3.1. Static Pressure
The cone segment is a critical zone for particle classification in hydrocyclones, serving as the key structure for flow field intensification, particle separation, and underflow regulation. Its geometric configuration directly determines the separation efficiency, processing capacity, and operational stability of the hydrocyclone, making it a vital research focus for hydrocyclone design and optimization [
20]. To investigate the effects of cone segment structure on the internal flow field of hydrocyclones, four configurations with single-, double-, triple-, and quadruple-cone structures were studied, respectively.
Static pressure is one of the core parameters of the internal flow field in hydrocyclones, directly reflecting fluid energy distribution and closely correlating with centrifugal separation efficiency, pressure loss, and equipment operational stability. Static pressure in hydrocyclones refers to the pressure of fluid in a relatively stationary frame of reference, representing the potential energy of the fluid, and together with dynamic pressure, it constitutes the total pressure. Its distribution is jointly determined by fluid rotational motion, flow channel geometric constraints, and the law of mass conservation [
21,
22]. The static pressure gradient is one of the core driving forces for particle radial migration, and coupled with centrifugal force, it governs the separation trajectory of particles. The static pressure distributions in the ZX plane of hydrocyclones with different cone segment structures are presented in
Figure 5. All cone segment configurations exhibit the typical radial gradient characteristic of static pressure in swirling flow fields: the wall region is a high-pressure zone, resulting from the pressure concentration effect induced by centrifugal force when the fluid is constrained by the wall with a large rotating radius; the central axis region is a low-pressure core zone, reflecting the essential upward reflux flow of the internal swirl; from the wall to the central axis, the static pressure decreases continuously in a gradient manner, conforming to the distribution law that pressure is positively correlated with the square of rotational angular velocity in swirling flow fields.
To microscopically characterize the effects of cone segment structure on static pressure, the static pressure along four diametral lines at Z = 30 mm, Z = 80 mm, Z = 120 mm, and Z = 160 mm was analyzed separately, with the results shown in
Figure 6. All axial cross-sections exhibit the typical U-shaped distribution of swirling flow fields: the static pressure in the wall region increases exponentially with the increase in radial distance to reach a maximum value, reflecting the pressure concentration effect under centrifugal force; the static pressure in the central region drops sharply to form a low-pressure valley, and negative pressure zones appear in most cross-sections, corresponding to the central air core region of the hydrocyclone, which is a hallmark feature of internal swirl reflux. The process of steep decline–valley–recovery of static pressure from the wall to the center corresponds one-to-one with the external swirl–vortex core–internal swirl structure of the flow field, in line with the energy conservation law of swirling flow fields.
The regulation of radial static pressure distribution by multi-cone configurations is mainly manifested as intensification and stratification of low-pressure zones: the low-pressure valleys in all axial cross-sections of the single-cone configuration are relatively narrow and shallow, with high uniformity of radial pressure gradients, only reflecting pressure changes caused by single-stage contraction; the growth rate of the width and depth of low-pressure valleys in the double-cone configuration with increasing axial height is significantly higher than that of the single-cone configuration, and the degree of separation from the single-cone curve gradually increases after Z = 80 mm; and among all axial cross-sections, the triple-cone configuration exhibits the widest and deepest low-pressure valleys. For instance, at Z = 120 mm, the width of the low-pressure valley in the triple-cone configuration is 1.5 times that of the single-cone configuration, and multi-layer low-pressure sub-valleys are formed in the range of Z = 80 mm~120 mm. This reflects the segmented intensification effect of the stepped contraction of the triple-cone on the pressure field, enabling more precise regulation of particle migration dynamics at different radial positions.
3.3. Axial Velocity
Axial velocity serves as the direct driving force for the axial migration of fluid inside hydrocyclones, and its directional and amplitude distribution governs the particle separation trajectory as well as the external swirl–internal swirl structure of the flow field [
24]. The axial velocity distributions in the ZX plane of hydrocyclones with different cone segment structures are presented in
Figure 9. As observed from the axial velocity contour plots, all cone configurations exhibit the core characteristic of bidirectional flow: positive axial velocity corresponds to the external swirl, where fluid migrates along the wall toward the underflow orifice, acting as the main channel for transporting coarse particles; negative axial velocity corresponds to the internal swirl, where fluid refluxes along the central axis toward the overflow orifice, serving as the core channel for carrying fine particles; and the zero-velocity envelope surface lies between the external and internal swirls, acting as the interface separating the two flow patterns, and its position directly determines the size of the separation space.
