Modeling unsteady multiphase flow in porous media  convergence problem
Hey everybody ,
I am solving a 1d , nonisothermal, 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 Underrelaxation techniques in a FDM , implicit coupled solvers using Tri diagonal matrix method. 2)Difference between explicit and implicit underrelaxation 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

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