Low flow rate in channel results in divergence
 

Dear friends,

I am working on DMFC simulation. I have developed a three dimensional two phase model and UDFs. Recently I tried to simulate 3*3 cm2 cell with single pass serpentine channel geometry, with 2.5/3.0 anode /cathode stoichiometry at 400 mA/cm2 operating current density. I used 0.3 and 0.5 under relaxation factors for pressure and momentum, respectively. Initially simulation goes well with decreasing residuals but after several hundreds of iterations momentum residual starts increasing and when it reaches around 1000 iteration momentum (mainly cathode side) residual increases to 10-4 and suddenly stops. For the same settings if I increase the flow rate by using large stoichiometry ratio such as 10.0/10.0 calculation converges well. I have problem only in the cathode side. Could anyone tell me the reason for divergence at low flow rate? I used pressure outlet condition at both cathode and anode. Thanks in advance.