Motor Driver Overheating Challenges in Robotic Applications

Introduction

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schematics-project

Fig. 1 – Experimental Setup

A motor driver (sometimes called a motor controller) is an electronic device that acts as an intermediate device between a microcontroller, a power supply or batteries, and the motors in robotics applications (Figure 1).

The physical size and weight of a motor driver can vary significantly, from a device smaller than the tip of your finger to a large controller weighing several Kg. As robotic designs and technologies evolve in the future, more powerful motor drivers will be required to accommodate these advances. It is expected that miniaturization will play an increasingly important role in near-term robotic applications as well. As power requirements increase, motor drivers must be designed to manage parallel overheating issues, while also maintaining a smaller footprint overall. Liquid immersion cooling is one way to successfully manage these concerns [1].

An experiment was conducted to demonstrate how overheating issues and limitations in contemporary motor drivers might be overcome with the introduction of liquid immersion cooling.

Discussion on Motor Driver Failure

Temperature management is a fundamental aspect of motor driver design due to its’ direct relation with component availability, whereas power and operation are related to system efficiency. When discussing traditional processors, the failure rate λ may be defined as the number of components failing per unit time [2]. Thus, component thermal reliability with a constant failure rate is expressed as [3]:

Modela

 

 

A temperature-related reliability model based on the mean time to failure (MTTF) for motor drivers is proposed where constant is as follows:

Modelb

The model above, derived from [4], is based on 5 component failure rates including the number of failures per million hours (C1 and C2), temperature factors (πt), quality factor (πQ), learning factor (πL), and environmental factor (πE), thus:

Modelc

 

 

As shown in the model above, temperature and environment each affect the overall reliability of motor drivers, with thermal behavior often influenced by robotic design.

Equipment Configuration and Setup
Fig 2. Syren 25A Motor Driver

Fig 2. Syren 25A Motor Driver

There are a large number of motors that work in conjunction with motor drivers, microcontrollers, and power supplies. These motors are used to move material, parts, tools, or specialized devices with various programmed motions in robotic units. For this experiment, a gear head motor with wheel operating at 45RPM/50mA @24VDC no load; 1A/7.5in-lb torque at stall was chosen.

The gear head motor was connected to a Syren 25A regenerative motor driver (Figure 2). The Syren 25A is one of the most versatile, efficient and easy to use dual motor drivers on the market. This motor driver can supply a single DC brushed motor with up to 25A continuously [5]. Syren 25A was the first synchronous regenerative motor driver in its class. The regenerative topology enables batteries to recharge whenever a robot is commanded to slow down or reverse. For the purposes of this experiment, power was added directly to the Syren 25A to power the motor using a variable power supply unit. Additionally, the motor driver has built-in thermistors that are designed to shut the motor driver off when input voltage exceeds 30VDC or the device gets too hot. For this experiment, a customized version of the device was used without thermistors so that a failure point could be established during overheating conditions. Prior to thermistor removal, the motor driver would regularly shut down when >24VDC input was introduced.

10VDC was initially supplied to each motor driver to determine which components on the device get hottest during standard operation. A thermal imaging camera was utilized to determine the motor driver’s IC hotspot (Figure 3).

Fig 3. Thermal Image - Syren 25A Motor Driver Powered Up

Fig 3. Thermal Image – Syren 25A Motor Driver Powered Up

Testing Procedures

Input voltage was introduced and maintained at 30VDC to an un-submerged motor driver (Motor Driver 1) and submerged motor driver (Motor Driver 2) during testing. Temperature levels of the IC hotspot and operation were monitored and recorded over a 3-hour period.

Fig 4. Power supplied to Motor Driver 1

Fig 4. Power supplied to Motor Driver 1

Fig 5. Motor Driver 2 immersed in dielectric cooling fluid at startup

Fig 5. Motor Driver 2 immersed in dielectric cooling fluid at startup

Results

The Syren 25A motor driver was designed to operate optimally with 6-24VDC input at a maximum threshold of 30VDC. Removal of the motor drivers’ thermal protection enabled us to continuously input this 30VDC threshold while monitoring temperature and operation to failure. Input voltage was maintained at 30VDC while recording the temperature of Motor Driver 1 and monitoring operation. Lab temperature was maintained at 20-20.28°C during testing. As input voltage was maintained, component temperature continued to increase, eventually leading to motor driver failure. At the 29-minute mark, Motor Driver 1 ceased to operate thereby cutting off the motor and wheel rotation. At this point, the motor driver was smoking and the device emitted a burned component smell. Decreasing the input power and waiting until the device cooled down did not turn the motor driver back on.

Motor Driver 2 was immersed in dielectric cooling fluid and 30VDC input voltage was applied at startup. Fluid temperature remained constant at 20.1°C. 30VDC input was maintained for a period of three hours reaching a maximum temperature of 35.7°C. During this 3-hour period, temperatures were recorded and operation was monitored. Although the voltage input was at the devices threshold, temperature of the device remained relatively constant during testing, thereby enabling continued operation in the device. There was no significant change in wheel rotation or motor stress over time.

chart Chart1

Discussion

Device failure experienced during Motor Driver 1 testing was expected due to thermal overload. This is indicative of the problems associated with overheating in robotics applications today, and the reason that thermistors are required on devices like the one utilized in this test. Alternatively, the results of Motor Driver 2 testing indicate that liquid immersion cooling is a viable thermal management option in robotics applications. Liquid immersion cooling has been proven to maintain operation and reliability in a similar fashion in other types of electronics devices [1]. This demonstrates a reliable method of overcoming thermal management challenges and increased power demands in future robotic development efforts. Additionally, liquid cooling can be used to increase maximum input/output thresholds of motor drivers thereby enabling more powerful robotic designs.

Reference

[1]     J. Carr and D. Sundin, Minimizing CPU overheating with liquid immersion cooling. Electronics Protection Magazine, 2014.

[2]     EPMSA, Guidelines to Understanding Reliability Prediction, 2005.

[3]     JEDEC, Failure Mechanisms and Models for Semiconductor Devices, JEP122F, 2010.

[4]     J. Carr and D. Sundin, Assessing the Corrosion Effect and Materials Compatibility of Electronics in Liquid Cooled Immersion Applications, 2015.

[5]     SyRen 10/Syren 25 motor driver user’s guide: Dimension Engineering, July 2007.

 

 

Component Reliability in Liquid Cooled Applications

Introduction

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Contemporary electronic devices and computing platforms are operated in a number of conditions and environments never considered just a few short years ago. Simultaneously, the demand for increased power and miniaturization has impacted their use and application in these new settings as well. Considerations such as overheating, corrosion, and reliability also must be factored in when developing modern equipment and technologies. Liquid immersion cooling is one option that may be considered when seeking out new ways to overcome engineering challenges such as increased power densities and advance power management requirements.

When evaluating liquid immersion cooling for electronics-related applications, corrosion is often a concern with regards to long-term reliability of components and overall operation. The degree of corrosion damage is dependent upon a number of factors such as the type of immersion fluid chosen, equipment exposure time, operating temperature, and the introduction of contaminants/moisture, to name a few.

In order to better understand the role that these factors might play in electronics immersion cooling, a series of experiments have been designed to better understand how overall equipment operation is impacted in varying applications. For instance, a previous test was designed to demonstrate how liquid immersion cooling is be used to mitigate overheating experienced during processor overclocking [1]. This paper is a continuation of these experiments and was conducted to evaluate any long-term affects on electronics components and secure digital cards (SD cards) when continually immersed in a controlled Opticool Fluid environment.

