Transpiration Cooling for Turbine Cooling

The specific power of gas turbine engines is fundamentally limited by the turbine combustion temperature. Progressively more elaborate methods have been employed to elevate this above the melting point of the metals employed in the engines. Coolant is taken from the compressor stages and used to reduce the temperature of the hot metal parts downstream of the combustion to an acceptable temperature.

The coolant requires compressing and the work done on it represents a direct source of loss of the engine. Currently gas turbines use about 30% of the air compressed to cool the hot stages of the engine.

Transpiration cooling is a potential improvement on the current state of the art. It involves passing coolant through fine passages that may resemble a porous material. The coolant flowing through the blade and the resulting effusion from its surface both serve to cool and protect the blade from the hot gasses.

Transpiration cooling requires new manufacturing technologies and deeper understanding of the coolant flow and the interaction with the main gas path of the engine. Transpiration cooling would enable a raising of the turbine entry temperature of 200C above what is currently possible increasing the specific power by up to 10%.

The coolant used in the engine could be reduced further increasing the efficiency significantly.
Transpiration cooling is also of interest to hypersonic vehicles such a space vehicles re-entering the upper atmosphere. Porous ceramics may be employed for this purpose. Although this project is confined to metal parts for gas turbines, experimental and numerical methods and coolant geometry may be transferrable.

Aims of the Project
Instrumentation and methods will be developed to measure the effusion of coolant through porous materials with and without cross flow. This will initially start with flat plates and will eventually be extended to blade samples.
Numerical models will be built and validated against experimental data.
Based on the operational requirements of the blade and the developing understanding of the coolant flow new geometries of coolant channels will be proposed.
Prof Nick Green at the High Temperature Research Centre in Birmingham will provide manufacturing expertise. Using traditional machining techniques he will supply flat plates for early study. A novel casting technique is proposed to make blade samples.

Areas of Novelty
Entirely novel coolant geometry is required to achieve the definition of transpiration cooling. There is scope for invention and novelty in this geometry.
The prototype blades will use totally novel manufacturing techniques which will permit new shapes and have constraints and issues not dealt with in the literature.
Novel instrumentation and techniques may be employed to measure the effusion building on the state of the art use of photosensitive paint, hot wire probes etc.

Research Theme
This project is aligned with the EPSRC sub-theme of energy efficiency under the main theme of energy.
This project is funded by the EPSRC.