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Old   March 5, 2008, 07:02
Default Phase change
  #1
Syed Shabbar Raza
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i have to model a problem in which water is converting into super heated steam in a pipe. how can i model it using fluent.

web links to tutorial for this types of problems are most welcome.
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Old   March 5, 2008, 08:52
Default Re: Phase change
  #2
Syed Shabbar Raza
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problem is simply.

water is entering a metalic pipe whose temperature is 800oC. through out its way, water converted in to steam and finally converted into superheated steam.

i think i have explained the problem, now i want to model it in fluent, so how could i. because water change its phase into steam, so how could i incooperated this change in phase into fluent
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Old   March 5, 2008, 10:08
Default Re: Phase change
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Robert
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Hi!

I have the same problem. I use the mixture/Cavitation model, but this model "lock" the pressure to the vaporization pressure, so the vapor doesn't expand, so it isn't cooling. Whith this model the water converted into steam, but doesn't cooled down.

So I think, our problem is the same. Read the forum topic "phase change" below. Unfortunatly nobody answer

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Old   March 5, 2008, 11:18
Default Re: Phase change
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Robert
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Well, the solution: The cavitation model isn't OK for this problem, You have to use the wet steam model.
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Old   March 5, 2008, 12:15
Default Re: Phase change
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Robert
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Well, the solution: The cavitation model isn't OK for this problem, You have to use the wet steam model.
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Old   March 6, 2008, 02:06
Default Re: Phase change
  #6
Syed Shabbar Raza
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thanks Robert,

i m not an expert user of fluent, so i ask u some thing more

if i define the fluid properties, like change of properties with respect to temperature, should it work ??
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Old   March 6, 2008, 09:30
Default Re: Phase change
  #7
Robert
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This is the first time, when I use this model, but I share my experience with You.

1. First calculate the flowfield with the pressure based model. Use only water or vapor fluid. So calculate a simple flowfield.

2. Switch to density based model.

3. Calculate a new solution. Note: These solutions aren't the final solutions, whit these solutions you create a better initial condition for the wet stem model.

4. Switch to wet steam model (DEfine/Models/Multiphase.../Wet steam)

5. Calculae the overheated solution.

This model set the material properties AUTOMATICALLY, so you haven't need to set any properties. This model will disable the material menu, because it set everything. If you use DPM, you have to set the material of discrate hase befor switch to wet steam model.

This model much slower like a "normal" multiphase models and as I see, it is very instable, so maybe you have to use smaller time step (10^-5 ; 10^-6), and maybe you have to reduce the under relaxation factors.

But as I saud, I haven't got too mauch experience with this model.

Bye
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Old   March 7, 2008, 01:25
Default Re: Phase change
  #8
Syed Shabbar Raza
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thanks robert, i will let u know my result after validation of out come.

Infact what i have done early to solve this problem is to define a new material using piecewise-linear and give it the properties of water then steam with respect to temperature. and then solve the unsteady solution.

what do u say about this approach. ??
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Old   March 7, 2008, 03:36
Default Re: Phase change
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Robert
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Sorry, but this approach looks bad. You defined a material, which properties "moving" on the line of water-vapor change. But the phase change strongly depending on the pressure to, not only the temperature. For example: the water boiling on 100 oC, when p=101325Pa, but if the pressure bigger, the boiling temperature is bigger, too.
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Old   March 7, 2008, 03:42
Default Re: Phase change
  #10
Syed Shabbar Raza
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Hmmm !!!!!!!!!

but when we choose a materail from fluent database, it didnt indicate any temperature or pressure for which it is takeing density, cp, visco, heat tranf coeff, etc,

it takes form operating condition ??
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Old   March 7, 2008, 03:47
Default Re: Phase change
  #11
Syed Shabbar Raza
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23.6.2 Wet Steam Flow Equations

The wet steam is a mixture of two-phases. The

primary phase is the gaseous-phase

consisting of water-vapor (denoted by the subscript v) while the

secondary phase is the liquid-phase

consisting of condensed-water droplets (denoted by the subscript l).

( and my dear u want to model inverse, i mean here fluent assume that the primary phase is steam, but in your case it is water and i can guess that u have given the boundary condition for water ( inlet condition),

so i think we should use inlet boundary condiition for outlet and vice versa

plz share your view

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