Heat Transfer is one of most important branch of engineering. It is invariably used everywhere around us. Various subjects like Applied Thermodynamics, Refrigeration and Air Conditioning are connected to heat transfer.
Need For Cooling:
Many portions of engines, such as gas turbine engines, become extremely hot during service. Some components are contacted by hot combustion gases whose temperatures exceed the melting points of the materials of construction of the components.
A number of techniques are used to allow the components to operate under such conditions. In one, the surface of the material is insulated by a protective thermal barrier coating.
In another technique, the component is actively cooled by a flow of cooling air that passes over its surface to allow it to continue functioning. High pressure turbine blades, for example, are typically hollow and have surface openings there through. Compressed cool air is passed into the hollow interior of the turbine blades and exits through the surface openings.
The air streams along the surfaces of the turbine blades to both cool the surfaces and provide a cool-air film layer between the hot combustion gas and the metal of the turbine blade. In a related approach, a jet of cool air may be directed against the surface of an article to be cooled.
Transpiration cooling has also been used. The article to be cooled is made to be porous. Compressed cooling air is forced through the porous article to remove heat.
Transpiration cooling has an advantage that the cooler air remains in contact with the material of the article for a relatively long period of time so that a significant amount of heat may be transferred into the air and thence removed from the article.
Types of Cooling:
- Convection Cooling.
- Film Cooling.
- Transpiration Cooling.
1. Convection Cooling:
In convection cooling, a coolant is forced along the underside of the material, or even perpendicular to the underside of the material (impingement cooling) to keep the upper surface temperature low. If surface coefficient of the heat transfer between the upper surface & the hot gas is very high, as it would be for high gas speed, the amount of coolant needed & the coolant speeds required may not permit an economical convection cooling design.
2. Film Cooling:
In film cooling, a layer of coolant is discharged from a slot, usually parallel to the upper surface & in the direction of the hot gas flow. This layer of coolant tends to insulate the surface from the gas , but its effectiveness continually decay s as distance increases because of the mixing between the film flow & the hot gas . As a result, it affords considerable thermal protection in a local region just downstream of the injection point. This decay of the film’s effectiveness can be overcome by adding additional downstream slots, spaced a finite distance apart.
3. Transpiration Cooling:
Now, here in transpiration cooling, the object to be protected is constructed of a porous material through which the coolant is forced, cooling the material as it passes through it, & then leaves at the surface to be protected & “blows” into the boundary layer. This blowing of the coolant into the boundary layer distorts the temperature field in that the local surface coefficient of heat transfer is reduced from the value it would normally have for a hot gas flowing over solid plate under the same condition.
