Thermal performance analysis of advanced cooling passages for gas turbine blades

The Ph.D. project here presented aimed at the design and realization of a rotating test rig for heat transfer measurements on internal cooling passages of gas turbine blades, with the final purpose to apply the developed heat transfer measurement methodology on a realistic geometry of blade cooling channels. The transient thermocromic liquid crystals (TLC) technique has been used for the measurements. This technique had to be adapted to the requirements of the test rig, hence allowing measurements in rotating conditions and with different temperature step evolutions imposed to the process fluid. Indeed, the adopted technique is based on the imposition of a temperature step on the fluid that laps the measurement surface, previously painted with thermocromic liquid crystals. The analysis of the surface temperature evolution given by the indication of the liquid crystals allows the evaluation of the surface heat transfer coefficient, that is a direct indicator of the heat transfer distribution of the analyzed surface. In order to replicate the same buoyancy effects induced on the flow by the Coriolis forces during rotation on this complex geometry, the transient measurements are performed with a cold temperature step on the coolant flow. The validation of both rig and methodology has been performed by tests on a simplified geometry of cooling channel, namely a square channel whit ribs normal to the flow direction installed only on a single wall. Liquid crystals with different activation temperatures were used. In order to stress the measurement chain, hence assess the reliability of the measurements, different temperature evolutions were imposed to the process fluid, also varying the rotating conditions. Test on a realistic cooling geometry were performed at different rotating conditions. The cooling scheme is made up of three passages connected by two 180 degree bend. Each passage has different aspect ratio and turbulent promoters configuration, also at the end of the first passage and along the trailing edge a flow extraction is imposed. In this thesis, the rig design and working principle are presented in details. In particular, the solutions adopted to generate the sudden cold temperature step, acquire the experimental data on board of the rotating test model and to control the experimental parameters during tests execution are described. The last chapters are dedicated to the presentation of the validation test and to comment the first results obtained on a realistic cooling channel geometry.