Structural optimization of vortex finder for a centrifugal air classifier

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Highlights

  • Further optimization of the air classifier with different vortex finders

  • Non-uniform slotted vortex finder enhances the particle separation

  • Optimal secondary washing air improves the particle classifying performance

  • Classifying sharpness index (K) of 1.72 is obtained

Abstract

To further improve the centrifugal air classifier performance, effects of the fully-inserted vortex finders with slots are investigated. Experimental results show that the classifier with the non-uniform slotted vortex finder has the higher particle selectivity, smaller cut size and narrower product particle size distribution than the classifier with the uniform slotted vortex finder and the base partly-inserted vortex finder. The classifier obtains the particle classifying sharpness of 1.72 when the primary and secondary gas velocities are 12 m/s and 2.4 m/s. The experimental curves present a same distorted S-shape with the fish-hook dip at about 4 μm. Simulation results indicate that the non-uniform slotted vortex finder could accelerate the swirling flow with the gas tangential velocity averagely increased by 3 m/s. The static pressure varies from −350 Pa to 1700 Pa. Meanwhile, the slotted vortex finder can reduce the influence of the short circuit flow on coarse particles. The DPM model can well predict the cut size with a maximum error of 4.9%, but not the fish-hook effect. The obtained results can provide a guideline for the vortex finder design of the centrifugal air classifier.

Introduction

The centrifugal gas classifiers are widely used in mineral, cement industry, agriculture, and resource recycling (Shapiro and Galperin, 2005; Lanzerstorfer, 2015). The classifiers’ performance is mainly determined by the structure design and operating parameters. Shapiro and Galperin (2005) noted that the gas flow in a centrifugal gas classifier is similar to that in a cyclone separator. Shah et al. (2009) have tried to improve the performance of the classifier by optimizing the vane settings. Fu et al. (2016) achieves orderly sorting of different sized particles at the outlet. A pneumatic type ultrafine classifier with a maximum diameter of 250 mm was presented by Morimoto and Shakouchi (2003). A gas classifier with an arrangement distributing the gas-particle flow was invented by Schwamborn and Smigerski (1999). Particles enter the classifier from the tangential gas-solid inlet, and this flow disturber can improve the particle dispersion near the classifier wall. Introducing the additional water or gas injection can also reduce the fine particles entrainment (Tao and Wang, 2009; Mi, 2012; Dueck et al., 2010; Golyk et al., 2011).

Johansson and Evertsson (2014) used CFD simulations to confirm an undesirable circulatory flow pattern and fine particle backflow in a centrifugal classifier. The fine particles are more easily influenced by the vortex finder diameter (Cepuritis et al., 2015), and decreasing the vortex finder diameter may decrease the cut size as reported in cyclone separators (Jihui et al., 2007; Ghodrat et al., 2010; Cuizhi et al., 2010). Sun et al. (2017a) optimized the inserted depth and diameter of the vortex finder and the corresponding operating parameters to improve the particle classifying performance.

To further increase the particle classifying efficiency, different fully-inserted vortex finders with slots are designed based on our previous work (Sun et al., 2017a). Then the flow field and the classifying performance of the air classifiers are investigated experimentally and numerically.

Section snippets

Vortex finder structures

The vortex finder configuration is displayed in Fig. 1. The base classifier is equipped with the partly-inserted vortex finder (PI-VF). To reduce the particle short-circuiting and increase the particle classifying time, the fully-inserted vortex finders with one slot and uniform four slots are designed, named non-uniform slotted vortex finder (NS-VF) and uniform slotted vortex finder (US-VF). The effective cross-sectional areas are identical for the classifiers. The dimensions are listed in

Variations of particle size distribution

The vortex finder structure has great influence on the particle size distribution (PSD) of the collected coarse fraction, as shown in Fig. 3a. The differential frequency of particle less than 30 μm in coarse fraction is 1%, which is greatly decreased than that in raw material. Thus, the classifier obtains a narrow PSD after the particle classifying process.

By analyzing the PSD of coarse friction and the raw material, the grade efficiency can be curved (Altun and Benzer, 2014; Guizani et al.,

Static pressure distributions

To further investigate the classifying mechanism of the classifiers, effects of the vortex finder structure on flow field are simulated. Fig. 6 shows the contours of the static pressure inside the classifiers with different vortex finders. The static pressure reaches its highest value of 1700 Pa near the wall and negative pressure zone appears in the central part of the classifier. This causes a progressing vortex core in the gas classifier as happens in cyclone separators (Lucca-Negro and

Conclusion

Effects of the vortex finder structure are investigated by experimental and numerical method. The conclusions are summarized as follows:

  • (1)

    Particle classifying efficiency increases obviously when the air classifier is equipped with the non-uniform slotted vortex finder. The classifier obtains the particle classifying sharpness index K of 1.72. Moreover, considering the wear of the fully inserted vortex finder with slots after a period of time, the vortex finder has to be regularly checked and the

Conflict of interest statement

We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled “Structural optimization of vortex finder for a centrifugal air classifier”.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (No. 51904088) and Scientific research project of colleges and universities in Hebei province (No. BJ2020042) and Innovation funding project for Postgraduates in Hebei Province (No. CXZZSS2020095).

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