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A Study of Two-Phase Cooling Methods for High Heat Flux Electronics

William Chow

University of California, San Diego

Jacobs School of EngineeringDepartment of Mechanical and Aerospace Engineering

MAE 221A

December 7th

, 2006

Abstract:

This paper investigates how two-phase heat transfer methods can be applied to the

cooling of high-power density electronic devices; specifically focusing on the methods whichinvolve the use of liquid droplet sprays and jets. Examples of current research work will be

discussed, as well as a detailed description of the importance and history of electronics

cooling. The conclusion of this report finds that, although its still in early stages of

development, two-phase liquid-vapor cooling techniques are a viable solution for future high-heat flux electronics, offering at least twice the heat dissipation of current mainstream cooling

systems.

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Introduction:

Figure 1: A computer processor silicon wafer during manufacturing.

Source: Intel Corporation

Recent improvements in semiconductor fabrication technology have produced smaller

and more powerful electronics, increasing the importance of efficient thermal management in

these components. Looking specifically at the computer processor industry, the seemingly

never-ending rise in processor speeds as well as the increasingly smaller manufacturing

processes has led to products with significant heat dissipation demands. Current computer

processors already possess heat fluxes of 50 W/cm2

to 80 W/cm2, and this number will only

increase as time goes on. Unfortunately, the performance of traditional air-cooling methods is

quickly becoming insufficient to handle these growing high heat flux requirements. Therefore,

new and more active methods of cooling these electronics must be developed.

The use of liquids, as an alternative to air, as the cooling medium appears to be the

most sensible solution to this dilemma. Newer, more efficient cooling systems involving

liquids will no doubt be prevalent in the future. In fact, several advanced thermal management

techniques that far exceed the heat flux requirements of modern electronics already exist.

However, they have yet to be simplified into the scalable and reliable form suitable for

consumer electronics. And with the increased demand for portability in todays market, the

performance of these techniques is no longer the only factor under consideration.

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This paper investigates the use of two-phase heat transfer practices in electronics,

specifically focusing on those which involve the use of liquid droplet sprays and jets. These

two-phase cooling methods are appealing than single-phase methods since the working fluid

is vaporized in one section and then condensed in another, leading to much higher heat

transfer coefficients. The application of liquid droplet sprays or impinging jets is

advantageous because it is one of the few methods that are able to provide direct cooling to

the surface of the microchip, while other liquid cooling methods can only provide indirect

cooling through conductive surfaces. The ultimate goal of this report will be to sum up the

current research progress for this cooling method and provide insight into how practical it will

be for widespread use.

Importance:

Proper thermal management in consumer electronics is very important. The immediate

effect of poor temperature control in these devices is twofold; not only does it degrade the

overall performance, but it will also affect the reliability of the component. If an electronic

device is not properly cooled, it will only be able to operate for short periods of time, greatly

detracting from its overall usability and may cause damage, shortening the lifespan of the

component. Because of this, it is essential that the temperature of the device stay within its

operating limits, which is only possible through effective heat diffusion methods.

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Figure 2: Apple Computer Inc. CEO Steve Jobs announcing the switch from PowerPC processors to Intel.

Source: Apple Computer Inc.

Another limitation caused by excess heat generation is that it directly limits the

performance potential of the component. A good example of this effect is in the case of

computer processors. As the speed of the processor is increased, the amount of heat the device

generates increases proportionally. Since this heat is typically dissipated by some external

cooling system, the performance of the processor is directly linked to how well this system

operates, so if it has limitations, those limitations will be reflected back onto the performance

of the processor. A good example of this comes from recent history; In the middle of 2005,

Apple Computer Inc. announced that they would stop using IBMs PowerPC processor in

their computers and switch to Intels Core Duo line of processors. The main motivation

behind this switch was the fact that IBMs processor ran at very high temperatures when

compared to Intels chip. Since most modern personal computers are still air-cooled using

fans and heatsinks, the IBM processor ultimately reached its limit; it could no longer increase

its performance without exceeding the heat flux capacity of its air-cooling system.

Looking generally at the realm of consumer electronics, allowing a device to exceed

its ideal operating temperature possesses even more consequences. Not only should the

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temperature be within the correct operating range for both performance and reliability, this

ideal temperature should be low enough for safe handling by the consumer. For example, a

common complaint about portable computers is that they tend to dissipate heat poorly,

causing the heat from its internal components to radiate through the outer casing, making the

surfaces hot to the touch. This is an obvious deterrent, for if the temperature were to reach an

unsafe level, the risk of injury would increase. Furthermore, the increasing demand for

portability in modern electronics only further emphasizes the need for more effective methods

of high-heat dissipation.

