UNILATERAL HEAT TRANSFER APPARATUS
United States Patent 3754594
Unilateral heat transfer apparatus comprises an evacuated heat transfer chamber having a variable pore size capillary structure therein with the pore size of the capillary structure in the evaporator portion of the heat transfer device being smaller than the pore size of the capillary structure in the condenser portion thereof. Further, the working fluid within the heat transfer device is limited to that amount which will saturate the capillary structure in the evaporator section only.
US Patent References:
HEAT PIPE FOR LOW THERMAL CONDUCTIVITY WORKING FLUIDS
Levedahl - September 1970 - 3528494
Capillary insert for heat tubes and process for manufacturing such inserts
Grover et al. - February 1967 - 3305005
/3686040.html
Grover et al. - August 1972 - 3686040
/3587725.html
Basiulis - June 1971 - 3587725
HEAT PIPES
Noren - October 1972 - 3700028
HEAT PIPE FOR LOW THERMAL CONDUCTIVITY WORKING FLUIDS
Levedahl - September 1970 - 3528494
Capillary insert for heat tubes and process for manufacturing such inserts
Grover et al. - February 1967 - 3305005
/3686040.html
Grover et al. - August 1972 - 3686040
/3587725.html
Basiulis - June 1971 - 3587725
HEAT PIPES
Noren - October 1972 - 3700028
Application Number:
05/220216
Publication Date:
08/28/1973
Filing Date:
01/24/1972
View Patent Images:
Export Citation:
Assignee:
Sanders Associates, Inc. (South Nashua, NH)
Primary Class:
Other Classes:
165/104.260
International Classes:
F28D15/04; F28D15/00
Field of Search:
165/32,105
US Patent References:
| 3666005 | SEGMENTED HEAT PIPE | May 1972 | Moore, Jr. |
Primary Examiner:
Davis Jr., Albert W.
Claims:
I claim
1. Unilateral hea4 transfer apparatus, comprising:
2. Unilateral heat transfer apparatus as recited in claim 1 wherein said capillary comprises first and second wicks coupled to one another to provide a continuous condensate flow path.
3. Unilateral heat transfer apparatus as recited in claim 2, said first and second wicks disposed in the evaporator and condenser portions of said chamber, respectively, with said first wick being thicker than said second wick in order to provide a working fluid storage volume.
4. Unilateral heat transfer apparatus as recited in claim 3 wherein said second wick comprises a single layer oF wire screen.
5. Unilateral heat transfer apparatus as recited in claim 3 wherein said first wick comprises a felted metal.
6. Unilateral heat transfer apparatus as recited in claim 3 wherein said first wick comprises a sintered metal.
7. Unilateral heat transfer apparatus as recited in claim 1, said closed evapOrative heat transfer chamber further including an adiabatic portion.
8. Unilateral heat transfer apparatus as recited in claim 7, the amount of said working fluid being limited to that necessary to saturate the capillary structure in the evaporator and adiabatic portions only.
9. Unilateral heat transfer apparatus, comprising:
1. Unilateral hea4 transfer apparatus, comprising:
2. Unilateral heat transfer apparatus as recited in claim 1 wherein said capillary comprises first and second wicks coupled to one another to provide a continuous condensate flow path.
3. Unilateral heat transfer apparatus as recited in claim 2, said first and second wicks disposed in the evaporator and condenser portions of said chamber, respectively, with said first wick being thicker than said second wick in order to provide a working fluid storage volume.
4. Unilateral heat transfer apparatus as recited in claim 3 wherein said second wick comprises a single layer oF wire screen.
5. Unilateral heat transfer apparatus as recited in claim 3 wherein said first wick comprises a felted metal.
6. Unilateral heat transfer apparatus as recited in claim 3 wherein said first wick comprises a sintered metal.
7. Unilateral heat transfer apparatus as recited in claim 1, said closed evapOrative heat transfer chamber further including an adiabatic portion.
8. Unilateral heat transfer apparatus as recited in claim 7, the amount of said working fluid being limited to that necessary to saturate the capillary structure in the evaporator and adiabatic portions only.
