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Review

The Secondary Flows in a Cyclone Separator: A Review

by
Chenwen Wang
*,
Yongshan Ma
and
Wenxuan Sui
Key Laboratory of Resource Environment and Sustainable Development of Oasis, College of Geography and Environmental Science, Northwest Normal University, Lanzhou 730000, China
*
Author to whom correspondence should be addressed.
Processes 2023, 11(10), 2935; https://doi.org/10.3390/pr11102935
Submission received: 1 September 2023 / Revised: 24 September 2023 / Accepted: 5 October 2023 / Published: 9 October 2023
(This article belongs to the Section Separation Processes)
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Abstract

:
A cyclone separator holds significant importance as the primary gas–solid separation apparatus in the industrial sector. Cyclone separators operate based on a fundamental principle, primarily harnessing the centrifugal force produced by the rotation of air in order to segregate solid particles from the gas stream and then collect them. In addition to the main vortex in the flow field, there are a number of secondary flows, which significantly impact the aggregation of fine particles and contribute to the heightened energy consumption. This paper provides a summary of the three secondary flows in a cyclone separator. These include the recirculation flow in the annular space, which is greatly influenced by the inlet particle concentration. Additionally, the short-circuit flow occurs beneath the vortex finder as a result of the collision between the incoming flow and the rotating flow. Furthermore, the eccentric circumfluence is defined as the deviation of the rotation center caused by the interaction between the upward and downward flows near the discharge. This paper aims to establish a theoretical framework to investigate the flow pattern tracking and the mitigation of secondary flows in order to enhance the operational efficiency of cyclone separators.

1. Introduction

In the field of industrial gas (liquid) and solid separation, cyclone separators are one of the most important dust removal instruments because they have a simple working principle, lower cost, fewer preconditions, and the ability to adapt to a wide range of working conditions [1,2,3]. Cyclone separators have broad application prospects [4,5,6,7,8]. First, cyclones can be used for gas–solid separation in almost all industrial fields, such as in ship unloading units, power stations, fluidized beds, and food processing, and in crushing, separation, grinding, and calcination in the mineral and chemical industries. Their separation performance directly affects the overall design of circulating fluidized bed boilers (CFBs), the system arrangement, and the performance of subsequent boiler operations. Second, cyclones can be used as gas–liquid separators for mist removal. Finally, a cyclone separator can also classify and collect solid dust particles according to their physical properties (such as the mass, density, or shape), and thus, it is a key instrument to reduce catalyst unit consumption and improve the economic efficiency of the catalytic plant in the petrochemical industry.
Although the theses of researchers such as Newton, Navier, and Stokes have laid a foundation for the theoretical study of cyclones, the study of cyclones is still in the developmental stage [9]. Thus far, the research on cyclone separators has gone through three stages [10,11].

1.1. Perceptual Stage

The first stage extended from the 1880s to the 1930s and was at the stage of gaining a perceptual understanding of cyclones. In this stage, a cyclone separator was primarily viewed as a dust collection machine that had the potential to take the place of the settling chamber; however, researchers did not devote a lot of effort into investigating the mechanisms at play. The fact that the cyclone separator had a particle size distribution that was primarily concentrated in the range of 40–60 μm greatly restricted the application scope for which it could be used.

1.2. Theoretical Models

The second phase of cyclone separator research extended from the 1920s to the early 1960s, and the main trend was a significant increase in scientific experiments and theoretical models. As the applicability of cyclones expanded, researchers gradually recognized the existence of very complex vortices and convergent flows inside the cyclone [12]. First, Ter Linden [13] experimentally investigated the three-dimensional flow field in a cyclone separator using a spherical Pitot tube and found that the tangential, axial, and radial velocity distributions of the flow field in the separator exhibited certain regularities and that there were large differences between the three. However, the limitations of the experimental equipment at that time made it difficult to accurately measure and record the internal flow field of the cyclone separator. Therefore, to better analyze the relationship between the cyclone separator flow field and the separation efficiency, researchers proposed two-phase separation theories based on these early experimental data, such as the time-of-flight model, equilibrium orbit model, and boundary layer model.

1.2.1. Time-of-Flight Model

Time-of-flight models, in analogy to gravitational settling, propose that individual particles are only considered to be collected when they are present in the cyclone long enough for the inertia to touch the wall [9,14]. Leith and Licht [15] calculated how far the particles must travel to be considered to be collected by assuming the precondition that turbulence leads to a homogeneous particle concentration contact with the radial distance of the vessel wall, and they were able to estimate the separation efficiency of several cyclone configurations with considerable accuracy. Clift et al. [16] achieved model improvement by changing the Leith and Licht model prediction equations and the residence time assumptions on which they rely. Dietz [17] divided the cyclone into several sections for separate rotating circle model fitting calculations to improve the accuracy of the calculations. Mothes and Löffler [18] and Li and Wang [19] improved the Dietz model by adding diffusion (turbulent dispersion) and particle re-entrainment parameters to calculate the separation efficiencies of a variety of cyclones. Zhao et al. [20] removed the assumption of a homogeneous state of particle concentration in the turbulent flow and re-examined the effect of the parameters of the radial particle concentration gradient on the rotating circle theory.

