Heat Transfer Measurements in a Turbine Cascade: Taking Advantage of the Thermal Bahaviour of a PSP Sensor

Film cooling experiments are commonly performed with Pressure Sensitive Paint (PSP). The oxygen sensitive probe molecules allow for measurement of the adiabatic film cooling effectiveness by detecting the concentration of a tracer-gas with different oxygen content. This is made possible by the analogy between transport of heat and mass. The technique is particularly advantageous when the high density of a cold coolant in gas turbines is simulated by the injection of CO2 into air flow (iso-thermal experiments). Thus the coolant has a different oxygen content compared to the main flow. However, the adiabatic effectiveness is only one of the quantities of interest in film cooling. In order to obtain the convective heat transfer coefficients, a temperature sensor is also required. Most PSP technologies possess some degree of temperature sensitivity that is typically taken into account and eliminated during aerodynamic pressure measurements. However, the unwanted temperature sensitivity can be turned into an advantage in film cooling studies where heat transfer through the wall surface is of interest. In this paper, the possibility to employ the moderate temperature sensitivity of a standard PSP formulation for heat transfer measurements is explored. An electrically heated foil attached on top of an acrylic wall imposes transient heat flux at the wall surface. The principle of the transient technique is to observe the time response of the surface temperature when a heating step is generated. The proposed thermography technique is based on the linear temperature sensitivity of the photoluminescent sensor for small temperature changes according to a calibration. Promising results with the proposed technique have been achieved on an engine realistic turbine vane platform. Advantages are full surface coverage with high spatial resolution which is important in film cooling studies. Moreover, the time required for measurements and data reduction is relatively short. Finally, use of a single PSP-coating ensures consistent surface quality during the measurement of all searched quantities – the adiabatic film cooling effectiveness and heat transfer coefficients.

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

Magnus Jonsson

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.

EPFL2010

Application of the Transient Heater Foil Technique for Heat Transfer and Film Cooling Effectiveness Measurements on a Turbine Vane Endwall

Dominique Charbonnier, Magnus Jonsson, Peter Ott

The paper presents an application of the transient heater foil measurement technique using thermochromic liquid crystals (TLC) to endwall heat transfer and film cooling investigations in a transonic turbine vane cascade. The film cooling configuration consists of an upstream slot, representing the leakage flow area between the interface of the combustor and the turbine, and several rows of cylindrical and fan-shaped holes within the passage. With the transient method chosen, the heat transfer and adiabatic film cooling effectiveness distributions can be obtained simultaneously together with the local heat release within the heater foil. Therefore, the heat release in the foil is not required to be uniform, and the foil can contain discrete holes in the film cooling configuration. Some new developments are presented, which are directed towards improved application of the transient heater foil method for such a complex configuration. This includes tailoring the foil heat-release distribution towards the expected heat transfer patterns, supported by numerical Finite-Element computations and the use of a double-TLC mixture for improved time-wise TLC indications. Additionally, CFDsimulations were used to evaluate the recovery temperature distribution through the vane cascade without film cooling. The experiments were performed in the linear cascade facility at the EPFL-Lausanne. A compressor provides a continuous air flow at near-ambient temperature regulated with heat exchangers. Carbon dioxide is used as coolant in order to achieve engine-representative density ratio between coolant and main flow. Multiple experiments with the same main and coolant flow settings but varying heat flux levels and coolant injection temperatures have been performed and simultaneously analysed using nonlinear regression analysis. The time required between successive experiments to return to homogenous initial conditions, as required by the transient method, has been analysed using an analytical solution for heat-on-heat-off conditions. This permits the model assumption of onedimensional conduction within a semi-infinite wall with a heat releasing layer on the top. Example results for cases without cooling, with film cooling from rows of discrete holes and the addition of slot film cooling are used to illustrate the benefit of the new approaches for the investigated vane cascade.

American Soc Mechanical Engineers, Three Park Avenue, New York, Ny 10016-5990 Usa2008

Detailed heat transfer distributions of narrow impingement channels for cast-in turbine airfoils

Alexandros Terzis

Gas turbine operation at elevated temperatures ensures increased thermal efficiency and useful power output. However, the industrial tendency to push further firing temperatures is limited by the capabilities of the current turbine blade cooling technologies which approach their limits. Nowadays, the capabilities of the foundry industry to produce high integrity and dimensionally accurate castings allow the production of integrally cast turbine airfoils with the so-called double-wall cooling technology. Cast-in turbine airfoils include a more distributed form of cooling, where the coolant can be injected in the form of impingement jets within the wall rather than the hollow of the airfoil increasing dramatically the heat exchanged capabilities. Narrow impingement cooling passages can be therefore generated close to the external hot gas flow. In this study detailed heat transfer distributions for all internal surfaces of narrow impingement channels are evaluated. The test models consist of a single row of five impingement jets investigated over a range of engine representative Reynolds numbers. Effects of jet-to-jet spacing, channel width and height and impingement jet pattern are independently examined composing a test matrix of 22 large-scale geometries. For the evaluation of the heat transfer coefficients, the transient liquid crystal technique is used considering also thermocouple thermal inertia effects. Given that the use of different impingement geometries could play an important role on the distribution of convection coefficients, and hence, on the homogeneity of blade metal temperatures, the experimental results of this study can be used along with the external heat load for the development of thermal design models for integrally cast turbine vanes and blades.