The efficiency of a turbine engine is enhanced by maximizing the proportion of gas that is properly directed into the rotating vanes or stationary impellers disposed throughout the engine. More particularly, air that flows through the arrays of rotating end stationary vanes contributes to the work performed by the engine, whereas air that escapes around the tips of the vanes performs no work and is lost.
The arrays of rotating turbine blades in the turbine engine are surrounded by a stationary shroud. The proportion of the gases that perform useful work in passing through the arrays of turbine blades can be increased by minimizing the clearance between the tips of the rotating turbine blades and the inner cylindrical surface of the stationary shroud.
Both the rotating turbine assembly and the cylindrical shroud surrounding it expand radially outwardly when subjected to increases in temperature and contract radially inwardly when the temperature decreases. However, the rotating turbine assembly generally is a massive structure, while the stationary shroud which surrounds the turbine typically is of comparatively low mass. As a result of these substantial physical differences, the turbine assembly and the shroud react quite differently to variations in temperature. More particularly, the shroud will expand radially outwardly much more quickly than the turbine assembly when subjected to an increase in temperature, and conversely the shroud will contract radially inwardly much more quickly than the turbine when temperatures decrease. Consequently, there is a tendency for a large tip clearance to be created for a period of time following an increase in temperature, such as the temperature increase that may occur during an acceleration of the engine. On the other hand, there is a tendency for the shroud to rub against the turbine during a period following a decrease in temperature, such as the decrease which occurs in conjunction with a deceleration.
The shroud often is cooled to reduce its rate of thermal expansion and to control the total growth achieved during steady state operation, thereby minimizing running tip clearance. This cooling typically is accomplished by removing air from the compressor and directing that air into channels formed in the shroud. Since the air extracted from the compressor has not yet passed through the combustor, it is significantly cooler than the combustion gases which approach the turbine assembly and the shroud. Therefore, the rate of thermal expansion of the shroud is reduced with a resulting decrease in tip clearance during conditions of temperature increase in the turbine engine.
Although the cooling of the shroud has a desirable effect during the transient conditions where expansion is likely, cooling has a negative effect when transient operating conditions cause the shroud to contract. For example, when the engine is undergoing a deceleration the shroud rapidly contracts radially inwardly. The cooling gases directed into the shroud can only accelerate this already rapid inward contraction. Therefore, to prevent rubbing during periods of deceleration it often has been necessary to build a greater cold tip clearance into the engine then would otherwise be desirable.
A secondary problem associated with cooling the shroud during periods of deceleration and low power operation is that the cooler air is being extracted from the combustor even though it is not required in the shroud. The extraction of this air from the combustor carries a price in terms of efficiency, in that work has been performed to compress this air, but the air is then being extracted to perform an unneeded cooling function rather than being directed to the combustor where it can continue to perform useful work.
Attempts have been made to control the amount of cooling air that is directed to the shroud. To the extent these attempts could be successful they could enable a smaller cold tip clearance with a resultant increase in efficiency during all operating conditions. Additionally to the extent these attempts could be successful, there could be a reduction in the amount of cooling air extracted from the compressor, thereby enabling this compressed air to be put to more useful purposes. However, the prior attempts to control shroud cooling have been extremely costly, inefficient and cumbersome.
In view of the above, it is an object of the subject invention to provide an efficient shroud construction to modulate the flow of cooling air to the shroud.
It is another object of the subject invention to provide a turbine engine construction in which cooling air to the turbine shroud is passively modulated.
It is an additional object of the subject invention to provide a passively modulated turbine shroud which is properly cooled without additional external equipment for controlling the flow of cooling air thereto.
It is still another object of the subject invention to provide a turbine shroud which can enable a smaller tip clearance under all operating conditions.
It is still an additional object of the subject invention to provide a turbine shroud which will undergo thermal expansion and contraction which closely approximates the expansion and contraction of the rotating turbine assembly.