Forced convection film boiling

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Dr Gideon Fareo, a lecturer from Johannesburg, has been a visitor to the Industrial Mathematics Group at the CRM since the start of April. He is funded by the CRM’s Dev-Math program and the University of the Witwatersrand. He has been working with Tim Myers on forced convection film boiling …

Boiling is a phase change process in which there is a formation of vapour bubbles either at a heating surface or in a superheated liquid layer next to the surface. These vapour bubbles form at nucleation sites, the number and location of which depend on the surface roughness of the heating surface, fluid properties and operating conditions. Film boiling, on the other hand, is a phenomena which occurs at a boiling stage, and in which a continuous layer of vapour covers the heating surface thereby keeping the liquid from making contact with the heating surface. Since water vapour conducts heat more poorly than does liquid water, the transfer of energy from the heating surface is diminished. This insulating property of vapour reduces the heat transfer coefficient in between the heating surface and the fluid medium.

In industrial processes, film boiling is encountered in various practices which include metallurgical, chemical and power engineering, as well as in nuclear power plants where water-based coolants are used. In general the formation of a vapour layer reduces heat transfer to the liquid, resulting in higher temperatures at the boundaries and possible overheating and melting of the cooling pipes. In the case of nuclear power plants, the vapour layer that forms does not absorb neutrons as well as liquid water does, even though neutron absorption is a major factor in achieving the much desired slowing down of nuclear reactions. Film boiling was a contributing factor in the Chernobyl nuclear disaster.

The problem of heat transfer in film boiling has formed the object of much theoretical research. Because the physics of boiling is highly complex, in that it involves the continuous coupled interaction between the fluid and heat variables, so far there has been no comprehensive theoretical framework developed that accurately predicts the boiling heat transfer coefficient.

In our research project, we have considered the problem of advective flow of water in a two-dimensional channel, whose boundaries are heated by a prescribed flux. The models describing the fluid and heat flow consisted of the lubrication equations, the heat equation with advection and the Stefan condition for conservation of energy at the fluid-vapour interface. Our ultimate goal in this initial project was to derive simple analytical expressions for the heat transfer coefficient, before and after film boiling. In later work we hope to extend the analysis in order to fully model the film boiling process and so determine the key factors. In this way we hope to be able to provide recommendations for industrial practitioners on ways to tackle vapour film formation.

Article written by Adewunmi Gideon Fareo (Adewunmi.Fareo@wits.ac.za)

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