 Device Reliability

As increased processing power and component miniaturization continues to evolve, thermal reliability and environmental factors will play an increasingly important role in future development efforts. Temperature is a fundamental aspect of systems design due to its’ direct relation with component availability whereas power and operation are related to system efficiency. When discussing electronic components, the failure rate λ is commonly defined as the number of components failing per unit time [2]. Thus, component thermal reliability with a constant failure rate is expressed as [3]:

Modela

 

 

A temperature-related reliability model based on the mean time to failure (MTTF) for processors has also been proposed where constant is as follows [4]:

Modelb

The model above is based on 5 component failure rates including the number of failures per million hours (C1 and C2), temperature factors (πt), quality factor (πQ), learning factor (πL), and environmental factor (πE), thus:

Modelc

 

 

As shown in the model above, temperature and environment each affect the overall reliability of microprocessors, with thermal behavior often influenced by the type of application being run.

Equipment Configuration and Setup
Fig 1. Device Submerged

Fig 1. Device Submerged

The Raspberry Pi computing device was selected for this test due to it’s condensed size and overclocking capability. The Raspberry Pi device is representative of ways that smaller component sizes are leading to more powerful computing. For example, the device has been used for media streaming, home automation, and robotic applications, to name a few [5]. Both the Raspberry Pi and SD card were tested and confirmed to be operating properly prior to the start of this evaluation. The Raspberry Pi unit and SD card were next immersed in Opticool Fluid coolant as shown in Figure 1.

5V was applied to power the device and a fluid temperature of 20.3°C was maintained for the duration of the experiment. The fluid was circulated using a 75 RPM motor.

Results

The test was conducted over a 6-month period. During this timeframe, the temperature of the fluid was monitored and maintained continuously. No changes were recorded during this time period. 5V was continuously supplied to power the device during the same period.

At the 6-month mark, the Raspberry Pi device and SD card were removed for inspection and testing. Neither the Raspberry Pi nor the SD card displayed any visual indication of surface corrosion as shown in Figure 2.

Figure 2: SD Card and Raspberry Pi condition after 6-month operation in Opticool Fluid

Figure 2: SD Card and Raspberry Pi condition after 6-month operation in Opticool Fluid

In addition to visual inspection, the functionality of both the Raspberry Pi and SD card was tested outside the fluid. Test results indicated that neither the Raspberry Pi device or SD card were affected by corrosion during the testing period. The devices continued to boot up and operate properly as shown in Figure 3. This indicates that Opticool Fluid has no adverse affects on various component types or SD cards over time.

Figure 3: Successful post-immersion operating system load.

Figure 3: Successful post-immersion operating system load.

Conclusion

As devices continue to shrink in size while increasing in power, performance improvements will continue to be plagued by increased power and cooling requirements. Furthermore, high temperatures negatively affect component reliability dependent on effective design and thermal management techniques. The results obtained during this experiment demonstrated that the integrity of various types of electronic components and devices are maintained over time in Opticool Fluid. This demonstrates a viable thermal management and design option for application developers and engineers.

Reference

[1]     J. Carr and D. Sundin, Minimizing CPU overheating with liquid immersion cooling. Electronics Protection Magazine, 2014.

[2]     EPMSA, Guidelines to Understanding Reliability Prediction, 2005.

[3]     JEDEC, Failure Mechanisms and Models for Semiconductor Devices, JEP122F, 2010.

[4]     J. Srinvasan, J., S.V. Adve, P. Bose, and J.A. Rivers, Exploiting structural duplication for lifetime reliability enhancement, ISCA’05, 2005.

[5]     Heath, N. (2012). 10 coolest uses for the Raspberry Pi. Retrieved July 14, 2014 from http://www.techrepublic.com/blog/european-technology/10-coolest-uses-for-the-raspberry-pi/

 

Minimizing CPU Overheating with Liquid Immersion Cooling

Introduction

Published: Electronics Protection, 12(3). 12-14
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to download a PDF version of this entry.

The Central Processing Unit (CPU) generates a great deal of undesirable heat in modern computing systems. The CPU is responsible for processing most of the data within systems and is often referred to as a computer’s central processor or simply processor. As data is processed within a system, heat is generated. Once heat thresholds are exceeded, CPUs are placed at risk of malfunction or permanent damage.

To overcome overheating, systems can be equipped with cooling systems that help to regulate temperature within a unit thus maintaining efficient operation. This paper examines the most common cooling technologies used today as well as a few that are currently in the early stages of research and development.

Any examination of CPU cooling, or any kind of electronics cooling for that matter, must take into account Moore’s law which predicts the number of transistors placed on integrated circuits will double every two years. As CPUs continue to decrease in size while exponentially increasing in power each year, adequate cooling methods will increasingly become an integral part of new design planning efforts. Figure 1 shows the evolution of module-level heat flux in high-end computers over several decades. As shown, module heat flux has continued to creep upward with each passing year.

Figure 1: Evolution of module level heat flux in high-end computers [1]

Figure 1: Evolution of module level heat flux in high-end computers [1]

An experiment was conducted to evaluate liquid immersion cooling designed to mitigate CPU overheating through the use of a commercially available synthetic petroleum fluid – Opticool Fluid.

Air Cooling

Air-cooling, which incorporates the use of fans, is currently the prevalent method of cooling CPUs in computing environments. It has several advantages including reduced cost, relatively low noise, and is free of piping elements, tubes and cables. [2] The main function of fans is to pump air so that heat is effectively carried away from the CPU assembly. Air pressure can vary by incorporating fans in series (placed on top of one another) or parallel configurations (side-by-side). [3] The serial setup increases the discharge pressure while the parallel setup increases the area coverage.

Based on several studies completed in recent years [1][2][4][5][6][7][8][9], air-cooling technology is insufficient to keep up with the growing requirements of CPU cooling in the marketplace. As a result of these deficiencies, a number of new cooling technologies have been developed while other advanced cooling technologies are currently under study.

Heat Transfer Overview

Fluid flow can be laminar (steady state) or turbulent, and heat might be transferred with and without phase change. In addition, the flow regime might be treated as Newtonian or non-Newtonian. Appropriate theoretical and empirical heat transfer equations have been developed for different velocity profiles, flow regimes and flow geometries. For the ideal case of fluid flow in a shell-and-tube exchanger, heat is transferred by radiation and convection to tubes via conduction through tube walls and by forced conduction from the internal wall surface to the bulk fluid. The basic equation governing all such heat transfer is

Q=UADelta T (Eq. 1)

where Q represents heat transferred in unit time, U represents overall heat transfer coefficient, A represents available surface area and ΔT represents temperature gradient between the source and the sink (or the inlet and the outlet). If multiple fluids or separating walls are used, then the overall coefficient U can be decomposed into individual coefficients h, each representing a particular medium. [1]

In instances of conductive heat transfer through several layers of materials, the thermal resistances can be added in series to obtain the total temperature gradient. This is shown in Figure 1 below, where xi represents media thickness and ki represent thermal conductivities:

Figure 1: Conductive heat transfer through composite media. Source: Coulson & Richardson, 1999, p. 391.

