13.1

Convection

CONCEPTS AT A GLANCE When heat is transferred to or from a substance, the internal energy of the substance can change, as we saw in Chapter 12. This change in internal energy is accompanied by a change in temperature or a change in phase, as suggested by the Concepts-at-a-Glance chart in Figure 12-23. The transfer of heat affects us in many ways. For instance, within our homes furnaces distribute heat on cold days, and air conditioners remove it on hot days. Our bodies constantly transfer heat in one direction or another, to prevent the adverse effects of hypo- and hyperthermia. And virtually all our energy originates in the sun and is transferred to us over a distance of 150 million kilometers through the void of space. Today’s sunlight provides the energy to drive photosynthesis in the plants that provide our food and, hence, metabolic energy. Ancient sunlight nurtured the organic matter that became the fossil fuels of oil, natural gas, and coal. This chapter examines the three processes by which heat is transferred: convection, conduction, and radiation. The Concepts-at-a-Glance chart in Figure 13-1, which is a modification of that in Figure 12-23, illustrates how these three methods of heat transfer are related to our previous study of heat and internal energy.

Figure 13-1 CONCEPTS AT A GLANCE Heat is transferred from one place to another by three methods: convection, conduction, and radiation. As discussed in Chapter 12, heat can change the internal energy of a substance, which results in a change in the temperature or phase of the substance. This shovel-snouted lizard lives in a desert environment. To reduce the amount of heat that is transferred into his body from the hot sand it lifts two of its feet into the air. (Michael & Patricia Fogden/Minden Pictures, Inc.)

When part of a fluid is warmed, such as the air above a fire, the volume of that part of the fluid expands, and the density decreases. According to Archimedes’ principle (see Section 11.6), the surrounding cooler and denser fluid exerts a buoyant force on the warmer fluid and pushes it upward. As warmer fluid rises, the surrounding cooler fluid replaces it. This cooler fluid, in turn, is warmed and pushed upward. Thus, a continuous flow is established, which carries along heat. Whenever heat is transferred by the bulk movement of a gas or a liquid, the heat is said to be transferred by convection. The fluid flow itself is called a convection current.

| Convection
| Convection is the process in which heat is carried from place to place by the bulk movement of a fluid.

The smoke rising from the volcanic eruption in Figure 13-2 is one visible result of convection. Figure 13-3 shows the less visible example of convection currents in a pan of water being heated on a gas burner. The currents distribute the heat from the burning gas to all parts of the water. Example 1 deals with some of the important roles that convection plays in the home.

Figure 13-2 The plumes of thick, gray smoke from this industrial fire rise hundreds of meters into the air because of convection. (Scott Shaw/The Plain Dealer/ © AP/Wide World Photos)

Figure 13-3 Convection currents are set up when a pan of water is heated.

| Example 1 | | | Hot Water Baseboard Heating and Refrigerators

Hot water baseboard heating units are frequently used in homes, where they are mounted on the wall next to the floor, as in Figure 13-4a. In contrast, the cooling coil in a refrigerator is mounted near the top of the refrigerator, as in part b of the drawing. The locations for these heating and cooling devices are different, because each location is designed to maximize the production of convection currents. Explain how.

Figure 13-4 (a) Air warmed by the baseboard heating unit is pushed to the top of the room by the cooler and denser air. (b) Air cooled by the cooling coil sinks to the bottom of the refrigerator. In both (a) and (b) a convection current is established.

