ModellingCondensation

in Automotive

Headlamps

ModellingCondensation

in Automotive

Headlamps

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Figure 1: Comparison Between an OldFashioned Glass Headlamp (top) and a New

Transparent Plastic Headlamp (bottom)

The environment inside an automotiveheadlamp has high thermal and low massexchanges with the external environment,causing condensation on the lens. Anacceptable headlamp design can only producedif this condensation can be disposed of in afixed time under severe thermal conditions.

Experimental studies are performed inclimatic chambers under highly controlledconditions, whilst long transient numericalsimulations are performed on large meshes inorder to capture the relevant physics of theproblem. This article outlines a new numericalmethod which has been used in order to studythe problem, and the results obtained whenapplied to real-world designs.

Jacopo Alaimo & Sergio Zattoni, Automotive Lighting Italia, Italy

Alberto Deponti, Fabio Damiani, Luca Brugali, & Lorenzo Bucchieri EnginSoft, Italy

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Jacopo Alaimo & Sergio Zattoni, Automotive Lighting Italia, Italy

Alberto Deponti, Fabio Damiani, Luca Brugali, & Lorenzo Bucchieri EnginSoft, Italy

Until a few years ago, lenseswere typically designed usingglass and covered by anoptical prism to obtain thecorrect light distribution (see

Figure 1). The increasedsophistication of moldingcapabilities has now led to the massproduction of large transparentplastic lenses. Curved shapes andtransparent surfaces have opened anew world for style solutions, but atransparent lens lets the eye see allthe way inside the headlamps.Today the observer has a free viewof the inside of the headlamp,highlighting even the slightestoptical fault, any thermal damage ofthe inner components and thepossible presence of water droplets.The presence of condensation insidethe headlamp is perceived by thecustomer as a lack of quality andreliability.

The most common solution fordecreasing condensation quantityand disposal time is represented bythe optimization of inner air flowsand of temperature distribution onthe main lens. Typically, at least twovent holes are present on theheadlamp housing; in order tooptimize their efficiency, it isimportant to find their optimalnumber and locations by performingnumerical and physical tests duringthe pre-industrialization phase. Untiltoday the solution to condensationformation has always been soughtby trial and error. This leads to agreat increase in time and costs. Theuse of appropriate numericalmethods and test rooms thereforebecomes a strategic tool fordecreasing production time and cost

and, in the near future, foroptimizing headlamp design withrespect to condensation formationand disposal.

From a fluid-dynamic point of view,an automotive headlamp can beconsidered as a cavity with lowmassflow interaction but highthermal interaction with the externalenvironment. One wall of the cavity,the lens, is transparent while theothers are opaque. Inside theheadlamp, there are one or morelamps and a number ofcomponents: reflectors, screens,caps, connectors, pipettes, etc.These components are used for thefunctionality of the headlamp butalso play a fundamental rolein the thermo-fluid-dynamic behavior of thefluid inside theheadlamp which is amixture of air and watervapour.

The headlamp can undergo thephenomena of heating and coolingbecause of internal and externalheat sources. The external heatsources or sinks are represented bythe external environmenttemperature or by the heat comingfrom the engine. The internal heatsource is represented by theswitched-on lamp, which heats upthe surrounding fluid and emitsradiation. Since the fluid inside theheadlamp is composed of a mixtureof air and water vapour, it changesdensity because of thermalevolution. Density differences arethe cause of internal convectivemotions which are always laminar.Since temperature is, in the final

analysis, the cause of the motion ofthe internal fluid, it is important toprecisely and accurately characterizeall of the components of theheadlamp. They are to becharacterized both from a thermaland an optical point of view in orderto model temperature, heat transferto surrounding fluid, radiationabsorption, emission and reflection.In addition, the assembly of allcomponents limits the space wherefluid can flow, hence determiningthe motion field inside theheadlamp.

All components should be modelledwith a geometrical detail which isadequate for the level of accuracyneeded for the fluid-dynamicresults. On the other hand, greatgeometric detail leads to a largemesh and, consequently, largecomputational costs. A trade-offbetween geometric detail andcomputational cost needs to beachieved. In addition to this,temperature evolution of theheadlamp may cause water phasechanges; in particular it may causewater condensation andevaporation on the lens which is amain issue for headlamp producersand the target of the present work.

The problem to be studied is atypical multi-phase problem inwhich it is important to properlydescribe the phase change betweenliquid water and water vapour. Inthis problem it is imperative tocapture the natural convectionvelocity field due to the differentdensity of fluid masses inside theheadlamp. For this reason it isimportant to account for gravity and

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buoyancy effects in the fluid. Sincethe motion field is driven by naturalconvection, the flow is laminar andno turbulence model is used.Another important phenomenonto be modelled is the heattransfer between walls and fluid,between different fluid massesand, particularly, the latent heatabsorbed by water evaporation andreleased by vapour condensation.Finally, when a switched on lamp isconsidered, thermal radiation is tobe accounted for.

