Turbine inner shell heating and cooling flow circuit

A method of operating a gas turbine having inner and outer shells, with the inner shell being radially movable relative to rotor bucket tip passage in the inner shell for flowing a thermal medium. A pair of passage portions are formed in each of the aft and forward inner shell sections with axially communicating passageways between the passage portions. A thermal medium, preferably from an off-turbine site, is provided for flow through the second-stage aft inner shell section, along axial passageways along the mid-line of the inner shell to a first passage portion of the forward inner shell section. Cross-over paths flow the thermal medium from the first passage portions to second circumferentially extending passage portions of the forward inner shell section, in turn, in communication with axial passageways extending from the forward section to the aft section. A second pair of passage portions flow the thermal medium to an outlet in the aft section. By controlling the temperature of the thermal medium for flow through the aft and forward sections of the inner shell, the clearance between the shrouds and the tips of the buckets for at least the first and second stages of the turbine is controllably adjusted.