Research on Flow Field Optimization and Performance Test of Vertical Honeycomb Wet Electrostatic Precipitator

Abstract

This study focuses on optimizing the flow field uniformity within a vertical honeycomb wet electrostatic precipitator (WESP), which is a critical prerequisite for achieving high particulate removal efficiency. For a vertical honeycomb WESP with an air capacity of 25,000 m³/h, the internal flow field is optimized by adjusting the opening ratio and aperture ratio of the airflow equalizing plate, installing additional deflector plates, and adding additional airflow equalizing plates at strategic locations. The optimization reduces the velocity relative standard deviation at the anode inlet section to 0.14. Through 1:1-scale equipment construction and testing, the particle concentration at the outlet is stabilized below 10 mg/Nm³, with an average removal efficiency of 95.88%—a 5.7% improvement over the original model. This study solves the design dependency on empirical guidance for vertical honeycomb WESP in the food industry, providing a green technology paradigm for low-carbon industrial emissions.

1. Introduction

For the exhaust gases generated from complex processes like crushing, mixing, pelletizing, and drying in food and animal feed production plants, achieving efficient and sustainable emissions has become a top priority. Traditional exhaust gas treatment technologies, such as bag filters, face critical sustainability limitations: their reliance on replaceable filter bags leads to high solid waste generation (30–50 kg/year for medium-scale plants) and increased carbon footprint from frequent replacements, conflicting with SDG (Sustainable Development Goal) 12 (Responsible Consumption and Production). In contrast, WESPs offer sustainable advantages: their high-voltage electric field ionization mechanism reduces energy consumption by 10–20% compared to bag filters, while the spray system enables closed-loop water circulation, addressing water resource scarcity (SDG 6). A vertical honeycomb WESP, with its modular design, further enhances sustainability by minimizing material usage (20% less steel than conventional ESP) and enabling 90% component recyclability, making it a key technology for green industrial transitions [1,2]. The advantage of the WESP is its efficient capture of small particles, which effectively removes pollutants such as PM_2.5 and PM₁₀ and meets strict environmental standards [3,4]. The pollutants in the exhaust gases of food and animal feed mainly include organic particles such as starch dust, feed pellets, and minor inorganic impurities. The particle size distribution is dominated by PM₁₀ (accounting for ~70%), with PM_2.5 accounting for ~20%–25%. These particles are hydrophilic, have a median diameter (d₅₀) of 5–15 μm, and are prone to agglomeration under high humidity conditions [5]. Vertical honeycomb WESP has significant advantages of good dust removal effect, small pressure loss, easy operation, low energy consumption, low maintenance costs, and a variety of designs to meet the needs of different occasions. The honeycomb WESP, as an important construction form of vertical ESP, is characterized by a unique honeycomb structure of the dust collection plate design. With its efficient intermittent flushing mechanism, compact layout, and small footprint, the honeycomb vertical ESP offers high space utilization, low material consumption, and easy installation and maintenance due to its modular design [6,7].

In the field of WESP, the internal structure of the precipitator is more complicated, and it is not suitable for industrial test optimization due to its large size and flue length [8]. With the rapid development of computer technology, computational fluid dynamics (CFD) technology has received more and more attention in recent years, and numerical simulation studies based on CFD have been widely carried out both at home and abroad, most of which focus on the optimization strategy of the flow field of the WESP [9,10,11]. A hexagonal collector plate was proposed as the anode device, and 42 discharge electrode shapes were selected; through numerical analysis, the optimization of the discharge electrode shapes was carried out by Hwang et al. [12]. The simulation results show that the installation of the deflector plate can significantly optimize the airflow organization and make its distribution more uniform and reasonable. Bian et al. [13] combined experimental observations with numerical simulation techniques and found that high humidity or low air pressure conditions can effectively reduce the onset voltage of the negative corona, and the cathodic corona voltage is also significantly reduced in this case. In addition, their research results also emphasized the significant advantage of the ion density of negative corona in the critical region, which is denser compared with positive corona. Ye et al. [14] derived the pressure drop formula of perforated plate and the linear relationship between the resistance of dust collection plate and the plate length through small-scale simulation, which verified the experimental validity of the simplified model. The optimized design adds perforated plates to make the flow field more uniform in the flue inlet and electric field area, which meets the design requirements, and the optimized scheme is feasible in practical engineering applications. Liu et al. [15] shortened the particulate matter moving distance to improve the dust removal efficiency by decreasing the spacing of the polar plates. These advancements align with the UN’s Sustainable Development Goals, particularly SDG 7 (Affordable and Clean Energy) and SDG 13 (Climate Action), by driving down industrial carbon emissions from air pollution control systems.