1.2.2. Equilibrium Orbit Model

The equilibrium orbit model, also known as the equilibrium force collection model, assumes that the cutting efficiency reaches fifty percent when the fluid traction (drag) acting on the surface of the particle volume is sufficient to keep the particles in a circular orbit. If the gravitational attraction is high, the particles suspended in the gas stream will settle and thus be collected; if the drag force caused by the rising gas stream is higher, the particles will escape from the overflow tube and will not be collected [21,22,23]. Later, Stairmand [24] modified the equilibrium force model using wall friction and fluid viscosity parameters, and Barth [25] used the particle settling velocity in still air to determine the fluid traction and considered the separation efficiency to be a function of the ratio of the particle settling velocity to the final velocity. In addition, Dirgo and Leith [26], Iozia and Leith [27], and Muschelknautz and Trefz [28] all suggested improvements to the equilibrium orbit theory, including fitting more experimental data and parameters to supplement and extend Barth’s theory, and they summarized the formulation and calculation of the cyclone pressure drop.

1.2.3. Boundary Layer Model

Leith and Dirgo [15,29] proposed that, assuming that the particle dispersion phase is homogeneous in any cross section inside the cyclone, the particles that are affected by the centrifugal forces are collected when they enter the laminar motion of the boundary layer of the separator wall. However, although the boundary layer model takes into account the effect of turbulent diffusion on small particles, it ignores the effect of the velocity changes in the radial direction of the airflow on the particle motion, and the particles are considered to have been collected by the cyclone separator once they enter the boundary layer of the vessel wall, which is obviously inconsistent with the experimental data.
Although these semi-empirical theoretical models have helped researchers to design trends for new cyclones, these early models are too crude, providing, at best, only reasonable predictions of the pressure drop and particle cut size, and they are too narrow in terms of their application and operating parameters [9]. First, these theories can only explain the modeling variables, and most parameters are simplified or neglected. However, the internal flow field of a cyclone separator is very complex, and the performance is significantly influenced by multiple parameters. For example, at a certain inlet particle concentration (up to 500 g/m3), the vortex and turbulence inside the cyclone decrease, and the pressure loss decreases, but the separation efficiency is improved, while higher inlet particle concentrations have the opposite effect [30]. In addition, the geometry of the vortex finder [31,32], which affects the cyclone’s performance, is not included in most models. Second, many models predict an increase in the separation efficiency with the increasing inlet gas velocity. However, secondary flows such as short-circuit flow can, in fact, affect the increase in the cyclone separation efficiency, and it is even possible that more small particles overflow out of the cyclone due to the short-circuit flow when the high-velocity gas stream enters [33,34], reducing the separation efficiency.

1.3. Ultrafine Particles

The third phase of cyclone research (from the 1960s to the present) was mainly driven by the implementation of emission regulations and policies [35]. The U.S. Public Health Service published the Compilation of Air Pollutant Emission Factors in 1968 (EPA, 2010). Subsequently, the U.S. Congress passed the Clean Air Act in 1970, which authorized the national ambient air quality standards (NAAQSs) and state implementation plans (SIPs). The need for cyclone capture and separation directly rose to ultrafine particles. Ramachandran et al. [36] used 98 published cyclone datasets and derived a geometry-based cyclone pressure drop model. Their statistical model predicts the pressure drop of the cyclone more accurately than the previous theoretical model. Elsayed and Lacor [37] derived the seven optimal geometrical parameters of the cyclone separator using the response surface method and the pressure drop of the cyclone separator using the pressure drop equation of Muschelknautz and Kambrock [38]. Qian and Zhang [39] also derived the natural vortex length of the flow field inside the cyclone separator using the response surface method. Li et al. [40,41,42] conducted a study in which they formulated mechanics-based models that incorporated the interaction of the fluid volume and discrete elements. Furthermore, a fluid–solid interaction model was created in order to examine the mechanism of multiphase free sink vortex vibration. In addition, as the application of computational fluid dynamics (CFD) has continued to be studied, our understanding of the internal flow field of the cyclone separator has been improved. For example, the airflow variation in the cyclone barrel, which is ignored by the semi-empirical model, can now be observed via CFD [43,44].
From the above studies, it is known that first, during the movement of the cyclonic flow inside the cyclone, there are mainly external vortices with downward gas movements and internal vortices with upward gas movements, as well as some secondary flows that interfere with the performance of the cyclone. These secondary flows mainly include the recirculation flow in the annular space, the short-circuit flow near the lower end of the vortex finder, and the eccentric circumfluence near the discharge port [45]. Second, the secondary flows not only lead to the disturbance of the main vortex inside the cyclone separator and increase the internal resistance loss, but also lead to the separated particles entering the inner vortex again and escaping from the overflow pipe, which seriously affects the gas–solid separation process of the cyclone separator. Thus, an in-depth study of the internal flow field characteristics of the cyclone separator, especially an exploration of the short-circuit flow, which affects the performance of the cyclone separator, is beneficial to the advancement of the cyclone separator to capture and separate superfine dust particles. The present research emphasis on the cyclone separator primarily revolves around investigating the optimized configuration, while there is a scarcity of comprehensive compilations and evaluations pertaining to the secondary flows within the cyclone separator.. Therefore, this paper provides a review of the secondary flow in the cyclone from two aspects: (1) the flow characteristics and formation mechanism of the secondary flow and (2) the effect of the configuration improvement on the secondary flow. The results of this study provide a theoretical basis for improvements in cyclone performance and engineering.

