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Old   December 26, 2015, 03:54
Question CEL Expression
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Shomaz ul Haq
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Dear all. Hi. Hope everyone is well and enjoying the holidays in good spirit. I wanted to ask whether the variables (e.g density) already available in CFX-Post are areaAve(variable)@boundary or do we need to area average them? Would be grateful. Thanks.
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Old   December 29, 2015, 06:08
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The CFX reference manual describes which variables are field variables and which are single values. For field variables you will need to average it back to a single value using some averaging function.
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Old   December 29, 2015, 12:51
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Thank you very much Glenn. I wanted to use velocity equation at inlet boundary condition and not a constant normal speed boundary condition as that is giving me constant Re number versus pipe length/axis in CFD-Post. I want to study how Re number changes and the different flow regimes (laminar, transitional, and turbulent) and boundary layer thickness and effect. Also I can't seem to find the relationship for velocity as a function of pipe length in counter flow double pipe heat exchanger. I have consulted numerous heat transfer articles and books. Do you know any relation of that kind? Would be grateful. Thanks.
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Old   January 2, 2016, 01:23
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I do not understand your question. For a new question please start a new thread and explain what you are doing, and include some images of the geometry and mesh.
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Old   January 2, 2016, 05:57
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Hi everyone. Hope all are well. Thanks Glenn. Actually I want to use a velocity equation at the inlet as a expression but don't know the relation of velocity as a function of pipe length. Wouldn't a variable velocity give me a non-constant graph of Re versus length. Would be grateful for help as I am stuck. Thanks.
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Old   January 3, 2016, 01:52
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I have moved this question to a new thread. Please start a new thread for a new question in future.

Please show an image of what you wish to do, and the equation you wish to implement.
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Old   January 3, 2016, 11:24
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Hi all. Thanks Glenn for starting this thread for me. Actually I started a new thread by the name of "Plotting of Graphs of Reynold Number Versus Length In Counter Flow Double Pipe in CFX". The issue is that I don't know what is the equation (as a function of pipe axial length) for velocity along the length of a counter flow double pipe heat exchanger. Without that I cannot plot Re number along the length of the pipe. I want to do that in CFX-Post but all I am getting is a constant Re line along the length of the pipe. I have seen in some research articles Re graph plotted along the length of the pipe in CFX. How did the investigators do that? Would be grateful for help. God bless you. Thanks.
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Old   January 4, 2016, 06:13
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Why does the velocity vary along the length? Is the flow transient? Or compressible? Or is there inlets or outlets along the length?
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Old   January 4, 2016, 07:39
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Hi Glenn. Thanks for the reply. The flow is steady state and incompressible. There is only one inlet at the entrance of the pipe and outlet at the exit of the pipe. There are no other inlets and outlets along the length. Is it because of being a steady state setup that I am getting constant Re along the length of the pipe. Would be grateful. God bless you. Thanks.
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Old   January 4, 2016, 07:42
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In that case won't you get a constant volume flow rate along the length of the device? So can you explain how the Re could vary along the length of the device?
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Old   January 4, 2016, 09:39
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Hi Glenn. Are you saying that because of my usage of steady state setup that I am getting constant Re along the length of the pipe? If I knew what type of behavior Re would show along the pipe I wouldn't be asking you. What would changing to transient setup do? Would be grateful for insight. God bless you. Thanks.
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Old   January 4, 2016, 18:35
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It looks like I do not understand what you are saying and you clearly do not understand what I am saying. Can you please describe what you are doing (using images), show your mesh and geometry and the CCL/output file.
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Old   January 16, 2016, 10:38
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Dear all. Hope you are all fine. Sorry couldn't continue with this post a few weeks back. Now I am back to the same problem of constant Re number on the length of the counter flow double pipe heat exchanger pipe. Glenn I am attaching images of Mesh and CFX-Solver as you requested. Would be grateful for help. Thanks.
Attached Images
File Type: jpg 1.jpg (74.5 KB, 12 views)
Attached Files
File Type: txt CFX_001.txt (125.0 KB, 5 views)
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Old   January 17, 2016, 00:26
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How are you defining Re? Are you using the pipe diameter as the length parameter or the distance along the pipe?
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Old   January 17, 2016, 04:58
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Dia of inner pipe.
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Old   January 17, 2016, 06:13
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Is the diameter constant along the length of the pipe? You have already said the flow rate is constant, and I assume the material properties are constant. If this is the case then the Re must be constant along the length of the pipe as well. Have I missed something?
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Old   January 17, 2016, 09:51
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Yes you are absolutely right. I don't think you are missing anything. But how can Re number change along the length of the heat exchanger with constant dia?
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Old   January 17, 2016, 17:25
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Re = density * velocity * length / viscosity

So if Re is changing then at least one of those terms is changing.
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Old   January 18, 2016, 00:59
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Thanks a lot Glenn. You have perfectly cleared my mind.
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