Turbine engines commonly operate at efficiencies ranging from about 30% to about 40%. The operational efficiency is less than the theoretical maximum because of losses that occur in the flow path. One of the major flow path losses is due to the leakage of hot combustion gases across the tips of the turbine blades. In particular, the leakage occurs across a space between the tips of the rotating turbine blades and the surrounding stationary structure such as the ring segments. This spacing is often referred to as the blade tip clearance.
Blade tip clearances cannot be eliminated because, during transient conditions such as during engine startup or part load operation, the rotating parts (blades, rotor, and discs) and stationary parts (outer casing, blade rings, and ring segments) thermally expand at different rates. As a result, blade tip clearances can actually decrease during engine startup until steady state operation is achieved at which point the clearances can increase, thereby reducing the efficiency of the engine. Thus, there is a need for controlling blade tip clearances in order to maximize the efficiency of a turbine engine.