## Fluids

#### Simulation of Water Diffusion through Porous Insulation Material

• Arun mathew
Subscriber

Hello,

I would like to simulate water diffusion through a porous media. Below are the details of the problem:

A pipe with a certain thickness is wrapped with a porous insulation material. Initially, there is some amount of water present at the outer boundary of the insulator. I need to determine the quantity of water content present at the outer surface of the pipe (or inner surface of the insulation) at certain time intervals. The top and bottom surfaces are perfectly sealed.

Could you please advise on how to solve this problem using Ansys Fluent software? Any help would be highly appreciated.

Thank you.

• Rob
Ansys Employee

Something like this?  https://forum.ansys.com/forums/topic/fluent-16/

I'd start by doing the tutorials (click on Help in Fluent) as that'll give you some guidance on how the solver works. You may need to determine more information on the boundaries, and also how to calculate porous coefficients (hint, the User's Guide is very good  https://ansyshelp.ansys.com/account/Secured?returnurl=/Views/Secured/corp/v231/en/flu_ug/flu_ug_chp_bcs.html   )

• Arun mathew
Subscriber

Based on my understanding, the simulation discussed in the link you provided may not closely match my problem, as there is no multiphase flow involved. My main challenge is to simulate water diffusion through a porous insulation layer. The temperature and water content at the outer boundary are assumed to be constant, and I need to determine the water content within the insulation layer. The inside temperature of the pipe is also known. If we adopt an axisymmetric approach, the problem can be represented as follows:

• Rob
Ansys Employee

If it's not multiphase then the water content of the porous material is "full", ie a volume fraction of 1. If there's no flow it's then a heat transfer calculation and I'd be using a calculator rather then CFD.  Hence my lack of understanding on what you're trying to model.

• Arun mathew
Subscriber

Thank you for your reply. I apologize for not explaining the problem clearly. Please forgive me for any confusion caused.

Let's simplify the problem to water diffusion through a porous medium. Suppose there is a pipe with insulation covering exposed to atmospheric conditions. Due to the condensation of water vapor on the outer layer of insulation, water is deposited there. The water then diffuses through the porous insulation layer and reaches the outside of the pipe. I would like to know the water content present at the outer surface of the pipe.

There will be a definite flow through the porous layer, but I am unsure whether it will be single-phase or multiphase flow.

Let's assume there is a certain quantity of water per unit area on the outer side. I would like to know what kind of inlet boundary needs to be applied.

• Rob
Ansys Employee

Which will be multiphase as the insulation starts out as being full of air and later on may contain water. It may also be phase change, doable but not recommended for a beginner (pressing the buttons is easy, understanding why you're pressing them is a little less so).

If you assume a mass of water you need somewhere for the air to escape. But a velocity boundary would sort of suitable, but likely to give the wrong result.

Take a very big step away from the computer. Sketch out the whole domain on paper. What are the physical phenomena that you're modelling? If you did the experiment where, and what flow rates, temperatures etc do/can you control? What do you want to measure in the model? Be specific, and work out controlled values against resulting values (for flow in a pipe I can set a pressure drop or a flowrate; I then measure the other).  Then look at where Fluent can help answer the above based on the inputs.

Then come back with what you think you need to model and we'll help more. There's a fine balance on here between helping and teaching: the former is quicker, the latter is much more useful longer term.