CFD simulation of cyclone separators to reduce air pollution

Development of a multi-stage fog droplet screening system based on the virtual impact principle

Accurately measuring fog droplet spectra is essential for understanding fog's formation, dissipation, and composition, which makes a challenge to the performance of droplet sampling and measurement systems. Standard particles such as glass beads are widely used to characterize their performance. However, the disparities between glass beads and fog droplets, including refractivity, size distribution, and composition, may lead to calibration errors. In this context, we developed a three-stage fog droplet screening system based on the virtual impact principle. We determined the Stokes number and the diameter of the acceleration nozzle through theoretical analysis. Subsequently, we utilized the computational fluid dynamics software Fluent to explore the influence of key system parameters on screening efficiency, including the diameter of the collection nozzle (D1) and the distance between the acceleration nozzle and the collection nozzle (S). The simulation results indicated that the screening efficiency improved with S. The best performance was achieved when D1 = 1.35 D0 and S = 1.90 D0 (where D0 represents the diameter of the acceleration nozzle), resulting in an average screening efficiency of 75.4%. Finally, we conducted experiments to validate the effectiveness of the screening system. The screening efficiency of each outlet was estimated at 42.2%, 66.1%, 84.0%, and 95.3%, with differences of 2.0%, 3.3%, 4.1%, and 4.7% compared to the simulations. The average screening efficiency was 71.9%, with a deviation of 3.5% from the simulation. These findings demonstrated that the screening system could provide an alternative technical apparatus for characterizing droplet sampling and measurement systems.

Sludge low-temperature drying with mainly non-phase change in mere seconds based on particle high-speed self-rotation in cyclone

Improper sludge treatment will cause serious environmental problems, and sludge drying is the key to effective treatment. Almost all the existing sludge drying technologies use heating to overcome the great latent heat of moisture vaporization, which leads to high drying energy consumption. In this study, based on the particle high-speed self-rotation in the cyclone and micro-interface oscillations, the cyclone self-rotation drying (CSRD) technology was developed. It can realize drying of the dewatered landfill sludge (DLS) and the urban sewage dewatered sludge (UDSS) with mainly non-phase change. The obtained results reveal that at low carrier gas temperatures (< 100 °C) and very short residence time (< 15 s), the moisture content of the DLS decreased from 53% to 6.85%, and that of the UDSS decreased from 67% to 18.92%. Through calculation, the proportions of moisture non-phase change removal during the CSRD process touched 68.94% and 63.39%, respectively. Based on the experimental studies, we proposed an enlarged industrial application program (50 t/d) for the UDSS drying by employing the CSRD technology. The operating cost was 159.69 CNY/t H₂O, showing prominent advantages. This study can provide guidelines for the practical application of CSRD technology and fill the scientific gap in the field of moisture non-phase change separation for sludge drying.

Removal of coke powders in coking wastewater using a hydrocyclone optimized by n-value

Coke powders in the coking wastewater generated by petroleum refining industry needs to be removed to achieve water reuse for lack of water resources. This study developed a decoking hydrocyclone in the closed coking wastewater circulation treatment system to remove coke powders, which was highly efficient and environmentally friendly. Computational Fluid Dynamics (CFD) method was carried out to study the tangential velocity distribution index n-value to guide design of decoking hydrocyclone and experiment was conducted to verify the coke powders removal effect. It was found that the increase of n-value is conducive to the improvement of coke powders separation efficiency. A decoking hydrocyclone with a cone angle of 15° and an inlet size of 4 × 6 mm is the optimum hydrocyclone and the recovery efficiency of coke powders is stable at more than 90%. It is the first time for hydrocyclone successfully applied to the removal of coke powders in coking wastewater in the decoking process of petroleum refining industry, in which the separation efficiency of coke powders is considerably improved.

Numerical and Experimental Analysis of Flow Pattern, Pressure Drop and Collection Efficiency in a Cyclone with a Square Inlet and Different Dimensions of a Vortex Finder

The paper presents results of numerical simulations and experimental investigations of a cyclone with a square inlet and different dimensions of a vortex finder. Investigations were conducted for five, different cyclone models. The main goal of the research was to determine the influence of cyclone vortex finder geometry (diameter, length) on key parameters for a cyclone operation, such as: gas flow pattern, gas velocity and pressure distribution, pressure drop and collection efficiency. The analysis of flow pattern inside the cyclone was carried out with the use of CFD (Computational Fluid Dynamics) simulations, verified using CTA (Constant Temperature Anemometry). Similarly, pressure distributions, pressure drop and collection efficiency for cyclones were determined numerically and with measurements. The study demonstrated that dimensions of the vortex finder have a significant effect on a cyclone performance. The numerical visualisations of flow showed some unfavorable and beneficial effects and phenomena that may occur in cyclones. Moreover, the smallest pressure drop (305 Pa) was predicted for the cyclone with a maximum diameter of the vortex finder (De = 0.105 m), the largest (358 Pa) when this diameter was the smallest (De = 0.075 m). The tests did not show any significant influence of the vortex finder length on the pressure drop in a cyclone. A different tendency was observed in the case of collection efficiency. The maximum value of this parameter (89.5%) was predicted for the cyclone with the shortest vortex finder (s = 0.060 m), minimum (85.5%) when the vortex finder was the longest (s = 0.220 m). In this case, however, tests did not show a significant impact of an outlet diameter on the collection efficiency. Results and conclusions presented in this paper can be practically used for design optimization of cyclone separators, and also to select their operating parameters.

