Chapter 27: Spray Dryer

This tutorial includes:

 

27.1. Tutorial Features

In this tutorial you will learn about:

  • Importing a CCL file in CFX-Pre.

  • Editing and creating boundary conditions in CFX-Pre.

  • Adding particles that evaporate.

  • Creating a domain interface in CFX-Pre.

  • Creating contour plots and inserting particle tracking in CFD-Post.

Component

Feature

Details

CFX-Pre

 

User Mode

General mode

Analysis Type

Steady State

Fluid Type

General Fluid

Domain Type

Single Domain

Boundary Conditions

Water Nozzle

Air Inlet

Outlet

Domain 1 Default

Domain Interface

Fluid Fluid

Timescale

Physical Timescale

Particle Coupling Control

Selected

Extra Output Variables List

Selected

CFD-Post

Plots

Contour Plots

Particle Tracking

 

27.2. Overview of the Problem to Solve

In this example, a spray dryer is modeled in which water drops are evaporated by a hot air flow. The goal of this tutorial is to observe the variation of gas temperature and mass fraction of water vapor, and of averaged values of mean droplet diameter and droplet temperature in the spray dryer, as well as the temperature and size of individual water drops as they travel through the spray dryer.

The following figure shows approximately half of the full geometry. The spray dryer has two inlets named Water Nozzle and Air Inlet, and one outlet named Outlet. The Water Nozzle is where the liquid water enters in a primary air flow at a mass flow rate of 1.33e-4 kg/s. The Air Inlet is for the swirling, drying air flow. The Water Nozzle inlet is located in the middle of the circular Air Inlet. When the spray dryer is operating, the inlets are located at the top of the vessel and the outlet at the bottom.

Periodic boundaries are used to enable only a small section of the full geometry to be modeled. The geometry to be modeled consists of a 9 degree section of the axisymmetric dryer shape. The relevant parameters of this problem are:

  • Static temperature at Water Nozzle = 300 K

  • Size distribution for the drops being created by the Water Nozzle is prescribed using discrete diameter values and associated fractions of the droplet mass flow rate.

  • Air Inlet mass and momentum axial component = 30 m/s (downwards along the axis of the spray dryer), Air Inlet mass and momentum radial component = 0 m/s, Air Inlet mass and momentum theta component = 10 m/s

  • Static temperature at Air Inlet = 423 K

  • Relative pressure at Outlet = 0 Pa

  • Normal speed of Water = 10 m/s

The approach for solving this problem is to first import a CCL file with the fluid properties, domain and boundary conditions in CFX-Pre. Minor changes will be made to the information imported from the CCL file. Boundary conditions and a domain interface will also be added. In CFD-Post, contour plots will be created to see the variation of temperature, mass fraction of water, average mean particle diameter of liquid water, and averaged temperature of liquid water in the spray dryer. Finally, particle tracking will be used for plotting the temperature of liquid water.

If this is the first tutorial you are working with, it is important to review the following topics before beginning:

 

27.3. Preparing the Working Directory

  1. Create a working directory.

    Ansys CFX uses a working directory as the default location for loading and saving files for a particular session or project.

  2. Download the spray_dryer.zip file here

  3. Unzip spray_dryer.zip to your working directory.

    Ensure that the following tutorial input file is in your working directory:

    • spraydryer9.gtm

  4. Set the working directory and start CFX-Pre.

    For details, see Setting the Working Directory and Starting Ansys CFX in Stand-alone Mode.

 

27.4. Defining the Case Using CFX-Pre

This section describes the step-by-step definition of the flow physics in CFX-Pre for a steady-state simulation.

  1. In CFX-Pre, select File > New Case.

  2. Select General and click OK.

  3. Edit Case Options > General in the Outline tree view and ensure that Automatic Default Domain is turned off.

  4. Click OK.

  5. Select File > Save Case As.

  6. Under File name, type SprayDryer.

  7. Click Save.

Later in this tutorial, you will import a template that sets up, among other things, a domain.

 

27.4.1. Importing the Mesh

  1. Right-click Mesh and select Import Mesh > CFX Mesh.

    The Import Mesh dialog box appears.

