Numerical simulation of flame-stabilization and combustion promotion by plasma jets in supersonic air streams

Abstract

The mechanisms of flame stabilization and combustion promotion by plasma jets, observed in scramjet bombustor experiments with transverse fuel injection, were investigated numerically, based on time-dependent, two-dimensional complete Navier-Stokes equations, with a six-species, eight-step reaction model combined with el'dovich chain reactions. Although most designs for scramjet combustors rely on the combination of a rearward-facing step and a plasma torch, the present study is conducted on a simple configuration without the step to make clearer the fundamental aspect of the mechanisms. The following results were obtained: o 1. A plasma jet with adequate input power and feedstock flow rate establishes a high-temperature recirculation zone with an adequate scale, which works for flame stabilization as that produced by the step. The plasma jet is also useful for the combustion promotion of the fuel jet, through the intensification of eddy diffusivity at its injection point and at the following downstream region. 2. The effectiveness of plasma jets in flame stabilization and combustion promotion increases with input electric power and feedstock flow rate, and the plasma jets with oxygen feedstock are more effective than those with air or hydrogen feedstock, as observed generally in scramjet combustor experiments. 3. A decrease of the plasma jet injection slit width, for a fixed input power and a fixed mass flow rate of feedstock, increases the effectiveness of plasma jets in some cases. 4. When the plasma jet injection slit is located downstream of the fuel injection slit, the required input power and feedstock flow rate for a desirable combustion progress are usually smaller than those when it is located upstream, in accordance with a previous supersonic-combustion experiment conducted without the step.