Visualization of boiling structures in high heat–flux pool-boiling

The present paper presents experimental results of observation of liquid–solid contact and bubble behaviors around the critical heat flux of saturated and subcooled pool boiling on a plate of single crystal sapphire. The observation was conducted from the backside of a rectangular boiling surface and also from the side and backside of a boiling surface with a narrow width. The main results obtained are summarized as follows. The bubble base area is almost dry and lateral coalescence of bubbles forms coalescent dry areas. As the wall superheat increases, liquid–solid contact becomes like a canal meandering through dry areas. The dependency on the surface superheat of the contact-line length density (CLLD) which is defined as the total length of the boundary between wetted and dry areas in a unit area is almost the same as the boiling curve, and the value of CLLD at CHF is not strongly dependent on boiling liquid and subcooling. The relation of the number density of dry areas and their equivalent diameter in the large dry area region at CHF is not dependent on boiling liquid and subcooling, and it is similar to that of dropwise condensation.