Three-dimensional velocity and concentration measurements of turbulent mixing in discrete hole film cooling flows [electronic resource]
Keywords: Mechanical Engineering.

Magnetic Resonance Velocimetry (MRV) and Magnetic Resonance Concentration (MRC) are used to measure the three-dimensional, three-component, time-averaged velocity and scalar concentration fields of ten different discrete hole film cooling configurations. Seven of these configurations feature variations in the mainstream flow, covering changes in streamwise pressure gradient, incoming boundary layer thickness, and injection wall curvature, as well as a baseline case on a flat wall with nominally zero pressure gradient and moderate boundary layer thickness. All configurations use a single film cooling hole with circular cross-section inclined 30 degrees and aligned with the streamwise direction of the mainstream flow. The remaining three configurations have nominal mainstream conditions, but include modifications to the film cooling hole to introduce three-dimensional complexity: a skewed film cooling hole, injected at a 30 degrees angle with the mainstream flow; an array of three film cooling holes that interact with one another; and a shaped hole with a non-circular cross-section that diffuses into an expanded exit. A separate water channel is constructed for each configuration, and each experiment is operated at a nominal blowing ratio of unity. The penetration of the jet of fluid from the film cooling hole into the mainstream flow, measurable in both the velocity and concentration fields, is sensitive to the thickness of the mainstream boundary layer at the point of injection. Evidence of this effect is seen in both the boundary layer and pressure gradient cases, with mainstream acceleration and deceleration due to the pressure gradients causing thinning and thickening of the boundary layer. Mainstream acceleration also strengthens the counter-rotating vortex pair (CVP), the dominant secondary flow feature for discrete hole film cooling flows. Increasing the strength of the CVP increases the tortuous path for fluid injected from the film cooling hole, but this effect is partially balanced by the stretching effect of the mainstream acceleration. The distinguishing feature of the skewed hole configuration is the development of a single dominant vortex that remains strong throughout the jet region in the mainstream flow. This single vortex preferentially entrains low concentration fluid from the mainstream and low velocity, high turbulence fluid from the boundary layer into one side of the jet region, causing asymmetric mixing and spread of the jet concentration and velocity contours. Mixing of low concentration fluid under the jet decreases the film cooling performance of the skewed jet as compared to the unskewed baseline geometry. The multihole experiment, having an array of three holes, is oriented with one central downstream hole and two flanking holes on either side upstream. The upstream holes are offset 2D on either side of the center hole, and located 3.07D upstream. Flow downstream of the holes is characterized by a CVP triplet, with one CVP emanating out of each hole. The flow between the CVP is a strong common down flow that brings jet fluid toward the bottom wall. This downward flow produces an increase in film cooling performance for the multihole case over the single hole. Superposition of concentration from the upstream and center holes produces a further increase in film cooling performance. The shaped hole differentiates itself from the other nine configurations tested in that the flow out of the hole does not initiate the formation of any strong secondary flows in the mainstream channel. The strong laidback fan-shaped expansion of the exit (12 degrees in both the streamwise and lateral directions) reduces the momentum of the fluid exiting the hole, such that its effect on the mainstream flow is negligible. As such, the jet fluid from the hole remains close to the wall after injection, significantly increasing the film cooling performance relative to non-shaped holes with circular cross-section. Finally, a high-fidelity large eddy simulation (LES) of the skewed hole case is used to evaluate several common models for turbulent scalar mixing. The Gradient Diffusion Hypothesis (GDH), Generalized Gradient Diffusion Hypothesis (GGDH), and Higher Order Generalized Gradient Diffusion Hypothesis (HOGGDH) are compared based on their abilities to capture the correct anisotropy of the turbulent scalar flux vector, as well as the influence of their modeling errors on the final concentration field. While the anisotropic GGDH and HOGGDH show improvements in the near-injection region over the isotropic GDH, further downstream the GDH better captures the concentration distribution at the wall.