| In this paper, the mechanism of atomization and vaporization process as well as mixture formation of dimethyl ether (DME) flash boiling spray are investigated numerically and experimentally in detail. The research contents focus on the analyses of mechanism of spray mixing process and the construction of spray models under flash boiling condition, and the multi-dimensional simulation of DME flash boiling spray is then performed.In order to establish the non-phenomenological model of flash boling spray, a computational program is written to simulate the process of bubble growth in uniformly superheated liquid. The program is validated by the experimental results of superheated water. On the base of calculating the main thermophysical properties of DME accurately, the computational program is used to simulate the process of DME vapor bubble growth under flash boiling condition and the study of influences of ambient pressure and fuel temperature on the DME bubble growth is carried out. The results show that according to the curve of time varying interface acceleration, the process of DME bubble gorwth can be divided into three domains: surface tension controlled growth, transition domain and heat transfer controlled growth. The bubble growth behaves differently in the three domains, which results from the interaction and competition between the hydrodynamic pressure and thermal feedback effect.To understand the atomization mechanism of superheated fuel further, the researching range is extended to the spray upstream boundary, namely the internal flow of injector nozzle. Accoding to actual situation of fuel injection of engine, the bubbly flow is confirmed as the flow pattern in the injector nozzle. Considering the known one-dimensional and multi-dimensional flow models can not reveal more details of bubble growth in the injector nozzle, the "internal flashing mode" and "external flashing mode" are combined effectively in this paper and an improved flow model for the superheated liquid in the injector nozzle is developed under reasonable hypotheses. The established one-dimensional model can not only simulate the process of bubble growth in the injector nozzel, but also supply the more accurate initial conditions for the multi-dimensional simulation of DME flash boiling spray. The validity of model is testified by the comparison between predicted and experimental results of initial spray cone angle.As a useful tool to analyse the stability of fluid flow, linear stability analysis method is adopted widely in the studies of atomization mechanism of fuel. Based on the linear stability analysis method, a dispersion equation which can describe the instability of flash boiling jet is derived in this paper and the instability of DME jet under normal and superheated conditions are analyzed in detail. Conclusions can be made that under normal condition, increasing the relative velocity and decreasing the liquid viscosity are able to improve the atomization of DME jet. The influences of surface tension and surrounding air density on the instability of DME jet are relatively complicated, which needs to be treated differently under different conditions. Under superheated condition, the more obvious the effect of flash boiling is, the more unstable the DME jet is. This is another important point for flash boiling spray in obtaining better atomization compared with common spray.A lot of experimental studies reveal that the spray is atomized primarily by bubble micro-explosion under flash boiling condition, but the exact atomization mechanism concerning when and how the bubble breakup operates is not very clearly identified up to now. Considering the influence of viscous stress additionally, the linear stability analysis method is adopted to deduce the more complete dispersion equation to represent the disturbance development during the bubble growth, and a new criterion for bubble breakup is established. The results show the bubble becomes more unstable with the increase of bubble Weber number and void fraction. The breakup void fraction is nearly constant at the lower bubble growth rate with different initial radius ratios of droplet to bubble, while the breakup time tends to be shorter with the increase of bubble growth rate or the decrease of initial radius ratio of droplet to bubble.For the original droplet vaporizaiton model in KIVA is not valid for the simulation of flash boiling spray, it is improved on the base of two-zone model. The formulations of superheated droplet's radius and temperature varying with time are obtained by conservation of mass and energy. Through coupling the bubble breakup model and the improved droplet vaporizaiton model with KIVA, two groups of DME flash boiling spray within a common-rail injection system are simulated numerically. The study indicates that the predicted spray tip penetration agrees with the experimental result basically, while the calculated sauter mean diameter (SMD) of droplet agrees with that of experiment well, which indicates indirectly that the bubble breakup model and the improved droplet vaporizaiton model are valid.The ultra high-speed video system is applied to conducted the experimental research on DME flash boiling spray, and the investigations of spray figure and macroscopical characteristic parameters of spray under different nozzle opening pressure, nozzle hole diameter and ambient pressure are carried out in this paper. The results show that the spray tip penetration increases with the enhancement of nozzle opening pressure,while the influences of nozzle hole diameter and ambient pressure on the spray tip penetration are relatively complicated, which result from the competition between the positive factors and passive ones. In addtion, decreasing the nozzle opening pressure and ambient pressure, or increasing the nozzle hole diameter, will increase the spray cone angle. Finally, the multi-dimensional simulation of DME flash boiling spray is performed and the comparison between calculation and experiment has a good agreement.
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