The Effect of Diluent on the Sustainability of MILD Combustion in a Cyclonic Burner

The present study investigates the characteristics of MILD/flameless combustion in a cyclonic lab-scale burner. Such a configuration is effective for achieving turbulent mixing in a very short time while allowing for a reasonably long residence time for the development of combustion reactions. These two constraints are mandatory in the case of MILD combustion processes (high inlet temperatures and diluted mixtures). Such operating conditions are achieved through massive heat/mass recirculation towards the fresh incoming mixtures by recycling the exhausted gases, featuring a process where chemical kinetics times are elongated because of the dilution levels. Thus, long residence times are needed to achieve a satisfying reaction progress, and the high inlet temperatures result in fast and efficient mixing between disproportionate flows to avoid the onset of oxidation reactions before achieving diluted conditions. Under these constraints, a lab-scale facility was designed and built. The oxidation processes of C3H8/O2 mixtures highly diluted in N2 or CO2 were investigated by varying the external parameters of the system, namely, the inlet temperature (up to 1300 K) and the mixture composition (from lean to rich mixtures). Several combustion regimes were experimentally identified. When the MILD regime was established, the combustion process became homogeneous within the burner without luminous emissions. To investigate the distributed nature of the MILD combustion processes, chemical simulations were performed under the assumption of a well-stirred reactor. For both the diluents, good agreement between the experimental and numerical results was obtained for MILD combustion conditions.