Macro Heat Exchanger Model Fluent Tutorial

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Introduction

In a typical heat exchanger core, the auxillary fluid temperature is stratified in the direction In a typical heat exchanger core, the auxillary fluid temperature is stratified in the direction of the

of the auxilauxillary fluid flow. lary fluid flow. As a As a resulresult, t, heat rejecheat rejection is tion is not constanot constant ovnt over the er the ententire core.ire core. In

In ANSYS FLUENTANSYS FLUENT the ﬂuid zone representing the heat exchanger core is subdivided intothe ﬂuid zone representing the heat exchanger core is subdivided into macr

macroscooscopic cells or pic cells or macromacros along the s along the auxilauxillary fluid path. lary fluid path. The auxillaThe auxillary fluid inlet ry fluid inlet tem- tem-perature to each macro is computed and subsequently used to compute the heat rejection perature to each macro is computed and subsequently used to compute the heat rejection from each macro

from each macro. . This approaThis approach provich provided a ded a realrealistic heat istic heat rejecrejection distributtion distribution over theion over the heat exchanger core.

heat exchanger core.

Results obtained from the heat exchanger model are very important in the design of cooling Results obtained from the heat exchanger model are very important in the design of cooling modules. So it is important to make sure that the heat exchanger model is used properly. modules. So it is important to make sure that the heat exchanger model is used properly. This tutorial demonstrates the following:

This tutorial demonstrates the following:

•

• Set up the heat exchanger model.Set up the heat exchanger model.

•

• Run the case inRun the case in ANSYS FLUENTANSYS FLUENT..

•

• Use a simple example and change many input parameters to see if you get expectedUse a simple example and change many input parameters to see if you get expected

results. results.

•

• Inherent limitations in the macro model.Inherent limitations in the macro model.

Prerequisites

This tutorial is written with the assumption that you have completed Tutorial 1 from the This tutorial is written with the assumption that you have completed Tutorial 1 from the ANSYS FLUENT

ANSYS FLUENT 13.0 Tutorial Guide, and that you are familiar with the13.0 Tutorial Guide, and that you are familiar with the ANSYS FLUENTANSYS FLUENT nav

navigatiigation pane and on pane and menmenu structureu structure. . Some steps in Some steps in the setup and the setup and solutisolution proceduron procedure e willwill not be shown explicitly.

not be shown explicitly.

In this tutorialyou will use the macro heat exchanger model. For details about this model, In this tutorialyou will use the macro heat exchanger model. For details about this model, see Section 6.1 The Macro Heat Exchanger Models in

see Section 6.1 The Macro Heat Exchanger Models in ANSYS FLUENTANSYS FLUENT 13.0 Theroy Guide13.0 Theroy Guide and Section 15.1 Overview and Restrictions of the Macro Heat Exchanger Models in

and Section 15.1 Overview and Restrictions of the Macro Heat Exchanger Models in ANSYSANSYS FLUENT

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Problem Description

A single pass heat exchanger is shown in Figure

A single pass heat exchanger is shown in Figure 11 (front and side view).(front and side view).

Figure 1: Single Pass Heat Exchanger Figure 1: Single Pass Heat Exchanger The following table shows the radiator performance data.

The following table shows the radiator performance data. A

Air ir InInlelet t TTemempeperraatuture re (T(Ta a inin) ) 4488.8.89 9 CC Coo

Coolalant nt InInlelet t TTememperperataturure(e(Tc Tc inin) ) 11115.5.56 56 CC Air M

Air Mass ass FloFlow Raw Rate (kte (kg/g/s) (ms) (mdot adot a) ) 1.11.14040 Cool

Coolanant t FloFlow w RatRate e (kg(kg/s/s) ) (md(mdot ot c) c) 2.82.87070 T

Tootatal l HeHeaat t RReejejecctitioon n (W(Waattttss) ) 557733445.5.996600

Table 1: Radiator Performance Data Table 1: Radiator Performance Data

Since the operating conditions (mdot c and mdot a) are the same as one of the heat Since the operating conditions (mdot c and mdot a) are the same as one of the heat ex-changer data, and since the operating inlet temperatures (Tc in and Ta in) are the same as changer data, and since the operating inlet temperatures (Tc in and Ta in) are the same as the ones used to obtain the data (48.89 and 115.56 C respectively), we should get the same the ones used to obtain the data (48.89 and 115.56 C respectively), we should get the same total heat rejection as the data, which is 57346 Watts. This will become more clear in the total heat rejection as the data, which is 57346 Watts. This will become more clear in the next tutorial.

next tutorial.

