Analysis of diffusion models in high speed reacting flow simulations

The purpose of this study was to analyze the effect of mass diffusion in a high speed reacting flow field. Diffusion processes have typically been neglected in high speed reacting flows as the time scales associated with convective and reactive processes are found to be much shorter than diffusive time scales. However, diffusion may become important in secondary flow structures such as shear layers. There are several diffusion models that can be used in reactive simulations. The benefits and numerical cost of each model are discussed along with implementations of the models in previous works. The effects of including thermal diffusion into the models are also discussed. Since thermal diffusion effects are expensive to simulate, it is important to know how they affect the accuracy of the simulation results. The High Speed Combustion and Detonation (HSCD) code was used to simulate a two dimensional reflected shock flow field in a reactive H2-O2 mixture. The goal was to evaluate the effect of diffusion on the flow field. The HSCD code was first validated with Cantera by analyzing the mass diffusion flux in a freely propagating flame. When Soret diffusion was included the HSCD code results matched Cantera well. Ignition delay times were compared with experimental results. The ignition delay times were found to be within the error bars of the experimental results. The two modes of ignition, strong and weak, were observed in the simulations. In the simulations, the strong ignition events were unaffected by diffusion effects while the weak ignition events were affected by diffusion. Weak ignition occurred earlier then predicted by constant volume combustion calculations. The diffusion model choice in the HSCD code noticeably changed the values for diffusion flux. The mass diffusion flux due to temperature gradients was found to vary for different temperatures at low initial temperatures while was unaffected by changes in temperature at high initial temperatures