Numerical Simulation of Performance of an Air–Water Separator with Corrugated Plates for Marine Diesel Engines

For low speed diesel engines under severe ocean conditions, high efficiency air–water separators must be equipped to separate excess moisture contained in the air and reduce the corrosion. Optimal design of air–water separator is an indispensable part in the development of marine main engines. In view of the complex gas–liquid two-phase turbulent motion within an air–water separator with corrugated plates, a mathematical model is established and numerical simulations of flow field, droplet trajectory and secondary transport are realized. The separation efficiency and pressure drop of the air–water separator under different structure parameters and different working conditions are studied. The results show that reducing the spacing of corrugated plate is helpful to improve the separation efficiency. The bending and hydrophobic hooks of the corrugated channel are important to improve the separation efficiency. The separation of droplet is mainly concentrated on the first two stages of the air–water separator, and the separation efficiency at the third stage is significantly reduced. Research results will further support corrugated plate theory, experimental research and optimization design of similar separators.

Design of Cyclone Separator Critical Diameter Model Based on Machine Learning and CFD

In this paper, the characteristics of the cyclone separator was analyzed from the Lagrangian perspective for designing the important dependent variables. The neural network network model was developed for predicting the separation performance parameter. Further, the predictive performances were compared between the traditional surrogate model and the developed neural network model. In order to design the important parameters of the cyclone separator based on the particle separation theory, the force acting until the particles are separated was calculated using the Lagrangian-based computational fluid dynamics (CFD) methodology. As a result, it was proved that the centrifugal force and drag acting on the critical diameter having a separation efficiency of 50% were similar, and the particle separation phenomenon in the cyclone occurred from the critical diameter, and it was set as an important dependent variable. For developing a critical diameter prediction model based on machine learning and multiple regression methods, unsteady-Reynolds averaged Navier-Stokes analyzes according to shape dimensions were performed. The input design variables for predicting the critical diameter were selected as four geometry parameters that affect the turbulent flow inside the cyclone. As a result of comparing the model prediction performances, the machine learning (ML) model, which takes into account the critical diameter and the nonlinear relationship of cyclone design variables, showed a 32.5% improvement in R-square compared to multi linear regression (MLR). The proposed techniques have proven to be fast and practical tools for cyclone design.

Numerical Study on Separation Performance of Cyclone Flue Used in Grate Waste Incinerator

The traditional treatment of waste incineration flue gas is mostly carried out in low temperatures, but there are some problems such as corrosion of the heating surface at high and low temperatures, re-synthesis of dioxins, and low efficiency. Therefore, it is necessary to remove the pollutants at high temperatures. For the grate waste incinerator, this study proposes an adiabatic cyclone flue arranged at the exit of the first-stage furnace of the grate waste incinerator to pre-remove the fly ash at high temperatures, so as to alleviate the abrasion and corrosion of the tail heating surface. In this paper, computational fluid dynamics (CFD) method is applied to study the performance of a cyclone flue under different structural parameters, and the comprehensive performance of the cyclone flue is evaluated by the technique for order preference by similarity to an ideal solution (TOPSIS) method. The results show that particle separation efficiency increases at first and then decreases with the increase of the vortex finder length, the vortex finder diameter, and the distance between vortex finder and gas outlet tube, while it decreases with the increase of the gas outlet tube diameter. The pressure drop increases with the increase of the vortex finder length, and the vortex finder diameter, while decreases with the increase of the distance between the vortex finder, the gas outlet tube, and the gas outlet tube diameter. In the scope of this study, when h1/a = 1.1, D1/A = 0.33, h2/A = 1.5, and D2/A = 0.50, the comprehensive performance of the cyclone flue is much better.

Study of Indoor Ventilation Based on Large-Scale [DNS by a Domain Decomposition Method

This paper presents a large-scale Domain Decomposition Method (DDM) based Direct Numerical Simulation (DNS) for predicting the behavior of indoor airflow, where the aim is to design a comfortable and efficient indoor air environment of modern buildings. An analogy of the single-phase convection problems is applied, and the pressure stabilized domain decomposition method is used to symmetrize the linear systems of Navier-Stokes equations and the convection-diffusion equation. Furthermore, a balancing preconditioned conjugate gradient method is utilized to deal with the interface problem caused by domain decomposition. The entire simulation model is validated by comparing the numerical results with that of recognized experimental and numerical data from previous literature. The transient behavior of indoor airflow and its complexity in the ventilated room are discussed; the velocity and vortex distribution of airflow are investigated, and its possible influence on particle accumulation is classified.

Effect of Inlet Air Volumetric Flow Rate on the Performance of a Two-Stage Cyclone Separator

It is very important to improve the cyclone separator separation efficiency for fine particles. On the basis of the Reynolds stress model (RSM), a new two-stage cyclone separation device is modeled and the model is simulated under six kinds of air volumetric flow rate conditions. The two-stage cyclone separator was then tested in the laboratory and the experimental data were compared with the simulation data. The results show that the RSM model can predict the performance of the two-stage cyclone separation device with high accuracy. Increasing of the air volumetric flow rate can not only improve the separation efficiency of the two-stage cyclone separator, but also effectively change the classification range. Because of the centrifugal force, even if the pressure drop is low, the 1st-cyclone can completely separate particles above 5.0 μm. When the air volumetric flow rate is more than 290 m³/h, the 2nd-cyclone can effectively separate the particles below 2.0 μm. The study also confirmed the nonlinear relationship between the pressure drop and the cut-off particle size and the maximum particle size. When the pressure drop exceeds a certain value, there is no longer any effect on the cut-off particle size and the maximum particle size.