Predicting convective heat transfer from Computational Thermal Manikin in urban outdoor environments

Abstract

Urban residents are increasingly encouraged to go outside for recreation and relaxation purposes, which may improve personal health and reduce building energy consumption. It is important to understand the thermal conditions of human body in urban outdoor environments. However, the urban wind conditions at the pedestrian level and their impact on the thermal comfort of people have not been thoroughly investigated to date. This study aims to predict the convective heat loss from human body subject to urban outdoor wind environments. Onsite wind measurements are carried out at 0.6 m, 1.2 m, and 1.8 m above the ground on three representative green lands in the coastal city of Sydney in Australia. Meanwhile, the effects of the wind velocity and turbulent conditions on the convective heat loss from human body are investigated using a computational thermal manikin (CTM) model, which is validated against published experimental data. Along with empirical equations derived from the CTM simulation, the wind data collected from onsite measurements is used for predicting the convective heat loss from human body in the outdoor wind environments. In total six groups of wind measurements have been carried out at each measurement sites over a period of four months (from March 2019 to June 2019). The time duration of each measurement is one hour and the sampling frequency is set to 20 Hz. Compared with the local meteorological data recorded at the seaside airport of Sydney, the wind speed in the city is at least 50% lower. To calculate the turbulence characteristics of the wind environment, we use a 1-min averaging period to generate the vertical wind profile of turbulent intensity and turbulence length scale. The correlations between the wind speed and wind turbulence characteristics at different measuring sites are examined. The turbulence intensity measured in this study matches with the reference range given in existing guidelines, while the measured turbulence length scale is much smaller than the value given in the guidelines. It is found that the empirical Von-Karman Spectra can be used to describe the frequency distribution of the turbulence at the pedestrian level in urban open space. The insight of this study regarding the vertical wind profile, turbulence intensity and turbulence length scale at the pedestrian height is beneficial for outdoor thermal comfort assessment. The results of the present CTM simulation show that the convective heat loss of most body segments increases with increasing wind velocity and turbulent intensity and decreasing turbulence length scale. Empirical correlations for predicting convective heat transfer coefficients as a function of the wind velocity, turbulent intensity and turbulence length scale are derived based on simple-geometry assumptions. It is found that, at a given wind velocity and over the ranges of the turbulence conditions from the field measurements, the variations between the high and low values of the convective heat transfer coefficients can be up to 67%. The results of the CTM simulation demonstrate the significance of capturing the turbulent wind conditions for accurately predicting the heat loss from human body for outdoor thermal comfort studies.