Elsevier

Powder Technology

Volume 348, 15 April 2019, Pages 65-69
Powder Technology

Short communication
A new volute design method for the turbo air classifier

https://doi.org/10.1016/j.powtec.2019.03.015Get rights and content

Highlights

  • A logarithmic spiral volute design method was proposed.

  • The distribution of the flow field was numerically simulated.

  • Single particle trajectory is simulated and tracked.

  • Theories about structure design of the classifiers were enriched.

Abstract

Volute profile plays an important role in guiding the airflow for a turbo air classifier. The traditional volute design is based on the method of analogue and there is a lack of uniform theoretical design basis and guidance. To meet the needs of guiding the airflow better, a logarithmic spiral volute design method is put forward based on the analyses of airflow motion in the volute and classification characteristics of the turbo air classifier. The numerical simulations on flow field distribution in the turbo air classifier show that the logarithmic spiral volute has a good effect on guiding the airflow and can form a well-distributed flow field in the classifier, which is conducive to improve the classification performance. The single particle trajectory verifies that the logarithmic spiral volute makes the airflow flow along the guide vanes circumferentially uniformly, forming a well-distributed flow field in the annular region of the classifier. So the classification results for the particles with the same size are in correspondence when the logarithmic spiral volute is applied. The numerical simulations of both gas phase and discrete phase indicate that the logarithmic spiral volute can provide a well-distributed flow field for the classification and this volute deign method for turbo air classifier is feasible.

Introduction

The turbo air classifier is widely used in the fields of fine chemical industry, ceramic and composite material manufacturing due to its simple structure and adjustable operating parameters [[1], [2], [3], [4]]. In recent years, with the development of powder technology, the performance of the classifier has been continuously improved, especially in the refinement and narrow distribution fields. The separation of coarse and fine particles is implemented in the turbo air classifier under the influence of the airflow field which influences classification performance of the classifiers directly. When the operating parameters are constant, the classifier structure determines the flow field distribution, even the classification performance. Comparing to the airflow distribution using positive bending, reverse bending and straight guide vanes, Huang Qiang concluded that the positive bending blades can reduce the swirl in the blade passages of the rotor cage, making the flow field well-distributed, improving classification accuracy, as well as reducing the cut size [5]. Liu Rongrong pointed out that the axial guide vanes with 2.5° are favorable to keep the flow field well-distributed and the classification force field is enhanced to improve the dispersion of the powders, which leads to an improved classification performance [6]. Sun Zhanpeng compared and analyzed the effect of air-inlet direction on the performance of classifier through powder classification experiments [7]. Zhao Dongmei did some researches on the influences of material feeding types on the classification performance [8]. Hideto Yoshida improved the air inlet structure of the cyclone separator and the new inlet provided a smaller cut size and a higher classification accuracy [9].

At present, the structure improvement of classifier is mainly focused on the rotor cage, guide vanes and spreading plate. However, there are few reports on the volute of the turbo air classifier. Before the air flows into the annular classification region, it will pass through the volute firstly. The structure of the volute determines the distribution of the airflow, the stability of the flow field and the classification performance. Since there is a lack of clear design method for the volute, the analogy method is often used to determine the volute profile of a turbo air classifier. The common volute profile types include involute, multi-arcs and approximate logarithmic spiral. However, in the turbine machinery such as centrifugal blower and gas turbine compressor, there is a set of complete volute design theory to standardize industrial applications [10]. In this paper, the volute design theory of turbine is introduced and applied to the turbo air classifier, combining its classification characteristics. A logarithmic spiral volute design method for the turbo air classifier is proposed. The well-distributed flow field of the air turbo classifier with the logarithmic spiral volute is feasible to improve the classification performance.

Section snippets

The formula derivation of volute profile

The structural scheme of the turbo air classifier used in the present study is shown in Fig. 1. Under central negative pressure, the airflow enters the annular classification region through the guide vanes from two horizontally symmetrical airflow inlets, forming a rotating velocity field. At the same time, the powder materials to be classified are fed onto the feeding plate, which is rapidly rotating together with the rotor cage. The particles are thrown outward into the annular region, which

Numerical simulation and analysis

To simulate the distribution of flow field in the turbo air classifier with this logarithmic spiral volute, Gambit 2.2.30 and ANSYS-FLUENT 17.0 were used to establish the model of a turbo air classifier and simulate its inner airflow field. The model was designed to describe the main classification regions, including the volute, the rotor cage and the center section. Structured Hex&Wedge meshes were applied to these regions and the Cooper algorithm was selected to create volumetric meshes.

Conclusions

A theoretical design method of logarithmic spiral volute profile is put forward, which can obtain a well-distributed flow field distribution in the annular region for the turbo air classifier. It is based on the design theory of the turbine machinery and the particles' motion characteristics in turbo air classifier. The logarithmic spiral volute profile can be expressed as the following formula:R=RheQ2πbRhVhtφ

Both the continuous phase and discrete phase simulations indicate that the logarithmic

Acknowledgement

This project is supported financially by the National Natural Science Foundation of China (No. 51204009).

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