Flash Boiling: Easy and Better Way to Generate Ideal Sprays than the High Injection Pressure

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. This new approach provides new opportunities for the DI engine combustion system design. Understanding the physical mechanisms involved in the flash boiling spray formation is crucial to achieve its full potential. In this study, the flash boiling spray from a multi-hole DI injector was investigated in a constant volume chamber using various laser diagnostic techniques. The macroscopic spray structure, flow field, droplet size and vapor concentration were obtained under typical conditions encountered in the DI engine operations. Non-dimensional analysis was used to correlate the spray characteristics under the wide range conditions. The results show that the dimensionless numbers governing the liquid spray formation, which include Reynolds number (Re) and Weber number (We), are no longer important and adequate to describe flash boiling spray formation process. It is found that the ratio between ambient pressure and liquid saturation pressure (P_a/P_s) plays an important role during the spray flash boiling with good correlations to the spray characteristics. Based on those correlations, a phenomenological model consisting of a set of empirical equations is proposed to quantitatively describe the flash boiling spray characteristics. Those equations reveal the unique mechanism related to the flash boiling spray formation, and explain why the flash boiling spray behaves so differently from the liquid spray.