Heat Transfer Analysis of Automotive Headlamp Using CFD Methodology

Transcription

1 Heat Transfer Analysis of Automotive Headlamp Using CFD Methodology Manoj Kumar S * N. Suresh Kumar R. Thundil Karuppa Raj Engineer, Dept. of CFD Manager, Dept. of CFD Professor, Dept. of Energy Mechwell Industries Ltd, Nasik Mechwell Industries Ltd, Nasik VIT University, Vellore Abstract- Nowadays, simulations through CFD technique is incorporated much earlier in the development stage of an automotive headlamp. Thereby, the automotive industries are able to produce an innovative and creative design with an enhanced performance standard within a reasonable time. The process of physical testing and its validation is available late in the development stage and are also very costly to be performed. Therefore, it is of great significance to simulate the risks during the design phase of the headlamp and henceforth, reduce the cost involved by carrying out proper thermal management studies through numerical analysis and simulations. The present study carries out an experimental analysis on a spherical automotive headlamp to determine the thermal characteristics and behaviour. Numerical analysis done through CFD technique is utilized to validate the experimental results. Thereby, showcase a simulation strategy using CFD methodology to determine the airflow and temperature distributions within a headlamp design. The methodology presents a fair agreement with the experimental and numerical analysis results. Keywords- Thermal behaviour; Headlight; Hot spot; Radiation; Computational fluid dynamics I. INTRODUCTION The source of lighting in vehicle headlamp systems has undergone enormous changes over the past few decades. Materials such as glass and metals have given way to high performance materials like durable plastics. Changes and improvements are certain to happen as requirements and necessities grow each year. Today, the headlamp industry is highly competitive and focuses more on cost reduction without sacrificing the customer driven requirements and styling characteristics. Modern day headlamps aim at improving the lighting and thermal distribution. Some factors like longer life span and aerodynamic characteristics can be obtained by utilizing materials like thermoplast reflectors along with plastic lens. Such materials are highly sensitive to high temperatures, so proper and efficient heat transfer analysis has to be done to determine the areas susceptible to thermal degradation. Vectis and RadTherm were used to carry out the numerical simulations and the two computational codes were used by a linking procedure. The approach was able to model the evaporation process under various conditions to provide an overall view of the entire physical phenomenon involved [1]. Heat transfer modes such as convection, conduction and radiation were modelled in Fluent to predict the wall temperature and airflow within the automotive headlights. Assumptions like 100% electric power from the bulb is converted to thermal energy are taken and a constant heat flux boundary condition on the filament surface was set. Hot spot determination and airflow analysis were carried out [2]. A new approach named mesh free simulation was utilized to analyse the thermal fluid field in a headlamp. A direct comparison was made between the conventional CFD and mesh free method. It showed 75% reduction in time for the process of model preparation while using the mesh free approach and it delivered the numerical results in a shorter period of time [3]. The methodology of Design of experiments was taken to study the influence of various geometric parameters of components inside the headlamp assembly. The approach was validated with CFD simulations and set of transfer functions were generated [4]. The surface heat transfer method was used to study the temperature of different parts and natural convection was analysed within a headlamp using CFD technique. Evaluation were carried out and CFD results were within + 10 degrees as compared with the experimental results [5]. The efficiency of FVM is evaluated in an absorbing, emitting and anisotropically scattering medium [6]. The paper is divided into various sections. First, two case studies will be discussed namely, thermal analysis on circular tubes and radiative heat transfer on rectangular enclosures filled with emitting-absorbing medium. Further, an experimental analysis carried out on spherical headlamp to determine the thermal distribution within the headlamp and heat transfer analysis carried out in fluent will be discussed along with the results and validation. II. CASE STUDY In this section, two thermal analysis studies have been carried out by utilizing CFD technique. These studies were done to verify the codes that are used for modelling the various heat transfer modes. First thermal analysis for the hollow tube was done and then, radiative heat transfer analysis for a rectangular enclosure filled with absorbing-emitting medium. A. Thermal analysis of smooth hollow tube In this study, heat transfer induced by the temperature differences in the hollow tube has been numerically analysed. A constant temperature of 450K was given as a boundary condition for the external surface of the tube. The hollow tube with a length of 72mm is made of commercial steel is having an inner diameter of 9mm and outer diameter of 11mm , IJIRAE- All Rights Reserved Page -90

