Tag Archives: heat exchangers

Fin Optimization in Heat Sinks and Heat Exchangers

(This article was featured in an issue of Qpedia Thermal e-Magazine, an online publication produced by Advanced Thermal Solutions, Inc. (ATS) 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.)

In electronics cooling, often separately managed Thermal/Mechanical (TM) and Software/Electrical (SE) engineering teams are finding themselves facing common challenges, as they are being driven towards similar business goals, such as product differentiation, company growth and profitability.

More so than ever today, these teams are being directed to find ways to increase component performance, particularly on highly populated boards within complex systems, at an acceptable cost of manufacturing. They are also discovering that their goals are being held back by governing specifications, environmental conditions, mechanical limitations and budget restrictions.

Heat Exchangers

Closeup of fin array on an ATS tube-to-fin heat exchanger. (Advanced Thermal Solutions, Inc.)

TM’s design thermal solutions based on airflow, envelope size, power dissipation, etc. and migrate (as expected) to the lower cost “standard solutions” whenever possible. If adequate margin is not met, reliability implications are more apparent as engineers will have to optimize solutions. This is because, in most cases, the form factor, layout, boundary conditions, etc. are set.

Thermal solutions become the gatekeeper, and in some cases, the determining factor in product deployment.

Many leading companies design their products by using technologies that will sustain long product life cycles for increased market share and brand awareness. As products are refined through the design cycle, thermal solutions may have to be optimized and this requires many investigations to be undertaken.

As the electronics industry continues to use components dissipating more and more power, new heat sink solutions must be able to accommodate large heat fluxes while keeping the same spatial dimensions [1]. Finned heat sinks and heat exchangers are largely employed in many engineering fields, and this demand spurs researchers into devising and testing new geometries for the heat sinks.

Engineers constantly try to develop new designs to enhance the performance of heat exchangers. One such effort is the design of the wavy fins to enhance the surface area.

Figure 1 shows a close up view of an extrusion type thermal solution where the profile has a feature of undulated fins. In general, a wavy fin heat sink should perform better under natural and forced convection due to the increased surface area created by the fins. This feature can easily be manufactured with a die. The “waviness” can be adjusted to increase surface area resulting in a positive impact on thermal performance.

Heat Exchangers

Figure 1. Close-Up View of Simply Wavy Fin Geometry [1]

Theoretical models have been devised to find the pressure drop and the heat transfer from wavy fin geometries. Figure 2 shows the schematic of a wavy fin.

Heat Exchangers

Figure 2. Schematic of a Wavy Fin Geometry [2]

In this figure, the fins are assumed to have a sinusoidal geometry where

λ = Wave length (m)
H = channel width (m)
S = channel height
2A = twice the amplitude of the wave

The shape of the curve is assumed to be:

The length of the curve can be found from the following equation:

Shah and London [3] came up with the following equation for the friction and Nusselt number in channels:

Where,
F = fanning friction factor
aspect ratio

The same equation applies for a wavy fin based on the correct length:

The Nusselt number for the straight fins and wavy fins is the same as long as the correct surface area is used:

The above equations are for the low Reynolds number.

For high Reynolds number Shapiro et. al [4] derived the following equations:

Where,
Dh = hydraulic diameter (m)
Reynolds number based on hydraulic diameter
L = half length of the channel (Le/2)
Pr = prandtl number
Dh = 2SH/(S+H)

The combined asymptotic for the friction and Nusselt number is as follows:

Figure 3 compares the results of the above analytical equations with the results from Kays and London [5]. In the graph, the Colburn j factor is shown and is defined as:

The results show that the experimental values of Shah and London are within 20% band of the values obtained from the above relations. The data is for the fin type 11.44-3/8W.

Heat Exchangers

Figure 3. f and j Values as a Function of Reynolds Number.[2]

Marthinuss et al. [6] reviewed published data for air-cooled heat sinks, primarily from Compact Heat Exchangers by Kays et al [5] and concluded that for identical fin arrays consisting of circular and rectangular passages, including circular tubes, tube banks, straight fins, louvered fins, strip or lanced offset fins, wavy fins and pin fins, the optimum heat sink is a compromise among heat transfer, pressure drop, volume, weight and cost.

