Film cooling effectiveness distribution on a gas turbine blade platform with inclined slot leakage and discrete film hole flows
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A five blade, linear cascade is used to experimentally investigate turbine blade platform cooling. A 30 inclined slot upstream of the blades is used to model the seal between the stator and rotor, and 12 discrete film holes are located on the downstream half of the platform for additional cooling. The film cooling effectiveness is measured on the platform using pressure sensitive paint (PSP). The mainstream Reynolds number is 3.1*105 based on the inlet velocity and the chord length of the scaled high pressure turbine blade. The upstream slot covers 1.5 passages with the coolant exiting the slot at the leading edge of the rotor blades. The length-to-width ratio (ls/w) of the slot is 5.7, and the slot flowrate varies from 0.50% to 2.0% of the mainstream flow. The discrete film cooling holes also have an inclination of 30, so the length-to-diameter (lf/d) ratio of each hole is 10. The blowing ratio of the coolant through the holes varies from 0.5 to 2.0, based on the mainstream exit velocity. Using PSP it is clear that the film cooling effectiveness on the blade platform is strongly influenced by the platform secondary flow through the passage. Increasing the slot injection rate weakens the secondary flow and provides more uniform film coverage. Increasing the freestream turbulence level was shown to increase film cooling effectiveness on the endwall, as the increased turbulence also weakens the passage vortex. However, downstream, near the discrete film cooling holes, the increased turbulence decreases the film cooling effectiveness (as reported for flat plate film cooling studies). Finally, combining upstream slot flow with downstream discrete film holes should be done cautiously to ensure coolant is not wasted by overcooling regions on the platform.
