Three-Dimensional Heat Transfer & Thermoelastic Deformation Predictions in Forward Lighting 2000-01-1396
The thermal performance of an automotive forward-lighting assembly is predicted with a computational fluid-dynamics (CFD) program. A three-dimensional, steady-state heat-transfer model seeks to account for convection and radiation within the enclosure, conduction through the thermoplastic walls and lens, and external convection and radiation losses. The predicted temperatures agree well with experimental thermocouple and infrared data on the housing. Driven by the thermal expansion of the air near the bulb surface, counter-rotating recirculation zones are predicted within the enclosure. The highest temperatures in the plastic components are predicted on the inner surface of the shelf above the bulb where airflow rising from the hot bulb surface impinges. The thermal model can be used to assess the importance of different heat-transfer mechanisms, such as the effect of coating on thermal-radiation energy redistribution within the enclosure and the result of bulb shield design on hot-spot location and temperature due to the change in airflow passage. One particularly interesting application of the method involves sensitivity studies (virtual design of experiments (DOEs)) that can help facilitate the development of headlamp design guidelines without requiring the costly fabrication and testing of multiple physical prototypes.
To address the issue of beam shift resulting from the thermal distortion of reflectors, a three-dimensional thermoelastic model was developed to predict local displacement and rotation of reflector surfaces. An in-house translator has been developed to translate the finite-element information and computed temperature distribution from a heat-transfer model into shell element. The model uses the thermal cycle as the input to thermo-structural analyses of the reflector. The elastic response resulting from the thermal cycle from room temperature to final steady-state temperature can then be passed to a ray-tracing package for assessing beam shift. Combined, these computational tools can often facilitate the development of business cases for headlamp component integration. For instance, one can choose a cost-effective material that responds to the thermal demands of a housing and bezel or housing and reflector and evaluate the option of integrating these components into a single molded part.