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Effect of Fouling on Nucleate Pool Boiling in Microgravity and Earth Gravity


 Nucleate pool boiling of water on clean and fouling surfaces was conducted in microgravity and earth gravity. The microgravity experiments were conducted in 8 s JAMIC drop shaft in Hokkaido of Japan. Platinum wires of 0.2 mm in diameter with or without fouling scale were used to provide uniform heat flux and measurement of the mean temperature of wires. The generated bubble volume was measured with high-speed video or CCD images.
 The more vigorous bubbling was observed on the fouling wire compared to that on the clean wire at a same heat flux both in earth gravity and microgravity. The enhancement of the bubbling was associated with the fact that the hydrophilic porous structure in the fouling scale provided the sufficient number of active sites for bubbling nucleation. The wettability of the surface with the fouling scale was much higher than that of the clean bare surface. The bubble departure diameter on the fouling wire was smaller due to the high wettability than that on the clean wire. The latent heat transportation ratio to the total heat flux was calculated with the generated bubble volume measured with high-speed video or CCD images. The ratio was approximately the same at the clean and fouling wires in spite of the apparent difference in bubbling behavior, but it was significantly affected with the gravity level. The ratio increased with an increase of the heat flux in the earth gravity but it remained at the smaller value in the microgravity. The nucleate heat transfer coefficient on the bare surface did not depend on the gravity levels although the bubbling behavior strongly affected with the gravity level.
 As the wire radius is small compared to the capillary length scale in microgravity, a growing and coalescing bubble sometimes completely covered the clean wire, evaporating all liquid in contact with the surface and inducing a transition to film boiling. However, on the fouling wire, many small bubbles were generated and sprang from the surface in various directions in microgravity. The spring out action of bubbles suppressed the transition to the film boiling on the fouling wire in the present experimental range.