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SAE TECHNICAL

PAPER SERIES 2008-01-0238

CFD Design Tool Improves HVAC Design and

Cuts Product Development Cycle Time

Meisen Li and LinJie HuangDelphi Thermal Systems

Reprinted From: CAD/CAM/CAE Technology, 2008(SP-2172)

2008 World CongressDetroit, MichiganApril 14-17, 2008

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ABSTRACT

In an effort to shorten and improve the efficiency of theproduct design process (PDP), Delphi has developed an

application specific CFD design tool that helps designengineers optimize designs prior to prototyping. Thisnew tool enables design engineers to set up an HVACmodule, powertrain cooling system, or heat exchangermodel for CFD analysis in relatively short time. Theability to evaluate and improve the design of the productat the very early stages of the development cycle greatlyreduces the need for design changes late in the process,which are expensive and time-consuming. Besidesreducing the design cycle, the new CFD tool has alsoreduced the model shop and testing expenses for eachdevelopment project with fewer prototypes. In this paper,a case study for using this tool during the HVAC moduledevelopment process in early stage is demonstrated.

INTRODUCTION

In the last fifteen years, Delphi has employed CFD todetermine the flow and thermal characteristics of theirthermal products and systems. Traditionally, CFDanalyses have to be performed by CFD experts duringthe late stages of the product development process.Unfortunately, this practice provided little benefit andopportunity for design engineers to develop and optimizetheir thermal product and system design concepts inearly PDP(Production Development Process). In an

effort to shorten and improve our PDP, We havedeveloped a product-specific CFD design tool aimed fordesign engineers to analyze their design concepts duringthe early phase of the product and system developmentprocess.

This product-specific CFD design tool is very easy to usewith minimal CFD knowledge and training. In the lastfive years, this product-specific CFD design tool hasbeen gaining its popularity from the design engineers.We are able to deploy the tool to most of designengineers at Delphi Thermal Systems to support their

product design and development. According to therecent record, this CFD tool is the integral part of PDPand has been used for all the production programs aDelphi Thermal System.

One of the biggest advantages for this CFD design toois able to provide design engineers a tool to evaluatetheir designs in the early phase of the productdevelopment cycle. It significantly reduces the need fodesign changes in the late stage of the process and leadto shorten the product development cycle and makesubstantial cost savings. Besides reducing the designcycle, the CFD design tool has also reduced theprototype “build and test” expenses for eachdevelopment project.

CFD DESIGN TOOL

Delphi CFD design tool was built on Flowizard fromFluent, Inc. A customized interface was created usingthe GUI customization functionality available inFlowizard, as shown in Figure 1. The customizationincorporated “best practices” for CFD modeling fromCFD experts and standard product design processesfrom design engineers for each product. The design tooguides the design engineer step-by-step through theprocess of analyzing a product or part through anexamination of airflow and thermal performance.

Figure 1 Delphi CFD Design Tool

2008-01-0238

CFD Design Tool Improves HVAC Design and Cuts Product

Development Cycle Time

Meisen Li and LinJie Huang

Delphi Thermal Systems

Copyright © 2008 SAE International

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Product-Specific Customization

Since each product has its own “best practices” for CFDmodeling and standard design procedure, thecustomization was created for each product or part, asshown in Figure 2. This made it possible for designengineers to input the geometry from their latest conceptdesign directly, enter information specific to the design,and do flow and thermal simulations in a very shortperiod of time. The interface limits the entry of designparameters to the range for which the model has beenvalidated, for ensuring its accuracy. To date, interfaceshave been created for design engineers to set up anHVAC system, Power-Train-Cooling system, or heatexchanger for analysis in a short time.

Figure 2 Menu for product selection

The customized interface walks the design engineersthrough the entire product-specific CFD process. Afterthe CAD geometry is imported, the CFD design toolautomatically performs the clean-up operation to thegeometry required for efficient CFD analysis. The design

engineer is then asked to graphically identify keycomponents of the geometry along with thecorresponding boundary conditions, as shown in Figure3. Depending on the specific type of product to beanalyzed, the mesh building routine includes guidelinesfor the type and size of the mesh in different areas of theproblem. Throughout the process, a model is createdthat takes advantage of the best practices developed byCFD experts and design guidelines developed by Delphiproduct experts.

Figure 3 “Best Practice” for each product

Accuracy and Speed

Mesh type has a strong effect on mesh count, which candirectly affect computational run-time. Two types omeshes, tetrahedral and hexahedra, are available in theCFD design tool. Tetrahedral mesh enables designengineers to mesh complex geometry automatically, buwith relatively larger mesh count. Hexahedra mesh withsame mesh size can significantly reduce the mesh countbut may fail to mesh most of complex geometries. In ouproduct-specific CFD processes, hexahedra mesh isused wherever are allowed. For instance, the meshcount for a typical HVAC module is about 1.5 millions fortetrahedral only meshes but 1 million with using ouHVAC CFD meshing process.