Figure 4 shows that if the goal is to get a higher value of heat transfer per unit of pressure drop, the straight fin is the best. Figure 5 shows that when heat transfer per unit height is of concern pin fin is the best.

Heat Exchangers

Figure 4. Profile Comparisons Based on Heat Transfer/Pressure Drop. [6]

Figure 5. Profile Comparisons Based on Heat Transfer/Volume. [6]

Sikka et al. [7] performed experiments on heat sinks with different fin geometries. Figure 6 shows 3 different categories of heat sinks tested. The conventional fins, such as straight and pin fins, are shown in (a); (b) shows the fluted fins and (c) shows the wavy fin design. The tests were done for both horizontal and vertical direction of air flow at natural convection and low Reynolds number forced flow. Table 1 shows the dimensional values of each of these heat sinks.

The last column shows the values of At/Ab (total surface area/base surface area).

Figure 6. (a) Traditional Fins, (b) Fluted Fins, (c) Wavy Fins. [7]

Table 1. Geometries and Dimensions of the Heat Sinks. [7]

The values of the Nusselt number were reported based on the following relation:

Figure 7 shows that for natural convection in the horizontal direction, the pin fin has the best performance. The fluted fins have, in general, a better performance compared to longitudinal fins. The lower graph in figure 7 shows that the wavy fins are essentially the same as the longitudinal fins.

Figure 7. Nusselt Number As a Function of Rayleigh Number for Natural Convection-Horizontal Direction. [7]

Figure 8 shows the natural convection cases for the vertical direction. The figure shows that heat transfer decreases for the pin fin, but increases for the plate fin. The pin fin still is better than the plate fin, but the difference is only 4-6%. Figure 8 also shows that the cross cut heat sink has the best performance. The bottom figure in 8 confirms that the wavy fins do not have much better heat transfer compared to plate fins.

Figure 8. Nusselt Number as a Function of Rayleigh Number for Natural Convection-Vertical Direction. [7]

Figure 9 shows the Nusselt number for forced convection over a horizontal plate as a function of Reynolds number. This figure indicates that, for very low Reynolds numbers, the cross fin is better than the pin fin; but, around Re = 2000, the situation reverses and the pin fin gets better than the cross cut heat sink. For low Reynolds numbers, the longitudinal pins are better than the wavy fins; but, at higher Reynolds numbers, the performance of the wavy fins gets better by almost 12-18%.

Figure 9. Nusselt Number as a Function of Reynolds Number for Forced Convection-Horizontal Direction. [7]

Figure 10 provides the Nusselt numbers for the vertical direction for forced flow. In comparing the results with the horizontal direction, the results are almost the same, with the difference being that the wavy fin heat sinks perform better than the plate fin heat sinks, by about 14-20%.

Figure 10. Nusselt Number as a Function of Reynolds Number for Forced Convection-Vertical Direction.[7]

The results presented in this article strengthen our understanding about how heat exchangers and heat sinks can be made more compact and efficient. The results show that the design of the fin field is still an issue and much remains to be investigated for optimization, depending on the conditions and application.

Further empirical testing is warranted for the evaluation of the effects of wavy fin heat sinks, as fine meshing and a high degree of confidence is not easily obtained through simulating these profiles using commercial CFD tools.

References:

1. Lorenzini, M., “Performance Evaluation of a Wavy-Fin Heat Sink for Power Electronics” Applied Thermal Engineering, 2007.
2. Awad, M., Muzychka, S., “Models for pressure drop and heat transfer in air cooled compact wavy fin heat exchangers”, Journal of Enhanced Heat Transfer, 18(3):191-207(2011).
3. Shah, R., London, A., “Advances in heat transfer, suppl. 1, laminar forced flow convection in ducts”, New York, Academic press, 1978
4. Shapiro, A., Sigel, R., Kline, S., “Friction factor in the laminar entry region of a smooth tube,” Proc., 2nd V.S.Nat. Congress of applied mechanics, PP. 733-741, 1954.
5. Kays, M., London,L., “Compact Heat Exchangers”, Third Edition, McGraw-Hill, 1984.
6. Marthinuss, E., Hall, G., “Air cooled compact heat exchanger design for electronics cooling”, Electronics cooling magazine, Feb 1st, 2004
7. Sikka, K., Torrance, K., Scholler, U., Salanova, I., “Heat sinks with fluted and wavy fins in natural and low-velocity forced convection”, IEEE, Intersoceity Conference, 2000.

