Numerical Analysis of Convection of High Power LED Street Light Radiator

With the development of LED technology, power LEDs have been rapidly developed in the fields of backlight, automotive, outdoor lighting, and commercial lighting. However, the output light flux of a single LED is currently low. For outdoor lighting, LED integration is required to achieve the desired brightness. In the photoelectric conversion of LED, only 10% to 20% of the electrical energy is converted into light output, and the rest is converted into thermal energy, and the heat is conducted through the LED substrate to the externally mounted heat sink for heat dissipation. In order to ensure the life and reliability of the LED street light, the junction temperature of the LED chip should be controlled below 120 °C. LEDs are used for road lighting or tunnel lighting. To meet the requirements of dust, water, lightning, wind pressure, etc., high-power LED street light radiators use natural convection, which is the best cooling method.

For the heat dissipation problem of high-power LED street lamps, domestic and foreign scholars or manufacturers have done a lot of work on the structure and materials of the radiator. Liu Jing et al. - The thermal resistance of the high-power LED illuminator was calculated by the thermal resistance method of the equivalent circuit, and the area of the heat sink was estimated. Then, the Icepak software was used for modeling analysis to change the geometric parameters of the heat sink structure. Analysis and comparison show that the change of fin height has the most obvious influence on the heat dissipation performance. Zhang Qi et al. used ANSYS finite element software to analyze the heat dissipation structure and analyzed the influence of different structural parameters of aluminum heat sink on its temperature field. Through simulation optimization, the quality of the heat sink is effectively reduced, and the structure of the heat sink is optimized. Hu Hongli et al. control the heat dissipation of LED lamps based on semiconductor thermoelectric elements and heat pipe technology, and add a waste heat recovery system with complex structure and many accessories, which affects the stability of its work. Zhang Xuefen has designed a variety of high-power LED radiator models, but for the simulation analysis of natural convection of each radiator, the average heat transfer coefficient is used for the surface. Although the calculation area only has the heat sink itself, the calculation amount is greatly simplified, the calculation time is reduced, and the heat sink design is facilitated. However, due to the complexity of the geometric structure, the average heat transfer coefficient must be repeatedly corrected by experiments and numerical calculations to be accurately obtained. L. Dialameh et al. optimized the three-dimensional numerical simulation of the finned radiator, and analyzed the distribution of the velocity of the air in different rib heights and rib spacing. Under different rib heights and rib spacing, the ribs were different. Average heat transfer coefficient.

The appearance of the conventional 50W LED street light radiator is shown in Figure 1. It is bulky, wastes metal materials and costs are high, which leads to the obstruction of industrial application of high-power LED street lamps. In this paper, the three-dimensional modeling and analysis of the radiator is carried out by Fluent software. The coupled heat transfer problem of the natural convection heat transfer in the large space is studied. The temperature field and the surrounding air flow vector during the heat dissipation process of the radiator are studied. The field has improved the structure of the radiator.

1 radiator analysis

1.1 Numerical analysis

2.1.1.1 Calculation domain

The establishment of 3D physical models, meshing, and the establishment of boundary conditions were all performed in the Fluent pre-processing software Gambit. The calculation domain of the model is shown in Fig. 2. The thickness of the substrate is 4mm, the bottom surface of the substrate is 270mm × 255mm, the thickness of the rib is 2mm, the maximum distance between the middle is 16mm, and the rest are 12mm. The height of the ribs is 32, 33, 33 from the outside to the middle. 34, 34, 35, 35, 36, 36 and 37mm.

In order to meet the accuracy of the natural convection coupling calculation of the radiator, the air flow domain must be large enough to accommodate the pressure inlet boundary conditions. However, the calculation domain is too large, and a dense mesh is required around the heat sink, which causes too many grids to be divided, insufficient computer resources (memory, CPU), and calculations are too slow. So we need to use the multi-layer grid method for the computation domain. In this way, the air flow area near the heat sink and the heat sink can be divided by a small grid unit interval, and the air flow area farther from the heat sink can be meshed. This can reduce the amount of calculation and shorten the calculation time.

