Study on Improving Pulsed-Jet Performance in Cone Filter Cartridges Using a Porous Diffusion Nozzle

Abstract

The new type of gold cone filter cartridge has dual functions of increasing filter area and enhancing pulsed-jet cleaning, but the issue of patchy cleaning remains to be addressed. This study further enhances the pulsed-jet cleaning performance of cone filter cartridges by employing a porous diffusion nozzle. The temporal and spatial distributions of pulse jet velocity and pressure under the condition of porous nozzles were investigated through numerical modeling. The variation law of pressure on the side wall of the filter cartridge was analyzed. The influence of jet distance of porous nozzles on pulsed-jet pressure and pulsed-jet uniformity was experimentally investigated. Dust filtration and cleaning experiments were conducted, and the filtration pressure drop, dust emission concentration, and comprehensive filtration performance were compared. It was found that the airflow jetted by the porous diffusion nozzle is more divergent than that of the common round nozzle. This results in a larger entrainment of the jet stream, a milder collision of the jet stream with the cartridge cone, and a slower overall velocity reduction. More airflow is generated into the filter cartridge and accumulated; the accumulated static pressure covers a larger range of the upper section of the filter cartridge, with a longer duration of static pressure. In the online dust filtration and cleaning experiment, compared with the condition of the common round nozzle, the porous nozzle can reduce the residual pressure drop by 27.0%, increase the filtration cleaning interval by a factor of 3.80, reduce the average dust emission concentration by 45.2%, and increase the comprehensive performance index Q_F by 5.2%. The research conclusions can provide references for the design and optimization of industrial filter cartridge dust collectors.

1. Introduction

In mining, cement manufacturing, chemical production, metal processing, wood processing, and other industrial production processes, there are numerous mechanical crushing, grinding, cutting, combustion, or chemical reaction processes that readily generate a large amount of dust suspended in the air for a long time [1,2,3]. Industrial dust, characterized by its small particle size, large specific surface area, and tendency to carry heavy metals and toxic substances, can be easily inhaled. Long-term exposure to high concentrations of dust can lead to respiratory diseases, such as pneumoconiosis, chronic bronchitis, and a decrease in lung function [4,5,6]. Additionally, some dust is explosive, such as coal dust and aluminum powder, which may cause dust explosions under certain concentrations and conditions, leading to major safety accidents [7]. Industrial dust can also pollute the surrounding environment, reduce air quality, and affect ecosystem stability. Harmful substances in dust may also enter the soil and water through sedimentation, causing secondary pollution and endangering the broader environment and public health.

Among dust control technologies, cartridge filtration technology has become a core component of industrial dust removal systems due to its advantages of high efficiency, energy saving, and simple maintenance [8,9]. The filter cartridge dust collector uses high-performance filter material, which can effectively collect submicron particles, with a filtration efficiency exceeding 99%. With the increasingly strict environmental regulations, filter cartridge filtration technology is becoming more and more important in industrial dust control and is a crucial technical support for achieving green manufacturing and sustainable development.

When the filter cartridge is in pulse cleaning, the jet airflow is unevenly distributed on the side wall of the filter cartridge, especially in the upper section of the filter cartridge [10,11,12,13], leading to dust residue and increase in resistance, which affects dust removal efficiency and the stable operation of the equipment. In recent years, a new type of cone filter cartridge has been proposed and studied [14,15,16]. The cone filter cartridge not only improves the filtration area per unit space but also promotes the uniform distribution of pulse jets in the cartridge by using the diversion and diffusion effect of the cone, thereby achieving a better dust cleaning effect.

Given the persistent problem of patchy (incomplete) cleaning in cone filter cartridges, many researchers have conducted numerous studies. Zhang et al. [15] investigated the differences in filtration and dust cleaning characteristics between common filter cartridges and built-in conical filter cartridges through numerical modeling. The results show that the built-in conical filter cartridge increases the filtration area, reduces airflow resistance, and enhances the pressure distribution of pulse jet airflow in the vertical direction in the filter cartridge, significantly improving the dust cleaning effect of the filter cartridge and prolonging its service life. The author Qiu et al. [17] investigated the influence of inner cone height on the pulse jet performance of pleated gold cone filter cartridges under common round nozzles by establishing numerical modeling. The results showed that the negative pressure zone at the top of the filter cartridge was obvious when the height of the gold cone was less than 660 mm, and disappeared when the height of the gold cone was between 760 and 860 mm, proving that increasing the gold cone height can improve the uniformity of static pressure energy distribution in pleated cone filter cartridges.

