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 tubes enclosing the turbine casing. The features of ACC systems make their cooling 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, cooling performance is affected by the target surface properties, such as its shape and roughness.

Starting from these considerations, the present work is aimed at the development of accurate and reliable tools to determine the heat transfer characteristics of ACC systems. To achieve this goal, a simple ACC circular duct is considered (16 mm outer diameter, 1 mm thickness), housing three square-edged cylindrical holes (1 mm diameter) aligned along the tube axis. The jets generated by such holes cool down a flat target surface, which is replicated by a stainless steel 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. In order to preserve the jet target surface properties, the heating foil is applied on the side of the target opposite to the jets, and temperature is also measured on this side by means of a 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. The proposed method is successfully validated by comparing the results with available literature data.

Experimental results have been employed as a benchmark for the development of a suitable numerical setup. In particular, both steady RANS and unsteady, scale resolving simulations have been carried out. Results revealed that, while both approaches are able to correctly replicate the average heat transfer values, only the scale resolving models provide the correct shape of its distribution, thus allowing to obtain a correct interpretation of flow and thermal phenomena for ACC systems.


Development of Experimental and Numerical Methods for the Analysis of Active Clearance Control Systems

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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