Boiling Visualization and Critical Heat Flux (CHF) Phenomena on PCB in a Saturated Pool at Various Surface Orientations

The onset of nucleate boiling (ONB) and critical heat flux (CHF) of saturated water on printed circuit board (PCB) that is immersed in a stainless steel pool were investigated at various surface orientations. Additionally, photographic analyses and image processing were conducted to observe the pool boiling phenomena and determine the bubble dynamics from upward to downward surface orientations (0°, 45°, 90°, 135°, 150°, and 180°). Results revealed that boiling commenced from downward to upward surface orientations (180° to 0°). When the heater was placed facing upwards (0° and 45°), the generated vapor bubble grew and coalesced together to form an elongated bubble which eventually departed from the heated surface due to the buoyancy in the vertical direction of the heated surface. In contrast to those orientations, at 90°, 135°, and 150°, the generated vapor bubble exhibited similar behavior except that it drifted along the heated surface before it departed at the most upper part of the heater. On the other hand, when the heater placed horizontally facing downward (180°), the generated vapor bubble grew and coalesced together until it ultimately covered the entire heated surface. Early ONB occurred when the heater was placed facing downwards (135°, 150°, and 180°). Moreover, it was observed that the CHF began when the heater was placed horizontally facing downward (180°) as the boiling commenced at this surface orientation. Furthermore, based on the photographic and image processing, the results revealed that the approximate bubble departure diameter with the heater facing upwards (0° and 45°) including at 90° was bigger compared to that of the heater facing downwards (135° and 150°). On the other hand, bubble frequency decreased with increasing surface orientation (0° to 90°) and increased as the surface orientation went beyond 90°. Moreover, it was observed that the bubble with the biggest departure diameter had the smallest departure frequency, and needs longer time to generate another bubble. Based on these results, it is apparent that the surface orientation significantly affects the ONB, CHF, and bubble dynamics on PCB in a saturated water pool.