Pool nucleate boiling heat transfer experiments were performed for water by using the well-controlled and -defined heat transfer surfaces. Artificial cavity(ies) was (were) created on the mirror-finished silicon plate of 0.525 mm thickness by utilizing the Micro-Electro Mechanical System (MEMS) technology. Each cavity had cylindrical shape. The diameter and the depth of the cavity were 10μm and 40μm, respectively. Experiments were performed in a range of a heat flux ∼6.0 × 104 W/m2 for distilled water. When the cavity interval was close, the horizontal and declining coalescence of bubble on the cavities were dominant. This vigorous bubble coalescence created strong convection. The heat carried by this convection took a main part in the heat transfer when cavities were close. As the cavity interval became wide, the horizontal and declining coalescence did not take place anymore. The coalescence was limited only to the vertical lift or no coalescence. In this situation, bubbles grew large on the cavities and absorbed latent heat sufficiently. Bubbles themselves took the main role of carrying heat away from the heat transfer surface when cavities were further apart.

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