Transition and heat transfer in highly accelerated rough-wall boundary layers

An experimental investigation was conducted to examine the effect of surface roughness on the convective heat transfer from a turbine vane airfoil. Both the effect of roughness on laminar-to-turbulent transition and on turbulent flow regions were of interest. The test apparatus was a large scale turbine vane cascade consisting of four vanes in a linear array. Detailed local surface heat transfer measurements were made on one of the vanes whose surface was roughened in different regions and to different degrees. The specific surface roughness conditions tested modeled those typically encountered on gas turbine airfoils. The effect of roughness in the high favorable pressure gradient flow on the pressure side of the vane was of primary interest.;The vane's suction side was very sensitive to the presence of surface roughness. The smallest roughness tested resulted in a laminar-to-turbulent transition occurring just 6.4 cm (0.22 axial chord) from the leading edge and with increases in the turbulent region Stanton number of 27 percent over that predicted for a smooth turbulent flow. The pressure side flow, dominated by a large favorable pressure gradient over much of its length, remained laminar for all but the largest roughnesses tested. In these latter cases, turbulent region Stanton numbers were about 30 percent higher than comparable smooth values.;An attempt was made to predict the measured heat transfer distributions using an existing model which accounts for the effects of roughness on turbulent flows. Although the predictions were in good agreement with the data on the suction side, the pressure side heat transfer data was over-predicted by as much as 50 percent. This suggests modeling roughness effects in high pressure gradients requires some additional considerations. The displacement thickness Reynolds number at transition with roughness is well correlated by the ratio of roughness size to displacement thickness, even with very large favorable pressure gradients.