60015 - Experimental and Computational Investigation of Film Cooling Performance and External Flowfield Effects Due to Impingement Coolant Feed in the Leading Edge of a Turbine Blade 

Showerhead film cooling holes at the leading edges of blades are typically fed by an internal impingement flow, where coolant enters the leading-edge chamber and impinges on the internal surface beneath the stagnation line to provide cooling protection in addition to the bore cooling and film cooling provided by the showerhead. The effect that this impingement flow has on the flow field inside the film cooling holes and subsequently on the exterior flow of coolant along the blade has not been studied thoroughly in the literature. Determination of the magnitude of and insight into the physics of this internal flow feed effect is useful not only as a tool to develop better cooling designs but also as a way to make more competent comparisons among results of different studies examining showerhead film cooling using different coolant feeds. Shaped holes were built into the leading edge of a scaled-up turbine blade model and tested in a low-speed recirculating wind tunnel facility. To isolate the effects due to the impingement flow field, adiabatic effectiveness and off-the-wall thermal field data were taken for the same hole geometry at the same test conditions using both a simulated plenum coolant feed and an engine-realistic impingement coolant feed. It was found that the impingement configuration had a moderately beneficial effect on adiabatic effectiveness performance on the order of 10%—a finding in agreement with the limited literature isolating its effect. Although an increase in conduction through the wall of the near-adiabatic test model could explain a portion of this increase in performance, additional flow field effects were hypothesized to have been the cause. This idea was bolstered by the apparent better attachment of the jets from the outer rows of holes at higher coolant injection rates. To investigate these hypothesized flow field effects further, a CFD RANS simulation was run. In a companion study to this one, RANS was shown to predict the behavior of the jet-mainstream interactions in the showerhead region reasonably well. Both the impingement configuration and pseudo-plenum configuration from the experiments were simulated for comparison. It was seen that, for the impingement configuration, flow remained better attached throughout the hole (both at the inlet and at the diffuser), leading to better attachment on the outer blade surface. This was especially true in the suction side holes at higher blowing ratios, as the pseudo-plenum caused significant separation in the holes while the impingement configuration caused very little separation.