Prof. Tassos G. Karayiannis

Centre for Energy efficient and Sustainable Technologies

Brunel University London

E-mail: tassos.karayiannis@brunel.ac.uk

Bio

Tassos Karayiannis studied at the City University London and the University of Western Ontario. He started his career as a researcher at Southampton University and later as a British Technology Group Researcher at City University. Subsequently he worked at London South Bank University and joined Brunel University London in 2005, where he is now Professor of Thermal Engineering Leader of the Two-Phase Flow and Heat Transfer Group and Director of the Energy Efficient and Sustainable Technologies Research Centre. Professor Karayiannis has carried out fundamental and applied research in a number of single-and two-phase heat transfer areas. Initially he worked on convective heat transfer and subsequently on the enhancement of pool boiling and condensation processes using high intensity electric fields. In parallel, he carried out extensive experimental work in pool boiling heat transfer with plane and enhanced surfaces. Professor Karayiannis has also been very actively involved with research in flow boiling in small to micro tubes and micro-multi-channels. This work involves fundamental studies as well as research leading to the design of high heat flux integrated thermal management systems.  He has published more than 260 chapters in books, papers and industrial reports. He chairs the Committee of the International Conference Series on Micro and Nanoscale Flows now in its 8th edition. He is a Fellow of the EI and the IMechE, Member of the Assembly for International Heat Transfer Conferences and the Chairman of the UK National Heat Transfer Committee.

Title

Fundamental Aspects of Pool Boiling Heat Transfer and Enhancement

Abstract

Pool boiling is one of the most effective modes of heat transfer, capable of transferring significant rates of heat from thermally active surfaces at small temperature differences between the surface and the fluid. Therefore, it finds a wide range of application in industrial processes, including cooling of nuclear reactors, evaporators in power producing and desalination plants, refrigeration and heat pump systems and in the chemical industries. The need to dissipate large amounts of heat from power electronic devices is also a more recent application as two-phase heat transfer outperforms current air or single-phase liquid systems, enabling at the same time a uniform temperature of the device to be cooled

The above summary clarifies the reasons for the continuous attention received by pool boiling heat transfer from the academic and industrial communities. Key areas of research aim to elucidate fundamental aspects relevant to the onset of nucleate boiling (ONB), established nucleate boiling and finally the occurrence of the Critical Heat Flux (CHF), i.e. the upper practical operational limit. Research in heat transfer enhancement techniques aims to reduce the temperature difference for the ONB, increase nucleate boiling heat transfer rates for a given surface-fluid temperature difference and increase CHF. Heat transfer enhancement techniques are classified as active and passive. Active techniques include the application of an external force in surface or fluid vibration or surface rotation or the use of a high intensity electric field – with the latter referred to as Electrohydrodynamic (EHD) Enhancement.  Passive techniques are based on surface profile designs and surface modifications in the form of rough, coated or treated surfaces.

The presentation will cover on-going research on boiling fundamentals, starting with the criteria for vapour entrapment and bubble generation, superheat required for heterogeneous nucleation, effect of surface topology and wettability plus a review and comparative presentation of models predicting bubble growth, departure and heat transfer rates. In the second part, and to start with, some reference will be made to EHD enhancement of pool boiling heat transfer. The rest of the presentation will focus on surface modification for heat transfer enhancement.  The requirement for benchmarking and evaluating the performance of smooth and subsequently enhanced surfaces will first be presented. The methods and designs employed by the research community to enhance heat transfer rates is then presented and discussed. The recommended designs, their complexity and their comparative performance in relation to smooth surfaces and surfaces modified by standard simple to implement techniques are then presented. The analysis performed includes two different fluids, namely water and FC-72 and two different surfaces, i.e. copper and silicon. This allows conclusions to be made covering a wide range of industrial applications.  We expect the presentation to contribute to the debate on pool boiling heat transfer enhancement, providing useful guidelines in developing, evaluating and recommending designs and their implementation in industrial applications.