Indoor-outdoor NOX modelling in a single-side naturally ventilated room in a real building in Madrid

Abstract

The specific objectives of this work are to assess a Computational Fluid Dynamics (CFD) modelling performance through data of one experimental campaign, to quantify the impact of outdoor NOX traffic emissions on indoor NOX concentration and to investigate the natural ventilation using the infiltrated NOX concentration decay and the Air Changes per Hour (ACHs). For this purpose, NOX transport phenomena in a single-side naturally ventilated room in a real building in Madrid is investigated through an Unsteady Reynolds-Averaged Navier-Stokes (URANS) approach and individual passive scalar transport equations for each source (parked vehicles with the engine idling, street traffic and urban background). The combination of this methodology with suitable boundary conditions properly reproduces the time evolution of Wind Speed (WS), Wind Direction (WD) and Turbulent Kinetic Energy (κ), both inside the streets and above the buildings, even for WS < 1 m s−1, and also outdoor and indoor NOX concentrations. In this case, urban background significantly contributes to the indoor concentration and idling vehicles just below the room can contribute between 10 % and 50 % to the total indoor concentration, even when their emissions are considerably smaller than those of traffic in nearby streets. The Indoor-Outdoor NOX concentration ratio (I/O) depends not only on the outdoor concentration (which, at the same time, depends on the atmospheric turbulence) but also on the ventilation (in this case, type Single-Sided Ventilation, SSV), showing a wide range of values. The impact of an indoor Heat Source (HS) below the open window on I/O is low, but under certain meteorological conditions, the stack pressure could be more important than the wind pressure. The time required to ventilate the room is 3.4 min. It has been obtained using the infiltrated NOX concentration decay and has been verified through the average ACH, ⟨ACH(t)⟩. These results provide a better understanding of the impact of the meteorological conditions in outdoor-indoor NOX exchange by natural ventilation, which is the main goal of this research.