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

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 U f is low, an attached laminar diffusion flame is stabilized at the burner rim with a premixed flame base. An increase of U f 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 U a and the water mist flow rate Q m . The stable lifted flame can be achieved by the dynamic balance of the local mean gas velocity U g and the mean turbulent flame propagation speed U p of the premixed flame formed at base of the lifted flame, and U p was estimated by the measurements of U g by a laser Doppler velocimeter. U p decreases with Q m and is of the order of 1 m/s at the blowout independently of U a and the mass fraction of water mist Y 0 . This value is nearly equal to 2.5 S L0 , where S L0 is the adiabatic laminar flame speed of a stoichiometric propane-air mixture. The liftoff height L increases both with U f and Q m . For the case without water mist, a unique correlation has been proposed in the previous studies between L and U f in a non-dimensional form using S L0 . 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. (c) 2021 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

The study investigates the effect of water mist on the liftoff and blowout of a propane/air coflowing diffusion flame. It was found that water mist deteriorates the stability of the flame, leading to blowout, while stability of the lifted flame depends on the dynamic balance between local mean gas velocity and mean turbulent flame propagation speed.