The present invention relates generally to gas turbine engines, and more particularly to a mechanical apparatus for controlling the clearance between a row of rotor blade tips and a surrounding shroud in a gas turbine engine.
A gas turbine engine includes a rotary compressor to compress the air flow entering the engine, a combustor in which a mixture of fuel and the compressed air is burned to generate a propulsive gas flow, and a turbine which is rotated by the propulsive gas flow and which is connected by a shaft to drive the compressor. The efficiency of a gas turbine engine, such as an aircraft jet engine, depends in part on the clearance or gap between the rotor blade tips and the surrounding engine casing shroud, such as the clearance between the engine's turbine blades and the engine's turbine casing and the clearance between the engine's compressor blades and the engine's compressor casing. If the clearance is too large, more of the engine air flow will leak through the gap between the rotor blade tips and the surrounding shroud, decreasing the engine's efficiency. If the clearance is too small, the rotor blade tips may strike the surrounding shroud during certain engine operating conditions.
It is known that the clearance changes during engine acceleration or deceleration due to changing centrifugal force on the blade tips and due to relative thermal growth between the rotor and the engine casing. For instance, upon engine acceleration, the thermal expansion of the rotor typically lags that of the engine casing, and upon engine deceleration, the engine casing contracts more rapidly than does the rotor.
Control mechanisms, usually of the mechanical or thermal type, have been proposed in the prior art to maintain a generally constant rotor-to-shroud clearance despite changing engine operating conditions. However, none are believed to represent the optimum design for controlling such clearance. Consequently, a need still remains for an improved apparatus for clearance control, one that will maintain a minimum clearance between the rotor blade tips and the engine casing shroud throughout the operating range of the engine and thereby improve engine efficiency by achieving more thrust with less fuel.