarified by the detailed analysis of direct numerical simulation (DNS) data of three-dimensional hydrogen-air turbulent premixed flame. SGS flame surface area strongly correlates with GS strain rate of turbulent velocity field, which is related to the cluster of coherent fine scale eddy. As the GS strain rate of the turbulent combustion field includes dilatation effects intrinsically, a physical consideration is introduced to give the lower limit of dilatation effects in GS. To represent the relation between GS flame surface area and GS strain rate, a new SGS combustion model is proposed under assumptions of fractal characteristics of flame fronts and the scale separation of turbulence. The results predicted by the proposed model coincide very well with DNS results.

DOI： 10.1299/jsmemecjo.2006.3.0_347

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Ltd  

DOI： 10.1016/j.proci.2004.08.249

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Direct numerical simulations of hydrogen-air turbulent premixed flames propagating in three-dimensional homogeneous isotropic turbulence are conducted to investigate effects of turbulence characteristics on the local flame structures. Calculations are conducted for the three cases : in the wrinkled flamelets, on the boarder of distributed reaction zones and well-stirred reactor regimes and in the well-stirred reactor regimes. The probability density functions of the curvature of flame fronts show the same distribution for all cases when it is normalized by the turbulence characteristics in the unburned side. The heat release rate increases linearly with the increase of the curvature and the increasing rates are nearly identical for all cases.

DOI： 10.1299/jsmeptec.2002.0_21

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Publishing type：Research paper (scientific journal)  

DOI： 10.1016/s1540-7489(02)80249-8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier Ltd  

DOI： 10.1016/S0082-0784(00)80252-0

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Japan Society of Mechanical Engineers  

DOI： 10.1299/kikaib.66.646_1527

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Other Link： https://ndlsearch.ndl.go.jp/books/R000000004-I5978490

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Japan Society of Mechanical Engineers  

DOI： 10.1299/kikaib.61.144

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DOI： 10.1080/00102209408935351

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Language：Japanese   Publisher：東京 : 日本流体力学会  

コレクション : 国立国会図書館デジタルコレクション > 電子書籍・電子雑誌 > 学術機関 > 学協会

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Other Link： https://ndlsearch.ndl.go.jp/books/R000000004-I031535993

Language：Japanese   Publisher：Combustion Society of Japan  

Turbulent combustion modelling is one of most difficult topics in turbulent combustion community. There are many models are proposed so far, and recent development in machine learning (ML) technique shed light onto the application of ML on turbulent combustion modelling. This paper introduces several examples of turbulent combustion modelling utilizing machine learning with varied approaches, in conjunction with the conventional modelling approaches. Several important issues arising when applying developed machine learning model are also mentioned.

DOI： 10.20619/jcombsj.63.203_37

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Other Link： https://ndlsearch.ndl.go.jp/books/R000000004-I031391126

Language：Japanese   Publisher：Combustion Society of Japan  

Change of velocity distributions, flame structures and pressure fluctuations corresponding to combustion oscillation onset in a methane/air turbulent swirling lean premixed combustion were investigated by simultaneous high-speed measurements of stereoscopic particle image velocimetry (SPIV), OH chemiluminescence and pressure fluctuation. Acoustic forcing was applied to make the coupling systems by means of acoustic interference clear. The transition process towards combustion oscillation was defined based on the root-mean-square (rms) values of pressure fluctuation p'_rms. Experimental conditions on flow rate, equivalence ratio, length of the combustion chamber and a degree of the swirler vane were set so as to be feasible to occur the transition process. Phase-based analyses of physical properties' fluctuation show that the transition process in all cases holds Rayleigh criteria. In the early stages of the transition process, dynamic instabilities were observed in sheer layers. Involved unburnt regions by vorticity growth were found in the latter half stage. Dynamic mode decomposition (DMD) for the velocity distributions around the beginning of the transition process revealed high-frequency asymmetry features in inner recirculation zones and low-frequency asymmetry features in outer recirculation zones.

DOI： 10.20619/jcombsj.2102

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Other Link： https://ndlsearch.ndl.go.jp/books/R000000004-I031946025

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Moderate or Intense Low-oxygen dilution (MILD) combustion was studied by utilizing data driven techniques. It was found that MILD combustion yields relatively small scalar gradients whereas intense reaction zones were predominantly observed. This could be caused due to local ignition and flame interactions in the combustion field. Such zones are identified in direct numerical simulation (DNS) field with appropriate thresholding of scalar gradient and reaction rate. Then, a data driven model for predicting local combustion modes was proposed and validated for large eddy simulation (LES) context, using filtered DNS data at various filter sizes.

DOI： 10.1299/jsmeted.2020.0049

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Heat transfer inside an engine has been studied for improving thermal efficiency by mitigation of the cooling loss. Flow near a piston top and heat flux on it were simultaneously measured with a microPIV and a MEMS thin film sensor in a motoring mode of an optical engine. It was confirmed that averaged heat flux over -15~15 CA deg. showed a positive correlation to the normal flow fluctuation, and that the instantaneous wall heat flux showed obvious correlation to the instantaneous normal velocity at 0.5 mm above the wall with a time lag of 200 s level. Moreover, a combination analysis of the wall parallel velocity and FFT spectrum of the heat flux showed that the fluctuation in the heat flux is composed of temperature unevenness of several millimeter scale and eddy motion of several hundred micrometer.

DOI： 10.1299/jsmeted.2020.0192

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

In order to understand the effect of velocity boundary layer on the heat loss in a tumble enhanced engine, simultaneous measurements of high-speed microscopic particle image velocimetry (PIV) near the piston top and a micro electro mechanical systems (MEMS) heat flux sensor on the piston surface was conducted in the motored optical engine with enhanced tumble flow. This engine was rotated at2000 rpm. Experimental results show the correlation between the velocity fluctuation and the peak value of heat flux, and the correlation coefficient shows higher value at 0.50 mm from the piston wall rather than 0.27 mm and 0.12 mm.

DOI： 10.1299/jsmeted.2020.0071

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

This paper describes the ignition and early flame propagation in a high-tumble SI IC engine under super lean-burn condition (excess air ratio (λ) of 2.0). To elucidate the detail process, a high-tumble optically-accessible SI IC engine operating at super lean and stoichiometric conditions were diagnosed by time-resolved OH-PLIF measurements. The engine was operated 2000 rpm with a port fuel injection of isooctane. The cross-sectional imaging of OH distribution was achieved at 12 kHz resulting in a temporal resolution of 1° crank angle at 2000 rpm. The results show that the OH radical is generated without any heat release by the spark discharge during approximately 15 ° crank angle after ignition timing in the super-lean condition. The OH distribution of these crank angles are scatted into small islandlike areas, and then subsequent flame propagation occurs.

DOI： 10.1299/jsmeted.2020.0185

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Language：Japanese   Publisher：Combustion Society of Japan  

DOI： 10.20619/jcombsj.62.200_79

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Other Link： https://ndlsearch.ndl.go.jp/books/R000000004-I030481181

Language：Japanese   Publisher：Combustion Society of Japan  

Planar laser induced fluorescence (PLIF) is useful tool to investigate flame structure, and a lot of attempts have been made to clarify characteristics of turbulent flames by introducing high repetition rate, multi-species or three dimensional measurements. In this manuscript, evaluations of flame displacement speed by using high speed PLIF, flame front curvature by using multi plane PLIF and chemical species distributions in thin reaction zones by using OH and formaldehyde PLIF are introduced. Experimental techniques have been assessed by using data of direct numerical simulation of turbulent premixed flames. Also a part of recent progresses in the techniques of PLIF for above topics are briefly introduced and discussed.

