Direct Numerical Simulation of Cavity-Stabilized Premixed Turbulent Flame

Air-breathing hypersonic flight vehicles are of great interest for military applications and cheaper access to space. Ramjet and supersonic combustion ramjet (scramjet) engines are designed to operate at such high-speeds by compressing air through a series of weak shocks in the intake. However, the high flow velocity with extremely short combustor residence times poses severe challenges on mixing and flame-holding with reduced combustion efficiency. A promising solution is to employ a cavity in which a recirculation region with relatively longer residence times can act as a stable ignition source by mixing radical species with fresh reactants. In this work Direct Numerical Simulation (DNS) of a scaled down aft linear-ramp cavity combustor is performed using a novel Immersed Boundary (IB) method. This is one of few DNS studies on an engineering scale combustion device and provides new information on the turbulent mixing processes and turbulence-chemistry interactions down to the smallest physical length scales, i.e. Kolmogorov length scales.

Critical to DNS is the resolution of all length scales and therefore understanding of the canonical laminar flame structure at the target conditions is of particular interest. A study on laminar premixed flames reveals that different laminar flame thickness definitions, at conditions relevant to ramjet/scramjet engines, have over an order of magnitude difference. This can have a significant impact on the choice of turbulent combustion closure models used in simulation of turbulent premixed flames.

To enable the simulation of non-mesh conformal geometries an IB method is implemented in the Sandia DNS code, S3D. This IB method is developed such that the use of S3D's higher-order numerical schemes even in the vicinity of the immersed boundary is possible. This is a key contribution to the use of IB in reacting DNS.

The DNS of the ramp cavity reveal significant production of turbulence by the cavity from the analysis of Favre averaged turbulent kinetic energy and dissipation rate. Mean OH species fluxes are found to transport OH through the recirculation zone towards the upstream end of the cavity, contributing significantly to flame stabilization. The transport of thermal energy and OH species by turbulent fluctuations are explored through the velocity-temperature and velocity-OH correlations. These revealed the transport of thermal energy and OH towards the reactant side of the flame front by turbulent fluctuations.