  2. Configure the following setting(s):

    Setting

    Value

    File name

    spraydryer9.gtm

  3. Click Open.

 

27.4.2. Importing the Evaporating CCL Drops Model Template

Ansys CFX Command Language (CCL) consists of commands used to carry out actions in CFX-Pre, CFX-Solver Manager and CFD-Post. The physics for this simulation such as materials, domain, and domain properties will be imported from a CCL file. Model library templates contain CCL physics definitions for complex simulations and are located in the <CFXROOT>/etc/model-templates/ directory. You will first analyze the evaporating_drops.ccl model template and then import it into the simulation.

Note:  The physics for a simulation can be saved to a CCL (CFX Command Language) file at any time by selecting File > Export > CCL.

  1. Browse to <CFXROOT>/etc/model-templates/, open evaporating_drops.ccl with a text editor, and take the time to look at the information it contains.

    The template sets up the materials water vapor H2O with a thermal conductivity of 193e-04 W/mK and water liquid H2Ol, which enters from the Water Nozzle. Note that the water data could also have been imported from the library in CFX-Pre. The template also creates a continuous gas phase named Gas mixture containing H2O and Air Ideal Gas and a binary mixture of H2O and H2Ol, which determines the rate of evaporation of the water. A domain named Domain 1 that includes the Gas mixture and H2Ol as a fluid pair as well as an inlet boundary is also specified in the CCL file. The inlet boundary is set up with a default static temperature of 573 K.

    Note:  When viewing a template file in a text editor, be careful to avoid modifying the original file.

  2. Select File > Import > CCL

    The Import CCL dialog box appears.

  3. Under Import Method, select Append. This will start with the existing CCL already generated and append the imported CCL.

    Note:  Replace is useful if you have defined physics and want to update or replace them with newly-imported physics.

  4. Browse to <CFXROOT>/etc/model-templates/ and select evaporating_drops.ccl.

  5. Click Open.

Note:  An error message related to the parameter Location will appear in the message window. This error occurs as the CCL contains a location placeholder that is not part of the mesh. Ignore this error message as the issue will be addressed when Domain 1 is being edited.

 

27.4.3. Editing the Domain

The fluid domain imported in the CCL file will be edited in this section.

  1. In the tree view, right-click Domain 1, then click Edit.

  2. Configure the following setting(s):

    Tab

    Setting

    Value

    Basic Settings

    Location and Type

    > Location

     

    B34

    Domain Models

    > Buoyancy Model

    > Option

     

     

    Buoyant

    Domain Models

    > Buoyancy Model

    > Gravity X Dirn.

     

     

    0.0 [m s^-2]

    Domain Models

    > Buoyancy Model

    > Gravity Y Dirn.

     

     

    -9.81 [m s^-2]

    Domain Models

    > Buoyancy Model

    > Gravity Z Dirn.

     

     

    0.0 [m s^-2]

    Domain Models

    > Buoyancy Model

    > Buoy. Ref. Density

     

     

    1.2 [kg m^-3] [a]

    Fluid Specific Models

    Fluid

    Gas mixture

    Fluid

    > Gas mixture

    > Fluid Buoyancy Model

    > Option

     

     

     

    Non Buoyant [b]

    Fluid

    H2Ol

    Fluid

    > H2Ol

    > Fluid Buoyancy Model

    > Option

     

     

     

    Density Difference

    1. The buoyancy reference density is set to 1.2 as representative of air.

    2. Because any natural convection in the gas can be neglected, you can set the fluid to non buoyant.

  3. Click OK.

 

27.4.4. Creating and Editing the Boundary Conditions

In this section, the Inlet and Domain 1 Default boundary conditions that were imported in the CCL file will be edited. Two boundary conditions, Air Inlet and Outlet will also be created for the spray dryer simulation.