The author Chen et al. [18] investigated the improvement effect of diffusion nozzles on the pulse jet cleaning performance of gold cone filter cartridges and modeled the jet performance by constructing a CFD (Computational Fluid Dynamics) numerical model. It was found that the improvement of pulsed-jet uniformity is mainly achieved by increasing the pressure in the upper section region of the filter cartridge. The combination of diffusion nozzles and gold cone filter cartridges has a synergistic effect on increasing the pulsed-jet pressure in the upper section of the filter cartridge, especially when the gold cone extends into the diffusion nozzle by 20–40 mm, the pulse jet intensity is 2.3 times that of the combination of common round nozzles and common filter cartridges. The author Su et al. [19] investigated the improvement of venturi nozzles on cone filter cartridge pulsed-jet cleaning performance by numerical modeling and experiment. It was found that the pulsed-jet intensity and uniformity of venturi nozzles increase gradually with the increase in jet distance from 150 mm to 550 mm. When the jet distance is 350 mm, the optimization effect of pulsed-jet performance is the best, and the pulsed-jet intensity and uniformity increase by 1.72 and 1.96 times. The author Li et al. [16] studied the improvement effect of annular-slit nozzles on the dust cleaning performance of guide inner cone filter cartridges and modeled the pulsed-jet performance using a CFD numerical model. It was found that the negative pressure in the upper section of the filter cartridge almost disappeared after using the slit nozzle, the pressure in the filter cartridge increased, and the pulsed-jet intensity increased; the jet intensity increased first and then decreased with the increase in jet distance, and pulsed-jet uniformity gradually improved. When the jet distance of the slit nozzle was 400 mm, the dust cleaning performance of the filter cartridge was the best, and the jet intensity increased by 44%.

The author Yang et al. [20] proposed a deflector-type diffusion nozzle to improve the pulse jet cleaning performance of a cone filter cartridge. Through numerical modeling, the effect of the diffusion nozzle on improving pulse jet velocity and pressure was studied, the influence of diffusion angle (θ) and diffusion distance (D) on the jet field of the conical filter core was discussed, and the jet intensity and dust removal uniformity were compared. Under the recommended diffusion nozzle parameters (θ = 70° and D = 40 mm), the intensity of the pulsed-jet is 1086 Pa, which is 5.4% higher than that under the condition of the common round nozzle, and the uniformity coefficient is 0.14, which is 60.0% better. For the upper section of the filter element, the pulse jet intensity is 1.39 times that of the common round nozzle.

This study proposes a novel approach by applying a porous nozzle to the pulse-jet cleaning of cone filter cartridges. The objective is to form a highly divergent jet that enhances air entrainment, resulting in a more uniform cleaning effect throughout the cartridge. It is thus expected to contribute to the development of superior pulse-jet techniques and improved jet performance for this specific application. An experimental system of a pulse jet of cone filter cartridges was constructed, and a numerical modeling model was established based on the physical system. The time-space distribution of pulse velocity and pressure under the condition of porous nozzles was investigated, and the pressure variation law of the filter cartridge side wall was examined. The influence of jet distance of porous nozzles on pulsed-jet pressure and pulsed-jet uniformity was analyzed. Dust filtration and dust cleaning experiments were conducted. The filtration pressure drops, dust emission concentration, and comprehensive filtration performance were compared. The research conclusions can provide references for the design and optimization of industrial filter cartridge dust collectors.

2. Experiment and Modeling

2.1. Experimental System

The schematic diagram of the dust collector experimental system is shown in Figure 1. The size of the main box body of the dust collector is 1225 × 750 × 1550 mm. A pleated cone filter cartridge is installed in the filtration chamber. The outer cylinder length of the filter cartridge is 660 mm, the outer diameter is 320 mm, the filter cartridge area is 7.89 m², the filter material thickness is 0.6 mm, and it is made of non-woven long staple cotton polyester material with a permeability of 80–100 m⁻²s⁻¹. The nozzle (a common round nozzle or porous nozzle) is installed directly above the filter cartridge. The porous nozzle features three concentric circles of diffusion holes, totaling 29 holes, each with a diameter of 5 mm. There are six innermost diffusion holes with a diffusion angle of 60°, nine middle diffusion holes with a diffusion angle of 75°, and 14 outer diffusion holes with a diffusion angle of 80°.

The basic operation process of the dust collector experimental system is as follows: under the action of the fan, the dusty airflow is sucked into the dust collector through the inlet, dust is collected on the outer surface of the filter cartridge in the filter chamber, the clean airflow passes through the filter cartridge to the cleaning chamber, and is discharged through the fan. When the filter cartridge needs to be cleaned, the electromagnetic pulse valve is triggered, high-pressure air from the air tank flows through the nozzle, and is injected into the filter cartridge. The dust on the outer surface of the filter cartridge is stripped by the reverse pulse jet airflow and deposited into the ash hopper.

