Experimental investigation of a high aspect ratio, low speed contra-rotating fan stage with complex inflow distortion

Abstract

This paper focuses on the response of a high aspect ratio, low speed contra-rotating fan with complex inflow distortion. The total pressure at the inlet is artificially distorted by means of two different sets of screens with different porosities to generate a hub-strong complex distortion and a tip-strong complex distortion. Detailed flow analyses were conducted for the design speed of rotor-1 in combination with off design speeds of rotor-2 both for the design and the peak pressure mass flow rates. In order to understand the extent of inlet distortion, the distortion sector was rotated circumferentially at intervals of 15° to cover the entire annulus. Detailed measurements of total pressure, static pressure, velocity components and flow angles were carried out at the inlet of the first rotor, between the two rotors and at the exit of the second rotor using three seven hole probes. The study reveals a few interesting aspects on the effect of complex inflow distortion on the flow behavior of a contra-rotating stage. The presence of a low porosity screen reduces the magnitude of axial velocity and generates higher spread of distortion near the localized region of placement of the screen. The hub-strong complex distortion has a greater effect of presence of this low porosity screen in both circumferential and radial directions. This leads to higher stagnation pressure variation at the inlet. On the other hand, for the tip-strong complex distortion case, the extent of distortion is observed to be higher in the circumferential direction towards the casing rather than radial. The localized improvement in the flow (in tip-strong inflow distortion) near the tip region improves the performance both in terms of pressure rise and efficiency of stage compared to hub–strong complex distortion.