Retardation of mixing and counter-gradient diffusion in a swirling flame

Abstract

Simultaneous measurements of velocity and temperature have been made with a two colour laser Doppler velocimeter and a compensated thermocouple in swirling and non-swirling turbulent diffusion flames confined in a tube. Their cross correlations together with other fluctuating properties were calculated, taking into account the quantities related to the turbulent radial flux of momentum and heat. The results are compared for the flames with and without swirl to determine the effects of the swirl. The results indicate the suppression of radial flux of momentum, and radial, counter-gradient diffusion of heat which retard mixing and combustion in swirling flames, no evidence of radial counter-gradient diffusion was observed in the non-swirling flame. In order to elucidate the phenomena, the magnitude of the source terms of the transport equations of turbulent fluxes in Favre-averaged form was estimated. The results indicate that the suppression of the radial momentum flux and counter-gradient diffusion in the swirling flame are dominated by the interaction of the fluctuation of velocity or temperature and the radial gradient of pressure induced by the circumferential velocity due to swirl.