DOI： 10.20619/jcombsj.62.200_143

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Other Link： https://ndlsearch.ndl.go.jp/books/R000000004-I030481381

Language：Japanese   Publisher：東京 : 日本伝熱学会  

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Other Link： https://ndlsearch.ndl.go.jp/books/R000000004-I030060690

Language：Japanese   Publisher：東京 : 日本計算数理工学会  

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Other Link： https://ndlsearch.ndl.go.jp/books/R000000004-I029627657

Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

To investigate near-wall flame behavior, two-dimensional direct numerical simulations (DNS) of methane/air and hydrogen/air premixed flames between two parallel walls are conducted under several conditions. Using the DNS results, the flame front position nearest to the wall and defined based on the OH gradient or temperature, and the flame displacement speed defined by the temporal evolution of the position are evaluated. The present analysis revealed following unique characteristics. (i) The flame speed takes a local minimum value near the wall. (ii) Under the conditions where the wall temperature equals unburnt gas temperature, this local minimum value corresponds to the laminar flame speed of a corresponding mixture.

DOI： 10.1299/jsmemecj.2019.j05112

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Other Link： https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-17H01247/

Language：Japanese   Publisher：Combustion Society of Japan  

In order to clarify the relationship between flame and turbulent flows in various combustion devices, it is one of the important factors to consider from the viewpoint of the fine scale structures of turbulent flow. In this article, direct measurement of turbulent fine structure by multi-plane stereoscopic particle image velocimetry (PIV) is introduced, and the measurement techniques, such as Stokes number of seeding particles, spatial resolution of PIV, calculation method of velocity gradients, are discussed from the fine scale structure of turbulent flow. Furthermore, the estimation method of turbulence characteristics by using single plane PIV is explained using the case of micro PIV for an optical gasoline engine in recent years.

DOI： 10.20619/jcombsj.61.197_231

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Other Link： https://ndlsearch.ndl.go.jp/books/R000000004-I029942998

Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

To investigate the flame propagation characteristics under a high efficiency spark ignition engine condition, a direct numerical simulation (DNS) of forced ignition of premixed mixtures in a constant volume vessel with a tumble flow has been performed for a lean Toluene Reference Fuel (TRF)-air mixture at a high exhaust gas recirculation (EGR) rate at high pressure. The wall heat flux evolution during flame-wall interaction is discussed based on DNS results.

DOI： 10.1299/jsmeted.2019.0164

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

DOI： 10.1299/jsmeted.2019.0078

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DOI： 10.1299/jsmemag.121.1196_4

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Other Link： https://ndlsearch.ndl.go.jp/books/R000000004-I029122443

Language：Japanese   Publisher：一般社団法人 日本機械学会  

DOI： 10.1299/jsmemag.121.1196_16

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Language：Japanese   Publisher：東京 : 日本流体力学会  

コレクション : 国立国会図書館デジタルコレクション > 電子書籍・電子雑誌 > 学術機関 > 学協会

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Other Link： https://ndlsearch.ndl.go.jp/books/R000000004-I029180116

Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

To investigate the flame propagation characteristics under a high efficiency spark ignition engine condition, a direct numerical simulation (DNS) of forced ignition of premixed mixtures in a constant volume vessel with a tumble flow has been performed for a lean Toluene Reference Fuel (TRF)-air mixture at a high exhaust gas recirculation (EGR) rate at high pressure. Main flame just after the ignition point shows flamelet-like structure, whereas flames that was split by turbulent motion seem to be in extinction process.

DOI： 10.1299/jsmeted.2018.0074

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

To investigate near-wall flame behavior including effect of the surface reaction, two-dimensional direct numerical simulations (DNS) of methane/air, hydrogen/air and n-heptane/air premixed ignition and propagation between two parallel walls are conducted under several thermochemical conditions. From the DNS results, the flame front position defined by the OH gradient, and the flame displacement speed defined by the temporal evolution of the position are evaluated. The present analysis revealed following characteristics. (i) The flame speed takes a local minimum value near the wall. (ii) Under the conditions where the wall temperature equals unburnt gas temperature, this local minimum value corresponds to the laminar flame speed of the initial thermochemical condition. (iii) These characteristics do not depend on the ignition position, equivalence ratio, fuel types and presence/absence of surface reaction.

DOI： 10.1299/jsmeted.2018.0010

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

To determine the ignition criteria and effects of turbulence on the localized ignition delay time, direct numerical simulations (DNS) of forced ignition of premixed mixtures have been performed for Toluene Reference Fuel (TRF)-air mixture at a high exhaust gas recirculation (EGR) rate and low equivalence ratio conditions at high pressure. A high-temperature kernel is imposed as an initial ignition kernel, and one-dimensional DNS have been preliminary performed to determine the ignition criteria of the laminar condition. It is found that ignition criteria of the laminar case are determined in terms of the ignition source energy and the heat conductivity from the ignition kernel regardless of fuel. Subsequently, two-dimensional DNS have been performed with an initial kernel size and temperature which yields successful ignition derived from the one-dimensional laminar cases, to clarify the influence of the turbulent flow on the ignition delay time. The two-dimensional DNS results are investigated to clarify the influence of the local and global strain rates on ignition delay time and the mechanism by which the turbulence influences the ignition kernel establishment. In the case of TRF-air mixture, when energy input is suitable, the relationship between local strain rate and ignition delay time holds regardless of fuel. However, the local strain rate alone dose not explain the deviated ignition delay time.

DOI： 10.1299/jsmemecj.2018.j0520303

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Language：Japanese   Publisher：Combustion Society of Japan  

This article introduces a research on thermoacoustic instability based on direct numerical simulation (DNS) of a hydrogen-air turbulent swirling premixed flame in a cuboid combustor. To investigate thermoacoustic instability and dynamic modes of a turbulent swirling premixed flame, dynamic mode decomposition (DMD) and spectral analysis are applied to results of the DNS. DMD of pressure field shows that the quarter-wave mode in longitudinal acoustics has the largest energy, while DMD of heat release rate field reveals that the quarter-wave mode in longitudinal acoustics is not a dominant mode in heat release rate field. It has found that oscillation modes caused by large scale vortical motions have large energy not only in pressure field but also in heat release rate field. Dynamic modes of the source term of classical acoustic energy equation are also discussed with the vortical motion and the resonance mode of the combustor.

DOI： 10.20619/jcombsj.59.187_16

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Language：Japanese   Publisher：Society of Automotive Engineers of Japan  

In this study, in-cylinder flow and turbulence characteristic induced by intake switching device, TCV (Tumble Control Valve) is investigated on a single cylinder optical engine with a transparent sleeve using high speed PIV. As a result of analysis, turbulent kinetic energy at the end of compression stroke increases by TCV. However it is clarified that an optimization of TCV is necessary because there is some cases which turbulence does not increase by some TCV specification. Since combustion duration on a metal multi cylinder engine decreases by optimal TCV specification, it is confirmed that TCV is an effective devise for enhancement of in-cylinder flow.

