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Standard k-e model: Calc, boundry conditions, etc

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Old   June 9, 2015, 11:19
Default Standard k-e model: Calc, boundry conditions, etc
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Hello, I am a Master student currently working on a thesis... Because of previous calculations and decisions the subject of this thesis has gone in a direction I am not very knowledgeable about, namely the in depth calculation/simulation of a heat exchanger. Therefor I come here looking for help and advice.

As stated before the problem concerns a heat exchanger, more specifically a fluid/gas double pipe counterflow heat exchanger (I am aware there are better types of heat exchanger for this purpose, but the decision about that is already made).
The gas flows through the inner pipe in a laminar flow, the fluid (water) flows through the annulus in a turbulent flow.

Data:

Temp, Gas_in= 873K
Temp, Water_in= 279K
Flow, Gas= 1.35e-5 kg/s
Flow, Water= 5 kg/s

Inner pipe:
d= 15.75 mm
D= 19.05 mm

Outer pipe:
d= 69.85 mm
D= 95.25 mm
Diameter through which the water flows into the inner pipe: 19.05 mm

I have chosen to do a CFD analysis using the standard k-e model for this problem, from the literature I've found about this it is necessary to define the boundary conditions...
http://www.cfd-online.com/Wiki/Standard_k-epsilon_model

I've calculated the speed of the fluids at the inlet using the area and the mass flow (u= 1.7m/s; v=0 for gas and u=0; v= 17.76m/s for water).
I've also read that Kappa and Epsilon in are also necessary and I've tried to calculate those of the gas (inner pipe) using fully developed pipe flow assumptions:
http://www.cfd-online.com/Wiki/Turbulence_intensity
http://www.cfd-online.com/Wiki/Turbulence_length_scale
And used those to fill in the "e= Cu* (k/l)" and "k= (3/2)*(Uref* TiČ)" formulas.
Which leads me to k= 0.02217 and e= 0.917

If someone could:
- tell me if I'm at least going in the right direction,
- give me something to compare these numbers to,
- tell me how I would go about doing the calculations for the fluid (water),
- point me in the direction of some literature where there is an actual application of this model(because so far I've only found theory or reports),
- help me account for dampening functions that might be needed because of the low Reynolds number of the gas,
- anything else you could think of that could help me understand this better.


I am also using Fluent to do the actual 'simulation' (which I will probably need help implementing as well), however my Promotor would like to see all of the underlying formulas etc to be in the thesis...

I'm sorry about the wall of text, but I felt it was needed in order to explain the situation and the problem.


Thanks in advance for any help you can give me in helping me understand / solve this problem and please let me know if additional information is needed.

Mstudent

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Old   June 11, 2015, 08:59
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Hi i have one question, you said the gas flow is laminar? why are you specifying turbulence properties to laminar flow.
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Old   June 12, 2015, 01:50
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Because I simply don't know how else to do it, the outer fluid flow is turbulent and the k-e model was chosen in order to describe the model...
So as far as I know that means that both flows need to be specified according to this model.
Please let me know if this is incorrect...
(this is supposed to be the "theoretical" side of a Fluent simulation)
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Old   June 16, 2015, 16:51
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sorry i never saw this message i replied to your new thread...i thought the problem description sounded familiar...ill be sure to check back this time
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