2. Geometric Structure

The basic shape and function of cyclone separators have not varied greatly over the past hundred years. From the earliest pot-bellied structure, there was a rapid evolution to more efficient designs with larger inlet volumes and simpler configurations. Currently, the most common type of cyclone used in academic research and industrial applications is the tangential single-inlet countercurrent cyclone, such as the Lapple cyclone shown in Figure 1.The geometric dimensions of the cyclone are typically characterized by a ratio that incorporates the cylinder’s diameter., e.g., xD (x is a numerical constant). These geometries include the following eight parameters: the cross-section width of inlet a; cross-section height of inlet b; vortex finder diameter, De; vortex finder depth, S; cylinder diameter, D; overall height, H; cylinder height, h; cone height, Hc; and discharge diameter, B. The design of these eight geometrical dimensions can directly affect the airflow and separation process of a cyclone separator [9].

2.1. Principle

The operational principle of a cyclone separator is characterized by its relative simplicity. As depicted in Figure 2, the tangential inlet allows for the air flow to be directed tangentially into the cyclone, resulting in the generation of a double swirling flow. In the outer region of the swirl, the gas exhibits a downward flow, commonly referred to as the “outer vortex”, whereas in the central region, an upward flow is observed, known as the “inner vortex”. The location along the axial direction where the outer vortex transitions completely into the inner vortex is commonly referred to as the “end” or “tail” of the vortex. The vertical distance between the bottom of the vortex finder and this “tail” is specified as the “nature vortex length” [46]. In the course of this action, the particles inside the dusty gas stream will be driven towards the wall of the apparatus as a result of centrifugal force. Following this, in tandem with the proximate airflow in the vicinity of the wall, they will be transported downhill into the dust discharge port of the cyclone separator.
The particles within a cyclone separator experience several forces, primarily including the mass force, fluid drag force, centrifugal force, buoyancy force, pressure gradient force, Basset force, and Saffman force [11]. However, the primary factors influencing the settling and collection of dust are the centrifugal force and the fluid drag force. The efficiency of the gas–solid separation process in a cyclone separator is affected by the relative slip between the airflow and the particles. This suggests that the drag force will exert a significant influence on the separation of particles, as evidenced by a numerical simulation [48]. In cases where the centrifugal force acting on the dust particles surpasses the drag force, the particles are primarily propelled to migrate towards the wall of the cyclone and are subsequently collected (Figure 3A). However, the impact of the drag force becomes more pronounced as the particles shrink in size below the threshold of the cut size. The movement of particles is entirely contingent upon the movements of the turbulence and gas (Figure 3B,C).

2.2. Parameters

Temperature. The dynamic viscosity of a gas stream within a cyclone is predominantly influenced by temperature. When the temperature rises, the viscosity of the gas stream also increases. Consequently, this elevated viscosity leads to an increase in traction, ultimately resulting in a reduction in the separation efficiency [49,50]. Brar and Wasilewski [51] conducted an analysis on the influence of elevated temperatures (ranging from 300 K to 1000 K) on the pressure drop and separation efficiency of cyclones. Their findings indicate a notable decrease in both the pressure drop and separation efficiency. Additionally, the researchers propose innovative cyclone designs that surpass the performance of the conventional square cyclone model, achieving an overall separation efficiency improvement of up to 60%. These novel designs exhibit enhanced efficiency even when subjected to higher temperature conditions, making them promising alternatives.
Velocity. The velocity of the inlet flow can exert a substantial influence on the process of cyclone separation. In the realm of industrial design, cyclone separators catering to high concentration feeds typically exhibit intake velocities ranging from 15 m/s to 18 m/s. According to the study conducted by Gimbun et al. [4], it was observed that a decrease in cyclone input air velocities leads to a significant increase in the pressure drop and a subsequent decrease in efficiency. Furthermore, it has been observed that cyclone separators have a maximum efficiency inlet airflow velocity threshold. It has been found that the separation effectiveness of cyclone separators decreases when the inlet air velocity regularly exceeds this threshold velocity [52].
Researchers have been interested in studying the geometrical parameters of cyclone separators.
Inlet. Baker and Hughs [53] found that the inlet shape of a cyclone separator has an effect on the separation efficiency, which increases as the inlet becomes shorter and wider. Misiulia et al. [54] studied the effect of the inclination of the inlet angle on the performance of cyclone separators, and they found that the highest separation efficiency was achieved when the inlet angle was inclined upward by 30°. Lim et al. [55] conducted an experimental comparison of two single-inlet cyclones with different inlet sizes and analyzed the separation efficiency of cyclone separators with two different inlet sizes. The effect of the inclination angle of the inlet on the performance of the cyclone separator was found to be the highest at an inlet inclination angle of 30° upward. Lim et al. [55] carried out an experimental comparison of two single-inlet cyclone separators with different inlet sizes to analyze the extent of the effect of the inlet size on the separation performance. Other researchers [47] changed the inlet structure of the cyclone separator and found that the internal flow field of a double-inlet cyclone separator was more stable than that of a single-inlet cyclone separator.
Vortex finder. Zhu et al. [56] conducted a series of experiments to investigate the impact of the insertion depth of the vortex finder on the separation efficiency of the cyclone separator. Their findings revealed that as the h/D and S/D ratios ranged from 0.75 to 4.5 and from 0.5 to 1.5, respectively, an increase in h resulted in a significant reduction in the pressure drop. Kim and Lee and Bogodage et al. [57,58] found that doubling the diameter of the vortex finder can increase the cut size of the particles by two times, and it can also affect the distribution of the combined vortices inside the cyclone flow field. El-Batsh [59], Ficici et al. [60], and Gimbun et al. [61] found that the maximum tangential velocity and the pressure drop increased as the diameter of the vortex finder decreased, and they decreased as the width and height of the cyclone inlet increased.
Discharge. Elsayed and Lacor [62] concluded that the discharge diameter, B, had little effect on the flow pattern. The maximum tangential velocity and pressure drop increased only slightly with the decreasing discharge diameter. However, Xiang et al. [63] explained that the effect of the discharge diameter on the pressure drop does not need to be evident until B is reduced to a certain level. In addition, the separating efficiency of the cyclone separator decreased as the diameter of the vortex finder was reduced.
Cone and accessories. Pandey et al. [64] studied the effects of different cone height designs on the cyclone separator and found that the pressure drop decreases with an increasing cone height. In addition, more scholars have changed the discharge structure by adding accessories such as an anti-mixing cone and a straight tube to the joint position of the cone and the dust discharge port in the cyclone separator to explore the changes in the performance of the cyclone separator. Moreover, the association of various dust collection devices with cyclone separators to enhance the entire particle separation process is also a current research hotspot [65].
In addition, the interaction between the gas and solid phases in the cyclone separator often involves not only a variety of physical changes such as heat transfer reactions, but there are also some complex and specific chemical changes, which lead to the entire cyclone separation process being more complex. Therefore, research on cyclone separators should take into account the influence of not only its own structural design on the separation performance, but also other factors, including the physical properties of the injected particles such as the particle size and density; the operating parameters; and the distribution of the internal flow field.

