A Study on the Thermal Performance of Automotive LED Head Lamps with Synthetic Jet

Transcription

1 A Study on the Thermal Performance of Automotive LED Head Lamps with Synthetic Jet S. W. Jang Department of Mechanical Engineering, Kongju National University, Gongju-si, Chungcheongnam-do, South Korea. Y. L. Lee* Dept. of Mechanical & Automotive Engineering, Kongju National University, Gongju-si, Chungcheongnam-do, South Korea. Abstract LED head lamps have a longer lifespan and better luminous efficacy compared to halogen bulbs. Recently, heat dissipation has emerged as an important issue due to the high power and more compact sizes of LED head lamps. This study adopted a forced cooling method using synthetic jet and optimized the thermal performance of LED head lamps by varying the shape of heat sinks and the types of PCBs. As a result, the optimal thermal performance of automotive LED head lamps was achieved with a two-sided PCB and an optimal heat sink design. Keywords: LED Head lamp, Synthetic jet, Heat sink, Junction temperature, Numerical analysis Introduction Automotive LED head lamps have a longer lifespan and better luminous efficacy compared to halogen bulbs. The replacement of halogen bulbs with LEDs will save a significant amount of energy. Given the growing preference for LEDs over halogen bulbs, automobiles are being equipped with LED head lamps. LEDs generate light energy by using 20 to 30% of input power and converting the remaining 70 to 80% to heat. The generated heat raises the temperature at the junction of LED and PCB[1]. The excess heat results in a lower luminous efficacy, the degradation of LED chips[2], short-circuit failure, and the separation of connections, which ultimately shorten the overall lifespan. To prevent these problems, it is necessary to ensure a high thermal performance when using LED head lamps. Extensive research has been conducted to enhance the thermal performance of LED headlights for high-efficiency automobiles. Yung et al.[3] optimized the thermal performance of an LED array package by using computational fluid dynamics and provided experimental validation. Jung et al.[4] developed LED headlights to replace existing automotive halogen bulbs. They presented an optimal thermal system through numerical analysis, and provided experimental validation by using prototypes. Jeong et al.[5] designed an optimal heat sink for the efficient cooling of the junction temperature of an LED module, and enhanced the cooling performance by improving the heat flow rate of horizontal fins. Christensen et al.[6] modified a high-power LED package array to optimize the performance, and selected a ceramic packaging architecture to lower thermal resistance. Wang et al.[7] applied thermoelectric cooling to automotive LED head lamps. They optimized the thermal performance by using a thermoelectric cooling device and a cooling fan. Kim et al.[8] improved the thermal performance by adding heat pipes to an LED package. They found that thermal resistance increases with input power and ambient temperature, but decreases with wind velocity. In addition, they presented a model based on a parallel circuit system and compared the analysis results to experimental data. Previous research on LED was mostly aimed at improving thermal performance for relatively low power headlights. However, the recent increase in demand for high-output LED headlights has led to a higher input power, which has caused problems in heat dissipation given the limited space for installation in automobiles. Against this backdrop, this study adopted a synthetic jet to improve the thermal performance of high-output LED head lamps and performed the optimization of PCB and heat sinks by using CFD. The critical junction temperature was set as 110 o C [4]. Numerical Analysis Three heat sink designs were considered in this study. Numerical analysis was performed to predict the junction temperature in relation to the heat sink design. As shown in Fig. 1, model 1 had a fin thickness of 1 mm, and a 3.3mm gap between the fins. Model 2 used helical fins with the same fin thickness to increase the heat transfer area. Model 3 involved fins with a thickness of 1.5mm and a gap of 2.8mm between the fins. Fig. 2 shows the 3D synthetic jet model of model 1. The mass flow outlet of synthetic jet was matched to the gap between the fins to facilitate air flow. As shown in Fig. 2, a two-sided model was considered to further lower the junction temperature. The angle between PCB surfaces was assumed to be 120 o. Numerical analysis was conducted with Ansys Fluent V14[9]. Both forced and natural convection were taken into account in the analysis. The other conditions included a heat generation rate of 19.2W by LED, a thermal resistance of 0.8 o C/W at the junction of LED and PCB, an emissivity of 3294

2 0.7, and a temperature of 25 o C. The synthetic jet flow rates were kg/s, kg/s, and kg/s, respectively. Figure 2: Schematic diagram of the synthetic jet model (a) Model 1 (b) Model 2 Figure 3: Schematic diagram of the two-sided PCB (c) Model 3 Figure 1: Schematic diagram of the heat sink model Results and Discussion Thermal analysis of the head lamp under natural convection This study first examined the thermal performance of the head lamp under natural convection without synthetic jet. Fig. 4 shows the temperature distribution for each of the three models. The temperature distribution is similar across the models, but the best junction temperature was achieved by model 1. As shown in Table 1, model 2 had the greatest heat transfer area. This indicates that the plate fins of model 1 have a smoother flow than the helical fins of model 2. The thermal performance of heat sinks is influenced not only by the heat transfer area, but also by the flow rate in the area surrounding the fins. Table 1: Heat transfer area and mass by model Models Heat transfer area Mass Model m kg Model m kg Model m kg 3295