2 The fluid flowing through tube is water and is having a temperature of 300K. The geometry creation and meshing was carried out in ICEM CFD. The mesh created for the tube is shown in Fig 1. The flow is assumed to be thermally developed at the inlet of the tube. The inlet velocity is given as an expression that was created on the basis of inlet boundary condition specified by Ozceyhan and Altuntop (2005). The expression given below defines the normal speed of water through the inlet of the tube, where vm is mean velocity of water at inlet. Vel = 2*vm*((1-(r0/r1)^2)); r0 = Sqrt((x^2+y^2)); r1 = [m]; vm = 0.3 [m s^-1]; Fig. 1 Mesh for hollow smooth tube The outlet is a zero gradient condition with static pressure as zero Pascal. The analysis was carried out in ANSYS CFX. The things to be considered for the study are the conduction within the tube material and convection happening from the inner surface of the tube to the fluid. The tube material is assumed to be homogenous and isotropic. Fig. 2 Comparison of temperature profiles , IJIRAE- All Rights Reserved Page -91

3 Fig. 2 shows a result comparison between the temperature profile plot obtained by Ozceyhan and Altuntop (2005) and CFD simulations. It could be seen that the minimum temperature obtained is 420K, when a mean inlet velocity of 0.3 m/s of water is passed through the tube. The results were validated with very good agreement with results of published work of Ozcheyhan and Altuntop (2005). The Fig. 3 shows the variation of temperature along the radial direction at different locations along the length of the tube. B. Radiative heat transfer of an enclosure Fig. 3 Temperature vs Radial distance plot In this case study, a radiative heat transfer analysis is carried out for a rectangular enclosure filled with absorbingemitting medium. The three dimensional enclosure is taken to be 2x2x4m. The enclosure is divided into 25x25x25 control volumes as shown in Fig 4. The angular discretization in θ and ϕ direction is taken as is 4x20. Fig. 4 Mesh for rectangular enclosure The boundary condition taken for wall z=0, is temperature of 1200K and an emissivity value of The temperature and emissivity value of wall at z=4 is 400K and 0.70 respectively. All the other walls are considered to be at a temperature of 900K and have an emissivity value of The temperature distribution is determined from the RTE is coupled with the energy equation with a heat source of q gen =5 kw/m 3. The Fig. 5 and Fig. 6 depicts the contours of radiation temperature for absorption coefficient of κ=0.5 m -1 and κ=1 m -1 at x=1m. It could be seen that, when κ is reduced from 1 to 0.5, there is an increase in the temperature value at z=4. This is because, when the absorption coefficient is less, the energy absorbed by the medium is less, so more heat flux is able to reach to the wall at z=4m. The values plotted shows very good agreement with values obtained by Lee et al [1994]. Thus, the DOM model is able to efficiently model the radiative heat transfer with temperature distribution with varying absorption coefficients in absorbing and emitting medium , IJIRAE- All Rights Reserved Page -92

4 Fig. 5 Contour of radiation temperature at x=1m for absorption coefficient κ= 0.5 m -1 Fig. 6 Contour of radiation temperature at x=1m for absorption coefficient κ= 1 m -1 III. EXPERIMENTAL ANALYSIS OF HEADLAMP As discussed in the previous sections, the three major modes of heat transfer, namely conduction, convection and radiation plays integral part in the heat transfer within an automotive headlamp. The thermal distribution and process of heating occurring within a headlamp is caused by the absorption of light radiated by the bulb, the heat carried away by the buoyancy driven flow and the transport of energy through the walls by heat conduction. So, in order to carry out a proper heat transfer analysis all three modes have to be considered. A. Experimental setup Experimental analysis is carried out on an automotive headlamp to determine the thermal characteristics and behaviour. The headlamp was placed in a room maintained at ambient temperature. The PT-100 RTDs along with data logger were used to measure the temperature developed within the headlamp surface. The RTDs were used which were having an accuracy of +2 0 C and it were pasted to reflector surface as well as to the lens. The volt and current were measured with the help of digital clamp multimeter. It was observed that there is a constant supply of power to the bulb and it was calculated to be 22W. The same was used for carrying out the numerical analysis on the headlamp by fixing a constant flux condition on the filament surface. After connecting all the necessary equipment, the headlamp was switched , IJIRAE- All Rights Reserved Page -93

