Yun Hua,Wen Nie,Qiang Liu,Xiaofei Liu,Chengyi Liu,Weiwei Zhou,Fengning Yu
College of Safety and Environmental Engineering,Shandong University of Science and Technology,Qingdao 266590,China
State Key Laboratory of Mining Disaster Prevention and Control Co-founded by Shandong Province and the Ministry of Science and Technology,Shandong University of Science and Technology,Qingdao 266590,China
ABSTRACT To define the diffusion behavior of harmful exhaust substances from diesel vehicles and support safety risk assessments of underground coal mines,we performed a multi-species coupling calculation of the emission and diffusion of harmful substances from a trackless rubber-wheel diesel vehicle.A computational fluid dynamics (CFD) model of the diffusion of harmful emissions was hence established and verified.From the perspective of risk analysis,the diffusion behavior and distribution of hazardous substances emitted by the diesel vehicle were studied under 4 different conditions;moreover,we identified areas characterized by hazardous levels of emissions.When the vehicle idled upwind in the roadway,high-risk areas formed behind and to the right of the vehicle: particularly high concentrations of pollutants were measured near the rear floor of the vehicle and within 5 m behind the vehicle.When the vehicle idled downwind,high-risk areas formed in front of it:particularly high concentrations of pollutants were measured near the floor and within 5 m from the front of the vehicle.In the above cases,the driver would not breathe highly polluted air and would be relatively safe.When the vehicle idled into the chamber,however,high-risk areas formed on both sides of the vehicle and near the upper roof.Forward entry of the vehicle caused a greater increase in the concentration of pollutants in the chamber and in the driver’s breathing zone compared with reverse entry.
Keywords:Diesel vehicle Harmful emissions Diffusion behavior CFD model Underground coal mine
Trackless diesel vehicles with rubber wheels are an important type of auxiliary transportation equipment in underground coal mines.While these vehicles improve the efficiency of mine transportation,they also produce serious exhaust pollution.In 2017,the International Agency for Research on Cancer (IARC) of the World Health Organization classified diesel engine exhaust as“highly carcinogenic” on its List of Category I carcinogens.This means that exhaust emissions from trackless rubber-wheel diesel vehicles are a serious threat to the health of any underground coal miners who are exposed to them [1-4].
Compared with the open space on the surface,underground coal mines are a special case.They comprise a limited and closed space with complex environmental conditions and less airflow that is unobstructed [5-9],which means that any harmful emission substances from rubber-wheel diesel vehicles cannot be rapidly diluted by airflow.Harmful emission substances can easily accumulate,especially in some cul-de-sacs,and may result in workers in mines being overexposed to these substances.To develop an efficient and economic control scheme for dealing with harmful pollutants from trackless rubber-wheel diesel vehicles,it is necessary to conduct a risk assessment of the diffusion of harmful emissions.Obtaining the diffusion behavior and distribution of harmful pollutants is an essential step in any risk assessment of an underground space [10].To date,many studies have reported vehicle emission dispersion behavior above-ground,which has some relevance for research into the diffusion behavior of diesel exhaust emissions in underground coal mines.Computational fluid dynamics (CFD) technology had been used to study the dispersion and spatial distribution of particles in street canyons and their nearby areas,taking into account the effects of different height ratios and wind directions [11-13].Kim studied the influence of vehicle emissions on nitrate aerosol concentrations in an urban street and concluded that controlling volatile organic compounds(VOCs)and NH3emissions may reduce particulate matter (PM) of 2.5 μm or less (PM2.5) more effectively than controlling nitrogen oxides(NOx) [14].Shi et al.evaluated a single vehicle and multi-vehicle emission diffusion model in urban street canyons and obtained the influence of vehicle-induced turbulence on carbon monoxide(CO) emissions and concentration distribution [15].However,there have been few studies into the emission and diffusion of harmful substances from diesel vehicles in underground coal mines,and only a few scholars have discussed this.Kurnia et al.evaluated the diffusion of carbon dioxide(CO2)from diesel engines in mining areas to provide a ventilation strategy for controlling the concentration of harmful gases [16].Zheng et al.used Euler’s method to conduct a series of studies of the distribution of diesel PM at a dead-end entry in an underground mine [17-20].Xu et al.studied the distribution and diffusion characteristics of diesel PM in an isolated area of an underground mine in the USA,and they used the Lagrange method to explore differences compared with the existing Eulerian simulation data [21].However,most of the previous research focused on the diffusion and distribution of a single harmful substance,with little discussion of the coupling diffusion of multi-species harmful substances.Diesel vehicle exhaust often synchronously produces a variety of harmful substances,and a variety of harmful substances are certain to interact and affect each other during the diffusion process.Furthermore,most researchers have not considered the influence of time conditions and have failed to outline the space-time distribution of harmful substances,resulting in imperfect conclusions.Therefore,it is necessary to study the coupling diffusion and space-time distribution of multi-species harmful substances in diesel vehicle exhaust,to improve the accuracy of results on the diffusion behavior of harmful substances and improve the relevant theories.