For more information about Advanced Thermal Solutions, Inc. thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.

#JustChilling: ATS Recirculating and Immersion Chillers for Liquid Cooling Systems

ATS Chillers

Advanced Thermal Solutions, Inc. has a line of recirculating and immersion chillers for conditioning the coolant in liquid cooling systems. (Advanced Thermal Solutions, Inc.)


ATS offers a variety of chillers, including the CHILL V and CHILL iM series, for conditioning the coolant in liquid cooling systems. The ATS-CHILL V series, including the ATS-Chill150V, ATS-Chill300V, and ATS-Chill600V, are re-circulating, vapor compression chillers that offers precise coolant temperature control using a PID controller. The ATS-CHILL iM is an immersion chiller for precise control of the bath temperature by immersing the evaporator in a fluid bath.

Learn more about ATS recirculating and immersion chillers in this recent blog post or in the video below:

ATS Liquid Cooling Products

In addition to chillers, ATS has a complete product offering for Liquid Cooling Closed Loop Systems, including flow meters, leak detectors, heat exhangers, and cold plates. ATS can also design off-the-shelf or custom liquid cooling systems to meet the thermal needs of a project.

ATS 3-Core design approach identifies the type of cooling required at the analysis level and informs the client of its options, saving cost and time on design iteration and simulation verification. Once it is determined that liquid cooling is the option to pursue, the ATS design team identifies all the required components of the liquid loop, as well its packaging requirements and integration in the system.

ATS offers a complete array of off-the-shelf liquid loop components that can be readily deployed or custom-designed to meet the thermal requirements of the system. Subsequent integration of the liquid loop into the system provides the customer with a turn-key option for thermal management of their system.

Don’t get burned! Take advantage of ATS expertise in liquid and air cooling to ensure proper thermal management for your project.

For more information about Advanced Thermal Solutions, Inc. thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.

Cold Plates and Recirculating Chillers for Liquid Cooling Systems

Recirculating Chillers

ATS cold plates and recirculating chillers can be used in closed loop liquid cooling systems for high-powered electronics. (Advanced Thermal Solutions, Inc.)


The miniaturization of high-powered electronics and the requisite component density that entails have led engineers to explore new cooling methods of increasing complexity. As a result, there is a growing trend in thermal management of electronics to explore more liquid cooling systems and the reintroduction, and re-imagining, of cold plate technology, which has a long history that includes its use on the Apollo 11 space shuttle.i

Thermal management of high-powered electronics is a critical component of a design process. Ensuring the proper cooling of a device optimizes its performance and extends MTBF. In order for a system to work properly, engineers need to establish its thermal parameters from the system down to the junction temperature of the hottest devices. The use of cold plates in closed loop liquid cooling systems has become a common and successful means to insure those temperatures are managed.

Cold plate technology has come a long way since the 1960s. At their most basic level, they are metal blocks (generally aluminum or copper) that have inlets and outlets and internal tubing to allow liquid coolant to flow through. Cold plates are placed on top of a component that requires cooling, absorbing and dissipating the heat from the component to the liquid that is then cycled through the system.

In recent years, there have been many developments in cold plate technology, including the use of microchannels to lower thermal resistanceii or the inclusion of nanofluids in the liquid cooling loop to improve its heat transfer capabilities.iii

An article from the October 2007 issue of Qpedia Thermal eMagazine detailed the basic components of a closed loop liquid cooling system, including:

• A cold plate or liquid block to absorb and transfer the heat from the source
• A pump to circulate the fluid in the system
• A heat exchanger to transfer heat from the liquid to the air
• A radiator fan to remove the heat in then liquid-to-air heat exchanger

The article continued, “Because of the large surface involved, coldplate applications at the board level have been straight forward…Design efforts for external coldplates to be used at the component level have greatly exceeded those for PCB level coldplates.”

