Application of Local Dust Removal and Wet String Grid Purification Device in Deep Buried Long Double-Hole Tunnel

Abstract

Dust pollution induced by blasting during tunnel construction via the drill-and-blast method poses a severe threat to workers’ health and construction safety. To address this issue, a wet chord grid dust removal and purification device adaptable to deep-buried long tunnels was developed in this study. The device integrates dust control and removal functions, featuring mobility, high purification efficiency, and water recycling capability. Through experimental tests, the optimal operating parameters of the system were determined: the dust removal efficiency reached a peak of 94.3% (laboratory optimal value from the basic parameter optimization test) when the frequency of the extraction axial flow fan was set to 30 Hz and the cross-sectional wind speed of the chord grid reached 3.34 m/s. The circulating water tank achieved the optimal water treatment performance under the conditions of a relative buried depth of 0.42 for the water inlet, a volume ratio of 1:2 for the sedimentation area to the clear water area, and a relative baffle height of 0.65. Numerical simulations based on CFD software (2021) revealed that the on-site dust removal efficiency of the device reached 79.86% and 87.9% under the working conditions where the tunnel face was 10 m and 100 m away from the connecting passage, respectively, which are in good agreement with the field measurement results. In the practical application at the Shierpo Tunnel of the Guangxi Tianba Expressway, the device achieved an average total dust removal efficiency of 78.4%, with 81.2% removal efficiency for PM₁₀ and 76.5% for PM_2.5, demonstrating excellent engineering applicability and dust removal performance for respirable dust. This study provides effective technical support and a theoretical basis for improving the construction environment of drill-and-blast tunnels.

1. Introduction

Dust pollution is a prominent problem in tunnel construction using the drill-and-blast method, and the control of respirable dust with high mass concentration has long been a tough challenge for the industry. On the one hand, dust accelerates the wear of construction equipment, reduces the accuracy of instruments, and shortens their service life. On the other hand, long-term inhalation of dust by construction workers is likely to cause pneumoconiosis [1]. Relevant statistics show that pneumoconiosis accounts for more than 80% of all occupational disease incidences [2]. Therefore, it is imperative to create a good working environment for workers, reduce their risk of occupational diseases, and improve construction efficiency.

Mechanical ventilation and dust extraction are widely used at construction sites to manage dust at the working face. Domestic and international scholars have also made revisions to numerous key parameters in ventilation systems [3,4,5,6,7]. However, as tunnel excavation projects tend towards greater length and scale, ventilation and dust extraction struggle to achieve ideal results. To reduce dust concentration at the working face and improve the tunnel working environment, various dust removal technologies have been developed and widely applied in industrial-scale tunnel construction, which can be mainly divided into the following categories:

Dry filtration dust removal technology: This technology intercepts dust particles through filter media such as filter bags and filter cartridges, with the advantages of no water consumption and high dust removal efficiency for dry dust. Representative commercial products include the Grydale JMS M-Series mobile dust collector (made in USA), which is widely used in open construction sites. However, its filtration efficiency for high-humidity respirable dust in tunnels is significantly reduced, and the filter element is prone to blockage and requires frequent replacement, resulting in high maintenance costs in long-term tunnel construction [8,9].

Electrostatic precipitation technology: This technology uses a high-voltage electric field to charge dust particles, which are then adsorbed by the electrode plate to achieve gas–solid separation. Typical industrial applications include KC Cottrell electrostatic precipitators, which have the advantages of low air resistance and large air handling capacity. However, the equipment has a large volume, poor mobility, and high requirements for the insulation performance of the power supply system, making it difficult to adapt to the narrow and complex construction environment of the tunnel excavation face [10,11].

Foam dust suppression technology: This technology covers the dust source through foam generated by a foaming agent, and wets and agglomerates dust particles. It has the advantages of good dust sealing performance and low water consumption, but the foaming agent will increase the construction cost, and the residual foam may affect the subsequent tunnel lining construction, with limited application in long-term continuous excavation [12,13].

Spray dust suppression technology: This technology is currently the most widely used dust control method in tunnel construction, with the advantages of low cost and simple operation. However, tunnel dust sources are easily disturbed by air currents, droplets and dust particles struggle to make contact, and the effective distance for wetting and coalescence is limited, significantly impacting spray dust suppression efficiency [14]. Recent research has focused mainly on aspects such as activated magnetized water spraying, pneumatic atomization, and ultrasonic atomization [15,16,17], but the above limitations have not been fundamentally solved.

Wet chord grid water film dust removal technology features high overall dust collection efficiency, low resistance, and stable performance. Spray droplets undergo wetting and coalescence with dust particles, and the dynamic water film formed by droplets impacting the chord grid surface enhances dust capture through dust-water binding. This technology shows promising application prospects in tunnel dust control [18]. Our previous study preliminarily verified the feasibility of wet chord grid technology for tunnel dust removal, optimized the key parameters of the air purification unit, and proposed a method to construct a dust control zone near the tunnel working face [19].

On the basis of the previous basic research, this study further develops a mobile integrated wet chord grid dust removal and purification vehicle for drill-and-blast tunnel construction, and carries out a series of expanded and in-depth research contents with essential differences from the previous study:

(1)

A systematic optimization experiment of the circulating water tank structure (the core component of the device) is carried out, and the optimal structural parameters are obtained to realize efficient recycling of water resources, which fills the gap in the previous study on the water treatment system of the device.

(2)

Full-scale CFD numerical simulation of airflow-dust coupled migration under two typical working conditions (tunnel face 10 m and 100 m away from the connecting passage) is carried out, revealing the dust control and diffusion law of the device in different excavation stages of deep-buried long tunnels.

(3)

Systematic on-site engineering application test in the Shierpo Tunnel is completed, the actual dust removal performance of the whole device under complex deep-buried long tunnel construction conditions is verified, and a targeted deployment strategy of the device in different excavation stages is proposed for engineering practice.

