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Extended surfaces, in the forms of longitudinal or radial fins or spines are ubiquitous
in applications where the need exists to enhance heat transfer between a surface and
an adjacent fluid. Applications range from very large scale, as with tubes in heat
exchangers, to the very small, as is the case for the temperature control of electronic
components.
At the fundamental level, the analysis of heat transfer from finned surfaces involves
solving second-order differential equations in a variety of coordinate systems. The
subject of extended surface heat transfer is one where analytical methods have been
very successful in providing design information for a variety of geometries, some of
which are very complex. As both primary and extended surfaces involve convective
exchange as a boundary condition, the convective heat transfer coefficient h, which
appears as a parameter in the solution, must be evaluated using standard analysis or
empirical correlations.
This coefficient can be modeled, most simply, as a constant, in which the governing
second-order differential equation is linear. When the more realistic definition of h
as a function of temperature is employed, the problem becomes nonlinear and is
considerably more difficult to solve. This nonlinearty is exaggerated when the solid–
fluid interface encounters a phase change in the form of evaporation or condensation.
The subject of convective heat transfer is included as a separate chapter in this work,
and this chapter provides some direction in evaluating the coefficient h. Two later
chapters are devoted entirely to the areas of boiling and condensation. |
