Problems in pressure when using mass flow rate outlet

rafiktharwat · May 30, 2011, 19:12

hi all,

i faced a problem in the pressure along car park simulation ,
when i use the exhust fans as outlet with mass flow rate

the pressure gradient is increasing by getting closer to the outlet
which is unacceptable solution

i used the supply fans as opening with pressure of 0 [pa]

i think i miss some physical inputs to have the correct solution.

what is the correct setup of the outlet and inlet to have more realistic pressure solution

note: fluid is Air at 25
ref pressure at 1 [atm]

thnx for your time and help

ghorrocks · May 30, 2011, 19:49

You use a boundary condition setup which matches what you are trying to model. The documentation has some comments about boundary condition choice which is worth reading as well.

And what do you mean by "the pressure gradient is increasing by getting closer to the outlet which is unacceptable"?

rafiktharwat · May 30, 2011, 20:18

i mean that the pressure before the outlet is greater than the pressure after the inlet

which i think its not realistc because the flow moves from the high pressure to the lower pressure not vice versa

my condition is to maintain only the outlet (Mass Flow Rate value)

thnx

rafiktharwat · June 1, 2011, 06:03

hi all

i want to ask if i use for outlet with mass flow rate

if i chose Shift Pressure option

would it help me to have a -ve pressure befor the outlet

and what about pressure profile blend value

thnx for help

ghorrocks · June 1, 2011, 08:04

Can you post an image of what you are seeing, which you think is not realistic?

kambireddy · June 1, 2011, 08:13

hiii,
Usually, i am using mass flow rate as outlet and TP = 0 as inlet for fans.

Does your fan is at open atmosphere? I means to say no pressure drop at in let and outlet. if it is so, you can use inlet Bcs as TP=0 and outlet as SP=0, then frm these you will get mass flow rate.

still it is not your cup of tea, if possible try to explain your problems in more details?

Glenn Horrocks feedback is more helpful...

rafiktharwat · June 1, 2011, 08:47

sorry for lack of details
and thanx for trying to help

i attached the car park i am working with

the openings are [ 0 pa]

the outlets are 11 [kg/s] each

the domain is air with pres ref of [0 pa]

in my opinion the pressure before the outlet has to be lower than the pressure after the inlet (opening)

acording to flow direction

sorry for mis understanding of me

and thank you for your help and time

kambireddy · June 1, 2011, 09:25

Check with Total pressure at both sides.
Apply Bernoulli equation on both sides and check it.

I think what every measuring is static pressure. it was same as flow through diffuser or diverging channel.

ghorrocks · June 1, 2011, 19:26

Also your reference pressure should be atmospheric pressure, not zero pressure. Unless this is a car park in space that is.

rafiktharwat · June 2, 2011, 06:04

i am sorry

the domain Reference pressure is [1 atm]

air @ 25 [C]

the opening boundry pressure is [0 pa]

thnx

A_Prakash · January 2, 2014, 10:14

I would have suggested making the Inlets as massFlow specified and outlet as Average Static Pressure of 0 or -1.
Probably your problem is solved by now. Did you manage to figure out what was wrong?

hmasenger · January 2, 2014, 18:21

I think you need to define a pressure inlet rather than opening.using mass flow BC and an opening sometimes leads to diverged results

May 30, 2011, 19:12	Problems in pressure when using mass flow rate outlet	#1
rafiktharwat New Member rafik Join Date: Jan 2011 Posts: 16 Rep Power: 15	hi all, i faced a problem in the pressure along car park simulation , when i use the exhust fans as outlet with mass flow rate the pressure gradient is increasing by getting closer to the outlet which is unacceptable solution i used the supply fans as opening with pressure of 0 [pa] i think i miss some physical inputs to have the correct solution. what is the correct setup of the outlet and inlet to have more realistic pressure solution note: fluid is Air at 25 ref pressure at 1 [atm] thnx for your time and help

May 30, 2011, 20:18	problems with the pressure	#3
rafiktharwat New Member rafik Join Date: Jan 2011 Posts: 16 Rep Power: 15	i mean that the pressure before the outlet is greater than the pressure after the inlet which i think its not realistc because the flow moves from the high pressure to the lower pressure not vice versa my condition is to maintain only the outlet (Mass Flow Rate value) thnx

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May 30, 2011, 19:49		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	You use a boundary condition setup which matches what you are trying to model. The documentation has some comments about boundary condition choice which is worth reading as well. And what do you mean by "the pressure gradient is increasing by getting closer to the outlet which is unacceptable"?

June 1, 2011, 06:03		#4
rafiktharwat New Member rafik Join Date: Jan 2011 Posts: 16 Rep Power: 15	hi all i want to ask if i use for outlet with mass flow rate if i chose Shift Pressure option would it help me to have a -ve pressure befor the outlet and what about pressure profile blend value thnx for help

June 1, 2011, 08:04		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	Can you post an image of what you are seeing, which you think is not realistic?

June 1, 2011, 08:13		#6
kambireddy New Member Redddy Join Date: Sep 2010 Posts: 25 Rep Power: 16	hiii, Usually, i am using mass flow rate as outlet and TP = 0 as inlet for fans. Does your fan is at open atmosphere? I means to say no pressure drop at in let and outlet. if it is so, you can use inlet Bcs as TP=0 and outlet as SP=0, then frm these you will get mass flow rate. still it is not your cup of tea, if possible try to explain your problems in more details? Glenn Horrocks feedback is more helpful...

June 1, 2011, 09:25		#8
kambireddy New Member Redddy Join Date: Sep 2010 Posts: 25 Rep Power: 16	Check with Total pressure at both sides. Apply Bernoulli equation on both sides and check it. I think what every measuring is static pressure. it was same as flow through diffuser or diverging channel.

June 1, 2011, 19:26		#9
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,871 Rep Power: 144	Also your reference pressure should be atmospheric pressure, not zero pressure. Unless this is a car park in space that is.

June 2, 2011, 06:04		#10
rafiktharwat New Member rafik Join Date: Jan 2011 Posts: 16 Rep Power: 15	i am sorry the domain Reference pressure is [1 atm] air @ 25 [C] the opening boundry pressure is [0 pa] thnx

January 2, 2014, 10:14		#11
A_Prakash Member Abdul Afoo Parkar Join Date: Oct 2012 Posts: 42 Rep Power: 14	I would have suggested making the Inlets as massFlow specified and outlet as Average Static Pressure of 0 or -1. Probably your problem is solved by now. Did you manage to figure out what was wrong?

January 2, 2014, 18:21		#12
hmasenger Senior Member hamed Join Date: Apr 2009 Posts: 148 Rep Power: 17	I think you need to define a pressure inlet rather than opening.using mass flow BC and an opening sometimes leads to diverged results