Patent Grant
10264707
The present invention relates to a compact flat plate pulsating heat pipe capable of controlling temperature without using external power in ultra-thin mobile devices with maximized portability, such as smart phones, table PCs, etc.
With performance improvement of microprocessors, electronic devices are becoming smaller and lighter. Recently, the demand for ultra-thin mobile devices with maximized portability, such as smart phones and table PCs, is rapidly increasing. These electronic devices typically have a stacked structure with very limited room for a cooling system. For this reason, the need for an effective cooling system has arisen.
A heat pipe is a micro cooling system and is a representative power-free thermal control device. Conventional heat pipes consist of a vacuumed and sealed tube containing a predetermined amount of working fluid. The inner wall of the sealed tube is composed of a capillary structure having working fluid passages for moving a working fluid by capillary action. A heat generating means or a heating means is installed outside the sealed tube to be arranged near an end portion of the sealed tube, so that the end portion of the sealed tube functions as an evaporator (heating unit) that evaporates the working fluid. Meanwhile, a heat-dissipating means or a cooling means is installed outside the sealed tube to be arranged near a opposite end portion of the sealed tube, so that the opposite end portion of the sealed tube functions as a condenser (cooling unit) that condenses the working fluid.
This heat pipe transfers heat from a heat generating means and a heat-dissipating means to a heating means and a cooling means and vice versa with the use of latent heat, which is concomitant with frequent liquid-vapor phase changes caused by an evaporator and a condenser. Therefore, this heat pipe exhibits higher heat transfer performance (thermal conductivity) than a case of using a general pure metal. Accordingly, a heat pipe is widely used as a key component for heat transfer in various products including heat exchangers, cooling devices, and heat transfer devices.
However, a conventional heat pipe has a wick (capillary structure) that is typically made of a porous material, such as a metal net, sintered metal powder, or metal fiber, and which has a predetermined thickness or thicker, for circulation of a working fluid. Generally, the thinner the heat pipe, the poorer the performance of the heat pipe. Therefore, there is a limit in application of a conventional heat pipe to ultra-thin electronic devices.
In order to address problems associated with heat pipes employing a wick, recently, a pulsating heat pipe (PHP) using a thin tube with a small diameter as a closed tube has been developed. The pulsating heat pipe evaporates, transfers, condenses, and returns a working fluid through capillary action without using a wick.
However, as to conventional pulsating heat pipes, study and research have been mostly focused on the shape of a single-turn PHP and only liquid such as water has been used as a working fluid. Therefore, there still remains a problem that it is difficult to obtain satisfactory heat transfer performance required for a cooling device for use in electronic devices.
Therefore, there is an urgent need for development of an improved flat plate pulsating heat pipe that has high heat transfer efficiency and which can stably operate in ultra-thin electronic devices.
As a reference, conventional flat plate pulsating heat pipes are disclosed in the following patent documents.
(Patent Document 1) Korean Patent Application Publication No. 10-2012-0042403
(Patent Document 2) Korean Patent No. 10-1250326
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to manufacture a flat plate pulsating heat pipe using Micro Electro Mechanical Systems (MEMS) technology and to provide a power-free high efficiency heat transfer system for use in small electronic devices such as mobile phones and laptop computers.
Another object of the invention is to provide a flat plate pulsating heat pipe having an optimum shape and size by evaluating thermal performance of a flat plate pulsating heat pipe having a single-turn loop or a multi-turn loop and a flat plate pulsating heat pipe having a single diameter channel or a dual diameter channel.
A further object of the present invention is to provide a flat plate pulsating heat pipe having optimum efficiency at a main operation temperature by evaluating characteristics in accordance with temperature ranges of a working fluid used in a flat plate pulsating heat pipe.
In order to accomplish the above object, the present invention provides a flat plate pulsating heat pipe including: a silicon lower wafer plate having a rectangular shape; an upper wafer plate bonded onto the silicon lower wafer plate; a capillary tube engraved in the silicon lower wafer plate and having a meandering closed loop shape; a working fluid in the capillary tube; and two through-holes provided to respective side surfaces of the silicon lower wafer to communicate with the capillary tube such that the working fluid is charged into and discharged from the capillary tube through the two through-holes.
The capillary tube may have a single-turn loop shape or a multi-turn loop shape, wherein in each case, the capillary tube may have a single diameter channel in which a portion of a channel from a condenser to an evaporator and a portion of the channel from the evaporator to the condenser have an equal diameter or a dual diameter channel in which a portion of a channel from the condenser to the evaporator and a portion of the channel from the evaporator to the condenser have different diameters.
