Compressor Blade Modelling Under Highly Negative Incedence

Abstract

This paper investigates performance prediction techniques for compressor blades operating under highly negative incidence angle which is typical during engine groundstarts or windmilling relights. Although this is a very frequently occurring situation during the life of an aero engine, turbomachinery components are rarely tested under those conditions in the sake of resource saving. However, performance engineers require some knowledge of generic blade loss coefficients under those conditions for the preliminary estimation of the groundstart or relight capability of the engine which is also linked to design decisions such as the volume of the combustion chamber. A blade element concept is employed to break a 3D compressor blade design down to a number of 2D cross sections and study them separately using a CFD derived 2D blade loss coefficient database. Several different ways to synthesize the 3D blade out of the 2D sections are herein presented based on different expressions of blade aerodynamic coefficients. An investigation based on the expressions of the aerodynamic coefficients is conducted in order to justify the applicability of the blade element theory at such off-design conditions. The most suitable parameter set to represent a three dimensional blade design by a number of radially stacked two dimensional profiles is identified. The analysis shows that the approach based on pressure change and tangential force coefficients can more adequately approximate the performance of the 3D blade and therefore can be safely employed for a preliminary off-design blade performance studies.