Design and Optimization of Horizontally-located Plate Fin Heat Sink for High Power LED Street Lamps Xiaobing Luo 1,2*, Wei Xiong1, Ting Cheng1 and Sheng Liu2, 3 School of Energy and Power Engineering, Huazhong University of Science & Technology, Wuhan, Hubei, 430074, China 2 Wuhan National Lab for Optoelectronics, Huazhong University of Science & Technology, Wuhan, Hubei, 430074, China 3 School of Mechanical Engineering, Huazhong University of Science & Technology, Wuhan, Hubei, 430074, China * Corresponding author, Tel: 86-13971460283, Fax: 86-27-87557074, Email:
[email protected]
1
Abstract When junction temperatures of the light emitting diode (LED) chips packaged inside LED lamps exceed their maximal limits, the optical extraction and the reliability/durability of the LED lamps will be jeopardized, therefore, thermal management is very important for high power LED street lamps. In this research, a design and optimization method of horizontally-located plate fin heat sink was presented to improve the heat dissipation of high power LED street lamps. To prove the feasibility of the present design and optimization method, a 112-W LED street lamp was co-designed by concurrent engineering of optical, thermal, and stress requirements. The engineering prototypes were manufactured and experimentally investigated. The experimental results demonstrated that the maximal heat sink temperature remained to be stable at 45°C when the ambient temperature was 25°C, the maximal temperature difference between steady state temperature and environment temperature for the heat sink was less than 21°C. Comparing the results achieved by the design with the ones by the experiment, it is found that the design and optimization method is feasible and works well for realizing the horizontally-located heat sink of such kind of high power LED street lamp. Nomenclatures exposed base area between two fins, [m2] Abp cross section area of the fin ≡ Ht, [m2] Ac a single fin surface area, [m2] Afin Gr Grashof number ≡ gβθδ3/ν2 H fin height, [m] L length of array, [m] g acceleration of gravity, m/s2 average fin heat transfer coefficient, [W/ m2K] hfin average heat transfer coefficient of exposed base hbp area, [W/ m2K] thermal conductivity of heat sink, [W/mK] kfin thermal conductivity of fluid, [W/mK] kf l characteristic dimension,[m] m fin parameter ≡ (hP/kA)1/2 ,[m-1] n number of plate-fins of the array Nu Nusselt number ≡ hδ/ kf P cross section circumference of the fin ≡ 2(H+t), [ m2] Pr Prandtl number fin perimeter ≡ 2(H+t), [m] Pfin expected heat dissipation, [W] QT
978-1-4244-4476-2/09/$25.00 ©2009 IEEE
Qbp Qfin Qhs Ra s t tb
heat dissipation of exposed base area, [W] heat dissipation of single fin, [W] actual heat dissipation of heat sink, [W] Raleigh number ≡ GrPr fin spacing, [m] fin thickness, [m] thickness of base plate, [m]
Greek Symbols β thermal coefficient of expansion, [1/K] θ excess temperature to the ambient temperature , [K] ν mean kinematic viscosity of fluid, [m2/s] ρ fin material density, [kg/m3] δ characteristic dimension, [m] Subscripts base plate bp fin fin f fluid. (air) hs heat sink Amb ambient Introduction In recent years, light emitting diode (LED) has begun to play more and more important role in many applications including back lighting for cell phones, LCD displays, interior and exterior automotive lighting such as headlights, large signs and displays, signals and illumination. [1] LED will soon be used in general illumination because of its distinctive advantages including high efficiency, good reliability, long life, variable colors and low power consumption. An expectation about high power LED is that it will be the dominant lighting technology by 2025[2]. In China, with the push of the government for more energy saving, the LED may be used earlier than this time. The estimation by Chinese authorities is that if LED dominates general lighting market in 2010, one third of the present lighting power consumption could be saved. One typical general lighting product of LED is LED street lamp, which is emerging in market, in particular in China. For modern LED street lamps, both optical extraction and thermal management are two critical factors for their high performance. In general, most of the electronic power of street lamp is converted into heat, which greatly reduces the chips’ luminosity. In addition, the high junction temperature of LED chips in the lamp will shift the peak wavelength, which will change the color of light. Narendran and Gu have experimentally demonstrated that the life of LEDs decreases with the increase of the junction temperature in an
854
2009 Electronic Components and Technology Conference
exponential manner.[3] Therefore, a low operation temperature is essential for LED chips in the LED street lamp. Since the market demands that LED street lamp have high power and small size, there is a contradiction between the power density and the operation temperature, especially when applications require LED street lamp operate at high power to obtain the desired brightness. [4] In terms of thermal management of LED street lamps, fin heat sinks are widely used because of their good reliability and low cost. There are few reports on the fin heat sink design for LEDs, but lots of professonals have done plenty of research on the fin heat sink for other electronic cooling. Typically, Culham and Muzychka developed a procedure which presented optimization of fin heat sink design parameters based on a minimization of the entropy generation associated with heat transfer and fluid friction. [5] All relevant design parameters for plate fin heat sinks, including geometric parameters, heat dissipation, material properties and flow conditions could be simultaneously optimized to characterize a heat sink that minimizes entropy generation and in turn results in a minimum operation temperature. Culham and Khan developed an analytical model to calculate the best possible design parameters for plate fin heat sinks using an entropy generation minimization procedure with constrained variable optimization. [6] The method characterized the contribution to entropy production of all relevant thermal resistances in the path