Simultaneously, to ensure effective suction and purification of dust at the working face, a dust control zone is created near the face using the airflow from the radiator. This effectively prevents un-purified dust from diffusing and accumulating in areas like the secondary lining trolley [20,21], thereby avoiding continued pollution of the tunnel construction environment. Furthermore, the dust control and removal devices are highly integrated into a single, self-propelled unit, forming an independent system. This minimizes interference with production operations and provides an important technical pathway for improving the working environment in drilling and blasting tunnel construction.

2. Project Overview

The Shierpo Tunnel on the Tianba Expressway has a total length of 4.732 km and a maximum burial depth of 430 m, which is a deep-buried long double-tube single-track tunnel with high engineering difficulty and complex construction conditions. As shown in the construction layout (Figure 1), a 30 m-long connecting passage is set between the left and right tunnels.

Twenty minutes after the tunnel blasting operation was completed, a total of 8 monitoring points were set up at the tunnel face, inside the connecting passage, and in the right tunnel section. All monitoring points were positioned at a height of 1.6 m above the ground and located along the tunnel centerline to reflect the actual conditions in the workers’ breathing zone. The total dust, PM₁₀ and PM_2.5 concentrations at each point were measured using an FCC-30 dust sampler and a real-time aerosol monitor, while wind speed values were simultaneously recorded with a TSI anemometer. The monitoring results, as shown in Figure 2, indicate that dust generated after blasting significantly accumulated in the tunnel face and connecting passage areas. Furthermore, the dust concentrations at all monitoring points exceeded the limits specified by current occupational health and safety technical standards, demonstrating the presence of significant dust pollution in the construction environment.

To further analyze the physical characteristics of the dust, the collected dust sample filter membranes were immersed and prepared into an aqueous solution. Particle size distribution was tested using an LS 13 320 laser particle size analyzer. As shown in Figure 3, the particle size of suspended dust in the tunnel air primarily ranged between 2 and 25 μm, with an average particle size of 8 μm. Among these, respirable dust with a particle size smaller than 10 μm accounted for 77.85% of the total dust. This type of dust can penetrate deeply into the human alveoli and deposit there, posing a serious health threat.

Long-term exposure of workers to a high-concentration dust environment not only easily induces occupational respiratory diseases such as pneumoconiosis, but also reduces visibility clarity, thereby increasing operational risks.

3. Development of Wet String Grid Purification Device

3.1. Device Structure and Working Principle

To effectively improve the air quality of the tunnel construction environment and reduce the dust concentration in the operation area, this study develops a wet chord grid dust removal device. Its overall structure is shown in Figure 4a. The device has a length of 3 m, a width of 1.8 m, and a height of 3.175 m, and integrates core components such as a tracked chassis, an air purification box, an extraction axial flow fan, a circulating water tank, an electric control box, and an operator cabin. The self-weight of the device is approximately 12 tons, and the total operating weight after adding hydraulic oil and circulating water is about 22 tons.

The ventilation and dust removal mechanism of the device (Figure 4a) follows a four-step sequential workflow, which is described clearly as follows:

• Dust control zone formation: The air flow emitted from the air outlet of the radiator forms an air curtain layer at a specific flow rate. Through entrainment and induction effects, the air curtain drives and organizes the surrounding air flow, forming a controllable closed dust control zone near the excavation face.
• Dust-laden gas extraction: The extraction axial flow fan provides negative pressure to suck the high-concentration dust-containing gas in the closed dust control zone into the air purification box, ensuring targeted collection of pollutants.
• Wet purification and clean air recycling: Inside the air purification box, fine water mist from spray nozzles and a stable water film on the wet string grid plates jointly capture dust particles. The purified, clean air is discharged back into the tunnel as recycled air, further diluting the dust concentration in the operation area.
• Sewage treatment and water recycling: Dust-laden sludge and sewage generated during purification are drained into the circulating water tank through the bottom sewage pipe. After sedimentation and filtration, the cleaned water is reused by the purification system, realizing high-efficiency recycling of water resources.

The main performance parameters of the device are shown in

Table 1. Equipment Performance Parameters.

Performance Parameters	Fan Power/kW	Handling Air Volume/m3/min	Internal Combustion Engine Power/kW	Circulating Water Tank Capacity/m3	Radiator Outlet Air Speed/m/s
Values	55	800	100	5	8

.

The core functional units of the system are the air purification box (Figure 4b) and the circulating water tank (Figure 4c). The air purification box is equipped with spray nozzles and a wet string grid plate structure. The fine water mist sprayed by the nozzles and the water film attached to the surface of the string grid work together to effectively intercept and capture the dust particles in the dust-containing air flow. Subsequently, the sewage containing dust flows into the circulating water tank through the bottom sewage pipe for purification treatment. To ensure the efficient and stable operation of the dust removal system, it is necessary to further explore the dust removal performance of the nozzle spray parameters and the wet string grid structure under different working conditions, and optimize the geometric design and hydraulic parameters of the circulating water tank accordingly to achieve the best effect of efficient dust-water separation and water resource recycling [22].

