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Aviation History
1955
1955 - 0559.PDF
ANNULAR JET Fig. 18 Annular pressure /er. Fig. 79 Noise reduction with annular pressure jet. (Southampton tests.) Fig. 20 Two-dimensional pressure jet. THE DISCUSSION Dr. F. B. Greatrex (Rolls-Royce) congratulated the lecturer on pre-senting an excellent paper which would be the classic reference on the subject for some time to come. He said that, with airscrews, a reduc-tion of disc loading was effective in reducing noise and wondered whether this could be applied to helicopters. He thought that the 12per cent thrust loss experienced with the cross-tail silencer was due to the sudden change of shape at the cross-tail and could be eliminated bycareful design. The principle was not unlike that of the corrugated silencer which was very selective in the frequency at which it was effec-tive, according to the number and size of the corrugations. His com- pany had achieved a 12 db noise reduction, with no thrust loss, byusing this device on a jet engine. Mr. G. H. Vokes (Voices, Ltd.) did not think that the direct measure-ment of two superimposed noises of different forms necessarily reflected the true effect on the human listener. The exhaust noise was a muchharsher sound, whereas blade noise was not so objectionable. Mr. A. Stepan (Pairey Aviation) thanked the lecturer for many usefulsuggestions. He felt that while it might be difficult to reduce sub- stantially the overall noise level of pressure-jets, an advantage mightbe gained by changing the burning pattern in such a way that the noise was shifted to a higher frequency. In this way a greater attenua-tion at distances would be achieved since it was well known that a high- frequency sound was less unpleasant than a low frequency. Mr. R. H. Whitby (B.E.A.) found it difficult to believe that the basicrotor noise was as high as had been shown on the curves. He had never found blade noise unpleasant and wondered whether (a) the curve ofacceptable noise was at fault or (b) the method of derivation of rotor noise was, perhaps, incorrect. On the WS-55 the noise level was 10 dblower on that side of the helicopter opposite the exhaust outlet, even when not fitted with a silencer. RATIO." 2-77 EQUIVALENT M-KJ ORCULAR JET ANNULAR JET 150 300 600 BOO 2400 4000300 600 1200 2400 4800 ftMO 200 400 BOO 1000 V00 6400400 800 000 1200 6400 12800 FREQUENCY BAND (c.p.«.) Mr. G. M. Lilley (College of Aeronautics) suggested that the use ofa number of smaller jet units on each blade might be a better solution than the use of one large unit as at present. He also asked why thelecturer had not made more reference to any problems which might arise from the use of higher tip-speeds, as supersonic blade-tip speedshad been thought by some to improve helicopter performance. Mr. J. Wootton (Hunting Percival) agreed with Mr. Whitby thatthe rotor blade noise was not unpleasant. The design tendency was, he said, for disc loadings to be increased but, with the application of free-lurbine engines to helicopters, some reduction in noise levels could be made in the hovering case by reducing the tip speed in this flight condi-tion. The tests so far carried out on his company's project had shown that the machine would not be noisy in operation, a feature which heattributed to higher mass flow and reduced exhaust-jet velocity. Mr. B. H. Arkell (technical author) asked whether, in view of thesimilarity between the lecturer's proposed two-dimensional tip-jet unit and the new Marquardt ramjet booster for helicopters, there were anyfigures available for comparison between the noise level of this latter unit and conventional jet units of similar thrust. Professor Richards, in reply to the discussion, affirmed his faith inthe noise measurement figures used as being a sound foundation on which to base the investigations. He agreed that the thrust loss on thecross-tail silencer could probably be obviated by a cleaner design. On the question of combustion noise, so many parameters were involvedthat it was difficult to make generalisations. A device to introduce swirling into the combustion chamber for better mixing had been testedand the resulting noise had been "terrific." Raising the noise fre- quency was a good suggestion—as, also, was the possibility of using anumber of smaller jet units in each blade in place of the single larger unit. He was horrified at the thought of helicopters having supersonicblade-tip speeds. A project for a supersonic airscrew had been aban- doned because its noise level would have been higher than that of areheated jet engine. He had no information on the noise level of the Marquardt ramjet but he thought that this type of design was on theright lines to achieve the maximum possible noise attenuation. ROCKET-MOTOR COOLING AT the British Interplanetary Society's meeting on April 2nd,• Mr. H. Ziebland, a Government scientist, spoke on Heat Transfer Problems in Rocket Motors. Many of the failures in rocket motors were^ueto faulty cooling, said the lecturer, and the assessment of heat transfer was a most important matter. Unfortunately both basic theory and research were lagging far behind the application, and as a result the estimation of heat transfer was a very approximate process. So far the temperatures and velocities obtaining in rocket motors had not been encountered in other processes and consequently existing data had to be extrapolated a great deal to meet rocket requirements. Furthermore, the theory in general use had been based on approximations which were far from true. It was in the transfer of heat from the combustion gases to the chamber walls that the greatest unknowns existed. The two pro- cesses of importance were convection and radiation, and large, errors in computation could occur in both. The amount of radia- tion from gases depended on their temperature, pressure and the "path length," a dimension proportional to the chamber diameter. Gas emissivities had only been determined for temperatures up to 1300 deg K and pressures up to one atmosphere, and usually these figures were extrapolated to rocket conditions. The errors arising were large, but in small rocket motors using fairly low temperatures the radiative component was appreciable only in the combustion chamber and, even then, amounted only to a few per cent of the total heat transfer. With larger motors and higher temperatures and pressures this component became progressively important and further investigation was essential. Only two attempts at measuring the emissivities of actual rocket gases had been reported and both of these indicated that the true emissivities were up to twice as great as those predicted by extrapolating existing experimental data. Forced convection had already been very widely studied because of its varied commercial applications, but the processes involved were very complex and still not completely understood. The usual theory assumed fully developed pipe-flow, a small temperature difference between the gas and the wall and no variation of gas properties with temperature. These assumptions were far from reality and, as a result, calculated values could be as much as fifty per cent lower than the experimental values. Various corrections could be applied, but more experimental results under closely controlled conditions were needed. A further phenomenon, known as dissociation, caused enhanced heat transfer when hydrogen-rich fuels were used at high com- bustion temperatures. This was the splitting up of gas molecules into simpler species, in particular molecular hydrogen to atomic hydrogen. The atomic hydrogen diffused rapidly into the bound- ary layer, where it recombined into molecular hydrogen, releasing heat and producing a high temperature zone which led to the increased heat transfer. The transfer of heat from the chamber wall to the coolant presented few problems, said Mr. Ziebland, although one interest- ing phenomenon—"nucleate boiling"—did occur. When the coolant temperature exceeded the local boiling point at the wall surface individual bubbles detached themselves from the wall, and at the same time there was a marked increase in the heat- transfer coefficient. It was thought that this played an important part in many rocket-motor cooling systems.
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