1 reply to this topic

[Mohamed Selim]

Question about heat transfer:

 

If i put a small ball of metal in a cup of hot water and leave them for some time, the metal ball temperature will rise.

what type of heat transfer is this process, if the ball doesn't move in water only in static state inside the cup of water?

is it convection or conduction transfer of heat, and why?

[Saml]

Assuming that you can introduce the ball in water fast enough so that temperature does not start to equilibrate, and slow enough as to not having any forced convection in the water.....the kind of the infinitesimal reversible piston on our thermodynamic classes.

 

If the cup is only a few microns bigger than the ball, it will be conduction. How you introduce the ball into a cup a few microns biggers is the same as the previously mentioned piston.

 

If the ball is hotter than the vapor pressure of the water it will initially be a phase change heat exchange, but from your question we should assume that the ball is colder thant the water.

 

Inside the ball it it will be conduction unless the metal is mercury.

 

If the water is hotter than the metal, the fraction of the heat transfer due to convection will be a function of the temperature difference. When the difference is large, convection will be predominant, while the limit when the temperature difference approach to zero is conduction. Here, the Grashoff number plays an imporant role.

 

And, finally, if the water is hot (above room temperature) but in the form of Ice seven or ten, it will be conduction.