Numerical Studies to understand an Effect of Rotor-Stator Axial Gap on the Performance of Transonic Axial Flow Compressor Stage Coupled with Circumferential Grooved Casing Treatment

Abstract

Axial flow compressors are the backbone of most of the gas turbine engines as they have capability to achieve higher per stage pressure ratio (PR) and swallow higher mass flow rate per unit frontal area. The flow in the transonic compressor stages is very complex phenomenon mainly in tip region due to many reasons like tip leakage flow, boundary layers, shocks and their interactions. One of such phenomenon is early stall emergence. From the past literature study it has been observed that the lesser axial spacing between rotor-stator is associated with increase in compressor efficiency and the stall margin can be improved by applying appropriate type of casing treatment. The present work focuses on the flow behavior in a single stage transonic axial flow compressor coupled with the circumferential groove (CCG) casing treatment with varying axial spacing between rotor and stator. The purpose of this study is to understand the flow phenomenon associated with the effect of varying axial spacing (Cax) between rotor and stator of compressor stage coupled with CCG in delaying stall inception and thus to improve stage stability using Computational Fluid Dynamics simulation approach. Attempt has been made to understand this complex phenomenon using commercial code ANSYS Fluent to perform steady state RANS simulation along with 3D implicit pressure based solver and SST K-ω Turbulence model. Grid independence study was carried out prior selecting the grid size. The steady numerical solutions were obtained to derive the performance map of compressor stage for three different operating speeds of 80%, 90% and 100%. The results have been verified with experimental values at 80% speed carried out at Axial Flow Compressor Research Facility (AFCR) available at Propulsion Division, CSIR-National Aerospace Laboratories, Bangalore. The CFD predictions showing very good agreement with the experimental results. Analysis for Base Line Model (BLM) having Solid Casing Wall (SCW) with 75% axial spacing between rotor-stator has been carried out initially to understand the flow behavior in tip leakage region and span wise region then the CCG type casing treatment with 10 grooves of the constant aspect ratio of 4 covering the 100% rotor blade tip chord. The axial spacing of 25%, 50%, 75% and 100% of the rotor axial chord between rotor and stator was varied to understand its effect. The axial spacing was varied by moving the stator and keeping the rotor position fixed. All the results obtained by applying CCG and varying axial spacing between rotor-stator has been compared with the results obtained for BLM having SCW. The results obtained from the study for SCW is showing very good agreement with the experimental results. The results identified from the computation study for four different axial spacing with casing treatment suggest that the CCG with 75%Cax is the best configuration for achieving highest stall margin improvement. It is observed that increase in axial spacing with casing treatment arrangement achieves higher improvement in stall margin as compare to decreased axial spacing with casing treatment designs. The detail methodology, flow behavior and cause of effects is explained in this project thesis. The present work has been carried out at Axial Flow Compressor Research Facility (AFCR), Propulsion Division, NWTC, CSIR-National Aerospace Laboratories, Bangalore.