cfd based complete engine cooling jacket development and analysis

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SAE TECHNICAL

PAPER SERIES 2007-01-412

CFD Based Complete Engine Coolin

Jacket Development and Analys

Aditya Mulemane and Ravindra SomMahindra and Mahindra, In

Powertrain & Fluid SysteConference & Exhibiti

Rosemont, IllinOctober 29-November 1, 20

Downloaded from SAE International by Automotive Research Association of India, Friday, August 01, 2014



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1

ABSTRACTThis paper discusses application ofComputational Fluid Dynamics (CFD) in thedevelopment and evaluation of a dieselengine cooling system.

Commercial CFD codes are effective indeveloping and analyzing engine coolingsystems including the complex cooling

jacket.

A complete cooling circuit model developedbased on 1D - 3D coupling is discussed.This approach is both cost and timeeffective. The coupled model enables theprediction of realistic flow rates through thecooling jacket outlets. Cooling jacket systemresistance is also determined.

The paper discusses various approaches forconducting heat transfer and thermalanalysis of engine crankcase and head.Boundary conditions for the thermal analysisare obtained from in-cylinder CFDsimulations for diesel spray and combustion.The phenomenon of nucleate boiling, itsmathematical modeling and its effect onheat transfer is discussed. It is observed thatat high operating temperatures, nucleateboiling occurs in regions around the exhaustport.

The simulations are validatedexperimentally. Good correlations areobserved with test results.

INTRODUCTIONBasic function of an engine cooling jacket isto dissipate that portion of heat generated inthe combustion chamber which is absorbedby the engine components like piston,cylinder liners, piston rings, cylinder head

and block. A well designed cooling systemmaintains coolant temperature in a welldefined range and ensures propercombustion, minimized blow-by and allowsthe engine lubricating system to functionproperly [1].

It is difficult to investigate the completebehavior of cooling jackets experimentallydue to complex geometries and flow paths.In the past some investigators and enginedevelopers have used engine cooling

jackets fitted with transparent acrylic boardsfor flow visualization [2]. However, thisapproach offers limited and localized views.

Computational Fluid Dynamics (CFD) hasmade rapid advances over the years and isnow used as an effective tool in the analysis

and visualization of fluid flows in complexsystems including the engine coolingjackets. Along with visualizing the flowdevelopment in the jacket passages, CFDtechniques are also applied in the predictionand estimation of temperature distributionover the entire engine block. It also helpsstudy and understand complex phenomenonthat commonly occur in cooling jackets likecavitation and nucleate boiling.

A large amount of time and effort is investedby engineers in trouble-shooting andoptimizing cooling jacket designs. CFDhelps in the development of the cooling

jacket by reducing the time and number ofprototypes involved.

This paper discusses a few concpets thatcan be used to interpret the behavior of thecooling jackets. The various approaches thatcan be used for engine block temperatureprediction based on CFD techniques are

2007-01-4129

CFD Based Complete Engine Cooling JacketDevelopment and Analysis

Aditya Mulemane and Ravindra SomanMahindra and Mahindra, India

Copyright 2007 SAE International




2

also presented. The discussions are basedon the development of a four cylinder enginecooling jacket. Commercial CFD codesIdeas-ESC and Fluent are used.

Cooling Jacket Design

Engine specifications along with engineblock shape and operating range determinethe cooling jacket layout.

A cooling jacket is not only intended to takeaway heat from the engine, but it shouldalso help the engine attain the optimaloperating temperature at the earliest [3].

The three components of a cooling jacketare the cylinder head, gasket and thecylinder crankcase. Figure 1 shows thecooling jacket for a four cylinder engine.

Figure 1: Engine cooling jacket geometry with theimportant components [2]

The cylinder head encounters very hightemperatures due to direct exposure to hotcombustion gases. Efficient cooling of theexhaust ports and the exhaust valve seatregions is required. Efficient cooling reduceswear and improves part durability. Also, the

injector nozzles need to be cooled efficientlyto improve the nozzle life and reduce needlewear and sticking, which other wise maylead to deterioration of engine performance.The jacket in the cylinder crankcase isresponsible for heat transfer from thecylinder liners and even distribution of flowto the head. Between the cylinder head andblock lies the engine gasket. The gasketconsists of small holes that act as conduits

between the block and head. The gasketholes are the most important features in theengine cooling jacket as they govern theflow and distribution of the coolant from theblock to the head.

Design ObjectivesIn order to achieve better engine cooling afew design guidelines need to be followed tomaintain the desired trends. Someguidelines are mentioned below.

