Tip clearance control for turbine blades

A method of controlling a temperature of a turbine casing of a gas turbine engine, the engine including an array of circumferentially spaced turbine blades disposed radially inwardly of the casing and circumscribed by a carrier section comprising a plurality of carrier segments, each carrier segment being disposed radially inwardly of the casing and radially outwardly of the turbine blades and each carrier segment including a pair of carrier walls including a radially outer carrier wall and a radially inner carrier wall, and the radially outer carrier wall comprising one or more portions facing the casing, wherein at least one of the one or more portions of the radially outer carrier wall is provided with one or more impingement apertures therein, including a main set of impingement apertures, for passage therethrough of air of a predetermined temperature from a feed source and into impingement onto the turbine casing, and each carrier wall extending between front and rear carrier ends, wherein the pair of carrier walls define therebetween one or more chambers for receiving therein heating or cooling air from the feed source via said front end, and each carrier segment further comprises one or more extension sections extending axially, relative to the engine's longitudinal axis, from a main carrier wall section via which the radially outer carrier wall is united with a remainder of the carrier segment, and wherein the or each extension section includes an extension wall that is provided with an extension set of impingement aperture(s) therein,

• wherein the method comprises, in the or each carrier segment:
• in a first mode, passing air of the predetermined temperature (i) from the feed source through the extension set of impingement aperture(s) to an extension impingement space, which is between the extension wall and the casing, and into impingement on the casing, and (ii) from the feed source through the main set of impingement aperture(s) to a main impingement space, which is between the radially outer carrier wall and the casing, and into impingement on the casing, so that the temperature of the casing is controlled in dependence on the predetermined temperature of the impinging airflow thereon, wherein, in the first mode, the air passing through the extension set of impingement aperture(s) travels to the main impingement space;
• and
• in a second mode, once the air has impinged onto the casing, exhausting the air radially outboard of the extension section from the main impingement space and from the extension impingement space, wherein, in the second mode, the air passing through the main set of impingement aperture(s) travels to the extension impingement space and then exhausts radially outboard of the extension section.