Measurement and Analysis of Underhood Ventilation Air Flow and Temperatures for an OffRoad Machine.pdf

Measurement and Analysis of UnderhoodVentilation Air Flow and Temperatures for an Off-Road Machine

Tanju Sofu and Fon-Chieh Chang, Argonne National Laboratory

Ron Dupree and Srinivas Malipeddi, Caterpillar, Inc.

Sudhindra Uppuluri and Steven Shapiro, Flowmaster USA, Inc.

Abstract

To gain insight into the ventilation needs for an enclosed engine compartmentof an off-road machine, a prototypical test-rig that includes an engine andother installation hardware was built. Well controlled experiments were con-ducted to help understand the effects of ventilation air flow on heat rejectionand component temperatures. An assessment of 1-D and 3-D simulationmethods was performed to predict underhood ventilation air flow and compo-nent temperatures using the experimental data. The analytical work involveddevelopment, validation, and application of these methods for optimized ven-tilation air flow rate in the test-rig. A 1-D thermal-fluid network model wasdeveloped to account for overall energy balance and to simulate ventilation andhydraulic system response. This model was combined with a 3-D CFD modelfor the ventilation air circulation in the test rig to determine the flow patternsand the distributed surface heat transfer. The tests conducted at Caterpillarand the complementary analyses performed at Argonne provide an opportu-nity to understand the isolated effect of ventilation air cooling on underhoodthermal management.

Introduction

Construction equipment and other types of heavy vehicles have common un-derhood thermal management challenges: restrictive enclosures and ever-increasing variety of heat sources. But off-road machines have rather uniqueadditional underhood thermal management issues such as

high auxiliary loads, severe operating conditions involving dust and debris, wide range of altitudes and temperatures,

374 T. Sofu et al.

lack of ram air, and increasingly restrictive sound regulations.

In addition to the coolingsystem design, the thermal man-agement challenge for a systemwith separate engine enclosure(as shown in Fig.1) is to main-tain acceptable underhood com-ponent temperatures in a rela-tively well sealed enclosure withlimited ventilation. The specificissues for underhood tempera-ture control are the ventilationair flow requirements and theeffect of ventilation on thermalbalance (e.g., cooling system de-sign). Typical underhood tem-peratures in a separated enginecompartment vary from 50C to 200C.

Since high underhood temperatures can reduce component durability andlife, the assessment of component temperatures is an important element of adesign cycle. These assessments are typically made during a conventionalcooling test. However, the measurement of large number of component tem-peratures for various configurations is not always feasible. Furthermore, thecooling test typically occurs during the later stages of the development cyclewhen major component relocation is not practical. Therefore, an analytical ca-pability to help understand the thermal conditions inside the separated enginecompartment is desirable for identification of possible hot-spots and assuranceof adequate air cooling.

To address these issues, a Cooperative Research and Development Agree-ment (CRADA) has been executed between Argonne National Laboratory andCaterpillar, Inc. for measurement and analysis of underhood ventilation airflow and temperatures. The experimental effort by the Caterpillar team has fo-cused on building a prototypical test-rig for an off-road machine engine, andconducting tests with controlled ventilation air flow rates from various inlet lo-cations to estimate the ventilation needs in an enclosed engine compart-ment[1]. The purpose of the analytical studies by the Argonne team (withmodeling support from Flowmaster USA) has been the assessment of varioussimulation methods that could be used in predicting underhood ventilation airflow and temperatures. The work involved development and validation ofcombined 1-D and 3-D simulation models of the Caterpillar test-rig for opti-mized ventilation air flow rate. Although the separated cooling system com-partments are unique to off-road machines, the Caterpillar tests and the com-plementary analyses provide an opportunity to understand the isolated effect ofair cooling on the engine performance for a wide range of heavy-vehicles.

Service Wall

Fig.1. Schematic of an off-road machine withseparated engine and cooling system compart-ments divided with service wall.

