Foreword
Since the United States Lumileds Lighting packaged the world's first high-power LED in 1998, high -power LEDs have been favored by researchers at home and abroad for their small size, high efficiency, long life, energy saving and environmental protection. As the input power of the LED chip continues to increase, the heat accumulation of the LED is more and more, and the thermal effect due to the temperature rise is gradually becoming apparent, thereby affecting the service life and reliability of the LED. Therefore, the reasonable thermal design of high-power LEDs and the improvement of their heat dissipation capability have become one of the key technologies to be solved urgently.
On the basis of introducing many advantages of LED light source, Wu Zhiqiang expounded its application status and development prospects. Tian Minghang et al. proposed the idea of ​​using heat pipe technology to dissipate heat from high-power LEDs. The principle structure of LED heat pipe radiators was designed, and the heat transfer mechanism, heat transfer route and thermal resistance of each heat transfer stage were qualitatively analyzed and quantified. analysis. Liu Yibing et al. analyzed the influence of the heating effect of the power LED to solve the heat dissipation problem from the perspective of improving the LED structure. It is pointed out that the use of packaging materials with excellent thermal conductivity is an important way to improve the heat dissipation efficiency, and the effects of sealing materials, bonding materials and heat dissipation substrates on heat dissipation are analyzed in detail.
Due to the sharp increase in temperature, the luminous efficiency and life of the LED lighting fixture have an irreversible effect. For every 10 °C increase in temperature, the light decay is 5 to 8%, and the service life is halved. Therefore, this article uses Icepak software to explore the heat dissipation of LED lighting fixtures.
Mathematical physics model
2.1 Description of the problem
The LED lighting fixture cabinet contains 34 LED heat sources (sealed in 1 cavity), 1 back plate, 34 fins, 3 fans, and 1 free opening. The fin and back plate are extruded with aluminum. The mass flow rate of each fan is 0.05 ng/s, the power of each heat source is 10W, and the heat transfer coefficient of the cavity is 15W/(m2.K). According to the design goal, when the ambient temperature is 20C, the back plate of the device should not exceed 90C. The model of the complete LED lighting fixture is shown in Figure 1.
LED lighting model
LED lighting model
2.2 Theoretical basis
2.2.1 Control equation
2.2.2 Calculation hypothesis
1) Air is considered to be an incompressible fluid.
2) The air inlet temperature is set to the ambient temperature.
3) Ignoring the contact thermal resistance of the fins and the backing plate, it is considered that the temperatures of the contact portions are the same.
4) The heat source is simplified into a two-dimensional heat source so that it is in close contact with the back plate, ignoring the contact thermal resistance between the heat source and the back plate, and the temperature of the contact portion is considered to be the same.
2.2.3 Evaluation index
As the temperature rises, the thermal stress on the component is increased. When the thermal stress exceeds the yield strength of the material, the material fails, resulting in a decrease in the performance of the component. Therefore, the highest temperature in the system is used as the index to evaluate the reliability of the finned radiator: Under the same conditions, the effective parameters are changed, and the highest temperature in the system is higher.
2.3 boundary conditions
(1) Environmental properties: the ambient temperature is set to 20 ° C, the pressure is 101325 N / m 2;
(2) Coolant properties: the coolant is air and the density is 1.225kg/m3;
(3) The outlet temperature is set to the ambient temperature and is a free opening; the flow direction is flowing out along the normal direction;
(4) The fin and back plate are made of aluminum extruded profile, the density is 2800.0kg/m3, the specific heat is 900J/kg.k, the conductivity is 205.0W/mK, and the conductivity is isotropic;
(5) The cabinet is a solid wall; all walls of the cavity are smooth and fixed, and the heat transfer coefficient is 15W (m2.K).
(6) The relaxation factor is set to pressure, momentum, temperature, viscosity, and mass force, and the corresponding values ​​are 0.3, 0.7, 1.0, 1.0, and 1.0, respectively.
2.4 meshing
As shown in Figure 2, a hexahedral unstructured mesh is used to divide the mesh. The number of grids is 163,816 and the nodes are 174,978.
3 results analysis and discussion
Check the airflow to get the Reynolds number 69197.6 and select the RNG turbulence model for calculation.
image 3 Temperature, velocity and pressure distribution on the XY plane at Z=0.05
Figure 3 (a) shows that due to the influence of air volume, the temperature near the side of the fan is significantly lower than the temperature at the exit, and the high temperature is concentrated near the exit area; Figure 3 (b) shows the air on the XY plane at Z = 0.05 m in the cabinet. The velocity vector shows that the air in the cabinet is greatly disturbed, and the disturbance near the fan is greater than the disturbance away from the fan. Figure 3(c) shows that the pressure distribution in the cabinet has obvious regional differences, close to the exit. The pressure is significantly higher than the pressure measured by the air volume.
Figure 4: Temperature and pressure distribution of the backing plate
Figure 4 (a) shows that the maximum temperature of the backing plate is 52.81 ° C, less than 90 ° C, so the amount of air at this time can cool these heat sources. In Fig. 4(b), the partial pressure is as high as 250N/m2, which causes a certain degree of impact on the root of the fin near the back plate, which generates a lot of noise and easily damages the fin, so the model needs to be optimized.
Summary and outlook
The simulation results show that the maximum temperature of the backing plate is 52.81C, which is less than 90C, so the air volume at this time can cool these heat sources. However, because the temperature and pressure distribution of the backing plate is somewhat concentrated, the model can be optimized in the next step.
The factors affecting the heat dissipation of LED lamps are: thermal resistance between structural members, surface convection coefficient, heat dissipation coefficient, air heat exchange, main heat dissipation channels, materials, heat transfer coefficient, heat sink heat dissipation area, particle distribution of aluminum substrate LEDs, heat sink shape Design, fin distribution, emissivity, external environment and other influencing factors [3]. In the thermal design simulation analysis, the factors such as the fin pitch distribution, the fin aspect ratio, the fin shape, and the total heat dissipation area of ​​the heat sink can be changed to explore the influence of the heat dissipation on the LED lamp.
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