Heat transfer via air cooling -- which airflow direction is best?

[freeelectron]

TL;DR Summary

Most efficient heat transfer based on airflow direction

I'm using a stainless steel pan and a fan to cool my coffee down as quickly as possible.
Usually, I have the fan blowing down towards the coffee, but this morning I flipped it over to suck the heat out of the coffee and I'm wondering which of the two is the most efficient, especially as, in the second case, if you lower the fan enough, you clearly start to feel a pulling force due to the vacuum.

Any thoughts?

Thanks

 

[256bits]

TL;DR Summary: Most efficient heat transfer based on airflow direction

if you lower the fan enough, you clearly start to feel a pulling force due to the vacuum.

In that case, your fan is just a little bit too low, as you are hindering the movement of air into the fan.
Put it much lower and you might be able to suck up some of the coffee into the fan too.

 

[jrmichler]

The rate of cooling is proportional to the rate of heat transfer. There are a number of variables that control the rate of heat transfer. The most important of those variables is the heat transfer film coefficient between the air and the coffee. The value of that film coefficient is proportional to the velocity of the air at the surface of the coffee. Faster air movement at the surface equals faster cooling.

It's easy to test this using a thermometer in the coffee and a clock.

 

[russ_watters]

Fans generate a low pressure area behind them that sucks-in air from all directions. On the discharge side they direct a coherent column of air. So for forced, open convection the column of air provides higher velocity at the thing you want to cool. Blow is better than suck.