Strong flow in the longitudinal direction(along the engines length) on the intakeside in the crankcase, along with a strongtransverse flow from the intake side to theexhaust side is desired.

Flow should be evenly distributed around allthe engine cylinders. A well distributed flow

means efficient heat transfer from theheated engine surfaces.

Flow stagnation needs to be avoided, sincestagnant regions lead to hot spots andeventually boiling.

Very high flow velocities and drasticpressure reductions lead to cavitation andsuch conditions should be avoided.Cavitation leads to material erosion.

The coolant pump located at the cooling

jacket inlet is responsible for maintaining aspecified pressure at the inlet. It is desired tohave a pressure drop as low as possible.The gasket-hole dimensions and theintricate geometry of the cooling jackettogether contribute to the pressure drop.

FLOW VISUALIZATION AND ANALYSISThe advantage of using CFD tools in cooling

jacket analysis is the ease with which theflow through the complex geometries can bevisualized. This section describes a fewvisual analyses that can be performed to

evaluate cooling jackets. The resultspresented are of a CFD optimizations studyon a 4 cylinder direct injection diesel enginewith a rated power of 100 hp. The coolantpump is designed considering 55% heatrejection to the cooling system. In optimizingthe cooling jacket of this engine, 14 majormodifications driven by CFD were madebefore the first prototype was built.




3

The computational domain for the CFDsimulation is developed by using tetrahedralcells and an appropriate boundary layermesh to account for the boundary layer flowwhich is critical in such cases. The solutionis at steady state with a simple k-

turbulence model.

Velocity VisualizationVelocity plots are good indicators of flowdevelopment and distribution. Figure 2shows the flow velocities on sectionsthrough the crankcase and the cylinderhead.

Figure 2: Flow distribution and development incooling jacket as velocity magnitude

Figure 2 helps establish the flow patterns. Alongitudinal flow pattern is observed. Thetransverse flow trend is also visible. The flowis observed to get around the first cylinder

transversely and reach the exhaust side inthe crankcase. Figure 2 (b) shows the flowdevelopment in the cylinder head. Asdesired the flow is directed using the gasketholes so that majority of it reaches theexhaust side which requires more cooling.The spots showing up in dark correspond tohigh coolant flow velocities as the coolantflows through the gasket holes.

Flow Path-linesAnother effective way of visualizing the flowdevelopment is using flow path-lines. Flowpath-lines help in visualizing flowdevelopment and verifying if all regions aresupplied with coolant. These also helpvisualize occurrence of re-circulation zones.Figure 3 shows the flow path-lines throughthe cooling jacket.

Figure 3: Path-lines showing flow development throughthe jacket and recirculation zones

Pressure VisualizationPressure is another indicator of the flowdistribution in the cooling jacket. Pressurecontours indicate the regions that contributeto increase in pressure drop. Also, lowerpressure indicates regions with tendency tocavitate. Figure 4 shows some pressureplots.

Figure 4: Pressure distribution in cooling jacket:(a) Crankcase and (b) cylinder head

Apart from pressure contour plots, thepressure drop of the coolant jacket can bedetermined. The pressure drop of thecooling jacket can also be used as a 1-Dinput in coupled models of the completecooling circuit. Figure 5 shows the system

resistance curve of the cooling jacket for theengine under discussion.

A lower pressure drop across the coolingjacket along with proper flow distribution isdesirable for better performance.




4

Figure 5: System resistance curve derived for thecooling jacket based on CFD study

Some of the key design features that wereimpacted by the CFD predictions are thenumber, size and layout of the gasket holes.

Figure 6 shows velocity contour plotsthrough some sections of the engine cooling

jacket for the first and the final iteration. Theimprovement in the flow is easily seen.

Figure 6: Velocity contours in critical sections comparedbetween the first and the final iteration

Cross drills are provided in the crankcase toconduct coolant flow from crankcase tocylinder head with desired distribution ofcoolant on the intake and the exhaust side inthe head. Figure 7 shows the flowdevelopment in a cross drill.

Based on similar optimization of othergasket holes a flow distribution of 80% on

exhaust side and 20% on intake side wasachieved in the cylinder head through thegasket holes.

Figure 7: Flow development in crankcase cross drill

COUPLED MODEL ANALYSISThe results presented so far are for 3D CFDmodels. In cases where only the cooling

jacket is the computational domain, the

conditions applied at the various coolingjacket outlets are approximations, based onexperience and knowledge of auxiliarydevices. However, conditions at the cooling

jacket outlets may be entirely different underactual engine operating conditions withvarious other components connected in thecooling circuit.

