Tutorial 17. Modeling Evaporating Liquid Spray Introduction In this tutorial, the air-blast atomizer model in ANSYS FLUENT is used to predict the behavior of an evaporating methanol spray. Initially, the air flow is modeled without droplets. To predict the behavior of the spray, several other discrete-phase models, including collision and breakup, are used. This tutorial demonstrates how to do the following: • Define a spray injection for an air-blast atomizer. • Calculate a solution using the discrete phase model in ANSYS FLUENT. Prerequisites This tutorial is written with the assumption that you have completed Tutorial 1, 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 The geometry to be considered in this tutorial is shown in Figure 17.1. Methanol is cooled to -10 ◦ C before being introduced into an air-blast atomizer. The atomizer contains an inner air stream surrounded by a swirling annular stream. To make use of the periodicity of the problem, only a 30 ◦ section of the atomizer will be modeled. Release 12.0 c ANSYS, Inc. March 12, 2009 17-1
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Tutorial 17. Modeling Evaporating Liquid Spray
Introduction
In this tutorial, the air-blast atomizer model in ANSYS FLUENT is used to predict thebehavior of an evaporating methanol spray. Initially, the air flow is modeled withoutdroplets. To predict the behavior of the spray, several other discrete-phase models,including collision and breakup, are used.
This tutorial demonstrates how to do the following:
• Define a spray injection for an air-blast atomizer.
• Calculate a solution using the discrete phase model in ANSYS FLUENT.
Prerequisites
This tutorial is written with the assumption that you have completed Tutorial 1, andthat 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
The geometry to be considered in this tutorial is shown in Figure 17.1. Methanol is cooledto −10◦C before being introduced into an air-blast atomizer. The atomizer contains aninner air stream surrounded by a swirling annular stream. To make use of the periodicityof the problem, only a 30◦ section of the atomizer will be modeled.
2. Change the periodic type of periodic-a to rotational.
Boundary Conditions −→ periodic-a −→ Edit...
(a) Select Rotational in the Periodic Type list.
(b) Click OK to close the Periodic dialog box.
3. In a similar manner, change the periodic type of periodic-b to rotational.
Step 2: General Settings
General
1. Check the mesh.
General −→ Check
ANSYS FLUENT will perform various checks on the mesh and report the progressin the console. Ensure that the reported minimum volume is a positive number.
3. Enable chemical species transport and reaction.
Models −→ Species −→ Edit...
(a) Select Species Transport in the Model list.
(b) Select methyl-alcohol-air from the Mixture Material drop-down list.
The Mixture Material list contains the set of chemical mixtures that exist in theANSYS FLUENT database. You can access a complete description of the react-ing system by selecting one of the pre-defined mixtures. The chemical speciesin the system and their physical and thermodynamic properties are defined bythe selection of the mixture material. You can alter the mixture material selec-tion or modify the mixture material properties using the Create/Edit Materialsdialog box.
(c) Click OK to close the Species Model dialog box.
When you click OK, ANSYS FLUENT will list the properties that are requiredfor the models you have enabled. An Information dialog box will open, re-minding you to confirm the property values that have been extracted from thedatabase.
(d) Click OK in the Information dialog box to continue.
(b) Select air-blast-atomizer from the Injection Type drop-down list.
(c) Enter 60 for Number of Particle Streams.
This option controls the number of droplet parcels that are introduced into thedomain at every time step.
(d) Select Droplet in the Particle Type group box.
(e) Select methyl-alcohol-liquid from the Material drop-down list.
(f) Enter 0, 0, and 0.0015 for X-Position, Y-Position, and Z-Position, respectively,in the Point Properties tab.
Scroll down the list to see the remaining point properties.
(g) Retain the default values of 0, 0, and 1 for X-Axis, Y-Axis, and Z-Axis, respec-tively.
(h) Enter 263 K for Temperature.
(i) Enter 8.5e-5 kg/s for Flow Rate.
This is the methanol flow rate for a 30-degree section of the atomizer. Theactual atomizer flow rate is 12 times this value.
(j) Retain the default Start Time of 0 s and enter 100 s for the Stop Time.
For this problem, the injection should begin at t = 0 and not stop until longafter the time period of interest. A large value for the stop time (e.g., 100 s)will ensure that the injection will essentially never stop.