The experimental system mainly consists of a dust feeder (LSC-6 type, range 0–450 g/min, Shanghai Chuanlingjidian Technology Co., Ltd., Shanghai, China), a compressed air tank (volume of 20 L), an electromagnetic pulse valve (DMF-Z-25 type, 1 inch, Shenchi Pneumatic Co., Ltd., Yueqing, China), a pulse controller (LC-PDC-ZC10D type, pulse duration range 0.01–0.99 s, Lingchuan Auto Technology Co., Ltd., Changzhou, China), a high frequency pressure acquisition subsystem (MYD-1530A Probe, pC/kPa, Mianyang Minyu Electronics Co., Ltd., Mianyang, China), a differential pressure recorder (DT-8920 type, range 0–6 kPa, Shenzhen Everest Machinery Industry Co., Ltd., Shenzhen, China), an online dust monitor (ZK-50 type, range 0–50 mg/m³, Zhongke Zhengqi (Beijing) Technology Co., Ltd., Beijing, China, and a thermal flowmeter (CKRSC-D150-F, range 0–5000 m³/h, Shanghai Chi Control Automation Instrument Co., Ltd., Shanghai, China). The ZK-50 type online dust monitor operates on the principle of alternating current electrostatic induction. Its measuring probe is vertically installed and aligned with the airflow direction within the exhaust duct of the dust collector. When dust particles collide with the probe, a trigger signal is generated, which is then converted into a dust concentration value. This signal is recorded directly by a computer system.

2.2. Numerical Modeling

2.2.1. Modeling

Due to the central symmetry of the filter cartridge dust collector, the system was simplified to a two-dimensional model to reduce computational complexity, and the area within the red dotted line box shown in Figure 1 was selected as the calculation domain of the numerical modeling. The simplified box size was 750 mm in diameter and 1400 mm in height. For the convenience of modeling and analysis, the nozzle structure was simplified in two dimensions. For the multi-orifice porous nozzle, equivalent area conversion was carried out to simplify the orifice, which not only retained the key structural features but also reduced the computational complexity. An unstructured grid was employed, with the number of elements ranging from 31,000 to 687,000.

In the calculation domain of the numerical modeling, the pressure inlet type boundary condition was applied at the nozzle inlet. The standard wall function with no-slip was applied at the side wall surface of the dust collector, and the pressure outlet type boundary condition was applied at the top surface of the dust collector. A symmetry condition was utilized, defined along the central axis of the filter cartridge.

2.2.2. Settings

In the numerical modeling, the Realizable k − ε turbulence model was used, and the combined pressure-velocity algorithm was chosen. The airflow in this region is considered ideal, unsteady, compressible, and isothermal turbulent. To explore the cleaning performance of cartridges caused by the reverse flow, no dust particle transport or coupling in the flow movement. Additionally, the influence of filter element deformation and dust cake is ignored.

The pressure-velocity coupling was achieved using the Coupled scheme. The Second Order Upwind scheme was employed for the momentum, turbulent kinetic energy (k), and turbulent dissipation rate (ε) equations to enhance the accuracy of the convective terms. The pressure interpolation was handled using the Second Order scheme. The Second Order Implicit scheme was used for time advancement to minimize numerical diffusion. A time step size (Δt) of 0.0005 s was determined through a time-step independence study to ensure the resolution of the rapid transient process during the pulse. The number of iterations per time step was set to 50 to ensure convergence, with scaled residual monitors for continuity and velocity set to 10⁻³.

The filter surface of the cartridge belongs to a porous medium. There are two modeling methods for the filter surface: porous zone and porous jump, with the former being selected here. There will be resistance when the air flows through this area, which will cause part of the energy loss. It is necessary to correct the energy equation and increase in viscosity loss and inertia loss. The energy loss equation is [13]:

$$S_i=\sum _{j=1}^3{D}_{ij}\mu v_j+\sum _{j=1}^3{C}_{ij}{\displaystyle \frac{1}{2}}\rho \left|v_j\right|v_j$$

(1)

where $S_i$ is the momentum source term, $D_{ij}$ and $C_{ij}$ are empirical matrices, and $\mu$ is the viscosity coefficient.

Considering the filter layer as a uniformly distributed porous material, the formula can be further simplified:

$$S_i={\displaystyle \frac{\mu }{\alpha }}{v}_i+C_2{\displaystyle \frac{1}{2}}\rho \left|v_j\right|v_j$$

(2)

where α is permeability and C₂ is the inertial resistance loss coefficient.

Based on experimental measurements, the pressure drop of the filter media (thickness 0.6 mm) was tested under a series of filtration air velocity conditions, and the values of 1/α and C₂ were obtained by fitting. The viscosity loss coefficient 1/α was 2.0 × 10¹¹ m⁻², and the inertial resistance loss coefficient was 675.53 m⁻¹ [20].

