Transonic Turbine Vane Wake Flows

Abstract

This paper summarizes the research on transonic turbine vane wake flows carried out in a Transonic Planar Cascade at the National Research Council of Canada between 1987 and 1995. The cascade used in the research is a large scale, continuously operating, inflow facility which was developed to study both flow phenomena and aerodynamics of turbine vanes. Research was initially directed at investigating the apparent redistribution of total temperature (energy) from the centre to the edge of the vane wake. This redistribution was found to be a real physical phenomenon that correlated extremely well with wake total pressure distributions indicating that the mechanism which redistributes the energy also has a direct effect on the losses associated with the vane wake. Both phenomena were found to be a function of Mach number in that the losses and the time-averaged total temperature difference between the centre and edge of the wake increased to a maximum as the Mach number approached 0.95 and then decreased by half as the Mach number was raised to 1.3. Following Kurosaka et al (1987) conclusion that the redistribution of energy behind a circular cylinder was caused by the vortices which were shed from the trailing edge an additional experimental program was initiated to study the details of the unsteady vane wakes by the use of high speed schlieren photographs and high frequency response transducers to measure unsteady static and total pressures. This research has confirmed that over the range of Mach numbers, from low to high subsonic, a von Karman vortex street is shed continuously from the vanes. What this research has revealed is that as the transonic regime is traversed, the von Karman vortex street still occurs but only as one of a number of different and highly transient vortex shedding patterns. This breakdown of the stable von Karman vortex street is associated with the migration of the origin of the vortices from the trailing edge of the vane to the nodal point formed by the trailing edge shock waves and the confluence of the two trailing edge shear layers. The cause of the redistribution of energy within the wake and of the high wake losses is the shedding of a continuous von Karman vortex street from the vane. In the subsonic flow regime the presence of a stable von Karman vortex street leads to high wake losses due to the depression of the base pressure while the redistribution of total temperature (energy) is caused by the combined pumping action of the vortices. The strength of the vortices and the above phenomena increase with increasing Mach number. As the transonic flow regime is encountered and the supersonic flow regime entered the coherent structures in the wake become unstable, less von Karman-like, and occur less frequently and the origin of the vortices migrates downstream. This leads to a significant elevation in base pressure and redistribution of energy, thereby implying that significant gains in engine stage efficiency can be realized by the destabilization or elimination of the von Karman vortex street from the vane wake.