Tag Archives: heat pipe

Technical Discussion of ATS Telecom PCB solution

Last year, Advanced Thermal Solutions, Inc. (ATS) was brought in to assist a customer with finding a thermal solution for a PCB that was included in a data center rack being used in the telecommunications industry. The engineers needed to keep in consideration that the board’s two power-dissipating components were on opposite ends and the airflow across the board could be from either side.

Telecom PCB

The PCB layout that ATS engineer Vineet Barot was asked to design a thermal solution for included two components on opposite ends and airflow that could be coming from either direction. (Advanced Thermal Solutions, Inc.)

The original solution had been to use heat sinks embedded with heat pipes, but the client was looking for a more cost-effective and a more reliable solution. The client approached ATS and Field Application Engineer Vineet Barot examined the problem to find the best answer. Using analytical and CFD modeling, he was able to determine that ATS’ patented maxiFLOW™ heat sinks would provide the solution.

Vineet sat down with Marketing Director John O’Day and Marketing Communications Specialist Josh Perry to discuss the challenges that he faced in this project and the importance of using analytical modeling to back up the results of the CFD (computational fluid dynamics).

JP: Thanks for sitting down with us Vineet. How was the project presented to you by the client?
VB: They had a board that was unique – where it would be inserted into a rack, but it could be inserted in either direction. So, we faced a unique challenge because airflow could be from either side of the board. There were two components on either side of the board, so if airflow was coming from one side then component ‘A’ would get hot and from the other side then component ‘B’ would get hot. The other thing was that the customer, who is a very smart thermal engineer, had already set up everything and he was planning on using these heat sinks that had heat pipes embedded in them. The goal was to try and come up with a heat sink that would do the same thing, hopefully without requiring the heat pipes.

JO: Can we talk for a second about the application? You mentioned that airflow was from either side, the board was going to be used in a data center or a telecom node?
VB: It was for a telecom company.

JP: Was there a reason he didn’t want to use a heat pipe?
VB: I think probably cost and reliability. We use heat pipes embedded in the heat sinks too, so it’s not a something we never want to use, but the client wanted to throw that at us and see if we had alternatives.

JP: Can you take us through the board and the challenges that you saw?
VB: As you can see from this slide, there are four main components and two of the hottest ones are on the edge. Airflow can be from right to left or left to right, so which one would be the worst-case scenario?

Telecom PCB

JO: From right to left, I think?
VB: Correct. This one is a straightforward one to figure out because not only is the component smaller but the power is also higher. Even though [air] can go both ways, there’s a worst-case scenario.

This was the customer’s idea – a straight-fin heat sink with a heat pipe and he put one block of heat pipe in there instead of two or three heat pipes that would normally be embedded in there. You can clearly see what the goal was. You have a small component in here, you want to put a large heat sink over the top and you want to spread the heat throughout the base of the heat sink. All the other components are also occupied by straight-fin heat sinks.

JO: Okay, at this point in the analysis, this is the rough estimate of the problem that you face?
VB: This is a straightforward project in terms of problem definition, which can be a big issue sometimes. This time problem definition was clear because the customer had defined the exact heat sink that they wanted to use. It’s not a bad heat sink they just wanted an improvement, cost-wise, reliability-wise.

This is the G600, which is the air going from left to right. The two main components are represented here and we want to make sure that the junction temperatures that the CFD calculated is lower than the maximum junction temperatures allowed, which they were. These heat sinks work. As we always like to do at ATS, we like to have two, independent solutions to verify any problem. That was the CFD result but we also did the analytical modeling to see what these heat sinks are capable of and the percent difference from CFD was less than 10 percent. Twenty percent is the goal typically. If it’s less than 20 percent then you know you’re in the ballpark.

(Advanced Thermal Solutions, Inc.)

(Advanced Thermal Solutions, Inc.)

JO: Do you use a spreadsheet to do these analytical modeling?
VB: HSM, which is our heat sink modeling tool, and then for determining what velocity you have through the fins, the correct way of doing this is to come up with the flow pattern on your own. You go through all the formulas in the book and determine what the flow will be everywhere or figure out what CFD is giving you for the fan curve and check it with analytical modeling. You can look at pressure drop in there, look at the fan curve and see if you’re in the ballpark. You can also check other things in CFD, for example flow balance. Input the flow data into HSM and it will spit out what the thermal performance is for any given heat sink. HSM calculations are based on its internal formulas.