Performance of the flat plate pulsating heat pipe varies according to a diameter difference between the two diameters of the dual diameter channel. When the diameter difference is small, the effect of the diameter difference is negligible, and thus thermal performance improvement cannot be accomplished with a small diameter difference. When the diameter difference is excessively large, forces are overly unbalanced and friction pressure loss abruptly increases in the channel having a smaller diameter, thereby negatively affecting the performance of the flat plate pulsating heat pipe. Therefore, the present invention features that the diameter difference is set such that the flat plate pulsating heat pipe can exhibit an optimum heat transfer performance.
The capillary tube may have a multi-turn loop shape including a single diameter channel in which a portion of a channel from a condenser to an evaporator and a portion of the channel from the evaporator to the condenser have an equal diameter, or including a dual diameter channel in which a portion of a channel from the condenser to the evaporator and a portion of the channel from the evaporator to the condenser have different diameters.
The capillary tube may be a combination of the single diameter channel and the dual diameter channel.
Since the working fluid exhibits thermal characteristics that vary according to operating temperatures of the evaporator, FC-72 is used as the working fluid when a main operating temperature of the evaporator is 100° C. or lower, and ethanol is used as the working fluid when the main operating temperature is higher than 100° C.
A performance index, i.e. figure of merit, that represents the performance of a flat plate pulsating heat pipe is developed to properly analyze and evaluate the thermal characteristics of a flat plate pulsating heat pipe. Using the performance index, a flat plate pulsating heat pipe with an optimum heat transfer performance is provided.
Since a flat plate pulsating heat pipe according to the present invention is manufactured using MEMS technology, the flat plate pulsating heat pipe can be used as an effective cooling system in ultra-thin microelectronic devices as well as in small electronic devices such as mobile phones and laptop computers. Therefore, the flat plate pulsating heat pipe also can be applied to smaller and lighter future electronic devices.
Since the flat plate pulsating heat pipe effectively transfers heat without using external electric power, it is unnecessary to take into account issues such as the storage capacity of an internal power source and connection to an external power source that must be taken into account when applying the flat plate pulsating heat pipe to electronic devices. Therefore, the flat plate pulsating heat pipe allows high flexibility in designing of electronic devices.
Particularly, since the flat plate pulsating heat pipe is a cooling system having a simple structure and independently operating without being connected to an external power source, the flat plate pulsating heat pipe does not require maintenance once installed.
Hereinafter, a flat plate pulsating heat pipe according to an exemplary embodiment of the present invention will be described in detail with reference to the accompanying drawings.
A heat pipe is a representative thermal control device that does not require supply of external electric power. However, a conventional heat pipe includes a wick structure having a predetermined thickness installed therein. The wick structure is provided for the purpose of circulation of a working fluid. Generally, as the thickness of a heat pipe decreases, the performance tends to deteriorate. Accordingly, conventional heat pipes have a limit in application to ultra-thin electronic devices. To overcome this problem, a pulsating heat pipe with no wick structure is suggested.
A pulsating heat pipe takes a form in which one capillary tube is bent in multiple turns, forming a closed loop. The closed loop is evacuated first, and then a working fluid is charged into the closed loop. In this case, a well-aligned slug-train unit consisting of liquid slugs and vapor plugs is formed in the closed loop. At this point, if heat is applied to one side of the closed loop, the aligned slug-train unit performs high-speed self-sustained oscillations and thus heat is transferred from an evaporator to a condenser by this motion.
As described above, a pulsating heat pipe has a simple structure and is equipped with no wick structure. Therefore, a pulsating heat pipe can be manufactured in a small size, and thus it can be suitably applied to microelectronic devices.
As illustrated in
A working fluid forms an oscillating flow (A) that oscillates up and down with small amplitude when the heat input is small. As the heat input increases, the amplitude of the oscillating flow correspondingly increases (B, C). When the heat input further increases, the working fluid does not oscillate but forms a circulation flow which moves in one direction (D, E).
When circulation motion occurs in the heat pipe, the heat pipe exhibits higher thermal performance. Due to the circulation flow, the working fluid cools down to a sufficiently low temperature while passing through a condenser, and then directly enters into an evaporator. Therefore, the circulation flow offers a higher heat transfer effect than the oscillating flow.
As illustrated in
The silicon lower wafer plate 100 is a 1 mm silicon wafer that is commonly used to manufacture MEMS. The upper wafer plate may be made of the same material as the silicon lower wafer plate 100, or may be made of glass that allows an inspector to check on the movement and state of a working fluid during testing. A representative example of the glass of the upper wafer plate is PYREX (registered trademark).