5 on. The headlamp reached a steady state condition within an hour and then, the temperature readings were taken. Fig. 7 shows the experimental setup carried out for the present study. Fig. 7 Experimental setup The RTDs were placed on a few locations as shown in Fig 8 within the reflector surface. A few numbers of experimental runs were performed to determine the consistency of measured readings. It was evident from the temperature measurements that high temperature regions are formed directly above the bulb surface and these are the regions that are susceptible to thermal degradation. Due to the strong influence of convective flow, temperature would be higher as we go along the upper surfaces of the reflector and lens. Fig. 8 RTD pasted on reflector Outer lens surface temperature was also measured with the help of infrared technique. Infrared camera Testo 881 with high quality wide lens was used for measuring the outer lens surface temperature. Before taking the temperature readings, the emissivity, temperature range of C were set and it has an accuracy of +2 0 C. The calibration of IR analysis was supported by the use of two RTDs placed on the surface of the outer lens. From the IR image, it was clear that high temperature region were obtained on the top side of the lens surface. Table I depicts the ambient and reflector surface temperature measured as the headlamp reached steady state condition. TABLE I Temperature measurement readings Cases RTD Ambient temperature Reflector temperature ( 0 C) ( 0 C) a b c d e f , IJIRAE- All Rights Reserved Page -94

6 IV. NUMERICAL ANALYSIS In this section, we discuss about the boundary conditions, geometry and mesh created to carry out the numerical simulation of automotive headlamp. Later, we have shown the results and validation obtained by utilizing the CFD methodology. A. Boundary condition Governing equations like conservation of mass, momentum and energy were solved along with radiative transfer equations for the simulation of heat transfer through an automotive headlamp using the FLUENT code. The material specifications and constants used for the analysis cannot be disclosed due to the Non-Disclosure agreement. The computational domain is divided in to fine control volumes. The Body force weighted scheme is taken for the pressure and second order upwind scheme for other parameters. For the coupling of pressure and velocity the SIMPLE algorithm is used. Gray model is used for the spectral approximation and the analysis was carried out at steady state condition for the three dimensional headlamp geometry. The flow within the headlamp is considered to be steady and laminar. The radiation model chosen for the analysis is Discrete Ordinate method. In this study, we have used a bulb with filament, which is supplied with a power of 22W as measured with the help of digital clamp multimeter. This is brought into the computational domain by fixing a constant heat flux for the filament, which is the actual heat source. The fluid enclosed within the headlamp and bulb is modelled as incompressible ideal gas. The cell zones will be the lens, reflector, bulb, headlamp fluid and bulb fluid. The bulb and lens surface will treated as semi-transparent walls and the reflector surface as opaque. Convective modelling is carried out by specifying the heat transfer coefficient on the reflector and lens outer surfaces. The theta-phi divisions were set as 5x5 and theta-phi pixels is kept as 3x3. Increasing these values will further increase the computational cost, which is not recommendable. B. Geometry and mesh The work initiated with CAD surface clean up and preparation of geometry. The main parts included for the analysis were the lens, reflector and the bulb with filament. Due to the complexity involved in the design of filament, it was modelled as cylinder of 2mm diameter. The geometry creation was carried out in Solidworks. The headlamp geometry is shown as in Fig 9. The parasolid model was then imported in Ansys ICEM CFD software for meshing. Fig. 9 Solidworks model of headlamp High quality hexa meshing is done for both fluid domain and solid domain of the headlamp. Fluid domain is having a total of elements and solid domain is having total of elements. Fig 10 and Fig 11 shows the fluid and solid domain mesh created for the analysis , IJIRAE- All Rights Reserved Page -95

7 Fig. 10 Fluid domain mesh Fig. 11 Solid domain mesh V. RESULT AND VALIDATION In this section, the validation of results between the experimental work and numerical simulation carried out by CFD methodology is illustrated. The thermal energy dissipated within the heat source is convectively transferred to the surrounding fluid and through the process of radiation, rest of the energy is transferred to the lens surface. The phenomena of conduction and radiation through semi-transparent walls, convection and radiation to the outer surfaces are the major factors for the distribution of wall temperature caused due to the heat transfer coefficient. The airflow distribution within the automotive headlamp and the temperature distribution through the lens and reflector surface obtained through the CFD simulations will be shown as well. Fig 12 shows the velocity distribution and the flow pattern followed by the fluid as a result of the convection and radiation effects. The dominant air convection can be observed from the velocity flow pattern within the bulb and headlamp, which is initiated by the density gradient , IJIRAE- All Rights Reserved Page -96

