models.cfd.water purification reactor

8/12/2019 Models.cfd.Water Purification Reactor

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Solved with COMSOL Multiphysics 4.3a

2 0 1 2 C O M S O L 1 | W A T E R P U R I F I C A T I O N R E A C T O R

Wate r Pu r i f i c a t i o n R ea c t o r

Introduction

Water purification for turning natural water into drinking water is a process constituted

of several steps. At least one step must be a disinfectant step. One way to achieve

efficient disinfection in an environmentally friendly way is to use ozone. A typical

ozone purification reactor is about 40 m long and resembles a mace with partial walls

or baffles that divide the space into room-sized compartments (Ref. 1). When waterflows through the reactor turbulent flow is created along its winding path around the

baffles towards the exit pipe. The turbulence mixes the water with ozone gas that

enters through diffusers just long enough to inactivate micro-pollutants. When the

water leaves the reactor, the remaining purification steps filter off or otherwise remove

the reacted pollutants.

In analyzing an ozone purification reactor, the first step is to get an overview of the

turbulent flow field. The results from the turbulent-flow simulation can then be usedfor further analyses of residence time and chemical species transport and reactions by

adding more physics to the model. The current model solves for turbulent flow in a

water treatment reactor using the Turbulent Flow, k-interface.

Model Definition

M O D E L G E O M E T R Y

The model geometry along with some boundary conditions are shown in Figure 1.

The full reactor has a symmetry plane which is utilized to reduce the size of the model.


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2 | W A T E R P U R I F I C A T I O N R E A C T O R 2 0 1 2 C O M S O L

Figure 1: Model geometry. All boundaries except the inlet, outlet and symmetry plane arewalls.

D O M A I N E Q U A T I O N S A N D B O U N D A R Y C O N D I T I O N S

Based on this velocity and the diameter of the inlet, which is 0.4 m/s, the Reynolds

number is

Here v is the kinematic viscosity. The high Reynolds number clearly indicates that the

flow will be turbulent. This means that the flow must be modeled using a turbulence

model. In this case, you will use the k-model which is often used in industrial

applications, much because it is both relatively robust and computationally inexpensive

compared to more advanced turbulence models. One major reason to why the k-

model is inexpensive is that it employs wall functions to describe the flow close to walls

instead of resolving the very steep gradients there. All boundaries in Figure 1except

the inlet, the outlet and the symmetry plane, are walls.

The inlet velocity is prescribed as a plug flow profile. The turbulent intensity is set to

5 %and the turbulent length scale is specified according to Table 3-3in the section

Re U L

-------------

0.1 0.4

1 10 6---------------------- 4 105= = =
http://../cfd/cfd_ug_fluidflow_single.pdfhttp://../cfd/cfd_ug_fluidflow_single.pdf


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Theory for the Turbulent Flow Interfacesin the CFD Module Users Guide. A

constant pressure is prescribed on the outlet.

Notes About the COMSOL Implementation

Three-dimensional turbulent flows can take a rather long time to solve, even when

using a turbulence models with wall functions. To make this tutorial feasible, the mesh

is deliberately selected to be relatively coarse and the results are hence not

mesh-independent. In any model, the effect or refining the mesh should be

investigated in order to ensure that the model is well- resolved.

Results and Discussion

Figure 2shows the velocity field in the symmetry plane. The jet from the inlet hits the

top of the first baffle which splits the jet. One half creates a strong recirculation zone

in the first chamber. The other half continues down into the reactor and gradually

spreads out. The velocity magnitude decreases as more fluid is entrained into the jet.

Figure 2: Velocity field in the symmetry plane.
http://../cfd/cfd_ug_fluidflow_single.pdfhttp://../cfd/cfd_ug_fluidflow_single.pdf


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Figure 3gives a more complete picture of the mixing process in the reactor. The

streamlines are colored by the velocity magnitude, and their width are proportional to

the turbulent viscosity. Wide lines hence indicate high degree of mixing. Theturbulence in this model is mainly produced in the shear layers between the central jet

and the recirculation zones. The mixing can be seen to be relatively weak in the

beginning of the reactor and that it increases further downstream.

Figure 3: Streamlines colored by velocity. The width of the streamlines are proportional tothe turbulent viscosity.

Reference

1. http://www.comsol.com/stories/hofman_water_purification/full/

Model Library path: CFD_Module/Single-Phase_Tutorials/

water_purification_reactor


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Modeling Instructions

From the Filemenu, choose New.

M O D E L W I Z A R D

1 In the Model Wizardwindow, click Next.

2 In the Add physicstree, select Fluid Flow>Single-Phase Flow>Turbulent Flow>Turbulent

Flow, k-(spf).

3 Click Next.

4 Find the Studiessubsection. In the tree, select Preset Studies>Stationary.

5 Click Finish.

G L O B A L D E F I N I T I O N S

Parameters

1 In the Model Builderwindow, right-click Global Definitionsand choose Parameters.

2 In the Parameterssettings window, locate the Parameterssection.

3 In the table, enter the following settings:

G E O M E T R Y 1

You can build the backstep geometry from geometric primitives. Here, instead, use a

file containing the sequence of geometry features that has been provided for

convenience.

1 In the Model Builderwindow, under Model 1right-click Geometry 1and choose Insert

Sequence from File.

