Numerical simulation of the impeller tip clearance effect on centrifugal compressor performance

Abstract

This thesis presents the numerical simulation of flow in centrifugal compressors. A three-dimensional Navier-Stokes solver was employed to simulate flow through two centrifugal compressors. The first compressor simulated was the NASA low speed centrifugal compressor (LSCC). The LSCC was selected as a test compressor because of the experimental data available. The numerically simulated data was compared to the experimental data to validate the flow solver. The numerical results were in good agreement with the experimental. The second compressor simulated was a Honeywell turbopump centrifugal compressor (HCC). The HCC was simulated for three tip clearances at six wheel speeds. Six operating conditions were investigated at each tip clearance and wheel speed. The data obtained from the HCC simulations was used to investigate the effect of tip clearance on compressor performance. Specifically, the total-total pressure ratio, mass flow rate and adiabatic efficiencies were tracked to see how each were affected by an increasing tip clearance. The results show that as the tip clearance was increased, the total-total pressure ratio, mass flow rate and adiabatic efficiencies all decreased for the same static-total pressure ratio. The pressure differences and temperature differences, from the pressure side to the suction side of the blade, and the vorticity were all explored as possible causes of this reduced performance. The pressure proved to be the biggest impactor on performance. The increased tip clearance let more flow leak across the blade tip. The leakage flow tried to homogenize the pressure difference across the blade. The overall effect resulted in a much lower pressure difference. The temperature remained nearly the same as the tip clearance increased. The vorticity increased, but this was also a result of the increased tip leakage flow. To aid in total-total pressure ratio predictions, the results for the HCC were put into compressor maps.