Effect of initial generating eddy height on temperature, velocity and air entrainment of fire whirl

Abstract

In this paper, we present an experimental effort to reveal the effect of the initially generated eddy height on the thermal, velocity, and air entrainment properties of the fire vortex plume. Experiments were performed by a fixed-frame facility with variable channel wall height (h), using propane as the fuel (20 cm in burner exit diameter, 50.0–100.0 kW in heat release rate). Results show that the growth rates of centerline excess temperature and axial velocity tend to increase with h in the continuous flame below h. It is found that the radial distribution of excess temperature is always of hump-type and little affected by h at lower levels, while the radial profile of axial velocity changes from the plateau-type into the hump-type as h is increased. In the upper continuous and intermittent flames, the radial profile of excess temperature evolves from the hump type into the unimodal type, and that of axial velocity develops into the unimodal type from the plateau type as h is increased. The growth rates of excess temperature radius and axial velocity radius with the normalized height decrease with h for the continuous and intermittent flames. The axial position corresponding to the significant increase in the growth rate moves upstream with the increase of the heat release rate (Q˙). Under small h, the tangential velocity within the vortex core decreases with height, suggesting that the rotational strength becomes weakened. It is found that the air entrainment is weak at the flame bottom within the initial generating eddy height, and the entrained air is insufficient for complete combustion. The axial mass flow rate increases obviously as the channel wall disappears. With the increase of h, turbulence suppression is enhanced, and its role in flame elongation becomes more significant. The reduction of flame height is probably attributed to the greater turbulent mixing length under lower initial generating eddy height.