Vapor-liquid distributions inside 100W-class loop heat pipes (LHPs) with and without a secondary wick have been observed using neutron radiography. Water is chosen as the working fluid in both cases to obtain pictures with good contrast. In the previous study (Okamoto et al., AIAA-2010-6031), a visualization result of a transient dry-out behavior in the primary wick is presented, whereas this paper has shown a result of detailed analysis on the liquid distribution in the evaporator in equilibrium state. Image processing has shown that the position of the vaporization area (vapor-liquid interface) in the fin-shaped portion of the primary wick shifts depending on the amount of heat load. The temperature trend indicates the fins are not totally dried, but in two-phase state even when the higher heat load is applied. In addition, the performance of secondary wick against gravity is also shown and the operation limit has been discussed based on a visual observation and a theoretical estimation.
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