Convection is heat transfer due to fluid flow. There are two types of convection: free or natural convection and forced convection. In natural convection, the fluid flow is caused by buoyancy force due to the density gradient. Forced convection, on the other hand, is caused by inertial force. Although the two may be different types of convection, both are dependent on the velocity of the fluid to determine the rate of heat flow. In order to qualify the fluid velocity, the following equation shows an empirical relation.
This relation is known as Newton’s law of cooling. h is the heat transfer coefficient and (TW-T?) is the difference between the wall temperature and the temperature of the surroundings. A is the area which is assumed to be invariant along the heat flow path. An example of natural convection is boiling water. The kettle is being heated up by the stovetop. However, in this case, the focus is on the heat flowing within the water. In other words, when the heat passes from the stovetop to the kettle, the water at the bottom heats up. At this time, the hot water rises and the cold water sinks, making a circular motion. This is due to hot water being lighter than cold water due to its particles being further apart. As a result, the hot water takes up more room but still contain the same amount of molecules as the cold water. An example of a forced convection is using the air conditioner in the summer or using the oven. This is when one uses an external source (usually electricity) to force convection. In the case of the oven, one uses electricity to forcefully raise the temperature of the oven through convection of air. The cold air closest to the heat source will heat up first