Study On System Level Thermal Management And Mechanism Of Heat Dissipation Enhancement For Tunnel LEDs

LED illumination system used in tunnel will not only effectively improve the safety for tunnel lighting, but also have great importance on energy saving and emission reduction. At present, removal of heat is one of the key factors that restrict the large-scale applications of tunnel LED illumination. Therefore, study on heat dissipation of LED is of great importance for research and application.This work was supported by National Natural Science Foundation of China Study on the high efficient heat dissipation mechanism based on the interface microstructure model and the multi-scale simulation of entire thermal transfer pathway (No.61077035) and Zhejiang Provincial Natural Science Foundation of China Study on heat dissipation mechanism of tunnel LED lighting system based on energy conversion principle (No. Z1110222).In this paper, heat dissipation performance of whole LED is analyzed, with a view to system level thermal management of LED. Based on Computational Fluid Dynamics (CFD) method, a whole three-dimensional mathematical model of LED luminaries and external fluid flow are constructed. The validity of mathematical model and the rationality of calculation parameters were verified through physical experiments. The following three aspects of working were simulated using this mathematical model. (1) Effect of special environment in tunnel to heat dissipation performance of horizontal rectangular fin arrays and effect of chip power to optimal installation height under conditions of natural convection. (2) Orientation effect on forced convection dissipation performance of horizontal rectangular fin arrays and louver fin. The influence of fins spacing and inlet velocity to orientation sensitivity of horizontal rectangular fin arrays also were researched. (3) The mechanism of tunnel LED heat transfer enhancement was explored based on field synergy principle.The followed conclusions were obtained through former study.(1) Under natural convection, in contrast to large space, the distance between arch-wall and LED has a large influence on heat dissipation performance of whole LED. With the decrease of the distance or increase of power, this influence will become more distinct. Take horizontal rectangular fin arrays for example, the influence scope of arch-wall to LED is 0.8H (52 millimeters) while the power is 60W. The influence scope increased to 4H (260mm) while the power is 150W. As the distance decreased from 2H (130mm) to 0.2H (13mm), the junction temperature raised by 8.5K, the average surface heat transfer coefficient of fin reduced by 1.1W/(m2·K).(2) Under forced convection, the heat dissipation performance of horizontal rectangular fin arrays has obvious orientation effect. The heat dissipation performance when tangential of velocity is parallel with fin will be better than that when tangential of velocity is vertical with fin. While the power is 150W and inlet velocity is 2.5 m/s, the junction temperature of latter has 18.3K higher than the former, and the average surface heat transfer coefficient of fin increased by 11.1 W/(m2-K).(3) With the increase of fins spacing, the orientation effect of horizontal rectangular fin arrays was weakened. While the power is 150W and inlet velocity is 2.5 m/s, as the fins spacing increased from 5mm to 15mm, the difference of average surface heat transfer coefficient when horizontal rectangular fin arrays is in two different directions reduced by 7.1 W/(m2-K).(4) With the decrease of inlet velocity, the orientation effect of horizontal rectangular fin arrays was weakened. While the power is 150W and fins spacing is 15mm, as the inlet velocity decreased from 3.0m/s to 0.5m/s, the difference of average surface heat transfer coefficient when horizontal rectangular fin arrays is in two different directions reduced by 8.8 W/(m2·K).(5) Compared with horizontal rectangular fin arrays, the disadvantage of orientation effect has been improved by louver fins. It is a heat-exchanger with excellent heat dissipation performance in two different directions. While the power is 90W and inlet velocity is 2.5m/s, the difference of junction temperature between two directions is 3.6K, and the difference of average surface heat transfer coefficient is only 3.3 W/(m2·K).(6) Field synergy principle indicate that, structure of flow field between fins and coordination of velocity and heat flow fields could be improved by optimizing the heat-dissipation structure, resulted in enhancement of heat dissipation performance. Therefore, it is an effective way of LED heat transfer enhancement through optimizing the heat-dissipation structure to improve coordination of velocity and heat flow fields.