On processes of turbulent exchange behind flame holders

Abstract

Flames in combustion chambers of jet engines are mostly stabilized by means of recirculation zones behind flame holders, which are in turbulent exchange of mass, momentum, and energy with their surroundings. The spreading of the main flame is rendered possible by the turbulent transport of heat from the flame at the boundary of the recirculation zone to the adjacent unburned gas. This exchange is determined by the geometry of the recirculation zones and the residence time of the gas particles within these zones. These quantities have been measured for recirculation zones behind axisymmetric flame holders, situated in a tube. The geometric quantities and the residence time are mainly determined by the flame-holder shape, the blockage ratio, the turbulence level in the approaching flow and the presence of a flame. Additionally, the mean residence time changes inversely proportional to the flow velocity. The presence of a flame increases the residence time considerably and, according to this result, reduces the exchange. The exchange velocity is presumably fixed by the flow velocity which exists at the boundary of the recirculation zone. According to this, those flame holders which deflect the flow to a greater extent produce the greater exchange. This result of measurements agrees with other experiments carried out by Mestre, after which these flame holders show the larger blow-off velocities and, thus, the better stabilization properties.