Numerical and Experimental Investigation of Riblet-Coating Impact on Centrifugal Compressor Stage Performance

Abstract

Abstract Rising energy costs and increasing efforts to reduce greenhouse gas emissions strengthen the demand for highly efficient operation of centrifugal compressors. This applies both to new machines as well as to compressor units with long operation times. One possible measure to reduce drag in flow channels and thus increase compressor efficiency are microstructured riblet-surfaces. Riblets imitate the structure of shark skin and are streamwise aligned grooved surfaces which diminish the viscous drag in turbulent boundary layers. This paper presents the results of a numerical and experimental investigation regarding the application of riblet structures in centrifugal compressor stages focusing on the aerodynamic performance. Steady-state computational fluid dynamics (CFD) calculations were used for the riblet design in dependence on the design operating conditions. Two different riblet sizes were identified for the application in diffuser and volute. The expected impact resulting from these riblet structures was calculated for compressor design and off-design operation. Subsequently, the riblet structure was applied on the compressor volute by a dual-cure coating-system consisting of a clear coat, which combines UV-curable resins with aliphatic polyurethanes. To allow for a sufficient accessibility for the manual coating process, the volute was manufactured featuring a vertically split geometrical design. A closed-loop centrifugal compressor test rig was utilized to investigate the stage performance characteristics at three different tip speed Mach numbers while accounting for tip speed Reynolds similarity. In the scale-model compressor stage, consisting of a shrouded impeller, vaned diffuser and a circular volute, static and total pressures as well as temperatures were measured to quantify the riblet impact. As a main result, the experimental data show an increase in polytropic efficiency of up to 0.5–1.4 %-points for selected operating points based on the riblet coating of the outlet volute. A drag reduction in the volute is indicated leading to lower total pressure losses.