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Aviation History
1948
1948 - 1905.PDF
NOVEMBER IITH, 1948 FLIGHT 581 O4 O-6 POWER RATIO8" O-8 1O PART LOAD POWER DESIGN POWER 1-2 1-4 Fig. 2. (1) Reaction turbine; assumed design PJP1=0.8 (4 stages). (2) Impulse turbine; assumed design PJP,=0.8 I stage). • (3) Impulse turbine; assumed design P2/Pl=0.38 (I stage). (4) Impulse turbine, assumed design P2/Pl—0.2S (2 stages). Variation of Part Load Efficiency of Turbines Running at Constant Speed. "choked " variation shown in Fig. i is gradual as the chokingflow is approached. The variation of efficiency at part-load, of a turbine runningat constant speed is illustrated in Fig. 2. The part-load effi- ciency when the turbine operates with constant outlet pressureand inlet temperature is always better than that obtained when operating with constant inlet pressure and inlet tempera-ture. When a turbine produces only a fraction of its optimum power, the densities of the gas flow are relatively less whenworking with a constant outlet pressure than when operating with constant inlet pressure. In the former case, the incidenceand deflection in each blade row must be relatively closer to the optimum values to produce a particularly part-load power,and thus the efficiency will also be correspondingly nearer to the optimum. The majority of cycles envisaged for the gas-turbine enginepermit the turbine component to operate with an approximately constant exhaust pressure. Fig. 2 shows that under these con-ditions, the best part-load efficiencies are given by single- stage impulse turbines designed for a large pressure-drop at theoptimum (design) point. Unfortunately this comparison is in- complete since no experimental characteristics exist for a highpressure-drop reaction turbine. Nevertheless the variation of part-load efficiency of low pressure-drop reaction and impulseturbines are similar, and it is conceivable that there will be little distinction between multi-stage impulse and reactiontypes designed for high pressure-drops. Although this com- parison leaves little to choose between impulse and reactionturbines, the absolute value of optimum efficiency is invariably greatest on reaction types. Since p reaction turbine developsless work per stage than an impulse turbine, the size and weight of a turbine designed for a particular work output willbecome greater as the degree of reaction is increased, and a com- promise between the size and efficiency often has to be made. cooled compression, reheated expansion, and heat exchangeSuch information enables the design performance of a par- ticular engine to be estimated, but there appears to be verylittle published work to help the designer to foretell what changes in performance are to be expected when particularengines operate at non-design conditions. In th4s lecture an attempt is made to include in a generalcomparison of part-load performance characteristics not only the simpler designs but also some of the more complex turbineengines which will be needed for land and marine applications. At the same time, by considering, in appropriate cases, theinfluence of changes in the assumed component performance features on the part-load operation of an engine, a broadeningof the basis of comparison is made possible. The Fuel Problem in Gas Turbines ~- : ."•- . By Peter Lloyd, M.A. A CCEPTING the principle that the gas turbine is potentially"• capable of consum'ing aS fuels, the author proceeds to investigate the range of possible liquids fuels and the fuelcharacteristics of greatest practical significance. Whilst certain properties of liquid fuels—as for example, ignition charac-teristics and molecular structure—are so complex that no corre- lation with gas-turbine combustion behaviour has been pos-sible,- yet other properties, mainly but not exclusively physical in nature, do exist for which the reverse is the case. Propertiesfalling in the latter category are those which show relatively little variation as between hydrocarbon types or as betweenisomers, and include carbon / hydrogen ratio, density, vapour pressure, viscosity, and inflammability limits. Theinfluence of these properties on the combustion process is dis- cussed in detail under the two main heads, (a) fuel injection andstabilization of flame, and (b) the burning of the droplets after ignition, and the nature of the exhaust products. 1/ Heat Flow in the Gas Turbine By A. G. Smith, B.Sc. TT^XPERIMENTAL data are presented on turbine-blade heat-«—' transfer, which is an important factor in the design of high-temperature turbines. Blade stagger has a considerableeffect on blade Nusselt number and on the rate of variation of Nusselt number with Reynolds number.. For nozzle blades,theoretical and experimental values of the Nusselt number agree fairly well. A method is proposed for the approximateestimation of Nusselt number by Reynolds analogy. The experimental data are used in the calculation of heat flow inblades cooled at the root, and in internally air-cooled blades. Other methods of cooling have been proposed, includingpartial admission of cold air or liquid through the nozzle ring; the theoretically advantageous "sweat" cooling byexuding coolant air through a porous skin, or the injection of coolant air through slits; root cooling might include modifica-tions, such as effective increase of blade conductivity by tubes of liquid embedded in the blade or reduction, by an insulatinglayer, of the amount of heat conducted to the blades; internal liquid cooling by forced or convective circulation is a possiblealternative to internal air-cooling. The Prospects of Land and Marine Gas Turbines Three-dimensional-flow Theories for Axial By Hayne Constant, M.A., M.l.Mech.E., F.R.S. IN his lecture the author considers the world conditions underwhich the gas turbine for. land and sea use is being developed, and attempts to assess its prospects. If some ofthe problems remaining, to be solved are overcome, it is pos- sible to foresee a wide field of usefulness for this prime mover.When the present world-shortage of liquid fuel is eased, the gas-turbine locomotive will become a strong competitor to thesteam locomotive, and, whatever the liquid-fuel position, it will shortly be able to meet the diesel-electric locomotive onmore than equal terms. The prospects for marine use turn on its ability to burn heavy residual oils. The Part-load Performance of Various Gas-turbine Engine Schemes By D. H. Maltinson, B.Sc., and W. G. E. Lewis, B.Sc.,G.I.Mech.E MUCH information is already available concerning the per-formance of the "constant-pressure" cycle both in its simple form and in its more complicated forms involving inter- • Compressors and Turbines By A. D. S. Carter, B.Sc., G.I.Mech.E. IT has long been known that the energy losses occurring inan 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 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 enables quantitative estimates to be made, are discussed at some length. Following this, the three-dimen- sional-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. B 25
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