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.