This study aims to bridge the gap between technical performance and sustainability in vertical honeycomb WESP design. Based on computational fluid dynamics, the internal flow field of the dust collector is optimized by changing the opening ratio (The ratio of the total opening area to the total area of the airflow distribution plate) and aperture ratio (The ratio of the opening diameter to the hole pitch) of the airflow homogenizer plate as well as installing additional deflector plates and airflow homogenizer plates, and the validated model is used for the optimization of the flow field of the vertical honeycomb WESP. The systematic approach is presented in Figure 1.

2. Geometric Model and Governing Equations

2.1. Geometric Model

A geometric model is established based on the structure of a vertical cellular WESP with a processing volume flow rate of 25,000 m³/h (recalculated to Normal Temperature and Pressure, NTP). The computational space of the geometric model is discretized, and the structured grid is used to divide the grid. To improve the reasonableness of the simulation results, encryption is carried out in the porous medium and at the junction of different cross-sections, and the model needs to be verified for irrelevance because there are certain changes between the simplified model and the original model. Considering the computational ability of the computer and the precision of the data, the pressure drop at the entrance and exit is selected as the index of grid irrelevance validation [16], and this paper compares 480,000, 700,000, 820,000, 1,530,000, 3,870,000, and 5,220,000, and six mesh division schemes for comparison, and the results are shown in Figure 2. From Figure 2, the pressure drop does not change much when the number of grids is greater than 1,530,000 under fixed working conditions, and the number of grids is selected as 1,530,000 to facilitate the calculation, and the grid is shown in Figure 3.

2.2. Governing Equations

The Reynolds number based on the hydraulic diameter of a single honeycomb channel and the average inlet velocity was calculated to be approximately 11,930, indicating a turbulent flow regime. According to the simulation of the internal flow field of the vertical honeycomb electrostatic precipitator, the fluid media exchange energy and momentum with each other, and turbulence occurs at the same time. The flow in the precipitator is mainly fully developed turbulence, and the uniform flow away from the wall dominates. The standard k-ε model is a suitable choice. Therefore, the standard k-ε model is used to solve the turbulence in the unit [17]. The control equations include the mass-conservation equation and the momentum-conservation equation.

The continuity equation (mass-conservation equation) is

$${\displaystyle \frac{\mathbf{\partial }\mathit{\rho }}{\mathbf{\partial }\mathit{t}}}+{\displaystyle \frac{\mathbf{\partial }}{\mathbf{\partial }{\mathit{x}}_{\mathit{i}}}}\left(\mathit{\rho }{\mathit{u}}_{\mathit{i}}\right)=\mathbf{0}$$

(1)

The momentum-conservation equation (N-S equation) is

$${\displaystyle \frac{\mathbf{\partial }}{\mathbf{\partial }\mathit{t}}}\left(\mathit{\rho }{\mathit{u}}_{\mathit{i}}\right)+{\displaystyle \frac{\mathbf{\partial }}{\mathbf{\partial }{\mathit{x}}_{\mathit{j}}}}\left(\mathit{\rho }{\mathit{u}}_{\mathit{i}}{\mathit{u}}_{\mathit{j}}\right)=-{\displaystyle \frac{\mathbf{\partial }\mathit{p}}{\mathbf{\partial }{\mathit{x}}_{\mathit{i}}}}+{\displaystyle \frac{\mathbf{\partial }}{\mathbf{\partial }{\mathit{x}}_{\mathit{j}}}}\left(\mathit{\mu }{\displaystyle \frac{\mathbf{\partial }{\mathit{u}}_{\mathit{i}}}{\mathbf{\partial }{\mathit{x}}_{\mathit{j}}}}-\mathit{\rho }{\mathit{u}}_{\mathit{i}}{\mathit{u}}_{\mathit{j}}\right)+{\mathit{S}}_{\mathit{i}}$$

(2)

The turbulence model adopts the standard k-ε model. The turbulent kinetic-energy equation is

$${\displaystyle \frac{\mathbf{\partial }}{\mathbf{\partial }\mathit{t}}}\left(\mathit{\rho }\mathit{k}\right)+{\displaystyle \frac{\mathbf{\partial }}{\mathbf{\partial }{\mathit{x}}_{\mathit{i}}}}\left(\mathit{\rho }\mathit{k}{\mathit{u}}_{\mathit{i}}\right)={\displaystyle \frac{\mathbf{\partial }}{\mathbf{\partial }{\mathit{x}}_{\mathit{j}}}}\left[\left(\mathit{\mu }+{\displaystyle \frac{{\mathit{\mu }}_{\mathit{t}}}{{\mathit{\sigma }}_{\mathit{k}}}}\right){\displaystyle \frac{\mathbf{\partial }\mathit{k}}{\mathbf{\partial }{\mathit{x}}_{\mathit{j}}}}\right]+{\mathit{G}}_{\mathit{k}}+{\mathit{G}}_{\mathit{b}}-\mathit{\rho }\mathit{\epsilon }-{\mathit{Y}}_{\mathit{M}}+{\mathit{S}}_{\mathit{k}}$$

