A. Basic equations

The main characteristic of compressible pipe flow is the change of the mean density in the flow direction. This can be caused by the pressure drop, by heat transfer, and/or by high mean velocities. Compressibility effects have to be considered when the mean pipe velocity is larger than about 30% of the speed of sound

The basic equations for general duct flows have been given in Section 15778B.

For simplification, the following assumptions will be made:

1. The cross-section area is constant. S = const

2. Buoyancy forces are neglected.

3. Velocity and density are constant across the cross section, i.e., α = 1, β =1.

4. Calorically perfect gases are considered [ see Section 15778D ].

5. The friction factor f̂ is constant, i.e., independent of the axial coordinate x. For variable f̂ (x), a mean value over the length considered has to be taken.

6. Wall shear stress τ_w and heat flux q̇_w (if it is nonzero) are constant around the perimeter.