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unraveling nucleate BOILing: MODEling, mesoscale simulatiONs and experiments

Periodic Reporting for period 1 - BOIL-MODE-ON (unraveling nucleate BOILing: MODEling, mesoscale simulatiONs and experiments)

Reporting period: 2019-05-20 to 2021-05-19

BOIL-MODE-ON is set within the context of a continuous miniaturisation of electronics components resulting in an ever increasing power density request. All of that energy is turned into heat to be dissipated through dedicated thermal management systems. The ability to dissipate these large amounts of heat while keeping the operating temperatures below prescribed thresholds is becoming critical to many applications ranging from integrated circuits, to X-ray medical equipment, and airplane avionics.
Classical air or liquid cooling methods have become inadequate for the most demanding recent applications. These limitations have spurred the transition from single-phase cooling solutions to two-phase thermal management systems. The heat transfer coefficient attainable can be dramatically enhanced by the use of two-phase systems employing boiling. The basic underlying idea is simple: nucleate vapour bubbles in the liquid in contact with the hot surface and take advantage of the latent heat of evaporation associated with boiling. Its implementation, however, faces a number of challenges, hindering the transition from laboratory research to commercial products.
The most fundamental difficulty is represented by the intrinsically multiscale nature of the boiling phenomenon: the large-scale features of the process — like the overall heat transfer, the flow pattern and the total pressure drop — are in fact strongly influenced by the small-scale characteristics such as the frequency of bubble nucleation, their size, and the release rate from the hot surface. Probably, the most elusive subprocess of boiling heat transfer, and nonetheless the most influential, is the boiling inception, namely, the very first stage of bubble formation, occurring at sub-micron length scales. The bubble nucleation rates (i.e. the number of bubbles formed per unit time and surface area), their spatial distribution on the hot surface, and the mean first passage times (i.e. the time to be awaited to observe a nucleation event), are necessary quantities in most of the semi-empirical models of boiling heat transfer, but difficult to be accessed via experiments.
BOIL-MODE-ON met the urgent need of a synergic effort on developing suitable theoretical models, specialised numerical simulations and accurate experiments, to make a real breakthrough on the understanding of the detailed mechanisms underlying the boiling inception at hot surfaces. The aim of the project is to investigate two of the major controlling mechanism of the nucleate boiling onset process: 1) the wetting properties of the hot surface; 2) the gas content dissolved in the liquid. The quantitative prediction of these effects is a daunting task, and remained an open problem before BOIL-MODE-ON.
The Fellow Dr. Magaletti developed a unique theoretical framework to analyse in full detail the process of bubble formation at hot walls during the (unsteady) boiling onset process. Specific numerical schemes have been devised and applied to analyse the influence of surface wettability and heating intensity on the boiling inception. The work carried out highlighted the importance on the overall boiling inception of the interaction between the nano-sized vapour regions forming due to thermal fluctuations: neighbouring vapour “embryos” coalesce with each other, extending their typical lifetime, and enhancing the probability of overcoming the critical size that effectively initiates the boiling process. This mechanisms finally explains the puzzling low onset superheat measured on ultra-smooth surfaces, where the classical mechanistic descriptions failed.
A model of the contact angle was developed to analyse surfaces with different wetting properties, from hydrophilic to hydrophobic materials. The simulations showed that nanoscale liquid stratification at the wall produces non-trivial behaviour for bubble nucleation. In particular, the expected catalyst effect of the wall disappears at moderate and strong hydrophilic surface, when the width of the stratification layer is comparable with the size of the critical bubble.
An innovative methodology has been also developed to address the thermodynamic limit of liquid water, a critical aspect to understand both boiling and cavitation processes. This approach enabled the investigation of the bubble formation even at very small superheats, where the application of brute-force simulations is unfeasible. This allowed to compare the most accurate experimental results on water cavitation in mineral inclusions, providing the thermodynamic limit of water in the whole range of temperatures.
The Fellow received specific trained on the experimental techniques for the characterisation of the nucleate boiling process, and improved the experimental apparatus available at the University of Brighton Labs in order to analyse the role of dissolved oxygen on the boiling onset temperature. The measurements carried out confirmed that the onset temperature is reduced when increasing the gas content.
Several dissemination activities, involving both the Academia and the Industry, have been carried out. In particular, two peer-reviewed articles have been published, two book chapters have been accepted (but not yet published at the date of writing this report), and other two articles have been submitted. The Fellow served as Organising Chairman for the 3rd edition of the International Workshop “Surface Wettability Effects on Phase Change Phenomena” (SWEP), organised in Brighton but held online due to the Covid-19 pandemic. Finally, a Webinar involving Industries have been delivered online few weeks after the end of the project.
BOIL-MODE-ON contributed to the knowledge advancement of bubble nucleation in superheated liquids by proposing innovative theoretical/simulative approaches. The proper description of thermal fluctuations in a continuum mesoscale setting turned out to be the key for having access to the complete dynamics of the nucleation event, up to time and length scales inaccessible to the more established Molecular Dynamics simulations. This has been possible by coupling the Diffuse Interface (DI) approach with the Fluctuating Hydrodynamics theory. This model enabled to assess the importance of the hydrodynamical couplings between the nuclei such as coalescence and competitive growing mechanisms, and of the wetting properties of the hot surface.
The project’s timeliness and relevance is not limited to thermal problems since vapour bubble nucleation is a fundamental phenomenon crossing the edges of many different fields, and ubiquitous in several natural phenomena. Fascinating examples are the spore release mechanism in the fern sporangium annulus, and the activation/inactivation of many biological ion channels. Bubble nucleation is also central in several technological applications. For example, in nanotechnology applications, the formation of bubbles is responsible for the low-frequency noise in solid-state nanopores used for DNA sequencing and biosensing. Moreover, the fundamental aspects underlying the phase change inception draws similarities in other industrial applications such as ice formation on aircraft, and phase changing material for energy storage. As a consequence, the outputs generated by this project is expected to positively affect all of these related fields.
Snapshots of the boiling inception and of the bubble growth and coalescence