Numerical and Experimental Analysis of Passive and Active Cooling Solutions for High-Power LED Light Sources

Effcient thermal management is one of the most important design considerations for LED applications. This thesis presents a systematic approach to the analysis, optimization, and comparison of two thermal solutions to support high-power LED street and flood lights. Both systems, one passive and one active design, were first experimentally investigated and then numerically improved by performing large-scale parametric studies. The passive solution consists of an oversized, free-hanging rectangular heat sink with straight fins, cooled by natural convection. The main aim was to study how the inter-fin base length influences the thermal performance. A total of 55 cases were examined experimentally, and the data were used to validate the numerical model. The results show that a shorter inter-fin base length can significantly enhance thermal performance, especially when the fins are along the longer base side. For the present case, the heat transfer coefficient was increased by up to 62.7%, and the thermal resistance was reduced by 36.7% to 0.29 K/W. It was further shown that the inter-fin base length greatly influences the optimal fin spacing. In addition, Nusselt correlations including a dimensionless geometrical parameter for the inter-fin base length, which are valid for a wide range of dimensions, were developed. The active cooling system is especially designed for high-power LED street lights. It is driven by a centrifugal fan placed inside a chamber at the lower part of the lamp post. The fan is connected to the lamp head via an internal pipe to form a closed-loop forced air cooling system, where the lamp post is used for heat dissipation. The experiments were conducted using a full-scale model with an overall height of 5 m. The design includes two different heat exchangers, which were separately modeled and analyzed. The first is a vertical double pipe single-flow heat exchanger integrated into the lamp post. A 2D-axisymmetric CFD simulation with Rayleigh numbers of over 10e10 was used to investigate the heat transfer characteristics of the lamp post for various flow conditions. The second is a horizontal counter-flow heat sink inside the lamp head, which was simulated as a 3D-model using ANSYS Icepak. The effect of geometric parameters and boundary conditions, such as the inlet position, fin thickness, and fin density, were analyzed in order to optimize the thermal performance of the cooling system. It was shown that the counter-flow heat sink with a higher fin density in the middle section can reduce thermal resistance. A direct thermal comparison of both cooling systems using a 150 W COB LED revealed that the passive system can keep the excess temperature of the LED close to 40∘C at an ambient temperature of 30∘C. Under the same conditions, the active cooling system can further lower the LED temperature by 8 to 13∘C. Based on an economical comparison of both cooling systems over a period of 24 years, it was concluded that, in its present configuration, the additional costs and increased complexity of the active system outweigh the performance improvements.