PM represents the main solid pollutant in diesel vehicle exhaust.Its main components are soot particles,which can cause respiratory diseases (in the short term) and cancer (in the long term) to coal miners who inhale them [22,23].Additionally,these particles tend to adsorb potentially toxic hydrocarbons from the exhaust gas on their surfaces,exacerbating respiratory damage.CO is a highly toxic gas,which if breathed in can lead to a lack of oxygen in the human body and cause other adverse reactions.People can suffocate and die if the concentration of CO exceeds 6×10-4[16].CO is often used as an index gas to predict the possibility of spontaneous combustion in underground coal mines.The CO contained in diesel exhaust may affect the accuracy of these predictions,resulting in false alarms and seriously affecting coal mine production.Therefore,in this study we explored the coupling diffusion behavior of these two typical gaseous and solid harmful substances.
To understand the diffusion behavior of harmful emissions from diesel vehicles in an underground space,it is necessary to carry out considerable field measurement work to obtain data relating to airflow velocity and the concentration distribution and diffusion range of various harmful substances at different times.Due to the restrictions caused by the complex field conditions,however,the taking of many such field measurements is not feasible.CFD simulation is therefore used,as it can replicate the complex multiphase physics of an environment full of obstacles,similar to that of the underground coal mine space.It has the advantages of easy operation,a high degree of visualization,low cost,and no limitations caused by the field environment,so it is regarded as an effective research method [24-26].In recent years,CFD methods have been widely used in mining industry to study the diffusion behavior of various harmful substances.Taraba and Michalec [27] used the commercial CFD software FLUENT to study the concentration of CO produced by the oxidation process of coal in the mined-out longwall (gob) area in a coal mine of the Czech Republic.Ren et al.developed a CFD model to obtain carbon dioxide(CO2)fringe behavior on the longwall face and concluded that gas drainage conducted through tailgate borehole can effectively solve the CO2accumulation[28].Wrona presented the results of CFD simulations of the change in distribution of CO2and methane (CH4) near a closed mining shaft under the predicted baric tendency [29].Hu et al.,Ren et al.,Zhang et al.,Ndenguma,and others mainly carried out CFD simulation research on the diffusion of dust particles in coal mines,and put forward some control measures to reduce the dust concentration to a certain extent [8,30-35].
The purpose of this study was to obtain the coupling diffusion behavior of typical harmful emissions from trackless rubberwheel diesel vehicles in underground coal mines,to support risk assessments and to provide a basis for developing appropriate control schemes.It will also provide a reference for research into the diffusion of multi-species pollutants.Therefore,we established a coupling diffusion CFD analysis model and investigated coupling diffusion and time-space distribution of CO and soot emissions from diesel vehicles under 4 common operating situations,i.e.,idling downwind in a cul-de-sac,idling upwind in a cul-de-sac,idling while driving forward into the chamber,and idling while reversing into the chamber;and the hazardous exposure areas were then identified.The validity of the CFD analysis model was verified by field measurements.
2.1.Mathematical model
The airflow in the roadway of underground coal mine is a continuous medium composed of many mass particles.As a continuous phase,its flow state is mostly turbulent.In this study,Euler method is adopted to describe the movement regularity,and realizable κ-ε model is selected as the continuous phase turbulence model [36].