(3)

The diffusion equation is

$${\displaystyle \frac{\mathbf{\partial }}{\mathbf{\partial }\mathit{t}}}\left(\mathit{\rho }\mathit{\epsilon }\right)+{\displaystyle \frac{\mathbf{\partial }}{\mathbf{\partial }{\mathit{x}}_{\mathit{i}}}}\left(\mathit{\rho }\mathit{\epsilon }{\mathit{u}}_{\mathit{i}}\right)={\displaystyle \frac{\mathbf{\partial }}{\mathbf{\partial }{\mathit{x}}_{\mathit{j}}}}\left[\left(\mathit{\mu }+{\displaystyle \frac{{\mathit{\mu }}_{\mathit{t}}}{{\mathit{\sigma }}_{\mathit{\epsilon }}}}\right){\displaystyle \frac{\mathbf{\partial }\mathit{\epsilon }}{\mathbf{\partial }{\mathit{x}}_{\mathit{j}}}}\right]+{\mathit{C}}_{\mathbf{1}\mathit{\epsilon }}{\displaystyle \frac{\mathit{\epsilon }}{\mathit{k}}}\left({\mathit{G}}_{\mathit{k}}+{\mathit{C}}_{\mathbf{3}\mathit{\epsilon }}{\mathit{G}}_{\mathit{b}}\right)-{\mathit{C}}_{\mathbf{2}\mathit{\epsilon }}\mathit{\rho }{\displaystyle \frac{{\mathit{\epsilon }}^{\mathbf{2}}}{\mathit{k}}}+{\mathit{S}}_{\mathit{\epsilon }}$$

(4)

1.

Porous-medium equation

The air-distribution plate used to make the flow field in the WESP uniform is composed of tens of thousands of units, which means that it may be infeasible to directly model the air-distribution plate. Therefore, it is necessary to simplify the air-distribution plate accordingly. The air-distribution plate can be regarded as a porous medium with finite thickness and directional permeability, and its pressure drop equation is

$$\mathit{\Delta }\mathit{P}=-\left({\displaystyle \frac{{\mathit{\mu }}_{\mathbf{1}}}{\mathit{\alpha }}}{\mathit{v}}_{\mathit{i}}+{\mathit{C}}_{\mathbf{2}}{\displaystyle \frac{\mathbf{1}}{\mathbf{2}}}\mathit{\rho }\left|\mathit{v}\right|{\mathit{v}}_{\mathit{i}}\right)\mathit{\Delta }\mathit{m}$$

(5)

The opening ratio of the air-distribution plate is 50%, so the air-distribution plate is designed as a porous-medium domain. The porous medium parameters for the perforated plate were set as follows: viscosity resistance coefficient $\mathit{\alpha }$ = 1 × 10⁷ m⁻², inertial resistance coefficient ${\mathit{C}}_{\mathbf{2}}$ = 100 m⁻¹, thickness $\mathit{\Delta }\mathit{m}$ = 0.01 m.

In a vertical honeycomb WESP, although the wettability of dust has a certain impact on its dust removal efficiency, it does not play a decisive role. The key factor that really determines the dust removal efficiency of a wet electrostatic precipitator is whether the airflow can be evenly distributed. Uneven airflow inside the precipitator will lead to local overflow, high-speed, or low-speed areas, reducing the dust removal efficiency. The flow field of the WESP is evaluated by the relative standard deviation of velocity, and the formula is as follows:

$${\mathit{\sigma }}_{\mathit{r}}=\sqrt{{\displaystyle \frac{\sum _{\mathit{i}=\mathbf{1}}^{\mathit{n}} {\left(\frac{{\mathit{v}}_{\mathit{i}}-\overline{\mathit{v}}}{\overline{\mathit{v}}}\right)}^{\mathbf{2}}}{\mathit{n}-\mathbf{1}}}}$$

(6)

According to the internationally accepted evaluation criteria for airflow distribution uniformity of dust collectors, such as the American RMS standard. When the relative standard deviation of velocity is less than 25%, the airflow distribution is considered qualified. When it is less than 20%, it is regarded as good. When it is less than 15%, it is deemed excellent [14].

According to Formula (6), the requirement for the airflow distribution uniformity at the entrance of the electric field (the entrance of the anode honeycomb tube) of the vertical honeycomb WESP designed in this article is ${\sigma }_r$ ≤ 0.2. The internal airflow uniformity is improved by adjusting the opening ratio of the air-distribution plate through CFD simulation.