Measurement and Analysis of Underhood Ventilation Air Flow 375

A U.S. EPA Tier II emis-sions level engine (Fig 2)was installed into amockup representing atypical medium size off-highway machine with afull engine enclosure sepa-rated from the cooling fanby a solid wall [1]. The en-closure was constructedfrom sheet metal andtightly sealed at all seams,but was not insulated. TheCAD model shown inFig.3 provides a perspectiveon the enclosure and in-let/outlet locations with respect to engine components. Consistent with a typi-cal off-highway machine with this size engine, the enclosure dimensions were100x140x140-cm

Experimental Study

3. The 30x30-cm2 inlet opening in front of the crank shaftwas used to supply ventilation air into the enclosure. A 30-cm diameter open-ing at the top was connected to a variable capacity blower to draw air from theenclosure, and the total flow rate throughout the enclosure was measured.

Since the highest underhood tem-peratures are expected to occur at thehighest engine loads, the engine wasmaintained at its rated speed andpower throughout the testing. In ad-dition, the test cell temperature waskept constant at 25C. Engine coolantand intake manifold temperatureswere maintained by laboratory heatexchangers and instrumented to con-trol the heat rejection closely.

Air and surface temperatures atvarious locations in the enclosure weremonitored. The other critical enginerelated temperaturessuch as coolant,oil, fuel, exhaust and intake manifoldtemperatureswere also measured inreal time. The total energy balance(energy in fuel vs. shaft work and heatrejection to coolant, aftercooler, ventilation, and energy in stack) was calcu-lated for each data point. All measurements were recorded after temperatures

Fig 2. Engine setup and enclosure frame without walls.

Fig.3. CAD model of engine and itscomponents relative to inlet/outlet loca-tions front view

Outlet

Front

Inlet

In

ta

ke S

id

e In

let

Ex

ha

ust S

id

e In

let

376 T. Sofu et al.

were stabilized. To allow the data to be scaled for different engine compart-ment configurations, the ventilation air flow rate was normalized with respectto the engine combustion air flow rate. This ratio of the ventilation air flowrate to the engine combustion air flow rate was also used as the basis of com-parisons with analytical results. The airflow ratio varied between 0.5 and 3.75.

Analytical Studies

Computer simulations can improve the understanding of interactions betweenthe engine subsystems[2]. The main purpose of this study has been an assess-ment of simulation methods that could be used in predicting underhood ven-tilation air flow field and temperatures for an off-road machine. The work in-volved development and validation of combined 1-D and 3-D simulationmodels of the Caterpillar test-rig. A 1-D thermal-fluid network model was de-veloped to account for overall energy balance and simulate cooling system re-sponse using the commercial software Flowmaster[3]. A 3-D underhoodmodel of the complex test rig was built using the commercial CFD softwareStar-CD[4] to determine the flow paths for the ventilation air system and thesurface heat transfer coefficient.

3-D CFD Analysis

Starting with a CAD model of the test rig, an unstructured hexahedral meshwas generated using Star-CDs underhood expert system module ES-Uhood.First the IGES surface definitions were extracted from the CAD model, andthen the ProSurf utility was used to generate a triangulated surface mesh.Starting from this mesh, surface fixing functions were used to merge the over-lapping surfaces, fill the open holes, generate feature lines, and create a newwrapped surface which captures the details of computational domainboundaries in 8 mm resolution (Fig.4a). This wrapped surface formed the basisof an extrusion layer through which the suitability of turbulence wall functionis assured. Although the flow is expected to separate over the complex enginegeometry, the inherent assumption of attached flow is made through the use oflogarithmic wall function since the integration to the wall is computationallyprohibitive. After filling the computational domain with regular brick cellswith gradual mesh refinement near the engine and enclosure surfaces, the vol-ume mesh was completed by cutting those hexahedral cells that intersect theextrusion layer (Fig.4b).

