Linear cascade wind tunnel testing of supersonic inflow and outflow turbine blades (test section design and flow visualization using schlieren techniques)

A linear cascade wind tunnel was developed to verify starting characteristics of supersonic inflow and outflow turbine cascade. Under the present near sonic conditions, the wind tunnel in itself is susceptible to become unstarted due to the blockage effect of the wind tunnel wall boundary layer. Hence, the present wind tunnel has special bypass flow passages, which were properly designed so as to remove the boundary layer without affecting the cascade flows. In addition, supersonic cascade flows may be affected by shock reflection stemming from the wind tunnel wall surface. Numerical simulations indicated that it is impossible to remove shock reflection just by sucking flows through a perforated wall surface if the incident shock wave is strong. In the present study, the wall surface was shaped wavy according to the flow turning induced by the incident shock wave, which shape was determined from the blade wake line predicted by numerical simulation of infinite linear cascade. The designed wind tunnel and non-reflective wall shapes were actually tested in the experiment, and shock patterns were visualized using Schlieren techniques. Test results indicated that the tested turbine cascade can be started as originally planned, and that shock reflection on the wind tunnel wall surface was clearly attenuated. The design methodology of supersonic turbine blade shape is confirmed to be valid regarding the starting characteristics at the designed inlet Mach number. Moreover, the wavy porous wall with suction is found to be effective to give the adequate cancellation of the reflected shock waves. © 2013 The Japan Society of Mechanical Engineers.