| Environmental protection,especially fine particle(PM2.5)pollution control,has become a crucial problem of public concern in China because the energy consumption,as well as the emission of environmental pollutants,has grown dramatically with the rapid development of economy and society,and the haze weather of important economic regions appears continually and frequently in the past 10 years.As one of the effective and reliable particulate control devices,the electrostatic precipitator(ESP)has been widely used for industrial gas cleaning with high collection efficiency.However,the industrial application has shown that ESP has low collection efficiency for submicron particles(particle diameter dp range from 0.1?m to 1?m)and ultrafine particles(dp less than 0.1?m).Since the control level of PM2.5 emission in haze pollution control has become increasingly strict,the improvement of fine particle collection efficiency has become a challenge for the purification equipment.The electrode structure and configuration,producing the electrical distribution and electrohydrodynamic(EHD)flow which directly impact on the charge,migration,and trapping of particles in the ESP channel,is the most basic and essential factors for the ESP collection efficiency.Therefore,electrical characteristics,EHD flow and particle behavior,as well as the coupling interactions among them,in ESP with different shaped electrodes are analyzed and discussed in detail by the numerical simulation and experimental evidence in this paper.Based on an improved numerical model of the electrical characteristics,seven different kinds of calculation models were established by C++ program and compared for simulating the corona discharge and the finite volume method was used to solve the distribution of electric field strength and space charge density in the ESP channels with different shaped electrodes.The distribution of electrical characteristics in the ESP channels with different shaped electrodes(six shaped discharge electrodes and five shaped collecting plates),which was computed by the reliable numerical model,was analyzed in detail.The results show that the corona characteristics caused by the discharge electrode structure determine the details of the space charge distribution.For six shaped discharge electrodes,there are two different patterns of space charge distribution in the channel.The influence of the collecting plate on the electrical characteristics near the collecting surface is significant,especially the bending of the collecting plate easily leads to a peak value of electric field intensity in the vicinity of the collecting plates.The W-type collecting electrode produces a nearly uniform space charge density along the collecting electrode,and the space charge density of Rod curtain plate possesses a large number of peaks,which are larger than that of the other four plate types.The electrical characteristics in the channel mainly depend on the discharge electrode structure,while the electrical characteristics near the collecting surface are more affected by the collecting plate.The auxiliary electrode in the tri-electrode ESP has a significant shielding charge effect,and the arrangement of the auxiliary electrode at the entrance of the channel is disadvantageous for the rapid charging of the particles entering the channel.Because of the coupling interactions between the mainstream and the electric wind generated in the discharge process,the EHD turbulent flow in the ESP channel becomes complicated.The turbulent model with the aid of User Defined Function(UDF)of FLUENT code is used to simulate the EHD turbulent flow in the ESP channel,and the reliability of the nine turbulence models of FLUENT code with the effect of the electric wind was tested by the experimental data.The patterns of EHD turbulent flow induced by the electric wind in the ESP channel with various combinations of the discharge electrode and the collecting plate were comprehensively simulated based on the validated model.The results demonstrate that the local vortexes near the discharge electrode and the collecting plate affect the charge and collection of particles.As the discharge electrode shape is only considered,the strength and range of the vortex at the downstream of the discharge electrode is larger than that near the collecting wall,and the vortex in the center of the ESP channel squeezes the main flow to the collecting plate.Squeezing the main flow toward the collecting surface by the downstream vortex in the channel with the Knife electrode is strongest.The intensity and range of the vortex produced by the Star-2 and Square discharge electrode is only half that by the other four discharge electrodes.The channels of five type plates with the rod wires exist the vortex near the collecting plate at the low inlet velocity(0.5 m/s),W-type plate offers the widest vortex area and the strongest vortex intensity induced by the secondary flow,and the maximum velocity around the discharge electrode.As the inlet velocity is increased to 1.0 m/s,the interaction between the primary flow and the electric wind still exists in the channel with the C-type plate and the electric wind accelerates the primary flow,while the influence of electric wind on primary flow becomes weaker in the W-type plate channel.For the channel with the rod-shaped plate,the dust-laden airflow at the center of the channel is squeezed by the electric wind to the collecting plate if the upstream of the electrode places no auxiliary electrodes at the entrance of the channel.On the contrary,no auxiliary electrodes at the outlet are placed at downstream of the discharge electrode and the dust-laden airflow in the center of the ESP channel is further accelerated by the electric wind and rapidly emitted along the main flow direction.The dynamic behavior of charged particles in the ESP channel was simulated by Lagrangian method and Eulerian method,respectively.The Lagrangian model was used to investigate the transport characteristics of the particles in the ESP with the different shaped electrodes based on the above electric and flow models.The results show that the downstream vortex of the discharge electrode squeezes the dust-laden airflow to the collecting plate which is beneficial to the particle deposition,while the vortex near the collecting plate pushes the main flow to the downstream which is not conducive to the particle deposition.For the submicron particles and ultrafine particles,the improvement of collection efficiency by the vortexes in the center of the ESP channels with six shaped discharge electrodes is significantly larger than that by the vortexes near the collecting wall,and the largest improvement of fine particle collection efficiency by the center vortexes is in the ESP channels with the Knife electrode.However,when the particle diameter is less than 0.1 ?m,the particles entering the downstream vortexes of discharge electrode increase significantly,while the center vortexes weak the moving of fine particles towards the collecting wall.The vortex near the collecting wall has a negative effect on the collection performance,and the C-type collecting plate takes the most serious influence,the influence of the Plate and Rod curtain type collecting plate is small.Compared with the flat-type electrode,the area of electric wind in the W-type electrode channel is large at the low inlet velocity(0.5 m/s),so the collection efficiency of W-type electrode is low.Keeping the most surface of the W-type collecting plate at the normal inlet velocity(1.0 m/s)in the low speed zone can improve the conditions of particle deposition.Compared with Lagrangian method,Eulerian method is more practical in engineering applications because of its relatively short time-consuming,low computer configuration requirements and suitable for simulating the mechanism of ultrafine particles.Therefore,the Eulerian numerical calculation model of charged particles in the ESP channel is established,and the applicability of the two methods and the reliability of the simulation data are analyzed.Firstly,the Eulerian model is used to investigate the interactions between the secondary flow of electric wind and charged particles in the wire plate ESP.The simulation results show that the secondary flow has a more significant effect on the flow pattern and the turbulence intensity in the ESP channel when the increase of NEHD number.The secondary flow has a negative effect on the particle collection efficiency,especially for particles with dp ? 1.0 ?m,which can make a maximum reduction of 4%,but its impact can be generally negligible for the large particle.Then,the flow characteristics and the particle dynamics behavior of the three-dimensional ESP with three shaped discharge electrode were simulated.The results show that the discharge needle of discharge electrode produces an electric wind similar to a high-speed jet and forms a complex airflow with the mainstream flow.Compared with the parallel flow arrangement of the discharge needle,the vertical arrangement of the discharge needle produces a more strong electric wind effect,which is more likely to bring the particles to the collecting surface and improve collection efficiency.The area between the two discharge needles at the discharge electrode is a weak region of charge,through which the dust-laden airflow moves toward the downstream of the discharge electrode,resulting in a significant reduction of the dust removal efficiency. |
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