The Coffee Cup Phenomena

The what Phenomena ? , Ahem !, I've just coined that as Coffee Cup Phenomena. So, what is it ?

I had taken up an industrial plant training at Manali PertoChemicasl Ltd., Chennai, India, during my 3rd Semester and I was guided by one Field Engineer who happened to be a graduate from Anna University, Chennai, India. This guy, one day, asked us (two of my friends were with me for the training)

"What are the mechanisms involved when you blow a cup of hot steaming coffee to cool it?"

Seems pretty simply and straightforward aint it ?

. I put this question to my kid(1oth std) cousin bro, he gave a neat reply -

"The coffee is at a higher temperature than the surrounding air, so heat flows from higher temperature to lower temperature and when we blow, it loses heat faster" , these were not his exact words, but 't was somewhat like this.

. I then, had a discussion with Srinivasa Ramanujam (a Unit Operations expert, whom I met in Orkut) , and here is the convo.


SR : the process which you described in your question is nothing but forced convection heat transfer which you will learn when you take heat transfer course.

In absense of blowing, the beverage which is at higher temperature will lose heat to relatively low temp air in atmosphere. The heat lost by the beverage will be picked up by the air and its temperature will increase (upto a limit called boundary layer). when this hot air is removed by blowing, more and more cool air will come into contact with the hot beverage surface which can remove more heat. while the absense of blowing is called natural convection heat transfer, blowing by external means is called forced convection heat transfer.

SR : what you are talking about is two-phase heat transfer coupled with mass transfer which depends on specific and latent heat of fluid(s) as well as concentration difference between them. your explanation is partly correct that some amount of hot beverage will be carried out while blowing (mass transfer), however the dominant mode is heat transfer due to high temperature difference.
for eg, during summer the water in reservoir will evaporate due to heat transfer, but during winter it is by mass transfer. you will learn more during your heat transfer course.


SR : juz now i had coffee with my collegue which stimulated my thoughts furthur. first of all, the mechanism involved in heat transfer from coffee to atmospheric air is more complex than described earlier, because coffee is not an ideal fluid. it is a mixture of milk, sugar and decoction of coffee bean so you will have fine solid coffee particles, water vapour and milk (which itself is a mixture of fatty acids and water). the water vapour which is trapped by coffee will help to retain the heat, by blowing we are releasing the latent heat stored in water vapour which will eventually bring down the coffee temperature. also the high temperature fluid will lose heat by convection to the atmosphere. moreover, the concentration difference between coffee and atmospheric air will carry away some mass (and heat stored in that mass). try to stir with a spoon instead of blowing and see what happens :).


SR : first of all, i appreciate your learning interest.
hot coffee is a mixture of sugar, fatty acids, fine coffee particles and water (and probably few more which i don't know!) each of which will boil at different temperature. the vapour composition is not an ideal gas and it depends strongly on temperature, density of the fluid and other properties. fine coffee particles will also be carried away by this vapour tha heating effect and hence its temperature will reduce.

I do not have my scraps as Mr.SR had deleted them and I think he is not in Orkut anymore.

Well, one explanation I can think of is this -

In any liquid system, homogenous or heterogeneous, there exists a Vapour Liquid Equilibrium (VLE). What this means is, in any container containing some liquid, there exists a default layer of the vapour phase (of the liquid) just above the liquid surface. This vapour and liquid phases of the substance are in equilibrium with each other. Why such kind of equilibrium exists and further explanation can be found in Mass Transfer Operations by Robert E.Treybal and numerous texts are available to cover this, its a fundamental topic.
So, in our cup of steaming coffee, there exists this vapour phase, or coffee vapours, just above the liquid surface, it is this vapour that we see as the steam. The coffee molecules, take up the inherent heat (the coffee is hot) and get converted to their vapour form. So, the steaming coffee has - a vapour phase that takes up the inherent heat in the coffee to sustain, the hot liquid layer from which molecules vapourise.
Now, we happily blow !, what happens ? We are actually blowing away the vapour molecules just above the liquid coffee layer. We blow, the vapour molecules go away. Now, VLE comes into action, since there has to be a vapour layer existing, more molecules from within the liquid take up the inbuilt heat (due to the coffee's high temp) and vapourise to the top, we blow again ! , these newly formed vapour goes off. Again, more molecules consume the inherent heat and become vapours that are blown away, thanks to us !. So what is happening, the coffee loses more and more of its inherent heat to maintain the VLE, in this process what happens ?the coffee loses heat and ..COOLS DOWN !!. Summarising -

-Vapour molecules - Liquid coffee exist at equilibrium
-We blow, vapour molecules go away
-More vapour molecules come up by consuming inherent heat to maintain VLE
-coffee loses its heat to sustain the VLE by providing heat for vapourisation of liq. molecules
-coffee loses heat and cools down.

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So, there, that is the Coffee Cup Phenomena !! It was nice to write about it, think about it!, such a simple, day to day activity, but it involves a beautiful mechanism , nature rocks !!

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Have a nice coffee..!!