Tag Archives: Integral Modeling

CFD With Analytical Modeling Gives ATS Edge

Posted on April 25, 2018 by | 3 comments

In January, Advanced Thermal Solutions, Inc. (ATS) and engineering simulation software leader Future Facilities announced that ATS had purchased multiple seats of 6SigmaET, an electronics thermal simulation software, adding to its CFD (computational fluid dynamics) capabilities.

CFD

ATS engineers are now using 6SigmaET to perform CFD on electronics cooling applications to find optimized thermal solutions for customers. (Advanced Thermal Solutions, Inc.)

In a joint press release from the two companies, ATS founder and CEO Dr. Kaveh Azar said, “We have decades of experience with a broad base of commercially available CFD tools. For the electronics thermal management analyses, 6SigmaET showed excellent agreement with our empirical and analytical modelling.

He added, “We were equally impressed with its ease of use and a short learning curve. Our engineering team was able to apply the tool to different levels of simulation extending from component to system level modelling. The speed of convergence and ease of use of 6SigmaET, have made it the first CFD software to use.”

6SigmaET becomes the lead thermal simulation software for ATS engineers dealing with standard electronics cooling challenges. ATS engineers will be able to quickly and efficiently simulate junction and ambient temperatures across boards and components or define airflow to find fan operating points or get a better understanding of pressure drop in a system.

Dr. Azar continued, “We always want to be working with the best breed of tools to deliver the innovative, high-quality and cost-effective thermal management and packaging solutions our customers expect. As a result, this addition is good news for our customers. The rich features of the 6SigmaET thermal simulation package not only enable us to do more when it comes to simulation, but also allows us to further deliver the solution to our clients in a shorter time interval. It is my highest compliment to 6SigmaET development team for putting together such a robust and effective software.”

Adding 6SigmaET to ATS CFD capabilities, which also includes FloTHERM from Mentor and Autodesk CFD (formerly CFdesign), enables engineers to save customers time in the design phase and makes it easier for ATS engineers to devise optimal thermal solutions.

ATS engineer Anatoly Pikovsky said, “Visual is definitely a great thing to have. If you look at this temperature map, for instance, you can look at the defined map and say right away, okay I have a very high temperature right in the middle.”

Pikovsky, who was working on Autodesk CFD to design a customized cold plate for a customer, demonstrated how the software allows for him to analyze the pattern of fluid flow through complex geometries that were imported from SolidWorks drawings. He used the software to show hot spots and fluid velocity and how small changes, such as the number of fins within the cold plate, could alter the results.

Field Application Engineer Vineet Barot explained that he used 6SigmaET on a board in which there was pressure drop coming from vents at the end of the board. In simulations, he was able to add fins to the heat sink without altering the fan operating point and quickly provide a thermal solution that was presented to a customer. He said, “If you had a standard 1-U chassis you can build it from scratch and run it in half an hour.”

While CFD continues to evolve to handle more complex problems, while also becoming easier to use for engineers, simulations are only part of the solution.

ATS engineers also perform analytical modeling, literally putting pen to paper with basic thermodynamic equations, to define the problem and provide a reference point for simulations. Coupling analytical and computer modeling is what sets ATS apart from its competitors because it ensures that thermal solutions provided by CFD are correct.

“CFD will give you a solution, whether it’s right or wrong, it will give you a solution,” Pikovsky said. “That’s the way it’s designed. Analytical coupled with CFD gives you a good reference point to know whether you’re in the ballpark.”

Analytical modeling also speeds up the process of finding an optimized solution. Rather than spending days or weeks plugging in different fin numbers and heights or trying numerous heat sink geometries, ATS engineers can define a small range of iterations, limiting the variables for CFD, to avoid countless simulations, each of which could take hours to run.

Pikovsky said, “Maybe you’ve designed a heat sink for certain airflow and you want to determine the number of fins. You can do it with CFD, but you start varying fins and it’s going to take you days. Analytical is great because you can determine the optimal number of fins and start CFD with that.”

CFD is a critical component of ATS thermal consulting and design services. 6SigmaET has quickly been adopted by ATS engineers as the lead software and been used in the design of thermal solutions for a number of customers in the past few months.

But, it is the combination of CFD with ATS engineers’ emphasis on analytical modeling that has made ATS a leader in the thermal management of electronics.

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.

