High power systems of various kinds have become even more widley deployed than in the past. From 15kW IGBT power systems, to 50kW high density power converters, to even higher wattage for computing racks. As the need for applications for systems with this power has grown, so too has the thermal management solutions that allows these systems to operate at their intended performance levels.
This webinar covers the newest strategies and technologies for these very high power applications. In this technology review webinar attendees will learn:
How the integration of the thermal architecture with electronic architecture can can significantly improve the effectiveness of thermal management.
The use of passive two phase thermosyphon loops for cooling data center racks
How to passively cool outdoor 7.2kW power supply electronics enclosures
The application of a closed rack cooling system based on pulsating heat pipes
The development of a loop heat pipe with kW-class heat transport capability
A new class of graphene enhanced heat pipe with heat dissipation characteristics 3.5x higher than copper
Using multi-stage heat pipe loops for cooling high heat density data centers
The application of pin finned cold plates for cooling high power density converters (HPDC)
Many more new and latest developments in thermal management for high power systems and components will be reviewed.
This webinar, presented by thermal management expert Dr. Kaveh Azar, Ph.D., will address all these issues and more. The webinar is one hour in length with time for questions and answers afterwards online and after the webinar concludes.
Most blade
servers for data and telecommunication systems use air to cool the high-power
chips inside. As the power level of these chips keep increasing, the pressure
is on thermal engineers to design ever higher performance air-cooled heat
sinks. In recent years, advancements in manufacturing of thinner heat pipes and
vapor chambers have enabled engineers to integrate the heat pipes and vapor
chambers into the blade server heat sinks.
A heat pipe
is a device with high thermal conductance that can transport large amounts of
heat with a small temperature difference between its hot and cold ends. The
idea of a heat pipe was first proposed by Gaugler [1]. However, only after its
invention by Grover [2, 3] in the early 1960s, were the remarkable properties
of heat pipes realized by scientists and engineers. It is now
widely used to transport heat from one location to another location or to smooth
the temperature distribution on a solid surface.
A heat pipe is a self-driven two-phase heat transfer device. A schematic view of a heat pipe is shown in Figure 1. At the hot section (evaporator), the liquid evaporates and turns to vapor. The vapor flows to the cold section (condenser) and liquefies there. The liquid is driven back from the cold section to the hot section by a capillary force induced by the wick structure. By using boiling and condensation, the heat pipes can transfer and spread the heat from one side to another side with a minimum temperature gradient.
Figure 1. Typical heat pipe. [4]
Vapor chambers are flat heat pipes with very high thermal conductance. They have flat surfaces on the top and bottom sides. See Figure 2, which can be easily attached to a heat source and a heat sink.
Figure 2. Vapor chamber. [5]
Just
like heat pipes, vapor chambers use both boiling and condensation to maximize
their heat transfer ability. A vapor chamber generally has a solid metal
enclosure with a wick structure lining the inside walls. The inside of the
enclosure is saturated with a working fluid at reduced pressure. As heat is
applied at one side of the vapor chamber, the fluid at locations close to the heat
source reaches its boiling temperature and vaporizes. The vapor then travels to
the other side of the vapor chamber and condenses into liquid. The condensed
fluid returns to the hot side via the gravity or capillary action, ready to
vaporize again and repeat the cycle.
In
electronics cooling, heat pipes are generally used to move the heat from
electronics to heat dissipation devices. For example, in a desktop computer,
multiple heat pipes are used to transfer heat from a CPU to an array of cooling
fins, which dissipate the heat to ambient environment through convection. Vapor
chambers are generally used to spread heat from a small size device to a larger
size heat sink, as it is shown in Figure 2. If used in server heat sinks, the
heat pipes and vapor chambers are both used to spreading the heat due to the
low profile and large footprint of the heat sinks.
Compared
to copper heat spreaders, heat pipes and vapor chambers have the following
merits.
First,
they have a much higher effective thermal conductivity. The pure copper has a
thermal conductivity of 401 W/m°C and the best conductive material of diamond
has a thermal conductivity of 1000-2000 W/m°C. The effective thermal
conductivity of a well-designed heat pipe and vapor chamber can exceed 5000 W/m°C,
which is an order of magnitude higher than that of pure copper. Second, the
density of the heat pipe and vapor chamber is much lower than that of copper.
Due to its hollow structure, the heat spreaders made by vapor chambers are much
lighter than those made of copper. These properties make them the ideal
candidate for high heat flux and weight sensitive heat spreading applications.
