CFD Study of Aggregative Fluidization of Gas-Solid System

Abu R. M. IasirM. M. HossenM. T. Sowgath

Department of Chemical Engineering BUET, Dhaka

Outline Introduction

Background

Model Equations

CFD process

Experiment

Simulation

Results

Conclusion

Purpose of simulation of gas-solid fluidized bed

Introduction

-Widely used in process industries-Complex flow-CFD simulation may reduce need for experimental analysis-Simulation results help sizing equipment

Regarding these a 2D fluidized bed is simulated using FLUENT6.3.26 and the result is compared with some experimental observations.

Fluidization

The operation by which fine solids are transformed into a fluid like state through contact with a gas or liquid.

Background

Two types of Fluidization

Particulate Fluidization Aggregative Fluidization

Background (Contd’)

What is CFD?

The science of predicting fluid flow, heat transfer, mass transfer, chemical reactions, and related phenomena by solving the mathematical equations which govern these processes using a numerical process.

Background (Contd’)CFD-how it works

Analysis begins with a mathematical model of a physical problem

Conservation of mass, momentum and energy must be satisfied

through the region of interest

Fluid properties are modeled empirically

Simplifying assumptions are made in order to make the problem

tractable(e.g., steady-state, transient, incompressible, inviscid,

two dimensional)

Provide appropriate initial and boundary conditions for the

problem

Two approaches for Multiphase flows

1.Euler-Lagrange• Used for DPM

-Spray dryers, coal and liquid fuel combustion2.Euler-Euler

• VOF model-Free Surface flows , filling, sloshing

• Mixture Model-Sedimentation ,Cyclone separator

• Eulerian Model-Bubble columns, risers, particle suspension , fluidized beds

Background

Model Equations

( ) ( )k k k k ki

i

v ot x

2

3

4

s g g g sd

s r

v vC

d v

2

4.80.63d

r

CRev

'

k k ki k k ki kj

j

g

k k k i k i ki kij k ki

i i

v v vt x

pg v v v

x x

Equation of continuity

Equation of momentum

Syamlal- O’ Brien Drag model

CFD Process

Geometry of problem is defined . Volume occupied by fluid is

divided into discrete cells.

CFD simulation is performed in three steps-

Preprocessing

CFD Process contd.Processing-

A physical model is defined Boundary conditions are defined which

involves specifying of fluid behavior and properties at the boundaries.

Equations are solved iteratively as steady state or transient state.

Post processing

Analysis and visualization of data

Software to Solve the Process

FLUENTCFXMFIXFLOTRAN

We used- GAMBIT 2.2.30 for preprocessingFLUENT 6.3.26 for processing and post processing

SPLASHTIDALGASPAND MANY MORE

Experimental Observation

11.5 in

Air at 0.3 m/s

Simulation SetupParameters Value RemarksParticle density 1050 kg/m3 ResinGas Density 1.225 kg/m3

AirParticle Diameter 0.3 mm Uniform distributionInitial solid volume fraction 0.6 FixedRestitution coefficient 0.9 -Initial bed height 5.5 inch Fixed Bed width 11.5 inch Fixed Inlet boundary condition` 0.3 m/s Velocity inletOutlet boundary condition 1 atm Pressure OutputGrid Spacing 0.05 inch FixedTime step 0.002 s SpecifiedConvergence allowance 0.001 Specified

Results

t= 0 s t=0.6s t=1.5s t=4s t=6s t=8s

Time Series of Volume fraction of solids Animation

Change of void fraction with respect to time and radial distance

Time(s) Radial distance(inch)

Conclusion Experimental phenomena is similar to that found in

literature Visualization of the simulation is almost similar to

experimental observation Difference in the bed expansion between

simulation(9 inch) and experiment(7 inch) is small Single diameter particle(0.3 mm) is used in

simulation but in experiment a range of diameter of particles(0.2-0.4 mm) is used.

Better simulation result could be found if multiple particle size distribution is used with enough time ,large iteration and smaller time steps.

Thank You