gambit_combustor_tutorial.pdf

MODELING A COMBUSTION CHAMBER (3-D)

4. MODELING A COMBUSTION CHAMBER (3-D)

In this tutorial, you will create the geometry for a burner using a top-down geometry con-

struction method in GAMBIT (creating a volume using solids). You will then mesh the

burner geometry with an unstructured hexahedral mesh.

In this tutorial you will learn how to:

Move a volume

Subtract one volume from another

Shade a volume

Intersect two volumes

Blend the edges of a volume

Create a volume using the sweep face option

Prepare the mesh to be read into FLUENT 5/6

4.1 Prerequisites

This tutorial assumes that you have worked through Tutorial 1 and you are consequently

familiar with the GAMBIT interface.

¨ Fluent Inc., Mar-06

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Problem Description

MODELING A COMBUSTION CHAMBER (3-D)

4.2 Problem Description

The problem to be considered is shown schematically in Figure 4-1. The geometry

consists of a simplified fuel injection nozzle that feeds into a combustion chamber. You

will only model one quarter of the burner geometry in this tutorial, because of the

symmetry of the geometry. The nozzle consists of two concentric pipes with radii of 4

units and 10 units respectively. The edges of the combustion chamber are blended on the

wall next to the nozzle.

12 4

Figure 4-1: Problem specification

4-2

¨ Fluent Inc., Mar-06

MODELING A COMBUSTION CHAMBER (3-D)

Strategy

4.3 Strategy

In this tutorial, you will create a combustion chamber geometry using the Ðtop-downÑ

construction method. You will create volumes (in this case, bricks and cylinders) and use

Boolean operations to unite, intersect, and subtract these volumes to obtain the basic

geometry. Finally, using the ÐblendÑ command, you will round off some edges to

complete the geometry creation.

For this model, it is not possible to simply pick the geometry and mesh the entire domain

with hexahedral elements, because the Cooper tool (which you will be using in this

tutorial) requires two groups of faces, one group topologically parallel to a sweep path,

and the other group topologically perpendicular. However, the rounded (blended) edges

fit in neither group. See the GAMBIT Modeling Guide for a more detailed description of

the Cooper tool. You need to decompose the geometry into portions that can be meshed

using the Cooper tool. There are several ways to decompose geometry in GAMBIT. In this

example, you will use a method whereby portions of the volume around the blend are split

off from the main volume. A detailed description of the decomposition strategy for the

combustion chamber is given below.

Note that there are several faces in the geometry for which the default meshing scheme is

the Pave scheme; most of these faces are perpendicular to the z direction. There are also

geometrical protrusions in the z direction, so this should be chosen as the main direction

for the Cooper meshing scheme. To make this possible, the paved faces in the x and y

directions (the two symmetry planes in the geometry shown in Figure 4-2) must be

changed to use the Submap or Map meshing scheme.

¨ Fluent Inc., Mar-06

4-3

Strategy

MODELING A COMBUSTION CHAMBER (3-D)

Figure 4-2: The two symmetry planes in the combustion chamber geometry

By default, GAMBIT selects the Pave meshing scheme for these two faces because each

has a rounded edge where the blend occurs. If you split off the rounded corners of both

faces and connect them through a volume, you can use the Submap meshing scheme on

the remaining faces, and hence the Cooper meshing scheme for the volume.

Instead of creating two faces, one on each symmetry plane, you will create a face at the

junction of the two blended edges (face A in Figure 4-3). This face will then be swept in

two directions onto the symmetry planes (creating faces B and C in Figure 4-3), to split

the volume into three parts. The three volumes can then be meshed individually using the

Cooper tool.

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¨ Fluent Inc., Mar-06

MODELING A COMBUSTION CHAMBER (3-D)

Strategy

Figure 4-3: Faces created at the blended edges and on the symmetry planes

This tutorial also demonstrates a few ways of controlling the mesh density and the mesh-

ing schemes used on individual faces. You will mesh the small quarter-circle face that

forms the second inlet with a Tri Primitive scheme and a finer mesh size. Similarly, you will

mesh the annular face of the primary inlet with a fine mapped mesh. To meet the require-

ments of the Cooper tool, you will also need to create a mapped mesh on the face between

these two faces. Finally, you will use the automatic Cooper tool to mesh the remaining

faces and the volume.

¨ Fluent Inc., Mar-06

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