Important Facts About Transpiration Cooling

There are 3 main types of cooling. First is film cooling. Second convection cooling and third is transpiration cooling method. Here we will see some facts about transpiration cooling method.

Principle of Transpiration Cooling:

1. In transpiration cooling, coolant coming from the reservoir flows through the porous wall towards the hot gas side wall towards the hot gas side wall surface. Passing through the porous structure the coolant absorbs the heat flux conducted into the solid material of the wall. At steady state operation a thermal equilibrium between the coolant and the solid wall is reached.
2. The coolant forms a film on the hot gas side surface. The coolant film partially absorbs the convective wall heat flux and thus reduces the heat flux conducted into wall to a certain amount, just like actual film cooling. The heated coolant at the film surface is transported downstream by the momentum of the hot gas flow, continuously replaced with fresh coolant flowing out of the wall.

Scope of Transpiration Cooling Method:

Transpiration cooling is a novel and efficient method of allowing the turbine blades and other parts within the hot section to operate at much higher turbine inlet temperatures.

By using a gaseous cooling medium, transpiration cooling could provide a higher cooling effect and larger coolant coverage than film cooling in the leading stagnation region, and on the side of the specimen at the same coolant injection flow rates.

But in the trailing stagnation region, the traditional coolant injection method through discrete film holes might be better than transpiration cooling, especially for turbine blades with thin trailing edges.

Finally, some of the disadvantages & problems associated with transpiration cooling are: Many of highly porous materials lack strength & machinability, in addition many of porous material exhibits a tendency to oxidized rather quickly, causing the pores to be blocked by oxide film & causing a coolant flow through the material to be reduced, resulting in material failure. Because of these problems, there has been a shift towards investigating better cooling schemes.

The transpiration of coolant fluid has great cooling effectiveness as a result of decrease of surface temperature and heat flux, as transpired fluid injected into the outer flow cools the surface as well as creates a moderate temperature gradient near the surface.

Increase of back face pressure corresponding to increase of transpiration mass flow causes the lower surface temperature and smaller heat flux. The cooling effectiveness per mass transpiration rate based on decrease of surface temperature is improved by smaller porosity, which shows the superiority of smaller porosity material for practical use.

Applications of Transpiration Cooling:

1. Fuel injection apparatus in Ram Jet engine needs good cooling system as it is present near to combustion in combustion chamber. Transpiration cooling performs good job in this phenomenon.

2. In thermal barrier coating transpiration cooling is provided successfully.

3. One of the very important application of transpiration cooling is cooling of turbine blades. This is widely used today for effective cooling of blades which in turn improves the life of turbine performance.

4. Transpiration cooling is also used in the rocket engines.

5. Even nature uses the principle of transpiration cooling in plants. T.C. phenomenon takes place at leaves of plants.