ENHANCED VAPORIZATION/CONDENSATION HEAT PIPE

Technical Field

The present invention relates to the method of and apparatus for transporting thermal energy and more particularly to the utilization of a heat pipe for* the transport of heat. More specifically, the present invention relates to the transport of heat utilizing a vaporization/condensation cycle enhanced by a reversi- hie chemical reaction.

Background of the Invention

Various techniques have been employed for trans¬ ferring or transporting thermal energy between a thermal source and a thermal sink- or load. One, tech- nique or means often used employs a heat pipe. The heat pipe is connected between the heat source and the heat sink and a transport medium therewithin is caused to flow between the two positions to transfer thermal energy from the source to the sink. A basic form of heat pipe employs a vaporization/condensation cycle or mode of operation for effecting the requisite thermal energy transfer. In that type of heat pipe, there is rapid heat transfer into the.-pipe resulting in vapor¬ ization of a liquid transport medium therein. The evaporated transport medium builds up sufficient pres¬ sure to be transported along the pipe and is then condensed at the heat sink position. The cycle is completed by returning the condensate to the evaporat¬ ing end by means of capillary or other action through a wick or other suitable means within the pipe.

Typically, the working fluid may be water, freon, methyl alcohol, acetone or the like. The heat of vaporization

is such that significant quantities of heat may be absorbed during the vaporization of the transport liquid and subsequently released at the heat sink dur¬ ing its condensation. Because the thermal energy transported in a vaporization/condensation type of heat pipe is trans¬ ported at the elevated temperatures of the vaporized transport medium, the opportunity for heat loss during transport by radiation, convection and/or conduction may be significant, particularly if the transport dis¬ tance is greater than tens of feet. In instances in which thermal energy is to be transported relatively long or significant distances, for instance from tens or hundreds of feet to as much as tens or hundreds of miles, and it is desired to minimize thermal losses during transport, chemical heat pipes can ^' be employed. In such, heat pipes, a reactant or reactants undergo a first chemical reaction at the heat source and a second chemical reaction at the heat sink. The re- actions are generally reversible, with the first being of an endothermic nature in which heat is chemically absorbed by the reaction process and with the second being exothermic in which, heat is chemically liberated during the reaction process. The reactant and/or reaction products may exist and be transported at tem¬ peratures which do not differ substantially from that of the environment, thereby greatly reducing the poten¬ tial for thermal loss from the system. In such chemical heat pipes, most of the thermal energy absorbed from the source occurs by virtue of the endothermic reaction, with relatively little heat being absorbed by evapora¬ tion. An example of one such chemical heat pipe is disclosed in copending U.S. application Serial No. entitled Self-driven Chemical Heat Pine by

A. S. Kesten and A. F. Haught, filed on even date here¬ with and assigned to the assignee of the present appli¬ cation.

Although heat pipes of the chemical reaction type may be particularly suited for t e long-distance trans¬ port of thermal energy, the sometimes simpler and less expensive vaporization/condensation type of heat pipe is used almost exclusively for situation in which the distance over which the thermal energy to be transported is relatively short, for instance less than tens of feet and for those situations in which the source temperature and/or the source-sink temperature differ¬ ence is insufficient for a suitable chemical heat pipe reaction. The vaporization/condensation type heat pipe is generally self-driven -and the rate of thermal energy transport is determined by the transport medium, by the relevant operating temperatures and by the geometry of the system. Generally, the rate of heat transport in a system in which the evaporation and condensation surface areas are relatively small will be less than that for which those surfaces are relatively large, other factors being equal. Various physical constraints and/or cost considerations may however, interfere with or prevent the provision of a vaporization/condensation heat pipe of sufficient physical capacity for the task intended.

Accordingly, it is a principal object of the pre¬ sent invention to provide a heat pipe of the vaporiza¬ tion/condensation type with enhanced operating capabilities. Included in this object is the provision of a method for enhancing the rate of thermal transport in vaporization/condensation heat pipes of particular and limited geometries.