A method of manufacturing a flat plate pulsating heat pipe according to the exemplary embodiment of the present invention is illustrated in
First, a photoresist layer is deposited on an upper surface of a rectangular silicon wafer 100 through a physical deposition process or a chemical deposition process (Step (a)).
The photoresist layer is patterned in the form of a capillary tube 300 having a closed loop shape that is bent at positions near respective ends of the silicon wafer 100. The patterning is performed by various methods, such as lithography, photolithography, electron beam lithography, ion ray lithography, X-ray lithography, and diamond patterning (Step (b)).
Next, dry or wet etching is performed to form the capillary tube 300 and through-holes 330 in an upper portion of the silicon wafer 100. In this process, dry etching and wet etching can be used in combination. In one embodiment of the present invention, deep reactive ion etching (Deep RIE), which is a representative combined wet and dry etching process, was used and an etching depth was 500 mm.
Next, the remaining photoresist layer on the silicon wafer is removed. An upper plate of glass is bonded onto the silicon wafer 100 to produce a flat plate pulsating heat pipe. In the bonding process, anodic bonding may be used because a bonding surface is very flat and smooth due to the nature of glass, whereby it is possible to produce a flat plate pulsating heat pipe with high bonding performance.
Finally, a working fluid is injected into the flat plate pulsating heat pipe through the through-holes 330 formed to pass through respective side end portions of the flat plate pulsating heat pipe, and then the through holes are sealed. In this process, when a working fluid or air is injected through one of the through holes and air is suctioned through the other one by a vacuuming apparatus, charging and discharging of a working fluid into and from the flat plate pulsating heat pipe can be effectively performed. That is, the structure with two through holes has an advantage over the structure with one through hole in that changing and discharging of a working fluid is faster and more precisely controlled.
The thermal characteristics of the flat plate pulsating heat pipe are largely affected by the shape of the capillary tube 300 engraved in the silicon wafer 100.
As illustrated in
Typically, a capillary tube engraved through dry etching or combined dry and wet etching tends to be a rectangular shape rather than a circular shape. For analysis on performance of a flat plate pulsating heat pipe, the rectangular capillary tube is considered to be an equivalent circular tube. The diameter Dh of the equivalent circular tube is calculated according to Equation 1.
where AC is area, perimeter is length of circumference, w is width of capillary tube, and hch is height of capillary tube.
In the present invention, to evaluate operational characteristics of a flat plate pulsating heat pipe, a test system shown in
With respect to the exemplary embodiment,
As illustrated in
When the working fluid is ethanol, as illustrated in
On the other hand, when the working fluid is FC-72, as illustrated in
This means that there is an optimum diameter difference between two channel diameters. The diameter difference at which a flat plate pulsating heat pipe exhibits optimum performance, suggested by
Equation 2
0.15≤ΔD/Davg≤0.35 [Equation 2]
where ΔD is difference between two diameters and Davg is average of two diameters.
A working fluid contained in a dual diameter channel of a multi-turn loop exhibits similar behaviors to a working fluid contained in a single diameter channel. That is, FC-72 exhibits better heat transfer performance when the input temperature is 100° C. or lower, but ethanol exhibits better performance when the input temperature is higher than 100° C.
The performance of a flat plate pulsating heat pipe having a multi-turn loop having a dual diameter channel is indirectly evaluated based on various indexes. Representation of the performance of a flat plate pulsating heat pipe varies according to an evaluator's perspective. Therefore, to conveniently and simply represent the performance of a flat plate pulsating heat pipe having a dual diameter channel structure, we use a figure of merit MPHP represented by a single numeric value and calculated according to Equation 3.
where ρ1 is density of liquid phase of working fluid, hfg is latent heat of vaporization, σ is surface tension, w1 is width of a larger channel, w2 is width of a smaller channel, μ1 is viscosity coefficient of a liquid phase of working fluid, x is vapor mass quality (mass of vapor with respect to mass of entire working fluid), h is height (depth) of a channel, Φ2L is a frictional multiplier (pressure difference required for two-phase flow/pressure difference required for liquid flow with same massflow rate.
In Equation 3, Φ2L is the frictional multiplier. It is a ratio of a pressure difference between pressures at respective ends of a heat pipe, required to activate motion of a working fluid when the heat pipe is filled with only liquid, with respect to a pressure difference between pressures at respective ends of the heat pipe, required to activate motion of a working fluid when the heat pipe is filled with liquid and gas. The frictional multiplier can be calculated using various models. A representative calculation model is Equation 4 using Martinelli parameter X. Normally, the frictional multiplier has a value within a range of from 0.001 to 10.
wherein C is Chisholm number.
