COMSOL 6.4 - Discharging Tank

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Discharging Tank

This tutorial model illustrates how to calculate the pressure drop and initial flow rate in a pipe system connected to water tank. The Pipe Flow interface contains ready-to-use friction models accounting for the surface roughness of pipes as well as energy losses in bends and valves.

Model Definition

Water from a tank flows through a total of 105 m of pipe to be discharged through an open ball valve. The pipes are 15 cm in diameter and made out of galvanized iron. The water level is 10 m above the point of discharge.

Figure 1: Water flows through a pipe system with two 90° bends and discharges through an open ball valve.

The model example is taken from Ref. 1.

In pipe networks, fittings, bends, valves, and so on, induce additional energy losses

characterized by loss coefficients, K. The Pipe Flow interface can include such resistances through the point features. This model uses two 90° bends and a Ball valve.

At the pipe inlet from the tank, see Figure 1, the pressure is taken as the atmospheric pressure at the top water surface in the tank plus the hydrostatic pressure due to the water column:

where g is the normal gravitational acceleration (m/s2) and the H the elevation height (m), the latter which is 25 m in this case. The entrance from the tank is rounded. At the system outlet to the right, atmospheric pressure p0 is specified. The pressure drop across the pipe system is computed for a constant water level H=H0.

Results and Discussion

Figure 2 shows the pressure drop over the pipe system for stationary solution, while Figure 3 shows the direction of flow and the Reynolds number.

Figure 2: Pressure drop across the pipe system.

Figure 3: The Reynolds number is 4.47·105,indicating that the flow is well in the turbulent regime.

The discharge rate is calculated to approximately 0.053 m3/s.

1. J.M. Coulson and J.F. Richardson, Chemical Engineering vol. 1, 4th ed., Pergamon Press, pp. 74–75, 1990.

2. S.W. Churchill, “Friction factor equation spans all fluid-flow regimes,” Chem. Eng., vol. 84, no. 24, p. 91, 1997.

Pipe_Flow_Module/Tutorials/discharging_tank

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 > > .

3

4

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In the tree, select > .

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

Polygon 1 (pol1)

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In the toolbar, 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. In the table, enter the following settings:


x (m)	y (m)
0	0
L1	0
L1	L2
V1	L2
L1+L3	L2

Now add Water from the Material Library. The material properties will apply to the entire model domain by default.

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In the toolbar, click

to open the window.

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Go to the window.

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In the tree, select > .

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Click the button in the window toolbar.

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In the toolbar, click

to close the window.

Water, liquid (mat1)

button in the toolbar.

Pipe Flow (pfl)

Next, specify the dimensions and surface roughness of the pipe. Note that you can add multiple features and assign them to different parts of a pipe network, should you have a system of made up of pipes with different characteristics.

Pipe Properties 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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From the list, choose .

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In the di text field, type Dh.

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Locate the section. From the list, choose .

Set boundary conditions for the inlet and use a feature to take gravity effects into account.

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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 pres text field, type 101325[Pa]+H0*g_const*pfl.rho.

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Select Point 1 only.

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From the list, choose .

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In the toolbar, click

and choose .

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Select Boundaries 1–3 only.

The default volume force is a gravity vector pointing in the negative y direction (downward).

3

button in the toolbar.

Next, add a number of point features to include the energy losses due to bends and the ball valve. The valve point may be difficult to select graphically with the mouse. Here, you can use the Selection list, as help to browse the points in a list.

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In the toolbar, click

and choose .

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Select Points 2 and 3 only.

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In the toolbar, click

and choose .

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Select Point 4 only.

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In the window for , locate the section.

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From the list, choose .

To help graphically indicate locations of pipe network elements, the physics symbols are available.

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In the window, click .

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In the window for , locate the section.

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Select the checkbox.

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Find the subsection. Click to display physics symbols for all features.

The model is now ready for solving. Lower the default tolerance to increase the accuracy of the solution.

Step 1: Stationary

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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 .

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In the text field, type 1e-6.

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In the toolbar, click

.

Default plots show the pressure drop in the pipe system, and the direction and velocity of the flow (Figure 2 and Figure 3). Modify the last plot to evaluate the Reynolds number and flow rate.

In the window, expand the > node.

Color Expression 1

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In the window, expand the > > node, then click .

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In the window for , click in the upper-right corner of the section. From the menu, choose > > .

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Click to expand the section. From the list, choose .

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In the window, right-click and choose .

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In the window for , locate the section.

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button. From the menu, choose > > .

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In the text field, type Qv=eval(pfl.Qv).

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In the toolbar, click

.

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button in the toolbar.