With the increase in the number of cone segments, the layered characteristics of axial velocity become more pronounced: for the single-cone configuration, the positive axial velocity zone is only concentrated near the wall, the range of the negative axial velocity zone is relatively narrow, and the zero-velocity surface is close to the central axis. This indicates that the flow channel contraction of a single cone can only form a basic bidirectional flow, resulting in a limited separation space (external swirl zone). For the double-cone configuration, the radial coverage of the positive axial velocity zone is expanded, the depth (axial extension length) of the negative axial velocity zone is increased, and the zero-velocity surface shifts toward the wall—reflecting the expansive regulation of the bidirectional flow structure by segmented flow channel contraction, leading to a significant enlargement of the separation space. For the triple-cone configuration, the stratification of positive and negative axial velocity zones is the clearest; the positive velocity zone covers most of the radial space of the cone segment, the amplitude of the negative velocity zone is more uniform, and the position of the zero-velocity surface is closer to the wall. This demonstrates that the stepped contraction of the triple cone can enhance the separation space of bidirectional flow while maintaining the orderliness of the flow field and avoiding flow pattern mixing.
To microscopically characterize the effects of cone segment structure on axial velocity, the axial velocity along four diametral lines at Z = 30 mm, Z = 80 mm, Z = 120 mm, and Z = 160 mm was analyzed separately, with the results shown in
Figure 10. All configurations with different numbers of cone segments follow the basic law of bidirectional flow (positive velocity in internal swirl and negative velocity in external swirl) across the four axial cross-sections, and the number of cone segments exerts the most significant influence on the internal swirl. For the single-cone configuration, the peak positive velocity of the internal swirl is only 5–6 m/s with narrow radial coverage; the upward driving force for fine particles is insufficient, making them prone to being entrained into the external swirl. The amplitude of the negative velocity of the external swirl is only −1 to −2 m/s with limited radial coverage, resulting in a narrow downward channel for coarse particles. For the double-cone and triple-cone configurations, the peak positive velocity of the internal swirl reaches 7–8 m/s with stable radial coverage; the reflux channel for fine particles is clear and well-powered. The amplitude of the negative velocity of the external swirl ranges from −1 to 0 m/s, and its radial coverage is wider than that of other configurations, providing sufficient separation space. For the quadruple-cone configuration, the peak velocity of the internal swirl is the lowest, with substantial velocity fluctuations along the axial direction and excessively narrow radial coverage, resulting in the poorest performance among the four configurations.
3.5. Turbulence Intensity
Turbulence intensity serves as a core parameter characterizing the degree of flow field disturbance, defined as the ratio of the root mean square of turbulent fluctuating velocity to the mean flow velocity. It directly reflects the intensity of energy dissipation and velocity fluctuation within the flow field. In hydrocyclones, the distribution of turbulence intensity exhibits a direct correlation with flow channel geometry: abrupt changes in flow channel configuration tend to generate high-turbulence zones, where energy dissipation concentrates, whereas gentle flow channel segments feature low-turbulence characteristics. The magnitude and spatial extent of turbulence intensity not only govern flow field stability but also influence classification accuracy through the mechanism of particle random diffusion—elevated turbulence disrupts the directional migration trajectory of particles, while moderate turbulence ensures orderly separation, rendering it one of the key indicators for balancing flow field stability and separation efficiency in hydrocyclone structural optimization [
27,
28,
29].
The turbulence intensity distributions in the ZX plane of hydrocyclones with different cone segment configurations are presented in
Figure 12. All cone configurations exhibit a universal distribution pattern characterized by localized high-turbulence zones superimposed on a background of overall low-turbulence flow: high-turbulence zones are concentrated at the bottom of the overflow pipe and the cylindrical-conical junction, corresponding to the flow channel transition zone where the fluid shifts from the cylindrical section to the cone segment. These regions represent focal points of energy dissipation induced by sharp velocity gradients and flow pattern transitions. Low-turbulence zones are distributed along the wall surface and near the underflow orifice, reflecting stable flow regions constrained by the flow channel geometry. The overall turbulence intensity magnitude ranges from 0.086 to 1.96, consistent with the inherent hydrocyclone flow field characteristic of local disturbance and global order.