3. Flow Field

The internal flow field of a cyclone separator is relatively complex. In general, the rotational motion in the cyclone separation space is generated by introducing gas through the inlet. The downward axial flow is called the outer vortex, and as the gas rotates downward, it also flows inward (radially) into the central core, changing into an inner vortex with an upward axial flow. Finally, it exits through the overflow tube at the top of the cyclone. In addition, the pressure gradient caused by the unsteady flow of the gas stream itself near the cyclone wall creates a secondary flow. The movement of the particles inside the cyclone is not only affected by the main vortices inside and outside, but is also disturbed by the generation of these secondary flows, which can even reduce the separation efficiency. Therefore, studies of the mechanism of the flow field in the cyclone separator cannot ignore the secondary flow.

3.1. Main Flows

Linden [13] used a spherical Bitot tube to determine the complex three-dimensional flow field inside a cyclone separator for the first time. In the tangential distribution of the cyclone velocity field, the tangential velocity exhibits a complex vortex structure comprising quasi-forced and quasi-free vortices. These two types of vortices are divided by the cylindrical surface of the extended surface of the overflow tube wall, i.e., the control surface (CS). Inside the CS is a flow field composed of the superposition of the quasi-forcing vortices and the source flow, while outside of the CS surface is a flow field composed of the superposition of the quasi-free vortex vortices and the sink flow. The quasi-forced vortices are cyclonic flows with the same tangential velocity distribution as the rotating rigid body, and the tangential velocity in this region increases linearly with the increasing radial distance. For the quasi-free vortices, the frictionless fluid rotates in such a way that the tangential velocity in this region decreases linearly with the increasing radial distance. In the axial distribution of the velocity field of the cyclone separator, the axial velocity is in the outer region of the downward-moving airflow and the axial region of the upward-moving airflow. Xu et al. [66] used Laser Doppler Anemometry (LDA) to determine the three-dimensional flow field inside the cyclone, and the test results presented a pattern that was basically consistent with Linden’s conclusions. However, they concluded that the existence of non-axial symmetry in the radial velocity distribution inside the cyclone was due to the fact that the center of the vortex was inconsistent with the center of the geometrical configuration caused by a single tangential inlet. This configuration is inconsistent with the center of the vortex. In the radial direction, the velocities are oriented outward in the center region and inward in the peripheral region, forming a source–convergence flow with a stagnant layer near the overflow pipe [67]. In summary, the flow field inside the cyclone is composed of two vortices with different properties, namely, quasi-free and quasi-forced vortices, as well as the superposition of source and sink flows with different flow directions.
The centrifugal force generated by the main flows in the internal flow field of the cyclone separator is the main driving force of the gas–solid separation, so the rotational strength of the main vortex directly affects the separation efficiency of the cyclone separator. In addition, the three-dimensional velocity distribution, frictional resistance, and turbulent energy dissipation in the internal flow field of the cyclone separator affect the rotational strength of the airflow, which exhibits a certain degree of attenuation in the axial direction, leading to the deviation between the theoretical and experimental values in the performance calculation of the cyclone separator.
Yazdabadi et al. [68] and Obermair et al. [69] found that there is an alternating vortex shedding phenomenon near the overflow tube in the cyclone separator, which will result in the end of the inner vortex being located not at the dust discharge port but at a certain place in the cone section. Therefore, they proposed the concept of a natural vortex length, which refers to the depth of the inner vortex and the influence area of the cyclone separator and is considered as the distance from the lower end of the overflow tube to the end of the vortex rotating flow in the cyclone separator [70]. The asymmetry of the inner flows can cause instability in the flow field inside the cyclone separator [71,72], which leads to the appearance of the phenomenon of a precession vortex core (PVC). Wu et al. [73] used Particle Image Velocimetry (PIV) and found that the PVC phenomenon exists in the vicinity of the overflow pipe and the dust outlet of the entire cyclone. Elsayed et al. [74,75,76,77,78] found that the vortex nucleus frequency decreases as the diameter of the overflow pipe increases and the PVC phenomenon decreases. Gao et al. [79] used the Q-criterion to create three-dimensional equivalent surfaces of vortices and found that the equivalent surfaces are twisted rather than circularly distributed around the axis of the geometric center. The equivalent diameter of the vortex surface decreases gradually downward along the axis, indicating that the carrying energy of the vortex gradually decays. Due to the presence of frictional resistance in the cylinder, the vortices near the sidewalls rapidly become smaller, and the energy loss in the vortex is elevated. In addition, the tendency of the vortex core center of the cyclone separator to deviate from the geometric center of the structure exhibits a process of gradual increase and then decrease until leveling off. During this process, the vortex can break up due to excessive growth, and the PVC phenomenon will result in significant energy loss. Therefore, ensuring the stability of the vortex structure and reducing the PVC phenomenon is conducive to the improvement in the separation efficiency of the cyclone separator and the reduction in the energy loss. Boysan et al. [80,81,82] utilized a Phase Doppler Particle Analyzer (PDPA) to explore the internal flow field changes in a cyclone separator and found that the addition of an inverted cone can effectively inhibit the oscillation of the vortex nuclei, which improves the non-axial symmetry inside the flow field and enhances the performance of the cyclone separator. Yoshida et al. [83,84,85,86] proposed a new method of fluid flow control for cyclone–submicron particle classification, which they found to enhance the fluid flow stability inside the cyclone separator. In conclusion, changing the configuration of a cyclone separator can affect the internal vortices of the cyclone separator to a certain extent, and a suitable optimized configuration can improve the stability of the main vortices inside the cyclone separator, thus improving the separation efficiency of the cyclone separator [87,88].