Figure 2: Conductive heat transfer through composite media. [2]

With reference to the above figure, the thermal gradient T1T4 is:

T_1-T_4=(x_1/(k_1 A)+x_2/(k_2 A)+x_3/(k_3 A))Q  (Eq. 2)

where Q becomes the ratio of total driving force to total thermal resistance per unit area. [2] In instances of convective transfer, which is often the principal mechanism in liquid cooling, the heat transfer coefficient may be expressed as a dimensionless relation known as the Nusselt number, or as a dimensional equation. Convection is distinguished between natural (fluid movement caused by the transfer process itself) or forced (fluid movement caused by an externally applied force); in the former, the Nusselt number for external spaces is

Nu={0.825+(0.387Ra^(1/6))/[1+(0.492/Pr)^(9/16) ]^(8/27) }^2  (Eq. 3)

where Ra, the Rayleigh’s number and Pr, the Prandtl number, are dimensionless numbers corresponding to the flow velocity and fluid properties. [3] When we have steady flow of fluid past an immersed flat plate, two boundary layers develop. The first is hydrodynamic, within which velocity profile changes from 0 at the plate surface to flow velocity at the outer boundary; the second is thermal in which temperature profile changes from the plate surface temperature to the fluid temperature. These boundary layers are shown in Figure 2 below:

Figure 2: Hydrodynamic and thermal boundary layers for steady flow of fluid past an immersed plate. Fluid temperature is T∞ and plate surface temperature is Tw; fluid velocity is u0. Entire plate is heated at top while there is an unheated length x0 at bottom. Source: McCabe, Smith, & Harriott, 1993, p. 332.

Figure 3: Hydrodynamic and thermal boundary layers for steady flow of fluid past an immersed plate. Fluid temperature is T∞ and plate surface temperature is Tw; fluid velocity is u0. Entire plate is heated at top while there is an unheated length x0 at bottom. [4]

For the particular case of forced convection with laminar flow and no phase change, the temperature gradient at the wall is given by

Eq 4  (Eq. 4)

where cp is the liquid’s specific heat, k is its conductivity, μ is its viscosity and ρ is its density. [4] The treatments of heat transfer for turbulent flow are more complex, but in the case of fully developed (or fully rough) flow of a Newtonian fluid, the velocity and temperature gradients within a tube have been observed to be parabolic. Empirical formulae for heat transfer with phase transition, for both natural and forced convection, have also been developed.

Raspberry Pi

Figure 4: Diagram of Raspberry Pi Model B [15]

Figure 4: Diagram of Raspberry Pi Model B [15]

The Raspberry Pi computing device was selected for this experiment due to it’s condensed size and overclocking capability. Figure 4 shows the layout of the unit. The Raspberry Pi device is representative of ways that smaller component sizes are leading to more powerful computing. The device has been used for media streaming, home automation, and robotic applications, to name a few [14]. The Raspberry Pi is typically powered by 5V and 1-1.5A and can be overclocked to a maximum 1000MHz. As with other computing devices, the Raspberry Pi tends to overheat when overclocked or when excess voltage is applied to the device. The maximum operating temperature for the Raspberry Pi CPU is 85°C. As this experiment was conducted, the CPU never exceeded this temperature.

The thermal image below (Figure 5) shows that the CPU is one of the largest heat emitters when the Raspberry Pi is running at normal capacity. During this study, we focused exclusively on CPU temperature measurements.

Figure 5: Thermal Image of Raspberry Pi Model B

Figure 5: Thermal Image of Raspberry Pi Model B

Experimental Design

The general layout of the experimental setup used for this study is shown in Figures 6 and 7. Baseline CPU temperatures were measured with standard 5V input while unit was overclocked at 800MHz, 900MHz and 1000MHz respectively. Laboratory temperature was maintained at 20°C throughout the duration of the experiment. The immersion fluid was maintained at this same temperature during immersion testing as well. Measurements were recorded at device startup and in subsequent 1-hour increments. CPU temperatures were measured for both air cooling and immersion cooling with overvoltage levels of 6V, 7V, and 8V applied during 3-hour periods for each test, respectively. Overclocking speed of 1000MHz was maintained for the duration of the overvoltage testing.

Figure 6: Raspberry Pi Model B computing device with standard 5V applied to power up.

Figure 6: Raspberry Pi Model B computing device with standard 5V applied to power up.

CPU temperatures were obtained via the Raspberry Pi unit itself by using the following command sequence upon login:

/opt/vc/bin/vcgencmd measure_temp
ENTER

Figure 7: Raspberry Pi immersed in Opticool Fluid and powered up for temperature measurements.

Figure 7: Raspberry Pi immersed in Opticool Fluid and powered up for temperature measurements.

Results

During baseline testing, 5V was applied to the Raspberry Pi during 3 overclocking tests. Results were as follows:

Startup 1-Hour 2-Hour 3-Hour
700 MHz 34.2° C 39.0° C 47.6° C 48.3° C
900 Mhz 36.3° C 48.2° C 48.7° C 51.9° C
1000 MHz 37.4° C 54.1° C 54.6° C 58.4° C

As shown above, the maximum CPU temperature at 1000MHz after 3-hours of operation was 58.4°C. Next, temperatures were recorded as overvoltage was applied to the unit. Maximum overclocking of 1000MHz was maintained throughout. The results were recorded and graphed (Figure 8).

Figure 8: CPU temperatures with Overclocking/Overvoltage

Figure 8: CPU temperatures with Overclocking/Overvoltage

During 36 hours of testing, the device maintained adequate operation however when 7-8V was applied to the unit, operation became extremely unstable. This occurred during both air and liquid cooling testing. This is likely due to a limitation of the CPU used for this board, however based on these findings, on-board processor power of the Raspberry Pi can be increased if a more powerful CPU were integrated into the design and combined with liquid immersion cooling.

Conclusion

As devices continue to shrink in size while increasing in power, performance improvements will continue to be plagued by increased power and cooling requirements. Furthermore, high temperatures negatively affect systems reliability as various components are exponentially dependent on operating temperatures. The results obtained during this experiment demonstrated how liquid immersion cooling can efficiently decrease CPU operating temperatures by as much as 50% or greater versus traditional air cooling. As discussed above, thermal management will become increasingly important in the future. Liquid immersion cooling with Opticool is a viable solution that may be applied in any number of applications including data center environments, underwater ROVs and electric engines, to name a few.

Reference

[1]        Chu, R.C.; Simons, R.E.; Ellsworth, M.J.; Schmidt, R.R.; Cozzolino, V., “Review of cooling technologies for computer products,” Device and Materials Reliability, IEEE Transactions on , vol.4, no.4, pp.568,585, Dec. 2004.

[2]        Mohammed, R.K.; Yi Xia; Sahan, R.A.; Pang, Y., “Performance improvements of air-cooled thermal tool with advanced technologies,” Semiconductor Thermal Measurement and Management Symposium (SEMI-THERM), 2012 28th Annual IEEE, vol., no., pp.354, 361, 18-22 March 2012.

[3]        http://en.wikipedia.org/wiki/Computer_cooling

[4]        Abbas, T.; Abd-elsalam, K.M.; Khodairy, K.H., “CPU thermal management of personal and notebook computer (Transient study),” Thermal Issues in Emerging Technologies Theory and Applications (ThETA), 2010 3rd International Conference, vol., no., pp.85, 93, 19-22 Dec. 2010.

[5]        Ye Li; Tong Zhengming; Huang Liping; Chen Hao, “Studies on heat transfer performances of a heat pipe radiator used in desktop PC for CPU cooling,” Materials for Renewable Energy & Environment (ICMREE), 2011 International Conference, vol.2, no., pp.2022, 2026, 20-22 May 2011.

[6]        Mohammed, R.K.; Yi Xia; Sahan, R.A.; Pang, Y., “Performance improvements of air-cooled thermal tool with advanced technologies,” Semiconductor Thermal Measurement and Management Symposium (SEMI-THERM), 2012 28th Annual IEEE, vol., no., pp.354, 361, 18-22 March 2012.

[7]        Chu, R.C.; Simons, R.E.; Ellsworth, M.J.; Schmidt, R.R.; Cozzolino, V., “Review of cooling technologies for computer products,” Device and Materials Reliability, IEEE Transactions on, vol.4, no.4, pp.568, 585, Dec. 2004.

