Mass Flow through a 1.5 Pipe @ 300kpa

[NateP]

Mass Flow through a 1.5" Pipe @ 300kpa

Hi,

I'm having some trouble with finding the "choke" point of a 1.5" diameter pipe.

Conditions:
300 kPa
383 deg K
1.5 in D

I've tried using this little guy, http://www.grc.nasa.gov/WWW/K-12/airplane/mflchk.html, with the Mach = 1, and only get ~1.5lb/min, which can't be correct. Can't figure out what I'm missing?

Thanks
Nate

 

[stewartcs]

Hi,

I'm having some trouble with finding the "choke" point of a 1.5" diameter pipe.

Conditions:
300 kPa
383 deg K
1.5 in D

I've tried using this little guy, http://www.grc.nasa.gov/WWW/K-12/airplane/mflchk.html, with the Mach = 1, and only get ~1.5lb/min, which can't be correct. Can't figure out what I'm missing?

Thanks
Nate

You need more information...take a look here and try again:

http://www.chem.mtu.edu/~crowl/cm4310/Chapter4b.pdf

CS

 

[NateP]

Thanks for the link,

P1 = P2 in my case, but having trouble understanding if this would fall under Adiabatic or Isothermal, but I take it has something to do with the velocity of the gas?

It looks like I need to start with finding the sonic velocity under the given conditions. Looking at the formula:

a = γ gcRgT /M

Which translates into:

Sq rt of ( Gamma * Grav constant * Ideal Gas Constant * Temp  / Molecular weight

Do any of these variables change under the given conditions? I can only seem to reference values at 20 deg C and 101 kPa...

 

[stewartcs]

Thanks for the link,

P1 = P2 in my case, but having trouble understanding if this would fall under Adiabatic or Isothermal, but I take it has something to do with the velocity of the gas?

It looks like I need to start with finding the sonic velocity under the given conditions. Looking at the formula:

a = γ gcRgT /M

Which translates into:

Sq rt of ( Gamma * Grav constant * Ideal Gas Constant * Temp  / Molecular weight

Do any of these variables change under the given conditions? I can only seem to reference values at 20 deg C and 101 kPa...

It's up to you to determine how you want to model the system. If the process is rapid, then the adiabatic assumption is valid. If the temperature of the gas is constant, the isothermal would be best. In reality it's somewhere in between the two.

In compressible flow the density of the gas typically changes by an appreciable amount. However, the 'gamma' term should capture that effect. Obviously the terms labeled 'constant' won't change, hence the name. The molecular weight won't change either. The temp was previously discussed as well as gamma.

CS

 

[stewartcs]

Thanks for the link,

P1 = P2 in my case, but having trouble understanding if this would fall under Adiabatic or Isothermal, but I take it has something to do with the velocity of the gas?
...

If P1 = P2 then no flow will occur. There must be a pressure differential in a horizontal pipe for fluid flow.

CS