JO: We effectively have a proprietary internal tool. We’ve made a conscious decision to use it.
VB: To actually use it is unique. Not everybody would use it. A lot of people would skip this step and go straight to CFD. We use CFD too but we want to make sure that it’s on the right path.

JP: What do you see as the benefit of doing both analytical and CFD modeling?
VB: CFD, because it’s so easy to use, can be a tool that will lead you astray if you don’t check it because it’s very easy to use and the software can’t tell you if your results are accurate. If you do any calculation, you use a calculator. The calculator is never going to give you a wrong answer but just because you’re using a calculator doesn’t mean that you’re doing the math right. You want to have a secondary answer to verify that what you did is correct.

JP: What was the solution that you came up with for this particular challenge?
VB: We replaced these heat sinks with the heat pipe with maxiFLOW™, no heat pipe needed. One of the little tricks that I used was to off-set the heat sinks a little bit so that these fins are out here and so the airflow here would be kind of unobstructed. And I set this one a little lower so it would have some fins over here, not much, that would be unobstructed. The G600 configurations worked out with the junction temperatures being below what the maximum requirement was without having to use any heat pipes for the main components. There is also a note showing that one of the ancillary components was also below the max. Analytical modeling of that was within 10-11 percent.

The final PCB layout with maxiFLOW heat sinks covering the hottest components on both ends of the board. (Advanced Thermal Solutions, Inc.)

The final PCB layout with maxiFLOW heat sinks covering the hottest components on both ends of the board. (Advanced Thermal Solutions, Inc.)

As you noted, this was the worst-case scenario, going from right to left and you can see because it’s the worst-case scenario this tiny little component here that’s 14 watts that’s having all this pre-heated air going into it, it’s junction temperature was exactly at the maximum allowed. That’s not entirely great. We want to build in a little bit of margin but it was below what was needed.

The conclusion here was that maxiFLOW™ was able to provide enough cooling without needing to use the heat pipes and analytical calculation agreed to less than 20 percent. We would need to explore some alternate designs and strategies if we want to reduce the junction temperature even further because that close to the maximum temperature is uncomfortable. The other idea that we had was to switch the remaining heat sinks, the ones in the middle, which are straight fin, also to maxiFLOW™ to reduce pressure drop and to get more flow through this final component.

(Advanced Thermal Solutions, Inc.)

(Advanced Thermal Solutions, Inc.)

JP: If you have an idea like that, is it something that you broach with the customer?
VB: They really liked the result. If this was a project where the customer said, ‘Yep, we need this,’ then we would have said here’s the initial result and we have an additional strategy. At that point the customer would have said, ‘Yeah this is making us uncomfortable and we need to explore further’ or they would have said, ‘You know what? Fourteen watts is a max and I don’t know if we’ll ever go to 14 watts or the ambient we’re saying is 50°C but we don’t know that it will ever get to 50°C so the fact that you’re at max junction temperature at the worst-case scenario is okay by us.’

JP: Do you always test for the worst-case scenario?
VB: It’s always at the worst-case scenario. It’s always at the max power and maximum ambient temperature.

JP: Was this the first option that we came up with, using maxiFLOW™? Were there other options that we explored?
VB: Pretty much. The way that I approached it was doing the analytical first. You can generate 50 results from analytical modeling in an hour whereas it takes a day and a half for every CFD model – or longer. These numbers here were arrived at with analytical modeling; the height, the width, the top width, were all from analytical modeling, base thickness to measure spreading resistance, all of that was done on HSM and spreadsheets to say this will work.

JP: Do you find that people outside ATS aren’t doing analytical?
VB: No one is doing it, which is really bad because it’s very useful. It gives you a quick idea if it’s acceptable, if this solution is feasible.

To learn more about Advanced Thermal Solutions, Inc. consulting services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.

The Ultimate Heat Pipe Guide: Selection for Performance

What are Heat Pipes?