In general, smaller mesh size will take longer run-timewith higher accuracy. In the design concept phase (earlystage) of HVAC module development process, in ordeto find out main design stream, the requirement foanalysis speed has higher priority than predictionaccuracy and is critical to the process. In thedevelopment phase (later stage of the process)

prediction accuracy is getting more important for definedesign. To meet those needs, design engineers canselect a different meshing level from the CFD designtool. For instance, with the priority on speed (largemesh size), ½ million meshes are typical for HVACmodule and the entire solution process would only behours. With priority on accuracy (smaller mesh size), themesh count for above case would rise to about 1 millionwith run-time of about a day on typical window desktopTo ensure the accuracy of the CFD analysis, higheorder of scheme is implemented in the tool.

Component model and data base

One of the difficulties for the design engineer to performCFD analysis is the selection of component models andthe corresponding performance data base. For instancelet’s look at heat exchanger modeling. How to modeevaporator airside pressure drop and thermaperformance? What type of CFD model should beemployed? What are the specific performance maps fothat evaporator?

Figure 4 Menus for component data base

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To make those processes seamlessly for the HVACdesign engineer, the component models for eachproduct in the tool are developed by component andCFD experts, and cataloged in the tool data base forselection, as shown in Figure 4.

CFD Validation

Understanding the accuracy and limitation of your CFDanalysis is critical to its success. To ensure the accuracyof a given product-specific CFD process, the CFDanalyses have to be validated with numerous standardtests. Following is some of the validation results forHVAC module CFD analysis:

1. One of the key specifications for dual-zone HVACsystem is left-to-right airflow balance. In Figure 5,the CFD prediction of left-to-right airflow balance iscompared with prototype performance test. Goodagreement has been found.

Figure 5 Validation I – Airflow balance on left and right

2. In Figure 6, the airflow rate at 9V of blower electronicvoltage for a given dual-zone HVAC system hasbeen analyzed and compared with prototypeperformance test. In the analysis, the fan/scroll wasmodeled using a fan-curve and was integrated withrest of HVAC system model. Airflow rate of theHVAC system for eleven (11) temperature valvepositions, from full cold to full hot, were predicted.The predicted airflow rates for all eleven temperaturevalve positions were compared with test result. Thecomparisons were acceptable.

Figure 6 Air flow with different temperature valveposition at 9V of blower electronic voltage

CASE STUDY: HVAC MODULE DESIGN

One of the basic functions of a HVAC module is todeliver conditioned air with desired flow rate andtemperature to specific location in a vehicle compartmento meet occupant thermal comfort. One of the keyspecifications to meet occupant thermal comfort is HVACtemperature linearity performance during the temperingmode. In the past, the development of HVACtemperature linearity performance was conductedthrough tests with hardware prototypes in a HVAC teststand. In general, hundreds of test points were requiredfor HVAC temperature linearity development. With theadvancements in CFD modeling technology, auto-mashers, and workstations speed, we are able tointegrate CFD design tool into HVAC developmenprocess. During the early stage of HVAC developmenprocess, CFD design tool has been employed toevaluate design concepts. During the HVACdevelopment stage, CFD design tool also allows designengineers to minimize airflow pressure drop, improvestemperature mixing in varies modes and temperaturedoor positions for a given HVAC design. Since the

analysis can be performed before the prototype isavailable, it significantly shortens the HVAC developmencycle and reduces the prototype cost. In this section, wewill illustrate how to employ the CFD design tool foHVAC temperature linearity development. In Figure 7the HVAC module model used for the case study isshown.

Figure 7 CFD model for the HVAC case study

Design optimization

During the HVAC module linearity development processdesign engineers are often required to evaluate theidesign concepts, compare different design proposalsand quantify the effects on performance for the differentdesign variables within a short period of time beforehaving prototypes. The CFD design tool provides designengineers with a tool to virtually “test” those designconcepts within a short time before hardware verificationTo illustrate how design engineers use the CFD designtool to support their development during the producdevelopment process, let’s use HVAC as an example. In

DUAL-ZONE MODULEAIRFLOW BALANCE

EXPERIMENT vs CFD ANALYSIS

0.0%

20.0%

40.0%

60.0%

80.0%

100.0%

DRV-SIDE PSS-SIDE

A I R F L O W B

A L A N C E , %

TEST

CFD

DUAL-ZONE Module Airflow @9V

Test Data vs Fan Model-CFD

0

50

100

150

200

250

0% 20% 40% 60% 80% 100%

Temp Valve Opening

A i r f l o w ,

C F M

TestFan Curve-CFD

Center-VentSide-Vent

Rear-Vent

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Figure 7 geometry of a typical HVAC module is shown,where the inlet geometry from the blower and outletgeometry to the ducts have been simplified to reduce themodeling complexity and computational run-time. Asone of the temperature linearity specification, minimumtemperature difference between center-vent and side-vent outlets (temperature spread), as well as minimumtemperature differences between center-vent and rear-vent outlets are required. To develop this specifictemperature linearity requirement, the width of

temperature valve seat plays an important role. In Figure8, typical temperature valve seat geometry is shown. Ingeneral, larger temperature valve seat tends to reducethe temperature differences between rear-vent andcenter-vent outlets, but will have the negative effect ontemperature spread and HVAC airflow pressure drop. Tooptimize the valve seat dimension for this HVAC moduledesign concept, design engineers can employ CFDdesign tool to quantitatively compare the effect of valveseat width on both HVAC module linearity performanceand air flow pressure drop.