The evaluation result of the performance of a flat plate pulsating heat pipe on the basis of the performance index MPHP shows that a flat plate pulsating heat pipe exhibits good thermal conductivity when the performance index MPHP is 1012 kg/(m·s) (W/m3) or higher as shown in
Meanwhile, a test system of
To measure surface temperatures of a flat plate pulsating heat pipe that can be installed at various inclination angles, multiple thermocouples 600 are respectively installed in a heat generator, evaporator, condenser, and heat insulator. Examples of thermocouples that can be used in the present invention include K-type and Omega. Temperatures measured by the thermocouples 600 are transmitted to a control computer 700 via a data acquisition device (DAQ) 760.
A hot wire 610, manufactured by coating a wire with Nichrom, is connected to a direct current (DC) power supply (E3631) 710 so that heat can be supplied to a heat generation unit of a flat plate pulsating heat pipe. A copper block 620 with a high thermal conductivity is installed to surround the condenser to deprive the condenser of heat. Constant temperature water supplied by a bath circulator (RW-0525G) 750 flows through the copper block 620.
To evaluate thermal characteristics in accordance with various inclination angles for a flat plate pulsating heat pipe that can be installed at various inclination angles, a support table 640 to support a flat plate pulsating heat pipe is provided. A rotation unit 650 is installed to rotate the support table and thus change the inclination angle of the flat plate pulsating heat pipe.
Preferably, a flat plate pulsating heat pipe is installed in a vacuum chamber 800 for more accurate and precise evaluation of thermal characteristics. The vacuum chamber 800 is vacuumed by a rotary pump 810 so that a vacuum state can be created and maintained. The inside of the vacuum chamber 800 is maintained at or below 0.01 torr.
One surface of the vacuum chamber 800 is provided with a glass window 820 to allow visual checking of behaviors of a working fluid contained in a flat plate pulsating heat pipe. A high speed camera 720 installed outside the vacuum chamber 800 takes an image of the flat plate pulsating heat pipe through the window 820. The high speed camera 720 captures an image of the flat plate pulsating heat pipe, and information of the captured image is transmitted as video data to the control computer 770.
Flat plate pulsating heat pipes according to the present invention are tested using the above-described test system and the results are summarized in
As illustrated in
Meanwhile, in the case of a single diameter channel, the thermal resistance is highest at an inclination angle of 0° and lowest at an inclination angle of 90° C. In the case of a dual diameter channel, as shown in
However, as shown in
That is, there is an optimum diameter difference between two channel diameters. The optimum diameter difference at which a flat plate pulsating heat pipe exhibits optimum performance satisfies the following condition:
Equation 5
0.3≤ΔD/Davg≤0.35 [Equation 5]
wherein ΔD is a difference between two diameters and Davg is the average between two diameters.
Meanwhile, as illustrated in
As illustrated in
Accordingly, in a case where a single diameter channel and a dual diameter channel are used in combination in a flat plate pulsating heat pipe, the thermal characteristics of the flat plate pulsating heat pipe largely vary in accordance with a proportion of the dual diameter channels with respect to all of the channels. To accurately and precisely evaluate the performance of a flat plate pulsating heat pipe, a performance index MPHP2, i.e. figure of merit, calculated according to Equation 6 is used.
where N is the number of dual diameter channels, Nt is the total number of all of the channels, ρ1 is density of liquid phase of working fluid, σ is surface tension, w1 is width of a larger channel of a dual diameter channel, w2 is width of a smaller channel of a dual diameter channel, hfg is latent heat of vaporization, w1 is width of a larger channel, w2 is width of a smaller channel, w is width of channel of a single diameter channel, μ1 is viscosity coefficient of a liquid phase of working fluid, C is a frictional coefficient, and h is height (depth) of a channel, wherein all units herein are SI base units.
The frictional coefficient C can be calculated in various ways. However, a fixed value of 4.0 is preferably used for easy calculation.
As illustrated in
As illustrated in
The test result also shows that the thermal characteristics of a flat plate pulsating heat pipe are not influenced by inclination angles when the performance index of the dual diameter channel is 2×105 kg/(m3·s) or higher. This means that a flat plate pulsating heat pipe has to be designed such that its performance index is 2×105 kg/(m3·s) or higher.