[8]        Chien-Yuh Yang; Chun-Ta Yeh; Pei-Kang Wang; Wei-Chi Liu; Kung, E.Y.-C., “An in-situ performance test of liquid cooling for a server computer system,” Microsystems Packaging Assembly and Circuits Technology Conference (IMPACT), 2010 5th International, vol., no., pp.1, 4, 20-22 Oct. 2010.

[9]        Song-Hao Wang; Guang-Yi Lee; Wei-Zhi Wang; Zhi-Yu Wang; Chyi-Shyan Tsai, “An Innovative Active Liquid Heat Sink Technology for CPU Cooling System,” Electronic Packaging Technology, 2007. ICEPT 2007. 8th International Conference on, vol., no., pp.1, 6, 14-17 Aug. 2007.

[10]     Halliday, D., Resnick, R., & Walker, J. (2014). Fundamentals of Physics, 10th Ed. Chichester: John Wiley & Sons, Inc.

[11]     Coulson, J. M. & Richardson, J. F. (1999). Fluid Flow, Heat Transfer and Mass Transfer, 6th Ed. Massachusetts: Butter worth–Heinemann.

[12]     Maloney, J. O. (2008). Perry’s Chemical Engineers’ Handbook, 8th Ed. New York: McGraw-Hill.

[13]     McCabe, W. L., Smith, J. C., & Harriott, P. (1993). Unit operations of chemical engineering, 5th Ed. New York: McGraw-Hill.

[14]     Heath, N. (2012). 10 coolest uses for the Raspberry Pi. Retrieved July 14, 2014 from http://www.techrepublic.com/blog/european-technology/10-coolest-uses-for-the-raspberry-pi/.

[15]     Raspberry Pi Model B Schematic. Retrieved July 15, 2014 from http://www.raspberrypi.org/help/faqs/.

CPU Cooling Technologies

Introduction

Click HERE to download a PDF version of this entry.

The Central Processing Unit (CPU) generates a great deal of undesirable heat in modern computing systems. The CPU is responsible for processing most of the data within systems and is often referred to as a computer’s central processor or simply processor. As data is processed within a system, heat is generated. Once heat thresholds are exceeded, CPUs are placed at risk of malfunction or permanent damage.

To overcome overheating, systems can be equipped with cooling systems that help to regulate temperature within a unit thus maintaining efficient operation. This paper examines the most common cooling technologies used today as well as a few that are currently in the early stages of research and development.

Any examination of CPU cooling, or any kind of electronics cooling for that matter, must take into account Moore’s law which predicts the number of transistors placed on integrated circuits will double every two years. As CPUs continue to decrease in size while exponentially increasing in power each year, adequate cooling methods will increasingly become an integral part of new design planning efforts. Figure 1 shows the evolution of module-level heat flux in high-end computers over several decades. As shown, module heat flux has continued to creep upward with each passing year.

Figure 1: Evolution of module level heat flux in high-end computers [1]

Figure 1: Evolution of module level heat flux in high-end computers [1]

This paper evaluates several CPU cooling methods that are currently available to mitigate overheating, with specific emphasis placed on liquid cooling technologies. The following section begins with the basic cooling technology most commonly used in the past – air-cooling. The paper next examines incremental improvements over air-cooling including the use of heat sinks, heat pipes, and a complete liquid cooling setup. We will also examine several advanced options that emerge from these basic system types such as the use of heat pipes for existing heat sinks, active liquid heat sinks, liquid immersion, and thermoelectric cooling technologies. Relevant data and comparison derived from a number of published papers are presented to support the provided information.

Air Cooling

Air-cooling, which incorporates the use of fans, is currently the prevalent method of cooling CPUs in computing environments. It has several advantages including reduced cost, relatively low noise, and is free of piping elements, tubes and cables. [2] The main function of fans is to pump air so that heat is effectively carried away from the CPU assembly. Air pressure can vary by incorporating fans in series (placed on top of one another) or parallel configurations (side-by-side). [3] The serial setup increases the discharge pressure while the parallel setup increases the area coverage.

Based on several studies completed in recent years [1][2][4][5][6][7][8][9], air-cooling technology is insufficient to keep up with the growing requirements of CPU cooling in the marketplace. As a result of these deficiencies, a number of new cooling technologies have been developed while other advanced cooling technologies are currently under study.

Heat Sink

One simple CPU cooling solution is accomplished via the use of a basic heat sink. A heat sink is a term for a component assembly that transfers heat generated within a solid material to a fluid medium such as air or a liquid. [4] A heat sink uses its extended surface (also called fins) to extend the surface of the material that is in contact with air or liquid. The main factors that affect the heat sink thermal performance are air velocity, choice of material, and fin design. A sample heat sink is shown in Figure 2.

Figure 2: Sample Heat Sink

Figure 2: Sample Heat Sink

An experiment [4] was completed that incorporates an aluminum alloy heat sink with 18 fins and a fan designed to produce a 0.026m2/s rate of airflow. The maximum temperature of the CPU reached a temperature of 39.83666°C and the heat removed from the configuration is 2-11 Watts (refer to Figure 3).

Figure 3: Time with respect to the temperature of processor

Figure 3: Time with respect to the temperature of processor

Heat Pipe Radiators

Another study [1] was conducted to extend the capability of heat sinks with fins. The research examined a heat sink that uses head pipes as a replacement for the fins. The primary difference is that the head pipes for the heat sink are hollow, while the fins are solid material. The assumption is that the heat pipes are geometrically equivalent to the solid pipes but possess a higher heat-transfer coefficient. An illustration of a heat sink that utilizes heat pipes is shown in Figure 4.

Figure 4: Heat sink that uses heat pipes

Figure 4: Heat sink that uses heat pipes

The study compared the thermal performance of a heat pipe radiator (i.e. heat sink) and the finned radiator. We can see in Figure 5 that the heat pipe radiator is an improvement over the ordinary finned radiator.

Figure 5: Heat pipe radiator v. finned radiator

Figure 5: Heat pipe radiator v. finned radiator

Researchers also studied the effects of varying the heat pipe diameter along with the number of heat pipes used. The results are shown in Figures 6 and 7 respectively. We can see that increasing the number and diameter of the pipes increases the thermal performance of the heat pipe radiator.

Figure 6: Varying the diameter of heat pipes

Figure 6: Varying the diameter of heat pipes

Figure 7: Varying the number of heat pipes

Figure 7: Varying the number of heat pipes

Heat Pipe

A heat pipe is an evacuated and sealed pipe that contains a small amount of working fluid and a wick structure. [6] A heat pipe is an additional structure that is added to a heat sink. Figure 8 shows the model of a heat pipe layout and an actual heat pipe is shown in Figure 9. This is a relatively simple design, as it involves no moving elements or additional tubes for the liquid. It also does not require a power source because it uses the natural convection of liquid for distribution. A heat sink with embedded heat pipes can offer thermal performance improvements of up to 20% when compared to a typical aluminum or copper base heat sink. [6]

Figure 8: Heat sink design with embedded heat pipe

Figure 8: Heat sink design with embedded heat pipe

Figure 9: Sample heat sink with heat pipe [Source: circuitremix.com]

Figure 9: Sample heat sink with heat pipe [Source: circuitremix.com]

Heat pipes offer three primary advantages. First, the heat pipes help to distribute heat thereby increasing thermal conductivity.  Second, they serve as a heat conductive path for transmitting heat from the base to another location so that heat can be managed within small CPU packages. Third, the heat sink effectively increases the conductivity and the efficiency of the traditional heat sink. [6]

The design of heat pipes can be varied to improve their performance as well. Figure 10 shows the performance of heat pipes with varying pipe diameter. The diameter is varied between 3mm, 4mm, and 6mm. The Y-axis, (Qmax●Leff) shows the amount of heat that the pipe can carry per meter length. We can see that performance of the heat pipe is improved as the diameter is increased. The heat pipe also has a higher efficiency rating as temperatures rise.

