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 LED street lamps, the junction temperature of LED chips 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 Xue powder designed a variety of high-power LED heat sink model, but each radiator in the simulation analysis of the natural convection, the surface thereof are used average heat transfer coefficient value. 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.
Figure 1 Outline of the LED street light 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 37 mm.
Figure 2 Schematic diagram of the numerical calculation model of the radiator
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 did a grid independence test, the standard is that the temperature on the radiator ribs in the adjacent two calculations does not exceed 1% of the surrounding vector flow field. The condition for calculating the convergence is that the residual between the two adjacent iteration steps is less than the given amount, the energy residual is 1×10-6, and the rest are 0.001.
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