HEAT-TRANSFER AND EFFECTIVENESS ON FILM COOLED TURBINE BLADE TIP MODELS

In unshrouded axial turbine stages, a small but generally unavoidable clearance between the blade tips and the stationary outer seal allows a a clearance gap leakage flow to be driven across the blade tip by th e pressure-to-suction side pressure difference. In modern high-tempera ture machines, the turbine blade rips am often a region prone to early failure because of the presence of hot gases in the gap and the resul tant added convection heating that must be counteracted by active blad e cooling. The blade tip region, particularly near the trailing edge, is often very difficult to cool adequately with blade internal coolant flow; and film cooling injection directly onto the blade tip region c an be used in an attempt to reduce the heat transfer rates directly fr om the hot clearance flow to the blade tip. An experimental program ha s been designed and conducted to model and measure the effects of film coolant injection on convection heat transfer to turbine blade tips. The modeling approach follows earlier work that found the leakage flow to be mainly a pressure-driven flow related strongly to the airfoil p ressure loaning distribution and only weakly if at all, to the relativ e motion between blade tip and shroud. In the present work the clearan ce gap and blade tip region is thus modeled in stationary form with pr imary flow supplied to a narrow channel simulating the clearance gap a bove a plane blade tip. Secondary film flow is supplied to the tip sur face through a line array of discrete normal injection holes near the upstream or pressure side. Both heat transfer and effectiveness are de termined locally ol er the test surface downstream of injection throug h the use of thin liquid crystal coatings and a computer vision system over an extensive test matrix of clearance heights, clearance flow Re ynolds numbers, and film flow rates. The results of the study indicate that film injection near the pressure-side corner on plane turbine bl ade tips can provide significant protection from convection heat trans fer to the tip from the hot clearance gap leakage flow.