In Equations (1)–(6), ${\mathit{x}}_{\mathit{i}}$—coordinates, $\mathit{\rho }$—fluid density (kg/m³), ${\mathit{u}}_{\mathit{i}}$—fluid velocity component in ${\mathit{x}}_{\mathit{i}}$ direction (m/s), $\mathit{p}$—pressure on the fluid (Pa), $\mathit{u}$—viscosity of air, $\mathit{\mu }$—dynamic viscosity of air (Pa·s), ${\mathit{G}}_{\mathit{k}}$—turbulent kinetic energy due to laminar velocity gradient (m²/s²), ${\mathit{G}}_{\mathit{b}}$—turbulent kinetic energy due to buoyancy (m²/s²), ${\mathit{Y}}_{\mathit{M}}$—pulsating expansion in compressible turbulence (m²/s²), ${\mathit{C}}_{\mathbf{1}\mathit{\epsilon }}$, ${\mathit{C}}_{\mathbf{2}\mathit{\epsilon }}$, ${\mathit{C}}_{\mathbf{3}\mathit{\epsilon }}$—constants, ${\mathit{\sigma }}_{\mathit{k}}$, ${\mathit{\sigma }}_{\mathit{\epsilon }}$—turbulent Prandtl number for kinetic energy $\mathit{k}$ (a model constant), turbulent Prandtl number for dissipation rate $\mathit{\epsilon }$ (a model constant), $\mathit{\Delta }\mathit{P}$—pressure drop, ${\mathit{\mu }}_{\mathbf{1}}$—liquid film thickness on the plate surface, $\mathit{\alpha }$—permeability of the porous plate, ${\mathit{C}}_{\mathbf{2}}$—internal resistance factor, $\mathit{\Delta }\mathit{m}$—porous medium thickness, ${\mathit{\sigma }}_{\mathit{r}}$—relative standard deviation of velocity, ${\mathit{v}}_{\mathit{i}}$—wind speed at measurement points, $\overline{\mathit{v}}$—average wind speed, and $\mathit{n}$—number of measurement points in the cross-section.

2.

Dust removal efficiency

According to the actual emission situation of animal feed processing plants and food processing plants, the vertical honeycomb WESP is installed after the desulfurization tower. Usually, the dust concentration is 50 mg/m³, and the dust concentration at the inlet of the designed WESP is 50 mg/m³. According to the “Comprehensive Emission Standard of Air Pollutants” in China and more stringent local emission standards, the particulate matter emission limit for industries such as feed processing is 10 mg/m³. The dust removal efficiency formula is

$$\mathit{\eta }={\displaystyle \frac{{\mathit{C}}_{\mathit{i}\mathit{n}}-{\mathit{C}}_{\mathit{o}\mathit{u}\mathit{t}}}{{\mathit{C}}_{\mathit{i}\mathit{n}}}}\times \mathbf{100}\mathit{\%}$$

(7)

In the above formula, $\mathit{\eta }$—dust removal efficiency, ${\mathit{C}}_{\mathit{i}\mathit{n}}$—inlet concentration, ${\mathit{C}}_{\mathit{o}\mathit{u}\mathit{t}}$—outlet concentration.

Therefore, the dust removal efficiency of the vertical honeycomb WESP is

$$\mathit{\eta }={\displaystyle \frac{\mathbf{50}-\mathbf{10}}{\mathbf{50}}}\times \mathbf{100}\mathit{\%}=\mathbf{80.0}\mathit{\%}$$

That is, under this working condition, to meet the national and local emission standards, the dust removal efficiency of the vertical honeycomb WESP needs to reach at least 80% to ensure that the particulate matter concentration of the emissions meets the requirements and effectively reduces the pollution to the atmospheric environment.

Although the electric field and particle charging mechanisms are critical to WESP performance, this study focuses primarily on flow field uniformity as a prerequisite for efficient particulate collection. Similar approaches have been adopted in prior studies where flow optimization preceded full EHD-coupled simulations [17].

3.

Boundary Conditions and Operating Parameters

Inlet Velocity: 2.5 m/s.

Turbulence Intensity: 5%.

Temperature/Humidity: 303 K, 60% relative humidity.

Spray Status: Off (if not activated in this simulation).

Outlet Condition: pressure outlet.

3. Results

3.1. Vertical Honeycomb Wet Electrostatic Precipitator Original Model Velocity Field

The original model design of this vertical honeycomb WESP is as follows: In the inlet bell-shaped smoke box, a layer of perforated plate is arranged along the airflow direction. The opening ratio of this perforated plate is 50%, and there is no guide plate in the fluid. The simulation results of this scheme are shown in Figure 4, Figure 5 and Figure 6.

