Abstract:
A unidirectional heat pipe (22) acting as a thermal diode or a heat switch, enables heat to be removed from electronic equipment (14) when temperatures outside of a missile envelope (12) are lower than those of the electronic equipment. When the temperature exterior to the missile envelope is greater than that of the electronic equipment, the heat therefrom is conducted to a package (16) containing phase change material which absorbs the heat from the electronic equipment, at which time the heat pipe acts as a thermal insulator. Heat is also removed from the PCM package by the heat pipe when exterior temperatures are lower than interior temperatures.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to continuous removal of heat from apparatus and transfer of the heat to a heat sink which is periodically subjected to being otherwise heated to temperatures exceeding that of the apparatus. 
     2. Description of the Prior Art 
     Many apparati using electronic equipment present problems for effective dissipation of its self-generated heat. Examples of such apparati include space and airborne vehicles, such as air-to-air missiles and supersonic air-to-ground missiles. Conventional systems are deficient in one or more particulars, and all suffer from the disadvantage that, once they are activated, their cooling capacity becomes depleted and used up, and cannot be reactivated or regenerated without major overhaul of the apparatus. Such systems and their limitations include expendable coolant bottles which are non-rechargable in flight, liquid cooling which requires connection to an aircraft, sensible heat sinks which are limited in their capacity and are non-rechargeable, and phase change material which has limited capacity and is of the type that is not capable of being recharged during operation. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes or avoids the above problems by providing a system by which a heat sink is passively coold by means of a unidirectional heat transfer mechanism, e.g., utilizing diurnal cycles or air during free flight. Specifically, heat is continuously removed from an apparatus and transferred by the heat transfer mechanism to the heat sink which, as indicated above, may be periodically subjected to heat from external sources and raised to temperatures exceeding that of the apparatus. Under such heating conditions, the heat from the apparatus is directed to a heat storage unit which stores the heat and which, when the heat sink temperature no longer exceeds that of the apparatus, is cooled and recharged through its coupling by the unidirectional heat transfer mechanism to the heat sink. 
     It is, therefore, a primary advantage of the present invention that it enables continuous cooling of electronic equipment. Other advantages include the capability of recharging the heat storage unit during operation of the apparatus in which the electronic equipment is used. 
     Other aims and advantages as well as a more complete understanding of the present invention will appear from the following explanation of exemplary embodiments and the accompanying drawings thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts a portion of a missile in which electronic equipment is thermally coupled to the thermal control system of the present invention; 
     FIG. 2 illustrates a more detailed view of the cooling system of the present invention; 
     FIG. 3 is a view taken along lines 3--3 of FIG. 2; and 
     FIG. 4 shows a modification of the cooling system of FIGS. 1 and 2. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, apparatus 10, which may be embodied as a front portion of a missile, includes electronic equipment 14 within its missile envelope 12. The electronic equipment is thermally coupled to a phase change material (PCM) package 16 by a unidirectional heat pipe assembly 18. While shown as being supported solely by the heat pipe assembly, both the electronic equipment and the PCM package are otherwise supported within the missile envelope. As illustrated, heat pipe assembly 18 includes a reservoir 20 and a pair of heat pipes 22, although it is to be understood that one, or more than two, heat pipes may be utilized. 
     As better shown in detail in FIGS. 2 and 3, reservoir 20 comprises a closed compartment having wick material 24 placed substantially on all of its surfaces and, in particular, on those surfaces 20a and 20b, respectively, adjacent electronic equipment 14 and PCM package 16 and on bottom surface 20c. As shown, wick material 24 has a continuous contact among all surfaces 20a, 20b and 20c. A working fluid 26 is contained within reservoir 20. 
     Heat pipe 22 comprises a tube 28 also having a double wick 30 (see also FIG. 3) therein along one of its interior surfaces from a first heat pipe end 22a to a second heat pipe end 22b, to act as a conduit for the working fluid in its liquid state. A space 32 within the heat pipe envelope and adjacent wick 30 acts as a path for the working fluid in its vapor state. In the practice of the invention, it is important that wick 30 at first end 22a does not physically contact wick 24 or other interior portions of reservoir 20. 
