Bubble behaviors during subcooled pool boiling in water and nonionic surfactant aqueous solution

A visualization study of the bubble behavior in pool boiling of ultra-pure water and surfactant aqueous solution (Triton X-114) under different heat fluxes was performed, using a narrow transparent bare FTO heating surface with various subcooling degrees of 5 degrees C, 25 degrees C and 35 degrees C, respectively. It is shown that with the decrease of liquid subcooling, bubble coalescence in ultra-pure water becomes obvious. Whilst with the increase of liquid subcooling, the bubble surface gets rough and unstable. In contrast, a newly discovered phenomenon of bubble bubble (BB) penetration occurs in the surfactant solution under the same working condition as that in water. BB penetration behavior is a bubble interaction without coalescence. This bubble behavior is believed to be the results of micro-convection due to the shrinkage of the bubble root, and the Marangoni effect due to the surface tension gradient. Combination of Marangoni effect and bubble surface movement causes the surfactant molecules to aggregate toward the bubble root, and it effectively promotes the nucleation of the next bubble. Owe to the rapid growth of newly nucleated bubbles, the bubble-bubble penetration and piercing behaviors happen during the growth stage. The newly nucleated bubble partially accelerates the first bubble departure, also interacts with the first bubble by forming a tiny vapor channel. This phenomenon should be one of the reasons of enhanced heat transfer using surfactant solutions. (C) 2020 Elsevier Ltd. All rights reserved.