In addition, as illustrated in
As described above, a flat plate pulsating heat pipe is designed on the basis of the performance index MPHP. Since the characteristics of the designed flat plate pulsating heat pipe are greatly influenced by operating temperatures and kinds of working fluids, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the technical spirit of the invention. Accordingly, the technical scope of the present invention should not be limited to the above descriptions but should be defined by the appended claims.
A flat plate pulsating heat pipe according to the present invention can be used in various industry fields, for example, in various electronic devices, small internal combustion engines, and small machinery. Since the flat plate pulsating heat pipe is power free and thus can be quasi-permanently used, the flat plate pulsating heat pipe is highly useful in industrial fields.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2014-0138168 | Oct 2014 | KR | national |
| 10-2015-0009528 | Jan 2015 | KR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/KR2015/005209 | 5/22/2015 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2016/060350 | 4/21/2016 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 20020007937 | Kroliczek et al. | Jan 2002 | A1 |
| 20030037910 | Smyrnov | Feb 2003 | A1 |
| 20080087406 | Asfia | Apr 2008 | A1 |
| 20090101308 | Hardesty | Apr 2009 | A1 |
| 20090323276 | Mongia | Dec 2009 | A1 |
| 20110067843 | Vasiliev, Jr. | Mar 2011 | A1 |
| 20110079022 | Ma et al. | Apr 2011 | A1 |
| 20110206077 | Schlie et al. | Aug 2011 | A1 |
| 20130044776 | Schlie et al. | Feb 2013 | A1 |
| 20130112375 | Choi et al. | May 2013 | A1 |
| 20130133871 | Ma | May 2013 | A1 |
| 20150072424 | Ma et al. | Mar 2015 | A1 |
| Number | Date | Country |
|---|---|---|
| 2002-081874 | Mar 2002 | JP |
| 20-0364898 | Oct 2004 | KR |
| 10-2005-0115504 | Dec 2005 | KR |
| 10-2010-0108978 | Oct 2010 | KR |
| 10-2011-0090491 | Aug 2011 | KR |
| 10-2011-0128539 | Nov 2011 | KR |
| 20110128539 | Nov 2011 | KR |
| 10-2012-0042403 | May 2012 | KR |
| 10-2012-0117411 | Oct 2012 | KR |
| 10-1250326 | Apr 2013 | KR |
| 201116793 | May 2011 | TW |
| Entry |
|---|
| Arab et al., “Experimental investigation of extra-long pulsating heat pipe application in solar water heaters”, Experimental Thermal and Fluid Science, vol. 42, 2012, pp. 6-15. |
| Charoensawan et al., “Closed loop pulsating heat pipes Part A: parametric experimental investigations”, Applied Thermal Engineering, vol. 23, 2003, pp. 2009-2020 |
| International Search Report (PCT/ISA/210) issued in PCT/KR2015/005209, dated Jul. 27, 2015. |
| Khandekar et al., “An insight into thermo-hydrodynamic coupling in closed loop pulsating heat pipes”, International Journal of Thermal Sciences, vol. 43, 2004, pp. 13-20. |
| Kwon et al., “Analysis of Pulsating Heat Pipe With a Dual-Diameter Tube”, HEFAT2014, 10th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Jul. 14-26, 2014, Orlando, Florida, 8 pages. |
| Kwon et al., “Operational characteristics of pulsating heat pipes with a dual-diameter tube”, International Journal of Heat and Mass Transfer, vol. 75, 2014, pp. 184-195. |
| Kwon et al., “Study on the Flow and Heat Transfer Characteristics of Pulsating Heat Pipe With Asymmetric Configuration”, Proceedings of the ASME 2013 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems, InterPACK2013, Jul. 16-18, 2013, Burlingame, CA, US, pp. 1-6. |
| Kwon et al., “Study on thermal performance of pulsating heat pipe with asymmetric configuration”, The Society of Air-Conditioning and Refrigerating Engineers of Korea, 2013 Summer Annual Conference, 2013, pp. 968-971. |
| Kwon et al., “Study on Thermal Performance of Pulsating Heat Pipes with a Dual-Diameter Tube”, The Korean Society of Mechanical Engineers, Thermal Engineering Part, 2014 Summer Annual Conference, 2014, pp. 244-245. |
| Kwon, Gi Hwan, “Study on the Thermal Performance of Pulsating Heat Pipe with Dual-diameter Channels”, School of Mechanical Engineering & Aerospace System Division of Mechanical Engineering, KAIST, 2014, 126 pages. |
| Number | Date | Country | |
|---|---|---|---|
| 20170245393 A1 | Aug 2017 | US |
