REDUCTION OF PRESSURE LOSSES IN A LINEAR CASCADE USING HERRINGBONE RIBLETS

This paper presents the results from the first experimental assessment of herringbone riblets in reducing total pressure losses in a linear cascade of diffuser blades. The experiments were undertaken at Re=1×105, M=0.13 and a free stream turbulent intensity of 2%. Three cascade configurations were examined at a blade incidence angle of 0.8˚; Case A: the baseline case without surface modification; Case B: blades with smooth strips; Case C: blades with ribleted strips. In Case A, flow separation starts at 24.1%c from the blade leading edge followed by a complete stall, resulting in significant total pressure losses as measured by a five-hole probe on a cross-flow plane downstream. Seven smooth or ribleted strips were adhered on the blade suction surfaces along their span in Case B and Case C. In comparison to Case A, the average total pressure loss coefficient is decreased by 6.4% and 16.8% in Case B and Case C, respectively. The velocity vectors leaving the cross-flow measurement plane also appear to be more uniformly distributed with the average flow turning angle being increased by 4˚ and 10˚ in Case B and Case C respectively, indicating that the extent of flow separation in the cascade has been reduced substantially. Furthermore, a pseudo sound power analysis of hot-wire data in the blade wake reveals a reduction in the noise level of 1.1dB and 1.6 dB, respectively. These results hence provide strong evidence that a profound aerodynamic improvement can be achieved in a cascade with the use of herringbone riblets.