Aspect Ratio Effects on Racetrack Shaped Film Cooling Holes

Round film cooling holes are commonly used in industry and have been extensively studied in the literature. The prominent counter rotating vortex pair (CVP) present in jets from round film cooling holes can significantly reduce film cooling effectiveness. Shaped holes, such as fan shaped diffuser holes, can be used to mitigate the effects of the CVP by increasing lateral spreading of the coolant, but these holes are more costly and difficult to manufacture. The cross section of the round film cooling hole can be stretched laterally to make a racetrack-shaped cross section. This simple variation of the round hole can be easily machined, and previous studies of similarly shaped oval holes show improved performance. In the present study, Magnetic Resonance Velocimetry (MRV) is used to measure the 3D, 3-component mean velocity field for racetrack shaped film cooling holes pitched at 30 degrees, with blowing ratio of unity, hydraulic diameter of 5.8 mm, and aspect ratios of 2 and 3. Three million data points on a cartesian grid with resolution of 0.6 mm in each direction are collected for each case. The scan domain begins 16 hole diameters upstream of injection and ends 27 hole diameters downstream of injection. The data are quantitatively compared with each other and to a previous MRV measurement of a round hole with the same hydraulic diameter and flow conditions. Increasing aspect ratio improves lateral spreading of the jet, potentially improving film cooling effectiveness. Separation underneath the jet immediately after injection is reduced with increasing aspect ratio. The counter rotating vortex pair and the effects of image vortices on jet lift-off and spreading are observed. The strengths of the counter rotating vortices are compared between the various aspect ratios. Detailed in-hole measurements and flow features are presented, informing how the racetrack hole may behave when used as an inlet to other shaped holes. The measured in-hole flow field shows the effects of asymmetric inlet conditions from the inlet plenum and separated flow within the hole due to sharp corners at the hole/plenum junction. More generally, MRV is a versatile technique that can be used to validate simulations and explore new hole shapes. MRV offers several advantages over laser-based techniques as it does not require optical access to take measurements and is simpler to set up. 3D velocity measurements can be taken for any geometry that can be 3D printed and data can be acquired in the span of several hours.