Patent Grant
10605539
A heat pipe can conduct heat from a heat source such as from an electronic device through vapor heat transfer. Typically, the heat pipe includes a working fluid, an evaporator section, and a condenser section. The working fluid is vaporized at the evaporator section. The vapor is received at the condenser section, whereupon the vapor is condensed to form a liquid working fluid. Capillary action returns the condensed working fluid to the evaporator section, thereby completing a cycle.
In a variable-conductance heat pipe (VCHP), the conductance of the heat pipe will vary depending on the operating temperature. This is typically achieved through a non-condensable gas, e.g., a noble gas such as helium, argon, nitrogen or the like, in an interior of the heat pipe. The non-condensable gas resides in passages adjacent to the condenser section. As the heat load from a heat source increases or as the evaporator temperature increases, the vapor pressure of the working fluid increases, forcing the non-condensable gas to compress and expose more of the condenser area. The dense vapor of the working fluid can then reach the exposed condenser surface for vapor condensation. On the other hand, when the evaporator is at a low temperature, the volume of the non-condensable gas increases, thereby increasing the blocked part of the condenser. The working fluid has a low vapor pressure allowing the component to warm up before the heat is removed. Due to the low vapor pressure, a relatively high volumetric flow rate would be needed to achieve a given amount of heat transfer. This high vapor flow rate can in turn facilitate maintaining the heat source at a relatively constant temperature despite a variation in the heat pipe's operating temperature.
It can be cumbersome to put an exact amount of non-condensable gas into a conventional VCHP. Moreover, adding passages for the non-condensable gas adjacent the condenser section can be difficult, costly, and hard to reliably replicate when making multiple units. Thus, there has developed a need for a heat pipe that can vary its conductance depending on the heat pipe's operating temperature, without solely relying on the non-condensable gas.
In some embodiments, a heat transfer device is provided for conducting heat from a heat source. The heat transfer device generally includes an evaporator for generating a vapor, a condenser in fluid communication with the evaporator, and a vapor flow restrictor interposed between the evaporator and the condenser, wherein the vapor flow restrictor can increase vapor pressure in at least a portion of the evaporator relative to vapor pressure in the condenser.
Also, in some embodiments, a heat transfer device is provided for conducting heat from a heat source, and generally includes an evaporator for generating a vapor, a condenser in fluid communication with the evaporator, and a vapor flow restrictor is interposed between the evaporator and the condenser, wherein the vapor flow restrictor is positioned adjacent the evaporator and includes a baffle with an orifice therethrough.
In some embodiments, a heat transfer device is provided for conducting heat from a heat source, and generally includes an evaporator, a condenser in fluid communication with the evaporator, and a vapor flow restrictor interposed between the evaporator and the condenser and adjacent the evaporator, wherein at least one of the evaporator and condenser comprises a wall having a wick disposed on at least a portion thereof, wherein a vapor flows between the evaporator and the condenser, and wherein the wick has a working fluid in contact therewith in a liquid form. The vapor flow restrictor comprises a baffle having an opening therethrough.
Also, in some embodiments, a heat transfer device is provided for conducting heat from a heat source, wherein the heat transfer device generally includes an evaporator for generating a vapor, and a condenser in fluid communication with the evaporator, wherein a vapor flow restrictor is interposed between the evaporator and the condenser, and is positioned adjacent the evaporator. The vapor is substantially free of non-condensable gas.
In some embodiments, a method of conducting heat from a heat source generally includes providing a heat transfer device that includes an evaporator for generating a vapor, a condenser in fluid communication with the evaporator, and a vapor flow restrictor interposed between the evaporator and the condenser, wherein the vapor flow restrictor is positioned adjacent the evaporator, and wherein vapor pressure can be increased in at least a portion of the evaporator relative to vapor pressure in the condenser.
Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.
Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limited. The use of “including,” “comprising” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “mounted,” “connected” and “coupled” are used broadly and encompass both direct and indirect mounting, connecting and coupling. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings, and can include electrical connections or couplings, whether direct or indirect.
As noted above, a working fluid resides within the heat transfer device 2 to facilitate heat transfer. Any number of fluids can be suitable as a working fluid so long as they have a liquid phase and a vapor phase. Suitable working fluids include, but are not limited to, water, ammonia, Freon, acetone, ethane, ethanol, heptane, methanol, potassium, sodium, hydrocarbons, fluorocarbons, methyl chloride, liquid metals such as cesium, lead, lithium, mercury, rubidium, and silver, cryogenic fluids such as helium and nitrogen, and other fabricated working fluids. The particular working fluid can be chosen depending on the operating temperature requirements, the material of the heat pipe wall 9, or upon preferences for the particular heat transfer device 2.
