Nu No for fully developed region

mina.p · March 5, 2014, 01:30

Hello
I have a problem, I am doing simulation of a laminar water flow in a 2D channel but I cannot reach the desired Nu=7.54. I don't know where the problem exists. could anybody help me

channel: 75mm (H) x 1500mm (L)
thickness=0.6 mm
Re=80
Dh (hydraulic diameter)= 75mm
T inlet=298K
T wall=400K
inlet b.c= normal speed according re=80
outle b.c=mass flow=p*u*A , A=0.6*75
but the Nu number i obtained is around 4.98

mvoss · March 5, 2014, 04:23

Please post a picture of the setup. Do you reach a converged solution?Did you do a mesh convergence study?

ghorrocks · March 5, 2014, 04:25

FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

JuPa · March 5, 2014, 06:07

The Nusselt number is the non-dimensional heat transfer coefficient, based on local thermal conductivity and length scale.

What reference temperature do you think CFX bases the heat transfer coefficient on?
And what reference temperature do you think literature bases the heat transfer coefficient on?

Don't use CFX's definition of the Nusselt number / heat transfer coefficient. I.e. if the literature base these values on some bulk temperature then use the bulk temperature in CFX.

mina.p · March 6, 2014, 02:08

would any body please help me???
i want to reach incropera Nu no 7.54 for constant wall tempof channel.
and i use this expression for Nu no:
((ave(Wall Heat Flux)@Line 128)/(400 [K]-(ave(T)@Line 128)))*(.15[m]/.6069[W m^-1 K^-1])
Dh=2h=2*.075=.15

http://www.imageupload.co.uk/TOs
http://www.imageupload.co.uk/TOx
http://www.imageupload.co.uk/TOz
http://www.imageupload.co.uk/TO8
http://www.imageupload.co.uk/TXK

ghorrocks · March 6, 2014, 03:56

There are three posts on this thread responding to your request which all suggest important things to consider. If you are going to ignore the advice you get on this forum then why bother posting on it?

You have a velocity inlet and a mass flow outlet. This is not good. You need to make one of these boundaries a pressure boundary. Read the section in the documentation about selection of boundary conditions.

mina.p · March 7, 2014, 02:39

dear Glenn
I change the mesh. b.cs(static pressure,opening for outlet), channel lenght and write an expression according Tbulk for Nu.
but always the same result

mvoss · March 7, 2014, 03:40

You should definitely check your boundary layer - i remember that to get the temperature gradient/wallheatflux right, you need a pretty super fine resolution at the wall. Like twice the resolution for the drag/lift-forces to remain constant when refining the mesh resolution.
please post some pictures.

March 5, 2014, 01:30	Nu No for fully developed region	#1
mina.p New Member mina Join Date: Nov 2013 Posts: 20 Rep Power: 12	Hello I have a problem, I am doing simulation of a laminar water flow in a 2D channel but I cannot reach the desired Nu=7.54. I don't know where the problem exists. could anybody help me channel: 75mm (H) x 1500mm (L) thickness=0.6 mm Re=80 Dh (hydraulic diameter)= 75mm T inlet=298K T wall=400K inlet b.c= normal speed according re=80 outle b.c=mass flow=puA , A=0.6*75 but the Nu number i obtained is around 4.98

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March 5, 2014, 04:23		#2
mvoss Senior Member Matthias Voß Join Date: Mar 2009 Location: Berlin, Germany Posts: 449 Rep Power: 20	Please post a picture of the setup. Do you reach a converged solution?Did you do a mesh convergence study?

March 5, 2014, 04:25		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,701 Rep Power: 143	FAQ: http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F

March 5, 2014, 06:07		#4
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 14	The Nusselt number is the non-dimensional heat transfer coefficient, based on local thermal conductivity and length scale. What reference temperature do you think CFX bases the heat transfer coefficient on? And what reference temperature do you think literature bases the heat transfer coefficient on? Don't use CFX's definition of the Nusselt number / heat transfer coefficient. I.e. if the literature base these values on some bulk temperature then use the bulk temperature in CFX.

March 6, 2014, 02:08		#5
mina.p New Member mina Join Date: Nov 2013 Posts: 20 Rep Power: 12	would any body please help me??? i want to reach incropera Nu no 7.54 for constant wall tempof channel. and i use this expression for Nu no: ((ave(Wall Heat Flux)@Line 128)/(400 [K]-(ave(T)@Line 128)))(.15[m]/.6069[W m^-1 K^-1]) Dh=2h=2.075=.15 http://www.imageupload.co.uk/TOs http://www.imageupload.co.uk/TOx http://www.imageupload.co.uk/TOz http://www.imageupload.co.uk/TO8 http://www.imageupload.co.uk/TXK

March 6, 2014, 03:56		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,701 Rep Power: 143	There are three posts on this thread responding to your request which all suggest important things to consider. If you are going to ignore the advice you get on this forum then why bother posting on it? You have a velocity inlet and a mass flow outlet. This is not good. You need to make one of these boundaries a pressure boundary. Read the section in the documentation about selection of boundary conditions.

March 7, 2014, 02:39		#7
mina.p New Member mina Join Date: Nov 2013 Posts: 20 Rep Power: 12	dear Glenn I change the mesh. b.cs(static pressure,opening for outlet), channel lenght and write an expression according Tbulk for Nu. but always the same result

March 7, 2014, 03:40		#8
mvoss Senior Member Matthias Voß Join Date: Mar 2009 Location: Berlin, Germany Posts: 449 Rep Power: 20	You should definitely check your boundary layer - i remember that to get the temperature gradient/wallheatflux right, you need a pretty super fine resolution at the wall. Like twice the resolution for the drag/lift-forces to remain constant when refining the mesh resolution. please post some pictures.