Design007 Magazine

October 2015 • The PCB Design Magazine 65 article lation in which a transient simula- tion can provide information on how fast and where the condensa- tion will disappear. It should also be able to show how evaporation could be improved with specific de- sign changes. Case Study The following is a real-world CFD analysis on an LED-based headlight (Figure 1) using the condensation and radiation models in FloEFD. In this experiment, the initial condi- tions for the model of the inside of the headlight were a temperature of 50°C, pressure at 1 atm, and relative humidiy of 95%. The external condi- tions were set at -10°C and 50% rela- tive humidity. The inner and outer surfaces of glass were assigned to be "wettable." The CFD software was used to simulate a warm headlight initially placed into a cold environ- ment, with the headlight off for 30 minutes and then turned on. Figures 3-5 show the product specifications used for this simulation. During the cooling phase, start- ing at about 1,000 seconds from the beginning of the 30 minutes, the temperature in various regions dropped below 0°C, and freezing (crystallization) of the film oc- curred. Between the start of the crystallization and the switching on of the bulb (1,800 seconds from the start) a minor decrease in the film mass caused by evaporation and sublimation was observed. Once the bulb was switched on, removal of the condensation film occurred. The processes of melting and subsequent evaporation lasted no more than 300 seconds. An ad- ditional 1,800 seconds were needed to obtain a steady thermal state of the headlight. Figure 6 shows the change in the film's thickness and contours over time. Although the condensation film mass and its average thickness decreased after switching on the bulb, the film thickness still increased on local regions on the glass. This was evidence of the vapor transport within the headlight's internal space; that is, the evaporated mass from the hot- ter regions of the surface is condensed on the relatively colder ones. Condensation film motion was observed as a ANALYzING CONDENSATION AND EVAPORATION IN HEADLIGHTS WITH THERMAL SIMuLATION Figure 4: Materials that need to be considered when creating the model. Figure 5: The leD specifications used in this model.

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