Liftoff height of turbulent diffusion flame in water-mist-laden coflowing air stream

Abstract

In the present study, the enhancement of the liftoff and blowout of a propane/air coflowing diffusion flame by water mist was experimentally investigated in the subsonic range. When the fuel injection velocity Uf is low, an attached laminar diffusion flame is stabilized at the burner rim with a premixed flame base. An increase of Uf leads to the liftoff of the attached flame, and eventually to the blowout of the lifted flame. The upstream laminar flame disappears at the liftoff and a turbulent lifted flame is stabilized in the downstream region. The stability of the lifted flame deteriorates with the increase of the coflowing air velocity Ua and the water mist flow rate Qm. The stable lifted flame can be achieved by the dynamic balance of the local mean gas velocity Ug¯ and the mean turbulent flame propagation speed Up¯ of the premixed flame formed at base of the lifted flame, and Up¯ was estimated by the measurements of Ug¯ by a laser Doppler velocimeter. Up¯ decreases with Qm and is of the order of 1 m/s at the blowout independently of Ua and the mass fraction of water mist Y0. This value is nearly equal to 2.5SL0, where SL0 is the adiabatic laminar flame speed of a stoichiometric propane-air mixture. The liftoff height L increases both with Uf and Qm. For the case without water mist, a unique correlation has been proposed in the previous studies between L and Uf in a non-dimensional form using SL0. By extending this correlation to the diffusion flame in the water-mist-laden air stream, the laminar flame speed influenced by water mist was appraised and compared with the simulation previously reported. Resulting flame speed reduction is 65% of that predicted by numerical simulation, presumably due to the partial evaporation of water mist.