The ever increasing performance requirements of modern aero-engines demand the development of effective ways to improve efficiency and reduce losses. Casing temperature control is particularly critical from this point of view, since thermal displacements directly affect the tip clearance and thus the associated aerodynamic losses. To achieve this goal, Active Clearance Control (ACC) systems have been introduced. These systems are usually based upon impingement cooling, generated by a series of circumferential perforated pipes enclosing the turbine casing. The features of ACC systems make their heat transfer performance not easy to predict: in fact, jets are fed by a duct-like plenum, where a strong upstream crossflow can occur, and are arranged in single or multiple rows, causing relevant jet-jet interactions. Moreover, considering real system hardware, the pipe assembly process may lead its actual position (in terms of distance and alignment with respect to target surface) to be different from the ideal one, which may significantly affect the system geometry and thus its cooling features.Starting from these considerations, in the present work the effect of the impingement holes relative distance and supply pipe position on the flow and cooling features of different ACC geometries has been experimentally investigated. All geometries present 0.8 mm circular impingement holes, arranged in a single row with various jet-to-jet distances. The jets generated by such holes cool a flat target surface, which is replicated by a metal plate in the experimental setup. Measurements are performed using the steady-state technique, obtained by heating up the target plate thanks to an electrically heated Inconel foil applied on the side of the target opposite to the jets. Temperature is also measured on this side using an IR camera. Heat transfer is then evaluated by means of a custom designed finite difference procedure, capable of solving the inverse conduction problem on the target plate.Jet Reynolds numbers ranging from 6000 to 10000 are investigated, as well as jet-to-target distances from 3 to 11 diameters. The effect of supply pipe orientation (i.e. rotation around its axis) is also studied. The outcomes are compared in terms of hole discharge coefficient and Nusselt number distribution on the target surface.

Effect of Jet-to-Jet Distance and Pipe Position on Flow and Heat Transfer Features of Active Clearance Control Systems

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2021/volume/84898

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