Novel cooling design for blade or vane trailing edges
Keywords: Turbomachinery
Vane
Pressure sensitive paint
Blade
Jet engines
Trailing edge
Turbine
Cooling
Film effectiveness

The trailing edge of the high pressure turbine blade and vane presents significant challenges to the turbine cooling engineer. This section endures very high temperature and mechanical stresses but aerodynamic efficiency requirements dictate that it must be tapered to the end with minimal thickness, making it difficult to cool internally. Pressure side film cooling using a slot has become a common and effective method to cool the trailing edge by creating a sheet of coolant which protects the trailing edge from the mainstream gas. However, continuous cooling slots are not mechanically robust and weaken the design, necessitating the use of interrupted slots with a "bridge" internally and a "land" externally. Manufacturing constraints also dictate that the slot cannot be too thin, resulting in excessive coolant flow unless the slot or feed passage are restricted.
The current research proposes a novel cooling slot design that uses cross corrugated channels on either side of the cooling slot. These improve internal cooling and increase pressure loss, allowing the flow to be restricted without reducing the slot height or introducing a restrictor into the feed passage which could be vulnerable to blockage. However, this design potentially has very poor film cooling performance due to the complex flow structures that exit the corrugated channels. Thus, it was necessary to investigate this weakness and recommend strategies to combat it.
The pressure sensitive paint technique for measuring adiabatic film cooling effectiveness was developed for use in a large scale simplified model of the trailing edge. Initial experiments were conducted to choose an appropriate land shape to be used with the slot and then a series of eight different cross corrugated geometries were tested at five blowing ratios ranging from 0.6 to 1.4 in increments of 0.2. These geometries encompassed two included angles of 90° and 120° and four different exit alignments. The experiments showed that the cross corrugated geometries did indeed perform very poorly in terms of film cooling effectiveness across the board. In an effort to remedy this, the cross corrugated slot designs were modified with exit shaping in such a way as to create a more uniform coolant flow at the slow exit.
In total, five slots with exit shaping were designed and tested in the facility. Exit shaping proved to be an effective solution to the issue of film cooling performance, increasing the effectiveness significantly on the cutback surface and increasing spanwise consistency across the slot as well. The best performing geometry was selected as the most optimised cross corrugated slot geometry with exit shaping and tested further using a more engine representative coolant to mainstream density ratio.
Overall film cooling performance using the optimised cross corrugated slot was improved beyond that of a simple rectangular slot by 6.6% up to a blowing ratio of 1.0. Trailing edge effectiveness was improved by 16.3% up to a blowing ratio of 1.0 and 5.4% up to a blowing ratio of 1.2. Simultaneously, pressure losses were significantly increased by the cross corrugated slots. Fanning friction factor was enhanced by between 15 and 25 times for an included angle of 90° and 50 to 90 times for an included angle of 120°. This shows that the cross corrugated slot geometry is a viable candidate for use in trailing edge slot cooling and can achieve great pressure losses without sacrificing film cooling performance.