3.2. Optimization of Air Purification Box

3.2.1. Dust Removal Mechanism of Wet String Grid Water Film

The air purification box adopts the wet string grid water film dust removal technology, and its mechanism is mainly based on the synergistic effect of inertial collision, interception, and diffusion effects. When the dust-containing air flow passes through the staggered string grid structure, it interacts with the water film formed on the surface of the grid plates. The dust particles are captured by the water film and discharged along with the water flow, thereby realizing gas–solid separation. The core of the dust removal performance lies in the collision and adhesion efficiency between the dust particles and the liquid film, which mainly depends on the following three physical mechanisms:

(1)

Inertial collision effect

This mechanism plays a leading role in capturing particles with a particle size greater than 1 μm. When the air flow deflects around the string grid, the dust particles deviate from the flow line due to inertial force, collide with the surface of the water film, and are captured. The key parameter to characterize the inertial collision efficiency is the Stokes number (S_tk), which is defined as follows:

$$S_{\mathrm{tk}}={\displaystyle \frac{{\rho }_{\mathrm{p}}{d_{\mathrm{p}}}^2{v}_{\mathrm{c}}}{18{\mu }_{\mathrm{g}}{D}_{\mathrm{c}}}}$$

(1)

In the formula, ρ_p is the density of dust particles (kg/m³); d_p is the diameter of dust particles (m); V_c is the characteristic velocity of the air flow in the string grid channel (m/s); μ_g is the dynamic viscosity of the gas (Pa·s); D_c is the characteristic size of the string grid (m).

The collision efficiency (η_impaction) is closely related to the Stokes number. The larger the S_tk value, the greater the inertia of the dust particles, the more likely the collision occurs, and the higher the efficiency.

$${\eta }_{\mathrm{impaction}}=f\left(S_{\mathrm{tk}}\right)$$

(2)

(2)

Direct interception effect

For particles with small inertia but a certain particle size, when the distance between their movement trajectory and the surface of the string grid is less than the particle radius, they will be intercepted and captured by the water film even if they move along the flow line. The interception parameter R is defined as:

$$R={\displaystyle \frac{d_{\mathrm{p}}}{D_{\mathrm{c}}}}$$

(3)

The interception efficiency (η_interception) is a function of R. Generally, the larger the R value (i.e., the larger the dust particles and the narrower the channel), the higher the interception efficiency.

$${\eta }_{\mathrm{interception}}=f\left(R\right)$$

(4)

(3)

Brownian diffusion effect

For submicron particles, Brownian motion is significant, causing them to deviate from the main flow line and diffuse to the surface of the liquid film. This process can be described by the Peclet number (Pe):

$$P\mathrm{e}={\displaystyle \frac{v_{\mathrm{c}}{D}_{\mathrm{c}}}{D}}$$

(5)

In the formula, D is the Brownian diffusion coefficient, which is calculated by the Einstein-Stokes equation:

$$D={\displaystyle \frac{k_{B}T}{3\pi {\mu }_{\mathrm{g}}{d}_{\mathrm{p}}}}$$

(6)

In the formula, K_B is the Boltzmann constant (1.38 × 10⁻²³ J/K); T is the absolute temperature (K).

The comprehensive collection efficiency η_single of a single string grid wire is the result of the combined action of the above three mechanisms, which can be expressed as:

$${\eta }_{\sin gle}=1-(1-{\eta }_{\mathrm{impaction}})(1-{\eta }_{\mathrm{interception}})(1-{\eta }_{\mathrm{diffusion}})$$

(7)

If the dust removal unit is composed of N string grid wires, and it is assumed that the collection efficiency of each grid wire is the same, the total dust removal efficiency η_total is:

$${\eta }_{\mathrm{total}}=1-{(1-{\eta }_{\mathrm{single}})}^N$$

(8)

3.2.2. Analysis of the Influence of Airflow Disturbance on Dust Removal Efficiency

The formation and stability of the water film on the surface of the string grid are affected by both the water supply pressure of the nozzle and the cross-sectional filtration wind speed. In the preliminary experiment of this study, it was found that due to the limited water supply pressure of the water pump and the presence of impurities in the water, if a small-diameter nozzle is used, it is prone to blockage, resulting in poor atomization effect; while using a large-diameter nozzle will produce droplets with too large a particle size, making it difficult to form a uniform and stable water film between the string grids. Finally, a low-pressure atomizing nozzle with a diameter of 2.0 mm was selected, and a better dust removal effect was achieved under the condition of a water supply pressure of 0.6 MPa.

In addition, the fan operating frequency directly affects the dust collection air volume, thereby changing the filtration wind speed (vc) of the string grid cross-section and having a significant impact on the dust removal efficiency. To systematically study this influence, a wet string grid filtration dust removal experimental platform, as shown in Figure 5, was built. The system includes a dust-generating device, a low-pressure atomizing nozzle, a string grid plate, a W-shaped water baffle, an extraction axial flow fan, and a dust sampler.

A TSI anemometer was used to measure the average wind speed of the string grid cross-section under the working conditions of fan frequencies of 20, 30, 40, and 50 Hz, and the results were 2.85, 3.34, 3.83, and 4.34 m/s, respectively. A dust sample with a mass of 2 g was accurately weighed and injected at the inlet of the system (Point 1). The dust mass was collected and measured by a precision balance at the inlet (Point 1) and the outlet (Point 2), respectively, and then the dust removal efficiency under different working conditions was calculated. The specific experimental data are recorded in

Table 2. Dust quality and dust removal efficiency at measuring points under different fan frequencies.

Fan Frequency/(Hz)	Average Air Velocity at the Grid Section/(m/s)	m1/(g)	m2/(g)	Dust Removal Efficiency	PM10 Removal Efficiency	PM2.5 Removal Efficiency
20	2.850	2.000	0.236	88.2%	90.5%	82.7%
30	3.340	2.000	0.114	94.3%	96.1%	91.8%
40	3.830	2.000	0.194	90.3%	92.4%	85.6%
50	4.340	2.000	0.210	87.5%	89.2%	82.1%

.

The dust removal efficiency ηc is calculated according to the following formula:

$$\eta =(1-{\displaystyle \frac{m_2}{m_1}})\times 100\%$$

(9)

In the formula, m₁ is the dust mass at the inlet (Point 1) (g), and m₂ is the dust mass at the outlet (Point 2) (g).

It can be seen from

Table 3. Boundary conditions.

