Discussion Forum

Hi all,

For my master thesis I'm trying to model the devolatization process of a biomass particle during pyrolysis. Hence, I'm dealing with a porous media. During this process some gas species will be produced (gasses en tar) . The transport of these gas species is by convection and diffusion inside the porous media. For the convection term i need the superficial velocity of the gas mixture (consisting out of gasses, tar and initially the inert gas). This velocity must be calculated using Darcy's Law, but so far I'm not able to compute this velocity using Comsol.

I'm using the 'Coefficient From PDE' interface to define all my equations. So I have an equations that calculates the density of each specie as a function of time and space (only 1D) and an equation for the Temperature as function of time and space. How can i use the ideal gas law to obtain the pressure?

P = rho*R*T / MW

(MW = molecular weight of gas mixture, rho is density of gas mixture, R is gas constant, T is temp)

For the Darcy velocity I need the pressure gradient, since my problem is only 1D it becomes d(P,r)

My first try was: make a variable P and then make a second variable with d(P,r).

But this gives strange results. Maybe the differentiation i did not in the right way? Because of Temperature and Density are both function of time and space (r)?

One i know the d(P,r) i can simply add another variable with: u = -k/mu * d(P,r), right? And then i can use this u in my convection equation?

Any help is more then welcome! Thank you in advance.

Kind regards,

Daan