Disclosure ^' of Invention

In accordance with the present invention, there is provided a method and apparatus for the enhanced transport of thermal energy utilizing a heat pipe operated principally in the vaporization/condensation mode. The heat pipe comprises a closed-circuit fluid conduit having a heat source position in heat exchange relation with the heat source and a heat sink position in heat exchange relation with the heat sink. Appro- priate catalysts are situated in or near the heat source and heat sink positions of the -conduit respec¬ tively. A transport fluid within the conduit is selected to enter the heat source position as a liquid, to be vaporized thereat and to be caused to chemically react at least partially, promoted by a catalyst, to thereby provide at least some reaction product. The reaction product and any unreacted vaporized transport fluid are transported to the heat sink position where the reverse chemical reaction is induced by s, change of conditions (e.g., lower temperature) and promoted by a suitable catalyst to reform transport fluid, accompanied by the generation of some thermal energy for release to the sink. The transport fluid vapor is condensed at the heat sink position thereby to release thermal energy to the sink and return the transport fluid to the liquid form. The liquid transport fluid is then finally returned to the heat source position for completing and repeating the cycle.

The transport fluid is selected such that for the temperature of operation at the heat source position and for the temperature drop between the source and sink, a significant portion of the total thermal energy removed fro the heat source occurs by vaporization of

the transport fluid. For example, the vaporization of the transport fluid prior to its endothermic reaction accounts for the absorption of at least 507 _β of the total thermal energy removed by that fluid and may range upwardly to 80% or more. By reacting the vapor¬ ized transport fluid to convert it to a reaction pro¬ duct, the transport fluid vapor is removed from the vicinity of the liquid surface from which it evaporates, thereby tending to enhance the net evaporation rate and thus the rate of heat absorption or transfer from the source.

In one embodiment, the temperature differential ^• between the heat source and heat sink positions is in the range of about 20-80 K. Isobutane and its reaction product n-Butane are examples of a transport fluid and reaction product respectively which provide the advantages of the invention, especially in heat pipes having size constraints and/or where the distance between the heat source and heat sink is relatively short.

Brief Description of Drawings

Fig. 1 is a diagrammatic representation of a .prior art vaporization/condensation heat pipe system;

Fig. 2a is an illustration of certain segments of the heat pipe of Fig. 1, shown aligned for graphical purposes; ^'

Fig. 2b is a graphical plot of the C—) pressure of the transport fluid in the heat pipe segments of Fig. 2a; Fig. 3 is a diagrammatic representation of the reaction-enhanced vaporization/condensation heat pipe of the invention;

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Fig. 4a is an illustration of .certain segments of the heat pipe of Fig. 3, shown aligned for graphical purposes ^" ; and

Fig. 4b -is a graphical plot of transport fluid and reaction product pressures in the heat pipe segments- of Fig. 4a.

Best Mode for Carrying Out the Invention

Referring to Fig. 1, there is diagra ntatically illustrated a conventional vaporization/condensation driven heat pipe* 8 of the prior art. A volatile liquid transport fluid A is vaporized at a heat source position comprised of an evaporator 12 in heat exchange relation with a heat source 14. The vapor A vap flows through conduit 16 to a heat sink position comprised of con- denser 18 in heat exchange relation with a heat sink 20. The vaporized liquid is condensed at condenser 18, thereby releasing to the sink 20 most of the heat acquired from the source. Some radiation, conduction and/or convection losses may be experienced, depending in part on the transport distance which is usually restricted to distances ^" of less than tens of feet. The condensate, A liq. , travels back to the evaporator 12 through a return conduit 22. The return conduit 22 may include a wick or the like for returning liquid 10 by means of capillary action or the like, as determined by the geometry and orientation of the heat pipe. Vapor movement is sustained by a negative pressure gradient between the source position and the sink position, established by having the condenser 18 at a lower temperature (and hence lower equilibrium pressure) than the evaporator 12.

