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The spray atomization process is composed by several breakup mechanisms and intensively investigated to develop a robust numerical model to accurately predict the droplet behaviours.The spray breakup can be divided into the primary and secondary breakup regime.The breakup mechanisms of these breakup regimes are various due to their different mechanisms.In the primary breakup regime,it is mainly affected by aerodynamic force,cavitation effects and the nozzle geometric designs.However,in the regime of secondary breakup,the major factor to affect spray characteristic is the aerodynamic force.In the primary breakup regime,the nozzle geometrical designs has been proved that it directly affects the spray characteristics.The nozzle inlet radius(r/D ratio)is the one of the most important nozzle parameters because of its closed relationship with cavitation formation and circulation zone inside nozzle.In the gasoline direct injection(GDI)injector,the cavitation has less effects on spray characteristics due to the lower injection pressure.However,the different circulation zone from different r/D ratios will directly affect the nozzle exit conditions such as velocity.Therefore,the effects of r/D ratio on spray characteristics in the primary and secondary regime are studied respectively by using ANSYS-Fluent and KIVA.Moreover,the nozzle exit results from nozzle flow simulation are used as the initial conditions for spray simulation to improve its accuracy.In this study,it indicates larger r/D ratio can increase the nozzle exit velocity and mass flow rate.However,due to the lower turbulent kinetic energy from larger r/D ratio,the spray atomization process slowes down.Also,the results from nozzle simulation can effectively improve the accuracy of the KH-ACT model due to the accurate perdition of turbulent parameters.The analysis of the breakup mechanisms show the turbulence induced breakup starts the initial breakup process in the primary breakup regime and aerodynamically induced breakup became a dominant role in the secondary breakup regime.