Fluidic Jet Barriers for the Reduction of Leakage Loss in Shrouded Turbines

Abstract

Tip leakage flows are a significant source of performance reduction in shrouded turbines. In this study, the leakage loss was addressed using the concept of fluidic jet barriers. The fluidic jet was employed:
- To form a seal so that less flow entered the leakage channel and passed through the blade passage instead.
- To impart a tangential momentum to the leakage flow and turn it towards the direction of the blade exit bulk flow.
This potential was explored using three dimensional steady state RANS CFD simulations that were first validated against experimental data. A commercial test campaign conducted on a leakage flow analysis cascade at Durham provided the data. The cascade data was then compared with its corresponding CFD analysis. Five turbulence models were tested and the model that matched the experiment most closely was selected. Then the fluidic jet was implemented on a best practice design of a shrouded rotor used in the industry. For this, first a baseline case was designed using the Durham Cascade as reference. Then it was modified to incorporate the fluidic jet. Four fluidic jet configurations were tested and the best performing configuration was selected. The jet pressure was gradually increased up to the overblown condition, i.e., when a part of the jet fluid turned upstream and entered through the inlet cavity.
As predicted, the fluidic jet was successful in increasing the blade work and reducing the mixing loss. Furthermore, it also improved the yaw angle distribution at the rotor exit. Therefore, the present study served to show for the first time, that fluidic jets can be used to address bypass effect as well as re-entry mixing loss in shrouded turbines. The favourable impact on the downstream incidence indicated that they also have the potential to reduce the secondary losses in downstream rows.