Flame Stabilization in Small Cavities

Abstract

This research is motivated by the necessity to improve the performance of ultracompact combustors, which requires flame stabilization in small cavities. An extensive computational investigation on the characteristics of cavity-stabilized flames is presented. A high-fidelity, time-accurate, implicit algorithm that uses a global chemical mechanism for JP8-air combustion and includes detailed thermodynamic and transport properties as well as radiation effects is used for simulation. Calculations are performed using both direct numerical simulation and standard k-e Reynolds-averaged Navier-Stokes model. The flow unsteadiness is first examined in large axisymmetric and small planar cavities with nonreactive flows. As with previous investigations on axisymmetric cavities, multiple flow regimes were obtained by varying cavity length (x/D o ) : wake backflow regime, unsteady cavity vortex regime, steady cavity vortex regime, and compressed cavity vortex regime. However, planar cavities only exhibit steady cavity vortex and compressed cavity vortex regimes. Two opposed nonaligned air jets were positioned in this planar cavity: the outermost air jet in coflow with the mainstream flow (i.e., normal injection). The fuel jet was injected either in coflow, crossflow, or counterflow with respect to the mainstream flow. Flow unsteadiness was observed to be relatively small for coflow- and crossflow-fuel-jet injection. By reversing the air jet positions (i.e., reverse injection), the flow unsteadiness is promoted regardless of fuel jet positioning. Finally, the effect of combustion and cavity equivalence ratio (φ CAV ) on flame unsteadiness is addressed. With normal injection (reverse injection), low and high φ CAV leads to low (high) and high (low) flame unsteadiness, respectively. Based on these results recommendations are provided to designers/engineers to reduce flame unsteadiness in these cavities.