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Integration of Surface Flux During Simulation
Posted 18.09.2012, 15:03 GMT-4 Chemical Reaction Engineering, Parameters, Variables, & Functions Version 4.2a 1 Reply
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Hello,
I am trying to model a heterogeneous reaction by specifying a Flux boundary condition that depends on the surface concentration, gamma, which changes with time: gamma(t) = gamma(t=0) - gamma_reacted(t). gamma_reacted(t) is just the integral from 0 to t of the surface flux. I see that the function timeint can only be evaluated after the simulation is finished, so I wanted to use a time discrete study and define gamma_reacted as a variable by adding timestep*flux to it at each step. Unfortunately I cannot define gamma_reacted = gamma_reacted + timestep*flux, but that is what I would like to do.
I am trying to model a heterogeneous reaction by specifying a Flux boundary condition that depends on the surface concentration, gamma, which changes with time: gamma(t) = gamma(t=0) - gamma_reacted(t). gamma_reacted(t) is just the integral from 0 to t of the surface flux. I see that the function timeint can only be evaluated after the simulation is finished, so I wanted to use a time discrete study and define gamma_reacted as a variable by adding timestep*flux to it at each step. Unfortunately I cannot define gamma_reacted = gamma_reacted + timestep*flux, but that is what I would like to do.
1 Reply Last Post 20.09.2012, 13:32 GMT-4
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Posted:
1 decade ago
I made some progress using the Transport and Adsorption Model included with COMSOL. I added Weak Form Boundary PDE with the dependent variable defined as the surface concentration, cs. I define the flux on my reactive surface as the reaction rate expressed in terms of this variable, R = k*c*(gammainf - cs). The expression for the Weak Form Boundary PDE is test(cs)*(R - cst) - I have no surface diffusion in my problem. If I use a user input velocity for the transport as in the sample model, the solver works and cs evolves with time from 0 to an equilibrium value, although this value is not gammainf as it should be. Also, the flux increases with time, which makes no sense because it should decrease as the reaction surface is saturated. I suspect it has something to do with defining the surface normal in the weak form expression because my model is 3D while the sample model is 2D. Additionally, if I define the velocity in the transport model as the solution to a coupled creeping flow solution, the solver gives me cs = gammainf at all times.