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Structural / CFD Heat Transfer
Cyprien Rusu
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Fluid Volume
Solid Part Solid Part
Fluid at Ambient Temperature
convection
FEA Model FEA Model (Full)
CFD Fluid/Solid Coupled Analysis Structural Heat Transfer
Analysis
Difference between Structural Heat Transfer and CFD Fluid/Solid
couple heat analysis
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Structural Heat Transfer Analysis
When ? -When the fluid temperature can be assimilated as uniform
around the solid part -When we investigate the behavior of
structural components only under heating (convection, heat
generation, radiation) -When we investigate the stress and
deformation caused to the part due to heat load (Thermal Stress
Analysis).
Solid Part
Fluid at Ambient Temperature
convection
FEA Model
Structural Heat Transfer Analysis
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CFD Heat Transfer coupled Analysis
Fluid Volume
Solid Part
FEA Model (Full)
CFD Fluid/Solid Coupled Analysis
When ? -When the distribution of the fluid around the object
need to be studied -To investigate the impact of the object on the
environment -To investigate natural cooling
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Analysis procedure For Structural thermal analysis
1. Fluid Volume (inside and outside) is not included in the
analysis
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Analysis procedure For Structural thermal analysis
2. Instead, You have to apply Convection to consider the
influence of the external fluid (attention: select only the faces
in contact with air)
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3. You can apply radiation inside the chip to consider the heat
generated by the chip on the other parts.
Analysis procedure For Structural thermal analysis
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Steady State & Transient Heat Transfer
You have 2 types of thermal structural analysis in NFX: Steady
State & Transient
I- Steady State Heat Transfer (Static/Heat Transfer > Heat
Load)
II- Transient Heat Transfer (Dynamic/Trans. Heat Transfer >
Transient Heat Load)
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Thermal loads and Boundary conditions
Thermal loads and boundary conditions for heat transfer analysis
used in midas NFX include temperature boundary conditions assigned
to nodes, heat flux applied to lines and surfaces, convection and
radiation, which are summarized in Table 6.3.1. Figure 6.3.1 shows
the load conditions and boundary conditions that can be considered
for heat transfer analysis in midas NFX.
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Heat Loads Prescribed nodal temperatures Prescribed nodal
temperatures are used to assign uniform known temperatures to a
model. In the same manner as forced displacements of structural
analysis, the temperature degrees of freedom of nodes to which
temperature conditions area assigned are eliminated from the global
degrees of freedom and influence the load vectors. Heat generation
Heat generation is used to simulate the quantity of heat generated
inside a solid. By inputting the rate of heat generation per unit
volume, , the effects of heat generation inside elements can be
obtained. Heat flux Heat flux expresses power per unit area or
energy per unit time and unit area. In midas NFX, heat flux can be
applied to nodes, faces and edges of elements. When heat flux is
assigned to a node, the area is determined through an additional
area factor value. When heat flux is applied to an edge, the area
is determined by using the thickness information of the edge or the
area factor provided by the user.
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Convection In midas NFX, natural convection conditions caused by
the difference between the ambient temperature and the surface
temperature can be applied to nodes, boundary faces and edges of
elements. The amount of heat exchange or heat flux caused by
convection is quite empirical. midas NFX provides two different
forms of heat flux relationship equations by convection.
Convection
The surface convection coefficient can be expressed as a
function of the surface temperature or ambient temperature. When
convection is assigned to a node, the area is determined through an
additional area factor. When convection is assigned to an edge, the
area is determined by using the thickness information of the edge
or the area factor provided by the user.
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Steady State Convection
Transient Convection
h ( film coefficient = convective heat transfer coefficient =
surface convection coefficient)
Fluid temperature around the object is defined here (Ambient
Temperature)
For Transient Analysis, fluid temperature can additionally be
function of the time
Steady- State Convection & Transient Convection
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Radiation Radiation Heat exchange by radiation takes place when
a difference between the surface and ambient temperatures exists.
midas NFX can apply heat exchange conditions caused by radiation.
The heat flux from radiation is expressed as,
Cavity Radiation Since heat exchanges occur in interaction
between individual surfaces in radiation analysis of the interior
of a cavity consisting in a number of surfaces, it takes a form
different from the radiation heat exchange with the ambient
temperature, which has been introduced previously. The heat flux
per unit area, which is transferred to the ith face by cavity
radiation, is expressed as follows:
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A radiation shape factor shows the level of radiation heat
exchange generated between two faces and is geometrically defined
as follows:
The above integration is valid only if a visibility relationship
between two points of both faces is maintained. Parts that do not
maintain such relationships are excluded from the integration. A
radiation blockage corresponds to such a case (Figure 6.3.3)in
which no radiation heat reaches certain faces due to the blockage
by a third object. In case of an enclosed cavity, the sum of the
radiation shape factors of all other faces relative to the given
ith face produces 1, which is a measure of accuracy in calculations
for radiation shape factors.
In case of an open cavity, the sum from the above equation
becomes smaller than 1 in which case radiation heat is transferred
to the ambient air. midas NFX provides a function to automatically
calculate radiation shape factors for specific three-dimensional
shapes.
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Steady State Convection
Transient Convection
Fluid temperature around the object is defined here
For Transient Analysis, fluid temperature can additionally be
function of the time
Steady- State Radiation & Transient Radiation
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Thermal Stress Analysis (Heat Structure coupled Analysis)
Heat Generation Condition: 0.01W/mm3
Convection Condition Ambient Temperature: 20 C Convection
Coefficient: 2e-5 W/mm2[T]
Nodal Temperature Results
Thermal deformation
Thermal stress
