micro loop

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Micro Loop Heat Pipe for Mobile Electronics Applications

CHAPTER 1

INTRODUCTION

Current mobile electronics, such as smart phones and tablet PCs, comprise many heat-generating devices, including CPUs, memory cards, communication ICs, wireless and

display control ICs, camera devices, and batteries. These devices have been miniaturized and designed for high-density packaging. As the heat-generating density of these

devices has increased, thermal management of the resulting heat generation has become a major problem. Mobile electronics show a complex thermal behavior due to their

usage under various circumstances. Therefore, the thermal design for these high-heat-generating mobile electronic devices affects their reliability and usability. Numerous

studies on potential cooling solutions for mobile electronic systems, including high conductivity materials [1], phase change materials [2], and heat pipe technologies [3,4],

have focused on distributing heat as evenly as possible in the devices. The heat-spreading performance of high conductivity materials, such as graphite sheets and clad metal

sheets, are limited by their thermal conductivity. Small, thin-layered phase change materials cannot sufficiently respond to the large degree of heat flux generated from high-

performance ICs. The use of heat pipe technology, which exhibits heat-transport performance superior to that of high conductivity materials, can offer effective solutions for

thermal diffusion within mobile devices. A loop heat pipe (LHP) and a capillary pumped loop (CPL) together form a passive two-phase heat transfer system that employs

capillary pumping of the working fluid and uses the latent heat of the vaporized fluid to transfer heat [5]. The advantages of this system, as compared with other two-phase

technologies such as the conventional heat pipe, the thermo syphon, and the vapor chamber, include its higher heat

transfer capability and low sensitivity to changes in orientation with respect to gravity. At present, studies are being conducted on the application of LHP technology for

cooling the electronics of PCs and servers [6,7]. Figure 1 shows our proposed concept for the thermal management of smart phones equipped with an LHP. This LHP

facilitates heat spreading by transporting the heat generated from the electronic components to low temperature spaces in the mobile devices without using an external power

source such as a pump. To apply the LHP to mobile electronics, a small, thin, and

compact system must be designed. Researchers have investigated small-size LHPs and CPLs using micro electro mechanical system (MEMS) technology as a candidate heat

transfer device for IC chip-level thermal management because the LHP and the CPL are more effective in heat transport than conventional heat pipes [8-12]. However, as the

LHP becomes thinner, its heat-transport performance decreases because thermo fluid flows are interrupted in fluid flow paths that have a smaller cross-sectional area. When

designing LHPs for thin, small devices, support for their thermal performance and inevitable downsizing is essential. To meet this requirement, we developed a thin micro

LHP (μLHP) for mobile electronic devices. In this study, we present our design and evaluation of the perfomance results for the proposed μLHP.

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Micro Loop Heat Pipe for Mobile Electronics Applications

CHAPTER 2

LITERATURE REVIEW

The most common coolants used in LHPs are anhydrous ammonia and propylene. LHPs are made by controlling the volumes of the reservoir carefully, condenser and vapor

and liquid lines so that liquid is always available to the wick. The reservoir volume and fluid charge are set so that there is always fluid in the reservoir even if the condenser

and vapor and liquid lines are completely filled. Generally small pore size and large capillary pumping capability are necessary in a wick. There must be a balance in the

wick pumping capability and the wick permeability when designing a heat pipe or loop heat pipe.

In a loop heat pipe, first the heat enters the evaporator and vaporizes the working fluid at the wick outer surface. The  vapor then flows down the system of grooves and then

goes to the evaporator and the vapor line towards the condenser, where it condenses as heat is removed by the radiator. The two-phase reservoir (or compensation chamber)

at the end of the evaporator is specifically designed to operate at a slightly lower temperature than the evaporator (and the condenser). The lower saturation pressure in the

reservoir draws the condensate through the condenser and liquid return line. The fluid then flows into a central pipe where it feeds the  wick. A secondary wick hydraulically

links the reservoir and the primary wick.

Heat pipes are excellent heat transfer devices but their sphere of application is mainly confined to transferring relatively small heat loads over relatively short distances when

the evaporator and condenser are at same horizontal level. This limitation on the part of the heat pipes is mainly related to the major pressure losses associated with the liquid

flow through the porous structure, present along the entire length of the heat pipe and viscous interaction between the vapor and liquid phases, also called entrainment losses.