Figure 10: Thermal Performance of Heat Pipes with Varying Diameter [6]

Figure 10: Thermal Performance of Heat Pipes with Varying Diameter [6]

Water Cooling

The next cooling technology to review is the basic liquid cooling system. A liquid-cooled system places a liquid-cooled heat exchanger in the heat source to extract heat and reduce air temperature. [7] Compared to air, water-cooling can provide almost an order of magnitude reduction in thermal resistance due to the higher thermal conductivity of water. Because of higher density and specific heat of water, its ability to absorb heat in terms of the temperature rise across the coolant stream is approximately 3500 times that of air. [7]

A study [8] was conducted to compare a liquid cooling system to a heat sink with heat pipe configuration discussed above. The heat sink used is the IBM power series server x3350 cooler. It contains a straight fin array with bended heat pipe heat spreader. [8] Cooling air was sucked by four hot swap fans and transferred out the left side of the system (Figure 11).

Figure 11: Cooling system for IBM power series server

Figure 11: Cooling system for IBM power series server

As for the liquid cooling system, an integrated cold liquid and pump liquid cooling system was used (Figure 12). The fan shown is an integration of a fan and a heat sink.

Figure 12: Assembly diagram of the liquid system

Figure 12: Assembly diagram of the liquid system

Three different types of heat sink-fans were used. The specifications of each fan are shown in Table 1.

Table 1: Detail Dimension of the Fan and the Fin Array Tested

Table 1: Detail Dimension of the Fan and the Fin Array Tested

The result of the experiment is shown in Figure 13. We can see that the liquid cooling system performs better than the existing integrated heat sink with heat pipe that the current IBM server is using.  We can also see there that the bigger the dimension of the fan, the better the thermal performance. This shows that the LC-1212 cooling system is the best cooling system for this experiment.

Figure 13: Cooling performance test results

Figure 13: Cooling performance test results

Aside from thermal performance, the noise level and the power consumption of the four cooling systems were also studied. The results are shown in Table 2 and Table 3. These results show that the liquid cooling system provides much higher cooling performance and lower power consumption coupled with lower system noise. The low noise level and low power consumption is due to the significant reduction of the speed of the fan. A large amount of air is not necessary in a liquid cooling system, which consequently lowers noise and power consumption. [8]

Table 2: Cooling performance and noise level test results

Table 2: Cooling performance and noise level test results

Table 3: Power consumption for each cooling system

Table 3: Power consumption for each cooling system

Active Liquid Heat Sink

A new design [9] was created in an effort to simplify the system and remove the external liquid pump of the typical liquid cooling system described above. It is composed of a liquid heat sink, liquid pump, a fan, and a radiator. This new system was called the Active-Liquid-Heat-Sink (ALHS). The main concept of this system is that the liquid heat sink actively pumps the cooling liquid in and out by itself, without the help from outside pumping system. [9] Figure 14 displays the layout of the liquid cooling system and the layout of ALHS. The new design integrates the liquid heat sink and the pump, which eliminates the housing needed for the liquid pump. This reduced the overall dimension of the cooling system. The driving torque of the fan motor is transmitted to the impeller through a magnetic coupling, which means that no extra motor is required to move the cooling liquid.

Figure 14: Layout of the liquid cooling system (left) and the ALHS (right)

Figure 14: Layout of the liquid cooling system (left) and the ALHS (right)

Figure 15 displays the actual images of the water block cooling system (left) and the ALHS (right). We can see that the ALHS is more compact and is actually comparable to the size of a typical heat sink system.

Figure 15: Regular water block v. ALHS

Figure 15: Regular water block v. ALHS

The study also compared the ALHS with three other systems: liquid-cooled system, heat pipe system, and an air-cooled system. We can see in the results shown in Figure 16 that ALHS has the best thermal performance compared to the three cooling systems testing.

Figure 16: Temperature v. Time

Figure 16: Temperature v. Time

Immersion Cooling

Immersion cooling is another advanced system of cooling CPUs where the coolant is in direct contact with the CPU itself. With this method, most of the contributors to internal thermal resistance are eliminated. Direct liquid immersion cooling offers a high heat transfer coefficient that reduces the temperature rise of the CPU surface above the liquid coolant temperature. [1]

One recent technique of immersion cooling is achieved through spray cooling where very fine droplets of liquids are sprayed directly onto the CPU. Cooling of the surface is then achieved through a combination of thermal conduction through the liquid contact with the surface and evaporation at the liquid-vapor interface. [1]

Intermittent spray cooling technique was investigated in a study by [11]. One reason for using intermittent spray cooling is because most systems have varying heat flux and the CPU needs to be cooled in a particular range only. Thus, the spray mechanism is only activated when the temperature of the CPU reaches a certain limit, and is in turn turned off when the temperature decreases to the lower threshold.

The result of this particular study is shown in Figure 17. The study found that for a high heat flux event, the spray cooling is turned on for a longer period. This is because it takes a longer time to cool the CPU. Researchers also observed that temperature fluctuations are minimized when the surface temperature is at a sufficient super heat, due to the cushioning effect provided by the evaporation of liquid on the surface when the spray cooling is off. [11]

Figure 17- Typical temperature transient and valve state

Figure 17: Typical temperature transient and valve state

De-ionized water was used as cooling for the intermittent spray cooling. Research [10] was also done to compare the heat transfer of different dielectric coolants used for immersion cooling. The thermal model used for the comparison of heat transfer is the annular rift geometry model (see [10] for details). The coolants that were studied include mineral oil (MIL), silicon oil (SIL), pentaeryt tetraester (PTE), silicate ester (SIE), and perflourcarbons (FPC). Their thermal properties are shown in Table 4.

Table 4: Thermal Properties of Dielectric Coolants

Table 4: Thermal Properties of Dielectric Coolants

Figure 18 displays the resulting thermal resistance of the different coolants in varying conditions. The results suggest that all coolants will show improved behavior with increasing temperature. This is because of the decreasing viscosities at higher temperatures that lead to higher Rayleigh and Nusselt numbers. The PTE has a higher thermal performance compared to other liquid as the temperature increases. On the other hand, the SIL and FPC achieved the lowest thermal resistance with lower temperatures. [10]

Figure 18: Results of the thermal modeling for different conditions

Figure 18: Results of the thermal modeling for different conditions

Thermoelectric Cooling

Another advance cooling system currently under study is the thermoelectric cooling system. Thermoelectric heat pumps perform the same as other cooling systems where the thermal energy is extracted from a region and then rejected to a heat sink. [4] All other systems use moving parts and require a working liquid whereas thermoelectric elements are all solid state. Passing a current through the heat pump generates a temperature differential across thermocouples. This transfers the heat from one side to another. The system is illustrated in Figure 19.

Figure 19: Thermoelectric Cooling System

Figure 19: Thermoelectric Cooling System

Researchers compared thermoelectric cooling to three other cooling systems: water-cooling, heat pipe, and heat sink. The results are shown in Figure 20. We can see from the results that the thermoelectric cooling system maintains the temperature of the CPU in an almost constant state.