Heat Pipes have been called Heat Superconductors! In this engineering article we’ll talk about what a heat pipe, how they are made, compare them with heat sinks, and talk about performance in various thermal management applications.

Diagram of a basic heat pipeFigure 1 Schematic View of a Heat Pipe [1]

Heat pipes are transport mechanisms that can carry heat fluxes ranging from 10 W/cm2 to 20 KW/cm2 at a very fast speed. Essentially, they can be considered as heat super conductors. Heat pipes can be used either as a means to transport heat from one location to another, or as a means to isothermalize the temperature distribution.

The first heat pipe was tested at Los Alamos National Laboratory in 1963. Since then, heat pipes have been used in such diverse applications as laptop computers, spacecraft, plastic injection molders, medical devices, and lighting systems. The operation of a heat pipe is described in Figure 1.

Sections of A Heat Pipe

A heat pipe has three sections: the evaporator, the adiabatic, and the condenser. The interior of the pipe is covered with a wick, and the pipe is partially filled with a liquid such as water. When the evaporator section (Le) is exposed to a heat source, the liquid inside vaporizes and the pressure in that section increases. The increased pressure causes the vapor to flow at a fast speed toward the condenser section of the heat pipe (Lc). The vapor in the condenser section loses heat to the integral heat sink and is converted back to liquid by the transfer of the latent heat of vaporization to the condenser. The liquid is then pumped back to the evaporator through the wick capillary action. The middle section of the heat pipe (La), the adiabatic portion, has a very small temperature difference.

Chart showing pressure drop distribution in a heat sinkFigure 2 Pressure Drop Distribution in a Heat Pipe [1]

Figure 2 shows the pressure drop distribution inside a heat pipe. In order for the capillary force to drive the vapor, the capillary pressure of the wick should exceed the pressure difference between the vapor and the liquid at the evaporator. The graph also shows that if the heat pipe is operated against the force of gravity, the liquid undergoes a larger pressure drop. The result is less pumping of the wick with reduced heat transfer. The amount of heat transfer decrease depends on the particular heat pipe.

Structure of a heat pipe

  1. Metallic pipe: The metal can be aluminum, copper or stainless steel. It must be compatible with the working fluid to prevent chemical reactions, such as oxidation.
  2. Working fluid: Several types of fluids have been used to date. These include methane, water, ammonia, and sodium. Choice of fluid also depends on the operating temperature range.
  3. Wick: The wick structure comes in different shapes and materials. Figure 3 shows the profiles of common wick types: axial groove, fine fiber, screen mesh, and sintering. Each wick has its own characteristics. For example, the axial groove has good conductivity, poor flow against gravity, and low thermal resistance.
    Conversely, a sintering wick has excellent flow in the opposite direction of gravity, but has high thermal resistance.

Different Wick Structures in a Heat PipeFigure 3 Different Wick Structures – From top to bottom: Sintered powder, fine fiber, wrapped screen, axial groove

Table 1 shows experimental data for the operating temperature and heat transfer for three different types of heat pipes [1].

Heat Pipes With Different Constructions and Operating ConditionsTable 1: Heat Pipes with Different Structures and Operating Conditions [1]

Certain factors can limit the maximum heat transfer rate from a heat pipe. These are classified as follows:

  1. Capillary Limit: Heat transfer is limited by the pumping action of the wick
  2. Sonic Limit: When the vapor reaches the speed of sound, further increase in the heat transfer rate can only be achieved when the evaporator temperature increases
  3. Boiling Limit: High heat fluxes can cause dry out.
  4. Entrainment Limit: High speed vapor can impede the return of the liquid to the condenser

A heat pipe has an effective thermal conductivity much larger than that of a very good metal conductor, such as copper. Figure 4 shows a copper-water heat pipe and a copper pipe dipped into an 80oC water bath. Both pipes were initially at 20oC temperature. The heat pipe temperature reaches the water temperature in about 25 seconds, while the copper rod reaches just 30oC after 200 seconds. However, in an actual application when a heat pipe is soldered or epoxied to the base of a heat sink, the effective thermal conductivity of the heat pipe may be drastically reduced due to the extra thermal resistances added by the bonding. A rule of thumb for the effective thermal conductivity of a heat pipe is 4000 W/mK.