 

27.4.4.1. Water Nozzle Boundary

The inlet boundary where the water enters in a primary air flow will be renamed and edited with the particle mass flow rate set consistent with the problem description. The particle diameter distribution will be set to Discrete Diameter Distribution, which will enable us to have particles of more than one specified diameter. Diameter values will be listed as specified in the problem description. A mass fraction as well as a number fraction will be specified for each of the diameter entries. The total of mass fractions and the total of number fractions will sum to unity.

  1. In the tree view, under Domain 1, right-click inlet, then click Rename. Set the new name to Water Nozzle.

  2. In the tree view, right-click Water Nozzle, then click Edit.

  3. Configure the following setting(s):

    Tab

    Setting

    Value

    Basic Settings

    Boundary Type

    Inlet

    Location

    two fluid nozzle

    Boundary Details

    Mass and Momentum

    > Option

     

    Normal Speed

    Mass and Momentum

    > Normal Speed

     

    10.0 [m s^-1]

    Heat Transfer

    > Option

     

    Static Temperature

    Heat Transfer

    > Static Temperature

     

    300.0 K

    Fluid Values

    H2Ol

    > Mass and Momentum

    > Option

     

     

    Normal Speed

    H2Ol

    > Mass and Momentum

    > Normal Speed

     

     

    10.0 [m s^-1]

    H2Ol

    > Particle Position

    > Number of Positions

    > Number

     

     

     

    500 [a]

    H2Ol

    > Particle Mass Flow

    > Mass Flow Rate

     

     

    3.32e-6 [kg s^-1] [b]

    H2Ol

    > Particle Diameter Distribution

    > Option

     

     

    Discrete Diameter Distribution

    H2Ol

    > Particle Diameter Distribution

    > Diameter List

     

     

    5.9e-6, 1.25e-5, 1.39e-5, 1.54e-5, 1.7e-5, 1.88e-5, 2.09e-5, 2.27e-5, 2.48e-5, 3.11e-5 [m]

    H2Ol

    > Particle Diameter Distribution

    > Mass Fraction List

     

     

    10*0.1

    H2Ol

    > Particle Diameter Distribution

    > Number Fraction List

     

     

    10*0.1

    H2Ol

    > Heat Transfer

    > Option

     

     

    Static Temperature

    H2Ol

    > Heat Transfer

    > Static Temperature

     

     

    300.0 K

    1. The number of representative drops was chosen to be 500 through experience of particle transport calculations.

    2. Note that this mass flow is only 1/40th of the total mass flow rate of water because only a 9 degree sector is modeled.

  4. Click OK.

 

27.4.4.2. Air Inlet Boundary

A second inlet in which the swirling, drying air flow will enter will be created with temperature component, mass and momentum axial, radial and theta components set consistent with the problem description.

  1. Select Insert > Boundary from the main menu or click Boundary  .

  2. Under Name, type Air Inlet.

  3. Click OK.

  4. Configure the following setting(s):

    Tab

    Setting

    Value

    Basic Settings

    Boundary Type

    Inlet

    Location

    air inlet

    Boundary Details

    Mass and Momentum

    > Option

     

    Cyl. Vel. Components

    Mass and Momentum

    > Axial Component

     

    -30.0 [m s^-1]

    Mass and Momentum

    > Radial Component

     

    0.0 [m s^-1]

    Mass and Momentum

    > Theta Component

     

    10.0 [m s^-1]

    Axis Definition

    > Option

     

    Coordinate Axis

    Axis Definition

    > Rotation Axis

     

    Global Y

    Heat Transfer

    > Option

     

    Static Temperature

    Heat Transfer

    > Static Temperature

     

    423.0 K

  5. Click OK.

 

27.4.4.3. Outlet Boundary

The outlet boundary will be created as an opening with pressure as specified in the problem description.

  1. Select Insert > Boundary from the main menu or click Boundary  .

  2. Under Name, type Outlet.

  3. Click OK.

  4. Configure the following setting(s):

    Tab

    Setting

    Value

    Basic Settings

    Boundary Type

    Outlet

    Location

    outlet

    Boundary Details

    Mass and Momentum

    > Option

     

    Average Static Pressure

    Mass and Momentum

    > Relative Pressure

     

    0.0 [Pa]

  5. Click OK.

 

27.4.4.4. Domain 1 Default

The Domain 1 Default boundary will be edited to use a heat transfer coefficient of 3.0 [W m^-2 K^-1] and an outside temperature of 300 K.