When conducting an in-depth analysis of the performance of a vertical honeycomb WESP, the first focus is on the distribution characteristics of its internal flow field. As clearly shown in the velocity streamline distribution diagram of the vertical honeycomb WESP model in Figure 5, when flue gas enters the interior from the inlet of the precipitator, since only one distribution plate acts to even distribute the airflow, the streamline distribution at this time is extremely uneven. The flue gas flows at high speed in the lower gas chamber and impacts the distribution plate, resulting in abnormally dense streamlines on the left and middle sides of the precipitator, while the right part is relatively sparse. This phenomenon of high and low speed zones, where the left and central parts are high-speed areas and the right side is a low-speed area, indicates the inhomogeneity of the flow field inside the precipitator. From the velocity distribution diagram of the anode inlet reference plane in Figure 6, even after the distribution effect of the distribution plate, the flow field distribution is not significantly improved. The boundary between high and low speed zones is very clear, with the left and central parts being high-speed areas and the right side being a low-speed area, and the overall flow field appears quite chaotic. Through calculation, the maximum velocity of the anode inlet reference plane is 2.4 m/s, the average velocity is 1.85 m/s, and the relative standard deviation of the velocity on this reference plane is as high as 46%, which far exceeds the conventional requirements for the uniformity of airflow distribution in the precipitator. This uneven flow field distribution has a direct impact on the performance of the precipitator, seriously reducing the dust removal efficiency.

According to the international common judgment standard of airflow distribution uniformity of dust collector, such as the American RMS standard, when the relative standard deviation of velocity is less than 25%, the airflow distribution is considered qualified; when it is less than 20%, it is regarded as good; and when it is less than 15%, it is regarded as excellent. Obviously, the relative standard deviation of the velocity at the anode inlet reference surface has a large gap with the relevant standards, which is far from the specified requirements and will directly reduce the dust removal efficiency of the vertical cellular WESP.

To improve this situation, the internal structure of the dust collector must be optimized and designed. Optimization measures include increasing or adjusting the number and position of the homogeneous plates to achieve more effective airflow distribution. In addition, adjustments to the geometry of the dust collector, such as adding a certain number of deflector plates at the flue gas inlet, may also help to improve the uniformity of the airflow.

3.2. Preliminary Optimization Scheme and Simulation Analysis of Vertical Honeycomb Wet Electrostatic Precipitator Original Model

3.2.1. Analysis of the Effect of Changing the Airflow Homogenizer on the Internal Flow Field

In the gas chamber of WESP, the opening ratio and aperture ratio of the flow distribution plates are the key factors influencing the internal gas flow field. In industrial applications, the opening ratio of the airflow distribution plate is usually controlled between 25% and 60%. Therefore, it was decided to start with adjusting the opening ratio of the homogenizing plate to improve the gas flow distribution. In FLUENT software (2023), different opening ratios, 25%, 30%, 35%, 40%, 45%, and 55%, were set, and iterative calculations were performed to observe the velocity streamlines and the velocity distribution cloud diagrams referenced to the anode inlet, so as to evaluate the effect of different opening ratios on the airflow homogeneity, and the specific cloud diagram results are shown in Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, Figure 13, Figure 14, Figure 15, Figure 16, Figure 17 and Figure 18.

Figure 7, Figure 8, Figure 9, Figure 10, Figure 11 and Figure 12 illustrate the velocity distribution at the anode inlet for different opening ratios. When the opening ratio is 55% (see Figure 7), the anode inlet is in the worst condition, with a severe high velocity zone near the center of the cross-section, with a maximum velocity of 2.95 m/s, and low velocity zones on both sides, with a significant velocity gradient, and the high velocity zones may lead to the escape of the dust and reduce the dust removal efficiency. When the opening ratio is 25% (see Figure 12), turbulence occurs in the local area. When the opening ratio is 40% (see Figure 9), the velocity distribution is relatively uniform, with no obvious high-speed or low-speed concentration area, and the airflow uniformity is better. From the cloud diagrams of six groups of anode inlet reference surfaces, it can be found that when the opening ratio is reduced, the scope of the high-speed area is reduced, but there is still a velocity gradient difference in the local area, and the velocity in the area above the cross-section is larger than the velocity in the area below.

Figure 13, Figure 14, Figure 15, Figure 16, Figure 17 and Figure 18 show the velocity traces corresponding to different opening ratios. When the opening ratio decreases (e.g., 30%~25%), the traces in (Figure 17 and Figure 18) show an increase in the zigzagging of the airflow path, and the local backflow phenomenon is obvious, which further verifies the existence of turbulence. In Figure 16, a slight separation of the airflow starts to occur internally, whereas when the openings are 40% (see Figure 15), the traces are uniformly distributed with no obvious vortex areas.

