Mechanism Of Three Dimensional Ventilation And Pollutant Dispersion Driven By Thermal Buoyancy In Urban Blocks

The rapid development of cities has brought serious challenges to the urban environment.With the improvement of life quality,people are more and more concerned about air quality.Good air quality and comfort have become the goal that people are chasing.In recent years,urban pollution and urban heat islands have become urgent problems to be solved.This article aiming at the air quality problem in urban blocks which studies the mechanism of ventilation and pollutant dispersion in three-dimension block driven by buoyancy.Due to the characteristics of low reflection and low specific heat capacity of buildings and streets in the city,they will emit heat outward after absorbing solar radiation.Especially in sunny and windless weather,the thermal buoyancy force caused by the heating of many building surfaces or other underlying surface structures condenses and becomes the main driving force of air flow in urban blocks.This article first introduces the cademic background,theoretical knowledge and significance of this topic,summarizes the current research status of urban ventilation and pollutant dispersion at home and abroad,and emphasizes the necessity and social value of research on urban ventilation.And then aiming at the factors of the scale of buildings,streets aspect ratio and block profile form that affect the ventilation of blocks.The ventilation and pollutant diffusion rules within the three-dimensional block are obtained under different working conditions by CFD.The simulation results show that the convergent flow is formed from the edge of the block to the center due to buoyancy,which promotes the heat dissipation of the street canyon at the edge of the block,but inhibits the heat dissipation of the street canyon at the center of the block.The temperature and the concentration of the pollutants has positive correlation with the scale of buildings and block form within three-dimensional block: when the buildings increases or the block profile form from the concave shape to a convex shape,the average temperature and concentration of pollutants are rising.This also shows that the larger the block size or the convex shape of the block,the poorer air quality and comfort level in the block.The temperature and the concentration of the pollutants has negative correlation with the street aspect ratios within three-dimensional block: As the aspect ratio of the street increases,the average temperature and pollutant concentration decrease.Finally,the simulation results are verified by scaling experiment.