DOI： 10.11351/jsaeronbun.48.65

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Other Link： https://ndlsearch.ndl.go.jp/books/R000000004-I027922549

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Due to the recent advancement of computational performance, large eddy simulation (LES) gains more and more attention for design and optimization of high efficiency and low emission combustion devices. To develop a turbulent combustion model for LES under a pressure rising condition such as IC engines, a fractal dynamic subgrid scale (FDSGS) combustion model is evaluated for turbulent combustion in a constant volume vessel using a filtered direct numerical simulation (DNS) result of hydrogen-air turbulent premixed flame. Flame surface area predicted with the FDSGS combustion model and conventional SGS models is computed, and then compared to flame surface area obtained from DNS data. The accuracy of the model is statistically evaluated. The results show that the FDSGS combustion model shows suitability to the combustion under a pressure rising condition, as well as constant pressure conditions. Furthermore, polynomial functions for δ_L, div(u)_L and δ_Δ/δ_L for the hydrogen-air premixed flame are proposed to simplify the computation of these quantities during LES, which are to be used in conjunction with the FDSGS combustion model. The predicted values agree reasonably well with the actual values. The results show that the functions together with the FDSGS combustion model can be applied to LES under pressure rising conditions.

DOI： 10.1299/jsmemecj.2017.j0520301

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The mechanism of flame-flame interaction is investigated using a one-dimensional direct numerical simulation of premixed methane-air flames propagating towards one another. At the beginning of the interaction process, the preheat zone structure is modified by the other incoming flame. However, this effect is not significant for inner-layer of the flame. In the final stage of the interaction, the reaction rate rapidly increases to 1.3 times of that of the corresponding non-interacting laminar flame while the value of scalar dissipation rate decreases. Also, the inner layer is significantly modified by this process. The higher reaction rate is produced due to the radical pool rapidly created by the diffusion and chemical reactions. This enhanced pool is compared with a well-stirred reactor result, and radicals and intermediate species show that the combustion mode transients to autoignition-like during the interaction.

DOI： 10.1299/jsmekanto.2016.22._gs0604-1_

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Direct numerical simulations (DNS) of ignition of an ultra-lean n-heptane/air mixture with high EGR at high pressure have been performed using a reduced kinetic mechanism to investigate the effect of the strain rate of turbulence on ignition criterion and its mechanism. An initial high-temperature region is set for ignition at the center of the computational domain. From one-dimensional DNS results, it is suggested that ignition is supported by the cool flame. Two-dimensional DNS results show that the ignition delay time becomes longer in proportion to the square of strain rate averaged in the high temperature region in the induction period.

DOI： 10.1299/jsmeted.2016.e231

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To investigate the characteristics of flame dynamics due to combustion oscillation, high-speed OH planar laser induced fluorescence (PLIF) measurements of methane-air turbulent premixed flame have been performed in a laboratory scale swirl combustor. The measurements are conducted under conditions where the total flow rate is 250 L/min, and the equivalence ratios are 0.70, 0.81, 0.90. Pressure fluctuation and CH chemiluminescence are simultaneously measured during the OH-PLIF. The dynamic mode decomposition (DMD) is applied to the series of OH-PLIF images. The mode corresponding to the dominant frequency of the pressure fluctuation is found to be stripe shaped in the upstream, while it shows a relatively large structure.

DOI： 10.1299/jsmeted.2016.e225

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Other Link： https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-16H06069/

Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

A subgrid scale combustion model for large eddy simulation (LES) of turbulent premixed combustion is developed. The model describes the flame surface area based on fractal characteristics of flame surfaces, and the inner cutoff is predicted by a correlating function with the Kolmogorov length scale. An expression is introduced to accurately estimate the Kolmogorov length scale. A priori and a posteriori tests with direct numerical simulation data of a turbulent jet premixed flame reveal that the present model accurately predicts the Kolmogorov length scale and the flame surface area and the LES using the model adequately predicts mean temperature distributions.

DOI： 10.1299/jsmeted.2016.e233

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Other Link： https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-16K18025/

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Other Link： https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-23226005/

Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

DOI： 10.1299/jsmemecj.2016.f051010

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Three-dimensional direct numerical simulation of a hydrogen-air turbulent premixed flame with decaying homogenous isotropic turbulence is conducted to investigate turbulence-flame interactions in a constant volume vessel. The detailed kinetic mechanism including 12 reacting species and 27 elementary reactions is used for the hydrogen-air reaction. Turbulence-flame interactions modeling is studied using a conventional flamelet model for reaction rate closure. The result shows that turbulence-flame interaction mechanism does not change significantly under pressure rising conditions, suggesting that this model could be also used in a pressure-evolving combustion system without further modifications.

DOI： 10.1299/jsmeted.2016.e224

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Language：English   Publisher：SAE International  

DOI： 10.4271/2015-01-1909

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DOI： 10.4271/2015-01-1910

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Direct numerical simulations of Taylor-Couette flow from Re = 8000 to 25 000 have been performed to understand the spectral characteristics of turbulence in the transition of the Reynolds number dependency of the mean torque near Re = 10 000. The velocity fluctuations are decomposed into the wavenumber components, and their contributions in the radial profiles of the Reynolds stress and the transmission of the mean torque are evaluated. In the net mean torque, the contribution of the azimuthally averaged component overtakes that of the remaining turbulent counterparts around Re = 15 000. Therefore, the torque transition can be explained by the competition between the contributions of azimuthally averaged Taylor vortex and the remaining turbulent fluctuations.

DOI： 10.1299/jsmefed.2015._1514-1_

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Language：Japanese   Publisher：Society of Automotive Engineers of Japan  

In this study, in-cylinder flow for a boosted GDI engine is investigated on an optical engine with a transparent sleeve using high speed PIV. A result of analysis of entire flow, a generated tumble flow in intake stroke is kept until the end of compression stroke. For investigation of turbulence, adequate time resolution is clarified and it is confirmed that cut-off frequency for a time filtering method is almost corresponded with a large time scale of flow, generated by piston motion. According to analysis, it is found that optimization of piston top shape is effective for enhancement of in-cylinder flow.

DOI： 10.11351/jsaeronbun.46.27

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Other Link： https://ndlsearch.ndl.go.jp/books/R000000004-I026070471

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

To investigate propagation characteristics of methane-air premixed flames at high pressure, constant volume vessel which can control turbulence characteristics, initial pressure and equivalence ratio has been developed, and high repletion rate OH planar laser induced fluorescence (PLIF) and CH chemiluminescence measurements are performed in the combustion field. The flame front and radius of flame are evaluated from OH PLIF and CH chemiluminescence independently. When radius of flame is large, flame speed becomes constant in laminar flame but increases in turbulent flame. Flame speed which is evaluated from OH PLIF images fluctuates significantly because of local turbulence characteristics and flame structure.

DOI： 10.1299/jsmeted.2015._f133-1_

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Three-dimensional direct numerical simulation of methane-air turbulent premixed planar flame propagating in homogenous isotropic turbulence is conducted to investigate local flame structure in thin reaction zones. GRI-Mech 3.0 is used to represent methane-air reactions. For a better understanding of the local flame structure in thin reaction zones, distributions of mass fractions of major species, heat release rate and temperature are investigated. The results show that in thin reaction zones, pixel by pixel product of CH_2O-OH or CH_2O-H images have good correlation with heat release rate and is more accurate to discern local flame structures than single OH images. The correlation between heat release rate, principal curvature and tangential strain rate are investigated.