3.2. Secondary Flows

The gas–solid separation inside a cyclone separator depends on the distribution of the main flows, but it is also affected by secondary flows. Analyzing the secondary flow has important theoretical significance and practical engineering value for improving the performance of the separator. The secondary flow inside a cyclone separator can be defined as the synthesis of the axial and radial velocities in the flow field of a cyclone separator, which is not conducive to particle removal and affects the separation performance of the various flow states [9]. These secondary flows mainly include the recirculation flow, the short-circuiting, and the eccentric circumfluence, as shown in Figure 4.

3.2.1. Recirculation Flow

Below the top plate of the cyclone separator are recirculating flows (with closed streamlines). Compared to the static pressure changes in the strong flow, the static pressure on the same cross section does not vary significantly with the radial distance, making it easy for a fluid with a higher static pressure on the outside to flow into this boundary layer and flow inwards, resulting in longitudinal circulation [89]. The longitudinal circulation will carry some of the particles that have been separated to the inner wall of the cyclone cylinder upward to the top plate, forming a layer of stagnant recirculation flow. Once other fine particles enter this area, they become difficult to separate, thus reducing the overall separation efficiency of the cyclone separator. The size of the recirculation flow in the cyclone is related to the concentration of the inlet particles. At a certain inlet concentration, the top ash ring decreases the separation efficiency of the cyclone separator. Under the experimental conditions of a higher inlet dust concentration, although the total amount of escaped particles in the cyclone separator is greater than that under a low concentration condition, and the concentration and range of the top ash ring are larger, it still exhibits a decreasing trend compared with the inlet dust concentration, and thus, the overall separation efficiency of the cyclone separator is still increased [90].

3.2.2. Short-Circuit Flow

Short-circuit flow exists near the end of the vortex finder [9], which will theoretically result in at least ten percent of the inlet airflow overflowing [91]. It not only dissipates more of the energy of the flow [92], but also affects the collection performance of the cyclone. The large radial centripetal velocity due to the presence of a short-circuit flow causes part of the airflow in the cyclone separator to enter the overflow tube directly without passing through the separation space in the lower part, and then the rising airflow carries this airflow out of the separator, especially for small particles. The results of the current study reveal that the radial velocity is more difficult to measure accurately, and the distribution exhibits non-axisymmetric and turbulent complexity, which has become one of the difficult problems in the study of the short-circuit phenomenon. Derksen et al. [72] pointed out that the short-circuit flow at the end of the inlet boundary layer of the cyclone separator, i.e., the short-circuit flow near the inlet cross-section, is stronger than that on the other side, which suggests that the inlet airflow may enhance the short-circuit flow. That is, airflow collisions only lead to short-circuit flow generation when inlet airflow is present.

3.2.3. Eccentric Circumfluence

Lucca-Negro and O’doherty [93] found that there is an eccentric longitudinal circumfluence near the discharge, that is, the downward airflow near the separator wall is purified with the center of the separator in the inner vortex of the ascending airflow convergence. The centrifugal force is separated to the cylinder wall with the downward flow of the particles downstream of the exhaust port and is involved in the upward inner vortex, thus causing the remixing of the particles. In the cyclone separator, at the bottom of the discharge near the center of the axial downward airflow and upward airflow center, overlap and deviation phenomena occur. The overlap of the airflow in the inertial force not only extends a certain distance downward along the axial direction, but also results in an upward axial airflow reflux for a certain distance. This is due to the reflux of the airflow and the inner field of the non-overlap phenomenon (eccentricity). The resulting asymmetry allows for spinning into the nucleus of the vortex to be formed and keep swinging, so the particles are re-entrained into the upward vortex, thus causing the particles to be mixed. As a result, the particles are re-involved in the upward overflow of the inner vortex, thus reducing the separation efficiency of the cyclone separator.
The experiments conducted in some studies have shown that the gas–solid two-phase flow inside a cyclone separator is very complex [94]. The main and secondary flows in the longitudinal section can coexist in the cross section and thus have effects on the gas flow pattern. The presence of these secondary flows not only increases the operating resistance of the cyclone separator but also greatly affects the separation efficiency of the cyclone separator. Therefore, weakening or eliminating the secondary flows in the cyclone separator has gradually become an important means of designing high-efficiency dust removal cyclone separators.

