A lean premixed ethylene-air flame in a backstep configuration is simulated on multiple grids using both direct numerical simulations (DNS) with reduced order kinetic mechanism and large eddy simulations (LES) with flamelet-based thermochemistry. The configuration includes preheated reactants and a recirculation zone that provides radicals and high temperature gases to stabilize the flame. Heat losses are present due to the proximity of cooled walls. The reacting flow obtained from DNS at different resolutions is first analyzed to investigate the property of heat transfer within the recirculation region. LES based on adiabatic flamelets with a correction of the heat capacity is then tested, and its ability to account for heat losses is compared to results obtained using a three-dimensional non-adiabatic flamelet approach. Mean fields and subgrid properties are compared to those obtained from DNS to assess the capability of the LES models. The results show that the non-adiabatic flamelet approach can predict recirculation region and temperature fields with good accuracy. The model with heat capacity correction is able to effectively correct the heat capacity behavior as observed by a priori comparisons. However, in the a posteriori context, it is observed to overestimate the temperature field, although the correct size of the recirculation region is predicted. The combined a priori and a posteriori analyses on the same configuration and at different mesh resolutions allow for a precise separation of modeling effects due to heat transfer at the wall and combustion closure, thus providing indications on the LES performance in the context of flamelets.

The coaxial injection of cryogenic nitrogen and warm hydrogen into a supercritical nitrogen atmosphere is studied numerically by means of well resolved large-eddy simulation (LES). Numerical data for two operating conditions are compared both quantitatively and qualitatively with a series of experiments [Oschwald et al., 35th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, 1999], A detailed thermodynamic analysis of phenomena associated with the binary mixing of nitrogen and hydrogen together with a systematic study of the thermodynamic state of cryogenic nitrogen prior to injection allows an assessment of uncertainties related to both numerical and experimental data. Trans- and supercritical mixing constitutes considerable challenges for numerical simulations with regard to physical modeling and numerical stability. We compare a fully conservative (FC) formulation to a quasi conservative (QC) formulation of the governing equation, which was recently proposed by Terashima and Koshi [J. Comput. Phys., 2012].

In a joint effort towards the development of robust numerical tools for Large-Eddy Simulations (LES) of the injection, mixing and combustion process in the high-pressure environments of liquid-propellant rocket engines, a consistent model of real-gas thermo-physical effects has been developed and implemented into two different CFD codes: INCA with a density-based approach and a pressure-based version of OpenFOAM. As part of the validation process, LES of trans- and supercritical nitrogen jets have been performed with different subgrid-scale modeling approaches. Despite the different code architecture and turbulence models, the numerical results are in excellent agreement with each other and with experimental reference data. Compared to experimental data, our results have the same or better quality as those from previous simulations.