A limitation on the performance of a heat pipe 8 results from the necessary trade-off among ^* - the require¬ ments for efficient vaporization at the source position,

condensation at the sink position, and mass transport between them. The vaporization rate in the evaporator 12 is proportional to the difference between the equi¬ librium pressure of the transport fluid A and its actual vapor pressure (i.e., (? ^e< ^ - P_)). Similarly, the condensation rate is proportional to the difference between the actual condenser pressure and its equilib¬ rium pressure (i.e., (P - P_ ^e< ))- In o er to maintain flow from source 14 to sink 20, the evaporator pressure must be greater than the condenser pressure. This leads to the inequality

It will be seen that the driving force for either evap¬ oration or condensation is limited to values less than (P _β ^δ< - P„ ^ec ) . Furthermore, an increase in one of them occurs at the expense of the other. For instance, lowering P _Λ to increase (P„ ^ec ** - P„) will lead to a smaller (Pc„ - Pc _Λ ") since Pc will be lowered. Hence, if there are constraints on the physical size of heat pipe components, these inequalities among the various pressures limit the possible vaporization/condensation rate and the accompanying heat transport.

Fig. 2a is an illustration of a portion of the prior art heat pipe 8 of Fig. 1, opened at the liquid return conduit 22 and arranged in straightline fashion for the purposes of the graphical presentation of the transport fluid pressure in Fig. 2b. The graph of Fig. 2b depicts the pressure of the transport fluid A of the heat pipe 8 expressed in arbitrary units of pressure and varying as a function of the location of the trans¬ port fluid within the heat pipe. Thus, in Fig. 2b, it will be observed that the maximum pressure of the transport fluid occurs in the evaporator 12 just down¬ stream of the liquid-vapor interface therein. The

pressure of the transport fluid is at minimum in the condenser 18 substantially at the interface at which it makes the transition from the vapor phase to the liquid phase. The drop in pressure across vapor-phase conduit 16 provides the driving force for the system a d a wick may return the liquid-phase transport fluid, A liq, from the condenser 18 to the evaporator 12.

The aforementioned limits on the performance of a conventional vaporization/condensation type of heat pipe can, however, be relaxed and its operation enhanced by the incorporation of a chemical reaction in the cycle. The heat pipe 38 of Fig. 3 represents such a reaction augmented vaporization/condensation type heat pipe. A volatile transport fluid, entering in the liquid phase, is vaporized and reacted at a heat source position comprised of the evaporative reactor 42 in heat exchange relation with heat source 14. The trans¬ port fluid is again designated A inasmuch as it may be the same as or may di fer from the transport fluid in the aforedescribed vaporization/condensation heat pipe of Fig. 1. The transport fluid A is first vaporized and then immediately endothermically reacted by the catalyst 54 in intimate contact therewith. The product or products, hereinafter referred to as reaction pro- duct B, is- transported in the gaseous or vapor phase through conduit 46 to the reactor condenser 48. To the extent the reaction in reactor 42 is not complete, some vaporized transport fluid A may also be transported to reactor condenser 48 via conduit 46. The reaction product B is exothermically catalytically reacted by a suitable catalyst 56 in reactor 48 to reform transport fluid A, which transport fluid is then condensed to the liquid phase and returned by a wick or the like through conduit 52 to the evaporator reactor 42 to complete and repeat the cycle.

An import.ant aspect of the above described cycle resides in the removal of the vaporized transport fluid A from the vicinity of the liquid surface of transport fluid A by means of the endothermic reaction in reactor 42. That removal of the vaporized transport fluid A serves to enhance the net evaporation rate of transport fluid A from the liquid supply thereof. In this parti¬ cular cycle of operation, and for the transport fluids selected, the heat of vaporization and thus the heat absorbed from the heat source by vaporization is greater and generally substantially greater than the heat of reaction. Therefore, this type of enhancement of the ■ net evaporation rate serves to enhance the operation of the heat pipe as a whole. The essential differences between the reaction augmented vaporization/condensation heat pipe 38 and the conventional heat pipe 8 arises from the fact that the rate ^; of evaporation and condensation depend only on the difference between the equilibrium vapor pres- sure, F _A ^e ^" -and the partial pressure of A, P» . Reduc¬ tion of the partial pressure P. at the evaporator interface leads to enhanced vaporization. The transport of gas or vapor from the heat source to the heat sink, on the other hand, depends on the difference between the total pressures at each location (i.e. P _tot -^tot )•

. e c It is no longer necessary, however, that P _Δ >P. .

