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Figure 14.1: Can Combustor Geometry

Compressed primary air is forced into the combustion chamber at 10 m/s through the main inlet at the base of the canister. Six swirl inlet vanes guide the incoming air into the canister and facilitate its mixing with pure methane for proper combustion. Methane is injected through six fuel inlets with a velocity

of 40 m/s. As the reacting mixture proceeds through the canister, secondary air is fed into the combustion chamber at a velocity of 6 m/s through six secondary air inlets downstream from the primary combustion zone. This helps increase the combustion efficiency and also cool the can walls as they are exposed to the hot reacting flow. The fuel and oxidizer enter the combustion chamber at 300 K.

In this tutorial, the quantitative analysis of the combusting mixture is performed and the following quantities are determined:

•The temperature distribution inside the combustor that burns methane in air

•The proportion of unburned fuel remaining at the combustor outlet

14.4. Setup and Solution

The following sections describe the setup and solution steps for this tutorial:

14.4.1.Preparation

14.4.2.Mesh

14.4.3.Solver Settings

14.4.4.Models

14.4.5.Boundary Conditions

14.4.6.Solution

14.4.7.Postprocessing for the Eddy-Dissipation Solution

To watch a video that demonstrates the steps below for setting up, solving, and postprocessing the solution results for diffusion-controlled combustion, go to:

• ANSYS Fluent: Diffusion Controlled Reacting Flow in a Can Combustor

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Setup and Solution

14.4.1. Preparation

To prepare for running this tutorial:

1.Download the edm_flamelet.zip file here.

2.Unzip edm_flamelet.zip to your working directory.

3.The file, combustor_poly.msh, can be found in the folder.

4.Use Fluent Launcher to enable Double Precision and start the 3D version of Fluent. Fluent Launcher displays your Display Options preferences from the previous session.

For more information about the Fluent Launcher, see starting Fluent using the Fluent Launcher in the Fluent Getting Started Guide.

5.Ensure that the Display Mesh After Reading option is enabled.

6.Run in Serial by selecting Serial under Processing Options.

14.4.2. Mesh

1.Read the mesh file combustor_poly.msh.

File → Read → Mesh...

As Fluent reads the mesh file, it will report the progress in the console.

2.Check the mesh.

Domain → Mesh → Check → Perform Mesh Check

Click OK and close the Information dialog box. The use of Warped-Face Gradient Correction will be selected later in the tutorial.

Fluent will perform various checks on the mesh and will report the progress in the console. Make sure that the reported minimum volume is a positive number.

3.Display the mesh.

Domain → Mesh → Display...

a.In the Options group, clear the Faces option and make sure that the Edges option is selected.

b.In the Mesh Display dialog box, select fuelinlet, inletair1, inletair2, outlet, wall-part-fluid, and wallvanes from the Surfaces selection list.

Tip

When selecting surfaces, it can be helpful to group surfaces by type by clicking and selecting Surface Type under Group By.

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c. Click Display and close the Mesh Display dialog box.

4.Examine the mesh Figure 14.2: Mesh Display of the Can Combustor (p. 484).

Figure 14.2: Mesh Display of the Can Combustor

The mesh consist of a fluid zone, canister wall, main air inlet, six guide vanes, six fuel inlets, six secondary air inlets, and a single outlet. All inlets of the combustor mesh are colored blue, and the outlet is colored red.

14.4.3. Solver Settings

1. In the Physics ribbon tab, retain the default setting of Pressure-Based (the Solver group).

Physics → Solver

14.4.4. Models

The fuel (methane) and oxidizer (air) undergo fast combustion (that is, the overall combustion rate is controlled by turbulent mixing). In this first part of the tutorial, the combustion reaction is considered

to be driven by turbulent diffusion, and it is modeled using the Eddy Dissipation model, which is suitable for modeling fast combustion.

1.Enable the standard - turbulence model.

