Evaporation and atomization characteristics of dual-fuel system under flash boiling conditions

Highlights: • Effects of T_f and P_a for dual-fuel spray on flash-boiling phenomenon are revealed. • Light fuel addition could help heavy fuel spray get flash-boiled. • P_a/P_s is not sufficient to characterize flash-boiling spray for dual-fuel system. • A new superheat index is proposed for dual-fuel system. -- Abstract: Flash boiling sprays are formed when injecting the fuel into an environment with the local pressure below the saturation pressure of the fuel. The formation of flash-boiling bubbles can enhance the atomization and evaporation of the fuel spray, thus enhancing fuel-air mixing characteristics. A significant factor impacting the performance of flash-boiling sprays is the co-evaporation of fuels with various volatilities, especially for combusting practical fuels. In the present study, two single component hydrocarbon fuels (i-pentane and n-undecane) with remarkably different boiling points were mixed with each other to examine the multi-components impact on the flash boiling properties of the resultant fuel spray. Experimental studies on spray structure with various blending ratio, fuel temperature, and ambient pressure were conducted in a constant volume chamber. The difference in spray penetration and collapse rate among various blending ratios obtained from high-speed Mie scattering measurements were compared. Results showed that fuel with a higher volume fraction of i-pentane had a higher tendency to achieve flash boiling as fuel temperature increase or ambient pressure decrease. Superheat index, i.e. the ratio of ambient pressure and saturated pressure was examined for predicting the flash-boiling phenomenon of the dual fuel system, was found no longer completely applicable for dual-fuel system. An equivalent saturation pressure considering both dewing and bubble point pressures, and molar fraction of light fuel component is proposed to calculate superheat index which can be used to predict flash-boiling behavior of a dual-fuel system.