DEM Tutorial

Modeling bubbling fluidized bed using DDPM+DEM Introduction The DEM collision model extends the DPM model in Fluent to model dense particulate flows. The model can be used with Dense DPM to account for effect of blockage of particles on primary phase solution. This is useful for modeling applications such as bubbling fluidized beds, risers, pneumatic conveying systems, and the flow of slurries. The DEM models is especially useful • •

When dealing with a wide particle size distribution When dealing with relatively coarse meshes

This document is a tutorial on the use of the DDPM model where collisions are modeled through DEM model.

Prerequisites This tutorial will not cover the mechanics of using the Dense DPM or DEM models. It will focus on the application of these models. For more information refer the ANSYS FLUENT User's Guide and Theory Guide. This tutorial is written with the assumption that you have completed Tutorial 1 from the ANSYS FLUENT 14.0 Tutorial Guide, and that you are familiar with the ANSYS FLUENT navigation pane and menu structure. Some steps in the setup and solution procedure will not be shown explicitly.

Problem Description In this tutorial we will model a bubbling fluidized bed and determine its behavior for a given superficial velocity. A rectangular bed of size 0.2m * 0.2m * 0.4m is initially charged with particles, and the superficial velocity of the gas is 0.5 m/s. The pressure drop across the bed is monitored. Schematic of the problem is shown in Figure.1. From the classic fluidization curve, if the superficial velocity of the inlet fluid is small, the bed is not fluidized and behaves like a packed bed. As the velocity of the fluid is increased, the bed begins to fluidize. One of the classical ways to understand the phenomena is the fluidization curve; here the pressure required to pump the fluid at the inlet is studied as a function of the superficial velocity. Under packed bed conditions, there is a linear increase in the pressure as the superficial velocity is increased. However, this increase begins to taper off as the condition of incipient fluidization is reached, and the pressure reaches a constant value (in a time averaged

sense). This constant pressure at fluidization conditions is sufficient to maintain the buoyant weight of the bed. In other words < ܲ >௜௡௟௘௧ × ‫ܣ‬௜௡௟௘௧ = Buoyant weight of bed In this tutorial we will perform simulations for a given superficial velocity where a bed is fluidized. It will be left upon the user to try with different superficial velocity to obtain the fluidization curve.

Figure.1: Schematic of problem description

Preparation A. Copy the files bed.msh, 92Kparcels.inj and view-0.vw to the working folder. B.

Use FLUENT Launcher to start the 3D version of ANSYS FLUENT. Note: For more information about FLUENT Launcher see Section 1.1.2 Starting ANSYS FLUENT using FLUENT Launcher in the ANSYS FLUENT 14.0 User's Guide.

C. Enable DoublePrecision in the Options list. Note: The Display Options are enabled by default. Therefore, after you read in the mesh, it will be displayed in the embedded graphics window.

Setup and Solution Note: All entries in setting up this case are in SI units, unless otherwise specified.

Step 1: Mesh a) Read the mesh file bed.msh. File
Read
Mesh...

Step 2: General a) Check the mesh. General
Check ANSYS FLUENT will perform various checks on the mesh and will report the progress in the console. Ensure that the minimum volume reported is a positive number. b) Enable the transient solver by selecting Transient from the Time list. General
Transient

Step 3: Models a) Multiphase model. Models
Multiphase
Edit… I. II. III.

Select Eulerian multiphase model. Enable Dense Discrete Phase Model. Retain other defaults and click OK.

Figure.2: Multiphase Model Panel

b) Discrete Phase Model. Models
Discrete Phase
Edit… I. Make sure that Update DPM Sources Every Flow Iteration. II. Enter 200 for Number of Continuous Phase Iterations per DPM Iteration. III. Make sure that Unsteady Particle Tracking is enabled. IV. Disable Track with Fluid Flow Time Step and enter 0.0002 for Particle Time Step Size (s). V. Enable DEM Collision under Physical Models tab. VI. Set Drag Law as Wen-Yu under Tracking Tab. VII. Set the following under Numerics tab. • Disable Accuracy Control. • Select implicit as Tracking Scheme. VIII. Click OK to close DPM panel.

Figure.3: Discrete Phase Model Panel c) Define Injection. Define
Injections…
Create I. Select file under Injection Type. II. Select phase-2 under Discrete Phase Domain. III. Enter 1e-8 for Stop Time (s). IV. Click File… button and select 92Kparcels.inj file from working folder. V. Click OK to close Set Injection Properties panel. VI. Click Close to close Injections panel.

Figure.4: Set Injection Properties Panel d) Set DEM collision laws. Models
Discrete Phase
Edit…
DEM Collisions… I. II. III. IV. V. VI. VII. VIII. IX. X.

Select dem-anthracite and click Set… This will open DEM Collision Settings panel. Select dem-anthracite – dem-aluminum from Collision Pairs. Retain spring-dashpot as Normal Contact Force and set friction-dshf for Tangential. Change spring-dashpot: k as 100 and spring-dashpot: eta as 0.5. Select dem-anthracite – dem-anthracite from Collision Pairs. Retain spring-dashpot as Normal Contact Force and set friction-dshf for Tangential. Change spring-dashpot: k as 100 and retain other settings. Click OK to close the panel. Click Close to close DEM Collisions panel. Click OK to close DPM panel.

