Civil Engineering Reference
In-Depth Information
field in the structures being studied. The analysis can include conduction,
boundary convection, and boundary radiation. It can also include cavity
radiation effects. In addition, the analysis can include forced convection
through the mesh if forced convection/diffusion heat transfer elements
are used. Uncoupled heat transfer analyses can include thermal interactions
such as gap radiation, conductance, and heat generation between contact
surfaces. The analyses can be transient or steady state and can be linear or
nonlinear. The analyses require the use of heat transfer elements. Uncoupled
heat transfer analysis is used to model solid body heat conduction with gen-
eral, temperature-dependent conductivity; internal energy (including latent
heat effects); and quite general convection and radiation boundary condi-
tions, including cavity radiation. Forced convection of a fluid through
the mesh can be modeled by using forced convection/diffusion elements.
Heat transfer problems can be nonlinear because the material properties
are temperature-dependent or because the boundary conditions are non-
linear. Usually, the nonlinearity associated with temperature-dependent
material properties is mild because the properties do not change rapidly with
temperature. However, when latent heat effects are included, the analysis
may be severely nonlinear.
Boundary conditions are very often nonlinear; for example, film coeffi-
cients can be functions of surface temperature. Again, the nonlinearities are
often mild and cause little difficulty. A rapidly changing film condition
(within a step or from one step to another) can be modeled easily using
temperature-dependent and field-variable-dependent film coefficients.
Radiation effects always make heat transfer problems nonlinear. Nonlinear-
ities in radiation grow as temperatures increase. ABAQUS (Standard) uses an
iterative scheme to solve nonlinear heat transfer problems. The scheme uses
the Newton's method with some modification to improve stability of the
iteration process in the presence of highly nonlinear latent heat effects.
Steady-state cases involving severe nonlinearities are sometimes more effec-
tively solved as transient cases because of the stabilizing influence of the heat
capacity terms. The required steady-state solution can be obtained as the
very long transient time response; the transient will simply stabilize the solu-
tion for that long time response.
Steady-state analysis means that the internal energy term (the specific
heat term) in the governing heat transfer equation is omitted. The problem
then has no intrinsic physically meaningful time scale. Nevertheless, you
can assign an initial time increment, a total time period, and maximum
and minimum allowed time increments to the analysis step, which is often
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