numerical stimulation of ventilated disc cooling … · · 2015-04-09numerical stimulation of...

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Int. J. Mech. Eng. & Rob. Res. 2015 Kiran C H, 2015

NUMERICAL STIMULATION OF VENTILATED DISCCOOLING EFFECT

Kiran C H1*

*Corresponding Author: Kiran C H, [email protected]

Braking system is one of the important control systems of an automotive. For many years thedisc brakes have been used in automobiles for safe retardation of the vehicles. During brakingenormous amount of heat will be generated and for effective braking sufficient heat dissipationis essential. The thermal performance of disc brake depends upon the characteristics of airflowaround the brake rotor and hence the aerodynamics is an important in the region of brakecomponents. This project aims at maximizing the airflow distribution across the rotor for betterheat dissipation and improving the cooling efficiency. A CFD analysis is carried out on the SkodaOctavia car braking system as a case study to make out the behaviour of airflow distributionaround the disc brake components using FLUENT software. The result obtained from this analysisgives an insight idea of the airflow distribution around the brake rotor in order to minimize thetemperature that affects the braking performance. Based on the results obtained, three newconcepts were generated by incorporating air ducts guiding the air towards brake rotor to enhancethe cooling effects. The results obtained for all the cases are analysed for effective cooling.From the results of temperature distribution it was observed that there is a considerable reductionin the max temperature generated during braking. 24 °C (611 °C to 587 ºC) decreased in maximumtemperature for concept1 and 41 °C for concept2 by properly guiding air towards brake rotorand 110 °C decreased was observed in cocept3 is achieved.

Keywords: Ventilated disc brake, Heat dissipation, CFD, Air flow, Cooling effect

INTRODUCTIONA braking system is one of the most importantsafety components of an automobile. It ismainly used to decelerate vehicles from aninitial speed to a given speed. In somevehicles, the kinetic energy is able to be

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Int. J. Mech. Eng. & Rob. Res. 2015

1 Department of Mechanical Engineering, Azad Institute of Engineering and Technology, Azad Technical Campus, Near CRPF Camp.,Lucknow 226002, India.

converted to electric energy and stored intobatteries for future usage. These types ofvehicles are known as electric or hybridvehicles. However, these kinds of vehicles stillneed a backup system due to sometimesinsufficient electric energy or failures which

Research Paper

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inevitably increase the cost of the vehicles. Sofriction based braking systems are still thecommon device to convert kinetic energy intothermal energy, through friction between thebrake pads and the rotor faces.

Excessive thermal loading can result insurface cracking, judder and high wear of therubbing surfaces. High temperatures can alsolead to overheating of brake fluid, seals andother components

Braking system is one of the most essentialmechanisms of a vehicle as shown in Figure1. It is implicit that the braking system must beable to eliminate the kinetic and potentialenergy to facilitate a safe deceleration.Generally, the methodologies l ikeregenerative-braking and friction-brakingsystem are used in a vehicle. Due to limitationsof Regenerative-braking system, friction

based braking system is universally adoptedfor retardation of vehicles. Friction-brakesfunction by transforming the vehicles kineticand potential energy into heat energyThe rateof heat generation in a friction-braking systemis a function of the vehicles mass, velocity andrate of deceleration. When the brakes areapplied a large amount of heat is generatedin a brake system accordingly the surroundingbrake components has to absorb the heatwithin a shorter period of time, but it is capableof storing only a limited amount of heatproduced during braking. In addition, theefficient dissipation of heat is instrumental tothe performance of the braking system.

If the temperatures go too high, problemsin braking system crop up from wear and tearof components by screeching and rapidvibration to permanent dysfunction of braking

Figure 1: Shows the Layout of Braking System

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systems. Figure 2 shows the heat generatedat the initial stage of brake rotor and padscomes in contact. When a sudden brake isapplied for a vehicle moving at a high velocitydue to friction, more heat is generated duringfrequent braking is shown in Figure 3. Someof the factors that affect disc brake due to hightemperature are brake fade; thermal judderand excessive component wear. Therefore, itis beyond doubt that the methods of coolingthe components of a brake system are to beimproved to minimize the risk of the problemsand enable safer vehicular movement.

Though at first, the heat is absorbed by thesurrounding components, later, continuedbraking radiates heat to the adjoiningcomponents through conduction andconvection through atmosphere. Conductionis an effective method of heat dissipation butcertain components get adversely affected.Sometimes when high temperatures are notcontrolled, they damage the tires of the vehicle.Therefore, convection to the atmosphere is theprincipal means to dissipate heat from thebrake rotor. Normally, the forward movementof the vehicle directs the cooling air at thebrake to transfer convection heat. So in orderto achieve maximum cooling of the brakes,airflow must be regulated and directed toappropriate areas.