Physics → Models → Viscous...

a.In the Viscous Model dialog box, select k-epsilon (2eqn) in the Model list.

b.Click OK to close the Viscous Model dialog box.

2.Enable chemical species transport and reaction.

Physics → Models → Species...

a.Select Species Transport in the Model list.

b.Select methane-air from the Mixture Material drop-down list.

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Setup and Solution

The Mixture Material list contains the set of chemical mixtures that exist in the ANSYS Fluent database. When selecting an appropriate mixture for your case, you can review the constituent species and the reactions of the predefined mixture by clicking View... next to the Mixture Material drop-down list. The chemical species and their physical and thermodynamic properties are defined by the selection

of the mixture material. After enabling the Species Transport model, you can alter the mixture material selection or modify the mixture material properties using the Create/Edit Materials dialog box.

c.Select Volumetric in the Reactions group box.

d.Select Eddy-Dissipation in the Turbulence-Chemistry Interaction group box.

The Eddy-Dissipation model computes the reaction rate under the assumption that chemical reaction is fast compared to transport of reactants in the combusting flow. That is, the reaction is controlled by diffusion.

e.Click OK to close the Species Model dialog box.

A Warning message appears in the console notifying you that ANSYS Fluent automatically enabled the energy equation required for the Species reaction model.

f.Click OK to close the Information dialog box.

14.4.5. Boundary Conditions

In this step, you will define the boundary conditions at the inlets and the outlet. The boundary conditions for fuelinlet, inletair1, inletair2 were defined as mass-flow-inlet in a meshing application. You will begin by changing the boundary condition type for these inlets to velocity-inlet.

Physics → Zones → Boundaries...

1. Organize the boundary conditions by type.

Setup → Boundary Conditions Group By → Zone Type

2. Under the Setup/Boundary Conditions tree branch, right-click inlet and select Type>velocity-inlet.

Setup → Boundary Conditions → inlet Type → velocity-inlet

The velocity-inlet boundary condition type is now assigned to all inlets.

3. Set the boundary condition for the fuel inlet.

Setup → Boundary Conditions → fuelinlet		Edit...
In the Velocity Inlet dialog box, configure the following settings.
Tab	Setting		Value
Momentum	Velocity Magnitude		40 m/s
Thermal	Temperature		300 (default)
Species	ch4 (Species Mass Fractions group box)		1

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4. Set the boundary condition for the primary air inlet.

		Setup → Boundary Conditions → inletair1		Edit...
In the Velocity Inlet dialog box, configure the following settings.
		Tab	Setting		Value
Momentum			Velocity Magnitude		10 m/s
Thermal			Temperature		300 (default)
Species			o2 (Species Mass Fractions group box)		0.23	[a]

a.Dry air is composed of 23% of oxygen and 77% of nitrogen, which is a bulk species in the mixture. ANSYS Fluent adds an appropriate amount of nitrogen at the boundaries to ensure that the sum of the mass fractions of the components is equal to unity.

5.Set the boundary condition for the secondary air inlet.

		Setup → Boundary Conditions → inletair2		Edit...
In the Velocity Inlet dialog box, configure the following settings.
		Tab	Setting		Value
Momentum			Velocity Magnitude		6 m/s
Thermal			Temperature		300 (default)
Species			o2 (Species Mass Fractions group box)		0.23

6. Set the boundary condition for the pressure outlet.

		Setup → Boundary Conditions → outlet		Edit...
In the Pressure Outlet			dialog box, configure the following settings.
		Tab	Setting			Value
Momentum			Gauge Pressure		0 Pa	[a]	(default)
			Backflow Pressure Specification		Total Pressure			[b]
					(default)
			Average Pressure Specification		(Selected)

a.The gauge pressure of 0 Pa means that the pressure equals the ambient pressure.

b.This setting ensures that if the backflow occurs, only pure nitrogen at 300 K enters the chamber, which will not affect the combustion reactions.

7.For wall-part-fluid, wallvanes and wallvanes-shadow retain the default stationary no slip adiabatic settings.

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