The effect of buoyancy in controlling vortex breakdown in swirling jets

Abstract

In this study we explore the effect of buoyancy in controlling vortex breakdown (VB) in swirling jets. The experimental apparatus consists of a vertical swirling water jet which discharges into a large tank, in which the temperature difference between the jet and its surrounding is controlled. Three non-dimensional parameters govern the flow: the jet exit Reynolds number, the swirl ratio and the Richardson number (buoyancy). Vector maps of the vertical mid-plane and horizontal cross-sections are obtained by PIV measurements. It is demonstrated that VB can be effectively suppressed (enhanced) by prescribing a negative (positive) temperature difference between the jet core and its surrounding fluid. Moreover, the experimental critical swirl ratio for the appearance of VB agrees with a value predicted by simple criterion. Finally, the transition of VB from a closed bubble to an open cone configuration is mapped in terms of the Reynolds and Richardson numbers.