recovering .... because dt_chem too large
 

I am constantly getting this printed in Converge status. I am running CI engine simulation with blended fuel. This is being repeated for almost all the main injection events and after.

[ 57.81%] ncyc 40962; Crank 364.341 (deg); dt= 1.250000000e-08 (dt_grow)
recovering .... because dt_chem too large: t_chem= 5.135885e-01 dt= 1.250000e-08 mult_dt_chem= 5.000000e-01 dt_chem= 2.846167e-09
[ 57.81%] ncyc 40963; Crank 364.341 (deg); dt= 1.250000000e-08 (dt_grow)
recovering .... because dt_chem too large: t_chem= 5.125824e-01 dt= 1.250000e-08 mult_dt_chem= 5.000000e-01 dt_chem= 2.890790e-09
[ 57.81%] ncyc 40964; Crank 364.341 (deg); dt= 1.250000000e-08 (dt_grow)
recovering .... because dt_chem too large: t_chem= 5.115406e-01 dt= 1.250000e-08 mult_dt_chem= 5.000000e-01 dt_chem= 2.938424e-09
[ 57.81%] ncyc 40965; Crank 364.341 (deg); dt= 1.250000000e-08 (dt_grow)
recovering .... because dt_chem too large: t_chem= 5.049359e-01 dt= 1.250000e-08 mult_dt_chem= 5.000000e-01 dt_chem= 2.946903e-09
[ 57.81%] ncyc 40966; Crank 364.341 (deg); dt= 1.250000000e-08 (dt_grow)
recovering .... because dt_chem too large: t_chem= 5.039253e-01 dt= 1.250000e-08 mult_dt_chem= 5.000000e-01 dt_chem= 2.999713e-09

This has caused the simulation too slow down a lot. By slowing down i mean in order of weeks. The simulation is stuck (progressing very slow) when the injection is happening. I decreased the mult_dt_chem from 0.5(default) to 0.1. but still no effect on speed (it is still being printed). The time step settings are default with min. time step of 1e-8 ( at some CAD this minimum time step is also forced as dt=dtmin forced by recover_chem). Any idea how to speed up the simulation? and get rid of this?My case setup is fine and I used the same for pure diesel. I am injecting 70000 parcels/nozzle with min cell size of 0.25 at nozzle having total mass of around 40mg. It would be appreciated. Thanks

Hello

The time step being limited by dt_chem is usually an indication that the temperature in a cell is increasing by a lot within a single time step and CONVERGE is recovering and limit the time step to prevent this from happening. Can you check your heat release rate profile and see if there is a rapid rise there? I would need to evaluate the case setup to help you further.

Please reach out to us at support@convergecfd.com so that we can look at the case setup and help you

Hello,
yes the HRR is rising in the main injection rapidly as seen in the attachment, though the total energy released is similar. can you give suggestions on how this problem can be tackled? I am using a validated spray model with KH-RT. Thanks

Hello,

Can you give me some more details about your spray setup? How many nozzles does you injector have? Can you confirm if you are using 70000 parcels per nozzle for a total injected spray mass of 40 mg for the injector?

Hello yes surely.
My setup has an injector with 8 nozzles. The total mass I am injecting is around 42mg and the parcels per nozzle are 70000. injection duration is around 16cads starting from -11aTDC. the injection pressure is adjusted at 1200 bars with a cd of 0.93. Other parameters like nozzle dia etc. are from experiments.
Thanks

Can you let me know if you have the new_parcel_flag = 1 for the KH breakup model?

yes i am creating child parcels. Also i have adjusted the constants from my previous experience with adjusting the same CFD model at different operating conditions. The spray modelling snap is attached here