Dynatron Corporation is an electronic cooling provider specializing in heat sink for servers. This article compares the thermal performance of its server heat sinks, some of which have integrated vapor chambers. Figure 3 shows the photos of two Dynatron 1U passive server heat sinks for Intel’s Sandy Bridge EP/EX Processors. The R12 is made of pure copper with skived fins. The R19 has a vapor chamber base and stacked copper fins. The heat sink specification is listed in Table 1. The R19 is 150g lighter than the R12.
Figures
4 and 5 show the thermal performance of R12 and R19 at different flow rates. At
10CFM, both heat sinks have a thermal resistance of 0.298ºC/W. When the flow
rate increases to 20CFM, the R19’s thermal resistance is 0.218ºC/W, which is
0.020ºC/W lower than that of R12.
Figure 6 shows the photos of two Dynatron 1U active server heat sinks for Intel’s Sandy Bridge EP/EX Processors. The R18 is made of copper with skived fins. The R16 has vapor chamber base and stacked copper fins. Both heat sinks use same blower. The heat sink specification is listed in Table 2. The R16 is 90g lighter than the R18.
Figure 6. Dynatron active heat sinks R18 (left) and R16 (right). [6]
Table 2. Dynatron active heat sink specification. [6]
Figures 7 and 8 show the thermal performance of R18 and R16 at different blower speeds. At 3000RPM, the R18 and R16 heat sinks have thermal resistance of 0.437ºC/W and 0.393ºC/W, respectively. When the blower speed increases to 6000RPM, the R18’s thermal resistance is 0.268ºC/W and the R16’s thermal resistance is 0.241ºC/W. The R16 is constantly able to outperform the R18 at different blower speeds and its thermal resistance is 10% lower than R18.
Figure 7. Dynatron R18 heat sink performance. [6]
Figure 8. Dynatron R16 heat sink performance. [6]
The
comparison of the Dynatron heat sinks shows that heat sinks with vapor chambers
have a slight thermal edge vis-a-vis its copper counterparts even though they
are light. This is true for both passive and active heat sinks.
Glover et al., for Cisco, have tested different heat sinks either with embedded heat pipes or vapor chambers for their servers and published their findings [7]. They tested five different heat sinks from different vendors, who utilized different manufacturing technologies to fabricate the heat sinks. The five heat sinks are similar in size: 152.4 x 101.6 x 12.7mm. Table 3 summarizes the physical attributes of these five heat sinks.
Figure 9-11 shows the three vapor chamber heat sinks with different vapor chamber structures and fin designs. Heat sink A-1 is an extruded aluminum heat sink with a vapor chamber strip. The 40 mm wide vapor chamber strip is embedded in the center of the base. It is the lightest one among five tested heat sink. Heat sink B-1 and C-1 have full base size vapor chamber and aluminum zipper fins.
Figure 9. Heat sink with vapor chamber A-1. [7]
Figure 10. Heat sink with vapor chamber B-1. [7]
Figure 11. Heat sink with vapor chamber C-1. [7]
Figures 12-13 show the two heat sinks with embedded heat pipes. Heat sink C-2 has heat pipes embedded inside its aluminum base. It uses zipper fins and has a copper slug in the middle of the base. Heat sink D-1 has three flat heat pipes embedded in its base. It has a copper plate as base.
Figure 12. Heat sink with heat pipes C-2. [7]
Figure 13. Heat sink with heat pipes D-1. [7]
Glover et al. tested the five heat sinks at different mounting orientation and air velocity. Table 4 presents the summary results of the heat sinks at 3m/s approach air velocity. The tested heat sinks were mounted horizontally with heater sources underneath the heat sink bases.
Table 4. Heat Sink Performance at 3 m/s with horizontal mounting position and bottom heating. [7]
The C-1 heat sink has the lowest thermal resistance; thus, its values are used as the benchmark for other heat sinks. The performance of heat sinks is purely design dependent. For vapor chamber heat sinks, the thermal resistance value varies from 0.19 to 0.23°C/W for 30 W of power. For heat sinks with heat pipes, the C-2 heat sink has a thermal resistance of 0.23°C/W, which matched with that of A-1 and B-1.
The D-1 heat sink has the highest thermal resistance, which is the result of inferior design and manufacture. However, the D-1 heat sink still has relatively low thermal resistance when it is compared to a regular heat sink without a heat pipe and vapor chamber.
Figure 14 shows the thermal resistance of the five heat sinks for 60W of input power at different air velocities. The C-1 heat sink performs best for all velocities and the D-1 heat sink’s performance is the worst.