3.3. Research Methods

As was previously mentioned, to deeply study the separation mechanism of the three-dimensional vortex flow and particles inside the cyclone separator, various corresponding assumptions have been made about the separation of the airflow and particles inside the separator, such as the modeling hypothesis [9]. However, the modeling hypothesis has limitations and cannot truly reflect the actual situation of the complex two-phase flow inside the cyclone separator. With the gradual development of science and technology, experimental studies and numerical simulations have become the main means for researchers to understand the flow field inside a cyclone separator.

3.3.1. Experiments

Experimental methods are the most traditional, intuitive, and effective technical means of academic inquiry through which findings can be analyzed and directly fed back through experimental data and phenomena. The most widely used techniques in the experimental observation of flow patterns within cyclonic separators include Bittor, 5-hole probe, Hot Wire Anemometers (HWAs), and contactless techniques such as LDA and PIV. There is latitude within categories and individual methods, which is further explored in Table 1, while the text below is meant to illustrate the most common experimental methods of cyclones observed in the literature.
Linden et al. [13,67] measured and analyzed the flow field within a cyclonic separation using a Bittor probe, and they determined and summarized the characteristics of the three-dimensional velocity field in the flow field. Their data results laid a foundation for subsequent research.
Gao et al. [103] used the multipoint pressure sensor and liquid tracer methods to investigate the pressure time series signals at different locations in a cyclone separator in the axial and radial directions, which revealed the location of the tail end of the vortex and its dynamic characteristics. Although the pressure transducer has a high sensitivity, it has the disadvantage of not being able to be read directly, and it is not convenient enough to go through the calculation operations such as converting the time series signals into velocity values.
However, with the development of experimental techniques, to pursue more accurate internal flow field data, the effect of the probe size on the experimental error of the turbulent flow field inside a cyclone separator became impossible to ignore, and more studies began to use contactless measurement methods. Mothes et al. [104] and Peng et al. [105] used LDA to measure the internal flow fields in several different cyclone separators and explored the relationship between the particle collection and the tangential velocity. Derksen et al. [76] used LDA to measure the flow field in an industrial cyclone and found that the strength of the spin-in vortex nuclei decreases as the diameter of the overflow tube increases. Obermair et al. [69] used LDA to determine that there is an alternating shedding of vortices near the outer wall of the overflow tube. The application of the LDA technique to the measurement of cyclone velocity fields has achieved significant success. It has clearly been successful in determining not only the variation in the velocity with time but also the amount of turbulence and the periodic fluctuations of the PVC. The only drawback is that the distribution of the tracer particles of the LDA may affect the measurement range, resulting in limited or even impossible measurements.
Subsequently, since PIV can be used not only to evaluate single-point velocity values but also to quantitatively assess the overall flow field state, it has replaced LDA as an important velocity measurement technique in experimental fluid dynamics and has led to many advances in analyzing physical quantities such as three-dimensional velocities, Reynolds stresses, and the turbulence strengths of various cross sections within the cyclone separator [97,98]. The non-contact measurement technique, namely, PDPA, has also been a popular technique in recent years. Gao et al. [79] used PDPA to measure the flow rate to analyze the change in the vortex structure in the flow field. They found that the equivalent diameter of the vortex surface is larger in the upper region of the cyclone column and decreases downward along the axis, and the vortex develops rapidly or even breaks up in this process. Fan et al. [100] used PDPA to identify the improvement in the vortex structure in the cyclone column.

3.3.2. Numerical Simulation

Computational fluid dynamics (CFD) is a computer and numerical method of solving the controlling equations of fluid dynamics. It is used to simulate and analyze fluid dynamics problems and can more accurately obtain the characteristics of the movement of a multiphase flow and the flow pattern [30]. The advantages of CFD are reflected in not only its ability to extract the flow field information needed by the researcher in a short period of time, but also its ability to track particles and obtain data results to achieve feedback for the purpose of the geometric improvement and performance optimization of the cyclone separator. This is conducive to the rapid assessment of the advantages and disadvantages of cyclone configurations, reduces the cost of the study, and shortens the study period. Therefore, the importance of numerical simulation studies is becoming more and more prominent.
Regarding cyclone flow field studies, Griffiths and Boysan [106,107] used CFD to simulate eddy turbulence and particle trajectories to summarize the characteristic laws of the gas–solid phases in turbulent flow. Alahmadi and Nowakowski [108] used improved shear stress and curvature correction (SSTCC), the Reynolds stress model (RSM), and k-ε turbulence modeling to simulate the airflow pattern inside the cyclone. Gronald and Derksen [109] used the tLarge Eddy (LES) and Reynolds-averaged Navier–Stokes (RANS) turbulence models to simulate the airflow inside the cyclone and compared the corresponding CFD results with he experimental LDA data. Another major research topic in the numerical simulation of cyclones is constitutive deformation, which was investigated by Duan et al. [110]. Siadaty et al. [111] investigated the effects of different geometrical variations of the cyclone on the entropy production change and effective energy loss using CFD. Alexander [112] investigated the effects of different geometrical parameters on the performance of a cyclone separator. Elsayed and Lacor [62,113] concluded that the cone variation and inlet variation greatly affect the cyclone separation efficiency. Xiang and Lee [114] found that increasing the height of the cyclone reduces the tangential velocity and decreases the separation efficiency. Safikhani and Mehrabian [115] determined the optimal height and depth of the vortex finder in the cyclone separator using CFD. In addition, CFD has great potential in predicting gas–particle (gas–solid two-phase) and particle–particle (solid–solid same-phase) interactions [116].
Therefore, numerical simulation is well suited to explain the complex secondary flow problems in simulated cyclone separators. Wang [47] used a numerical simulation to investigate the formation mechanism of short-circuit flow in a cyclone separator and was able to experimentally verify the reliability of the simulation method (Figure 5); they also determined that the airflow loss caused by short-circuit flow can be accurately calculated by using numerical simulation software. Dong et al. [45] then carried out a particle simulation on this basis and found that the smaller the particles are, the faster the inlet air velocity and the greater the efficiency loss caused by the short-circuit flow. Misiulia et al. [92] used CFD to study the effect of the inlet flow velocity on the secondary flow, and they found that the recirculation and short-circuit flow decreased with an increasing flow rate, and that the short-circuit flow had the greatest effect on the separation efficiency of the cyclone separator.
The use of CFD research data allows researchers to attain a deeper understanding of and summarize the flow field laws, particle behavior, and practical tractability in rotating flow fields.

