Application of Photoluminescent Measurement Techniques for Quantitative Assessment of Turbine Film Cooling

Abstract

Reduction of the specific fuel consumption of gas turbines can be achieved by maximizing the turbine inlet temperature. However, this requires a detailed understanding of the cooling performance in order to ensure safe and economical operation of the engine. This is because the resulting temperature distributions in the blade walls dictate the thermal stresses and life-time of the hot section of the engine. Film cooling performance depends on a large number of parameters including turbulence making it complex to predict numerically. Moreover, the harsh flow conditions found in turbines situated directly downstream of the combustion chamber makes it difficult to obtain high resolution measurements of the relevant quantities. Therefore, simplified experiments are employed in order to simulate the engine situation by matching the most important flow similarity parameters. More specifically, the large difference in temperature between hot-gas and cooling air leads to considerable difference in density between the film cooling jets and hot-gas. This can be simulated at ambient temperatures by injecting a heavy foreign gas into a less dense main-flow of air. This thesis aims at demonstrating the applicability of photoluminescent paints as sensing element for film cooling measurements. When this type of sensor is excited by ultraviolet light, the photoluminescent process leads to the emission of red-shifted light. The luminescence depends on oxygen pressure as well as temperature at the coating surface. A commercially available Pressure Sensitive Paint (PSP) is chosen for experimental characterization of the two fundamental quantities of film cooling: Adiabatic film cooling effectiveness Heat transfer coefficients The adiabatic film cooling effectiveness is a measure of the mixing between coolant and mainflow. This quantity corresponds to concentration of gas species according to the analogy between heat and mass transfer. An oxygen-free coolant acts as tracer gas as it dilutes the oxygen of the main-flow, allowing for detailed measurements of the cooling gas concentration with PSP. The corresponding convective heat transfer coefficient is measured by imposing step changes in heat flux and tracing the transient temperature response of the wall. For this purpose, the same PSP-sensor is employed for thermography measurements by taking advantage of the inherent temperature sensitivity of this technology. An engine-realistic test case illustrates the capability of the technique under complex flow conditions. It allows to demonstrate that measurements with high spatial resolution can be obtained over large portions of the surface within a short period of time. It turns out that the combination of oxygen pressure and temperature sensors, in the form of photoluminescent paint, is particularly advantageous in compressible flow experiments with moderate heat transfer at the wall. Another important advantage of the technique is the possibility to separate the adiabatic film cooling effectiveness from the wall heat transfer effects, without applying another coating for separate experiments. This ensures high geometrical tolerance of the small-scale cooling holes of the experimental models. However, the technique is limited by the maximum applicable temperature of the photoluminescent system making it first of all suitable for cold-flow measurements. While both pressure and temperature sensitive paints have previously been employed in film cooling studies, the proposed combination of those techniques is unique in that it allows for simultaneous, yet decoupled measurement of the two fundamental quantities in film cooling. In the present thesis, it is shown that the application of the presented technique allows to validate numerical flow models. Furthermore, conclusions can be transferred to the modelling of real engines in the form of technology trends. Ultimately, this allows to optimize film cooling performance and reduce the fuel consumption of gas turbines.