Simulation and measurement of heat flow in recessed LED fixtures mounted in a UL box

This is an effort to use natural convection inside a room to enhance the heat flow from a ceiling-mounted recessed LED fixture. Thermal performance of recessed fixtures is evaluated experimentally in a wooden box specified by Underwriters Laboratories, called a UL box. The UL box is approximately 0.67 × 0.62 × 0.46 m and is used to emulate an insulated ceiling for testing purposes. The UL box is suspended above open air with a recessed fixture mounted in the bottom and with ceiling insulation poured inside the box above the fixture. We describe two different fixture designs, one that is flush against the ceiling and a “partially-recessed” one that protrudes downward into the air. Thermal measurements show that the partially-recessed fixture is able to dispatch more heat and more light than the flush-mounted fixture for a given operating temperature. Using finite-element analysis of axi-symmetric compressible flow, we simulate convective heat flow in the air below the fixture, coupled with the conductive heat flow inside the fixture, and insulation in the UL box. These simulations show how convection of air below the ceiling is the key to improved thermal performance. In each case, the dominant paths of heat flow are identified and quantified in the simulation output. This enables a plausible description of the primary mechanism and the key role that convection plays in the improved thermal performance of the partially-recessed design.