9. Unilateral heat transfer apparatus, comprising:
Description:
BACKGROUND OF THE INVENTION
Heat pipes capable of transferring large quantities of heat with a very small temperature difference between a heat source and a heat sink in one direction only are known. U.S. Pat. No. 3,613,774, assigned to the assignee of the present invention, describes a unilateral heat transfer device having a capillary structure only in the evaporator portion of the heat pipe such that heat added to the evaporator produces vaporization of a volatile liquid from the capillary structure whereby the vapors travel from the evaporator to a condenser under a slight pressure gradiant. The removal of the latent heat of vaporization at the condenser causes the vapors to condense and the condensate to return by gravity to the region of the capillary structure where it is returned to the evaporator to complete the cycle. Since there is no capillary structure disposed adjacent the heat output end of the device, a heat source near the output does not reverse the operation of the device, and flow of heat back into the object to be cooled is inhibited. This device requires gravity to return the condensate from the condenser portion of the heat pipe to the capillary structure. There are many applications where a unilateral heat transfer device is required which will operate in any orientation or in a gravity free environment, and, thus, gravity cannot be used to return the condensate to the capillary structure, one such application being in outer space.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide a new and novel unilateral heat transfer device.
It is another object of this invention to provide a unilateral heat transfer device which will work independent of gravitational forces.
Briefly, a unilateral heat transfer device, and more particularly a heat pipe, is provided having a non-uniform capillary structure therein. The capillary structure comprises a small pore size wick in the evaporator portion of the heat pipe and a relative y large pore size wick in the condenser portion thereof. Also the working fluid within the heat pipe is limited to that amount necessary to saturate the capillary structure in the evaporator section only. When heat is applied to the evaporator section of the heat pipe the working fluid is evaporated as in an ordinary heat pipe, the vapors travel to the condenser section, condensation takes place and the condensate is returned by the capillary structure to the evaporator section. When heat is applied to the condenser section of the heat pipe, only a very small amount of working fluid is available, and once this is dried out, heat transfer in this direction ceases so that substantial heat transfer can take place only from the evaporator section to the condenser section, and not in the reverse direction.
BRIEF DESCRIPTION OF THE DRAWINGS
The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description, taken in conjuntion with the accompanying drawing, in which:
FIG. 1 is a schematic cross-sectional illustration of a unilateral heat pipe in accordance with the present invention; and
FIG. 2 is a schematic cross-sectional illustration of a second embodiment of a unilateral heat transfer device.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring now to FIG. 1, the present invention is schematically illustrated in its fundamental form. A closed evacuated container 10 has a capillary structure in the form of a pair of wicks 12 and 14 disposed on the inside surface thereof. Wick 12 is disposed on the inside surface of the condenser portion 16 of the heat transfer device and extending part way into the adiabatic portion 18. Wick 14 is disposed on the inside surface of the evaporator portion 20 and also extends into the adiabatic portion 18. It is not necessary for successful operation of the device that both of the wicks 12 and 14 extend into the adiabatic section as long as one of them does so in order to provide a continuous path for return of condensate.
Wicks 12 and 14 are of different pore sizes with the smaller pore size wick being in the evaporator section. The wick in the evaporator section is also relatively thick in order to provide a working fluid storage volume. The wick does not need to be thick in the condenser section, in fact, from a heat transfer point of view and for liquid storage reasons, it should be as thin as possible.
In the embodiment of FIG. 1, the wick is essentially a hybrid wick consisting of one layer of wire screen in the condenser section and a portion of the adiabatic section, and a sintered or felted metal in the remainder of the heat pipe. This combination forms thick and thin wick sections as well as make the different pore size requirement relatively easy to satisify. The wick is filled only with sufficient working fluid to completely saturate the wick in the evaporator section (and perhaps a portion of the adiabatic section).
When heat is supplied to the evaporator section 20, the working fluid is evaporated as in an ordinary heat pipe. The vapors travel to the condenser section, condensation takes place, and the condensate is returned by the wick to the evaporator section.
As the heat input iS reduced, the heat transfer is, of course, automatically reduced until both the evaporator and condenser are at the Same temperature. At this point, the majority of the liquid phase of the working fluid is drawn into and stored in the evaporator portion of the wick due to the force imbalance between the condenser and evaporator owing to the difference in capillary pore sizes in the two regions. If due to heat input to the condenser section that section is raised to a higher temperature than the evaporator section, the small amount of fluid retained in the condenser section is rapidly evaporated and transferred to the other end of the heat pipe where it is retained and stored. Thus, no liquid remains in the condenser (now turned would be evaporator) to provide for heat pipe operation. The only mode of heat transfer between the two sections then reduces primarily to conduction down the heat pipe wall with minor second order effects of natural convection in the eat pipe vapor and radiation. This principle of operation affords a very rapid response as a thermal diode since the amount of liquid which must be evaporated to dry the condenser is very small, and this will occur immediately after the temperature at the evaporator falls below the temperature at the condenser.
