Untitled - Aerobib - Universidad Politécnica de Madrid

54 CHAPTER 2. TRANSPORT PHENOMENA IN GAS MIXTURES
The following Table 2.4 gives, as an example, the values of the theoretical and
experimental viscosity coefficients for a mixture of oxygen and carbon monoxide.
%O
T (K) 0 42.01 77.33
300.06
Experimental 1 776 1 900 1 998
Calculated 1 771 1 896 2 003
500.06
Experimental 2 548 2 741 2 908
Calculated 2 539 2 717 2 871
Table 2.4: Viscosity coefficient (10 7 µ gr cm −1 s −1 ) for a mixture of O 2 -CO.
For mixtures of more than two components Buddemberg and Wilke give the
following semi-empirical expression which represents experimental results with good
approximation
[µ] 1
= ∑ i
X 2 1
Xi
2 + 1.385 ∑ X i X j
[µ i ] 1 j≠i
. (2.52)
RT
pM i [D ij ] 1
This expression allows calculation of the viscosity coefficient of a mixture when those
corresponding to the species are known as well as the diffusion coefficients of the
same when taken by couples.
In the preceding formulae substitution of molar fractions by mass fractions is
straightforward by making use of relation
X i = M m
M i
Y i . (2.53)
Fig. 2.7 gives the law of variation as a function of the composition of the viscosity
coefficients for some gas mixtures.
2.4 Thermal conductivity
Theory of heat conduction shows 26 that heat transfer at a point of a material medium
is defined by a vector ¯q of heat flux
¯q = q 1 ī 1 + q 2 ī 2 + q 3 ī 3 . (2.54)
Physical meaning of ¯q is the following: q i is heat flux per unit surface and per unit
time through a surface element normal to axis x i .
26 See, i.e., Ref. [16], pp. 3 and following.