3 Thermal analysis of the head lamp under forced convection Synthetic jet was applied to each heat sink to determine if there is an improvement in thermal performance. Fig. 5 presents the junction temperature of each model under a synthetic jet flow rate of kg/s. The junction temperature was o C for model 1, o C for model 2, and o C for model 3. Model 1, which had the best thermal performance, was the only model that satisfies the critical junction temperature. For model 1, the junction temperature under a synthetic jet flow rate of kg/s was lower than that of natural convection by approximately 35 o C. Next, the effect of mass flow rate of the synthetic jet on thermal performance was studied. Fig. 6 shows the junction temperature of each model in relation to changes in the mass flow rate of synthetic jet. With an increasing mass flow rate, the junction temperature tends to decrease linearly. For model 1, when using synthetic jet with a flow rate of 0.001kg/s, the junction temperature can be lowered up to 93.6 o C. Figure 5: Junction temperature of each model under synthetic jet Figure 4: Temperature contour for each model during natural convection Figure 6: Junction temperature of each model with a varying mass flow rate of synthetic jet 3296

4 Thermal analysis of the two-sided head lamp with synthetic jet Fig. 7 shows the streaklines of the two-sided headlamp with synthetic jet. In Fig. 7(a), the streaklines head upward due to the effect of natural convection immediately after passing the heat sink. In Fig. 7(b) and (c), the streaklines flow smoothly by following the heat sink and PCB as the synthetic jet flow rate increases. As such, a higher synthetic jet flow rate is expected to enhance the cooling effect. Changes in the junction temperature of the two-sided LED head lamp were also examined in relation to the flow rate of synthetic jet. As shown in Fig. 8, the junction temperature drops with an increasing synthetic jet flow rate. When the synthetic jet flow rate is 0.001kg/s, the junction temperature decreases up to 90 o C, which is approximately 20 o C lower than the critical junction temperature. The junction temperature was improved over that of the one-sided PCB model by 3.4 o C. Figure 7: Streaklines of the two-sided PCB model under synthetic jet Figure 8: Junction temperature of the two-sided head lamp with a varying mass flow rate of synthetic jet Conclusion To improve the thermal performance of high-output LED head lamps, this study optimized various components such as the synthetic jet, PCB, LED module, and heat sinks. The analysis of this study led to the following conclusions: 1. For natural convection, model 1 had the best junction temperature of o C, but failed to satisfy the critical junction temperature. The head lamp considered in this study cannot achieve optimal thermal performance without forced convection. 2. When using synthetic jet, the junction temperature decreases linearly with the synthetic jet flow rate. Model 1 satisfied the critical junction temperature with a junction temperature of 93.6 o C. 3. A two-sided PCB head lamp was introduced to further enhance the cooling efficiency, and the junction temperature improved down to 90.2 o C. The combination 3297

5 of synthetic jet, two-sided PCB and model 1 had the junction temperature lowered by approximately 20 o C. As future work, prototypes of the proposed LED head lamp need to be manufactured to validate the thermal performance. Acknowledgement This work was supported by the Human Resources Development program(no ) of the Korea Institute of Energy Technology Evaluation and Planning(KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy. References [1] Shen, Q., Sun, D., Xu, Y., Jin, T., Zhao, X., 2014, "Orientation effects on natural convection heat dissipation of rectangular fin heat sinks mounted on LEDs", International Journal of Heat and Mass Transfer, Vol. 75, pp. 462~469. [2] Shin, D. H., Baek, S. H., Ko, H. S., 2016, Development of heat sink with ionic wind for LED cooling, International Journal of Heat and Mass Transfer, Vol. 93, pp. 516~528. [3] Yung, K. C., Liem, H., Choy, H. S., Cai, Z. X., 2014, Thermal investigation of a high brightness LED array package assembly for various placement algorithems, Applied Thermal Engineering, Vol. 63, pp. 105~118. [4] Jung, E. D., Lee, Y. L., 2015, "Development of a heat dissipating LED headlamp with silicone lens to replace halogen bulbs in used cars", Applied Thermal Engineering, Vol. 86, pp. 143~150. [5] Jeong, M. W., Jeon, S. W., Kim, Y. C., 2015, Optimal thermal design of a horizontal fin heat sink with a modified-opening model mounted on an LED module, Applied Thermal Engineering, Vol. 91, pp. 105~115. [6] Christensen, A., Graham, S., 2009, Thermal effects in packaging high power light emitting diode arrays, Applied Thermal Engineering, Vol. 29, pp. 364~371. [7] Wang, J., Zhao, X. J., Cai, Y. X., Zhang, C. Z., Bao, W. W., 2015, " Experimental study on the thermal management of high-power LED headlight cooling device integrated with thermoelectric cooler package ", 2015, Energy Conversion and Management, Vol. 101, pp. 364~371. [8] Kim, L., Choi, J. H., Jang, S. H., Shin, M. W., 2007, " Thermal analysis of LED array system with heat pipe", Thermochimica Acta, Vol. 445, pp [9] Ansys Fluent version 14, 2013, Ansys inc., User's Manual version