Experimental StudiesALIT Condensation Test RoomALIT (Automotive Lighting Italia)Condensation Test Room is a metalroom with a volume of about 30m3.Glass windows allow the techniciansto follow the ongoing tests. By usingdedicated hardware, it is possible tocontrol all the main variables relatedto the condensate disposal processsuch as:

• Heat Transfer Coefficient (HTC) onheadlamp boundary walls;

• Internal and external air relativehumidity (RH);

• Internal and externalair temperature;

• Pressure and air flowfields in the proximity of

ventilation pipettes;

• Mission profile reproductionaccounting for engine inducedtemperature and wind speed;

• Interaction between headlamp-engine assembly.

The ALIT condensation test room isprojected to control all the mainfactors involved in the HTCdistribution. It is possible to controlexternal air RH and temperature;moreover, an air speed of up to80Km/h can be produced along thelongitudinal car axis. Inside theroom, an engine box mock-upreproduces the effects of theaverage temperature produced bythe engine. Since the HTC is

influenced by aerodynamic effectstoo, the engine box mock-up

reproduces the car shape.At present the effects notreproducible arerepresented by pressureand air flow fields inside

the engine box. Indeed, geometricand thermodynamic effects of theengine are still too complex to bereproduced. Nevertheless, a goodapproximation is obtained by usingan average temperature inside theengine box mock-up.

Measurement devicesA major problem related to thecondensation issue is represented bythe difficulty of an objectivecondensation tracking. Indeed largevariations in condensation layerthickness as well as in water dropletsdiameters may occur, and this has adirect influence on the human eyeperception. The use of a standardphotographic camera with flashusually highlights even the smallesttraces of condensation which maynot be visible by human eye. At thesame time, it is not possible tomeasure a continuous distribution ofthe dew point. Several temperatureand humidity probes are presentinside ALIT condensation test room,these are located in the free-areazone and inside the engine boxmock-up. Moreover, it is possible toplace thermal couples and moisture

Figure 2: Thermal Maps on the Lens at Two Different Times

Figure 3: Condensate on a prototype specifically designed for enhancing condensation Figure 4: Temperature and Relative Humidity Graphsin ALIT Condensation Test Room

conservation law for internal energyis:

where Cp is the water latent heatfor vaporization/condensation. Massand energy sources are applied only

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meters inside the headlamp in orderto get data. Finally, the temperaturedistribution on the lens is tracked bymeans of an infra-red camera.Combining these data together withphotos and videos of condensationdistribution it is possible to track thedew point line. At present it is notpossible to measure condensationthickness.

Test ResultsThe outputs of the condensation testare:

• thermal maps and videos shotusing infra-red camera (Figure 2);

• condensation images and videosshot using photographic camerawith flash (Figure 3);

• temperature andrelative humiditygraphs measuredby the thermalcouples andmoisture metersplaced inside theheadlamp, insidethe engine boxmock-up and in theexternal environment(Figure 4).

From Figure 3 it can be noticed thatcondensation tends to accumulate onthe outer side of the headlamp (leftside in the figure) which is thecoldest part of the lens, as shown byFigure 2.

Numerical SimulationsThe Numerical MethodWhen a switched-on lamp is to bemodelled, a radiation model has tobe used in order to compute thesource term for the energy equationand the radiative heat flux at walls. Inthis case, the Discrete Transfer model

is used for the directionalapproximation and the Grey model isused for the spectral approximation.The Gray model assumes that allradiation quantities are nearlyuniform throughout the spectrum,consequently the radiation intensityis the same for all frequencies. TheDiscrete Transfer model assumes thatthe scattering is isotropic. Theswitched-on lamps aremodelled by imposing thetemperature data comingfrom experimentalmeasurements. In theevaporation/condensation modelconsidered, the liquid phase is notmodelled directly. Instead, theevaporation/condensation processesoccurring on the lens are modelled

by means of suitable mass andheat sources for the continuity

and thermal equations. Themass source term applied tothe conservation law forwater vapour mass in thegas is:

Here m is the water mass perunit area transferred between

liquid and gas, A is the area of theelement face where evaporation andcondensation processes occur, Al isthe total area of the surface whereevaporation and condensationprocesses occur, L is the typicallength scale of the process, μ is thediffusivity of water vapour in the air,considered equal to the air dynamicdiffusivity, e is the water massfraction at equilibrium, mf is thewater mass fraction and Sh is theSherwood number. The air volumefraction is the complement to unityof the computed vapour volumefraction. The energy source due tophase change applied to the

at surfaces whereevaporation/condensation processesoccur. In the framework of thisevaporation/condensation model, itis possible to define the water massper unit area lying on the lens as:

Here, the space and timedependency of the water mass perunit area is explicit. This variableallows for precise tracking of theamount of condensation lying on thelens. Moreover, in the case ofevaporation, the local mass sourcehas to be null where local watermass per unit area is null; this isachieved by a local control of themass source term. Mass and energysources are implemented in ANSYSCFX by means of properly definedfunctions and variables using theCEL language. The analyses were runusing an upwind advection schemeand the first order backward Eulertransient scheme. The time step andthe convergence criteria were chosenin order to minimize thecomputational time withoutcompromising result quality andmethod robustness.

Figure 5: A Typical Tetra-Mesh Optimized for Thermal andCondense Simulations.

Figure 6: Velocity Vectors on a Vertical Plane Passing Through theLamps (note that vectors are colored with temperature distribution)

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The Computational MeshSolid and fluid domains werediscretized using a thetra-prismmesh. In particular, prism layerswere used inside each of the soliddomains and outside of the rearbody, the lens and the lamps. Atotal of about 1.750.000 elementswere used to discretize the entireheadlamp. (Figure 5)

Initial and Boundary ConditionsAt the start the lamps are switchedoff, the temperature is 6°C and therelative humidity is 95%. At thebeginning of the simulation, thelamps are switched on. After 20minutes rain starts. After 40 minutesrain stops and a wind of 30 km/hstarts blowing until the end of thesimulation at 60 s. These conditionsare simulated by varying externaltemperature and relative humiditytogether with HTC on the lens. Theinitial and boundary conditions usedin the simulation are summarized inTable 1.

ResultsThe simulation was run on 32parallel CPUs with OS LinuxCENTOS. The computational timewas roughly 12 days. In Figure 6velocity vectors on a vertical planepassing through the lamps ispresented; note that vectors are

colored with temperaturedistribution. The strong buoyancyeffect caused by the switched onlamps can be appreciated.

ConclusionsBecause of difficulties in measuringcondensation mass on the lens, atpresent, only a qualitativecomparison can be made; in Figure7 such a comparison is presented. Itcan be noticed that the two resultsare in good agreement highlightinga region of condensationaccumulation in the outer side ofthe headlamp. It has to behighlighted that some sensitivityanalyses showed a strongdependency on initial and boundaryconditions demonstrating thecomplexity of the phenomenonunder study and the need ofstrongly controlled experimentalconditions. Due to the complexity ofthe problem, and the fact thatnumerical simulations are to beperformed over long time periodand on large meshes, highcomputational power is needed.Nevertheless, numerical simulationsare able to give detailed informationon the thermo-fluid-dynamics of theheadlamp, taking into account thecondensation/evaporationphenomena that may occur on thelens. Moreover, by superimposing

numerical results and condensationimages taken from the experimentaltests, it is possible to correlateresults and to get importantinformation about the condensationissue in terms of distribution andthickness of the water layer. Thecombined use of numerical andexperimental studies is a powerfultool for optimizing headlamp designand obtaining high performanceheadlamps.

References• ANSYS CFX-Solver Modelling Guide.

• ANSYS CFX-Solver Theory Guide.

• Perry, R.H. and Green, D.W. (Editors)(1997). Perry's Chemical Engineers'Handbook, 7th Edition, McGrawhill.

• Kreith, F. and Bohn, M.S. (2001).Principles of Heat Transfer, ThomsonLearning.

• Chenavier, C. (2001) ThermalSimulation in Lighting Systems - 5Days / 5 Degrees. PAL SymposiumDarmstadt, 2001.

• Preihs, E. (2006). Analytic Solution andMeasurements of Condensation insidea Headlamp, COMSOL Conference2006.

• Nolte, S. and Maschkio, T. (2007).Development of a Software Tool forthe Simulation of Formation andClearance of Condensation in VehicleHeadlamps, LLAB.

• Schmidt, T. (2008). Nanotechnologiessurface modifications for anti-fogapplications in automotive lightingand sensor serial production, SAE2008

Figure 7: Qualitative Comparison Between Numerical and Experimental Results

Table 1: Initial and Boundary Conditions

AUTHOR INFORMATIONJacopo Alaimo I Automotive Lighting Italyjacopo.alaimo@al-lighting.com

Alberto Deponti I EnginSoft Italya.deponti@enginsoft.it