| Author: | Li, Zhengtong |
| Title: | Optimal urban design for outdoor thermal comfort and air quality |
| Advisors: | Wen, Chih-yung (AAE, ME) |
| Degree: | Ph.D. |
| Year: | 2022 |
| Subject: | City planning -- Environmental aspects Insulation (Heat) Air quality management Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Mechanical Engineering |
| Pages: | xxv, 183 pages : color illustrations |
| Language: | English |
| Abstract: | Air pollution and urban heat island become worldwide growing concerns, which can directly affect public health. Optimal urban design is a promising method to create a more thermally comfortable and cleaner (less air pollution) outdoor urban environment. However, most of the previous papers focused on either thermal comfort or air quality only; only very limited studies addressed them simultaneously. Under certain cases, the impacts of some urban designs on thermal comfort and air quality present opposite trends. This thesis therefore performs a systematic investigation of optimal urban design on thermal comfort and air quality under the same framework to identify critical design parameters. To achieve the research objective, two sub-works are carried out: 1) exploring the influence of urban morphology, including the frontal area density of urban arrays and height-asymmetric street canyon configurations, 2) investigating the effects of local mitigation strategies, including the building setbacks and tree plantings. First, the investigation on frontal area density λF suggests that with an increase in λF, the physiologically equivalent temperature (PET) decreases above most of the sidewalks during the daytime, while only a steady reduction of air quality is observed above the west and east sidewalks of spanwise streets. According to the multivariable regression analysis for Hong Kong, the building density should have a λ value between 0.82 and 0.84 to realize PET <38 °C and CO concentration < 30000 µg/m3 at the same time in the daytime in June. Second, the investigation on height-asymmetric street canyon configurations indicates that for the step-up canyon (the upwind building is lower than downwind building), a higher upwind building is found to produce a hotter air temperature only at a low wind speed and pollute more severely at both high and low wind speeds, compared with its lower upwind building counterpart. In contrast, for the step-down canyon (the upwind building is higher than downwind building), a higher downwind building is found to produce cooler air temperatures at both high and low wind speeds and accumulate more pollutants only at a low wind speed, compared with its lower downwind building counterpart. On the other hand, at the high wind speed, both air quality and thermal environment are better in the step-up canyon than in the step-down canyon. However, at the low wind speed, the air quality is higher in the step-down canyon than the step-up canyon, while the step-up canyon still provides a better thermal environment than the step-down canyon. Third, several design parameters of building setbacks are considered, i.e., the dimensionless height (HHS/W) and dimensionless width (DHS/W) for the horizontal setbacks (HS), as well as the dimensionless length (LVS/L) and dimensionless width (DVS/W) for the vertical setbacks (VS), where W and L are the street width and length. The research on the building setback demonstrates that the horizontal building setbacks are advocated within the low-rise street canyon, which simultaneously improves the thermal comfort and air quality. By manipulating its dimensionless vertical cross-section area SHS (increasing SHS = HHS/W×DHS/W) and its dimensionless aspect ratio HHS/DHS (lowering HHS/DHS), the average PET can decline by up to 2.1 °C and the average pollutant concentration can reduce by up to 66% at the two-side pedestrian level. The vertical building setbacks are more suitable for creating a better outdoor environment for the high-rise street canyon. The dimensionless horizontal cross-section area SVS (= LVS/L×DVS/W) should be as large as possible so that the average PET can decrease by up to 0.7°C and the average pollutant concentration can reduce by up to 35% at the two-side pedestrian level. Fourth, the investigations on the tree plantings suggest that increasing LAD (from 0.5 to 2) results in a significant reduction of air temperature (up to 1.5 °C), while it increases gaseous concentrations by up to 370%. The trees with LAD ≤ 0.5 are advocated since they hardly worsen the air quality but still induce a 0.5-1℃ reduction in air temperature. Increased Htrunk/H causes a lower concentration but a weaker cooling effect. Once Htrunk/H > 0.375, trees hardly increase concentrations compared to tree-free cases. The trees with Htrunk/H ≥ 0.375 are suggested which still declines air temperature by up to 1.5℃. Increasing Wspacing/Wcanopy causes less pollutant accumulation but a poorer cooling effect. When Wspacing/Wcanopy ≥ 2, trees almost do not worsen the air quality. The trees with Wspacing/Wcanopy ≥ 2 are recommended which still causes a 1℃ decrease in air temperature. Besides, the influence of lateral entrainment is investigated. The results of this analysis demonstrate that lateral entrainment could conditionally reduce the pollutant concentration of low-rise canyons. This reduction, which is affected by lateral entrainment, is confined in a range of approximately 2.5 times the street width from the street ends. In contrast, the lateral entrainment causes a more pronounced reduction in the pollutant concentrations of the high-rise canyons. Besides, all three strategies can considerably facilitate the lateral entrainment, leading to a significant reduction in the cross-section pollutant concentrations (by up to 76%) and therefore a significant reduction in the personal intake fraction P_IF of the residents (by up to 81%). |
| Rights: | All rights reserved |
| Access: | open access |
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