The 2012 International Conference on Green Technology and Sustainable Development (GTSD2012) A STUDY ON ENHANCING HEAT TRANSFER EFFICIENCY OF LED LAMPS Thanhtrung Dang 1 , Vanmanh Nguyen 1 , Nhatlinh Nguyen 1 , Tansa Nguyen 1 , Quocdat Vu 1 , Dinhvu Tran 1 , Vanchung Ha 1 , Jyh-tong Teng 2 , and Ngoctan Tran 2 1 Ho Chi Minh University of Technical Education, Vietnam 2 Chung Yuan Christian University, Taiwan ABSTRACT This paper presented investigations for enhancing heat transfer efficiency of LED Lamp, using numerical and experimental methods. The solver of numerical simulations – COMSOL – was developed by using the finite element method. The results obtained from numerical simulation were in good agreement with those obtained from the experimental data, with the maximum percentage error being less than 8%. In addition, an optimization on heat transfer phenomena of LED lamps was also done in the study. KEYWORDS: Temperature, heat transfer, efficiency, heat sink, LED. 1. INTRODUCTION Nowadays, light emitting diode (LED) has become more popular because it needs only low consumption in electricity, but it can provide high luminosity. LEDs are more energy efficient than other conventional lamps for two reasons - they require less energy to operate than incandescent and fluorescent bulbs and they supply more lighting capability per watt than incandescent bulbs. The increased efficiency equates to lower energy costs and less environmental impacts. However, LED's working temperature should be accounted for. It is estimated that approximately 70-85% LED power is converted into heat. High operating temperature would reduce the LED lifetime and brightness. With high power LEDs, they could generate more heat. Many cooling methods have been used to dissipate heat from LED lamps. The normal methods are using natural convection by adding additional surface area to be in contact with the environment which is at lower temperature. One effective way to increase the contact area is by attaching a heat sink to the heat source; in this case, the heat source is the LED lamp [1]. Heat sinks are devices which enhance heat dissipation from a hot surface, usually for the case of a heat generating component, to a cooler ambient. Alvin et al. [1] studied thermal resistance of extruded fin heat sink on LED lamp. In their study, the most significant factor affecting the thermal resistance value between LED and heat sink is the heat sink mounting pressure, followed by thermal interface material and heat sink materials. However, the study did not compare the influence of heat sink configurations on the overall thermal resistance for the LED system. Luo et al. [2] presented temperature estimation of high-power light emitting diode street lamp by a multi-chip analytical solution. In their study, the fin-heat-sink is still the predominant method used in the lighting industry due to its highest reliability and lowest cost. Heat pipe [3, 4] is becoming a good option for emerging high power LEDs. Thermal analysis of high power LED array packaging with microchannel cooler was done by Yuan et al. [5]. Weng [6] studied advance thermal enhancement and management of LED packages by using the FEM modeling technique for simulating the LED package with different heat slug, PCB, cooling condition and chip size. In [7, 8], liquid The 2012 International Conference on Green Technology and Sustainable Development (GTSD2012) metals were used as the coolants to enhance heat transfer for heat sinks. Liu [9] presented structural optimization of a microjet based cooling system for high power LEDs. Several numerical and experimental investigations were done in [10-12] on the behaviors of heat transfer and pressure drop for microchannel heat sinks and heat exchangers. In their study, DI water was used as a working fluid. Based on reviews of the above literatures, it is essential to study the heat transfer behaviors of the LED heat sink, using both numerical and experimental methods. For the present study, air was used as the working fluid and the influence of configuration of LED heat sink on heat transfer characteristics was investigated. In the following sections, two cases will be discussed for the LED heat sink: (1) the case with natural convection and (2) the case with forced convection. 2. METHODOLOGY 2.1 Numerical simulation The governing equations in this system consist of the continuity equation, momentum equations, and energy equation [10-12]. The equations can be expressed by u/t+(u)u=[-pI+(u+(u)T)]+F (1) u = 0 (2) CpT/t+(-T)=Qi-CpuT (3) where  is dynamic viscosity,  is density, u is velocity field, p is pressure, I is the unit diagonal matrix, F is body force per unit volume (F x = F y = F z = 0 N/m 3 ), Q i is internal heat generation, T is temperature, C p is specific heat at constant pressure, and  is thermal conductivity. Numerical study of the behavior of the LED heat sinks with 3D heat transfer was done by using the COMSOL Multiphysics software, version 3.5. The algorithm of this software was based on the finite element method. The generalized minimal residual (GMRES) method was used to solve for the present case and shown in more detail in [1, 10-12]. For this study, air was used as the working fluid. No internal heat generation was available. Boundary condition for the