2 Browse to the models Model Library folder and double-click the file models/cfd/

water_purification_reactor_geom_sequence.mph .

3 Click the Build Allbutton.

The model geometry is now complete (see Figure 1).

M A T E R I A L S

Mater ial Browser

1 In the Model Builderwindow, under Model 1right-click Materialsand choose Open

Material Browser.

Name Expression Description

u_in 0.1[m/s] Inlet velocity


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2 In the Material Browserwindow, locate the Materialssection.

3 In the tree, select Built-In>Water, liquid.

4 Right-click and choose Add Material to Modelfrom the menu.

T U R B U L E N T F L O W , K -

Inlet 1

1 In the Model Builderwindow, under Model 1right-click Turbulent Flow, k-and

choose Boundaries>Inlet.

2 Select Boundary 1 only.3 In the Inletsettings window, locate the Boundary Conditionsection.

4 In the LTedit field, type 0.07*0.2[m].

5 Locate the Velocitysection. In the U0edit field, type u_in.

Symmetry 1

1 In the Model Builderwindow, under Model 1>Turbulent Flow, k-right-click

Boundariesand choose Symmetry.2 Select Boundary 3 only.

Outlet 1

1 Right-click Boundariesand choose Outlet.

2 Select Boundary 28 only.

M E S H 1

1 In the Model Builderwindow, under Model 1click Mesh 1.

2 In the Meshsettings window, locate the Mesh Settingssection.

3 From the Element sizelist, choose Coarser.

Size 1

1 Right-click Model 1>Mesh 1and choose Edit Physics-Induced Sequence.

2 In the Model Builderwindow, under Model 1>Mesh 1right-click Size 1and choose

Disable.

Boundary Layer Properties 1

1 In the Model Builderwindow, expand the Model 1>Mesh 1>Boundary Layers 1node,

then click Boundary Layer Properties 1.

2 In the Boundary Layer Propertiessettings window, locate the Boundary Layer

Propertiessection.


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3 In the Number of boundary layersedit field, type 2.

4 In the Thickness adjustment factoredit field, type 6.

5 In the Model Builderwindow, collapse the Mesh 1node.

6 Click the Build Allbutton.

Confirm that the mesh matches the figure.

Next, solve for the flow field. This takes approximately 15 minutes on a quad-core

desktop computer.

S T U D Y 1

In the Model Builderwindow, right-click Study 1and choose Compute.

R E S U L T S

The following steps reproduce Figure 2.

Create a data set that corresponds to the non-wall boundaries.

Data Sets

1 In the Model Builderwindow, under Resultsright-click Data Setsand choose Surface.

2 Select Boundaries 1, 3, and 28 only.


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Velocity (spf)

1 In the Model Builderwindow, expand the Results>Velocity (spf)node.

2 Right-click Slice 1and choose Disable.

3 In the Model Builderwindow, click Velocity (spf).

4 In the 3D Plot Groupsettings window, locate the Datasection.

5 From the Data setlist, choose Surface 3.

6 Right-click Velocity (spf)and choose Surface.

7 In the Surfacesettings window, locate the Datasection.

8 From the Data setlist, choose Surface 2.

9 Locate the Coloring and Stylesection. From the Coloringlist, choose Uniform.

10 From the Colorlist, choose Gray.

11 Right-click Velocity (spf)and choose Surface.

12 Right-click Velocity (spf)and choose Arrow Surface.

13 In the Arrow Surfacesettings window, locate the Coloring and Stylesection.

14 From the Arrow lengthlist, choose Logarithmic.

15 Select the Scale factorcheck box.

16 In the associated edit field, type 1.4.

17 In the Number of arrowsedit field, type 300.

18 From the Colorlist, choose White.

Velocity (spf)

1 In the Model Builderwindow, under Resultsclick Velocity (spf).

2 In the 3D Plot Groupsettings window, click to expand the Titlesection.

3 From the Title typelist, choose Manual.

4 In the Titletext area, type Velocity field.

5 Click the Plotbutton.

R E S U L T S

Proceed to reproduce Figure 3as follows.

1 In the Model Builderwindow, under Results>Velocity (spf)right-click Surface 1and

choose Copy.

3D Plot Group 4

1 Right-click Resultsand choose 3D Plot Group.


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2 In the Model Builderwindow, under Resultsright-click 3D Plot Group 4and choose

Paste Surface.

3 Right-click 3D Plot Group 4and choose Streamline.

4 Select Boundary 1 only.

5 In the Streamlinesettings window, locate the Streamline Positioningsection.

6 In the Numberedit field, type 45.

7 Locate the Coloring and Stylesection. From the Line typelist, choose Ribbon.

8 In the Width expressionedit field, type spf.nuT*1[s/m].

9 Select the Width scale factorcheck box.

10 In the associated edit field, type 100.

11 Right-click Results>3D Plot Group 4>Streamline 1and choose Color Expression.

12 In the Color Expressionsettings window, click to expand the Rangesection.

13 Select the Manual color rangecheck box.

14 In the Minimumedit field, type 0.

15 In the Maximumedit field, type 0.1.

16 In the Model Builderwindow, click 3D Plot Group 4.

17 In the 3D Plot Groupsettings window, click to expand the Titlesection.

18 From the Title typelist, choose Manual.

19 In the Titletext area, type Streamlines colored by velocity. Width

proportional to turbulent viscosity..

20 Click the Plotbutton.


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