Evaluation of natural ventilation using age of air

This paper presents the evaluation of natural ventilation in a building with cross ventilation using parameters associated with age of air in computational fluid dynamics (CFD) simulations with a RANS turbulent model. The parameters associated with the age of air are used for air quality evaluation inside buildings. The age of the air is the time that a parcel of air molecules has stayed inside the building since it has entered through one of the building openings. The parameters associated with the age of air are the local mean age of air and the spatial average of the age. With these two parameters, the ventilation efficiency is obtained. The calculation methodology for the parameters is presented using ANSYS Fluent CFD software. A numerical model experimentally validated with a laboratory scale model (1/15) is used. Simulations are carried out for two scales of the building, laboratory scale and full scale, maintaining geometric, kinematic and dynamic similarities. The characteristic time of each scale is obtained and it is demonstrated that the parameters normalized with the characteristic time have the same value in both scales. A comparison of the parameters associated with the age of air with other parameters commonly used for the evaluation of natural ventilation is included. Palabras clave: Natural ventilation; Cross-ventilation; Age of air; Computational fluid dynamic; CFD; RANS; Coupled simulation. INTRODUCTION In studies of natural ventilation in buildings, the use of age of air [s] and air exchange rate [s-1] as evaluation parameters is reported (Jin et al., 2016; Montazeri and Montazeri, 2018). The age of air is defined as the time that a parcel of air has stayed inside the building since it has entered through one of the building openings (Etheridge and Sandberg, 1996). is defined as the inverse of nominal time constant ( ), where [m3/s] is the volumetric flow rate across the openings of the building and [m3] the interior air volume. The inverse of ! is known as the air changes per time unit (ASHRAE, 2015). By definition, ! does not consider the air distribution, i.e. the inlet jets, recirculation zones and stagnation points. While the residence time does consider the air distribution inside the building, due to its definition as the average of the age of air in the exhausts. To implement the parameters associated with the age of air in studies of natural ventilation, the Group of Energy in Buildings at IER-UNAM develops a methodology using CFD simulations. Thus in this work, the methodology implementation on cross ventilation cases is presented. AGE OF AIR THEORY In this section a summary of the age of air theory by Etheridge and Sandberg (1996) is presented. In the age of air analysis, three periods of times are defined: internal age ! [s], residual lifetime ! [s] and residence time ! [s] . is the time that a parcel of air inside the building has elapsed in the building since its entrance, ! is the remaining time that a parcel of air will spend in the building until its leaving and ! is the time when a parcel of air leaves the room, i.e. is the sum of the internal age and the residual lifetime