Figure 14. Heat sink thermal resistance at 60 W. [7]
The pressure drop across the heat sink at different air velocities was also measured and the results were plotted in Figure 15. The B-1, C-1 and C-2 heat sinks have similar fin structures. Therefore, their pressure drop is similar, too. The pressure drop of the A-1 and D-1 heat sinks are similar and higher than the other heat sinks. This is because the A-1 heat sink has thicker fins and the D-1 heat sink has a thicker base.
Figure 15. Heat sink pressure drop. [6]
Because the heat pipes and vapor
chambers use capillary force to drive liquid back from the condensation section
to the evaporation section, their thermal performance is prone to orientation variation.
Glover et al. also investigated the effects of the mounting orientation on the performance
of the five heat sinks. They found the effect of the orientation is design
dependent and is the result of both the wick structure and the entire heat sink
assembly construct.
The heat sink specification from
Dynatron Corporation and the test results from Cisco, show that the server heat
sinks with embedded heat pipes or vapor chamber have a better thermal
performance than their copper counterparts. The heat sinks with embedded heat
pipes or vapor chamber are also lighter than the pure copper heat sinks, which
make them more suitable for applications which are weight sensitive. If the
cost of such heat sinks is justified, they are definitely good candidates for
server cooling applications.
References
Gaugler, R. S., US Patent 2350348, Appl. 21 Dec, 1942. Published 6 Jun. 1944.
For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.
Advanced Thermal Solutions, Inc. (ATS) is hosting a series of monthly, online webinars covering different aspects of the thermal management of electronics. This month’s webinar will be held on Thursday, Nov. 29 from 2-3 p.m. ET and will cover the role of heat pipes and vapor chambers in heat transfer. Learn more and register at https://qats.com/Training/Webinars.
(This article was featured in an issue of Qpedia Thermal e-Magazine, an online publication dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine.)
The discussion generally arises as to what would be the heat pipe performance as a function of liquid fill ratio and vacuum. Before we show some results, let’s see how a heat pipe works. The thermodynamic cycle of a heat pipe is shown in figure 1 in a T-S diagram. [1]
Fig. 1 – Thermodynamic cycle of a heat pipe. [1]
Liquid at state 1 enters the evaporator and after absorbing the heat vaporizes to a mixture at 2 or to a saturated vapor at 2’. This vapor travels through the length of the heat pipe and enters the condenser at state 3. This vapor after losing its heat in the condenser exits at state 4 which is saturated liquid and upon travelling through the wick loses its temperature until it reaches point 1, which the cycle begins. If one looks at the phase diagram for a liquid, for example water in figure 2, it is apparent that the state of the liquid should be very close to the liquid vapor line in order for the liquid to promptly changes phase from liquid to gas upon heating. The triple point of water is at 4.58 Torr at temperature of 0.0075℃.
Fig. 2 – Phase diagram of water.
The state of liquid pressure should be in the region below atmosphere (vacuum) and above the complete vacuum. To obtain lower operating temperature the heat pipe pressure should be decrease and vice versa. The state of liquid after the condenser has to be saturated liquid, so the wick can create the liquid motion.
Lin, et al. [2] have shown that maximum heat transfer in a heat pipe is an exponential function of vacuum pressure according to the following formula:
Where,
Qmax,0 = maximum heat transfer at 0 pressure (no condensable gas)
PNCG = pressure of the non-condensable gas
∆Pcg = pressure drop difference between capillary and gravity
This equation clearly shows that by decreasing vacuum pressure, Qmax increases.
Another important factor is the amount of liquid in the heat pipe which is commonly called the fill ratio or inventory. If there is too much liquid, evaporation will not happen or delayed, and if there is not enough liquid, the dry out condition will happen. The rule of thumb is the volume of the liquid should be higher than the volume of the pore volume of the wick.
Figure 3 shows that as the pressure decreases from 10 Torr to 1 Torr the Qmax increases. The graph also shows that as the inventory (fill ratio) increases from 0.7 ml to 1.1 ml, Qmax peaks at 0.8 ml. This corresponds to a fill ratio of 26.4%, which is the ratio of the liquid volume to total volume of the heat pipe when it is empty. This graph shows the importance of fill ratio. If the fill ratio is not optimized as is shown for example for 1 Torr, Qmax drops from 8 W to 4 W, a 50% drop that can be catastrophic for the application. Mozumder et al. [3], in their experiment, measured the thermal resistance of a heat pipe for different fill ratios and power.