Experiment Comparing Speed of Heat Transfer Between a Heat Pipe and a Copper PipeFigure 4. Experiment Comparing Speed of Heat Transfer Between a Heat Pipe and a Copper Pipe [1].

Heat pipe manufacturers generally provide data sheets showing the relationship between the temperature difference and the heat input. Figure 5 shows the temperature difference between the two ends of a heat pipe as a function of power [2].

Temperature Difference Between the Evaporator and the Condenser in a Heat PipeFigure 5. Temperature Difference Between the Evaporator and the Condenser in a Heat Pipe [2]

Types of heat pipes

There are many heat pipe shapes in the market, but the most common are either round or flat. Round heat pipes can be used for transferring heat from one point to another. They can be applied in tightly spaced electronic components, such as in a laptop. Heat is transferred to a different location that provides enough space to use a proper heat sink or other cooling solution. Figure 6 shows some of the common round heat pipes available in the market.

Typical Round Heat Pipes in the Market.Figure 6. Typical Round Heat Pipes in the Market.

Flat heat pipes (vapor chambers) work conceptually the same as round heat pipes. Figure 7 shows a flat pipe design, they can be used as heat spreaders. When the heat source is much smaller than the heat sink base, a flat heat pipe can be embedded in the base of the heat sink, or it can be attached to the base to spread the heat more uniformly on the base of the heat sink. Figure 8 shows some common flat heat pipes.

Conceptual Design Schematic of a Flat Heat Pipe

Figure 7. Conceptual Design Schematic of a Flat Heat Pipe

Commonly Used Flat Heat Pipes

Commonly Used Flat Heat Pipes

Figure 8. Commonly-used Flat Heat Pipes

Although a vapor chamber might be helpful in minimizing spreading resistance, it may not perform as well as a plate made from a very high conductor, such as diamond. A determining factor is the thickness of the base plate. Figure 9 shows the spreading resistance for 80 x 80 x 5 mm base plate of different materials with a 10 x 10 mm heat source. The vapor chamber has a spreading resistance that is better than copper, but worse than diamond. However the price of the diamond might not justify its application. Figure 9 also includes the spreading resistance from the ATS Forced Thermal Spreader (FTS), which is equal to that of diamond at a much lower cost. The FTS uses a combination of mini and micro channels to minimize the spreading resistance by circulating the liquid inside the spreader.

Thermal Spreading Resistances for Different Materials

Thermal Spreading Resistances for Different Materials

Importance of an Heat Pipe

Heat pipes have a very important role in the thermal management arena. With projected lifespans of 129,000-260,000 hours (as claimed by their manufacturers), they will continue to be an integral part of some new thermal systems. However, with such problems as dry out, acceleration, leakage, vapor lock and reliable performance in ETSI or NEBS types of environments, heat pipes should be tested prior to use and after unsatisfactory examination of other cooling methods have been explored.

Have you got a question on heat pipes or their application? How about an interest in bringing ATS’s team of experienced thermal engineers into one of your projects?  Reach us by visiting ATS Heat Pipe Page  or email us at ats-hq@qats.com or give us a call at 781-769-2800

References:
1. Faghri, A. Heat Pipe Science and Technology Taylor & Francis, 1995.
2. Thermacore Internation, Inc., www.thermacore.com.
3. Xiong, D., Azar, K., Tavossoli, B., Experimental Study on a Hybrid

How Does the Wick and Orientation of a Heat Pipe Affect its Performance?

Heat pipes are used in a wide variety of applications, particularly in the aerospace, military and consumer electronics industries. A heat pipe is self-driven, two phase device used to transport heat from one end to the other. The liquid evaporates and turns to vapor on the heat pipe’s hot end (also known as the evaporator) where it then flows to the cold end (also known as the condenser) and liquifies. wick_type_structuresThe heat transfer ability of a heat pipe is determined by its diameter, fluid type, wick structure, and orientation. The Qpedia article, “How Wicks and Orientation Affect a Heat Pipes Performance” will review the different types of wicks and other factors that need to be considered when selecting a heat pipe for specific applications.

