Annular Flow of Compressible Fluid
Annular Flow of Compressible Fluid
(OP)
Anyone have experience with flow equations for natural gas in a circular pipe flowing around an innerduct (annular flow). Want pressure drop for given flowrate with initial pressure of 10 to 60 psig. Pipe is 1 to 6" IPS plastic, cast iron or steel. Innerduct is plastic 1/8" to 1" CTS.





RE: Annular Flow of Compressible Fluid
Find the area of the annulus to calculate the velocity, but use the hydraulic diameter to find the friction coefficient and friction losses.
Hydraulic diameter = 4 x (wetted flow area / wetted perimeter). Then treat it as if it's an independent pipe.
For a true circular concentric annulus, the hydraulic diameter works out to the width of the annular space.
You'll need a formula for friction factor which uses the variable surface roughness characteristics of your different materials. I use "colebrook" which needs an iterative solution but you can input the hydraulic diameter into the relative roughness section.
The rest is standard hydraulics. For the DP I suggest the isothermal compresible flow. There are innumerable reference on this site to papers but, like others, I'd start with the Crane publication 410.
That all sounds too simple !!
If I've misunderstood your question please say so.
RE: Annular Flow of Compressible Fluid
RE: Annular Flow of Compressible Fluid
[[ID{outer)+OD(inner)]^2*[ID(outer)-OD(inner)]^3]^(1/5)
You use the result of this equation just like it was an actual diameter of a conduit. It gives pretty good results for flow and friction calculations.
RE: Annular Flow of Compressible Fluid
RE: Annular Flow of Compressible Fluid
"Accurate" is such an elusive term in vertical multi-phase flow which is what most gas-field wellbore flow tends to be.
RE: Annular Flow of Compressible Fluid
Kevin