A blacksmith wishes to cool his hot piece of steel as rapidly as possible. He has a bucket of ice-water and a bucket of oil (at room temperature). Which bucket should he dump his steel into?

Ice water is the better choice of the two, assuming the steel is hotter than 100 degrees C.

Btw, it's Fourier's law of heat transfer that says that the rate of heat transfer is proportional to the temperature gradient, not thermo.

Using either bucket does not change the fact that the steel loses heat by means of conduction, assuming you don't move the steel too much. It only changes the boundary condition of the governing differential equation (Laplace Operator on the temperature is zero, assuming roughly constant properties).

The boundary condition is Newton's law of cooling, q''=h*deltaT or q=h*A*deltaT. It is true that the fact that water evaporating will cause the steel to contact vapor (100 degrees C) slightly more.

But because of this, the mechanism of heat transfer has changed from natural convection to natural convection with boiling/evaporation. This means that the h is greatly increased (typically about 100-10,000 times larger), which more than balances the change in T (~50 degrees or so). Of course, other fluid properties have to be taken into account such as viscosity, density, thermal diffusivity, heat capacities, and beta (which I won't go into detail).

When the lower portion of the steel touches the water, the water vapor will rise up and take away heat from the higher portions steel (which I assume to be hotter than 100 degrees). Then the upper portion of the steel will be cooled down completely as it's submerged into the bucket. Although the water vapor will only cool the steel for a few seconds, this will make it a better choice.

And for those of you who are still in doubt, you can check anywhere that using water with evaporation (equipment safety issues though) or steam with condensation is one of the best ways of heat transfer. That's why it's used in most industrial plants, not only because it's cheap.

Posted by np_rt on 2003-04-14 10:24:53