Modeling unsteady multiphase flow in porous media - convergence problem
 

Hey everybody ,
I am solving a 1-d , non-isothermal, multiphase , transient problem inside a porous media using fortran as a compiler . In all ; mass , momentum conservation equation for each phase , 2 species conservation equations , 1 energy equation are being solved .
Initially i took the time step to be 5*10^(-10) , for allowing the convergence within a particular time step . But my prof told me , that it's impractical for a flow within a fuel cell's gas diffusion layer (Porous media ) . So i gave relaxation factor value to be 0.0002 and increased the time step to 5*10^(-6) . but now the solution is producing NAN values after converging within certain time steps . What should i do ?
Any sort of help would be appreciated . i would be highly thankful if anyone could suggest good reads related to the undermentioned topics :
1)How to implement Under-relaxation techniques in a FDM , implicit coupled solvers using Tri diagonal matrix method.
2)Difference between explicit and implicit under-relaxation technique , and their limitations and their uses.
3) Can the relaxation factors be different for different variables in a coupled solver or a seggregated solver .

Thanks in advance :)
Vaibhav Kumar

come on guys, anyone ... any sort of help would be appreciated