Heat Spreading with Copper, Silicon and Heat Pipes

Power dissipation is a drastic issue to be tackled due to the continued integration, miniaturization, compacting and lightening of electronics systems [1]. Heat spreaders are not only chosen for their thermal performance; other design parameters include weight, cost and reliability. Depending on the application, different priorities will influence the design parameters. For example, weight and reliability are important for a space application. This article covers heat pipe technology, including a discussion of the different types of heat pipes. Additionally, the article provides comparisons between aluminum, silicon, copper and heat pipe-based heat spreaders.

Tubular Heat Pipes
A heat pipe is a heat transfer device that uses two phase flow to transfer heat energy. A heat pipe system is composed of a sealed, evacuated container, partially filled with a liquid so that liquid/vapor equilibrium is obtained. A wick structure or a specific envelope shape enables efficient capillarity. Heat applied to the evaporator section by an external source is conducted through the pipe wall and wick structure where it vaporizes the working fluid. The resulting vapor pressure drives the vapor through the adiabatic section to the condenser, where it condenses, releasing its latent heat of vaporization to the heat sink. The capillary force created by the menisci in the wick pumps the condensed fluid back to the evaporator section. This provides the driving force for liquid in the heat pipe. The operating principle as described here is shown schematically in Figure 1. More details on wicks and the orientation dependency for the performance of heat pipes can be found in [2].

fig1

Figure 1

Flat Heat Pipes
In order to dissipate very high heat flux densities, the required heat sink must often be larger than the devices [3]. Temperature gradients occur in the heat sink base due to the heat spreading resistance in the material. The results are hot spots and a non-uniform heat flux at the heat sink level. Consequently, the heat sink performance is reduced. A method for lowering the spreading resistance in the heat sink base is to use higher conductivity materials as the base material, such as copper. Alternatively, materials with higher thermal conductivity than the heat sink can be embedded on the heat sink base. The added material can be copper or even diamond.

Yet another choice to the aforementioned material is the use of flat heat pipes. A flat heat pipe functions like a convectional tubular heat pipe, the main difference being the form the wick takes to enable liquid distribution over a wide surface area [4]. The operating principle is quite different because the evaporator and the condenser are on opposite faces of the heat pipe [3], as shown in Figure 2. As in a conventional heat pipe, the capillary wick transports the liquid to the heated region. The benefits of a flat heat pipe include multi- component array temperature flattening, multi-component array cooling and its additional use as a module wall or mounting plate [4].

fig2

Figure 2

fig3

Figure 3

Avenas et al. have published data comparing heat pipe performance with that of plain copper and plain silicon [3]. Each comparison features the same dimensions between the heat pipe and the plain solid material. Figure 3 shows that there is an average improvement of 56% against plain copper and an average 36% against plain silicon.

Micro Heat Pipes using Silicon as the Heat Pipe Material
Babin [5] states that a micro heat pipe is a heat pipe in which the mean curvature radius of the liquid-vapor interface is comparable in magnitude to the channel hydraulic radius. Unlike tubular and flat heat pipes, micro heat pipes do not contain a wick material. The capillary force necessary for transporting the condensate to the evaporator is attributed to the sharp edges in the grooves inside the heat pipe structure. Therefore, the design of the capillary structure is critical to the maximum heat transfer rate of the micro heat pipe. Triangular, rectangular, star and rhombus groove micro heat pipes have been explored in different applications.

fig4

Figure 4

Hopkins et al. [6], Plesch et al. [7] and Cao et al. [8] provide theoretical and experimental results on rectangular grooved copper heat pipes with water as their working fluid. Plesch et al. [7] and Launay et al. [9] have published data for silicon-based micro heat pipes using water or methanol as the working fluid. It was found that the effective conductivity of the heat pipe increased by 10 % [7] to 300 % [9] when compared with pure silicon.

fig5

Figure 5

The flat heat pipe discussed in the previous section had dimensions of 127 x 76 x 5 mm [3]. Micro heat pipes that have been discussed in the literature can be in the order of 50 x 50 x 1 mm [1]. A schematic of a micro heat pipe is shown in Figure 4. Figure 5 presents a top view of the capillary wick structure in the micro heat pipe.

