specifying interface boundary condition
Need help !!
I need to specify the following interface boundary condition : (viscosity1)*d(velocity1)/d(x)=(viscosity2)*d(velocity2)/d(x) where 1 and 2 represents two domains containing different fluids the interface is planer 
Is the x direction prependicular to the surface? Then isn't this simply the shear stress on one side of the interface is the same as the other?
I cannot think of an easy way to implement this with two domains. You could do it as a multiphase free surface model, but freeze the volume fraction field using expert parameters. This would match the shear stresses across the interface. 
yes, the xdirection is perpendicular to the interface
Actually, I am new to this software, please explain in detail Thank you for the reply 
Set up a multiphase simulation with one fluid as one phase and the other fluid as the other phase. Use a single domain. Set an initial conidtion on the volume fraction for the location of the two fluids. Use the expert parameter "solve volume fraction" set to false so the solver does not update the volume fraction field.
But I suspect the interface will actually move, so you could probably leave the volume fraction on as default and let the solver sort it out. Do the CFX tutorials on free surface flow to understand setting it up. 
Multiphase models take 23 days to solve just momentum equations. And here we have to solve momentum, mass tranfer and reaction equations too. So, I am trying it in single phase
Actually I am using single phase model with two domains. And the boundary condition is to given at the interface of the two domain. Could you please help on this. 
That's why I say to freeze the volume fraction equation and the run time will not be too much longer than a single phase model.

Is it posssible to run a simulation including mass transfer and reaction in free surface model (multiphase) in CFX

In general yes, but I do not know specifically what you are doing so cannot say for sure on your case.

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