Microbubble emission boiling in scbcooled pool boiling and the role of Marangoni convection in its formation

An experimental apparatus was designed and fabricated to visually investigate microbubble emission boiling (MEB) phenomena characterized by its extremely high heat dissipation capacity and potential application in the field of thermal engineering. The heating element was a cone copper block with an upper cylindrical section of 10 mm in diameter. A high-speed video camera (Photron: Fastcam SA5) was employed for recording the scenario of MEB. Visualized results show that both the subcooling and noncondensable gas have significant effects on MEB. Remarkable MEB could be observed when the liquid subcooling exceeds 25 K, with a maximum heat flux about 9 MW/m2 at the subcooling of 60 K. In order to explore the mechanism of MEB, numerical simulations were carried out to obtain the velocity field near a single vapor film on the same heating surface to that in the experiments with the software of “FLUENT”. Simulation results indicates that there exists Marangoni convection around the film under subcooled condition, which is influenced significantly by subcooling as well as thermal properties of the liquid. Strong Marangoni convection around the vapor film and the condensation at interface would induce interfacial waves, likely resulting in the collapse of the film in the case of high subcooling. As a result, it is reasonable to conclude that Marangoni convection is probably one of the key factors triggering the occurrence of MEB.