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Aviation History
1956
1956 - 1821.PDF
FLIGHT, 28 December 1956 COCKPIT SIOES AND FLOOR 985 KEEPING HEAT IN ITS PLACE Some Basic Problems of Insulation By L. F. E. COOMBS THE aircraft gas turbine, like all other heat engines, makesits presence felt by radiating heat from its external surfaces.Unlike the reciprocating engine, which also radiates heat, the gas turbine (particularly in the form of a turbojet) gives offheat from 100 to 200 sq ft of surface; the "hot" parts of a piston engine are limited to the comparatively smaller area of the exhaustmanifolds and pipes. The heat emitted by the external surface of a jet engine is inthe form of electro-magnetic energy, which can be passed from the radiating surface into and through any medium such as airor metal. Such energy is variously described, depending on its source temperature, as infra-red, visible light, ultra-violet, X-rays;each type occupies a band of frequencies in the energy spectrum. At normal temperatures the heat of an engine is emitted asinfra-red rays, which are invisible to the human eye. When tem- perature begins to approach the condition at which the surfacebegins to glow visibly, light rays are emitted. The radiant energy emitted travels outwards until it reaches amedium or body which is usually at a lower temperature than its starting point. The receiving body or surface absorbs some ofthe energy and reflects the remainder. The proportion between the absorbed and reflected energy depends on the surface charac-teristics of the receiver; and this aspect of the problem introduces complications, as most surfaces which are subjected to bombard-ment by heat energy exhibit a variation in reflectivity with time. Very few metals are resistant to oxidation by heat; even stainlesssteels suffer surface oxidation after only a short period of exposure to high temperature. Ability to absorb a proportion of the energy is measured inrelation to the emissivity of the surface. The latter is itself related to the emissivity of a reference surface termed a "black body,"which absorbs all the radiant energy to which it is exposed. Thus, the ability to absorb energy is compared with a "black body" underthe same conditions. In addition to the change in characteristics caused by surface changes, such as corrosion, the surface emissivityis affected by its own "body" temperature, so there is not always a direct relationship between emissivity and absorption under theconditions obtaining in the vicinity of a jet engine. The problem which has to be overcome by both airframe andengine designers is the prevention of damage to the aircraft struc- ture from the radiated engine heat. Basically there are three coursesthat can be taken: (1) Intersperse a flow of air; (2) install some form of "material" insulation; (3) use a structural metal, such asstainless steel or Nimonic, which will maintain its strength at elevated temperatures. The use of a flow of air around a jet engine is, up to a point, asimple method of insulation, but there are two major objections to such an easy way out. First, the air used for cooling is notavailable in large enough quantities during ground running, and during flight it represents a loss of power through added drag; thethrust produced by the difference in temperature between the upstream end and the exit of the moving column of air does notbalance the drag loss. Secondly, as aircraft speeds increase the designer does not want to have to cope with additional entrydesign problems. When an aircraft is intended f6r operations at flight speedswhich will exceed Mach 0.8, or thereabouts, the designer has to contend with kinetic heat induced into the external surfaces ofthe airframe. Both engine and kinetic heat are similar in that they adversely effect the airframe structure, but they differ inintensity and quantity. Engine heat is represented by tempera- tures of (depending on location) 500 deg C to 1,000 deg C con-centrated around the engine bay or pod, whereas kinetic heating affects large areas of the airframe skin, albeit at a somewhat lowerrange of temperatures. Both kinetic and engine heat are dependent on time for theeffects they can induce in adjacent structures and components. In a conventional transonic aircraft the effects of kinetic heat areretarded by time, so the structure can act as a large-capacity heat absorber. Of course, a problem does exist in that there is atemperature gradient from the external surfaces to the interior bulk of the airframe which could overstress the primary structural AFTERBURNER NOZZLEACTUATOR MAINWHEEU WELLS'* AND DOORS ACCESSORY BAY This diagram illustrates the probable disposition of some of the heat- insulating blankets in a hypothetical supersonic fighter. Components are not necessarily precisely to scale. members, so some form of skin insulation is also needed. Thetype of insulation discussed here is, however, primarily intended for component insulation, and therefore it is assumed that theairframe under consideration has unlimited heat capacity and resistance to kinetic heat. The heat effects of a jet engine are moresevere because the temperatures are higher and are maintained throughout the flight, so the structure adjacent to an engine willbe subject to both engine and kinetic heating at the same time. With a "hot" surface temperature of 600 deg C to 1,000 deg Cthe heat effects can only be delayed. Even although at time zero the "hot" temperature is say 600 deg C and the temperature ofthe structure one foot away is 60 deg C a time will eventually come, dependent upon the heat resistance and capacity of the ':•structure, when the structural temperature will approach 600 deg C. The time to reach equilibrium may vary from a few minutes to a 'few hours; it is dependent on the material of the structure and the rate at which the heat is being carried away by a heat "sink"—a medium which has virtually unlimited capacity, such as air. In practice the aircraft manufacturer is not always able to employmaterials which combine reasonable cost, workability, acceptable weight and resistance to heat. Here we are primarily concernedwith resistance to heat. The conventional materials of construc- tion, the aluminium alloys, lose much of their strength whensubjected to temperatures in excess of 150 deg C. As an example, after one hour at 200 deg C Duralumin might show a reduc-tion in strength of 28 per cent. After 1,000 hours the strength might be down to only 30 per cent of the "cold" value. Though1,000 hours is possibly an unrepresentative time, as flights will rarely exceed 12 hours, 1,000 hours might represent 200 changesin temperature—so it is reasonable to suppose that the strength would be even less than 30 per cent after 1,000 hours of operation. In an endeavour to reduce the adverse effects on adjacentstructure the designer can, as has been shown, provide a "boundary layer" of cooling air around the engine. Unfortunatelythere are limits to the amount of air that can be circulated in this way. The "tube" of air represents not only wasted space but 'also unwanted frontal area and drag. In addition, air taken on board for cooling is subject to kinetic heating; at present-dayspeeds the air temperature within the intake duct could be 100 deg C. Therefore, if alloys with a temperature limit of only150 deg C are still to be used for structures then large quantities of air must be passed to give adequate protection. A small amountof air cooling is acceptable, and usually takes the form of an annular space between |in and lin wide. It should be noted thatthe protection offered by a moving airstream extends only to reducing the temperature of the "hot" surface of the engine. Witha temperature of 200 deg C—the air is progressively heated as it passes downstream alongside the engine—there is not much reduc-tion on the 600 deg C; and in any case the radiant heat, which is a function of the fourth power of the absolute temperature, islittle affected by the air stream when the distance traversed is less than an inch. In the combustion chamber of a gas turbineair is used in a similar manner, and with success, to protect the chamber walls; but the axial distance traversed by die air, and thequantities involved do not compare with the conditions surround- ing the engine itself. The slight protection offered by an air shroud has to be sup-plemented by using an additional form of insulatfon. As in an electrical circuit, the more discontinuities and resistances intro-duced in series the lower the conductance. By balancing the drag-cost of an air shroud with the weight and cost of a materialinsulation, the designer endeavours to provide both a compromise and composite of protection for the structure and the componentsadjacent to a hot engine. Having determined the amount of air that can be afforded forcooling, the designer has to adopt some form of material insulation
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