[Ice man]

Hi every one

I simulate turbulent cross flow finned tube heat exchanger shown below by using fluent, and study the same case experimentally.

I've tow problems

1. There are two way to calculate Re number (Re based on duct hydraulic diameter or pipe hydraulic diameter). I think the second one is common but it require up to 200000 to gain turbulent, and i can't get this value experimentally, so what is your suggestion?
2. Nusselt number (Nu= h* hydraulic diameter/K)

I want to know how to calculate hydraulic diameter of the pipes with its fins.

I'm looking for answer or any site can answered me please

thank you

[ghost82]

Hi,
your question is not clear (at least for me)...where you want to calculate Re number?What's your 2nd problem?

[Ice man]

-depending on the model's image which way is the right to measure Reynold number ( based on duct hydraulic diameter or pipe hydraulic diameter).
-the second problem the hydraulic diameter of pipe is simple to calculate, but how to calculate the hydraulic diameter taking in consideration the fins.

[Bionico]

Hello Ice man,
I suggest using pipe diameter for internal flow Re (water), and fin diameter in order to calculate external flow Re (air) in your boundary conditions.

Best regards

[Ice man]

Originally Posted by Bionico

Hello Ice man,
I suggest using fin diameter in order to calculate external flow Re (air) in your boundary conditions.

Do you have any reference or research do that("fin diameter in order to calculate external flow") please ?

[Bionico]

Actually I based the reply on my experience: usually in heat transfer the Caracteristic Dimension of the system is the fin dimension (lenght for longitudinal fin, diameter for radial fin).
I'm sure there are lot of articles about these kind of systems.

Regards

[Ice man]

Thank you so much Bionico
final question please:I need justification to write that in my thesis, do you have any idea about the reason of using fin diameter. Is this as a type of safety factor?

[Bionico]

Well, characteristic parameters aren't always objective: it depends on which dimension (or temperature, pressure...) you think is representative of your system.
About heat exchanger with extended surfaces (fins) I'm sure that Heat transfer Theory suggests using the lenght of the fin as characteristic lenght because boundary layer and turbulent parameters depend on this dimension.
Another example: if you simulate an aerodynamic profile the characteristic lenght is the chord of the wing, etc...
In your case I would choose diameter for water (in internal flow hydraulic diameter is usually used in correlations, or the radius too) and fin-diameter for air.
I have only a doubt: does the air flow in a duct or are you simulating an external system (open space)? In the first case you might also choose the hydraulic diameter of the duct, taking into account that it isn't circular (anyway I prefer fin lenght because the main physical phenomenon in your system is the heat transfer).

Sorry for my bad english

Regards

[Ice man]

Thank you so much man, you lost more and more of your time because me.
I was found some article support your opinion, and an other calculate hydraulic diameter as below
Dh=4a/b
where:
a:Ratio of free flow area (up, bottom, and sides of heat exchanger) to the stagnation area(where air collide the heat exchanger).
b:Total surface area for one side of heat exchanger to the total volume of heat exchanger.

Do you have any idea?

[Bionico]

Ok, it seems that there are some more specific formulas about your problem (what I said in previous post was referred to a more general theory about fins and heat transfer with extended surfaces): well, you can choose this one, it sounds good!

Best Regards

[Ice man]

OK man,
thank you for your effort