Temperature measurements near heating surface at high heat flux in subcooled pool boiling

The authors conducted measurements of liquid-vapor structure in the vicinity of a heating surface for subcooled pool boiling on an upward-facing copper surface by using a conducting probe method, and reported that the macrolayer dryout model is the most appropriate model of the CHF and the reason why the CHF increase with increasing subcooling is most likely that a thick macrolayer is able to form beneath large vapor masses and the lowest heat flux of the vapor mass region shifts towards the higher heat flux. To establish a mechanistic model of the CHF for subcooled boiling, therefore, it is required to clarify effects of local subcooling on liquid-vapor behaviors in the vicinity of a heating surface. This paper presents local temperatures close to a heating surface measured with a micro-thermocouple probe at high heat fluxes for water boiling on an upward surface in the range of subcooling 0 K to 40 K. An effective subcooling defined as a local subcooling during a waiting period of large vapor mass formation, which is important parameter to examine behaviors of bubbles forming macrolayer, was estimated from detected bottom peaks of temperature fluctuations with time. It was found that the effective subcooling adjacent to the surface remains at considerably lower values than the bulk liquid subcooling. This means that the bulk liquid subcooling has a limited effect on behaviors of primary bubbles from nucleation to coalescence. An empirical correlation of distribution of the effective subcooling was proposed to contribute to quantitative modeling of the CHF for subcooled boiling.