Page 84 - Modelling in Transport Phenomena A Conceptual Approach

Chapter 4

Evaluation of Transfer

Coefficients:

Engineering Correlations

Since most engineering problems do not have theoretical solutions, a large portion
of engineering analysis is concerned with the experimental information which is
usually expressed in terms of engineering correlations. These correlations, however,
are limited to a specific geometry, equipment configuration, boundary conditions,
and substance. As a result, the values obtained from correlations are not exact and
it is possible to obtain two different answers from two different correlations for the
same problem. Therefore, one should keep in mind that the use of a correlation
introduces an error in the order of f 25%.
Engineering correlations are given in terms of dimensionless numbers. For ex-
ample, the correlations used to determine friction factor, heat transfer coefficient
and mass transfer coefficient are generally expressed in the form

f = f (Re)
Nu = Nu(Re,Pr)
Sh = Sh(Re,Sc)

In this chapter, some of the available correlations for momentum, energy, and
mass transport in different geometries will be presented. Emphasis will be placed
on the calculations of force (or, rate of work), heat transfer rate and mass transfer
rate under steady conditions.

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