Khellil Sefiane

School of Engineering, Institute for Multiscale Thermofluids,  University of Edinburgh,

 Edinburgh, EH9 3FD, United Kingdom

Professor Khellil Sefiane, PhD, HDR, FRSC and FInstP is a Professor and chair of Thermo-Physical Engineering in the School of Engineering at the University of Edinburgh, Scotland, United Kingdom. He is the Head of the research Institute for multiscale Thermofluids at the University of Edinburgh. Professor Sefiane is a vice President of the UK Heat Transfer Committee. He is the UK editor for the International Journal of Heat and Mass Transfer. He has been associate editor for the International Journal of Multiphase Flows and the ASME Journal of Heat Transfer.

He held honorary appointments as Adjunct Professor at the University of Toronto, Canada (2008-2014), Visiting Professor at Kyushu University in Japan and World Premier International Professor at the International Centre for Carbon Neutral Energy Research (I2CNER) at Kyushu in Japan (2015-2019), Shanghai Jiao Tong University, China (2020) and Pretoria University, SA (2021). He is Fellow of the Royal Society of Chemistry, FRSC, and Fellow of the Institute of Physics, FInstP. Professor Sefiane has been research active for the last 25 years in various areas related to multiphase flows, heat transfer, microfluidics, interfacial phenomena and phase change. He has published more than a 250 journal papers in international journals. He has been recipient of the prestigious Institute of Physics (IoP) award (2009) for his work on droplets wetting and evaporation. He holds an ExxonMobil fellowship and Global Research Award, both awarded by the Royal Academy of Engineering, London. Professor Sefiane is member of numerous international scientific committees of experts in heat transfer and multiphase flows (ICHMT, EUROTHERM).

Title: Boiling and Bubbles Dynamics from Artificial Nucleation Sitessystems.

Abstract: Nucleate pool boiling plays a crucial role in thermal management systems such as high power systems and electrical devices including electric vehicles, photovoltaics, and supercomputers. Unlike single-phase heat transfer, nucleation pool boiling two-phase-change can attain higher heat-transfer coefficients with smaller interface temperature increase and effective area for heat transfer due to the high-energy transfer resulting from the phase-change process. A great amount of research has focused on bubble growth and departure as a convenient and effective method to investigate the performance of the boiling mechanisms.

In this lecture, bubble behaviour during the boiling process, incorporating nucleation, growth, coalescence and departure, is studied. This latter being an important phenomenon affecting heat transfer and heat removal. Observing the bubble behaviour is a crucial method to understand the boiling heat transfer mechanism. This work studies the dynamics of single bubbles nucleating and departing from isolated artificial cavities with different diameters on different wettability coated surfaces. The experiments were conducted with FC-72 under saturation pressures. The bubble behaviour during nucleation was obtained by analysing high-speed videos. During the whole bubble growth period, Bubbles growth shape is spherical and its only contact with the boiling surface is by what we have termed the narrow ‘Vapour Bridge’ after an initial growth period. The contact area size is affected by the cavity diameter, with a larger bubble departure diameter for a larger cavity mouth. In addition, the higher saturation pressure will result in smaller bubble departure diameter. The bubble departure diameter is found to be nearly constant with different degrees of superheat for similar cavity diameter and saturation pressure. The bubble departure frequency increases linearly with increasing degree of superheat. The nucleation pool boiling from an isolated artificial cavity with 70 µm diameter on a horizontal silicon substrate. The bubble dynamic with SiO2, FDTS and Glaco coated surface materials were compared to investigate the effect of surface characteristics on bubble vertical coalescence. The single- and multi- vertical coalescence were observed at the boundary between the departure bubble and the initial bubble. In addition, vertical coalescence appears earlier on FDTS surface than it appears on SiO2 and Glaco coated surfaces. It is due to the faster growth of the bubble at the initial period on FDTS coated surface. It is also pointed out that the vertical coalescence of the bubble has a limited effect on bubble departure diameter.

We will demonstrate the effect of a wide range of surface wettability on the nucleation, growth, detachment and coalescence of bubbles.