Turbulent kinetic energy equation (κ equation):
where ρ is the fluid density;κ the turbulent kinetic energy;ε the turbulent dissipation rate;tthe time;uithe fluid velocity component,i=1,2,3;xithe axial coordinate;xjthe radial coordinate;μ the viscosity coefficient of the fluid;μtthe turbulent viscosity;σκthe turbulent Prandtl number of the κ equation,take the empirical value σκ=1.0;andGκ the generation term of turbulent kinetic energy caused by the average velocity gradient,
whereEis the modulus of the mean rate-of-strain tensor;andEijthe strain rate tensor.
The turbulent energy dissipation rate equation (ε equation) is:
where σεis the turbulent Prandtl number of the ε equation,take the empirical value σε=1.2;C1a function of the time scale ratio of the turbulence to the mean strain;C2the model coefficients,take the empirical valueC2=1.9;v the kinematic viscosity;η the dynamic viscosity coefficient of the fluid;Cμ a function related to the average change rate and turbulent flow field (κ and ε);A0andASare the model constants,take the empirical valueA0=4.0;φ the displacement phase shift (radians);Wthe model constant;U* the streamwise mean velocity;the time-mean rotational rate tensor observed from the reference frame of angular velocity ωk.For a rotation-free flow field,the second term in the square root of the aboveU*calculation equation is 0,and this term introduces the effect of rotation;Ωijthe mean rate of rotation tensor;the vorticity tensor;εijkthe Levi-Chivita factor;and ωkthe angular velocity.
In this study,the soot particles phase volume fraction is less than 10%.The movement of soot particles in roadway airflow is a two-phase flow of thin gas and solid particles.In order to calculate the path of soot particles in airflow,the discrete phase model(DPM) in Lagrangian coordinate system is adopted,and the interaction and influence between airflow and soot particles are considered [37,38].
According to the balance of forces acting on soot particles,the motion equation of soot particles in the Lagrangian coordinate system can be obtained:
whereuis the fluid velocity;upthe particle velocity;gxthe gravitational acceleration in thexdirection;ρpthe density of the particle;Fxthe additional forces;FDthe particle viscous resistance;Dpthe particle diameter;Rethe relative Reynolds number;CDthe drag coefficient;and α1,α2and α3the constants,and they are given according to the experimental results of smooth spherical particles.
The other method to defineCDis:
whereb1,b2,b3andb4are the constants,and they are given according to the equation of Turton and Levenspiel.
The shape factor φ is defined as:
wheresis the surface area of the volume ball of the same size as the particle;andSthe actual surface area of the particle.
The size of soot particles is less than micron,so the Stokes resistance formula is adopted:
whereCcis the Cunningham correction coefficient;and λ the molecular average free path.
The CO gas in the exhaust gas is produced by the incomplete combustion of diesel in the diesel engine of the trackless rubberwheel vehicle.After being emitted,the CO mainly migrates with the airflow in the roadway,and the process of dilution by airflow is mainly convective diffusion.The mass fractionYiof each substance is estimated by the convection-diffusion equation for theithsubstance [30,33,39].The conservation equation can be expressed as:
whereRiis the net production rate of the chemical reaction;Sithe additional production rate due to the discrete phase and the defined source term;the fluid velocity;andthe material diffusion flux due to the concentration gradient.Since there is no chemical reaction between CO gas and downhole air,it is only a mixture of two substances,so bothRiandSiare set to 0.
In turbulent flow,mass diffusion is calculated as follows:
whereSctis the turbulent Schmidt number;Di,mthe mass diffusion coefficient for speciesiin the mixture;andDtthe effective mass diffusion coefficient due to turbulence.In the mixed flow of air and CO,,wherehiis the sensible enthalpy of the speciesi.This diffusion has an important effect on the enthalpy field,it cannot be ignored.In particular,when the Lewis number for all substances is far from 1,ignoring this term can lead to serious errors.material diffusion leads to enthalpy transfer
wherekis the thermal conductivity;andcpthe specific heat capacity.
