Best Practices Workshop: Heat Transfer

Overview

•

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:

• Newer STAR-CCM+ Features for CHT • Wall Treatments and Near-Wall Meshing • Internal Heat Generation

• Thermal Contact Resistance • S2S Thermal Radiation

New Simulation

• Start a new STAR-CCM+ session • Under File, click New Simulation

Import CAD Model

• 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

Examine CAD Model

• 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

Delete Tube Body

• 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

Create Sketch

• Right-click on the top surface of

the board to highlight it (see adjacent image) and select

Create Sketch

• This allows you to draw a new

sketch on the planar surface defined by the board top surface

• Click the Create Rectangle

button and create a rectangular sketch as follows:

• Lower left corner: (-0.07, -0.07) • Upper right corner: (0.25, 0.07)

Extrude Block

• 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

Extract External Volume & Delete Original Block

• 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

Best Practices:

Geometry & Meshing

• The current best practice for conjugate heat transfer is to use a conformal

mesh

• 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

• 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

Best Practices:

Conformal vs. Non-Conformal

Conformal

Indirect Mapped Interfaces Demo

• 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

Best Practices:

Thin-Walled Bodies

• 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

• 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

• 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

Shell Modeling Demo

• New approach available in STAR-CCM+ v7.02: Shell Modeling

• 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

Imprint Bodies

• 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

Rename Surfaces

• We will now rename some of

the surfaces

• 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

Create Geometry Parts

• 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

Surface Repair

• 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

Surface Repair

• 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

Create Regions from Parts

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,

Define Mesh Continuum

• 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

• 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

Best Practices:

Wall Treatments

models

• Three wall treatment options are available in

STAR-CCM+:

– 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

Best Practices:

Prism Layer Meshing

• Prism layers are mainly used to resolve flow boundary layers, so they are not

needed at flow boundaries (e.g. inlets, outlets)

• 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

• Activate the Interface Prism Layer Option at all fluid-solid interfaces • 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

Best Practices:

Estimating y+

•

We generally wish to target a specific value of y

for the near-wall mesh,

where:

•

The wall shear stress

t

can be related to the skin friction coefficient:

•

The skin friction coefficient can be estimated from correlations

–

For a flat plate:

–

For pipe flow:

n y u y * = + r t _w u* » 2 / 2 U C w f

r

t

Example: Estimating y+

• 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

• Using the friction coefficient correlation for internal flow:

• The definition of the friction coefficient is used to compute the wall shear stress:

• The wall stress is used to compute u*:

• We will target a y+ of 80, so:

3 5 / 1 9.06 10 Re 039 . 0 2 -´ = Þ = _f D f _C C 2 2 _/2 1.192 N /m U C w _w f = _r Þt = t s m u* _» w _{= 1.009 /} r t 4 10 743 . 4 Re_D = ´

Mesh Reference Values

• 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

Modify Boundary Types

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

Interface Prism Layers

• 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

Disable Prism Layers in Solid Regions

• 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

Generate Mesh

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

Examine Mesh

the mesh as shown

• Although fairly

coarse, the mesh density is adequate for the purposes of this demonstration

• The resulting volume mesh consists of

Air Physics Continuum

• 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

Copper Physics Continuum

• 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 >

Silicon Physics Continuum

• 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 >

Modify Region Physics Continua

• 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

Box Volume Report

• 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

Best Practices:

Internal Heat Sources

• Internal heat sources can be applied within materials in two different ways

• Volumetric sources • Interfacial sources

• Volumetric sources are applied within the volume of a region

• 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

• Enable Energy Source Options under interface’s Physics Conditions

• Specify Method (constant, table, field function, user code) under interface’s Physics

Best Practices:

Thermal Contact Resistance

contact is not perfect

• 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

• Conduction is purely one-dimensional (no in-plane conduction)

• Specify Method (constant, table, field function, user code) under interface’s Physics

Box Heat Source Field Function

• 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

Box Heat Source

• 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

Board-Chip Interface Heat Generation

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

Box-Board Thermal Contact Resistance

• 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

Best Practices:

S2S Thermal Radiation

• 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

– Radiation patches are groups of boundaries which form

a continuous portion of a surface

– View factor F_ij is the proportion of radiation leaving a

patch i that strikes another patch j

j i A A ij j i i ij dAdA R A F i j

ò ò

Best Practices:

S2S Radiation Patches

•

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

• e.g. a patch/face proportion of 25.0 (a

commonly-used value) would correspond to

• Each color is a different patch • Note that multiple cell faces form

Best Practices:

S2S Radiation Properties

• Thermal radiation properties to be specified for non-participating media are:

– Emissivity e

– Absorptivity a

– Reflectivity r – Transmissivity t

• From energy conservation, a + r + t = 1 • From Kirchhoff’s Law, a = e

– 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

Set Patch/Face Proportion

• 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

Copper Sink Emissivity

• 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

Inlet & Outlet Boundary Conditions

• 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

1/h

Best Practices:

Thermal Boundary Conditions

• For wall boundaries which are not solid-fluid or solid-solid

interfaces (i.e. true boundaries), the following choices are available:

– 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

• Boundary conditions are specified as “radiation temperatures” • Radiation temperatures do not have to be the same as the

Board Thermal Boundary Conditions

• 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

Set Maximum Steps & Run Analysis

• 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

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

Best Practices:

Heat Transfer Coefficients

•

The convective heat transfer coefficient (HTC) is defined as:

•

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

– 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

(

)

T

T

q

h

-¢¢

=

Best Practices:

STAR-CCM+ HTCs

• 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

• Local Heat Transfer Coefficient:

• 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

Heat Transfer Coefficient

Can have

negative

Specified y+ Heat Transfer Coefficient (y+ = 100)

Summary

•

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:

• 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