As the evaporator section is heated to a temperature above that of the condenser section, normal heat pipe operation is resumed. A heat pipe is, thus, provided which is fabricated very simply and since the wick is placed over the entire condenser section, continuous condensate pickup is assured under all gravity conditions. Also, since the amount of working fluid is limited, the proper locatOn of the working fluid is assured, thus providing an adequate supply of fluid to the evaporator at all times.
A second embodiment of a thermal diode constructed according to the invention is set forth in FIG. 2 and comprises an evacuated container having wicks 32 and 34. The wick 32 in the condenser portion 36 of the heat pipe (and part of the adiabatic section 37) is of relatively large pore size and as thin as possible, while the wick 34 in the evaporator portion of the heat pipe 38 (and part of the adiabatic section 37) has a relatively small pore size and is relatively large. The condenser section 36 is 18 inches long, the evaporator section 3.34 inches, and the adiabatic section 37, that is the remaining portion of the device, is 3.15 inches in length. The thickness of the walls of the housing 30 is 0.020 inches, and the housing is stainless steel.
Any conventional heat pipe working fluid may be employed, and in the embodiment of FIG. 2 ammonia was used. Condenser wick 32 comprises a single layer of 100 mesh stainess steel screen plus a small arterial wick of 0.0625 inches in diameter while the evaporator wick is a nickel felted metal wick 0.125 inches thick. The pore size in the condenser section is 114 microns and that in the evaporator sections 97.53 microns. If desired, the pore sizes can be reduced in order to pump the liquid working fluid further. As in the previous embodiment, the felt and metal wick in the evaporator and a portion of the adiabatic section must be capable of storing all of the liquid working fluid in the heat pipe.
While I have described above the principles of my invention in accordance with specific apparatus, it is to be clearly understood that the description is made only by way of example and not as a limitution of the scope of my invention as set forth in the accompanying claims.
Heat pipes capable of transferring large quantities of heat with a very small temperature difference between a heat source and a heat sink in one direction only are known. U.S. Pat. No. 3,613,774, assigned to the assignee of the present invention, describes a unilateral heat transfer device having a capillary structure only in the evaporator portion of the heat pipe such that heat added to the evaporator produces vaporization of a volatile liquid from the capillary structure whereby the vapors travel from the evaporator to a condenser under a slight pressure gradiant. The removal of the latent heat of vaporization at the condenser causes the vapors to condense and the condensate to return by gravity to the region of the capillary structure where it is returned to the evaporator to complete the cycle. Since there is no capillary structure disposed adjacent the heat output end of the device, a heat source near the output does not reverse the operation of the device, and flow of heat back into the object to be cooled is inhibited. This device requires gravity to return the condensate from the condenser portion of the heat pipe to the capillary structure. There are many applications where a unilateral heat transfer device is required which will operate in any orientation or in a gravity free environment, and, thus, gravity cannot be used to return the condensate to the capillary structure, one such application being in outer space.
SUMMARY OF THE INVENTION
Accordingly, it is an object of this invention to provide a new and novel unilateral heat transfer device.
It is another object of this invention to provide a unilateral heat transfer device which will work independent of gravitational forces.
Briefly, a unilateral heat transfer device, and more particularly a heat pipe, is provided having a non-uniform capillary structure therein. The capillary structure comprises a small pore size wick in the evaporator portion of the heat pipe and a relative y large pore size wick in the condenser portion thereof. Also the working fluid within the heat pipe is limited to that amount necessary to saturate the capillary structure in the evaporator section only. When heat is applied to the evaporator section of the heat pipe the working fluid is evaporated as in an ordinary heat pipe, the vapors travel to the condenser section, condensation takes place and the condensate is returned by the capillary structure to the evaporator section. When heat is applied to the condenser section of the heat pipe, only a very small amount of working fluid is available, and once this is dried out, heat transfer in this direction ceases so that substantial heat transfer can take place only from the evaporator section to the condenser section, and not in the reverse direction.
