Performance Limiting Flow Processes in High-State Loading High-Mach Number Compressors; Final technical rept. 15 Dec 2004-14 Dec 2007

摘要

In high-stage loading high-Mach number (HLM) compressors, counter- rotating pairs of discrete vortices are shed at the trailing edge of the upstream blade row at a frequency corresponding to the downstream rotor blade passing frequency. This is a consequence of an alternating change in blade loading caused by the passage of shocks or compression waves emanating from the downstream rotor. While this effect is present at any Mach number the combination of high loading and high Mach number mean that the attenuation of the rotor pressure field with upstream distance is much less than at either lower loading or subsonic Mach numbers, i.e., the upstream extent of rotor influence is a generic feature for HLHM compressors. Computations and physical arguments have shown that the pitchwise location at which these discrete vortices enter the rotor passage has a substantial impact on the rotor performance, for example work input and entropy (loss) generation. The features of the effect have been characterized by a reduced frequency parameter defined as the ratio of convective time for the vortex to travel from trailing edge of the upstream blade-row to the leading edge of downstream rotor to vortex shedding time (rotor blade passing time). Thus a change in inter-blade-row spacing, rotor wheel speed, through-flow velocity in the intrastator /vane- rotor gap, rotor pitch, and shock angle would result in rotor performance change. The flow in a two- dimensional diffuser subjected to a wake and jet is used to provide a physical context explaining the response of the rotor performance to the discrete wakes. This provides a more direct avenue for establishing the scaling of rotor performance change, appropriately non- dimensionalized, in terms of the reduced frequency, Mach number and characteristics of discrete vortices (i.e. its strength and Size).