To more accurately assess the airflow homogeneity at different opening ratio, 100 data points were selected on the inlet to the anode section reference surface for calculation, and the results are shown in Figure 19. From the figure, when the opening ratio of the homogenizing plate is increased from 25% to 55%, the values of the relative standard deviation of the velocity are 45%, 35%, 28%, 25%, 37%, and 58%, respectively. Among them, the airflow homogeneity reaches the qualified level when the opening ratio of the homogenizing plate is 40%, which indicates that the airflow homogeneity is optimized at this opening ratio. Through the optimization of the single-layer flow distribution plate for the vertical honeycomb WESP, the RSD (Relative Standard Deviation) at the inlet to the anode section has decreased from 0.46 to 0.14, a 70% reduction; this also indicates that a uniform flow field distribution can ensure that the dust in the flue gas maintains a more stable and orderly motion state within the electric field. On the one hand, it can ensure that the dust has enough time to interact with the electric field and be fully charged; on the other hand, it is conducive to the effective capture of the charged dust by the dust collector plate and reduce the dust escape, thus improving the dust removal efficiency of the whole vertical honeycomb WESP.

After in-depth analysis and optimization of the original model of the vertical honeycomb WESP, a series of measures were taken to improve the uniformity of the airflow distribution, including structural optimization of the inlet flue, the inlet flare, the outlet flue portion, and the interior of the main body of the WESP. Two deflector plates were added at the elbow of the inlet flue, an improvement aimed at adjusting the flow direction and velocity of the airflow to reduce the high velocity and low velocity zones in the flow field and achieve a more uniform airflow distribution.

The simulation results of the optimized flow field show that these improvements have achieved significant results. Comparison of Figure 20 shows that the optimized inlet flue streamline distribution is significantly improved, and the uniformity of the airflow is enhanced behind the homogeneous plate and in the inlet region of the anode plate. The local high-speed zone (edge high-speed) that may exist before optimization is effectively suppressed, and the flow path tends to be straight after optimization as can be seen in Figure 21.

3.2.2. Analysis of the Effect of Changing the Aperture Ratio of the Airflow Homogenizing Plate on the Internal Flow Field

When discussing the influence of the airflow distribution plate on the airflow field inside the WESP, the aperture ratio and aperture size of the airflow equalization plate play a crucial role. Especially for the airflow distribution plate adjacent to the electrostatic precipitator, its design parameters are directly related to the overall performance and efficiency of the precipitator. In this paper, we focus on the influence of the airstream averaging plate on the airflow field inside the WESP and take the circular aperture averaging plate with 40% aperture ratio as the research object and investigate the effect of aperture size on the characteristics of the airstream averaging through numerical calculations.

To evaluate the airflow homogenization under different aperture sizes more intuitively, 100 data points were selected on the anode inlet reference surface, and the relative standard deviation of the velocity under each aperture size was calculated. When the aperture of the homogenized plate was increased from 30% to 70%, the relative standard deviation values of velocity were 32%, 30%, 27%, 23% and 25%, respectively, and the relative standard deviation values of velocity showed a certain trend with the increase in the aperture ratio. When the aperture ratio of the plate is increased from 30%, the values of relative standard deviation of velocity may decrease, indicating that the homogeneity of the airflow has improved to some extent. However, when the aperture ratio continues to increase from 60% to 70%, the relative standard deviation of the velocity starts to increase again, which means that the airflow homogeneity deteriorates again. When the aperture ratio is 60%, the relative standard deviation of the velocity is optimized, and an improvement of the dust removal efficiency can be achieved. The curves of pressure drop (pressure difference between inlet and outlet) versus velocity for different aperture ratios are derived from Figure 22. As the air passes through the perforated plate, the pressure decreases in the region where the backflow occurs, indicating that the backflow downstream of the perforated plate leads to a loss of pressure. It can also be seen from Figure 22 that the pressure drops increases with velocity, and the smaller the perforated plate ratio, the greater the pressure drop. The inflection point occurs when the orifice ratio increases to 70%, which is related to the intensification of the backflow phenomenon at high openings, leading to an increase in energy consumption, and further verifies the effect that the relative velocity standard deviation value is optimized when the orifice ratio is 60%, which improves the efficiency of the dust removal.

The results show that the aperture ratio of the homogenizing plate significantly affects the airflow homogeneity and pressure drop performance of the WESP, which can effectively control and optimize the flow field distribution inside the WESP. When the aperture ratio is 60%, the relative standard deviation of the velocity is the best, and the pressure drop is low, which is the optimal choice for the current design. Reasonable setting of the homogenizing plate is essential to achieve uniform distribution of the airflow, which helps to improve the dust removal efficiency, reduce energy consumption, and extend the service life of the equipment.

