Thermal transfer through membrane cushions analyzed by Computational Fluid Dynamics

Building envelopes built of membrane cushions are sometimes called the new face of architecture. This new and innovative building technology stands out due to an increasing growth worldwide. Because of its newness some standards, which are long time established for other types of construction, are not available for membrane cushions. There are no methods available how to calculate the thermal transfer through the membrane cushion in everyday engineering practice. Existing standards refer to windows and can not be used for membrane cushions because of different material, shape, dimensions and curvature. Deeper knowledge about what occurs inside the cushions and its effects on thermal transfer is not available. Even basic understanding about processes inside and around the cushions is often missing, which leads to problems in the whole area of building physics. The thermal transfer through this building part depends mainly on the airflow within the cushion and the surface to surface radiation. Both mechanisms can be calculated with commercial Computational Fluid Dynamics (CFD) tools. This simulation technique was used for the study. Within the study presented the natural convection inside membrane cushions was investigated. Parameter studies with different temperature boundary conditions as driving potential, different cushion volumes and inclinations provide insight into varying flow patterns resulting from the different conditions. The dimensions of the simulation model correspond to the dimensions of an experimental set-up for a membrane cushion tested under realistic outdoor conditions. Different states of flow patterns can be defined as result of the parameter variations. A general description of these states leads to basic generalizations about natural convection and surface to surface radiation within the membrane cushion. The numeric fluid mechanics study provides additional insight which can lead to improved constructions and adaptation of existing codes and standards. A comparison with measurements in membrane cushions tested under real conditions shows similar flow patterns and temperature fields for measurement and simulation. The results of this study can help to develop a simple method for the determination of the thermal transfer through the membrane cushion and a resulting U-value.