Velocity Inlet	Press-in Duct Outlet Inlet1/(m/s)	Dust Collection Inlet Inlet2/(m/s)	Radiator Outlet Inlet3/(m/s)	Device Outlet Inlet4/(m/s)	Service Tunnel Entrance Inlet5/(m/s)
Measured Results	12	−15	8	25	0.5

that as the fan frequency increases from 20 Hz to 50 Hz, the dust removal efficiency first increases and then decreases, reaching a maximum of 94.3% at a frequency of 30 Hz. This optimal efficiency value is obtained from the laboratory-scale parameter optimization test of the air purification unit, which is consistent with the basic test results of our previous study [19]. The reason is that when the wind speed is moderate, the liquid film is stable and the gas–liquid contact time is sufficient, so the inertial collision and interception effects are enhanced; when the wind speed further increases, the liquid film is prone to breakage or loss, resulting in shortened gas–liquid contact time and reduced collection efficiency. Therefore, there exists a critical filtration wind speed that optimizes the dust removal efficiency.

3.3. Optimization of Circulating Water Tank

3.3.1. Experimental Design and Method

As the core component of the wet string grid dust removal and purification system, the structural design of the circulating water tank significantly affects the particle separation performance. Reasonable setting of the water baffle and the burial depth of the water inlet pipe can effectively extend the particle sedimentation path, suppress short-circuit flow and turbulence, thereby improving the sedimentation efficiency and promoting the recycling of water resources.

The main body of the experimental system is made of acrylic plates (Figure 6), which mainly includes a water baffle, a water inlet, a water outlet, an intelligent self-priming pump, a water meter, and a sewage preparation device. To simulate the actual suspended particles, M25 strength cement mortar was prepared according to the ratio of cement:medium sand:water = 6:10:4. After standing for 3 days to solidify, it was crushed and sieved to obtain particles with the target particle size.

A JZ-280A self-priming pump was used in the system to realize water circulation, and a 099-type rotor cold water meter was used to measure the flow rate. A WGZ-200B turbidity meter was used with a sampling bottle to measure the turbidity of different water areas. The volume ratio of the sedimentation area to the clear water area is 1:2. The experiment focuses on investigating the influence of the relative height of the water baffle (0.50, 0.65, 0.80) and the burial depth of the water inlet pipe (0, 0.42, 0.85) on sedimentation. The water area is divided into the upper part (0–20 cm), the middle part (20–40 cm), and the bottom part (40–70 cm), and 9 groups of experiments were carried out according to the two-factor and three-level design.

The experimental steps are as follows:

① Fill with clean water and pre-run for 10 min;

② Soak the water baffle for more than 3 h to prevent water absorption and deformation;

③ Add self-made suspended particles to simulate sewage;

④ Measure the turbidity of each water area and the water outlet every 10 min;

⑤ Sample and measure according to the standard and record the data, with a total duration of 30 min;

⑥ Clean the water tank and analyze the data after each group of experiments.

3.3.2. Analysis of the Influence of Geometric Parameters on the Sedimentation Efficiency of Suspended Particles

To explore the sedimentation efficiency of the circulating water tank in treating suspended particles under different working conditions, the concentrations of suspended particles at the water inlet and outlet were compared. The sedimentation efficiency η_d is defined as:

$${\eta }_{\mathrm{d}}=(1-{\displaystyle \frac{C_2}{C_1}})\times 100\%$$

(10)

In the formula, C₁ is the concentration of suspended particles at the water inlet (g/L), and C₂ is the concentration of suspended particles at the water outlet (g/L).

The sedimentation efficiency results under different working conditions are shown in Figure 7. When the relative burial depth of the water inlet pipe is 0, under the condition that the relative height of the water baffle is 0.5, the sedimentation efficiency first increases and then decreases with the increase in the volume ratio; when the relative height is 0.65, the sedimentation efficiency is positively correlated with the volume ratio; when the relative height is 0.8, it shows a trend of first decreasing and then increasing. A horizontal comparison shows that under this burial depth, the sedimentation efficiency when the relative height of the water baffle is 0.65 and 0.8 is better than that when it is 0.5, indicating that the particles mainly diffuse in the upper part of the water tank, and a higher water baffle is more conducive to improving the sedimentation efficiency.

When the relative burial depth of the water inlet pipe is 0.42, the sedimentation efficiency corresponding to the water baffle height of 0.5 still first increases and then decreases with the volume ratio, while the sedimentation efficiency corresponding to the heights of 0.65 and 0.8 is positively correlated with the volume ratio. A vertical comparison shows that the sedimentation efficiency of each working condition under this burial depth is better than that under the burial depth of 0, indicating that appropriately increasing the burial depth of the water inlet pipe is beneficial to improving the sedimentation performance.

When the relative burial depth of the water inlet pipe increases to 0.85, the sedimentation efficiency corresponding to the water baffle heights of 0.5 and 0.8 first increases and then decreases with the volume ratio, while the sedimentation efficiency corresponding to the height of 0.65 increases slowly. A horizontal comparison shows that the efficiency decreases significantly when the water baffle height is 0.8, indicating that an excessively large burial depth of the water inlet pipe and an excessively high water baffle are not conducive to particle sedimentation and will reduce the overall operating efficiency of the circulating water tank.

4. Numerical Simulation of Airflow-Dust Coupled Migration of Wet String Grid Dust Removal and Purification Device

4.1. Physical Model and Working Condition Setting

According to the actual working conditions of the tunnel, a physical model with a 1:1 scale was established, as shown in Figure 8. Figure 8a is a single-head forced ventilation model when the tunnel face is excavated 10 m; Figure 8b is a dust removal model combining single-head forced ventilation and the wet string grid purification device when the tunnel face is excavated 10 m, and the device is installed in the connecting passage; Figure 8c is a single-head forced ventilation model when the tunnel face is excavated 100 m; Figure 8d is a dust removal model combining single-head forced ventilation and the wet string grid purification device when the tunnel face is excavated 100 m, and the device is installed at the tunnel face.