between source and sink as well as the contribution to viscous dissipation associated with fluid flow on the boundaries of the heat sink. Teertstra et al. proposed an analytical model to predict the average heat transfer rate for forced convection, air cooled plate fin heat sinks for use in the design and selection of heat sinks for electronics applications. [7] By using a composite solution based on the limiting cases of fully-developed and developing flow between isothermal parallel plates, the average Nusselt number could be calculated as a function of the heat sink geometry and fluid velocity. Lee obtained an analytical simulation model to predict and optimize the thermal performance of bidirectional fin heat sinks in a partially confined configuration. [8] Sample calculations were carried out, and parametric plots were provided, the results illustrated the effect of various design parameters on the performance of a heat sink. Azar et al. developed a narrow channel heat sink, which was a departure from the micro channel heat sink, and could be used for air cooling of high power components. [9] In the above references, most of them have focused on the vertically-located plate fin heat sink design, however, in LED street lamp, the heat sink base should be located in the horizontal direction. For horizontally-located heat sink, the design and optimization have more difficulty because the gravity direction is along the fin height. In this case, the heat transfer coefficient between the fins is difficult to determine. Such a situation brings much trouble for the LED engineers, especially when they design LED lamps. A simple engineering solution to horizontally-located heat sinks is strongly demanded for LED industry. In this paper, a new design method for horizontallylocated plate fin heat sinks was presented and the corresponding code was developed. Fin height, fin thickness
and fin spacing of horizontal plate fin heat sinks were designed and optimized with the aim of maximal heat dissipation and least material. The method is based on the empirical equations and easy for engineers. Based on the method, heat sink for a 112 watts LED street lamp was designed and manufactured, the experimental studies on the 112 watts street lamp were conducted. The comparison between experiment and design demonstrates that the method works well for horizontally-located fin heat sinks. Design and Optimization of Horizontally-located Fin Heat Sink 1. Design Model For the heat sink of LED street lamp, it is usually designed as horizontally-located plate fin heat sink, as shown in Figure.1. Although the heat transfer coefficient is comparatively low in natural convection (usually less than 10W/Km2), the plate-fin natural convection heat sinks offer distinctive advantages in cost and reliability.
(a)
(b)
(c) Fig.1. Horizontally-located rectangular plate fin heat sink—(a) horizontal configuration; (b) side elevation; and (c) Top view. In the design and optimization of the horizontally-located plate fin heat sinks, heat transfer coefficient is a key factor. However, the averaging heat transfer coefficient is associated with the fin dimensions, which are the optimization factors and they are strongly coupled. In addition, due to the overlapping of the boundary layers between the adjacent fins, it is difficult to solve boundary layer equations, correspondingly, it is very difficult to calculate the fin heat transfer coefficient during computation process. For the application of horizontally-located plate fin heat sink in LED, heat is generated by the LED chips and then conducted through aluminum alloy base board and finally dissipated to the surroundings by convection. To simplify the heat transfer and optimize the heat sink of high power LED street lamp, there are certain assumptions: 1）The material is
855
2009 Electronic Components and Technology Conference
isotropic; and 2 ） The spreading and contact resistance is neglected in heat sink. 2. Design and Optimization Method Figure 1 shows the heat sink dimensions and their indications. The total heat Qhs that a heat sink can dissipate is expressed by the formula: (1) Qhs= Qbp+n·Qfin where Qbp is the heat that is dissipated by the exposed base of the heat sink and is defined by formula (2) , Qfin is the heat that is dissipated by fins of the heat sink and is defined by formula (3): (2) Qbp= hbp·(n-1)·θbp·Abp where hbp is the average heat transfer coefficient of the exposed base area, n is the number of plate-fins of the array, θbp is the excess temperature from the heat sink base to the ambient temperature, Abp is the surface area of the exposed base and could be defined as: (3) Abp=s·L where s is the fin spacing, L is the fin thickness. Qfin=hfin·Afin·θbp (4) where hfin is the averaging heat transfer coefficient of the heat sink, Afin is the surface area of a single fin and could be defined as: (5) Afin=2(H·t+L·H+L·t/2) where H is the fin height, and t is the fin thickness. For most of the applications, especially for the heat sink design for LED street lamp, the heat transfer and the approximate temperature difference between the heat sink and the environment are given, the heat dissipation are designed to meet the amount of heat dissipation and the heat sink mass. Since most plate fin heat sinks are produced with extrusion aluminum alloys [6], with considerations of manufacturability and strength, the ranges of fin parameters should be as follows; 1) fin thickness is between 1mm to 3mm; 2) fin spacing is between 1mm to 15mm; and 3) fin height is between 25mm to 50mm. In order to obtain the averaged heat transfer coefficient of the heat sink, the total heat dissipation area can be divided into two parts. One is from exposed base area and the other is from fin array. A. Heat Dissipation from Exposed Base Area 1）When the ratio of fin spacing to fin height is less than 0.28, the flow inside the fins is enclosed space natural convection. In this case, the characteristic dimension is the height of enclosed space. If the value in the square brackets of Eq. (8) is negative, Nusselt number of base plate Nubp is replaced with 1. Eq. (6) to Eq. (7) are available when the Raleigh number of base plate Rabp
