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Digital News Gets Support from ATS

With Thermal Engineer day approaching (7/24) we here at ATS would like to thank all of the PR firms and digital news magazines who covered our new clipKIT campaign.

thermal technology - digital news - electronics businessAs a token of our appreciation, we have provided a link to our customers and viewers to download our clipKIT data sheet for all your attachment needs. HERE.

 

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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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.

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ATS’ Dr. Camil Ghiu to Present at coolingZONE-13

CZ13_speaker_camil

ATS’ own, Dr. Camil Ghiu, will be presenting “Driving Towards 0.1oC/W In Compact Air Cooled Heat Sinks: Advancements In Flow Management And Air Jet Impingement Cooling” at the Thermal Management Industry International Summit: coolingZONE-13. The Summit will be held in Boston, Massachusetts, October 21-23, 2013.

Considering the widespread use of compact systems, such as the 1U platform, and the drive to reduce costs from the system and deployment view points, air cooling continues to be sought for thermal management of such systems. The decrease in size of the new generation of electronic devices imposes a severe constraint on their incorporated thermal management devices. In this context, the development of low thermal resistance heat sinks (0.1 oC/W) for cooling compact electronics systems (1U form factor) continues to be a challenge for the thermal management community.

Dr. Ghiu’s presentation will present recent developments in designing compact heat sinks using advanced air flow management. Two main approaches will be presented, including heat sink design implementing jet impingement and sectional heat sinks. Both design approaches have been explored at ATS, and the experimental data and simulation results will be presented for further discussion.CZ13_HP

coolingZONE-13 is the premiere engineering conference for the thermal management industry. Leading experts from academia and the electronics cooling industry will present emerging technologies in the most crucial areas of thermal engineering. A wide range of topics will be discussed, including liquid cooling, advanced heat sink and heat pipe design, thermal interface materials, data center cooling and analysis, CFD, and vapor compression cooling. Keynote speakers this year are Dr. Vincent Manno of Olin College, Dr. Marc Hodes of Tufts University and Dr. Kaveh Azar, CEO of Advanced Thermal Solutions, Inc. Additional speakers and exhibitors from Laird, CD-Adapco, Aavid, Cradle-CFD, Schneider Electric, and Future Facilities will also be presenting at the conference.

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ATS Offers Arrow Customers a Half-Day of Free Access to its Thermal Characterization Lab

arrow_ats

As part of the new distribution agreement between Arrow Electronics and Advanced Thermal Solutions, Inc., ATS is offering a half-day of free, no-obligation use of its unique Thermal Characterization Laboratory to Arrow customers. The Thermal Characterization Lab, located at ATS headquarters in Norwood, MA, allows engineers to perform thermal testing on heat sinks, fans and fan trays, PCBs, blades, enclosures, or complete systems. Experienced engineers, board and system designers can perform the tests themselves, or consult with an ATS thermal engineer at no cost during their 4 hours of laboratory time.

ATS’ Thermal Characterization Lab features a full range of research-quality instruments, including open and closed loop wind tunnels, for ambient and elevated temperature testing, all with PC-driven controls and automated data collection. The lab is also outfitted with a full array of the company’s sensor systems and thermocouples, which can be used to characterize electronic products under variable airflow and temperature conditions.

Liquid Crystal Thermography

In addition, the lab also features a JEDEC approved component thermal testing facility for conducting multitude of device level testing per JEDEC standards. The facility also provides a complete liquid crystal and IR thermography systems for non-invasive temperature mapping to 0.1oC with one micron-level spatial resolution; and a liquid cooling facility for complete testing and characterization of cold-plates, cooling effect and proof of concept testing.

 

“Most of today’s electronics have thermal situations that can turn into big problems if left alone. The easiest, lowest cost way to manage this is to conduct an accurate thermal characterization of the problem at hand,” said Kaveh Azar, Ph.D., President and CEO of Advanced Thermal Solutions, Inc. “If you have the right facility and associated know-how, you can often complete your test in a half-day, then you can readily assess what is the best thermal solution for your application. For engineers short on time and resources, we believe this free use of ATS’ Thermal Characterization Lab could be very helpful.”

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To contact ATS for more information on this opportunity, please call 781-769-2800, email ats-hq@qats.com or visit www.qats.com.