Numerical study of film cooling in hypersonic flows
In this thesis, a numerical study of film cooling in hypersonic laminar and turbulent flows has been performed using an in-house Navier-Stokes solver. The aim of this computational work is to investigate the mechanism and effectiveness of film cooling in hypersonic laminar and turbulent flows. Hypersonic flow over a flat plate without film cooling was first studied to provide a reference datum to check the effectiveness of film cooling. For laminar film cooling (M¥ = 9.9), three different primary flow conditions were first used for validation. The inclusion of the development of the flow in the plenum chamber upstream of the slot was found to provide better heat prediction than a uniform boundary condition at the slot exit. Detailed information of the flow field including velocity profile, Mach contour, temperature contour and heat transfer rate was presented. The mechanism of film cooling has been revealed according to the plots of calculated velocity profiles, Mach contours and temperature contours downstream of the slot. The coolant fluid was found to affect the primary boundary layer in two ways: 1) initially a separate layer established by the coolant fluid itself in the near slot area, 2) later a mixing layer between the primary and coolant flow streams. Then five coolant injection rates between 2.95 x 10-4 and 7.33 x 10-4kg/s and three slot heights 0.8382, 1.2192, 1.6002 mm, were examined in hypersonic laminar film cooling. For turbulent film cooling (M¥ = 8.2), for the geometry used in the experiment, the injection at an angle of 20° was found to be appropriate. Different turbulence models including Wilcox's k - w model. Menter's baseline and SST model have been tested. It is concluded that the Wilcox's k - w turbulence model with dilatation-dissipation correction provides the best heat prediction. Again, five coolant injection rates varies from 5.07 x 10-4 to 30.69 x 10-4 kg/s and three slot heights (the same as studied in the laminar film cooling) were studied to check the influence on film cooling effectiveness. Both the coolant and the primary flow were air. Film cooling was found to be an effective way to protect wall surfaces that are exposed under a high heat transfer environment especially in hypersonic laminar flow. Increasing the coolant injection rate can obviously increase the film cooling effectiveness. Again, this works better in laminar flow than in turbulent flow. The coolant injection rate in turbulent flow should be considered to be high enough to give good heat protection. Slot height in both laminar and turbulent flows under the flow conditions in this study was found to be less important, which means other factors can be considered in priority when constructing film cooling systems. With the application of curve fitting, the cooling length was described using power laws according to curve fitting results. A two-equation film coating model has been presented to illustrate the relation between the film cooling effectiveness and the parameter x/(h/m). For film cooling effectiveness in log-log coordinates, a second-order polynomial curve can be used to fit the laminar flows, whilst a straight line is suitable for the turbulent flows.