Author: Xie, Yongxin
Title: A quantitative analysis and modeling of human thermal sensation and comfort in outdoor spaces
Advisors: Niu, Jianlei (BSE)
Mak, Cheuk-ming (BSE)
Degree: Ph.D.
Year: 2020
Subject: Environmental engineering
Human comfort
Urban climatology -- Tropics
Hong Kong Polytechnic University -- Dissertations
Department: Department of Building Services Engineering
Pages: xxvi, 238 pages : color illustrations
Language: English
Abstract: To be involved in the outdoor environment is human nature, especially for the residents living in the cities. However, their desire of outdoor activities are hindered by the uncomfortable thermal conditions in the outdoors. For the residents living in the cities located in the subtropical and tropical areas, the humid and warm to hot outdoor conditions are those causing uncomfortable feelings. In recent years, cities located in the subtropical and tropical areas are experiencing an extended warm-biased period. The high-rise buildings built in the high-density cities weaken the wind environment and thus intensify the heat island effect. The building's structure, the arrangement of building clusters and vegetation such as trees and grass, and the infrastructures in human-height in the outdoor environment can have a significant influence on the micro-thermal environment. Different arrangements can create a various micro-thermal environment. Previous studies lack a clear understanding of the complex outdoor thermal environment and its influence on the residents. Therefore, this thesis aims at providing knowledge in a better understanding of the outdoor thermal environment and its influence on the thermal perception of actual users. The research goal will be achieved through numerical modeling based on the physiological parameters and statistical modeling using a large amount of field survey data. Three sub-works are included in this thesis to achieve the research goal. Namely, (1) investigating the application of the CBE model in the outdoor environment from the aspects of wind and solar sensitivity; (2) the development of a model for accurate prediction of thermal sensation in the outdoor environment based on measured skin temperature; (3) locating the thermal neutral and thermal comfort ranges of meteorological parameters in Hong Kong through statistical modeling. The thesis is based on a large amount of field measurement data of different micro-thermal environments using a microclimate station and survey response from actual users. The collected data covered four seasons. The parameters including air temperature (Ta, °C), globe temperature (Tg, °C), relative humidity (RH, %), wind speed (v, m/s), wind direction, black globe temperature (Tb, °C), long-wave irradiance (Ql, W/m2), and short-wave irradiance (Qs, W/m2) were collected simultaneously. At the same time, the human subjects were invited to experience the specific outdoor conditions. The physiological parameters, such as core and skin temperature, were collected simultaneously for certain experiment settings. A multi-nodal thermal regulation model developed by the University of California-Berkeley targeted at the prediction of thermal sensation and thermal comfort in the transient and asymmetry thermal environment was selected for the prediction of thermal perception in the outdoor environment and the prediction accuracy was first investigated through the comparison of the field surveyed thermal response of human subjects. The preliminary study points out that human subjects were highly sensitive to the outdoor wind and solar environment. The human subjects were highly sensitive to the changing wind speed in the low-radiation conditions. The CBE model failed to predict such a high sensitivity. Besides, the human subjects had a higher tolerance to high air temperatures in outdoor environments than indoors when the solar radiation was acceptable, but the UCB model over-predicted the thermal sensation in such conditions. Both the field survey results and the predictions by the CBE model showed that subjects were more sensitive to wind speed in hotter environments while they were the least sensitive to solar radiation in neutral thermal conditions.
Physiological parameters such as local and overall skin temperatures were used in the CBE model as bridges to link the measured meteorological parameters and the prediction of thermal perception. Therefore, to identify the causes of prediction error, the field measured local and overall skin temperatures were compared with the simulated skin temperatures from the CBE model using the meteorological parameters as input. The measured and simulated skin temperatures were similar to each other merely in the range of 32.5 to 34.0 °C. The prediction gap existed when the human body was experiencing cold and hot conditions. In the comparison between the relation of field-measured mean and local skin temperatures and overall and local thermal sensations, it is discovered that there was a wide range of mean and local skin temperatures corresponded to the thermal neutral range. Such a phenomenon was not observed in the prediction results from the CBE model due to the setting of 'set-point'. A discussion about the usage of 'set-point' was introduced. Due to the characteristics of fluctuating wind environment in the outdoors and human subjects' adaptation, we propose replacing 'set-point' with 'null-zone'. The range of 'null-zone' was determined for different genders and applied to the calculation of local thermal sensation in the CBE model. Including the forehead as one of the dominant parts other than chest, abdomen, back, and pelvis in the logic of determining overall thermal sensation was another development. The prediction accuracy improved to 93.7% for the revised model. The collected 1600 human subject responses from the field survey with the concurrent measurement results of meteorological parameters were used for the statistic modeling of locating thermal neutral and comfort ranges. Probit analysis was used for searching for the thermal neutral range of Hong Kong residents in a year span. Logistic regression was used for locating the meteorological parameter ranges for thermal neutral and comfort conditions. The results from Probit analysis showed residents had difficulties in determining their actual thermal feelings near the thermal neutral status when using the nine-point thermal sensation scale to describe their thermal feelings. The logistics regression models for thermal neutrality and thermal comfort were built using the combination of meteorological parameters. The results of the regression models showed that wind and solar radiation had an interaction effect with air temperature in determining thermal sensation and thermal comfort. Wind can effectively offset the negative effect of solar radiation in summer when the air temperature was lower than 31 °C. The thermal comfort condition allowed a higher limit of solar radiation than the thermal neutral condition when the air temperature was lower than 31 °C. The present thesis investigates the human thermal perception in the outdoor environment. The findings in the present thesis contribute to a better understanding of creating a comfortable outdoor thermal environment. The revised CBE model can help to give an accurate prediction of thermal sensation in the outdoor thermal environment. The results from logistic regression modeling provide the reference of thermal neutral and comfort ranges for the planners and designers in the subtropical cities.
Rights: All rights reserved
Access: open access

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