EXPERIMENTAL INVESTIGATION OF ADDITIONAL LOSS ASSOCIATED WITH INCOMING WAKES IN LOW-PRESSURE TURBINE CASCADES

The present paper aims at experimentally investigating the influence of wake-passing frequency, Reynolds number and suction surface diffusion rate on the additional loss due to incoming wakes to the profile loss in low-pressure turbine (LPT) cascades and discussing a design guide for reducing the additional loss. Using a moving-bar mechanism, the unsteady effects of incoming wakes were measured in a low-speed linear turbine cascade facility. The wake-passing frequency was varied by adjusting the moving-bar speed. The different airfoils with different surface velocity distributions were tested to examine the effect of suction surface diffusion rate. For each test case in an unsteady flow condition, the additional loss due to incoming wakes was derived by subtracting the profile loss from the measured total pressure loss across the cascade. Here the profile loss was estimated by using the friction drag force, which was calculated by the measured surface velocity distribution, and the pressure drag force, which was calculated by the measured surface pressure distribution and the predicted base pressure. The resultant additional loss includes all kinds of losses associated with incoming wakes, such as mixing loss of incoming wakes enhanced in the blade passage and unsteady interaction loss between incoming wakes and surface boundary layers. It was demonstrated that, in an unsteady flow condition, a substantial performance improvement was obtained in the entire Reynolds number range by applying the surface velocity distribution with no laminar separation to a low solidity blade row.