Diffuser reversed flow

macdhui · September 27, 2015, 05:51

Hi,

I know the subject of ‘reversed flow’ has been brought up several times, and I have diligently searched previous threads, however I cannot find a solution to my issue.

I am trying to simulate an eductor that uses gas as the motive inlet, and a liquid phase as the suction inlet, both of which are pressure inlet BCs. Due to the study parameters, I have supersonic flow just following the nozzle, but I am continuing to get reversed flow at the end of the diffuser which has a pressure outlet BC.

“reversed flow in XXX faces on pressure-outlet YY”

So far I have tried the following solutions:

Extending the outlet further away (6-7x the diameter of end diffuser)
I cannot use ‘outlet’ as a BC because I have compressible, two-phase flow
Closer matching of each phases’ temperature specification (which you need to specify when using pressure outlet) to the temperatures earlier in the diffuser
Lengthening of the elements near the exit
Running the diffuser with a less structured mesh
Lowering the angle of the diffuser (from 7deg, to 5deg, to 3deg)

Model information:

Standard K-epsilon, per phase
Ideal gas compressibility
2-phase eulerian mixture model, with implicit body forces
Pressure-velocity coupled solution
First order upwind for density, momentum, volume fraction, turbulent kinetic energy, dissipation rate, energy
Low courant number (=20), and have halved the URF for all other factors

Any help/advice to solve the reversed flow issue would be amazing!

Cheers,
Macdhui

Far · September 27, 2015, 11:42

There are two possible causes:

1. Fluent gives the warning without reason (mostly for external flows), where velocity is almost zero.

2. For internal flow, best choice is to place the outer boundary faraway from the main flow.

macdhui · October 8, 2015, 04:13

Thanks Sijal.

I have placed the outlet boundary 10x the internal diameter away from the exit of the diffuser, but I still get this issue. Should I extend it even further or do I need to investigate other sources of the reverse flow?

Macdhui

Far · October 8, 2015, 04:28

can you show some pics at the outlet domain (vector plots )

macdhui · October 8, 2015, 04:48

Sijal - I have PM'd you.

Many thanks

September 27, 2015, 05:51	Diffuser reversed flow	#1
macdhui New Member T Join Date: May 2015 Posts: 3 Rep Power: 10	Hi, I know the subject of ‘reversed flow’ has been brought up several times, and I have diligently searched previous threads, however I cannot find a solution to my issue. I am trying to simulate an eductor that uses gas as the motive inlet, and a liquid phase as the suction inlet, both of which are pressure inlet BCs. Due to the study parameters, I have supersonic flow just following the nozzle, but I am continuing to get reversed flow at the end of the diffuser which has a pressure outlet BC. “reversed flow in XXX faces on pressure-outlet YY” So far I have tried the following solutions: Extending the outlet further away (6-7x the diameter of end diffuser) I cannot use ‘outlet’ as a BC because I have compressible, two-phase flow Closer matching of each phases’ temperature specification (which you need to specify when using pressure outlet) to the temperatures earlier in the diffuser Lengthening of the elements near the exit Running the diffuser with a less structured mesh Lowering the angle of the diffuser (from 7deg, to 5deg, to 3deg) Model information: Standard K-epsilon, per phase Ideal gas compressibility 2-phase eulerian mixture model, with implicit body forces Pressure-velocity coupled solution First order upwind for density, momentum, volume fraction, turbulent kinetic energy, dissipation rate, energy Low courant number (=20), and have halved the URF for all other factors Any help/advice to solve the reversed flow issue would be amazing! Cheers, Macdhui borahdj2010 likes this.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
About Some Concepts:Laminar flow, turbulent flow, steady flow and time-dependent flow	Jing	Main CFD Forum	8	October 5, 2018 17:02
ask for help about the reversed flow	yuhehuan	FLUENT	3	January 9, 2013 05:30
Reversed flow at pressure outlet	Seeker Phil	FLUENT	9	January 2, 2010 05:21
reversed flow at velocity inlet / mass flow inlet	ib	FLUENT	1	March 26, 2007 13:11
reversed flow	Sastry	FLUENT	5	March 22, 2007 08:05

September 27, 2015, 11:42		#2
Far Super Moderator Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	There are two possible causes: 1. Fluent gives the warning without reason (mostly for external flows), where velocity is almost zero. 2. For internal flow, best choice is to place the outer boundary faraway from the main flow.

October 8, 2015, 04:13		#3
macdhui New Member T Join Date: May 2015 Posts: 3 Rep Power: 10	Thanks Sijal. I have placed the outlet boundary 10x the internal diameter away from the exit of the diffuser, but I still get this issue. Should I extend it even further or do I need to investigate other sources of the reverse flow? Macdhui

October 8, 2015, 04:28		#4
Far Super Moderator Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54	can you show some pics at the outlet domain (vector plots )

October 8, 2015, 04:48		#5
macdhui New Member T Join Date: May 2015 Posts: 3 Rep Power: 10	Sijal - I have PM'd you. Many thanks