Influences of Freestream Turbulence on Flame Dynamics in a Supersonic Combustor

Abstract

Turbulent flame dynamics in an ethylene-fueled model scramjet is experimentally investigated in Mach 4.5 freestream flows with a total temperature of 2600 K using a pulsed arc-heated hypersonic wind-tunnel facility at the University of Notre Dame. Gaseous ethylene fuel is injected into the combustor through a supersonic nozzle, and is autoignited by high-enthalpy flows that are compressed and decelerated by a train of shock waves through the scramjet isolator/combustor. The turbulence level of the freestream is manipulated to investigate the influence of freestream turbulence on the flame dynamics in the supersonic combustor. General flame behavior is observed in time-averaged ethylene-flame-chemiluminescence images, whereas the detailed turbulent flame structures are resolved by imaging instantaneous ground-state hydroxyl radical (OH) distributions using the planar-laser-induced-fluorescence technique, wherein a planar laser sheet is projected into the scramjet in the counterstreamwise direction to illuminate the entire internal flow region, including the isolator and supersonic combustor of the model scramjet. Enhanced freestream turbulence can improve fuel–air mixing efficiency and, therefore, determine the flame dynamics, such as the turbulent burning velocity and flame geometry. A direct impact of the freestream turbulence is clearly seen, that being a significant combustion enhancement in the supersonic flow.