heat sinks was a constant room temperature at 30 ºC. There are three models to be used for simulation of LED heat sink, as shown in Fig. 1: (1) without any crevice, (2) with one crevice, and (3) with two crevices. The substrate material for heat sinks is aluminum having the thermal conductivity of 237 W/(mK), density of 2,700 kg/m 3 , and specific heat of 904 J/(kgK) [13, 14]. a) LED heat sink without crevice (Model 1) b) LED heat sink with one crevice (Model 2) c) LED heat sink with two crevices (Model 3) Figure 1. Models of LED heat sinks The 2012 International Conference on Green Technology and Sustainable Development (GTSD2012) 2.2 Experimental setup The experimental system includes a power supply, a temperature measurement unit, a fan, and a velocity measurement unit, as shown in Fig. 2. The heat dissipation patterns – fin aluminum heat sink - were tested under different heat transfer modes: natural convection and forced convection. The LED with a power supply of 7W was used in this study. Accuracies and ranges of testing apparatuses are listed in Table 1. Table 1. Accuracies and ranges of testing apparatuses Testing apparatus Accuracy Range Thermocouples  0.1 C 0 100 C Velocity meter  1 % 0  50 m/s Figure 2. A photo of the experimental system Experimental data obtained from the LED heat sinks were under the constant room temperature condition of 30 ºC. For the case with natural convection, air velocity was measured at 0.1 m/s; for forced convection, air velocity was measured at 1.2 m/s. At the middle fin of the heat sink, five thermocouples were soldered on the top of fin to obtain the temperature readings. 3. RESULTS AND DISCUSSION 3.1 Natural convection condition a. For Model 1 Heat Sink Configuration For experiments carried out in this study, with LED capacity of 7 W and air velocity of 0.1 m/s, heat transfer from the LED through the heat sink was constant; the bottom temperature of heat sink was measured to be 49 ºC. Fig. 3 shows temperature profiles of heat sink and air for model 1 heat sink configuration. Figure 3. Temperature profiles of heat sink and air for Model 1 heat sink configuration 0 10 20 30 40 50 0 20 40 60 80 Temperature of the middle fin, o C Fin length of LED, mm Numerical Experimental Figure 4. Comparison between numerical and experimental results for Model 1 heat sink configuration Comparison between numerical and experimental results for Model 1 heat sink configuration is shown in Fig. 4. It is observed that the results obtained from the numerical simulation are in good agreement with those obtained from the experimental data, with the maximum diffrence of 4.6%. The difference is due to the error in temperature measurements caused by temperature sensors which were soldered at the outer rims of the fins while The 2012 International Conference on Green Technology and Sustainable Development (GTSD2012) the numerical results indicated more exact phenomena taken place in the air surrounding the heat sink. b. For Model 2 Heat Sink Configuration For the same experimental condition above, with air velocity of 0.1 m/s, the bottom temperature of heat sink was measured to be 50.4 ºC. Temperature profiles of heat sink and air for Model 2 heat sink configuration are shown in Fig. 5. Fig. 6 shows the comparison between numerical and experimental results. Figure 5. Temperature profiles of heat sink and air for Model 2 heat sink configuration 0 10 20 30 40 50 0 20 40 60 80 Temperature of the middle fin, o C Fin length of LED, mm Numerical Experimental Figure 6. Comparison between numerical and experimental results for Model 2 heat sink configuration c. For Model 3 Heat Sink Configuration With the same conditions, the bottom temperature of heat sink was measured to be 49.7 ºC. The Fig. 7 shows temperature profiles of heat sink and air for Model 3 heat sink configuration. Figure 7. Temperature profiles of heat sink and air Model 3 heat sink configuration 0 10 20 30 40 50 0 20 40 60 80 Temperature of the middle fin, o C Fin length of LED, mm Numerical Experimental Figure 8. Comparison between numerical and experimental results for Model 3 heat sink configuration Comparison between numerical and experimental results for Model 3 heat sink configuration is shown in Fig. 8. It is also indicated that the numerical and experimental results are in good agreement. From Figs. 3-8, for the natural convection case, it is observed that the bottom temperature of heat sink for Model 1 heat sink configuration was the lowest. It is due to the fact that Model 1 heat sink configuration has the largest heat transfer area. The 2012 International Conference on Green Technology and Sustainable Development (GTSD2012) 3.2 Forced convection condition Experiments for forced convection condition were done on Model 3 heat sink configuration by using a fan with an air velocity of 1.2 m/s. For this case, the bottom temperature of heat sink was measured to be 38.5 ºC. Figure 9 shows the comparison between numerical and experimental results for the case with forced convection. It is also indicated that the numerical and experimental results are in good agreement, with the maximum discrepancy of the temperature estimated to be less than 8 %. From Figs. 4-9, it is shown that the heat transfer capability obtained from the case with forced convection is higher