Thrust of an Air-Augmented Water jet with a Converging-Diverging Nozzle

Abstract

This study is concerned with predicting the performance of an air-augmented waterjet with a converging-diverging nozzle. The device under study is one in which compressed air is injected into the high-pressure water stream leaving the water pump. A mixture of water and finely dispersed air bubbles is generated in a constant-area mixing chamber, which joins the water-pump outlet. The two-phase bubbly mixture enters the coverging-diverging thrust nozzle and expands under the action of a negative pressure gradient. The two-phase nozzle flow is analyzed through application of the conservation laws for a single air bubble and an incremental control volume over a nozzle section. Three different models which describe the heat transfer between the air bubbles and the water are considered. Results indicate that thrust augmentation is most effective for low pump-outlet pressures and for high mass flow ratios in the range permitting bubbly fow. The solution of the nozzle flow was accomplished by specifying the velocity distribution in the nozzle and calculating the nozzle geometry. Several expressions were used to calculate sonic velocity and it is shown that discrepancies exist between the computed results of the different expressions. Also none of the expressions place the sonic velocity at the throat. Thus, it is shown that further work is needed in determining sonic speed in two-phase compressible mixtures.