Introduction To Molecular Transport

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Introduction to Molecular Transport
(CHEG 301 : 9/4/14)

Molecular Transport Processes:
Molecular transport processes are concerned with the transfer of a given property or entity by molecular movement through a system or medium (e.g., fluid or solid)
Transferred property:
§ mass,
§ thermal energy (heat), or
§ momentum
Transfer of property is due to a difference in concentration of the property from one region to an adjacent region density (a.k.a. molecule closeness) gases liquids

solids

rate of property transfer

1

Molecular Transport Processes:

rate of transfer process =

driving force resistance Flux of the property

y = −

Γ

≡

amount of property transferred per unit of time per unit cross-sectional area perpendicular to the z direction of flow

Diffusivity
(proportionality constant)
§ If the process is at steady state, then

ψ =

=

=

= −

G
G

G

(G − G )
−

Newton’s Law of Viscosity:
Pressure
Surface forces
Shear stress

On flowing fluid
Body forces

:

Gravity

g
When a fluid is flowing over a rigid surface, the velocity of that fluid (in the fluid) varies as a function of position.

2

Newton’s Law of Viscosity:
Whether in a pipe or between two flat plates, we distinguish two regimes (or types) of flow
§ Laminar flow o with no cross currents perpendicular to the direction of flow, nor eddies or swirls of fluid o happens at low velocities when the fluid flows without lateral mixing
§ Turbulent flow o at higher velocities, where eddies form and lead to lateral mixing
In laminar flow, viscosity is the property of fluid which gives rise to forces that resist the relative movement of adjacent layers In the fluid.

Newton’s Law of

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