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TECHNICAL PAPERS: Electronic Cooling

# Novel Design of a Miniature Loop Heat Pipe Evaporator for Electronic Cooling

[+] Author and Article Information
Randeep Singh1

Energy CARE Group, School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, P.O. Box 71, Bundoora, Victoria 3083, Australiarandeep.singh@rmit.edu.au

Energy CARE Group, School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University, P.O. Box 71, Bundoora, Victoria 3083, Australia

Masataka Mochizuki

Fujikura Ltd., 1-5-1 Kiba, Koto-ku, Tokyo 135, Japan

1

Corresponding author.

J. Heat Transfer 129(10), 1445-1452 (Feb 09, 2007) (8 pages) doi:10.1115/1.2754945 History: Received May 16, 2006; Revised February 09, 2007

## Abstract

Miniature loop heat pipes (mLHPs) are coming up with a promising solution for the thermal management of futuristic electronics systems. In order to implement these devices inside compact electronics, their evaporator has to be developed with small thickness while preserving the unique thermal characteristics and physical concept of the loop scheme. This paper specifically addresses the design and testing of a mLHP with a flat evaporator only $5mm$ thick for the cooling of high performance microprocessors for electronic devices. A novel concept was used to achieve very small thickness for the mLHP evaporator in which the compensation chamber was positioned on the sides of the wick structure and incorporated in the same plane as the evaporator. This is unlike the conventional design of the flat evaporator for mLHP in which the compensation chamber, as a rule, adds to the overall thickness of the evaporator. The loop was made from copper with water as the heat transfer fluid. For capillary pumping of the working fluid around the loop, a sintered nickel wick with $3–5μm$ pore radius and 75% porosity was used. In the horizontal orientation, the device was able to transfer heat fluxes of $50W∕cm2$ at a distance of up to $150mm$ by using a transport line with $2mm$ internal diameter. In the range of applied power, the evaporator was able to achieve steady state without any temperature overshoots or symptoms of capillary structure dryouts. For the evaporator and condenser at the same level and under forced air cooling, the minimum value of $0.62°C∕W$ for mLHP thermal resistance from evaporator to condenser $(Rec)$ was achieved at a maximum heat load of $50W$ with the corresponding junction temperature of $98.5°C$. The total thermal resistance $(Rt)$ of the mLHP was within $1.5–5.23°C∕W$. At low heat loads, the mLHP showed some thermal and hydraulic oscillations in the transport lines, which were predominately due to the flow instabilities imposed by parasitic heat leaks to the compensation chamber. It is concluded form the outcomes of the present investigation that the proposed design of the mLHP evaporator can be effectively used for the thermal control of the compact electronic devices with high heat flux capabilities.

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## Figures

Figure 1

Schematic of the experimental prototype and test layout for the mLHP. (a) Bottom view of the mLHP showing temperature measurement points and the heater location. (b) Top view of the mLHP showing different loop components and condenser fan location. (c) Side view of the mLHP showing the evaporator thickness and the heater location relative to the evaporator active zone.

Figure 2

mLHP Evaporator. (a) Cross-sectional details of the mLHP evaporator. (b) Sectional view of vapor removal channels.

Figure 3

Planar view of the mLHP evaporator showing the position of the evaporation zone and CC

Figure 4

Startup of mLHP at 20W input heat load

Figure 5

Figure 6

Interface temperature versus applied heat load

Figure 7

mLHP startup at 10W input heat load showing notable temperature oscillations at the evaporator outlet and exit of the vapor and liquid line

Figure 8

mLHP evaporator to condenser thermal resistance versus applied heat load

Figure 9

Total thermal resistance versus applied heat load

Figure 10

Evaporator thermal resistance versus applied heat load

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