An Experimental Study of Heat Transfer Coefficients and Friction Factors in Airfoil Leading Edge Cooling Cavities Roughened with Slanted Ribs

In turbine blade design, the use of turbulators in airfoil cavities has been a preferred means to cool the metal temperatures within the airfoil. Temperatures in the turbine section of a jet engine can easily reach beyond material temperature capability limits and without any internal cooling, the turbine blades will begin to creep and eventually lead to engine failure. The introduction of turbulators has provided a means to increase the heat transfer coefficient within the airfoil cavities and help promote turbulence and better mixing to facilitate convective cooling. In this study, 4 different test rigs were experimented upon with each test rig assessing 3 different turbulator blockage ratios (e/Dh). Each test section's cross section was based on leading edge cavity geometry scaled up from a "real-life" airfoil. Turbulators were placed along the backwall and also along the leading edge nose. The backwall turbulators had rounded corners and staggered, and were placed 45° along the surface of the wall. The nose turbulators also had rounded corners and staggered, but, unlike the wall turbulators, were placed at 90° along the nose surface. To determine the reference temperature of the measured wall and nose surfaces, liquid crystals were used. The liquid crystals were laid on top of the wall and nose surfaces on one wall of the test section. Electric foil heaters were placed beneath the liquid crystals to simulate a heated wall boundary condition. The remaining walls were insulated from the environment to simulate adiabatic conditions. For this study, the heat transfer coefficient, friction factors, enhancement factors, and thermal performance were calculated based on experimental data collected on the backwall and nose surfaces. Upon conclusion of this study, it was found that: (a) Rig 1 has the highest thermal performance at the nose at all blockage ratios. Rig 3A has the highest thermal performance at the backwall at low and high blockage ratios. (b) Rig 1 had the highest friction factor across the range of Reynolds Numbers. Rig 2 had the lowest. (c) As the blockage ratio increased, so did the heat transfer coefficient and friction factors. It was noted, however, in some cases, that as the blockage ratio increased to the maximum blockage the heat transfer benefit was reduced. (d) The turbulator spacing was suggested to have a potential impact on the overall heat transfer coefficient as demonstrated by looking at the results between rigs 2 and 3A and 3B. (e) To validate the test results and trends seen from this experiment, it is recommended that a CFD analysis be performed on each test section.