Turbulent Flow Cyclone Separator 3D

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F l ow i n a C y c l o n e S e p a r a t o rs o l v e d w i t h F EM L AB 3 . 1

F l ow i n a C y c l o n e S e p a r a t o r

Introduction

This model examines the air flow in a cyclone separator, a device that removes solid particles or moisture droplets carried in an air stream. The separator forces the air to swirl in a cyclonic fashion. The resulting centrifugal force deposits liquid droplets and solid particles on the container wall where they flow down into the collecting chamber. The air leaving the separator through the tube in the middle is free of particles. This technique is common in a broad range of industries from small separators (in the centimeter scale) to large separators (tens of meters across).

This model examines the turbulent flow in a cyclone and performs a particle-tracing study to determine the threshold particle mass for separation from the air flow.

Model Definition

Figure 1 shows the cyclone geometry for this study

Inlet

Outlet

Collection chamber

. It uses the k-ε model to describe

Figure 1: Cyclone-separator geometry.

F L O W I N A C Y C L O N E S E P A R A T O R | 1

turbulent flow in the reactor. The equations for the momentum balances and continuity are

ρt∂

∂U ∇ η ρCµσk-------+

k2

ε------⎝ ⎠

⎛ ⎞ ∇U ∇U( )T+( )⋅⋅– ρU ∇⋅ U ∇P+ + 0=

∇ U⋅ 0=

where ρ denotes the density of the fluid (in units of kg m-3), U represents the averaged velocity (m s-1), η is the dynamic viscosity (kg m-1 s-1), P is the pressure (Pa), k gives the turbulent energy (m2 s-2), ε is the dissipation rate of turbulence energy (m2 s-3), and Cµ is a model constant. You get the turbulence energy by solving the equations

ρt∂

∂k ∇ η ρCµσk-------+

k2

ε------⎝ ⎠

⎛ ⎞ ∇k⋅– ρU ∇k⋅+ =

ρCµk2

ε------ ∇U ∇U( )T+( )2 ρε,–

and you find the dissipation by solving the equations

ρt∂

∂ε ∇ η ρCµσε-------+

k2

ε------⎝ ⎠

⎛ ⎞ ∇ε⋅– ρU ∇ε⋅+ =

ρCε1Cµk ∇U ∇U( )T+( )2 ρCε2ε2

k-----.–

The model constants in the above equations come from experimental data and are set to the values in the following table:

CONSTANT VALUE

Cµ 0.09

Cε1 0.1256

Cε2 1.92

σk 1.0

σε 1.6

At the inlet the air flows at a rate of 1 m s-1; it also has the forced condition of flowing straight out at the top, in other words, you set the tangential velocity is zero. For all other boundaries, use the logarithmic wall function boundary condition.


Results and Discussions

Figure 2 shows the modulus of the velocity field and the turbulent kinetic energy in the cyclone. The top plot shows that the highest velocities arise at the entrance and in the beginning of the exit pipe. The bottom figure shows that the flow is very turbulent at the entrance to the vertical exit pipe as well as where the inlet flow hits the opposite wall.

Figure 2: Slice plot of the modulus of the velocity vector (top) and turbulent kinetic energy (bottom).


A streamline plot of the velocity field within the cyclone illustrates the fluid motion. Two such plots appear in Figure 3.

Figure 3: Flowlines depicting the flow in the cyclone separator. The top image shows four trajectories, each with an individual color. The bottom image shows the pressure as the color plot on the flowlines.


Turbulent Flow Cyclone Separator 3D

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