  1. In the tree view, right-click Domain 1 Default, then click Edit.

  2. Configure the following setting(s):

    Tab

    Setting

    Value

    Boundary Details

    Heat Transfer

    > Option

     

    Heat Transfer Coefficient

    Heat Transfer

    > Heat Trans. Coeff.

     

    3.0 [W m^-2 K^-1]

    Heat Transfer

    > Outside Temperature

     

    300.0 [K]

  3. Click OK.

 

27.4.5. Creating a Domain Interface

A domain interface will be created to connect the Domain 1, periodic1 and periodic 2 regions. The two sides of the periodic interface, periodic1 and periodic 2, will be mapped by a single rotational transformation about an axis.

  1. Select Insert > Domain Interface. Accept the default name.

  2. Configure the following setting(s):

    Tab

    Setting

    Value

    Basic Settings

    Interface Type

    Fluid Fluid

    Interface Side 1

    > Domain (filter)

     

    Domain 1

    Interface Side 1

    > Region List

     

    periodic1

    Interface Side 2

    > Domain (filter)

     

    Domain 1

    Interface Side 2

    > Region List

     

    periodic 2

    Interface Models

    > Option

     

    Rotational Periodicity

    Interface Models

    > Axis Definition

    > Rotation Axis

     

     

    Global Y

    Mesh Connection

    Mesh Connection Method

    > Mesh Connection

    > Option

     

     

    Automatic

  3. Click OK.

 

27.4.6. Setting Solver Control

  1. Click Solver Control  .

  2. Configure the following setting(s):

    Tab

    Setting

    Value

    Basic Settings

    Convergence Control

    > Max. Iterations

     

    100

    Convergence Control

    > Fluid Timescale Control

    > Timescale Control

     

     

    Physical Timescale

    Convergence Control

    > Fluid Timescale Control

    > Physical Timescale

     

     

    0.05 [s] [a]

    Convergence Criteria

    > Residual Type

     

    RMS

    Convergence Criteria

    > Residual Target

     

    1.E-4

    1. Based on the air inlet speed and the size of the dryer.

  3. Click OK.

 

27.4.7. Setting Output Control

In this section, two additional variables, H2O1.Averaged Mean Particle Diameter and H2O1.Averaged Temperature will be specified. These variables will be used when viewing the results in CFD-Post to understand the flow behavior.

  1. Click Output Control  .

  2. Configure the following setting(s):

    Tab

    Setting

    Value

    Results

    Extra Output Variables List

    Selected

    Extra Output Variables List

    > Extra Output Var. List

     

    H2Ol.Averaged Mean Particle Diameter, H2Ol.Averaged Temperature [a]

    1. Click the Ellipsis icon   to open the Extra Output Variable List dialog box, then select H2Ol.Averaged Mean Particle Diameter and H2Ol.Averaged Temperature, holding the Ctrl key. Click OK.

  3. Click OK.

 

27.4.8. Writing the CFX-Solver Input (.def) File

  1. Click Define Run  .

  2. Configure the following setting(s):

    Setting

    Value

    File name

    SprayDryer.def

  3. Click Save.

    CFX-Solver Manager automatically starts and, on the Define Run dialog box, Solver Input File is set.

  4. Quit CFX-Pre, saving the simulation (.cfx) file.

 

27.5. Obtaining the Solution Using CFX-Solver Manager

When CFX-Pre has shut down and the CFX-Solver Manager has started, obtain a solution to the CFD problem by following the instructions below.

  1. Ensure that the Define Run dialog box is displayed.

    Solver Input File should be set to SprayDryer.def.

  2. Click Start Run.

    CFX-Solver runs and attempts to obtain a solution. At the end of the run, a dialog box is displayed stating that the simulation has ended.

