Influence of Fuel Boiling Point on Discharge Characteristics of Superheated Hydrocarbon Liquid Jets

Abstract

An experimental study was conducted to investigate the effect of fuel boiling point on the hydraulic characteristics of high-temperature liquid jets, simulating injection of fuel used as coolant in the active cooling systems of a hypersonic flight vehicle. Two hydrocarbon fuels were specially created to have higher boiling points than conventional aviation fuels. The fuels were heated to close to 573 K (300 degrees C) using an induction heater at an upstream pressure of up to 1.1 MPa, and discharged to atmospheric downstream pressure conditions through a plain orifice nozzle of diameter 0.7 mm. The fundamental hydraulic characteristics represented in C-d with respect to T-inj at three injection pressure conditions for the three fuels show that the temperatures at which C-d begins to decrease are very close to each fuel's boiling or bubble point and remain almost constant for each fuel even when increment P is varied. In the relationship between C-d and Re, the discharge coefficients, which are almost identical regardless of fuel and Delta P conditions in relatively low ranges of Re, begin to deviate and decrease sharply as Re increases, due to the collapse of the mass flow rate induced by the choked cavitation. The present results also confirm that the effect of fuel boiling point on thermal cavitation at temperatures above the boiling point is well correlated with the relationship between C-d and cavitation number, and the degree of choked cavitation as quantified by the cavitation numbers collapses to almost the same line, even for fuels with different boiling or bubble points.