Determining non-equilibrium physical time scales

[Stephen Faris]

Good afternoon,

Does Cantera allow for determining the physical time scale of equilibration of an air composition (a composition in non-equilibrium) at a given temperature and pressure?

I just dowloaded/installed Cantera with the MATLAB interface for Mac. I have been coming to understand the equilibrate method for use with air and have been comparing results to NASA's CEAM software (Chemical Equilibrium Applications for MATLAB). I find good agreement between both software in the equilibrated species concentrations, but CEAM (I am interested in the shock tube problem) does not consider the physical time scale to arrive at equilibrium -- it assumes the air equilibrates instantaneously behind the shock, or CEAM may allow the assumption that the air remains frozen behind the shock and will not equilibrate at all.

CEAM's shock tube problem requires initial velocity, temperature and pressure of free stream air. If I use the CEAM resultant values of instantaneous equilibrium temperature and pressure behind the shock (not the initial values immediately in front of the shock, and not the frozen values immediately behind the shock, but the final downstream equilibrium values) as the Cantera values of temperature and pressure that are to be held constant for an equilibrated composition, I find Cantera species concentrations agree well with CEAM.

All that said, how may I use Cantera to investigate similar problems, particularly to determine the amount of physical time required to equilibrate the composition beginning from the moment the temperature and pressure are set? 

Respectfully,

Stephen Faris

[Matei Radulescu]

Hi Stephen,

If all you want to know is the chemical equilibration behind a steady shock, the problem is essentially a ZND detonation problem: compute the chemical composition evolution while conserving mass momentum and energy for inviscid steady flow.  The Shock and Detonation Toolbox 

https://shepherd.caltech.edu/EDL/PublicResources/sdt/

permits you to do these type of calculations.    

If you just need an approximate estimate for this relaxation, you can evolve a constant pressure combustor initialized at the post-shock conditions and monitor the evolution of your favourite species to get its relaxation to steady state.  This will not be as accurate as above, but since there is no significant thermicity in the air equilibration, it will do.  

Hope this helps,

Matei Radulescu

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