2.2.Physical modeling and mesh division
According to the actual situation at the site,Solidworks was used to establish a physical model of equal scale,including the roadway,trackless rubber-wheel diesel vehicle,belt conveyor,gas exhaust pipe,air-blowing duct,and vehicle meeting chamber,i.e.,a total of 6 components (Table 1).The trackless rubber-wheel diesel vehicle studied in this study was a WC5E (B) type,a medium-sized carrier vehicle used to transport materials and small machinery and equipment.It is a commonly used model of vehicle in underground coal mines.The tailpipe is located below the codriver’s seat and has a rectangular shape.The established physicalmodel is shown in Fig.1.Then,ICEM-CFD was used to mesh the established physical model,and the mesh used in the numerical simulation was verified using a grid independence test.The final physical model after mesh division is shown in Fig.2.
Table 1 Details of physical model parameters.
Fig.1.Physical model before mesh filling.
Fig.2.Physical model after mesh filling.
When trackless diesel vehicles with rubber wheels were rapidly loading and unloading goods or avoiding pedestrians in the cul-desac,they often stopped with the engine idling instead of stopping the engine.According to the different driving directions of diesel vehicles in the roadway,there were two situations: idling downwind and idling upwind.When diesel vehicles were driving in the roadway,due to space restrictions,they had to temporarily enter the meeting chamber connected to the roadway and idle for a period of time when they met other vehicles,operators,or in emergency situations.Based on the different way of driving into the chamber,there were also two situations: idling when driving forward into the chamber and idling when reversing into the chamber.In the above 4 cases,diesel vehicles were equivalent to a fixed source of pollution,continuously emitting harmful exhaust substances to the surrounding environment and thus polluting the underground working environment.
In this section,FLUENT software was used to conduct CFD simulation and analysis of the coupling diffusion and time-space distribution of soot particles and CO in exhaust gas from diesel vehicles under the above 4 common conditions.The simulated operating conditions were set according to the actual situation on-site.The ventilation conditions of the vehicle operating environment are as follows.A local compressed-air ventilation system was set in the intake airflow roadway;this included a local compressed-air fan and an air duct and was used to supply air to the fully mechanized excavation face.The roadway considered in this study was a section of the entire roadway in which the trackless rubber-wheel diesel vehicle passed.This section had no local ventilation systems or fans,and it was nearly 2 km away from the air outlet of the compressed-air ventilation system;therefore,the airflow here was relatively uniform (i.e.,evenly distributed).Additionally,in order to facilitate the analyses,we assumed that the harmful components did not experience any physical absorption or chemical changes during the emission process.Key parameters and initial conditions of the CFD model are shown in Table 2.
In Table 2,the mass flow of CO (M1) was calculated based on:
Table 2 Key parameters and initial conditions of the CFD model.
Table 3 Setting of the measuring points.
wherem1is the gas mass concentration,mg/m3;Mthe relative molecular mass of the CO,andM=28 g/mol;Vcthe gas volume concentration,10-6;tthe temperature,℃;Pthe pressure,Pa;andQthe total volume flow of exhaust gas,m3/s.
The mass flow of soot particles (M2) was calculated based on:
wherem2is the mass concentration of soot particles,mg/m3.
Notably,the values ofVc,Q,m2,tandPwere obtained from field measurements.They were directly measured using a Portable Emission Measurement System (Type: Gasboard-9805).The total mass flow rate of the exhaust gas was also directly measured.Then,we can obtain the mass fraction of CO by the ratio of the mass flow rate of CO to the total mass flow rate.The particle size data were obtained by field sampling and then analyzed in the laboratory.In particular,we collected the exhaust of the trackless rubberwheel diesel vehicle on site,and then analyzed the particle size distribution using a Mastersizer 3000 Laser Particle Size Analyzer in the laboratory,obtaining the maximum,minimum,and median particle sizes.
3.1.Airflow migration conditions of trackless diesel vehicles with rubber wheels under different conditions
It can be seen from Fig.3a and b that the airflow was relatively uniform in the area far away from the diesel vehicles.Due to the obstruction caused by the vehicle’s body,some fluctuations in the airflow occurred when it passed the diesel vehicle,resulting in the relative disorder of airflow near the vehicle’s body.In addition,due to the wall-attached effect,some wind flowed along thesurface of the vehicle’s body,at a speed below approximately 0.2 m/s.Influenced by the incoming airflow from the front,the flow direction of the airflow at the tailpipe was shifted: it followed the incoming airflow from the front and flowed to the rear of the vehicle(in the case of idling upwind)and to the front of the vehicle(in the case of idling downwind).The flow rate near the tailpipe was about 1.2 m/s.In the above two cases,the airflow speed in the roadway showed little overall change,mostly within 0.1 to 0.6 m/s.
