CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Temperature drop ideal gas choked flow.

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   June 30, 2023, 07:05
Lightbulb Temperature drop ideal gas choked flow.
  #1
New Member
 
Laurens
Join Date: Sep 2018
Location: the Netherlands
Posts: 20
Rep Power: 7
LJA_Koers is on a distinguished road
Hi all,

I'm trying to make a transient simulation of the choked flow of methane in a piping system. I want the flow to become choked when the outlet pressure is reduced to atmospheric pressure (1atm). With a heat capacity ratio of 1.32, the critical pressure ratio of methane becomes 0.542. Hence the inlet pressure is set to 1 atm / 0.542 = 1.845 atm. And the speed of sound of methane at atmospheric pressure is 453.6 m/s.

To see if the flow is indeed choked at these conditions. I ran a steady state simulation and used the ideal-gas equation of state and an inlet and backflow temperature of 26.85C (isothermal throttling). Running the pressure based solver.

The result in the velocity contour shows a maximum velocity magnitude of 484 m/s, exceeding the critical velocity. To find the cause of this, I plotted the density and subsequently temperature plots to find that the temperature in the jet exiting the orifice drops to -26C. A 53C drop in temperature!

This setup is supposed to simulate the throttling of an ideal gas and thus be isothermal, yet the result is not.

Does anyone know what causes this?
And in addition, are there any other tips for modeling throttling/choked flow?

Thanks in advance.
Attached Images
File Type: jpg velocity-contour.jpg (34.1 KB, 9 views)
File Type: jpg temp-contour.jpg (30.4 KB, 9 views)
LJA_Koers is offline   Reply With Quote

Old   June 30, 2023, 10:59
Default
  #2
Senior Member
 
CFDKareem's Avatar
 
Kareem
Join Date: Nov 2022
Location: New York
Posts: 101
Rep Power: 3
CFDKareem is on a distinguished road
I would try and set the density behavior of the gas more explicitly, i.e. don't use the ideal gas model. You should be able to find the density information for methane and use the polynomial or piecewise polynomial for the density. I don't think the ideal gas formulation will capture the temperature correctly.
__________________
Please like the answer if it helped!

Video Tutorials and Tips: https://www.youtube.com/@cfdkareem/featured
CFDKareem is offline   Reply With Quote

Old   June 30, 2023, 19:48
Default
  #3
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,631
Rep Power: 65
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
These results look right.


Gas expansion is not isothermal, the temperature should drop, except over very long distances for enough heat transfer to take place so that the gas returns to its original temperature. This is approximately what you have at the exit. If you want an isothermal gas, don't use an ideal gas.


The heat capacity ratio for an isothermal gas is 1. Clearly, 1.32 is not 1.
LuckyTran is offline   Reply With Quote

Old   July 3, 2023, 06:04
Default
  #4
New Member
 
Laurens
Join Date: Sep 2018
Location: the Netherlands
Posts: 20
Rep Power: 7
LJA_Koers is on a distinguished road
Hi LuckyTran,

You're right. I should have read up more on the matter. The wiki on choked flow just mentions the static pressure and density drop.

I am however still unable to get the choked flow behavior, i.e the limited massflow,when I reduce the outlet pressure. I'm getting shock diamonds and Mach-numbers greater than 1.
LJA_Koers is offline   Reply With Quote

Old   July 4, 2023, 03:30
Default
  #5
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,631
Rep Power: 65
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
You will find Mach numbers greater than 1 because the critical pressure ratio in the wiki is for 1D flow and doesn't account for 3D effects. Real flows choke well before this pressure ratio. However, you should be able to find a Mach=1 disc near the throat.


Plot the Mach number contour and make an isosurface of M=1 or just make an isosurface of the sonic velocity.


Btw you can push more mass flow through a nozzle by increasing the upstream pressure. Choked only means that the downstream pressure becomes decoupled. If you change your upstream BC to 2 atm for example, you'll get more flow. This is a super duper important to understand because the choked pressure ratio for 3D bodies is more like 0.9 (not 0.5). In other words, you can lower your inlet pressure to about 0.7 atm and still find Mach discs and this is very real.


In summary, it is choked. You just need to convince yourself.
LuckyTran is offline   Reply With Quote

Reply

Tags
choked, flow, gas, ideal, temperature

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
choked mass flow value pamstad CFX 2 October 2, 2019 04:02
Domain Reference Pressure and mass flow inlet boundary AdidaKK CFX 75 August 20, 2018 06:37
Calculation of the Governing Equations Mihail CFX 7 September 7, 2014 07:27
Transient natural gas flow description Leila FLUENT 0 November 29, 2003 17:06
ideal gas law in turbulent flow Achim Siemens 0 March 26, 2003 15:38


All times are GMT -4. The time now is 10:15.