Marangoni effects on near-bubble microscale transport during boiling of binary fluid mixtures

In this study, boiling experiments of 2-propanol/water mixtures in confined gap geometry under various levels of gravity were conducted to examine the Marangoni effects on near-bubble microscale transport. Full boiling curves were obtained and two boiling regimes—nucleate boiling and pseudo film boiling, and the transition condition, critical heat flux, identified. In the nucleate boiling regime, small bubbles were generated, condensed quickly in the bulk liquid, so that the agitation in the system resulted in great heat transfer performance. As the superheat level increased, the bubbles coalesced to a large vapor slug that was constrained by the gap geometry and started to blanket the heated surface. The maximum heat flux, (i.e. the critical heat flux, CHF), was then reached. The pseudo film boiling regime was reached by further increasing the superheat level, creating a slow, fluctuated bubble where boiling only occurred at its base perimeter because the heated surface was dried out.; The temperature field created within the parallel plate gap resulted in evaporation over the portion of the vapor-liquid interface of the bubble near the heated surface, and condensation near the cold surface. This scenario produced a heat pipe effect inside the constrained bubble. For positive mixtures, Marangoni forces were proven to greatly enhance heat transfer under reduced and terrestrial gravity when compared with the boiling of distilled water. However, the presence of the gap geometry caused a premature occurrence of CHF conditions, and deteriorated heat transfer at high superheated temperatures. The influence of the confined space was particularly significant when greater Marangoni forces were presented under reduced gravity conditions. It caused the value of the CHF for x = 0.025, which corresponded to weaker Marangoni forces, to be greater than that of x = 0.015 with a 6.35 mm gap. This demonstrates the complex interaction that these three factors—Marangoni effects, gravity level, and gap size—have on heat transfer.; A major contribution of this study is that it provides a parametric database, further expanding the knowledge in the field of heat transfer to make more optimal use of coolant composed of binary mixtures in various thermal applications.