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Posted in CFD, Consulting, Modeling, Simulation

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How to Achieve Localized Cooling with Cold Plates

Posted on January 16, 2018 by | 6 comments

Many applications in electronics cooling require a cold plate to remove heat from discrete components laid out on a board. In these circumstances, it is more efficient that the liquid does not completely fill the cold plate, but is only transferred to areas that need to be cooled.

With this kind of design, the required volumetric flow rate of the coolant will be significantly lower than if the entire cold plate was filled with liquid. The schematic for a typical example of this scenario is given in Figure 1. [1]

Localized Cooling With Cold Plates

Figure 1. Schematic of a Board with a Localized Area of Heat Dissipation . [1]

In Figure 1, areas A, B, C and D must be cooled for the components dissipating from 5 to 15 W/cm2. The other areas, designated as open, have components that interfere with the cold plate and must be avoided in the design. Two designs were considered for this case: a drilled hole and a press-fit tube. Figure 2 shows the drilled hole concept.

As can be seen, there are multiple small holes around the heat dissipating components under the cold plate surface. Large holes are machined to interconnect the smaller holes. A technique called gun drilling was used for machining the long holes. The entire cold plate was made from a copper block.

Cold Plates

Figure 2. Schematic of the Drilled Hole Cold Plate Design. [1]

Figure 3 shows the press-fit tube design. In this approach, a copper tube with high thermal conductivity is routed through the areas of heat transfer and either brazed or epoxied to the aluminum cold plate base. This design is considerably lighter and cheaper than the drilled hole design.

Cold Plates

Figure 3. Schematic of the Press-Fit Tube Cold Plate Design. [1]

To analyze the performance of this cold plate configuration, simple analytical tools can be used for a standard cold plate design. A brief summary of the equations is described here. To analyze the problem, we first have to calculate how much flow is going through the cold plate, and evaluate the pressure drop of the flowing fluid.

Pressure drop is calculated from:

Where
Um = bulk mean fluid velocity (m/s)
f = fanning friction factor
Awet = wetted surface area of the tube
Ac = cross section of the tube
K = loss coefficients related to turns, sudden expansion and contraction, etc.

The friction factor was obtained from the following equation which is in satisfactory agreements for the laminar, turbulent and transition regimes [2]

Where

Where ν is the kinematic viscosity of the fluid (m2/s) and P is the wetted perimeter of the tube.

For the heat transfer calculation, the Nusselt number can be calculated from standard correlations in the literature for fully developed flow. The Nusselt number is related to the heat transfer coefficient as:

Where
Kf = fluid conductivity

For thermally developing flow the following correlation can be used: [3]

Where
Num = mean Nusselt number
Nu∞ = fully developed Nusselt number
L = duct length

Then the convective resistance can be calculated as:

Where
hm = mean heat transfer coefficient

For the tube fitted design the overall thermal resistance is made of four components: convection, tube conduction resistance, epoxy conduction resistance and the cold plate. It is stated as:

Where
Rh = convection resistance
Rtube = conduction resistance of tube walls
Repoxy = conduction resistance of the epoxy
Rcoldplate = conduction resistance of the cold plate

For the drilled design the overall thermal resistance can be written as:

If the heat transfer coefficient is based on the local fluid temperature, then a caloric resistance must be added based on the fluid mass flow rate. The effective heat transfer coefficient is then:

Where
ṁ = mass flow rate (kg/s)
Cp = fluid heat capacitance (kJ/kg·K)

Figure 3 shows the pressure drop of the two designs as a function of water flow rate. It can be seen that with a water flow rate up to 1.89 l/min (0.5 GPM) the pressure drop between the two designs is almost the same, but at higher flow rates the drilled design’s pressure drop exceeds the tube design. The sharp 90-degree turn of the drilled holes, which lead to a higher loss coefficient, is the major contributor to the higher pressure drop.

Figure 4. Total Pressure Drop of the Drilled Design and the Tube Design as a Function of the Volumetric Flow Rate. [1]

Figures 5 and 6 show the effective heat transfer coefficient of the two designs as a function of flow rate. The bend and sharp increase of the curves around 0.95 l/min (0.25 GPM) is due to the flow transitioning from laminar to turbulent. The drilled hole design shows effective convection heat transfer between 7,000 and 27,000 W/m2K for the range of flow between 0 and 7.56 l/m (2.6 GPM). The press-fit tube design on the other hand shows a lower effective heat transfer coefficient of between 6,000 and 17,000 W/m2K.

