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Aviation History
1958
1958 - 0763.PDF
FLIGHT, 6 June 1958 779 Combustion and Propulsion FURTHER ABSTRACTS OF PAPERS AT AGARD MEETING Part 3 ON this and the next page are a further and concludingseries of summaries of the papers given at the recent"colloquium" given in Sicily by AGARD—the NATO Advisory Group for Aeronautical Research and Development. The first two instalments appeared in our issues of May 23 and 30. A joint paper, Jet Engine Noise Reduction, by Mr. Newell D. Sanders and Mr. Warren J. North was, in essence, an N.A.C.A. statement of an acoustic scene divisible into five categories: nuisance to communities near airports; aircraft structural damage from high sound pressures, a problem increasing with thrust; hearing, and other body-function, injury to ground per- sonnel; crew and passenger inconvenience and discomfort; and ground manoeuvring nuisances. Noise research, started in Great Britain and intensified in the U.S.A., is at present concentrated on the take-off community nuisance and the suppression of turbulent fluctuations in the jet-mixing region. Professor M. J. Lighthill's basic quadruple relationship P=Kp0 AVs/t£o (P, sound power; p0, density of surrounding medium; a«, speed of sound in surrounding medium; A, jet cross- sectional area; V, efflux velocity; K, dimensionless constant) makes it possible to calculate the effect of jet velocity on the sound radiated. Experiments proved Lighthill's "eightii power law" for plain jets, except for afterburning and rockets which have proved less noisy than predicted, but the supersonic sound source is not yet understood—akhough it does seem that temperature is im- material. In N.A.C.A. tests, the Greatrex "crenelated" nozzle was extra- polated as a row of rectilinear oudets with a variable ratio between their width and the space between each. At zero spacing they act as a single nozzle, while wide apart they become individual nozzles without interaction, and in neither case is there any noise reduc- tion. Intermediately there is a ratio of optimum interaction, e.g. 2 for a pressure ratio of 2. Nozzles longer than six times the spacing cause excessive thrust loss and give poor noise reduction. The next tests were upon deeply fluted nozzles, with and without centrebody, giving thrust losses of 3 and 4 per cent and noise attenuation of 4 and 3 decibels respectively. This led to the Boeing tubular-cluster nozzle, which was considered by the company to be easier to fabricate despite the difficulty of fairing die internal flow passages, with a noise reduction of 3 decibels. Douglas Aircraft initiated experiments with an ejector nozzle to promote mixing, which N.A.C.A. took up, combining a corrugated nozzle with an ejector shroud. The total noise power was reduced by 8 db, with a reduction of 12 db in the direction of maximum intensity, at about 45 deg to the jet axis. The ejector gave static thrust augmentation, but this fell away rapidly with speed, to a loss at quite low speed—probably before take-off. Nozzle drag was investigated on a full-scale nacelle up to M = 0.5 in a 20ft tunnel and on l/5di scale models in the 6ft x 8ft transonic tunnel Generally, the nozzles tested showed a drag at M = 0.5 less than 3 per cent of the thrust. The ejector caused no alteration in the nacelle drag-rise Mach number, but between M = 0.8-0.86, the cruise range, the drag coefficient increased by 60 per cent. Most effective suppressor was the "lobe" nozzle with ejector shroud (8 db) but the thrust loss was 6.5 per cent—which could be sub- stantially reduced by shroud retraction in flight. An alternative method of noise reduction is to design the engine with a low jet velocity, eidier a by-pass or by a reduction in turbine inlet temperature. The test-bed aspects of silencing were very fully presented in a paper L'Insonorisation des Banes d'Essais de Moteurs d'Aviation: Aspects Techniques,Financiers et Physiologiques by M.P-Ducrot, A. Riehn, Dr. G. Roumilhac and Dr. H. Souvras of SNECMA. With a wealth of illustrations and a lengdiy bibliography—both remarkably international and including several Flight sectioned drawings. The paper ended with a brief account of the work of the medical staff at Melun-Villaroche in investigating physiological damage from noise. Effects vary greatly with individuals, but there are general trends: workers responsible for a loud noise are little affected, manual workers employed directly in the neighbourhood of noise experience an aggravation of subjective symptoms, mainly debility and irritability; intellectual workers, and those with responsibility, experienced early-morning insomnia (waking at 3 or 4 a.m. and only falling asleep again at the moment of rising) in as many as 70 per cent of the cases, while in many cases there was severe debility accompanied by loss of weight and arterial tension. In animal experiments a rabbit exposed to an ultrasonic source at an intensity of 160 db