DOI： 10.1299/jsmeted.2015._f144-1_

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Direct numerical simulation is employed to investigate the turbulent flow characteristics and their effect on local flames for mean reaction rate modelling of turbulent swirling premixed flames. Two swirl number conditions having significant effects on the formation of a central recirculation zone in the combustor are considered. The mean turbulent and heat release rate fields suggest that the heat and radicals from the hot products trapped in the recirculation zones are mixed with the reactants, not only through scalar dissipation rate (i.e.scalar gradient) but also by small-scale processes of turbulence relevant to turbulent kinetic energy dissipation rate. Therefore, both scalar and velocity gradients have a strong influence on the chemical reactions through mixing of cold reactant and hot products. A conventional flamelet and non-flamelet models are used to estimate the mean reaction rate, and to study the balance between these two mixing mechanisms. Although both models show a qualitative agreement with the DNS results, these models compensate their limitations each other, depending on the local turbulence and thermochemical conditions. A simple approach is proposed to exploit the advantages of these two models by considering the balance of two mixing mechanisms based on the chemical and turbulence time scales.

DOI： 10.1299/jsmecmd.2015.28._244-1_

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It is known that Taylor-Couette flow shows a transition in the Reynolds number dependency of the mean torque near Re 〜 10000. To investigate the turbulent vortical structures in the transition state, direct numerical simulations of Taylor-Couette flow with fixed outer cylinder have been conducted in the different Reynolds numbers from Re = 8000 to 25000. As the Reynolds number increases, the fine scale eddies are more populated in a large part of the domain and they tend to take more various angles. The joint probability functions of the tilting angles of eddies with respect to the tangential direction and their radial positions exhibit preferential distributions, which is due to the shear layer between the large scale Taylor vortex motions. The present results suggest that the distribution and the orientation of the fine scale eddies in this Reynolds number range are greatly affected by the Taylor vortex structure.

DOI： 10.1299/jsmemecj.2015._j0550301-

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Simultaneous CH-OH planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (SPIV) measurements at 10 kHz have been conducted in a methane-air turbulent jet premixed flame. Around the flame tip in the high Reynolds number condition, the fine scale unburned mixture islands are ejected downstream. The enhancement of the turbulent burning velocity is statistically analyzed by the local consumption rate, which is estimated from the OH and CH PLIF images with the assumption of the spherical and pillar shapes of the unburned mixture. The most probable consumption rates with different methods give significantly higher burning velocity than the laminar.

DOI： 10.1299/jsmefed.2015._1027-1_

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To investigate local flame structures of turbulent swirling premixed flame, direct numerical simulation (DNS) of hydrogen-air swirling premixed flame with swirl numbers of S = 0.6 and 1.2 in a cuboid combustor is conducted by considering a detailed kinetic mechanism. Heat release rate of turbulent swirling premixed flame is lower than that of laminar flame in the same inflow condition. For S = 0.6, the heat release rate distribution conditioned by reaction progress variable, which is defined by temperature in this study, has a peak at the reaction progress variable where laminar flame with same premixed gas takes the maximum heat release rate. However, for S = 1.2, the conditioned heat release rate distribution is much lower than the laminar condition in the region of low reaction progress variable, and that takes the maximum in high reaction progress variable region. Analysis on contribution of elementary reactions on heat release rate reveals that elementary reactions which contribute to heat release rate in low temperature region are disturbed because of low mass fraction of H for the case of S = 1.2. Furthermore, relations between tangential strain rate at flame front and heat release rate are investigated to understand local flame structures in detail.

DOI： 10.1299/jsmemecj.2014._j0570301-

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Language：English   Publisher：The Japan Society of Mechanical Engineers  

Three-dimensional direct numerical simulation of methane-air turbulent premixed planar flame propagating in homogenous isotropic turbulence is conducted to investigate local flame structure in thin reaction zones. GRI-Mech 3.0 is used to represent methane-air reactions. For a better understanding of the local flame structure in thin reaction zones, distributions of mass fractions of major species, heat release rate and temperature are investigated. To clarify effects of turbulence on the local flame structure, the statistical characteristics of flame elements are also revealed. In the thin reaction zones, the mean flame thickness increases about 3% that of laminar flame, although most of the flame elements are stretched in the tangential direction. An asymmetrical dependence of flame thickness on flame front curvature is observed.

DOI： 10.1299/jsmeted.2014._i124-1_

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Other Link： https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-24760160/

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

To investigate fractal characteristics of turbulent premixed flames with mean shear, direct numerical simulations of hydrogen-air turbulent jet and V-shape premixed flames are conducted. From fractal analysis with box-counting methods, it is revealed that fractal dimensions of flame surfaces of jet and V-shape flames increase in streamwise direction. The fractal dimensions calculated with a three dimensional box-counting method reach about 2.52-2.57 in downstream region where turbulence is fully developed by mean shear. An expression proposed by Shim et al. gives good predictions for inner cutoffs of turbulent premixed flames with mean shear and in a high pressure condition when flame surfaces are sufficiently distorted by turbulence.

DOI： 10.1299/jsmeted.2014._i123-1_

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Direct numerical simulation (DNS) has been carried out to clarify the characteristics of turbulent statistics of Taylor-Couette flow in high Reynolds number around Re ≈ 10000. Reynolds number, Re, which is defined by the differences of inner-wall and outer-wall and rotating speed of inner cylinder, is set in the range from 8000 to 20000, where the turbulent characteristics are expected to change referring to Wendt's empirical formula. In this study, turbulent statistics are obtained by averaging in azimuthal direction, axial direction and time in fully developed state, and their characteristics are discussed with budgets of the transport equation for each Reynolds stress component. As a result, the intensity of diagonal component of Reynolds stress do not show monotonic increase in wall units when Reynolds number rise. The reason of large Reynolds stress in axial direction at Re = 20000 is also discussed using contributions of each term in the transport equation.

DOI： 10.1299/jsmekanto.2014.20._20306-1_

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To investigate thermoacoustic instability and dynamic modes of turbulent swirling premixed flame, dynamic mode decomposition (DMD) is applied to results of direct numerical simulation (DNS) of hydrogen-air turbulent swirling premixed flame in a cuboid combustor. DMD of pressure field reveals that the quarter-wave mode in longitudinal acoustics has the largest energy and that there is differences between dominant frequencies in spectral analysis of time-series pressure data at one point on a wall and DMD of time-series pressure field data. DMD of heat release rate field reveals that the quarter-wave mode in longitudinal acoustics is not a dominant mode in heat release rate field and oscillation mode caused by large scale vortical motions has large energy not only in pressure field but also in heat release rate field. Interactions between dynamic modes of pressure and heat release rate are also discussed.

DOI： 10.1299/jsmekanto.2014.20._10301-1_

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Language：Japanese   Publisher：東京 : 日本ガスタービン学会  

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Language：Japanese   Publisher：東京 : 日本伝熱学会  

コレクション : 国立国会図書館デジタルコレクション > 電子書籍・電子雑誌 > 学術機関 > 学協会

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Direct numerical simulations of autoignition have been conducted to clarify effects of length scale in flow and temperature field and initial temperature fluctuations on turbulent combustion mechanisms in homogeneous charge compression ignition engines. Methane-air mixtures at high pressure with spatial inhomogeneity of temperature are investigated by considering a detailed kinetic mechanism. For the case with lower temperature fluctuations, local autoignition starts later at smaller length scale due to larger effects of turbulent and molecular diffusion before the autoignition, while effects of length scale is relatively weak for high temperature fluctuations. Lagrangian fluid particle tracking method is conducted to identify combustion regimes under the different length-scale conditions.