4. Conclusions

In this review, the investigation direction and hot topics are summarized in terms of the working principle and geometrical structure based on the current research. Then, despite the fact that the working principle and geometric structure of the cyclone separator are relatively straightforward, the experimental and simulation results from recent years indicate that its internal flow field is highly turbulent and complex. To improve the separation performance of cyclone separators and the small particle separation efficiency, a comprehensive examination of the internal flow field and airflow motion of cyclone separators is essential. The primary conclusions of this review are as follows:
(1)
The performance of cyclone separators has witnessed notable advancements in two primary aspects following the initial two stages of development. Firstly, these separators have demonstrated enhanced adaptability to a broader spectrum of operational conditions, encompassing high temperatures and pressures. Secondly, they have exhibited greater efficiency in the collection of ultrafine particles. There are two types of airflow in a cyclone separator: primary flow and secondary flow. Secondary flows not only facilitate the transportation of fine particles towards the overflow or their redistribution, but also contribute to the increases in energy consumption and industrial cost.
(2)
The study of the geometrical enhancement of cyclone separators has evolved from basic modifications of components such as the inlet, vortex finder, cone, and dust discharge to the incorporation of various fittings in specific areas such as the top plate, inner wall, and inlet. Additionally, there has been exploration into the implementation of multiple cyclone separators in series. These advancements have demonstrated an enhancement in the separation efficiency of fine particles to some extent, but they may also lead to increased energy consumption.
(3)
Because the secondary flow is primarily formed by the superposition of radial and longitudinal flows, it is difficult to experimentally measure, and numerical simulation is the predominant research method at present. Nevertheless, the turbulence models exhibit significant variations in terms of computational requirements and time consumption, as well as disparities in accurately representing the intense rotational flow within a cyclone separator. Numerous investigations have been conducted to assess and contrast the suitability and dependability of various turbulence models in simulating the flow characteristics within cyclone separators, utilizing empirical data. The findings consistently demonstrate that the RSM model and the LES model exhibit superior agreement with the experimental data.
(4)
Regarding the reduction in the secondary flow, a cyclone separator’s collection performance can be enhanced through geometric deformation according to its formation position.
However, there are still numerous obstacles in the research on secondary flows:
(1)
Approaches employed in the investigation of secondary flows. The gas-phase flow within a cyclone separator exhibits intricate and extremely turbulent behavior. As the vortex strength intensifies, a significant interdependence between the axial and tangential velocity components emerges. Consequently, this coupling poses challenges in terms of experimental monitoring. The secondary flow has a strong correlation with the axial and radial velocity profiles. However, obtaining precise and timely input on the variations in radial velocity throughout the whole gas-phase flow field is challenging using experimental methods alone. While simulations can complement experimental data, there remains a dearth of empirical evidence to substantiate certain aspects, such as the existence of a threshold value. Specifically, it is unclear to what extent the reduction in the three secondary flows will cease to impact the cyclone separator’s performance.
(2)
The correlation between the proportion of secondary flow and the effectiveness of separation. Prior research has indicated that the enhancement of separation efficiency for ultrafine particles can be achieved by attenuating or eliminating the secondary flow deficit. However, there exists a dearth of quantitative analyses regarding the proportion of secondary flow and the existence of a critical point beyond which the reduction in secondary flow ceases to impact the separation efficiency.
(3)
The interplay between the main and secondary flows. Numerous scholarly investigations have examined the two phenomena in isolation. However, it is imperative to recognize that the formation of a secondary flow and the presence of a primary flow are inherently interconnected. Certain optimized configurations just focus on mitigating the effects of secondary flow while neglecting to address the intricacies of the main flow.
In future research, the analysis of cyclone separators could be extended to include other factors such as materials and broader operational conditions. Furthermore, a more detailed exploration of the interactions between particles within the flow field, encompassing phenomena such as vortex patterns, particle aggregation, and settling dynamics, has the potential to yield a more comprehensive and systematic comprehension of this intricate issue. Finally, the lack of actual experimental data for comparison with numerical results is a limitation of studies on secondary flows. This constraint may impede the direct use of research findings in practical engineering projects.

Author Contributions

Conceptualization, C.W.; writing—original draft preparation, C.W.; writing—review and editing, C.W.; visualization, Y.M.; reference editing, Y.M. and W.S.; supervision, C.W.; project administration, C.W.; funding acquisition, C.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by the Natural Science Foundation of Gansu Province (21JR7RA155) and the Youth Foundation of Northwest Normal University (NWNU-LKQN2021-13).

