Best Practices Workshop:
Heat Transfer
Overview
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This workshop will have a ‘mixed’ format: we will work through
a typical CHT problem in STAR-CCM+, stopping periodically to
elucidate best practices or demonstrate new features
In particular, the following topics will be highlighted:
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Imprinting, Meshing and Interfaces for CHT
Newer STAR-CCM+ Features for CHT
Wall Treatments and Near-Wall Meshing
Internal Heat Generation
Thermal Contact Resistance
S2S Thermal Radiation
Thermal Boundary Conditions
Heat Transfer Coefficients
HTBP-2
New Simulation
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•
Start a new STAR-CCM+ session
Under File, click New Simulation
–
Click OK
HTBP-3
Import CAD Model
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Right-click on Geometry
> 3D-CAD Models and
select New
In 3D-CAD, right-click
on 3D-CAD Model 1
and select Import >
CAD Model
In the file browser
window, select the file
Cooled_Board.x_t
HTBP-4
Examine CAD Model
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•
•
After the surface has been
imported, you should see a
scene like that shown to the
right
The CAD model consists of
several components
mounted on a ‘board’ that
has a cooling water tube
running through it
We will delete the cooling
water tube and create a
model for air cooling of the
components
HTBP-5
Delete Tube Body
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•
To delete solid body representing
the tube, right-click on Bodies >
Tube and select Delete
The next step is to extract an air
domain around the board – this
will be done on the following few
slides
HTBP-6
Create Sketch
•
Right-click on the top surface of
the board to highlight it (see
adjacent image) and select
Create Sketch
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•
Click the Create Rectangle
button and create a rectangular
sketch as follows:
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•
•
This allows you to draw a new
sketch on the planar surface
defined by the board top surface
Lower left corner: (-0.07, -0.07)
Upper right corner: (0.25, 0.07)
Click OK
HTBP-7
Extrude Block
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•
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To create an extruded block,
right click on Sketch 1 and
select Features > Create
Extrude
In the Extrude window, set
the Distance and Body
Interaction as shown, then
click OK
A new body named Body 9
has been created
HTBP-8
Extract External Volume & Delete Original Block
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•
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Now we will extract the air
domain from the extruded
body that was just created
Right-click on Bodies >
Body 9 and select Extract
External Volume, then click
OK
A new body has been
created; rename it to Air
Delete Body 9, since it is no
longer needed
HTBP-9
Best Practices: Geometry & Meshing
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The current best practice for conjugate heat transfer is to use a conformal
mesh
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Conformal meshes have faces that match exactly one-to-one at interfaces
This ensures that heat transfer occurs smoothly across the interface
Requires imprinting of geometry surfaces on each other
Conformal meshes can only be generated by the Polyhedral Mesher (though it is
not guaranteed!)
Alternative approach is to use non-conformal meshes with in-place interfaces
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Matching will be extremely good, if not perfect, along flat interfaces
Non-matching faces at interfaces are most likely to occur on curved interfaces
with dissimilar mesh densities on either side
Interface matching can be improved by adjusting the Intersection Tolerance
(default is 0.05): higher values should result in more faces matching, though
values that are too large can adversely impact mesh quality
The Trimmed Mesher will always generate non-conformal meshes
HTBP-10
Best Practices: Conformal vs. Non-Conformal
Conformal
NonConformal
HTBP-11
Indirect Mapped Interfaces Demo
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New approach available in STAR-CCM+ v7.02: Non-conformal meshes with
indirect mapped interfaces
Improves interface matching robustness and likelihood of 100% matching
Currently available for fluid/solid and solid/solid interfaces (not yet compatible with
fluid/fluid interfaces or finite-volume stress)
HTBP-12
Best Practices: Thin-Walled Bodies
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When in-plane conduction can be neglected, contact interfaces can be used at
fluid-solid or solid-solid interfaces, and baffle interfaces can be used at fluid-fluid
interfaces
When in-plane conduction is important, the Thin Mesher and Embedded Thin
Mesher are available for meshing thin-walled bodies
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Both will generate a prismatic type mesh in geometries that are predominantly thin or
have thin structures included in them
The Thin Mesher will produce a non-conformal mesh
The Embedded Thin Mesher will produced a conformal mesh under certain conditions,
generally when the thin region is completely embedded within another region
New approach available in STAR-CCM+ v7.02: Shell Modeling
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Allows for the simulation of thin solids where lateral (in-plane) conductivity is important
Automatically created from a boundary – new shell region and interfaces are generated
Single or multiple shell layers may be modeled
