GPPS-2018-47 OPTIMIZATION OF HIGH SUBSONIC , HIGH REYNOLDS NUMBER AXIAL COMPRESSOR AIRFOIL SECTIONS FOR INCREASED OPERATING RANGE

In this paper an increase of up to 4 % in surge margin while having more than a one percent increase in peak isentropic efficiency for an optimized stator of a high subsonic axial compressor was achieved. Using a simplified design method coupled with an optimization algorithm, only stacked stator spanwise blade elements were optimized and remaining the same rotor. Subsonic axial compressor can be exposed to different inflow situations along blade height with circumferential changes. In the present study, the optimization of each airfoil section is done by only varying three main parameters, which reduces computational time. The chosen approach combines a generalized parabolic arc mean camber with the so-called Class Function / Shape Function Methodology. The comparison to the original stator airfoil sections shows an increase in operating range of up to 22 % with 33 % lower minimum losses. A stator only investigation confirms the increased performance for all optimized airfoil sections. INTRODUCTION In recent years, several optimization methods for turbomachinery blade profiles have been developed. This includes parameterized airfoil sections with spline functions coupled with a normal-distributed random search code and a gradient method with the two-dimensional blade-to-blade flow solver MISES (Koeller et al., 1999). An increase of stall margin with lower pressure losses over the entire larger operating range was observed in comparison to controlled diffusion airfoils. In experimental investigations the widening in low-loss operating range and increased stall margin for those high Reynolds numbers airfoils were confirmed by Kuester et al. (1999). Similar achievements were made by Sieverding et al. (2004) with a genetic algorithm. For profile generation two fourth order Bezier splines with two linear functions as camber line for profile generation were utilized. Again, the flow solver MISES was implemented within the optimization procedure. Both approaches share a large number of geometric parameters for optimization, which is, in general, widely used in profile optimization techniques. In Giesecke et al. (2017) a geometry parameterization technique combined MISES and a single-objective genetic optimization algorithm with a suitable objective function. The geometry definition consists of a generalized parabolic arc line (Schlichting and Truckenbrodt, 1979) and the socalled Class Function / Shape Function Methodology (CSM) based on Kulfan and Bussoletti (2006) for aircraft shapes as a thickness distribution. Furthermore, in a comparison with high-performance profiles of the above-mentioned literature, a close agreement was shown. This approach results in a profile shape defined by only three optimization parameters and enables designs with large operating ranges. The developed airfoils were designed for modern compressor middle and end stages of industrial gas turbines, which operate at high Reynolds numbers and moderate subsonic Mach number. The corresponding Reynolds number is in the order of 2 to 6 x 10 6 . This is much higher compared to aero engine profile, which operate at Reynolds number of about 0.6 to 1.2 x 10 6 (Schreiber et al., 2000). The present study aims at applying the chosen approach on profiles operating at large Reynolds number. In particularly, the stator airfoil sections of a high subsonic, high Reynolds number axial compressor are optimized and the resulting stage performance is presented.