(k) Enter 0.0035 m for the Injector Inner Diameter and 0.0045 m for the InjectorOuter Diameter.
(l) Enter -45 degrees for Spray Half Angle.
The spray angle is the angle between the liquid sheet trajectory and the injectorcenterline. In this case, the value is negative because the sheet is initiallyconverging toward the centerline.
(m) Enter 82.6 m/s for the Relative Velocity.
The relative velocity is the expected relative velocity between the atomizing airand the liquid sheet.
(n) Retain the default Azimuthal Start Angle of 0 degrees and enter 30 degrees forthe Azimuthal Stop Angle.
This will restrict the injection to the 30-degree section of the atomizer that isbeing modeled.
The lower half of the dialog box will change to show options for the tur-bulent dispersion model.
ii. Enable Discrete Random Walk Model and Random Eddy Lifetime in theStochastic Tracking group box.
These models will account for the turbulent dispersion of the droplets.
(p) Click OK to close the Set Injection Properties dialog box.
(q) Click OK in the Information dialog box to enable droplet coalescence.
(r) Close the Injections dialog box.
Note: In the case that the spray injection would be striking a wall, you shouldspecify the wall boundary conditions for the droplets. Though this tutorialdoes have wall zones, they are a part of the atomizer apparatus. You need notchange the wall boundary conditions any further because these walls are notin the path of the spray droplets.
(b) Retain the default selection of point from the Style drop-down list.
(c) Select Particle Variables... and Particle Diameter from the Color by drop-downlists.
This will display the location of the droplets colored by their diameters.
(d) Select injection-0 from the Release from Injections selection list.
(e) Click Display and close the Particle Tracks dialog box.
(f) Restore the 30–degree section to obtain the view as shown in Figure 17.7.
Graphics and Animations −→ Views...
i. Click the Define button to open Graphics Periodicity dialog box.
ii. Click Reset and close the Graphics Periodicity dialog box.
iii. Close the Views dialog box.
(g) Use the mouse to obtain the view shown in Figure 17.7.
Figure 17.7: Particle Tracks for the Spray Injection After 200 Iterations
The air-blast atomizer model assumes that a cylindrical liquid sheet exits the atom-izer, which then disintegrates into ligaments and droplets. Appropriately, the modeldetermines that the droplets should be input into the domain in a ring. The radiusof this disk is determined from the inner and outer radii of the injector.
Note: The maximum diameter of the droplets is about 10−4 m or 0.1 mm. This isslightly smaller than the film height. The inner diameter and outer diameterof the injector are 3.5 mm and 4.5 mm, respectively. Hence the film heightis 0.5 mm. The range in the droplet sizes is due to the fact that the air-blastatomizer automatically uses a distribution of droplet sizes.
Also note that the droplets are placed a slight distance away from the injector.Once the droplets are injected into the domain, they can collide/coalesce withother droplets as determined by the secondary models (breakup and collision).However, once a droplet has been introduced into the domain, the air-blastatomizer model no longer affects the droplet.
Step 9: Postprocessing
1. Create an isosurface of the methanol mass fraction.
Surface −→Iso-Surface...
(a) Select Species... and Mass fraction of ch3oh from the Surface of Constant drop-down lists.
(b) Click Compute to update the minimum and maximum values.
(c) Enter 0.002 for Iso-Values.
(d) Enter methanol-mf=0.002 for the New Surface Name.
(e) Click Create and the close the Iso-Surface dialog box.
Figure 17.8: Full Atomizer Display with Surface of Constant Methanol Mass Fraction
4. Save the case and data files (spray3.cas.gz and spray3.dat.gz).
File −→ Write −→Case & Data...
Summary
In this tutorial, a spray injection was defined for an air-blast atomizer and the solutionwas calculated using discrete-phase model in ANSYS FLUENT. The location of methanoldroplet particles after exiting the atomizer and an isosurface of the methanol mass frac-tion were examined.
Further Improvements
This tutorial guides you through the steps to reach an initial solution. You may be ableto obtain a more accurate solution by using an appropriate higher-order discretizationscheme and by adapting the mesh. Mesh adaption can also ensure that the solution isindependent of the mesh. These steps are demonstrated in Tutorial 1.