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Unformatted text preview: Chapter 17 Steady Heat Conduction Chapter 17 STEADY HEAT CONDUCTION Steady Heat Conduction In Plane Walls 171C (a) If the lateral surfaces of the rod are insulated, the heat transfer surface area of the cylindrical rod is the bottom or the top surface area of the rod, 4 / 2 D A s π = . (b) If the top and the bottom surfaces of the rod are insulated, the heat transfer area of the rod is the lateral surface area of the rod, A DL = π . 172C In steady heat conduction, the rate of heat transfer into the wall is equal to the rate of heat transfer out of it. Also, the temperature at any point in the wall remains constant. Therefore, the energy content of the wall does not change during steady heat conduction. However, the temperature along the wall and thus the energy content of the wall will change during transient conduction. 173C The temperature distribution in a plane wall will be a straight line during steady and one dimensional heat transfer with constant wall thermal conductivity. 174C The thermal resistance of a medium represents the resistance of that medium against heat transfer. 175C The combined heat transfer coefficient represents the combined effects of radiation and convection heat transfers on a surface, and is defined as h combined = h convection + h radiation . It offers the convenience of incorporating the effects of radiation in the convection heat transfer coefficient, and to ignore radiation in heat transfer calculations. 176C Yes. The convection resistance can be defined as the inverse of the convection heat transfer coefficient per unit surface area since it is defined as R hA conv = 1/ ( ) . 177C The convection and the radiation resistances at a surface are parallel since both the convection and radiation heat transfers occur simultaneously. 178C For a surface of A at which the convection and radiation heat transfer coefficients are h h conv rad and , the single equivalent heat transfer coefficient is h h h eqv conv rad = + when the medium and the surrounding surfaces are at the same temperature. Then the equivalent thermal resistance will be R h A eqv eqv = 1/ ( ) . 179C The thermal resistance network associated with a fivelayer composite wall involves five single layer resistances connected in series. 1710C Once the rate of heat transfer Q is known, the temperature drop across any layer can be determined by multiplying heat transfer rate by the thermal resistance across that layer, ∆ T QR layer layer = 171 Chapter 17 Steady Heat Conduction 1711C The temperature of each surface in this case can be determined from ( ) / ( ) ( ) / ( ) Q T T R T T QR Q T T R T T QR s s s s s s s s = → = = → = + ∞ ∞  ∞ ∞  ∞∞ ∞∞ 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 where R i ∞ is the thermal resistance between the environment ∞ and surface i....
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 Spring '08
 Chung
 Heat Transfer, Ri

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