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Applications of a coupled multizone-CFD model to calculate airflow and contaminant dispersion in built environments for emergency management.

Article Excerpt
INTRODUCTION

The attacks of September 11, 2001, on the World Trade Center, and the following anthrax cases spawned serious concerns about various possible terrorist attacks in built environments, including release of chemical, biological, and radiological warfare agents (CBRWA) in buildings and subways (Stenner et al. 2001). The anthrax attacks via letters in Florida, New York City, and Washington, DC, caused five deaths and affected over 20 other people in 2001 (Ko 2003). In March of 1995, the Japanese cult Aum Shinrikyo used Sarin to attack the Tokyo subway system, causing 12 deaths and injuring hundreds. Once inside a building, CBRWA could disperse quickly through the HVAC system. Therefore, to design a built environment that protects its occupants from these threats, it is crucial to know how CBRWAs are dispersed after their release.

A popular tool to predict CBRWA dispersion is a multizone network model, e.g., CONTAM (Walton and Dols 2003) and COMIS (Feustel 1999). Kowalski et al. (2003) used CONTAM to evaluate the performance of air-cleaning and disinfection systems against CBRWAs for a 40-story office building. Their results indicated that the office building could be protected by using affordable air filters combined with ultraviolet germicidal irradiation. A multizone airflow simulation helped Li et al. (2005) discover that natural ventilation may have helped circulate SARS viruses at a building complex in Hong Kong. Multizone models use simple flow mass balance equations, so they need little computing time but assume a well-dispersed contaminant in a zone (or a room).

However, Schaelin et al. (1994) and Upham (1997) noticed that the results of a multizone model may not be accurate enough when the well-mixing assumption is used for cases with nonuniform distributions of contaminant concentrations. Wang and Chen (2007b) studied the nonuniform distribution of a contaminant in a four-zone building and found that the well-mixing assumption could cause significant inaccuracy in the calculation of contaminant distribution. The problem is especially severe in large spaces with contaminant dispersion at a transient state. Since the flow resistance to define a subzone for a large space is unknown, a multizone model must simulate a large space as a single zone, which causes apparent errors for building ventilation designs, as found by Jayaraman et al. (2004) and Wang and Chen (2007b). To provide more detailed contaminant information in a zone, especially for a large-space simulation, more sophisticated models, such as computational fluid dynamics (CFD), are used.

CFD can provide very detailed information on contaminant transport in a zone. Boris (2002) used CFD to simulate atmospheric contaminant dispersion in downtown Portland, Oregon, and provided superb spatial resolution of contaminant concentrations. Zhai et al. (2003) applied CFD to predict CBRWA dispersion in buildings and determined the best locations for CBRWA sensors. However, compared to multizone models, CFD calculations normally take hours or even days of computing time. CFD is a very slow tool for computing CBRWA dispersion during a CBRWA release (Settles 2006).

A better approach would integrate a multizone model with a CFD model. The CFD model is only applied to zones where the well-mixing assumption of multizone methods fails. The coupled multizone-CFD model can thus reduce the computing time as compared to a CFD-only simulation of a whole building and can provide more accurate results than a multizone simulation. Jayaraman et al. (2004) showed that the personal exposure to contaminants calculated by a coupled multizone-CFD model is more reasonable than that of a multizone model, especially for buildings with large spaces. Wang and Chen (2007b) demonstrated that a coupled multizone-CFD model can predict better contaminant distributions than a multizone model in an experimental chamber with four rooms. However, most of the previous studies are limited to simple two-dimensional cases and/or idealized conditions at steady state. Since airflow and contaminant dispersion in built environments under a CBRWA attack are transient and three-dimensional, it is necessary to examine further the capability of a coupled multizone-CFD model in such scenarios.

This study demonstrates a coupled multizone-CFD model, which uses a stable integration method (Wang and Chen 2007a) to simulate airflow and contaminant dispersion in a complex three-story building under a CBRWA attack. The simulations are for both steady and transient states to show the problems of the well-mixing assumption for a large space, to evaluate the effectiveness of emergency ventilation, to determine the optimal placement of sensors, and to determine evacuation routes for building occupants.

METHODS

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