Numerical Simulation of Boiling from a Single Reentrant-Cavity
In this paper, results from numerical simulations of pool boiling of R-134a from single reentrant-cavities are presented. The simulations allow to study heat transfer and flow characteristics inside the cavities. Simulations were performed in 2D with circular cavities for several bubble cycles until a quasi steady-state was obtained and in 3D with square cavities for one bubble cycle. The results show that liquid films inside the cavities can significantly increase heat transfer performance. With circular cavities, no liquid backflow into the cavities was observed for all pore diameters and dryout occurred after one bubble cycle. With square cavities, thin liquid films establish in the edge regions due to capillary forces. These films connect the liquid pool with the liquid inside the cavities and allow backflow of liquid even during bubble growth. For boiling processes being governed by evaporation from thin liquid films in the vicinity of the three-phase contact line, heat transfer coefficients decrease with increasing heat flux. The results suggest that with careful design significant improvements of heat transfer coefficients are possible, not only with structures with subsurface tunnels as employed on tubes in industrial heat exchangers, but also with single reentrant-cavities.