| Author: | Zhou, Siqi |
| Title: | Human body convective heat transfer under dynamic pedestrian-level wind |
| Advisors: | Niu, Jianlei (BEEE) Yu, Yichen (BEEE) |
| Degree: | Ph.D. |
| Year: | 2026 |
| Subject: | Heat -- Transmission Human body Human comfort Microclimatology Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Building Environment and Energy Engineering |
| Pages: | xxix, 184 pages : color illustrations |
| Language: | English |
| Abstract: | As global warming intensifies, extreme heat events are sweeping the globe with unprecedented frequency and intensity, threatening the lives of hundreds of millions. The demand for developing cool and sustainable cities has never been greater. Thermal comfort models serve as a vital tool for this effort, as they quantify the complex relationship between environmental parameters and thermal sensation, thereby providing the theoretical foundation for developing a reliable thermal environment evaluation system. However, the predictive performance of these models remains limited in the outdoor environment, likely due to the oversimplification of the dynamic conditions. Wind, affecting the heat balance by enhancing convective heat transfer over the human body, is among the most dynamic environmental parameters. However, the influence of wind on heat transfer is often oversimplified in conventional thermal comfort models, as the prevalent models typically employ correlations that depend solely on mean wind speed. Outdoor wind is turbulent and inherently non-stationary, exhibiting a time-varying mean speed due to synoptic trends. Although field measurements clearly show this, the effects of these components on convective heat-transfer coefficients remain largely unknown. This research comprehensively evaluated the effect of dynamic outdoor wind on convective heat transfer over the human body, aiming to improve the accuracy of the thermal comfort model. It is organised into three sub-studies that systematically employ an advanced thermal manikin as the primary measurement instrument: a) A field study to simultaneously monitor typical pedestrian-level wind and measure convective heat transfer over the human body. b) A wind tunnel study to quantify the effects of wind fluctuations at different frequencies on the convective heat transfer. c) A wind tunnel study to evaluate mixed convection over the human body under urban weak wind conditions. Previous studies on how turbulence affects convective heat transfer over the human body were conducted mainly in wind tunnels or climate chambers, where test-facility constraints led to relatively low turbulence intensity, small turbulence integral length scale, and fixed prevailing wind direction. To address this gap, a field study was conceived to monitor the pedestrian-level wind in actual outdoor settings and to measure the convective heat transfer coefficient (hc) of a thermal manikin at the same time. The results show that both longitudinal and lateral turbulence intensity significantly enhance whole body hc. The integral length scales found at the two sites were larger than a typical manikin dimension, and the whole body hc peaked when these two scales were comparable. As the first field study to quantify convection heat loss of a thermal manikin exposed to real-life urban boundary layer wind, it is demonstrated crucial to consider realistic turbulence characteristics in the field when evaluating hc over a human body for urban microclimate design. To further quantify the effect of non-stationary turbulent wind observed in the above field measurement on hc under a controlled setting, such flows were reproduced in a wind tunnel using an active shutter and a passive grid. Meanwhile, a thermal manikin was employed to determine the hc over the human body. The findings indicate that while high-frequency turbulence intensity significantly increases hc, hc does not change notably with fluctuation amplitude in the low-frequency range. In outdoor settings, turbulence intensity can be overestimated by more than half if the synoptic trend is not removed. Therefore, detrending dynamic flow is critical for accurately calculating turbulence intensity; otherwise, the whole body's hc could be overestimated by up to 40%. Furthermore, a reduction in wavelength of low-frequency fluctuation component in dynamic flow contributes to enhancing hc, while no more than 4% in typical outdoor pedestrian-level wind conditions. To improve the accuracy of the predicted hc in prototype pedestrian-level urban environment, an equivalent wind speed which accounts for the effects of turbulence and synoptic trend has been proposed to adjust the wind speed input in current thermal comfort models. The field measurements also reveal a frequently occurring yet understudied regime: low-speed but turbulent winds induced by high-density urban layouts. In such low wind speed environments, a high skin-to-ambient temperature difference (ΔT), often caused by strong radiation, can lead to mixed convection supplanting forced convection as the dominant heat transfer mechanism. Nevertheless, the mixed convective heat transfer over the human body remains poorly understood. To investigate this, an experiment was conducted in a multi-fan wind tunnel, equipped with a passive grid and a perforated plate to generate low wind speed across multiple turbulence intensity levels. A thermal manikin was used to measure the hc across different ΔT. The findings show that under the combined conditions of low wind speed (< 1 m/s) and low turbulence intensity (< 8%), increasing ΔT significantly enhances the hc of the lower body, by over 10% at ΔT = 12 °C, whereas the upper body's hc exhibits negligible change. Conversely, hc does not vary with ΔT when TI exceeds 14%, likely because the thermal boundary layer is more easily disrupted by the highly turbulent flow. Among the prevailing thermal comfort models, the Fiala model is the only one that explicitly accounts for mixed convection conditions when predicting the convection and evaporation heat loss. Yet it substantially overestimates the whole body hc by more than 37%. Therefore, this study proposes a new set of hc prediction formulas for local body segments to improve the prediction accuracy in outdoor mixed convection conditions. This research systematically investigates convective heat transfer over the human body under pedestrian-level wind conditions, spanning commonly observed skin-to-ambient temperature differences, mean wind speeds, low-frequency fluctuations, and high-frequency turbulence (in a high-density subtropical city). The proposed binned hc prediction formulas provide a more accurate representation of heat transfer in realistic urban wind environments, thereby improving the predictive accuracy of existing thermal comfort models. These advancements support the optimisation of pedestrian thermal comfort, contributing to the design of cooler, more resilient urban spaces in a warming world. |
| Rights: | All rights reserved |
| Access: | open access |
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