On Simulation and Modeling of Turbulent Non-Premixed Reacting Shear Flames at Low and High Pressure

A database of simulations of fundamental turbulent H2/O2 and H2/Air shear layer flames has been developed using the direct numerical simulation technique in which all length and time scales of a flow are fully resolved without resort to turbulence modeling. The formulation includes the fully-compressible form of the governing equations, a pressure dependent detailed chemical kinetics scheme (9 species, 19 steps), a cubic real gas equation of state, realistic property models, and generalized heat and mass diffusion models. Simulations were conducted for a temporally developing shear layer geometry and cover a wide range of initial pressures (1 atm \u3c P \u3c 125 atm) and flame Reynolds numbers (850 \u3c Re \u3c 4500). These supercomputing simulations are the largest yet performed for high pressure conditions to the author\u27s knowledge. Computational requirements for the largest simulations included mesh sizes of ~3/4 billion grid points and ~2.2 million CPU-hrs conducted on nearly 4,000 CPU cores on Clemson\u27s Palmetto cluster. The database was then used to perform several analyses relevant to turbulent combustion modeling at large pressure. The first analysis examines H2/O2 and H2/Air simulations at initial pressures of 100 atm and 35 atm, respectively, in the context of large eddy simulation (LES). Specific attention is given to the subgrid mass flux vector as this term is unclosed and often neglected in LES simulations. Results suggest the subgrid mass flux vector to be non-negligible in localized regions of large temperature subgrid scale (sgs) variance and filtered sgs scalar dissipation. This term is also shown to diminish, but remain significant, at higher Reynolds numbers as well. The next analysis focuses on the effects of differential diffusion (DD) on the overall flame structure of flames as a function of pressure (1 atm The final analysis focuses on the characteristic length and time scales that identify those regimes consistent with flamelet models in the highest Reynolds number H2/O2 and H2/Air flames. A chemical time scale analysis shows the rate-limiting behavior to be strongly related to the local fuel consumption/production rate. Also, the significant amount of local extinction/re-ignition in the H2/Air flame leads to partially premixed behavior during the unsteady development of the flame and alters the flame\u27s response to a given scalar dissipation field. By utilizing the scalar dissipation a priori, good agreement between reactive scalars and mixture fraction is observed for those scalars with reaction zones on the order of the Kolmogorov length scales; consistent with commonly held flamelet theories. However, temperature and several minor species, such as the hydrogen radical, do not correlate as well with mixture fraction (especially for the H2/Air flame). This behavior is attributed to the stricter requirement for these scalars to obtain reaction zones comparable to the Kolmogorov scale. The results of this study provide a wealth of information required for improving predictive turbulent combustion models; particularly at large pressures for which there are many applications but relatively little fundamental studies or model validations