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Aviation History
1948
1948 - 2005.PDF
NOVEMBER 25TH, 1948 FLIGHT 041 Ramjet Performance Part II of Canadian National Research Council Report (Continued from page 611) THE second section of the reportdeals with the combustion pro-cesses, and states that all obser- vations on the combustion of liquid hydrocarbons indicate that the visible combustion takes place only at a certain ignition temperature at a measurable velocity and after a given time called the ignition lag. The fuel injected: in the combustor does not immediately react with the oxygen in the air to form carbon-dioxide and water vapour, but highly active intermediate products—atoms, radicals and activated molecules—are formed in the fuel vapour resulting from the heat available in the combustor. These particles, by collision with the oxygen molecules, help to bring about the chemical reaction, whilst . the energy released by the collision helps to accelerate the rate of reaction. Following more closely the physical and chemical processes taking place in a combustor, the course of events is as follows: First, a high-pressure liquid fuel is injected in the airstream. Ato- mization of the fuel then occurs, the droplets having a velocity relative to the air. Due to friction, heat conduction and radiation from hot gases, evaporation then takes place froin the surface of the droplets. : Breakdown Details It has been shown that the droplets are formed by the continuous disinte- gration of fragments of the original fuel film which break ofi irregularly in the form of cords and sheets. The breaking- up region of the film approaches the orifice as the liquid pressure increases. yVith liquids of low viscosity, the films show considerable oscillations as the pressure is increased. Viscosity, how- ever, damps down these oscillations and retards the breaking-up process. Re- duced surface tension results in an increased tendency of the film to form cords at its extremities, which generate into strings of droplets. Regarding evaporation, it was found that, although the heat of evaporation does not vary much between the different hydrocarbons, the temperature at which this heat must be supplied varies considerably and has a large effect on the performance of the com- bustor. The one dimensional problem of heat addition in the ducts has been com- pletely solved. It has been found that heat "may be added in a duct until the Mach number on a certain cross-section (most often the outlet) reaches unity. In order to attain high combustion intensities (defined as the heat released in a combustor per unit volume and time) several methods have been sug- gested, the most important of which are (i) the use of catalytic combustion and, (ii) evaporation and injection of the fuel vapour. No work on catalytic com- bustion has been reported as yet, although the proposition seems rather attractive. Regarding the method of fuel evapora- tion, it has been found that aerated B 29 , kerosene vapour can be burned by up- stream injection. The air was mixed with the fuel in order to reduce its tendency to "crack" and form carbon, .but later experiments have shown that, if the vapour temperature is kept below 450 deg. C, air mixing is unnecessary. Flame stability persisted up to an air velocity of 200ft/sec. The use of altitude chambers has in- dicated that the performance of the " combustor is adversely affected as the static pressure is decreased. A part of this loss is associated with poor atomiza- tion by virtue of the low fuel flows required. Moreover, there is also sufficient evidence to indicate that there is a fundamental loss due to the increase of the ignition lag and other reasons. On the subject of fuels which may be burned in ramjets, the report gives a generalized survey of the properties of a wide variety of elements, and concludes that the only liquid hydride which :s stable at ordinary temperatures, and which is superior in gravimetric calorific value to liquid hydrocarbon fuels, is stable pentaborane. From comparative cycle calculations it was found that, owing to its lower gravimetric calorific value, aluminium has a 50 per cent higher specific fuel consumption than kerosene: on a volu- metric basis~ however, the reverse is true. The possibility of the production of boosted fuels by the use of isotopes and nuclear methods is in its infancy, and it is as yet too early to foresee any results from which reliable conclusions may be drawn. The author of tht- report then gives a general peribrrriance analysis trom which, for given conditions ol operation, the optimum distribution losses will result and, in dealing with regulation, states that, due to lack of a hot super- sonic tunnel for investigations, very littlfc is known of the operation of ramjets at points other than the design point. From tests in a cold supersonic tunnel, and from theoretical analysis, it has been established that, when a supersonic ram- jet operates in a region of smaller Mach number than that for which it was designed, rapid rise in specific fuel con- sumption occurs, although when it operates at higher Mach numbers, the rise in specific fuel consumption is not so pronounced. In a simple ramjet, the variable quantities for control are three in number, namely, the fuel quantity, the inlet area and the propelling nozzle area. The fuel quantity fixes the maximum temperature. The variation of the inlet area aims at the achievement of an opti- mum external drag and inlet pressure recovery, and finally, the exit area is used to vary the thrust. Unfortunately, this is not always feasible, because, at supersonic flight speeds, critical condi- tions at the entry and exit may appear, and this places a restriction on the inde- pendent variation of the three para- meters. Finally, it may be added that sim- plicity, reliability, duration of flight and manufacturing costs are decisive factors for the selection of the quantities to be regulated. APOLLO PROGRESS HAVING now been moved into thefinal assembly shed at Bagintou, the AW 55 Apollo is busily being groomed in readiness for its initial flight trials. These can be expected early in the New Year. As we go to press, two engines have been delivered and are ready for installation, whilst the remain- ing two Mambas are expected to be delivered within the near future. Pre- liminary functioning trials of the under- carriage installation have been started, and although the elevator and rudder are in position, the ailerons and flaps have yet to be installed. The attractive- ness of the aircraft, which was so readily apparent from the models and full-scale mock-up, is, if anything, emphasized in physical actuality. • The second Apollo (chronologically) is almost complete in major component break-down state. This aircraft, how- ever, will not fly; it is destined for vibra- tion and strength tests. The third air- craft, i.e., the second flying prototype, is now in course of construction in the jigs. During our visit to Baginton, we naturally enquired of other activities and learned that, although the Nene- engined AW 52 is now undergoing in- spection and sundry modifications—it hai/incr cornT>1eted about thirtv hours' flying—the Derwent-powered version is now flying at Bitteswell and, of course, is continuing the research programme basically common to both of these machines. In addition, flight trials with the AW 52G glider are now to be resumed, a new- tug, a Lancaster, having been acquired. The flight trials with the glider are, of course, complementary to those with the turbojet 52s, and in some respects a glider offers certain advantages over a powered aircraft for fundamental flight research. For example, no allowance need be made for the windmilling drag of engines, which quality will, of course, vary with speed and altitude, and for sundry aerodynamic investigations this simplification makes the aerodynami- cist's job a good deal less involved. A new departure in Armstrong Whit- worth activity is, in common with that of many other aircraft companies to- day, the design and manufacture of guided missiles. This work is being inaugurated at the company's Whitley establishment under the leadership of Dr. Leslie Newman. Naturally enough, very little can be said at this stage about the nature of the work being undertaken, other than that it is concerned with guided missiles and rockets for naval and anti-aircraft use.
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