|Ph.D Thesis||Department of Aerospace Engineering|
|Supervisor:||Prof. Cohen Jacob|
|Full Thesis text|
Vortex breakdown (VB) is a remarkable phenomenon which may occur in swirling flows. It is characterized by a sudden deceleration of the flow near the axis and the formation of a stagnation point accompanied by the divergence of the stream surface. Downstream of the stagnation point a region of reverse flow is formed and the wake of the expanded vortical structure may undergo large-scale velocity fluctuations. As such, VB can have either detrimental effects, for example over delta wings at high angle of attack, or beneficial ones as a flame stabilizer utilizing the fast spreading and rapid mixing of the incoming flow with it's surrounding. Thus there is a strong need for better understanding of the phenomenon in an attempt either to prevent the breakdown or to promote it.
In this experimental study we explore the effect of buoyancy on VB in swirling jets and its possible use to controlling VB. Three parameters govern the flow: the flow rate, the angular momentum of the fluid and the temperature difference between the jet and the ambient fluid. These respectively correspond to the non-dimensional 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 of 150 < Re < 600. Swirl is imparted onto the jet in a rotating chamber whereas the temperature difference between the jet and its surrounding 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 the particle image velocimetry 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 is found to be in good agreement with a simple criterion, originally derived by Billant, Chomaz & Huerre (1998) 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 is exhibited using two different interchangeable rotating chambers.