History:

The traditional method for cooling modern electronics is through air-cooling. The heat

generated by the integrated circuits present in the device needs to be dissipated in order to

keep the component at a safe and reliable operating temperature. The idea behind the use of

air-cooling is that it allows this heat generated to be dispersed into the surroundings through

convection. Early electronic devices were relatively simple and consequently did not possess

high rate of power consumption, which allowed them to be passively cooled using only

natural convection. Simply attaching a heatsink to these components as a method of

increasing heat transfer area was enough to keep it in an ideal operating range.

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Figure 3: A modern fan & heatsink computer processor cooler.

Source: Zalman Tech.

However, as the technology behind these electronics components improved, the

amount of heat produced became a side effect of the increased complexity and power

consumption of these devices. To offset this increase in temperature, more active cooling

methods have been created. The addition of larger heatsinks and electric fans has allowed the

use of forced convection to improve heat transfer rates. The use of a fan allows the hot air

around the device to be pushed away, while drawing in cooler air from the surrounding

environment. In fact, this fan and heatsink combination is the most widely used method for

cooling high power density electronics. However, as these integrated circuits become smaller

and more intricate, the heat flux necessary to cool them quickly becomes out of the reach of

these simple cooling systems.

Current alternatives to air-cooling include the use of liquids as the cooling medium.

Water-cooling is one method, commonly found in large data processing systems, where water

instead of air flows over a heatsink attached to the electronic component. Because water

possesses a higher heat capacity compared to air, it is able to transmit heat away from the

source at a much higher rate, resulting in more efficient cooling and a lower temperature

difference. Other, more uncommon cooling methods include submersion cooling, where the

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components are placed directly into a dielectric fluid, allowing passive cooling between the

device and the working fluid. Again, due to the liquids higher heat capacity, this method

offers greater heat transfer than even active air-cooling methods.

Jet impingement cooling has always been an attractive alternative to common cooling

methods, mainly because it is capable of very high heat removal rates. It is currently used in

many industrial applications, including, but not limited to, metal sheet cooling, cooling of

turbine blades and temperature regulation in high powered lasers. Jet impingement, as well as

other two-phase cooling methods allow for higher rates of heat dispersion when compared to

single-phase cooling due to the large amount energy absorbed by the phase change of the

liquid. Adapting this mechanism for use in electronic systems would offer a significant

improvement over traditional air-cooling or even more recent single-phase liquid cooling

techniques.

Examples of Current Research Work:

Two-phase cooling methods involving a liquid-vapor phase change have always been

an appealing option for removing heat from high power density electronics because their heat

transfer coefficients are typically high. Although these techniques have long been used in

many industrial applications, they only have recently become a viable for high-powered

thermal management solutions, and therefore, many studies have been aimed directly at this

type of application.

Fabbri et al. [1] conducted a comparative study of the application of two-phase

cooling methods on high heat flux electronics using sprays and microjets. An experiment was

conducted using HAGO nozzles and orifice plates to create droplet sprays and arrays of

microjets. The experimental setup diagram is shown in Figure 4. The jets possessed diameters

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ranging from 69 to 250 m, and used deionized water with jet Reynolds numbers ranging

from 43 and 3813. The experiment concluded that the microjet arrays were superior to the

spray nozzles, since they required less pumping power per unit of power removed, as well as

the fact that the pressure drop needed by the HAGO nozzles quickly reached values that were

impractical for normal applications. Using a system consisting of a 4 by 6 array of microjets

of water of 140 m resulted in heat fluxes as high as 300 W/cm2

for a surface temperature of

80 C.

Figure 4: Schematic of the cooling experiment done using droplet sprays and arrays of microjets.

Source: Fabbri et al. [1]

Additional research in the area of two phase cooling systems has been done using

microsprays. Amon et al. [2] developed an integrated droplet impingement cooling device

specifically designed for removing heat fluxes from computer chips. The system of

embedded droplet impingement for integrated cooling of electronics (EDIFICE), consisted of

microspray nozzles designed to produce droplets 50-100 m in diameter, along with a specific

texturing pattern on the heat transfer surface meant to promote droplet spreading, as seen in

Figure 5. The experiment also investigated the effect of different nozzle orifice shapes, and

their effect in promoting uniform heat transfer from the surface of the component. The final

experiment was conducted on a notebook computer processor using the EDIFICE system with

a swirl type nozzle orifice and HFE-7200 dielectric coolant as the working fluid. The results

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of the prototype experiment gave a uniform heat flux of 45 W/cm2

over the surface of the

processor, at a mass flux of 32.2 g/cm2min.

Figure 5: Schematic of the EDIFICE cooling system which makes use of microspray droplets.

