Effects of Temperature Variation among Droplets in an Evaporating Fuel Spray,
UNIVERSITY OF SOUTHERN CALIFORNIA LOS ANGELES DEPT OF MECHANICAL ENGINEERING
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It is well known that the boiling point of a typical fuel blend is different from those of the pure components. For an ideal mixture of liquids at a given state the new boiling point can be calculated either theoretically or can be obtained from experimental data e.g. ASTM distillation curve. The ASTM distillation data for JP-4, for example, show that the boiling point can change by about 250 C depending upon the degree of evaporation. Droplets of different diameters in a spray undergo different degrees of evaporation and hence have different boiling temperatures. The purpose of the present paper is to investigate the effects of temperature distribution in the spray on the local overall fuel-air ratio. Sprays of both JP-8 and JP-4 ASTM distillation data are used as examples in a flowing, one-dimensional stream at 20 atm and 680 K. The droplets undergo an initial phase of unsteady heating and finally steady evaporation follows when the wet-bulb temperature is reached. The results of this study show that operating conditions the temperature distribution in a fuel spray can have a significant effect on the local fuel-air mixture and hence on the performance of an airbreathing propulsion system. The use of a constant mean boiling temperature rather than the variable boiling temperature will result in significant errors.