Reasoning and Solution An important goal for a heating system is to distribute heat throughout a room. The analogous goal for the cooling coil is to remove heat from all of the space within a refrigerator. In each case, the heating or cooling device is positioned so that convection makes the goal achievable. The air above the baseboard unit is heated, like the air above a fire. Buoyant forces from the surrounding cooler air push the warm air upward. Cooler air near the ceiling is displaced downward and then warmed by the baseboard heating unit, leading to the convection current illustrated in Figure 13-4a. Had the heating unit been located near the ceiling, the warm air would have remained there, with very little convection to distribute the heat.
Within the refrigerator, the air in contact with the top-mounted coil is cooled, its volume decreases, and its density increases. The surrounding warmer and less dense air cannot provide sufficient buoyant force to support the colder air, which sinks downward. In the process, warmer air near the bottom is displaced upward and is then cooled by the coil, establishing the convection current shown in Figure 13-4b. Had the cooling coil been placed at the bottom of the refrigerator, stagnant, cool air would have collected there, with little convection to carry the heat from other parts of the refrigerator to the coil for removal.

The physics of heating and cooling by convection.

Another example of convection occurs when the ground, heated by the sun’s rays, warms the neighboring air. Surrounding cooler and denser air pushes the heated air upward. The resulting updraft or “thermal” can be quite strong, depending on the amount of heat that the ground can supply. As Figure 13-5 illustrates, these thermals can be used by glider pilots to gain considerable altitude. Birds such as eagles utilize thermals in a similar fashion.

Figure 13-5 Updrafts, or thermals, are caused by the convective movement of air that the ground has warmed.

The physics of “thermals.”

It is usual for air temperature to decrease with increasing altitude, and the resulting upward convection currents are important for dispersing pollutants from industrial sources and automobile exhaust systems. Sometimes, however, meteorological conditions cause a layer to form in the atmosphere where the temperature increases with increasing altitude. Such a layer is called an inversion layer because its temperature profile is inverted compared to the usual situation. An inversion layer arrests the normal upward convection currents, causing a stagnant-air condition in which the concentration of pollutants increases substantially. This leads to a smog layer that can often be seen hovering over large cities.

The physics of an inversion layer.

We have been discussing natural convection, in which a temperature difference causes the density at one place in a fluid to be different from that at another. Sometimes, natural convection is inadequate to transfer sufficient amounts of heat. In such cases forced convection is often used, and an external device such as a pump or a fan mixes the warmer and cooler portions of the fluid. Figure 13-6 shows an application of forced convection that is revolutionizing the way in which the effects of overheating are being treated. Athletes, for example, are especially prone to overheating, and the device illustrated in Figure 13-6 is appearing more and more frequently at athletic events. The technique is known as rapid thermal exchange and takes advantage of specialized blood vessels called arteriovenous anastomoses (AVAs) that are found in the palms of the hands (and soles of the feet). These blood vessels are used to help dissipate unwanted heat from the body. The device in the drawing consists of a small chamber containing a curved metal plate, through which cool water is circulated from a refrigerated supply. The overheated athlete inserts his hand into the chamber and places his palm on the plate. The chamber seals around the wrist and is evacuated slightly to reduce the air pressure and thereby promote circulation of blood through the hand. Forced convection plays two roles in this treatment. It causes the water to circulate through the metal plate and remove heat from the blood in the AVAs. Secondly, the cooled blood returns through veins to the heart, which pumps it throughout the body, thus lowering the body temperature and relieving the effects of overheating.

Figure 13-6 An overheated athlete uses a rapid-thermal-exchange device to cool down. He places the palm of his hand on a curved metal plate within a slightly evacuated chamber. Forced convection circulates cool water through the plate, which cools the blood flowing through the hand. The cooled blood returns through veins to the heart, which circulates it throughout the body.

The physics of rapid thermal exchange.

Figure 13-7 shows the application of forced convection in an automobile engine. As in the previous application, forced convection occurs in two ways. First, a pump circulates radiator fluid (water and antifreeze) through the engine to remove excess heat from the combustion process. Second, a radiator fan draws air through the radiator. Heat is transferred from the hotter radiator fluid to the cooler air, thereby cooling the fluid.

Figure 13-7 The forced convection generated by a pump circulates radiator fluid through an automobile engine to remove excess heat.

Copyright © 2007 John Wiley & Sons, Inc. All rights reserved.