Measurement of nuclei seeding in hydrodynamic test facilities

Microbubble populations within the test section of a variable-pressure water tunnel have been characterised for various operating conditions. The tunnel was operated with demineralised water and artifcially seeded with microbubbles from an array of generators located in a plenum upstream of the tunnel contraction. The generators produce a polydisperse population of microbubbles 10–200 μm in the diameter. The microbubbles are generated from supersaturated feed water within a confned turbulent cavitating microjet. The generator and tunnel operating parameters were systematically varied to map the range of nuclei concentrations and size ranges possible in the test section. Microbubbles were measured with Mie Scattering Imaging (MSI), an interferometric sizing technique. A new method was introduced to calibrate the detection volume and extend the dynamic range of the MSI. The acquisition and processing of microbubble measurements with MSI have a fast turn-around such that nuclei concentration measurements are approaching real time. Estimation of the total bubble concentration was within 5% of the sampled concentration after only 100 detections but 10⁴ were necessary for full histogram convergence. The tunnel is operated with water at low dissolved gas content to ensure all injected microbubbles dissolve and do not complete the tunnel circuit. As a result of this, the injected population is altered by dissolution as well as pressure change during the short residence between plenum and test section. The transformation is shown to be complex, changing with tunnel operating conditions. The measured test section nuclei populations were found to follow a power law for the higher concentrations. Test section nuclei concentrations of 0–24 mL⁻¹ can be achieved through variation of generator and tunnel operating parameters.