MIcrochannels.

abadiee · July 4, 2010, 14:01

I am simulating microchannels in FLUENT. I have a single channels with rectangular crosssection. Hydrolic diameter is 0.0015, Length is 1 cm.
BC: Inlet> Velocity inlet
Outlet> Outflow
Bottom wall > Heat flux: 100000 w/m2, Th:0.0005m
Upper and side walls: Adiabatic, Th:0.0005m
But What I get in pressure contour is negative pressure inside the channel which does not make any sense.
Can I have your opinions on the issue please.

thecfduser · July 4, 2010, 15:19

Hi
the pressure in the N.S equation appears as a gradient. What is important, in an incompressible flow, is the pressure difference and not the pressure.
The pressure will depend on the point in wich u choose the reference pressure (in operating pressure window of Fluent).
Anyway, i think ure simulation is not well posed.
Choose pressure outlet instead of outflow, with pressure=0 on that boundary.
U will get positive pressure inside the microchannel, and ure simulation will be good.

abadiee · July 4, 2010, 16:10

Thank you.
The problem with pressure outlet is that the fluid is not going to atmosphere pressure. It is in a closed circle and I choose outflow to see how much the outlet pressure is. When I choose pressure outlet with 0 Pascal. the outlet pressure is 0 which is not sensible. Even I changed the operation condition but no changed in negative pressure. The gradient is absolutely correct but how can I have the absolute pressure?

thecfduser · July 4, 2010, 16:30

With outflow, u assume that ure fluid tempearture dont change anymore at the outlet...(dT/dx=0)....
When u put outlet pressure with 0 Pa, it is the gauge pressure that is 0. To get the absolute pressure, u must know the pressure at least in a point of ure domain (the pressure biggest derivative is 1st order in NS equations).
so in a resume, if u dont know the absolute pressure in a point of ure domain, u cannot get the absolute pressure at ure outlet.
(if u choose a pressure outlet, Fluent will not consider anymore the operating condition. The operating pressure will be calculated from ure outlet).
If u didnt undertsand, write to me again

July 4, 2010, 14:01	MIcrochannels.	#1
abadiee New Member Amir Join Date: Jul 2010 Location: The UK Posts: 2 Rep Power: 0	I am simulating microchannels in FLUENT. I have a single channels with rectangular crosssection. Hydrolic diameter is 0.0015, Length is 1 cm. BC: Inlet> Velocity inlet Outlet> Outflow Bottom wall > Heat flux: 100000 w/m2, Th:0.0005m Upper and side walls: Adiabatic, Th:0.0005m But What I get in pressure contour is negative pressure inside the channel which does not make any sense. Can I have your opinions on the issue please.

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July 4, 2010, 15:19		#2
thecfduser Senior Member karine Join Date: Nov 2009 Posts: 158 Rep Power: 16	Hi the pressure in the N.S equation appears as a gradient. What is important, in an incompressible flow, is the pressure difference and not the pressure. The pressure will depend on the point in wich u choose the reference pressure (in operating pressure window of Fluent). Anyway, i think ure simulation is not well posed. Choose pressure outlet instead of outflow, with pressure=0 on that boundary. U will get positive pressure inside the microchannel, and ure simulation will be good.

July 4, 2010, 16:10		#3
abadiee New Member Amir Join Date: Jul 2010 Location: The UK Posts: 2 Rep Power: 0	Thank you. The problem with pressure outlet is that the fluid is not going to atmosphere pressure. It is in a closed circle and I choose outflow to see how much the outlet pressure is. When I choose pressure outlet with 0 Pascal. the outlet pressure is 0 which is not sensible. Even I changed the operation condition but no changed in negative pressure. The gradient is absolutely correct but how can I have the absolute pressure?

July 4, 2010, 16:30		#4
thecfduser Senior Member karine Join Date: Nov 2009 Posts: 158 Rep Power: 16	With outflow, u assume that ure fluid tempearture dont change anymore at the outlet...(dT/dx=0).... When u put outlet pressure with 0 Pa, it is the gauge pressure that is 0. To get the absolute pressure, u must know the pressure at least in a point of ure domain (the pressure biggest derivative is 1st order in NS equations). so in a resume, if u dont know the absolute pressure in a point of ure domain, u cannot get the absolute pressure at ure outlet. (if u choose a pressure outlet, Fluent will not consider anymore the operating condition. The operating pressure will be calculated from ure outlet). If u didnt undertsand, write to me again