1997-02-24

Predicting the Thermal Performance of an Automotive Fog Lamp Reflector 970912

The thermal performance of an automotive fog lamp reflector is predicted with a computational fluid dynamics program. The 2D, steady-state heat transfer model accounts for convection and radiation within the enclosure, conduction through the reflector walls, external convection and radiation losses, and transmission through the lens. A good comparison of the predicted reflector temperatures with experimental thermocouple and infrared data suggests that the specular component of the inner metal coating plays a secondary role in the overall heat transfer and that a detailed thermal model of the bulb is not required. The radiant exchange with the tungsten filament and the conductive energy losses through the bulb connections are accounted for by specifying an appropriate heat flux at the bulb surface, and the transmission of radiant energy through the lens is modeled with an appropriate heat sink. Driven by the thermal expansion of the air near the bulb surface, 4 counter-rotating recirculation zones are predicted within the reflector enclosure. The highest temperatures are predicted at the intersection of these zones on the inner surface of the shelf above the bulb. The thermal model can be used to assess the importance of different heat-transfer mechanisms. For example, sensitivity studies show that thermal radiation within the reflector enclosure redistributes the energy so that the wall temperatures are more uniform. In addition, thermal radiation losses to the environment surrounding the reflector need to be included in the thermal model; otherwise, the predicted wall temperatures are higher than the experimental data.

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