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Heat exchanger - crazy temperature

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Old   July 3, 2014, 07:11
Default Heat exchanger - crazy temperature
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Dawid
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Hello,
I want to simulate heat exchanger. Model pipe in pipe. I want to air flowing with 40C in inside pipe and air 900C i outside pipe. What i get is... weird.
I don't know where the problem could be.
Those are setting i made:
-Model: Energy On, Viscous Laminar
-Cell Zone Conditions: Materials inside pipes air, materials of pipes solid aluminium
-Boundary Conditions: inlets of hot/cold pipes are set up on massflow inlet, 0.01kg/s and 900C/40C temperature
oulets are set up to outflow
I am learning fluent for 2 months and maybe i am doing something stupid (and simple i hope so).
Yours faithfully,
Dawid Pogorzelski
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Old   July 11, 2014, 11:37
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David
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At first, i think it could probably be mesh-related, your mesh seems course. What are the dimensions of your problem? The residuals have reached convergence? You could give us a little sketch. Check also again your BC, it is a good practice.

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Old   July 17, 2014, 08:28
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Hello, thank You for reply. I have changed mesh size. Now elements are much smaller. Unfortunately it didn't change anything. This is 2D problem. I think residuals have reached convergence. What kind of sketch do you mean? Project? I have checked my BC many many times

here is link to my project
project It is created in asnys workbench 14.

I hope together we will find solution
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Old   July 18, 2014, 07:18
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I meant a simple drawing with your bounday conditions and how you have set them up in Fluent.

pd: Also post here your residuals and the result of the new mesh
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Old   August 1, 2014, 07:55
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ok, i am not sure but i have found reason. It could be velocity inlet sot up i diffrent direction than normal to boundries. So far is good. I hope nothing more will get crazy from now. Thank you for your patience and help
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Old   August 5, 2014, 04:48
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Dawid
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Hello again,
After little succes in 2D simulation(based on your help) I decided to create 3D simulation. And again i have strange temperatures. Before (it's shame, i know) the problem was boundary conditions (direction of specification method wasn't normal to boundry). Now i have different problem. In "hot entry" i have great 1013K and in "cold" entry i have beatiful 313K. The problem is that i have 4,83e3K in couple points on "out" wall, and somewhere -14K. I did this time everything as in 2D simulation. I hope it's some stupid and easy mistake but i don't know where to look for it. Please help?

Here are:
-main problem, weird temperatres on surface of inside pipe:


-mesh (with highlighted inner pipe)


-residuals (first 100 are for laminar viscous model, after is standard k-e)


-my boundary conditions
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Old   August 5, 2014, 09:36
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Hi, it's clear from the residual image that solution is not converged.
Why don't you setup a structured hexa mesh? It is quite simple for your geometry.
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Old   August 6, 2014, 21:13
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Are you going with steady solver?
You should use unsteady solver when handling heat transfer problem
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