Software Option for Plus
versions
Thermal / Field Analysis
The LUSAS Thermal / Field option
contains extensive facilities for both simple and advanced steady
state, and transient thermal / field analyses. By combining the LUSAS
Thermal / Field option with other appropriate LUSAS options, heat
transfer due to conduction, convection and radiation can be analysed.
In addition, the effects due to phase change of material may also be
included.
The LUSAS Thermal / Field option offers
a powerful set of thermal link elements for solving analyses which
include the following material characteristics:
 Isotropic materials
 Orthotropic materials
 Specific heat
 Convective heat transfer
coefficient.
 Radiative heat transfer coefficient
 Rate of internal heat generation
 Latent heat flow due to phase change
 Temperature dependence
A large selection of boundary
conditions and loadings are available and include:
 Prescribed temperature
 Heat flux or rate of heat generation
or absorption
 Convection between surfaces or to
the environment
 Radiation between surfaces or to
environment
 Environmental or initial
temperatures
 Impermeable boundaries for seepage
flow
 Temperature dependent properties
Thermal surfaces are used to model heat
transfer across gaps; heat transfer by contact when a gap closes; heat
transfer to the general environment, and heat transfer by radiation
exchange. Gap and contact heat transfer is modelled with a thermal gap
that is defined by thermal properties and thermal surfaces.
The Thermal / Field option enables
conduction and convection to take place by specifying an environmental
temperature and a heat transfer coefficient on any surface. Unlike
many other systems there is no need to use complex nodal links to
define the convective surface. In LUSAS, a face load is applied to a
chosen surface in a single action.
When the LUSAS Nonlinear
and Thermal / Field options are combined gap radiation can be modelled.
Radiation exchange may be modelled using a radiation surface defined
by any number of thermal surfaces. Planes of symmetry that cut through
the radiation enclosures may be defined and obviate the requirement to
model the whole structure. Radiation surfaces also allow for the
calculation of diffuse view factors.
View factors are used to express the
fraction of radiative energy leaving an emissive segment of a thermal
surface which is incident on a receiving segment. Surface emissivity
can also be defined in accordance with the material used.
When a material changes state it is
accompanied by either a liberation or absorption of latent heat.
Thermal modelling of the transition state is done in LUSAS using the
latest enthalpy approach ensuring that the temperature at a point does
not pass through the phase change temperature without including the
effect of the phase change in the analysis.
For both linear and nonlinear analyses,
LUSAS allows variable time stepping to be used in transient thermal
problems providing efficient and accurate results. A choice of time
integration schemes is available. Note that for
analyses within the timedomain the Dynamic
software option is necessary.
The solution of
problems involving temperature dependent material properties and
loading is possible when a LUSAS Nonlinear
option is used to set a nonlinear
control. Temperature dependent material properties are
defined in a table in which any of the material parameters may vary
with temperature. LUSAS linearly interpolates the material parameters
from the table at the required temperature for each integration point
within each element.
In some types of problem the
temperature distribution may significantly affect the material
properties. This will occur when a material undergoes a phase change
or the material yield stress is reduced as the temperature increases.
In such problems the temperature distribution evaluated from a thermal
analysis must be coupled to a structural analysis. When the Dynamic and Thermal / Field options are
combined, semi or fully coupled analyses can be carried out.
A semicoupled analysis is carried out
when the thermal solution will not be significantly affected by any
changes in geometry. In this type of problem the thermal and
structural analyses are run separately. Structure temperatures at
predefined time steps are defined, and a stress analysis, based upon
these temperature loadings, is performed to obtain the final results.
Fully coupled analyses are carried out
by running the thermal and structural analyses simultaneously. A fully
coupled analysis ensures that the correct material parameters are used
at the iterative level which is required for particularly sensitive
problems. In a fully coupled analysis the temperatures from a thermal
analysis are used as input to the stress analysis and the
displacements from the stress analysis are used to update the geometry
in the thermal analysis.
In addition to the powerful contouring,
graphing and plotting features in LUSAS Graphics, a large number of
specific thermal results processing features are available including:
 Total flow history
 Gap and environmental flow
 Radiation flow between segments
 Radiation flow to environment
 Total nodal flow
See also
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