Transient Buoyancy-Induced Flow and Heat Transfer in Rotating Compressor Cavities

The next generation of aeroengines will feature compressors with increasing pressure ratios and smaller engine cores. Maintaining high efficiencies will require increased sensitivity to reduced blade tip clearances, governed by strong buoyancy-induced flow and heat transfer within the rotating cavities formed by the discs to which the blades are attached. The inherently unsteady flow within these cavities is three-dimensional and unstable. Thermal stresses in the discs are governed by forced and natural convection across large differences in temperature, conjugate heat transfer, centrifugal forces, and disrupted by mass exchange between the core air and an axial cooling throughflow at low radius. The thermo-fluid-dynamics has further complexity during accelerations or decelerations in aeroengine transients. The engine design process requires expedient and reliable aerothermal models to predict the transitory temperatures of the discs, and hence the thermal growth of the rotor and tip clearance. This paper presents, for the first time, a theoretical model to predict compressor cavity transient heat transfer and temperatures from first principles. The reduced-order model was created in close partnership with an experimental programme using an innovative rig designed specifically to explore buoyancy-induced flow in compressor cavities. Unsteady pressure, temperature and heat flux data were collected in the rotating frame of reference under controlled boundary conditions for two engine-representative open-cavity configurations. The predicted temperature, mass flow and heat flux results were consistent with measured values within experimental uncertainty as shown by RMSE analysis. The research has identified sub- and super-critical flow regimes governed by the Rossby number and enthalpy exchange with the axial-throughflow. Fundamental insight has been established, including the presence of large-scale structures formed from buoyancy-induced convection as well as that induced purely by throughflow interaction. In collaboration with Rolls-Royce, this study has been framed in the practical context of providing expedient solutions appropriate for transient thermo-mechanical codes during the engine design process.