Recirculating Cells and Vortex Structure in Shear Layers of Double Coaxial Pipe Jets

Abstract

Flow of double coaxial pipe jets has been experimentally and numerically studied. Utilizing the tandem-type hot-wire technique and smoke-wire technique, the present investigation confirms the existence of recirculating cells around the axis of circular coaxial jets when the velocity ratio of inner jet to outer jet is small. It is found that the cells expand when the velocity ratio decreases and the outer pipe length increases. Quantitative agreement is found between the hot-wire mean velocity measurements and the numerical calculations by the standard k-ε model along the jet axis in the presence of cells. Several nozzles with outer pipe lengths of L/D₀=0 to 2 are employed to examine interaction of inner and outer vortices. It is found that the correlation of velocity fluctuations between the inner and outer mixing regions increases markedly when the outer pipe lengths are L/D₀=0.5 to 1. The measured distributions of phase-average velocity perturbation and vorticity show the coherent structures in the inner and outer mixing regions of the jets.