free convection inside tube

Agis · March 12, 2013, 07:24

Hello!! i am new in CFX and a need your advise in a simulation i am trying to perform. My geometry consists of a vertical conical tube. All i want to do is to set different values for temperature at the top and the bottom boundary in the same time and as a result create recirculation inside tha tube. My working fluid is water. The geometry dimensions are:

3 microns diameter at the bottom
1 micron diameter at the top
2.5 mm length.

Any advice on the boundary conditions I should use?

JuPa · March 12, 2013, 07:51

Hi! I suggest you visit my thread: http://www.cfd-online.com/Forums/cfx...er-guides.html as I have a similar problem setup.

Your units of length are microns! Are you sure about this?

As you have 3 microns diameter at the bottom and 1 micron diameter at the top your aspect ratio is not homogeneous. I'm not sure if Rayleigh Bernard theory will cover non homogeneous aspect ratios.

I suggest you read Fundamentals of Heat and Mass Transfer by Incropera and DeWitt, namely Chapter 9.
Also, read Physical Fluid Dynamics by Tritton chapters 4, 14, 16, 17 and 22. The book is quite short and those chapters can be read in a few hours of good reading.

ghorrocks · March 12, 2013, 18:17

As stated in the previous thread already quoted, the key non dimensional number is the Rayliegh number. I suspect the Ra number here is quite low meaning the flow is laminar and dominated by conduction.

The bounadry conditions you describe sound trivial to set up. Have you done the tutorials?

Agis · March 13, 2013, 04:09

I have solved the most of the tutorials. I am quite familiar on the free convection theory but i am still learning. Please give me at first some advice on the kind of boundary conditions you would use in an enclosure similar to this supposedly that the aspect ratio is low and the Rayleigh convection is valid. i think wall at different constant temperatures for the top and bottom plates and adiabatic wall for the rest boundary. the run could be both steady state and transient?

ghorrocks · March 13, 2013, 07:02

The boundary conditions must match what you are modelling. I have no idea what you are modelling so cannot help. If you describe what you are modelling we might be able to help you.

JuPa · March 13, 2013, 07:10

Please provide an annotated diagram. The annotations should state what your boundary conditions are.

March 12, 2013, 07:24	free convection inside tube	#1
Agis New Member Join Date: Mar 2013 Posts: 15 Rep Power: 13	Hello!! i am new in CFX and a need your advise in a simulation i am trying to perform. My geometry consists of a vertical conical tube. All i want to do is to set different values for temperature at the top and the bottom boundary in the same time and as a result create recirculation inside tha tube. My working fluid is water. The geometry dimensions are: 3 microns diameter at the bottom 1 micron diameter at the top 2.5 mm length. Any advice on the boundary conditions I should use?

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March 12, 2013, 07:51		#2
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 15	Hi! I suggest you visit my thread: http://www.cfd-online.com/Forums/cfx...er-guides.html as I have a similar problem setup. Your units of length are microns! Are you sure about this? As you have 3 microns diameter at the bottom and 1 micron diameter at the top your aspect ratio is not homogeneous. I'm not sure if Rayleigh Bernard theory will cover non homogeneous aspect ratios. I suggest you read Fundamentals of Heat and Mass Transfer by Incropera and DeWitt, namely Chapter 9. Also, read Physical Fluid Dynamics by Tritton chapters 4, 14, 16, 17 and 22. The book is quite short and those chapters can be read in a few hours of good reading.

March 12, 2013, 18:17		#3
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	As stated in the previous thread already quoted, the key non dimensional number is the Rayliegh number. I suspect the Ra number here is quite low meaning the flow is laminar and dominated by conduction. The bounadry conditions you describe sound trivial to set up. Have you done the tutorials?

March 13, 2013, 04:09		#4
Agis New Member Join Date: Mar 2013 Posts: 15 Rep Power: 13	I have solved the most of the tutorials. I am quite familiar on the free convection theory but i am still learning. Please give me at first some advice on the kind of boundary conditions you would use in an enclosure similar to this supposedly that the aspect ratio is low and the Rayleigh convection is valid. i think wall at different constant temperatures for the top and bottom plates and adiabatic wall for the rest boundary. the run could be both steady state and transient?

March 13, 2013, 07:02		#5
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	The boundary conditions must match what you are modelling. I have no idea what you are modelling so cannot help. If you describe what you are modelling we might be able to help you.

March 13, 2013, 07:10		#6
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 15	Please provide an annotated diagram. The annotations should state what your boundary conditions are.