Studies on Spray Behavior of a Pressure Swirl Atomizer in Transition Regime

Abstract

Hollow-cone sprays produced by pressure-swirl atomizers find application in a wide range of propulsive systems. During throttling and under low-thrust requirements, however, the spray pattern may change from a fully developed hollow cone to a solid (collapsed) spray resulting in an increase in pollutant emissions, thermal spikes, and decrease in efficiency. Also, the solid and developing cone regimes are susceptible to flow instabilities. The transition regime from a collapsed to a developing hollow-cone spray from a pressure-swirl atomizer has been investigated in this paper. The present atomizer uses a helical insert having a swirl number of 3.6. At low injection pressures (ΔP inj ) the spray cone angle did not change appreciably with increasing ΔP inj as a result of the dominating surface tension forces. However, with increasing pressure, the conical sheet opens up. Under these conditions, the spray cone angle increases monotonically with injection pressure, finally leading to fully developed hollow-cone regime. Experiments were also carried out to determine spatial drop-size distribution and patternation. Average drop sizes were found to decrease from 150 to 100 μm for a twofold increase in injection pressure drop, along with a narrower drop-size spectrum with the transition from collapsed to developing cone regime. Increasing axial distance resulted in a narrower spray drop-size spectrum with slightly higher drop sizes. This could be attributed to the combined effect of coalescence, evaporation, and aerodynamic influences on drops. The drop size along the spray axis was found to be higher in comparison to the periphery for the collapsed spray regime, with a reversed trend in case of higher pressures. Mass distribution obtained using a mechanical patternator clearly reveals the flow features during the transition from collapsed regime to the developing cone regime.