Referring also to
The illustrated baffle 110 has a baffle diameter D1. The orifice 120 defines a vapor passageway, and has an orifice diameter D2. In some embodiments, the orifice 120 diameter D2 varies from 0.5% to 15% of the baffle 110 diameter D1, although the orifice 120 diameter D2 can be even less than 0.5% of the baffle 110 diameter D1. Accordingly, in embodiments employing a flow restrictor baffle 110, the conductance of the heat pipe 2 can be varied depending at least in part upon the temperature without solely relying on non-condensable gas, as will be explained further below. Moreover, in some embodiments, the orifice diameter D2 may be required to have a particular tolerance dependent on the application. For example, one application may require a tolerance of approximately ±0.01 mm, while another application may allow a tolerance of approximately ±0.1 mm.
In some embodiments, the baffle 110 may have a shape other than circular (e.g. oval, square, rectangular, or other regular or irregular shapes) in which cases the cross-sectional dimensions may be expressed in terms other than diameter, for example the lengths of major and minor axes or the cross-sectional area of the baffle 110. Similarly, the orifice 120 may have a shape other than circular (e.g. oval, square, rectangle, or other regular or irregular shape) in which cases the cross-sectional dimensions may be expressed in terms other than diameter, for example the lengths of major and minor axes or the cross-sectional area of the orifice 120. In still other embodiments, the orifice 120 may include more than one opening, i.e. the orifice 120 may include two or more openings in the baffle 110, e.g. if a screen is used as part of the vapor flow restrictor 100 (see below). In general, it is the total area of the opening(s) of the orifice(s) combined that has the greatest impact on performance, whereas loss coefficients based on different sizes and shapes of the orifice(s) have a secondary effect. The size of the orifice 120 generally depends on the power that it will transmit and the desired operating temperature. The dimensions of the heat transfer device/heat pipe 2 will depend on these factors as well as other factors including, without limitation, the length of the heat transfer device/heat pipe 2, the properties of the wicking material that is used, and the operating orientation of the device.
At high ambient temperatures, vapor pressure of the working fluid inside the heat pipe 2 increases. The dense vapor of the working fluid passes through the orifice 120 to reach the condenser 7 for vapor condensation. The high vapor pressure of the working fluid means that a given amount of heat transfer would require a relatively low vapor flow rate. Because the vapor flow rate is low, the orifice 120 does not substantially restrict the flow of the vapor. The heat pipe 2 thus operates at full capacity and can efficiently cool a heat source. As such, the temperature differential between the evaporator 5 and condenser 7 of the heat pipe 2 is relatively low at high operating temperatures.
At low ambient temperatures, vapor pressure of the working fluid decreases. This means that a relatively high vapor flow rate would generally be required for a given amount of heat transfer. The orifice 120 in this case restricts or chokes vapor flow. This has the effect of operating the heat pipe 2 at a reduced capacity. The vapor flow restrictor 100 increases vapor pressure in at least a portion of the evaporator 5 relative to vapor pressure in the condenser 7, thereby increasing the pressure differential. This has the effect of also increasing the temperature differential between the evaporator 5 and the condenser 7 compared to what the temperature differential would be absent the vapor flow restrictor 100. In sum, the temperature differential is low at high ambient temperatures, and high at low ambient temperatures. As a result, the heat source adjacent the evaporator 5 can be maintained at a relatively constant temperature despite a variation in the temperature of the condenser 7 in the heat pipe 2.
The variable conductance of the heat pipe 2 can be beneficial where the temperature of the condenser 7 varies due to environmental conditions. For example, the condenser 7 may be located outdoors. Moreover, the heat pipe 2 that includes the condenser 7 may be sealed during summertime when the humidity is high. When that heat pipe 2 is operating in winter and the condenser 7 is exposed to an ambient temperature of about 4° C., the humidity captured in the enclosure surrounding the device being cooled may condense on an external surface of the heat pipe 2. The condensation could be desirably reduced if the external surface of the heat pipe 2 is maintained at a higher temperature. The vapor flow restrictor 100 can facilitate maintaining the heat pipe 2 at a higher temperature when the ambient temperature is low. As described above, this is achieved by running the heat pipe 2 at a reduced capacity when the ambient temperature is low.