DOI： 10.1299/jsmeted.2013.309

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DOI： 10.1299/jsmecmd.2013.26._208-1_

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Peta-Scale Direct Numerical Simulation (DNS) of wall turbulence are conducted by taking advantage of "K Computer". In the previous DNS of turbulent channel flows, Reynolds number based on the friction velocity and the half height of channel, Re_τ, has reached about 2500 owing to developments of supercomputers and numerical algorithms, while bulk Reynolds number, Re_b, is at least hundred thousand, which is corresponds to Re_τ ? 5000, in real thermal and fluid machines in industrial application. In this study, DNS of turbulent channel flows are carried out at high Reynolds number up to Re_τ = 5000 to reveal dynamic interactions between large- and small-scale structures and construct DNS database as the "standard solution" to verify turbulent model calculation in industrial utilization.

DOI： 10.1299/jsmefed.2013._0507-01_

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DOI： 10.1299/kikaib.79.2193

DOI： 10.1299/transjsme.18-00051_references_DOI_7JpMa76wpat4o4vGixVfUp0jwgC

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Direct numerical simulation (DNS) of methane-air turbulent jet premixed flame has been conducted to clarify characteristics of local burning velocity. A detail kinetic mechanism including 49 reactive species and 279 elementary reactions is used to represent the methane-air reaction. In this study, we investigate effects of definition of flame front and fluid velocity on local burning velocity. The calculated displacement speed at flame front is strongly affected by the definition of fluid velocity. It is difficult to describe dynamic characteristics of turbulent premixed flame in strong turbulence by the flamelet concept.

DOI： 10.1299/jsmeted.2013.305

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DOI： 10.1299/jsmecmd.2013.26._206-1_

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Direct numerical simulation of methane-air turbulent plane jet premixed flame have been conducted by considering detailed kinetic mechanism and temperature dependence of thermal and transport properties. In this study, we focus on relation between local flame structures and production mechanisms of NO, especially prompt NO, in turbulent shear flow. In the turbulent plane jet premixed flame, unburnt mixture islands are frequently discharged from the main flame. It is clarified that generation of unburnt mixture islands locally enhances production of prompt NO near the disconnection region of the flame surface. The production mechanisms of prompt NO are investigated in terms of reaction rate of elementary reactions and concentrations of major species. In the generation process of unburnt mixture islands, the state of almost incomplete combustion is formed and it results in increase of CO concentration. As a result, production of H and NNH is promoted. Consequently, prompt NO is increased by the elementary reaction related with H and NNH during the disconnection process of the flame surface.

DOI： 10.1299/jsmemecj.2013._j057024-1

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In this study, a turbulent jet flame is studied using simultaneous high speed laser diagnostics. Information from CH and OH PLIF, as well as stereoscopic PIV was obtained on a single plane at a frequensy of 10 kHz. Analysis of the OH PLIF and velocity data using Extended POD, as well as OH LIF data using DMD, showed that in this particular flame the average of the signal is by far the main component of the signal, with no outstanding strong feature. Such diagnostics and post-processing methods open new perspectives on the experimental study of flow-flame interactions and flame dynamics.

DOI： 10.1299/jsmeted.2012.359

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Simultaneous multi-plane measurement of planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (PIV) to investigate three-dimensional structures of turbulent premixed flames is introduced in this manuscript. Wavelength-based dualplane stereoscopic PIV, which can provide nine components of velocity gradients, was developed and combined with dual-plane CH PLIF and single-plane OH PLIF. The simultaneous measurement was applied to methane-air turbulent premixed flames in the swirl-stabilized combustor. three-dimensional flame structures including the spire structure which has been shown by the previous direct numerical simulation (DNS) of turbulent premixed flame was clarified. Magnitude of strain rate, which is evaluated from nine velocity gradients, shows high value in unburnt region except for the region engulfed by burnt gas. Moreover, quad-plane PLIF which has been developed to investigate threedimensional flame structure in more detail is introduced in this manuscript.

DOI： 10.1299/jsmefed.2012.613

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dynamic SGS combustion model has been developed based on fractal features of flame surface of turbulent premixed combustion and the scale separation of high Reynolds number turbulence. The model dynamically determines the local fractal dimension of the flame surface and calculates the local flame surface area. The local turbulent burning velocity is then obtained with the surface area assuming the flamelet theory. The model has been evaluated through a series of static tests on filtered DNS data of hydrogen-air premixed flames that propagate in homogeneous isotropic turbulence. The results demonstrate the accuracy of the present model under the corrugtated flamelet regime and close to the boundary between the corrugated flamelet and the thin reaction zones regimes, i.e. the model is fully applicable to the conditions in the corrugated flamelet regime. Furthermore the test results are compared with those obtained using several conventional SGS combustion models. Considerable improvement of accuracy is observed compared with the conventional models.

DOI： 10.1299/jsmemecj.2012._j054011-1

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Language：Japanese   Publisher：東京 : 日本流体力学会  

コレクション : 国立国会図書館デジタルコレクション > 電子書籍・電子雑誌 > 学術機関 > 学協会

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Three-dimensional direct numerical simulation of hydrogen-air turbulent premixed flame in a larger constant volume vessel at relatively high Reynolds number has been conducted considering the detailed kinetic mechanism to clarify effects of pressure increase on dynamic characteristics of turbulent premixes flame. The DNS results in the larger volume vessel of this study and in a smaller volume vessel of our previous study have been compared to reveal effects of the vessel's size. The mean pressure in both the vessel reaches about 3 atm at the end of rapid combustion. Heat release rate of each flame element is augmented since the pressure increase makes flame thickness thin. The local pressure rise due to dilatation also enhances turbulence and finer scale vortices appear, which makes flame surface more complicated and results in increase of the flame surface area. The shape of the flame front becomes complicated and the meximum value becomes higher due to increase in pressure. By identifying flame elements in turbulence, the statistical characteristics are also discussed quantitatively.

DOI： 10.1299/jsmeted.2012.363

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Language：Japanese   Publisher：Combustion Society of Japan  

With the increase of resources of supercomputer system, handing and visualization of huge DNS data of turbulent combustion become more important. In this paper, current state in the data handing is discussed based on the state of art DNS. Even though the speed of internet is improved, data transfer time of DNS data from the remote supercomputer is still enormous, which prevents active researches. The effective utilization of scientific visualization is also prevented by the data transfer time. To resolve this issue, cloud-type data analysis center will be required to use expensive huge DNS database effectively. As for the visualization of DNS data, several defects in visualization without mature consideration are described especially for contour surfaces and volume rendering. The inappropriate applications of these techniques result in misreading of physics. The developments in acquisition system and laser cause similar problems in experimental researches. The practical realizations of 3D technique and virtual reality may contribute further developments in turbulent combustion researches though the scientific visualization.

DOI： 10.20619/jcombsj.54.169_158

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Direct Numerical Simulation (DNS) of hydrogen-air turbulent premixed planar flames in homogeneous isotropic turbulence has been conducted to investigate local flame structures in thin reaction zones. The detail kinetic mechanism including 12 reactive species and 27 elementary reactions is used to represent the hydrogen-air reaction. DNS is conducted at Re_λ= 37.4 and 60.8, where Re_λ denotes Reynolds number based on Taylor micro-scale (λ). In the thin reaction zones, heat release rate is highly fluctuating in low temperature region. The high heat release rate regions are surrounded by the low heat release regions and are shown like pocket and reed-shape structures. In the unburned side, the radical fingering of HO_2 is also generated, which suggests that the strong coherent-fine scale eddies in the unburnedside transport reactive species in the preheat zone into the unburned side for the case of thin reaction zones.

