The Influence of Single Bubble Growth and Bubble Coalescence on Boiling Heat Transfer

Boiling processes are widely used since decades. Nevertheless, the heat transfer phenomena are still not sufficiently understood. The subject of the presented work summarizes a generic experimental investigation of the boiling process performed for single and interacting bubbles at artificial cavities. Influences of these small \nscale events on the macroscopic boiling processes are discussed. The experimental setup consists of a 2-phase fluid container with FC-72 as working fluid. A thin stainless steel foil acts as Joule heater. Single or multiple bubbles are generated on top of the foil. Optical measurement techniques enable the measurement of the bubble’s shape and the temperature of the foil as well as the subsequent calculation of heat flux profiles. In case of single bubble nucleation a clear influence of the temperature of the heated surface just before the \ninitial nucleation is observed. Depending on the conditions a liquid microlayer was formed or not, and the role of contact line evaporation versus microlayer evaporation changes. The experiments with bubble coalescences show a clear dependency of the coalescence frequency on the system pressure and the nucleation site distance. Probability distributions are derived. The evaluation of the transient heat flux profile during bubble coalescence shows, that the coalescence event leads to an increased heat transfer coefficient. However, these observations could not be confirmed on heaters with a large number of active artificial nucleation sites.