Film Cooling in Unsteady Flow With Separation Bubble

Abstract

This study gives a detailed experimental evaluation of film cooling characteristics in unsteady flow with a separation bubble. The research project is divided into two phases. In the first phase, which is presented here, only the variation of the velocity caused by upstream blades is simulated in the experiments while the free-stream turbulence intensity is retained at a constant low level. The experiments are carried out on a flat plate with superimposed pressure distribution typical of turbine blading. A contoured wall opposite the flat plate generates this pressure distribution with a strong adverse pressure gradient, which induces a separation bubble in the middle of the plate. The measurements are conducted in an open-circuit, low-speed, suction-type wind tunnel, which can generate periodically pulsating flow. The flat plate is 1000 mm in length and has a width of 400 mm. The cooling air enters the test section through a row of 7 cylindrical film-cooling holes with sharp edges and an inclination angle of 35 degrees. The film cooling holes, which are 8 mm in diameter and have a pitch to diameter ratio of 3:1, are located in the middle of the flat plate. The main objective is to investigate the influence of the separation bubble on the cooling air flow and different film cooling parameters under periodically unsteady flow conditions. Therefore, measurements of the flow velocity and temperature using hot and cold wire anemometry for different boundary conditions were carried out. The results show that the periodic changes of size and shape of the separation bubble in a film cooled flow field under unsteady flow conditions are still dominated by the superimposed periodically changing pressure distribution. The incoming cooling air influences the separation bubble in two ways. On the one hand, the separation bubble is displaced by the film cooling jet, which means that it is only present upstream of the air injection point and on the other hand the separation bubble is thicker directly in front of the incoming film cooling jet because of the superimposed pressure field upstream of the jet. The results of flow temperature measurements show a small low-temperature area upstream of the film cooling jet at the position of the separation bubble.