Lesson 4 - Convection
This lesson deals with the principles and application of a form of heat transfer called convection. More specifically, it describes the heat transfer mechanisms that involve convection and calculation methods for heat transfer rates determined by convection and phase changes.
Preface
The transfer of heat in liquids or gases is caused by the bulk movement of the fluid. This process is called convection or convective heat transfer.
Liquids and gases both behave in a fluid-like manner, and therefore throughout this lesson we will use the term ‘fluid’ for both liquid and gas.
As temperature increases in a fluid, the molecules have more energy and move around faster and further away from each other. The increase in space between molecules results in a lower density. If you recall, density is mass over volume. So when molecules are further apart there is less mass in a given volume, resulting in a lower density.
Convection is responsible for many everyday heating and cooling operations. Let’s look at a room heater as an everyday example. The heater supplies hot air to a room. The hot air is less dense than the colder air, so it travels to the top of the room (ceiling). The cold air drops to the bottom of the room (floor) to replace the warmer air. This cycle continues for as long as there is a temperature difference somewhere in the room. This is an example of natural convective heat transfer.
Natural convection occurs well, naturally, while forced convection requires an outside force like a fan or pump.
It is more common to find a heat transfer example in nature that involves more than one mode of heat transfer. Often conduction and convection are two transfer methods that are found working together.
An example to illustrate this is boiling an egg. Conduction is occurring between the stove top and the pot because there is direct contact between the heating element and the pot. The pot is then transferring heat to the water to cause it to boil. The circulation of boiling water in a heated pot is an example of convective heat transfer. The water closest to the heating source (the bottom of the pot) becomes hotter, is less dense and moves to the top, replacing the cooler water that was on top. At the fluid solid interface between the water and the egg, heat is transferred via conduction. The heat transfer from the stove top through the pot, through the water and finally to the egg results in cooking the egg. And there you have it – conduction and convection working in unison to transfer heat!