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When heated fuel is injected into an ambient environment below its saturation pressure, the fuel could reach superheated state and experience flash boiling. Comparing with the non-flash boiling spray, namely the single phase liquid spray, flash boiling spray is characterized by its nature of two phase flow, due to vapor bubbles constantly generating inside the liquid phase. The behavior of those microscopic scale bubbles could introduce prompt spray atomization and vaporization, resulting in dramatically different spray characteristics. Comparing with the sprays generated via a high pressure injection system, the flash boiling spray has much shorter penetration, wider spray angle, more uniformly distributed mass, quicker evaporation, and smaller drop sizes, etc., which are ideal for the direct-injection (DI) gasoline and diesel engine applications without the hassle and the high cost associated with the high pressure injection system.

Liquid and vapor envelopes of sprays from a multi-hole fuel injector operating under closely-spaced double-injection conditions were investigated using a combination of high-speed schlieren and Mie scattering imaging. The effects of mass split ratio and dwell time between injections on liquid and vapor penetration have been investigated under engine-like pressures and temperatures. For the conditions evaluated, the results indicate that closely-spaced double-injection generally reduces liquid and vapor penetration.