Turbine Blade - Cooling

Cooling

For same pressure ratio at high maximum temperature thermal efficiency is high. But the high temperature can lead to the damage of the turbine. So the cooling of blade becomes essential.

METHODS OF COOLING

Cooling of components can be achieved by air or liquid cooling. Liquid cooling seems to be more attractive because of high specific heat capacity and chances of evaporative cooling but there can be problem of leakage ,corrosion, choking,etc. which works against this method. On the other hand air cooling allows to discharge air into main flow without any problem. Quantity of air required for this purpose is 1-3% of main flow and blade temperature can be reduced by 200-300°C. There are many types of cooling used in gas turbine blades; convection, film,transpiration cooling, cooling effusion ,pin fin cooling etc which fall under the categories of internal and external cooling. While all methods have their differences, they all work by using cooler air (often bled from the compressor) to remove heat from the turbine blades.

INTERNAL COOLING

Convection cooling It works by passing cooling air through passages internal to the blade. Heat is transferred by conduction through the blade, and then by convection into the air flowing inside of the blade. A large internal surface area is desirable for this method, so the cooling paths tend to be serpentine and full of small fins.the internal passages in the blade may be circular or elliptical in shape. Cooling is achieved by passing the air through these passages from hub towards the blade tip. This cooling air comes from air compressor. In case of gas turbine the fluid outside is relatively hot which passes through the cooling passage and mixes with the main stream at the blade tip.

impingement cooling A variation of convection cooling, impingement cooling, works by hitting the inner surface of the blade with high velocity air. This allows more heat to be transferred by convection than regular convection cooling does. impingement cooling is used in the regions of greatest heat loads. In case of turbine blades, leading edge has maximum temperature and thus heat load. Impringement cooling is also used in mid chord of the vane. Blades are hollow with a core. There are internal cooling passage. Cooling air enters from the leading edge region and turns towards the trailing edge..

EXTERNAL COOLING

film cooling Film cooling (also called thin film cooling)is a major type of cooling which works by pumping cool air out of the blade through small holes in the blade. This air creates a thin layer (the film) of cool air on the surface of the blade, protecting it from the high temperature air. The air holes can be in many different blade locations, but they are most often along the leading edge. A United State Air Force program in the early 1970s funded the development of a turbine blade that was both film and convection cooled, and that method has become common in modern turbine blades. There are orifice on the surface through which the cool air flows on the surface and makes a film on the surface which acts as a barrier to heating and provides effective cooling. Beside cooling blade surface it decreases heat transfer from metal surface to the hot fluid. One consideration with film cooling is that injecting the cooler bleed into the flow reduces turbine efficiency. That drop in efficiency also increases as the amount of cooling flow increases. The drop in efficiency, however, is usually mitigated by the increase in overall performance produced by the higher turbine temperature.

Cooling effusion Blade surface is made of porous material which means having infinite number of small orifices on the surface. Cooling air is forced through these porous holes which forms a film or cooler boundary layer. Besides this uniform cooling is caused by effusion of the coolant over the entire blade surface .

Pin fin cooling In the narrow trailing edge film cooling is used to enhance heat transfer from the blade. There is array of pin fins on the blade surface. Heat transfer takes place from this array and through side walls. As the coolant flows across the fins with high velocity, the flow separates and wakes are formed. Many factors contribute towards heat transfer rate among which type of pin fin and spacing between fins are the most significant.

Transpiration cooling It is similar to film cooling in that it creates a thin film of cooling air on the blade, but it is different in that air is "leaked" through a porous shell rather than injected through holes. This type of cooling is effective at high temperatures as it uniformly covers the entire blade with cool air. Transpiration-cooled blades generally consist of a rigid strut with a porous shell. Air flows through internal channels of the strut and then passes through the porous shell to cool the blade. As with film cooling, increased cooling air decreases turbine efficiency, so that decrease has to be balanced with improved temperature performance.