Elliptical pin fins as an alternative to circular pin fins for gas turbine blade cooling applications: Part 2: Wake flow field measurements and visualization using particle image velocimetry

Extensive wake flow field measurements and visualizations are conducted using particle image velocimetry (PIV) inside the wakes of the elliptical and circular pin fin arrays in order to better understand the flow physics and the loss mechanisms of these devices. The true-mean velocity field inside the wake two diameters downstream of the pin fin arrays is obtained by collecting and ensemble averaging a large number of PIV samples in the midplane of the test section. Additional experiments are also conducted inside the very near wake of the pin fins in order to visualize instantaneous flow field features. The results of the study reveal that the circular pin fin array creates a large low momentum wake region when compared to the elliptical pin fin arrays. It is observed from the flow visualization inside the wake that this kind of a very large momentum deficit is created due to the early separation of the flow from the circular fins in the second row. In the case of elliptical fins, however, the flow stays attached to the fin surface and the separation occurs very close to the downstream stagnation point on the surface which in turn results in a very small low momentum wake region behind the elliptical pin fin arrays. The mean turbulent kinetic energy levels from the PIV measurements show very high turbulence levels in the wake of the circular fin arrays compared to the elliptical fins. However, the smaller momentum deficit inside the elliptical pin fin wakes results in higher local Reynolds numbers inside the wake when compared to the circular pin fin wakes. This in turn helps to keep the endwall heat transfer enhancement levels close to the circular fin arrays although the turbulence levels are much lower in this region.