This is mostly due to the interfacial resistance and tube wall conduction. In the drilled design example, these two resistances do not exist. In a real application, the pumping of fluid is constrained by the pump and its characteristic curve. Even though the drilled hole shows a higher heat transfer coefficient for the same flow rate, the extra pressure drop caused by the drilled design may have a lower flow rate hence lowering the heat transfer coefficient.

Figure 5. Effective Heat Transfer Coefficient of the Tube Design as a Function of Flow Rate for Different Regions on the Plate. [1]

Figure 6. Effective Heat Transfer Coefficient of the Drilled Hole Design as a Function of Flow Rate for Different Regions on the Plate. [1]

Figure 7 shows another cold plate design, this one by Lytron. [4] In this design, the extended-surface cold plate material and micro-channel aluminum extrusion are sandwiched between aluminum sheets. The entire assembly is welded using vacuum brazing. It is all aluminum, which makes it very light weight. The flexibility of this design allows the placement of cooling channels in different positions to enable localized cooling.

Figure 7. Lytron Vacuum Brazing of a Cold Plate for Localized Cooling. [4]

The above analytics show that the performance of a cold plate for localized cooling can be calculated using a simple analytical tool. The designer then has to consider such factors as weight, manufacturing, cost and thermal performance to decide the best option for his or her design. The characteristic of the pump has a paramount effect on the design and cannot be neglected.

References:

1. Seaho, S., Moran, K. and Rearick, D. (IBM Corporation) and Lee, S. (Aavid Engineering), Thermal Performance Modeling and Measurements of Localized Water Cooled Cold Plate, http://www.aavidthermalloy.com/technical/papers/pdfs/water.pdf
2. Churchill, S., Comprehensive Correlating Equations for Heat, Mass and Momentum Transfer in Fully Developed Flow in Smooth Tubes, Ind. Eng. Chem. Fundam., Vol.16, 1977.
3. Al-Arabi, M., Turbulent Heat Transfer in the Entrance Region of a Tube, Heat Transfer Eng., Vol. 3, 1982.
4. http://www.lytron.com

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.

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Case Study: Thermal Management in Harvard Medical School Tissue Analysis Instrumentation

Posted on October 4, 2016 by | Leave a comment

Designers of today’s highly advanced medical diagnostic equipment must overcome many of the same thermal challenges common to telecommunications, industrial and information technology electronics.

In addition, medical diagnostic devices present unique design issues and boundary conditions that factor into thermal solutions. These include isothermal and cyclic temperature demands, precise test repeatability, and maintaining the patient’s safety and comfort.

These kinds of issues were presented by Harvard Medical School to the experts at Advanced Thermal Solutions, Inc. (ATS) when it needed a cooling solution for the diagnostic equipment it was relying on for the analysis and observation of human tissue samples in a controlled laboratory setting. This was the school’s Frozen Tissue Microarrayer System.

ATS engineers had to provide thermal solutions to meet a range of design goals:

• Provide long-term temperature control for tissue samples embedded in an optimum cutting temperature fluid.
• Create a cooling system to maintain tissue samples below -70°C for six hours.
• Ensure operator visibility of the samples.
• Eliminate humidity and frost within the system to prevent sample contamination.

ATS Cooling Solutions

ATS engineers developed highly effective thermal solutions to meet all the design requirements of the diagnostic equipment. A reservoir in the device holds the liquid cooling medium and tissue samples are loaded through an opening at the top. Through a duct, cool air is circulated over the top of the samples to maintain temperature and humidity requirements.

As seen in Fig. 1 (below), the diagnostic system consisted of:

• Frozen tissue coring machine (on the right in the photo)
• Tissue sample loading area at the top of the cooling system (seen on the left)
• Duct system (on both sides of system) to circulate cool air
• Ice/alcohol reservoir at the system’s bottom to contain the cooling medium

Harvard Case Study

Figure 1. Prototype system created by ATS engineers for Harvard Medical School laboratory. (Advanced Thermal Solutions, Inc.)

Conduction Cooling Design

In operation, tissue samples are loaded into removable aluminum cassettes that fit tightly into a metal receiver (top left, Figure 2). The receiver contacts the cassette on five sides which allows for cooling of the samples by conduction. The receiver is lowered into a reservoir containing a slurry of dry ice and ethyl alcohol. Here the receiver is maintained at a constant temperature until the dry ice evaporates. The reservoir is double-walled and insulated to extend the evaporation time of the dry ice.

