Effects of the prolonged vertical tube on the separation performance of a cyclone

Abstract

This article aims at the gas flow into the dustbin of conventional cyclones, the prolonged cyclone (attaching a vertical tube at the bottom of the dust outlet) is proposed by some researchers, which can make flow with dust enter into the tube and separate further. The Reynolds stress transport model (RSTM) has been employed to predict the gas flow fields of the conventional and prolonged cyclones. The tangential velocity, axial velocity profiles and turbulent kinetic energy profiles are presented, and the downward flow rates into the dustbin of the three cyclones are compared. The separation performances of these three cyclones are tested. The result indicates that the tangential velocity, axial velocity and turbulent kinetic energy in the dustbin reduce greatly when the prolonged vertical tube attaching into the dust outlet, which can avoid the re-entrainment of already separated dust effectively. Furthermore, the prolonged vertical tube increases the separation space of dusts. The downward flow rate into the dustbin of the prolonged cyclone decreases compared with the conventional cyclone. The experimental results show that the prolonged vertical tube can improve the separation efficiency by a slightly increased pressure drop. However, for an even longer tube, the separation efficiency is slightly reduced. Thus, there is an optimal tube length for a given cyclone.

Introduction

The cyclone is a two-phase device for the separation of dispersed particles from their carrying fluid flow. It is now commonly employed in industry for the removal of dust dispersed solid particles which have a difference in density from that of air. In comparison with other types of dust removal devices, such as the electrostatic, fabric and wet collectors, the cyclone has the advantage of being simple in design, very reliable in performance and considerably low cost in maintenance. The cyclone also possesses a relatively large effective collection region and a relatively low-pressure drop. However, it is not very efficient and cost effective for the separation of very fine particles.

A dustbin is attached to the dust outlet for conventional cyclones. Some experiments had indicated that much gas flow entered into the dustbin through here. However, because the bottom of the dustbin is stifled, the gas flow will return and re-enter into the separation space, which will disturb some separated particles and bring them into the inner core vortex, and lead to so-called “re-entrainment”, thus will reduce the separation efficiency of the cyclone. Hoffmann et al. [1] and Obermair et al. [2], [3] have attached a vertical tube into the dust outlet, which can make flow with dust enter into the tube and separate further, and they have conducted detailed experiments on cyclones of different dust outlet geometries and found that these parts have important influence on separation efficiency of the cyclones. However, determining the influence of different dust outlets geometries and operation conditions on the separation efficiency of cyclones by means of experiments will waste a lot of time and resources. On the other hand, with the rapid development of the computer and computational fluid dynamics (CFD) techniques, the use of numerical simulations to predict the performance of the cyclone has received much attention and it is at present under intensive development [4], [5], [6], [7], [8], [9], [10], [11]. An evident advantage of CFD calculations with respect to experiments is that a large number of flow and geometry variables can be varied at relative low costs. Therefore, this paper used CFD simulation to study the gas flow fields of the conventional cyclone and the prolonged cyclones. In addition, comparison of the separation performances of the three cyclones is made to reveal how the length of the vertical tube influences their performances.