Measurement of internal heat transfer distributions using transient infrared thermography

The aim of this work is to develop a suitable technique to measure internal heat transfer coefficient distributions of a high conductivity device (e.g. a gas turbine blade). The technique implements the acquisition of external full surface temperature data of the geometry during the thermal transient evoked by internal cooling, in concurrence with geometry material property details, part wall thickness and internal fluid temperature. With the help of an infrared camera the external temperature history of the geometry is recorded during the whole thermal transient. Infrared data is combined with cooling flow network modeling to provide the boundary conditions for a finite element model representing the geometry. An inverse transient conduction model of the geometry is executed with updated internal heat transfer coefficient distribution until the convergence is reached with the experimental measured external wall temperature at a given time. The first iteration of internal heat transfer coefficient distribution is calculated by using lumped thermal capacitance model. Then a root finding algorithm is used for further iterations until convergence is achieved. The technique is validated on a canonical test case composed by a straight circular duct. The experimental setup consists of an oven which contains ceramic heaters, an air intake system used for flowing air inside the duct and a data acquisition system consisting of Coriolis flow meter, thermocouples and an infrared camera. First the duct is heated to uniform temperature in the oven, then an air is introduced into the duct in order to induce a thermal transient. Reynolds number of 25000, 50000 and 75000 are used for different tests. The outcomes of different tests are compared with available literature data, obtaining a good agreement and thus validating the employed experimental approach.