In the process of pulse dust cleaning, the pressure of the airflow will experience a process of first increasing and then decreasing when it is ejected from the nozzle. To accurately describe this change, a high-frequency pressure sensor was installed 10 mm below the nozzle during pulsed-jet cleaning to measure the change of inlet pressure in real-time. By processing and analyzing the measured data, a time-dependent pressure function was fitted. For a porous diffusion nozzle, the function of the pressure at the nozzle inlet P_e (kPa), as a function of time t (s) is given by Equation (3):

$$P_e=\left\{\begin{array}{l}0,\\ 29.06\ast sin(130.89\ast t-3.14)+29.06,\\ -204\ast t+65.55,\\ 21.72\ast cos\left(98.17\ast t-10.60\right)+21.72, \\ 0,\end{array}\begin{array}{l}t<0.0010\\ 0.0010\le t<0.1641\\ 0.1641\le t<0.1843\\ 0.1843\le t<0.2382\\ 0.2382\le t\end{array}\right.$$

(3)

2.2.3. Model Validation

To verify the grid, three different sizes were generated for the model of the dust removal system with porous nozzles, and the corresponding grid numbers were 31 k, 82 k, and 687 k. The modeled pressure at the middle observation point in the cartridge was compared with the experimental value from one measurement, as shown in Figure 2. There is no significant difference among the three size mesh models in pressure. After considering the continuity and computational efficiency of the cloud images displayed by numerical modeling, the mesh size of 82 k was selected for numerical modeling.

Comparing the modeling results with the experimental results, the fluctuation of experimental values is larger, and the pressure ending time is earlier than that of the modeling, which is due to the vibration of the filter wall under the influence of pulse airflow in the experiment, causing the fluctuation of the sensor. Additionally, the pulsed jet dissipates more rapidly in the experiment, leading to an earlier cessation. Generally, the modeled values are close to the experimental values, the variation trend is consistent, and the peak pressure is close; therefore, the modeling results are considered reliable.

2.3. Modeling Scheme Design

(1)

The time-space distribution of velocity and static pressure under the operating conditions of porous nozzles is investigated using numerical modeling. The improvement mechanism of the flow field with the porous nozzle is analyzed by comparing the effects of common round nozzles and porous diffusion nozzles.

(2)

The effect of jet distance on pulsed-jet pressure is studied by testing the variation of peak pressure of porous nozzles at jet distance JD = 150–650 mm under the condition of inner cone height HC = 560, 660, 760 mm. The effect of porous nozzles on pulsed-jet performance is analyzed by comparing them with common round hole nozzles (shortened to round nozzles) and deflector-type diffusion nozzles (shortened to deflector nozzles).

The performance of a pulse jet can be generally expressed by pulsed-jet intensity and pulsed-jet uniformity. The average value of positive peak pressure at each observation point is often used as the index of pulsed-jet intensity, and the coefficient of variation, C.V., of peak values is often used as the index of pulsed-jet uniformity [10].

(3)

An online filtration and dust cleaning experiment is carried out to investigate the improvement effect of porous nozzles on the operational performance of the filtration dust collector, and to analyze the changes in filtration pressure and dust concentration of the dust collection system before and after using porous nozzles.

The inlet dust concentration was set at 10 g/m³ and the filtration velocity was maintained at 1.67 cm/s. This experiment adopted a fixed-resistance cleaning mode, where cleaning started when the filtration pressure drop reached the maximum allowable filtration pressure drop of 500 Pa. The initial tank pressure for cleaning was 0.5 MPa, and the pulse duration was 0.15 s.

The dust removal performance of the filter cartridge dust collector needs to comprehensively consider the dust filtration efficiency and energy consumption of dust cleaning. Therefore, a comprehensive performance evaluation index Q_F is adopted [21], as shown in Equation (4):

$$Q_F={\displaystyle \frac{-\mathit{ln}(C_{out}/C_{in})}{∆P·Q+(P_0-P_1)·V_t·n/t}}$$

(4)

where $C_{in}$ and $C_{out}$ are dust emission concentrations at the inlet and outlet, $∆P$ is the average filtration pressure drop, $Q$ is the filtration air volume, $P_0$ and $P_1$ are the compressed air pressures before and after the pulse jet, $V_t$ is the air tank capacity, $n$ is jet times, and $t$ is pulse duration. The larger the Q_F value, the better the comprehensive filtration performance of the filter cartridge.

