Nonisothermal Pipe Flow
Next, specify the physics feature settings, including the flow and temperature conditions, as well as the friction correlations to be applied to the pipe segments.
Pipe Properties 1
1
In the Model Builder under Nonisothermal Pipe Flow, click Pipe Properties 1.
2
In the Settings window for Pipe Properties locate the Pipe Shape section.
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From the Shape list, choose Circular.
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In the di text field, type 20[mm]
3
In this example, the default Churchill friction model can be applied. In many of the Settings windows in the COMSOL Desktop, there is an Equation section that you can expand. This shows you the equations employed by the current setting. In this case, the correlations for the default friction factor model Churchill is displayed. Also, if you are interested in the underlying theory of any Settings window, press F1 or the Help button . This takes you to the documentation section describing the current Settings window.
4
Locate the Flow Resistance section. From the Surface roughness list, choose Thermoplastics (0.0015 mm).
Pipe Properties 2
Now you can assign additional different properties to a new group of pipe segments.
1
In the Physics toolbar click Edges and choose Pipe Properties.
2
In the Settings window for Pipe Properties, locate the Edge Selection section.
3
From the Selection list, choose Feed and Return Pipes.
4
In the Settings window for Pipe Properties locate the Pipe Shape section.
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From the list, choose Circular.
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In the di text field, type 50[mm].
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From the Surface roughness list, choose Thermoplastics (0.0015 mm).
Temperature 1
1
In the Model Builder under Nonisothermal Pipe Flow, click Temperature 1.
2
In the Settings window for Temperature locate the Temperature section. In the Tin text field, type 5[degC].
Wall Heat Transfer 1
The Wall Heat Transfer feature accounts for the thermal resistance between the fluid inside the pipe and the surrounding medium, and it is applied to all sections of the system submerged below the water level.
1
In the Physics toolbar click Edges and choose Wall Heat Transfer.
2
From the Selection list, choose Submerged pipes. The first two segments of the main feed and return pipes are situated above the water surface and are therefore and are not included in this feature.
3
In the Settings window for Wall Heat Transfer locate the Heat Transfer Model section. In the Text text field, type T_pond(z).
The Wall Heat Transfer feature consists of three components: the internal film resistance, which accounts for heat transfer from the fluid to the wall; the wall layer, which models heat conduction through the pipe material; and the external resistance, which represents heat transfer from the outer wall surface to the external environment.
Internal Film Resistance 1
In the Physics toolbar, click  Attributes and choose Internal Film Resistance. This adds an automatic calculation of the heat transfer coefficient, based on the Nusselt number.
Wall Layer 1
Next, define the properties of the polyethylene pipe walls by adding a wall layer.
1
In the Physics toolbar, click Attributes and choose Wall Layer.
2
In the Settings window for Wall Layer locate the Specification section.
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From the k list, choose User defined.
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In the k text field, type 0.46[W/m /K].
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From the Δw list, choose User defined.
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In the text field, type 2[mm].
More than one Wall Layer can be added if you want a multilayered pipe wall.
External Film Resistance
1
Click Attributes and choose External Film Resistance.
The external flow velocity of 0.2 m/s represents the mild pond current. This velocity is sufficient to classify the heat transfer outside the pipes as forced convection.
2
In the Settings window for External Film Resistance locate the Specification section.
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In the External film heat transfer model list, External forced convection should now be active by default.
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From the Surrounding fluid list, choose Water, liquid.
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In the uext text field, type 0.2[m/s].
The temperature dependent material properties of water are all taken from the Materials node added earlier.
Inlet 1
1
In the Physics toolbar click Points and choose Inlet .
2
Select Point 1 only.
3
In the Settings window for Inlet locate the Inlet Specification section.
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From the Specification list, choose Volumetric flow rate.
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In the qv,0 text field, type 4[l/s] (as in liters/second).
The outlet condition is already set in the default node Pressure 1.
Heat Outflow 1
1
In the Physics toolbar click Points and choose Heat Outflow .
2
Select Point 2 only.