Mass flow

gustavosimiema · August 5, 2015, 07:59

Just one more question, as the inlet will be divided by 4 ... I will use a quarter of my previous model... The inlet velocity (BC) I should also divide by 4, correct?

fresty · August 5, 2015, 17:33

theoretically yes as volumetric flow is directly additive (per my humble understanding)... this is something you could always check i guess.. not sure how fluent defines symmetry bc option.. it may be possible that once symmetry planes are defined the velocity input is considered for the entire inlet area rather than the 1/4th.. however just go through the below for an insight..

https://www.sharcnet.ca/Software/Flu...ug/node258.htm

gustavosimiema · August 11, 2015, 08:27

Hi,

I increased the number of holes and than my solution doesn´t converge anymore. There are reverse flow in practically all pressure outlets. Each hole I called as one outlet. I have a 500 mm pipe lenght and 19 holes (2mm Diameter). The pipe has a square sectional area (L=30mm).
Laminar flow , steady state.

As there is the presence of reversed flow, it means that the pressure inside the gasfeed is lower than the pressure out of the Domain? If it´s correct how can I identify that in the Simulation? How to see the pressure inside the pipe and out of the Domain?

gustavosimiema · August 11, 2015, 11:10

One additional Information, if I increase the velocity inlet this Problem (reverse flow in the outlet) vanish out...
What´s the Explanation for that?

fresty · August 12, 2015, 02:45

What are your outlet BCs? Have you specified the Outlet pressure & Operating Pressure? what are the wall conditions of the pipe?
The local pressure drop at a certain region could also result in reversed flow.. in your case the local pressure drop increase at the outlets due to increase in local velocity seems sufficient to have caused reversed flow by increase in dynamic pressure loss..which you probably should expect firstly because of low pressure flow and secondly as you mentioned earlier that the pressure difference to drive the flow is only 1 Pa (please correct me if i got it wrong).. by increasing the velocity you're helping the flow to have enough momentum to pass through even if the pressure drop is the same (in physical terms)..
You could use an expression to see the pressure at inlet as well as at outlet however you could always get an approximate idea from contours considering the issue is quite prominent already..

gustavosimiema · August 12, 2015, 02:54

The Outlet BC is pressure outlet = 0 Pa. Operating pressure = 1Pa. You are right, the pressure difference to drive the flow is 1 Pa.
How can I use this expressions? Where are them?

Another addittional Information... This Problem of reversed flow doesn´t appears with just 7 holes in the same conditions. It´s the same Explanation? With 7 holes I have less dynamic loss?

fresty · August 12, 2015, 03:05

Quote:

Originally Posted by gustavosimiema

The Outlet BC is pressure outlet = 0 Pa. Operating pressure = 1Pa. You are right, the pressure difference to drive the flow is 1 Pa.
How can I use this expressions? Where are them?

I am not really well versed in Fluent UDFs or expressions, however, it shouldn't be hard to find them with a quick search in the forum.. they should be something like (not the right syntax for sure).. area-avg(pressure)@location ...

Quote:

Originally Posted by gustavosimiema

Another addittional Information... This Problem of reversed flow doesn´t appears with just 7 holes in the same conditions. It´s the same Explanation? With 7 holes I have less dynamic loss?

Yes that would be the reason.. the increase in velocity (momentum) is necessary to cover the increase in number of outlets..

gustavosimiema · August 12, 2015, 03:13

Now I just made a test to see if it works accordingly. I reduced the outlet from 2mm to 1.5mm. Then I could observe that the mass flow rate was the same because the discharge area reduced but the velocity increased to Balance and also the pressure inlet increased.

So maybe to vanish with the reversed flow maybe a solution is to increase the Diameter of the holes, than the velocity will reduce and also the pressure drop. Am I right in my way of thinking?

fresty · August 12, 2015, 03:23

Yes i believe that should be the case.. however, you could compare your test situation with another by increasing the outlet diameter and see the physical changes in the system. It would always be more or less a trade off situation and you would have to find the optimum design point between change in diameter/ no. of holes/ dynamic loss etc. (given that your pressure difference to drive the flow remains same).

gustavosimiema · August 12, 2015, 03:51

One Thing that I could observe is that the mass flow rate (kg/s) is almost the same in all holles. This can be explained by the fact that is a steady Simulation, flow is fully developed and the pressure difference that drive the flow is small? Or do you have a different opinion?
Because I could not see any effect off back flow because of the fact that the sides are closed.

gustavosimiema · August 12, 2015, 04:18

Also the viscosity effect is almost negligible, so there is not many loss in the pipe, right?

gustavosimiema · August 12, 2015, 11:31

Please see the attachment:

The first one is with 7 holes and the second one with 19 holes. Both in the same conditions. From the pressour contour can be seen that both has negative gauge pressure in the outlet. But why just in the second one with 19 holes the reversed flow appeared? I cannot understand that...

gustavosimiema · August 17, 2015, 08:36

Hi,

I increase the Diameter of the outlet to find the "borders" of my Problem. When I increased, started to happen "reversed flow". It means that for my purpous, this configuration didn´t work well with larger diameter. Therefore the convergence (1e-6) is not achieved anymore. This is what I would expect, since I´m reducing the internal pressure. My question is: How can I quantificate this amount of reversed flow? How can I evaluate the way it is affecting the gas flow?

