Modélisation et caractérisation numérique et expérimentale de la ventilation et du confort thermique dans un espace fermé à l'aide de diffuseurs à conduits perforés uniformes et non uniformes

Abstract

Abstract: The HVAC system is one of the main parts of each building and uses about 50% of its total energy, which equals 10 to 20% of the total energy consumption of the developed countries. Incidentally, people spend almost all of their time in closed spaces, and to have a comfortable environment, as well as a vigorous protective effect against contagious diseases, like COVID-19, well-conditioned buildings are necessary. It was shown that the protective effect of a well-conditioning system is as high as five times in comparison to the face-covering and other proposed procedures. In this context, the age of air, as well as the type of filtration systems in closed spaces, became the critical criteria to compare the capability of ventilation systems. So, any improvement in the energy consumption of the sub-systems drastically saves energy. As a practical solution, perforated duct diffusers (PDDs) benefit from the equal static pressure alongside the duct, which results in a more uniform air distribution pattern with a lower inlet airflow, i.e., lower energy consumption. The more efficiency of the mixing, the less energy is required by the system. Generally, this problem bifurcates into two classic physical problems, namely the airflow inside the duct and the injected flow into the free stream. Yet, there is no theoretical solution for the combined problem. However, to investigate the physics of phenomena and create a comprehensive database, one requires both numerical modeling and empirical correlation (objective). To quantify the amount of energy-saving using PDDs a numerical model was developed at the isothermal and heating mode. Based on the ASHRAE standard, the required airflow for four different geometries has been calculated. In order to perform numerical simulations for these geometries, k-ε Realizable turbulence models with 4.3 and 4.6 million mesh elements for the isothermal and heating mode have been preferred over the tested two-equation turbulence models. Thermal comfort indices such as the age of air, terminal velocities, air-change effectiveness, air exchange efficiency, effective draft temperature, and thermal efficiency were calculated for all the cases under various initial conditions, like air change per hour. To validate the results, an office room facility was equipped for full-scale experiments akin to numerical simulation. The details of each approach are explained consecutively. Both models have high accuracy for air velocity (94.81%) and temperature (96.75%). The results determine an up to 30% decrease of the residence time of the infectious nuclei in the breathing zone, with 20 to 35% lower required airflow (energy consumption), and more than 90% occupant’s satisfaction. Besides the significant energy saving, high rate of satisfaction, and strong protective effect of these new prototypes, namely non-uniform PDDs, determining the local age of air for each diffuser under the procedure of ASHRAE standard is extremely expensive. Given the hazardous side effects of the tracer gas, a special collector is required, and it’s rather extravagant to spend tens of thousand CAD for each test. Developing an accurate numerical model following the ASHRAE standard methods would decrease the design cost drastically. The outcome of this project, the so-called design diagrams, will be transformed into commercial software, which all rights will be reserved by the University of Sherbrooke and its industrial partner.