Fig.3.Airflow migration when the trackless diesel vehicle with rubber wheels under different conditions in mine.
As illustrated in Fig.3c,when diesel vehicles entered the meeting chamber and the airflow in the roadway was flowing to near the chamber,part of the airflow would continue to flow forward,while a small part would flow into the chamber.Due to the influence of entrainment by the airflow in the roadway,two obvious eddy fields formed at the spaces between the upper part of the vehicle body and the end of the chamber,and between the vehicle body and the end of the chamber.When reversing into the chamber,the airflow migration in the chamber was similar to that seen with the forward entry,mainly forming a large eddy field above the vehicle body,and the airflow showed a state of spiral linear flow in the rear space of the vehicle (Fig.3d).The airflow migration state inside the chamber was also affected by the diesel vehicle’s body.In these two cases,the wind speed in the chamber was mostly less than 0.12 m/s,which was less than the airflow speed in the roadway.
Fig.4.Airflow-soot particles-CO coupling diffusion situation.
Fig.5.Coupling diffusion and space-time distribution of soot particles and CO in exhaust gas of diesel vehicles when idling upwind.
3.2.Coupling diffusion of soot particles and CO in diesel vehicle exhaust under different conditions
According to the actual situation in the field,most underground diesel vehicles did not idle for more than 5 min at a time.Therefore,the simulation time in this section was set as 5 min.In the entire process of coupling the diffusion of harmful pollutants(soot particles and CO) in the exhaust gas,the harmful substances were first emitted from the exhaust outlet at their initial velocity.Then,the diffusion behavior was mainly affected by the airflow in the roadway or chamber,and thus the airflow played a dominant role.These phenomena are shown in Fig.4a and b.The left-and righthand sides of a person when facing the head of the diesel vehicle were defined as the left and right-sides of the vehicle,respectively,to facilitate the description of the process of harmful substances migration.
Fig.6.Contour of concentration distribution of hazardous substances at 1.55 m height of personnel’s breathing zone when idling upwind.
3.2.1.Idling in the roadway
Fig.5a and b show the diffusion of soot particles and CO,respectively,when diesel vehicles were idling upwind in the roadway.For the convenience of observation and analysis,soot particles and CO are shown separately.It can be seen that after being emitted from the exhaust outlet,harmful substances mainly migrated following the airflow in the roadway.With increasing time,the diffusion range gradually increased.Within 120 s,the harmful substances had diffused 52 m along thex-axis.In general,the farther away from the diesel vehicle,the lower the concentration of harmful substances.In the process of migration of harmful substances,the right front wheel obstructed them and formed a diversion,and two high-concentration pollution belts formed on either side of the wheel.Most of the soot particles in the pollution zone were at a concentration of more than 140 mg/m3,more than ten times the Chinese national limit of 10 mg/m3,and the CO concentration also exceeded the Chinese national regulation of 2.4×10-5.From the high-concentration pollution zone to the surrounding area,the concentration of harmful substances showed a gradually decreasing gradient.Whentwas changed from 100 to 300 s,the concentration of CO reached 1.12×10-4within the range of 14 m from the tailpipe,which seriously exceeded the standard.As can be seen from Fig.6a,fromt=16 s,in the 1.55-m-high breathing zone,soot particles of more than 10 mg/m3were mostly distributed behind the vehicle.Whent=178 s,most of the particles diffused to the roadway outlet,and after aboutt=200 s,the concentration distribution was essentially unchanged.The diffusion distance of particles more than 10 mg/m3along thex-axisl1showed a linear relationship with timet1,and the mathematical relationship between the two was fitted asl1=0.448t1-6.038.The range of exceeding the CO concentration standard,in the local area about 4.8 m(along thex-axis forward)behind the rear of the vehicle,mostly did not exceed 3.6×10-5(Fig.6b),while the maximum concentration range of particles was distributed in this local area,at more than 120 to 180 mg/m3.
