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30523_Ward's World+MGH Heat Transfer.2

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Heat Transfer (continued) to the snow on the roof. Conduction is the only mechanism for the transfer of heat through an opaque solid. Some heat may be transferred through transparent solids, such as glass, quartz, and certain plastics, by radiation. In moving fluids, conduction is supplemented by convection, and if the fluid is transparent, by radiation. Conduction heat transfers can be quantified using Fourier's Law, which includes the important thermal physical property thermal conductivity. The thermal conductivity of a material is an indication of how effectively the material transfers heat. The thermal conductivities of materials vary widely, and thermal conductivity is highest for metals, lower for nonmetals, still lower for liquids, and lowest for gases. Any material which has a low conductivity may be considered to be a thermal insulator. Solids that have high conductivities can be used as thermal in- sulators if they are distributed in the form of granules, powders, fibers, or foam. Using dispersed materials like foams increases the path length for heat transfer and at the same time reduces the effective cross-sectional area of heat transfer, both of which decrease the heat transfer. Mineral wool, glass fiber, diatoma- ceous earth, glass foam, Styrofoam, corkboard, Celotex, and magnesia are all examples of such materials. Other applications use high thermal conductivity materials to increase the rate of heat transfer, such as thermal adhesives used to attach fins to microchips. A high thermal conductivity adhesive increases the heat transfer from the hot chip to the fins, and the fins increase the area for convection heat transfer. Convection Like conduction, heat transfer by convection is due to mo- lecular motion. However, during convection molecules are also transported, or moved, due to fluid motion. Convection is quantified using Newton's Law of Cooling, which includes the heat transfer coefficient. Like the thermal conductivity, the heat transfer coefficient is a function of thermophysical proper- ties of the fluid; however, it is also a function of the physical characteristics of the system, such as the velocity of the fluid and the shape of the flow field. Diffusion is when molecules are transported due to gradients in the fluid (such as temperature gradients). Advection is when molecules are transported by bulk motion of the fluid, such as water flowing in a river. Dif- fusion and advection can occur simultaneously in a fluid, such as when the water in a river is colder at the bottom of the river and hotter on top. When a fluid is in contact with a surface at a temperature different from the fluid temperature, conduction will transfer heat from the hot surface to the cold fluid (or from a cold surface to a hot fluid). The motion of the fluid will then transfer the heat within the fluid. Continuing with the river ex- ample, conduction will occur from the hotter soil at the bottom of the riverbed to the cooler water in the river. Convective heat transfer between a surface and a fluid cannot occur without conduction at the interface between the fluid and the surface. The motion of the fluid may be entirely due to differences in density (resulting from temperature differences in the fluid), as in natural convection (also called buoyant convection), or due to mechanical means (such as a fan), as in forced convection. Convection heat transfer is vital to an enormous number of engineering applications and systems found in nature. For example, most of the heat supplied to a room from a steam or hot-water radiator is transferred by convection. The cooling and heating provided in a vehicle for the passenger compartment is predominantly due to convection heat transfer. The heat from the combustion of natural gas in a hot water heater to the wa- ter tank is by convection. Water in lakes and ponds freeze first on the surface and not at the bottom due, in part, to natural convection. Ice cools a drink in a glass, in part, by convection, and soup is heated in a pot by forced convection if the soup is stirred and by natural convection if the soup is not stirred. Heat transferred by convection can involve single-phase fluids (for example, gases or liquids) or fluids that undergo a phase change. Phase changes can dramatically increase the heat transfer due to the additional energy required to vaporize or condense a fluid, for instance via the latent heat of vaporiza- tion, solidification, sublimation, or crystallization. As another example, the human body can be cooled to lower than ambi- ent temperature by evaporation of sweat from the skin. Dry ice (solid phase carbon dioxide) can absorb large quantities of heat by sublimating the carbon dioxide. Heat extracted from the products of combustion in a boiler is transferred through a metal tube wall and converts the water inside the tube from liquid to vapor steam. Radiation All materials, regardless of temperature, emit radiation in all directions. Transfer of energy by radiation is unique in that no conducting substance is necessary, as is required with conduc- tion and convection. In other words, radiation heat transfer can occur in a vacuum. This unique property makes possible the transfer of energy from the Sun to the Earth. Radiation heat transfer is quantified by the Stefan-Boltzmann Law and can be interpreted using wave theory, in the same way as light. Radia- tion heat transfer occurs by interaction of surfaces at different temperatures, and radiation can be absorbed, reflected, or transmitted. The properties that characterize effectiveness at + ward ' s science

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