Hello Muhammad,

Can you provide a little more information? Such as the application, physics and geometry? To my knowledge I don't believe it is possible to assign two different materials to a single domain, but hopefully someone else here on RG can give you a conclusive answer.

Are you indeed trying to define a single domain as two different materials or are you trying to define a different material property at a domain boundary which is a common boundary of two domains?

If you are trying to define a porous domain, you can use the boolean operators to define two complex domains within a single simple domain (air gaps in a solid). You can then assign different material properties to each. COMSOL does also have a tutorial on a porous reactor and I have provided the links for you.

http://www.comsol.com/model/porous-reactor-with-injection-needle-25

https://www.comsol.com/blogs/modeling-reacting-flow-porous-media/

Thanks Jules L Hammond for the detailed answer, I have added details if you can guide me as per details I will be grateful..

Thanks for adding some details Muhammad.

Have you calculated the Reynolds number? I have only worked with the microfluidics module where the low Reynolds number simplifies things somewhat. When working with multiple phases I prefer the phase field method but you should really read up on the differences (between phase field and level set) if modelling the interface becomes important.

You didn't really give any indication of the characteristic length nor advise what modules you have access to so it's a little tricky to recommend a module.

Maybe have a look at the CFD module which explains a lot of COMSOL's capabilities:

http://www.comsol.com/cfd-module

I understand that there is a 'pipe flow' module which contains 'transport of diluted species in pipes' (dsp) and 'reacting pipe flow' (rpfl) physics and have provided links for you. 

There is also a model in the application gallery showing a laminar static mixer, again I have provided the link.

I do recommend that you check the application gallery (use the search field or filter by module) to see if there are any similar applications. Over the past few years I have seen the popularity of COMSOL grow and consequently there are already many brilliant models to use as a starting point or to compare strategies.

http://www.comsol.com/models

I hope this helps. Again, the more detail you can provide about the model the more useful an answer we can provide.

P.s. When starting out, I recommend using the least number of dimensions possible; it will be much quicker to define the physics and quicker to compute. Some other great  tips are here: http://evgeni.org/oldfish/Top_nine_hints_for_working_with_Comsol

https://www.comsol.com/pipe-flow-module

http://www.comsol.com/video/simulating-pipe-channel-network-flow-comsol