Investigation On The Migration Law Of High-Temperature Fine Particles Associated With Heat Process And Its Deposition Characteristics In The Respiratory Tract

Solid particulate matter is a common pollutant in industrial buildings.Inhalable particulate matter(PM₁₀,that is,particulate matter with an aerodynamic equivalent diameter?10?m)enters the respiratory tract and accumulates in the lungs,which is the main cause of industrial pneumoconiosis.As one of the common and serious complications of pneumoconiosis patients,chronic obstructive pulmonary disease(COPD)is expected to become the third leading cause of death in the world in 2030.At present,with the rapid development of industrialization in China,the intensity and total amount of harmful pollutants released in industrial processes have increased significantly.Besides,in high-polluting industrial buildings such as metallurgy,machinery,and foundry,the discharge of unorganized harmful particles is still widespread.Therefore,the indoor environment of industrial buildings is worse than that of civil buildings and outdoor atmospheric spaces.Workers are exposed to high concentrations of particulate pollutants for a long time,which results in a sharp increase in the infection rate of respiratory diseases.The heat source at the industrial site is usually also a pollution source.The harmful particles released by the pollution source will diffuse under the action of the hot-air flow.According to the releasing time,the industrial processes can be roughly divided into two categories.One is similarto the metal welding process,which emits a certain amount of hot-air in a short time,carrying a large number of high-temperature particles with different diameters,and then a high-temperature gas-solid two-phase flow cloud is formed under the action of local hot-air.Another one,similar to the pouring process,a floating jet is formed under the induction of a high temperature heat source,which will continuously release harmful particles to the surrounding environment for a period of time.These particles usually carry a series of toxic or carcinogenic chemicals,and their particle size ranges from nanometers to micrometers.The particles can also be irregularly shaped non-spherical particles.Compared with normal temperature particles in civil buildings,the movement process of these industrial particles is more complicated,and their concentration is also higher.However,the existing ventilation system design for different industrial processes mainly relies on experience,which greatly increases the difficulty of design and the uncertainty of particulate control.In addition,due to the difference in dynamic characteristics between particulate matter and air,and the interference of heat sources and equipment on site,it is inevitable that some particulate matter will diffuse and escape.Therefore,in-depth study of the movement and concentration evolution characteristics of fine particles in typical high-pollution emission processes,and analysis of their transmission process in the workers'respiratory tract,is of great significance for the systematic evaluation of industrial environmental safety.Based on this background,this article focuses on the environmental movement behavior of particles in industrial buildings and combined with physical and chemical characteristics of particles,conducts systematic research on the dynamic mechanism of fine particle migration and deposition in the respiratory tract during typical emission processes by using theoretical analysis,experimental test and numerical simulation methods.Specifically,it is to determine the quantitative relationship between the diffusion distance of emitted fine particles under the action of instantaneous hot air flow and different influencing factors,reveal the time evolution of particulate concentration driven by buoyancy in different areas of the room,clarify the dynamic mechanism of non-spherical particle transport and deposition in the respiratory tract,and propose a theoretical prediction model for particle deposition rate in obstructed airways.Consequently,it can provide a theoretical reference basis for personnel exposure assessment and ventilation system optimization design in industrial environment.The analysis of the migration characteristics of the fine particles emitted under the action of the instantaneous hot-air flow during the welding process shows that,the conversion of heat and kinetic energy promotes the two-phase flow.The temperature of the particles and the air flow will drop sharply in a short time,and their speed will increase first and then slowly decrease.Therefore,in the early stage of particle emission,it is necessary to prevent the two-phase flow from burning the upper respiratory tract of the human body,and the closer the two-phase flow to the center,the higher the flow rate and temperature,the greater the risk of human exposure.During the two-phase flow,the particles will depart from the airflow in both vertical and horizontal directions,and the more heat the process emits instantaneously,the higher the maximum average velocity that the particles can obtain,and the better the following performance between the particles and the airflow.Different influencing factors change the diffusion area of high-temperature particles through energy conversion and particle dynamics in the initial stage,and all have significant effects on the horizontal and vertical diffusion distance of particles.Moreover,the diffusion area of the particles in different directions will increase with the initial temperature,the initial velocity,and the release time increasing,but decrease with particle diameter increasing.Therefore,the harm of fine particles under the influences of multiple factors must be considered comprehensively on site.For this reason,within the given range of different variables,a prediction model of the maximum diffusion radius of fine particles is developed by multiple regression analysis.Furthermore,for the pouring process,by revising the existing thermal stratification theoretical model of natural ventilation driven by buoyancy,two types of prediction models for the concentration evolution of the emitted fine particles driven by buoyancy are established.It is found that,the results obtained from the numerical solution of the theoretical model in this paper are in better agreement with the experimental datas.The existing models are only a few special cases,which confirms that the non-uniform three-layer model proposed by present work is more general for predicting transient buoyancy-driven natural ventilation.When the dimensionless effective ventilation area a is a constant,the ratio of 1-?and?