Numerical Simulation of a Direct-Acting Piezoelectric Prototype Injector Nozzle Flow for Partial Needle Lifts

Actual combustion strategies in internal combustion engines rely on fast and accurate injection systems to be successful. One of the injector designs that has shown good performance over the past years is the direct-acting piezoelectric. This system allows precise control of the injector needle position and hence the injected mass flow rate. Therefore, understanding how nozzle flow characteristics change as function of needle dynamics helps to choose the best lift law in terms of delivered fuel for a determined combustion strategy. Computational fluid dynamics is a useful tool for this task. In this work, nozzle flow of a prototype direct-acting piezoelectric has been simulated by using CONVERGE. Unsteady Reynolds-Averaged Navier-Stokes approach is used to take into account the turbulence. Results are compared with experiments in terms of mass flow rate. The nozzle geometry and needle lift profiles were obtained by means of X-rays in previous works. Simulations are able to properly capture the relationship between instantaneous partial needle lifts and the corresponding rate of injection. The difference in mass flow rates between simulations and experiments is below 5%, although experiments show some oscillations that are not predicted by the model employed here. In order to simulate the experimental rising slope of the injected mass, the pressure evolution at the inlet boundary condition had to be modified, increasing the pressure from the discharge value up to the injection pressure. This modification in the upstream pressure value seems to be in accordance with the literature. Cavitation phenomenon and flow detachment have been observed downstream the needle seat for low lift values. The converging shape of the orifices overrides cavitation inside them, and also allows the flow to reattach even when the length to diameter ratio is below 10. However, fuel vapor is still present at the orifices exit for low needle lift cases, which affects velocity and fuel distributions within the sprays.