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-   -   Heat Exchanger Model (http://www.cfd-online.com/Forums/fluent/83409-heat-exchanger-model.html)

sak2129 December 27, 2010 16:34

Heat Exchanger Model
 
To anyone who can help:

I searched this form for issues similar to the one I am facing, but wasn't able to find a solution.

I am modeling a tube bank using the heat exchanger model and porous media. Basically, I have defined a mass flow inlet at a square boundary. The primary fluid travels in a rectangular duct and eventually hits the heat exchanger where it loses some energy to the auxiliary fluid. The auxiliary fluid is specified with just one pass, going from the bottom of the duct to the top.

I am using the heat rejection option with the NTU method. Here are the findings from what I have done so far:
1) The pressure drop in the air heater is as expected
2) The heat rejected from the primary fluid is as specified
3) The auxiliary fluid inlet temperature (computed by Fluent) is unreasonable (negative at times)
4) When I print the heat exchanger report, Fluent keeps printing the constant specific heat for the auxiliary fluid even if I read an UDF for enthalpy.

My questions are as follows:
1) If I am getting unreasonable values for the auxiliary fluid temperature, how do I fix my solution to ensure my primary fluid side calculations are correct?
2) Is there a special way to make Fluent use the UDF that you specify under the auxiliary fluid tab? Also, is there a way of verifying that Fluent actually used your UDF instead of taking constant specific heat for the calculations?

If anyone can assist, it'd be much appreciated. Thank you!

lucio December 30, 2010 20:40

the UDF for auxiliary fluid can only be used in the simple-effectiveness-model !so you couldn't use it in the NTU model.

tushar.apatil December 8, 2014 08:28

Query
 
How do we calculate no. of passes/rows & /columns for a given model in NTU method, in fluent??

Quote:

Originally Posted by sak2129 (Post 288609)
To anyone who can help:

I searched this form for issues similar to the one I am facing, but wasn't able to find a solution.

I am modeling a tube bank using the heat exchanger model and porous media. Basically, I have defined a mass flow inlet at a square boundary. The primary fluid travels in a rectangular duct and eventually hits the heat exchanger where it loses some energy to the auxiliary fluid. The auxiliary fluid is specified with just one pass, going from the bottom of the duct to the top.

I am using the heat rejection option with the NTU method. Here are the findings from what I have done so far:
1) The pressure drop in the air heater is as expected
2) The heat rejected from the primary fluid is as specified
3) The auxiliary fluid inlet temperature (computed by Fluent) is unreasonable (negative at times)
4) When I print the heat exchanger report, Fluent keeps printing the constant specific heat for the auxiliary fluid even if I read an UDF for enthalpy.

My questions are as follows:
1) If I am getting unreasonable values for the auxiliary fluid temperature, how do I fix my solution to ensure my primary fluid side calculations are correct?
2) Is there a special way to make Fluent use the UDF that you specify under the auxiliary fluid tab? Also, is there a way of verifying that Fluent actually used your UDF instead of taking constant specific heat for the calculations?

If anyone can assist, it'd be much appreciated. Thank you!



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