1.1.2 Calculation method

The bottom surface of the heat sink substrate continuously supplies heat, and the joint between the substrate and the heat sink ribs is a coupling problem of heat conduction convection heat exchange, and the ribs and the surrounding air undergo natural convection heat exchange. Therefore, the problem is approximately regarded as a three-dimensional, steady-state, constant-physical, coupling problem of heat conduction and convection heat transfer with an internal heat source. The radiation heat transfer caused by the temperature difference is negligible in the calculation process. Due to the floating force caused by the temperature difference, the Boussinesq hypothesis is introduced in the calculation: 1) the viscous dissipation in the fluid is neglected; 2) the physical properties except the density are It is a constant; 3) the density only considers the terms related to the volume force in the momentum equation, and the density in the other items is treated as a constant. In numerical calculations, the heat sink and large space use the whole field discrete, whole-field solution method to combine the heat transfer process in solid and fluid as a unified heat transfer process. The calculation region uses the finite volume method to solve the discrete equations of the governing equations on the co-located grid, and the κ-ε two-equation model solves. The literature indicates that in order to ensure the continuity of the physical heat flux density between the solid and fluid coupling interface, the specific heat capacity in the solid uses the value of the specific heat capacity in the fluid zone. Solving the SIMPLE algorithm using pressure-velocity coupling, the convection terms in the momentum and energy equations are all second-order welcome style, and the pressure term is PRESTO! format. We have done an assessment of grid independence. The criterion is that the temperature on the radiator ribs in the adjacent two calculations does not exceed 1% of the surrounding vector flow field. The conditions for calculating convergence select two adjacent iteration steps. The residual difference is less than the given amount, the energy residual is 1×10-6, and the rest are 0.001.

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1.1.3 Boundary conditions

The bottom surface of the heat sink substrate is assumed to be an equal heat flow boundary condition, given according to the power and the area of the bottom surface of the substrate. The natural convection heat transfer of the ribs on the radiator is the coupling calculation surface, and the boundary condition is determined according to the wall function method. The radiator is a natural convection heat transfer in a large space. The six faces of the large space of the calculation domain are set as pressure inlet boundary conditions, and the ambient pressure is one atmosphere.

1.1.4 Calculation results

When the heating power of the heat sink is 50W, the heat flux density is calculated as follows: q=Q/A, where q is the heat flux density, Q is the heat flux, and A is the bottom surface area of the substrate. When the ambient temperature is 23 ° C, the steady-state temperature field of the radiator rib and the bottom surface of the substrate is numerically calculated as shown in FIG. 3 and FIG. 4 . At this time, the average temperature of the heat sink fins was 39 ° C, and the maximum temperature of the bottom surface of the substrate was 53 ° C.

In order to reduce the influence on the flow field during the experiment, the thermocouple was taken out from above the radiator. In order to determine the solid surface temperature of the main part of the heat sink, a total of 17 thermocouple measurement points were placed on the heat sink. The thermocouples No.1 and No.2 are arranged on the rib bottom and the rib top in the geometric center of the radiator, and the thermocouples No.3 and No.4 are arranged on the rib bottom and the rib top on the front end surface of the middle fin of the radiator, and the thermocouples 5 and 6 are arranged. The rib bottom and the rib top in the middle of the third rib from the left side of the heat sink, the thermocouples No. 7 and No. 8 are arranged on the rib bottom and the rib top of the front end face of the third rib from the left side of the heat sink, and the thermocouples No. 9 and No. 10 A rib bottom and a rib top disposed in the middle of the outermost rib of the left side of the heat sink. The thermocouples 11 to 17 are symmetrically arranged along the symmetry line on the bottom surface of the substrate. The electric heating plate is powered by the voltage regulator and the voltage regulator. When the maximum temperature of the bottom surface of the heat sink substrate is less than 0.5 °C within 10 min, we believe that the heating amount of the electric heating plate and the heat dissipation amount of the heat sink are balanced. Thereafter, the temperature values of the respective measurement points are collected.

1.3 Comparative analysis of numerical calculations and experimental results

In this paper, the experimental heating power interval is 20W, which is carried out in the range of 30-110W, and the maximum temperature of the substrate bottom surface is 41, 55, 67, 78 and 87 °C, respectively. The corresponding values calculate the maximum temperature of the bottom surface of the substrate to be 41, 53, 65, 75 and 88 ° C, respectively. From the experimental and numerical calculation results, it can be seen that as the heating power increases, the maximum temperature of the substrate bottom surface of the heat sink also increases, and changes linearly. The comparison between the experimental results and the numerical calculation results is shown in Fig. 6. The relative error rate is in the range of 1%, indicating that the numerical analysis results are reliable.