Conflicts of Interest

The authors declare that they have no conflict of interest.

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Figure 1. The Lapple cyclone.
Figure 1. The Lapple cyclone.
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Figure 2. Vortex in the cyclone [47].
Figure 2. Vortex in the cyclone [47].
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Figure 3. Simulated results of the normalized average forces in the radial direction and of the separation efficiency as a function of particle size [48]. The efficiency of particle separation is represented by the black line, while the magnitudes of the two forces in the radial direction are depicted by the green and blue lines, respectively. In Region (A), the centrifugal force is considerably greater than the drag force, resulting in the complete collection of all particles. In Region (B), it is observed that as the particle size decreases, the alteration in the centrifugal force is relatively minimal, whereas the drag force exhibits a continuous increment. In the context of particles located in Region (C), it can be observed that the drag force experiences a sudden increase, surpassing the centrifugal force by a considerable margin.
Figure 3. Simulated results of the normalized average forces in the radial direction and of the separation efficiency as a function of particle size [48]. The efficiency of particle separation is represented by the black line, while the magnitudes of the two forces in the radial direction are depicted by the green and blue lines, respectively. In Region (A), the centrifugal force is considerably greater than the drag force, resulting in the complete collection of all particles. In Region (B), it is observed that as the particle size decreases, the alteration in the centrifugal force is relatively minimal, whereas the drag force exhibits a continuous increment. In the context of particles located in Region (C), it can be observed that the drag force experiences a sudden increase, surpassing the centrifugal force by a considerable margin.
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Figure 4. Schematic diagram of the secondary flows in a cyclone separator.
Figure 4. Schematic diagram of the secondary flows in a cyclone separator.
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Figure 5. Comparison of the tangential velocity distribution of the numerical results and experimental data [46], X(Y)/R represents the dimensionless radial positions, v denotes velocity, and vin = 16.00 m/s. The dots and lines represent the experimental and simulated data of the tangential velocity, respectively. Black represents the data on the straight line perpendicular to the inlet of the cyclone separator, and red represents the data on the straight line parallel to the inlet of the cyclone separator. (a) shows the comparison between the numerical simulation and the experimental data of the tangential velocity when the height of the cylinder is 0.5 m, and (b) shows the comparison between the numerical simulation and the experimental data of the tangential velocity when the height of the cylinder is 0.2 m.
Figure 5. Comparison of the tangential velocity distribution of the numerical results and experimental data [46], X(Y)/R represents the dimensionless radial positions, v denotes velocity, and vin = 16.00 m/s. The dots and lines represent the experimental and simulated data of the tangential velocity, respectively. Black represents the data on the straight line perpendicular to the inlet of the cyclone separator, and red represents the data on the straight line parallel to the inlet of the cyclone separator. (a) shows the comparison between the numerical simulation and the experimental data of the tangential velocity when the height of the cylinder is 0.5 m, and (b) shows the comparison between the numerical simulation and the experimental data of the tangential velocity when the height of the cylinder is 0.2 m.
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Table 1. Summary of experimental studies based on different approaches.
Table 1. Summary of experimental studies based on different approaches.
Common ApplicationTechniquesMethodAvailable Data AccuracyUsabilityKey Examples
Flow fieldContactBittorTotal pressure, static pressureLowLow—the obtained pressure data are usually used to calculate the air velocity based on Bernoulli’s equationRefs. [13,67]
5-hole probeTotal pressure, static pressure, velocityMediumHigh—pressure and velocity data can be obtained simultaneouslyRef. [95]
Pressure sensor Total pressure, static pressure, High Low—instantaneous readings are unstableRef. [96]
HWA*Tangential velocity,
axial velocity		MediumHigh—instantaneous data on axial and tangential velocities can be obtained depending on the direction of placementRefs. [45,55]
Non-contactLDA*Velocity,
linear or vibratory trajectories 		MediumMedium—the addition of tracers can be a distraction; speed flows have the potential to result in significant inaccuracies within the dataset; operating the instrument is a highly intricate task; the experimental platform for cyclone separators necessitates the utilization of transparent or semi-transparent components, hence augmenting the expenses associated with conducting testsRefs. [97,98,99]
PIV*Three-dimensional velocities,
Reynolds stresses, turbulence strengths, and other parameters of the flow field 		High
PDPA*Refs. [79,100]
Separation efficiencyOn-lineAutomatic soot detector ConcentrationHighHigh—real-time quantification of particle concentration at both the inlet and outflow of the cycloneRef. [101]
Off-lineWeighingLowLow—insufficient collection of particles results in significant inaccuraciesRef. [101]
Laser particle size analyzerParticle mass fractions, grade efficiencyHighHigh—the instrument is easy to operate and simple to calculateRef. [102]
* HWAs: Hot Wire Anemometers. LDA: Laser Doppler Anemometry. PIV: Particle Image Velocimetry. PDPA: Phase Doppler Particle Analyzer.
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Wang, C.; Ma, Y.; Sui, W. The Secondary Flows in a Cyclone Separator: A Review. Processes 2023, 11, 2935. https://doi.org/10.3390/pr11102935

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Wang C, Ma Y, Sui W. The Secondary Flows in a Cyclone Separator: A Review. Processes. 2023; 11(10):2935. https://doi.org/10.3390/pr11102935

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Wang, Chenwen, Yongshan Ma, and Wenxuan Sui. 2023. "The Secondary Flows in a Cyclone Separator: A Review" Processes 11, no. 10: 2935. https://doi.org/10.3390/pr11102935

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