With increasing number of cone segments, the high-turbulence zones exhibit distinct characteristics of magnitude attenuation and spatial contraction. The single-cone configuration features the highest turbulence intensity magnitude, with core zone values ranging from 1.93 to 1.94, coupled with the widest radial coverage that occupies most of the upper-mid region of the cone segment; this phenomenon reflects that the one-step flow channel contraction of the single-cone configuration induces abrupt velocity gradient changes, leading to concentrated energy dissipation and intense flow field disturbance. The double-cone and triple-cone configurations display a significant reduction in high-turbulence zone magnitude, with core values ranging from 1.61 to 1.87 for the double-cone and from 1.47 to 1.82 for the triple-cone; the radial extent of these high-turbulence zones contracts to the vicinity of the cone segment central axis, demonstrating that segmented flow channel contraction decomposes a single abrupt transition into multiple gradual steps, resulting in more uniform velocity gradients and dispersed energy dissipation. The quadruple-cone configuration exhibits a further reduction in high-turbulence zone magnitude, with core turbulence intensity ranging from 1.41 to 1.96, yet the radial coverage of these zones expands slightly; this observation indicates that excessive segmentation in the quadruple-cone configuration triggers cumulative effects of flow channel perturbations, leading to a renewed increase in local velocity gradients.
Multi-cone configurations achieve turbulence intensity regulation through segmented flow channel contraction: the triple-cone configuration confines the magnitude and spatial extent of high-turbulence zones within an optimal range while ensuring overall flow field stability, which constitutes the core flow field foundation underpinning its superior separation performance. In contrast, the elevated turbulence intensity of the single-cone configuration and the turbulence resurgence in the quadruple-cone configuration both compromise the orderliness of the separation process.
To characterize the effects of cone segment structure on turbulence intensity at a fine scale, the turbulence intensity along four diametral lines at Z = 30 mm, Z = 80 mm, Z = 120 mm, and Z = 160 mm was analyzed, with the results presented in
Figure 13. For all cone configurations, the turbulence intensity exhibits a universal evolutionary trend involving magnitude escalation, subsequent stabilization, and final fluctuation convergence, with the triple-cone configuration consistently demonstrating optimal flow field stability. Across the entire axial range, the turbulence intensity of the triple-cone configuration remains within an optimal interval that balances maximum fluctuation suppression and minimum energy consumption; its fluctuation amplitude is 30–40% lower than that of the single-cone configuration and 15–20% higher than that of the quadruple-cone configuration, which simultaneously inhibits particle random diffusion and preserves sufficient centrifugal separation driving force. This balance is particularly pronounced in the outlet region at Z = 160 mm, where the turbulence intensity peak of the triple-cone configuration is 14% lower than that of the single-cone and 6% higher than that of the quadruple-cone configuration, achieving an optimal trade-off between energy consumption and discharge efficiency.
3.6. Air Core
The air core is a cylindrical gaseous core structure formed by the entrainment of ambient air into the central low-pressure zone inside a hydrocyclone. Its diameter and axial morphology are jointly determined by the intensity of the central low-pressure field and the flow channel geometry, serving as a direct indicator of hydrocyclone flow field stability. In swirling flow fields, the central negative pressure generated by high-speed fluid rotation draws in ambient air to form a continuous air core; the diameter of the air core directly reflects the intensity of the central low-pressure field, with stronger negative pressure inducing greater air entrainment and a thicker air core. Meanwhile, the axial fluctuation of the air core is correlated with flow field orderliness: a stable and uniform air core signifies a gentle central low-pressure field and distinct bidirectional flow boundaries, facilitating directional particle migration; conversely, an excessively expanded or violently fluctuating air core compresses the separation space and disrupts flow patterns, thereby degrading classification accuracy. Accordingly, the air core represents one of the key flow field characteristics for evaluating hydrocyclone separation performance [
30,
31].