DOI： 10.1299/jsmemecj.2012._j054012-1

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Other Link： https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-24760160/

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Other Link： https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-24760160/

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Other Link： https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-24760160/

Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

High repetition rate simultaneous planar laser induced fluorescence (PLIF) of CH and OH radicals and stereoscopic particle imaging velocimetry (PIV), both at 10 kHz, was developed. The high repetition simultaneous system is applied to methane-air turbulent jet premixed flame to show the performance of developed system. The obtained simultaneous CH and OH PLIF images well represent flame dynamics in turbulent jet premixed flame. Simultaneously obtained three-component velocity vectors also represent relation between fluid velocity and flame dynamics. From temporal analyses, the injection frequency of the large-scale unburnt mixture island is clarified. The creation of small scale unburnt mixture islands from the large scale ones, which may contribute the enhancement of turbulent burning velocity, is also presented. For general understanding of flame structure, Proper Orthogonal Decomposition (POD) analysis has been applied to the OH PLIF images.

DOI： 10.1299/jsmemecj.2012._f051002-1

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Language：Japanese   Publisher：Gas Turbine Society of Japan  

To develop the theoretical strategy for controls of turbulent combustion, sound generation mechanism in turbulent reactive flows has been investigated from direct numerical simulation (DNS). The turbulent combustion controls based on local flame structure were demonstrated on the swirl-stabilized combustor. The controls of a combustor were conducted by adding secondary fuel injection. The combustion noise can be reduced about 1OdB by adding continuous 1% secondary fuel injection and about 5dB more by controlling frequency of a secondary fuel injection. The relation between local flame structure and pressure fluctuation was discussed by using the simultaneous measurement of CH-OH PLIF and pressure. The combustion-induced oscillation is well correlated with the beat frequency observed in pressure, and the global characteristics of flame fronts are also correlated with the beat frequency. By adding this beat frequency to the secondary fuel injection, the maximum noise reduction can be achieved. The mechanism of noise reduction by a secondary fuel injection was also clarified by combined laser diagnostics and DNS.

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

A dynamic model of the subgrid scale wrinkling factor for large eddy simulation (LES) of turbulent combustion is developed. The model is based on that proposed by Charlette et al. in the thickened flame model context. This model is modified to be capable of treating weak turbulent cases. Two and three-dimensional LES of V-flames are performed with this modified dynamic model. The results of the two-dimensional simulations are compared to those obtained from two-dimensional simulations without the dynamic formulation and those of the three-dimensional simulation to an experiment conducted with the combustion chamber of the identical geometry. These comparisons demonstrate the availability of the dynamic formulation and that the formulation is especially useful when the appropriate values for the model constant to calculate the subgrid scale wrinkling factor are not empirically available.

DOI： 10.1299/jsmekanto.2011.17.457

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Direct numerical simulation (DNS) of hydrogen-air premixed flame with swirling flow is conducted to investigate dynamic characteristics of turbulent and flame structures in a micro-scale combustor. Size of the combustor is assumed to be 15.0 mm in streamwise direction and 10.0 mm X 10.0 mm cross-section. DNS is carried out under a swirl number: S = 0.6. Large-scale helical vortical structures are generated near the inlet of combustion chamber, and a lot of fine-scale vortices appear downstream. Interactions between dynamics of large-scale vertical structures and pressure fluctuations in the combustor are focused on. The large-scale coherent structures generated near the inlet grow, pass through regions of high heat release rate and disappear. The period of this transition corresponds to the period of the pressure oscillation on side wall of the combustor. Fluctuations of heat release rate induced by progress of the large-scale vortical structures influence pressure fluctuations. These results suggest that controlling the occurrence of the large-scale eddies is important for suppression of pressure fluctuations.

DOI： 10.1299/jsmemecj.2011._j056045-1

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Language：Japanese   Publisher：The Heat Transfer Society of Japan  

DOI： 10.11368/nhts.2011.0.189.0

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Direct numerical simulations of autoignition have been conducted to clarify turbulent combustion mechanisms in homogeneous charge compression ignition engines. Methane-air mixtures at high pressure, 51.3 atm, with spatial inhomogeneity of temperature are investigated by considering a detailed kinetic mechanism. For the case with smaller temperature fluctuations, autoignition occurs in almost whole domain. On the other hand, for the case with larger temperature fluctuations, autoignition occurs at high temperature region and flame propagates from the ignition kernel. The early autoignition and the flame propagation with increasing pressure inhibit the drastic rise of the heat release rate and shorten the ignition delay time due to the increased pressure.

DOI： 10.1299/jsmeted.2011.137

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Language：Japanese   Publisher：The Heat Transfer Society of Japan  

DOI： 10.11368/nhts.2011.0.47.0

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Direct numerical simulations of chemically reacting turbulent mixing layer have been conducted to investigate effects of heat release on turbulent structures and fractal characteristics of turbulent scalar mixing. Fine scale eddies in roller structure and braid region are suppressed by dilatation induced by heat release. As the turbulent transition occurs, the fractal dimension increases linearly and reaches around 2.4. The development of the fractal dimension delays as a result of slower development of chemically reacting turbulence with heat release.

DOI： 10.1299/jsmeted.2011.71

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Direct numerical simulation (DNS) of a hydrogen-air turbulent premixed flame propagating in homogeneous isotropic turbulence was conducted to investigate the local flame structure and fractal characteristics in high Reynolds number turbulence. A detail kinetic mechanism including 12 reactive species and 27 elementary reactions was used to represent the hydrogen-air reaction. In high Reynolds number turbulence, very complicated flame structures induced by strong coherent fine scale eddy were generated and strong correlation between the inner cutoff and the ratio of diameter of coherent fine scale eddy to laminar flame thickness was observed.

DOI： 10.1299/jsmeted.2011.113

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Language：Japanese   Publisher：The Heat Transfer Society of Japan  

DOI： 10.11368/nhts.2011.0.46.0

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Three-dimensional direct numerical simulations of turbulent plane jet premixed flames with different turbulent intensity and different shear caused by difference of inflow velocity between unburnt and burnt gas have been conducted to investigate shear effects on flame structures of turbulent premixed flames. Detailed kinetic mechanism including 12 reactive species and 27 elementary reactions is considered to represent reactions of hydrogen-air mixture. For weak shear flow case, eddy structure is produced by velocity difference in burnt gases. This difference is caused by inflow burnt gas and produced burnt gas and forms shear flow along the edge of generated OH radical. For strong shear flow case, Kelvin-Helmholtz rollers are induced by the large velocity difference between unburnt and burnt gases, and the rib structures are formed in the braid region. The vortices generated by shear flow distort flame structure in large-scale and the fine scale eddies which are imposed on inflow wrinkle flame front.

