COMSOL 6.4 - Outgassing Pipes

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Outgassing Pipes

This benchmark model computes the pressure in a system of outgassing pipes with a high aspect ratio. The results are compared with a 1D simulation and a Monte-Carlo simulation of the same system from the literature.

Model Definition

The system consists of a long circular tube with a single change in cross section. A constant outgassing flux of 3×10-12 Torr-l/cm2 is emitted from the walls of the pipes. Two pumps are attached to the system, one directly on the pipe and the other via an additional length of pipe. Both pumps operate at a pump speed of 30 l/s. The model geometry, along with the location of the two pumps, is shown in Figure 1.

Figure 1: Model geometry. The location of the two pumps is indicated. All other surfaces outgas at a constant rate of 3×10-12 Torr-l/cm2.

Results and Discussion

Figures 2, 3 and 4 show the molecular flux, number density and pressure respectively on the surfaces of the pipes.

Figure 2: Molecular flux on the surface of the pipes.

Figure 3: Number density on the pipe surfaces.

Figure 4: Pressure in the pipes.

Figure 5: Pressure distribution along the top surface of the pipes. The results are compared with those from Ref. 1 and Ref. 2.

The pressure distribution in the pipes is in good agreement with the distributions given in Ref. 1 and Ref. 2.

Molecular_Flow_Module/Benchmarks/outgassing_pipes

1. J. Howell, B. Wherle, and H. Jostlein, “Calculation of pressure distribution in vacuum systems using a commercial finite element program,” Proc. 991 IEEE Particle Accelerator Conference (APS Beam Physics), vol. 4, pp.2295–2297, 1991.

2. R. Kersevan, “Analytical and numerical tools for vacuum systems,” CERN Accelerator School, Silken Park Hotel San Jorge, Platija d’Aro, Spain, 16-24 May 2006.

Modeling Instructions

From the menu, choose .

In the window, click

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In the window, click

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In the tree, select > > .

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In the tree, select > .

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Define model parameters.

Global Definitions

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In the window, under click .

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In the window for , locate the section.

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In the table, enter the following settings:


Name	Expression	Value	Description
T0	293.15[K]	293.15 K	Temperature
Mn0	0.028[kg/mol]	0.028 kg/mol	Molar mass
ps	30[l/s]	0.03 m³/s	Pump speed

Define the geometry.

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In the window, under click .

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In the window for , locate the section.

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From the list, choose .

Cylinder 1 (cyl1)

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In the toolbar, click

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In the window for , locate the section.

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In the text field, type 2.

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In the text field, type 400.

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Locate the section. From the list, choose .

Cylinder 2 (cyl2)

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In the toolbar, click

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In the window for , locate the section.

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In the text field, type 5.

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In the text field, type 100.

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Locate the section. In the text field, type 400.

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Locate the section. From the list, choose .

Cylinder 3 (cyl3)

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In the toolbar, click

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In the window for , locate the section.

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In the text field, type 2.

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In the text field, type 2.

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Locate the section. In the text field, type 100.

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In the text field, type -2.

Cylinder 4 (cyl4)

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In the toolbar, click

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In the window for , locate the section.

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In the text field, type 2.

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In the text field, type 100.

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Locate the section. In the text field, type 300.

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In the text field, type -100.

Cylinder 5 (cyl5)

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In the toolbar, click

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In the window for , locate the section.

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In the text field, type 2.

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In the text field, type 4.

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Locate the section. In the text field, type 98.

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Locate the section. From the list, choose .

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Cylinder 6 (cyl6)

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Right-click and choose .

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In the window for , locate the section.

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In the text field, type 298.

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Cylinder 7 (cyl7)

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In the toolbar, click

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In the window for , locate the section.

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In the text field, type 2.

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In the text field, type 2.

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Locate the section. In the text field, type 300.

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In the text field, type -2.

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In the toolbar, click

and choose .

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In the window for , locate the section.

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Clear the checkbox.

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Click in the window and then press Ctrl+A to select all objects.

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button in the toolbar.

