Flow Randomness and Tip Losses in Transonic Rotors

Abstract

The results of a test program incorporating a dual hot-wire aspirating probe to radially survey the exit flowfields of two axial-flow transonic fans are presented. The probe measures time-resolved total temperature and total pressure, from which the instantaneous isentropic efficiency is calculated. A technique for quantifying the randomness in an unsteady turbomachine flowfield is developed. Randomness is quantified by blade wake aperiodicity (variation of blade passage period) and non-uniformity (variation of blade wake shape). A method of ensemble averaging instantaneous data is presented which produces an identifiable blade passage wake profile even in a random flowfield where traditional techniques often fail. The flowfield randomness for the two fans is shown to correlate well with the respective tip losses. Results also show that the overall efficiency of the best blade on a fan rotor can be 0.5% higher than the performance of the rotor overall average or as much as 0.7% higher than the worst blade on the rotor.