Numerical investigation of single-sided natural ventilation and interunit dispersion in multistory buildings

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Numerical investigation of single-sided natural ventilation and interunit dispersion in multistory buildings

 

Author: Ai, Zhengtao
Title: Numerical investigation of single-sided natural ventilation and interunit dispersion in multistory buildings
Degree: Ph.D.
Year: 2015
Subject: Natural ventilation.
Tall buildings -- Environmental aspects.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Building Services Engineering
Pages: xxiv, 200 pages : illustrations ; 30 cm
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2816328
URI: http://theses.lib.polyu.edu.hk/handle/200/8071
Abstract: Interunit dispersion through opened windows in residential buildings has been attracted more and more attentions since it was first identified in Hong Kong during the outbreak of SARS in 2003. This dispersion is a dangerous pollutant transport route, especially because of the short transport distance and time as well as the possibility of involving the transport of infectious aerosols. Previous studies regarding interunit dispersion were limited to either the solely upward spread or the analyses of envelope concentrations. This thesis therefore provides a systematic investigation of interunit dispersion in multistory buildings with rooms and openings, namely with natural ventilation. Only single-sided natural ventilation was considered, as it is more common in practice than cross ventilation in densely urban areas like Hong Kong. Obviously, an accurate prediction of single-sided natural ventilation is the prerequisite of the successful prediction of interunit dispersion. To achieve the research objective, three sub-works were carried out: (a) an evaluation of predictive methods of single-sided natural ventilation, (b) quality assessments and improvements of CFD prediction of coupled indoor and outdoor flow and dispersion, and (c) CFD simulations of interunit dispersion in multistory buildings.Several predictive methods of single-sided natural ventilation were evaluated and compared to identify an appropriate predictive method. On-site measurements of ventilation performance were conducted in several multistory residential buildings in Hong Kong, which reveal that previous empirical models established based on simple building models are not reliable to use in multistory buildings, as they cannot account for the effect of room location. The model scale and availability issues as well as the significant difference in ventilation characteristics between rooms in a multistory building post a great difficulty to experimental methods. However, the CFD method is particularly suitable for predicting single-sided natural ventilation in multistory buildings, despite that its quality must be seriously assured.
The quality of CFD prediction of coupled indoor and outdoor flow and dispersion was assessed and improved. Most of these works have not or not comprehensively been investigated in existing literature. Based on previous high-fidelity experimental data, important influencing factors of both RANS and LES simulation of flow and dispersion around an isolated building were examined and the appropriate selections were recommended. Such sensitivity analyses of influencing factors were conducted also for CFD simulation of single-sided natural ventilation and the implications for its accurate prediction were summarized. Particularly, a simple method of refining the near-wall mesh was proposed and a homogeneous ABL, based on a roughness modified two-layer near-wall model, was developed. Some important issues, such as the inflow fluctuating algorithm of LES simulation, were investigated fundamentally, where novel and useful findings were obtained. These findings provide supplementary information for developing new or updating the current best practice guidelines.Taking into account wind directions, interunit dispersion in multistory buildings was investigated using both RANS and LES models. A tracer gas was adopted to simulate gaseous and fine particulate pollutants. Using RANS models, the mean dispersion routes were determined and the corresponding reentry ratios from each unit to other units were calculated. Typical dispersion routes and reentry quantities were summarized. Many reentry ratios appear to be within the range from 5.0% to 10.0%, confirming that the interunit dispersion is an important pollutant transmission route. LES results reveal more complex dispersion mechanisms, broader dispersion scopes, and higher dispersion uncertainties than RANS results. The time scales of interunit dispersion are comparable with those of natural ventilation. They are however much smaller than the survival times of most pathogens under ordinary physical environments, implying that the interunit dispersion is highly dangerous. These findings contribute to enhanced understanding of interunit dispersion mechanisms and improved infectious intervention strategies.

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