The effect of buoyancy on vortex breakdown in a swirling jet

The purpose of this experimental study is to explore the effect of buoyancy on vortex breakdown (VB) in swirling jets. Three non-dimensional parameters govern the flow: the jet exit Reynolds number, the swirl ratio and the Richardson number (buoyancy). The experimental apparatus consists of a vertical swirling water jet which discharges into a large tank. Moderate values of the Reynolds number are used, in the range 150 <= Re <= 600. Swirl is imparted to the jet in a rotating chamber whereas the temperature difference between the jet and its surroundings is established by passing the jet through a heat exchanger, immersed in a circulating water bath with a controlled temperature. Vector maps of the vertical mid-plane and horizontal cross-sections are obtained by particle image velocimetry (PIV) measurements. It is demonstrated that VB can be effectively suppressed (enhanced) when there is a negative (positive) temperature difference between the jet core and its surrounding fluid. The experimental critical swirl ratio for the appearance of VB is found to be in good agreement with a simple criterion, originally derived by Billant, Chomaz & Huerre (J. Fluid Mech., vol. 376, 1998, p. 183) for isothermal swirling jets and extended here to include buoyancy effects. The transition of VB from a closed bubble to an open cone configuration is mapped in terms of the Reynolds and Richardson numbers. Finally, the effect of the upstream velocity field on the critical rotation rate for the onset of VB and its configuration is exhibited using two different interchangeable rotating chambers.