4.2. Mathematical Model and Control Equations

The wind speed in the tunnel is not high and the pressure change is small, so the compressibility of air can be ignored. Therefore, in the calculation, the air flow in the tunnel is regarded as three-dimensional incompressible steady viscous turbulent flow. The high Reynolds number k-ε model is used as the turbulent flow model. The mathematical model includes the continuity equation, momentum equation, and k-ε model equation [23,24].

Incompressible continuity equation:

$${\displaystyle \frac{\mathsf{\partial }\rho {\overline{u}}_i}{\mathsf{\partial }{x}_i}}=0$$

(11)

Incompressible momentum equation:

$${\displaystyle \frac{\mathsf{\partial }(\rho {\overline{u}}_i)}{\mathsf{\partial }t}}+{\displaystyle \frac{\mathsf{\partial }(\rho {\overline{u}}_i{\overline{u}}_j)}{\mathsf{\partial }{x}_i}}=\rho f_i-{\displaystyle \frac{\mathsf{\partial }p}{\mathsf{\partial }{x}_i}}+{\displaystyle \frac{\mathsf{\partial }}{x_i}}(\mu {\displaystyle \frac{\mathsf{\partial }{\overline{u}}_i}{\mathsf{\partial }{x}_i}})-{\displaystyle \frac{\mathsf{\partial }(\rho {\overline{u^{\prime }}}_i{\overline{u^{\prime }}}_j)}{\mathsf{\partial }{x}_j}}$$

(12)

Realizable k-ε turbulence model:

$$u_t=\rho C_{\mu }{\displaystyle \frac{k^2}{\epsilon }}$$

(13)

K equation:

$${\displaystyle \frac{\mathsf{\partial }k}{\mathsf{\partial }t}}+{\overline{u}}_j{\displaystyle \frac{\mathsf{\partial }k}{\mathsf{\partial }j}}=v_t\left({\displaystyle \frac{\mathsf{\partial }{\overline{u}}_i}{\mathsf{\partial }{x}_j}}+{\displaystyle \frac{\mathsf{\partial }{u}_j}{\mathsf{\partial }{x}_i}}\right){\displaystyle \frac{\mathsf{\partial }{\overline{u}}_i}{\mathsf{\partial }{x}_j}}-{\displaystyle \frac{\mathsf{\partial }}{\mathsf{\partial }{x}_j}}\left[\left({\displaystyle \frac{v_t}{{\sigma }_k}}-v\right){\displaystyle \frac{\mathsf{\partial }k}{\mathsf{\partial }{x}_j}}\right]-\epsilon$$

(14)

ε equation:

$${\displaystyle \frac{\mathsf{\partial }\epsilon }{\mathsf{\partial }t}}+{\overline{u}}_j{\displaystyle \frac{\mathsf{\partial }\epsilon }{x_j}}=-C_{{\epsilon }_1}\left(\mathsf{\partial }{\overline{u}}_i{\overline{u}}_j\right){\displaystyle \frac{\mathsf{\partial }{\overline{u}}_i}{\mathsf{\partial }{x}_j}}-{\displaystyle \frac{\mathsf{\partial }}{\mathsf{\partial }x}}\left[\left({\displaystyle \frac{v_t}{{\sigma }_{\epsilon }}}-v\right){\displaystyle \frac{\mathsf{\partial }\epsilon }{\mathsf{\partial }{x}_j}}\right]-C_{{\epsilon }_2}{\displaystyle \frac{{\epsilon }^2}{K}}$$

(15)

In Equations (11)–(15), ρ is the fluid density (kg/m³); u_i and u_j are the components of the velocity vector (m/s); p is the pressure acting on the fluid element (Pa); μ is the dynamic viscosity (Pa·s); μ_t is the turbulent (eddy) viscosity (Pa·s); k is the turbulent kinetic energy (m²/s²); and ε is the turbulent dissipation rate (m³/s²). σ_k and σ_ε the turbulent Prandtl numbers for the k and ε-equations, respectively. According to relevant experimental validations, the model constants are set as follows: C_μ = 0.09, C_ε1 = 1.44, C_ε2 = 1.92, σ_k = 1.0, σ_ε = 1.3.

4.3. Boundary Conditions and Grid Division

The boundary conditions and various parameters are shown in Table 2 and Table 3. The dust particle distribution coefficient is determined with reference to Figure 3. The grid quality affects the accuracy of the simulation results, so it is crucial to verify the grid independence, as shown in Figure 9.

The Computational Model Setup, DPM Settings, Injection Parameters are shown in

Table 4. Parameter settings of model.

Computational Model Setup			DPM Settings		Injection Parameters
Solver Type	Turbulence Model	DPM Model	Injection Frequency	Time Step	Injection Source	Particle Material	Size Distribution	Particle Size Range
Pressure-Based	Standardk-Ɛ	On	20	1	Tunnel Face	SiO2	R-R	10−6/10−4
Turbulence Model	Gravity Acceleration	Time Type	Total Number of Steps	Particle Shape	Median Particle Diameter	Mass Flow Rate	Spread Parameter	Tracking Model
Off	−9.81	Transient	1800	Spherical	10−5	0.06	3.05	Random Walk Model

. Particle injection rate: The particle injection rate of 0.06 kg/s is calculated based on the actual blasting conditions of the Shierpo Tunnel: the average dust production per blasting is about 120 kg, and the dust diffusion time after blasting is 1800 s, so the average mass flow rate of dust in the tunnel is about 0.06 kg/s, which is consistent with the on-site measured dust concentration data.