For the applications involving transfer of large heat loads over long distances, the thermal performance of the heat pipes is badly affected by increase in these losses. For the

same reason conventional heat pipes are very sensitive to the change in orientation in gravitational field. For the unfavorable slopes in evaporator-above-condenser

configuration, the pressure losses due to the mass forces in gravity field adds to the total pressure losses and further affect the efficiency of the heat transfer process.

As a result of these limitations, different solutions involving structural modifications to the conventional heat pipe have been proposed. Some of these modifications

incorporate arterial tubes with considerably low hydraulic resistance for liquid return to the heat source (arterial heat pipes), while others provide spatial separation of the

vapor and liquid phases of the working fluid at the transportation section (separated line heat pipes).

Though these new forms of heat pipes are able to transfer significant heat flows and can increase heat transport length, they remain very sensitive to spatial orientation

relative to gravity. To extend functional possibilities of two-phase systems towards applications involving otherwise inoperable slopes in gravity, the advantages provided by

the spatial separation of the transportation line and the usage of non-capillary arteries are combined in the loop scheme.

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Micro Loop Heat Pipe for Mobile Electronics Applications

This scheme allows heat pipes to be created with higher heat transfer characteristics while maintaining normal operation in any directional orientation. The loop scheme

forms the basis of the physical concept of Two-Phase Loops (TPLs).

Loop heat pipes were patented in USSR in 1974 by Yury F. Gerasimov and Yury F. Maydanik (Inventor's certificate № 449213), all of the former Soviet Union. The patent

for LHPs was filed in the USA in 1982.

The first space application occurred aboard a Russian spacecraft in 1989. LHPs are now commonly used in space aboard satellites including; Russian Granat, Obzor

spacecraft, Boeing’s (Hughes) HS 702 communication satellites, Chinese FY-1C meteorological satellite, NASA’s ICESat. LHPs were first flight demonstrated on the

NASA space shuttle in 1997 with STS-83 and STS-94. Loop heat pipes are important parts of systems for cooling electronic components.

CHAPTER 3

SYSTEM ARCHITECTURE

We fabricated a μLHP by diffusion bonding using six copper plates 0.1 mm thick. Figure 1 shows a schematic of the μLHP’s six copper plates, with two surface and four

inner layer sheets. An evaporator section, a vapor transport line, a liquid return line, a condenser line, and a thermal diffusion plate were formed by chemical etching on the

copper. We designed the μLHP to be 107 mm × 58 mm, with a thickness of 0.6 mm for the evaporator and 1.0 mm for the vapor line, so that it can be fitted into the casing of

a smartphone. Table 1 lists the design parameters of the μLHP’s main components.

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Micro Loop Heat Pipe for Mobile Electronics Applications

Fig 1 – Schematic of µLHPs copper plates

Figure 2 shows an inner layer sheet of the evaporator section, in which we etched holes 200 μm in diameter in staggered positions such that the holes in the four inner layer

sheets formed a three-dimensional mesh structure for communication between each layer. Figure 3 shows a crosssectional SEM image of the evaporator section’s mesh

structure after diffusion bonding between the six copper plates. Each hole in the inner layers links with every other hole in every direction such that capillary forces develop

in the pore structure formed by the holes in each layer to circulate the working fluid. This structure functions as a wick in the evaporator of the μLHP. We also arranged a

similar mesh structure in the liquid line to assist in the return of the subcooled liquid to the evaporator. In addition, the evaporator section in the inner layers has vapor

grooves between the mesh structures to remove the vapor phase of the working fluid that is vaporized from the evaporator to the vapor line, as shown in Fig. 2. In a

conventional LHP evaporator, an LHP container is metal (e.g., stainless-steel, copper, aluminum) and a porous wick must be prepared separately. In general, wick materials

of nickel, copper, stainless, ceramic, or polymer resin are

selected.

When an evaporator is assembled, the contact between the inside of the container and the wick must be tight so that there is an efficient heat flux flow from the heat source to

the wick through the container. To meet performance standards, the assembly process is one of the key technologies in LHP fabrication and the cost of assembly is relatively

high. However, our proposed μLHP has a relatively cost-effective manufacturing process, because it uses a chemical-etching

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Micro Loop Heat Pipe for Mobile Electronics Applications

process on thin metal plates to prepare fine patterns in all the parts, including the LHP’s container (surface sheets) and its wick structure (inner layer sheets), as shown in Fig.

1. The μLHP fabrication process is characterized by an absence of any joint line between its parts. Also, when designing multiple shapes of the loop layout for a large copper

plate for each layer, we can fabricate a number of μLHPs in one batch using a fine alignment technique and diffusion bonding of the copper plates.

