Abstract
The present numerical investigation identifies quantitative effects of fundamental controlling parameters on the detachment characteristics of isolated bubbles in cases of pool boiling in the nucleate boiling regime. For this purpose, an improved Volume of Fluid (VOF) approach, developed previously in the general framework of OpenFOAM Computational Fluid Dynamics (CFD) Toolbox, is further coupled with heat transfer and phase change. The predictions of the model are quantitatively verified against an existing analytical solution and experimental data in the literature. Following the model validation, four different series of parametric numerical experiments are performed, exploring the effect of the initial thermal boundary layer (ITBL) thickness for the case of saturated pool boiling of R113 as well as the effects of the surface wettability, wall superheat and gravity level for the cases of R113, R22 and R134a refrigerants. It is confirmed that the ITBL is a very important parameter in the bubble growth and detachment process. Furthermore, for all of the examined working fluids the bubble detachment characteristics seem to be significantly affected by the triple-line contact angle (i.e., the wettability of the heated plate) for equilibrium contact angles higher than 45°. As expected, the simulations revealed that the heated wall superheat is very influential on the bubble growth and detachment process. Finally, besides the novelty of the numerical approach, a last finding is the fact that the effect of the gravity level variation in the bubble detachment time and the volume diminishes with the increase of the ambient pressure.
Highlights
Boiling heat transfer is encountered in a wide field of applications, ranging from everyday life applications to more complex, industrial applications
Since the superheated bulk liquid thermal boundary layer thickness determines how much heat is stored in the fluid layer in the vicinity of the heated plate, it was deemed appropriate for a parametric study to be conducted, aiming to identify the effect of the initial thermal boundary layer (ITBL) thickness, on the bubble growth and detachment process
It is strongly suggested that the bulk liquid thermal boundary layer thickness should be measured and reported in future experimental studies, since it comprises a required input for the successful numerical simulation of nucleate boiling processes
Summary
Boiling heat transfer is encountered in a wide field of applications, ranging from everyday life applications to more complex, industrial applications. With the growing computing capabilities and the amount of available computing resources, as well as with the rapid development of modern numerical methods for the simulation of multi-phase flows, the numerical simulation of boiling heat transfer has become possible for a wide range of applications as well as spatial and temporal scales. Most of the existing open-source, in-house, and especially commercial CFD codes have adopted a Eulerian multi-phase flow approach, based on a two-fluid model. An enhanced VOF-based numerical model that utilises a smoothing technique in order to suppress the development of spurious velocities in the vicinity of the interface, which was previously presented, validated and applied to the investigation of adiabatic bubble dynamics in the work of Georgoulas et al [36], is further extended for the simulation of diabatic, liquid-vapour flows with phase change. The validated and optimised version of the model is further applied for the conduction of a wide range of parametric numerical experiments, identifying the effects of the initial thermal boundary layer (ITBL) thickness, the surface wettability (triple-line contact angle), the plate superheat and the gravity level on the bubble detachment characteristics
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