LITERATURE REVIEWMuch attention has been focused on improvingthe thermal performance of brake discs.Numerical simulations and ComputationalFluid Dynamics (CFD) are commonly appliedto brake disc thermal performance analyses.Many experimental studies have also beenconducted to measure the air flow andtemperature field inside the discs underbraking operations. It has been demonstratedthat CFD simulation results have achievedgood agreement with those based onexperimental studies. By study of literature, wecame to know that, Numerical simulationinvestigation can be done by properly guidingthe airflow from the front end of a car towardsthe wheel. Conventional method is consideredas the best method to dissipate heat becauseit’s dependent on speed which can becontrolled. Improvements can be done bymodifying the dimensional characteristics ofventilated vane disc rotor to enhance good

Figure 2: Shows Heat GeneratedBetween Rotor and Pad

Figure 3: Shows Overheatingof Brake Pad

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airflow and heat dissipation characteristics.The component of disc brake assembly triesto transfer better heat to the surrounding air.The front end design approach influence theimportance of bumper and grill design. Thisproject aims at maximizing the airflowdistribution across the rotor for better heatdissipation and improving the coolingefficiency.

ANALYTICAL MODELFor fluid flow and heat transfer analysis acrossbrake rotor, the governing equations, such ascontinuity equation, momentum (NavierStokes) equation and energy equations areused in CFD for solving the solution

Continuity Equation: A continuity equationis a based on the principle of conservation ofmass. For steady state it states that the massof fluid entering a fixed control volume eitherleaves that volume or accumulates within it isconstant. It is thus a “mass balance”requirement posed in mathematical form, andis a scalar equation.

∂ρ∂t +

∂(ρ. u)∂x +

∂(ρ. v)∂y +

∂(ρ. w)∂z = 0 ...(1)

Momentum (Navier Stokes) Equations:The momentum equation is a statement ofNewton’s second law and relates the sum offorce acting on an element to its acceleration.Hence F = ma which forms the basis of themomentum equations. The three differentmomentum equations (x, y and z), altogethercomprise the Navier Stokes equations thatdescribe the flow of incompressible fluids.

ρ∂u∂t

+ ρu∂u∂x

+ ρv∂u∂y

+ ρw∂u∂z

= ρgx −∂p∂x

+ μ∂2u∂x2 + μ

∂2u∂y2 + μ

∂2u∂z2

ρ∂v∂t

+ ρu∂v∂x

+ ρv∂v∂y

+ ρw∂v∂z

= ρgy −∂p∂x

+ μ∂2v∂x2 + μ

∂2v∂y2 + μ

∂2v∂z2

ρ∂w∂t

+ ρu∂w∂x

+ ρv∂w∂y

+ ρw∂w∂z

= ρgz −∂p∂x

+ μ∂2w∂x2 + μ

∂2w∂y2 + μ

∂2w∂z2

..(2)

Energy equations: The continuity andmomentum equations are adequate insituations, where the fluid is incompressibleand the temperature differences are small. Ifthe heat flux occurs (temperature not constant)an additional equation (energy) is enabled. Anenergy equation is a scalar equation. It has noparticular direction associated with it. Thisequation demonstrates that, per unit volume,the change in energy of the fluid moving througha control volume is equal to the rate of heattransferred into the control volume plus the rateof work done by surface forces plus the rate ofwork done by gravity.∂∂t ρe +

12 ρv2 +

∂∂x ρue +

12 ρuv2 +

∂∂y ρve +

12 ρvv2 +

∂∂z ρwe +

12 ρwv2 =

k∂2T∂x2 +

∂2T∂y2 +

∂2T∂xz2 − u

∂p∂x + v

∂p∂y + w

∂p∂z +

μ u∂2u∂x2 +

∂∂x v

∂v∂x + w

∂w∂x + v

∂2u∂y2 +

∂∂y v

∂u∂y + w

∂w∂y + w

∂2u∂z2 +

∂∂z v

∂v∂z + u

∂u∂z

+2μ∂2u∂x2 +

∂u∂y∂v∂x

+∂2v∂y2 +

∂v∂z∂w∂y

+∂2w∂z2 +

∂w∂x

∂u∂z

+ ρugx + ρvgy + ρwgz

...(3)

