Proposal for a Simplified CFD Procedure for Obtaining Patterns of the Age of Air in Outdoor Spaces for the Natural Ventilation of Buildings

Abstract

Built urban environments modify the air quality of the natural ventilation processes in buildings. The experimental assessment of outdoor air change (OACH) processes is complex due to the air volumes involved. The quality of the OACH can be analysed through the ventilation efficiency concept, which has been extensively studied in order to characterize the ventilation of indoor spaces. Ventilation efficiency through the age-of-the-air concept assesses the air-change capacity of a certain space. A procedure formulation for obtaining an adequate control domain (CD) to evaluate outdoor air change efficiency (OACE) is proposed. A methodology in two phases is presented: first, an evaluation stage, in which the outdoor air behaviour patterns are studied; then, a second phase where an ideal control domain (ICD) is obtained in order to comparatively assess several cases in similar urban environments. The outcomes determine a proposed ICD for evaluating the design of efficiently ventilated buildings in open urban built environments. The methodology of validation proposed simplifies the assessment of the building shape and its impact on the quality of the OACH by the OACE. The suggested ICD has dimensions of L = 5H; D = 3H; and T = 2H. ICD enables the representation of the air behaviour and fulfils the imposed requirements to evaluate efficiency with a mean accuracy lower than 0.6%.

Impacts of Urban Layouts and Open Space on Urban Ventilation Evaluated by Concentration Decay Method

“…Moreover turbulent fluctuations induce upward and downward fluxes across the open street roofs, representing a large amount of fluid particles leave and re-enter UCL space. Thus, similar to the indoor environment [35,36], fluid particles flowing across UCL boundaries can be divided into two groups, one is for air leaving UCL space for never returning again and the other for air returning or revisiting UCL space (it has been in the UCL space before). The first group is much more significant to UCL ventilation and pollutant dilution.…”

“…where C(0) = 1.225 × 10 −7 kg/m 3 is a uniform initial concentration, Vol is the entire volume of UCL space, <C(t)> is the spatial mean concentration in the entire UCL space, and 3600 denotes one hour or 3600 s. Moreover, the concentration decay method can be used to predict urban age of air (τ p s) at a given point (see Equation (9)) [32,36]:…”

“…Similar to indoor ventilation [32][33][34][35][36], the key point of applying these ventilation indices is based on the UCL ventilation processes as below: The surrounding external air is relatively clean and can be transported into urban areas to aid pollutant dilution with physical processes of horizontal dilution, vertical transport, recirculation of contaminants, and turbulent diffusion. In this sense, the tracer gas technique originated from indoor ventilation sciences has been used to predict outdoor ventilation by analyzing the final steady-state pollutant concentrations or the transient concentration decay history, which illustrates the processes of pollutant dilution and ventilation.…”

“…In particular, the air change rate per hour is one of the most widely used indoor ventilation concepts [32][33][34][35][36], calculated by ACH = 3600Q T /Vol, where Q T is the total volumetric flow rate and Vol is the room volume. Later it is adopted to quantify the volumetric air exchange rate of the entire UCL volume per hour, including two-dimensional street canyon ventilation assessment [37,38] and three-dimensional UCL ventilation modeling [39][40][41][42][43][44].…”

“…It depends on several time scales, i.e., the overall turn over time (air volume (Vol) divided by the total volumetric flow rate (Q T )), purging time for air moving from entrances to exits, time constant for local vortices and turbulent mixing. The concentration decay method has been widely used to predict the actual ACH and age of air in indoor environments in which the concentration temporal profile accords with the exponential decay law and the concentration decay history shows the pollutant dilution and ventilation processes in room ventilation [32][33][34][35][36]45,46]. However, to date, this method has not been introduced to quantify the actual or net ACH index and age of air in UCL models.…”