Bubbly Flows through a Convergent-Divergent Nozzle

Abstract

Characteristics of bubbly flow with a small void fraction through a vertical, two-dimensional, converging-diverging nozzle are investigated experimentally and numerically. Emphasis is placed on the mechanism for large velocity slip near the nozzle throat, where the pressure gradient is very large. Bubble velocities are measured by taking double-exposure photographs with stroboscopic light sources having a flash duration of a few μ sec. The pressure distribution of the mixture along the nozzle axis is measured by semiconductor pressure transducers. The local liquid velocity is determined through continuity equations of gas and liquid in conjunction with the measured data of pressure distribution and experimental conditions at the nozzle inlet and exit. The power spectrum density of the pressure fluctuations is measured to investigate some instabilty of the bubbly flow, which is believed to be inherent to the velocity slip. It is proved that the numerical results using Wijngaarden's model equations agree well with the experiments. The characteristics of flow instability are explained according to the theoretical predictions of Morioka et al.