Automated Optimization of the Non Axisymmetric Casing Endwall of a Fan Rotor

Abstract

Abstract In this study the end wall casing of a fan rotor is modified in an automated optimization process in order to achieve a higher efficiency and to gain surge margin. The casing end wall is allowed to take non-axisymmetric shapes by means of different parameters describing the contour. The control points defining the contour have only the possibility to move upwards or downwards in the radial direction. Given these non-axisymmetric shapes and that the simulations are conducted in the steady state, this study is a theoretical case not realizable in reality. The main aim is the understanding of the influence of end wall shaping on the flow and what the flow characteristics would be if the losses were minimized. The DLR in-house CFD code TRACE is used. The used optimization tool is AutoOpti, a DLR developed tool, which is based on a genetic algorithm speeded up by surrogate models. During the optimization two operating points are considered: One is calculated at a fixed backpressure (OP1) representing the operating point at maximum stage efficiency for the smooth casing and the other one (OP2) is an operating point near the surge limit calculated at a fixed outlet mass flow regulated by a PID controller. The optimization delivered some improvements that can be seen in the reduced losses in the tip area of the OP1 as well as in the surge margin. It resulted as well in particular surface distortion patterns on the contour responsible for a change in the static pressure distribution in the tip area. The study mainly focuses on the aerodynamic explanation of these improvements analyzed in different members.