Fig.7.Coupling diffusion and space-time distribution of soot particles and CO in exhaust gas of diesel vehicles when idling downwind.
When a diesel vehicle idled downwind in the roadway,as shown in Fig.7,areas where hazardous substances exceeded the standard were distributed in front of the vehicle.The distribution of harmful pollutants in the exhaust gas in the roadway was as follows:the concentration of harmful substances gradually decreased in two directions:from the floor along thez-axis and from the tailpipe along thex-axis.There was a pollution zone,with a high concentration of harmful substances,within a 2-m width near the roadway floor,and this migrated to the side of the roadway during the diffusion process;the concentration of soot particles in the pollution zone reached 180 mg/m3or more (Fig.7a).Most of the CO also exceeded 2.4×10-5and reached more than 1×10-4within about 16 m from the exhaust outlet along thex-axis (Fig.7b).As can be seen from Fig.8,when a diesel vehicle was idling downwind,the overall concentration of hazardous substances in the breathing zone section was lower than that when a diesel vehicle was idling upwind.However,the concentration distribution trend of both was consistent,i.e.,the diffusion distance and concentration of pollutants showed an increasing trend with time.According to Fig.8a,the diffusion distance of soot particles more than 10 mg/m3(l2) also showed a linear relationship with time(t2),and the mathematical relationship between them wasl2=0.4375t2-3.993.Before aboutt=240 s,the CO standardexceeding zone was within 4.8 m in front of the vehicle,and the CO concentrations were in the range of 2.4×10-5to 2.8×10-5.Aftert=240 s,a standard-exceeding zone appeared near the roadway wall on the right side,64 m away from the front of the vehicle,as shown in Fig.8b.The highest particle concentration,ranging from 80 to 140 mg/m3,was within 5 m in front of the vehicle.
Fig.8.Contour of concentration distribution of hazardous substances at 1.55 m height of personnel’s breathing zone when idling downwind.
It can be seen from Figs.5-8 that when diesel vehicles were idling upwind and downwind in the roadway,the drivers were not polluted by harmful exhaust gas and were relatively safe.However,when idling upwind,there were a lot of areas to the rear of the vehicle and on the right side of the vehicle that exceeded the limit.In particular,in the two high-concentration pollution zones near the rear roadway floor of the vehicle and within 5 m behind the vehicle,the soot particles and CO in these areas far exceeded the national standard,making them high-risk areas.The areas near the upwind side and left side of diesel vehicle were essentially unpolluted,which made them relatively safe areas.When idling downwind,there was an exceeding-limit zone ahead of the vehicle.Special attention should be paid to the high-concentration pollution zones near the roadway floor and within 5 m in front of the vehicle,which had exceeded the national standard and were considered to be high-danger zones.If the idling downwind time exceeds 240 s,attention should also be paid to the exceedinglimit zone near the roadway wall on the right side,64 m away from the vehicle.The upwind side,left side,and right side of the diesel vehicle were not polluted,making them relatively safe areas.
3.2.2.Idling in the meeting chamber
When a diesel vehicle idled in the meeting chamber,the diffusion of harmful exhaust was mainly affected by the airflow in the chamber.After being emitted from the exhaust outlet,the two types of pollutants migrated along with the airflow,and their migration trend was roughly the same.However,because the airflow migration in the chamber was more complicated than that in the roadway,and there were more eddies,the migration of CO and soot particles was more complicated than that in the roadway.
As shown in Fig.9,after being emitted,harmful pollutants gradually escaped from the bottom of the vehicle and spread around the vehicle body.There were many areas in the chamber that exceeded the standard,especially at the bottom of the vehicle and on both sides of the vehicle body,where there were highdanger areas with serious pollution,and soot particles and CO concentrations were more than 120 mg/m3and 7.6×10-5,respectively.After aboutt=70 s,CO on both sides of the body presented a concentration distribution pattern similar to a“semicircle”,with a gradient distribution from high to low around the center of the semicircle;this semicircle gradually expanded over time.The soot particles on the left side of the vehicle were similar to the semicircular concentration distribution of CO.The concentration distribution on the right side of the vehicle was irregular,first gradually increasing,and then the number of particles gradually decreased,and the concentration gradually decreased.With the passage of time,the harmful substances on the left side of the body spread to upper side of the vehicle and the end of the chamber.Whent=145 s,the harmful substances began to spread into the roadway from the chamber,and the harmful substances in the chamber had spread to the vicinity of the roof.Whent=300 s,after the harmful substances had diffused to the end of the chamber,the reflux phenomenon appeared due to the influence of eddies.
