Study On Pollutant Dispersion Behavior In Street Canyon Fires Under Coupling Effects Of Cross Wind Flow And Thermal Plume

With the continuous development of urbanization in China and the increase of construction density in cities,various street canyons have been established in cities.The street canyon is a narrow and long street with buildings lining up continuously along both sides,and it can be considered as the most typical structural unit in modern cities.In addition,with the increase of urban population,the number of cars in cities is increasing annually,which leads to a high risk of car fires in the street canyon.In the event of the car fires in the street canyons,the thermal buoyancy induced by the fire can drive the hot,toxic pollutants to rise vertically and lead to pollutants diffusing radially.When the cross wind（perpendicular to both the axis of the street canyon and the longitudinally arranged buildings）passes over the street canyons,the pollutants dispersion will become more complicated.With the increase of the cross wind velocity,part of pollutants escaping the street canyon may be diverted downstream,and re-entrained into the street canyon after impinging into the leeward building,which is called the re-entrainment phenomenon.This phenomenon can worsen the pollution levels in the street canyon,which can seriously threaten the health and safety of the residents.Consequently,the study on pollutant dispersion behavior in street canyon fires under coupling effects of cross wind flow and thermal plume is of great reference value for evaluating the pollution level in the street canyon.In response to the shortcomings in the previous studies,this numerical study investigated the dispersion behavior of the fire induced pollutants in street canyons considering atmospheric pressures,transverse fire locations and aspect ratios of street canyons.In addition,in order to validate the accuracy and reliability of the FDS,the small-scale wind tunnel experiments were conducted,the data from previous small-scale wind tunnel experiments and full-scale tunnel fire experiments were collected to validate the predicted results of the street canyon flow field,street canyon fire smoke dispersion and fire temperature distribution under low air pressure in the FDS simulation.The main research works in this study are as follows:The effects of ambient pressure on pollutants dispersion behavior in the idealized street canyons were investigated in this study.The results showed that when the fire occurred in the street canyons under 60 kPa～100 kPa of the ambient pressures,the smoke plume development could be divided into four regimes,i.e.,buoyancy dominated regime,transition regime,critical regime and inertial force dominated regime.The smoke diffusion under relatively larger ambient pressure was more susceptible to the cross wind flow.The dimensionless ambient pressure P^*was introduced to quantify the effect of ambient pressure on critical wind velocity u_cr,and a prediction model of critical wind velocity was proposed.Further,considering the influences of ambient pressures and entrainment coefficient,a uniform correlation was proposed to predict the temperature rise ΔT_z,0 at different heights in the plume region.Besides,the Froude number Fr was introduced to characterise the competition between the inertial force of the cross wind and the fire induced thermal buoyancy.The Froude number with the critical wind velocity was insensitive to the ambient pressure.A good correlation for predicting the Fr_cr value was given based on the critical wind velocity u_cr and the temperature rise ΔT_z,0.The influences of the transverse fire source locations on the indoor and outdoor pollutants dispersion behavior in the idealized street canyons were investigated in this study.Three fire scenarios are defined according to the transverse location of the fire source,i.e.,scenario 1（fire source near the windward building）,scenario 2（fire source in the middle of the canyon）,and scenario 3（fire source near the leeward building）.The results showed that a re-entrainment phenomenon appeared when the wind velocities reached the critical values in scenarios 1 and 2,but it didn’t occur in scenario 3.Fire source location significantly influenced the critical reentrainment velocity.The critical velocity in scenario 1 was approximate 1.2 m/s～1.5 m/s larger than that in scenario 2.With the increase of wind velocity,the critical Froude number gradually decreased and tended to constant values with values of 0.47（scenario 1）and 0.37（scenario 2）.When the wind velocity was large,more compartments could be affected in the upper floors in all of the three scenarios,and smoke is distributed in an inverted triangle within the buildings.According to different aspect ratios（W/H）of street canyons,10 street canyon types with aspect ratios ranging from 0.5 to 2 were selected to investigate the effect on the indoor and outdoor pollutants dispersion behavior in the street canyons.The results showed that when the cross wind velocity was larger,the smoke filled the street canyon with the aspect ratio of 0.5,and smoke concentration was relatively lower in the street canyon with the aspect ratio of 2,which indicated that the street canyon with the aspect ratio of 2 had a greater ventilation capacity.The critical wind velocities for different aspect ratios could be divided into three regions,i.e.,the linear growth region（Region 1）,the constant region（Region 2）and the linear growth region（Region 3）.The dimensionless critical wind velocity u^*was introduced to standardize the critical wind velocities in the two fire scenarios.And a prediction model of dimensionless critical wind velocity u^*was proposed.In addition,there was a good positive correlation between the critical Froude number Fr_cr and the aspect ratios of the street canyons.The prediction model of the critical Froude number Fr_cr for different aspect ratios was proposed.