Part of Lesson Plan: Methods of Heat Transfer
Activity Overview / Details
Understanding how heat moves from one object to another is one of the basic premises of understanding fire behavior. Knowing how heat transfers will assist the firefighter in estimating fire growth which in turn will assist in their ability to fight the fire.
Heat moves from warmer objects to cooler objects. Therefore in order for heat transfer to occur, objects must be at different temperatures. The rate at which heat transfers is dependent on a couple of tangibles. Those include the materials involved and how they conduct heat. The transfer of heat energy is measured as energy flow.
There are three ways in which heat transfers from one object to another. They are Convection, Conduction, and Radiation.
Conduction: The best description of this type of heat transfer is "point to point" heat transfer. In other words it is the transfer of heat from one body to another through direct contact. Increased molecular activity and collisions between molecules of a substance causing conduction to transfer energy through a particular object. The density of the object influences the rate of transfer. The denser the object, the more readily it will conduct heat. Heat flow is also influenced by the difference in teperature of the object being heated and the heat source as well as the thermal conductivity of the material.
One of the easiest examples of conduction is heating a metal object such as a piece of rebar or a metal pipe. While heating one end of the metal object with an open flame, that object will eventually transfer heat to the other end through conduction.
Convection: this is the transfer of heat by the movement of heated fluids or gases in an upward direction. As a fire grows heat will begin to move through hot smoke and gases. The hot smoke and gases then transfer heat to cooler solid surfaces, and air. Use the example of a convection oven with the students as most will be familiar with the ovens in their homes.
Another example that can be used is with an open flame such as a candle. Move your hand over the candle until you start feeling the heat. Point out that the heat is moving in an upward motion and that heat will continue to increase as long as you keep your hand near the flame.
Radiation: This is best described as heat transfer through electromagnetic waves with no intervening medium. Some examples of electromagnetic waves are; radio waves, x-rays, and light rays. Radiation becomes one of the most dominant forms of heat transfer as a fire grows. Radiation contributes to more exposure fires than the other methods. Radiant heat is also a significant contributor to fire development as it leads to a flashover stage.
Radiant heat can be influenced by several factors. Distance between objects will reduce the ability of radiant heat to transfer. Objects that are darker have the ability to absorb heat more effectively than objects that are lighter in color. Since radiation occurs through an electromagnetic wave, it moves in a straight line at the speed of light. The best example of this is the sun. As we feel the heat from the sun we can interrupt this heat transfer by putting an object between our body and the sun thus interupting the electromagnetic waves.
Another great example to employ is using an open flame. Move your hand close to the flame from the side until you can feel the heat.