Simulation of a Scramjet Combustor: A Priori Study of Thermochemistry Tabulation Techniques

Abstract

Large eddy simulations (LES) of a scramjet combustor are reported in this paper. The case under study is a cavity-based combustion chamber that is experimentally studied at the US Air Force Research Laboratory. The chamber is fed by eleven injectors. The computational domains are either simplified including only one or two injectors or complete with the 11 injectors. A good agreement is found between experimental data (velocities measured by PIV) and results from the LES if the kinetic used is chosen with care. A high temperature is found inside the cavity promoting a reactive zone located in the mixing layer where the flow velocity is high. At this location, the combustion occurs first in a diffusion dominated regime followed by the efficient burning of a well mixed mixture (rich then lean). A significant diffusion dominated burning is also found inside the cavity, mostly at the interface between the two recirculation zones. The simulation of the complete geometry revealed a transverse phenomenon on the temperature and mixing fields, but which had nevertheless little effect on the comparison with the available experimental data. A tabulation of the chemistry based on a premixed flamelet library without compressibility effects has been tested a priori on the results of the simulation with one injector. Good results on temperature and $\hbox {H}_2\hbox {O}$ fields are found. Significant localized discrepancies appeared on CO and $\hbox {CO}_2$ fields due to the complexity of the combustion regimes, while compressibility effects were found to be weak for the configuration studied.

Appendix: Progress Variable

The construction of progress variable needs to satisfy two conditions:

• (1) the progress variable C must be a monotonically increasing function of the physical coordinate X;

• (2) all tabulated quantities must be injective functions of the progress variable C.

The verification is done for $\phi \in \left[ 0.4,5 \right]$ in Fig. 35 for (I) and in Fig. 36 for (II).

Fig. 35

Progress variables $C_1$ (left) and $C_2$ (right) in physical space at equivalence ratio $\phi \in \left[ 0.4,5 \right]$: blue, green and red colors correspond respectively to lean, stoichiometric and rich mixtures

Fig. 36

Tabulated temperature and major combustion product mass fractions against progress variables $C_1$ (left) and $C_2$ (right) at equivalence ratio $\phi \in [ 0.4,5 ]$: blue, green and red colors correspond respectively to lean, stoichiometric and rich mixtures