Figure 20: Comparison between processor temperature and time for all systems

Figure 20: Comparison between processor temperature and time for all systems

Summary

This paper has reviewed different cooling systems beginning with the basic fan-based air-cooling systems commonly used today. Incremental system upgrades were next introduced from one cooling system to another. These include a common cooling method that utilizes a heat pipe cooling system composed of a radiator, heat pipe and a fan, as well as liquid-cooled systems that add a liquid exchanger to the heat pipe cooling design. A system was also presented that involves replacing the typical solid fins of the radiator with a hollow material (head pipes) to increase thermal conductivity. Additionally, the ALHS cooling system was introduced which was created to reduce the size of a typical liquid cooling system. The paper also discussed how CPU immersion could be utilized to remove most internal thermal resistance. The intermittent spray cooling technique was explored which is a type of immersion cooling sometimes used in an effort to maintain the temperature of CPUs within a specified operating range. An alternative technique (thermoelectric cooling system) designed to maintain the CPU in a steady state was explored as well. This method is unique relative to other cooling systems because it uses solid-state materials.

With each system upgrade, CPU cooling and overall thermal management is improved. It is important to note that each upgrade brings new complexity to designs that directly relates to the cost of the selected cooling system. With the current rate of increasing heat flux existing within the semiconductor industry today (and in the future) however, engineers can no longer afford to ignore the benefits that new CPU cooling techniques offer. In the end, the selection of a specific cooling system depends on the application of the CPU. As CPUs continue to advance in terms of computing power in the years ahead, usage of advanced cooling systems such as those outlined within this paper will become increasingly commonplace. 

Reference

[1]        Chu, R.C.; Simons, R.E.; Ellsworth, M.J.; Schmidt, R.R.; Cozzolino, V., “Review of cooling technologies for computer products,” Device and Materials Reliability, IEEE Transactions on , vol.4, no.4, pp.568,585, Dec. 2004.

[2]        Mohammed, R.K.; Yi Xia; Sahan, R.A.; Pang, Y., “Performance improvements of air-cooled thermal tool with advanced technologies,” Semiconductor Thermal Measurement and Management Symposium (SEMI-THERM), 2012 28th Annual IEEE, vol., no., pp.354, 361, 18-22 March 2012.

[3]        http://en.wikipedia.org/wiki/Computer_cooling

[4]        Abbas, T.; Abd-elsalam, K.M.; Khodairy, K.H., “CPU thermal management of personal and notebook computer (Transient study),” Thermal Issues in Emerging Technologies Theory and Applications (ThETA), 2010 3rd International Conference, vol., no., pp.85, 93, 19-22 Dec. 2010.

[5]        Ye Li; Tong Zhengming; Huang Liping; Chen Hao, “Studies on heat transfer performances of a heat pipe radiator used in desktop PC for CPU cooling,” Materials for Renewable Energy & Environment (ICMREE), 2011 International Conference, vol.2, no., pp.2022, 2026, 20-22 May 2011.

[6]        Mohammed, R.K.; Yi Xia; Sahan, R.A.; Pang, Y., “Performance improvements of air-cooled thermal tool with advanced technologies,” Semiconductor Thermal Measurement and Management Symposium (SEMI-THERM), 2012 28th Annual IEEE, vol., no., pp.354, 361, 18-22 March 2012.

[7]        Chu, R.C.; Simons, R.E.; Ellsworth, M.J.; Schmidt, R.R.; Cozzolino, V., “Review of cooling technologies for computer products,” Device and Materials Reliability, IEEE Transactions on, vol.4, no.4, pp.568, 585, Dec. 2004.

[8]        Chien-Yuh Yang; Chun-Ta Yeh; Pei-Kang Wang; Wei-Chi Liu; Kung, E.Y.-C., “An in-situ performance test of liquid cooling for a server computer system,” Microsystems Packaging Assembly and Circuits Technology Conference (IMPACT), 2010 5th International, vol., no., pp.1, 4, 20-22 Oct. 2010

[9]        Song-Hao Wang; Guang-Yi Lee; Wei-Zhi Wang; Zhi-Yu Wang; Chyi-Shyan Tsai, “An Innovative Active Liquid Heat Sink Technology for CPU Cooling System,” Electronic Packaging Technology, 2007. ICEPT 2007. 8th International Conference on, vol., no., pp.1, 6, 14-17 Aug. 2007.

[10]      Lenke, R.U.; Christoph, M.; De Doncker, R.W., “Experimental characterization of immersion-cooled devices at elevated ambient temperatures,” Power Electronics Specialists Conference, 2008. PESC 2008. IEEE, vol., no., pp.493, 497, 15-19 June 2008.

[11]     Somasundaram, S.; Tay, A.A.O., “Intermittent spray cooling — Solution to optimize spray cooling,” Electronics Packaging Technology Conference (EPTC), 2012 IEEE 14th, vol., no., pp.588, 593, 5-7 Dec. 2012.

Coefficient of Thermal Expansion

Introduction

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Paraffins such as those found in Opticool Fluid may be used as heat transfer mediums in electrical cooling and thermal systems, in liquid phase cooling of electronic components, in thermal energy storage and a host of similar applications 1,2,. Although air and water have long been employed as cost-effective heat transfer agents, paraffins have the advantage of being good electrical insulators which is important in applications where removal of heat by convection – that is to have the fluid directly in contact with heated electronic components – is desired.

Paraffins have better thermal insulating capability than air-cooled systems. Their other notable properties that make paraffins useful for cooling applications include thermal and chemical stability, non-toxicity, biodegradability, and low cost.

One particular property found within paraffins involves a low coefficient of thermal expansion. Coefficient of thermal expansion is defined as the change in volume of a material per one degree Celsius change in temperature. This is an intensive property and is unique for each substance.

For applications requiring fluid to be contained in pipes or containers, this property is important to consider in choosing the fluid suited for each application. As the fluid is exposed to varying temperatures, its volume also changes and the container or pipes should be designed to accommodate this volume changes to ensure proper functioning. This helps to avoid build-up of hydraulic pressure and spills during equipment servicing as well. Because the coefficient of thermal expansion for paraffins are much lower than that of air, their use as coolants offer many advantages to modern systems designers in the unending pursuit of technological miniaturization.

The coefficient of thermal expansion of a particular substance is precisely determined via experimental methods such as dilatometric and pycnometric procedures. The first method measures the changes in volume of the object using a length sensitive device and requires that the objects are highly elastic, mostly solids or solid-like material such as pastes. The second method relies on the fact that changes in volume can be indirectly obtained from the densities of materials as temperatures change 3. Both methodologies are extremely laborious to perform 3, however, if the fluid is well-researched and their densities are known for a certain temperature of interest, the coefficient of thermal expansion of the fluid can be estimated. Thus, volume changes can be calculated as well in the following manner.

To obtain the equation for the Coefficient of Thermal Expansion, we begin with the Ideal Gas Law:

Fig1a1

From the above equation, we have the Volumetric Coefficient of Thermal expansion, beta, as: Fig1a2 This is the coefficient of Thermal expansion, changing with Temperature.

For a finite change of Temperature, we can get the approximate value of the Thermal expansion coefficient as:

Fig1a3

It should be noted here that for Fluids, beta or Coefficient of Volumetric Thermal expansion is used; while for Solids alpha or Coefficient of Linear Thermal expansion is commonly used. With that, we will have the following formula for the fluid as:

Figi1a1

And for a solid as:

Figi1a2

For an application that uses a Solid as a container for the Fluid, we will be dealing with Volume of the Solid. Thus, obtaining the relationship of alpha and beta as applied to solid will be useful, and the approximate relationship will be as follows:

Fig2a1

With the relevant equations outlined, we can now apply these to a practical application. We will start by calculating the required size of a container needed to accommodate a volume of a particular liquid (Paraffin) over a known temperature range. We will use the graph shown here to indicate the temperature range of operation (given that the density at said points are known):

Fig2a2

Fig3a

For further analysis, we will calculate the Void space (volume) of the Container when the liquid completely cools down. This is the state #3 in the above graph. It should be noted that the locations of the said temperature points are arbitrary as long as density at that point is known.  For this analysis, we assume that T1 = T3 (that applies to the full temperature range of the Fluid).

