Introduction to Heat Transfer

The transport of thermal energy can be broken down into one or more of three mechanisms conduction--heat transfer via atomic vibrations in solids or kinetic interaction amongst atoms in gases1 convection - - heat rapidly removed from a surface by a mobile fluid or gas and radiation—heat transferred through a vacuum by electromagnetic waves. The discussion will begin with brief explanations of each. These concepts are important background in the optical measurement of temperature (optical pyrometry) and in experimental measurement of the thermally conductive behavior of materials. 

Heat transfer by conduction can be most simply stated as heat flows as a result of a temperature difference  

Steady state heat transfer refers to the condition where the rate of heat flowing into one face of an object is equal to that flowing out of the other. If, for example, a slab of metal were placed on a hot-plate, the heat flowing into the metal would initially contribute to a temperature rise in the material, until ultimately a linear temperature gradient formed between the hot and cold faces, wherein heat flowing in would equal heat flowing out and steady state heat transfer would be established. The time involved before steady state conditions axe encountered is dependent on the thermal requirements, that is, the total heat capacity of the material. A useful constant, therefore, in depicting transient, or non-steady state heat transfer is the thermal diffusivity  

Thermal conductivity does not remain constant with temperature. For gases, the thermal conductivity increases with (the square root of) temperature. The atoms in a higher tem- 

Discontinuities in the lattice such as vacancies, impurities, or grain boundaries also act to scatter phonon propagation, hence a lower thermal conductivity is expected in solids containing these defects at cryogenic temperatures. Whichever mecha- 

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Natural convection occurs when a solid surface is in contact with a fluid of different temperature from the surface. Density differences provide the body force required to move the flmd. Theoretical analyses of natural convection require the simultaneous solution of the coupled equations of motion and energy. Details of theoretical studies are available in several general references (Brown and Marco, Introduction to Heat Transfer, 3d ed., McGraw-HiU, New York, 1958 and Jakob, Heat Transfer, Wiley, New York, vol. 1, 1949 vol. 2, 1957) but have generally been applied successfully to the simple case of a vertical plate. Solution of the motion and energy equations gives temperature and velocity fields from which heat-transfer coefficients may be derived. The general type of equation obtained is the so-called Nusselt equation hL I L p gp At cjl... 

EDE, A. j.. An Introduction to Heat Transfer, Pergamon Press, Oxford, 1967... 

Butterworth, D. (1977) Introduction to Heat Transfer, Engineering Design Guide No. 18 (Oxford U.P.). 

In the Nusselt number, the term (q/AT), the rate of heat transfer per unit area of heat exchanger per unit temperature driving force, is known as the heat transfer coefficient and is given the symbol h. The heat transfer coefficient is used to characterise heat transfer rates. Heat transfer processes are described in more detail in Chapter 4, 11 An Introduction to Heat Transfer . 

Butterworth—Introduction to Heat Transfer, Oxford University Press. 

The third chapter covers convective heat and mass transfer. The derivation of the mass, momentum and energy balance equations for pure fluids and multi-component mixtures are treated first, before the material laws are introduced and the partial differential equations for the velocity, temperature and concentration fields are derived. As typical applications we consider heat and mass transfer in flow over bodies and through channels, in packed and fluidised beds as well as free convection and the superposition of free and forced convection. Finally an introduction to heat transfer in compressible fluids is presented. 

Arpaci VS, Kao S-H, Selamet A (1999) Introduction to heat transfer. Prentice Hall, Upper Saddle River... 

F. P. Incropera and D. R DeWitt, Introduction to Heat Transfer, Wiley, New York, 1990. 640 F. Eisner, 3rd Int. Cong. AppL Meek, Stockholm, 1930. 

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