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Seung Ook Kim(김승욱),Jacqueline H. Chen,Chun Sang Yoo(유춘상) 한국연소학회 2015 KOSCOSYMPOSIUM논문집 Vol.2015 No.5
The characteristics of turbulent lifted non-premixed hydrogen jet flames under various coflow conditions have widely been investigated due to their relevance to practical applications. Three 3-D DNSs of turbulent lifted hydrogen/air jet flames in heated coflows near auto-ignition limit are performed to examine the stabilization mechanisms and flame structure of turbulent lifted jet flames.
A numerical study of the diffusive-thermal instability of opposed nonpremixed tubular flames
Bak, Hyun Su,Lee, Su Ryong,Chen, Jacqueline H.,Yoo, Chun Sang Elsevier 2015 Combustion and Flame Vol.162 No.12
<P><B>Abstract</B></P> <P>The diffusive-thermal (D-T) instability of opposed nonpremixed tubular flames near extinction is investigated using two-dimensional (2-D) direct numerical simulations together with the linear stability analysis. Two different initial conditions (IC), i.e. the perturbed IC and the C-shaped IC are adopted to elucidate the effects of small and large amplitude disturbances on the formation of flame cells, similar to conditions found in linear stability analysis and experiments, respectively. The characteristics of the D-T instability of tubular flames are identified by a critical Damköhler number, <I>Da<SUB>C</SUB> </I>, at which the D-T instability first occurs and the corresponding number of flame cells for three different tubular flames with different flame radii. It is found that <I>Da<SUB>C</SUB> </I> predicted through linear stability analysis shows good agreement with that obtained from the 2-D simulations performed with two different ICs. The flame cell number, <I>N</I> <SUB>cell</SUB>, from the 2-D simulations with the perturbed IC is also found to be equal to an integer close to the maximum wavenumber, <I>k</I> <SUB>max</SUB>, obtained from the linear stability analysis. However, <I>N</I> <SUB>cell</SUB> from the 2-D simulations with the C-shaped IC is smaller than <I>k</I> <SUB>max</SUB> and <I>N</I> <SUB>cell</SUB> found from the simulations with the perturbed IC. This is primarily because the strong reaction at the edges of the horseshoe-shaped cellular flame developed from the C-shaped IC is more likely to produce larger flame cells and reduce <I>N</I> <SUB>cell</SUB>. It is also found that for cases with the C-shaped IC, once the cellular instability occurs, the number of flame cells remains constant until global extinction occurs by incomplete reaction manifested by small <I>Da</I>. It is also verified through the displacement speed, <I>S<SUB>d</SUB> </I>, analysis that the two edges of the horseshoe-shaped cellular flame are stationary and therefore do not merge due to the diffusion–reaction balance at the edges. Moreover, large negative <I>S<SUB>d</SUB> </I> is observed at the local extinction points while small positive or negative <I>S<SUB>d</SUB> </I> features in the movement of flame cells as they adjust their location and size towards steady state.</P>
Kim, Seung Ook,Luong, Minh Bau,Chen, Jacqueline H.,Yoo, Chun Sang Elsevier 2015 Combustion and Flame Vol.162 No.3
<P><B>Abstract</B></P> <P>Two-dimensional direct numerical simulations (DNSs) of ignition of lean primary reference fuel (PRF)/air mixtures at high pressure and intermediate temperature near the negative temperature coefficient (NTC) regime were performed with a 116 species-reduced mechanism to elucidate the effects of fuel composition, thermal stratification, and turbulence on PRF homogeneous charge compression-ignition (HCCI) combustion. In the DNSs, temperature and velocity fluctuations are superimposed on the initial scalar fields with different PRF compositions. In general, it was found that the mean heat release rate increases slowly and the overall combustion occurs rapidly with increasing thermal stratification regardless of the fuel composition. In addition, the effect of the fuel composition on the ignition characteristics of PRF/air mixtures was found to be significantly reduced with increasing thermal stratification. Chemical explosive mode (CEM) and displacement speed analyses verified that nascent ignition kernels induced by hot spots due to a high degree of thermal stratification usually develop into deflagration waves rather than spontaneous auto-ignition at reaction fronts and as such, the mean