Flame inhibition/suppression by water mist: Droplet size/surface area, flame structure, and flow residence time effects

The variation of optimal water droplet size on flame inhibition/suppression is investigated numerically by exploiting the distinct flow residence time of laminar methane-air premixed flames and counterflow non-premxed flames. For a gas inflow stream saturated with water vapor, it is shown that the relevant flow residence time of a premixed flame is governed by the droplet trajectory through the preheat region, while for a non-premixed flame, the relevant time scale is governed by the convective-diffusive layer relative to the saturated air-stream. Assuming no velocity lag between the gas and droplets at the inflow, a factor of four difference in droplet residence time is related to the predicted optimal droplet sizes of 6.5 mu m for premixed flames and 20 mu m for non-premixed flames. For droplets larger than the optimal size, the flame strength (defined as the square of the normalized burning velocity for premixed flames or normalized extinction strain rate for non-premixed flames) is shown to decrease linearly with increasing specific droplet surface area. As the water droplets approach the optimal size, the linear correlation stated above fails for both premixed and non-premixed flames; this failure can have major implications for the design of water-mist fire suppression systems. A comparison of present predictions and previously reported experimental data is also presented. (c) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.