Page 153 - Modelling in Transport Phenomena A Conceptual Approach
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Chapter 5
Rate of Generation in
Momentum, Energy and
Mass Transfer
In Chapter 1, the generation rate per unit volume is designated by R. Integration
of this quantity over the volume of the system gives the generation rate in the
conservation statement. In this chapter, explicit expressions for !J? will be developed
for the cases of momentum, energy, and mass transport.
5.1 RATE OF GENEMTION IN MOMENTUM
TRANSPORT
In general, forces acting on a particle can be classified as surface forces and body
forces. Surface forces, such as, normal stresses (pressure) and tangential stresses,
act by direct contact on a surface. Body forces, however, act at a distance on a
volume. Gravitational, electrical and electromagnetic forces are examples of body
forces.
For solid bodies Newton’s second law of motion states that
Summation of forces Time rate of change of )
acting on a system >=( momentum of a system (5.1-1)
in which forces acting on a system include both surface and body forces. Equa-
tion (5.1-1) can be extended to fluid particles by considering the rate of flow of
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