Currently permit only isotropic thermal conductivity
HTBP-13
Shell Modeling Demo
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New approach available in STAR-CCM+ v7.02: Shell Modeling
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Allows for the simulation of thin solids where lateral (in-plane) conductivity is important
Automatically created from a boundary – new shell region and interfaces are
generated
Single or multiple shell layers may be modeled
Currently permit only isotropic thermal conductivity
HTBP-14
Imprint Bodies
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•
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To create a conformal mesh
we need to imprint the bodies
on each other
Select all of the bodies (using
the Shift key), then right-click
and select Boolean > Imprint
Accept the default Imprint
Type (Precise) and the click
OK
HTBP-15
Rename Surfaces
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We will now rename some of
the surfaces
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•
We could also wait until
later to do this, but it is most
convenient to do it now
Right-click on the short side
of the Air body that is closest
to the Board, select Rename
and set the name to Inlet
Similarly, rename the
opposite side to Outlet
Click on Close 3D-CAD
HTBP-16
Create Geometry Parts
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To convert the 3D-CAD
model to geometry parts,
right-click on 3D-CAD
Models > 3D-CAD
Model 1 and select New
Geometry Part
In the Part Creation
Options popup window,
accept the defaults by
clicking OK
Note that a part has been
created corresponding to
each CAD body
HTBP-17
Surface Repair
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Using the Shift key,
select all of the parts,
then right-click and
choose Repair
Surface…
In the Surface
Preparation Options
window, accept the
defaults by clicking
OK
HTBP-18
Surface Repair
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In the surface repair too,
window, click on Surface
Diagnostics…
In the Diagnostics Options
popup window, click OK
Note that the only problem
areas in the surface are
Poor Quality Faces and
Close Proximity Faces
•
These can be easily
fixed using the Surface
Remesher, so no surface
repair is required
HTBP-19
Create Regions from Parts
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We can now create regions
from the geometry parts in
preparation for meshing
Select all of the parts, then
right-click and select Assign
Parts to Regions…
Set the Region Mode,
Boundary Mode and Feature
Curve Mode as shown then
click Create Regions
Note that multiple regions,
boundaries and interfaces
have been created
HTBP-20
Define Mesh Continuum
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To begin the meshing process,
start by defining a new mesh
continuum and associated mesh
models
Right-click on Continua and
select New > Mesh Continuum
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A new mesh continuum named
Mesh 1 has been created
Right-click on Continua > Mesh
1 and choose Select Meshing
Models…
Select the Surface Remesher,
Prism Layer Mesher and
Polyhedral Mesher as shown
HTBP-21
Best Practices: Wall Treatments
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Wall treatment models are used in conjunction with RANS
models
Three wall treatment options are available in STARCCM+:
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High-y+ wall treatment: equivalent to the traditional wall
function approach, in which the near-wall cell centroid
should be placed in the log-law region (30 ≤ y+ ≤ 100)
Low-y+
wall treatment: suitable only for low-Re turbulence
models in which the mesh is sufficient to resolve the viscous
sublayer (y+ » 1) and 10-20 cells within the boundary layer
All-y+ wall treatment: a hybrid of the above two approaches,
designed to give accurate results if the near-wall cell
centroid is in the viscous sublayer, the log-law region, or the
buffer layer
First grid point, 30 < y+ < 100
Viscous sublayer
First grid point y+ ~ 1
Not all wall treatments are available for all RANS models
HTBP-22
Best Practices: Prism Layer Meshing
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Prism layers are mainly used to resolve flow boundary layers, so they are not
needed at flow boundaries (e.g. inlets, outlets)
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Set proper boundary types prior to meshing and STAR-CCM+ will
automatically disable prism layers at all flow boundaries
Prism layers are mainly used to resolve flow boundary layers, so they are not
generally required within solids
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Activate the Interface Prism Layer Option at all fluid-solid interfaces
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Disable prism layers within all solid regions
The All-y+ Wall Treatment offers the most meshing flexibility and is
recommended for all turbulence models for which it is available (most of them)
Follow the guidelines on y+ for different wall treatments as outlined on the
preceding slide
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Build and run a coarse ‘test’ mesh to help estimate the proper near-wall mesh size
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Estimate the y+ value for your problem using the procedure on the following slide
HTBP-23
Best Practices: Estimating y+
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We generally wish to target a specific value of y+ for the near-wall mesh,
where:
t
u* y
y+ =
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•
n
u* »
w
r
