Heat Transfer (Unit-6) Boiling and Condensation IIT lecture videos

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Heat and Mass Transfer

Prof. S.P. Sukhatme

Department of Mechanical Engineering

Indian Institute of Technology, Bombay

Lecture No. 29

Boiling and Condensation-1

Today we will begin with the topic of condensation and boiling; we will be studying the heat transfer during this change of phase processes. Now, first of all, the obvious question is where are we interested in, what situations are we interested in studying heat transfer during condensation and boiling. Well, if you look, say at a power plant - any thermal power plant - you got change of phase occurring when water is converted into steam in a boiler or sometimes in an evaporator and you got change of phase occurring when the steam which has been expanded through a turbine is condensed in a steam condenser. Or you, if you look, say at a refrigeration system - vapor compression refrigeration system - there also we have change of phase occurring when the refrigerant is condensed in a condenser after having given up its heat in the evaporator. So, there are numerous such situation; I have just mentioned 2 where we are interested in knowing the heat transfer rates during change of phase that is either during condensation or during boiling. So, our purpose in this topic when we are studying this topic is to derive expressions for the heat transfer coefficient in some situations for some geometries and with the help of these expression or with the help of these correlations, one is able to design the equipment in which either condensation or boiling is taking place.

Lecture No. 30

Boiling and Condensation-2

We were, when we stopped last time, we were studying heat transfer during film condensation on a vertical plate. I was deriving an expression for the heat transfer coefficient and following the derivation which was first done by Nusselt long ago in 1960. And if you recall where we stopped after making a certain number of simplifying assumptions, we had applied this Newton’s second law of motion, that is one of our basic fundamental laws, derived an expression for the velocity profile in the liquid fill. Then, we had derived an expression for the mass flow rate in the film, the liquid mass flow rate in the film and then taken the differential of that to show that if you go from a section z down to a section z plus dz, then the mass flow rate increases by the amount -differential amount - and we had an expression for that and that is where we stopped. So, let me just put down those expressions again for, to recollect.

Lecture No. 31

Boiling and Condensation - 3

Last time we derived the expression for the heat transfer coefficient during condensation of a saturated, pure saturated, vapor on vertical plate. The expression we derived was first derived by Nusselt and after deriving the expression based on the fundamental laws of conservation of mass, Newton’s second law of motion and the first of law thermodynamics, after deriving that simple expression we then went on to use it to solve a numerical problem of condensation on a vertical tube. Now, today we will extend that result further; we will use a similar result for condensation on horizontal tubes. So today, we are going to take up film condensation on horizontal tubes.

Lecture no. 32

Boiling and Condensation 4

Last time we have started with the topic of boiling and I, first of all, I said I described to you when boiling will occur, then I classified two types, two types of boiling; I said there is pool boiling and flow boiling and then we focused our attention on pool boiling and I identified various regimes or various regions in pool boiling and I drove a typical pool boiling curve while identifying those regions. What were those regions? They were first of all the natural convection region, then the nuclear boiling region and the third one was the film boiling region. Even in the film boiling region, there were 2 parts, one part which I called the transition boiling region in which we have a transition from nucleate to stable film boiling and the second part which are called as stable film boiling. I also identified one important point on that pool boiling curve and I said that point is called that peak heat flux or the critical heat flux and I told you why that point is important.