Mass Transfer Effects of Free-Stream Turbulence and Horseshoe Vortex Formed at the Upstream Edge of Film Cooling Jets About a Cylindrical Surface

Abstract

The leading edge of a turbine blade was simulated as a circular cylindrical surface. The effect of free-stream turbulence on the mass transfer upstream edge of the injection hole normal to the cylindrical surface has been investigated by using a naphthalene sublimation technique. The effects of injection location, blowing ratio on the Sherwood number distribution were examined as well. The free-stream Reynolds number based on the cylinder diameter is 53,000 and other conditions investigated are: free-stream turbulence intensities of 0.5%, 3.9% and 8.0%, injection locations of 40, 50 and 60 degrees from the front stagnation point of the cylinder, and blowing ratios of 0.5 and 1.0. The role of the horseshoe vortex formed upstream edge of the normally injected jet is discussed in detail. When the coolant jet is injected at 40 degrees, the mass transfer upstream of the jet is not affected by the coolant jet at all. On the other hand, when the injection hole is located beyond 50 degrees the mass transfer upstream edge of the injection hole suddenly increases due to the formation of the horseshoe vortex, but it decreases as the free-stream turbulence intensity increases because the strength of the horseshoe vortex structure is weakened. The role of the horseshoe vortex is confirmed by placing a rigid rod at the injection hole instead of issuing the jet. In the case of the rigid rod, the Sherwood number upstream of the injection hole is much larger due to the intense influence of the horseshoe vortex. The variation in the strength of the horseshoe vortex structure with free-stream turbulence intensities was observed by a flow visualization.