[amin_gls]

hi guys
i want to explicitly set density for a variable composition mixture ,what should i change in cfx pre to be able set it manualy . by default cfx determines density by mass fraction ..
thanks so much ..

[ghorrocks]

Can you explain what you want to do? The density of multi component mixtures is already determined by the solver from the density of the compoenents and their mass fractions. What do you need to explicitly set?

[amin_gls]

thanks glenn. i know it that density is determined by solver , i have a variable composition mixture ,copper and h2so4 .i dont want to determines density by solver ,in ANSYS CFX-Pre User's Guide , in chapter 24 in topic variable composition mixture and mixture properties tab i read it {When you create a fixed composition, variable composition, or a reacting mixture, then the fluid properties are
determined by mass averaging the properties of the component materials. In some cases, the ideal mixture rule used
by the CFX-Solver may not be representative of the mixture properties. You can override the individual
thermodynamic and transport properties by enabling the appropriate toggles and directly specifying the mixture
properties. For details, see Mixture Properties (Fixed and Variable) (p. 36) in the ANSYS CFX-Solver Modeling guide }
i need to specify directly the density of mixture .
just this . how? i dont know!!!.
thanks if you can help me.

[ghorrocks]

The multi component model is designed for mixtures of gasses. You appear to have a solute and a liquid - is this correct? Copper dissolved in H2SO4 (Please correct me if I am wrong).

[cfdonlineuser]

Why don't you just use a single material for the mixture eg water and set the density directly to a constant value as you require. Alternatively set the mass fraction to zero for one of the species then the other species density becomes the mixture density

[amin_gls]

Originally Posted by ghorrocks

The multi component model is designed for mixtures of gasses. You appear to have a solute and a liquid - is this correct? Copper dissolved in H2SO4 (Please correct me if I am wrong).

thats right .

[ghorrocks]

For solar salt bath heating (which seems similar to your application) a typical approach is to use an additional variable to model the salt concentration and make th density a function of the AV. Would this approach work for you?

[amin_gls]

thanks .. i want to simulate buoyancy in a box that there is a source of copper in one wall and a sink in another
the material between this two wall is variable composition material ..
if i use additional variable ror copper . is the buoyancy force calculated ?

[ghorrocks]

Not by default, I think. You will need to include a function for the material density as a function of copper concentration.

[Elechi]

Please I want to know what an additional variable is as mentioned by @ghorock, I have a mixture of steam and water and I want to use variable composition mixture to couple both material... in this process I intend to make the water(liquid) as a gas material in Ansys before making the variable composition mixture, please will my approach work?

If my approach is wrong, please I want to ask if I can use a homogeneous binary mixture to mix both fluids(mentioned above) using a single phase model?

Thank you.

[Opaque]

If you are trying to model wet-steam, you are better off using a homogeneous binary mixture if you want to model it using a single phase approach.

The alternative is to use a multi-phase approach with two materials: liquid water, o water vapor.

I would not use a variable composition mixture approach.

[Elechi]

I am trying to model the vaporization of water as it gets heated when it flows in a tube. Please guide me @opaque.

Thank you for you're response.

[Opaque]

The simplest model is the single-phase approach with equilibrium phase change.

Use a homogeneous binary mixture, set the composition to the initial state, run the simulation.

You should see the formation of vapor as the water is heated. Once you are comfortable with the solution (converged and mesh independent), compare the solution to existing experimental data, and decide afterwards.

[Elechi]

@Opaque, Thank you very much.