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A flash boiling model is being developed to capture the effects of bubble nucleation and growth inside an injector. This model will be applied to the study of flash boiling fuel injector sprays in gasoline direct injection (GDI) automotive engines. There is potential for flash boiling to reduce drop sizes, increase spray cone angles, and ultimately produce more favorable combustible fuel mixtures for a GDI engine than before. In order for that potential to be realized, however, more work needs to be done to understand how flash boiling can be promoted by heating the fuel and introducing additional nucleation sites. With this concern in mind, a quasi one-dimensional nozzle flow model is being developed to help quantify the effects of various methods that could be used to promote flash boiling. This model features a discrete mass group approach to capture the effects of a bubble size distribution on bubble growth and void fraction development. Simulations were made for a converging water nozzle, and preliminary results show that capturing a bubble size distribution is important for multiple and distributed sources of bubble nucleation. Future work will involve coupling this internal nozzle model with a flashing spray model to predict the characteristics of flashing GDI sprays.