Formation and growth of vapour bubbles

Abstract

Vapour bubble formation has been attributed as the driving factor behind natural phenomena such as geyser formation and volcanic eruptions. The strong forces associated with the formation of bubbles has been utilised for ultrasonic cleaning. The high heat fluxes dissipated from surfaces during bubble formation and growth has seen pool boiling attracting significant interest in thermal management systems. On the other hand, the explosive failure of pressurised containers and wear of turbomachinery caused by cavitation bubbles highlight the deleterious effects of vapour bubbles on industrial processes. Understanding the formation and growth behaviour of vapour bubbles is therefore an important open problem, and current theoretical models remain incomplete. The understanding of homogeneous vapour bubble growth is currently restricted to asymptotic descriptions of their limiting behaviour. While attempts have been made to incorporate both the inertial and thermal limits into bubble growth models, the early stages of bubble growth have not been captured. By accounting for both the changing inertial driving force and the thermal restriction to growth, an inertio-thermal model of homogeneous vapour bubble growth is developed, capable of accurately capturing the evolution of a bubble from the nano- to the macro-scale. These model predictions are compared with: a) published experimental and numerical data, and b) new molecular simulations, showing significant improvement over previous models. This work utilises molecular dynamics (MD) simulations to investigate nanoscale vapour bubble growth. MD is a technique where the future positions and momenta of the molecules in a system are determined according to a defined intermolecular potential used to compute forces between molecules. A velocity-verlet algorithm is then used to integrate Newton’s equations of motion to calculate future molecule positions.

In practise, vapour bubbles typically form heterogeneously, on a solid surface where the barrier to nucleation is lower. The majority of the industrial and scientific interest in the the study of vapour bubbles has, therefore, been investigating the formation and growth of heterogeneous bubbles. Much research has been performed on the effects of the surface on the formation and ultimate detachment of the bubbles. However, the role that the surface plays in determining the growth of the bubble is still poorly understood. Currently, theoretical understanding of heterogeneous vapour bubble growth is limited to hemispherical bubbles or completely spherical bubbles next to a heated surface. By accounting for the effect of the surface on both the geometry of the bubble and on the available thermal energy, the homogeneous inertio-thermal model is extended to capture the effect of surface wettability on heterogeneous vapour bubble growth. Using molecular simulations, the effect of bubble geometry on growth rate is investigated. This is achieved by modifying the strength of the fluid-solid intermolecular forces to obtain different bubble contact angles. The resulting bubble growth simulations showed good agreement with theoretical predictions.

These heterogeneous bubble growth simulations show the formation of a non-evaporating layer (NEL) of molecules underneath bubbles on wetting surfaces. The NEL has been shown to alter the interfacial free energy balance at the three phase contact line, causing a change in the contact angle behaviour of the bubbles. The subsequent effect of this layer on the growth rate of vapor bubbles is analysed in this work. An energy balance criterion is developed to predict the formation of the NEL, accounting for the potential contributions from the solid-fluid and fluid-fluid interactions. This analysis highlights how the non-continuum nature of the fluid under the bubble on hydrophilic surfaces plays a vital role in determining the bubble shape and subsequent dynamics.

The effect of surface wettability on vapour bubble nucleation has been widely studied due to its particular importance to two-phase thermal management systems. Classical nucleation theory (CNT) predicts that nucleation occurs preferentially on hydrophobic surfaces due to a reduced energy barrier. While there have been many investigations highlighting the agreement of experimental data to classical nucleation theory, molecular simulation results have observed preferential nucleation on hydrophilic surfaces. These surprising results arise from the methodology used in the simulations, where the results become strongly dependant on non-equilibrium and temporal effects. Using an isothermal-isobaric ensemble, this work shows how the predictions of CNT can be recovered in molecular simulation. Further testing investigates the role of surface roughness on nucleation. Distinct regimes are identified for bubble growth from surface cavities.

Wetted cavities are shown to not significantly alter nucleation behaviour, while dewetted cavities are shown to reduce the temperature required for sustained bubble growth. It is hoped that the findings presented in this thesis help improve the understanding of the fundamental physical processes responsible for the formation and growth of vapour bubbles. These findings, have application to help improve the performance and design of micro- and nano-fluidic devices, as well as two-phase thermal management systems.