Fig4a

Specific Example

For the case of Paraffin (C16-C28), we have the Following:

Fig5a

Conclusion:

Mineral oils are a mixture of hydrocarbons constituting saturated straight chain and cyclic hydrocarbons as well as aromatic hydrocarbons. Paraffins are purified portions of mineral oils and consist primarily of “paraffinic” or saturated straight chain hydrocarbons. Paraffins are also further classified to many fractions having specific flash/fire points, depending upon the number of carbons in the backbone of the constituent hydrocarbon for a certain fraction. In real-world cooling applications, densities of a specific paraffin fraction at a certain temperature should be accurately identified in order to accurately estimate the coefficient of thermal expansion and volume changes using the equations above. While some values may be obtained via textbooks and information found on the Internet, more reliable data may be obtained from suppliers or manufacturers for each fluid under evaluation.

Reference
  1. N. Ukrainczyk, S. Kurajica, and J. Sipusie. “Thermophysical Comparison of Five Commercial Paraffin WSaes as Latent Heat Storage Materials.” Chem. Biochem. Eng. . 24 (2) 129-1377 (2010).
  1. B. Zalba, J. Marin, L. Cabea and H. Mehling. “Review on thermal energy storage with phase change: materials, heat transfer analysis and application.”Applied Thermal Engineering 23 (2003) 251-283.
  1. M. Schimmelpfenning, K. Weber, F. Kalb, K.-H. Feller, T. Butz and M. Matthai. “Volume expansions of paraffins from dip tube measurements.”

Heat Capacity Overview

Heat-CapacityOne of the most important characteristics of a good heat transfer medium is a high heat capacity, or “specific heat”.

What, exactly, is the heat capacity of a material, and how does it affect cooling of a circuit board or in a data center?  Heat capacity is an intrinsic characteristic of a material, and refers to the amount of heat, measured in joules or calories, that must be input into a material in order to raise its temperature by a certain amount.  Different materials hold different amounts of heat (again, measured in joules or calories), even when they’re at the same temperature.  Think of heat capacity as the “thermal mass” of a material, if you will.

A heat capacity means that a relatively small mass of fluid carries a large amount of energy away, per unit temperature drop.  A fluid with a lower heat capacity would need a greater temperature drop or greater flow rate with more heat exchanger surface to transfer the same amount of heat away.

 

Reference:

Sundin, D. (2013). Discussion on Specific Heat. Retrieved May 4, 2014, from http://dsiventures.com/2013/09/04/discussion-on-specific-heat/

Liquid Cooling – Theory and Application

Introduction

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A growing number of engineering applications at different scales, from nuclear reactors to industrial heat exchangers to electronic devices to micro machinery, have thermal management concerns. Depending upon the requirement, cooling is commonly achieved by air or liquids, with each coolant category having its own suitability, advantages and disadvantages. Liquid cooling offers advantages of rapid and efficient heat removal from a source, often with a lower thermal gradient, due to high specific heat capacities of many engineering fluids. Liquids, and especially water, are also sometimes used in evaporative cooling applications, where their high latent heat of vaporization allows removal of large quantities of heat in confined spaces. This paper broadly outlines various types of liquid coolants, their theoretical basis and performance characteristics, as well as their applications.

Theoretical Framework

Fluid flow can be laminar (steady state) or turbulent, and heat might be transferred with and without phase change. In addition, the flow regime might be treated as Newtonian or non-Newtonian. Appropriate theoretical and empirical heat transfer equations have been developed for different velocity profiles, flow regimes and flow geometries. For the ideal case of fluid flow in a shell-and-tube exchanger, heat is transferred by radiation and convection to tubes via conduction through tube walls and by forced conduction from the internal wall surface to the bulk fluid. The basic equation governing all such heat transfer is

Q=UADelta T (Eq. 1)

where Q represents heat transferred in unit time, U represents overall heat transfer coefficient, A represents available surface area and ΔT represents temperature gradient between the source and the sink (or the inlet and the outlet). If multiple fluids or separating walls are used, then the overall coefficient U can be decomposed into individual coefficients h, each representing a particular medium. [1]

In instances of conductive heat transfer through several layers of materials, the thermal resistances can be added in series to obtain the total temperature gradient. This is shown in Figure 1 below, where xi represents media thickness and ki represent thermal conductivities:

Figure 1: Conductive heat transfer through composite media. Source: Coulson & Richardson, 1999, p. 391.

Figure 1: Conductive heat transfer through composite media. [2]

With reference to the above figure, the thermal gradient T1T4 is:

T_1-T_4=(x_1/(k_1 A)+x_2/(k_2 A)+x_3/(k_3 A))Q  (Eq. 2)

where Q becomes the ratio of total driving force to total thermal resistance per unit area. [2] In instances of convective transfer, which is often the principal mechanism in liquid cooling, the heat transfer coefficient may be expressed as a dimensionless relation known as the Nusselt number, or as a dimensional equation. Convection is distinguished between natural (fluid movement caused by the transfer process itself) or forced (fluid movement caused by an externally applied force); in the former, the Nusselt number for external spaces is

Nu={0.825+(0.387Ra^(1/6))/[1+(0.492/Pr)^(9/16) ]^(8/27) }^2  (Eq. 3)

where Ra, the Rayleigh’s number and Pr, the Prandtl number, are dimensionless numbers corresponding to the flow velocity and fluid properties. [3] When we have steady flow of fluid past an immersed flat plate, two boundary layers develop. The first is hydrodynamic, within which velocity profile changes from 0 at the plate surface to flow velocity at the outer boundary; the second is thermal in which temperature profile changes from the plate surface temperature to the fluid temperature. These boundary layers are shown in Figure 2 below:

Figure 2: Hydrodynamic and thermal boundary layers for steady flow of fluid past an immersed plate. Fluid temperature is T∞ and plate surface temperature is Tw; fluid velocity is u0. Entire plate is heated at top while there is an unheated length x0 at bottom. Source: McCabe, Smith, & Harriott, 1993, p. 332.

Figure 2: Hydrodynamic and thermal boundary layers for steady flow of fluid past an immersed plate. Fluid temperature is T∞ and plate surface temperature is Tw; fluid velocity is u0. Entire plate is heated at top while there is an unheated length x0 at bottom. [4]

For the particular case of forced convection with laminar flow and no phase change, the temperature gradient at the wall is given by

Eq 4  (Eq. 4)

where cp is the liquid’s specific heat, k is its conductivity, μ is its viscosity and ρ is its density. [4] The treatments of heat transfer for turbulent flow are more complex, but in the case of fully developed (or fully rough) flow of a Newtonian fluid, the velocity and temperature gradients within a tube have been observed to be parabolic. Empirical formulae for heat transfer with phase transition, for both natural and forced convection, have also been developed.

Types of Liquid Coolants and Performance Characteristics

A wide variety of liquid coolants have been developed for various applications in industrial and technological designs. These include water (perhaps the most widely used), molten salts, metallic fluids, thick pastes, dielectric fluids and nano composite fluids. Selection of the coolant often depends upon whether it directly comes into contact with the heated surface or not. If there is no contact, water is often used in many cases due to its ready availability. In instances where there is direct contact, then the dielectric constant of the selected fluid plays a role in its selection. For example, microelectronic devices with a high power output (above 100W/cm2) can be cooled by single and multiphase liquid metals owing to the latter’s high vapor pressure and thermal conductivities. The liquid metal alloy Ga68In20Sn12 shows much higher heat transfer coefficients when compared to water, while the liquid alloy Ga61In25Sn13Zn1 exhibits excellent thermal performance while removing heat from a metallic surface. [5]

These are shown graphically in Figure 3 below:

Fig 3aFig3b

Figure 3: Changes in ratios of heat transfer coefficients of Ga68In20Sn12 compare to water as flow changes from laminar to turbulent (left); thermal performance of in terms of heat flux removed (right). Ts is plate surface temperature and Tf is fluid inlet temperature. [5]

Physical and thermal characteristics of a number of such liquid metal alloys are shown in Table 1 below:

Table 1: Thermal and physical properties of various liquid metals employed in heat transfer. Source: Miner & Ghoshal, 2004, p. 507.