was killed in ten minutes by congestion and swelling of the brain and viscera. Four papers were presented in Group IV, the Combustion Session, the one which perpetuated the original purposes of the Panel, starting with Recent Progress in Flame Theory by Mr. D. B. Spalding of Imperial College. Until 1953, when Avery, Longwell, Bragg and van Heerden published separately their three basic ideas, the reasons for the extinction of steady-flow flames at high velocity or low pressure were not clear. It is only possible to study flame phenomena by simplifying the whole flame complex into separate groups of processes—fluid mechanics, heat and mass transfer, chemical reaction—and to treat the gas state and chemical features locally. Even so, combustion theory is too much for busy men to study and so Mr. Spalding offered seven "U.H.T.s"—"use- ful half-truths": The aerodynamic half-truth: In a given steady-flow combustion chamber, the streamline pattern depends only on overall fuel-air ratio (OFAR); the mixing half-truth: The local fuel/air ratio (LFAR) at any point in a combustion chamber of given shape depends only on the overall fuel/air ratio, and the position of the point; the thermodynamic half-truth: In a given two-stream flow, the state of the local gas mixture is completely specified by the local fuel/air ratio and the local reactedness, r, the relation between the properties being that for adiabatic steady- flow mixing; the chemical half-truth: The local volumetric reaction rate in a flame depends only on the local fuel/air ratio and the local reactedness; the two-stream half-truth: The state of a two- stream steady flame in a given chamber depends only on its dimen- sionless loading, L2, and the overall fuel/air ratio; the one-stream half-truth: The state of a one-stream steady flame in a given chamber depends only on its dimensionless loading, Li; The super- position half-truth: The reactedness at any point in a steady flame is equal to the sum of the increments of reactedness which would be caused at that point by the volumetric reaction rates prevailing at each increment of flame volume, if that incremental volume alone were reacting. These seven U.H.T.s were explained and elaborated, after which six examples were discussed to make the case tiiat today's prob- lems are no longer restricted to laminar flame propagation and the homogeneous reaction zone. There is hope now that theory can be used to predict the results of a change in chamber geometry and that the chemist can use flame theory much more. There are three outstanding tasks: for die engineer to study models of engine- like geometry to see if separate fuel and air induction is better than pre-mixing; for the aerodynamicist to study turbulent flows involv- ing simultaneous jet-mixing, recirculation and chemical reaction; and for the chemist to devise experimental flame arrangements that are amenable to theoretical analysis, are easily set up, con- trolled and measured, and give widely differing results according to die reaction-kinetic scheme involved. Mr. Melvin Gerstein, assistant chief, propulsion chemistry divi- sion of Lewis Flight Propulsion Laboratory, N.A.C.A., pre- sented a Review of Some Recent Combustion Experiments. He described a number of basic combustion experiments to show the status of knowledge of die chemical processes in flames. Reaction rates and orders derived from a number of flame properties are quite similar, suggesting that similar chemical reactions are in- volved. Certain inconsistencies exist, however, which may indicate changes in mechanism with alterations in fuel/oxidant ratio, par- ticularly for hydrocarbon flames. A large part of the reaction in a hydrocarbon/air flame does not appear to involve the original fuel molecule, which breaks down early in die flame reaction zone. Unfortunately, many flame-investigations have been made under widely different experimental conditions, so that quantitative com- parisons are difficult to make. It would be desirable to study some reasonably simple reaction to determine whether a consistent re- action order and activation energy can be derived from flame velocity, quenching distance, flashback-velocity gradient, flame structure and odier properties. The reaction constants derived from tiiese properties could dien be compared quantitatively widi die known reaction kinetics. Role des Recherches Fondamentales dans I'Etude des Foyers d'Aviation by M. Claude Foure of ONERA had a remarkable bibliography of 111 items. He pointed out that die study of aviation combustion chambers calls for research into phenomena already of recognized importance; injection, vaporization and com- bustion of clouds, flame stabilization and propagation, spon- taneous ignition of combustible mixtures, homogeneous reactions, and internal aerodynamics. The general metiiod of similarity helps in the analysis of the effects of altitude and scale where aero-
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