Shroud Cooling with Blade Rotation Using Discrete Holes and an Upstream Slot

Shroud film cooling above the blade row in a low-speed rotating turbine facility is investigated with coolant injected through an upstream slot in combination with injection through angled discrete holes on the shroud. Measurements of the film-cooling effectiveness are performed with coolant injection from the slot alone, from discrete-shroud holes alone, and from combined slot and discrete-shroud holes. Blowing ratios for the discrete holes are in the range of 1.0 to 3.0, whereas those from the slot are in the range of 0.5 to 2.0. The tests are performed at a scaled-down design rotation speed of 550 rpm using liquid crystal thermography. The results for the slot-only cooling tests show increasing cooling effectiveness up to a blowing ratio of 1.25, followed by decreasing effectiveness due to jet liftoff at the higher blowing ratios. When compared on the basis of blowing ratios, slot cooling provides a higher area-averaged film-cooling effectiveness up to a blowing ratio of 2.4, after which the shroud hole cooling for this configuration provides a higher area-averaged effectiveness. The results for the combined cooling show improvement in the area-averaged film-cooling effectiveness for all blowing ratios studied over the individual cooling configuration results at the same blowing ratios. With combined cooling, there is better penetration of the coolant further downstream of the region covered by the slot-only and discrete-hole-only configurations.