Semi-Empirical Approach to Predicting Temperatures of a Non-Opaque Surface

The methods used in predicting the thermal profile of a high-pressure mercury arc lamp housed in an electronics enclosure are discussed. These types of enclosures are typically operated in harsh environments. High temperature and low pressure air is typically used to cool the electronics inside. A lamp, which dissipates roughly one half of the total power within the box, must be cooled sufficiently so as to not affect the performance of the circuit cards. Since the majority of the heat being transferred from the lamp’s center arc tube to its surrounding atmosphere (to the lamp housing and then to the circuit cards inside the electronics box) is via radiation, a Computational Fluid Dynamics (CFD) code with a radiation solver was essential to drive the design. Fluent Icepak was chosen as a capable code for electronic box type problems. However, lcepak does not account for radiative transfer through non-opaque surfaces. Since the lamp housing is very transmissive in the infrared at certain wavelengths, the energy equations could not be solved using only analytical techniques. Therefore, tests were conducted that first characterized the thermal performance of the lamp and then predicted the energy that was conducted and absorbed by the glass housing (made up of a reflector and front cover). The remaining power was then assumed to be transmissive in nature. In the computational model, powers were iteratively applied to various locations on the lamp housing until the model matched the empirical results. Once the lamp model was characterized, it could be used to drive the design of any type of enclosure in any type of environment.