Structural design of distributed high power LED street light radiator

1 question raised

As a new generation of environmentally-friendly solid-state lighting source, LED's luminous efficiency has been greatly improved after decades of development. It has low power consumption, long service life, safety and environmental protection, small volume and power, and easy light distribution. A series of advantages [1]. With the development of technology, the integration of LED chips is getting higher and higher. The higher the power of the LED, the greater its heat flux density. If the heat cannot be dissipated from the inside of the chip in time, the heat inside the chip will continue to accumulate, causing the junction temperature of the chip to continuously rise. Too high a junction temperature will cause a series of problems such as drift of light wavelength, reduction of light extraction efficiency, and accelerated aging of the chip [2-4]. Therefore, in order to ensure that the LED can work at normal temperature, the generated heat must be dissipated in time. At present, the luminous efficiency of LEDs is only 10% to 20%, and the remaining 80% to 90% of energy is converted into thermal energy [5], so the heat dissipation of LED chips becomes crucial.

Many scholars have studied finned radiators. Hung-YiLi, Shung-MingChao [6] studied the performance of plate-fin radiators in staggered flow, considering the Reynolds number of cooling air, the influence of fin height and width on the thermal resistance and pressure drop of the radiator. Dong-Kwon Kim [7] et al. studied the effect of fin thickness variation on the heat dissipation performance of the heat sink in the direction of vertical fluid flow, indicating that increasing the fin thickness along the vertical fluid flow direction can reduce the thermal resistance of the heat sink. . Avram Bar-Cohen [8] et al. studied the optimal array of fins for the least material and the effect of different materials on the heat dissipation of the finned heat sink.

In the study, an optimized heat sink was used for optimization. The ANSYS finite element software was combined with the orthogonal design method to optimize the structural parameters of the distributed 200W Led Street Light radiator, as shown in Figure 1. The normal operating temperature of high-power LED lamps is below 65 °C, so the main purpose of this paper optimization is to ensure that the LED lamps work at normal temperature, and on this basis, the quality of the heat sink is lighter.

Figure 1 Distributed LED street light

2 radiator analysis model

2.1 Basic form of heat transfer

There are three basic ways of heat transfer in nature: heat transfer, heat convection, and heat radiation. In the LED cooling system, these three heat transfer methods exist. For reasons of reliability and cost, street light radiators generally use natural convection heat dissipation. Therefore, the analytical model of the LED heat sink can be simplified into a heat transfer heat transfer model that does not consider thermal convection and heat radiation. In this way, heat dissipation can be satisfied without considering heat convection and heat radiation, so in reality, it must meet the heat dissipation requirements.

The heat flux of convective heat transfer can be expressed by Newton's cooling formula

Where A is the heat dissipation area, h is the surface heat transfer coefficient of convective heat transfer, Δt is the temperature difference, and the specified temperature difference is always positive [9].

It can be known from equation (1) that the magnitude of the convective heat transfer heat flow is related to the heat dissipation area, the temperature difference, and the surface heat transfer coefficient. Therefore, the heat dissipation area can be increased to enhance the heat dissipation performance of the heat sink.

2.2 Establishment of the radiator model

The structure of the heat sink is shown in Figure 2. The overall size is 300mm × 600mm × 110mm. It has a spatially symmetrical structure, so only 1/2 of the temperature field is simulated for analysis. The substrate of the heat sink is curved, and a 6×20 LED array is formed on the substrate (1W, 3W, 1W, 1W, 3W, W1 array from left to right in the figure); there are 5 pairs of fins, and the lower left of the fins The edge is on the cylindrical surface; the two cylindrical holes in the middle serve as a support, and the arc-shaped structure on the top plays an auxiliary role. Compared with the traditional radiator, the radiator has the following two advantages: 1 the fins of the radiator are horizontal, dust and dirt are not easy to accumulate on the fins, and it can also prevent rain, which can ensure the heat dissipation of the radiator. effect. The conventional heat sink has fins that are vertical, and dust and dirt can easily accumulate on the fins, which affects the heat dissipation effect of the heat sink. 2 The fins of the radiator are horizontal, making full use of aerodynamics, and the buoyancy of the air can reduce the bending torque of the street lamp to its pole. This can ensure the safety performance of LED street lights.