DOI： 10.1299/jsmemecj.2011._j056044-1

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Language：English   Publisher：The Visualization Society of Japan  

Detailed understanding of turbulent combustion phenomena is very important to develop high efficiency combustors which contribute to resolve global environmental issues. Since turbulent combustion involves strong interactions between turbulence and combustion reactions, highly-sophisticated measurement techniques both for fluid motion and flames are required. For fluid velocity, state-in-art dual-plane stereoscopic particle image velocimetry (PIV) is shown and its importance in turbulent combustion researches is discussed. For flame measurements, advanced planar laser induced fluorescence (PLIF) techniques such as simultaneous CH-OH PLIF, double-pulsed CH PLIF and simultaneous dual-plane CH/single-plane OH PLIF are presented and investigations on local flame structure, global/local flame dynamics and three-dimensional flame structures are surveyed. Finally, current state and challenges of multi-dimensional/multi-variables laser diagnostics which consist of the advanced PLIF and color-based dual-plane stereoscopic PIV are presented.

DOI： 10.3154/jvs.31.15

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Other Link： https://ndlsearch.ndl.go.jp/books/R000000004-I10928301

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Direct numerical simulations (DNSs) of temporally developing mixing layer with non-reactive and reactive scalar transport up to Re_<ω,0>=900 and Sc=30.0 have been conducted to investigate fractal geometry of scalar surfaces in turbulence. Effects of Reynolds number (Re), Schmidt number (Sc) and chemical reaction on scalar mixing are discussed by applying fractal analysis for scalar surfaces. For higher Sc, two fractal dimensions D_1 and D_2 can be defined, while only one fractal dimension D can be defined for lower Sc. The fractal dimension D_1 corresponds to relatively large scales. D_1 gives close agreement with D of lower Sc case in the inertial subrange and is around 2.5. The fractal dimension D_2 corresponds to small scales and shows larger values. D_2 depends on Schmidt number and rises to about 2.8 for Sc=30.0. As for the case with chemical reaction, all fractal dimensions show lower values. In all cases, D, D_1 and D_2 are independent of Reynolds number.

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To investigate relations between hierarchical structure of turbulence and GS-SGS energy transfer, direct numerical simulation databases of homogeneous isotropic turbulence for Re_λ=175.4 and Re_λ=222.7 have been analyzed. The energy transfer from GS to SGS shows high value where strain rate in GS is high and the eddies in SGS exist. SGS models of Smagorinsky model (SM), dynamic Smagorinsky model (DSM) and coherent structure Smagorinsky model (CSM) are evaluated in terms of GS-SGS energy transfer. Energy transfer by DSM does not show good correlation with that by filtered DNS data even in the inertial subrange. GS-SGS energy transfer described by SM and CSM shows strong correlation with DNS results in the inertial subrange. However SM tends to overestimate spatial average of GS-SGS energy transfer.

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Direct numerical simulation (DNS) of turbulent Taylor-Couette flow at high Reynolds number has been conducted to investigate dynamics of fine vortical structures near inner and outer walls. Reynolds number based on rotating speed of inner cylinder, u_<in>, and gap width, d=r_<out>-r_<in>, is 12000 and the axial length of the computational domain, h, is set to be 5d. In high-Reynolds-numbers Taylor-Couette flow, many fine scale eddies elongated in the azimuthal direction appear in whole flow fields and fine scale eddies parallel to the axial direction are also formed in sweep regions between large scale Taylor vortices. The generation mechanisms of these fine scale vortices, especially parallel to the axial direction, are discussed.

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Language：Japanese   Publisher：東京 : 日本流体力学会  

コレクション : 国立国会図書館デジタルコレクション > 電子書籍・電子雑誌 > 学術機関 > 学協会

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Other Link： https://ndlsearch.ndl.go.jp/books/R000000004-I10707187

Language：Japanese   Publisher：Combustion Society of Japan  

To relate actual turbulent premixed flames observed in many applications with turbulent combustion diagram, experimental method for evaluations of turbulence characteristic length and Reynolds number based on Taylor microscale are discussed in detail. As for the hot wire anemometer, importance of energy spectrum and probability density function of velocity derivatives are shown for the accurate prediction of Taylor microscale. In the case of particle image velocimetry (PIV), spatial resolution should be in the same order of those of direct numerical simulation (DNS) of turbulence (less than 3-4 times the Kolmogorov length scale) to obtain Taylor microscale accurately. Current state and perspective of DNS of turbulence and turbulent combustion are discussed with feature trend of the fastest supercomputer in the world. Based on the perspective of DNS of turbulent combustion, possibility of perfect simulations of IC engine is shown. In 2020, the perfect simulation will be realized with 30 billion grid points by 1EXAFlops supercomputer, which requires 4 months CPU time. The CPU time would be reduced to about 4 days if several developments are achieved in the current fundamental researches.

DOI： 10.20619/jcombsj.52.159_39

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Other Link： https://ndlsearch.ndl.go.jp/books/R000000004-I10588864

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Three-dimensional direct numerical simulations of turbulent plane jet premixed flames have been conducted to investigate local structures of turbulent premixed flames in shear flow. Detailed kinetic mechanism including 12 reactive species and 27 elementary reactions is considered to represent reactions of hydrogen-air mixture. Eddy structure is produced by velocity difference in burnt gases. This difference is caused by inflow burnt gas and produced burnt gas and forms shear flow along the edge of generated OH radical. In the downstream region of jet flame, strong fine scale eddies distort flame surface intricately, and heat release rate tends to be high where flame front is convex towards burnt side. The unburnt mixture islands appear behind flame front and enhance turbulent burning velocity.

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Direct numerical simulation (DNS) of hydrogen-air turbulent premixed flame with swirling flow in a cuboid combustion chamber is conducted to investigate hydrodynamic behaviors, characteristics of flame structures and heat transfer properties of a micro gas turbine combustor. The size of the combustion chamber is 15.0mm×10.0mm×10.0mm. To investigate swirling effects on flow and combustion field, DNS of both cold and reactive flows is carried out under two swirl number conditions: S=0.6 and 1.2. Large-scale coherent vortical structures such as ring-shaped or helical structures are generated near the inlet of chamber, and lots of small-scale vortices appear downstream. Flame front tends to be entrained by large-scale coherent vortical structures. Distribution of instantaneous heat flux on walls depends on swirl number. However, for both swirl number cases, the ratio of total heat loss on the walls against total heat release converges to a constant value.

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Language：Japanese   Publisher：The Heat Transfer Society of Japan  

DOI： 10.11368/nhts.2010.0.15.0

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Language：Japanese   Publisher：The Heat Transfer Society of Japan  

DOI： 10.11368/nhts.2010.0.10.0

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Language：Japanese   Publisher：The Heat Transfer Society of Japan  

DOI： 10.11368/nhts.2010.0.14.0

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Language：Japanese   Publisher：The Heat Transfer Society of Japan  

DOI： 10.11368/nhts.2010.0.16.0

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Other Link： https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-18106004/

Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Effects of turbulence characteristics on three-dimensional structure and fractal characteristics of flame front were investigated by direct numerical simulation (DNS) of hydrogen-air turbulent premixed flame in high Reynolds number turbulence. The detail kinetic mechanism including 12 reactive species and 27 elementary reactions was used to represent the hydrogen-air reaction. Flame structure in high Reynolds number was very complicated due to strong coherent fine scale eddy and unburned mixture appeared behind flame front. Fractal characteristics of flame front were also investigated by three-dimensional box counting method. Strong correlation between the inner cutoff and the ratio of diameter of coherent fine scale eddy to laminar flame thickness has been observed.