3. Results and Discussion

3.1. Temporal and Spatial Distribution of Jet Airflow Modeling

Under the conditions of jet distance JD = 250 mm, air tank pressure PT = 0.5 MPa, and pulse duration = 0.15 s, the space-time distribution of gas flow velocity and static pressure under the operating conditions of porous nozzles was investigated by numerical modeling, as shown in Figure 3.

After the pulsed jet is triggered, the airflow is ejected downward from the porous nozzle, and the jet flow entrains the surrounding air and flows into the interior of the filter cartridge. Due to entrainment, the jet flow swirls over the cartridge opening. Due to the obstruction and diversion of the gold cone in the middle of the filter element, the jet flow disperses into the filter element. Due to the blockage of the jet flow on the filter surface of the filter cartridge, the pressure energy of the jet flow entering the filter cartridge is weakened, and the converted static pressure energy is gradually accumulated. A spatial distribution of static pressure in the filter cartridge is formed in which the static pressure is higher in the lower section and smaller in the upper section. The accumulated static pressure reaches relative stability at t = 0.020 s and lasts until t = 0.160 s, that is, the pressure dissipates at the end of the jet.

Overall, the jet flow from the porous diffusion nozzle exhibits an entrainment effect and a static pressure distribution (decreasing from bottom to top) similar to those observed with the common round nozzle and the deflector-type diffusion nozzle [20].

However, compared with the round nozzle, the porous nozzle and deflector nozzle jet more divergent airflow, so that the jet airflow has a larger entrainment contact area with the surrounding air and produces a larger entrainment amount. Moreover, the collision between the jet and the gold cone is milder when the jet collides with the gold cone, and the overall velocity of the jet decreases slowly, thus avoiding the violent collision and energy dissipation between the jet and the gold cone under the condition of the round nozzle. This allows the porous nozzle to generate more airflow into the cartridge interior to accumulate, and the accumulated static pressure covers a larger area of the cartridge upper section, with a longer duration of static pressure.

Compared with the deflector nozzle, the diffusion effect of the porous nozzle is greater, mainly because there is a larger negative pressure area under the deflector nozzle, and even in the optimized flow-guiding cone angle case [20], the diffusion effect of the deflector nozzle is still inferior to that of the porous nozzle.

Further observe the pressure distribution on the filter cartridge wall, as shown in Figure 4, which is the pressure evolution along time of five monitoring points (the observation points O1–O5 are equally spaced in the vertical direction of the filter cartridge inner wall, see Figure 1 for specific positions).

The duration of positive pressure of the pulsed airflow in the cartridge with the porous nozzle is 0.235 s (t = 0.01–0.245 s), longer than that in the case of the round nozzle (0.16 s (t = 0.01–0.17 s)) [20]. Moreover, the static pressure increasing rate of the jet flow produced by the porous nozzle is slower, and the peak value is smaller. This is mainly due to the relatively larger airflow resistance of the porous nozzle. At the pulse jet start-up stage, the accumulation velocity of airflow in the filter cartridge is not as good as that of the round nozzle, and thereby the static pressure accumulation velocity is lower. Moreover, the higher airflow resistance of the porous nozzle causes more high-pressure air to remain in the jet pipe during pulsing, and the high-pressure airflow can still be temporarily accumulated between the pulse valve and the porous nozzle until the pulse valve is closed, resulting in the phenomenon that the jet time is longer than that of the round nozzle.

It can be further observed that the pressure difference between the observation points on the cartridge side wall is smaller for the porous nozzle than for the round nozzle. In the case of the round nozzle, the pressure difference between the observation points is large, especially the pressure peak value (374 Pa) and the pressure in the stable stage (~101–129 Pa) at O1 observation point are much lower than those at O5 observation point (the peak pressure is 1311 Pa, and the pressure in the stable stage is 1050–1277 Pa, which are about 3.5 and 9.9–10.4 times). The peak pressure and steady pressure at the O5 observation point in the porous nozzle case are about 2.8, 2.8–3.5 times those of the O1 observation point. This improvement can be attributed to the divergent effect of the porous nozzle on the jet flow and the further diffusion effect of the filter cartridge gold cone on the airflow, realizing the direct impact of the jet flow on the filter surface in the open area of the filter cartridge (especially the O1 observation point position). On the other hand, the porous nozzle produces more entrained air with a larger contact area with ambient air and enters the cartridge with a larger flow cross-section and a milder form, achieving better static pressure accumulation and covering more filter surface (especially to the O2 observation point).

3.2. Experimental Test of the Effect of Jet Distance

Numerical modeling demonstrates the feasibility of the porous nozzle for improving pulsed-jet performance in terms of flow field characteristics. Further experiments were conducted to measure the pulsed-jet pressure and investigate the factors influencing the improvement in pulsed-jet performance.

