Tip injection is applied in high pressure gas turbine blades to improve the tip surface heat transfer, while it also alters the flow fields in the tip gap and near the tip regions. This paper evaluates the aerodynamic performance of tip injections for a linear turbine cascade. A previously investigated winglet shroud tip without (WS) and with seals (WSS) and a flat tip are considered as datum cases. Five jet holes are distributed on the winglet shroud tips but they are not constructed on the flat tip geometry. Four injection mass flow ratios, Mr,d, of the injection mass flow rate to the mainstream mass flow rate being 0.1%, 0.2%, 0.3% and 0.5% are examined using both experiments and CFD, while three additional Mr,d including 0.7%, 0.9% and 1.0% are further numerically studied. Influences of tip injections on loss changes under various jet mass flow ratios are pinpointed via analyzing the entropy generation rate and energy loss coefficient.
For Mr,d being 0.3%, the jet fluid penetrates into the near-tip region and enhances the upper passage vortex, especially for the WSS case due to the blockage effect of the seals. More severe velocity gradients and larger entropy generation rates are observed in the cascade for the WS and WSS tips with the tip jet (simply named by WSJ and WSSJ respectively). Compared with the flat tip, the WS and WSS tips reduce the energy loss coefficient by 18.98% and 33.86% respectively, while the WSJ and WSSJ bring smaller decrements of 15.89% and 27.08% separately.
Contrast to the energy loss changes, tip injection can help prevent the over-tip leakage (OTL) flow from entering into the tip gap. For Mr,d being 0.3%, the WSJ and WSSJ decrease the OTL mass flow rates at a gap inlet plane by 16.97% and 65.37% respectively relative to the flat tip. When compared to the corresponding non-injection WS and WSS cases, the WSJ and WSSJ achieve further reductions of 11.43% and 29.00% separately. However, in the WSJ and WSSJ cases, the OTL mass flow rate at a tip exit plane is not noticeably lessened as it also includes the increased injection mass flow apart from the leakage main fluid.
With the increase in jet mass flow ratio (Mr,d), the aerodynamic performance of the cascade with WSJ and WSSJ is gradually deteriorated. Particularly, the energy loss coefficient of the injection cases even becomes larger than that of the flat tip when Mr,d exceeds 0.7%. This change trend of the energy loss is also confirmed by an one-dimensional loss model analysis for the mixing process between the injected and the main streams.