Although
As noted previously, the vapor flow restrictor 100 is arranged so as to permit fluid to flow past it. In the illustrated embodiment, the vapor flow restrictor 100 includes a plurality of tabs 130. Although
As described above, the heat pipe 2 comprises a wick 25 disposed on at least a portion of the wall 9. The working fluid flows between the evaporator 5 and the condenser 7 via the wick 25. In some embodiments, both the evaporator 5 and the condenser 7 have the wick 25 disposed therein. The baffle 110 of the vapor flow restrictor 100 has a perimeter 150, which in some embodiments is in contact with the wick 25.
In some embodiments, the vapor flow restrictor 100 comprises at least a portion of the wick 25. The wick 25 can comprise an inner surface 160 spaced apart from the wall 9, wherein at least a portion of the inner surface 160 tapers in a direction along the wall 9. In particular, the wick 25 can be generally hourglass-shaped, i.e. includes a constriction in at least one location, where the orifice is associated with the constriction, with opposite end portions that are wider than the constriction. The hourglass shape of the wick 25 can be achieved by having the thickness of the wick 25 vary in a direction along a cylindrical wall 9. Alternatively, the wall 9 of the heat pipe 2 can be shaped to at least partially define the vapor flow restrictor, such as by having an hour-glass shape with a constant-thickness wick 25 or a varying-thickness wick on an inside surface thereof. Such heat pipe shapes can define an integral vapor flow restrictor (e.g., at the neck of the hourglass shape) having any of the features described above, or can be used in conjunction with a separate flow restrictor (e.g., also at the neck of the hourglass shape) as described and illustrated herein.
The heat transfer device in this embodiment is a multipart heat pipe 2′ that includes a primary part 170 and a secondary part 180 branching from a primary part 170. The primary and secondary parts 170, 180 can assume any suitable geometric forms, including, but not limited to, a cylindrical, a conical, a pyramidal, an ellipsoidal, a regular polyhedral, and an irregular polyhedral shape, derivatives thereof, and combinations thereof. Moreover the primary and secondary parts 170, 180 can have any relative sizes. For example, in some embodiments, the secondary part 180 is substantially the same size as the primary part 170. In other embodiments, however, the secondary part 180 can be sized smaller or larger relative to the primary part 170. In the illustrated embodiment, the secondary part 180 extends from the primary part 170 at a perpendicular angle. In other embodiments, however, the secondary 180 can extend from the primary part 170 at an acute angle, e.g., generally giving the appearance of a y shape. Although
The secondary part 180 may be, for example, an auxiliary condenser. The multipart heat pipe 2′ in this embodiment includes an orifice 120 between the primary part 170 and the secondary part 180 which regulates vapor flow into the secondary part 180. Condensed working fluid is permitted to return from the secondary part 180 to the primary part 170, for example at the junction between the primary part 170 and the secondary part 180. Referring to
Referring to
Accordingly, the vapor flow restrictor 100 of the present invention variably restricts the flow of vapor depending upon the temperature of the multipart heat pipe 2′. As described above, this has the effect of maintaining the heat source at a relatively constant temperature despite a variation in the temperature inside the primary part 170 of the multipart heat pipe 2′.
Thus, by placing an orifice between the primary and secondary parts of a multipart heat pipe assembly (or, more generally, between the evaporator and condenser of a heat pipe, loop heat pipe, thermosiphon, or other heat transfer device), the flow of vapor to the secondary part can be restricted when the temperature of the primary part is below a desired trigger point. The orifice is generally sized to achieve choked flow (the sonic limit) in the orifice. This limits the amount of vapor flow through the orifice thus restricting to an acceptably low level the amount of heat that flows to the secondary part when the primary part is below the trigger temperature. When the temperature of the primary part rises above the trigger temperature, the vapor density increase drops the Mach number in the orifice to the point where the flow is no longer choked. Thus, the orifice can sustain a significant flow rate of vapor so that the primary and secondary parts of the heat pipe are nearly isobaric and isothermal. Because the vapor pressure curve as a function of temperature is steep for most heat pipe working fluids, especially close to their freezing temperature, this thermal diode behavior is sharp enough to be useful in practical applications.
The heat transfer device in this embodiment may be a loop heat pipe, a loop thermosiphon, or a thermosiphon 2″, where the evaporator 5 is connected to the condenser 7 in a closed loop. In this embodiment, the working fluid may return from the condenser 7 to the evaporator 5 via gravity, with or without a wick. In one embodiment, the vapor flow restrictor 100 is placed in the vapor line between the evaporator 5 and condenser 7. Placing the vapor flow restrictor 100 close to the evaporator 5 would be preferred to minimize heat losses from the vapor transport line. The loop heat pipe 2″ optionally includes a check valve 190. The check valve can regulate or propel the flow of the working fluid and/or vapor so that the liquid and/or vapor of the working fluid are permitted to move in one direction only and/or toward a predetermined direction. In heat transfer devices such as these which have a continuous circuit, the vapor flow restrictor 100 can be placed at a point between the evaporator and condenser, downstream from the evaporator, for example, in the vapor transport line between the evaporator and condenser.
