Heat Transfer to an Actively Cooled Shroud With Blade Rotation

Abstract

An experimental study of the shroud heat transfer behavior and the effectiveness of shroud cooling are undertaken in a single-stage turbine at low rotation speeds. The shroud consists of a periodic distribution of laterally oriented cooling holes that are angled at 45 deg to the shroud surface in a repeating circumferential pattern and has five unique hole pitches in the axial direction. Measurements of the normalized Nusselt number and film cooling effectiveness are done using liquid crystal thermography. These measurements are reported for the no-coolant case and nominal blowing ratios (BRs) of 1.0, 1.5, 2.0, 2.5, and 3.0. The tests are performed at an inflow Reynolds number of 17,500 corresponding to a scaled down design rotation speed of 550 rpm, and two off-design speeds imposed by a motor: (1) a rotation speed below the design speed (400 rpm) and (2) a rotation speed above the design speed (700 rpm). The results at the design speed show that increasing the BR increases the area-averaged film cooling effectiveness, while the Nu/Nu0 in the shroud hole region decreases. As the rotor speed is changed from the design speed, the high Nu/Nu0 region migrates on the shroud surface. This migration affects the coolant coverage in the shroud hole region resulting in increased coolant coverage at below-design rotation speeds and decreased coolant coverage at above-design rotation speeds. At all rotation speeds, as the BR increases, the area-averaged film cooling effectiveness in the shroud hole region increases. Decreasing the circumferential shroud coolant hole spacing changes the lateral heat transfer profile from a periodic sinusoidal distribution for a shroud hole spacing of P/D = 10.4 to a more even distribution for a smaller shroud hole spacing (P/D = 4.8).