Extinguishment of counterflow diffusion flame stabilized in turbulent airflow by polydisperse water mist

In the present study, the effect of turbulence on extinguishment of counterflow diffusion flame by polydisperse water mist was investigated experimentally. A counterflow methane-air diffusion flame was stabilized in the forward stagnation region of a porous cylinder. Water mist was supplied by a pressure-type atomizer. Uniform turbulence was produced in the airflow by a grid. Mist characteristics were measured by a phase Doppler particle analyzer and turbulence properties by a hot wire anemometer. For laminar flames, the critical stretch rate at extinguishment decreases with the mass fraction of water mist and the extinguishment is not affected by the Sauter mean diameter D-32 even when D-32 is as large as 70 mu m. Turbulence reduces significantly the critical bulk (mean flow) stretch rate at extinguishment. Effect of turbulence on extinguishment is relevant to the turbulence stretch rate, which is the inverse of the Kolmogorov time scale. Sum of the bulk stretch rate and the turbulence stretch rate is the total stretch rate that actually exerts on turbulent flame. If the relative turbulence intensity v'/V is as low as 0.06 and D-32 is smaller than 30 mu m, extinguishment of turbulent diffusion flame is expressed uniquely by the total stretch rate, which coincides with that of laminar flame. On the other hand, for large water mist, D-32 > 30 mu m or high turbulence intensity, v'/V = 0.10, non-uniformity of mist dispersion appears, of which scale is of the order of integral scale of turbulence. The locally extinguished flame by a spot of dense water mist is reignited by surrounding flame and the suppression effectiveness of water mist is deteriorated when the turbulence intensity is high or the mist diameter is large. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.