Turbulence Model: A standard k- model isselected with reference to Rolf Krusemann andGerald Schmidt (1995). It is a two-equationturbulence model derived from ReynoldsAveraged Navier Stokes modeling. K is theturbulent kinetic energy, defined as thevariance of the fluctuations in velocity and isthe turbulence eddy dissipation rate at whichthe velocity fluctuations dissipate. This semiempirical model is robust and economic, mostwidely used in industrial and practicalengineering turbulent flow problems, alsosuitable for parametric studies. The turbulencekinetic energy (k) and rate of dissipation () is

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obtained from the following transportequations;

For turbulent kinetic energy:

∂∂t

(ρk) +∂∂xi

(ρkui) =∂∂xj

μ +μtσt

∂k∂xj

+ Gx − ρε + Sk

...(4)

And for rate of dissipation:

∂∂t

(ρε) +∂∂xi

(ρεui) =∂∂xj

μ +μt

σε+∂ε∂xj

+ C1εεk Gk − C2ερ

ε2

k + Sε

...(5)

The turbulent viscosity, is computed bycombining K and as follows,

휇푡 = 휌퐶휇푘2

휖 …(6)

Brake Heat TransferFigure 4 shows the schematic shape of thedisk and the pad in sliding contact is shown.As it is shown disk is like an annulus and padis like a partial annulus. The brake systemclamps the pads through the calliper assemblyby brake fluid pressure in the cylinders. Rotarymotion of the disk causes a sliding contact

between the disk and the pad and generatesheat.

For calculation of heat generation due tofriction, rate of dissipated heat via frictionshould be taken into account. This is all to dowith the calculation of friction force and rate ofwork done by friction force. For calculation offriction force, the pressure distribution at thecontact surface of the disk and the pad shouldbe determined. Here, two types of pressuredistribution are taken into account.

In the contact area of brake components;the pads and the disk; heat is generated dueto friction. For calculation of heat generationat the interface of these two sliding bodies’ twomethods is suggested:

1. At the basis of law of conservation of energythe kinetic energy of the vehicle duringmotion is equal to the dissipated heat aftervehicle stop.

2. By knowing the friction coefficient, pressuredistribution at the contact area, geometriccharacteristics of the pad and the disk,relative sliding velocity and duration ofbraking action one can calculate the heatgenerated due to friction.

Brakes are essentially a mechanism tochange the energy types. When a car is movingwith speed, it has kinetic energy. Applying thebrakes, the pads or shoes that press againstthe brake drum or rotor convert this energy intothermal energy. The cooling of the brakesdissipates the heat and the vehicle slowsdown. This is all to do with the first law ofthermodynamics, sometimes known as the lawof conservation of energy that states thatenergy cannot be created nor destroyed; it canonly be converted from one form to another. In

Figure 4: Shows Schematic Shape of theDisk and the Pad in Sliding Contact

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the case of brakes, it is converted from kineticenergy to thermal energy:

The different modes of heat dissipation are;

1. Conduction through the brake assemblyand hub

2. Radiation to nearby components

3. Convection to the atmosphere

Conduction is an effective mode of heattransfer but it affects the adjoining componentsin terms of damaging the seals, bearing, etc.

Radiation heat transfer has the maximumeffect of temperature that is to be controlledand it is estimated as negligible during normalbraking conditions. So the convection isconsidered as primary means of heatdissipation from the brake rotor to theatmosphere. The Figure 5 shows theschematic form of heat transfer mechanism ina disc brake system.

As for conduction through surfaces, the heatflow can be expressed by Fourier’s law ofconduction as follows:

Qcond = −kAdTdx …(7)

where, Qcond is the heat transferred, k is thethermal conductivity, A is the area of contactand T is the temperature.

Similarly, the heat is transferred byconvection, also known as Newton’s law ofcooling is governed by,

Qconv = hA(Ts − T∞) ...(8)where, Qcond = Rate of heat transfer (W)

h = convection heat transfer coefficient (W/m2K)

A = surface area of the rotor (m2)

Ts = surface temperature of the brake rotor(°C)

T∞= Ambient air temperature (°C)

GEOMETRIC MODELING ANDCFD ANALYSISIn this chapter the study of airflow distributionis carried out on the existing disc brake ofSkoda Octavia passenger car as case studyin order to reduce the temperature by guidingair towards brake rotor. The overall dimensionsof car body and brake components areillustrated in Table 1.