The wall shear stress tw can be related to the skin friction coefficient:
tw
Cf =
rU 2 / 2
The skin friction coefficient can be estimated from correlations
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–
For a flat plate: C f 0.036
=
2
Re1L/ 5
For pipe flow:
Cf
0.039
=
2
Re1D/ 5
HTBP-24
Example: Estimating y+
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For our electronics cooling problem, we will use an inlet velocity of 15 m/s. Using
the air domain height of 5 cm as the characteristic length, along with the
properties of air, we find Re D = 4.743 ´104
Using the friction coefficient correlation for internal flow:
Cf
=
2
•
The definition of the friction coefficient is used to compute the wall shear stress:
Cf =
•
0.039
Þ C f = 9.06 ´ 10 -3
1/ 5
Re D
tw
Þ t w = 1.192 N / m 2
2
rU / 2
The wall stress is used to compute u*:
u* »
•
tw
= 1 . 009 m / s
r
We will target a y+ of 80, so:
u* y
y =
Þ y = 1 . 25 mm
n
+
HTBP-25
Mesh Reference Values
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•
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Right-click on Continua > Mesh
1 > Reference Values and
select Edit…
Set the mesh values as shown in
the adjacent screenshot
Note that by using two prism
layers with a total prism layer
thickness of 2.5 mm and a
stretching factor near 1, we will
achieve a near-wall prism layer
thickness close to our estimated
value of 1.25 mm
HTBP-26
Modify Boundary Types
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To prevent prism layers
being generated on the inlet
and outlet boundaries
(where they are not
needed), we will modify
some boundary types
Select Regions > Air >
Boundaries > Inlet
In the Properties window,
set the Type to Velocity
Inlet
Similarly, select the Outlet
boundary and set its Type to
Pressure Outlet
HTBP-27
Interface Prism Layers
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Since prism layers are not
needed we will activate
prism layer growth at the
interfaces, but disable prism
layers in the solid regions
Under Interfaces, select all
interfaces, then right-click
and select Edit…
Under Mesh Conditions >
Interface Prism Layer
Option, tick the Grow Prisms
from Interface box
HTBP-28
Disable Prism Layers in Solid Regions
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•
Under Regions, select all of
the solid regions (i.e. all
except Air), then right-click
and choose Edit…
Under Mesh Conditions >
Customize Prism Mesh, set
Customize Prism Mesh to
Disable
HTBP-29
Generate Mesh
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Generate the remeshed surface
and volume mesh using the
Generate Volume Mesh button on
the Mesh Generation toolbar:
Before proceeding check the mesh
quality by selecting Mesh >
Diagnostics… from the top menu,
then clicking OK in the Mesh
Diagnostics popup window
The results at the right show that
the mesh quality is very good
HTBP-30
Examine Mesh
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Make a few plots of
the mesh as shown
Although fairly
coarse, the mesh
density is adequate
for the purposes of
this demonstration
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The resulting volume mesh consists of
approximately 108K cells
HTBP-31
Air Physics Continuum
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Under Continua, change the
name of the Physics 1
continuum to Air
Right-click on Continua > Air
and choose Select Models…
Select the physics models as
shown
HTBP-32
Copper Physics Continuum
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Right-click on Continua and select New > Physics
Continuum
Change the name of the newly-defined continuum
to Copper
Select the physics models as shown in the
screenshot below
Right-click on Continua > Copper > Models >
Solid > Al and select Replace with…
Select Material Databases > Standard > Solids >
Cu (Copper) to change the material properties
HTBP-33
Silicon Physics Continuum
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Right-click on Continua and select New > Physics
Continuum
Change the name of the newly-defined continuum
to Silicon
Select the physics models as shown in the
screenshot below
Right-click on Continua > Silicon > Models >
Solid > Al and select Replace with…
Select Material Databases > Standard > Solids >
Si (Silicon) to change the material properties
HTBP-34
Modify Region Physics Continua
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Select all of the regions
except for Air and Sink
In the Properties window,
set the Physics Continuum
to Silicon
Similarly, select the Sink
region and change its
Physics Continuum to
Copper
HTBP-35
Box Volume Report
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We will now create a report
to output the volume of the
Box region
Right-click on Reports and
select New Report > Sum
Rename this new report to
Box Volume
Define the properties of the
report as shown
Note that a new field
function named Report:
Box Volume has been
automatically defined
HTBP-36
Best Practices: Internal Heat Sources
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Internal heat sources can be applied within materials in two different ways
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Volumetric sources
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Interfacial sources
Volumetric sources are applied within the volume of a region
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Enable Energy Source Options under region’s Physics Conditions
Specify Method (constant, table, field function, user code) under region’s Physics
Values
Input values have units of power per unit volume
Interface heat sources are applied at a fluid-solid or solid-solid contact interface