In order to capture the ventilation air flow distribution at the enclosure in-let accurately, a large inlet plenum (not shown in Fig.4) was also included inthe model to represent ambient conditions (pressure and temperature). Thedesired flow rate through the enclosure was assured by imposing a proportionaluniform flow field at the plenum inlet as the boundary condition. The enclo-sure outlet pipe was considered much longer than what is shown in Fig.4 and


its top end was treated as a standard outlet boundary. The final CFD modelconsists of 1.34 million fluid cells, with a 3 mm thick extrusion layer sur-rounding the engine and enclosure surfaces to give a maximum y+ value of 200for airflow ratio of 1.5.

Fig.4. CFD mesh of the test rig (a) cutaway view of the surface mesh (b) a cross section of thevolume mesh.

The ventilation air flow field in the test rig and the convective heat transfercoefficient for the solid surfaces were obtained using the commercial CFDsoftware Star-CD. An initial parametric study for inclusion of the buoyancyforce in the thermal-fluid calculations revealed that the effect of density varia-tions on the overall flow and temperature fields is negligible. Thus, the venti-lation air flow field was simulated as a steady incompressible flow with energyequation using the high-Re number k-epsilon turbulence model with loga-rithmic wall functions.

As the most basic two-equation model, k-epsilon model is believed to pro-vide a reasonable approximation of the time-averaged flow distribution overthe surface of the engine and its components in the test rig. A set of transientcalculations were also studied to investigate temperature fluctuations observedduring the experiments and assure that the calculated flow field is steady withno oscillations. The results indicated negligible difference between the tran-sient and steady state solutions. Five different inlet locations, each for five air-flow ratios, were studied with the CFD model; however, only the results offront inlet configuration (shown in Fig.4) are discussed here. The calculationswere performed on a linux cluster.

Front

Inlet

Outlet

Outlet

378 T. Sofu et al.

1-D Network Flow Analysis

The complete thermal system analyzed with the network flow model is a col-lection of different thermal subsystems of an off-road machine engine includ-ing the air, coolant, and oil loops. The model consists of 1-D descriptions ofthese three loops combined with a lumped parameter approach to characterizethe thermal interactions between them through the engine structure as themajor conduction paths (Fig.5). This approach simplifies the complex enginesystem by discretizing it based on known heat transfer paths under steady-stateconditions; i.e., the heat generated from combustion is considered to be trans-ferred to various discrete surface points on the engine using specified conduc-tion paths. This 1-D network flow model served as a tool to analyze the inter-actions of the engine with the ventilation air, coolant, and oil loops forpredicting the complete thermal system performance.

Fig.5. 1-D network flow model of the test rig for front-inlet configuration.

Air flow paths in the 1-D model are based on 3-D simulation results. In theair loop, the entering ventilation air is considered to gain heat as it passesthrough individual surface points on the engine as shown in Fig.5. In the oil


loop, after losing heat through the oil pan, the flow splits into three separatebranches (the turbo, the cylinder head, and the engine block) before returningto the sump. In the coolant loop, the water cools the lubrication oil in the oilloop and circulates inside the engine block and the cylinder head. The radiatoris simply modeled as a source with constant flow rate and with known inlettemperature.

Interface between the 3-D CFD and 1-D Network Flow Models

Fig.6 shows the schematic of the sequential analyses with the 1-D networkflow and 3-D CFD models. The 1-D model requires flow rates and inlet tem-peratures as the boundary conditions in the air and coolant loops and oil pumpspeed in the oil loop to account for overall energy balance and predict the en-gine component temperatures. In the 3-D thermal analysis, these predictionsare prescribed as surface temperature boundary conditions for various enginecomponents and enclosure walls, and they are used to calculate ventilation airflow field and temperatures. The results of the 3-D CFD analysis are, in re-turn, provided back to the 1-D model to improve component temperaturepredictions by modifying the air flow paths and heat transfer coefficients be-tween the engine components and ventilation air. The typical values of esti-mated heat transfer coefficients between the engine components and ventila-tion air are found to vary in the range from 10 to 50 W/m2-K.