  3. Select Post-Process Results.

  4. If using stand-alone mode, select Shut down CFX-Solver Manager.

  5. Click OK.

 

27.6. Viewing the Results Using CFD-Post

In this section, contour plots located on one of the periodic regions of the spray dryer will be created to illustrate the variation of temperature, water mass fraction, liquid water average mean particle diameter and liquid water averaged temperature. Finally, particle tracking will be used for plotting the temperature of liquid water. Particle tracking will trace the mean flow behavior in and around the complex geometry of the spray dryer.

 

27.6.1. Displaying the Temperature Using a Contour Plot

A contour plot located at the Domain Interface 1 Side 1 region will first be created, and used to show the temperature variation through the spray dryer.

  1. Right-click a blank area in the viewer and select Predefined Camera > View From -Z.

    This ensures that the view is set to a position that is best suited to display the results.

  2. From the main menu, select Insert > Contour.

  3. Set the name to Temperature Contour. Click OK.

  4. Configure the following setting(s):

    Tab

    Setting

    Value

    Geometry

    Location

    Domain Interface 1 Side 1

    Variable

    Temperature

  5. Click Apply.

  6. When you have finished, right-click the contour you just created in the tree view and select Hide.

 

27.6.2. Displaying the Water Mass Fraction Using a Contour Plot

A contour plot located at the Domain Interface 1 Side 1 region will be created and used to show the H2O.Mass Fraction variation through the spray dryer.

 

27.6.3. Displaying the Liquid Water Averaged Mean Particle Diameter Using a Contour Plot

A contour plot located at the Domain Interface 1 Side 1 region will be created and used to show the H2Ol.Averaged Mean Particle Diameter variation through the spray dryer.

  • Repeat steps 1-6 in the Displaying the Temperature Using a Contour Plot section. In step 3, change the contour name to H2Ol Averaged Mean Particle Diameter Contour. In step 4, change the variable to H2Ol.Averaged Mean Particle Diameter. Click the Ellipsis icon   to open the Variable Selector dialog box in order to see the entire variable list.

 

27.6.4. Displaying the Liquid Water Averaged Temperature Using a Contour Plot

A contour plot located at the Domain Interface 1 Side 1 region will be created and used to show the H2Ol.Averaged Temperature variation through the spray dryer.

 

27.6.5. Displaying the Liquid Water Temperature Using Particle Tracking

This section outlines the steps for using particle tracking to trace the variation of the water temperature.

  1. Right-click a blank area in the viewer and select Predefined Camera > Isometric View (Z up).

    This ensures that the view is set to a position that is best suited to display the results.

  2. From the main menu, select Insert > Particle Track.

  3. Set the name to H2Ol Temperature. Click OK.

  4. Configure the following setting(s):

    Tab

    Setting

    Value

    Color

    Mode

    Variable

    Variable

    		H2Ol.Temperature

  5. Click Apply.

    From the contours and particle tracks, notice that the water droplets entering the spray dryer through the Water Nozzle recirculates in the region between the two inlets before merging in with the stream of hot air entering the spray dryer through the Air Inlet. Based on this flow of the water drop, the temperature of the hot gas coming from the Air Inlet decreases as the process takes place. During the spray drying cycle, the air transfers its thermal energy to the liquid water drops, leading to evaporation. As the air carries the thermal energy by convection, liquid water droplets that are close to the Air Inlet see their temperature increase, which leads to evaporation, resulting in a decrease in droplet diameter and an increase in the amount of water vapor.

 

27.6.6. Displaying the Diameter of a Water Drop Using Particle Tracking

This section outlines the steps for using particle tracking to trace the variation of water droplet diameter.

  1. Repeat steps 2-5 in the Displaying the Liquid Water Temperature Using Particle Tracking section. In step 3, change the name to H2Ol Mean Particle Diameter. In step 4, change the variable name to H2Ol.Mean Particle Diameter

    From the water drop diameter particle track, you can see that as the air from the Air Inlet transfers its thermal energy to the liquid water, the diameter of water drops decreases as they evaporate. So when the water drop move away from the Water Nozzle, its diameter decreases as a function of the temperature increase.

  2. Quit CFD-Post, saving the state (.cst) file at your discretion.