     In order to insure physical contact between heat pipe envelope 28 and missile envelope 12, a conforming heat transfer plate 34 is secured by welding or other acceptable means to the heat pipe envelope at heat pipe second end 22b and is secured by rivets 36 or other means of affixation to missile envelope 12. 
     Phase change materials placable within package 16 are selected to meet the cooling requirements of specific electronic compenents. For example, there are waxes with melting temperatures in the range 20° C. to 120° C. Examples of such waxes, organic materials and other inorganic materials, whose melting points are placed in parentheses, include n-heptadecane C 17  H 36  (21.7° C.), n-heneicosane C 21  H 44  (40° C.), paraffin wax (54.4° C.), methyl fumarate (CHCO 2  CH 3 ) 2  (10° C.), gallium Ga (30° C.), bismuth-lead-tin, 52.2 Bi+32 Pb+15.5 Sn (96° C.). 
     Cooling media external to missile envelope 12 may include forced convection, cold plate, ram air, and diurnal cycles. 
     In operation, during the cooling cycle, heat is removed from both electronic equipment 14 and PCM package 16, thus causing the phase change material therein to undergo a phase change, for example, from a liquid state to a solid state, i.e., the phase change material is solidified. Under these conditions, since electronic equipment 14 and PCM package 16 are hotter than missile envelope 12, working fluid 26 is evaporated and moves as a vapor from reservoir 20 to heat pipe 22 for condensation therein adjacent heat transfer plate 34. The working fluid vapor thus condenses and moves back into reservoir 20 through wick 30. Because of the lack of physical contact between wick 30 and wick 24 or other interior portions of reservoir 20, the condensed fluid will collect and drip from wick 30 into the reservoir. Such dripping occurs in any orientation of heat pipe assembly 18 except when the open end 22a of heat pipe tube 18 in the reservoir is gravitationally higher than its closed end at plate 34. However, because there can be at least two heat pipes 18, such as illustrated in FIG. 1, there will be at least one heat pipe whose open end is gravitationally lower than its closed end. Thus, heat transfer is always assured. 
     In addition, the open end of tube 28 is shown to extend within reservoir 20 beyond the wall thereof shown without wick 24. Such an arrangement has been found to have the distinctively ameliorative advantage of preventing liquid working fluid from being forced into tube 28 during the exertion of high gravitational loads directed towards the open tube end. Such liquid would accumulate on that reservoir wall 20d having no wick 24 and about the exterior portion of tube 28 extending within the reservoir. 
     When missile envelope 12 is hotter than equipment 114, heat pipe 22 cannot act as a heat transfer device. For example, during supersonic flight, or during the heat of day, the heat pipe, which acts as a thermal diode, switches off, inasmuch as its end 22b at plate 34 is hotter than its end 22a in reservoir 20 and is at a temperature at least equal to the vaporization temperature of the working fluid. In particular, working fluid at its end 22b at plate 34 is vaporized and the vapor condenses in the reservoir. Because of the absence of physical contact between wick 30 at end 22a and the reservoir, the working fluid as a liquid is trapped in the reservoir. Thus, heat pipe 22 becomes a thermal insulator. Under such conditions and during operation of the electronic equipment, its generated heat is absorbed by the phase change material in package 16, with reservoir 20 and its wick 24 acting as a heat pipe. During this portion of the operating cycle, some or all of the phase change material is liquified. 
     During subsonic flight when the temperature outside the missile envelope is lower than that of equipment 14 and the vaporization point of the selected working fluid, heat pipe 22 becomes operable to transfer heat in parallel from electronic equipment 14 and PCM package 16 to missile envelope 12 through plate 34, thus cooling the electronic equipment and recharging the PCM package by solidifying its phase change material. 
     While FIG. 2 depicts reservoir 20 to be sandwiched between electronic equipment 14 and PCM package 16, FIG. 14 illustrates modifications in the arrangement by placing PCM package 16 intermediate or in series with electronic equipment 14 and reservoir 20. 
     It is to be understood that, while package 16 contains a phase change material, other materials may be used, such as a metal block, so that package 16 would operate as a sensible heat sink. 
     Although the invention has been described with reference to particular embodiments thereof, it should be realized that various changes and modifications may be made therein without departing from the spirit and scope of the invention.