Advanced CFD using ANSYS FLUENT-Cyclone Separator

Introduction

The purpose of this challenge is to to perform CFD Analysis on a cyclone seperator. We will be running the simulation as two cases with each containing multiple Sub-Cases

Case 1:Perform an analysis on the given cyclone separator model and apply four different boundary condition types at the inlet i.e. reflect, trap, escape and wall-jet and submit images on your findings on the different conditions

Case 2:Vary the number of particles through the inlet and run the simulation for 3 cases. Report the number of particles that pass through or remain in the cyclonic separator. Post images to show the same. For this question, use the “Reflect” option as the boundary condition for the Discrete Phase

Cyclone Seperator

A cyclonic separation is a method of removing particulates from an air, gas or liquid stream, without the use of filters, through vortex separation. When removing particulate matter from liquid, a hydrocyclone is used; while from gas, a gas cyclone is used. Rotational effects and gravity are used to separate mixtures of solids and fluids. The method can also be used to separate fine droplets of liquid from a gaseous stream.

A high speed rotating (air)flow is established within a cylindrical or conical container called a cyclone. Air flows in a helical pattern, beginning at the top (wide end) of the cyclone and ending at the bottom (narrow) end before exiting the cyclone in a straight stream through the center of the cyclone and out the top. Larger (denser) particles in the rotating stream have too much inertia to follow the tight curve of the stream, and thus strike the outside wall, then fall to the bottom of the cyclone where they can be removed. In a conical system, as the rotating flow moves towards the narrow end of the cyclone, the rotational radius of the stream is reduced, thus separating smaller and smaller particles.

Laminar flow Approach

The collection efficiency of a single particle size can be determined by a semiemperical approach, developed by Lapple (1951), in which the gas flow is considered laminar.

Assuming Laminar Flow the fractional collection efficiency is defined as

where η is the fractional efficiency, N is the effective number of turns, ρ is the particle density, d is the particle diameter, V is the gas velocity, μ is the gas viscosity, Q is the volumetric flow rate of gas, and H and W are defined as in Fig. 8.8. This equation has one main deficiency in that it predicts that there is a finite value of d_p beyond which the collection efficiency is always 100%, whereas experimental evidence shows that efficiency approaches 100% asymptotically with increasing particle diameter.

Iozia and Leith

Any design method based on theory depends on the accurate prediction of efficiency and pressure drop. Dirgo and Leith(1985) developed an optimization program to investigate design changes that would improve cyclone separator design.Owing to the poor prediction by the efficiency and pressure drop theories used, substantial improvement in performance was not shown by pilot scale cyclones designed according to the program. Thus, efficiency and pressure drop theories with better predictive capabilities are needed. Subsequently, Iozia and Leith (1990) developed an improved method to predict cyclone aerodynamic cut diameter, Aerodynamic cut diameter is the particle size collected with 50% efficiency. Iozia and Leith claimed that their method works better than the theories of Lapple (1950), Leith and Licht (1972), and Dietz (1981).

Leith and Licht

The laminar flow model has limitations in that the gas flow in the cyclone is neither laminar nor fully turbulent; therefore, other models have been developed. A theory developed by Leith and Licht has proved useful in practical cyclone design. In their model, cyclone collection efficiency is given by

where

and

where D1 is the cyclone diameter in meters, T is the gas temperature in Kelvin, D is the particle diameter in centimeters, ρ is the particle density in g/cm³, Q is the volumetric flow rate in m³/s, μ is the gas viscosity in g/cm-s and K is a geometric configuration parameter that depends only on the relative dimensions of the unit.

Pressure Drop Empirical Model.

Pressure drop is an important operating parameter because increased pressure drop results in higher operating costs. Higher cyclone efficiencies typically come with higher pressure drop; therefore, a compromise must be made between collection efficiency and pressure drop. Often, a cyclone must be designed around a given system pressure drop. There are several pressure drop equations in the literature but most have not been found to be more accurate than the equation developed by Lapple:

where ΔP is the pressure drop (Pa), ρ the gas density (kg/m³), V_g the gas velocity (m/s) and H_v the number of velocity heads defined by

where H, W, and D_e are defined and K is an empirical constant with a value of 16 for a tangential inlet cyclone and 7.5 for one with an inlet vane

Procedure

Open the spaceclaim module and import the Model. Extract the volume of the Cyclone seperator.

Open the meshing module and mesh the volume as required. Name inlet and outlet

Open the fluent module.Select the viscous model. And setup the required settings.

Open the post processing module and setup the required contours.

Results

Case 1: Different Boundary Conditions,All cases have Mesh of size 10mm

Reflect

Numbers tracked: 364, Escaped:294,Incomplete:70

Trap

Numbers Tracked:364,Escaped:291,Incomplete:73

Escape

Numbers Tracked:364,Escaped:291,Incomplete:73

Wall-Jet

Numbers Tracked:364,Escaped:292,Incomplete:72

Case 2: Different Mesh sizes

12mm Mesh Size

Numbers Tracked:226,Escaped:208,Incomplete:18

10mm Mesh Size

Numbers Tracked:364,Escaped:294,Incomplete:70

9.15mm Mesh Size

Number tracked:384,Escaped:379,Incomplete:5

Conclusion

The higher the mesh refinement The more the number of particles in the cyclone seperator

The higher the mesh refinement the less time the particles remain in the cyclone seperator.

Escape Boundary condition has the most number of escaped particles from the cyclone separator.