| Author: | Dai, Yuwei |
| Title: | A study of pollutant dispersion and natural ventilation in multistory buildings in urban environment |
| Advisors: | Mak, Cheuk-ming (BSE) |
| Degree: | Ph.D. |
| Year: | 2020 |
| Subject: | Hong Kong Polytechnic University -- Dissertations Air -- Pollution Fluid dynamics Natural ventilation |
| Department: | Department of Building Services Engineering |
| Pages: | xxiii, 173 pages : color illustrations |
| Language: | English |
| Abstract: | As it is associated strongly with human health, the pollutant dispersion in urban environments has attracted particular attention. Recently, a special mode of pollutant transmission, known as interunit dispersion, has gained popularity. Interunit dispersion illustrates the cross-transmission between apartment units within the same multistory building. This airborne transmission mode is highly risky because of the relatively short dispersion distances and transportation time as compared to other modes. Previous studies regarding pollutant dispersion were limited to either an isolated building or purely outdoor environments. This thesis provides a systematic investigation of pollutant dispersion with natural ventilation in the urban environment with both scaled outdoor experiment and CFD simulations. To achieve the research objectives, three sub-works were conducted: (a) a scaled outdoor experiment of natural ventilation and interunit pollutant dispersion in street canyons; (b) quality assessments and improvements of CFD simulations of airflow and pollutant dispersion in building arrays and (c) CFD prediction of interunit dispersion with surrounding building effect. A scaled outdoor experiment was conducted to examine the interunit dispersion characteristics in consecutive two-dimensional street canyons. Tracer gas (CO₂) was continuously released to simulate the pollutant dispersion routes between the rooms in street canyons. The wind velocity, wind direction, air temperature, and tracer gas concentrations were monitored simultaneously. Two important parameters, the air exchange rate and re-entry ratio, were analyzed to reveal the ventilation performance and interunit dispersion of the rooms in the street canyons. Based on the real-time weather conditions. It was found that the ventilation performance of the source room varied according to the room location. The tracer gas was mainly transported in the vortex direction inside the street canyon, and the highest re-entry ratio was observed at the room nearest to the source room along the transportation route. In addition, under real weather conditions, the rooms in the street canyon have a high possibility of occasionally experiencing a high re-entry ratio. A systematic evaluation in CFD simulations of four computational and two physical parameters was conducted on pollutant dispersion in building arrays, including turbulence models, grid resolution, discretization of time step size Δt, length of sampling period, aspect ratio of the arrays, and release rate of tracer gas. For concentration simulations, LES model gave the most accurate results but still had limitations in areas near the source, whereas DES and RANS models underperformed in some areas. In addition, a larger aspect ratio did not necessarily result in a higher concentration field than a smaller ratio. An increase in the tracer gas release rate did not change the general dispersion characteristics, but it still affected the concentration distribution in the areas near the source and resulted in a larger polluted area. Re independence criteria were assessed by CFD simulations with parameters of non-dimensional air velocity, pollutant concentration, ventilation rate, and re-entry ratio. Two sets of critical values were proposed: ReH based on the building height equal to 4.8×10⁴ for the outdoor environment, and ReW based on the opening height equal to 1.4×10⁴ for the indoor environment. The concentration field was more difficult to meet the Re-independent requirement which were ReH=7.9×10⁴ and ReW=3.0×10⁴. For the non-dimensional ventilation rate and re-entry ratio, the critical value for the Re independence was ReW=1.4×10⁴ for both the isolated buildings and the building arrays. CFD simulations were also conducted to investigate the influence of such changed airflow patterns on interunit dispersion characteristics around a multistory building due to wind effect. The presence of upstream building(s) could disrupt the strong impingement of approaching flows but brings a more complex and irregular airflow pattern around the downstream multistory buildings, leading to a more scattered distribution of re-entry ratio values among different units and uncertain dispersion routes. Generally, the tracer gas concentration in most units were lower than those in an isolated building, although very high concentrations were found in some specific areas. |
| Rights: | All rights reserved |
| Access: | open access |
Files in This Item:
| File | Description | Size | Format | |
|---|---|---|---|---|
| 991022379156803411.pdf | For All Users | 4.72 MB | Adobe PDF | View/Open |
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