Preparation

1. Copy the ﬁles

1. Copy the ﬁles wedge.msh.gzwedge.msh.gz andand rad.tabrad.tab to the working folder.to the working folder. 2. Use

2. Use FLUENT LauncherFLUENT Launcher to start theto start the 3D3D version of version of ANSYS FLUENTANSYS FLUENT.. 3. Enable

3. Enable Double-PrecisionDouble-Precision in thein the Display OptionsDisplay Options list.list. For more information about

For more information about FLUENT LauncherFLUENT Launcher see Section see Section 1.1.2 Starting ANSYS FLU-1.1.2 Starting ANSYS FLU-ENT Using FLUFLU-ENT Launcher

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Setup and Solution

Ste

Step 1: p 1: MesMeshh

1.

1. ReaRead the mesh ﬁled the mesh ﬁle,, wedge.msh.gzwedge.msh.gz..

Figure 2: Mesh Figure 2: Mesh

Step 2:

Step 2: GeneGeneral Settingral Settingss

1.

1. RetaRetain the in the defadefault solveult solver settings.r settings. General

General

2.

2. ChecCheck the meshk the mesh.. General

General−−→→CheckCheck

3.

3. Scale thScale the grid.e grid. General

General−−→→Scale...Scale...

(a)

(a) SelecSelectt mmmm fromfrom Mesh Was Created InMesh Was Created In drop-down list.drop-down list. (b)

(b) ClicClickk ScaleScale and close theand close the Scale MeshScale Mesh dialog box.dialog box.

Ste

Step 3: p 3: ModeModelsls

1.

1. EnaEnable thble thee Energy EquationEnergy Equation.. Models

Models−−→→ EnergyEnergy−−→→Edit...Edit...

You will keep the ﬂow laminar. You will keep the ﬂow laminar.

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2. Enable the

2. Enable the Heat ExchangerHeat Exchanger model.model. Models

Models−−→→ Heat ExchangerHeat Exchanger−−→→ Edit...Edit...

(a)

(a) EnabEnablele UngroUngrouped uped MacrMacro o ModelModel.. (b) Enable the

(b) Enable the Macro Model GroupMacro Model Group.. (c)

(c) ClicClickk Deﬁne...Deﬁne... next tonext to UngroUngrouped uped Macro ModelMacro Model..

i. Enable

i. Enable Fixed Inlet TemperatureFixed Inlet Temperature from thefrom the OptionsOptions group.group. ii. Enter

ii. Enter 115.56115.56 forfor Auxiliary Fluid TemperatureAuxiliary Fluid Temperature.. iii.

iii. EnteEnterr 48.8900148.89001 forfor Primary Fluid TemperaturePrimary Fluid Temperature.. iv. Click on the

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A.

A. ClicClick onk on Read...Read... B.

B. SelSelect ﬁlect ﬁlee rad.tabrad.tab and clickand click OKOK.. C. The

C. The Heat Transfer Data TableHeat Transfer Data Table dialog box is updated.dialog box is updated. D.

D. ClicClickk OKOK.. v. Click the

v. Click the GeometryGeometry tab.tab.

A. Set

A. Set Number of PassesNumber of Passes toto 11.. B. Set

B. Set Number of Rows/PassNumber of Rows/Pass toto 11.. C.

C. RetRetainain 11 forfor Number of Columns/PassNumber of Columns/Pass.. D. In the

D. In the Auxiliary Fluid Inlet Direction (height)Auxiliary Fluid Inlet Direction (height) group set thegroup set the X, Y,X, Y, andand ZZ values to

values to 0, 0, -1-1,, andand 00 respectively.respectively. E.

