Effect of Injection Pressure on Nozzle Internal Flow and Jet Breakup under Sub-Cooled and Flash Boiling Test Conditions 2019-01-0286

Injection pressure plays a vital role in spray break-up and atomization. High spray injection pressure is usually adopted to optimize the spray atomization in gasoline direct injection fuel system. However, higher injection pressure also leads to engine emission problem related to wall wetting. To solve this problem, researchers are trying to use flash boiling method to control the spray atomization process under lower injection test conditions. However, the effect of injection pressure on the spray atomization under flash boiling test condition has not been adequately investigated yet. In this study, quantitative study of internal flow and near nozzle spray breakup were carried out based on a two-dimensional transparent nozzle via microscopic imaging and phase Doppler interferometery. N-hexane was chosen as test fluid with different injection pressure conditions. Fuel temperature varied from 112°C to 148°C, which covered a wide range of superheated conditions. Injection pressure was varied from 3MPa to 5MPa. The effect of injection pressures on nozzle internal flow and jet breakup is analyzed experimentally. As a result, decreasing injection pressure leads to smaller droplet size and better spray breakup near the nozzle exit under flash boiling test conditions. It is because lower injection pressure enhances the growth of bubbles inside of the nozzle and the spray primary breakup near the nozzle. It can be inferred that although higher injection pressure can increase the spray velocity near nozzle, the primary breakup is controlled by flash boiling bubbles inside of the nozzle. Therefore, higher injection pressure leads to better secondary breakup.