The micro heat pipe discussed by Ivanova et al. [1] is a heat pipe with a capillary structure which is able to assure heat spreading from the dissipative components to the metallic frame in two directions. Silicon was chosen due to the critical weight requirement of the system. The heat pipe had a mass lower than 6 g. Integration of a micro heat pipe provides better transfer of the heat flux dissipated by the components to a cooler (e.g. cool box, heat exchanger) and then reduces the thermal resistance between the component and the cooler [1]. The temperature on the substrate is homogenized and the occurrence of hot spots is eliminated.

fig6

Figure 6

Figure 6 shows the maximum temperature at the chip level versus the input power for three configurations: an empty micro heat pipe, a silicon plate and a filled micro heat pipe. The water temperature in the copper cold plate is fixed at 50°C. The operational heat pipe has a capacity to spread more than 70 W/cm2 with a temperature on the resistor level less than 120°C. For the pure silicon spreader only 20 W/cm2 of heat flux can be achieved.

Summary
This article has discussed the use of heat pipe heat spreaders and micro heat pipes for heat spreading applications. Their performance was compared to that of pure silicon and copper heat spreaders. For heat pipe heat spreaders there was an average improvement of 56% and 36% respectively when compared to equivalent silicon and copper heat spreaders. When a micro heat spreader was used, the effective conductivity of the heat pipe increased from 10% [7] to 300% [9] when compared with pure silicon. A micro heat pipe design by Ivanova et al achieved 70 W/cm2 of heat dissipation where the equivalent silicon design was only able to dissipate 20 W/cm2. It has been shown that heat pipes are an interesting alternative to plain solid material heat spreaders. Their application becomes more common as the component size diminishes. Their use in space applications, where the outside ambient temperature is around -60°C, is highly recommended.
 

This article originally appeared in Qpedia Thermal eMagazine, Volume 6, Issue 11, November 2010.

round or flat heat pipes for electronics cooling

References:
1. Ivanova, M., Lai, A., Gillot, C., Sillon, N., Schaeffer, C., Lefèvre, F., Lallemand, M. and Fournier, E., Design, Fabrication and Test of Silicon Heat Pipes with Radial Microcapillary Grooves, SEMI-Therm, 2006.
2. Azar et al., How Wicks and Orientation Affect Heat Pipe Performance, Qpedia Thermal E- Magazine, August 2009.
3. Avenas, Y., Gillot, C., Bricard, A. and Schaeffer, C., One the Use of Flat Heat Pipes as Thermal Spreaders in Power Electronics Cooling, Power Electronics Specialists Conference, 2002.
4. Dunn, P. and Reay, D., Heat Pipes, Pergamon Press, 2nd Edition, 1978.
5. Babin, B., Peterson, G. and Wu, D., Steady-state Modeling and Testing of a Micro Heat Pipe, J. Heat Transfer, Vol. 112, 1990.
6. Hopkins, R., Flat Miniature Heat Pipes With Micro Capillary Grooves, Journal of Heat Transfer, Vol. 121, Feb. 1999.
7. Plesch, D., Bier, W., Seidel, D. and Schubert K., Miniature Heat Pipe for Heat Removal from Microelectronic Circuit, American Society of Mechanical Engineers, Vol. 32, 1991.
8. Cao, Y., Gao, M., Beam, J. and Donovan B., Experiments and Analyses of Flat Miniature Heat Pipes, Journal of Thermophysics and Heat Transfer, Vol. 11, 1997.
9. Launay, S., Sartre, V. and Lallemand, M., Experiments of Silicon Capillary Grooved Micro Heat Pipe, Applied Thermal Engineering, Vol. 24, 2004.

Understanding Loop Heat Pipes

Looped heat pipes (LHPs) are two-phase heat transfer devices that employ the same capillary pumping of a working fluid as used in conventional heat pipes. LHPs can transfer heat efficiently up to several meters at any orientation in the gravity field. When placed horizontally, this distance can extend to several tens of meters.