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August 5, 2015, 07:59		#21
gustavosimiema Senior Member Gustavo Join Date: Jun 2015 Posts: 121 Rep Power: 10	Just one more question, as the inlet will be divided by 4 ... I will use a quarter of my previous model... The inlet velocity (BC) I should also divide by 4, correct?

August 5, 2015, 17:33		#22
fresty Senior Member Join Date: Aug 2014 Location: UK Posts: 213 Rep Power: 12	theoretically yes as volumetric flow is directly additive (per my humble understanding)... this is something you could always check i guess.. not sure how fluent defines symmetry bc option.. it may be possible that once symmetry planes are defined the velocity input is considered for the entire inlet area rather than the 1/4th.. however just go through the below for an insight.. https://www.sharcnet.ca/Software/Flu...ug/node258.htm

August 11, 2015, 08:27		#23
gustavosimiema Senior Member Gustavo Join Date: Jun 2015 Posts: 121 Rep Power: 10	Hi, I increased the number of holes and than my solution doesn´t converge anymore. There are reverse flow in practically all pressure outlets. Each hole I called as one outlet. I have a 500 mm pipe lenght and 19 holes (2mm Diameter). The pipe has a square sectional area (L=30mm). Laminar flow , steady state. As there is the presence of reversed flow, it means that the pressure inside the gasfeed is lower than the pressure out of the Domain? If it´s correct how can I identify that in the Simulation? How to see the pressure inside the pipe and out of the Domain?

August 11, 2015, 11:10		#24
gustavosimiema Senior Member Gustavo Join Date: Jun 2015 Posts: 121 Rep Power: 10	One additional Information, if I increase the velocity inlet this Problem (reverse flow in the outlet) vanish out... What´s the Explanation for that?

August 12, 2015, 02:45		#25
fresty Senior Member Join Date: Aug 2014 Location: UK Posts: 213 Rep Power: 12	What are your outlet BCs? Have you specified the Outlet pressure & Operating Pressure? what are the wall conditions of the pipe? The local pressure drop at a certain region could also result in reversed flow.. in your case the local pressure drop increase at the outlets due to increase in local velocity seems sufficient to have caused reversed flow by increase in dynamic pressure loss..which you probably should expect firstly because of low pressure flow and secondly as you mentioned earlier that the pressure difference to drive the flow is only 1 Pa (please correct me if i got it wrong).. by increasing the velocity you're helping the flow to have enough momentum to pass through even if the pressure drop is the same (in physical terms).. You could use an expression to see the pressure at inlet as well as at outlet however you could always get an approximate idea from contours considering the issue is quite prominent already..

August 12, 2015, 02:54		#26
gustavosimiema Senior Member Gustavo Join Date: Jun 2015 Posts: 121 Rep Power: 10	The Outlet BC is pressure outlet = 0 Pa. Operating pressure = 1Pa. You are right, the pressure difference to drive the flow is 1 Pa. How can I use this expressions? Where are them? Another addittional Information... This Problem of reversed flow doesn´t appears with just 7 holes in the same conditions. It´s the same Explanation? With 7 holes I have less dynamic loss?

August 12, 2015, 03:13		#28
gustavosimiema Senior Member Gustavo Join Date: Jun 2015 Posts: 121 Rep Power: 10	Now I just made a test to see if it works accordingly. I reduced the outlet from 2mm to 1.5mm. Then I could observe that the mass flow rate was the same because the discharge area reduced but the velocity increased to Balance and also the pressure inlet increased. So maybe to vanish with the reversed flow maybe a solution is to increase the Diameter of the holes, than the velocity will reduce and also the pressure drop. Am I right in my way of thinking?

August 12, 2015, 03:23		#29
fresty Senior Member Join Date: Aug 2014 Location: UK Posts: 213 Rep Power: 12	Yes i believe that should be the case.. however, you could compare your test situation with another by increasing the outlet diameter and see the physical changes in the system. It would always be more or less a trade off situation and you would have to find the optimum design point between change in diameter/ no. of holes/ dynamic loss etc. (given that your pressure difference to drive the flow remains same).

August 12, 2015, 03:51		#30
gustavosimiema Senior Member Gustavo Join Date: Jun 2015 Posts: 121 Rep Power: 10	One Thing that I could observe is that the mass flow rate (kg/s) is almost the same in all holles. This can be explained by the fact that is a steady Simulation, flow is fully developed and the pressure difference that drive the flow is small? Or do you have a different opinion? Because I could not see any effect off back flow because of the fact that the sides are closed.

August 12, 2015, 04:18		#31
gustavosimiema Senior Member Gustavo Join Date: Jun 2015 Posts: 121 Rep Power: 10	Also the viscosity effect is almost negligible, so there is not many loss in the pipe, right?

August 17, 2015, 08:36		#33
gustavosimiema Senior Member Gustavo Join Date: Jun 2015 Posts: 121 Rep Power: 10	Hi, I increase the Diameter of the outlet to find the "borders" of my Problem. When I increased, started to happen "reversed flow". It means that for my purpous, this configuration didn´t work well with larger diameter. Therefore the convergence (1e-6) is not achieved anymore. This is what I would expect, since I´m reducing the internal pressure. My question is: How can I quantificate this amount of reversed flow? How can I evaluate the way it is affecting the gas flow?