The profiles of the air core in the ZX plane of hydrocyclones with different cone segment configurations are presented in
Figure 14. All cone configurations exhibit a consistent axial evolutionary pattern characterized by initial contraction, subsequent stabilization, and final expansion. Within the axial range of Z = 0–50 mm, the air core diameter undergoes rapid contraction, reflecting the enhancement of rotational kinetic energy and the incipient formation of the central low-pressure zone as fluid enters the hydrocyclone. In the axial interval of Z = 50–200 mm, the air core diameter stabilizes, corresponding to the equilibrium stage between the central low-pressure field and the overall flow field. Beyond Z = 200 mm up to 350 mm, the air core diameter expands significantly, exhibiting a distinct peak value that reflects the intensification effect of the low-pressure zone in the flow field near the underflow orifice. In the axial range of Z = 300–350 mm, the single-cone, triple-cone, and quadruple-cone hydrocyclones exhibit a bottom expansion phenomenon, which is absent in the double-cone configuration. This phenomenon arises because the segmented contraction of the double-cone configuration moderates flow channel abruptness, resulting in a central low-pressure field intensity in the underflow orifice region that is weaker than that of the single-cone, triple-cone, and quadruple-cone configurations, thereby precluding the formation of a pronounced terminal air core expansion.
The number of cone segments exerts a direct influence on the fluctuation degree of the air core diameter, and consequently on the size of the effective separation space. The single-cone configuration features minimal air core diameter fluctuation in the main cone segment, yet undergoes substantial expansion at the underflow orifice, compressing the terminal separation space. The triple-cone configuration exhibits gentle air core diameter fluctuation across the entire axial range, with moderate expansion at the underflow orifice and a smaller peak diameter, yielding sufficient and stable effective separation space. In contrast, the quadruple-cone configuration experiences violent air core diameter fluctuation in the main cone segment, coupled with significant expansion at the underflow orifice, which drastically compresses the separation space and tends to restrict particle migration trajectories.
The stable and moderately sized air core of the triple-cone configuration not only ensures the fine particle reflux channel of the central internal swirl flow but also avoids excessive air core expansion that would compress the separation space. The terminal air core expansion of the single-cone configuration and the main section fluctuation of the quadruple-cone configuration both disrupt flow field orderliness, leading to increased fine particle entrainment in the underflow or coarse particle misplacement in the overflow. The air core of the triple-cone configuration maintains stable and moderate characteristics throughout the entire separation process, balancing the central reflux channel and the effective separation space, which constitutes the core flow field support underpinning its superior separation performance.
3.7. Efficiency Curve
Classification efficiency serves as the core indicator for evaluating the separation performance of hydrocyclones, typically defined as the percentage of the mass of particles of a specific size that are effectively separated into the underflow relative to the total mass of particles of the same size in the feed. The curve depicting its variation with particle size directly reflects the separation capability of hydrocyclones for particles across different size fractions. In engineering practice, the morphology of the classification efficiency curve is directly correlated with classification accuracy: a steeper curve indicates a stronger discriminative ability of the hydrocyclone for the target separation particle size and thus higher classification accuracy. In contrast, the typical S-shaped distribution, characterized by low efficiency in the fine particle range and near-100% efficiency in the coarse particle range, embodies the inherent separation feature of hydrocyclones—prioritizing the separation of coarse particles while allowing fine particles to be easily lost with the overflow. This curve morphology is therefore a critical criterion for assessing the structural rationality and operational efficiency of hydrocyclones.
The classification efficiency curves of hydrocyclones with different cone segment configurations are presented in
Figure 15. All configurations exhibit the typical S-shaped growth trend, which can be divided into three characteristic size intervals based on particle diameter: in the fine particle interval, the classification efficiency rises rapidly but remains at a low level (<20%), reflecting that fine particles tend to enter the overflow with the internal swirl flow and are difficult to be effectively separated. The sub-coarse particle interval shows a linear increase in classification efficiency, representing the core interval for evaluating classification accuracy, where the efficiency differences among different configurations are most pronounced. In the coarse particle interval, the classification efficiency stabilizes and approaches 100%, indicating that coarse particles can be efficiently captured by the external swirl flow, and the influence of structural variations on their separation capability is minimal. The triple-cone configuration achieves the highest classification efficiency across the entire particle size range; particularly in the sub-coarse particle interval (e.g., for 20 μm particles), its efficiency is nearly 20% higher than that of the single-cone configuration, with the steepest curve slope, demonstrating its superior classification accuracy for the critical separation particle size. The efficiency curves of the single-cone and double-cone configurations have relatively gentle slopes, with slow efficiency improvement in the sub-coarse particle interval, reflecting their insufficient capability to capture fine particles and low classification accuracy. The quadruple-cone configuration exhibits the lowest efficiency in the fine particle interval, and its efficiency growth in the sub-coarse particle interval lags behind that of the triple-cone configuration, which indicates that excessive segmented contraction induces flow pattern mixing and conversely degrades the separation accuracy of fine particles.
