Methods Of Heat Transfer :
The transfer of energy as heat occurs in three ways: (1) by conduction (2) by convection and (3) by radiation
Heat transfer by conduction occurs when energy is transmitted by direct contact between the molecules of a single body or between the molecules of two or more bodies in good thermal contact with each other. In either case, the heated molecule communicate their energy to the other molecules immediate adjacent to them. The transfer of energy from molecule to molecule by conduction is similar to that which takes place between the balls on a billiard table, wherein all or the some part of the energy of motion of one ball is transmitted at the moment of impact to the other balls that are struck.
When one end of a metal rod is heated over a flame, some of the heated energy from the heated end of the rod will flow by conduction from molecule to molecule through the rod to the cooler end. As the molecule of the rod at the heated end absorb energy from the flame, their energy increases, and they move faster and through a greater distance. The increased energy of the heated molecules causes them to strike against the molecules immediately adjacent to them. At the moment of impact and because of it, the faster moving molecules transmit some of their energy to their slower moving neighbors, so that they too begin to move rapidly. In this manner, energy passes from molecule to molecule from the heated end of the rod to the cooler end of the rod. However, in no case would it be possible for the molecules furthest from the heat source to have more energy than those at the heated end.
As heat passes through the metal rod, the air immediately surrounding the rod is heated by conduction. The rapidly vibrating particles of the heated rod strike against the molecule of air that are in contact with the rod. The energy so imparted to the air molecules causes them to move about at a higher rate and communicate their energy to other nearby air molecules. Thus, some of the heated supplied to the metal rod is conducted to and carried away by the surrounding air.
If the heat supply to the rod is interrupted heat will be continued to carried away from the rod by the air surrounding the rod until the temperature of the rod drops to that of the air. When this occurs, there will be no temperature differential , the system will be in equilibrium, and no heat will be transferred.
The rate of heat transfer by conduction, as previously stated, is in direct proportion to the difference in temperature between the high and low temperature parts. However all materials do not conduct heat at the same rate. Some materials, such as metal, conduct heat very rapidly, whereas others, such as glass, wood, and cork, offer considerable resistance to the conduction of the heat. Therefore, for any given temperature difference, the rate of heat flow by conduction through different materials of the same length and cross section will (.) with the particular ability of the various materials to conduct heat. The relative capacity of a material to conduct heat is known as its conductivity. Materials that are good conductors of heat have a high conductivity, whereas materials that are poor conductors have a low conductivity and are used as heat insulators.
In general, solids are better conductors of heat than liquids, and liquids are better conductors than gases. This is accounted for by the difference in the molecular structure. Since the molecules of a gas are widely separated, the transfer of heat by conduction, that is, by direct contact between the molecules is difficult.
Heat transfer by convection occurs when heat moves from one place to another by means of currents that are set up within some fluid medium. These currents are known as convection currents and result from change is density that is brought about by the expansion of the heated portion of the fluid.
When any portion of the fluid is heated, it expands, and its volume per unit of mass increases. Thus the heated portion becomes convection currents set up in a vessel of water when the vessel is heated at bottom center.
Lighter, rises to the top, and is immediately replaced by a cooler, heavier portion of the fluid. For example, assume that a tank of water is heated at the bottom in the center.(Fig 2.2) The heat from the flame is conducted through the metal bottom of the tank to the water inside. As the water adjacent to the heat source absorbs heat, its temperature increases, and it expands. The heated portion of the water, being lighter than surrounding water, rises to the top and is replaced by cooler, denser water pushing in from the sides. As this new portion of water is being heated, it too rises to the top and is replaced by cooler water from the sides. As this sequence continues, the heat is distributed throughout the entire mass of the water by means of the convection currents established within the mass.
Warm air currents, such as those that occur over stoves and other hot bodies, are familiar to everyone. (Fig. 2.3) illustrates how convection currents are utilized to carry heat to all parts of a heated space.
Heat transfer by radiation occurs in the form of the wave motion similar to light waves wherein the energy is transmitted from one body to another without the need for intervening matter. Heat energy transmitted by wave motion is called radiant energy.
It is assumed that the molecules of a body are in rapid vibration and that this vibration sets up a wave motion in the space surrounding the body. Thus the internal molecular energy of the body is converted into radiant energy waves. When these energy waves are intercepted by another body of matter, They are absorbed by that body and are converted and are converted into its internal energy.
The earth receives heat from the sun by radiation. The energy of the sun’s molecular vibration is imparted in the form of radiant energy waves to the space surrounding the sun. The energy waves travel across billions of miles of space and impress their energy upon the earth and upon any other material bodies that intercept their path. The radiant energy is absorbed and transformed into internal energy, so that the vibratory motion of the hot body (the sun) is reproduced in the cooler body (the earth).
All materials give and absorb heat in the form of radiant energy. Any time the temperature of a body is greater than that of its surroundings, it will give more heat by radiation than it absorbs. Therefore, it losses energy to the surroundings, and its internal energy decreases. When no temperature difference exists, the energy exchange is in equilibrium, and the body neither gains or nor loses energy.
Heat transfer through a vacuum is impossible by either conduction or convection, since these processes, by their very nature, require that matter be the transmitting media. Radiant energy, on other hand, is not dependent upon as a medium of transfer and therefore can be transmitted through a vacuum. Furthermore, when radiant energy is transferred from a hot body to a cold through some intervening media such as air, the temperature of the intervening media is unaffected by the passage of the radiant energy. For example, heat is radiated from a “warm“ wall to a “cold” through the intervening air without having any appreciable effect upon the temperature of the air. Since the molecules of the air are relatively few and widely separated, the waves of radiant energy can easily pass between them, so that only a very small part of the radiant energy is intercepted and absorbed by the molecules of the air. By far, the greater portion of the radiant energy impinges upon and is absorbed by the solid wall, whose molecular structure is much more compact and substantial.
Heat waves are very similar to light waves, differing from them only in length and frequency. Light waves are radiant energy waves of such lengths as to be visible to the human eye. Thus, light waves are visible heat waves. Whether heat waves are visible or invisible depends on the temperature of the radiating body. For example, when metal is heated to a sufficiently high temperature, it will “glow”, that is, emit visible heat waves (light).
When radiant energy waves either visible or invisible, strike a material body, they may be reflected, refracted, or absorbed by it, or they may pass through it to some other substance beyond.
The amount of radiant energy that will pass through a material depends on the degree of transparency. A highly transparent material, such as clear glass or air, will allow most of the energy to pass through to the material beyond, whereas opaque materials, such as wood, metal, and cork, cannot be penetrated by radiant energy waves, and none will pass through.
The amount of radiant energy that is either reflected or absorbed by a material depends on the nature of the material’s surface, that is, its texture and its color. Material with a light-colored, highly polished surface, such as a mirror, reflect a maximum of radiant energy, whereas materials with rough, dull, dark surface will absorb the maximum amount of radiant energy.