Effects of recess depth and turbine reaction on the recessed tip thermal load in turbine blade cascades

Abstract

Both tip leakage loss and thermal load of a turbine blade need to be kept as low as possible. In this investigation, detailed thermal load and near-wall flow distributions for recessed tips of different hst/s (recess depth-to-span ratio) in a turbine cascade of outlet-to-inlet velocity ratio (VR) = 2.01 are obtained with the naphthalene sublimation method and oil film technique, respectively. The results show that there is no repetitive local thermal load change on the recess floor, contrary to the flat tip case. As the recess deepens, the average thermal load on the recess floor decreases consistently, whereas the mass-averaged loss tends to decrease, to become minimal for hst/s = 1.88%, and then to increase. Present recessed tip flow models based on the flow visualizations clearly explain the reason for these tendencies. The deep recessed tip of hst/s = 5.63% has higher leakage outflow rate and lower near-wall momentum over the recess floor than the shallow recessed tip of hst/s = 1.88%. The comparisons of the previous data for VR = 1.2 and 2.4 with the present one show that as VR increases, the average thermal load on the recess floor as well as on the flat tip tends to increase, regardless of hst/s. VR (turbine reaction) is considered as the most important parameter that influences tip average thermal load in different turbine blade rows.