Water drainage in HVAC piping systems for automotive industry
Overview: In HVAC systems for automotive industry, air taken by the fan is cooled by the evaporator to reduce humidity; a portion of this cold air is heated by the heater core. Since the temperature of hot and cold airflows is fixed by the A/C system, the temperature of air in the cabin is controlled by mixing in the HVAC unit. While this issue is now well understood and resolved, remains another one related to condensation, more precisely liquid drainage in small pipes forming heat exchangers.
The challenge: The primary focus of this study was to compare results obtained using another industrial code by the company for pipe water drainage simulations using an independent code (in this case TransAT). The main issue of interest in the simulations was related to liquid hold–up in the pipe as a function of the contact angle and pipe inclination (90o & 5o), for both wetting 80o and non–wetting 10 o equilibrium contact angles.
TransAT Solution: TransAT has been successfully employed for microfluidics, free-surface flows in general, where surface tension and capillary effects are important. This is true for both Medtech and Microtech problems. Thanks to its dynamic contract angle model (DCA) implemented within the in-house Level-Set approach, the code could predict the rate of drainage (volume of water remaining in the 1mm diameter pipes) for both pipe inclinations (90o almost vertical, down to 5 o, almost horizontal), for wetting and non-wetting conditions. These findings were in line with the company lab. Data, and contradict the earlier results obtained by the company by the CFD codes in use internally.
The work extended to treat more complex HVAC geometries, including heat transfer through the louvered fins, as shown in the lowest panel.
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