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Old   June 22, 2005, 19:09
Default Heat Exchanger
  #1
walid
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Hi

Is there any one who simulated heat exchangers or condenser ?

I am looking to validate the simulation results with an experimental data for temperature and velocity fields in the tube side.

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Old   June 23, 2005, 06:28
Default Re: Heat Exchanger
  #2
stex
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Here I am. Simulation of heat exchanger is my job.

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Old   June 23, 2005, 18:49
Default Re: Heat Exchanger
  #3
walid
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Dear Stex,

Thank you for your reply on my request. Actually, I am doing a study on estimate the efficiency of steam heat exchanger condenser and I use some design data for real condensers in industry.

First, I tried to make it as simple as possible so I start to simulate a tube with considering the inlet fluid temperature and saturation steam temperature along with relevant information about flow rate and dimensions. I have got the outlet fluid temperature which should be predicted by CFX.

However, I noticed when the wall temperature constant the fluid reaches the wall temperature before passing 1/3 of tube length which contradicts the design data.

To over come this, I start to do trail and error procedures to locate actually wall temperature which match the design outlet fluid temperature but it does not work and results were as before. It seems the wall temperature is quite different from the saturation steam temperature.

I would be grateful if you can help me on this Is there any wrong on my assumptions?

Best regards
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Old   June 23, 2005, 19:33
Default Re: Heat Exchanger
  #4
Glenn Horrocks
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Hi,

Is your simulation steady state? If so then you might not yet have fully converged the simulation.

Glenn Horrocks
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Old   June 24, 2005, 05:58
Default Re: Heat Exchanger
  #5
walid
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Hi Glenn

Yes, it is in st. st. and the convergence residual was 1E-4 (set default in CFX).

Walid
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Old   June 26, 2005, 19:16
Default Re: Heat Exchanger
  #6
Glenn Horrocks
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Hi,

I'll bet it is not converged. What are the imbalances? If they are not small then you are not converged. Converging on imbalances AND residuals is useful in CHT simulations.

Glenn Horrocks
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Old   June 27, 2005, 04:07
Default Re: Heat Exchanger
  #7
stex
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Sometimes you have fast convergence (residuals) due to a simple structured mesh but if you plot the imbalance you find a high value. Try to restart the run and minimize the imbalance...

Good luck
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Old   July 1, 2005, 10:04
Default Re: Heat Exchanger
  #8
Walid
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Dear Friends,

Thank you for your contribution on this forum. Actually, I have checked the imbalance values after the solver finished the run. Their values are very small usually and they are ;

U-mom -0.004% V-mom -0.0032% W-mom 3.045% p-mass -0.1149% H-energy 2.6218%

I tried to run my case without switching energy on. The results were close to the above values.

Again thank you for your help.

Regards
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Old   July 3, 2005, 20:44
Default CFX Failure ... can be???
  #9
Walid
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Is it possible that CFX can failure in producing reasonable data on simulation a heat exchanger?

Can this be as the flow system very narrow inside the tube side?

As known, the industrial HEs usually are designed with tube ratio (length/diameter) of 500 or greater.

Or is it the turbulence model?

Regards

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Old   July 11, 2005, 09:15
Default Re: CFX Failure ... can be???
  #10
funster
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It is interesting that you are basing your simulations on a single tube. Previously I have done simulations looking at complete Steam Condensers where individual tubes were not modelled. We would model the effects of the tube bundles representing them as porous areas with heat sinks. th e3 dimensionality of the heat exchanger tubes was taken into account using empirical correlations, so that the effectiveness of the heat exchanger tubes diminished further into the condenser away from their inlet points. The Inundation effects were also modelled within the tube bundells. The steam was modelled as a scalar. Effects of poor design were often seen through the build up of air pockets causing poor heat exchange across certain areas of tubes. Air off take was also modelled as often poorly performing condensers require the removal of air from the system. Otherr factors that were looked at were the back pressure to the turbine. This again was a big performance criteria, and often the aim was to try and reduce this.

Studies have been conducted looking at temperatures withing individual heat exchanger tubes. This enabled the mapping of potential air pockets withing tube bundles.

Baffles withing the condensers were also modelled, often being the causes of the problems within older designs of condensers.

Hoep this helps and if you have any questions drop me a line.

Stuart
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Old   July 12, 2005, 08:32
Default Re: CFX Failure ... can be???
  #11
Walid
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Funster,

Thank you for yours contribution. I impressed by your way in tackling the difficulties of simulation of heat exchanger. My aim is to determine the accurate temperature and velocity fields for the tube heat exchanger.

If you have previous case in simulating a condenser please share your finds with CFX community and I would like to know if you published your results.

Best Regards
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Old   July 26, 2005, 08:06
Default Re: CFX Failure ... can be???
  #12
funster
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This was a while back and I know longer work in the field of CFD - long story !!

But the work I did was for an Engineering Consultancy Company called Mott MacDonald under a guy called Norman Rhodes. He used to publish papers on this and did a great deal of heat exchanger / condenser simultations including a few MSR's - Moisture Separator's in Nuclear Plants.

Alas I don't have any of his papers any more but he was a good man to talk to and was very good at explaining technical points. The guys at CFX -uk would be able to point you in his direction but last I herd he was working over in the states for Motts. The company may now be called Hatch Mott MacDonald ? like I said its been a while.

Stuart
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Old   August 1, 2005, 18:52
Default Re: CFX Failure ... can be???
  #13
Walid
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Thanks Funster for your help.

Regards
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