Simultaneous visualization of bubbles and dry spots for pool and forced convective boiling on a horizontal heater and pool boiling CHF model development

Abstract

A new experimental attempt is made to simultaneously observe the dynamic behaviors of bubbles and dry spots in the vicinity of boiling surface for pool and forced convective boiling. Based on the direct observation results for CHF mechanism, an effective nucleate boiling limitation model is proposed to predict nucleate boiling heat transfer as well as CHF for pool boiling. From the visualization results, the formation of bubbles and dry spots occurs simultaneously. This indicates that, when a bubble nucleates and grows at a nucleation site, a dry spot is formed under the corresponding bubble. Therefore, they should be considered as synchronized identity rather than an independent one. Also, the dry spot density is equal to the active site density in discrete bubble regime. At the situation of CHF for pool boiling, the fraction of the dried area of the heater surface is above 70%. However, the heater surface temperature is stably maintained due to the nucleate boiling in the strip-shape wetted region existing around the edge of the vapor film. At just after CHF, the nucleate boiling region still exists but the rewetting region is extinguished by an agglomeration of nucleating bubbles. Then, dry spots under the agglomerated bubbles grow, eventually resulting in the dryout of all heater surfaces. For forced convective boiling, the formation of bubbles and dry spots occurs simultaneously as observed as in pool boiling. Nucleation on the heater surface is suppressed as mass flux increases, and the active site density correlation for the pool boiling does not incorporate the forced convective boiling data. At the situation of forced convective boiling CHF, the fraction of the dried area of the heater surface is above 85 %. Also, the heater surface temperature is stably maintained due to the nucleate boiling at the wetted region around the large vapor clots. This implies that the CHF triggering mechanism for convective flow boiling is not much different from that obse...