Numerical simulation of pool boiling: the effects of initial thermal boundary layer, contact angle and wall superheat

Boiling heat transfer has been a subject of extensive investigation during the last decades. Since the subprocesses in nucleate boiling, involve quite complex physics, the development of comprehensive correlations and/or models has not been possible so far. However, more recently, numerical simulations of the boiling process have proven to be capable of reliably predicting bubble dynamics and heat transfer characteristics. In the present paper, heat transfer and phase-change are coupled with a previously improved and validated Volume Of Fluid (VOF) model for adiabatic bubble dynamics. The model is initially verified with an existing analytical solution for cases of evaporating bubble growth in a superheated liquid domain. Moreover, the predictions of the proposed model regarding the bubble detachment characteristics are also validated against available experimental data on pool boiling of refrigerants. The validated and optimised version of the model is further applied for the conduction of a wide range of parametric numerical simulations, identifying the effects of the initial thermal boundary layer thickness, the contact angle between the liquid/vapour interface and the heated plate, as well as the plate superheat, on the bubble detachment characteristics. It is found that the bubble growth and detachment characteristics are highly sensitive to the initially developed thermal boundary layer thickness, following a linear relationship. This has a strong implication on the experimental activities, since in many cases it is not clear at which time the initial measurements of the pool boiling characteristics have been carried out with respect to the time scale to reach the quasi steady-state condition of the thermal boundary layer, linked to the natural convection. As for the imposed contact angle effect, a threshold value is identified below which, the effect on the bubble detachment characteristics is minimal while above this value the influence is quite significant. Moreover, the bubble detachment characteristics follow an exponential increase with the corresponding increase in the superheat of the heated plate.