Exploring liquid cooling loops at the board or the component level, according to the author, requires an examination of the heat load and junction temperature requirements and ensuring that air cooling will not suffice to meet those thermal needs.iv

To read the full article on “Closed Loop Liquid Cooling for High-Powered Electronics,” click http://coolingzone.com/blog/wp-content/uploads/2017/01/Qpedia_Oct07_Closed_Loop_liquid_cooling_
for_high_power_electronics.pdf
.

Chillers provide additional support for liquid cooling loops

In order to increase the effectiveness of the cold plate and of the liquid cooling loop, recirculating chillers can be added to condition the coolant before it heads back into the cold plate. The standard refrigeration cycle of recirculating chillers is displayed below in Fig. 1.

Chiller,s Cold Plates

Fig. 1. The standard refrigeration cycle for recirculating chillers. (Adavanced Thermal Solutions, Inc.)

Several companies have introduced recirculating chillers to the market in recent years, including ThermoFisher, PolyScience, Laird, Lytron, and Advanced Thermal Solutions, Inc. (ATS). Each of the chiller lines has similarities but also unique features that fit different applications.

In order to select the right chiller, Process-Cooling.com warns that it is important to avoid “sticker shock” because of testing conditions that are ideal rather than based on real-world applications. The site suggests a safety factor of as much as 25 percent on temperature ranges to account for environmental losses and to ensure adequate cooling capacity.v

The site also noted the importance of speaking with manufacturers about the cooling capacity that is needed, the required temperature range, the heat load of the application, the length and size of the pipe/tubing, and any elevation changes.

“Look for a chiller with an internal pump-pressure adjustment,” the article stated. “This feature enables the operator to dial down the external supply pressure to a level that is acceptable for the application. Because the remaining flow diverts internally into the chiller bath tank, no damage will result to the chiller pump or the external application.”

When trying to decide on the right size chiller for your particular application, there are several formulas that can help make the process easier. Bob Casto of Cold Shot Chillers, writing for CoolingBestPractices.com, gave one calculation for industrial operations. First, determine the change in temperature (ΔT), then the BTU/hour (Gallons per hour X 8.33 X ΔT), then calculate the tons of cooling ([BTU/hr]/12,000), and finally oversize by 20 percent (Tons X 1.20).vi

Not every application will require industrial capacity, so for smaller, more portable chillers, Julabo.com had a secondary calculation for required capacity (Q).

Q=[(rV cp)material+(rV cp)bath fluid]ΔT/t

In the above equation, r equals density, V equals volume, cp equals constant-pressure specific heat, ΔT equals the change in temperature, and t equals time. “Typically, a safety factor of 20-30% extra cooling capacity is specified for the chilling system,” the article continued. “This extra cooling capacity should be calculated for the lowest temperature required in the process or application.”vii

Comparison of Industry Standard Recirculating Chillers

Recirculating Chillers

Applications for liquid cooling systems with chillers

Recirculating chillers offer liquid cooling loops precise temperature control and coupled with cold plates can dissipate a large amount of heat from a component or system. This makes chillers (and liquid cooling loops in general) useful to a wide range of applications, including applications with demanding requirements for temperature range, reliability, and consistency.

Chillers have been part of liquid cooling systems for high-powered lasers for a number of years to ensure proper output wavelength and optimal power.viiiix To ensure optimal performance, it is important to consider safety features, such as the automatic shut-off on the ATS-Chill 150V that protects against over-pressure and compressor overload. Other laser-related applications include but are not limited to Deep draw presses, EDM, Grinding, Induction heating and ovens, Metallurgy, Polishing, Spindles, Thermal spray, and Welding.x

Machine hydraulics cooling and semiconductors also benefit from the inclusion of chillers in liquid cooling loops. Applications include CVD/PVD, Etch/Ashing, Wet Etch, Implant, Inductively Coupled Plasma and Atomic Absorption Spectrometry (ICP/AA), Lithography, Mass Spectroscopy (MS), Crystal Growing, Cutting/Dicing, Die Packaging and Die Testing, and Polishing/Grinding.xi

One of the most prominent applications for liquid cooling, heat exchangers, cold plates, and chillers is in medical equipment. As outlined in an ATS case study,xii medical diagnostic and laboratory equipment requires cyclic temperature demands and precise repeatability, as well as providing comfort for patients. For Harvard Medical School, ATS engineers needed to design a system that could maintain a temperature of -70°C for more than six hours. Using a cold plate with a liquid cooling loop that included a heat exchanger, the engineers were able to successfully meet the system requirements.

