Experiments and numerical simulation on the suppression of explosion of propane/air mixture by water mist

Effect of water mist on minimum ignition energy (MIE) and lower and upper flammability limits (LFL and UFL) of C3H8–air mixtures were investigated by experiments and numerical simulation. Ignition experiments were performed in a combustion tube. Polydisperse water mist with a Sauter mean diameter of D32 = 21 µm was generated by a piezoelectric atomizer. Mixture was ignited by an exploding wire heated instantaneously by electrical discharge. Ignition probability follows the cumulative probability distribution function of energy density, i.e., energy per unit volume of the wire. Water mist increases minimum ignition energy density (MIED) defined at 50% ignition probability when the equivalence ratio φ < 1.0 and φ > 1.3. However, for 1.0 < φ < 1.3, the suppression effectiveness was scarcely observed. In numerical simulation, water mist was assumed to be an imaginary ideal gas and evaporation process was expressed by a chemical reaction. Ignition energy is added in the flame kernel at the center and the flame propagates spherically if ignited. This model can predict MIE of the mixture without water mist reasonably. MIE increases nonlinearly with the mass fraction of water mist Y0 and flammability range narrows. At Y0 = 0.146 and beyond, the mixture of any φ becomes unable to be ignited. Relative suppression effectiveness factors Se and Ss are newly introduced for experiments and simulation, respectively. Comparison of Se with Ss shows that the water mist is not so effective as expected by simulation. The life time of a droplet before evaporation is much longer for the water mist of D32 = 21 µm than the residence time in the flame, and only a part of water mist can evaporate in the flame zone. To make full use of the suppression potential of water mist, the mist diameter should be sub-5 µm.