Dear Sumit Sharma

For the outer pipe, if you set the volume fraction of the nano-particles to zero, this would act as a single phase fluid.

Hope this works for you.

First you have to create two different zones, i.e. inner and outer, while creating your mesh then you have to set  the inlet boundary conditions separately for both the zones. For inner zone you have to provide the volume fraction of the nano-particles at inlet boundary condition of inner zone, as per your problem definition. For outer zone, you have to provide the inlet boundary condition as zero volume fraction of nano-particles and one for water. I hope this will work.

Dear Morteza Anbarsooz and Sanjeev Kumar,

Thank you very much for your reply and suggestion. I will try on that and hoping for the good result.

You can create (water + nano particle) fluid and put it inside inner tube and put water (only) inside outer tube .

This will work 

Dear Birendra Rajan, There are two different phases on is fluid (water) and other one is solid (nano particle). you can not flow these two phases as you mentioned in your comment. You need to use multiphase model > mixture> no. of phases-2> ok, then you need to define the phases, which phase is solid and which one is fluid. after this you need to create interface with coupled wall option.   

After all this, you need to define the cell zone and boundary conditions. 

For the boundary condition you need to define values for each phase for each zone as per your requirements.

Dear Sumit,

 We can create a mixture of water + nano particle by injecting specific nano material with their sizes and volume . This mixture u can flow in the inner tube and water u can flow in the outer tube.