3.2.3. Analysis of the Effect of Adding a Second Airflow Homogenizing Plate on the Internal Flow Field

To further optimize the flow field distribution inside the WESP and to improve the dust removal efficiency, it was decided to add a second uniform plate in the lower air chamber in front of the first uniform plate (see Figure 23). This decision was based on an in-depth analysis of the current uneven flow field distribution, and a more uniform airflow distribution was achieved by increasing the number of homogenizing plates.

In FLUENT software, the effect of two homogenizing plates with different openings on the flow field was simulated. The openings and aperture ratios of the two homogenizing plates were finely tuned. The openings of the second homogeneous plate were set to 40% and 35% for both scenarios to be examined, while keeping the aperture ratio at 60%. Velocity streamlines for both scenarios and velocity distribution cloud maps at the anode inlet reference surface were obtained through iterative calculations.

The simulation results when the second homogeneous plate was opened with 40% opening ratio are shown in Figure 24 and Figure 25.

The simulation results show that the uniformity of the entire flow field is significantly improved compared with the flow field line diagram before optimization in the case of 40% openings of the uniform plate. The improvement of the flow field is especially obvious at the rear of the uniform plate and the inlet area of the anode plate, and the distribution of the flow lines is further close to the uniform state, which plays a key role in the improvement of the dust removal efficiency of the WESP. Through the synergistic rectification effect of the deflector plate and the second uniform plate, the velocity and pressure in the flow field are effectively balanced, which makes the airflow distribution of the WESP more uniform. The relative standard deviation of the velocity at the anode inlet reference surface was calculated to be reduced to 20%, which achieves the standard of good airflow distribution, but does not achieve the excellent result. Based on this result, the opening ratio of the second homogeneous plate will be changed and set to 35% for the simulation, and the simulation results are shown in Figure 26 and Figure 27.

After exhaustive simulation and analysis, it is found that the uniformity of the flow field is significantly improved when the opening ratio of the second uniform plate is set to 35%.

The optimized CFD model is shown in Figure 26. After the addition of the second uniform plate, the distribution of the flow lines tends to be uniform, and the vortex phenomenon is basically eliminated, and the addition of the uniform plate significantly promotes the uniform distribution of the flow field. After flue gas from the inlet crosses the deflector plate, its distribution state is optimized and becomes more uniform, thus effectively reducing the probability of reflux phenomenon caused by the uneven distribution of airflow. Combined with Figure 27, Figure 28, Figure 29 and Figure 30, it can be found that when the flue gas passes through the first homogeneous plate, as can be seen from the velocity cross-section in Figure 29, the velocity of the airflow is significantly reduced, and under the action of the two homogeneous plates, the vortex in the electric field has been basically eliminated, and the airflow is able to pass through the electric field uniformly, which is essential for improving dust removal efficiency.

Figure 27 demonstrates that the uniformity of the flue gas at the anode inlet reference surface has been significantly improved after optimization, and the relative standard deviation of the velocity has been reduced to 14%, which is an effective reduction of 32% compared with the original model and meets the requirement of excellent design, and this value fully proves the effectiveness of the flow field optimization scheme. With the addition of the orifice plate, the maximum velocity at the inlet section of the anode tube is reduced by 21% from 2.4 m/s to 1.9 m/s, and the average velocity is also reduced from 1.85 m/s to 1.22 m/s. This change not only helps to reduce the impact and wear of the airflow but also improves the overall stability and durability of the dust collector, and the homogeneous flow field prolongs the retention time of the dust in the electric field area, thus increasing the charging and trapping efficiency. and trapping efficiency. Combined with the optimization of the relative standard deviation values of the velocity, it is expected that the dust removal efficiency will be increased by more than 5%.

In summary, the flow field of the WESP was successfully optimized by fine tuning the opening ratio of the homogeneous plates and the aperture ratio, as well as increasing the number of homogeneous plates to reduce the impingement of the airflow on the homogeneous plates. The next step is to verify the accuracy of the simulation results through experimental tests, and the combination of numerical simulation and engineering practice has set an example for the fine optimization of WESP.

3.3. Vertical Honeycomb Wet Electrostatic Precipitator Experimental Test Analysis

To verify the reliability of the simulation, the results of the flow field simulation optimization were applied to the project for experiments by building a one-to-one vertical cellular WESP, as shown in Figure 31. The model diagram is shown in Figure 32.

The WESP was operated at a voltage of 60 kV and current of 200 mA. The spray system was set to intermittent mode with a flow rate of 10 L/min.