^A e ^A c

In principle, it is possible for the partial pressures to be such that P. <P. ^ec and P _A >? _A ^eα -. Thus, the e c c e magnitudes of the driving force factors, (P. ^e - ? _Λ ) e e and (P^ - P _A ^q , can be much larger than would be c c possible in a conventional case and heat transport can be enhanced.

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Reference is made to the following simplified example for an illustration of a basic form of the invention. Assuming that the reaction near the evapo¬ rator 42 is instantaneous and goes to completion, and 5 that the reaction near the condenser 48 regenerates a finite amount of transport fluid A, then the partial pressure, P _Δ . , of A at the liquid-gas interface of the

^A e evaporator 42 will be zero. Moreover, the evaporiza- tion rate will attain it maximum possible value, pro-

10 portional to P _Δ ^ec . The rate of mass addition due to

^A e vaporization must balance the mass flow from the source position to the sink position, and the rate of conden¬ sation must equal the rate of vaporization. The total pressures at evaporator 42 and condenser 48 will adjust

15 to satisfy these constraints in accord with the equili¬ brium composition within the condenser reactor 48. An equivalent conventional vaporization/condensation heat pipe (i.e. equivalent temperatures and surface areas for evaporation and condensation, ^' and equivalent flow

20 passages between source and sink) will have a finite pressure of transport fluid A at the evaporator 18 interface, rather than zero. Hence, the evaporization rate (and heat transport) will be higher for the reac¬ tive case.. The correspondingly higher condensation

25 rate means that the pressure, P. , of transport medium

A at the condenser 48 interface, ^c also will be higher for the reactive case. Since P.t.ot.. > PA. and P.t.o_t > e

£.t__.o_t_. , the total pressure in condenser 48 will be c higher for the reactive case, and generally so will 30. the total pressure in the evaporator 42.

For intermediate cases in which the.endothermic reaction is incomplete, the pressures and heat trans¬ port values will be intermediate those at the extremes

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of no reaction (conventional) and complete reaction. The same general relationships will exist for compari¬ sons with the conventional case: (1) heat transport - greater, (2) evaporator partial pressure, P _Δ - lower,

^A e (3) evaporator total pressure, P _tDt - generally e higher, (4) condenser total pressure, P - higher, c and (5) condenser partial pressure, P. - higher.

^A c Fig. 4a illustrates the reaction-enhanced heat pipe 38 of Fig. 3, opened at the liquid return conduit 52 and arranged in straight line fashion for the graphical presentation of the transport fluid pressure and partial pressures depicted in Fig. 4b. The inter¬ mediate case described in the immediately foregoing paragraph is portrayed in Fig. 4b in which the total fluid pressure, ■P _tota *-_• in heat pipe 38 is comprised of a transport fluid A component, P. , (shown in Dashed line) and a reaction product component, P _g , (shown in dotted line) . It will be observed that while the general profile of total pressure, ^ -, , is similar, but somewhat higher, than that of the conventional heat pipe 8 depicted in Fig. 2b, the pressure of transport fluid A is rapidly decreased by reaction at catalyst 54. Had an even more complete reaction of transport fluid A occurred, as is possible, the pressure P _Δ of that fluid could have dropped to near zero and the pressure P-g of reaction product would have similarly increased.

In 3x1 ideal case, the reaction augmented heat pipe would involve chemistry in which both, the tr-ans- port medium A and the reaction product B are stable, but are easily reversibly reacted by catalytic reaction with only a small temperature change between the source and sink positions. Whereas conventional heat pipes

