You can only use two boundary conditions for one boundary only if they appear at different times not at once. For different time allocating of Boundary Conditions you need an if clause type notation (C*(X>Y)). This means C happens only if X is bigger than Y. Hope it was helpful

I am not expert in this field

Hi, Mohsen!

To give the correct answer, it is  you needed more information about the formulation of problem.

What are the boundary conditions are given on other remaining boundary surfaces?

If all the boundariy conditions are specified as Newman ones, the decision is accurate to constant for. Unambiguity occurs when  Dirichlet boundary conditions are given at one point of the boundary surface. Only at this point both types of boundary conditions are specified at the same time.

Dest regards,

Alexander Yakunin

You can apply only one boundary condition to a boundary at a time. That is, you cannot have two boundary conditions acting on the same boundary at the same time.

More precisely, there can be only one boundary condition per physics acting on a boundary at a time.

PS. Your images are neither showing nor downloading.

COMSOL solve differential equations and you cann't apply two boundary conditions on the same boundary. I think the problem is mathematically incorrect. For example for solving differential equations you can apply pressure of velocity. One more thing you can apply two boundary conditions if you solve hydraulic and thermal problem on the same time

You can only use two boundary conditions for one boundary only if they appear at different times not at once. For different time allocating of Boundary Conditions you need an if clause type notation (C*(X>Y)). This means C happens only if X is bigger than Y. Hope it was helpful

Applying Neumann and Dirichlet boundary condition is indeed possible but not straightforward to implement in COMSOL. I also don't know whether it is physically possible to achieve. 

However your current equation does not require that. I believe your question relates to equation 25 in the figure you attached in your comment. It seems like you have a system with many species in it. 

What you have in your system is Dirichlet boundary condition at r=R1 for CO2 and H2S, and Neumann boundary condition for species other than CO2 and H2S. Different species are represented by different independent variables in a mathematical model. So at any boundary it is quite easy to specify concentration (Dirichlet) for some species and flux (Neumann) for leftover species.

This question is not in my field

I've solved the problem. Thanks for your advices.

Hi Mohsen,

Can you please tell me, how to solve two boundary conditions on the same edge in COMSOL. I also got stuck at the same problem.

Thanks in advance

I am trying to do something similar. In a 2D problem, I need to impose zero velocities at the walls and dP/dn=0 (n is the normal vector of the wall). The variable is (u,v,P), (u,v) are the velocity field and P the pressure.

I have defined the matrix c, beta and a in the PDE, and do not get the way to set the Neuman boundary condition mentioned above.