Numerical Investigation On Influence Of Surface Thermal Radiation For Outdoor Thermal Comfort In Street Canyon

With the increasing demand for quality urban living,the comfort and livability of the outdoor thermal environment is now a key concern for architects and engineers.Surface radiation characteristics have a certain influence on the air temperature and radiation field in the outdoor thermal environment,which in turn causes a change in the thermal comfort of the residents in the outdoor space.However,there is a paucity of quantitative analysis of the significance of surface radiation characteristics on the outdoor thermal environment.This paper quantifies the effect of thermal radiation on the thermal comfort inside street canyon,using an ideal array of street canyon as the target building.The correlation between the radiation characteristics of different surfaces in the street canyon and the thermal comfort is investigated in order to provide a reference for the setting of radiation boundaries in engineering design using numerical simulation to analyze the outdoor thermal environment.The content and results are as follows.1)A classical wind tunnel test was selected to validate the RNG k-εmodel and the computational setup used in this study.The results show that the numerical simulation results are similar to those of the wind tunnel tests.It is shown that the numerical simulation method used is suitable for simulating the thermal environment around buildings under the influence of thermal buoyancy.2)Taking an ideal array of street valleys in a typical city of Shanghai in a hot summer climate zone as an example,a base case was set up with common surface materials.After completing the mesh independence verification,the distribution of the thermal environment around the buildings was analyzed by combining evaluation indicators such as gust wind speed,air temperature,radiation temperature and the mean wind speed change rate.Then,the changes in thermal comfort around the streets under the influence of thermal radiation were also compared.The results show that the mean wind speed change rate ranged from-95%to 145%,while the wind speed still less than the threshold value in the wind comfort standard.The average value of the mean wind speed change rate for the three calculation areas was 9.5%after considering thermal radiation.The air temperature increased by an average of 2.7°C,with an average rate of change of 7.7%.Therefore,the influence of solar radiation should not be oversimplified when using numerical simulation for outdoor thermal environment assessment.3)Taking equivalent wind speed,air temperature,radiation temperature and physiological equivalent temperature(PET)as test indicators,an orthogonal test was set up.It has 8 factors and 3levels of 27 trials.Short-wave absorption rateαand long-wave emissivity rateε_e for different surfaces(windward,leeward,roof,ground)in the streets were selected as factors,each set to three levels.To determine the degree of influence and significance of the radiation characteristics on the thermal environment,the test result was analyzed using range analysis and analysis of variance(ANOVA).Results indicate that for all evaluation indicators the most influential and significant factor is the ground shortwave absorption rate,followed by the leeward shortwave absorption rate.Subsequently,a single-factor analysis of the most significant factor was performed.The results show that changes in the shortwave absorptivity rate of the ground alter the velocity and temperature distributions.The ground shortwave absorptivity rate is proportional to the equivalent wind speed and inversely proportional to the air temperature,radiation temperature and PET.Thermal comfort is better when the ground shortwave absorptivity rate increases.4)The thermal environment in the street canyons between the optimum case and the worst case was compared.The results show that the wind speed in the calculation area of the optimum case increases by 0.04 m/s,and the average values of air temperature and radiation temperature decrease by 1.86°C and 8.1°C respectively.The rates of change in wind speed,air temperature and radiation temperature were 2.4%,5.1%and 18.5%respectively.The average value of PET of the optimum case is on 5.8°C lower than that of the worst case,with a rate of change at 13.4%.The magnitude of the difference between the PET of the two cases amounts to about 1 thermal sensation level in the original PET classification and about 0.5 thermal sensation levels in the modified PET classification.Therefore,in the planning of buildings in hot summer regions,attention should be paid to the choice of wall radiation characteristics to achieve better outdoor thermal comfort.