Effect of Roughness and Unsteadiness on the Performance of a New Low Pressure Turbine Blade at Low Reynolds Numbers

Abstract

This paper presents a study of the performance of a high-lift profile for low pressure turbines at Reynolds numbers lower than in previous investigations. By following the results of Coull et al. (2008, "Velocity Distributions for Low Pressure Turbines," ASME Paper No. GT2008-50589) on the design of high-lift airfoils, the profile is forward loaded. The separate and combined effects of roughness and wake passing are compared. On a front loaded blade, the effect of incidence becomes more important and the consequences in terms of cascade losses, is evaluated. The experimental investigation was carried out in the high speed wind tunnel of Whittle Laboratory, University of Cambridge. This is a closed-circuit continuous wind tunnel where the Reynolds number and Mach number can be fixed independently. The unsteadiness caused by wake passing in front of the blades is reproduced using a wake generator with rotating bars. The results confirm that the beneficial effect of unsteadiness on losses is present even at the lowest Reynolds number examined (Re 3 = 20,000). This beneficial effect is reduced at positive incidence. With a front loaded airfoil and positive incidence, the transition occurs on the suction side close to the leading edge and this results in higher losses. This has been found valid for the entire Reynolds range investigated (20,000 ≤ Re 3 140,000). Roughening the surface also had a beneficial effect on the losses but this effect vanishes at the lower Reynolds numbers, i.e., (Re 3 ≤ 30,000), where the surface becomes hydraulically smooth. The present study suggests that a blade with as-cast surface roughness has a lower loss than a polished one.