WANG Zhiyuan¹， SUN Zhanpeng^1，2， YANG Huandi¹， ZHANG Kaixuan¹， YAO Yang¹， YUE Sujia¹

1.School of Mechanical Engineering， Hebei University of Science and Technology， Shijiazhuang 050018， China；

2.Hebei Additive Manufacturing Industry Research Institute， Shijiazhuang 050018， China

Abstract

Objective　Vortex technology is widely used in industries such as petrochemicals， mineral processing，and environmental protection. Gas-solid two-phase vortex flow is an important component of this technology，playing a crucial role in gas-solid separation and particle classification. Particle movement directly affects equipment performance，and understanding particle motion within classifiers is key to its development.This study focuses on analyzing particle group motion characteristics inside the classifier under different operating conditions to advance particle grading techniques.

Methods　Infrared thermal imaging and visible light imaging techniques were used to observe the movement of particle groups in both the feed and near-wall areas of a double vortex classifier. The study explored the movement morphology，velocity， and temperature field distribution of particle groups under different inlet velocities and particle size distributions.To verify the feasibility of using infrared thermal imaging to study particle motion，preliminary experiments were conducted to capture the movement of particles falling in an open space. Preliminary results showed that the infrared thermal imaging could effectively capture the particle acceleration process，with an acceleration value of about 9.5 m/s²， consistent with gravitational acceleration. This demonstrated the viability of infrared thermal imaging for studying particle motion. The infrared radiation characteristics of objects enabled real-time， efficient，and non-contact imaging of high-temperature particle groups in the classifier，providing valuable experimental data for revealing the mechanisms underlying particle classification in complex environments.

Results and Discussion　Accelerated by vortex flow， the distribution range of particles increased rapidly in the circumferential direction. At the same time， the particles moved centrifugally and centripetally along the radial direction， generating a greater radial temperature gradient in the thermal images. The thermal imaging area stabilized toward a dynamic balance with more particles being fed. Particle motion varied notably with different sizes. Fine particles （5~10 μm） formed a subhigh temperature region downstream of the feed area， while particles sized 20~45 μm and 45~100 μm had the largest radial distribution ranges. Coarse particles （20~45 μm） did not form sub-high temperature regions. Some fine particles exhibited extended residence times in the annular space. Part of the fine particles moved centripetally under drag force， forming high concentration regions below the exhalation pipe. Other fine particles moved gradually to the wall at 0.546 s， indicating an unfavorable centrifugal motion. The high-concentration particle region rapidly expanded due to the 20~45 μm particles，with larger and smaller particles quickly exiting the target area. A large number of cut-size particles aggregated in the annular region of the feed pipe at 0.884 s， where centrifugal and drag forces were balanced， resulting in the longest residence time. Particles sized 45~100 μm aggregated at the wall under the centrifugal force. With the increase in inlet velocity， vortex intensity rose，increasing both the centrifugal and drag forces. This led to an increased radial motion rate of particles， reducing the high-concentration area. At an inlet velocity of 14 m/s，fine particles formed large-scale agglomerates and deviated from the annular region of the feed pipe. As the inlet velocity further increased to 18 m/s， vortex core oscillations became irregular， leading to increased asymmetry and instability，and large-scale particle agglomerates broke apart due to vortex flow.

Conclusion　The innovative application of infrared thermal imaging technology provides a significant advancement in the visualization and quantitative analysis of particle group motion， promoting the study of particle dynamics. The following conclusions are drawn.1）The initial particle groups rotated more rapidly under the influence of airflow，leading to significant large-scale agglomeration of fine particles（5~10µm） in the downstream area of the feeding zone，where they migrate towards the wall. Particles sized 20~45 µm tended to accumulate in the annular space of the feed tube due to their proximity to the cutoff size， resulting in significantly prolonged residence times，exceeding 0.884 seconds. Coarse particles （45~100µm） were propelled toward the wall in a much shorter time. Particle clusters of varying sizes formed bundles at horizontal angles ranging from 26.5° to 27°beneath the inlet. As particle size increased， the stability of these bundles decreased，with some disintegrating into smaller clusters.2） With the increase in inlet velocity， the intensity of vortex flow within the classifier rose，increasing centrifugal and aerodynamic drag forces on the particle groups，which accelerated their movement while reducing the area occupied by high-concentration particles.Thermal imaging identified high-concentration regions and boundaries of particle distribution. A quantitative analysis of the circumferential expansion speed of particle populations with varying sizes revealed that the movement rate did not exhibit a simple relationship with inlet air speed. Notably， fine particles demonstrated the highest circumferential movement rates. Accelerated by vortex，fine particles（5~10 μm） formed large-scale agglomerates.Coarse particles（45~100 μm） moved to the wall in the shortest time.The circumferential motion rate of fine particles reached a maximum of 2.9m/s at an inlet velocity of 14 m/s，about three times that of coarse particles.3） The interaction between particle size and flow field intensity significantly affected the heat transfer characteristics of particle clusters. Heating rates for clusters ranged from approximately 200 to 800℃/s， while cooling rate varied from about 150 and 400℃/s. At an inlet velocity of 10m/s， there was minimal difference in cooling rates and maximum temperatures among particles of different sizes， but heating rates for particles sized 20~45 µm were roughly double that of the other two sizes. As the inlet velocity increased， vortex intensity rose， which enhanced cooling rates across particle sizes. When the inlet air velocity further escalated to 18 m/s， resulting in increased flow field intensity， both heating and cooling rates for coarse particle groups remained relatively stable， and the temperature differential during heat transfer for fine particle groups reduced significantly. Consequently， their heating and cooling rates experienced a sharp decline until reaching a minimum value.4） There are still limitations in the experimental equipment and techniques. Particles with low concentrations in the edge regions cannot be captured by the infrared camera，resulting in reduced accuracy in the circumferential motion rate of fine particles.

Keywords： vortex flow field； airflow particle classifier； particle group motion； thermal imaging

Get Citation:WANG Zhiyuan， SUN Zhanpeng， YANG Huandi， et al. Particle group motion in a double vortex classifier［J］. China Powder Science and Technology， 2025， 31（3）： 1-13.