Tag Archives: electronics cooling

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.”

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.

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].

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. ■

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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ATS Expands in the COTS Market with its Wind Tunnel Sale to the US Navy

ATS has continued to expand in the COTS industry because of its expertise in resolving thermal challenges through its consulting services, cooling solutions, and thermal test instruments. Most recently, the CWT-107 open loop wind tunnel was sold to the United States Navy for use in their Research Development Labs.



CWT-107 Open Loop Wind Tunnel

The CWT-107 is a research quality wind tunnel designed for multiple PCB and component level testing. It is used in air flow characterization and flow visualization, thermal resistance measurements and generation of P-Q curves. The large test section (24″ x 2″ x 7″) is designed to accommodate multiple PCBs, as seen in a typical ATCA chassis. The wind tunnel can also be used to characterize different heat sink sizes for natural and forced convection cooling. Additionally, multiple heat sinks can be tested side by side to determine their thermal performance in the same environment.

The following video is a brief demonstration and walk through of the CWT-107 Open Loop Wind Tunnel:

To learn more about the CWT-107 and how ATS products can be utilized in COTS applications, please visit www.qats.com.

Want to keep up to date on ATS products, services, free engineering webinars and our monthly Qpeda Thermal eMagazine? Subscribe to ATS

ATS Officially Granted Access for Government Contracts

COTS Advanced Thermal Solutions, Inc., (ATS), is now officially in active status with the United States Federal Governments System for Award Management (SAM). The System for Award Management (SAM) is the Official U.S. Government system that consolidated the capabilities of CCR/FedReg, ORCA, and EPL, for vendors doing business with the Federal government.

Military electronics systems include communications, weapons, reconnaissance, targeting and evasion, delivery and functionality. Power dissipation in these devices is at an all-time high and thermal management is becoming even more of a critical issue. Along with power and heat challenges, engineers must factor in remote locations, rough handling and temperature extremes, in which systems must continue to function reliably. Further, many military systems require small form factors leaving less room for conventional heat dissipation solutions. ATS has already received its several contracts with the US Government and expects to expand its current role in the military, defense, and aerospace markets.

Want to keep up to date on ATS products, services, free engineering webinars and our monthly Qpeda Thermal eMagazine? Subscribe to ATS

Qpedia, Official Media Sponsor of coolingZONE-13, Offers 30% Discount on Esteemed Book Series


Qpedia, the official media sponsor of coolingZONE-13, has just announced a special promotion offering a 30% discount on the widely read Qpedia Book Series. The promotion will take place at coolingZONE-13, the Thermal Management Industry International Summit in Boston, Massachusetts, October 21st -23rd, 2013. Additionally, the books will be available to purchase through coolingZONE’s online bookstore , with a 25% discount, until the conference ends on October 23. These books provide an expert resource for engineering professionals, students, educators and others who want to learn the latest theories and applications in the electronics cooling field.

The Qpedia Book series is a four volume set of highly technical articles, written and published by Advanced Thermal Solutions, Inc. The authors include Dr. Kaveh Azar, the company’s president and CEO; and Dr. Bahman Tavassoli, its chief technologist. Both Drs. Azar and Tavassoli are internationally recognized experts in the field of thermal management.

The four volume set contains over 250 in-depth articles filled with technical information, fundamental calculations and thoughtful analysis, printed in rich color in a hardbound book. They discuss the most critical areas of electronics cooling, covering a wide spectrum of topics in the telecom, aerospace and defense, embedded computing, medial, automotive, and semiconductor industries.

The articles, inspired by real-life scenarios, solve the thermal management challenges that today’s engineer is faced with. Drawn from personal experience, the veteran authors pass on knowledgeable examples of problem solving techniques that can be applied by all thermal and mechanical engineers. To order the complete book set, or individual volumes, please visit www.coolingzone.com/cart.

coolingZONE-13, the Thermal Management Industry International Summit, is a leading conference for all technical professionals in thermal management, electronics cooling, heat transfer and energy transport fields. The conference gathers recognized experts and technology companies in thermal management, providing solutions for the thermal engineering challenges of the future. Dr. Kaveh Azar, CEO of Advanced Thermal Solutions and Qpedia’s Editor-in-Chief, will be opening the conference with a keynote address on “The State of the Art in Thermal Management – From Vacuum Tubes to Super Computers”. Other keynote addresses include “Redefining Engineering as a Profession of Innovation” by Dr. Vincent Manno of Olin College and “Galinstan-Based Cooling of Microelectronics: Beyond Tuckerman and Pease?” by Dr. Marc Hodes of Tufts University. To learn more about coolingZONE-13, and to register for the conference, please visit: www.coolingzone.com

Next Webinar Shows How to Properly Measure and Analyze Temperatures in Electronic Systems

ATS WebinarsThe upcoming webinar “How to Perform and Understand Temperature Measurement in Electronic Systems” will be held this Thursday, September 12, 2013 at 2pm ET. The free, prerecorded technical presentation will deepen attendees understanding of the importance of temperature measurement in electronic systems. Attendees will learn about each of the instruments needed for measuring temperature and interpreting temperature data. Key locations will be identified where thermal testing should be conducted in order to obtain the most accurate and actionable results.

The webinar will be taught by Dr. Kaveh Azar, CEO of Advanced Thermal Solutions, Inc. Since 1985, Dr. Azar has been an active participant in the electronics thermal community and has served as the organizer, general chair and the keynote speaker at national and international conferences sponsored by ASME, IEEE and AIAA. In addition, he has been the recipient of the IEEE SEMITHERM Significant Contributor Award in the thermal management of electronics systems.

Dr. Kaveh Azar

Dr. Kaveh Azar

Dr. Azar has been an invitee to national bodies such as NSF, NIST and NEMI for organizing government and industry research goals in electronics cooling. He has also been an adjunct professor at a number of universities, including Northeastern University, and lectures worldwide in analytical and experimental methods in electronics cooling.

He holds more than 36 national and international patents, and has published more than 75 articles, 3 book chapters and a book entitled Thermal Measurements in Electronics Cooling. Dr. Azar has also served as the editor in chief of Electronics Cooling Magazine, the premier resource for practitioners in the field of electronics thermal management, from the publications founding in 1995 to 2006.