
[Sponsors] 
September 27, 2015, 10:05 
Convergence Problem simpleFoam with reaction

#1 
New Member
Join Date: Apr 2015
Posts: 2
Rep Power: 0 
Dear Foamers,
I hope this is the correct Forum for my Problem... I'm trying to implement reaction into simpleFoam to simulate the steadystate of a radical polymerisation. As a first testsetup I simplify my case:  the case is laminar and incompressible  density and viscosity of the fluid do not change with reaction  no diffusion  convection happens at constant velocity  2Dflow with zeroGradient walls > species concentration changes 1D In the simpleloop I first calculate the reaction rates of the various reactions and then the source term for each species. The component balance is then solved in YEqn.H (according to reactingFoam). However, in my solver, Yi denotes the species concentration in [mole/m³] instead of the mass fraction. Calculations of UEqn.H and pEqn.H were not changed. I started with simplified reactions: Initiator > 2*Radical //Initiator decomposition Radical + Monomer > Radical //Propagation Radical + Radical > Dead chain //Termination The chain length of the growing radical is not important, since I'm doing all my calculations with concentration and not mass fractions. the rate constants are first assumed constant, they are in the correct order of magnitude: k_d = 7.8e4 1/s k_p = 27 m³/mol/s k_t = 3e4 m³/mol/s the Source terms are: r_ini = k_d * c(ini); r_rad = 2*k_d*c(ini)  2*k_t*c(rad)² r_mon = k_p*c(mon)*c(rad) Without termination (if I set k_t=0), the solver works fine, and I checked the initiator concentration is correct (I have a predici curve for comparison) so there seems to be no major (theoretical) fault in the setup. However, if I turn on the termination reaction, the solution does not converge. Here are the things I tried so far:  reducing the time step  refining the mesh  change the convective Schemes for Yi (even upwind did not work) I am grateful for any help or hints to stabilize my solver! Thank you! Last edited by unicat; September 28, 2015 at 03:50. Reason: Changed attachments 

September 29, 2015, 10:03 

#2 
New Member
Join Date: Apr 2015
Posts: 2
Rep Power: 0 
I attached the plots of the initial residuals for both cases (with and without termination).
I have now gained further understanding of my problem: With termination, the r_rad reaction becomes very small (in the order of magnitude of 10e20) which leads to unphysical negative concentrations of this species. Since the Monomer and radical are coupled, the monomer does also not converge. I defined the reaction rate of the radical species as follows (without units): r_rad = 2* 7.8e4 * c(ini)  2 * 3e4 *c(rad)² The first term is always in the order of magnitude of 1e3 and c(ini) converges. The second part however is sensitive to changes of rad because of the square. So I'm thinking, some way of "dampening" the solution progress could help, what do you think? How can I do that? 

Tags 
convergence problem, polymerisation, simplefoam 
Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
a problem with convergence in buoyantSimpleFoam  skuznet  OpenFOAM Running, Solving & CFD  5  February 19, 2014 05:30 
SimpleFoam convergence problem with really simple simulation  mayank.dce2k7  OpenFOAM Running, Solving & CFD  2  November 19, 2013 06:28 
Convergence problem for P1 & Energy  HP  FLUENT  5  May 21, 2005 15:01 
Chem. Reaction and Ignition Setting in ICE Problem  Tulus  CDadapco  0  March 28, 2005 00:02 
chemical reaction  decompostition  La S. Hyuck  CFX  1  May 23, 2001 00:07 