By capturing the side view of the distribution of hazardous substances in the driver’s breathing zone at different moments (the cross-section wasx=50.78 m),as shown in Fig.10,from aboutt=75 s,an area of more than 10 mg/m3appeared in the driver’s breathing zone,and from aboutt=100 s,CO concentrations began to exceed 2.4×10-5.With increasing time,the exceeded-limit zone in the driver’s breathing zone gradually expanded.Aftert=120 s,most areas had soot that was more than 170 mg/m3,and drivers were therefore in a high-risk area with serious pollution.Whent=200 s,the CO concentration at the driver’s location was more than 4.8×10-5,and whent=300 s,the CO concentration was more than 1.12×10-4,which posed a serious threat to the driver’s health.
As can be seen from Fig.11,when a diesel vehicle reversed into the chamber,harmful substances were emitted,which first mainly diffused to either side of the vehicle,then entered the roadway through the corner connecting the chamber and roadway,and diffused following the roadway airflow.As can be seen from Fig.11,the high-risk area was still mainly distributed on either side of the vehicle,with soot particles and CO up to 180 mg/m3and more than 1.12×10-4,respectively.The distribution of harmful substances on the right side of the vehicle was triangular in shape,which was mainly due to the influence of the eddy field on the right side of the vehicle.The closed streamline of airflow led most harmful substances limited into the area.As time went by,the triangle gradually expanded.Due to the influence of airflow near the roof,a high concentration of pollutants gradually spread along the chamber roof from the top vertex of the triangle.There was also a high danger area,up to 130 mg/m3,near the roof,and att=300 s,this had diffused about 6 m from the rear of the vehicle.A relative safety area within the range of 7 m from the end of the chamber,which had no pollutants,appeared.
Fig.9.Coupling diffusion and space-time distribution of soot particles and CO when diesel vehicles were idling driving forward into the chamber.
Fig.10.Driver’s breathing zone pollution situation when the diesel vehicle idled driving forward into the chamber.
Fig.11.Coupling diffusion and space-time distribution of soot particles and CO when the diesel vehicle idled reversing into the chamber.
Fig.12 shows the concentration distribution of exhaust pollutants at different times in the driver’s breathing zone,sectionx=49.27 m,when idling reversing into the chamber.It can be seen that the pollution range and concentration of harmful substances in the sectionx=49.27 m gradually expanded with time.According to Fig.12a,at aroundt=215 s,the standard-exceeding zone where soot particle concentration was more than 10 mg/m3began to appear in the driver’s breathing zone.Whent=255 to 300 s,the area more than 10 mg/m3accounted for a large part,and the maximum was more than 180 mg/m3.While the CO concentration in the breathing zone gradually increased with time,it did not exceed 2.4×10-5during the entire diffusion process,which meant the standard was not exceeded at any time (Fig.12b).
Fig.12.Driver’s breathing zone pollution situation when the diesel vehicle idled reversing into the chamber.
By comparing Figs.9-12,it can be concluded that in the two cases(Case 1:the diesel vehicle reverses into the chamber at idling speed;and Case 2:the diesel vehicle drives forward into the chamber at idling speed),the pollution range and concentration of exhaust gas in the chamber in Case 1 were both lower than those in Case 2.The concentration and range of pollution in the roadway caused by the former was greater than those of the latter.This showed that Case 1 was conducive to spreading out harmful substances in a timely manner,and it was not easy for them to accumulate in the chamber.In Case 1,the time when the soot particles began to exceed the standard at the driver’s location was 215 s,which was later than the 75 s seen in Case 2,indicating that the driver’s breathing zone would not exceed the standard in a short time after reversing into the chamber and was relatively safe.Compared with Case 1,the concentration of hazardous substances in the driver’s breathing zone was higher with Case 2.Furthermore,CO did not exceed the standard in the driver’s breathing zone throughout the entire process in Case 1.Therefore,from the perspective of protecting the driver’s health,and to accelerate the harmful exhaust substances in the chamber diffusing out through the roadway,reversing into the chamber was better than driving forward into the chamber.Thus,if the actual situation permits,it is recommended to reverse into the chamber for parking.