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Enable Energy Source Options under interface’s Physics Conditions
Specify Method (constant, table, field function, user code) under interface’s Physics
Values
Input values have units of power per unit area
HTBP-37
Best Practices: Thermal Contact Resistance
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Thermal contact resistance is often important between parts in which the
contact is not perfect
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Depends on factors such as surface roughness, flatness and cleanliness, as well as
interstitial materials and contact pressure
Results in a temperature discontinuity at the interface
Can be modeled in STAR-CCM+ but contact resistance values must be supplied by
the user (i.e. STAR-CCM+ cannot predict these values)
‘Contact’ resistance can also be specified at a fluid-solid interface (e.g. to model a
thin coating or fouling layer)
Contact resistance is applied at contact interfaces
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Conduction is purely one-dimensional (no in-plane conduction)
Specify Method (constant, table, field function, user code) under interface’s Physics
Values
Input values have units of m^2-K/W
HTBP-38
Box Heat Source Field Function
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Next, we define a new field function to set
the box volumetric heat source
Right-click on Tools > Field Functions
and select New
Rename the newly-created field function to
Box Heat Source
Set the Function Name to BoxHeatSource
and set the Dimensions as shown
Set the field function Definition as shown
•
This will be used to distribute 70 W of
power uniformly throughout the Box region
volume
HTBP-39
Box Heat Source
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Select Regions > Box > Physics
Conditions > Energy Source Option
In the Properties window, select
Volumetric Heat Source for the
Energy Source Option
Right-click Regions > Box > Physics
Values and select Edit…
In the edit window, select Physics
Values > Energy Source and set the
Method to Field Function and the
Scalar Function to Box Heat Source
HTBP-40
Board-Chip Interface Heat Generation
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Under Interfaces, select all of the
interfaces and make sure that the
Type for each is set to Contact
Interface
Next, right-click on the Board/Chip
interface and select Edit…
Under Physics Values > Heat Flux,
set the Value to 50000 W/m^2
This will provide the specified
interface heat generation rate with
the fraction of heat traveling into the
Chip and Board regions according
to their respective thermal
resistances
HTBP-41
Box-Board Thermal Contact Resistance
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A thermal contact resistance will be
applied between the Board and Box
regions
Select Interfaces > Board/Box >
Physics Values > Contact
Resistance > Constant
In the Properties window, set the
Value to 1.e-4 m^2-K/W
HTBP-42
Best Practices: S2S Thermal Radiation
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Used to simulate gray (wavelength-independent)
diffuse thermal radiation exchange between surfaces
forming a enclosure
Medium between surfaces must be non-participating
When all solids are opaque in a CHT problem (a
common occurrence), thermal radiation needs to be
activated only for the fluid domain(s)
Requires the definition of radiation patches and the
availability of view factors between these patches
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Radiation patches are groups of boundaries which form
a continuous portion of a surface
View factor Fij is the proportion of radiation leaving a
patch i that strikes another patch j
1
Fij =
Ai
ò ò
Ai
Aj
cos q i cos q j
p Rij2
dAi dA j
HTBP-43
Best Practices: S2S Radiation Patches
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Default behavior is that each cell face
corresponds to a patch
This can lead to a prohibitive number of
patches on larger models
Number of patches can be controlled using
the patch/face proportion or by specifying a
target total number of patches
The patch/face proportion specifies the
(approximate) percentage of each patch
occupied by one cell face
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Each color is a different patch
Note that multiple cell faces form
each patch
e.g. a patch/face proportion of 25.0 (a
commonly-used value) would correspond to
each patch consisting of 4 cells faces, on
average
HTBP-44
Best Practices: S2S Radiation Properties
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Thermal radiation properties to be specified for non-participating media are:
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–
–
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From energy conservation, a + r + t = 1
From Kirchhoff’s Law, a = e
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Emissivity e
Absorptivity a
Reflectivity r
Transmissivity t
Kirchhoff’s law states that the emissivity of a surface at temperature T equals
the absorptivity by that surface of radiation from a black body at the same
temperature
Therefore, Kirchhoff’s law is not true in general for radiation from surfaces at
differing temperatures, but it is usually assumed to be valid
Note that for opaque surfaces (t = 0), assuming Kirchhoff’s Law to be valid
and using energy conservation, only one independent radiation property (e)
needs to be specified
HTBP-45
Set Patch/Face Proportion
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We will now set the patch/face
proportion which will control the
