Mohammad Rahiminejad
For many decades, ventilated cavities in wall assemblies of buildings have been essential for creating moisture-resilient constructions by allowing airflow within the air gap to promote drying. In addition to that, the airflow in the cavity can contribute to the overall performance of the wall structure. Similar to the wall assemblies with traditional claddings, photovoltaic (PV) panels in the Building-integrated photovoltaic (BIPV) façades can be separated from the building envelope by an air gap that is created for ventilation purposes. In this study, the impact of the thermo-hydrodynamic behavior of the airflow in the naturally ventilated cavity behind traditional (passive) and BIPV (active) façades on the performance of the entire wall assembly is numerically and experimentally investigated. Due to the complexity of the problem caused by the contribution of several parameters, as a first step, a comprehensive literature review is performed to specify the effective factors on the air change rate in the ventilated cavity behind common wall assemblies. As the second step, the steady-state analysis is carried out within the framework of a research project ASHRAE 1759-RP funded by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), in which the plausible definitions of the thermal resistance of a vertical ventilated air-space behind external cladding systems are defined using theoretical and applied formulations. Moreover, based on the uncertainty analysis of measurements, the modifications of the relevant standardized method (ASTM C1363-19) to account for the airflow effects on the thermal performance of the wall assembly are proposed. In the third step, the problem is further explored in the transient conditions by developing a 2D numerical in-house code validated against experimental measurements. Accordingly, the performance of wall structures with different thermal inertia having passive (traditional) and active (BIPV) external claddings are numerically investigated. In the last step of this thesis, the problem is experimentally discovered, and the thermo-hydrodynamic performance of ventilated wall structures of the test facility is investigated using data collected during a full-year measuring campaign. The results of this study show that the thermo-hydrodynamic behavior of the airflow behind external claddings is a complex phenomenon. The results of the steady-state analysis reveal that, under some conditions, the air-space could be able to resist the heat flux passing the wall assembly even more than the external cladding. Moreover, the uncertainty analysis sheds light on the fact that accurate instruments are needed to reduce the uncertainty of measuring the thermal resistance of the cavity. The results of the transient numerical analysis show that the overall performance of a ventilated wall structure dynamically changes depending on the diurnal variation of the outdoor/indoor conditions. Moreover, it is shown that replacing the passive cladding with an active façade could strongly affect the hydrodynamic and thermal performances of the entire wall assembly. The results of the experimental measurements pointed out that the airflow in the ventilated cavity has a major impact on the yearly performance of a wall structure; therefore, further investigations are required in future studies to evaluate the possibility of controlling the air change rate in the cavity using a fan-assisted system.
EPFL2022