Disable the analysis of the geometry as the remaining small geometric details can be kept.

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In the window, click .

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In the window for , locate the section.

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Clear the checkbox.

Add interpolation functions for benchmark comparisons.

Interpolation 1 (int1)

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In the toolbar, click

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In the window for , locate the section.

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Browse to the model’s Application Libraries folder and double-click the file outgassing_pipes_howell.txt.

Interpolation 2 (int2)

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In the toolbar, click

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In the window for , locate the section.

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Browse to the model’s Application Libraries folder and double-click the file outgassing_pipes_kersevan.txt.

Set up the physics and boundary conditions.

Free Molecular Flow (fmf)

Molecular Flow 1

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In the window, under > click .

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In the window for , locate the section.

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In the Mn,G text field, type Mn0.

Surface Temperature 1

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In the window, click .

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In the window for , locate the section.

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In the T text field, type T0.

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In the window, click .

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In the window for , locate the section.

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From the list, choose .

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Locate the section. From the list, choose .

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In the toolbar, click

and choose .

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Select Boundaries 10 and 12 only.

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In the window for , locate the section.

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From the list, choose .

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In the SG text field, type ps.

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Right-click and choose .

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In the window for , locate the section.

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Select Boundary 26 only.

Mesh the geometry.

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In the toolbar, click

and choose .

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Select Edges 9, 10, 12, 13, 15, 18, 22, 26, 29, 30, 32, 34, 37, 38, 43, 44, 46, 48, 54, 60, 63, 64, 67, 69, 72, 73, 75–77, and 79–81 only.

This rather long list includes all of the circular edges where different cylinders are connected in the geometry. A shortcut is to copy the selection list from these instructions and then click the button. An alternative is to create an selection, where you can select the checkbox to reduce the number of clicks required.

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In the window, click .

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In the window for , locate the section.

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From the list, choose .

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In the toolbar, click

and choose .

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Select Boundaries 2–5, 18–21, 24, 25, 31, 34, 37–40, and 42–45 only.

This boundary selection comprises the sides of all of the long cylinders in the geometry. As for the edge selection above, an selection with the checkbox can reduce the number of clicks required.

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Right-click and choose .

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Select Edges 3, 51, 65, and 74 only.

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In the window for , locate the section.

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In the text field, type 80.

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In the window, right-click and choose .

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Select Edge 31 only.

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In the window for , locate the section.

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In the text field, type 160.

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Free Triangular 1

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In the toolbar, click

and choose .

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In the window for , locate the section.

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From the list, choose .

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In the toolbar, click

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Total Pressure (fmf)

Plot the pressure profile.

1D Plot Group 4

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In the toolbar, click

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In the window for , click to expand the section.

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From the list, choose .

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Locate the section.

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Select the checkbox. In the associated text field, type Position (cm).

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Select the checkbox. In the associated text field, type Pressure (torr).

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Right-click and choose .

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In the window for , locate the section.

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In the text field, type fmf.ptot.

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Locate the section. From the list, choose .

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In the text field, type x.

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Locate the section. From the list, choose .

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Select Edges 7, 19, 33, 35, 49, 70, and 82 only.

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Click to expand the section. Select the checkbox.

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From the list, choose .

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In the table, enter the following settings:


Legends
COMSOL 3D Angular Coefficient

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Right-click and choose .

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In the window for , locate the section.

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In the text field, type int1(x/1[cm]).

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Locate the section. In the table, enter the following settings:


Legends
Howell 1D Conductance Model

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Right-click and choose .

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In the window for , locate the section.

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In the text field, type int2(x/1[cm]).

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Locate the section. In the table, enter the following settings:


Legends
Kersevan 3D Monte Carlo

Pressure Profile

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In the window, under click .

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In the window for , type Pressure Profile in the text field.

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Locate the section. From the list, choose .

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Locate the section. Select the checkbox.

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In the text field, type 1e-10.

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In the text field, type 1e-8.

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Select the checkbox.

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In the toolbar, click

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Compare the resulting plot with that in Figure 5.