Figure 5 shows the change of peak pressure at three observation points (P1–P3, the points equally spaced in the vertical direction of the inner wall of the filter cartridge, see Figure 1) in the porous nozzle case under the condition of cone height (HC = 560, 660, and 760 mm) and jet distance (JD = 150–650 mm).

The pressure at each observation point increases first and then decreases in JD (except when HC = 560 mm, the upper section of the filter cartridge (P1 observation point) only decreases in JD), and the peak value of pulsed-jet pressure at each observation point is the largest when the jet distance is 200 mm. This is mainly due to the fact that, in the smaller jet distance range, with the increase in JD, the external airflow entrained by the airflow ejected from the nozzle increases, the airflow entering the interior of the filter cartridge increases, and the pulsed-jet intensity increases. An excessive jet distance will cause the jet flow to exceed the opening section of the filter cartridge, resulting in a decrease in the pulsed-jet pressure in the filter cartridge. Because the dispersion angle of the porous nozzle itself is large, the phenomenon that the jet flow exceeds the opening section of the filter cartridge begins to occur at a small jet distance. The peak value of pulsed-jet pressure at each observation point is maximum when the jet distance is 200 mm. However, for the round nozzle, the air dispersion angle is small, and the peak pressures increase in JD in the studied range [21].

The dispersion angle of the jet flow produced by the deflector nozzle is between that of the round nozzle and that of the porous nozzle, and the static pressure peak value produced by the jet flow and the gold cone varies with the jet distance more complicatedly [21]. Figure 6 shows how the pulsed-jet pressure increases for two types of diffusion nozzles under the influence of JD and HC. The porous nozzle produces a more significant pressure increase in the upper section of cartridges with larger cone heights (HC) at smaller jet distances (JD). For example, when JD is 150–450 mm and HC = 760 mm, the peak pressures at P1, P2, and P3 observation points are increased by 2.72, 2.23, and 1.74 times on average compared with those at the round nozzle; when HC = 660 mm, they are increased by 2.58, 2.24; and 1.74 times, respectively; when HC = 560 mm, they are increased by 2.09, 2.25; and 1.76 times. The pressure rise of the deflector nozzle in the upper section of the small HC cartridge is more obvious under the condition of small JD. For example, when the jet distance is 150–450 mm and HC = 560 mm, the peak pressures at P1, P2, and P3 observation points are increased by 3.75, 1.30, and 1.12 times, respectively, compared with those at the round nozzle, when HC = 660 mm, they are increased by 2.32, 1.87, and 1.39 times, respectively, when HC = 760 mm, they are increased by 2.05, 2.18, and 1.72 times.

The results indicate that both porous and deflector nozzles enhance airflow entrainment, allowing more airflow to enter the cartridge to accumulate higher and larger static pressure in the cartridge. Moreover, the jet air collides with the gold cone before entering the filter cartridge, which intensifies the diffusion of the airflow and makes the static pressure nearby, especially in the upper section of the filter cartridge, larger. However, the porous nozzle has a stronger diffusion effect on airflow than the deflector nozzle and can entrain more ambient air in a larger range. Thus forming a larger cross-section, the velocity distribution is more uniform, and mild airflow into the filter cartridge. The porous nozzle is, therefore, suitable for use at a smaller jet distance, where the airflow would otherwise exceed the cartridge opening. A smaller jet distance is also advantageous for the miniaturization of the pulse nozzle device.

To further characterize the pulsed-jet performance of the nozzle, the variation of pulsed-jet intensity and uniformity at the above jet distance was calculated, as shown in Figure 7.

The pulsed-jet intensity of the porous nozzle increases first and then decreases with JD increasing, and reaches a maximum at JD = 200 mm. The pulsed-jet intensity in HC = 560, 660, and 760 mm filter cartridges was 2481, 2790, and 2903 Pa, respectively, which were 2.17, 3.30, and 3.24 times higher than those in the case of the round nozzle. The pulsed-jet intensity of the porous nozzle applied to HC = 560, 660, and 760 mm filter cartridges is 2.72, 2.81, and 2.68 times that of the round nozzle [21]. The comparison shows that when the cone height is larger, the improvement effect of the porous nozzle is better.

In terms of pulsed-jet uniformity, when HC = 560 mm and JD < 500 mm, the pulsed-jet uniformity in the porous nozzle case is better than that in the round nozzle case. And when HC increases to 660 mm and 760 mm, the porous nozzle brings better jet uniformity than that of the round nozzle. This is mainly because the porous nozzle can effectively raise the pressure in the upper section of the filter cartridge, which is low under the condition of the round nozzle. The pulsed-jet uniformity in the porous nozzle case is generally better than that in the deflector nozzle case [21].

