Mass Flow through a 1.5 Pipe @ 300kpa

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Discussion Overview

The discussion revolves around determining the "choke" point for mass flow through a 1.5-inch diameter pipe under specific conditions of pressure and temperature. Participants explore the implications of adiabatic versus isothermal flow and the necessary calculations for sonic velocity.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant, Nate, expresses confusion about calculating the choke point for a 1.5" pipe at 300 kPa and 383 K, noting that the results from a specific online tool seem incorrect.
  • Another participant, CS, suggests that more information is needed and provides a link to additional resources.
  • Nate questions whether the flow should be modeled as adiabatic or isothermal, indicating uncertainty about how the gas velocity affects this decision.
  • CS explains that the choice between adiabatic and isothermal modeling depends on the nature of the process and mentions that in compressible flow, density changes significantly, but constants like molecular weight do not change.
  • CS also points out that if the inlet and outlet pressures are equal (P1 = P2), no flow will occur, emphasizing the need for a pressure differential for fluid flow.

Areas of Agreement / Disagreement

Participants do not reach consensus on the modeling approach (adiabatic vs. isothermal) and the implications of pressure conditions for flow. The discussion remains unresolved regarding the correct assumptions and calculations needed for the choke point determination.

Contextual Notes

Participants reference specific conditions and formulas but express uncertainty about variable changes under the given conditions, particularly regarding temperature and pressure. The discussion highlights the complexity of modeling fluid dynamics in this context.

NateP
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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
 
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NateP said:
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
 
Last edited by a moderator:


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:
Code:
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...
 


NateP said:
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:
Code:
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
 


NateP said:
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
 

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