CFD validation for flyash particle classification in hydrocyclones

Abstract

The investigation pertains to establishing a simulation methodology for understanding the flyash classification characteristics of a 76 and 50 mm diameter hydrocyclone where the work was carried out using commercially available CFD software. Comparative results on the simulated and experimental water throughput, split values are presented. Results indicted that there is a good match in water split between the experimental and simulated values with error values below 10% at different hydrocyclone designs. Further a discussion is made on the flow features at comparable ratio of cyclone diameter to spigot opening in the 76 and 50 mm designs. The vertical core region around the cyclone axis having static pressure equal to or below the atmospheric pressure is examined to be increasing in diameter from bottom of the spigot opening till the interface where vortex finder joins the main cylindrical cyclone body and remains more or less similar at the vortex finder outlet. The diameter of this zone at the spigot outlet is 0.6 and 5.4 mm at 3.2 and 9.4 mm spigot openings in case of 50 mm diameter hydrocyclone. The diameter of the core at spigot outlet is found to be around 9.2 and 11 mm at 15 and 20 mm for spigot openings in case of 76 mm diameter hydrocyclone.

Classification of flyash particulates is simulated through discrete phase modeling using particles injection technique and the simulated results are further validated with suitably performed experiments. With 50 mm diameter hydrocyclone, reasonable predictions are observed at 9.4 mm spigot opening. Considerable deviation in particle distribution points with this hydrocyclone is observed at narrowest spigot diameter of 3.2 mm. The simulated values of d₅₀ in case of 50 mm diameter hydrocyclone are 8 and 10 μm at 9.4 and 3.2 mm diameter spigot openings. Better predictions are obtained with 76 mm diameter hydrocyclone at both 10 and 15 mm diameter spigot openings. Similarly, the simulated d₅₀ values are 14 and 20 μm at 15 and 10 mm diameter hydrocyclones. Possible reasons for deviations in the results relating the spigot opening, solids concentration at the underflow and in turn role of slurry viscosity on the air core diameter are proposed.

Introduction

Hydrocyclone is a versatile unit operation for applications in liquid–liquid separation, liquid–solid separation, air–solid separation, and solid–solid separation. Due to several industrial advantages, like good separation efficiency, ease in operation, high throughput, less maintenance, less floor space requirement etc., the hydrocyclones are very popular in large number of industrial sectors like mineral processing, environmental, food processing, chemical engineering etc. The works related to understanding the principles began only in mid fifties (Kelsall, 1952) though the unit was originally invented in late 18th century. Kelsall’s studies on the axial, radial and tangential velocity profiles formed the basis for subsequent research on hydrocyclones. Due to the complex phenomenon involved in analyzing flow behavior coupled with non-availability of high-speed computational systems most of the research works till the recent past were focused on the empirical modeling (Lynch and Rao, 1975, Plitt, 1976). These models are material specific and their applications are limited to the boundary conditions within which the model suitability is evaluated. However, the advent of high speed computational systems, in last couple of decades made researchers focus performance simulations using Computational Fluid Dynamics (CFD) techniques (Pericleous and Rhodes, 1986, Hsieh and Rajamani, 1988, Monredon et al., 1992, Rajamani and Milin, 1992, Dyakowski and Williams, 1993, Dyakowski et al., 1994, Dyakowski et al., 1999, Malhotra et al., 1994, Hargreaves and Silvester, 1990, Devulapalli and Rajamani, 1996, Griffths and Boysan, 1996, Slack and Wraith, 1997, Slack and Boysan, 1998, Stovin and Saul, 1998, Suasnabar and Fletcher, 1999, Slack et al., 2000, Slack et al., 2003, Ma et al., 2000, Nowakowski et al., 2000, Nowakowski et al., 2004, Grady et al., 2002, Grady et al., 2003, Nowakowski and Dyakowski, 2003, Schuetz et al., 2004, Cullivan et al., 2003, Cullivan et al., 2004, Delgadillo and Rajamani, 2005, Narasimha et al., 2005, Narasimha et al., 2006, Udaya Bhaskar et al., in press). Gradual evolution has taken place from 2D axi-symmetric studies of hydrocyclones to 3D non-axisymmetric geometry simulation methodologies. Any numerical technique needs development of suitable methodology and extensive validation with the experimental data before its real applications in design and performance simulation. Among the reported studies, the validation was mostly carried out with water flow characteristics and only a few of them are related to solids separation behavior. The present study involves simulation of water flow behaviour at different design conditions of 50 and 76 mm diameter hydrocyclone and particle size distribution behaviour under both simulated and experimental conditions. Simulation was carried out using commercially available CFD software ‘Fluent 6.1.22’. The simulation results on cyclone cut size d₅₀ values at different spigot openings are validated with experimental data generated in the laboratory treating flyash at different concentration of solids in the feed slurry.