Rosin–Rammler distribution parameters: The spread parameter of 3.05 is obtained by non-linear fitting of the on-site dust particle size distribution test data (Figure 3), which has been fully verified by the laser particle size analysis experiment, and can accurately reflect the actual particle size distribution characteristics of the tunnel dust.

Discrete Random Walk (DRW) model: The DRW model (random walk model) is enabled in the simulation, with the number of eddy interactions set to 10, and full turbulence fluctuation is considered. This setting fully considers the influence of turbulent eddies on the dispersion of small dust particles in the highly turbulent tunnel flow, ensuring the physical authenticity of the dust diffusion simulation.

The grid division of Figure 8d includes low-quality (249,637 units), medium-quality (783,518 units), and high-quality (1,091,336 units). Under the three different grid division schemes, there are large differences in the simulation results between the low-quality grid and the medium-quality and high-quality grids. Considering the computer performance and simulation error, the medium-quality grid scheme is adopted.

4.4. Analysis of Simulation Results

4.4.1. Working Condition of 10 m Excavation

Figure 10 shows the evolution law of the tunnel airflow field when the tunnel face is 10 m away from the connecting passage.

It can be seen from Figure 10a that after the air flow is emitted from the outlet of the forced ventilation duct, it flows around the side wall of the right tunnel, and after hitting the tunnel face, it flows into the service passage along the connecting passage. It can be seen from Figure 10b that the airflow field near the tunnel face, in the connecting passage, and in the left tunnel is turbulent, affected by the jets from the dust collection port of the device and the air outlet of the radiator.

To understand the dust removal effect of the wet string grid purification device, three time points, namely T = 30 s, 60 s, and 90 s, were selected to compare and analyze the coupled diffusion process of airflow and dust particles before and after the device is turned on. The dust distribution at different times is shown in Figure 11. It can be seen from Figure 11 that the horizontal jet from the air outlet of the radiator creates a dust control zone, which effectively controls the dust near the tunnel face, and the mass of dust diffused into the connecting passage and the left tunnel is less than that when the device is not turned on. The comparison of dust mass concentration before and after the device is turned on is shown in Figure 12.

It can be seen from Figure 12a,b that within 0–90 s, the dust concentration at the front end of the dust collection port of the device when it is turned on is higher than that when it is not turned on, and the dust concentration at the rear end of the air outlet of the device when it is turned on is lower than that when it is not turned on. This indicates that most of the dust is suppressed in the dust control zone by the device, and the purified recycled air mixes with the dust-containing dirty air in the tunnel to reduce the dust concentration in the tunnel. It can be seen from Figure 12c that when the airflow is stable at T = 90 s, a dust concentration monitoring point is set every 10 m in the cross-section at the human breathing height (from the right tunnel face to the left tunnel air outlet), and the dust concentration at each monitoring point decreases after the device is turned on.

β = (G₁ − G₂)/G₁ × 100%

(16)

In Equation (6), β represents the dust reduction efficiency, G₁ is the dust concentration at the monitoring point without the device, and G₂ is the dust concentration at the monitoring point with the device in operation. According to Equation (16), when the tunnel face is 10 m away from the cross passage, the dust removal efficiency of the device is approximately 79.86%.

4.4.2. Working Condition of 100 m Excavation

Figure 13 shows the evolution law of the tunnel airflow field when the tunnel face is 100 m away from the connecting passage. When the wet string grid purification device is not installed, the wind speed of the airflow originally moving near the side wall of the air duct decreases, and the dust accumulates in the right tunnel; after starting the wet string grid purification device, the recycled air discharged from the air outlet entrains the air to move towards the left tunnel, which can ensure air circulation and airflow circulation.

Figure 14 shows the dust distribution at different times when the tunnel face is 100 m away from the connecting passage. At T = 30–60 s, after the device is applied, the dust is mainly concentrated at the front end of the device, and part of the dust escapes into the connecting passage; at T = 90 s, the recycled air released by the device has entrained the dust into the left tunnel, and the dust concentration is lower than that of single-head forced ventilation.

At T = 90 s, the dust at the tunnel face under the single-head forced ventilation state has not diffused into the service passage. To ensure the accuracy of the simulation results, T was extended to 200 s. It can be seen from Figure 15a that after the device is turned on, the dust concentration at the front end of the dust collection port is lower than that when the device is not turned on during T = 60–200 s; it can be seen from Figure 15b that after the device is turned on, the dust concentration at the rear end of the air outlet of the device is lower than that when the device is not turned on during T = 0–200 s; this indicates that installing the device around the tunnel face has a significant dust reduction effect. When the airflow is stable at T = 200 s, the dust reduction efficiency of the device is 87.9%.

5. Analysis of On-Site Application Effect of Wet String Grid Purification Device

5.1. Field Test Scheme and Results

To verify the performance of the wet string grid dust removal and purification device under the actual construction conditions of the tunnel, this study selected the Shierpo Tunnel of Tianba Road to carry out on-site application tests. During the test, according to the matching relationship between the fan frequency and the cross-sectional wind speed, the filtration wind speed of the string grid cross-section was controlled at approximately 3.3 m/s (corresponding to a fan frequency of 30 Hz) to achieve the optimal dust removal efficiency of the system. The device was deployed in the tunnel face areas 10 m and 100 m away from the connecting passage to carry out on-site dust removal operations. The arrangement of dust monitoring points followed the scheme described in Section 2, and the dust concentration data at the breathing zone height were collected systematically.

The on-site application results show (Figure 16) that the wet string grid dust removal and purification device can effectively suppress the high-concentration dust pollution generated during the construction of long tunnels using the drill-and-blast method. Relying on its gas-water coupling purification mechanism and highly integrated mobile design, the device significantly reduces the dust mass concentration in the operation area of the tunnel, especially in the breathing zone, with an average dust reduction efficiency of 78.4%, and the working environment is significantly improved. This device provides a green, safe, and efficient dust control technical equipment for drill-and-blast tunnel construction.

