Civil Engineering Reference
In-Depth Information
additional temperature degrees of freedom 12, 13, etc., through the thick-
ness. Boundary conditions can be specified as functions of time by referring
to amplitude curves. For purely diffusive heat transfer elements, a boundary
without any prescribed boundary conditions (natural boundary condition)
corresponds to an insulated surface. For forced convection/diffusion ele-
ments, only the flux associated with conduction is zero; energy is free to
convect across an unconstrained surface. This natural boundary condition
correctly models areas where fluid is crossing a surface (as, e.g., at the
upstream and downstream boundaries of the mesh) and prevents spurious
reflections of energy back into the mesh.
Thermal loading in a heat transfer analysis comprises concentrated heat
fluxes, body fluxes, and distributed surface fluxes; average-temperature
radiation conditions; convective film conditions; and radiation condi-
tions (film properties can be made a function of temperature) as well as
cavity radiation effects. Predefined temperature fields are not allowed
in heat transfer analyses. Boundary conditions should be used instead
to specify temperatures, as described earlier. The thermal conductivity
of the materials in a heat transfer analysis must be defined. The specific
heat and density of the materials must also be defined for transient heat
transfer problems. Latent heat can be defined for diffusive heat transfer
elements if changes in internal energy due to phase changes are important.
Thermal expansion coefficients are not meaningful in an uncoupled heat
transfer analysis problem since the deformation of the structure is not
considered.
The heat transfer element library in ABAQUS (Standard) includes (1)
diffusive heat transfer elements, which allow for heat storage (specific heat
and latent heat effects) and heat conduction; (2) forced convection/diffusion
heat transfer elements; (3) shell heat transfer elements; and (4) the first-order
heat transfer elements (such as the two-node link, four-node quadrilateral,
and eight-node brick), which use a numerical integration rule with the inte-
gration stations located at the corners of the element for the heat capacitance
terms and for the calculations of the distributed surface fluxes. First-order
diffusive elements are preferred in cases involving latent heat effects since
they use such a special integration technique to provide accurate solutions
with large latent heats. The second-order heat transfer elements use con-
ventional Gaussian integration. Thus, the second-order elements are to
be preferred for problems when the solution will be smooth (without latent
heat effects) and usually give more accurate results for the same number of
nodes in the mesh.
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