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