Source: Anon et al. [2]

Mathematical Background:

The physical theory behind the heat dissipation process using impinging droplets is

very complex and not yet fully understood, so other examples of research work have focused

more on the mathematics behind these two-phase heat transfer methods. Early research done

by Jiji and Dragan [3] investigated the heat transfer effects of single-phase jets impinging on

microelectrical heat sources. Experiments were conducted using square arrays of 1, 4 and 9

jets, FC77 as the test fluid and square heat sources 12.7 12.7 mm in size. From the results of

this experiment, the area-averaged Nusselt number was correlated as function of the Reynolds

and Prandtl Numbers:

1 2 1 33.84Re Pr 0.08 1nL

n

LNu N

= +

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Where n is the jet diameter, L is the length of the heat source, and Nis the number of jets.

Another important conclusion from their research was the notion that the surface temperature

of the heat source became more uniform as the distance between the jets and the surface

decreased.

For arrays of multiple jets, there is a difference in heat transfer at the stagnation points

and the radial flow regions on the surface. Research done by Womac et al. [4] first

investigated two different jet configurations, one with 2 2 jets, and another with 3 3 jets.

Using both water and FC77 as the cooling fluid, experiments were conduction with Reynolds

numbers for both laminar and turbulent flow, ranging from 500 to 20,000. The rate of heat

transfer increased for cases of increasing jet velocity, but very significant heat transfer losses

were found when the fluid flow rate was decreased in the case of very low flow rates.

However, to account for the differences due to the stagnation points on the heat transfer

surface, an area-weighted method was used, and the resulting correlation for average Nusselt

number was found to be:

( )*0.5 0.579 0.4*0.516 Re 0.344 Re 1 Pr iL r Li

L LNu A A

L

= +

r

Where Ar = Ndi2/4L

2, Vi = (vn

2+2gz)

0.5, di = (Vndn

2/Vi)

0.5, and L

*= (2

(1/2)+1)s-di/4 is an

estimate of the average distance associated with radial flow.

An important aspect of evaluating the performance of a phase-change cooling system

is the critical heat flux. The critical heat flux is the maximum heat flux on the boiling curve of

the cooling liquid. If the critical heat flux is exceeded, a significant increase in the

temperature of the object will occur. Because of this, the critical heat flux turns out to be the

upper design limit for any cooling systems of this type. Nakayama et al. [5] researched the

boiling of jet impinging flows using FC72 as the working fluid. The conclusions from this

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research found that increases in jet velocities or the number of nozzles lead to higher values

for the critical heat flux. Critical heat flux values of up to 200 W/cm2

were obtained using a

system of multiple jets. In comparison, the heat flux needed by future high-power electronics

is estimated to be near100 W/cm2, showing that phase-change cooling systems are capable of

dissipating almost twice the heat required.

Personal Research Ideas:

Since so many experiments have already been conducted to determine the

performance of these two-phase cooling systems, if I were to perform my own, I would

choose to investigate the reliability of such systems. Current cooling systems, such as the fan

and heatsink combination for personal computers, have a lifespan of at least 40,000 hours. So

even if a two-phase jet impingement cooling system was adapted for mainstream use, it would

still need to meet the reliability of current solutions, despite its increase in cooling

performance.

For an experiment, I would propose setting up a system similar to those already used

in current research work. Since the general consensus is that jet impinging systems perform

better than sprays, I would choose to focus on the reliability of that type of system. However,

instead of system with variable specifications, I would choose fixed system parameters, such

as the number of jets, the jet Reynolds number, the heat source area, and so on. These

parameters would be chosen so that the resulting heat flux capacity is something reasonable

(>100 W/cm2), so that the reliability results can be representative of how an actual,

mainstream system would perform. So the final experiment would simply leave the system

running at set operating specifications, to determine if such a system could match the

reliability of the cooling methods already in use today.

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References:

[1] Fabbri, M., Jiang, S., and Dhir, V. K., 2005, A Comparative Study of Cooling of High

Power Density Electronics Using Sprays and Microjets, ASME Journal of Heat Transfer,

127, pp. 38-48.

[2] Amon, C. H., Yao, S. C., Wu, C. F., Hsieh, C. C., 2005, Microelectromechanical System-

Based Evaporative Thermal Management of High Heat Flux Electronics, ASME Journal ofHeat Transfer, 127, pp. 66-75.

[3] Jiji, L. J., and Dagan, Z., 1987, Experimental Investigation of Single-Phase MultijetImpingement Cooling of an Array of Microelectronic Heat Sources, Proceedings of the

International Symposium on Cooling Technology for Electronic Equipment, W. Aung, ed.,

Hemisphere Publishing Corporation, Washington, D.C., pp. 333351.

[4] Womac, D. J., Ramadhyani, S., and Incropera, F. P., 1993, Correlating Equations forImpingement Cooling of Small Heat Sources With Single Circular Liquid Jets, ASME

Journal of Heat Transfer, 115, pp. 106115.

[5]Nakayama, W., and Copeland, D., 1994, Heat Transfer from Chips to Dielectric Coolant:Enhanced Pool Boiling Versus Jet Impingement Cooling, J. Enhanced Heat Transfer, 1(3),pp. 231243.