The performance of the cooling circuit interms of pressure drops is evaluated on astatic test bed. Tests are carried out undervarying pump operating speeds. Thepressure drop values recorded at variouslocations are useful in understanding thebehavior of the system. The simulationresults are correlated with the experimentalresults.

To capture effects of back-pressures actingduring actual engine operation and tocorrelate it with the engine test results, theCFD study of the cooling jacket is extendedto complete cooling circuit based on acoupling between 1D lumped model of theauxiliary components and the actual 3DCFD model for the pump and cooling jacket.

This is represented schematically in Figure8. The thermostat is modeled as fully open.The Multiple Reference Frame (MRF)method is used to model the coolant pump.This replicates the actual test setup. Thecomputational domain can be seen in Figure9.




5

Figure 8: Schematic of the engine cooling circuit.

Figure 9: Computational domain replicating the testdomain

Results and DiscussionsThe velocity vectors of the flow through thecoolant pump are shown in Figure 10.

The pressure drop data of the cooling circuitaccessories like the oil cooler, and theradiator obtained experimentally are shownin Figure 11. These are used in the 1Dmodels of the coupled system.

Figure 10: Velocity vectors of flow in Coolant pump

SYSTEM RESISTANCE CURVE FOR COOLING CIRCUIT COMPONENTS

RADIATOR

OIL - COOLER

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120 140 160

DISCHARGE (LPM)

PRESS

UREDROP(kPa)

Figure 11: Experimental system resistance curve forcooling circuit accessories.

Figure 12 compares the normalized flow-rates through major cooling jacketboundaries under open and closed loopconditions. The observed difference in flowrates is due to the more realistic

representation of the system in case of theclosed loop. This demonstrates the effect ofthe complete circuit on flow rates at theoutlets of the jacket and hence the need tosimulate the entire circuit.

NORMALIZED FLOW RATE COMPARISON

0.9

0.91

0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1

NO

RMALIZEDFLOWR

ATE

JACKET INLET OIL-COOLER OUTLET THERMOSTAT OUTLET

Figure 12: Normalized flow rate comparison betweenopen and closed loop simulation

The pump performance curve obtained bysimulating flow in the coolant pump by theMRF approach is shown in Figure 13.

COOLANT PUMP PERFORMANCE AT 3900 RPM (ENGINE RPM 3000)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

80 100 120 140 160 180 200 220

DISCHARGE (LPM)

PRESSUREDROP(ba

r)

Figure 13: Pump performance curve derived fromsimulation




6

Correlation ResultsThe simulation results are compared withthe engine cooling circuit test results. Thetypical locations of pressure measurementare Pump inlet, Pump outlet, Thermostathousing, Circuit top hose, top hose, cylinder

block and cylinder head. The thermostat ismaintained in a fully open condition.

Pressure drop values are measured in theCFD model at locations similar to the testsetup. Some of the pressure drop values arecompared. Figures 14 to 16 show somecomparisons between the test and thesimulation results.

PRESSURE DROP BETWEEN BLOCK AND HEAD

0

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120 140 160

TOP HOSE FLOW (LPM)

PRESSUREDROP(mbar)

TEST RESULTS

SIMULATION

RESULTS

Figure 14: Comparing simulation and test results forpressure drop between block and head.

PRESSURE DROP BETWEEN WATER PUMP-OUT AND THERMOSTAT

0

200

400

600

800

1000

1200

1400

1600

0 20 40 60 80 100 120 140 160

TOP HOSE FLOW (LPM)

PRESSUREDROP(mbar)

TEST RESULTS

SIMULATION

RESULTS

Figure 15: Comparing simulation and test results forpressure drop between pump out and thermostat

From the above figures, it is seen that thesimulation results correlate well with the testresults. However, some deviation isobserved. Primarily, it is observed that in allcases, the simulated pressure drops arelower than the experimental values. Due tosmooth surfaces, flow resistance is lower insimulation and hence the predicted velocityis higher. Since pressure head is inverselyproportional to velocity head, predicted

pressure values are lower leading to lowerpressure drop values.

PRESSURE DROP BETWEEN TOP AND BOTTOM HOSE

0

40

80

120

160

200

0 20 40 60 80 100 120 140 160

TOP HOSE FLOW (LPM)

PRESSUREDROP

(mbar)

TEST RESULTS

SIMULATION

RESULTS

Figure16: Comparing simulation and test results forpressure drop between top and bottom hose

The pump performance along with thecooling jacket pressure drop data obtainedby CFD can be directly used in a 1D modelof the cooling system to determine flowthrough the cooling system at any engineRPM.

