A study on the influence of film hole manufacturing variations and blockages on turbine aerofoil cooling, and on turbine heat transfer

Thesis

Intricate cooling methods in high pressure turbine blades (HPTB) are continually being developed and perfected in order to keep up with rising turbine entry temperatures (TET). The challenge for turbine cooling engineers is to design film cooling systems for HPTBs which allow them to withstand the harsh thermal conditions for as long as possible. Due to the application of super-alloys and intricate geometries, the manufacturing process of a single HPTB is highly expensive, therefore this challenge is of financial nature for turbine manufacturers. Thus it is imperative that cooling mechanisms operate at their predicted level. Predictions of film cooling hole performances are based on pristine film hole geometries.

This research addresses the question of what happens when film hole geometries are altered due to inaccuracies in manufacturing processes or in-service conditions leading to blockages of film cooling holes and how this impacts film cooling effectiveness. Deviations in film holes have the potential to adversely affect film cooling effectiveness and heat transfer, and the full impact is currently not well addressed in the literature. Therefore, it is important to characterise this issue and determine the performance of altered film hole geometries. Manufactured and engine-run HPTBs were analysed and representative deviations were identified and modelled in Computer-Aided Design (CAD). Film hole geometries on the leading edge, pressure surface, and trailing edge were then 3D-printed and tested in the high-velocity 2D linear cascade. Film effectiveness measurements were taken using pressure sensitive paint (PSP) and flow testing of individual geometries was performed to characterise the effect of geometry alterations on coolant mass flow rate. The performance of altered film holes was then compared to design-intent film holes. Of the blockages tested in pressure surface fan holes, most were found to negatively affect film effectiveness either by reducing mass flow rate or creating narrower films, thereby leaving larger parts of the blade exposed to hotter mainstream gases. Manufacturing deviations in a trailing edge cutback region were studied by looking at tolerances of the film hole axis location and cutback step height. Manufacturing variations underperformed the baseline, ranging from a drop of 10-35 % in performance compared to the baseline geometry. Manufacturing offsets in diffuser section of fan-shaped holes on the pressure surface and on the leading edge were found to increase film effectiveness. The manufacturing offsets increased lateral spreading of the coolant, and therefore increased the cooled surface.

RANS CFD analysis was used to complement the experimental findings. The numerical results were used to provide an understanding of the flow field that was caused by the film hole geometry variations and how this might affect the film effectiveness results found in the PSP experimental campaigns.

Authors: von Mueller, T

• DOI: 10.5287/ora-9rkr85yab
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English