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### Gas Pipe

The gas pipe element is a pipe element with constant cross section (Figure 50).

Figure 50: Geometry of the Gas Pipe element

The friction parameter is determined as

| (12)

for laminar flow ( Re) and

| (13)

for turbulent flow. Here, is the diameter of the material grains at the surface of the pipe and Re is the Reynolds number defined by

Re | (14)

where is the liquid velocity and is the kinematic viscosity. A gas pipe is described by the following parameters (to be specified in that order on the line beneath the *FLUID SECTION,TYPE=GAS PIPE card):

- A: cross section of the pipe
- D: diameter of the pipe
- L: length of the pipe
- : grain diameter at the pipe surface
- 0 (adiabatic) or 1 (isothermal)
- not used
- form factor

The default gas pipe is adiabatic, i.e. there is no heat exchange with the pipe. Alternatively, the user may specify that the pipe element is isothermal. This means that the static temperature does not change within the pipe. In that case the energy equation in one of the two end nodes of the element is replaced by a isothermal condition.

The form factor is only used to modify the friction expression for noncircular cross sections in the laminar regime as follows:

| (15)

Values for for several cross sections can be found in [10].

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