Fig.6. Schematic of combined 1-D and 3-D simulations.

Boundary Conditions:

Coolant flow rate and

inlet temperature

Oil pump speed

1-D Network Flow Model

using FLOWMASTER

(All four loops)

3-D CFD Model

using STAR-CD

(only for ventilation air

flow inside enclosure)

Output:

Surface temperatures

Air temperatures

Oil and coolant temps.

Model Improvements


Air flow rate and inlet

temperature

Output:

Ventilation air flow paths

and heat transfer rates

between engine and air


Coolant flow rate and

inlet temperature

Oil pump speed

1-D Network Flow Model

using FLOWMASTER

(All four loops)

3-D CFD Model

using STAR-CD

(only for ventilation air

flow inside enclosure)

Output:

Surface temperatures

Air temperatures

Oil and coolant temps.

Model Improvements


Air flow rate and inlet

temperature

Output:

Ventilation air flow paths

and heat transfer rates

between engine and air

380 T. Sofu et al.

Results and Validation

Energy Balance

Over the entire rangeof testing, approxi-mately 96% of thetotal fuel energy (cal-culated based on fuelconsumption) was ac-counted for. The dis-tribution of fuel en-ergy between the shaftwork and heat rejec-tion through exhaustsystem, coolant, com-pressed air aftercooler,and ventilation air isshown in Fig 7. Theventilation air flowrate was varied fromhigh flow to low flowin small steps. Thefigure indicates thatheat rejection through the ven-tilation air in the engine com-partment is only a small frac-tion of the overall energybalance. The unaccounted en-ergy in this test (about 4% oftotal energy) is attributed tothe energy convected from ex-terior of the enclosure walls.

A comparison of the meas-urements and 1-D model pre-dictions for the enclosure outletair temperature as a function ofairflow ratio is provided inFig.8. As the airflow ratio in-creases, the enclosure outlettemperature stabilizes. Thisimplies that, after reaching theinflection point at around anairflow ratio of 2.5, the enclo-sure heat rejection increases linearly with mass flow.

Fig 7: Effect of airflow ratio on different heat loads

for front inlet opening.

Fig.8: Comparison of ventilation air tempera-tures at enclosure outlet as a function of airflowratio.

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0 1.0 2.0 3.0 4.0

Airflow Ratio

Norm

alized T

em

perature

Calculated

Experimental


3-D CFD Results and System Restriction

As examples of the results obtained with the CFD model, the ventilation airflow field and temperature distributions are shown in Fig. 9 on a vertical planethrough the enclosure front inlet. The results indicate that the most significantpressure drop takes place near the inlet and outlet restrictions. Consistent withthe experimental observations, the results indicate a well mixed flow inside theenclosure with no significant difference in component temperatures for differ-ent ventilation inlet locations.

Fig.9. The calculated ventilation air flow field and temperature distributions on a vertical planethat intersects the front inlet.

The comparison of the ex-perimental and 3-D model pre-dictions for pressure dropthrough the test rig is shown inFig.10 as a function of airflowratio. The y axis is the normal-ized pressure drop for flowthrough the enclosure. A goodagreement for such system re-striction curves is the first indi-cation that CFD model capturesthe flow field accurately. Theother comparisons (air tem-peratures throughout the enclo-sure) are consistent with the ex-perimental values when accuratesurface temperatures are speci-fied as the boundary conditions. Fig.10. System restriction curve comparisons for

front inlet.

0.0

0.2

0.4

0.6

0.8

1.0

0.0 1.0 2.0 3.0

Airflow Ratio

No

rm

alized P

ressu

re D

rop

Calculated

Experimental

382 T. Sofu et al.

Air and Fluid Temperature Comparisons

The various temperatures in the 1-D model are calculated based on the enginecomponent dimensions and the heat transfer coefficients at the solid-fluid in-terfaces as input. Some physical dimensions for the internal loops of the enginewere supplied by Caterpillar and others were interpreted based on CAD data.A comparison of measured and calculated ventilation air, coolant water, andoil temperatures is shown in Fig. 11. Most of the predictions with the 1-Dnetwork model (including surface temperatures) are within 10% of the ex-perimental values. For a complex network of engine and its thermal subsystemsof coolant, oil, and ventilation air, these small discrepancies are considered arespectable degree of accuracy.