than that obtained from the case with natural convection case: at the same room temperature condition and LED power supply capacity, the bottom temperature of LED heat sink is reduced from 49.7 to 38.5 ºC. 0 10 20 30 40 50 0 20 40 60 80 Temperature of the middle fin, o C Fin length of LED, mm Numerical Experimental Figure 9. Comparison between numerical and experimental results for Model 3 heat sink configuration with forced convection case 4. CONCLUSION Numerical and experimental studies have been performed on three LED heat sinks with different configurations. In natural convection case, the heat transfer capability obtained from the heat sink without crevice was higher than those obtained from the heat sinks with crevice or crevices. The heat transfer capability obtained from the case with forced convection is higher than that obtained from the case with natural convection case: at the same room temperature condition and LED power supply capacity, the bottom temperature of LED heat sink is reduced from 49.7 to 38.5 ºC. Furthermore, the results obtained from the experiments were in good agreement with those obtained from the numerical simulations, with the maximum discrepancy of the temperature estimated to be less than 8 %. 5. ACKNOWLEDGEMENTS The supports of this work by (1) the projects (Project Nos. 54-11-CT/HD-CTTB and 38- 12-CT/HĐ-CTTB) sponsored by New Product & Technology Center (NEPTECH) – Hochiminh City Department of Science and Technology of Vietnam, (2) the project (Project No. T2012-16TĐ /KHCN -GV) sponsored by the specific research fields at Hochiminh City University of Technical Education, Vietnam, (3) the project (Project Nos. NSC 99-2221 -E-033-025 and NSC 100-2221 -E-033-065) sponsored by National Science Council of the Republic of China in Taiwan, and (4) the project (under Grant No. CYCU-98-CR -ME) sponsored by the specific research fields at Chung Yuan Christian University, Taiwan, are deeply appreciated. 6. REFERENCES [1] Christian Alvin, Jyh-tong Teng, and Thanhtrung Dang, Thermal Resistance Analysis of Extruded Fin Heat Sink on LED Lamp, The International Electron Devices and Materials Symposium 2011 (IEDMS2011), Taipei, Taiwan, Nov 17-18, 2011, P-C-19, pp. 1-4 [2] X.B. Luo, W. Xiong, T. Cheng, and S. Liu, Temperature estimation of high-power light emitting diode street lamp by a multi-chip analytical solution, IET Optoelectron, 3, 2009, pp. 225–232 [3] L. Kim, J.H. Choi, S.H. Jang, and M.W. The 2012 International Conference on Green Technology and Sustainable Development (GTSD2012) Shin, Thermal analysis of LED array system with heat pipe, 6th Symposium of the Korean Society of Thermophysical Properties, Seoul, 2006, pp. 21–25 [4] Zirong Lin, Shuangfeng Wang, Jiepeng Huo, Yanxin Hu, Jinjian Chen, Winston Zhang, and Eton Lee, Heat transfer characteristics and LED heat sink application of aluminum plate oscillating heat pipes, Applied Thermal Engineering, 31, 2011, pp. 2221-2229 [5] L.L. Yuan, S. Liu, M.X. Chen, and X.B. Luo, Thermal analysis of high power LED array packaging with microchannel cooler, 7th International Conference on Electronics Packaging Technology, Shanghai, 2006, pp. 574–577. [6] C.J. Weng, Advanced thermal enhancement and management of LED packages, International Communications in Heat and Mass Transfer, 37, 2009, pp. 245–248. [7] Y. Deng and J. Liu, A liquid metal cooling system for the thermal management of high power LEDs International Communications in Heat and Mass Transfer, 37, 2010, pp.788–791. [8] K.Q. Ma and J. Liu, Liquid metal cooling in thermal management of computer chips, Front. Energy Power Eng. China, 1, 2007, pp. 384–402. [9] S. Liu, J.H. Yang, Z.Y. Gan and X.B. Luo, Structural optimization of a microjet basedcooling system for high power LEDs, Int. J.Therm. Sci. 47, 2008, pp. 1086–1095. [10] Thanhtrung Dang, Ngoctan Tran and Jyh-tong Teng, Numerical and Experimental investigations on heat transfer phenomena of an aluminium microchannel heat sink, Applied Mechanics and Materials, 145, 2012, pp. 129-133 [11] Ngoctan Tran, Thanhtrung Dang and Jyh-tong Teng, Numerical and experimental studies on pressure drop and performance index of an aluminum microchannel heat sink, 2012 IEEE International Symposium on Computer, Consumer and Control (IS3C2012), June 4-6, 2012, Taichung City, Taiwan, pp. 252-257 [12] Thanhtrung Dang and Jyh-tong Teng, Comparison on the heat transfer and pressure drop of the microchannel and minichannel heat exchangers, Heat and Mass Transfer, 47, 2011, pp. 1311-1322. [13] J.P. Holman, Heat transfer, Ninth Edition, McGraw-Hill, New York, 2002 [14] COMSOL Multiphysics version 3.5 (2008) – Documentation. Contact Thanhtrung Dang, Ph.D. Tel: +84913606261 Email: trungdang@hcmute.edu.vn . percentage error being less than 8%. In addition, an optimization on heat transfer phenomena of LED lamps was also done in the study. KEYWORDS: Temperature, heat transfer, efficiency, heat sink, LED. . Thanhtrung Dang, Ngoctan Tran and Jyh-tong Teng, Numerical and Experimental investigations on heat transfer phenomena of an aluminium microchannel heat sink, Applied Mechanics and Materials,. International Conference on Green Technology and Sustainable Development (GTSD2012) A STUDY ON ENHANCING HEAT TRANSFER EFFICIENCY OF LED LAMPS Thanhtrung Dang 1 , Vanmanh Nguyen 1 , Nhatlinh