Fig. 3 – Qmax as a function of vacuum pressure and fill ratio for a heat pipe. [2]
Figure 4 shows that as the fill ratio increases from dry run to 85% (in their experiment fill ratio is defined as the volume of liquid to the volume of the evaporator section), thermal resistance decreases, but then increase with further increase of fill ratio.
Fig. 4 – Heat pipe thermal resistance as a function of fill ratio. [3]
The aforementioned arguments demonstrate that the fill ratio and vacuum pressure are very important in the proper design of a heat pipe. There is not much data in the literature about the effect of these two factors on the performance of the heat pipe. And it appears that most heat pipe manufacturers either resort to a try and error procedure or use the information from past experience. This topic needs further research.
Advanced Thermal Solutions, Inc. (ATS) is an industry-leader in heat pipe technology and has recently expanded its offering of high-performance, off-the-shelf heat pipes to provide the broadest selection on the market. Use the heat pipe selection tool to find the right fit for your project and avoid the cost and time for custom solutions. Learn more at https://qats.com/Products/Heat-Pipes.
For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.
Thermal management innovations need to match the rapid pace at which the electronics industry is advancing. As consumers demand new and more powerful devices or greater amounts of information at faster speeds, cooling solutions of the past will not be enough. Today’s cooling solutions must be smaller, lighter, and offer higher performance, but also need to be cost-effective, meet demanding project specifications, and be reliable for many years.
Advanced Thermal Solutions, Inc. (ATS) understands the importance of creating cutting-edge thermal solutions for its customers and has geared its thermal design capability and its research and development to match the innovations taking place in electronics design.
An ATS engineer assembles a rig for testing cold plates in one of ATS’ six state-of-the-art labs. (Advanced Thermal Solutions, Inc.)
To meet the need for innovative solutions, ATS engineers are hard at work in the company’s six state-of-the-art laboratories at the ATS headquarters, located in Norwood, Mass. (south of Boston). Thermal issues of all kinds are recognized, broken down, and resolved and cooling solutions are designed, simulated, prototyped, and rigorously tested in these research-grade facilities.
When someone thinks of a research lab, the initial picture is scientists in white coats working for major corporations, such as IBM, Microsoft, or Google, but the development of new ideas is an essential tool for any company in the technology field. Working with empirical tests in a lab environment pushes concepts from the white board or the computer screen to reality. There comes a time when engineers need to produce tangible data to ensure that a design works as planned.
ATS thermal engineers are no different. They use state-of-the-art instruments and software in each of the six labs to conduct a long list of characterization, quality-assurance, and validation tests. In addition to finding custom cooling solutions for customers, ATS engineers produce thermal management products for commercial uses, including a variety of next generation heat sink, heat pipe, vapor chamber, and liquid cooling designs.
Engineers test ATS instruments using a wind tunnel and sensors in the Characterization Lab. (Advanced Thermal Solutions, Inc.)
Among the most common tests performed in the ATS labs are:
• Measurements of air velocity, direction, pressure and temperature;
• Characterization of heat sink designs, fans and cold plates
• Flow visualization of liquid and air flow
• Image visualization characterization using infrared and liquid crystal thermography.
Smoke flow visualization tests run in ATS wind tunnels demonstrate how air flows through a system. (Advanced Thermal Solutions, Inc.)
Heat pipes and vapor chambers are increasingly common cooling solutions, particularly for mobile devices and other consumer electronics, and ATS engineers are working to expand the company’s offerings for these solutions and to develop next generation technology that optimizes the thermal performance of these products. This research involves advanced materials, new fabrication methods, performance testing, and innovative designs that are ready for mass production.
ATS engineer Vineet Barot sets up a thermal imaging camera for temperature mapping studies in the lab. (Advanced Thermal Solutions. Inc.)
As liquid cooling technology has grown, ATS has met this demand with new instruments and lab capabilities, such as the iFLOW-200™, which measures a cold plate’s thermal and hydraulic characteristics, and full liquid loops to test ATS products under real-world conditions.
ATS engineer Reza Azizian (right) works with intern Vladislav Blyakhman on a liquid cooling loop in the lab. (Advanced Thermal Solutions, Inc.)
The labs at ATS are up to even the toughest electronics cooling challenges that the company’s global customers present. Thanks to its extensive lab facilities, ATS has provided thousands of satisfied customers with the state-of-the-art thermal solutions that they demand.
For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.