The receiver also features integral fins that increase surface area for drawing heat downward from the base of the cassettes into the icy slurry (bottom left, Figure 2). These fins are based on the same ATS heat sink design principles used in the company’s high performance maxiFLOW™ heat sinks.

Using analytical modeling, ATS engineers determined that 10 fins were the optimal number for cooling the cassette receiver and its contents. CFD simulations also showed that the 10-fin concept resulted in an optimal design. The engineers validated their analytical and CFD results through empirical testing. It was determined that extending 10 fins into the slurry provided the cooling performance to maintain tissue sample temperatures below the -70°C threshold for 9.75 hours.

Further temperature testing using thermocouples showed only a 2.5°C difference between the coldest points at the bottom of the fins and the tissue samples in the cassette. This proved that the design overcame thermal conduction resistance and could effectively maintain the samples below their critical temperature.

Figure 2. Temperature testing with thermocouples demonstrated that the temperature difference between the bottom of the fins and the top of the cassette, through three intervening layers, was only 2.5°C. This proved that the thermal design was successful. (ATS)

Figure 2. Temperature testing with thermocouples demonstrated that the temperature difference between the bottom of the fins and the top of the cassette, through three intervening layers, was only 2.5°C. This proved that the thermal design was successful. (ATS)

Figure 3. Using a heat sink-specific thermal resistance network ATS determined that the optimal number of fins was 10. (ATS)

Figure 3. Using a heat sink-specific thermal resistance network ATS determined that the optimal number of fins was 10. (ATS)

Convection Cooling Design

The above conduction cooling design provided only part of the solution. There were additional needs to maintain the temperature at the top of the samples and to decrease the relative humidity of the cool air from the ambient air in the lab. ATS engineers designed a convection cooling system to fulfill these requirements.

A heat exchanger was installed with its fins in the dry ice/alcohol slurry and its other side extending into a duct to cool the air passing over it. This approach uses the same cooling medium for both convection and conduction to ensure there is no temperature differential throughout the sample and that the sample is as isothermal as possible.

Air is pushed by a counter-rotating fan through the duct and into the heat exchanger. The heat exchanger forms a thermal link between this air and the slurry mixture. The heat exchanger was designed with an optimum balance between its surface area and the resulting pressure drop to ensure the fan was operating with the most effectiveness.

Once the air passes the heat exchanger, it moves through the ducts and into a diffuser at the top of the system. The diffuser disperses the air over the sample creating a barrier between the tissue and the ambient environment of the lab so outside moisture and heat are not transferred in.

The ATS engineers tested this design using an array of thermocouples and ATS hotwire anemometer Candlestick Sensors connected to an ATS ATVS-2020, a temperature and air velocity scanner. They determined there was too much mixing between the air flowing over the samples and ambient air. The diffuser was redesigned with a new connection to the duct and an optimized outlet radius (see Figure 4).

In the ducts, a molecular sieve desiccant housed in a honeycomb structure was used to reduce the dew point of the air to -84.4°C, which was well below the -72°C air temperature in the duct.

Figure 4. Initial testing led to a redesign of the air diffuser to prevent ambient humidity from mixing with the air over the tissue samples. (ATS)

Figure 4. Initial testing led to a redesign of the air diffuser to prevent ambient humidity from mixing with the air over the tissue samples. (ATS)

Conclusions

ATS engineers performed a final series of tests of the Frozen Tissue Microarrayer System using Candlestick Sensors, thermocouples and the ATVS 2020 scanner. The tissue temperature stayed constant over the required six-hour period and well below the -70°C threshold. In fact, testing determined that the tissue temperature remained below the threshold for nearly eight hours before warming above a usable temperature (Figure 5). The multi-part cooling system was a success, meeting the original design objectives provided by Harvard Medical School.

Figure 5. Final testing showed that the ATS cooling design kept tissue temperature (shown in blue in the graph above) below the -70°C threshold for more than the required six hours. (ATS)

Figure 5. Final testing showed that the ATS cooling design kept tissue temperature (shown in blue in the graph above) below the -70°C threshold for more than the required six hours. (ATS)

The process of designing cooling solutions for the Frozen Tissue Microarrayer demonstrated that thermal design practices used throughout electronics cooling can be applied in the medical device industry. Fin efficiency, thermal resistance, and pressure drop calculations are standard regardless of the application. Thermal solutions should be considered early in the design process so they can be incorporated into the overall system as efficiently as possible.