An illustrative embodiment of the vapor flow restrictor is described in greater detail below.
The size of the orifice in the baffle can be calculated based on the sonic velocity of the working fluid vapor inside the heat pipe, where the working fluid in this embodiment is water. The evaporator in this particular example is designed to operate at a temperature between 22° C. and 50° C. A standard heat pipe, which does not vary its conductance in this temperature range, maintains a more or less constant temperature differential between the evaporator and the condenser, where the temperature of the condenser is a function of the cooling fluid, e.g. water or air, that is applied to the condenser. Thus, the temperature of a heat source associated with the evaporator of a standard heat pipe would vary according to the temperature of the condenser, which for certain applications is undesirable. To address this issue, a vapor flow restrictor can be used which permits the heat pipe to transmit the maximum power (i.e. heat removing capability) when the evaporator is at the highest temperature (in this case, 50° C.) and less power when the evaporator is at lower temperatures. The temperature differential between the condenser and the evaporator/heat source is thus variable, thereby maintaining the heat source at a relatively constant temperature (i.e. within the desired operating range of 22° C. to 50° C.). When the heat source (and hence the evaporator) is less than 50° C., less power (heat energy) is transmitted to the condenser due to the vapor flow restrictor and therefore the heat source stays within the operating range of 22° C. to 50° C.
Relevant properties of steam at 50° C. are listed in the following Table 1.
For the heat pipe to transmit a maximum power of 30 watts (equivalent to 102.4 BTU/hr), the area A of the orifice is calculated as follows:
If the orifice is round, A=πr2, and
Therefore, the diameter D2 of the orifice 120 in this case is D2=2r=0.0292 in.
The amount of power transmitted at 22° C. with this orifice can be calculated as follows. Relevant properties of water vapor/steam at 22° C. are listed in the following Table 2.
Thus, a heat transfer device with an orifice diameter D2 of 0.0292 inches, which transmits 30 watts when the evaporator is at 50° C., transmits only 7 watts when the evaporator is at 22° C. The power transmitted by the heat transfer device 2 is therefore variable, and as a result, the heat source can be maintained at a relatively constant temperature.
Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of one or more independent aspects of the invention as described.
This application is a continuation of U.S. application Ser. No. 13/473,755, filed May 17, 2012, which claims priority to U.S. Provisional Patent Application No. 61/645,906, filed May 11, 2012, the contents of each of which are incorporated herein by reference in their entireties.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3414050 | Anand | Dec 1968 | A |
| 3750745 | Moore, Jr. | Aug 1973 | A |
| 3776304 | Auerbach | Dec 1973 | A |
| 3965970 | Chisholm | Jun 1976 | A |
| 4162394 | Faccini | Jul 1979 | A |
| 4170262 | Marcus et al. | Oct 1979 | A |
| 4437510 | Martorana | Mar 1984 | A |
| 4785875 | Meijer et al. | Nov 1988 | A |
| 4883116 | Seidenberg et al. | Nov 1989 | A |
| 4921041 | Akachi | May 1990 | A |
| 5044426 | Kneidel | Sep 1991 | A |
| 5566751 | Anderson et al. | Oct 1996 | A |
| 5647429 | Oktay et al. | Jul 1997 | A |
| 5667003 | Mandjuri-Sabet | Sep 1997 | A |
| 6167955 | Van Brocklin et al. | Jan 2001 | B1 |
| 6571863 | Liu | Jun 2003 | B1 |
| 7272941 | TeGrotenhuis et al. | Sep 2007 | B2 |
| 9810483 | Bilski et al. | Nov 2017 | B2 |
| 20070095506 | Hou et al. | May 2007 | A1 |
| 20070204975 | Liu et al. | Sep 2007 | A1 |
| 20070235165 | Liu et al. | Oct 2007 | A1 |
| Entry |
|---|
| Koplow, “A Fundamentally New Approach to Air-cooled Heat Exchangers,” Sandia National Laboratories, Jan. 2010, 48 pages. |
| Number | Date | Country | |
|---|---|---|---|
| 20180216896 A1 | Aug 2018 | US |
| Number | Date | Country | |
|---|---|---|---|
| 61645906 | May 2012 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 13473755 | May 2012 | US |
| Child | 15804400 | US |