heat release rate becomes more distributed over time. These analyses also revealed that the fuel composition effect vanishes as the degree of thermal stratification is increased because the deflagration mode of combustion, of which propagation characteristics are nearly identical for different PRF/air mixtures, becomes more prevailing with increasing degree of thermal stratification. Ignition Damköhler number was proposed to quantify the successful development of deflagration waves from nascent ignition kernels; for cases with large ignition Damköhler number, turbulence with high intensity and short timescale can advance the overall combustion by increasing the overall turbulent flame area instead of homogenizing initial mixture inhomogeneities.</P>
Doubly conditional moment closure modelling for HCCI with temperature inhomogeneities
Salehi, Fatemeh,Talei, Mohsen,Hawkes, Evatt R.,Bhagatwala, Ankit,Chen, Jacqueline H.,Yoo, Chun Sang,Kook, Sanghoon Elsevier 2017 Proceedings of the Combustion Institute Vol.36 No.3
<P><B>Abstract</B></P> <P>This paper presents a doubly conditional moment closure (DCMC) as an <I>a posteriori</I> predictive modelling tool for ignition of mixtures with large thermal stratification in homogeneous charge compression ignition (HCCI) conditions. Double conditioning is applied on enthalpy and its dissipation rate. The performance of the DCMC model is evaluated using a number of previously reported direct numerical simulations (DNSs) with various fuels. The DNSs modelled ignition of various lean homogeneous mixtures with a high level of temperature inhomogeneities. The selected cases exhibit a prevalence of deflagration mode of combustion as opposed to a spontaneous ignition-front mode, which has proven challenging for previous singly CMC. In all simulations, DCMC solver is run in a stand-alone mode with certain terms, such as the probability density functions of enthalpy and dissipation rate, being provided using the DNS input. The DCMC results are in a very good agreement with the DNS data, and are significantly improved compared with a singly conditional moment closure. A set of <I>a posteriori</I> DNS-DCMC tests is also performed to demonstrate importance of various terms in the doubly CMC equations. These tests first reveal that the effects of the cross dissipation and sources of enthalpy and dissipation rate (which lead to convective terms in conditional space) are insignificant and these terms can be safely neglected from the DCMC equations. The significance of this result is that the main unclosed models that would be needed for satisfactory results in a practical simulation of an engine would be the joint probably density function of enthalpy and its dissipation rate and the dissipation rate of dissipation rate.</P>
Sankaran, Ramanan,Hawkes, Evatt R.,Yoo, Chun Sang,Chen, Jacqueline H. Elsevier 2015 Combustion and Flame Vol.162 No.9
<P><B>Abstract</B></P> <P>Direct numerical simulations of three-dimensional spatially-developing turbulent Bunsen flames were performed at three different turbulence intensities. The simulations were performed using a reduced methane–air chemical mechanism which was specifically tailored for the lean premixed conditions simulated here. A planar-jet turbulent Bunsen flame configuration was used in which turbulent preheated methane–air mixture at 0.7 equivalence ratio issued through a central jet and was surrounded by a hot laminar coflow of burned products. The turbulence characteristics at the jet inflow were selected such that combustion occured in the thin reaction zones (TRZ) regime. At the lowest turbulence intensity, the conditions fall on the boundary between the TRZ regime and the corrugated flamelet regime, and progressively moved further into the TRZ regime by increasing the turbulent intensity. The data from the three simulations was analyzed to understand the effect of turbulent stirring on the flame structure and thickness. Statistical analysis of the data showed that the thermal preheat layer of the flame was thickened due to the action of turbulence, but the reaction zone was not significantly affected. A global and local analysis of the burning velocity of the flame was performed to compare the different flames. Detailed statistical averages of the flame speed were also obtained to study the spatial dependence of displacement speed and its correlation to strain rate and curvature.</P>