The Interdependence of Spray Characteristics and Evaporation History of Fuel Sprays

Previously developed calculation procedures [1, 2] based on evaporation analysis are used to predict the variation of JP-5 fuel spray characteristics (mean drop diameter, D_m, and drop-size distribution parameter, n) with time during evaporation in hot air. The method takes full account of transient effects occurring during the heat-up period. The results show that both D_m and n increase with time, but the changes are more significant for sprays having small initial values of n. The time to vaporize a certain percentage of spray mass is proportional to the square of the initial mean diameter, D_mo. The effect of the initial value of n, is that a spray having a larger value of n_o will reach its 90 percent evaporation point faster, but a smaller value of n_o will give a shorter 20 percent evaporation time. Based on these calculations, a general method for estimating the time required for any liquid fuel to attain any given percentage of spray mass evaporation is proposed. Although the method was developed for quiescent mixtures of fuel drops and air, it can be applied to many practical combustion devices (for example, a gas turbine combustor fitted with airblast atomizers) where it is reasonable and sufficiently accurate to assume a low relative velocity between the fuel drops and the surrounding air or gas.