Computational analysis of swirling flows in a pipe

Abstract

The vortex breakdown of a swirling jet flow entering a finite-length pipe is studied in this thesis. The theories of Rusak and co-authors which provide fundamental tools to predict the first occurrence of breakdown and simulate the flow behavior are applied. To demonstrate the ideas, the detailed experimental data of Novak and Sarpkaya (2000) are used, specifically, the upstream (inlet) axial and circumferential velocity profiles ahead of the breakdown (stagnation) point. The critical swirl ratios, ω0 and ω1, that respectively form the necessary and sufficient conditions for the occurrence of breakdown in a swirling jet flow, are computed from the ordinary differential equations of the problem. It is found that for the upstream velocity profiles ω0 = 0.5607 and ω1 = 1.35196. The swirl level in the experiment of Novak and Sarpkaya (2000) was ω = 1, and it shows that vortex breakdown may occur downstream of the inlet in the vortex flow field, as indeed is found in the experiments. Moreover, the experiments provide flow profiles along the whole pipe which are compared with simulation results based on Granata (2014) for a swirling flow in a pipe that has the same inlet conditions. An agreement is found between the simulated results and the experimental data all along the pipe from the upstream inlet state up to the breakdown point. Behind the breakdown point, no concise agreement is found which may be due to the high turbulence in the high-Re experimental flow or a result of non-full convergence of simulated results. The present theoretical analysis and simulations shed light on the breakdown process of swirling jet flows in pipes.