Experimental investigation of cavitation in a cylindrical orifice

Abstract

The atomisation of liquid jets is crucial to a wide range of applications including industrial processing, automotive fuel injectors and agriculture. While, the presence of cavitation in plain orifice atomisers is known to enhance this process, the complex flow mechanisms that result within the nozzle, such as periodic cavitation shedding, are not fully understood.

A new cavitation research rig was designed and built to investigate the influence of cavitation on atomization in large-scale plain orifice nozzles. Refractive index matching with an 8.25mm diameter acrylic nozzle using aqueous sodium iodide as the test fluid allowed clear optical access of the flow structures near the orifice wall. High resolution images of the flow structures recorded using high-speed visualisation were digitally processed to extract cavitation cloud shedding frequencies, collapse lengths, re-entrant jet motion and spray angles across a range of Reynolds numbers from 48,000 to 220,000 and cavitation numbers, K, from 1.1 to 3.5. Nozzle discharge coefficient was also measured across the entire experimental range.

Periodic shedding of cavitation clouds was found to occur for the partial cavitation regime. Spectral analysis revealed the frequencies associated with this shedding to vary linearly with K, from approximately 500Hz at K=1.8 to approximately 2500Hz at K=2.1. The mechanism behind this periodicity appears to be the motion of the re-entrant jet, largely driven by the adverse pressure gradient around the flow reattachment zone.

An inherent instability in the flow was observed during the transition from partial cloud cavitation to developed cavitation at approximately K=1.8. It is believed this instability is related to the slope of the pressure gradient in the stream-wise direction. Spray angles were significantly enhanced by the presence of cavitation in the nozzle, with the angle directly related to the distance between the collapse region and the nozzle exit.

Transition from hydraulic flip to supercavitation was regularly observed for cavitation numbers below K=1.7. This unusual tendency was shown to be related to the vertical inclination of the nozzle allowing liquid to flow back down the nozzle under the action of gravity and thus disturb the separated jet. Extensive discharge coefficient measurements closely follow 1D model predictions from the literature except for the region of periodic cavitation shedding where the model under-predicts the value.