Research On Cross-household Transmission Characteristics Of Indoor Pollutants In Naturally Ventilated Buildings Under Typical Urban Form

In urban environment,indoor pollutants in natural ventilated buildings diffuse through the windows,which may enter other rooms of the building and cause inter-unit dispersion,When the pollution source involves pathogenic microorganisms,it will bring some risk of infection to indoor persons in other rooms.In recent years,this special airborne transmission route has been widely concerned by scholars,but previous studies are limited to the isolated building or single structure form building.In urban form,the surrounding buildings and building structure form will affect the flow field around buildings,and then change the inter-unit dispersion route,which needs more in-depth research.In this paper,the indoor and outdoor coupling numerical simulation of indoor pollutant dispersion characteristics of single-sided natural ventilation buildings in typical two-dimensional and three-dimensional urban form is carried out,and the inter-unit dispersion route is analyzed by combining with the indoor pollutant concentration,re-entry ratio and other index parameters.Firstly,the indoor and outdoor coupling numerical simulation of air flow and pollutant dispersion in the two-dimensional ideal street canyon four story single-sided natural ventilation building under wind pressure is carried out by using computational fluid dynamics(CFD)software.The indoor and outdoor flow field and pollutant concentration field are analyzed.The natural ventilation capacity of the room and the route and infection risk of pollutant inter-unit dispersion are discussed in combination with the index parameters such as air exchange rate(ACH),pollutant concentration,re-entry ratio(R_k)and infection risk.The effects of different reference wind speed(U_ref)and aspect ratio(AR)of street canyon were compared.The results show that:(1)the ventilation capacity of the rooms on the windward side of the street canyon is generally greater than that on the leeward side,and it increases with the increase of the reference wind speed U_ref of the incoming flow.The ventilation capacity of the rooms on the windward side of the shallow street canyon and the regular street canyon with AR?1 is higher than that of the rooms on the deep street canyon with AR=2 and AR=3;(2)The lowest concentration of pollutants in the indoor breathing height of the pollution source room occurs in the fourth floor room on the windward side,and the highest concentration mainly occurs in the fourth floor room on the leeward side,the lowest floor room on the leeward side and the windward side;(3)the outward diffusion of indoor pollutants is mainly affected by the vortex flow in the street canyon.When the pollutants are released in the room on the windward side of the street canyon,they are mainly diffused downward,and when they are released in the room on the leeward side of the street canyon,they are mainly diffused upward.In the diffusion direction,the room closest to the source room has a higher re-entry ratio.For the shallow street canyon and regular Street canyon with AR?1,the re-entry ratio of the second floor room on the windward side below the source room in the third floor on the windward side is more than 30%when U_ref=0.5m/s,and more than 10%when U_ref=2m/s;(4)The source room always own the greatest risk of infection,which depends on the natural ventilation capacity of the source room;For shallow street canyon and regular street canyon with AR?1,the infection risk of non-source rooms was lower,which was less than 30%when U_ref=0.5m/s and less than 7%when U_ref=2m/s,and the distribution was basically consistent with the room re-entry ratio distribution;when the wind speed was low,the number of source rooms leading to high risk of infection in non-source rooms in deep street canyon increased,and the inter-unit dispersion range of source rooms increased,resulting in cross street canyon transmission.Then,the indoor and outdoor air flow and pollutant dispersion of four story single-sided natural ventilation buildings with different structure forms in three-dimensional urban form under the effect of wind pressure are numerical simulated by CFD.The indoor and outdoor flow field and pollutant concentration field are analyzed.The natural ventilation capacity and pollutant dispersion characteristics of the room are discussed in combination with the index parameters of air exchange rate,pollutant concentration,re-entry ratio and the possible inter-unit dispersion route.The results show that:(1)the building structure form has a significant effect on the overall room ventilation capacity on the windward side of the building,but has a weak effect on the overall room ventilation capacity on the leeward side,and the overall room ventilation capacity on the windward side of the building is higher than that on the leeward side except for the courtyard form;(2)the building structure form has a significant effect on the outdoor flow field of the windward side of the building,thus changing the inter-unit dispersion route,but has a subtle effect on the leeward side.(3)On the windward side,the indoor pollutants of slab and courtyard buildings mainly diffuse downward,the street canyon building on the contrary,the indoor pollutants of semi-closed buildings mainly diffuse horizontally and obliquely;on the leeward side,the indoor pollutants of the four types of buildings mainly diffuse upward from the second and third floor source rooms in the middle.The research results can deepen people's understanding of the dispersion mechanism of indoor pollutants in natural ventilated buildings in urban environment,and help relevant departments to formulate more effective prevention strategies for respiratory infectious diseases.