Subsubsection Heat Transfer
Heat can flow from one substance to another only when a temperature difference exists. Heat flow can occur only from a higher temperature to a lower temperature. When two objects at different temperatures are placed in contact with each other (or near each other), heat flows from the warmer object to the cooler one until both are at the same temperature. Heat transfer occurs at a faster rate when there is a larger temperature difference between the two objects. As the temperature difference approaches zero, the rate of heat transfer approaches zero.
Conduction, radiation, and convection are usually considered to be the three methods by which heat transfer can occur. It is more accurate, however, to consider conduction and radiation as the two basic methods of heat transfer and to consider convection separately as a special process which involves movement within a mass of fluid.
Conduction.
Conduction is the method by which heat flows from a hotter to a colder substance when there is physical contact between the two substances. For example, consider a cold metal bar which is held firmly against a piece of red-hot metal. In a short time, the end of the bar which is not touching the hotter metal will have become too hot to hold. We say that heat has been conducted from molecule to molecule, throughout the entire bar. The process of conduction will continue as long as there is a temperature difference between the two ends of the bar.
Radiation.
Radiation is a mode of heat transfer that does not require any physical contact between the warmer substance and the cooler substance. For example, a person sitting near a hot stove is warmed by radiant heat even though the air between the person and the stove may remain cold. Similarly, radiant heat from the sun warms the earth without warming the space through which it passes.
Convection.
At the molecular or sub-molecular level, heat transfer takes place through both the processes of conduction and radiation. If we use the term “heat transfer” in a somewhat different way, we may also include convection as a mode of heat transfer. However, it is important to understand the difference between convection and the basic heat transfer processes of conduction and radiation.
If we put a hot brick into a wheelbarrow and wheel it across the street, we have in one sense “transferred” heat. However, any heat transfer that takes place between the brick and its surroundings while we are wheeling it across the street will be by conduction and by radiation. Therefore, it would really be more accurate to say that we have “transported” the brick and all its contained thermal energy from one side of the street to the other.
Convection occurs only in fluids – liquids, gases, and vapors – not in solids, such as the brick we have just transported in the wheelbarrow. Convection is the transportation or movement of some portions of a fluid within the total mass of the fluid. As this movement occurs, the moving portions of the fluid transfer their contained thermal energy from one part of the fluid to another. The effect of convection is thus to mix the various portions of the fluid; the part that was at the bottom of the container may move to the top, or the part that was at one side may move to the other side. As this mixing takes place, heat transfer occurs from one part of the fluid to another and between the fluid and its surroundings. But this heat transfer, like any other heat transfer, takes place by conduction and by radiation. In other words, convection transports portions of the fluid; conduction and radiation transfer the thermal energy.
Convection serves a vital purpose in bringing the different parts of the fluid into close contact so that heat transfer can occur. Without convection, there can be little heat transfer from or within fluids. Most fluids are poor conductors of heat when they are not in motion.
What causes this transportation of a mass of fluid? In the case of natural convection, the movement is caused by differences in the density of different parts of the fluid. The differences in density are usually caused by unequal temperatures within the mass of fluid. For example, as the air over a hot radiator is heated, it becomes less dense and therefore begins to rise. Cooler, heavier air is drawn in to replace the heated air, and convection currents are thus setup. Another example of natural convection is the circulation of water in a natural circulation boiler. As the water in the generating tubes is heated, it expands and becomes much lighter (less dense) than the cooler water in the downcomers. Therefore the hotter and lighter water rises, while the cooler and heavier water flows downward. The resulting circulation of water in the boiler is thus clearly an example of convection currents established by differences in temperature (and therefore differences in density) in various parts of the fluid.
In the case of forced convection, some mechanical device such as a pump or a fan produces the movement of the fluid. When the main feed pump moves feedwater toward the boiler, the water is transported by forced convection. The flow of combustion gases through a boiler is partly by natural convection and partly by forced convection. Natural convection occurs because the gases of combustion are hotter and lighter than air, so they tend to rise and go up the stack. Forced convection is also involved in this process, however, because the forced draft blowers supply an air pressure which increases the rate at which the combustion gases travel across the tubes and up the stack. When you stir a cup of hot coffee, you are forcing convection and thus increasing the rate of heat transfer. Natural convection currents would be set up in the coffee if you did not stir it, because differences in density would occur as some portions of the coffee cooled before others. If you want to cool the coffee rapidly, use forced convection (stirring). If you do not want it to cool so rapidly, wait for natural convection to do the job.
In summary, then, we use the term convection to describe the transportation, or loosely, the “heat transfer” of a mass of fluid and its contained thermal energy. However, the processes by which any substance gains or loses thermal energy are most accurately described in terms of conduction and radiation.
