Heat transfer measurements in rotating turbine blade cooling channel configurations using the transient thermochromic liquid crystal technique

Abstract

The turbine blades of modern gas turbine engines are equipped with sophisticated cooling schemes. Cooling air is bled from the compressor and passed through internal convective cooling channels with the aim to keep the blade metal temperatures within safe limits. In an effort to increase overall engine performance and efficiency, turbine inlet temperatures are being steadily increased while simultaneously the cooling air consumption is reduced. A reduction in cooling air mass flow however increases the effects of rotation on the internal flow structures and thus the heat transfer distribution of a rotating cooling channel. These effects are Coriolis forces and rotational buoyancy forces that may increase heat transfer significantly at one surface compared to the non-rotating case, while simultaneously the heat transfer may drastically be reduced at the opposite surface. For future cooling scheme developments it is vital to understand the influence of these rotational effects on the heat transfer distribution in order to derive resulting temperature distributions inside the turbine blade. A well established measurement technique is the transient thermochromic liquid crystal (TLC) technique that is applied in various investigations of non-rotating cooling channels to evaluate locally resolved heat transfer data. The aim of this thesis is to apply this measurement method for the investigation of rotating cooling channels in order to assess the influence of the rotational effects on the heat transfer distribution. A test rig has been developed that was specifically designed to apply the transient TLC measurement method to rotating cooling channels. Several challenges had to be mastered in the process. First of all, the test model and all rotating components had to be designed or selected to withstand the specified centrifugal forces. Second, a new co-rotating camera unit had to be developed to allow the TLC signal to be captured during rotation. Exemplary results of a test campaign with a total of 32 experiments are discussed. The results are presented as locally resolved distributions of Nusselt number ratios as well as line and area averaged data. Furthermore, for each rotating experiment also a corresponding stationary experiment was conducted. This allowed a direct visualization of the rotational effect on the heat transfer distribution by calculating the local normalized Nusselt number ratio between the rotating and the non-rotating experiment.