I am running an opposed jet case, where a stream of fresh reactants is opposed to a stream of burnt products. I am looking at the effects of stratification on the flammability limits of methane.
In some cases, in the reaction zone, the heat release is about 10^3-10^4 lower than compared to my reference case, which is far from the lean or rich flammability limits. Since I am sitting on or close to those limits, it is important for me to define one (or multiple) parameter for which I can say that the fuel is indeed burning.
Here is my reasoning right now, but it might be incomplete or even erroneous. To check if the fuel is indeed burning, I have been looking at the distribution of key radicals. If the mass fraction of say H2, does not have peak in the reaction zone and only diffuses from one stream to another, then I can definitely say there is no flame. But a contrario, if there is a peak, can I definitely say there is a flame? And how big does this peak needs to be compared to its corresponding value in the reactant and products stream? (I am guessing at least one full order of magnitude).
If there is a standard definition of a flame in numerical combustion, please share it, preferably with an attached reference.
Is this a pure chemical kinetics simulation or a CFD study of opposed jets? What we in my organization usually do in the CFD simulation of furnaces is to use local CO concentrations as an indicator for the flame shape within the combustion chamber. A common threshold for methane-air combustion is 2000 ppm CO (dry). If the local value of CO is higher, we consider it to be within the flame, if it's below, we consider it to be outside of the flame. This is more of a rule-of-thumb, but it works reasonably well for us.
Are you simulating a premixed flame of methane formed somewhere at the interface of hot burned products jet and cold premixed reactants jet?
You can consider the species like CH2O, CH, and OH as flame marker. CH2O is probably a marker species for ignition, while CH and OH are likely flame markers for CH4 flames. I am not sure about OH as a flame marker for the limit flames on the rich side, but many DNS studies use CH as the flame marker.
This configuration sounds like what is conventionally called "counterflow". I did a quick search and found lots of papers for methane/air, but not with products as the opposing stream. I suggest you perform a thorough search using that keyword.
It also looks like a premixed case, so you should ignore anything which is a "diffusion" flame (a non-premixed case). Unfortunately, a search for "premixed" also gets "non-premixed"!
For the computation case, usually we could say that there is a flame when the local temperature has a rise about 500K, compared with the initial temperature.
It is known that very near to the flammability limits the reaction rate vanishes exponentially, but does not become zero. So there is a continuous transition between burning and non-burning, although in practice the flame is either seen as burning or not, because the transition is quite rapid.
From these observations, we can derive that there is no strict separation between "flame" and "no flame", so some engineering rule has to be used (it is similar to the edge of a boundary layer, which is defined at the location where the velocity differs only by 1% from the freestream). I would support a rule like the one given by Chitralkumar.
The problem needs to be defined more precisely before you can receive a useful meaningful answer. Please define the stoichiometry of the fresh mixture, temperature of burnt products that you are setting in your calculations of OJDF.. There is a golden principle (not so golden though!) hat minimum flame temperature is about 1000 C. If this helps do use it. If not, you need to define the problem in some detail.
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