To verify the reliability of the numerical simulation model, the simulated dust concentration values at each monitoring point under the two working conditions (10 m and 100 m tunnel face advancement) were compared with the actual field measurement data. The numerical simulation results are in good agreement with the field-measured data, and the relative error between the simulated dust concentration and the measured value at each monitoring point is within 10%. The simulated dust removal efficiency of the device is 79.86% (10 m working condition) and 87.9% (100 m working condition), which is highly consistent with the field-measured average efficiency of 78.4%. The small error is mainly due to the simplification of the tunnel structure in the numerical model and the fluctuation of the actual airflow field in the tunnel during the field test. The good consistency between the simulation results and the measured data fully verifies the reliability and accuracy of the numerical model established in this study.

Based on the on-site test results, the following optimization measures are recommended in practical applications: In the early stage of tunnel excavation (depth ≤ 50 m), the device should be installed in the connecting passage to prevent dust from diffusing into the service passage; in the deep-buried section construction stage (depth > 50 m), it is recommended to install the device close to the tunnel face to strengthen the dust source control and purification effect. In addition, the circulating water tank and the string grid plate should be cleaned and maintained regularly to prevent nozzle blockage and uneven water film distribution caused by sludge accumulation or water quality changes, thereby ensuring the long-term stable operation of the device.

5.2. Comparative Evaluation with Commercial Dust Removal Systems

To objectively clarify the application advantages and limitations of the developed device, this section compares the key performance parameters of the device with three typical commercial dust removal systems widely used in tunnel construction, including the Grydale JMS M-Series dry dust collector, Aigner ECCOAIRJET wet dust collector, and KC Cottrell electrostatic precipitator. The specific comparison results are shown in

Table 5. Boundary conditions.

Performance Indicators	Developed Wet Chord Grid Device	Grydale JMS M-Series (Dry)	Aigner ECCOAIRJET (Wet)	KC Cottrell Electrostatic Precipitator
Handling Air Volume	800 m3/min	600~1200 m3/min	500~1000 m3/min	1000~5000 m3/min
Dust Removal Efficiency (Respirable Dust)	Average 78.4% on-site; 94.3% laboratory optimal	85~90% (dry dust); <60% (high-humidity dust)	75~85% on-site	80~90% (dry dust); <70% (high-humidity dust)
Rated Power	55 kW (fan) + 100 kW (engine)	75~132 kW	45~90 kW	150~500 kW
Mobility	Tracked mobile, integrated design, adaptable to narrow tunnel face	Wheeled mobile, large turning radius, not suitable for tunnel face	Fixed installation, poor mobility	Fixed large-scale equipment, no mobility
Water Recycling Capability	Yes, recycling rate >85%	No water consumption	No recycling, continuous water supply	No water consumption
Space Occupation	3 m × 1.8 m × 3.175 m, compact structure	4.5 m × 2.2 m × 3.5 m	Large installation space	Large floor area, not suitable for tunnel excavation section
Core Applicable Scenario	Deep-buried long tunnel excavation face, mobile dust source control	Open construction sites, fixed dust sources	Tunnel fixed section ventilation	Large-scale fixed space dust removal

.

Academic Discussion on Advantages and Limitations

Core Advantages of the Developed Device

Compared with commercial systems, the developed device has three core competitive advantages in the application scenario of drill-and-blast tunnel excavation face:

High integration of dust control and dust removal: The device integrates the dust control air curtain and wet purification unit, which can realize the closed control of the dust source at the tunnel face and centralized purification, solving the problem that traditional commercial systems can only passively extract dust but cannot control dust diffusion.

Strong adaptability to tunnel construction environment: The tracked mobile design and compact structure make the device suitable for the narrow and complex environment of the tunnel excavation face, and can move forward with the excavation progress, which is not available for fixed or large mobile commercial equipment.

High efficiency for respirable dust in high-humidity environment: The wet chord grid water film technology has stable dust removal efficiency for high-humidity respirable dust in the tunnel, which avoids the problem of efficiency reduction in dry filtration and electrostatic precipitation systems in high-humidity environments; at the same time, the water recycling function greatly reduces water consumption, which is suitable for long tunnel construction with limited water supply.

Limitations of the Developed Device

Compared with dry dust collectors, the device will produce dust-laden sludge, which requires subsequent disposal.

Compared with large electrostatic precipitators, the handling air volume of the device is more suitable for the local dust control of the excavation face, and cannot meet the full tunnel ventilation and dust removal needs.

The device has higher requirements for water quality, and the nozzle and chord grid need regular maintenance to prevent scaling and blockage.

5.3. Research Limitations and Applicability

Before the Conclusions Section, this section comprehensively discusses the engineering limitations and applicable conditions of the developed device, to provide a more comprehensive reference for engineering applications and subsequent research.

5.3.1. Freezing Risk in Cold Climates

The current version of the device is mainly designed for temperate and subtropical climate regions, and the operability of the wet scrubbing system will be limited in long-term low-temperature cold regions. When the ambient temperature is below 0 °C, the water in the circulating water tank, pipeline and nozzle may freeze, leading to the failure of the spray system and the reduction in dust removal efficiency. For cold region applications, we have proposed a corresponding improvement scheme: adding an electric heating module with temperature control for the circulating water tank and water pipeline, adding an insulation layer for the purification box and pipeline, and using an environmentally friendly antifreeze fluid for the circulating water under extreme low-temperature conditions, to ensure the stable operation of the system in cold climates.

