Foreword
The rapid development of LED technology has greatly promoted its application in the field of lighting, and the development of high-power LED lighting products has become a hot spot. However, since the LED itself generates a large amount of heat and is a temperature sensitive device, the junction temperature rise will affect the LED efficacy, light color (wavelength), color temperature, light shape (light distribution), forward voltage, maximum injection current, etc. Lightness, chromaticity and electrical parameters as well as reliability, etc. Therefore, heat dissipation design is one of the key technologies for LED lighting product development.
At present, LED lighting products mostly adopt natural convection cooling (heat dissipation). Compared with the forced convection cooling method, the natural convection-cooled electronic products account for a large proportion of the radiation heat exchange, and the heat dissipation is also greatly affected by the ambient wind speed. Therefore, the analysis of ambient temperature (which has a greater impact on thermal radiation) and the effect of wind speed on the heat dissipation of natural convection-cooled LED lighting products are of great importance for thermal analysis and thermal testing of LED lighting products.
1. Thermal simulation model
The thermal simulation model used in this study is shown in Figure 1.
The thermal simulation conditions are as follows:
Ambient temperature: 25.0oC;
Thermal radiation background temperature: 25.0~C;
The LED heat source model is replaced by a copper (thermal conductivity: 398W/(m · K)) cylindrical module, and the heat generation is 4.25W;
The LED base and the heat sink material are aluminum alloy AA6063 (thermal conductivity: 201W/(m·K)); the outer cylindrical surface of the base is attached to the inner cylindrical surface of the heat sink, regardless of the contact thermal resistance between the two;
In addition, the upper and lower end cap materials of the products in Fig. 1(f) and Fig. 1(g) are plastic, and the thermal conductivity is set to 0.3 W/(m·K).
2 The effect of ambient temperature on the heat dissipation of LED lighting products
The thermal emissivity experiment was carried out by changing the material emissivity and the heat sink structure in the thermal simulation model. The results are shown in Table 1.
From the data in Table 1, the following conclusions can be drawn:
(1) For Experiment 1, the surface emissivity of the material was 0, that is, heat radiation was not considered. At this time, as the ambient temperature increases, the temperature difference between the heat source and the environment increases. The reason is that as the ambient temperature increases, the properties of the air will change to some extent, which will cause a decrease in the surface heat transfer coefficient of convective heat transfer.
For example, for natural convection heat transfer in large spaces, the horizontal plate heat faces up and down, and the average surface heat transfer coefficient under uniform heating is calculated as follows (1)
Where h is the surface heat transfer coefficient of convective heat transfer, w/(m 2 ·K); Nu is the Nusselt number; Gr is the Grachef number, which represents the ratio of the lift force to the viscous force; λ is the heat conduction of the air. Coefficient, w / (m · K); L is the characteristic length, m; Pr is the air Prandtl number; is the volumetric expansion coefficient of air, 1 / K; q is the heat flux density of the convective heat transfer surface, w / m 2 ; for the kinematic viscosity of air, m / s 2 ; 曰, m is a constant.
As the ambient temperature increases, the Pr and αν=1/T of the air decrease, A and ν increase, and the remaining parameters do not change, so that the Nu decreases, that is, the surface heat transfer coefficient h decreases.
(2) With the increase of ambient temperature, the radiation heat transfer on the outer surface of the lighting product gradually increases, the radiation heat transfer is strengthened, and the proportion of the radiation heat exchange to the total heat of the heat source gradually increases, as shown in Fig. 2.