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can operate over temperature drops of a few degrees, somewhat higher temperature drops are necessary to realize the significant enhancement afforded by the invention. However, the temperature drops required by the invention, generally in the range of 20-80°K, are normally significantly below the temperature drops of 100-200°K and greater required by classical chemical heat pipes. Heat source position temperatures of 280- 380 are representative, but they are by no means limiting and may be. lower or substantially higher. For applications in which efficient cooling is provided to a high temperature device and there is relatively little concern for heat loss during trans¬ port, a relatively large temperature drop may be accom- odated by the heat pipe of the invention. If, however, the application is the transport of heat while τπimmi.z- ing degradation in its quality, it may be necessary to raise the temperature of the catalyst surfaces in the evaporator reactor 42 (or shift the equilibrium by other means) to promote reaction in the proper direc¬ tion. If the reaction is not highly energetic only a small penalty on efficiency occurs. Further, note that the reaction augmentation depends more strongly on complete dissociation at the source than on complete recombination at the sink. Thus, smaller temperature differences can be achieved by accepting incomplete recombination in the condenser,reactor 48, which will manifest itself primarily as higher total pressure in the system. The enhanced vaporization/condensation heat pipe 38 of the invention will typically transport most of the heat, i.e. 50-95% or more, as the heat of vaporiza¬ tion of the transport medium, whereas the classical chemical heat pipe typically transports most of the

heat, i.e. 50-95% or more, in chemical form via the heat of reaction, and the conventional vaporization/ condensation heat pipe transports all of the heat as the heat of vaporization. In accordance with a preferred embodiment of the invention, an exemplary and particularly suitable reaction comprises the iso erization reaction of isobu¬ tane. Isobutane is transported as a liquid to evapo¬ rator 42 where it is first vaporized and then catalyti- cally endothermically reacted to form reaction product B, in this instance n-butane. Depending on the operat¬ ing temperatures, the isobutane may be completely reacted or it may be only partially reacted such that some vaporized isobutane is also transported through heat pipe conduit 46 to condenser 48. At condenser 48 the n-butane is catalytically reacted exothermically, liberating some heat to sink 20 and reforming isobutane. The vaporized isobutane is then condensed, releasing an even greater quantity of heat to sink 20, and the resulting liquid isobutane is conducted back to evapo¬ rator 42 through conduit 52. A suitable material for catalysts 54 and 56 is aluminum chloride on alumina. The reaction may be expressed as :

(CH ₃ ) ₂ CH CH ₃ > G ₄ H _1Q The heat of vaporization of isobutane is about 5 Kcal/ mole and the heat of reaction is about 1.6 Kcal/mole, such that more than 70% of the thermal energy absorbed from the heat source is by vaporization of the isobu¬ tane even for complete reaction. Consideration is now given to a series of compari¬ sons between a conventional vaporization/condensation heat pipe (as in Fig. 1) employing isobutane as trans¬ port fluid A and a reaction-enhanced vaporization/ condensation heat pipe (as in Fig. 3) also -employing

isobutane as the transport fluid A. These comparisons are expressed in the following table in which a 300°K sink temperature was assumed for all cases and a total pressure of 8.5 atmospheres (maximum without condensing n-butane) was provided. It was necessary to vary the capacity of the system somewhat to obtain the same pressure in each instance, resulting in the small apparent inconsistencies in the pressures for the con¬ ventional case.

S i _π sSs ^* Jτrrυτε SHEET

From the table above, it is seen that for rela¬ tively small differences between source and sink tem¬ peratures, i.e. -_Υ = 20 - 50 K, the reaction-enhanced case is capable of transporting heat at a 207o faster rate than the conventional case at the same temperatures. The increase in the heat transport rate of reaction- enhanced case over the conventional case is provided both by an increased vaporization rate and by the heat of reaction. It will be appreciated, however, that the contribution by the heat of reaction to the total heat transported is relatively small, such that the heat transported by the heat of vaporization of the isobutane comprises more than 80% of the total heat transported. The third, fourth and fifth examples in the fore¬ going Table represent additional heating of the source catalyst 54 beyond the source temperature to shift the equilibrium point of the reaction and thereby promote a more complete reaction while minimizing the degrada- tion in quality of the heat transported. The secondary values in the first colum of the Table are catalyst temperatures. The secondary values in the last two columns of the Table represent heat transport rates and enhancements respectively resulting from heat supplied at both temperatures . An additional source of heat

(not shown) would be required to heat the catalyst to the desired temperature. Such additional heat might represent an additional energy cost, but the quantity of that additional heat would normally be small rela- tive to the heat received from the waste heat source and its cost may be minimized if it were usefully recovered at the heat sink. The heat of vaporization remains the dominant mechanism for transporting heat.

Although this invention has been shown and described with respect to detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the claimed invention.