Liquid cooling with chillers are also being used for medical imaging equipment and biotechnology testing in order to provide accurate results. ATS CEO and President Dr. Kaveh Azar will discuss the “Thermal Management of Medical Electronics” in a free webinar on Jan. 26 at 2 p.m. For more information or to register for the webinar, click https://www.qats.com/Training/Webinars.

Conclusion

Closed loop liquid cooling systems are not new but are gaining in popularity as heat dissipation demands continue to rise. Using cold plate technology with recirculating chillers, such as the ATS-Chill150V, ATS-Chill300V, and the ATS-Chill600V, to condition the coolant in the system can offer enhanced heat transfer capability.

Portable and easy to use, ATS vapor compression chillers are air-cooled to eliminate costly water-cooling circuits and feature a front LED display panel that allows users to keep track of pressure drop between inlet and outlet and the coolant level. They each use a PID controller.

Recirculating Chillers

For more information about Advanced Thermal Solutions, Inc. thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.

References
i http://history.nasa.gov/SP-287/ch1.htm
ii https://heatsinks.files.wordpress.com/2010/03/qpedia_0309_web.pdf#page=12
iii http://www.sciencedirect.com/science/article/pii/S0142727X99000673
iv https://www.qats.com/cpanel/UploadedPdf/Qpedia_Thermal_eMagazine_0610_V2_lorez1.pdf#page=16
v http://www.process-cooling.com/articles/87261-chillers-evaluation-and-analysis-keys-to-selecting-a-winning-chiller?v=preview
vi http://www.coolingbestpractices.com/industries/plastics-and-rubber/5-sizing-steps-chillers-plastic-process-cooling
vii http://www.julabo.com/us/blog/2016/sizing-a-cooling-system-control-temperature-process-heating-operations
viii http://www.laserfocusworld.com/articles/print/volume-37/issue-6/features/instruments-accessories/keeping-your-laser-cool0151selecting-a-chiller.html
ix https://www.electrooptics.com/feature/keeping-it-cool
x http://www.lytron.com/Industries/Laser-Cooling
xi http://www.lytron.com/Industries/Semiconductor-Cooling
xii https://www.qats.com/cms/2016/10/04/case-study-thermal-management-harvard-medical-school-tissue-analysis-instrumentation/

How to Calculate Heat Loads for Liquid Cooling Systems

A series of calculations can be used to find the thermal loads in common liquid cooling systems. Calculations of this nature are needed to predict the performance of liquid cooling systems, which are effective but complex thermal management solutions. Several equations must be calculated to fully understand the behavior of a liquid cooled system, and ATS is providing these to engineers via personal instruction and in a paper available free from the company’s website, Qats.com.

IIn the paper, which appears in the company’s e-magazine, ATS considers a liquid cooling system as a closed loop system with three major components: cold plate, heat exchanger and pump. The cold plate is typically made from aluminum or copper, and is attached to the device being cooled. The plate usually has internal fins which transfer heat to the coolant flowing through them. This fluid moves from the cold plate to a heat exchanger where its heat is transferred to the ambient air via forced convection. The final part of the cooling loop is the pump, which drives the fluid through the loop.

A series of equations is provided to predict the final temperature of the device being cooled. The first of these equates the surface temperature of this device with the product of the power dissipated by the device times the thermal resistance of the cold plate (and its thermal interface material), added to the temperature of the water entering the cold plate.

The sequence of calculations factors in specifications from the cold plate, heat exchanger and pump. The result is a solution for the device temperature as a function of cold plate resistance. In the example cited by ATS, a cold plate thermal resistance of less than 18 degrees C/W is required to cool an Intel Xeon 5492 processor in a 25C temperature environment.

Liquid cooling is an important and expanding practice in the electronics industry. It is important to understand the impact on performance of all three major parts of liquid cooling loops (cold plate, heat exchanger and pump) to ensure an acceptable level of performance at the lowest cost.

Instructions for calculating load for liquid cooling systems are available on Qats.com in the pages of Qpedia, the thermal management emagazine from ATS. More information is also available by calling 1-781-949-2522.