About messing
 

How to do node to node connectivity in Ansys 18 between solid and fluid domain. Does anyone know?

You can do that in geometry editing by SpaceClaim/DesignModeler or you can also do that in the ANSYS mesher.
- For SpaceClaim http://help.spaceclaim.com/2015.0.0/...edTopology.htm
- For DesignModeler https://www.sharcnet.ca/Software/Ans...dTopology.html
- For ANSYS Meshing https://www.sharcnet.ca/Software/Ans...ode_merge.html

Queries regarding ANSYS
 

How to give flux in the interface, can you tell?

Well since you can't do that in ANSYS Meshing & Geometry part, you need to assign it as a boundary condition in your solver software (FLUENT, CFX, AIM, etc...).

After messing when I go to set up in boundary conditions that part should be a wall so that I can give flux but it creates an interface where I unable to give boundary condition.

Since there is no schematic of your problem, I am just going to assume things.
I think you are trying to simulate not the interface but different zones between solid and fluid, not fluid/fluid.
So if this is your case, you have to change your zones to fluid and solid according to your problem then you can assign an interface as a wall in your solver.
Another possibility is that you have a fluid and solid zone separately but the solver automatically created the connection between them as an interface where you should then change it to the wall manually.

Plz see the attachment, and green colour part is my part where I suppose to give flux, but it creates an interface. Outside the tube, it is the air domain.

still, I am unable to change the interface to the wall.

Sorry for the late reply,
But as I said above you can change your boundary conditions in your solver settings.
Even if the solver creates automatically the interface in the beginning, by simply naming the surface in ANSYS Meshing with Named Selection you can assign another boundary condition to that spot in your solver.