Turbomachinery Wakes: Differential Work and Mixing Losses

Abstract

In this paper the mixing of stator wakes in turbomachinery is considered. An extension is made to the existing model of Denton (1993, ASME J. Turbomach., 115, pp. 621–656) which addresses the effects of acceleration before mixing. Denton showed that if a total pressure wake was accelerated, mixing loss diminished, and vice versa. Here a total temperature wake is shown to exhibit a reverse trend. An attempt is also made to understand better the work transfer process between a stator wake and a rotor. The paper concentrates on axial turbines, but a brief look at compressors is included. It is argued that the free-stream work is not the same as the wake work, and the concept of “Differential Work” is introduced. A simple steady velocity triangle based model is proposed to give an estimate of the ratio of wake work to free-stream work (μ, see later). The model is compared to an unsteady CFD result to offer some verification of the assumptions. It is concluded that the rotodynamic work process tends to reduce total pressure wake depths in turbines and compressors and therefore mixing losses. The mixing loss due to total temperature wakes is less strongly affected by the differential work process. [S0889-504X(00)00801-1]