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The increased electrical currents used to drive light emitting diode (LED) cause significant heat generation in the solid state lighting (SSL) system. As the temperature will directly affect the maximum light output, quality, reliability and the life time of the SSL system, thermal management is a key design aspect in terms of cost and performance. Particularly for consumer SSL system, natural convection cooling is cheaper and more reliable than the forced air cooling heat sinks. Although with less efficiency, natural convection heat sink is a good compromise between economy and thermal performance of SSL systems. In this work, the thermal performance of two geometrically different passive heat sink designs for consumer SSL applications is numerically simulated. The heat sink performance is simulated for two orientations: LED up and LED down orientation. Simulation runs for the two designs at the two orientations, in order to investigate the thermal performance of the heat sinks with natural convection cooling. Meanwhile, the radiation effect is considered. With passive cooling, the natural convection plays an important role, and results show that if free ambient air flow is blocked by the heat sink design and the performance reduces considerably. Furthermore, the volume of free air in the luminaire is expected to have significant impact to the heat sink thermal performance. Therefore, the thermal performance for different volumes of luminaire enclosures is also investigated in this work. To analyze the modeling results, a straightforward calculation of the thermal resistance between the LED junction and the environment is applied. In the results, the thermal resistances of LED junction to environment decreases but the air velocity gradually increases with the increasing luminaire volume. In conclusion, although more study is needed for validation of the optimal volume and shape for natural convection, the results in this work can already be used to guide the design of luminaires. In future work, the simulation on real model of bulb or luminaires will be applied to investigate what extend designs based on natural convection and radiation principles can be exploited to manage the LED junction temperature.