Liquid crystal thermography for the thermal analysis of gas turbine blades internal cooling systems

The present work focus on the analysis of the transient liquid crystal thermography, which is employed to accomplish spatially resolved heat transfer performance on cooling channel of gas turbine blades. This methodology has already been implemented to its early stage in the rotating channel test facility of the Turbomachinery and Energy Systems Laboratory of the University of Udine; however, several aspects are still unsettled. Therefore, the main objectives of this thesis is to address the accuracy and validation of the transient thermography technique with the particular approach developed at the University of Udine. With these aims, transient thermography tests are carried out in a ribbed cooling channel on both static and rotating conditions. Even if a very common channel geometry has been chosen as a study case, no reliable experimental data were found in the open literature for validation purposes. In order to overcome this lack, the heat transfer data necessary to perform the comparison are achieved with the better-established liquid crystal thermography in steady-state approach. This work addresses further development and improvement of the test facility to make possible the implementation of the steady-state methodology. Moreover, a complex iterative numerical procedure is set up to estimate the heat losses that are the major cause of the lack of accuracy in the steady-state thermography measurements. Part of the work was also dedicated to the definition of the best calibration methodology to take when liquid crystals are exploited as temperature indicators in transient thermography; especially, when liquid crystals with activation temperatures below ambient one are used, as in the present case. The results clearly show that the temperature evolution approach must be preferred to the previously used calibration method (gradient temperature approach). The results for all the rotation conditions provided by the two experimental approaches are in good agreement, representing the evidence of the validation of the transient thermography. Nevertheless, this work suggests a possible method to estimate the uncertainty of the heat transfer coefficient values in transient experimental approach, and this is done by a sensitivity analysis to the variation of the most important experimental parameters. Furthermore, the influence of two uneven channel wall heating conditions on the local heat transfer distribution is investigated by means of the steady-state technique. The results show that the uneven thermal conditions have negligible impact on the stationary case, but they significantly affect the heat transfer when the rotation takes place. This can be due to the different buoyancy effects that in turns affects the secondary flow structures, and consequently, the local heat transfer. Anyway, additional investigations are required to better understand the reasons why of this behaviour.