Effect of water mist on minimum ignition energy of propane/air mixture

Abstract

Water mist is anticipated to be an alternative of halogenated hydrocarbon fire suppressants because it has high thermal and chemical potential to prevent the ignition of a combustible mixture and it is environmentally acceptable. The minimum ignition energy (MIE) is an important property for designing safety standards and understanding the ignition process of combustible mixtures. In the present investigation, the effect of polydisperse water mist on MIE of propane/air mixture is investigated experimentally. Instead of the electric spark or laser-induced spark, the mixture is ignited by a fine Nichrome wire which is heated and melted electrically. Ignition energy of propane/air mixture is measured from the energy released by melting of a fine Nichrome wire. The discharge duration of wire melting is longer than that of electric spark or laser induced spark, resulting in the larger MIE. The ignition probability follows the cumulative probability distribution function of net discharged energy. The minimum energy density, i.e. energy per unit volume is found to depend on the energy discharge duration. Volumetric energy release rate (VERR), i.e. energy released per unit volume and unit time, is the dominant factor for the ignition probability. For lean propane/air mixtures, water mist increases minimum VERR by 50% when water mist mass fraction Y0 is 0.17 as compared to Y0=0. The ignition probability vanishes when Y0>0.2. On the other hand, for rich mixtures, the reduction of the minimum VERR at ignition is small but definite. The minimum VERR for wire melting is one or two orders of magnitude larger than that calculated for electric spark due to the longer ignition delay. Water mist is more effective in suppressing the explosion of combustible mixtures for lean mixtures than for rich mixtures.