The high pressure turbine shroud and the high pressure blade form the portion of the flow-path where the gas stream internal energy is converted to horsepower. If shrouding does not fit tightly to the HP blade tip, gas can go over the blade tip and escape without sharing its energy with the rotor. This loss of efficiency has led mechanical designers to design all kind of gadgets or tricks to minimize this gap. A major part of this problem arises from transient and steady state positioning of the HP blade.
As a function of changing engine speed and temperatures, deformation of the HP turbine disk alters the blades position in the flow-path. To match this variation, the engine control meter compressor bleeds air, which is piped to the HP turbine shroud. Alternate heating and cooling cause the shroud support structure to expand and contract to meet changing rotor deflections and, thereby, improve turbine efficiency. This process is called active turbine clearance control.
