Page 159 - Mechanical Engineers' Handbook (Volume 4)

148 Heat-Transfer Fundamentals

Figure 1a The temperature dependence of the thermal conductivity of selected solids. 1

and 2, respectively. Insulating materials, which are used to prevent or reduce the transfer of
heat between two substances or a substance and the surroundings, are listed in Tables 3 and
4, along with the thermal properties. The thermal conductivities for liquids, molten metals,
and gasses are given in Tables 5, 6 and 7, respectively.

1.2 One-Dimensional Steady-State Heat Conduction
The steady-state rate of heat transfer resulting from heat conduction through a homogeneous
material can be expressed in terms of the rate of heat transfer, q,or q T/R, where T
is the temperature difference and R is the thermal resistance. This thermal resistance is the
reciprocal of the thermal conductance (C 1/R) and is related to the thermal conductivity
by the cross-sectional area. Expressions for the thermal resistance, the temperature distri-
bution, and the rate of heat transfer are given in Table 8 for a plane wall, a cylinder, and a
sphere. For a plane wall, the heat transfer is typically assumed to be one dimensional (i.e.,