An Analysis of Heat Transfer in LED Luminaires.

摘要

The promise that light-emitting diode (LED) technology holds for long life and significantly reduced maintenance costs for lighting luminaires may not be realized if the heat at the LED junction is not managed well. Lighting luminaires can operate in a variety of thermal environments where, in the best case, there is significant airflow surrounding the luminaires, or, in the worst case, they are placed inside a thermally insulated cavity. A worst-case example is the recessed downlight in an airtight, insulated ceiling. Presently, commercial LED luminaires often include large metal heat sinks at the back end with the expectation that these will keep the LED junction temperature low.;The objective of this thesis study was to understand the heat transfer mechanisms in an LED luminaire, namely, a recessed LED downlight in an insulated ceiling, by analyzing and comparing the effectiveness of two passive thermal management schemes. In the first part of the study, a passive cooling method emphasizing radiation-induced cooling was analyzed using an experimentally validated numerical model. Computational fluid dynamics (CFD)-based simulations were performed to investigate the effects of emissivity, surface area, and view factors on reducing heat sink temperature. Results showed that an extended surface below the ceiling and a high surface emissivity are two key factors that could effectively improve the thermal performance of the heat sink. In the second part of the study, a passive cooling method emphasizing convection-induced airflow was analyzed in a similar fashion to the first method, where CFD-based simulations were carried out to investigate the effectiveness of natural convection-induced air circulation in an asymmetric geometry for improving heat transfer. The scheme consisted of a plate-fin heat sink and an inverted U channel. Results showed that fin spacing, fin height, and outlet configuration impacted the heat sink temperature.;In the final step, these two methods were compared by designing two LED recessed luminaires with similar dimensions and heat sources, and conducting CFD simulations. Comparisons showed that the radiation-induced method results in a lower heat sink temperature than the convection-induced air circulation method. Further numerical simulations were conducted on the radiation-induced method to understand the effects of trim width, wall thickness, and contact thermal resistance between joint faces on the heat sink temperature. Results showed that a lower heat sink temperature could be achieved by increasing trim width, increasing wall thickness, and removing contact thermal resistance.;The final conclusion of this study shows that large metal heat sinks attached to LED luminaires designed for insulated ceilings are ineffective at keeping the LED junction temperature low. Instead of a large finned heat sink at the back, an extended trim below the ceiling with high emissivity can keep the LED junction temperature much lower.