An Experimental Investigation of Confined Cyclonic Flows and Ensuing Jet

Abstract

Confined cyclonic flows are being explored for their application in propulsion due to several advantages rendered by confined vortices which include, increased fuel residence time, turbulence, and propellant mixing, resulting in improved combustion efficiency. A number of analytical models have been presented that attempt to establish the inner flowfield of the confined cyclonic flows. However, in the absence of experimental data, these models remain unvalidated. Therefore the research work in this dissertation is motivated by the need to experimentally characterize the fluid dynamics of the vortex flows in the cyclonic chamber. Additional research focused on the swirling jets emanating from the cyclonic chamber and the role of vorticity on the jet characteristics. Particle image velocimetry measurements were taken for a modular hydro cyclone with tangential injection as an analog of the cold flowfield of cyclonic chambers. The analysis of the velocity profiles showed the bidirectional vortex behavior in the complete range of the operating conditions. The trend of variation in the characteristic parameters of the tangential velocity profile such as the core thickness, maximum tangential velocity, and wall boundary layer were found in agreement with the analytical solution. However, two additional flow features in the form of the high exit-bound velocity region of the head-induced flow and the reverse flow region were established. The vortex breakdown and resulting reverse flow region were eliminated by allowing passive flow entrainment or active injection from the axial head port. Proper orthogonal decomposition revealed the presence of coherent structures in the vicinity of the head injection port that suggests efficient mixing of the injected fluid. In addition to head port configuration, the exit contraction ratio was also found to significantly affect the flow. The influence of both the exit contraction ratio and injection area can be characterized together with the help of Swirl no, whereas the effect of the chamber aspect ratio can be adequately characterized with the vortex Reynolds number, V, and the off-swirl number, κ. Jet emanating from the cyclonic chamber was observed to be swirl-dominated, indicating a strong influence of the fluid rotation inside the chamber that resulted in reduced axial momentum and rapid decay. The configuration and operating condition of the axial head port greatly influenced the flowfield of the jet. Opening of the head port and subsequent injection reduced the axial adverse pressure gradient, thereby eliminating the reverse flow region along the chamber centerline. However, the unconstrained expansion of the swirling flow in the radial direction outside the chamber led to the bubble-type vortex breakdown of the jet. Further increase in the axial injection ratio, inhibited the breakdown and an axial jet was observed. The proper orthogonal decomposition showed a progressive increase in the mean modal energy with a higher fraction of axial injection.