The outer surface of the radiator in contact with air is set to natural convection, and the convective heat transfer coefficient is 10 W/(m 2 ·K). The two cylindrical holes of the radiator are supported by two brackets, and the inner surface is not in contact with air, and can be defined as adiabatic. Due to the sealing effect of the lampshade, the remaining heat sink surface is also defined as adiabatic. The material of the heat sink is aluminum alloy 104, and its thermal conductivity is 147 W/(m2·K). The ambient temperature was set to 35 °C. The size of the LED heat source is much smaller than the size of the heat sink substrate, so the LED heat source can be simplified to a point heat source. The finite element model of the heat sink is shown in Figure 3.

3 Optimization design and result analysis of radiator structural parameters

3.1 Orthogonal test design

The orthogonal experimental design method is a high efficiency, fast and economical design method. In this paper, the influence of different structural parameters of the radiator on its temperature field is studied. In order to reduce the scale of operation, orthogonal experiments are designed to perform multiple thermal analysis on the radiator structure. The fin thickness, fin spacing, fin outer contour radius and substrate thickness were selected as design variables, and the maximum temperature of the heat sink and the quality of the heat sink were used as experimental indicators. Taking into account the mold design of the heat sink and the size limit of the entire LED street light, determine the range of values of each design variable: fin thickness 2 ~ 4mm, fin spacing 6 ~ 8mm, fin outer contour radius 176 ~ 180mm, substrate thickness 5 to 9 mm. Each variable takes 3 levels, so the test is a 4-factor 3 level test with an L9 (34) orthogonal table. The factor and level setting table is shown in Table 1.

Table 1 Factor and Level Settings Table

3.2 Analysis of test results

The test results are shown in Table 2.

Table 2 Test results table

The results of the orthogonal test simulations in Table 2 were analyzed by the range analysis method, as shown in Tables 3 to 6.

In the scope of the simulation, the influence of various structural parameters of the radiator on its mass and heat dissipation performance can be known by the extreme difference of the test indicators by the various factors in Tables 3 to 6. The order of influence of various factors on the quality of the heat sink is: fin thickness, substrate thickness, fin outer contour radius and fin spacing; the order of influence of each factor on the maximum temperature of the heat sink is: fin spacing, fin Thickness, fin outer contour radius, and substrate thickness.

The optimal level combination of each factor can be determined by the average of the test indicators at different levels of each factor in Tables 3-6. For the test index of radiator quality, the optimum level combination is: A1 B3 C1 D1; for the test index of the maximum temperature of the radiator, the optimum level combination is: A1 B1 C3 D1.

Table 3 Range analysis of fin thickness versus test index

Table 4 Range analysis of fin spacing to test indicators

Table 5 Range analysis of the outer radius of the fin to the test index

Table 6 Analysis of the range of substrate thickness versus test index

The combination of optimization levels analyzed by the above two indicators is not exactly the same. For factors A and D, there is no doubt that A1 and D1 are taken; for factor B, the influence on the maximum temperature of the heat sink is ranked first, for heat dissipation. The effect of mass is ranked last, so the B factor takes B1; for factor C, the order of influence on the mass and maximum temperature of the heat sink is the same, but when the C factor is taken as C1, the mass of the heat sink is lower than when the C3 is taken. 3.1%, and the maximum temperature of the radiator increased by 1.2%, so the C factor takes C1. In summary, the optimal level combination of each factor is A1 B1 C1 D1, which is the No. 1 model structure in Table 2. Figure 4 shows the temperature field distribution of the heat sink of the A3 B2 C1 D3 combination. The analysis of the other combinations is similar.

Figure 4 A3 B2 C1 D3 combination temperature field distribution / K

4 Conclusions and prospects

The ANSYS finite element thermal analysis combined with orthogonal test method is used to analyze the influence of different structural parameters on the steady-state temperature field of distributed high-power LED street light radiators, and a better combination of parameters is obtained. This not only ensures the heat dissipation of the radiator, but also helps the company to reduce its cost and obtain better economic benefits. Therefore, this optimization method has a very important role and significance for the promotion of LED lamps.

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