New observation of unsteady jets impingement caused by nozzle cavitation at varying ambient gas pressures

Previous studies showed that liquid sheet formed by impinging jets from two identical circular nozzle orifices break up in different patterns, in terms of closed rim, open rim, wave breakup, and catastrophic breakup. In this study, we examine the effect of ambient gas pressure pc from 0.11 to 5.0 MPa on the impinging jet atomization characteristics. Ambient gas density is fixed for varying pc conditions to exclude its influence on hydrodynamic instability. Results show that at high pc, the liquid sheet formed by impinging jets exhibit similar breakup regimes as reported in the literature. When pc decreases in a certain range, new impinging jet atomization behaviors are observed. The jets impinging process becomes highly unsteady as evidenced by the notable random displacement of the jet center axis, leading to offset and misaligned impingement, rotation of the liquid sheet and significant oscillations of the atomization behaviors downstream. We prove that the observed unsteady sheet breakup is caused by cavitation formed in the nozzle. The calculated cavitation number K show that at critical Kcr1=1.38 where the significant jet oscillations and unsteady impingement are observed, the nozzle discharge coefficient becomes to decrease, which indicates the onset of cavitation in nozzles. At low critical Kcr2=1.08, the unsteady impingement disappears as the cavitation induced perturbations of jet axis is attenuated. While the breakup length remains insensitive to variations in ambient pressure, the atomization angle sharply decreases by up to 20% at the onset of cavitation. These findings clarify the decoupled effect of ambient gas pressure and nozzle cavitation on impinging jet atomization, providing guidance for injector design to avoid potential engine startup failures and combustion instabilities.