To compare the results of the original model and the optimized results of the flow field simulation, the original model was tested, and the results of the particulate emission are shown in Figure 33.

The inlet concentrations of the particulate matter in the three tests were 92 mg/m³, 85 mg/m³, 105 mg/m³, with an average inlet concentration of 94 mg/m³, and the outlet concentrations were 8.6 mg/m³, 8.1 mg/m³, 8.9 mg/m³, with an average outlet concentration of 8.53 mg/m³, and the dust removal efficiencies were 90.1%, 90.5%, 91.5%, with an average dust removal efficiency of only 90.67%, respectively. The average dust removal efficiency was only 90.67%. Although the export concentration of particulate matter meets the emission requirements, but due to the single-layer homogenizing plate has a general effect on the homogenization of the airflow, resulting in the flue gas from the flue to the lower chamber of the high and low velocity areas, which directly leads to the general efficiency of the dust removal.

For the optimized vertical cellular electrostatic precipitator exhaust gas organized emission test results are shown in Figure 34, Figure 35, Figure 36 and Figure 37.

The following is known through the results of many field tests:

The inlet concentration of the vertical cellular WESP fluctuates in the range of 95.6–158 mg/m³, and the outlet concentration of particulate matter fluctuates in the range of 3.8–7.4 mg/m³, which is stable and less than 10 mg/Nm³. According to Figure 38, the removal efficiency of particulate matter fluctuates from 95% to 97%. From Figure 33, in the second test, the concentration of particulate matter at the inlet of the dust collector is 126 mg/m³, and the concentration at the outlet is only 3.8 mg/m³, which meets the requirement of ultra-low emission, and the removal efficiency is as high as 97%. The average particulate removal efficiency is 95.88%, much higher than the design requirement of 80%, which is better than the traditional dust removal equipment.

4. Overall Evaluation

(1)

Methodological Integration

The study effectively combined CFD simulation with experimental validation to optimize the field of WESP flow. By adjusting the opening ratio of airflow homogenizing plates (40%–60%), installing deflector plates, and adding a second homogenizing plate, the relative standard deviation of velocity at the anode inlet was reduced from 46% (original model) to 14% (optimized model), meeting the excellent standard (σᵣ ≤ 0.15). This demonstrates the feasibility of CFD-driven optimization in resolving flow field non-uniformity, a critical challenge in WESP design.

(2)

Performance Enhancement

The experimental results showed that the optimized WESP achieved an average dust removal efficiency of 95.88%, significantly higher than the original model (90.67%) and the design requirement (80%). The outlet particle concentration stabilized below 10 mg/Nm³, fulfilling strict emission standards for food and feed processing industries.

(3)

Limitations and Implications

While this study focuses on vertical honeycomb WESP for medium air capacity (25,000 m³/h), the proposed optimization strategies (e.g., multi-plate homogenization, deflector plates) can be generalized to other WESP configurations or industries with high particulate and aerosol emissions. Future research may explore the impact of water spray parameters and electrode spacing on particles capture efficiency under varying humidity conditions.

(4)

Contributions to Environmental Sustainability

The optimized vertical honeycomb WESP achieves an outlet particulate matter concentration ≤ 10 mg/Nm³. Compared with traditional bag filters, it reduces the frequent replacement demand caused by filter bag clogging (approximately 30% annual reduction in filter bag replacement), thereby decreasing industrial solid waste generation. Taking a device with a processing air volume of 25,000 m³/h as an example, it can reduce PM2.5 emissions by about 12.8 tons annually, effectively contributing to regional air pollution prevention and control.

5. Conclusions

(1)

Based on the CFD method, the maximum velocity of the anode tube inlet section of the vertical honeycomb WESP is reduced from 2.4 m/s to 1.9 m/s, and the average velocity is reduced from 1.85 m/s to 1.22 m/s by changing the aperture rate and aperture ratio of the airflow homogenizing plate, and increasing the deflector plate and airflow homogenizing plate, which helps to reduce the impact and abrasion of the airflow and improve the overall stability and durability of the dust collector. model by 32% (from 0.46 to 0.14), reaching an excellent standard. This optimization helps to reduce the impact and wear of the airflow on the structure and improve the overall stability and durability of the dust collector.

(2)

After many tests, the concentration of particles at the outlet of the vertical cellular WESP is less than 10 mg/Nm³, and the removal efficiency is 95.88%, which is 5.7% higher than that of the original model.

(3)

The optimization scheme verified by numerical simulation proved to be feasible through engineering practice, which not only saved on the research cost but also provides meaningful guidance for engineering applications.

(4)

This study paves the way for integrating CFD-based sustainable design into broader industrial sectors, supporting global transitions toward low-carbon, resource-efficient manufacturing.