CFD Simulation of Airflow in Ventilated Wall System Report #9

The objective of this report was to examine air movements in vinyl and brick ventilation cavities in detail, using a state of the art CFD commercial modeling tool. The CFD activity was planned to proceed the other activities in order to develop insight on the important magnitudes of scales occurring during ventilation air flow. This information generated by the CFD model was to be used to modify (if necessary) and to validate the air flow dynamics already imbedded in the hygrothermal model for the computer-based air flow simulation procedures. A comprehensive program of advanced, state-of-the-art hygrothermal modeling was then envisaged mainly to extend the knowledge to other wall systems and at least six representative climatic areas. These data were then to be used to provide the basis for the development of design guidelines. CFD results provided timely and much needed answers to many of the concerns and questions related to ventilation flows due to thermal buoyancy and wind-driven flow scenarios. The relative strength between these two mechanisms. Simple correlations were developed and are presented in the report providing the overall pressure drop, and flow through various cavities under different exterior solar and temperature scenarios. Brick Rainscreen Wall: It was initially expected that a 50 mm cavity would offer reduced pressure drops and increased air flow compared to a 19 mm cavity. However, these models showed that the size of the ventilation slots through the wall are the limiting factor rather than the cavity depth. Of course, once the slots are enlarged beyond a certain point, this could change. The effects of natural convection within the air cavities, driven by the temperature difference across the cavity, were shown to be less important than the external wind speed (for a wind direction normal to the wall surface), when wind action is present. Vinyl Rainscreen Wall: The CFD model of the vinyl rainscreen wall was simpler than that for the brick wall. Constant wall temperatures were used rather than conjugate heat transfer. Although this is appropriate for a thin surface with little heat capacity, it does mean that an empirical correlation between solar radiation (and perhaps wind speed) and vinyl temperature is required to use these results appropriately. The results developed from this CFD model were correlated to weather parameters and construction details so that they can be incorporated into ORNL s advanced hygrothermal models MOISTURE- EXPERT