Numerical study of influence of lateral bubble merger on bubble dynamics and heat transfer during nucleate pool boiling on a horizontal substrate under a constant imposed heat flux

• Numerical simulations of lateral bubble merger with coupled substrate conduction. • Transient conduction and fluid side heat transfer together affect the waiting time. • Lateral bubble merger has a strong influence on the waiting time. • Cavity spacing affects the bubble departure frequency and heat transfer rate. In the present work, three-dimensional numerical simulations are performed for nucleate pool boiling, with coupled transient conduction in the solid substrate, to study the effect of lateral merger of bubbles from two adjacent cavity sites on bubble dynamics and heat transfer, at boiling inception. Level set method is used to track the liquid-vapor interface, and finite-difference schemes are used to discretize the governing equations. The waiting time is determined as the time taken for the cavity site temperature to recover to the nucleation temperature after bubble departure. The present study is done for pool boiling of saturated water at atmospheric pressure on a 7 mm x 7 mm portion of a 1 mm thick horizontal stainless-steel substrate having two identical cavity sites. The spacing between the two cavity sites is parametrically varied. A uniform, time-invariant heat flux is imposed at the bottom of the substrate. The solution of the coupled transient conduction in the solid substrate shows that lateral bubble merger results in a significant variation in the waiting time and in turn the bubble departure frequency with cavity spacing. The vapor production rate and heat transfer coefficient, both averaged over one bubble cycle, for cases in which the two bubbles merge laterally are smaller than those in which the bubbles do not merge. The average heat transfer coefficient over one bubble cycle for the largest cavity spacing is 15 % higher than that for the smallest cavity spacing investigated in this work.