Table 1: Thermal and physical properties of various liquid metals employed in heat transfer. [5]

At the other end of the complexity scale, molten salts of fluorides, chlorides and nitrates are used for heat removal from the core of some nuclear reactors. A primary heat exchanger uses these salts for removing heat directly from the dissolved fuel, while a secondary heat exchanger containing pressurized steam is used to remove heat from the primary loop. The primary performance characteristic of such coolants is their ability to remove non-uniformly generated heat through both radiated and convective heat transfer. [6]

Recently, a great deal of research has been conducted into coolants that have a nano disperse phase, for application in complex electronic circuits and devices. Nano particles of copper or aluminum, upon being dispersed into a liquid phase, create coolants that have not only better thermal and rheological properties, but also substantially improved thermal conductivity and no extra pressure drop. The nano disperse phase constitutes less than 1% of total volume; if a molten metal alloy is used as the bulk liquid carrier, then thermal conductivities improve by up to 2.5 times. [7] This is shown in Figure 4 below:

Figure 4: Increased thermal conductivities when different nano particle species are dispersed in liquid-gallium coolant. Keff is thermal conductivity of the nano phase and Kf is that of the base phase. Source: Ma & Liu, 2007, p. 255.

Figure 4: Increased thermal conductivities when different nano particle species are dispersed in liquid-gallium coolant. Keff is thermal conductivity of the nano phase and Kf is that of the base phase. [7]

Similarly, multi-walled carbon nano tubes (MWCNTs) dispersed in water or ethylene glycol at 0.1-0.4wt.% have shown much greater heat exchange capacities when used as radiator coolant fluids. [8] Finally, low viscosity synthetic hydrocarbon dielectric fluids have successfully been used to cool electronic circuitry, high-power vacuum tubes, underwater hydraulics, and electric drivetrain motors.

Existing and Future Applications

It is not possible to mention all the varied applications of liquid coolants, thus only a few specialized and technological ones will be mentioned. The reason that liquid cooling is especially efficient for electronic components with high heat fluxes is that it can be used as a heat sink with micro-channels. Micro rectangular and trapezoidal grooves on silicon wafers can be designed with geometries optimized for rapid and efficient heat removal. The Reynolds number for flows within such channels is usually above 10,000 and pressure drops, which depend on their aspect ratios, are between 490 and 2940 Pa. The design of such a micro-channel block having an aspect ratio of 7.7 is shown in Figure 5 below:

Figure 5: Dimensions of a micro-channel heat sink (left); the actual assembly (right). Source: Chiu et al., 2011, p. 35, 36.

Figure 5: Dimensions of a micro-channel heat sink (left); the actual assembly (right). [9]

Another common application area is the use of coils or ducts containing fluids in order to dissipate heat from electromagnetic coils. Low viscosity fluids are used with copper tubes having a large number of windings and improved heat dissipation that improves the electromagnetic field strength. [10] Liquid immersion of circuitry via synthetic paraffins and isoparaffins have proven to be technologically viable alternatives for thermal management as well. Due to the reasons discussed above, liquid cooling is employed extensively in server farms, for individual high performance computers, and in supercomputing environments.

Liquid cooling is also expected to play an important role in many future applications such as robotics, quantum computing, high sensitivity optical and radio telescopes, and in astro-engineering. For example a combination of gadopentetic acid and D2O was used as a heat sink to cool a quantum system consisting of glycine and glutamate. [11] Cryogenic cooling systems consisting of liquid helium, nitrogen or other fluids are regularly employed to increase the sensitivity of telescopes and many other astronomy and physics equipment. Another novel application approach currently under research is the liquid cooling of personal, wearable garments. Also referred to as liquid cooled garments (LCGs), these mostly use water as the coolant. They are outfitted with a pump and a heat exchanger, where heat carried away from the body surface is exchanged with ice or another temperature sink. [12]

Conclusion

Liquid cooling technology is an important part of modern engineering applications, both at industrial and personal levels. Many different fluids have been developed for different application purposes, and research is ongoing to identify the material properties of novel liquid coolants. It is expected that their use will increase in the near future and lead to more powerful and efficient electronics and other devices.

Reference

[1]    Halliday, D., Resnick, R., & Walker, J. (2014). Fundamentals of Physics, 10th Ed. Chichester: John Wiley & Sons, Inc.

[2]    Coulson, J. M. & Richardson, J. F. (1999). Fluid Flow, Heat Transfer and Mass Transfer, 6th Ed. Massachusetts: Butter worth–Heinemann.

[3]    Maloney, J. O. (2008). Perry’s Chemical Engineers’ Handbook, 8th Ed. New York: McGraw-Hill.

[4]    McCabe, W. L., Smith, J. C., & Harriott, P. (1993). Unit operations of chemical engineering, 5th Ed. New York: McGraw-Hill.

[5]    Miner, A. & Ghoshal, U. (2004). Cooling of high-power-density microdevices using liquid metal coolants. Applied Physics Letters, 85(3), 506-508.

[6]    Qian, L. et al. (2010). Numerical research on natural convection in molten salt reactor with non-uniformly distributed volumetric heat generation. Nuclear Engineering and Design, 240, 796–806.

[7]    Ma, K.-Q. & Liu, J. (2007). Nano liquid-metal fluid as ultimate coolant. Physics Letters A, 361, 252–256.

[8]    Teng, T.-P. & Yu, C.-C. (2013). Heat dissipation performance of MWCNTs nano-coolant for vehicle. Experimental Thermal and Fluid Science, 49, 22–30.

[9]    Chiu, H.-C. et al. (2011). The heat transfer characteristics of liquid cooling heatsink containing microchannels. International Journal of Heat and Mass Transfer, 54, 34–42.

[10]  Ricci, L. et al. (2013). A current-carrying coil design with improved liquid cooling arrangement. Review of Scientific Instruments, 84, 065115 1-4.

[11]  Elias, Y. et al. (2011). Heat-bath cooling of spins in two amino acids. Chemical Physics Letters, 517, 126–131.

[12]     Yazdi, M. M. & Sheikhzadeh, M. (2014). Personal cooling garments: a review. The Journal of The Textile Institute, 1-20.

Welcome

Welcome to my new research site. Here, I’ll be posting research progress updates, helpful learning resources, etc. It is my hope that this will grow into a repository of papers, videos, project details, discussions and more. Most content will focus on the use of liquid immersion cooling for energy and technical applications. I don’t expect this site to grow overnight – research takes time and will likely continue to be a work in progress. Ultimately I do hope that those of you reading will find some interesting and useful stuff here.

As technology continues to shrink in size while becoming more powerful with each passing year, overheating issues will continue to present an impediment to progress. As the great Albert Einstein quote points out below, we can’t continue to approach the problem of technological overheating as we have in the past (i.e. air)  – it simply isn’t viable. I believe that liquid immersion cooling is an option that can be used to overcome many of the challenges we’re going to face in the years ahead. They warrant a closer look if nothing else. The research that is to be chronicled on this site will be focused on determining whether or not I’m right. I look forward to the journey.

einstein