heat-transfer-best-practices-nov2012_pe.pdfat transfer best practices nov2012 pe

7/25/2019 Heat-Transfer-Best-Practices-Nov2012_PE.pdfat Transfer Best Practices Nov2012 PE

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Best Practices Workshop:

Heat Transfer


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HTBP-2

Overview

This workshop will have a mixed format: we will work througha typical CHT problem in STAR-CCM+, stopping periodically to

elucidate best practices or demonstrate new features

In particular, the following topics will be highlighted:

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


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New Simulation

Start a new STAR-CCM+ session

Under File, click New Simulation

Click OK


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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


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HTBP-5

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 componentsmounted 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 thecomponents


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HTBP-6

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


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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) Click OK


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HTBP-8

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

has been created


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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 nolonger needed


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HTBP-10

Best Practices: Geometry & Meshing

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

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

(default is 0.05): higher values should result in more faces matching, thoughvalues that are too large can adversely impact mesh quality

The Trimmed Mesher will always generate non-conformal meshes


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Best Practices: Conformal vs. Non-Conformal

Conformal

Non-Conformal


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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

fluid/fluid interfaces or finite-volume stress)


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Best Practices: Thin-Walled Bodies

When in-plane conduction can be neglected, contact interfaces can be used atfluid-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

Currently permit only isotropic thermal conductivity


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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


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Imprint Bodies

To create a conformal mesh

we need to imprint the bodies

on each other

Select all of the bodies (using

the Shiftkey), then right-click

and select Boolean > Imprint

Accept the default Imprint

Type (Precise) and the click

OK


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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 sideof the Airbody 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


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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 toeach CAD body


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HTBP-18

Surface Repair

Using the Shiftkey,

select all of the parts,

then right-click and

choose RepairSurface

In the Surface

Preparation Options

window, accept the

defaults by clicking

OK


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HTBP-19

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 SurfaceRemesher, so no surface

repair is required


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Create Regions from Parts

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 interfaceshave been created


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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 MesherandPolyhedral Mesher as shown


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HTBP-22

Best Practices: Wall Treatments

Wall treatment models are used in conjunction with RANSmodels

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 (30y

+ 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

Not all wall treatments are available for all RANS models

First grid point, 30 < y+ < 100 Viscous sublayer

First grid point y+ ~ 1


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HTBP-23

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 Build and run a coarse test mesh to help estimate the proper near-wall mesh size

Estimate the y+ value for your problem using the procedure on the following slide


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Best Practices: Estimating y+

We generally wish to target a specific value of y+ for the near-wall mesh,where:

The wall shear stress tw 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

yuy

*

=+r

twu *

2/2UC w

f r

t

=

5/1Re

036.0

2 L

fC =

5/1Re

039.0

2 D

fC =


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HTBP-25

Example: Estimating y+

For our electronics cooling problem, we will use an inlet velocity of 15 m/s. Usingthe 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 1006.9

Re

039.0

2

-== fD

fC

C

2

2 /192.1

2/mN

UC w

wf == t

r

t

smu w /009.1* =r

t

mmyyu

y 25.1*

==+n

410743.4Re =D


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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 estimatedvalue of 1.25 mm


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HTBP-27

Modify Boundary Types

To prevent prism layersbeing 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 toPressure Outlet


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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

UnderMesh Conditions >

Interface Prism Layer

Option, tick the Grow Prismsfrom Interface box


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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

Disable


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HTBP-30

Generate Mesh

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 thatthe mesh quality is very good


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Examine Mesh

Make a few plots ofthe mesh as shown

Although fairly

coarse, the mesh

density is adequate

for the purposes of

this demonstration

The resulting volume mesh consists ofapproximately 108K cells


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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

shown


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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 >

Cu (Copper) to change the material properties


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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 >

Si (Silicon) to change the material properties


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HTBP-35

Modify Region Physics Continua

Select all of the regions

except for Airand Sink

In the Properties window,

set the Physics Continuumto Silicon

Similarly, select the Sink

region and change its

Physics Continuum to

Copper


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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 beenautomatically defined


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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 regions Physics Conditions

Specify Method (constant, table, field function, user code) under regions PhysicsValues

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 interfaces Physics Conditions

Specify Method (constant, table, field function, user code) under interfaces Physics

Values

Input values have units of power per unit area


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Best Practices: Thermal Contact Resistance

Thermal contact resistance is often important between parts in which the

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 interfaces Physics

Values Input values have units of m^2-K/W


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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 toBox 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 regionvolume


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HTBP-40

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

Methodto Field Function and the

Scalar Function to Box Heat Source


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HTBP-41

Board-Chip Interface Heat Generation

Under Interfaces, select all of theinterfaces 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 thermalresistances


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HTBP-42

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

Value to 1.e-4 m^2-K/W


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HTBP-43

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 beactivated 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 Fij is the proportion of radiation leaving apatch i that strikes another patch j

jiA A

ij

ji

i

ij dAdARA

Fi j

= 2coscos1

p

qq


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HTBP-44

Best Practices: S2S Radiation Patches

Default behavior is that each cell facecorresponds 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 atarget 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 toeach patch consisting of 4 cells faces, on

average

Each color is a different patch

Note that multiple cell faces form

each patch


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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 Kirchhoffs Law, a = e Kirchhoffs 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, Kirchhoffs 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 Kirchhoffs Law to be validand using energy conservation, only one independent radiation property (e)needs to be specified


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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

consisting of 4 cells faces, on average


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HTBP-47

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 Airregion

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


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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

to 15.0 m/s


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HTBP-49

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Best Practices: Thermal Boundary Conditions

For wall boundaries which are not solid-fluid or solid-solidinterfaces (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

temperature of the flow crossing the boundary Radiation temperature should represent the temperature to

which radiation from the model is transmitted


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HTBP-50

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

Methodto Convection

Under Physics Values, set the

Ambient Temperature to 300

K and the Heat TransferCoefficient to 100 W/m^2-K


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HTBP-51

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

For examples, see the slides that follow


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HTBP-52

Convergence History


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HTBP-53

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 layerthere!


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HTBP-54

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 differentheat 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 onthe value of the boundary condition

The above will be true only for particular choices of the fluid temperature

( )fluidwallwall

TT

qh

-

=


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HTBP-55

Best Practices: STAR-CCM+ HTCs

Heat Transfer Coefficient: 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 Recommended as best practice - combines the best features of the Heat Transfer

Coefficient and the Local Heat Transfer Coefficient


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HTBP-56

Heat Transfer Coefficient

Can havenegativevalues


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HTBP-57

Local HTC


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HTBP-58

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

Recommended


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Summary

We have worked through a simplified conjugate heat transferproblem 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 Thermal Boundary Conditions

Heat Transfer Coefficients

heat-transfer-best-practices-nov2012_pe.pdfat transfer best practices nov2012 pe

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