E. SimilaSimilarlyrly, in the, in the Pass-to-Pass Direction (width)Pass-to-Pass Direction (width) group set thegroup set the X, Y,X, Y, andand ZZ values to

values to 1, 0,1, 0, andand 00 respectively.respectively. vi. Click on the

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A.

A. EnEnterter35593559 forfor Auxiliary Fluid Speciﬁc Heat (j/kg-k)Auxiliary Fluid Speciﬁc Heat (j/kg-k).. B.

B. EnEnterter2.8700012.870001 forfor Auxiliary Fluid Flow Rate (kg/s)Auxiliary Fluid Flow Rate (kg/s) (mdot c).(mdot c). C.

C. EnEnterter115.56115.56 forfor Inlet Temperature (c)Inlet Temperature (c) (Tc in).(Tc in). D.

D. ClicClickk ApplyApply and close theand close the Ungrouped Macro Heat ExchangerUngrouped Macro Heat Exchanger dialog boxdialog box (d) Click

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Step 4:

Step 4: BounBoundary Conditdary Conditionsions

1.

1. Set the boundary conSet the boundary conditionditions for the inlet.s for the inlet. Boundary Conditions

Boundary Conditions−−→→ inletinlet−−→→Edit...Edit...

(a)

(a) EntEnterer 1.141.14 forfor Mass Flow RateMass Flow Rate (Mdot a).(Mdot a). (b)

(b) SelSelectect Normal to BoundaryNormal to Boundary fromfrom DirecDirection tion SpeciﬁcSpeciﬁcation ation MethodMethod drop-down list.drop-down list. (c)

(c) ClicClick on thek on the ThermalThermal tab and entertab and enter 48.8948.89 forfor Total TemperatureTotal Temperature (Ta in).(Ta in). (d)

(d) ClicClickk OKOK to close theto close the Mass-Flow InletMass-Flow Inlet dialog box.dialog box. 2.

2. Set the boundary conSet the boundary conditionditions for the outlet.s for the outlet. Boundary Conditions

Boundary Conditions−−→→ outletoutlet−−→→Edit...Edit...

(a)

(a) ClicClick on thek on the ThermalThermal tab and entertab and enter 48.8948.89 forfor Backﬂow Total TemperatureBackﬂow Total Temperature.. (b)

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Step 5:

Step 5: SoluSolutiontion

1.

1. Set the solution parSet the solution parameteameters.rs. Solution Methods

Solution Methods

(a)

(a) SelecSelectt Green-Gauss Cell BasedGreen-Gauss Cell Based from thefrom the GradientGradient drop-down list.drop-down list. 2. Select only

2. Select only EnergyEnergy from the list of equations.from the list of equations. Solution Controls

Solution Controls −−→→Equations...Equations...

3.

3. EnabEnable the le the plottplotting of ing of residresiduals during the calculatuals during the calculation.ion. Monitors

Monitors−−→→ ResidualsResiduals −−→→Edit...Edit...

(a)

(a) EnsurEnsure thate that Print to ConsolePrint to Console andand PlotPlot are enabled.are enabled. (b)

(b) DisabDisablele Check ConvergenceCheck Convergence in the group of in the group of EquationsEquations.. (c)

(c) ClicClickk OKOK to close theto close the Residual MonitorsResidual Monitors dialog box.dialog box. 4.

4. InitiaInitialize the solutlize the solution.ion. Solution Initialization

Solution Initialization

(a)

(a) SelecSelectt inletinlet from thefrom the Compute fromCompute from drop-down list.drop-down list. (b) Click

(b) Click InitializeInitialize 5.

5. CalcuCalculate for 20 iteratilate for 20 iterations.ons. Run Calculation

Run Calculation−−→→ CalculateCalculate

Step 6:

Step 6: PoPostprocstprocessiessingng

1.

1. CompuCompute the total heat rejectite the total heat rejection rate.on rate. Reports

Reports−−→→ Heat ExchangerHeat Exchanger−−→→ Set Up...Set Up...

