Analysis of combustion modes in a cavity based scramjet

Abstract

Large eddy simulations (LES) of non reactive and reactive flows in a cavity-based scramjet combustor configuration from the U.S Air Force Research Laboratory (AFRL) are performed. These simulations feature a 22 species and 206 reactions chemical scheme for ethylene/air. The ability of LES to reproduce the main features found in the experiment is first emphasised such as the average velocity field and the stability of the combustion for the case studied. The influence of the mesh resolution and of the thermal wall condition on the simulation results is also investigated along with the soundness of the use of a laminar model for the filtered source terms. The results of the simulations with the finest grid (resolution of 100 micrometers in the flame region) are then employed to gain understanding in the flame dynamics. This reactive simulation shows the persistence of the two recirculation zones already present in the non reactive flow. The globally high temperature into the cavity helps to sustain a reactive zone located in the mixing layer above the cavity. Combustion first occurs in a diffusion dominated regime followed by the efficient burning of a well stirred mixture (rich then lean). A significant diffusion dominated burning is also found inside the cavity. The links between the residence time inside the cavity and the efficiency of the combustion are explored along with the velocity/heat release correlation.

Stabilisation and extinction mechanisms of flames in cavity flameholder scramjets

“…All these results are consistent with other literature articles for which RANS (Reynolds Average Navier-Stokes) [16] or different RANS-LES hybrid approaches [17] have been tested. Further comparisons were made on the velocity and wall pressure profiles for the MHF case and a good agreement was found by comparison with the experimental data [10,12].…”

“…A no-model assumption is used to compute the chemical source terms as in [8,18]. It has been verified in [10,12,31] that these assumptions do not modify the results and scenarios described hereafter. The use of the quasi-laminar-chemistry approach has been found acceptable in [5] for small scramjet-like problems.…”

“…It becomes then a formidable challenge to predict such behaviors and the use of numerical simulations appears as a necessary tool [5]. In particular, the use of Large-Eddy Simulation (LES) for supersonic combustion is now becoming relevant [6,7,8,9,10] even if for highspeed combustion, the choice of turbulent-combustion models can be challenging [11,12] since the majority of models in literature have been developed for low-speed combustion. Besides, combustion can occur near or in the distributed-reaction-like regime [5], i.e.…”

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

“…In such a device, air flows at a supersonic velocity over a cavity that traps hot products coming from the decomposition of fuel and air by means of a primary vortex positioned inside the cavity. The reaction zone is located between the air flow and the cavity, and a secondary vortex acts as a fuel pool mixed with hot burned products in a dead zone at the front of the cavity [34]. Again, the effects of compressibility can play a major role in the creation of a look-up table.…”

“…An array of 11 fuel injectors are evenly distributed on the aft cavity wall. This experiment has already been studied by LES (Large-Eddy Simulations) or RANS (Reynolds Average Navier-Stokes) [2,16,17,34]. Particle image velocimetry are available inside the cavity [37].…”