Compared with the reported research results of hydrocyclone structural optimization, the triple-cone configuration in this study shows significant advantages in comprehensive performance: Ghodrat et al. optimized the cone type and achieved a pressure drop of about 2.0 × 105 Pa with a classification efficiency of 75% for 20 μm particles and Yang et al. designed a double-cone hydrocyclone with a steepness index of about 0.48 and a cut size of 15.2 μm, while the triple-cone configuration in this study achieves a classification efficiency of more than 90% for 20 μm particles with a lower pressure drop (1.8 × 105 Pa), a higher steepness index (0.55) and a reasonable cut size (17.5 μm). In addition, compared with the single-cone hydrocyclone commonly used in engineering, the triple-cone configuration reduces the split ratio by 46% and increases the steepness index by more than 30% while slightly increasing the pressure drop, which realizes the synergistic optimization of separation accuracy and energy consumption.
The differences in efficiency curves can be directly correlated with the flow field analysis presented earlier. The triple-cone configuration features a stable zero-velocity envelope surface, a moderately sized air core, and low turbulence intensity, which collectively ensure the directional migration of particles, thus yielding optimal efficiency in the sub-coarse particle interval. The quadruple-cone configuration suffers from excessive intensification of the low-pressure field and turbulence resurgence, which trigger flow pattern mixing and result in low efficiency in the fine particle interval. The single-cone configuration is afflicted by high turbulence intensity and unstable bidirectional flow boundaries, leading to insufficient classification accuracy in the sub-coarse particle interval.
Pressure drop refers to the pressure difference between the inlet and outlet of a hydrocyclone, directly reflecting the energy loss during operation; a higher pressure drop indicates greater energy consumption of the equipment. The split ratio is defined as the percentage of the mass of underflow material relative to the total feed mass, reflecting the mass distribution ratio of material between the overflow and underflow. An excessively high split ratio typically implies the loss of fine particles into the underflow, accompanied by low classification accuracy, whereas an excessively low split ratio may indicate the entrainment of coarse particles into the overflow. These two parameters represent the core trade-off indicators for energy consumption control and separation performance in hydrocyclone design: pressure drop determines the operational cost of the equipment, while the split ratio reflects the rationality of material distribution. Their synergistic variation can directly characterize the capability of structural design to balance low energy consumption and high separation accuracy.
The profiles of pressure drop and split ratio in the ZX plane of hydrocyclones with different cone segment configurations are presented in
Figure 16. The pressure drop exhibits an overall increasing trend with the rise in the number of cone segments: the single-cone configuration yields the lowest pressure drop of approximately 1.35 × 10
5 Pa, while the quadruple-cone configuration achieves the highest pressure drop, approaching 1.9 × 10
5 Pa. This observation indicates that multi-stage contraction increases flow channel resistance, thereby elevating operational energy consumption. In contrast, the split ratio presents a trend of first decreasing and then slightly rising with the increase in the number of cone segments, followed by a slight decline, which reflects the regulatory effect of segmented contraction on material distribution in the hydrocyclone: the single-cone configuration has the highest split ratio of about 5.2%; the double-cone configuration drops sharply to 1.8% due to the first-stage segmented contraction; the triple-cone configuration rises slightly to 2.8% with the optimal flow field synergy effect; and the quadruple-cone configuration decreases again to 2.0% as excessive segmentation causes flow field disturbance and partial fine particle entrainment. This phenomenon demonstrates that appropriate segmented contraction can effectively reduce the entrainment of fine particles in the underflow and optimize the rationality of material distribution, while excessive segmented contraction will cause flow field disturbance, leading to a slight rebound or secondary decline of the split ratio and failing to further improve the material distribution effect.
