Simulation of flow through conical diffusers with and without inlet SWIRL using CFD

One of the major problems encountered in the operation of hydraulic turbines (such as Francis turbines) is a rotating vortex rope which forms in the draft tube under part load conditions. Overall goal of the present research is to study the formation of this oscillating vortex rope using CFD and understand the fundamental mechanisms governing this flow phenomenon. A systematic step by step CFD approach is chosen starting from the simplest to the most complicated flow. The current CFD study reported here therefore aims at studying flows in conical diffusers with and without swirl as a simplified draft tube flow. Two test cases are considered, one is flow with inlet swirl and the other without swirl in a conical diffuser. CFD simulations were carried out using five different turbulence models, namely standard, realizable and RNG k-ε (along with the enhanced wall treatment for near-wall region), SST k-ω and the Reynolds stress model (RSM). Wall pressure coefficient along the diffuser, streamwise and circumferential mean velocity, turbulent kinetic energy (TKE) and Reynolds stress profiles are compared with the experimental data as well as CFD results from literature. It is shown that the moderate levels of swirl cause improvement in the pressure recovery in the diffuser as much as 15%. Also, the standard k-ε and RSM models perform best in predicting turbulent swirling flow behavior. Profiles of the streamwise velocity obtained from these models are in relatively good agreement with the experimental data (with maximum deviation of 25%), while the predictions of the SST k-ω show as much as 60% difference. Also, there is only 8% difference between the level of Reynolds stress obtained from the standard k-ε model and those from the experimental data. Overall, however, all turbulence models need to be improved in order to fully capture the details of the swirling flow in a diffuser and certainly the flow in a draft tube of a hydroturbine where vortex rope breakdown and/or boundary layer separation occurs.