By comparing the flow field characteristics and pulsed-jet performance of the porous nozzle with that of the round nozzle, the main mechanism of the porous nozzle improving the pulse pulsed-jet performance of the gold cone filter cartridge is summarized as shown in Figure 8.

Similar to the deflector-type diffusion nozzle, the porous diffusion nozzle increases the entrainment in ambient air by increasing the diffusion angle range, allowing more airflow to accumulate in the cartridge than in the round nozzle condition, resulting in a higher and larger range of static pressure overall. At the same time, better diffusion also allows the jet flow to better cover the upper section of the filter cartridge, which is often a blind area under round nozzle conditions.

The diffusion effect of porous nozzles is stronger than that of deflector nozzles, and more ambient air can be entrained in a larger range. Thus forming a larger cross-section, the velocity distribution is more uniform, and mild airflow into the filter cartridge. Therefore, the violent collision between jet airflow and the gold cone is avoided, which occurs under the condition of the deflector nozzle. Especially when the gold cone is high (above the cartridge opening), this violent collision will cause the jet flow to dissipate too much energy before entering the cartridge, resulting in a reduction in overall pulsed-jet intensity. Of course, if the porous nozzle is applied to a cartridge with a lower cone height (cone height lower than cartridge height), jet air can significantly increase in static pressure in the upper section observation point area and overall static pressure.

In the smaller range of jet distance, with the increase in JD, the external airflow entrained by the airflow ejected from the porous nozzle increases, the airflow entering the interior of the filter cartridge increases, and the pulsed-jet intensity increases. An excessive jet distance will cause the jet flow to exceed the opening section of the filter cartridge, resulting in a decrease in the pulsed-jet pressure in the filter cartridge. Since the dispersion angle of the airflow jetted by the porous nozzle itself is larger, the corresponding preferred jet distance is smaller.

3.3. Filtration and Dust Cleaning Performance

An online filtration and dust cleaning experiment was further carried out to investigate the improvement effect of the porous nozzle on the dust collector operational performance, as shown in Figure 9.

In terms of filtration pressure drop, the residual pressure drops of the first five pulse jets were 347, 385, 392, 408, and 421 Pa, respectively, with an average of 391 Pa when using the round nozzle. When the porous nozzle is used, the first five residual pressure drops are 255, 267, 284, 300, and 310 Pa, respectively, with an average pressure drop of 284 Pa, which is 0.73 times that of the round nozzle. The average residual pressure drop for the deflector nozzle was 236 Pa, 0.60 times that of the round nozzle. The lower the residual pressure drop, the higher the cleaning degree of the filter cartridge. Therefore, the use of porous nozzles and deflector nozzles is beneficial to improving the cleaning performance of the filter cartridge, and the cleaning effect of the porous nozzle is found to be weaker than that of the deflector nozzle.

In terms of cleaning interval (the time between two pulse jets), the first five cleaning intervals for the round nozzle were 162, 103, 73, 64, and 48 s, respectively, with an average of 90 s. The average cleaning interval is 342 s when the porous nozzle is used, which is 3.8 times that of the round nozzle, and the average cleaning interval is 437 s when the deflector nozzle is used, which is 4.85 times that of the round nozzle. A longer cleaning interval means a decrease in pulse jet frequency. Therefore, the use of diffusion-type nozzles can save energy and prolong the service life of filter cartridges, but the corresponding cleaning interval of porous nozzles is shorter than that of deflector nozzles.

The dust emission concentration in the initial stage of filtration is relatively stable with time and remains below 1.0 mg/m³. The dust emission during the pulse-jet cleaning phase is a key factor limiting the overall dust removal efficiency. For the round nozzle, the peak dust emission concentrations corresponding to the first five times of dust cleaning are 2.62, 2.10, 2.11, 1.85, and 1.63 mg/m³, respectively, with an average of 2.06 mg/m³. The average peak dust emission concentration is 3.67 mg/m³ when using the porous nozzle and 5.26 mg/m³ when using the deflector nozzle. It can be seen that the dust emission concentration is high in the case of using the porous nozzle and the deflector nozzle, which is due to the improvement of the pulse pulsed-jet intensity of the two nozzles. More dust is separated from the filter cartridge during dust cleaning, resulting in higher dust concentration at the filtration upstream. Additionally, the filter material is subjected to greater vibration, resulting in more open pores. These factors collectively contribute to higher instantaneous peak dust emission concentrations. It is obvious that the control of dust emission concentration by the deflector nozzle is not as good as that by the porous nozzle during pulse jetting.