8 Fig. 12 Velocity flow distribution Fig. 13 clearly depicts that the heat source develops buoyancy driven flow that transports the heat energy convectively. A maximum velocity of 0.3 m/s is obtained near to the upper part of the reflector and very low velocity can be noticed on the bottom side of the reflector and bulb region. Fig. 13 Velocity flow pattern within the bulb and headlamp As the filament surface dissipates heat to the surrounding fluids, its density decreases, and its starts to flow upwards. The fluid flow pattern within the headlamp follows a closed loop. Due to the substantial increase in the temperature of the air around the bulb, it flows to the upper side of the headlamp through the reflector curvature. Fig. 14 Reflector surface temperature , IJIRAE- All Rights Reserved Page -97

9 Then it turns and follows the lens topology and at last, flows to the bottom part. Due to this air movement pattern, it can be noted that the hot spot locations can be formed on the top side of the lens and reflector of a headlamp. Fig 14. depicts the temperature profile formed on the reflector surface of the spherical headlamp. High temperature of C is obtained on the top side of the reflector with a minimum temperature of 42 0 C formed on the bottom side of the reflector. These results obtained through CFD simulations correlate well with the RTD readings taken on the reflector surface during physical testing. Fig. 15 Comparison of IR image and CFD result on outer lens Fig 15. shows the comparison of the temperature distribution obtained through IR analysis and CFD simulation. It can be seen from the temperature profile pattern for both the IR image and CFD simulation, that the location of the high temperature region is exactly matching and it is formed on the top side of the lens surface. The magnitude of temperature obtained through numerical simulation can be clearly validated with the IR image. A slight deviation in temperature was noticed on the bottom side of lens, where in experiment, the temperature at the bottom was observed to be 56 0 C, whereas as through CFD it was obtained to be 51 0 C. Overall, there is a fair agreement between the experimental and numerical results with a maximum deviation not exceeding 5 0 C. The temperature distribution obtained in the reflector surface also has good correlation to the physical testing temperature measurements with a maximum temperature obtained on the top side of the reflector. TABLE II Comparison of temperature at reflector surface cases Experiment ( 0 C) CFD ( 0 C) Deviation ( 0 C) a b c d e f It can be observed that the regions susceptible to thermal degradation can occur in headlamps along the top half of the reflector or lens. Thus care must be taken while designing the headlamp geometry and proper heat transfer analysis should be carried out to detect the risks early in the design stage. VI. CONCLUSION A comprehensive methodology to carry out the heat transfer analysis of an automotive headlamp has been showcased here. All the three modes of heat transfer have been fairly embraced in an accurate way to carry out an efficient thermal analysis. CFD simulations on the models have shown a fair agreement with the experimental work. It was observed that hot spots can occur on the upper side of the lens and reflector surface, thus this has to be taken into account while designing headlights. Further studies on effectiveness of vent needs to be carried out. The innovative CAE techniques like CFD can thus be used to predict the thermal behaviour and air flow pattern with in complicated headlamp structures before the actual prototype is being manufactured. Thereby, the CFD technique provides a cost effective and thermally optimized headlamp model to the market , IJIRAE- All Rights Reserved Page -98

10 REFERENCES [1] H.P Bensler, A. de Risi,., C. Freek, Heath D and Montefrancesco. E (2005), Investigation of thermal and fluid characteristics in Automotive headlights, SAE Technical paper, [2] Costa Edinilson alves and Santos Luis dos (2006), Wall temperatures and Air flow prediction in automotive headlights utilizing the CFD methodology, SAE technical paper [3] Sato Harumi, Takaki Nakamura, Minoru Inoue, Yoshiichi Ozeki and Tsunehiro saito (2011), Thermal fluid analysis by a mesh free simulation- part 1 Analysis of the thermal fluid field in a headlamp based on the real 3D-CAD model, SAE Technical paper, [4] Tankala Triloka, Sudhakar R.Marur and Jim Wilson (2006), Rapid thermal predictions for headlamp reflectors, SAE Technical paper, [5] Shiozawa Touichirou, Masatoshi Yoneyama, Kiochi Sakakibara, Shuichi goto, Norihisa Tsuda, Tetsuo saga and Toshio kobayashi (2001), Analysis of natural convection inside an Automotive headlamp by using CFD and temperature prediction of the parts by SHT method, SAE Technical paper, [6] H.S. Lee, Chai J.C., and Patnakar S.V (1994), Finite volume method for radiation heat transfer, Journal of Thermophysics and Heat Transfer, 8, pp [7] Ozceyhan Veysel and Necdet Altuntop (2005), Heat transfer and thermal stress analysis in grooved tubes, Sadhana, 30, Part 4, pp , IJIRAE- All Rights Reserved Page -99