Chemical Engineering Journal 151 (2009) 39–45 Contents lists available at ScienceDirect Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej Numerical investigation of performance characteristics of a cyclone prolonged with a dipleg Fuat Kaya ∗ , Irfan Karagoz 1 Uludag University, Department of Mechanical Engineering, 16059 Bursa, Turkey article info Article history: Received 23 May 2008 Received in revised form 19 January 2009 Accepted 26 January 2009 Keywords: Swirling flows Separation efficiency Numerical simulation Lagrangian method abstract Numerical modeling of a particle separation process is carried out to understand the gas-particle two- phase flow field inside a cyclone prolonged with a dipleg and results of the numerical simulations are compared with experimental data to validate the numerical results. The flow inside the cyclone separator is modeled as a three-dimensional turbulent continuous gas flow with solid particles as a discrete phase. The continuous gas flow is predicted by solving Navier–Stokes equations using the differential RSM tur- bulence model with nonequilibrium wall functions. The second phase is modeled based on a Lagrangian approach. Analysis of computed results shows that the length of the dipleg considerably influences the cyclone separation efficiency rather than the cyclone pressure drop, especially for lower inlet velocities in relatively short cyclones, by providing more separation space. © 2009 Elsevier B.V. All rights reserved. 1. Introduction An efficient removal of particles from two-phase flows is essential to develop advanced separation technologies. Cyclone separators are widely used for this purpose, due to their sev- eral advantages such as simple design, absence of moving parts, low manufacturing and maintenance costs. Entrance of flow into cyclone can be axial or tangential through inlet section, which can be in different shapes for each cyclone. Cyclone separators oper- ate under the action of centrifugal forces. Fluid mixture enters the cyclone and makes a swirl motion and, due to the centrifugal forces, the particles in the flow gain a relative motion in the radial direc- tion and are separated from the main flow. It is difficult to analyze this problem, since this swirling flow is very complex, and there are many parameters influenced this flow. The main performance char- acteristics of a cyclone separator are collection efficiency, fractional efficiencies and pressure losses. Many experimental and theoretical studies performed on this difficult problem provide semiempirical models ranging from simple [1–4] to more comprehensive models [5,6] for the prediction of a cyclone performance. Due to the fact that the fluids dynamics of cyclones are com- plex, including highly turbulent structure and limitations in the usage of empirical models, optimization of cyclone performance by improving the cyclone efficiency and minimizing the pressure drop was essentially based on experiments rather than theoretical studies [7–14]. However, the computational fluid dynamics (CFD) ∗ Corresponding author. Tel.: +90 224 2941997; fax: +90 224 2941903. E-mail address: fkaya@uludag.edu.tr (F. Kaya). 1 Tel.: +90 224 2941911; fax: +90 224 2941903. of cyclones has exploded recently with advances in computer capa- bilities, numerical methods and software, and with the advent to the field of a large number of investigators [15–23]. Although many works have been carried out to investigate the influence of different geometric parameters such as cyclone length, inlet and outlet pipe geometries etc. on the performance of cyclones, there has been little work concerning the dust outlet geometries. Obermair et al. [24] performed cyclone tests with five different dust outlet geometries to find the influence of the dust outlet geometry on the separation process. They showed that sepa- ration efficiency can be improved significantly by changing the dust outlet geometry, and they reported that further research is needed to clarify precise effects of dust outlet geometry. The effect of a dipleg was posed and investigated by several researchers [25–27]. It is well known that in a reverse flow cyclone, the outer vortex flow weakens and changes its direction at a certain axial distance from the vortex finder. This axial magnitude has been called the “natural length” or the “vortex length” of the cyclone, and the axial position is referred to as “the end of the vortex”. The influence of the dipleg on the vortex and flow characteristics was investigated by Gil et al. [27], and they found that the vortex end was related not only to inlet and vortex finder geometry, but also to inlet gas velocity and solid loading. They also found that a small amount of “underflow” drawn through the dust exit causes the vortex end to move down the dipleg and a higher dust loading caused the end of the vortex to move up the dipleg. Hoffmann et al. [28] studied the effect of cyclone length on separation efficiency and pressure drop, experimentally and the- oretically, by varying the length of the cylindrical segment of a cylinder-on-cone cyclone. They showed that the separation effi- ciency improves for certain ratio of L/D. However, they found an 1385-8947/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2009.01.040