Numerical Simulation of the Influence of Viscosity on the Performance of a Two-Stage Centrifugal Separator

Document Type : Research Article

Authors

1 Dept. of Mining and Metallurgy, Yazd University, Yazd, Iran

2 Dept. of Mechanical Engineering, Yazd University, Yazd, Iran

10.29252/anm.2019.12439.1406

Abstract

Summary
In this research, computational fluid dynamics (CFD) and discrete phase model (DPM) were used to investigate the influence of medium viscosity on the flow field and the performance of a two-stage centrifugal separator. Volume of fluid (VOF) and Reynolds stress model (RSM) were used to tack the liquid-liquid interface and turbulence modeling, respectively.
 
Introduction
Two-stage centrifugal separators are the most novel generation of gravity separators that are increasingly used in the beneficiation of coal and minerals. However, the flow behavior and pattern in these separators are very complex and the mechanism of separation in these two-stage separators and the effects of different parameters on their performance has rarely been reported. The purpose of this paper is to review the influence of medium viscosity on the fluid flow and the performance of the separator. On this basis, an experimental apparatus was built. In this research, a CFD and discrete phase model (DPM) has been performed. The air core pattern and size, the pressure drop across the separator, the velocity and pressure fields, and the turbulence intensity were studied in response to the changing parameters.
 
Methodology and Approaches
An experimental setup including a 70 mm Two-stage centrifugal separator that was made of transparent Plexiglas and the complementary components such as two tanks, pumps, and instruments were manufactured to perform the physical experiments. The effects of viscosity were changed in three different simulations run. Using the transparent body for the separator, the internal flow pattern including the air core behavior was recorded and the required data were consequently extracted via the image processing technique. The pattern and the size of the air core in each stage of the Tri-Flo separator and the particle fate were used for the validation of CFD simulation results. The CFD simulation was performed with two Intel(R) Xeon(R) CPU E5-2630 v3 @ 2.40GHz, 32 GB RAM memory, and 2 TB hard-disc memory using Ansys Fluent version 18.0. The duration of each simulation was about 144 hours.
 
Results and Conclusions
With an increase in the medium viscosity, the performance of the separator was decreased as a consequence of decreasing the tangential velocity, and the air volume fractions inside the separator. Moreover, a high viscosity increases the drag force and consequently the medium recovery into the floating stream. Such an effect consequently increased the separation density of the separator. When the medium viscosity was changed from 1×10-3 to 3.09×10-3 Pa.s, the medium recovery of the first and the second stages of the separator were increased by about 56% and 23.7 %, respectively. Ecart probable (Ep) values for the first and second stages were also increased by 98% and 104%, and the air volume fractions were decreased by about 38.73% and 29.35%, respectively.

Keywords

Main Subjects

Full Text

جداکننده دو مرحله‌ای گریز از مرکز در سال 1982 توسط رووف توسعه داده شد[1]. این جداکننده از دو محفظه استوانه‌ای در قالب یک دستگاه عملیاتی تشکیل شده است. جداکننده دو مرحله‌ای گریز از مرکز مانند سیکلون‌های واسطه سنگین برای جدایش ماده با ارزش از ناخالصی‌های همراه آن، از نیروی گریز از مرکز استفاده می‌کند. از تفاوت‌های بارز جداکننده دو مرحله‌ای نسبت به سیکلون واسطه سنگین، می‌توان به عدم نیاز به پمپ برای ورود ماده معدنی به محفظه جدایش و نیز قابلیت تولید همزمان سه محصول در یک دستگاه جداکننده اشاره کرد. جداکننده در هر مرحله دارای دو دهانه مماسی ورودی و خروجی در انتهای پایینی و بالایی است. وقتی سیال واسطه از طریق دهانه مماسی پایینی جداکننده تحت فشار ثابت به بخش حلزونی شکل وارد شود، در طول مسیر یک جریان چرخشی که به سمت بالا حرکت می‌کند، ایجاد می‌نماید. سیال با حرکتی چرخشی به سمت بالا حرکت می‌کند تا به دیواره انتهای بالایی جداکننده برخورد ‌کند که در این حالت بخشی از آن از طریق دهانه غوطه‌ور تخلیه می‌شود و مازاد آن نیز در خلاف جهت چرخش اولیه و در داخل جریان اولی به سمت پایین حرکت می‌کند و از طریق دهانه شناور خارج می‌شود. بنابراین جریان سیال در داخل جداکننده، از دو مارپیچ تشکیل شده است که بر خلاف جهت یکدیگر حرکت می‌کنند. مارپیچ داخلی به لایه باریکی از هوا که از سمت دریچه خوراک به سمت دریچه شناور جریان دارد، محدود می‌شود. مارپیچ بیرونی از کناره‌های ظرف به سمت خروجی غوطه‌ور حرکت می‌کند. وقتی ذرات ماده معدنی در فشار اتمسفر به محفظه اول جداکننده خوراک‌دهی می‌شود، گرداب اولیه تشکیل شده و حرکت چرخشی سیال، ذرات ورودی را به سرعت، بر مبنای اختلاف چگالی تقسیم می‌کند. ذراتی که چگالی کمتری نسبت به واسطه چرخنده دارند نمی‌توانند به داخل سیال نفوذ کنند و از طریق دهانه مرکزی پایینی جداکننده (جریان شناور) تخلیه می‌شوند. جریان خارج شده از این دهانه چگالی پایین‌تری نسبت به واسطه ورودی دارد. ذرات با چگالی بالاتر نسبت به واسطه چرخنده به داخل واسطه نفوذ کرده و به سمت دیواره جداکننده دو مرحله‌ای گریز از مرکز ته‌نشین می‌شوند و از طریق دهانه مماسی بالایی جداکننده (جریان غوطه‌ور) خارج می‌شوند[3-1].

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Journal of Analytical and Numerical Methods in Mining Engineering
Volume 10, Issue 23
August 2020
Pages 91-106

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History

  • Receive Date: 19 June 2019
  • Revise Date: 21 October 2019
  • Accept Date: 30 October 2019

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