_cis significantly affected by the buoyancy combination coefficient?,but dose not change its trend.The initial outdoor temperature only changes the absolute temperature of the thermal stratification,but has no effect on the height of the thermal stratification.Besides,the instantaneous temperature of thermal stratification will increase with the effective ventilation area A^*,the room height H and the buoyant flux B of the heat source increasing,but decrease with the room area S increasing.During the evolution of the particulate concentration,the thermal stratification interface?₀where the vertical velocity in the room is zero,and the fresh air layer interface?are not the same interfaces.The original pollution layer is devided into two regions by?₀,the thickness of which is?-?₀and?₀-?_a,respectively.Under the pure displacement model,the fresh air layer concentration C_kis always equal to C_a+C_f,while the original pollution layer concentration C_ldecays with time until reaching a stable concentration of C_a+C_f.The upper particle concentration C_uincreases sharply in the initial stage,and then slowly decreases on this basis before reaching its stable value of C_a+C_f+C_s.For the uniform mixing model,the changes in particle concentration of booth the upper and lower layers are more gentle,and the stable values are C_a+C_fand C_a+C_f+C_s,respectively.It shows that the mixing characteristics of lower pollutants will affect the characteristics of indoor pollutant stratification and concentration evolution,but it does not change the indoor particle concentration distribution at the stable moment.In addition,the concentration of particles in the upper layer should be greater than that in the lower layer at any time during the ventilation process,and the larger a is,the faster the particle concentration decays and the higher the efficiency of pollutant discharge.Through testing,it is found that industrial metal dust is usually composed of spherical,ellipsoidal,cuboid,cylindrical and irregular cone-shaped particles,with well-developed gaps on the surface and large specific surface area.In addition,the metal dust contain a large amount of oxides of elements such as Fe,Al,Si,and heavy metal elements such as Mn,Ti,and Cr,which increases the risk of workers suffering from various occupational diseases.The deposition efficiency?of particles in the respiratory tract not only depends on the shape coefficient?,but also depends on the specific shape.On the whole,the larger the?is,the higher?of particles is,reflecting that irregular non-spherical particles are more likely to be transported to deeper positions in the respiratory tract,which may do greater harm to respiratory health.The difference of?between non-spherical particles increases with the particle diameter or respiratory volume increasing,and it is more significant in G3?G6 respiratory tract than the G9?G12 respiratory tract.The difference in the local deposition rate of particles mainly occurs at the bifurcation of the respiratory tract,especially the first-stage bifurcation B3 and B9,and the maximum difference exceeds 30%and 20%,respectively.In addition,for the G3?G6 respiratory tract,the smaller the?of the particles,the more dispersed the particle deposition distribution,which can eventually cover the outer area of the respiratory tract,and it becomes more obvious with the respiratory volume and particle size increasing.However,when the particle size increases,the gravity effect plays a more significant role,and the particle deposition distribution may have an opposite trend of change in the G9?G12 respiratory tract.The results of the gas-solid two-phase movement in the obstructed respiratory tract show that,the distribution of the flow field in the respiratory tract is asymmetry affected by COPD,and the greater the respiratory tract obstruction rate?,the more severe the patient's local hypoxia.When?=0.8,the relative hypoxia rate is larger than 90%.Under the same labor intensity,workers with obstructive pulmonary disease will have faster breathing,and the stronger the labor intensity,the greater the absolute hypoxia of the patient,and the higher the possibility of asthma.It is also found the obstruction of the respiratory tract does not change the deposition mechanism of particles,but it has a significant impact on particle deposition form.Specifically,the number of particles deposited on the healthy side increases,while the number of particles deposited on the diseased side decreases.The particles are distributed asymmetrically,and the asymmetry of the deposition distribution will become high with the increase of the?,labor intensity or d_p.Airway obstruction does not change the variation of the total deposition efficiency?_tof particles with the Stokes number or the gravity sedimentation factor?,but the larger the?is,the smaller the total deposition efficiency?_twill be.According to the numerical calculation results,a theoretical prediction formula for the deposition efficiency of particles in the obstructed respiratory tract is developed under the combined action of inertial collision and gravity.The applicable range of the empirical formula is:0<St<0.3,and 0<?<0.01.