number of radiation patches in the
model
Select Regions > Air > Physics
Values > Patch/Face Proportion
and set the Patch/Face Proportion to
25.0
This will have the effect of each patch
consisting of 4 cells faces, on average
HTBP-46
Copper Sink Emissivity
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Next we will set the emissivities of the
surfaces
Note that since all solids are opaque,
thermal radiation is active only for the
air region, so all radiation properties
are set within the Air region
We will assume the silicon emissivity
to be 0.8 (the default value) and the
copper emissivity to be 0.1
Select Regions > Air > Boundaries
> Default (Air/Sink) > Physics
Values > Surface Emissivity >
Constant and set the Value to 0.1
HTBP-47
Inlet & Outlet Boundary Conditions
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Right-click on Regions > Air >
Boundaries > Inlet and select Edit…
The Static Temperature and
Radiation Temperature can be left at
their default values of 300 K in this
case
Set the value of the Velocity Magnitude
to 15.0 m/s
HTBP-48
Best Practices: Thermal Boundary Conditions
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For wall boundaries which are not solid-fluid or solid-solid
interfaces (i.e. true boundaries), the following choices are
available:
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–
–
•
1/h
Adiabatic walls (zero heat transfer)
Fixed wall heat flux
Fixed wall temperature
Convection: see adjacent image
For S2S thermal radiation, the domain must form an
enclosure, so flow boundaries (e.g. inlets, pressure outlets)
must have environmental patches and boundary conditions
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Boundary conditions are specified as “radiation temperatures”
Radiation temperatures do not have to be the same as the
temperature of the flow crossing the boundary
Radiation temperature should represent the temperature to
which radiation from the model is transmitted
HTBP-49
Board Thermal Boundary Conditions
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We set the side and bottom
boundaries of the Board region
to have convective heat
transfer boundary conditions
Right-click on Regions >
Board > Boundaries > Default
and select Edit…
Under Physics Conditions >
Thermal Specification, set the
Method to Convection
Under Physics Values, set the
Ambient Temperature to 300
K and the Heat Transfer
Coefficient to 100 W/m^2-K
HTBP-50
Set Maximum Steps & Run Analysis
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Click on Stopping Criteria > Maximum
Steps and set the Maximum Steps to 300
Run the analysis:
After the analysis is complete, make some
plots of the results
•
For examples, see the slides that follow
HTBP-51
Convergence History
HTBP-52
Wall y+
•
Note that our estimate
resulted in y+ values in the
proper range for the high-y+
wall treatment
•
•
However, the values are a
bit low compared to our
targeted value
Many of the lower y+ regions
are in areas where the flow is
impinging on the surface or
has separated from the
surface
•
There is no boundary layer
there!
HTBP-53
Best Practices: Heat Transfer Coefficients
•
The convective heat transfer coefficient (HTC) is defined as:
¢
q¢wall
h=
(Twall - T fluid )
•
•
The only term not clearly specified is the fluid temperature, i.e. the
fluid temperature where?
The choice of fluid temperature may be used to define different
heat transfer coefficients
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–
–
–
Some definitions may be more useful than others
For turbulent forced convection, we would like the HTC to depend on the
Reynolds’ number, fluid properties and geometry
There may also be some sensitivity to the type of boundary condition (i.e.
fixed temperature vs. constant heat flux), but the HTC should not depend on
the value of the boundary condition
The above will be true only for particular choices of the fluid temperature
HTBP-54
Best Practices: STAR-CCM+ HTCs
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Heat Transfer Coefficient:
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Local Heat Transfer Coefficient:
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Uses the computed wall heat flux, wall temperature, and a fluid temperature
specified by the user
Does not account for local variations in fluid temperature
Uses definitions from the wall treatment to compute a heat transfer coefficient
These definitions effectively use the near-wall fluid cell temperature
May have some sensitivity to near-wall mesh size
Specified y+ Heat Transfer Coefficient:
•
•
•
•
Uses a fluid temperature at a specified y+ value
Accommodates local fluid temperature variation effects
Eliminates sensitivity to near-wall mesh size
Recommended as best practice - combines the best features of the Heat Transfer
Coefficient and the Local Heat Transfer Coefficient
HTBP-55
Heat Transfer Coefficient
Can have
negative
values
HTBP-56
Local HTC
HTBP-57
Specified y+ Heat Transfer Coefficient (y+ = 100)
Recommended
HTBP-58
Summary
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We have worked through a simplified conjugate heat transfer
problem with a number of features typically encountered in
real industry problems
Best practices for the following topics have been
demonstrated and discussed:
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•
•
•
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Imprinting and CHT Interfaces
New STAR-CCM+ v7.02 Features for CHT
Wall Treatments and Near-Wall Meshing
Internal Heat Generation
Thermal Contact Resistance
Thermal Radiation
Thermal Boundary Conditions
Heat Transfer Coefficients
HTBP-59