[JakinJ]

Hi,
I am fairly new to ANSYS CFX and this is my first time using multi-phase or multi-component model. I am trying to simulate mixing of Acetone in Water (as a note to users Acetone is miscible with water).
Information Regarding the Domain:
The Fluid domain consists of a T pipe section with water entering the domain from the two T inlets (Pipe Diameter:3.175 mm). There is a needle of 0.337 mm inserted into the T pipe at the center of the T from where acetone is injected. The Fluid Domain and the setup Figure is shown in the Figure below:


I am defining two continuous fluids: water and acetone for the domain and have not selected Homogeneous Model and have kept the Free Surface Model to None.

Also, I am using Mixture model as the Interface Transfer Model with interface length scale 1mm [This was the default setting]

Boundary Conditions:
Water Inlet Flow Rate at the two Inlets: 0.003635 kg/sec [Water Volume Fraction: 1 and Acetone Volume Fraction: 0]

Acetone Inlet Flow Rate at the Needle Inlet: 0.000572 kg/sec [Water Volume Fraction: 0 and Acetone Volume Fraction: 1]

Pipe Outlet: Set as Opening with Opening Pressure of 0 atm relative, Since I don't know the exact volume fraction of Acetone and water at the outlet, I have assigned the volume fractions for them based on their inlet mass flow rates and its taking ratio with the total mass flow rate.

I have run the simulation with this setup and have also run a simulation by including another fluid called water-acetone mixture (created as fixed composition mixture in materials containing acetone and water with the mass fraction of acetone and water determined again from their inlet mass flow rate). Both these simulations run without any error but I am not sure which one is the correct method or if both methods are wrong.

I was not able to find any tutorials to simulate multi-fluid mixing. Also, I read in previous threads that additional variables won't be useful in this case since it has two fluid with different properties.

My end goal is to determine the mixing time and length for acetone in water. Kindly inform me if the way I am setting up my model is correct and if not then what would be the correct option and how could I approximately determine the mixing time for the two fluids?

Thank You.

[ghorrocks]

Before you do any simulation you need to get the basic physics correct. So if you are looking at a water/acetone mixture and they are miscible then everything is in the same phase - it is not a multiphase model like you have set up. There are two basic options to model this:
1) additional variable - you use the additional variable as a tracer to track its motion. This is a very quick and easy way of doing it, but does not easily allow you to model physical property changes with concentration or allow chemistry.
2) multicomponent mixture - you define a multicomponent mixture of water and acetone, and the amount if defined my mass fractions. This approach is still reasonably simple (much simpler than multiphase models!) but is a little more involved than option (1). It also allows physical properties to change with concentration changes and can be used for chemistry modelling if that is relevant.

From what you are describing a multicomponent mixture approach sounds like a good way forwards, and the model is single phase (not multiphase).

[JakinJ]

Dear Glenn,

Thank you for your prompt reply and sorry for the confusion. Actually I had set up my simulation as a multicomponent mixture model. I just had a doubt regarding setup for multicomponent mixture. Do I need to include water, acetone and the fixed component mixture of acetone and water in the fluid domain or just the mixture? I am confused because if I only have mixture then I could explicitly specify water and acetone at the inlets.
Also, is it fine to use fixed component mixture, because I read that acetone doesn't react with water but it just dissolves and mixes with water. So I am guess the mixture won't change the composition of acetone and water over time and that specifying the mass fraction of acetone and water for the mixture should take care of it.

[ghorrocks]

Everything you say in this bit is regarding multiphase models:

I am defining two continuous fluids: water and acetone for the domain and have not selected Homogeneous Model and have kept the Free Surface Model to None.

Also, I am using Mixture model as the Interface Transfer Model with interface length scale 1mm [This was the default setting]

So you need to remove all these multiphase models as they are not appropriate physics.

You will need to define a new material as variable composition mixture with two components in the materials list, water and acetone. Then you just use this fluid as a single phase fluid in the normal simulation set up, with the exception that you need to define mass fractions at boundaries and initial conditions. Also note you probably want to set one of the equations as a constraint equation and you might want to set a diffusivity and turbulence properties.