Heat Transfer and Effectiveness on Film Cooled Turbine Blade Tip Models

Abstract

In unshrouded axial turbine stages, a small but generally unavoidable clearance between the blade tips and the stationary outer seal allows a clearance gap leakage flow to be driven across the blade tip by the pressure-to-suction side pressure difference. In modern high temperature machines, the turbine blade tips are often a region prone to early failure because of the presence of hot gases in the gap and the resultant added convection heating that must be counteracted by active blade 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 can be used in an attempt to directly reduce the heat transfer rates from the hot clearance flow to the blade tip. An experimental program has 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 the airfoil pressure loading distribution and only weakly, if at all, to the relative motion between blade tip and shroud. In the present work the clearance gap and blade tip region is thus modeled in stationary form with primary flow supplied to a narrow channel simulating the clearance gap above a plane blade tip. Secondary film flow is supplied to the tip surface through a line array of discrete normal injection holes near the upstream or pressure side. Both heat transfer and effectiveness are determined locally over the test surface downstream of injection through the use of thin liquid crystal coatings and a computer vision system over an extensive test matrix of clearance heights, clearance flow Reynolds numbers and film flowrates. The results of the study indicate that film injection near the pressure-side corner on plane turbine blade tips can provide significant protection from convection heat transfer to the tip from the hot clearance gap leakage flow.

Effects of the novel rib layouts on the tip leakage flow pattern and heat transfer performance of turbine blade

“…In a flat blade tip case, Kwak and Han 16 found that the coolant formed a film near the tip wall to isolate the reattachment of the high temperature leakage flow. Kim and Metzger 17 observed that the coolant aggravates the convective heat transfer of the blade tip within a certain range in comparison with the no coolant case. Ma et al.…”

Investigation on heat transfer of a rotor blade tip with various film cooling holes arrangements and groove depths

“…Meanwhile, improving the thermal environment along the blade tip is generally accomplished through impingement cooling and film cooling. Researches have been done in blowing from the tip by Kim and Metzger, 2 Kim et al, 3 Kwak and Han, 4,5 Ahnet al, 6 Christophel et al, 7 Acharya et al, 8 and Hohlfeld et al 9 They also summarized the influence of cooling holes geometry and configuration. Huitao Yang et al 10 performed numerical prediction of film cooling and heat transfer with different film holes arrangements on the plane and squealer tip of a gas turbine blade.…”