A material property denoted by λ and defined by *Fourier's Law*, which for one-dimensional conduction in an isotropic medium is:

The value of λ is temperature dependent and is usually determined experimentally by methods based on Fourier's Law, i.e.

where, for example, is measured for an imposed value of dT/dx. Details of the range of methods used to determine λ for solids, liquids and gases are given by Tye (1969) and Maglic, Cezairliyan and Peletsky (1984, 1992). Values of λ are catalogued by Touloukian and Ho (1970 to 1977), an extensive data bank compiled by the Thermophysical Properties Research Center at Purdue University and continually extended and updated by CINDAS at the same university. Other readily available data books are those by Kaye and Laby (1986) and Perry and Green (1984).

Figure 1 illustrates the very wide range of values of λ for solids, liquids and gases at normal temperatures and pressures.

The temperature dependence of λ values for the three states of matter is exemplified in Figures 2 and 3.

The thermal conductivity of a metal can usually be correlated with temperature, over a restricted range, using an expression such as:
λ = λ_{0}(a + bθ + cθ^{2})
where θ = T – T_{ref} and λ_{0} is the thermal conductivity at a reference temperature T_{ref}.

The thermal conductivity of a nonhomogeneous solid is usually dependent on the apparent bulk density and as a general rule increases with increasing temperature and increasing bulk density.

Figure 3 shows the temperature dependence of λ for some saturated liquids and vapors, and gases, of engineering importance. λ for most liquids decreases with increasing temperature. The exception is water, which exhibits increasing λ up to about 150°C and decreasing λ thereafter. Water has the highest thermal conductivity of all liquids except for liquid metals.

The thermal conductivity of gases increases with increasing temperature but is essentially independent of pressures for pressures close to atmospheric. λ values for steam exhibit strong pressure dependence.

Methods for the estimation of λ values outside the range of published values and for the λ values of liquid and gas mixtures are described by Reid et al. (1987).

#### REFERENCES

CINDAS, Centre for Information and Numerical Data Analysis and Synthesis, Purdue University, 2595 Yeager Road, West Lafayette, IN 47906, USA.

Kaye, G. W. C. and Laby, T. H. (1986) *Tables of Physical and Chemical Constants*, 15th edn., Longmans Scientific and Technical, Harlow, UK.

Maglic, K. D., Cezairliyan, A. D. and Peletsky, V. E. (1984, 1992) *Compendium of Thermophysical Property Measurement Methods*, Vols. 1 (1984) and 2 (1992), Plenum Press, New York.

Perry, R. H. and Green, D. W. (1984) *Perry's Chemical Engineers’ Handbook*, 6th edn., McGraw-Hill.

Reid, R. C, Prausnitz, J. M., and Poling, B. E. (1987) *The Properties of Gases and Liquids*, 4th edn., McGraw-Hill, New York.

Touloukian, Y. S. and Ho, C. Y. 1970 to 1977. Thermophysical Properties of Matter, *The TPRC Data Series* (13 volumes), Plenum Press, New York.

Tye, R. P. (1969) *Measurement of Thermal Conductivity*, Vols. 1 and 2, Academic Press.