Several crucial technological applications are limited by cooling efficiency, such as datacentre thermal management, fuel cells, as well as a variety of fields in the aerospace and the automotive industry. Advanced cooling technologies need to be applicable to radically different systems spanning an enormous range of scales, from microelectronics to huge power plants. The economic impact of advancing research in this field is huge. In this context, passive means of heat removal exploiting the large latent heat associated with phase change, without the need of externally driven systems like pumps, are extremely promising and robust. It is well known that the thermal energy transfer from a heated surface into a liquid can be considerably enhanced through the formation of vapour bubbles.
The ideal solution is to anticipate the Onset of Nucleate Boiling (ONB) as much as possible by reducing the onset temperature, and to delay the Critical Heat Flux (CHF) condition by increasing the maximum Heat Transfer Coefficient (HTC).
However this is a complex task since it depends on the frequency of bubble nucleation, their size, and the release rate from the hot surface. These characteristics of the bubbly flow can be influenced in a non-trivial way through properly tailoring chemical and geometrical surface characteristics and by exploiting liquids with specific physical-chemical properties. Moreover, no matter how carefully the system degassing procedure is applied, in any practical application a small amount of gas or impurities is always dissolved in the cooling liquid. Nanobubbles could appear near the wall and could act as nuclei for bubble growth.
A synergic effort on developing accurate experiments, suitable theoretical models and specialized numerical simulations, is required in order to make a real breakthrough on understanding the detailed mechanisms underlying heat transfer during phase change. This is of paramount importance in the particular case of nucleation phenomena (like e.g., boiling, cavitation, ice formation) where a detailed investigation is still an extremely challenging task and the available models are not yet able to definitively explain intriguing experimental results.
One of the major difficulties with boiling is the need of tracking the interface separating the two phases. Phase change adds complexity to the numerical description, requiring heat and mass transfer across the interface. Moreover, the phase change inception, i.e. the very first appearance of the vapour phase into the liquid, cannot be captured by direct application of both VOF or DI methods, requiring the vapor nuclei to be prescribed at the beginning of the simulations, with the risk of altering the nucleation rate. The real challenge is, instead, to provide a physically-ground methodology capturing the nuclei spontaneous appearance, at the phase change inception.
The PhD project has the main goals of 1) developing a multiscale technique to numerically address the boiling problem, 2) capturing the thermofluidic dynamics from the bubbles inception, their growth and detachment, 3) taking into account the conjugated thermal problem through coupling between fluid and solid domain, 4) Investigate experimentally and numerically the effect of the gas content dissolved in the liquid on the onset of nucleate boiling.
You will be asked to support the development of a reduced (cylindrical) FDI model for the simulation of the nucleation on flat solid surfaces. Afterwards, this numerical tool will be used to analyse the effect of different wall wettabilities. In a second phase, you will support the mathematical application of Rare Event Techniques to the continuum FDI model. Finally, you will learn the Volume-of-fluid approach in order to upscale the simulations from tens of nanometres to the micrometre scale.