There is no very clear international specification for the definition of the flameless combustion state, and the one that is more recognized is the study of Wunning and Sons, who obtained the relationship between the internal circulation rate Kv and the temperature of the flue gas under the diffusion combustion mode of methane, where the mixing and combustion are carried out simultaneously, through a large number of experiments.
The axial evolution and radial diffusion of the fuel jet MF and the oxidizer jet MA induce a coiling effect of the surrounding air ME, so that the central zone of the jet is gradually diluted by combustion products (CO2 and H20) and non-reacting gases (N2) to achieve "low oxygen" combustion conditions.
Kv is the ratio of the circulating flue gases (CO2, H2O and N2) sucked by the combustion jet to the mass flow rate of the incoming jet (fuel and air).
KV=ME/ (MF+MA)= (MJ-MF-MA) / (MF+MA)
In addition, there are some other methods of determining this. For example, the temperature rise is used to determine whether the combustion is in a flameless combustion state, and Kumar et al. concluded that the combustion can be defined as flameless combustion if the temperature rise in the reactor is normalized to a spatial temperature change of about 15%. The inlet temperature is higher than the self-ignition temperature and the temperature rise is higher than the inlet temperature, which is called high temperature combustion.
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