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Discs and Drums: The Thermo-Fluid Dynamics of Rotating Surfaces

This paper reviews long-term experimental and theoretical research programmes concerned with flow and heat transfer over the large rotating surfaces, commonly discs, often drums but sometimes conical, used to support the blades in turbomachinery. The account begins with a geometry found in turbomachinery from the oldest steam plant to the most modern gas turbine, in which a disc rotates near to a stationary, usually coaxial, member. The flow in the intervening ‘wheel-space’ is well understood, but external conditions can affect the extent and nature of ingress from the surrounding fluid. In the gas turbine this fluid is the mainstream hot gas, an inflow of which could have serious consequences, so that the study of ingress has become the principal subject of research for rotor-stator systems and recent work is fully reported here. In many turbo-machines, especially compressors, adjacent coaxial surfaces rotate together and thus enclose a cavity subject to unusual forces in which a wide range of flow regimes can obtain. The precise form depends largely on whether the cavity allows a net radial inflow or outflow of fluid or whether the only access and egress are from near the axis of the system, the so-called ‘axial through-flow’ case. Systems with a net radial flow, inward or outward, are well understood. In their absence, the flows are often four dimensional, varying with time and in the three space coordinates. Such regimes remain incompletely understood although recent congruence between experimental and theoretical studies is encouraging. Finally, attention is turned to surfaces nearer parallel than orthogonal to the axis of rotation, as in the drums used in older steam turbines and commonly in compressors. Here the main concern has been with the effect of stationary blading, where the close clearance between the blading and the rotating surface modifies the boundary layers and thus the friction and heat transfer on the latter.