The development of the LHP was driven mainly by a limit of conventional heat pipes in which the wick system abruptly decreases its heat transfer capacity, if the evaporator is raised higher than the condenser. This need was acutely felt in aerospace applications where the heat generated by the electronics had to be transferred efficiently away for dissipation purposes. But the device needed to be much less sensitive to changes in orientation in the gravity field. Figures 1a and 1b show the schematic of an LHP [1].

The development of looped heat pipes dates from 1972.    Qpedia_0508_Loop_Heat_Pipes_Figure1Figure 1. Schematic of Principle of Operation of a Loop Heat Pipe [1, 2].

The first such device, with a length of 1.2m, a capacity of about 1 kW, and water as its working fluid, was created and tested successfully by the Russian scientists Gerasimov and Maydanik from the Ural Polytechnic Institute. With heat needing to be transported over a longer distance, and because the working fluid circulation in a heat pipe is directly proportional to the surface tension coefficient and inversely proportional to the effective pore radius of the wick, a different system for heat transport was required when the evaporator was above the condenser. This is shown in Figure 1.

The capillary head must be increased to compensate for pressure losses when the liquid is moving to the evaporator while operating against gravity. This can only be done by decreasing the effective pore radius of the wick. However, the increase in hydraulic resistance is approximately proportional to the square of the pore radius. As a result, it has not been possible to build a heat pipe of sufficient length that is capable of operating efficiently against gravity. Thus, there was incentive to develop LHPs, and they are now finding further application in modern electronics.

As stated, a number of limits impact the performance of an LHP. Qing et. al. [3] performed a detailed investigation of three key parameters on the performance of a looped heat pipe for use in cryogenics applications. This LHP is shown in Figure 2.

1) Effect of Wick Pore Size – It is well known that the maximum capillary pressure produced by the primary wick depends on both the effective pore size and the surface tension of the working fluid. In general, the smaller the pore size and the larger the surface tension, the higher the maximum capillary pressure. A smaller pore size will also result in larger flow resistance which will limit heat transfer capability. The pore sizes considered were 2 and 10 μm.
Figure 2. Schematic of an LHP for Cryogenics Application [3].
When the pore size of the primary wick is larger (10mm), the heat transfer capability of the LHP can reach 26 W only when a smaller reservoir (60cc) is used. Its ability to operate against gravity is greatly weakened. With a wick pore size of 2mm, the LHP can transfer a heat load of 26 W under horizontal orientation no matter what size reservoir volume is used.

Qpedia_0508_Loop_Heat_Pipes_Figure2Figure 2. Schematic of an LHP for Cryogenics Application [3].

2. Effect of Reservoir Size – It is interesting to see how the LHP will function with different reservoir sizes. As shown in Figure 3, the combination of gravity and reservoir size has a direct impact on the heat transfer capability of the LHP. Under adverse gravity, the heat transfer capability of the LHP is 12 W using the larger reservoir and only 5W using the smaller one.Qpedia_0508_Loop_Heat_Pipes_Figure3
Figure 3. Heat Transfer Capability of LHPs with 2mm and 10mm Pore Diameters in Horizontal Orientation [3].

3. Effect of Working Fluid – Fluids have different surface tensions that impact the heat transport capability of the LHP.

Figure 4 demonstrates this capability: Qpedia_0508_Loop_Heat_Pipes_Figure4
Figure 4. Heat Transfer Capability of an LHP When the Working Fluid is Oxygen [3].

Though not shown in Figure 4, when the working fluid is oxygen instead of nitrogen, the heat transfer capability can be up to 50 W under horizontal orientation with the other experimental conditions remaining the same.

LHP Applications
This discussion has highlighted the functionality and importance of design parameters on the performance of LHPs. While this discussion concerns an aerospace application, LHPs have been used for standard electronics as well. Maydanik gives several examples where miniature LHPs are used for microelectronics [1]. Figure 5 shows the “use of flat disk-shaped evaporators in LHPs. The scheme and the external view of such evaporators 10 and 13mm
thick, whose thermo-contact surface is made in the form of a flange 45 mm in diameter for fixing the heat source. The results of development of ammonia LHPs 0.86m and 1m long with a vapor and a liquid line 2mm in diameter equipped with such evaporators of stainless steel. In trials the devices demonstrated serviceability at any orientations in 1-g conditions. The maximum capacity was, respectively, 90–110 W and 120–160 W, depending on the orientation, and the value of the minimum thermal resistance 0.30 K/W and 0.42 K/W.”

