Transitional Flows Within Cooled HP Turbines

Abstract

Abstract All modern gas turbines, from a small business jet to a large civil engine, make use of film cooling within the HP turbine. Transition on the blade surfaces has a large impact on blade heat transfer and profile loss. Many studies have considered the role of freestream turbulence and surface roughness on transition, yet no systematic study of the role of film cooling on transition has been reported. This paper shows that film cooling is as potent at causing transition as roughness typically found on the suction surface of end-of-life engines, and more so than freestream turbulence. For the blades tested, film cooling induced transition begins at the rear of the suction surface, the transition point moving forward to the film cooling hole with a doubling in Reynolds number. Averaging the acceleration parameter over a fixed distance downstream of the cooling hole collapses the transitional behaviour for varied film ejection locations. A model is proposed to capture the Reynolds sensitivity of film cooling induced transition. The simultaneous presence of freestream turbulence and roughness as well as film cooling, are shown to enhance transition compared to any mechanism in isolation, despite the localised nature of the film cooling holes. It is likely that the combined effect of the mechanisms is maximised when each mechanism in isolation would result in transition at a similar Reynolds number. Therefore a method of mapping each transition mechanism in isolation is demonstrated to identify which mechanism is dominant, the proximity of other mechanisms and hence their likely enhancement of transition. By testing combinations of film cooling, roughness and freestream turbulence typical of a cooled HP rotor the transitional region is found to cover the range 300,000 < Rex < 1,000,000, where transition will be primarily dependent on surface roughness and film cooling. Below Rex = 300,000 transition is likely via a laminar separation bubble and above Rex = 1,000,000 the suction surface boundary layers will be fully turbulent. Large civil engines sit within the top of the transitional range at cruise and smaller engines span the entire range between cruise and take-off conditions.