Experimental study on fundamental phenomena of boiling using heat transfer surfaces with well-defined cavities created by MEMS (Effect of spacing between cavities)

Pool nucleate boiling heat transfer experiments were performed for water using heat transfer surfaces having unified cavities. Cylindrical holes of 10 mu m in diameter and 40 mu m in depth were formed on a mirror-finished silicon wafer of 0.525 mm in thickness using Microelectromechanical systems technology. The test heat transfer surface was heated by a semiconductor laser beam. Experiments were conducted in the range of up to 4.54x10(4) W/m(2). The temperature of the back side of the heat transfer surface was measured by a radiation thermometer. When the spacing between cavities was S/L-c < 0.8, the horizontal and declining coalescence of bubbles on the neighboring cavities were dominant. Strong thermal and bubble coalescence interactions between nucleation sites were observed in this situation. This vigorous bubble coalescence created strong convection. The heat carried by this convection accounted for a large part of the heat transfer. As the cavity interval became wide, S/L-c >= 1.2, the horizontal and the declining coalescence of the bubbles ceased. The coalescence was limited to the vertical or no coalescence. The thermal and bubble coalescence interactions between the nucleation sites became quite low, to the extent of being negligible. The bubbles themselves were key in carrying heat away from the heat transfer surface.