Experimental study on velocity field between two adjacent blades and gas–solid separation of a turbo air classifier

•Classification experiments for a turbo air classifier•Velocity field between two adjacent blades measured by PIV technique•Separation mechanism of a turbo air classifier•Effect of velocity field on separation efficiency for a turbo air classifier

The velocity field between two adjacent blades in a turbo air classifier was measured using the particle image velocimetry (PIV) technique equipped with a self-developed synchronizer. It was found that vortex swirling flow existed between the two blades or in the annular region between the impeller and the side wall. Under a given condition, the swirling intensity was higher while the swirling zone was smaller in the horizontal plane of the upper stream. As the impeller rotational speed S increased, the swirling became more intensive and the vortex center moved toward the side wall; when total air flow rate Q increased, the swirling intensity increased but the swirling center barely changed. The performance experiments showed that the overall classification efficiency η increased with S firstly and then decreased with the further increase of S. Correspondingly, non-monotonic variation trend was found for the cut size of the passing through particles d50. At a higher inlet solid concentration Cs, the dependence of d50 on S was more profound. d50 decreased significantly with the increasing Cs when Cs was smaller than a critical value and then barely changed when Cs kept increasing. For the tested turbo air classifier, when Q = 750 m3∙ h− 1, the minimum d50 was ~ 32 μm, occurring at the condition of Cs ≈ 0.08 kg∙m− 3 and S ≈ 120 rpm. The classification performance could be well explained by the velocity field near the entrance of the impeller and particle aggregation in the classifier.

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