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How to estimate mass flux through pipe

How to estimate mass flux through pipe

How to estimate mass flux through pipe

(OP)
Hello,

I am trying to estimate the mass flux (m dot) of air (ideal gas, compressible flow) through adiabatic pipe.
Can this be done without involving the velocity or Mach number? Forgive my ignorance...

I can get the following: inlet pressure, inlet density, pipe cross-sectional area, pipe length and outlet pressure.
If I could get an approximate formula for m_dot, that would still be good.

I found one expression for m_dot here (http://www.cheresources.com/content/articles/fluid...),
but it has the Mach number in it.

Many thanks for your help. It could very well be that this cannot be done without the velocity...

RE: How to estimate mass flux through pipe

First, a pipe cannot be anything but adiabatic. What can change is the flow within the pipe.

Second, virtually all real pipe flow is suitable for an incompressible assumption. Basically, if your velocity is below 0.6 Mach then as long as downstream density is more than 90% of upstream density then all of the incompressible arithmetic works.

Third, "mass flux" is a term that is defined many different ways by many different authors. There is not really an agreed upon definition for the term. Typically when I see any term with a "dot" over it I treat it as the time derivative of whatever is under the dot. So I would take "m dot" as the time derivative of mass which is usually taken as dm/dt = dq/dt*ρ = q(dot)*ρ = v*A*ρ. Mostly that is called "mass flow rate" not "mass flux"

If you have done something exceptional to get to very high velocity gas flow in a pipe, then there is no easy breezy way to deal with it. Compressible gas flow will generally have Mach numbers and often some much more difficult terms to determine.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

RE: How to estimate mass flux through pipe

Further to above, if you are trying to find "flux" or "flow rate" then you need something that is described by time, generally velocity (e.g m/sec) or volumetric flowrate (e.g. m3/sec at some defined pressure).

Therefore I cannot see how you can estimate, calculate or find m dot without knowing velocity or some other time dependat information. Your 5 bits of data are all "static" figures which could be used to calculate velocity and therefore mass or volume flow. As ZDAS04 says, depending on the relative inlet and outlet pressures you can can calualte quite easily - there are many equations and online calculations available for low differences (<10%), for higher differences you really need a good simualation program.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

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