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Researches on Evaporation Enhancement and Heat Leak Alleviation in Copper-water Loop Heat Pipe (LHP)

Author: XuJiaYin
Tutor: XuHong
School: East China University of Science and Technology
Course: Power Engineering and Engineering Thermophysics
Keywords: Copper-water LHP Powder sintering wick Evaporation enhancement Heatleak Charging ratio
CLC: TK124
Type: PhD thesis
Year: 2013
Downloads: 34
Quote: 0
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Abstract


Copper-water LHP can simultaneously present the highly efficient evaporation of the copper wick and high latent heat of water vaporization. Therefore, it is an effective two-phase heat transfer device, which features a passive heat transport system without moving parts. LHP has promising prospects in applications such as the cooling of high-power electronic chips and Light-Emitting Diodes (LEDs). However, massive heat leak resulted from traditional evaporator with a copper wick leads to a slow start-up process, poor heat transfer performance, which limits the application in electronic thermal management.In this paper, researches on evaporation enhancement and heat leak alleviation in the copper-water LHP were carried out. Main works and results are as follows:(1) A cylindrical evaporator without vapor deprime, which characterizes the eliminations of the contact thermal resistance and the chance of vapor deprime, was proposed. It was experimentally testified that the cylindrical evaporator could achieve a fast start-up with a low superheat, avoid the appearance of vapor deprime at high heat load operation, and improve the stability and reliability of the LHP.(2) Within the evaporating zone, the wick and the compensation chamber of a flat LHP evaporator, visual observations of the flow were realized by a transparent evaporator cover. It was found that:1) some bubbles were accumulated on the upper surface of the wick during the incipient stage of the start-up process at low heat load, which resulted in a slow start-up process.2) Heat pipe effect was shown in the compensation chamber during the steady operation at high heat loads.(3) With the combination of experiment and numerical simulation, the effect of average particle diameter of the sintered copper wick on heat transfer was studied. The influences of average pore size on heat leak and start-up characteristics were also discussed. The effect of copper wick average particle diameter on the distribution of the two-phase in the evaporator, the efficiency of heat transfer and the velocity of the vapor flow were analyzed during the operation at different heat loads, which concluded that heat leak was sensitive to the average particle diameter of the powder. When the average particle diameter of the copper powder is139.2μm among three kinds powder, the LHP could reach a fast start-up, and the heat transfer capacity of500W was reached during the operation. (4) The effect of charging ratio on copper-water LHP heat transfer characteristics was investigated. Visualization research on working fluid circulation in flat LHP evaporator was also performed, taking advantage of high-speed camera. Due to low charging ratio, strong temperature oscillation resulted from the alternative appearance of dry up and wet in the flat evaporator was observed during operation. The experimental results demonstrated that obvious heat leak was alleviated by the high charging ratio, which generated in a fast start-up and a high heat transfer capacity. The optimal charging ratio for the cylindrical LHP ranged from69.6to78.3%.(5) In order to address massive heat leak in copper-water LHP during low heat load start-up and high heat load operation, nickel layer with low thermal conductivity was introduced to manufacture a new two-layer copper-nickel composite wick. When the thickness ratio of copper layer to nickel layer was3:2, efficient evaporation and low heat leak were achieved simultaneously at the same time. With the combination of experiment and numerical simulation, the mechanism of heat leak alleviation from the two-layer copper-nickel composite wick was also studied. It was found that nickel layer of the wick could prevent bubbles from creating on the upper surface of the wick during the incipient start-up process, speed up the start-up process and reduce the time for start-up process by37.7%. No heat pipe effect was shown in the compensation chamber during the operation at a high heat load of120W, and the evaporator wall temperature was about10℃lower than before.

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CLC: > Industrial Technology > Energy and Power Engineering > Thermal engineering, heat > Thermal Engineering Theory > Heat Transfer
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