Unsteady Work Transfer Within a Turbine Blade Row Passage

Abstract

There is evidence in the literature of strong static pressure pulsation on the suction and pressure sides of turbine aerofoil rows experiencing unsteady wake interaction from upstream blade rows. This evidence is both computational and experimental. It has been proposed that this unsteady pulsation causes an unsteady work transfer between the wake and free-stream fluid. It has also been proposed that such a work transfer process may cause a reduction in downstream mixing losses and, thus, an improvement in turbine efficiency. While the literature has provided evidence of the existence of such pulsations, there is no clear explanation of why they occur. This paper addresses the above topic from an analytic perspective; a one-dimensional, incompressible, linear solution to the governing differential equation is used to shed light on this behavior. While the model lacks a lot of physical detail, the pulsation effect is captured, and it is shown that the unsteady pressure attenuates the amplitude of the unsteady total pressure through the transfer of work. The significance of reduced frequency is also clearly demonstrated with convection dominated flow at low reduced frequency and unsteady work dominated flow at high reduced frequencies. The model allows the specification of the amplitude and phase of the down stream static pressure perturbation. This variability is shown to have a significant effect on the attenuation of the unsteady total pressure. New insight is provided in to the much studied topic of “clocking” in axial turbines.