вторник, 29 декември 2009 г.

HEAT TRANSFER

          Heat can be transferred by three processes: conduction, convection, and radiation. Conduction is the transfer of heat through a solid object; it is this process that makes the handle of a poker hot, even if only the tip is in the fire. Convection transfers heat through the exchange of hot and cold molecules; this is the process through which water in the kettle becomes uniformly hot even though only the bottom of the kettle contacts the flame. Radiation is the transfer of heat via electromagnetic (usually infrared) radiation; this is the principal mechanism through which a fire warms a room.
            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 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. For example, heat is transferred predominantly  by conduction through the brick wall of house, a pan of water on a stove is largely heated by conduction, and the earth receives heat from the sun almost wholly by radiation.
            Conduction
            The only method of heat transfer in opaque solids is conduction. If the temperature at one end of a metal rod is raised by heating, heat is conducted to the colder end. The exact mechanism of heat conduction in solids is not entirely understood, but it is believed to be partially due to the motion of free electrons, which transport energy in a temperature difference is applied. This theory helps to explain why good electrical conductors also tend to be good heat conductors. It was not until 1822 that the French mathematician Jean-Baptiste Joseph Fourier gave precise mathematical expression to what is now called Fourier's law of heat conduction. This 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 know the rate at which heat will be conducted through a solid across which a known temperature difference exists. Sophisticated mathematical techniques are required to establish this, especially if the process varies with time, the phenomenon then being known as transient-heat conduction. With the aid of analogue and digital computers, these problems are now being solved for bodies of complex geometry.
            Convection
            If a temperature difference arises within a liquid or a gas, then fluid motion will almost certainly occur. This transfers heat from one part of the fluid to another, a process called convection. The motion of the fluid may be natural or forced. If a liquid or gas is heated, its density or mass per unit volume generally decreases. If the liquid or gas is in a gravitational field, the hotter, less dense fluid rises while the colder, denser fluid sinks. This kind of motion, due solely to the non-uniformity of the fluid's temperature, 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 laws of mechanics.
            Suppose, for example, that water in a pan is heated from below. The liquid closest to the bottom is warmed by heat conducted through the bottom of the pan. It expands and its density decreases; the hot water as a result rises to the top and some of the cooler fluid descends towards the bottom, thus setting up a circulatory motion. The cooler liquid is again heated by conduction; the warmer liquid at the top loses its heat by conduction and radiation into the air at the top of the pan. Similarly, in a vertical gas-filled chamber, such as the air space between two window panes in a double-gazed 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 along the wall, drawing cooler air back to the radiator from the rest of the room.Because of the tendency of hot air to rise and of cool air to sink, radiators should be placed neither 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 draught in a chimney. Convection also determines the movement of large air masses above the earth, the action of the winds, the formation of clouds, ocean currents, and the transfer of heat from the interior of the sun to its surface.
                        Radiation
            Radiation 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, but the only satisfactory general account of electromagnetic radiation is quantum theory. In 1905 Albert Einstein proposed that radiation sometimes displays quantized behaviour: in the photoelectric effect the radiation behaves like tiny "bullets", called photons, rather than as waves. The quantized nature of energy had been postulated before Einstein's paper, and in 1900 the German physicist Max Planck had used quantum theory and the mathematical formalism of statistical mechanicsto drive a fundamental law of radiation. The mathematical expression of this law, called Planck's distribution, relates the intensity of radiant energy emitted by a body at a given wavelength to the temperature of the body. 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 radiation exactly according to Planck's law. Real bodies emit at a somewhat reduced intensity.
            The contribution of all frequencies to the radiant energy emitted is called the emissive power of the body, the amount of energy emitted by unit surface area per unit 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 proportionallity factor is called the Stefan-Boltzmann, who, in 1879 and 1884 respectively, discovered the fourth power relationship for the emissive power. According to Planck's law, all substances emit radian energy merely by virtue of having temperature above absolute zero. 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, bull, 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 thus maximizing the net heat tuansfer into the contents of the pot.
            Some substances, including many 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 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 diaplacement 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 micrometer (millionths of a merte), multiplied by the temperature of the body in Kelvins, is equal to a constant, 2878. This fact, together with the transmitting properties of glass mentioned above, explains the warming of the greenhouses. Radiant energy from the sun, predominantly of visible wavelengths, is transmitted through the glass and enters the greenhouse. The energy emitted by the contents of the greenhouse, however, which emit primarily at longer, infrared, wavelengths, is not transmitted out through the glass. Thus, although the air temperature outside the greenhouse may de low, the temperature inside the greenhouse will be much higher because there is a sizable net heat transfer into it.
            In addition the heat transfer processes that result in raising or lowering the 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 re-erterig 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 heating produced by the friction of the atmosphere is used up in melting the heat shield and not in raising the temperature of the capsule.

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