Development of a cavitation susceptibility meter for nuclei size distribution measurements

Cavitation inception in practical flows is invariably heterogeneous as nucleation sites are provided by microbubble populations. Microbubbles grow explosively, filling with vapour, when exposed to a critical pressure which is size dependent. The detection of physical bubble activations in a known pressure field can therefore be used to measure bubble size distributions. The nuclei population within a test flow can be measured using a venturi and by counting the number of activations or events using the acoustic emission from each bubble collapse in the downstream pressure recovery region. Such devices are known as Cavitation Susceptibility Meters (CSMs).

The development, calibration and operation of a CSM for use in the cavitation tunnel at the Australian Maritime College is described. The minimum pressure in the CSM is reduced in steps by increasing the flow rate or decreasing tunnel static pressure to activate increasing numbers of smaller nuclei in order to provide a cumulative size distribution. Simultaneous flow rate measurement permits nuclei volumetric concentration as well as venturi throat pressure to be determined.

The concentration measurement is shown to have an uncertainty of less than 0.5%, while the critical pressure has an uncertainty of approximately 5%. The volume measurement and timing uncertainties for flow rate calibration are found to account for 81% of this uncertainty. Sample nuclei distribution measurements are presented, showing critical pressures as low as 100 kPa below vapour pressure, corresponding to an equivalent bubble diameter of 1 µm in the test section.