Study On Outdoor Thermal Radiation’s Impact On Thermal Environment And Thermal Comfort With Different Subsurface In Guangzhou

In an outdoor solar radiation environment,the thermal comfort of the human body is largely influenced by the thermal radiation of the surrounding environment,and the thermal radiation factor appears in the form of mean radiation temperature（MRT）,so the accurate measurement of MRT is essential to understand and evaluate the outdoor thermal comfort.In addition,the human body is closely related to the ground in outdoor spaces,and ground radiation has a significant effect on MRT;however,the effect of ground radiation may vary due to differences in environment and materials.Therefore,this paper investigates the radiation characteristics and thermal comfort of different outdoor subsurfaces in Guangzhou area and clarifies the accurate measurement method of MRT,which is important for optimizing the outdoor microclimate thermal environment.The study was carried out in Guangzhou between December 2021 and September 2022,and four different outdoor environments of limestone（reflectivity 0.4）,asphalt（reflectivity0.1）,concrete（reflectivity 0.35）,and grass（reflectivity 0.2）were selected and measured under shading and non-shading conditions.The measurement process was combined with questionnaire research,and finally 2682 valid questionnaires were obtained,including 911 in spring,963 in summer and 808 in winter.First,through the analysis of outdoor ground radiation characteristics,it is found that the surface temperature of different lower mat surfaces has a good positive correlation with the received radiation intensity,and is basically proportional to the reflectivity.In addition,the long wave（L↑）and short wave（K↑）in ground radiation have obvious seasonal changes（summer>spring>winter）,where K↑is closely related to reflectivity,and the smaller the reflectivity,the smaller the fluctuation of K↑on the lower mat surface.the size of L↑is determined by daytime incident radiation,and it does not decrease significantly at night with the decrease of K↓.Ground radiation(R_str,g)has a significant effect on MRT(R_str,g/S_str≈50%).The hourly average maximum MRT variation caused by R_str,gunder unshaded conditions is8.1℃ in winter,9.3℃ in spring,and 7.1℃ in summer,while it varies within 3℃ in the shaded environment.Secondly,compare the methods of measuring MRT,the results show that:when the total short-wave radiation flux W_sumkabsorbed by the human body is less than 50W/m²,the calculation error can be controlled within±2℃ using the original ISO formula;when the total short-wave radiation flux W_sumkabsorbed by the human body is 50～150W/m²,increasing the wind speed measurement frequency to 10HZ can improve the accuracy to±2℃ range;when The total short-wave radiation flux W_sumkabsorbed by the human body exceeds 150W/m²,if the MRT accuracy requirements are low（acceptable error±5℃）,the wind speed measurement frequency can be increased to 10HZ,if the MRT accuracy requirements are high（acceptable error±2℃）,it is necessary to make further corrections to the original formula.Again,the effect of thermal comfort on different outdoor lower cushion surfaces was investigated,and it was found that temperature and radiation were the main factors affecting the thermal perception of outdoor personnel in outdoor environment,while humidity was the weakest;under the same temperature,the difference in thermal perception of personnel on different lower cushion surfaces was smaller at shaded nights;comparing daytime under shaded and unshaded conditions and at unshaded nights,it was found that the difference in thermal perception mainly came from MRT.Finally,the thermal environment of different underlayment surfaces was evaluated based on PET,and it was found that the neutral PET in different seasons were:asphalt>grass>concrete>limestone,in the opposite order of reflectivity of subsurfaces.Therefore,materials with higher reflectivity and lower solar radiation absorption should be used as much as possible in the selection of outdoor sub-bedding surfaces.On this basis,the PET indexes of different sub-bedding surfaces are corrected based on the measured mean thermal sensory voting MTSV.The differences between the prediction model of PET calculated by ENVI-met software and the measured model of PET obtained in this study are also analyzed and corrected to provide reference for optimizing the software calculation results.This study clarifies the radiation characteristics of different outdoor subsurfaces and their effects on personnel thermal comfort,and proposes a method to improve the accuracy of outdoor MRT measurements,which has important implications for the optimization and design of outdoor microclimate thermal environments in Guangzhou.