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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,
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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.
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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
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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
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Measurement and Analysis of Underhood Ventilation Air Flow
377
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
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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
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Measurement and Analysis of Underhood Ventilation Air Flow
379
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
Boundary Conditions:
Air flow rate and inlet
temperature
Output:
Ventilation air flow paths
and heat transfer rates
between engine and air
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
Boundary Conditions:
Air flow rate and inlet
temperature
Output:
Ventilation air flow paths
and heat transfer rates
between engine and air
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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
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Measurement and Analysis of Underhood Ventilation Air Flow
381
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
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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.
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Measurement and Analysis of Underhood Ventilation Air Flow
383
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.