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Research Papers: Evaporation, Boiling, and Condensation

Investigations of Biporous Wick Structure Dryout

[+] Author and Article Information
Qingjun Cai, Ya-Chi Chen

Teledyne Scientific & Image Company, 1049 Camino Dos Rios, Thousand Oaks, CA 91360qcai@teledyne.com

J. Heat Transfer 134(2), 021503 (Dec 13, 2011) (8 pages) doi:10.1115/1.4005099 History: Received January 05, 2011; Revised August 31, 2011; Published December 13, 2011; Online December 13, 2011

Dryout in a heat pipe evaporator is caused by insufficient condensate supply through the wick structure. Dryout is generally considered a failure of the heat pipe operation. However, traditional dryout theory may not fully explain the heat and mass transport limitations in the biporous (biwick) wick structure due to new mass transfer mechanisms, such as liquid splash at high heat flux, and vapor bubble/jet occupation of liquid transport passages. This article investigates the dryout phenomenon in carbon nanotube (CNT) based biwick structure. The incipience and expansion of the dryout zone on the CNT biwick structure are visualized. Variation of the evaporator temperatures at various heat fluxes is measured to characterize the temperature responses on the biwick dryout. Results based on both visualization and measurement show that dryout of CNT biwick structures is affected by vapor flow induced droplet splash and vapor occupation of liquid transport passages, which reduces the liquid supply to the hottest region and creates a local dry zone. On the curves of heat flux versus the evaporator temperature, dryout can be defined as the appearance of the inflexion point during the heating period, and associated with the existence of a large temperature hysteresis in a heating and cooling cycle. Experimental measurement also shows that over 12% of the liquid by volume is lost without being phase changed, due to high-speed vapor flow induced liquid splash. Liquid splash and interactions between vapor and liquid flows also increase the pressure drop weight in the evaporator over the system loop and result in more notable heating area effect on biwick structures when compared with traditional monowick structures.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 1

Experimental test samples: (a) biwick structure composed of cylindrical CNT pillars; (b) biwick structure composed of straight CNT stripes; and (c) biwick structure composed of zigzag CNT stripes

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Figure 2

Platinum heater and thermometer: (a) heating area is 2 mm × 2 mm and the substrate size is 12 mm × 12 mm; (b) heating area is 10 mm × 10 mm and the substrate size is 20 mm × 20 mm

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Figure 3

Experimental test system of the CNT biwick structure phase change

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Figure 4

Experimental setup of the CNT biwick sample in the test chamber: a stainless steel cylinder passes through the electrical connection window of the test chamber, and three stainless steel condenser panels control system temperature

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Figure 5

Measurement setup of the liquid splash volume ratio in high heat flux phase change

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Figure 6

Heat flux versus the evaporator temperature of the pillar-like CNT biwick structure using the 10 mm × 10 mm heater: the hysteresis indicates a dry and rewet process of the wick structure

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Figure 7

Wick dryout of the cylindrical CNT pillars: (a) dispersed dryout (dark spots) occurs on the center of the heating area (point C in Fig. 6); (b) a large dryout zone appears (point D in Fig. 6)

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Figure 8

Dryout of the zigzag CNT biwick structure using the 10 mm × 10 mm heater: Dryout occurs at the upper heating area where the liquid transport is the most difficult

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Figure 9

Dryout and heat flux versus the evaporator temperature of pillar-like CNT biwick structure using the 2 mm × 2 mm heater: dryout appears at the inflexion point of the curvature, and the temperature hysteresis ends without rewet of the biwick structure

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Figure 10

Visualization of phase change and dryout zones at high heat flux (938 W/cm2 , point D in Fig. 9)

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Figure 11

Heat flux versus the evaporator temperature of CNT stripe-like biwick structure using the 2 mm × 2 mm heater: the early stage of dryout or the inflexion point may appear when heat flux is increased to 770 W/cm2

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Figure 12

Dryout image analysis of CNT stripe biwick structure: (a) evaporation and boiling at 370 W/cm2 (point B in Fig. 1) and (b) dryout occurs at 770 W/cm2 (point D in Fig. 1)

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Figure 13

Volume ratio of splashed liquid versus heat flux of the tripe-like CNT biwick structures (heating area 2 mm × 2 mm)

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