DOI： 10.1299/jsmeted.2010.119

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Three-dimensional direct numerical simulations (DNS) with detailed kinetic mechanism have been conducted for statistically-planar, freely-propagating turbulent flame and turbulent V-flame of hydrogen-air mixture in relatively high Reynolds number turbulence. DNS of each configuration was performed in two numerical conditions which are characterized by the difference of Reynolds number. From these results, eigen values of strain tensor are evaluated to investigate characteristics of the strain field near the flame and alignment of the principal strain and flame. Strong dependency of turbulent-flame interaction on Reynolds number and convection are revealed. Interesting feature of high Reynolds number turbulent flame is described as well.

DOI： 10.1299/jsmeted.2009.11

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DOI： 10.1016/j.tca.2009.06.009

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DOI： 10.1007/s12206-009-0608-9

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DOI： 10.1299/jtst.4.494

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Language：Japanese   Publisher：Combustion Society of Japan  

Turbulent premixed combustion has been discussed focusing on theoretical expectation of the flame structure in turbulence. To investigate combustion regimes of the turbulent premixed flames, clear definitions of reference length and velocity scales are introduced both for a laminar premixed flame and fluid turbulence. Based on the relevant reference scales, derivation of the combustion diagram for turbulent premixed flames is introduced by treating turbulent premixed combustion as an interaction between two physical phenomena: laminar premixed flame and turbulence. The characteristics of combustion diagrams by Borghi [2] and Peters [3,4] and their differences are presented, and drawback and advantage of each diagram are shown based on the physical insights obtained from recent progresses in turbulence and turbulent combustion researches.

DOI： 10.20619/jcombsj.51.158_295

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Other Link： https://ndlsearch.ndl.go.jp/books/R000000004-I10496516

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Three-dimensional direct numerical simulations (DNS) with detailed kinetic mechanism have been conducted for statistically-planar, freely-propagating turbulent flame and turbulent V-flame of hydrogen-air mixture in relatively high Reynolds number turbulence. DNS of each configuration was performed in two numerical conditions which are characterized by the difference of Reynolds number. From these results, eigen values of strain tensor are evaluated to investigate characteristics of the strain field near the flame and alignment of the principal strain and flame. Strong dependency of turbulent-flame interaction on Reynolds number and convection are revealed. Interesting feature of high Reynolds number turbulent flame is described as well.

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Language：Japanese  

Investigation of sound generation mechanism in flow fields is important for reduction of aerodynamic sound. The detailed investigation of sound sources requires velocity measurements with high temporal and spatial resolutions. In this study, sound sources in turbulent mixing layer have been investigated by time-resolved stereoscopic particle image velocimetry (PIV). Powell's sound source and Reynolds stress component of Lighthill's sound source have been evaluated from experimental results and compared with those obtained from the previous direct numerical simulation. It is shown that estimation of Powell's sound source is effective for the analysis of aerodynamic sound source in restricted measurement regions, and that evaluation of Reynolds stress component is useful for the analysis if measurable velocity is limited in two components. In addition, spectral analysis for temporal variations of vorticity and sound sources reveals that frequency of sound generated in mixing layer is dominated by the generation of large-scale vortical structures.

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Direct numerical simulations of turbulent premixed flame in confined vessel have been conducted to investigate heat losses and quenching mechanism of turbulent premixed flames near the wall. Near-wall behaviors of hydrogen/air premixed flames and heat transfer characteristic were investigated by considering detailed kinetic mechanism and temperature dependence of transport and thermal properties. In confined vessel, the frame is affected by the pressure wave which caused by the flame ignition and the flame propagation, and the flame displacement speed of the appearance decreases remarkably. However, the turbulent burning velocity at that time doesn't decrease because of an increase of fluid velocity normal to the flame.

DOI： 10.1299/jsmeted.2009.9

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Language：Japanese   Publisher：The Heat Transfer Society of Japan  

DOI： 10.11368/nhts.2009.0.34.0

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

To investigate propagation of methane-air premixed flame in constant volume vessel, constant volume combustion chamber which can control turbulent characteristic and is applied by optical measurement has been developed and time-resolved measurements of CH chemiluminescence have been conducted. When flame front approaches near wall region, CH chemiluminescence vanishes and flame quenching occurs. Distance between flame front and wall when flame quenching occurs (quenching distance) is smaller in stoicheiometric condition than lean condition. Displacement speed of flame front above the ignition point decreases gradually as flame front approaches the wall, and corresponds to laminar burning velocity approximately just before flame quenching.

DOI： 10.1299/jsmeted.2009.7

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Language：Japanese   Publisher：一般社団法人 日本物理学会  

DOI： 10.11316/jpsgaiyo.64.1.2.0_275_1

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Statistical properties of a coherent structure function F_<CS> are shown with outstanding DNS databases for homogeneous isotropic turbulence and turbulent channel flows. The function F_<CS> is defined as the second invariant of a velocity gradient tensor divided by the magnitude of the velocity gradient tensor. Despite the different Re_λ, the pdf of F_<CS> shows good agreement. For the channel flows, the pdf at different y^+ coincides very well for different Re_γ, Moreover, the conditional average and number fraction are given as a function of y^+. This statistical properties of F_<CS> would be used for a wall modeling of large-eddy simulation.

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To investigate fine scale structure of turbulent flows, high resolution dual-plane stereoscopic PIV system was applied to velocity measurements of a turbulent jet. This system provides all three velocity components and nine velocity gradients with high spatial resolution (2.8η×2.8η×4.1η). From the velocity gradients, vorticity vectors, second invariant of velocity gradient tensor and energy dissipation rate are obtained exactly. These properties allow an eddy identification, which has been used in the analysis of direct numerical simulation (DNS). The probability density functions (pdf) of the diameter and maximum azimuthal velocity of the detected eddy show peaks at D/η≈8 and u_<θ.max>/u_κ≈1. These characteristics of the fine scale eddy coincide with those obtained from DNS.

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DOI： 10.1007/s00348-008-0487-3

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DOI： 10.1007/s00348-008-0482-8

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Language：English   Publisher：The Japan Society of Mechanical Engineers  

Direct numerical simulation (DNS) of ignition/propagation and auto-ignition of turbulent premixed flames has been conducted to investigate turbulent combustion mechanism in PCCI and HCCI engines. DNS are conducted for hydrogen/air, methane/air and n-heptane/air mixtures by considering detailed or reduced kinetic mechanism. From DNS of ignition/propagation of homogeneous and inhomogeneous mixtures, effects of turbulence characteristics, fuel species and pressure on the ignition and propagation processes are clarified. The ignition delay of the premixed mixture is correlated with mean strain rate near the ignition kernel. For high intensity and fine scale turbulent field, multiple-point ignition is observed even for a single initial ignition kernel, which is caused by the local suppression of radical production by turbulence. From DNS of auto-ignition of turbulent premixed flames, effects of turbulence and inhomogeneity of mixture on the auto-ignition process are investigated. Even for the homogeneous mixture, temperature in the auto-ignition process fluctuates in space, while this fluctuation slightly affects on global combustion process of the mixture. As for the hydrogen, inhomogeneity effects are not significant due to molecular diffusion of hydrogen. For hydrocarbons, however, inhomogeneity of the mixture changes auto-ignition process. Form these DNS with different combustion field, turbulent combustion mechanism in PCCI and HCCI engines is discussed.

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DOI： 10.1016/j.ijheatfluidflow.2008.02.009

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