Convective heat transfer is the transfer of heat through means of convection, the transfer of heat through fluid motion. Convective heat transfer can occur in one of two major ways: through natural convection and through forced convection. These two methods of heat transfer are distinguished by the use, or lack of, external devices to move the heated fluid around. In either case, the transfer of heat occurs by a fluid-- such as a liquid, gas, or a rheid-- flowing over a solid mass.
One example of convection is water boiling in a pot. As the water near the heat source gets warmer, the molecules spread apart. This causes the water to become less dense and more buoyant. The more buoyant water rises to the top of the pot and cooler water takes its place. The cooler water will then get heated by the heat source, become less dense, and rise to the top.
This process results in a current called a convection current. In the case of water boiling, it becomes more visible when something like macaroni is in the pot-- one will see the macaroni moving up and down with the convection current. Boiling water is an example of natural convection. In this system, nothing except the varying densities of the water causes the convective heat transfer. In fact, transferring heat through a natural convection works because, in general, heated liquids have lower densities than cooler liquids.
In contrast to convective heat transfer caused by natural convection is heat transfer caused by forced convection. Forced convection differs from natural convection because of the use of an external mechanism which moves the fluid around. This method of convective heat transfer does not need the varying densities of fluids to work. Some examples of external mechanisms used in forced convection are pumps and fans. The heart is a natural example of such a device used in forced convection.
The circulatory system is one of the ways humans and some animals regulate their body temperatures. A heart is essentially a pump, and blood is a heated fluid. The heart pumps the heated fluid through blood vessels and toward the skin. Heat is then taken away from the body through various means. This is one of the reason that many desert animals have large ears— blood is able to circulate through the ears, giving off its heat to the environment through the various heat transfer methods.