Effects of Casing Roughness on Aerodynamic Performance in a Turbine Cascade

Abstract

To investigate effects of casing roughness on aerodynamic performance especially secondary loss, four cases of experiment conducted in a linear turbine cascade. Smooth endwall case was the baseline. The others were roughness cases of 0.001, 0.002 and 0.005 of ks/C size of roughness (sand grain) adhered to the endwall. One-dimensional hot wire anemometry of constant temperature anemometry applied for measurement of incoming endwall boundary layer and turbulence intensity. Total pressure and flow angle measured by 5-hole probe at the 1.2 axial chord (Cx) length downstream. Incoming endwall boundary layer thickness was increased but thickening boundary layer effect was negligible considering 200mm of blade span while momentum thickness grew significantly. Mass averaged loss coefficient at 1.2Cx downstream of blade leading edge was increased by 38% from smooth case to 0.005 of ks/C size of rough case. This increment was mostly due to stronger secondary motions of passage vortex and shearing motions between it and trailing filament and shed vortex which induced by higher level of incoming normal vorticity inside of incoming boundary layer as bigger size of roughness adhered. Increased roughness size on the endwall made greater velocity deficit toward the endwall so that incoming normal vorticity was grown. Flow exit angles of free stream were increased but those remained within measuring uncertainty. Mass averaged deviation angle was also reduced. But deviation angle difference was not considerable by considering over 100º of free stream turning angle. Therefore bigger incoming normal vorticity was the main reason of stronger streamwise voriticity rather than turning angle at the downstream according to classical secondary flow theory. Comparing inlet loss and downstream loss for smooth and rough cases, net increase in loss across the blade row was increased which was not similar as reported by many researchers for thin and thick boundary layer previously. It meant additional loss was raised through blade row. Mixing loss was thought to be the reason of additional loss due to higher turbulence intensity inside incoming boundary layer.