| The current requirements for lowering the weight of the low-pressure turbines of modern aircraft engines have resulted in high-loaded blades with strong adverse pressure gradients over the aft suction surfaces. The stronger adverse pressure gradients increase the risk of boundary layer separation and the accompanying higher losses. In some cases the separated boundary layer fails to reattach to the surface resulting in very high profile losses. This thesis investigates the aerodynamic performance of a very highly-loaded low-pressure turbine airfoil, PAKD-A.; Measurements were made in a low speed linear cascade wind tunnel under steady flow conditions. The first investigation is to examine the influences of the typical levels of Reynolds number and freestream turbulence intensity on the blade loadings and the profile losses at the midspan of the blade. The second investigation is to examine the extent of the two-dimensional flow in the cascade by means of flow field measurements at inlet and outlet of the cascade across the full spanwise extend of the blade. Comparisons are made with the results for a more lightly-loaded airfoil, PAK-B, that was designed for the same inlet and outlet flow conditions.; It was found that PAKD-A has higher profile losses than PAK-B as expected. Both airfoils were found to have separation bubbles on the suction surface. The significant difference between them is that the PAKD-A separation bubble appears to "burst" and fails to reattach at a particular Reynolds number, while the length of the PAK-B separation bubble continues to increase with decreasing Reynolds number. Flow field measurements showed that all the measurements performed at midspan of cascade in the current study are essentially two-dimensional. |
