Active tip-clearance control

Active clearance control (ACC) is a method used in gas turbines to improve fuel efficiency. This is achieved by dynamically controlling the turbine tip clearance.
During normal cruise flight, the engine is exposed to many loads such as intense heat and centrifugal force. This causes expansion of certain components and alters the gap between the turbine casing and tips of the spinning turbine blades. The amount of air leaking around, past the edge of the blades without passing through them is critical to engine performance and fuel efficiency. For this reason since the late 1960s, blade tip sealing has taken on a prominent role in aircraft engine design. The ACC system dynamically controls the high pressure turbine (HPT) blade clearance. This can be achieved in numerous ways.
Background.
HPT (high pressure turbine) blade tip clearance has a significant impact on fuel burn and emissions.
Blade tip sealing has been a challenging problem since the development of the gas turbine engine. It is such because the clearance between the blade tips and surrounding casing (shroud) tends to vary due primarily to changes in thermal and mechanical loads on the rotating (turbine wheel) and stationary (stator, turbine casing) structures.
Using ACC gives significant benefits in cruise fuel burn, range, and payload capability for long range aircraft.
Basic system overview.
One common active clearance control consists of the ACC valve which mixes hot and cold air from the compressor exit and the bypass duct, respectively, to a desired temperature. The air is routed to flow through tubes surrounding the casing at each turbine stage. This air expands or contracts the turbine case and in doing so, it maintains the accurate clearance between the turbine case and the blade tip. This clearance should be maintained accurately which is essential for the engine efficiency and its performance. The ACC valve opening is adjusted automatically by the FADEC system depending on the thrust lever position.
Categories.
HPT clearance control systems can be categorized as passive and active, active being controlled via a hydro-mechanical control or via FADEC (full authority digital engine control). The systems can be further classified as either thermal or mechanical.