Figure.5: DEM Collision Settings Panel

Step 4: Phases a) Set phase-2. Phases
Phase-2
Edit... I.

Deselect Volume Fraction Approaching Continuous Flow Limit and click OK.

Step 5: Operating Conditions Define
Operating Conditions… a) Enable Gravity and set Z component as -9.81 m/s2. b) Specify Operating Density to be 1.225 kg/m3.

Step 6: Boundary Conditions Boundary Conditions a) Set boundary conditions for inlet. I. Select inlet from zone list and click Edit… while phase is mixture. This will open Velocity Inlet panel for mixture phase. II. Go to DPM tab and select Discrete Phase BC Type to reflect and DEM Collision Partner as dem-aluminum. III. Click OK to close this panel. IV. Select phase-1 from Phase drop down list and click Edit…. V. Enter 0.5 for Velocity Magnitude (m/s) and click OK to close the panel. b) Set boundary conditions for outlet. I. Select outlet from zone list and click Edit… while phase is mixture. This will open Pressure Outlet panel for mixture phase. II. Go to DPM tab and select Discrete Phase BC Type to reflect and DEM Collision Partner as dem-aluminum. III. Click OK to close this panel. c) Retain default settings for wall.

Step 7: Solution Methods Solution Methods a) Select Green-Gauss Node Based from Gradient.

b) Select QUICK for Momentum and Volume Fraction Spatial Discretization.

Step 8: Solution Controls Solution Controls a) Set Under-Relaxation Factors for variables as given below. I. Pressure: 0.9 II. Momentum: 0.2 III. Volume Fraction: 1 IV. Discrete Phase Sources: 1

Step 9: Monitors Monitors
Surface Monitors
Create… a) b) c) d)

Create monitor of Area-Weighted Average of Static Pressure on inlet surface. Enable Plot and Write. Set X Axis as Flow Time and Get Data Every 1 Time Step. Click OK to close the panel.

Figure.6: Surface Monitor Panel

Step 10: Solution Initialization Solution Initialization a) Initialize with default settings. Click Initialize.

Step 11: Calculation Activities Calculation Activities
Execute Commands
Create/Edit… We will define four commands in this step which will save images of particle tracks colored by particle velocity magnitude at specified interval. These images can be clubbed together to create animation. a) Set Defined Commands to 4. b) Set execute commands as shown in the Figure.7 below. c) Set settings for saving images as shown in Figure.8. File
Save Picture…

Figure.7: Execute Commands Panel

Figure.8: Save Picture Panel

Step 12: Run Calculation Run Calculation We will perform this step in three stages. First we will perform calculation for single time step to inject all particles in the domain. We would then set post-processing parameters which would be used to save image files at specific intervals as entered in Step 11. In the second stage, we

would run the calculation for two seconds of flow time with Execute Commands enabled. In the last stage, case will be run for two more seconds without Execute Commands. Set Time Step Size (s) as 0.001. Set Number of Time Steps as 1. Set Reporting Interval as 5. Click Calculate. Read view-0 from file view-0.vw from working folder. Display
Views…
Read… f) Create iso-surface of y-coordinate=0. Name it as y=0. Surface
Iso-Surface… g) Display contour of phase-2 volume fraction on iso-surface y=0. Make sure Filled and Node Values are enabled. Graphics and Animations
Contours
Set Up… h) Set Light settings. Make sure that Light On and Headlight On are enabled. Select Lighting Method as Gouraud. Display
Lights… i) Set particle track settings as shown in Figure.9. Graphics and Animations
Particle Tracks j) Click on Attributes… button under Track Style and select Parcel Diameter as shown in Figure.10. k) Click on Filter by… button and select Y-Coordinate and set Filter-Min, Filter-Max as shown in Figure.11. l) Click Display.

a) b) c) d) e)

Figure.9: Particle Tracks Panel

Figure.10: Particle Sphere Style Attributes Panel

Figure.11: Particle Filter Attributes Panel m) Save the case file as fbed-first-t-step.cas.gz. n) Run calculation for 2000 time steps. o) Disable Execute Commands by setting Defined Commands to 0 under Execute Commands panel. p) Run calculation for 2000 time steps. q) Save case and data as fbed-final.cas.gz and fbed-final.dat.gz.

Step 13: Results a) Figure.12 shows contour plot of secondary phase volume fraction, contour plot of DPM Concentration and Particle tracks from the final data file. Notice that results are close. b) Figure.13 shows plot of pressure drop across the bed. Notice that mean value is close to the pressure drop equivalent for buoyant weight of the bed which is 981 Pa.

Figure.12: Results from final data file. A. Contour plot of secondary phase volume fraction B. Contour plot of DPM Concentration C. Particle Tracks colored by secondary phase volume fraction

Figure.13: Monitor plot for pressure drop across the bed