3D velocity and scalar field measurements of discrete hole film cooling flows [electronic resource]
Keywords: Mechanical Engineering

Three-dimensional mean velocity field measurements from magnetic resonance velocimetry (MRV) are used to study the flow fields in the interaction region between the film cooling jet and the mainflow, as well as inside the film cooling hole, for different film cooling hole inclination angles and velocity ratios. The vorticity field created by the interaction of the in-hole vorticity and surface boundary layer vorticity is discovered to resemble a streamwise series of horseshoe vortices inclined in the forward direction. A streamwise-normal view of this field reveals the traditionally identified counter-rotating vortex pair (CVP) which is detrimental to film cooling. The in-hole flow feature of a counter-rotating vortex pair, which affects the main flow field, is also identified for inclined film cooling holes. Tracking of the coolant exiting the film cooling hole is achieved through 3D measurement of the coolant concentration in the mainflow using magnetic resonance concentration (MRC). Through the scalar analogy, concentration measurements are related to temperatures and the adiabatic surface effectiveness of the tested film cooling cases is measured. Shallower inclination angles and low velocity ratio jets are seen to produce coolant jets which remain closer to the surface and provide good film cooling. However, low velocity ratios correspond to low coolant flux which limits cooling performance due to high turbulent mixing. It is desirable to produce a film cooling hole which creates similarly advantageous flow fields at higher velocity ratios which increase coolant flux and endure more mixing before falling below effective cooling levels. This is attempted through shaped holes. MRV studies of three traditionally shaped film cooling holes, which diffuse laterally and/or in the forward direction over a final length of the hole, are done to evaluate the flowfield developed by different diffusion angles and diffusive section lengths. Increased exit area is seen to reduce the momentum of the exiting jet and produce coolant flows which remain closer to the surface. However, in-hole measurements of the two more conservative diffuser-shaped holes show uneven flow through the diffusing section of the hole and room for improvement. The most extreme diffuser-shaped hole shows marked decrease in the strength of the mainfield CVP. In addition to conventional film cooling holes, MRV is done on novel hole shapes to ascertain whether they will perform well as film cooling holes. A round hole which lofts into an exit section modeled on two intersecting yawed holes is studied, which creates a pair of CVPs which lead to central vortices spinning counter to a conventional CVP. Two non-circular cross-section holes, a spanwise-stretched oval and a rounded triangle pointing streamwise, are tested in hopes of decreasing streamwise vorticity in the mainflow. The rounded triangle shapes leads to a very lifted coolant jet and increased vorticity, but the oval shape creates a coolant jet with a more complex vorticity field which remains attached and creates a beneficial velocity profile along the surface. MRC was done on the oval shaped hole, because of the promising flow features observed in the MRV results. Comparison of the surface effectiveness to all round hole cases and a single diffuser-shaped hole case show marked improvement in film cooling using an oval shaped hole over traditional round and diffuser-shaped holes.