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2 Analysis of natural convection process of radiator

During the natural convection of the heat sink, the bottom surface of the substrate continuously supplies heat. Due to the good thermal conductivity of the heat sink material, the heat causes the heat sink temperature to continuously increase. The air around the heat sink is heated, the density becomes smaller, and the density of the air away from the heat sink is poor, resulting in buoyancy. In the heat dissipation process, the numerical calculation results can obtain the velocity vector field in the YZ plane as shown in Fig. 7. It can be visually seen that the air is directly escaping from the periphery of the radiator when the bottom surface of the substrate of the radiator is under the action of buoyancy, and the air is directly moved upward from the periphery of the radiator, but cannot enter the radiator rib to cool the radiator. When the amount of heating of the electric heating plate is balanced with the amount of heat dissipated by the heat sink, the radiator fins become isothermal walls. The speed is caused by the temperature difference, and the small speed causes the floating force of the air to be less than the viscous force. The cold air around the radiator moves up from the periphery of the radiator, and when it finally mixes together, a large stagnation area is formed above the radiator ribs. It can be seen from the velocity vector diagram of the XZ plane in Fig. 8 that two small eddies are formed at both ends in the stagnation region in the direction of the ribs, preventing the surrounding air from entering the radiator ribs. Moreover, because of the viscous force, the air flow rate in this stagnation region is very small, so under such a structure, the ribs of the heat sink do not fully exert the heat dissipation effect of natural convection.

3 radiator structure improvement

The heat dissipation intensity of natural convection depends not only on the flow rate, temperature difference and fluid properties, but also on the synergy between the velocity field and the temperature field. From the analysis of numerical calculation results, in order to improve the heat dissipation capability of the heat sink and reduce the maximum temperature of the bottom surface of the substrate, there are two methods: (1) making the heat sink bigger, the larger the heat sink volume, the larger its heat capacity, The larger the heat dissipation area, the equivalent of reducing the unit heat flux density. However, the disadvantage is that it increases the cost and wastes the metal material; (2) by changing the air turbulence flow field line, the synergy between the velocity field and the temperature field is better. The original radiator model is processed as shown in Fig. 9, so that the air can be circulated up and down in the middle of the radiator under the action of buoyancy, and the ribs are spoiled to increase the air turbulence area. This not only destroys the stagnation area above the radiator, but also increases the air flow and more fully cools the radiator.

In order to analyze the heat dissipation capability of the new structure radiator, the comparative experiment verified the reliability of numerical calculation, so we use the same mathematical model, meshing, calculation method and boundary conditions to numerically calculate and analyze, which is time-saving, efficient and low-cost. The calculation results show that the temperature distribution of the substrate bottom surface of the new structure of the heat sink is as shown in FIG. At the same power, although the heat-receiving area of the bottom surface of the substrate is reduced, the unit heat flux density of the bottom surface is increased. However, the maximum temperature of the bottom surface of the substrate of the heat sink is still 5 °C lower than the original model. The average heat transfer coefficient of the ribs was also increased from 5.1 W/(m2.K) to 6.0 W/(m2.K). From the XZ plane, the speed vector diagrams 11 and 12 on the YZ plane, it can be seen that the new structure realizes the air circulation up and down in the middle of the radiator, increasing the air circulation, and the maximum speed when the heated air disturbs the middle fin of the radiator. Only about 0.9m / s, under this new structure, when the working environment is under windy conditions, the heat transfer effect will be enhanced, and the heat dissipation effect will be better. This new structure is simple to process, reducing the weight of the radiator and the total metal consumption, and is also convenient for automated production and installation.

4 Conclusion

In this paper, the Fuent software is used to calculate the coupling value of the natural convection cooling of the high-power LED street light radiator in large space. The heat dissipation process is analyzed and the following conclusions are drawn: (1) The numerical calculation results are in good agreement with the experimental results, indicating the reliability of the calculation method; (2) the numerical calculation is better, more scientific and more than the experimental one. It is convenient to analyze the heat dissipation process of the radiator; (3) The new structure of the radiator designed in this paper allows the air to flow up and down in the middle of the radiator, increase the air circulation, reduce the temperature of the bottom surface of the substrate, and improve the average heat transfer coefficient of the rib; (4) The processing pitch of the bottom surface has a significant influence on the heat dissipation capability of the heat sink.

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