Untitled - Aerobib - Universidad Politécnica de Madrid

Chapter 2
Transport phenomena in gas
mixtures
2.1 Introduction
When the state or composition of a mixture of gases are not uniform, a transport
of mass, momentum and energy takes place between different points of the mixture
tending to level the initial differences.
At the neighborhood of each point transport depends on the state and composition
of the mixture and on the lack of uniformity from which it arises, according
to laws dealt with in the present chapter. Even though these phenomena arise from
molecular motion their description can be made through phenomenological variables
except when the gas is very rarefied or when the variations in state and composition
within distances comparable with the molecular mean free path are large. 1 If such
cases are excluded, gas can be assimilated to a continuous medium where at each of
its points density, pressure, temperature, mass fractions and velocities of the species
and mixture are definable. Under such conditions, let P , Fig. 2.1, be a point, fixed or
in motion at a given velocity ¯v, and dσ a surface element linked to it. Transport dF
of mass of one of the species, or of momentum or energy of the mixture, through dσ
during time dt is of the form
dF = f dσ dt, (2.1)
where f is the transport per unit surface and per unit time. f is a function of the orientation
of dσ, defined by unit vector ¯n normal to dσ, of the variables that determine
the state and composition of the mixture at the point and of their derivatives. If P
1 See chapter 3 §1.
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