The generated heats of high-power LED illumination will increase the LEDs’ working temperatures which may degrading the illumination efficiency and lifetimes of LED devices. In this study, the liquid cooling of directly immersed LEDs in different ambient of air or liquids is investigated by the experimental measurement methodology. For LEDs which are operated in series, the generated heats are dissipated into and stored in ambient fluids, then, the casing of lamp (usually acted as a heat sink structure) further conduct these heats and dissipate it into atmospheric air finally. In the experiments, different powers are sequentially input into the LEDs and different fluids, including air, silicone oil, and FC-40, respectively, are filled to cool the LEDs which are directly immersed in these fluids. Temperatures are measured by thermocouples and the IR thermal imager, respectively. Fluid-to-air thermal resistances and fluid temperatures are estimated by a proposed resistance equation. The calculated temperatures are within 10% of error as compared to experimental results. This resistance model is a very good way to quickly examine the fluid-to-air resistances if we want to design the liquid-cooled package of LED illumination.

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