THERMAL ANALYSIS OF COOLINGJACKETSApart from flow analysis, thermal analysis isimportant to predict the temperaturedistribution and the structural durability ofthe engine block. 3 approaches to study theheat transfer in cooling jackets arediscussed. These approaches, along withtheir advantages and limitations aresummarized in Table 1. Inputs required are

derived from test results or from in-cylindercombustion and under-hood thermalanalysis.

APPROACH 1 APPROACH 2 APPROACH 3

Dirchlet

Conditions

Planar

Conduction

Conjugate

Heat Transfer

INPUTS1.Surface

Temperatures

1. Surface

Temperatures

2. Underhood

HTC and T

1. Surface

heat flux

2. Underhood

HTC and T

OUTPUTS

1. HTC

2. Fluid

temperatures

1. HTC

2. Fluid

temperatures

3. Solid

temperatures

1. Solid

temperatures

ADVANTAGES1. Simple to

setup

1. Simpler

than #3

2. Lesser

iterations

than #1

1. No iterations

2. Highly

accurate

LIMITATIONS

1.Highly

iterative

2. Lower

accuracy

1. Greater

assumptions

2. Lower

accuracy

1. Physical

limits for

solving

Table 1: Approaches for solving heat transferin cooling jackets




7

Approach 1In some cases, it may not be possible tomodel and solve the complete engine blockalong with the cooling jacket due to thecomplexity. In such cases the thermalproblem can be solved by applying the

Dirichlet boundary conditions along with theflow properties of the cooling jacket.

Surface temperatures on the solid sidealong with the flow rates of the coolant arethe only boundary conditions. The output isin the form of Heat Transfer Coefficients(HTCs) and the fluid temperatures.

Though this approach is easier to setup andquicker to solve, the determination of thetemperature at the interface between thecoolant and the cooling jacket walls isiterative between a structural code (used for

solving thermal durability) and the CFDcode. Few iterative loops of simulation areneeded before the thermal data converges.

Approach 2Planar conduction conditions can be appliedon the engine walls by assuming anapproximate thickness. This approachmakes the heat transfer solution morerealistic, at the same time maintaining themodel simplicity. The temperaturedistribution on the wall along with the flowproperties are the inputs. The outputs are

similar to that of approach 1.

This approach needs lesser number ofiterative loops compared to approach 1.Figure 17 shows the sample temperaturedistribution on the fluid layer immediatelynext to the wall and on the wall, predictedusing this approach. The HTCs obtained areshown in Figure 18. The distributions shownare for a single cylinder.

Figure 17: Temperature (deg K) distribution on the fluidlayer and the wall in the cooling jacket block

Figure 18: Heat Transfer Coefficients(W/m2-k) basedon wall functions

Approach 3In the third approach, the problem is treatedas a case of conjugate heat transfer. It is thebest method and is highly accurate. Sinceboth flow and heat transfer are solved

simultaneously modeling and meshing of theentire engine block required. This makes theapproach tedious and time consuming.

There are no iterative loops involved in thisapproach and the actual thermal conditionsare applied. The HTCs and the airtemperatures external to the engine blockare obtained from the under-hood thermalanalysis. Temperatures and HTCs of thefluid layer adjacent to the liner are appliedas boundary conditions. These are obtainedfrom in-cylinder simulations. Figure 19shows the temperature distribution obtainedby the conjugate heat transfer approachapplied to a cylinder head.

Figure 19: Temperature distribution obtained by

Conjugate heat transfer on a cylinder head

NUCLEATE BOILINGAs we discuss thermal issues in enginecooling jackets, an important aspect to beaddressed is the occurrence of coolantvapors due to localized boiling of the coolantin very high temperature zones. Thisphenomenon is referred to as Nucleateboiling.




8

When heat stressed metal surfaces exceedthe thermal capacity of the coolant material,the coolant begins to boil forming a vaporlayer at the surface. This vapor layer acts asinsulation and prevents efficient heat

transfer from the hot engine surfaces to thecoolant. This causes localized over-heatingand coolant vaporization. This isrepresented schematically in Figure 20.

Figure 20: Schematic representation of occurrence ofNucleate Boiling [4]

Boiling ModelModels are available to predict nucleationand boiling in commercial CFD codes.

Numerical simulation of boiling is a case ofmultiphase flow. Multiphase models likeDiscrete phase model (DPM), Mixturemodel, Volume of Fluid model and EulerianMultiphase Flow model are available. Thesub-cooled nucleate boiling model based onthe Mixture model is used in the study. This

model doesnt consider phase separation.The velocity of the vapor phase is equal tothat of the bulk fluid. This makes the modelapplicable to convective flows, similar to theone encountered in engine cooling jackets.