Qpedia_0508_Loop_Heat_Pipes_Figure5
Figure 5. Photo and Schematic of Flat, Disk-Shaped Evaporators in an LHP [1].

Another design is shown in Figure 6, where miniature LHPs are made from stainless steel and copper and the working fluids are ammonia and water . The ammonia LHP has a 5mm diameter evaporator with a titanium wick, and 2mm diameter lines for vapor and liquid.. The water LHP is equipped with a 6mm diameter evaporator and 2.5mm diameter lines. The effective length of the devices is about 300mm.

Qpedia_0508_Loop_Heat_Pipes_Figure6
Figure 6. Miniature LHPs [1].

Each has a finned condenser, 62mm long, whose total surface is about 400cm2. The condensers are cooled by a fan providing an air flow rate of 0.64 m3/min, at a temperature of 22 ± 2°C.
Tests show that the maximum capacity of the ammonia LHP is 95 W at an evaporator wall temperature of 93°C. The maximum capacity for the water LHP was not achieved, but at the same temperature it was equal to 130 W. The minimum thermal resistance values of the LHP, 0.12 K/W and 0.1K/W, were obtained at heat loads of 70 W and 130 W, respectively. It should be noted that the ammonia LHP demonstrated a higher value of for heat transfer coefficient in the evaporator, which reached 78,000 W/m2K at a heat flow density of 21.2 W/cm2 at the surface of an interface with an area of 4 cm2. For the water LHP, these values were, respectively, 31,700 W/m2K and 35 W/cm2. In this case, at the surface of the evaporator’s active zone, the heat flow density was much higher. For the ammonia LHP it was 44.5 W/cm2, and for the water was 69.1 W/cm2 [3].

Qpedia_0508_Loop_Heat_Pipes_Figure7
Figure 7. Photo and Schematic of a CPU Cooler Based on an LHP [4, 5].

Another example of LHPs in microelectronics is shown in Figure 7. Here, an LHP was designed for cooling a 25-30 W processor with a total weight of 50g. This LHP was based on copper-water with an evaporator diameter of 6mm.
In conclusion, LHPs may resolve many of the drawbacks seen in conventional heat pipes and provide additional capabilities. As shown by Maydanik, the capillary mechanism, in conjunction with the reservoir size and the use of different fluids, can bring significant advantages that may not readily be seen in heat pipes. Some of these include:

  • the use of fine-pored wicks,
  • maximum decrease in the distance of the liquid motion in the wick,
  • organization of effective heat exchange during the evaporation and condensation of a working fluid, and,
  • maximum decrease in pressure losses in the transportation (adiabatic) section.

Along with the advantages gained from LHPs, the use of liquids in electronics and potential operational instability must be considered carefully. Operational instability, if not managed, could conceivably create thermal cycling on the electronics component being cooled. As with heat pipes,operational dry out or the loss of fluid due to leakage could render the LHP inoperable. Otherwise, LHPs appear to be an attractive supplement to the arsenal of cooling options available to the design engineer. ■

References:
1. Maydanik, Y.., Loop Heat Pipes, Applied Thermal Engineering, 2005.
2. Muraoka, I., Ramos, F., Vlassov, V., Analysis of the Operational Characteristics and Limits of a Loop Heat Pipe with Porous Element in the Condenser, International Journal of Heat and Mass Transfer, V44, 2001.
3. Mo, Q., Jingtao, L., Jinghui, C., Investigation of the Effects of Three Key Parameters on the Heat Transfer Capability of a CLHP, Cryogenics V47, 2007.
4. Chang, C., Huang, B., Maydanik, Y., Feasibility of a Mini LHP for CPU Cooling of a Notebook PC, Proc. of 12th Int. Heat Pipe Conference, Moscow, Russia, May 2002.
5. Pastukhov, V., Maydanik, Y., Vershinin, C., Korukov, M., Miniature Loop Heat Pipes for Electronic Cooling

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