BRIEF DESCRIPTION OF THE DRAWINGS
The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description, taken in conjuntion with the accompanying drawing, in which:
FIG. 1 is a schematic cross-sectional illustration of a unilateral heat pipe in accordance with the present invention; and
FIG. 2 is a schematic cross-sectional illustration of a second embodiment of a unilateral heat transfer device.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring now to FIG. 1, the present invention is schematically illustrated in its fundamental form. A closed evacuated container 10 has a capillary structure in the form of a pair of wicks 12 and 14 disposed on the inside surface thereof. Wick 12 is disposed on the inside surface of the condenser portion 16 of the heat transfer device and extending part way into the adiabatic portion 18. Wick 14 is disposed on the inside surface of the evaporator portion 20 and also extends into the adiabatic portion 18. It is not necessary for successful operation of the device that both of the wicks 12 and 14 extend into the adiabatic section as long as one of them does so in order to provide a continuous path for return of condensate.
Wicks 12 and 14 are of different pore sizes with the smaller pore size wick being in the evaporator section. The wick in the evaporator section is also relatively thick in order to provide a working fluid storage volume. The wick does not need to be thick in the condenser section, in fact, from a heat transfer point of view and for liquid storage reasons, it should be as thin as possible.
In the embodiment of FIG. 1, the wick is essentially a hybrid wick consisting of one layer of wire screen in the condenser section and a portion of the adiabatic section, and a sintered or felted metal in the remainder of the heat pipe. This combination forms thick and thin wick sections as well as make the different pore size requirement relatively easy to satisify. The wick is filled only with sufficient working fluid to completely saturate the wick in the evaporator section (and perhaps a portion of the adiabatic section).
When heat is supplied to the evaporator section 20, the working fluid is evaporated as in an ordinary heat pipe. The vapors travel to the condenser section, condensation takes place, and the condensate is returned by the wick to the evaporator section.
As the heat input iS reduced, the heat transfer is, of course, automatically reduced until both the evaporator and condenser are at the Same temperature. At this point, the majority of the liquid phase of the working fluid is drawn into and stored in the evaporator portion of the wick due to the force imbalance between the condenser and evaporator owing to the difference in capillary pore sizes in the two regions. If due to heat input to the condenser section that section is raised to a higher temperature than the evaporator section, the small amount of fluid retained in the condenser section is rapidly evaporated and transferred to the other end of the heat pipe where it is retained and stored. Thus, no liquid remains in the condenser (now turned would be evaporator) to provide for heat pipe operation. The only mode of heat transfer between the two sections then reduces primarily to conduction down the heat pipe wall with minor second order effects of natural convection in the eat pipe vapor and radiation. This principle of operation affords a very rapid response as a thermal diode since the amount of liquid which must be evaporated to dry the condenser is very small, and this will occur immediately after the temperature at the evaporator falls below the temperature at the condenser.
As the evaporator section is heated to a temperature above that of the condenser section, normal heat pipe operation is resumed. A heat pipe is, thus, provided which is fabricated very simply and since the wick is placed over the entire condenser section, continuous condensate pickup is assured under all gravity conditions. Also, since the amount of working fluid is limited, the proper locatOn of the working fluid is assured, thus providing an adequate supply of fluid to the evaporator at all times.
A second embodiment of a thermal diode constructed according to the invention is set forth in FIG. 2 and comprises an evacuated container having wicks 32 and 34. The wick 32 in the condenser portion 36 of the heat pipe (and part of the adiabatic section 37) is of relatively large pore size and as thin as possible, while the wick 34 in the evaporator portion of the heat pipe 38 (and part of the adiabatic section 37) has a relatively small pore size and is relatively large. The condenser section 36 is 18 inches long, the evaporator section 3.34 inches, and the adiabatic section 37, that is the remaining portion of the device, is 3.15 inches in length. The thickness of the walls of the housing 30 is 0.020 inches, and the housing is stainless steel.
Any conventional heat pipe working fluid may be employed, and in the embodiment of FIG. 2 ammonia was used. Condenser wick 32 comprises a single layer of 100 mesh stainess steel screen plus a small arterial wick of 0.0625 inches in diameter while the evaporator wick is a nickel felted metal wick 0.125 inches thick. The pore size in the condenser section is 114 microns and that in the evaporator sections 97.53 microns. If desired, the pore sizes can be reduced in order to pump the liquid working fluid further. As in the previous embodiment, the felt and metal wick in the evaporator and a portion of the adiabatic section must be capable of storing all of the liquid working fluid in the heat pipe.
While I have described above the principles of my invention in accordance with specific apparatus, it is to be clearly understood that the description is made only by way of example and not as a limitution of the scope of my invention as set forth in the accompanying claims.