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-   -   pipe in pipe heat exchanger (http://www.cfd-online.com/Forums/fluent/70637-pipe-pipe-heat-exchanger.html)

JohannV December 1, 2009 07:33

pipe in pipe heat exchanger
 
Hi all. I have to simulate a gas burner and compare the data with some experimental data’s.
The burner is heating a glass pipe with an air flow in it.

But before I will start to simulate the whole process I want to simulate some basic models.
So I thought the pipe in pipe heat exchanger would be a nice introduction.

I have two pipes, in the outer pipe I have air: u_air=10m/s, t_air=300C, Re_air=4000.
In the inner pipe is water: u_water=1m/s, t_water=20C, Re_water=50000.

I use the standard k-e model, boundary layer on both faces on the separating pipe.
The simulation take place in 2 dimensions.

I thought it is enough to show a result but it is not.
My problem is that the separating pipe is keeping the initialized temperature and no heat flux is going from the hot ait to the water through the pipe.

What is wrong in my model?
Do I need the heat exchange model?

Thanks’ for you help.

Chris D December 1, 2009 11:08

Quote:

Originally Posted by JohannV (Post 238303)
Hi all. I have to simulate a gas burner and compare the data with some experimental datas.
The burner is heating a glass pipe with an air flow in it.

But before I will start to simulate the whole process I want to simulate some basic models.
So I thought the pipe in pipe heat exchanger would be a nice introduction.

I have two pipes, in the outer pipe I have air: u_air=10m/s, t_air=300C, Re_air=4000.
In the inner pipe is water: u_water=1m/s, t_water=20C, Re_water=50000.

I use the standard k-e model, boundary layer on both faces on the separating pipe.
The simulation take place in 2 dimensions.

I thought it is enough to show a result but it is not.
My problem is that the separating pipe is keeping the initialized temperature and no heat flux is going from the hot ait to the water through the pipe.

What is wrong in my model?
Do I need the heat exchange model?

Thanks for you help.

Do you mean that there is absolutely no heat transfer from the air to the water, or that there is not much heat transfer from the air to the water? It seems to me (though I could be completely wrong) that a glass pipe is not going to transfer heat very well and the water temperature isn't going to rise very much.

Numerically, are you using shadow zones, or are you directly solving for the temperature in the walls of the pipe?

JohannV December 1, 2009 16:41

Hi Chris thank you for you answer. You are right; glass is not that good in heat conduction.
But I think I found my mistake. I use an aluminium pipe and the heat conductivity is very high compare to glass. So the heat is very fast conducted to water wich has a very high heat capacyty value.

My mistake was the wrong interpretation of the results. I expected a clear temperature gradient in air and water. But the water temperature was only increased by 0.1K.
I think the results shows in the right direction, but how can I be sure that the results are right?:confused:


What can I do to improve the model? Another turbulence model? Mesh refinement? Residuals?

I also calculated a analytical solution of the problem, the solution was: t_outflow_water_analytical=20.006C, t_outflow_air_analytical=360C

To compare with the results of Fluent
t_outflow_air_fluent=430C, t_outflow_water_fluent=20,1C

There is not much what I can improve in the analytical solution, so I think the results from fluent are (wrong) or what would you say?

What do you mean by this question?
Quote:

Originally Posted by Chris D (Post 238340)

Numerically, are you using shadow zones, or are you directly solving for the temperature in the walls of the pipe?

My model is build up from the outer radius to the inner as follows: outer_wall, fluid_air, air_alu_pipe_wall_shadow, air_alu_pipe_wall, alu_pipe_solid, water_alu_pipe_wall_shadow, water_alu_pipe_wall, water_alu_pipe_shadow_wall, water_fluid



Tomorrow I will post my mesh and the case file

Thank you for you help.
Johann

JohannV December 3, 2009 03:53

1 Attachment(s)
Hello,
After a calculation with double precision I got quite low residuals.:)
But one think I dont like. The temperature distribution in the aluminium tube (attached) is linear. How can I change this to a logarithmical distribution?:confused:

Thanks for answering.


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