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Aviation History
1957
1957 - 1766.PDF
856 FLIGHT, 29 November 1957 AMPLIFIER COMPONENTS FILLER HOLE RADIATION SHIELD PASSIVE REFRIGERATION A New Heat-Insulating Technique A CONTEMPORARY problem of rising speeds in thesupersonic regime is how to get rid of the internallygenerated heat of electronic equipment in the high ambient temperatures associated with aerodynamic heating. AtMach 1 at 40,000ft, for example, aircraft skin temperature will stabilize at about 40 deg F, but at Mach 2 at the same heightwill rise to 250 deg F. Temperature, in fact, increases exponenti- ally with speed, and at Mach 3 an aircraft's skin temperaturewill reach 630 deg F—and the air surrounding the electronic components will be at well above their maximum workingtemperature. The trend towards ever-smaller electronic com- ponents has not resulted in a corresponding reduction in thequantity of heat to be dissipated, and more watts have now to be dispersed in smaller spaces than ever before. There are various ways of insulating equipment to provide thenecessary temperature protection, all of which provide an arti- ficial atmosphere in the electronic compartment. At speedswhere the ambient temperature is low compared with that of the component, ram air can be used for blast cooling, or thetemperature range can be extended by tapping bleed air from the engine compressor and cooling it before it is introduced intothe equipment bay. Where ambient temperatures are high, it is sometimes possible to dissipate the generated heat by means ofliquid cooling, and it is intended to make widespread use of the thermal inertia of the fuel in the tanks of aircraft or missiles forthis purpose. But particularly in missile application, where components haveto be protected for a limited time, dynamic cooling may be difficult to apply and uneconomical from a weight and installa-tion point of view. Another method, which has so far received only limited attention, is to make use of the latent heat of fusionof various compounds to absorb heat from the ambient atmos- phere and from the components themselves. Some details ofthis system of "passive refrigeration" were recently given in a paper entitled Protection of Electronic Components during High-temperature Transients using Heat-storage Materials and read to the 1957 Electronic Components Symposium in Chicago byMr. Duane M. Trones, who is a development engineer of the Minneapolis-Honeywell Regulator Company. Some use of thistechnique is now being made by the company. The properties necessary for a storage material of this type(said Mr. Trones) are a high latent heat of fusion, good stability in temperature cycling, non-corrosiveness and low electrical con-ductivity; and it has been found from investigation that the organic compounds—principally the fatty acids, higher alcoholsand fatty esters—are likely to be the best materials. An applica- tion was described in Mr. Trones' paper where a unit had beenprotected for 19 min at 350 deg F ambient, using only two ounces of storage material. The principle of passive refrigeration is quite simple. Thediagram on the left of Fig. 1 shows the condition where a pro- tected unit with some internal heat dissipation is at its maximumsteady-state operating temperature; the temperature profile would be as indicated. The internally-generated heat flows through thestorage material, through the air film and out to the ambient air. GLASS BALLS STORAGE MATERIAL AIR GAP Fig. / (left). Temperature profiles at low and high ambient temperatures. Fig. 2 (right). Time-temperature relationship of heat-storage material. ujT^ COMPONENT STORAGE MATERIAL SOLID •\/\/\ri/ THERMAL INSULATING BUSHING :. A miniaturized amplifier surrounded by heat-storage material. The can and outer shield are polished to reduce radiation, and separated by glass balls to reduce heat flow from conduction. i i-r The difference between the ambient temperature Ta and thecomponent temperature T c is the gradient that will be experiencedwith the internally-generated heat flowing through the thermal resistance of the storage material and the air film. The particu-lar insulating material used is chosen to have a melting tempera- ture (Tf) just above the maximum operating temperature of thecomponent. It will thus be solid as long as T c is not exceeded.If the ambient temperature is then suddenly raised to a new temperature Ta'—which exceeds the maximum operating tem-perature of the component—the profile will be as shown on the right of Fig. 1. Under these conditions the heat-storage materialacts as a thermal reservoir accepting heat from the ambient air and also from the unit. The temperature of this material remainsconstant at the fusion point until it is completely melted, when it will have absorbed the total latent heat required to change itsstate. The component temperature will remain substantially at Tc while the storage material is being melted and this time repre-sents the period of protection that can be provided. It can readily be calculated. Fig. 2 shows the time-temperaturerelationship during this period with step increases and decreases of temperature such as might be encountered by an aircraft whichperformed a high speed dash. The heat pick-up from the ambient, Mr. Trones went on tosay, is a function of conduction, convection and radiation. Taking as an example the protected unit shown in the photograph, heamplified each of these features. The conductive heat-flow is along the thermal bushings, the spacers between the shield andthe can, and the air jacket, where gaseous conduction takes place. In the latter case, a layer of still air—one of the best insulatorsprovided by nature—blankets the major part of the can. Heat is transferred from the ambient to the shield and also to the bottomof the can by convection, but there is no convection path from the shield to the can. The air jacket set-up by the radiationshield is too thin to allow convective currents and attenuates both conductive and convective heat flow. To reduce the radiationemissivity factor the shield and the can of the unit are chrome- plated and polished. The maximum temperature that the unit can stand indefinitelywithout any melting of the storage material is 200 deg F, but it was found that the component temperature could be heldpractically constant at 350 deg F for 19 min, and at 500 deg F for nine min. Greater protection timesmay be expected at higher altitudes, because the heat-pick-up due to con-vection would be reduced. A minor snag is that the radiation shield de-presses the maximum allowable am- bient in the normal operating range.The advantages of heat-storage high-temperature protection, Mr-Trones concluded, were that it was inexpensive, since common chemicalscosting an insignificant amount o-~ tne cost of the protected equipment couldbe used; that it was non-expenaabie and could be used repeatedly witaowrefilling; and that it was self-contained and did not require auxiliary e!5uJP"ment that would take up valuable TIME " space and add to the weight. STORAGE MATERIAL HELTINC
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