5.3.2. Stability of Water Film at High Wind Speeds

The experimental results show that the dust removal efficiency of the device increases first and then decreases with the increase in the cross-sectional wind speed of the chord grid, and the optimal efficiency is achieved at 3.34 m/s; when the wind speed exceeds 4.0 m/s, the dust removal efficiency decreases significantly. The physical mechanism behind this phenomenon is: when the cross-sectional wind speed exceeds the critical value, the aerodynamic shear force of the high-speed airflow will cause the breakage of the continuous water film on the surface of the chord grid, and the droplet entrainment effect will reduce the effective coverage of the water film, thus weakening the inertial collision and interception capture effect of dust particles. For high-pressure ventilation tunnels with high background wind speed, the fan frequency of the device should be controlled within 30–40 Hz to avoid excessive wind speed leading to performance degradation, and the optimal wind speed range of 3.0–3.5 m/s should be maintained.

5.3.3. Toxic Sludge Management and Disposal

The device separates dust from water through sedimentation, and the dust will accumulate at the bottom of the circulating water tank in the form of sludge. For the management and disposal of the sludge:

Sludge discharge frequency: Under normal continuous operation, the sludge discharge cycle of the device is set to once a day, and the sludge at the bottom of the tank is cleaned through the sewage outlet.

Transportation and disposal: The cleaned sludge is transported out of the tunnel by the construction muck truck together with the blasting muck, and the disposal is carried out in accordance with the local construction solid waste environmental regulations.

Toxicity control: Before the formal disposal of the sludge, the heavy metal content and toxicity of the sludge should be tested according to the type of blasted rock. In the on-site application of the Shierpo Tunnel, the main component of the sludge is silicate rock dust, which is non-toxic and meets the requirements of ordinary construction waste disposal. For tunnels with potentially toxic rock formations, the sludge should be treated as hazardous waste in accordance with relevant regulations.

5.3.4. Water Supply and Scaling Issues

Water supply adaptability: The water recycling rate of the device can reach more than 85% under the optimal working conditions, which greatly reduces the water consumption. For tunnel construction areas without fixed water sources, the water demand of the device can be met by transporting water with a construction water truck, which has good adaptability to the construction environment of deep-buried long tunnels.

Nozzle scaling and maintenance: The scaling of the nozzle and chord grid caused by water hardness will affect the atomization effect and the stability of the water film, thus reducing the dust removal efficiency. To solve this problem, the device is equipped with a 50 μm water filter at the inlet of the circulating water pump to filter impurities in the water; at the same time, it is recommended to clean and maintain the nozzle and chord grid plate once a week under normal operation, to avoid scaling and blockage, and ensure the long-term stable operation of the system.

5.3.5. Discussion on the Discrepancy Between Laboratory and On-Site Efficiency

The laboratory test shows that the maximum total dust removal efficiency of the device can reach 94.3% under optimal operating parameters, while the average on-site efficiency in the Shierpo Tunnel is 78.4%, with a significant discrepancy. The main reasons for this difference are analyzed as follows:

Complexity of the actual tunnel environment: The laboratory test is carried out in a stable and closed experimental platform, with uniform and stable dust concentration and airflow field; while in the actual tunnel, the airflow field is highly turbulent, affected by the press-in ventilation, construction equipment, and tunnel structure, resulting in partial dust escape from the dust control zone, which reduces the overall dust removal efficiency.

Dust particle size distribution difference: The dust sample used in the laboratory test is standard dust with a fixed particle size distribution, while the actual tunnel dust has a wider particle size distribution, with a higher proportion of fine PM_2.5 particles, which are more difficult to capture, resulting in lower overall efficiency.

Instability of operating parameters in field application: In the actual construction process, the fan frequency, water supply pressure, and other operating parameters will fluctuate with the construction progress, and it is difficult to maintain the optimal state for a long time; in addition, the water film on the chord grid surface will be uneven due to the vibration of the device during movement, which will also reduce the dust removal efficiency.

Dust re-entrainment in the tunnel: In the actual tunnel, the purified air discharged from the device will cause slight re-entrainment of the settled dust on the tunnel wall and ground, which will lead to an increase in the measured dust concentration and a decrease in the calculated dust removal efficiency.

The above analysis shows that the optimal laboratory efficiency reflects the maximum dust removal capacity of the device, while the on-site efficiency reflects the actual performance of the device in real engineering environments. The average on-site efficiency of 78.4% has reached the advanced level of similar mobile dust removal equipment for tunnel excavation faces, fully verifying the engineering practicality of the device.

6. Conclusions

Based on experimental research, numerical simulations, and field application of the self-developed wet string-grid dust removal and purification device for drill-and-blast tunnel construction, the main conclusions of this study are drawn as follows:

(1)

A mobile integrated wet string-grid dust removal and purification device with the combined functions of dust control, dust removal and water circulation is successfully developed, which can form a closed dust control zone near the tunnel face to realize efficient capture and purification of dust, and effectively reduce the tunnel dust concentration to improve the construction working environment.

(2)

The optimal operating parameters of the device and its core components are determined through systematic experiments: the air purification box achieves a maximum dust removal efficiency of 94.3% at a fan frequency of 30 Hz and a string-grid cross-sectional wind speed of 3.34 m/s; the circulating water tank reaches the optimal sedimentation performance under the conditions of a water inlet relative buried depth of 0.42, a baffle relative height of 0.65 and a sedimentation area-clear water area volume ratio of 1:2, which guarantees the stable operation and water resource recycling of the whole system.

(3)

The device shows strong adaptability to different tunnel excavation depth conditions and excellent engineering practicality: numerical simulation results show its dust removal efficiency reaches 79.86% and 87.9% when the tunnel face is 10 m and 100 m away from the connecting passage, respectively, and the average dust removal efficiency in the on-site application at the Shierpo Tunnel reaches 78.4%, which provides a reliable and efficient technical solution for dust pollution control in similar deep-buried long drill-and-blast tunnel construction projects and has significant environmental and occupational health benefits.