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Radiology => Electromagnetic Radiation => Heat Transfer
Heat Transfer
INTRODUCTION
Heat Transfer, in physics, process by which energy in the form of heat is exchanged between bodies or parts of the same body at different temperatures. Heat is generally transferred by convection, radiation, or conduction. Although these three processes can occur simultaneously, it is not unusual for one mechanism to overshadow the other two. Heat, for example, is transferred by conduction through the brick wall of a house, the surfaces of high-speed aircraft are heated by convection, and the earth receives heat from the sun by radiation.
CONDUCTION
This is the only method of heat transfer in opaque solids. If the temperature at one end of a metal rod is raised by heating, heat is conducted to the colder end, but the exact mechanism of heat conduction in solids is not entirely understood. It is believed, however, to be partially due to the motion of free electrons in the solid matter, which transport energy if a temperature difference is applied. This theory helps to explain why good electrical conductors also tend to be good heat conductors . Although the phenomenon of heat conduction had been observed for centuries, it was not until 1882 that the French mathematician Jean Baptiste Joseph Fourier gave it precise mathematical expression in what is now regarded as Fourier's law of heat conduction. This physical law states that the rate at which heat is conducted through a body per unit cross-sectional area is proportional to the negative of the temperature gradient existing in the body.
The proportionality factor is called the thermal conductivity of the material. Materials such as gold, silver, and copper have high thermal conductivities and conduct heat readily, but materials such as glass and asbestos have values of thermal conductivity hundreds and thousands of times smaller, conduct heat poorly, and are referred to as insulators. In engineering applications it is frequently necessary to establish the rate at which heat will be conducted through a solid if a known temperature difference exists across the solid. Sophisticated mathematical techniques are required to establish this, especially if the process varies with time, the phenomenon being known as transient-heat conduction. With the aid of analog and digital computers, these problems are now being solved for bodies of complex geometry.
CONVECTION
Conduction occurs not only within a body but also between two bodies if they are brought into contact, and if one of the substances is a liquid or a gas, then fluid motion will almost certainly occur. This process of conduction between a solid surface and a moving liquid or gas is called convection. The motion of the fluid may be natural or forced. If a liquid or gas is heated, its mass per unit volume generally decreases. If the liquid or gas is in a gravitational field, the hotter, lighter fluid rises while the colder, heavier fluid sinks. This kind of motion, due solely to nonuniformity of fluid temperature in the presence of a gravitational field, is called natural convection. Forced convection is achieved by subjecting the fluid to a pressure gradient and thereby forcing motion to occur according to the law of fluid mechanics.
If, for example, water in a pan is heated from below, the liquid closest to the bottom expands and its density decreases; the hot water as a result rises to the top and some of the cooler fluid descends toward the bottom, thus setting up a circulatory motion. Similarly, in a vertical gas-filled chamber, such as the air space between two window panes in a double-glazed, or Thermopane, window, the air near the cold outer pane will move down and the air near the inner, warmer pane will rise, leading to a circulatory motion.
The heating of a room by a radiator depends less on radiation than on natural convection currents, the hot air rising upward along the wall and cooler air coming back to the radiator from the side of the bottom. Because of the tendencies of hot air to rise and of cool air to sink, radiators should be placed near the floor and air-conditioning outlets near the ceiling for maximum efficiency. Natural convection is also responsible for the rising of the hot water and steam in natural-convection boilers and for the draft in a chimney. Convection also determines the movement of large air masses above the earth, the action of the winds, rainfall, ocean currents, and the transfer of heat from the interior of the sun to its surface.
RADIATION
This process is fundamentally different from both conduction and convection in that the substances exchanging heat need not be in contact with each other. They can, in fact, be separated by a vacuum. Radiation is a term generally applied to all kinds of electromagnetic-wave phenomena. Some radiation phenomena can be described in terms of wave theory (see Wave Motion), and others can be explained in terms of quantum theory. Neither theory, however, completely explains all experimental observations. The German-born American physicist Albert Einstein conclusively demonstrated (1905) the quantized behavior of radiant energy in his classical photoelectric experiments. Before Einstein's experiments the quantized nature of radiant energy had been postulated, and the German physicist Max Planck used quantum theory and the mathematical formalism of statistical mechanics to derive (1900) a fundamental law of radiation. The mathematical expression of this law, called Planck's distribution, relates the intensity or strength of radiant energy emitted by a body to the temperature of the body and the wavelength of radiation. This is the maximum amount of radiant energy that can be emitted by a body at a particular temperature. Only an ideal body (blackbody,) emits such radiation according to Planck's law. Real bodies emit at a somewhat reduced intensity. The contribution of all frequencies to the radiant energy emitted by a body is called the emissive power of the body, the amount of energy emitted by a unit surface area of a body per unit of time. As can be shown from Planck's law, the emissive power of a surface is proportional to the fourth power of the absolute temperature. The proportionality factor is called the Stefan-Boltzmann constant after two Austrian physicists, Joseph Stefan and Ludwig Boltzmann, who, in 1879 and 1884, respectively, discovered the fourth power relationship for the emissive power. According to Planck's law, all substances emit radiant energy merely by virtue of having a positive absolute temperature. The higher the temperature, the greater the amount of energy emitted. In addition to emitting, all substances are capable of absorbing radiation. Thus, although an ice cube is continuously emitting radiant energy, it will melt if an incandescent lamp is focused on it because it will be absorbing a greater amount of heat than it is emitting.
Opaque surfaces can absorb or reflect incident radiation. Generally, dull, rough surfaces absorb more heat than bright, polished surfaces, and bright surfaces reflect more radiant energy than dull surfaces. In addition, good absorbers are also good emitters; good reflectors, or poor absorbers, are poor emitters. Thus, cooking utensils generally have dull bottoms for good absorption and polished sides for minimum emission to maximize the net heat transfer into the contents of the pot. Some substances, such as gases and glass, are capable of transmitting large amounts of radiation. It is experimentally observed that the absorbing, reflecting, and transmitting properties of a substance depend upon the wavelength of the incident radiation. Glass, for example, transmits large amounts of short wavelength (ultraviolet) radiation, but is a poor transmitter of long wavelength (infrared) radiation . A consequence of Planck's distribution is that the wavelength at which the maximum amount of radiant energy is emitted by a body decreases as the temperature increases. Wien's displacement law, named after the German physicist Wilhelm Wien, is a mathematical expression of this observation and states that the wavelength of maximum energy, expressed in micrometers (millionths of a meter), multiplied by the Kelvin temperature of the body is equal to a constant, 2878. Most of the energy radiated by the sun, therefore, is characterized by small wavelengths. This fact, together with the transmitting properties of glass mentioned above, explains the greenhouse effect. Radiant energy from the sun is transmitted through the glass and enters the greenhouse. The energy emitted by the contents of the greenhouse, however, which emit primarily at infrared wavelengths, is not transmitted out through the glass. Thus, although the air temperature outside the greenhouse may be low, the temperature inside the greenhouse will be much higher because there is a sizable net heat transfer into it.
In addition to heat transfer processes that result in raising or lowering temperatures of the participating bodies, heat transfer can also produce phase changes such as the melting of ice or the boiling of water. In engineering, heat transfer processes are usually designed to take advantage of these phenomena. In the case of space capsules reentering the atmosphere of the earth at very high speed, a heat shield that melts in a prescribed manner by the process called ablation is provided to prevent overheating of the interior of the capsule. Essentially, the frictional heating produced by the atmosphere is used to melt the heat shield and not to raise the temperature of the capsule.
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