It has long been known that the energy losses occurring in an axial compressor or turbine cannot be fully accounted for by the skin-friction losses on the blades and annulus walls. The difference, usually termed secondary loss, is attributed to miscellaneous secondary flows which take place in the blade row. These flows both cause losses in themselves and modify the operating conditions of the individual blade sections, to the detriment of the overall performance. This lecture analyses the three-dimensional flow in axial compressors and turbines, so that, by appreciation of the factors involved, possible methods of improving the performance can readily be investigated. The origin of secondary flow is first examined for the simple case of a straight cascade. The physical nature of the flow, and theories which enable quantitative estimates to be made, are discussed at some length. Following this, the three-dimensional flow in an annulus with a stationary blade row is examined, and, among other things, the influence of radial equilibrium on the flow pattern is noted. All physical restrictions are then removed, and the major factors governing the three-dimensional flow in an actual machine are investigated as far as is possible with existing information, particular attention being paid to the influence of a non-uniform velocity profile, tip clearance, shrouding, and boundary layer displacement. Finally the various empirical factors used in design are discussed, and the relationships between them established.
One of the main features characterizing gas-turbine technology has been the aerodynamic approach adopted towards such items as the axial compressor and turbine, the ducting components, and so forth. This has necessitated a lot of research into many basic fluid dynamic problems which had hitherto received little or no attention. Equipment has had to be constructed and special techniques developed or adapted to meet the new requirements. This lecture gives an account of such work, with particular reference to the methods adopted at the National Gas Turbine Establishment (N.G.T.E.). In the first part of the lecture the simple cascade tunnel has been described. Its design involves many features encountered in work of this nature, and touches on many basic principles. Following this, some special forms of cascade tunnel are described. For example, heat-transfer tunnels, an icing tunnel, and annular tunnels are considered as departures from the conventional. Following a few notes on ducting research, the final section deals with general instrumentation techniques. The measurement of air pressures and direction is considered, together with the measurement of gas and wall or blade-surface temperatures. Some mention is made of boundary layer investigation and high-speed flow visualization. An apparatus for determining the potential flow past blades is also described. All these have been selected as examples representing techniques which have contributed in some way to our knowledge, and hence to the development of the gas turbine.
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