The sub-cooled nucleate boiling model usedis based on the wall flux partitioning modelproposed by RPI (Rensselaer PolytechnicInstitute) [7]. Based on this model:

Wall heat flux = Single phase heat flux + Quenchingheat flux + Evaporation heat flux

(1)

This flux partition model helps determine therate of vapor generation at the superheatedwall.

At the vapor-liquid interface, evaporationrate is determined by net energy flux acrossthe interface. Net heat flux addition causesevaporation and removal, causes

condensation. Net heat flux across theinterface wall is determined by the bulk fluidtemperature, vapor temperature andinterface heat transfer coefficient [8].

The rate of vapor formation is given by:

))0,max(.(

)("

lspl

wEisllvvl

TTcL

Aq

L

ATThm

+

+

=>

&

(2)

The rate of vapor formation in free stream(neglecting heat transfer at superheatedsolid wall) would be,

L

ATThm isllvvl

)( =

>&

(3)

The interface heat transfer coefficient isdetermined based on the Ranz-Marshallcorrelation:

33.05.0PrRe6.02+=Nu (4)

Solid temperature and the thermalconductivity determine the wall heat transferflux in conjugate heat transfer cases.

)("

lwsolidw TThq = (5)

Using this model, various parameters can bemeasured. Figure 21 shows the vaporfraction through a section. Nucleate boilingoccurs in a region adjacent to the exhaustports.

Figure 21: Contours of vapor phase through a section inthe coolant volume

Velocity plots of Figure 22 through the samesection (as shown in Figure 21), indicatethat the region where the vapor phase is




9

observed corresponds to a stagnationregion.

Figure 22: Stagnation flow regions corresponding tovapor film sites

CONCLUSIONSThis paper summarizes the variousapproaches that can be used in the flow

visualization in engine cooling jackets.

The paper describes 1D-3D coupling tosimulate the entire engine cooling circuit..

The simulation predictions correlate wellwith the test results validating the developedprocedure.

Different approaches for the thermalanalysis of the engine block for use instructural durability are described in thiswork.

The paper highlights the phenomenon ofnucleate boiling and its prediction using CFDcodes. Poor flow in regions corresponding tostagnation zones and adjacent to hotsurfaces act as initiation sites for nucleateboiling.

Using CFD almost 14 major designmodifications were carried out on the cooling

jacket of the engine under study. Theaverage flow was improved from 1 m/s to 2m/s based on these changes. Gasket layoutchanges helped achieve a distribution of80% on exhaust side and 20% on intakeside in the head.

NOMENCLATURE

vlm >& Rate of vapor formation

lvh Interface heat transfer coefficient

T Temperature

iA Area of liquid vapor interface

L Latent heat per unit volume

wA

Area of superheated wall

"

Eq Evaporative heat flux at super

heated wall

pc

Specific heat at constant pressure

Nu Nusselt Number

Re Reynolds Number

Pr Prandtl Number

"

wq Wall heat transfer flux

solidh Solid conduction heat transfer

coefficient

Subscript l Liquid state

Subscript v Vapor state

Subscript s Saturated state

ACKNOWLEDGEMENTSThe authors would like to express theirgratitude to Dr. Arun Jaura, Sr. Vice-President Product Development, Mahindraand Mahindra for granting permission topublish this work. Technical support fromMr. Manish Kulkarni, Fluent India is alsohighly appreciated.

REFERNCES[1] Robert N. Brady, Modern Diesel

technology, Prentice Hall, 1996.

[2] Shibata, T., Matsui, H., Tsubouchi, M.,and Kasturada, M., Evaluation of CFDTools Applied to Engine Coolant FlowAnalysis, Mitsubishi Motor Techanicalreview, 2004.

[3] Laramee, R.S., Garth, C., Doleisch, H.,Schneider, J., Hauser, H., and Hagen,




10

Hans., Visual Analysis and Exploration ofFluid Flow in a Cooling Jacket, Proceedingsof IEEE Visualization, pg 623 630, October2005.

[4] Evans Cooling Systems Inc., Engine

Cooling Without The Water, DieselProgress, September 2000.

[5] Fluent 6.2 User Guide, Fluent Inc., 2005.

[6] Ideas ESC User Guide, UGS Inc.,2005.

[7] Basu, N., Troshko, A., and Nurnberg, G.,Modeling of Two-phase Flow and Boilingwith Fluent, RELAP5 UGM., July 2003.

[8] Narumunchi, S.V.J., Hassani, V., andBharatan, D., Modeling Single-Phase and

Boiling Liquid Jet Impingement Cooling inPower Electronics, National RenewableEnergy Laboratory Technical Report No.NREL/MP-540-38787, September 2005.


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