Compared with the average dust emission concentration, the dust emission concentrations corresponding to the common round nozzle, the porous nozzle, and the deflector nozzle are 0.62, 0.34, and 0.39 mg/m³, respectively, in the first 1200 s of operation. Compared with the round nozzle, the dust emission concentration of the porous nozzle and deflector nozzle decreased by 45.16% and 37.10%. Although both diffusion nozzles increase in the instantaneous dust emission concentration, they reduce the cleaning frequency, and the duration of the instantaneous concentration is very short compared with a cleaning cycle, so the total dust emission concentration is low under the condition of the diffusion nozzle. Among them, the porous nozzle is more effective at controlling the overall dust emission concentration than the deflector nozzle.

Observing the dust residue on the outer surface of the filter cartridge after the pulse dust cleaning experiment (Figure 10), it is found that the porous nozzle case has less residual dust than the ordinary round nozzle case. And the residual dust is more uniform along the length of the filter cartridge in the porous nozzle case. This phenomenon also proves that the porous nozzle improves the pulsed-jet intensity of the cone filter cartridge and improves the pulsed-jet uniformity, and the pulsed-jet uniformity is better than that of the deflector nozzle.

Furthermore, the comprehensive performance index Q_F value is calculated, see Figure 10b for comparison. The Q_F value is 0.203 in the porous nozzle case, which is 5.2% higher than that of the round nozzle. It is 0.212 in the deflector nozzle case, which is 9.8% higher than that of the round nozzle. The comprehensive operational performance in the porous nozzle case is found to be between those of the round nozzle and the deflector nozzle.

The Q_F index represents dust collection efficiency per unit time and unit energy consumption. It serves as a convenient metric for quantifying filtration performance throughout both filtration and dust cleaning cycles. While the Q_F value is used as one of the reference indicators, specific scenarios and requirements must be taken into account. For instance, the emphasis may be placed on further reducing dust emission concentrations, or alternatively, on energy saving and consumption reduction when dust emission levels already comply with standards.

Based on the action mechanism and test results of the porous nozzle and the deflector nozzle, the analysis suggests that the porous nozzle is recommended for industrial sites where the dust emission concentration is difficult to meet the standard or where lower emissions are required. The porous nozzle is designed with moderate pulsed-jet intensity and good dust cleaning uniformity, which effectively reduces the dust emission concentration at the moment of dust cleaning and can meet more stringent environmental protection standards.

For places where the dust removal efficiency has been kept good and is expected to achieve energy saving and consumption reduction, the deflector nozzle is recommended. The deflector nozzle can keep a high dust removal efficiency, and at the same time, the dust cleaning times required for a long dust cleaning interval are few; the energy loss of pulse jet is reduced, thereby realizing the goal of energy saving, contributing to reducing the operation cost and improving the overall economic benefit.

4. Conclusions

This study investigated the improvement in pulsed-jet performance of cone filter cartridges achieved by using a porous nozzle. The temporal and spatial distributions of pulse jet velocity and pressure are studied. The influence of porous nozzle jet distance on pulse pulsed-jet pressure and pulsed-jet uniformity is analyzed. The filtration pressure drops, dust emission concentration, and comprehensive filtration performance are compared by dust filtration and filter cleaning experiments. The main conclusions are as follows:

(1)

Compared with the common round nozzle, the airflow injected by the porous diffusion nozzle is more divergent, and the divergent effect is stronger than that of the deflector nozzle. This results in a larger entrainment of the jet stream, a milder collision of the jet stream with the gold cone, and a slower overall velocity reduction. More airflow is generated into the filter cartridge and accumulated, and the accumulated static pressure covers a larger range of the upper section of the filter cartridge and persists for a longer duration.

(2)

Under the condition of the porous nozzle, the peak pressure at each observation point and the overall pulsed-jet intensity in the cone filter cartridge increase first and then decrease with the increase in JD. The corresponding pulsed-jet intensity in HC = 560, 660, and 760 mm filter cartridges is 2.17, 3.30, and 3.24 times that in the round nozzle. At larger cone heights (>660 mm), the pulsed-jet intensity of the porous nozzle is higher than that of the deflector nozzle. The improvement effect of the porous nozzle on pulsed-jet uniformity is generally superior to that of the deflector nozzle.

(3)

In the online filtration and dust cleaning experiment, the residual pressure drop in the porous nozzle case is reduced by 27.0%, the filtration cleaning interval is increased by a factor of 3.80, the average dust emission concentration is reduced by 45.2%, and the comprehensive performance index Q_F is increased by 5.2% compared with that of the round nozzle. The improvement effect of the porous nozzle on comprehensive performance is weaker than that of the deflector nozzle, but the control of dust emission is better. It is suggested that porous nozzles should be used in places where dust emission concentration is difficult to meet standards or where emission requirements are stricter. For places where the dust removal efficiency is relatively high and more attention is paid to energy saving and consumption reduction, the deflector nozzle should be the preferred option.