Experimental Measurements of Vortex Breakdown with non-Isothermal Inflow

Abstract

Vortex breakdown occurs in many flow applications such as weather, aerodynamics, swirl combustors, and more, introducing unsteadiness that is often undesired. While vortex breakdown has been previously investigated extensively, researchers have struggled to efficiently characterize these flows due to the flow sensitivity to intrusive probes, and there is no real consensus regarding the process and reason of its formation. Recently, researchers have discovered the blue whirl, a silent and efficient flame which they believe is a mode of vortex breakdown, and suggested mechanisms of stabilization and prevention of it through temperature control. This thesis uses modern time-resolved measurement techniques to investigate breakdown at smaller temporal and spatial scales than previously researched, and shows the effect of heat addition at the inflow of a vortex generator on the onset and behaviour of breakdown modes. Smoke flow visualization experiments were preformed to identify heating and swirl rate effects on vortex flow, and the resulting swirl angle of the flow was measured. Decreasing the incoming swirl was found to delay or fully suppress the formation of a breakdown bubble. Increasing the inlet temperature had a similar effect due to the buoyancy effects on the flow increasing the axial velocity thus reducing the effective swirl. 3D Particle Tracking Velocimetry was used to obtain time-resolved flow fields of the vortex flow and breakdown with and without heating. Individual velocity profiles and velocity fields are presented, showing that flow behavior is dependent on much smaller scales than previously researched. Proper Orthogonal Decomposition is used to isolate energetically dominant modes and determine whether higher order modes are significant to the flow.