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Gas turbine performance is highly dependent on turbine inlet temperature, which often exceeds the working limitations of the materials involved. Film cooling is a widely used technology enabling highly efficient gas turbine cycles, where relatively cold air is injected as a film on the airfoil surfaces protecting the airfoils from the hot combustion gasses. Film cooled turbines exist in highly unsteady environments due to interactions between stationary and rotating components, and film cooling further complicates the flow. There is limited understanding of the unsteady nature of film cooling flows, resulting in limited ability to predict heat transfer and metal temperature on the components of a gas turbine. The goal of this work is to increase understanding of turbine cooling technology by examining time-accurate and time-averaged behaviors of the cooling flows.This dissertation incorporates experimental and computational analysis of pressure and heat transfer on an industry scale high-pressure turbine stage. Experimental measurements of pressure and heat transfer were performed on a turbine stage installed in the Turbine Test Facility at the Gas Turbine Laboratory. This facility is uniquely equipped to examine unsteady pressure and heat transfer on turbine stages operating at design corrected conditions. Heat transfer measurements are compared for multiple different cooling configurations on the rotating airfoils. Data are analyzed on time-averaged and time-resolved bases, and the results highlight cooling benefit differences among the various cooling hole shapes and coolant flow rates.Computational models of the turbine stage are also employed with steady and unsteady RANS modeling techniques. Experimental data are used for boundary conditions in the computational models as well as to evaluate the accuracy of the models. Comparisons of experimental and steady computations of film cooled turbines often result in poor agreement due to the complexity of film cooling flows. Unsteady modeling in this work utilizes time-marching sliding mesh and harmonic balance techniques to efficiently resolve unsteadiness. The results identify several important physical forces driving unsteadiness in film cooling flow and in blade heat transfer that are crucial to model accuracy. Accounting for unsteadiness in turbine simulations improves simulation quality without drastically increasing the computational resource requirements. Film cooling mass flow rate and film cooling hole shape have different cooling effects on different areas of the blade. More advanced diffuser shaped film cooling holes provide a beneficial reduction in heat transfer on the suction surface of the blade, but due to differences in coolant flow rates, pressure surface heat transfer increases with shaped film cooling holes. Pressure surface heat transfer had an optimal reduction in heat transfer at the nominal cooling flow rate, but suction surface heat transfer continually decreased with cooling mass flow over the conditions used in this study. Evidence of film cooling unsteadiness is experimentally and computationally documented, and both film cooling hole shape and coolant mass flow rate influence unsteadiness in measured heat transfer. Shaped film cooling holes provide a cooling benefit to the blade through coolant steadying effects in addition to reductions in coolant velocity. Computational analysis of pressure surface film cooling flow indicates that unsteady pressure gradients on the blade surface generate lateral sweeping motions in the coolant streams. Since the diffusing shaped film cooling holes generate increased lateral spread of the coolant, there are significant increases in time-averaged film coverage achieved with the shaped film cooling holes compared to the baseline round holes. Computational analysis of the film cooling helps explain disconnects between turbine operation and the parameterizations of shaped film cooling performance from simpler experiments. Significant increases in computational and experimental agreement are achieved by performing unsteady simulations resolving vane blade interactions. Unsteady simulations in this study identified the generation of a moving separation bubble on the suction surface leading edge, which was not observed in steady simulation. Vane wakes cause a large variation in flow incidence on the blade leading edge, causing the formation of the separation bubble and leading to large levels of unsteadiness in pressure and heat transfer. The effects at the leading edge are also amplified through coupling between the unsteady pressure and the blade showerhead cooling. An increased understanding and appreciation of the unsteady performance of various film cooling geometries is a foundational piece of continued gas turbine development. |
