Riblet application in compressors: Towards efficient blade design?

Nowadays, modern axial compressors have already reached a very high level of development. The current study is focused on the question, if the application of riblets on the surfaces of a highly efficient modern compressor blade can be a further step towards more efficient blade design. Therefore, a highly loaded compressor cascade has been designed and optimized specifically for low Reynolds number conditions, as encountered at high altitudes and under consideration of the application of riblets. The optimization was performed at a Mach number of 0.6 and a Reynolds number of 1.5×105. Two objective functions were used. The aim of the first objective function was to minimize the cascade losses at the design point and at incidence angles of +5 and −5 degrees. The intention of the second objective function was to achieve a smooth distribution of the skin friction coefficient on the suction side of the blade by influencing the blade curvature in order to apply riblets. The MISES flow solver as well as the DLR optimizer “AutoOpti” were used for the optimization process. The developed compressor cascade was investigated in the transonic cascade wind tunnel of DLR in Cologne, where the Reynolds number was varied in the range of 1.5×105 to 9.0×105. Furthermore, the measurements were carried out at a low turbulence level of 0.8 percent and at a high turbulence level of 4 percent, representative for high pressure compressor stages. The measurement program was divided into two parts. The first part consisted of the investigation of the reference cascade. In the second part of the study riblets were applied on suction and pressure side of the cascade blades; two different manufacturing techniques, a rolling and a coating technique were applied. The rolling technique provides riblets with a width of 70 μm and the coated riblets have a width of 50 μm. The wake measurements were performed using a 3-hole probe at midspan of the cascade in order to determine the resulting losses of the reference blade and the blades with applied riblets. The detailed analysis of the measurements shows, that the riblets have only a slight influence on the viscous losses in the case of the compressor application in this study. Finally, these results are discussed and assessed against the background of feasibility and effort of riblet applications within the industrial design and manufacturing process