Author: Li, Jianong
Title: The measure of outdoor thermal comfort with static-dynamic metrics for microclimate-friendly urban designs
Advisors: Niu, Jianlei (BEEE)
Mak, Cheuk-ming (BEEE)
Degree: Ph.D.
Year: 2022
Subject: Recreation areas -- Environmental aspects
Public spaces -- Environmental aspects
Human engineering
Environmental engineering
Hong Kong Polytechnic University -- Dissertations
Department: Department of Building Environment and Energy Engineering
Pages: xxxv, 266 pages : color illustrations
Language: English
Abstract: The combined effects of faster global warming and serious urban heat island in overpopulated cities like Hong Kong lead to the acute heat hazard in urban microclimatic environment with high-density built-up areas. It has particular negative impacts on the engagement in outdoor activities and outdoor space usage by citizens, physical and mental health, as well as the energy use in buildings. Recently, under the frustrating impacts of novel coronavirus disease 2019 (COVID-19), citizens were highly recommended to stay in open outdoor places with good ventilation for both physical and mental health. Thus, in face of the challenges brought about by climate change and epidemic spread, it is urgent than ever to design thermally comfortable outdoor places where citizens spend their everyday lives to mitigate the effects of climate change and improve citizen's life quality.
Based on the urgency of creating microclimate-friendly spaces in high-density urban environment, the objective of this thesis is to address the following four important issues/questions. 1. How to evaluate outdoor thermal comfort in perspectives of sun and wind conditions, and how to optimize their combination for a comfortbale outdoor place welcoming relaxation? 2. What are the limitations of the popular thermal comfort indices in evaluating the outdoor thermal comfort? 3. How do frequent exposures to cool-biased and hot-biased condition cycles influence the outdoor thermal comfort on the summer days, and how can these cycles be utilized for comfort and cooling strategy? 4. What are the influences of fluctuating wind and solar radiation on pedestrians in the urban continuum on hot summer days, and what are the suggested fluctuations for comfortable sidewalks design? 5. How to develop a metric to predict dynamic outdoor thermal comfort in diverse microclimate environments?
The second and third parts of this thesis concentrates on the study of the outdoor thermal comfort when people's thermal state thought to be near to steady in a microclimate environment. The acceptable microclimate environments were explored, as well as the strategies of optimizing sun and wind conditions for comfortable microclimate environments under a wide air temperature range. Plenty of simultaneous physical measurements of temperature (Ta,°C), relative humidity (RH, %), wind speed (v, m/s), wind direction, black globe temperature (Tg, °C), long-wave and short-wave irradiance, and questionnaire surveys about subjective thermal perceptions were carried out to study over 1500 subjects' thermal responses to microclimate thermal conditions. The experiments were conducted in typical outdoor places on a university campus under various seasons in Hong Kong.
The second part proposed a conception of desirability of sun and wind conditions when people spending leisure time outdoors. The evaluation of outdoor thermal comfort using the Universal Thermal Climate Index (UTCI) can be improved by considering the subjective desirability of sun and wind conditions. Based on the desirability of sun and wind conditions, sun acceptability and wind acceptability were prosed using the utility function to determined acceptable UTCI ranges. The third part demonstrated that the mean radiant temperature (Tmrt) and the wind speed (v) directly influenced subjective perceptions but whose effects were not well estimated by UTCI. A combined sun and wind condition index (SWI) together with a Tmrt_v diagram were proposed to optimize the combinations of the sun and wind conditions for outdoor thermal comfort across a wide air temperature range.
The fourth part of this thesis concentrates on transient thermal comfort of people who are being exposed to frequent spatiotemporal step changes in outdoor microclimate environments. It is one of dynamic natures of outdoor thermal environments. The fourth part revealed the significant influences of step change frequencies on transient and adapted thermal perceptions. The greater thermal satisfaction and decreased need for comfort can be achieved by higher frequency of step changes or more opportunities to cool-biased environments. A UTCI-based new index equivalent UTCI* is proposed to depict sensational temperature in microclimate environments where people are subjected to frequent spatiotemporal step changes. With a higher alternating frequency, there was reduced thermal dissatisfaction with hot summer days and a lower comfort requirement for shade, as well as the upper limit of acceptable UTCI* approaching 43.7°C.
One another dynamic aspect of outdoor thermal environments was the highly spatiotemporal fluctuating solar radiation and wind flow produced by complex urban continuum during movement. Such fluctuating microclimate can bring instantaneous pleasant or annoyed stimulus to pedestrians, which may influence the overall thermal comfort in both physiological and psychological aspects. Therefore, the fifth parts of this thesis concentrated on the dynamic thermal comfort of pedestrians strolling in the urban continuum while subjected to fluctuating solar radiation and wind flow.
The simultaneous mobile physical measurements of Ta, RH, v and Tg, and collection of physiological (skin temperature and local sweating rate) and instantaneous perceptual (questionnaire) responses from 70 college-age subjects were conducted in two different urban continua. Subjects were required to walk and have a rest in each measurement within 1.5 hours on typical summer days. A perceptible fluctuation index (PFI) was proposed to describe the apparent fluctuating wind flow and solar radiation during experiments.
The fifth part confirmed that, there was a mixed thermal perception responding to fluctuating sun and wind conditions on hot summer days, in which the hot and cool warnings appeared at skin alternately, regardless of the heat accumulated inside the body. Impacts of fluctuating v and fluctuating Tmrt on transient thermal perceptions cannot be reflected by UTCI. The fluctuation of mean skin temperature was found highly related to the fluctuating v and Tmrt, which became the basis of integrating the fluctuation of v and Tmrt. This integrated fluctuation significantly influenced the needs of cooling and the possibility of pleasant or annoyed feelings.
Moreover, possibility of pleasant feelings was influenced by mixed fluctuating v and Tmrt and sweating rate, less than 50% of subjects were found annoyed by changes in v and Tmrt. The minimum v triggering pleasant feelings, the threshold of Tmrt irrigating people, and acceptable sequential change in mean skin temperature were explored under different fluctuating v and Tmrt. By regulating the fluctuation of v and Tmrt in a proper way and considering their mixed impacts, a cooling effect equivalent to lowering air temperature by 1.0-2.0 °C, as well as the continuously acceptable thermal environments, can be achieved. A new framework to evaluate the dynamic outdoor thermal comfort and predict thermal sensation was proposed by taking into account fluctuating v and Tmrt, physiological thermal load in addition to UTCI and DTS.
The obtained results can guide the evaluation of outdoor thermal comfort in near steady state and dynamic state, and suggest the further modification of thermal comfort models by carefully taking into account the impacts of sun and wind conditions on heat transfer process and thermal perceptions in the urban continuum. It is expected that the results in this research could be significant for understanding the actual impacts of outdoor solar and wind conditions on thermal comfort, as well as for providing criteria of how to regulate and combine the sun and wind environments through urban building morphologies for outdoor thermal comfort. Furthermore, it can inspire urban designers to adapt present urban designs to ones with fluctuating variables that can be adjustable to create microclimate-friendly environments, meeting the needs of users and mitigating the impacts of urban heat island and global warming.
Rights: All rights reserved
Access: open access

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