Experimental and numerical investigation of flame speed retardation by water mist

Water mist has been recognized as an alternative of halogenated hydrocarbon fire suppressants due to its high thermal and chemical effects on the flame speeds. In the present study, the effects of fine water mist on laminar flame speeds of propane-air mixtures are investigated both experimentally and numerically. In experiments, the laminar flame speeds are measured using a single jet-plate configuration for the cases with and without water mist. The numerical simulation is also performed using the PREMIX and OPPDIF codes in CHEMKIN package. To include the phase change with evaporation, the evaporation process is assumed as a chemical reaction of which rate constant follows the Arrhenius law. The laminar flame speeds without water mist increase with stretch rate for all the equivalence ratios tested and are in fairly good agreement with the previous experiments. When the water mist is added, the numerical simulation predicts the positive dependence of flame speed on stretch rate similar to the case without water mist, whereas in the experiments the flame speeds decrease with stretch rate. In the stagnation flow field, the large radial acceleration of the flow induces the mist droplet accumulation around the stagnation stream line, leading to the negative dependence of flame speed on stretch rate. Numerical simulation reveals the thermal, dilution and chemical effects of water mist on laminar flame speed, and the chemical effect is found to be small but cannot be neglected. The water mist reduces the rates of chemical reactions involving the radicals such as O, H and OH, which have the positive sensitivity of flame speed. Furthermore, three-body chain terminating reactions involving H2O are enhanced. These reactions have large negative sensitivities of flame speed due to high chaperon efficiency of H2O.