Influence of stator-rotor gap on axial-turbine unsteady forcing functions

This paper investigates the effects of stator-to-rotor axial gap on the two-dimensional propagation of pressure disturbances due to potential-flo w interaction between the blade rows and viscous-wake effects from upstream blade rows in axial-turbine-blade rotor cascades. Results are obtained by modeling the effects of the upstream stator viscous wake and potential-flowfield on the downstream rotor flowfield, and computing the unsteady flowfields in the rotor frame. The amplitudes of the two types of disturbances for typical turbines are based on a review of available experimental and computational data. The potential-flowfield is modeled as a sinusoidal pressure disturbance of amplitude 4% of the local pressure across the stator trailing edges that decays downstream. This potential-flow disturbance from upstream is affected by the potential-flowfield of the downstream cascade. The velocity wake is modeled as a Gaussian velocity defect of varying amplitude and width, depending on the stator-rotor gap between the blade rows. The wake amplitudes and widths are based on conservation of loss of incoming momentum to the rotor due to the wake. The axial gap between rotor and stator is varied to show how the two disturbances propagating in different directions reinforce or counteract each other at different stator-rotor gaps. The corresponding forces on rotor blades are computed for typical values of reduced frequency. Analyses of this type will enable turbomachinery designers to predict (and with geometric design modifications to reduce) the unsteady stresses acting on turbomachiner y blades.