Although the single-cone configuration features the lowest energy consumption, its high split ratio suggests substantial loss of fine particles into the underflow, resulting in low classification accuracy, which represents a typical low-energy-consumption but low-performance operation mode. The triple-cone configuration exhibits a moderate increase in pressure drop compared with the single-cone configuration, but its split ratio decreases significantly. Combined with the classification efficiency curves discussed earlier, it achieves optimal efficiency in the sub-coarse particle interval, realizing a balance between moderate energy consumption and high separation performance. The quadruple-cone configuration has the highest pressure drop, yet its split ratio does not undergo further optimization, indicating that excessive segmented contraction only increases energy consumption without delivering continuous improvement in material distribution, corresponding to a high-energy-consumption mode with marginal performance gains.
The combination of pressure drop and split ratio of the triple-cone configuration corresponds to the optimal interval of classification efficiency. The moderate pressure drop ensures sufficient separation driving force, while the rational split ratio reduces fine particle loss; their synergistic effect underpins high classification accuracy. In contrast, the low pressure drop coupled with high split ratio of the single-cone configuration, as well as the high pressure drop paired with low split ratio of the quadruple-cone configuration, both lead to degraded classification performance due to the imbalance between energy consumption and material distribution. The triple-cone configuration achieves the optimal trade-off among the four designs; its combined pressure drop and split ratio not only guarantee separation driving force but also control material loss, making it the optimal structure that balances energy efficiency and separation performance.
The cut size is defined as the particle size corresponding to a classification efficiency of 50% in a hydrocyclone, directly reflecting the separation threshold of the equipment, i.e., the minimum particle size that can be effectively separated. The sharpness index is a quantitative metric characterizing the steepness of the classification efficiency curve; a higher value indicates a stronger discriminative ability of the hydrocyclone between target and non-target separation particle sizes, corresponding to higher classification accuracy. These two parameters jointly constitute the core quantitative indicators for evaluating the classification performance of hydrocyclones: the cut size defines the threshold range of separation, while the sharpness index determines the precision of separation. Their synergistic variation can comprehensively reflect the regulatory effect of structural design on classification accuracy.
The profiles of the cut size and sharpness index in the ZX plane of hydrocyclones with different cone segment configurations are presented in
Figure 17. The cut size exhibits an increasing trend with the rise in the number of cone segments: the single-cone configuration has a cut size of approximately 14.5 μm, while the quadruple-cone configuration reaches nearly 18 μm, indicating that an increase in the number of cone segments shifts the separation threshold of the hydrocyclone toward the coarse particle range. The sharpness index shows a trend of first increasing and then decreasing with the number of cone segments, peaking at approximately 0.55 for the triple-cone configuration and reaching the minimum for the single-cone configuration, which reflects that there exists an optimal range for classification accuracy with respect to the variation in the number of cone segments.
The single-cone configuration features the smallest cut size but the lowest sharpness index, demonstrating that although it can separate relatively fine particles, its ability to discriminate between target and non-target particle sizes is weak, resulting in low classification accuracy. The triple-cone configuration achieves the highest sharpness index, with its cut size maintained within a reasonable range of approximately 17.5 μm, indicating that it achieves the strongest discriminative ability for particles of different sizes while retaining a moderate separation threshold, thus yielding optimal classification accuracy. The quadruple-cone configuration has the largest cut size, but its sharpness index is lower than that of the triple-cone configuration, which reflects that excessive segmented contraction increases the separation threshold yet disrupts flow field orderliness, leading to a decline in classification accuracy.
An increase in the number of cone segments enhances the central low-pressure field and bidirectional flow structure, thereby elevating the separation threshold. The trend of the sharpness index first rising and then falling embodies the balance between flow field intensification and flow pattern stability: the triple-cone configuration achieves an optimal match between these two aspects, and the combination of its high sharpness index and reasonable cut size constitutes the core quantitative support for its superior classification efficiency. Both the weak discriminative ability of the single-cone configuration and the excessive intensification of the quadruple-cone configuration compromise the classification performance.