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An experimental study of CO2/H2O dilution effects on CH4-air-O2 turbulent flames in a swirl burner is investigated. Combustion products (CO2, H2O...) reintroduced with the fresh gases allow a significant reduction of NOx. This beneficial effect has been demonstrated in some industrial applications (engines, turbines), however, it has been little studied and is not taken into account in the numerical simulation codes. The motivations of oxygen enrichment are related to the high thermal efficiency and the increase of CO2 concentration in the flue gases which improves the CO2 capture efficiency. The present work focuses on pollutant emissions (NOx and CO), topology and flame stability using OH* chemiluminescence technique, and flow fields with PIV system. The burner configuration consists of two concentric tubes with a swirler placed in the annular part supplying the oxidant (air-O2 with H2O/CO2) flow rate. The CH4 is injected radially from the central tube throughout eight holes. The swirling flame takes place inside a combustion chamber with a square cross-section of 48 x 48 cm2 and a height of 1 m that operated at atmospheric pressure. The measurements are performed for different operating conditions: from 21 % to 30 % of O2 enrichment, from 0 to 20% of CO2/H2O dilution, from 0.8 to 1.4 of swirl. The results show that the oxygen enrichment and CO2/H2O dilution significantly affects the flame stability and pollutant emissions. The PIV measurements allow a fine description of 3D aspects of the flow including the central recirculation zone.

Effects of exhaust gas on CH4-air-O2 turbulent swirling flames