Transport phenomena

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In physics, chemistry, biology and engineering, a transport phenomenon is any of various mechanisms by which particles or quantities move from one place to another. The laws which govern transport connect a flux with a "motive force". Three common examples of transport phenomena are diffusion, convection, and radiation. The science of transport phenomena is a great complement to rheological study of Newtonian fluids.

There are three main categories of transport phenomena:

An important principle in the study of transport phenomena is analogy between phenomena. For example, mass, energy, and momentum can all be transported by diffusion:

  • The spreading and dissipation of odours in air is an example of mass diffusion.
  • The conduction of heat in a solid material is an example of heat diffusion.
  • The drag (physics) experienced by a rain drop as it falls in the atmosphere is an example of momentum diffusion (the rain drop loses momentum to the surrounding air through viscous stresses and decelerates).

The transport of mass, energy, and momentum can also be affected by the presence of external sources:

  • An odour dissipates more slowly when the source of the odor remains present.
  • The rate of cooling of a solid that is conducting heat depends on whether a heat source is applied.
  • The gravitational force acting on a rain drop counteracts the drag imparted by the surrounding air.

All these effects are described by the generic scalar transport equation.

The generalized method adopted for solving transport phenomena problems start with quantity analysis for any given system as:

(Rate of quantity IN) - (Rate of quantity OUT) + (Rate of Production of the quantity) = (Rate of Accumulation of the Quantity)

The transferring quantity here can be momentum, energy or mass. For example, during momentum transport analysis for a freely falling film of a Newtonian liquid, gravitational force is counted as a factor increasing momentum in the system; and the momentum dissipation will be in the form of fluid moving out of the system, and work losses. 1

The same equations governing convection in heat transfer can be applied to convection in mass transfer. When studying complex transport phenomena problems one must use tools from continuum mechanics and tensor calculus and often problems can be expressed as partial differential equations.

In solid state physics, the motion and interaction of electrons, holes and phonons are studied under "transport phenomena". 2

In biomedical engineering, some transport phenomena include thermoregulation, perfusion, and microfluidics.

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References

  1. ^ R. Byron Bird, Warren E. Stewart, Edwin N. Lightfoot, Transport Phenomena (1960) John Wiley & Sons, New York, ISBN 0-471-07392-X, pp.42-48
  2. ^ J. M. Ziman, Electrons and Phonons: The Theory of Transport Phenomena in Solids (Oxford Classic Texts in the Physical Sciences)

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  • This page was last modified on 11 December 2008, at 19:54.

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