The experts at ATS used traditional thermal calculations, CFD simulations, empirical testing, and its leading-edge heat sink technology to successfully design the thermal solution for this medical equipment application. The ATS design allowed Harvard Medical School to test tissue samples while meeting its strict requirements.

To learn more about the design, watch the video below:

Download a PDF of this case study at http://www.qats.com/cms/wp-content/uploads/Harvard-Medical-case-study.pdf.

Visit www.qats.com, call 781-769-2800 or email ats-hq@qats.com to learn more about ATS and its Thermal Management Analysis and Design Services.

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Integral Modeling Is First Step for ATS Engineers

Posted on August 25, 2016 by | 2 comments

Integral Modeling

ATS engineers utilize integral or analytical modeling as a first step to solving thermal management issues in a design. [Advanced Thermal Solutions, Inc.]

In July, Future Facilities, a CAD software company, released the results of a survey it conducted of more than 350 electrical engineers (the link to the story is below) on how thermal management relates to reliability in electronics design. The survey coincided with the release of the company’s newest version of its thermal simulation software 6SigmaET.

Forty percent of the surveyed engineers believed thermal simulations for their projects to be too time-consuming or complex. Sixty-two percent of the engineers said that they would rather over-design a project than optimize thermal performance in the design process. In fact, 33 percent of the engineers called thermal issues an “irritation” and would prefer to not deal with them.

Tom Gregory, Product Manager at 6SigmaET, concluded, “It’s clear that a lot of engineers still don’t feel comfortable creating thermal simulations of their designs, a fact which is not being helped by the complex nature of most thermal simulation tools currently on the market.”

The engineers at Advanced Thermal Solutions, Inc. (ATS), a leading-edge engineering and manufacturing company focused on the thermal management of electronics, have long demonstrated that thermal solutions are a critical component to electronics design and that incorporating thermal management early in the design process will lead to a more cost-effective and reliable product.

By incorporating thermal management into the design process engineers optimize time between failure for individual components as well as the overall system. They actually reduce the cost of the system by limiting the need to overdesign it. Well thought out thermal solutions increase the likelihood that the final design will succeed and meet the specifications that were set out at the beginning of the project.

The survey results pointed to CFD analysis as the jumping off point for thermal solutions. But an easier and more efficient way to start the process is with an integral or analytical model, using pencil and paper or a spreadsheet.  In its 3-Core Design Process, ATS has utilized integral modeling as its first step to quickly and easily provide first order solutions and determine whether a design will succeed in meeting its thermal requirements.

Integral modeling, as Dr. Kaveh Azar, founder, President, and CEO of ATS, explained in a webinar (the link is below), utilizes standard equations based on the basic laws that govern thermal engineering: Conservation of Mass, Conservation of Momentum, Conservation of Energy, and Equation of State (i.e. the Ideal Gas Law).

Determining pressure, temperature, and air velocity differentials throughout a system and plugging those numbers into equations that most engineers will remember from undergraduate and graduate training will define the problem that will be faced in designing the system.

Dr. Azar said, “When I focus on integral modeling as I go through the process, you’ll see how easy it is and how broad-spectrumed the applications of these are and this is going to form the first foundation for any kind of analysis that we do in electronics cooling.”

Integral modeling is applicable to any domain and will give a substantiated, independent model to ensure the system is built within the proper parameters. Taking this early step saves time and money that may have been wasted on designing a system that ultimately would not work. Integral modeling also establishes parameters under which the system can be built to save costs after deployment.

Dr. Azar explained, “If we design it for the worst case scenario, we always have the adequate margins and as a result have lesser cost of deployment.”

It is a competitive market. Integral modeling is a quick first step to ensure thermal solutions are part of a design to save on component and system costs. A few quick calculations will have a major impact on the project’s bottom line.

The survey results from Future Facilities can be found at http://www.thermalnews.com/main/news/40-percent-of-electronics-engineers-find-thermal-simulation-too-complex-and-time-consuming.

For more information about the importance of integral modeling and practical applications, watch the webinar with Dr. Kaveh Azar of Advanced Thermal Solutions, Inc. below:

Does the process of thermal design for your next project seem daunting?  Contact us.  ATS offers a  free four-hour consultation in its lab.  Email ATS at ats-hq@qats.com.

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