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Aviation History
1958
1958 - 0717.PDF
30 May 1958 733 Combustion and Propulsion POINTS FROM PAPERS AT AGARD MEETING Part 2 IN Flight last week we summarized three of the papers givenat the recent "colloquium" held in Sicily by the NATOAdvisory Group for Aeronautical Research and Development. Below are four further summaries, and others will appear in thenear future. M. Emile Le Grives, of ONERA contributed Le Probleme duStato-Fusee, what must be the first detailed statement of the ram/rocket, a scheme for a very-high-altitude M = 4.0 dual-powermissile engine. This consists of a ramjet duct, in the centrebody of which is mounted a liquid rocket (50/50 furfuryl alcohol andxylidine with nitric acid) which would run fuel-rich so that die "waste" products would be burnt with air induced into the ductby the rocket efflux and speed ram. A schematic configuration for a ram-rocket. M. Le Grives, with some mathematics and a wealth of curves,examined the essential performance parameters under constant conditions corresponding to supersonic flight at constant altitude.A simple reference rocket, with a fuel consumption at a constant ratio to the ram/rocket was used as a "control." The calculationsalso included a factor representing the weight of the rocket instal- lation in the ramjet. Both the net thrust and s.f.c. are worse forthe ram/rocket than for ramjet and rocket operating as separate units; but these conclusions were reached without allowing forcombustion stability or the choice of the optimum fuel and oxidant for the configuration. Sip. Luigi G. Napolitano, Mr. Paul A. Libby and Mr. AntonioFerri reviewed Recent Work on Mixing at the Polytechnic Insti- tute of Brooklyn. Theoretical and experimental work on mixingflows was described and two general problems were discussed; the free mixing of semi-infinite streams, and mixing in the presenceof a wall. In the first, the cases of uniform streams and of non- uniform streams involving separately streamwise pressure gradi-ents and vorticity were considered. The fully developed free tur- bulent mixing of two different gases was explained and governingequations for two different assumptions with respect to turbulent transport were derived. Solutions for unity values of turbulentPrandtl and Schmidt numbers and for a parabolic density-velocity relationship have been obtained by high-speed machine calcula-tions. On the basis of the numerical results it was concluded that turbulent mixing-velocity profiles were substantially independentof the nature of the gases, of the free-stream density ratio and of the Mach numbers. Approximate solutions to these types of non-homogeneous turbulent mixing were presented. The possibility of single parameter solutions of homogeneous compressible laminarmixing in the presence of streamwise pressure gradients was demonstrated and the pertinent equations derived. The influenceof outer vorticity on the characteristics of the laminar and turbu- lent interaction of an incompressible, constant-vorticity streamwith fluid at rest was also determined. In the second problem, the various zones of the interactionbetween the mixing region and the boundary layer on the wall were discussed. For the first, wherein an inviscid core betweenthe two viscous regions exists, a solution was presented. Pre- liminary results of an experimental investigation of compressible,iso-energetic mixing in the presence of a wall were also presented. Shockwave and Flame Interactions were discussed by Mr.George Rudinger, principal physicist of the Cornell Aeronautical Laboratory, Inc. Part of the material in this paper was basedupon work under the U.S. Office of Naval Research Project SQUID. High-speed photography (6,700 frames/sec) by Dr. G. H.Markstein of a flame subjected to shock disturbances in a vertical shock tube was used for experimental observation from whichtheoretical analyses of the phenomena occurring when pressure waves in a duct interact with a flame front were examined. It wasobserved that, when a Shockwave interacts with a laminar flame, the latter emits pressure waves for a short time after the interactionin addition to the immediately established transmitted and reflected waves. The break-up of the flame due to shock acceleration wasdemonstrated by Schlieren photographs. The subsequent increase in flame surface is accompanied by an increased rate of combus-tion associated with the emission of secondary pressure waves— these being shown by Schlieren streak records. Many flames are highly turbulent, containing pockets of burned and unburned gassurrounded by relatively large zones of differential density. These pockets have a different response to wave accelerations than thesurrounding gases, which result in distortion of the flame and a different burning rate—and may be a secondary source of pressurewaves. If the burning zone is short enough to be approximated to aplane-surface discontinuity normal to the flow direction, combus- tion can be specified by the amount of heat released and the effec-tive burning velocity. The boundary conditions at a flame front can be derived in terms of two parameters and computing pro-cedures are given for the interaction of pressure waves with a flame front. Most of these procedures are lengthy, but it is oftenpossible to simplify them. Assumptions for the combustion process do not affect application of these procedures. A review of approxi-mate analytical solutions which can be derived by the method of small perturbations was given. Mr. Rudinger concluded with aplea for carrying out similar studies with burners and research into the study of extended burning regions which can no longerbe approximated to a plane flame front. Great Britain was well represented in Group III, Noise, byProfessor E. J. Richards' extensive and practical paper Some Thoughts on Noise Suppression Nozzle Design, in which hepresented a clear picture of the task, the present field of knowledge, and the solutions attempted to date. While the parameters of drag are so well understood today thatits value can be calculated to within 1 or 2 per cent, the physical picture of the relationship between the kinetic energy of a jet andits total radiated sound energy (perhaps 0.01 per cent) is not. This has resulted in two diametrically opposite lines of attack; funda-mental studies of noise from circular jets to confirm Lighthill's theories; and blatantly ad hoc experiments of every imaginablenozzle shape. "Indeed, it is understood that the initial impetus to nozzle-shape variation arose not from elaborate theory but fromthe noise reduction noticed when a mechanic slipped his finger into a model jet in operation!" As a first approximation, acoustic power varies as the eighthpower of the jet velocity and the square of the nozzle diameter, after which allowance has to be made for downstream convectionof the eddies—for which basic data are not available—since noise is generated in the turbulent mixing zone between the jet and thesurrounding air. The various nozzles are designed to achieve the most rapid mixing-in of cold air and therefore the most rapiddeceleration of the jet efflux. This must be done with a minimum of thrust loss. The most successful general line of attack is tobreak the jet into multiples of smaller size, which raises the general frequency of the noise to a value where it does not carry.The Boeing multiple nozzle (as on the first 707s), the cross-sfit nozzles for Fairey pressure-jets, and the "tea strainer" for theOlympus 101 are examples. This last requires a long shield to (Above) The noise-suppress- ing nozzle for the Bristol Olympus 101 series, men- tioned by Prof. Richards. (Right) Another suggestion for reducing jet noise—and, possibly, improving specific fuel consumption: a tree- turbine fan ejector. DUCT"
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