Calculation for Input Parameters: The heatflux is calculated for the car moving with avelocity 22.22 m/s (80 kmph) and the followingis the calculation procedure.

Data given:

Mass of the vehicle = 1330 kg (Table 2)

Initial velocity (u) = 22.22 m/s (80 kmph)

Final velocity (v) = 0 m/s

Figure 5: Shows the Heat TransferMechanism in Brake System

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Brake rotor diameter = 0.270 m

Assuming, full load condition (100% brakingcondition) Energy generated during braking =44324.46J

Stopping distance (D) = 35.94 m

Heat generated/volume (q) = 1.5 x 107 W/m3

Geometric ModelThe vehicle used for the simulation is SkodaOctavia 1.9TDI, a four door passenger car.Knowing the flow is to be analysed thegeometric modelling is constructed using CADsoftware tool (CATIA V5R16) as shown inFigures 6 and domain creation is shown inFigure 7.

Meshing (Pre-Processor)To output the solution, the above tasks arecarried out with the interaction between theuser and the computer. This stage is done withthe software Hyper Mesh, linked to the

All Dimension are in mm

Overall length 4512

Overall width 1731

Overall height 1431

Wheel base 2512

Ground clearance 134

Front track 1513

Rear track 1494

Kreb weight 1330 Kg

Front brakes Disc brakes

Rear brakes Drum brakes

Front suspension McPherson Sturt with wish bonearms

Rear suspension Compound link crank axle withtorsion stabilizer

Brake disc (size) 270 Dia x 22

Brake pads 115 x 50 x 20

Tires 196/65 R15

Wheel size 6J x 15"

Tables 1: Shows the Specificationof Skoda Car

Figure 6: Shows 3D Geometric Model of Car

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Figure 7: Shows the Domain Creation

Figure 8: Shows the Fine Mesh Around the Wheel and Disc Brake

Figure 9: Shows the Fine Tetra Mesh Around the Surface of Body

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FLUENT software are shown in Figures 8, 9and 10.

CFD AnalysisCFD is primarily used as a design aid forpredicting the performance characteristics ofequipment involving fluid flow and heat transfer.It’s capability to achieve fast and reliableconvergence by solving the equationsprecisely.

Solver Setting and ImportanceTurbulent intensity and viscosity was to specifythe turbulence specification method and isdefined as the ratio of root mean square ofthe velocity fluctuations to the mean flowvelocity expressed by,

I = 0.16(Re )−18 ...(9)

Reynolds number

Re =ρνlμ

...(10)

where ‘’ is the density of fluid (Kg/m3)

‘’ is the velocity of fluid (m/s)

‘l’ is the length of the obstacle (m)

‘’ is the co-efficient of viscosity

Figure 10: Shows the Complete Meshing of Domain

The Reynolds number and Mach number iscalculated and mentioned in the followingTable 2.

For Velocity 22.22 m/s

Mach number 0.1

Reynolds number 686497

Turbulent intensity 2.6

Table 2: Illustrates the SolverParameters

Domain length 22500 mm

Domain width 6800 mm

Domain height 4500 mm

Table 3: Shows the Overall Dimensionof Domain Creation

Solver Segregated

Formulation Implicit

Time Steady state

Velocity formulation Absolute

Pressure discretization Standard

Momentum discretization First order upwind

Turbulent kinetic energy First order upwind

Specific dissipation rate (omega) First order upwind

Pressure velocity coupling SIMPLE

Table 4: Shows the Input Parameterfor Solution

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Component Brake Rotor Brake Pad

Material Grey cast iron Asbestos

Density Kg/m3 7100 3500

Co-efficient of heat (Cp)j/Kg-°K 45 800

Thermal conductivity (K)W/m-°K 46 4

Table 5: Shows the Material Propertiesof Brake Rotor and Pad

The material properties for disc rotor andpad such as thermal conductivity, specific heatand density are illustrated in the followingTable 5.

Solution for Existing ModelFigure 13 shows the contours of statictemperature distribution for the baseline modelacross the brake rotor. It is observed that themaximum temperature 610.64º.

Figure 11: Shows the Boundary Condition

Figure 12: Shows Velocity Vectors for Baseline Model at a Velocity 22.22 m/s

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Figure 13: Shows Airflow Pattern at the Lateral Section and Temperature Contoursof Ventilated Brake Rotor

Figure 14: Shows Meshing of Concept1

Figure 15: Shows Meshing for Concept2

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Approach of Design ImprovementDue to friction the higher temperature in thebrake rotor thermal stress are generated thataffects higher thermal stresses are induced athigh temperature that affects the performanceduring braking. The affects may leads to crack,brake judder, spots, etc., and in order tominimize the high temperature, the threedifferent concepts are generatedinconsideration of brake cooling performance.The concepts are shown in Figures 14 and 15and concept3 is as same geometric ofconcept2 but change of material of brake rotorto ALMMC.

