Flash boiling spray comparison at fixed superheat index: independent control of fuel temperature and ambient pressure

Flash boiling is a promising technique for enhancing fuel spray, even in the absence of a high-pressure injection system. The superheat index (SI), defined as the ratio of ambient pressure (Pa) to saturated vapor pressure (Ps), serves as a representative criterion for classifying the flash boiling regime. It is found that the spray characteristics remain constant at the same SI, regardless of the fuel temperature (Tf) and Pa. However, it is anticipated that Tf and Pa influence the sub-processes of flash boiling spray, such as bubble nucleation, plume expansion, and vapor-induced breakup, in different ways. In this work, numerical and experimental approaches were combined to systematically investigate the effects of Tf and Pa. The pressure and vapor volume fraction (α) of the in-nozzle flow were obtained through simulations. Subsequently, the spray morphology, velocity field, vorticity field and droplet size distributions were measured using particle imaging velocimetry (PIV) and structured laser illumination planar imaging–laser induced exciplex fluorescence/Mie scattering (SLIPI-LIEF/MIE) techniques. It was found that under identical SI conditions, cases with higher Tf and Pa exhibited more intense in-nozzle heterogeneous nucleation and bubble growth, resulting in a higher α compared to lower Tf and Pa. Additionally, the higher Tf and Pa cases showed shorter plume width and penetration, lower radial velocity, and more small, weak vortices within the flow field. Under transitional flash boiling conditions, higher Tf and Pa result in larger and less uniformly distributed droplets, whereas under flare flash boiling conditions, higher Tf and Pa result in smaller and more uniformly distributed droplets.