Figure 8. Geometry of temperature door valve seatfor a typical HVAC module

In Figure 9, the temperature distributions at center-vent,side-vent, and rear-vent outlet with three different valveseat widths are shown. As the valve seat widthincreases, the temperatures of center-vent outletdecrease while the temperature of rear-vent outletincreases which lead to the temperature differencebetween center-vent and rear-vent outlets decreases.On the other hand, as the valve seat width increases, thetemperature difference between center-vent and side-vent increases.

In Figure 10, the air flow velocity distribution around thevalve seat in the HVAC module is shown. With 20 mmvalve seat width, the velocity magnitude is high near theseat. In general, larger valve seat improves the airmixing in the module but also lead to increase air sidepressure drop and could potentially increase air-bornenoise level in the module.

In Figure 11, the air temperature distribution near thevalve seat is shown. Due to the valve seat blockage, thehot air zones behind the valve seat increases as thevalve seat width increases. As the results, the

W = 6 mm

W = 10 mm

W = 15 mm

W = 20 mm

Figure 9 Effect of valve seat dimension ontemperature distributions of HVAC modules

Valve seatValve seat

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temperature difference between the center-vent andside-vent outlets increases as valve seat widthincreasing.

Figure 12 Effect of valve seat width on HVACmodule outlet temperature difference

The effects of valve seat width on the outlet temperaturedifferences are summarized in Figure 12. In the figure,the x coordinate is the valve seat width in mm and the ycoordinate is the outlet temperature differences. Theresults show that the temperature difference betweencenter-Vent and Rear-Vent outlets (blue line) decreasesas the valve seat width increases, but much effectivewith valve seat width less than 10 mm. On the other

hand, the valve seat width has linearly effect on thetemperature difference between center-Vent and Side-

Vent outlets. Based on the CFD analysis, 10 mm valveseat width will provide the best compromise.

The effect of the valve seat width on HVAC module airpressure drop is shown in Figure 13. The result showsthat valve seat width has minimum effect on HVACmodule airflow pressure drop.

W= 6 mm

W = 20 mm

Figure 10 Air flow velocity distributionsnear the valve seat

W= 6 mm

W = 20 mm

Figure 11 Temperature distributionsnear the valve seat

0

100

200

300

400

0 5 10 15 20 25

Valve seat width (mm)

D p ( P a )

Figure 13 Effect of valve seat width on HVAC

module air flow pressure drop

Valve seat

Valve seat

0 5 10 15 20 25

Valve seat width, mm

T e m p D i f f e r e n c e , C

CenterVent - RearVent

CenterVent-SideVent

Valve seat

Valve seat

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CONCLUSION

In this paper, Delphi CFD design tool has beenintroduced. Since the tool is aimed for non-CFDspecialists with product-specific customization and built-in component data base, we are able to integrate it intoour product development process with great success.

Using the HVAC case study, the benefit of this CFDdesign tool in the HVAC product development processhas been presented. With the CFD design tool andstandard CFD process, design engineers can mastercertain CFD tasks to support their product developmentprocess and significantly shorten the productdevelopment cycle with substantial cost saving benefit.

REFERENCES

1. M. Cartwright, L. Huang, “HVAC system Design andOptimization Utilizing Computational Fluid

Dynamics”, 97VTMS-101, VTMS III, April 1997.

2. L. Huang and T. Han, “Validation of 3-D PassengerCompartment Hot Soak and Cool-Down Analysis forVirtual Thermal Comfort Engineering”, SAE Paper2002-01-1304, Mar. 2002.

3. A. Farag and L. Huang, “CFD Analysis andValidation of Automotive Windshield De-Icing

Simulation”, SAE Paper 2003-01-1079, Mar. 2002.4. L. Huang, “Application Specific CFD Tool for Design

Engineers”, Keynote Speech at Fluent AutomotiveCFD Summit 2002, June 2002.

5. L. Huang, “CFD Technology in Delphi: Moving into21

stCentury”, Keynote Speech at Fluent Automotive

CFD Summit 2004, June 2004.6. L. Huang, “Delphi Cut Cycle Time Automatically”

Automotive Engineering International, July 2005.7. Fluent Inc., “Introduction to Renormalization Group

Method and Turbulence Modeling,” Fluent Technica

Memorandum , TM-107, Jan. 1993.

CONTACT

Dr. Meisen LiDelphi Thermal & Interior200 Upper Mountain RoadLockport, NY, [email protected]

Dr. LinJie HuangDelphi Thermal & Interior200 Upper Mountain Road

Lockport, NY, [email protected]

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