(a)

(a) SelecSelectt Computed Heat RejectionComputed Heat Rejection from the list of from the list of OptionsOptions.. (b) Click

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(c)

(c) CloClose these the Heat Exchanger ReportHeat Exchanger Report dialog box.dialog box. The total heat rejection rate(Q) is

The total heat rejection rate(Q) is 57336.7557336.75 Watts. This is the same value as the heat Watts. This is the same value as the heat exchang

exchanger er data.data. 2.

2. CompuCompute the air outlet temperatute the air outlet temperature.re. Reports

Reports −−→→ Surface IntegralsSurface Integrals −−→→ Set Up...Set Up...

(a)

(a) SelecSelectt Area-Weighted AverageArea-Weighted Average from thefrom the Report TypeReport Type drop-down list.drop-down list. (b)

(b) SelSelectect TemperatureTemperature andand Static Static TTemperatureemperature from thefrom the Field Field VaVariableriable drop-down list.drop-down list. (c)

(c) SelSelectect outletoutlet from the list of from the list of SurfacesSurfaces.. (d)

(d) ClicClickk ComputeCompute The

The Area-Weighted AverageArea-Weighted Average temperature istemperature is 372.0141 k372.0141 k( ( Ta outTa out)).. (e)

(e) CloClose these the Surface IntegralsSurface Integrals dialog box.dialog box. 3.

3. SavSave the case and data ﬁles (e the case and data ﬁles (wedge1.cas.gzwedge1.cas.gz andand wedge1.dat.gzwedge1.dat.gz).). File

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Further Improvements

1.

1. Use morUse more macroe macros.s. (a)

(a) ReaRead the case and data ﬁle (d the case and data ﬁle (wedge1.cas.gzwedge1.cas.gz andand wedge1.dat.gzwedge1.dat.gz).). Models

Models−−→→ Heat ExchangerHeat Exchanger−−→→Edit...Edit...

i. Click

i. Click Deﬁne...Deﬁne... next tonext to Ungrouped Macro ModelUngrouped Macro Model.. ii. Click on

ii. Click on GeometryGeometry tab.tab. iii. Set

iii. Set Number of Rows/PassNumber of Rows/Pass toto 6060 andand Number of Columns/PassNumber of Columns/Pass toto 7070.. iv. Click

iv. Click ApplyApply and close theand close the Ungrouped Macro Heat ExchangerUngrouped Macro Heat Exchanger dialog box.dialog box. (b) Close the

(b) Close the Heat Exchanger ModelHeat Exchanger Model dialog box.dialog box. (c)

(c) Run calcRun calculatioulation.n. Run Calculation

Run Calculation−−→→CalculateCalculate

(d)

(d) CompuCompute the total heat rejectite the total heat rejection rate.on rate. Reports

Reports −−→→ Heat ExchangerHeat Exchanger−−→→Set Up...Set Up...

The total heat rejection rate(Q) is

The total heat rejection rate(Q) is 56815.0556815.05 Watts. the result is almost the same.Watts. the result is almost the same. There is only 0.9% under-prediction in total heat rejection. Since, you are scaling There is only 0.9% under-prediction in total heat rejection. Since, you are scaling each small macro, there are numerical ineﬃciencies.

each small macro, there are numerical ineﬃciencies. (e)

(e) DisplDisplay the ay the temperatemperature contoture contours.urs. Graphics and Animations

Graphics and Animations −−→→ ContoursContours−−→→Set Up...Set Up...

i. Enable

i. Enable FilledFilled from thefrom the OptionsOptions group box.group box. ii. Select

ii. Select TemperatureTemperature andand Static TemperatureStatic Temperature from thefrom the Contours of Contours of drop-downdrop-down list.

list.

Temperature contours at the outlet gives variation as expected. Temperature contours at the outlet gives variation as expected. 2. Similarly read the case and data ﬁle again and set the

2. Similarly read the case and data ﬁle again and set the Number of Rows/PassNumber of Rows/Pass toto 3030 and

and Number of Columns/PassNumber of Columns/Pass toto 3535..

The results are still the same, but slight improvement due to reduced numerical The results are still the same, but slight improvement due to reduced numerical inef- ﬁciencies.

ﬁciencies.