Fig 2 - Evaporator section of an inner layer sheet.

Fig 3 - Cross-sectional SEM image of the mesh structure

in the evaporator section after diffusion bonding between the copper plates.

CHAPTER 4

EXPERIMENTAL SETUP

We evaluated the μLHP’s heat transport characteristics by infrared thermography and measured the temperature using thermocouples in each part of the μLHP for the

applied heat input to the vaporator. All tests were conducted with water as the working fluid. We also evaluated the ravitational effect on thermal performance by changing

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Micro Loop Heat Pipe for Mobile Electronics Applications

the μLHP orientation and the amount of the charged working fluid used in the μLHP. Figure 5 shows a photograph of the μLHP test configuration. The heat flux to the

evaporator comprised the average amount of heat that passed through the 15 mm × 15 mm copper heater block in contact with one side of the evaporator. A 100 mm × 65

mm copper plate, 3 mm thick, was in contact with the condenser section of the μLHP. In the tests, the condenser was not actively cooled. The heat exchange between the

condenser and ambient air occurred by free convection. The thermal resistance Rec

was calculated as follows:

Rec = (

Tevp

– Tcond

)/Qin

Where: Qin is heat input for the evaporator

Tevp is the temperature of the evaporator

Tcond is the average temperature of the condenser from the condenser inlet to the outlet.

The thermal resistance Rec corresponds to the heat transport ability of the μLHP.

CHAPTER 5

RESULT & DISCUSSIONS

Figure 4 compares the temperature profiles of the evaporator (EVP) and the condenser inlet (CND-IN) with two μLHP prototype models in the start-up test at a heat input of

2.5 W for a horizontal orientation. When the height of the vapor line was 0.6 mm, the evaporator temperature continued to rise, and the condenser inlet temperature does not

rise. We observed no heat transfer when operating the μLHP with a vapor line thickness of 0.6 mm. However, when the vapor and condenser lines of the loop were increased

to between 1.0 mm and 1.2 mm, the condenser inlet temperature rose rapidly. This shows that the vapor phase of the working fluid comes through the condenser inlet from

the evaporator. We confirmed that, at this point, the evaporator temperature suddenly decreased and the μLHP transferred heat from the evaporator to the condenser. The

pressure drop of the vapour line can be decreased by a factor of ten by expanding the thickness of the flow path from 0.6 mm to 1.2 mm. Therefore, we conclude that the

loop operation strongly influences the pressure drop of the vapor and condenser lines. Figure 5shows an infrared photo of the steady state operation of the μLHP at a heat

input of 5 W. We observed an obvious difference in temperature between the vapor transport line and the liquid return line. From this measurement, at steady-state μLHP

operation, we predicted that there would be little heat conducted back from the evaporator to the liquid line.

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Micro Loop Heat Pipe for Mobile Electronics Applications

Figure 4: Temperature profile of the evaporator and

condenser inlet of two μLHPs in a start-up test with a heat

input of 2.5 W for a horizontal orientation

Figure 5: Infrared photo of the steady state operation of the

μLHP at a heat input of 5 W.

CHAPTER 6

CONCLUSION

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Micro Loop Heat Pipe for Mobile Electronics Applications

In this paper, a new solution of thermal management using LHP was proposed to equalize thermal imbalances in mobile electronic devices. We developed a μLHP, which

can be embedded in smartphones and tablet PCs, using a chemicaletching and diffusion-bonding process on thin copper plates. The μLHP was designed in a size of 107 mm

× 58 mm, with a thickness of 0.6 mm for the evaporator and 1.0 mm for the vapor line. We found the operation of the μLHP to strongly depend on the pressure drop in the

vapor and condenser lines. Thermal resistance between the evaporator and condenser of 0.8 K/W was achieved at a heat input of 5 W, with an evaporator temperature of 50.5

°C, and a minimum resistance was 0.32 K/W at 15 W. In the steady state operation of the μLHP, the heat leak from the evaporator to the liquid line was approximately 11%

of the applied input power. Experimental results showed a quick response to periodic power input and a long term stable operation of the μLHP. We also confirmed the

existence of a slight dependence of the μLHP

performance on its operating orientation. This study has demonstrated that the μLHP is a promising option for the thermal management of mobile electronics applications..

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Micro Loop Heat Pipe for Mobile Electronics Applications

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