ABOUT THE DISTRIBUTION OF THE MACH NUMBER AND LOSSES UNDER CONSIDERATION OF THE DIFFUSION FACTOR: PROFILE DISTRIBUTION AND LOSSES FOR LOW PRESSURE TURBINE PROFILES WITH HIGH DIFFUSION FACTORS UNDER STEADY FLOW CONDITIONS

摘要

The results of this investigation consist of two linear cascades at high diffusion factors. The present measurements for each low pressure turbine profile were conducted at midspan under a range of Reynolds- and exit Mach numbers. The exit Mach number was varied in a range covering low subsonic up to values where a transonic flow regime on the suction side of the blade could be expected. The variation of the exit Mach number was also used to create different locations of the maximum Mach number and to evaluate the resulting total pressure losses. This work focuses on two profiles with a diffusion factor in a range of 0.18 ≤ DF ≤ 0.22, which is considered as a comparable level for the two cascades. The profile A is a front-loaded design and has shown no obvious flow separation on the suction side of the blade. Compared to the profile A the design B is a more aft-loaded profile which indicates flow separation on the suction side for all investigated Reynolds numbers. The integral total pressure losses were evaluated by wake traverses downstream of the profile. To determine the isentropic Mach numbers and the character of the boundary layer along the suction side of the profile, static pressure tappings and measurements with a flattened Pitot probe were carried out. Numerical studies were also conducted to investigate further the influence of a reduced turbulence intensity on the boundary layer of the suction side of design B. The results show that the optimum of the integral total pressure losses are significantly dependent on the Reynolds number. Therefore a correlation between the maximum Mach number on the suction side and the integral total pressure losses has been successfully established. A significant change of the turbulence intensity at the inlet of the cascade leads to shift of the location of the maximum Mach number. It also results in an equivalent change of the total pressure losses, which has been predicted by the trend line. However, the trend lines, which are based on the data of the integral total pressure losses of an attached boundary layer, are not able to predict the integral total pressure loss or the location of the maximum Mach number on the suction side of the blade since an open separation bubble occurs.