This invention relates generally to turbine engine stator assemblies, and more particularly, to apparatus and method for controlling operating clearance between a stationary shroud surface in a turbine engine stator assembly and a rotating surface of juxtaposed blading members.
Forms of an axial flow turbine engine, typically a gas turbine engine, include rotating assemblies radially within stationary assemblies that assist in defining a flowpath of the engine. Examples include a rotary compressor assembly that compresses incoming air, and a rotary turbine assembly that extracts power from products of engine fuel combustion. Such assemblies comprise stages of rotating blades within a surrounding stator assembly that includes a shroud surface spaced apart from cooperating surfaces of the rotating blades. Efficiency of a turbine engine depends, at least in part, on the clearance or gap between the juxtaposed shroud surface and the rotating blades. If the clearance is excessive, undesirable leakage of engine flowpath fluid will occur between such gap resulting in reduced engine efficiency. If the clearance is too small, interference can occur between the rotating and stationary members of such assemblies, resulting in damage to one or more of such cooperating surfaces.
Complicating clearance problems in such apparatus is the well known fact that clearance between such turbine engine assemblies changes with engine operating conditions such as acceleration, deceleration, or other changing thermal or centrifugal force conditions experienced by the cooperating members during engine operation. Clearance control mechanisms for such assemblies, sometimes referred to as active clearance control systems, have included mechanical systems or systems based on thermal expansion and contraction characteristics of materials for the purpose of maintaining selected clearance conditions during engine operation. Such systems generally require use of substantial amounts of air for heating or cooling at the expense of such air otherwise being used in the engine operating cycle. Provision of an improved means for active clearance control that reduces the need for engine flowpath fluid for such heating or cooling could enhance engine efficiency.