[Shomaz ul Haq]

Dear all,

Hello, Hope you are all doing great. I am modeling nanofluids in ANSYS CFX and wanted to ask whether I can use "Dispersed Solid" in "Option, Morphology, Fluid and Particle Definition" along with "Fluid Domain" as "Domain Type" in "Basic Settings" tab in "Domain". The domain in my case is hot water. I want to check whether introduction of nanoparticles in hot water increases the heat transfer further to the hot water and consequently to the cold water. The hot water with nanoparticles is flowing in pipe and cold water is flowing in another pipe in a "double pipe heat exchanger". Also I have defined nanoparticles (Al203) as a pure substance in my case. Is it correct or do I need to further make another "Fixed Component Mixture" in "Option" in "Basic Settings" tab in "Material" using both hot water and nanoparticles and then use that in "Domain" as a "Fluid and Particle Definition". Anybody's help would be highly appreciated. God bless you all.

Sincerely,
Shomaz ul Haq

[ghorrocks]

First of all - the multiphase model is not usually appropriate for nanoparticle models. The particles are too small and molecular level effects become important (eg Brownian motion, Van Der Waals forces etc) and these are not modelled in the multiphase model.

How do you expect the particles to increase the heat transfer? What physical mechanism does it do this by?

[Shomaz ul Haq]

Thanks Ghorrocks. The nanoparticles cause heat transfer by increasing the thermal conductivity and heat transfer coefficient of nanoflud. Also at the fluid flies towards the end of the tube, heat transfer increases and heat is transferred from hot water-alumina nanofluid to cold water.

[Shomaz ul Haq]

Also physical mechanism for heat transfer is convection.

[ghorrocks]

If the thermal conductivity of the mixture increases simply because the particles have a higher thermal conductivity and displace a lower thermal conductivity fluid then simply model it as a single phase fluid with a thermal conductivity as a function of an additional variable (which will be the particle concentration). No need for a multiphase model.

Also at the fluid flies towards the end of the tube

I do not understand this comment.

[Shomaz ul Haq]

Sorry I meant "flows" not "flies". I meant as the fluid particles enter the fully developed region heat transfer increases. Can you tell me how to mode a single phase fluid with a thermal conductivity as a function of an additional variable (particle concentration). Would be grateful. Thanks.

[ghorrocks]

First define an additional variable for the particle concentration. Make it a convective variable, and diffusive if you want to model diffusion.

Then define a new material as a copy of water (or whatever your base fluid is), and make the thermal conductivity of the fluid a function of AV concentration.

[Shomaz ul Haq]

Thanks. How can I define an additional variable for particle concentration and make it convective or diffusive in CFX-Pre? Would be grateful as I haven't done this before. Thanks.

[ghorrocks]

The CFX documentation gives a pretty good description of how to do it and what the options mean.

[Shomaz ul Haq]

Thanks ghorrocks. Where is the CFX documentation. Are you referring to the CFX portion in "ANSYS Help 15.0"? Please help me where can I find informaton on how to model an additional variable and make thermal conductivity a function of it. God bless you.

[ghorrocks]

Yes, that is correct. Do a search of the documentation. There is lots of important information in the documentation about using the software so I recommend you spend some time getting familiar with it.

[Shomaz ul Haq]

Thanks Ghorrocks. I have explored the documentation of additional variables. I wanted to ask you something that if I use an unspecified additional variable (the only way an additional variable can be defined for as an algebraic equation) for "volume fraction of nanoparticles", I have come across a problem. It states that I can "assign an expression to an additional variable,", but can I use an additional variable in an expression/formula? Would be grateful for reply. Thanks.

[ghorrocks]

If you know the volume fractions of the nanoparticles you should use a volumetric AV. If you know the mass fractions you should use a specific AV. These two options allow the choice of transport and diffusion equation.

[Shomaz ul Haq]

Hi all. I wanted to ask how can I model nanofluid behavior use single-phase fluid with enhanced thermophysical properties (thermal conductivity and viscosity). I want to add volumetric additional variable of volume concentration of nanoparticles. Where should I add volumetric additional variable in CFX and how? I am modeling double pipe heat exchanger. Would be grateful. Thanks.

[ghorrocks]

Add it as an additional variable. These are described in the CFX Modelling guide and CFX Pre guide.

[Shomaz ul Haq]

How to add volume concentration as an additional variable? Do I need to use some empirical correlation/equation already established by previous investigators or do you mean something else? Would be grateful. Thanks.

[ghorrocks]

Assuming the nanoparticles convect and/or diffuse with the flow then yes, use an additional variable to model them.

[Shomaz ul Haq]

So I should use that AV in some correlation established by a researcher? I don't understand what do you mean by "nanoparticles convect and/or diffuse with the flow"?

[ghorrocks]

I think we are getting distracted by details here when you do not appear to have worked out what you are doing. In CFD you need to determine what you are modelling, then work out the important physics, then choose simulation models to capture that physics. Do not assume that nanoparticles means you need to use an AV - it depends on what is happening in your flow.

So work out what you are modelling, what the important physics is, and only then consider what models CFX has available to model it.

[Shomaz ul Haq]

Hey all. I wanted to ask how can I use variables (e.g. thermal conductivity) of different materials (e.g. water and solid particles of Al2O3) in an expression. For example when I used the name "thermal conductivity" in an expression and tried to evaluate it in CFX-Pre the expression asked me the value of thermal conductivity considering it as an independent variable. The whole purpose was to get the value in the expression by just using that variable. I have searched a lot on ANSYS Help but couldn't find specifically how to address properties. Please tell me is there a specific syntax to use. Would be grateful. Thanks.