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Wind-induced natural ventilation through openings with small wind pressure differences was examined using large eddy simulation (LES) modelling. This study focused on cases in which the ventilation rate is predicted using a standard Orifice equation. The purpose of this study was to clarify how the ventilation rate is underestimated in such cases for both single-sided and double-sided openings, and to clarify the difference between the effective ventilation rate (purging flow rate, PFR) and bulk airflow rate (AFR), which have not been sufficiently and systematically understood. A simple cubic-room model with two openings was analysed using LES by varying the opening position after validation, and the ventilation rate, velocity field, and wind pressure coefficient were compared with experimental results. The PFR was determined by tracing particles, and the AFR was obtained based on the instantaneous velocity over the openings. The AFR predicted by the Orifice equation was underestimated when the difference in the mean wind pressure coefficient (Delta(C-p) over bar) was less than 0.1. The main feature of this study was to show the ventilation effectiveness defined by the PFR divided by AFR, which was approximately 70-80%, 60%, and 90% for the double-sided openings, single-sided openings on the lateral side, and windward and leeward sides, respectively. Another feature was to propose a method for estimating the AFR reflecting two key phenomena, namely pulsating flow and eddy penetration. In addition, a simple equation considering the standard deviation of wind pressure coefficient difference (sigma(Delta Cp)), and external local velocity around an opening (V-vic) was obtained.

Heating, ventilating, and air-conditioning (HVAC) systems usually supply air, which is a mixture of fresh air from the outdoor environment, and return air from rooms via the ventilation ductwork. This air reduces the heat load and cost impact of air conditioning using outdoor air. This recirculation of room air in air-conditioning systems is reasonable in terms of energy saving; however, the deterioration of air quality might be a concern because of the recirculation of contaminated room air. Here, we numerically investigate the effect of pollutant recirculation/return on the formation of concentration distributions of local pollutants in indoor environments when the mixing ratio of recirculated air in the HVAC system changes. We discuss the detailed structure of the formation mechanism of local pollutant concentration distributions using various indices for indoor ventilation efficiency in simplified room models. Among the indices, visitation frequency and net escape probability are the ones that directly assist in evaluating the recirculation/return characteristics of indoor pollutants. As a result, when the proportion of air that is recirculated becomes large, the number of pollutants returning to a target local domain, the visitation frequency, increases exponentially, and the net escape probability-which directly expresses the probability of pollutant discharged from the target domain-is close to zero.

The mechanism of structure formation of non-uniform pollutant concentration distributions in indoor environments have been investigated over the past several decades to determine effective ventilation designs. In this study, numerical analyses of local pollutant concentration distributions in indoor environments are performed based on computational fluid dynamics techniques. In particular, the influence of gas phase chemical reactions and turbulent diffusion on the formation of the local pollutant concentration is parametrically analyzed, and the structures are quantitatively investigated using the index for ventilation efficiency, namely the net escape velocity (NEV) concept. The NEV concept represents the substantive velocity scale, combining advection and diffusion velocity of pollutant at a point, and functions as an index to determine the pollutant concentration at that point. Sensitivity analyses as functions of the first Damköhler number (Da) and the turbulent Schmidt number (Sct) were performed, and the influence of Sct on pollutant concentration distributions was more significant compared with that of Da. When Sct was changed from 0.5 to 1.0, the significant NEV increase, especially that in the stagnant flow region, could be attributed to the enhanced pollutant discharge efficiency due to turbulent diffusion in addition to convective flow. Thus, NEV could be used to quantitatively assess the changes in pollutant concentrations accompanying the change in Da or Sct.

A concept of ventilation efficiency, Net Escape Velocity (NEV), developed by authors presents the net and integrated velocity of contaminant transport by convection and diffusion effect at a point within the room. The NEV is the effective ventilation rate with a velocity scale determining the contaminant concentration at a target point and can be expressed by vector and scalar quantities. It is the most important characteristic of NEV concept. An expanded concept of NEV (NEV*), under an assumption that the inflow flux of a contaminant on the control volume is a contaminant generation, was proposed. We believe that the NEV and NEV* distributions can provide helpful information for ventilation design to control contaminants. The purpose of this study was to demonstrate the advantage and contribution of NEV* to current ventilation design procedure by using numerical analysis. Here, it was evaluated by the NEV* that the contaminant removal performances of local ventilation systems which are a kitchen exhaust hood in a kitchen environment, a push-pull hood in an industrial environment and an adsorptive building material in a test chamber. The distributions of the NEV* as vector quantities under the different flow and diffusion fields were analyzed to investigate contaminant leakage across the hypothetical boundaries of the control target domain of the local ventilation hood and to investigate the contaminant concentration reduction performance of the adsorptive building materials.