RESULTS AND DISCUSSIONFigure 16 shows the contours of statictemperature distribution for the baseline modelacross the brake rotor. The maximumtemperature observed is 610.64 ºC in betweenthe pad and the rotor.

Figure 17 shows the contours of statictemperature distribution for the modifiedmodel across the brake rotor. The maximumtemperature observed is 586.21 ºC in betweenthe pad and the rotor.

Figure 16: Shows Contoursof Temperature Distribution Across

Baseline Model


Concept1


Concept2


Concept3

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The Figure 18 shows the contours of statictemperature distribution for the modifiedmodel across the brake rotor. The maximumtemperature observed is 569.45 ºC in betweenthe pad and the rotor.

Figure 19 shows the contours of statictemperature distribution for the concept3model across the brake rotor. The maximumtemperature observed is 271.86 ºC in betweenthe pad and the rotor.

CONCLUSIONA detailed study of the flow across the discbrake rotor and the suggestion for the newdesign concepts has been carried out.Following are the conclusions based on theanalysis results.

The results achieved through CFD withacceptable accuracy for the baseline and themodified concepts help in understanding andvisualizing the airflow across the brake rotor.The brake rotor serves as energy dissipater,so to achieve this better additional air is guidedto provide an adequate cooling. The modifieddesign concepts are found to be effective interms of temperature reduction with increasedairflow. Even a material change in the brakerotor yields better results than the modifiedconcepts. Using together the Al metal materialmatrix composites rotor and the modifiedconcept2 may yield better cooling.Considering the costs of composites material,the concept2 is an appropriate choice for moreairflow than compared to concept. Half loadresult looks more practically than full loadresults.

From the results of temperature distributionit was observed that there is a considerablereduction in the max temperature generated

during braking. 24 °C (611 °C to 587 ºC)decreased in maximum temperature forconcept1 (air duct attached from bumper todisc rotor) and 41 °C for concept2 (wish bonetype) by properly guiding air towards brakerotor.

SCOPE FOR FUTURE WORKIn addition to steady state analysis transientanalysis can be carried out to study the heattransfer across the disc brake. Further the studyof the heat transfer around the disc brake rotormay be done considering the radiation. Furtherinvestigation through CFD analysis can becarried out considering the rear disc braktostudy the airflow distribution.

REFERENCES1. Christer Bergstrom and Anders Jerhamre

(2001), “Numerical Study of Brake DiscCooling Accounting for Both AerodynamicDrag Force and Cooling Efficiency”, SAETechnical Paper.

2. Del Amno Nieto O, Gonzalez HernandezM I and Canibano Alvarez E (2006),“Development of a MultidisciplinaryMethodology for Brake Disc Design”, XInternational Congress on ProjectEngineering, September 13-15,Valencia.

3. Galindo-Lopez C H and Tirovic M (2008),“Understanding and Improving theConvective Cooling of Brake Discs withRadial Vanes”, Department of Automotive,Mechanical and Structures Engineering,Cranfield University, Cranfield, UK.

4. Gupta R B (2001), A Text Book ofAutomobile Engineering, 6th Edition,Published by Satya Prakashan, New Delhi.

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5. Jack Williams (1985), “An AutomotiveFront-End Design Approach forImproved Aerodynamic Design”, SAETechnical Paper.

6. Michael D Hudson and Roland L Ruhl(1997), “Ventilated Brake Rotor AirflowInvestigation, University of Illinois atUrbana Champaign”, SAE Paper,pp. 10-33.

7. Rolf Krusemann and Gerald Schmidt(1995), “Analysis and Optimization ofDisc Brake Cooling via ComputationalFluid Dynamics”, SAE Paper, pp. 07-91.

8. Talati F and Jalali far S (2008),“Investigation of Heat TransferPhenomena in a Ventilated Disc BrakeRotor with Straight Radial RoundedVanes, Asian Network for ScientificInformation”, Journal of AppliedSciences, pp. 3583-3592.

9. Warren Chan (2007), “Analysis of HeatDissipation in Mechanical BrakingSystems”, Department of Mechanical andAerospace Engineering, University ofCalifornia, San Diego 2007-07.

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