The Flow Field Within an Axial Flow Compressor at Extremely High Flow Coefficients

Abstract

This article describes an experimental investigation of the flow structures occurring in a three-stage axial flow compressor during fourth quadrant operation in the incompressible flow regime. In fourth quadrant operation, the flow coefficient exceeds the design value to such a degree that the pressure difference between the compressor inlet and outlet becomes negative, and the compressor acts as a badly designed turbine. The pressure rise characteristic curve thus extends into the fourth quadrant of the compressor map. A three stage axial flow compressor, with a mass flow rate of 2.7 kg/s and a pressure ratio of 1.022 was investigated. The design rotor tip Mach number is 0.2. Three operational points within the fourth quadrant were investigated, at flow coefficients of 0.665, 0.747 and 1.024. A five hole conical probe and a 50 μm diameter inclined hot film anemometer were used in this investigation. Radial traverses downstream of rotor rows and a time-dependent area traverse downstream of the first stage stator were performed. Three-dimensional steady-state and time-dependent numerical Navier-Stokes solutions for single blade passages in each blade row for each of the cases are compared with experimental work. Large wakes were observed downstream of all stator rows, as a result of significant flow separation on stator blades. The area fraction of the flow passage affected by the wakes increases as the flow coefficient increases. Flow through rotor blade-passages is heavily affected by the blade position relative to upstream stator wakes. Due to the effect of the stator wakes on downstream blading, time-dependent solutions using the nonlinear harmonic approximation were found to agree better with experimental results than steady-state solutions using mixing planes between blade rows.