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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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Qpedia Thermal eMagazine June 2013

Qpedia Thermal eMagazine June 2013

Qpedia Thermal eMagazine, Volume 7, Issue 6, has just been released and can be downloaded at: http://www.qats.com/Qpedia-Thermal-eMagazine/Back-Issues.

This month’s featured articles include:

Enhancing Heat Sink Performance Using Thermoelectric Coolers

With the increase in the power dissipation of components and the parallel reduction of their size, engineers and researchers across the globe have been predicting that the era of air cooling might come to an end. Even though in some applications, with very high power dissipations such as IGBTs, air cooling may not be adequate and liquid cooling is a must; air cooling will continue to be the first choice for most electronic cooling applications for many years to come. Advances in air cooling continue to extend its use and the implementation of thermoelectric coolers (TECs) in heat sink applications is one such effort.

Immersion Liquid Cooling for Servers in Data Centers

Data center designers and operators have invented many ways to improve the data center’s thermal efficiency, such as optimizing the rack layout and air conditioner location, separating cold aisles and hot aisles, optimizing the configuration of pipes and cables in under-floor plenum, introducing liquid cooling to high-power severs. While the above methods can improve the data center heat load management, they cannot dramatically reduce the Power Usage Effectiveness (PUE). This article reviews two relatively new solutions: active single-phase immersion cooling technology proposed by Green Revolution Cooling (GRC) and a passive two-phase immersion cooling technology proposed by the 3M Company.

Industry Developments: Piezoelectric Cooling

Piezoelectric fans and jets must overcome various materials, thermal and mechanical challenges to become widely used in electronics cooling, but because they consume just 1/150 of the electricity of circular fans, run with little noise and have no parts that will wear out, they remain of great interest. In this article, a number of issues are addressed, including the inverse effect of the piezoelectric phenomena and dual piezoelectric cooling jets.

Technology Review: Innovative Cold Plate Designs, 2007 – 2012

In this issue our spotlight is on innovative cold plate designs. There is much discussion about its deployment in the electronics industry, and these patents show some of the salient features that are the focus of different inventors.

& Cooling News featuring the latest product releases and buzz from around the electronics cooling industry.

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Some Basic Principles of Wind Tunnel Design

Wind tunnels generate uniform air flows, with low turbulence intensity, for thermal and hydraulic testing. These devices have been around for more than a century, and are used in many industries, including aerospace, automotive, and defense. They also play a key role in electronics thermal management. Wind tunnels are made in different shapes and sizes, from just 30 cm long to large enough to contain a passenger airplane. But the basic idea behind all wind tunnels is universal.

There are two basic kinds of wind tunnels. One is the open type, which draws its air from the ambient and exits it back to the ambient. This kind of wind tunnel provides no temperature controls. The air follows the ambient temperature. The second type of wind tunnel is the closed loop wind tunnel, whose internal air circulates in a loop, separating it from outside ambient air. The temperature in a closed loop wind tunnel can be controlled using a combination of heaters and heat exchangers. Air temperatures can be varied from sub-ambient to over 100oC. Figure 1 shows a schematic of a closed loop wind tunnel.

In general, closed loop wind tunnels are made with the following sections:

1-Test section

2-Settling chamber

3-Contraction area

4-Diffuser

5-ÂBlower assembly

6-Heater/heat exchanger assembly

Figure 1. Schematic of an ATS Closed Loop Wind Tunnel.

A good quality wind tunnel will have a flow uniformity of 0.5-2% and turbulence intensity of 0.5-2%. It should provide temperature uniformity within 0.1-0.5oC at the inlet of the test section [1].

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To achieve uniform, high quality flow in the test section, the settling chamber and the contraction area are used to smooth the flow. The role of the settling chamber, which is upstream of the contraction area, is to eliminate swirl and unsteadiness from the flow. The settling chamber includes a special honeycomb and a series of screens. As long as a flows yaw angles are not greater than about 10o, a honeycomb is the most efficient device for removing swirl and lateral velocity variations and to make the flow more parallel to the axial axis [2]. Large yaw angles will cause honeycomb cells to stall, which increases the pressure drop and causes non-uniformity in the flow. For large swirl angles, screen meshes should be placed before the honeycomb. For swirl angles of 40o, a screen with a loss factor of 1.45 will reduce yaw and swirl angles by a factor of 0.7. Several screens are needed upstream of the honeycomb to bring the swirl down to 10o.

Using a honeycomb will also suppress the lateral components of turbulence. Complete turbulence annihilation can be achieved in a length of 5-10 cell diameters [2]. Honeycombs are also known to remove the small scale turbulence caused by the instability of the shear layer in front of them. This instability is proportional to the shear layer thickness, which implies a short honeycomb has a better ratio of suppressed turbulence to that generated.

Screens break up large eddies into smaller ones which decay faster. They lower turbulence drastically when several screens are placed in a row. Screens also make flow more uniform by imposing a static pressure drop which is proportional to velocity squared. A screen with a pressure drop coefficient of 2 removes nearly all variations of longitudinal mean velocity. Low open area screens usually create instabilities. In general, screens should have openings larger than 57%, with wire diameters about 0.14 to 0.19 mm. Sufficient distance is needed between multiple screens to stabilize static pressure from perturbation. This distance is typically a percentage of the settling chamber diameter.

The contraction area is perhaps the most important part of a wind tunnel’s design. Its main purpose is to make the flow more uniform. It also increases the flow at the test section, which allows flow conditioning devices to be at lower flow section with less pressure drop. Batchelor used the rapid distortion theory and estimated the variation in mean velocity and turbulence intensity [3]

A considerable number of shapes have been investigated for contraction, including 2-D, 3-D and axisymmetric shapes with various side profiles.

The shape of the contraction can be found using potential flow analysis. Consider the axisymmetric contraction shown in Figure 2 [4]

Figure 2. Schematic of an Axisymmetric Contraction [4].

The design of a wind tunnel is a lengthy process and, as shown above, it requires extensive knowledge and experience in both theory and construction. A novice might attempt to construct a tunnel, but considering the time spent, it might not be justified economically. Wind tunnel design also depends on economic and space constraints. Larger wind tunnels allow more space to have all the conditioning elements in place. A space-constrained wind tunnel must compromise some features at the cost of reduced flow quality, but can still be acceptable for practical engineering purposes.

References

  1. Azar, K., Thermal Measurements in Electronics Cooling, Electronics Cooling Magazine, May 2003.
  2. Bell, J. and Mehta, R., Design and Calibration of the Mixing Layer and Wind Tunnel, Stanford University, Department of Aeronautics and Astronautics, May 1989.
  3. Batchelor, G., The Theory of Homogeneous Turbulence, Cambridge University Press, 1953.
  4. Edson, D. and Joao, B., Design and Construction of Small Axisymmetric Contractions, Faculdade de Engenharia de Ilha Solteira, Brazil, 1999.