3. Read the case and data ﬁle and set

3. Read the case and data ﬁle and set Number of Number of RowsRows/Pa/Passss toto 3131 andand Number of Number of Columns/Pass

Columns/Pass toto 3636..

There is about 0.94% under-prediction of total heat rejection, which is tolerable, but There is about 0.94% under-prediction of total heat rejection, which is tolerable, but the conto

the contour of ur of air tempair tempereraturature is e is not so not so gogoodod. . In concluIn conclusion choosion choose these the Number of Number of Rows/Pass

Rows/Pass and and Number of Columns/PassNumber of Columns/Pass (and uniform grid) so that each macro is the(and uniform grid) so that each macro is the same size.

same size.

Note:

Note: This limitation is inherent in the macro-based models.This limitation is inherent in the macro-based models. 4. Read the case and data ﬁle and set

4. Read the case and data ﬁle and set Number of Number of RowsRows/Pa/Passss toto 120120 andand Number of Number of Columns/Pass

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Note:

Note: Now you have more macro than available cells. So some macros will be empty.Now you have more macro than available cells. So some macros will be empty. If you click

If you click ApplyApply you will get a message for each empty macro. The end result you will get a message for each empty macro. The end result is not realistic.

is not realistic.

Note:

Note: This limitation is inherent in the macro-based models.This limitation is inherent in the macro-based models. 5.

5. PrediPredict coolant inlet temperaturct coolant inlet temperature for e for a given total heat a given total heat rejecrejection.tion.

Note:

Note: The The heheat at exchexchanganger er rrepeport ort givgives es a a tottotal al heheat at rrejeejectiction on of of 5.73367e+0045.73367e+004 for for coolant inlet temperature of

coolant inlet temperature of 115.56115.56. In this exercise you will predict coolant inlet . In this exercise you will predict coolant inlet temperature for a given total heat rejection. The way the algorithm works is that temperature for a given total heat rejection. The way the algorithm works is that it will predict total heat rejection starting from an initial guess of coolant inlet it will predict total heat rejection starting from an initial guess of coolant inlet temperature. If predicted total heat rejection is diﬀerent from the targeted value, temperature. If predicted total heat rejection is diﬀerent from the targeted value, it will adjust the coolant inlet temperature accordingly for next iteration.

it will adjust the coolant inlet temperature accordingly for next iteration. (a)

(a) Read the casRead the case and data ﬁle.e and data ﬁle. (b)

(b) MakMake changee changes to s to the heat exchathe heat exchanger model.nger model. Models

Models −−→→ Heat ExchangerHeat Exchanger−−→→Edit...Edit...

(c)

(c) ClicClickk Deﬁne...Deﬁne... next tonext to UngroUngrouped uped Macro ModelMacro Model.. i. Select

i. Select Fixed Heat RejectionFixed Heat Rejection from thefrom the OptionsOptions group box.group box.

ii. Click on

ii. Click on Auxiliary FluidAuxiliary Fluid tab and settab and set Heat RejectionHeat Rejection andand Initial TemperatureInitial Temperature toto 57886.7

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(d)

(d) EnsurEnsure e that under-that under-relaxrelaxation factoation factor r of energy isof energy is 11.. Solution Controls

Solution Controls

(e)

(e) InitiaInitialize using valize using value computelue computes from the s from the inlet.inlet. Solution Initialization

Solution Initialization

i. Select

i. Select inletinlet from thefrom the Compute fromCompute from drop-down list.drop-down list. (f)

(f) Run caRun calculalculation.tion. Run Calculation

Run Calculation−−→→CalculateCalculate

It will take about six iterations for residuals to go below 1e-17. After converging It will take about six iterations for residuals to go below 1e-17. After converging it will predict Tc in =

it will predict Tc in = 343.15 K343.15 K, which is way oﬀ., which is way oﬀ.

Summary

The tutorial shows how to set up the heat exchange model, run the case in

The tutorial shows how to set up the heat exchange model, run the case in ANSYS FLUENTANSYS FLUENT and comp

and comparare e the resulthe results. ts. SpecSpecial care ial care mumust st be be taktaken en whewhen n usiusing ng the macrthe macro-bo-baseased d heaheatt exchanger models.