The Influence of Droplet Evaporation on the Fuel-Air Mixing Rate in a Burner

Abstract

The importance of droplet evaporation in the overall fuel-air mixing process in liquid fuel spray flames is examined with a series of experiments in a simple atmospheric pressure burner burning a range of hydrocarbon fuels. Two types of fuel atomizers were studied—air-assist and pressure jet—which have substantially different operating characteristics. Fuel-air mixing rates were determined from time-average oxygen concentrations measured with overall burner operation stoichiometric. It is shown that with air-assist atomizers, the kinetic energy of the atomizer jet determines the mixing rate intensity for both liquid and gaseous fuels. Since the droplet evaporation time is much less than the mixing time, the details of the evaporation process are not important and the jet length scale and kinetic energy govern the mixing process. With pressure jet atomizers, the characteristic evaporation and mixing times are comparable. The evaporating fuel drops create fuel vapor concentration nonuniformities on a scale much smaller than the fuel jet scale. Mixing rate intensities comparable to those obtained with air-assist atomizers can, therefore, be achieved with much lower turbulent kinetic energy dissipation rates. With pressure jet atomizers, both the kinetic energy of the fuel jet and the evaporation characteristics of the fuel droplets control the initial fuel-air mixing rate.