CHAP. 10: TRANSPORT PROCESSES [CONTENTS] 334
U Main unit:J K−^1 m−^1 s−^1.
10.2.3.1 Dependence on state variables.
According to kinetic theory, the thermal conductivity in gases at low and medium pressures in-
creases with temperature and does not depend on pressure, see section10.5.3, relation (10.22).
The behaviour of real gases qualitatively corresponds to kinetic theory results. Only at high
pressures the thermal conductivity of gases increases with pressure.
Thermal conductivity of liquids is higher than that of gases. It decreases with temperature
and (slightly) increases with pressure.
Thermal conductivity of solids is higher than that of liquids; in metals it is usually higher
than in non-metals. Diamond exhibits an anomalously high thermal conductivity.Example
Let us consider a dual window of an area of 4 m^2. The inside window has the temperature of
the room 25◦C, the outside window has a temperature of –10◦C. The space between the two
windows is 20 cm. What is the amount of heat that passes through the air layer between the
two windows in one hour? The thermal conductivity coefficient of air under the given conditions
isλ= 2. 41 × 10 −^2 J K−^1 m−^1 s−^1.