Transferring Heat through Convection: Natural versus Forced - dummies

Transferring Heat through Convection: Natural versus Forced

By Steven Holzner

Thanks to physics, we know that convection can be either natural, where heat rises on its own, or forced, where you control the movement of the heat.

You may have heard the maxim “heat rises,” which is all about convection. However, a more accurate statement is that “hot fluid rises.” In substances where convection is free to take place — that is, in gases and liquids — hotter material naturally ends up on top and cooler material ends up on the bottom because of the higher buoyancy of the hotter material.

If your house has two floors, you often end up with the bottom floor being cooler than the top floor. The warmer air rises by buoyancy (since it is less dense than cooler air), which drives the convection. Physicists refer to this type of convection as natural convection because it isn’t externally driven.

To understand how natural convection works, look at the microscopic picture. Any substance is made up of molecules, tiny particles that zip around at varying speeds. When a gas or liquid becomes hot, its molecules move faster. If you have a heating element that contacts the bottom of the substance — such as a wood stove at the bottom of a room or a stove element that’s heating a kettle of water — the molecules near the heating element become hot. Hotter molecules have more kinetic energy and so can zip around faster and hit other molecules harder.

Because they move faster and hit harder, hotter molecules make the substance in their immediate area less dense. That is, they have more energy with which to push other molecules out of the way. The molecules that have been hit also have more energy to push other molecules out of the way, so the substance in the immediate vicinity of the heating element becomes less dense.

A unit volume of material that’s less dense weighs less than a unit volume of the surrounding material, and if that material is a gas or liquid, the less dense stuff rises. Because the denser material has more mass per volume, it sinks under the influence of gravity.

Sometimes natural convection is the opposite of what you want. With forced convection, you control the movement of the warm or cool fluid, often using a fan or pump.

For example, take a room on a cold winter’s day. Because heat rises, the hotter air in the room drifts up to the ceiling, while the cooler air in the room settles near the bottom of the room, where you are. So in time, all the hot air in the room collects near the ceiling, and all the cold air collects near the floor. Although you were originally quite cozy, you may now be getting pretty cold — all as a result of natural convection.

What can you do? You can turn on your room’s ceiling fan in reverse! Ceiling fans force the air to circulate, so the hot air near the top of the room moves downward. The warmer air at the top of the room now ends up at the bottom of the room again, where you are. Just make sure you choose a low speed so you don’t create a breeze.

You find forced convection all around you. The fans in a desktop computer, for example, cause forced convection (and the lack of room for a fan in some laptop computers has caused plenty of overheating problems). Refrigerators use fans to blow away heat, again relying on forced convection.