A basic question I just cant understand, On blowing air to a hot dish, egs: soup, why does it cool faster than than the air. And why is the air we blow cold? Isnt the temp. of our body warmer than the air outside?

Question

anonymous · Answer

@Mani_Jha Help?

anonymous · Answer

I think it's because the air we are exhaling is a lower temperature than the soup we are blowing. The heat transfers away from the hot dish to the air we blow on it because it is trying to reach an equilibrium temperature with its surroundings.

mani_jha · Answer

The air we blow is actually warm. Just blow some air into the skin of your hand and feel it.

anonymous · Answer

@ManiJha Exactly. Then why does it become cold?

stormfire1 · Answer

This is pretty simple.  Soup serving temperatures range from around 140-170F.  The air you breathe out is going to be very close to your internal body temperature (combined with the ambient air - venturi effect).  The large temperature difference between them is why the soup cools faster if you blow on it.

stormfire1 · Answer

BTW: You should avoid using your body as temperature gauge.  The air you blow isn't any warmer than your body just because it feels like it is...it's just warmer than your skin.

anonymous · Answer

This can best be explained by convection, which is one of the three heat transfer modes. 

Conduction is the transfer of heat within a solid medium. 
Convection is the transfer of heat by the motion of fluids. 
Radiation is the transfer of heat by electromagnetic radiation. 

Let me present a simple explanation of convective heat transfer. 

In general, there are two "types" or classifications of convection: natural and forced. 

Let's consider a bowl of soup at, as stormfire1 pointed out, 140F. A typical room can said to be at 75F. We know that for heat transfer to occur, there has to be a temperature gradient (difference in temperature) which we have here. If we simply place the bowl of soup on the table, the soup will warm the air in the room that is very close to the surface of the soup. This warm air is now less dense than the surrounding air in the room and will consequently rise. When the air rises, it takes thermal energy, which it got from the soup, with it. This removes thermal energy from the soup and makes the soup cooler. This process is dubbed natural convection and is a direct result of buoyancy forces. This effect can be observed on a hot day if you look at the hood of a car. You can actually see the air moving as the difference in density of the air causes the light to refract. 

Now, let's consider that we blow on the soup. Our soup is still at 140F, but we are blowing air that is 96F across it. We still have the temperature difference needed for heat transfer to occur. This time, however, since the air is being forced across the surface. More mass per unit time of air is being moved across the surface of the soup because we are actively blowing it across the surface. Each unit of mass will again take with it thermal energy from the soup, and since more units of mass pass over the soup, more heat will be removed. Albeit that each unit of mass takes with it less thermal energy than in the natural convection case, the net effect is a greater heat transfer rate (units of thermal energy moved per unit time). This is dubbed forced convection. 

As a rule, forced convection will have greater heat transfer rates than natural convection given the same temperature gradient. 

This is why we have fans. The fan actually heats the air in the room (by means of doing shaft work on the system, see the First Law of Thermodynamics), but because it actively moves the air across our bodies, it removes thermal energy from our bodies at a greater rate than if the air were still, which is why we feel cooler.

vincent-lyon.fr · Answer

The heat transfer does not take place between soup and air.

Air warmer than the sea will actually cool the water down if the wind is blowing.

The cause is the moving air blows away the water vapour in equilibrium with the liquid water underneath. Equilibrium is broken so liquid water will have to evaporate to restore this equilibrium. Evaporation needs energy that is taken from the liquid which will cool down as a result.

stormfire1 · Answer

@eashmore: Very nice answer...for some reason I completely forgot to consider the forced air difference and the actual heat transfer mechanism.