(a) Air Temperatures

0

0.2

0.4

0.6

0.8

1

Ex

hau

st S

id

e F

ro

nt

In

tak

e S

id

e F

ro

nt

Ex

hau

st S

id

e R

ear

In

tak

e S

id

e R

ear

EC

M A

rea

Fro

nt P

late A

rea

No

rm

alized

T

em

peratu

res

Experimantal

Calculated

(b)Coolant and Oil Temperatures

0

0.2

0.4

0.6

0.8

1

Water to

E

ng

in

e

Water fro

m E

ng

in

e

Oil to

C

oo

ler

Oil fro

m C

oo

ler

Oil to

B

earin

g

Oil to

S

um

p

No

rm

alized

T

em

peratu

res

Experimantal

Calculated

Fig.11. Comparison of temperatures between measured data and model predictions: (a) ventila-tion air temperatures, (b) coolant and oil temperatures.

Although the discrepancies are generally small, the attempts to resolve themare part of the overall modeling effort to provide a better description of theunderhood system. For example, based on the CFD results, the discrepancy forthe exhaust-side rear ventilation air temperature is attributed to a local recir-culation zone in that region. However, since the estimated temperature is smalland its impact on overall temperature distributions is negligible, a modificationto the network flow model for the front inlet configuration is not consideredto be essential.


Conclusions

Experiments were conducted to gain insight into the ventilation air flow needsfor an enclosed engine compartment of an off-road machine. These laboratoryexperiments were well controlled to provide good accuracy and to draw im-portant conclusions on minimum ventilation flow requirements for maintain-ing acceptable underhood temperatures. About 96% of the total fuel energywas accounted for during the test. Underhood temperatures in the areas ofconcern are found to be generally stabilized near an airflow ratio of two. Dataobtained were also used to provide boundary conditions and validation infor-mation for simulation methods.

A combined 1-D and 3-D simulation methodology was developed for op-timization of engine compartment ventilation air flow. The air flow field andthe rate of heat transfer between engine and ventilation air inside the enclosurewere determined with the 3-D CFD simulations. A 1-D network model wasbuilt by discretizing the various fluid paths and the solid metal structure in thesystem. Once the ventilation air flow paths and heat transfer coefficients weredetermined with CFD, the 1-D network model with reduced complexity wasused to simulate thermal interaction of the engine structure with the air, cool-ant, and oil flow. The results indicate that the temperatures and distributedheat rejection rates can be estimated within reasonable accuracy when 3-D and1-D models are used in combination.

Acknowledgements

This work was completed under the auspices of the U.S. Department of En-ergy Office of FreedomCAR and Vehicle Technologies. The submitted manu-script has been created by the University of Chicago as Operator of ArgonneNational Laboratory (Argonne) under Contract No. W-31-109-ENG-38with the U.S. Department of Energy.

References

[1] Srinivas R. Malipeddi, Underhood Thermal Management Guidelines,Jan 2003, Caterpillar Internal Document.

[2] C. Hughes, et.al, Heavy Duty Truck Cooling System Design Using Co-Simulation, SAE Technical Paper Series 2001-01-1707, Proceeding ofVehicle Thermal Management Systems Conference & Exhibition, Nash-ville, TN, May 14-17, 2001.

[3] D. S. Miller, Internal Flow Systems, 2nd edition, Flowmaster Interna-tional Ltd., published by BHR Group Limited, 1996.

[4] Star-CD, Version 3.150A, CD-adapco Group, Melville, NY.

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Measurement and Analysis of Underhood Ventilation Air Flow and Temperatures for an OffRoad Machine.pdf