CFD Modeling and Design of an Air Heater Mixing System in a Gypsum Board Dryer

Abstract: The process of manufacturing gypsum boards in a Gyptech gypsum plant involves, among other things, drying of the boards to remove excess water. It is here crucial that the drying occurs uniformly over the boards, as the gypsum properties and consequently the quality of the end product relies on it. Uniform drying can be ensured by having a uniform temperature of the air ejected over the gypsum boards, which in turn is facilitated by having a static mixer in the air heater channel. In the present work, the air heater channel has been modeled using Computational Fluid Dynamics (CFD) and the purpose has been to improve the mixing capabilities of the static mixer while a low pressure drop is maintained. A large portion of the work was spent on reducing and developing an appropriate domain that could be afforded given the large size of the dryer, the steep temperature gradients therein, and the relatively limited computer resources. The final model was compared to experiments, where the pressure drop was in good agreement; 89 Pa compared to the measured value of 100 Pa. The standard deviation in temperature was overpredicted however, something that is considered to be due to the domain simplifications and due to the use of steady state Reynolds-Averaged Navier-Stokes turbulence modeling. Two unsuccessful attempts of improving the static mixer included variations of some parameters of the current mixer and utilizing a second mixer downstream of the first. Other but entirely different concepts proved to be more promising; one including a variant of the corrugated plate mixer, one based on guide vanes, and one that was a modified version of the original static mixer. These reduced the standard deviation in reference to the original mixer in the CFD model by 68%, 13.8% and 22.9% respectively, and produced only moderate pressure drops. The results have shown the potential of some concepts, and have indicated that more large scale rearrangement and large scale vortices may achieve better mixing, in comparison to the original mixer. However, it can not be claimed that the optimal dimensions have been established for any of the concepts, as the number of cases that have been examined are limited.