To verify the accuracy of the CFD model,an exhaust particle analyzer and a CO gas detector were used to monitor the concentration of soot particles and CO emitted by diesel vehicles under different conditions in the 2306 return airflow roadway of Xiahuo Coal Mine,Shanxi Province,China.The measured results were then compared with the simulation results.As shown in Fig.13,when diesel vehicles were idling upwind and downwind in the roadway,the measuring points were set in the same way.A total of 5 measuring sections were set,with two measuring points set in each section,giving ten measuring points in total.The 10 measuring points were represented byA,B,C,D,E,F,G,H,IandJrespectively.The measuring points for diesel vehicles driving forward and reversing into the chamber at idling speed were also set up in the same way,with a total of 7 measuring points.The 7 measuring points were represented bya,b,c,d,e,fandgrespectively.Table 3 shows the details of how the measuring points were set.In 4 cases,the concentration of soot particles and CO at each point was measured during the idle operation of the diesel vehicle for 5 min.
Fig.13.Schematic diagram of measurement points layout at underground site and actual measurement situation.
Fig.14.Field measurement results of soot particles and CO concentration.
Fig.14(continued)
As can be seen from Fig.14,under the 4 conditions of diesel vehicle idling,the relative errors between the measured value of soot particle concentration at each measurement point in the field and the CFD simulation value were below 10.85%.It can be seen that most of the measured values of CO were more than the simulated values,possibly because a small proportion of CO came from the penetration of coal wall and diffused into the airflow of roadway during the actual measuring process.The relative errors between the measured values and the CFD simulation values of CO gas concentration were mostly below 11.6%.Overall,the relative errors between the measured values and the simulated values of soot particles and CO concsentrations were small and within the allowable range,indicating that the CFD simulation was essentially consistent with the results measured in the field.Therefore,the CFD model adopted was correct.
By establishing a CFD model of multi-species coupling diffusion of harmful emissions from trackless rubber-wheel diesel vehicles,the diffusion and space-time distribution of harmful emissions under 4 conditions were acquired.Field measurement data further verified the accuracy of the CFD model.
(1) In the roadway,when a diesel vehicle idles upwind,there are several areas that exceed the safe limit values to the rear and the right side of the vehicle.In particular,there are two pollution zones with a high concentration of harmful substances,near the rear roadway floor by the vehicle and within 5 m behind the vehicle,which are considered to be high-risk zones.When idling downwind,there is an area that is above the safe limit in front of the vehicle,so special attention should be paid to the high-concentration pollution zone near the roadway floor and within a range of 5 m in front of the vehicle,which are high-risk areas.In the above two cases,the driver’s zone has not been polluted and is relatively safe.
(2) When idling in the chamber,the high-risk areas caused by harmful emissions mainly occur on either side of the vehicle and near the upper roof.When driving forward into the chamber,harmful substances essentially spread throughout the entire chamber,while when reversing into the chamber there is a relatively safe area within the 7 m range from the end of the chamber.Forward entry caused more pollution in the chamber and the driver’s breathing zone than reverse entry.Therefore,it is recommended that vehicles reverse into the chamber and idle.
(3) Underground workers should try to avoid areas where there is a high risk of encountering harmful exhaust gas and should take shelter in a safe area.It is suggested that the idling time should be reduced as much as possible to reduce harmful emissions from diesel vehicles,especially in the chamber with its relatively poor airflow.If the idling time is too long,a driver will be exposed to hazardous material exceeding the recommended safe standard.Operators should wear protective equipment,especially the driver and other workers operating in high-risk areas.The ventilation system should be optimized to accelerate the removal of harmful substances and reduce the time that they can accumulate.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Nos.52174191 and 51874191),the National Key R&D Program of China(No.2017YFC0805201),Qingchuang Science and Technology Project of Shandong Province University (No.2020KJD002),and Taishan Scholars Project Special Funding (No.TS20190935).