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The aim of this work is to investigate the effect of exhaust gas recirculation (EGR: water vapor and CO2), with and without O2 enrichment, on non-premixed turbulent flames stabilized on a swirl burner. The motivation includes CO2 capture applications using O2 and CO2, combustion of biogas that containing CO2 and the use of EGR or H2O in certain industrial applications to reduce pollutant emissions. Experiments were carried out on a coaxial swirl burner placed in a combustion chamber of 25 kW nominal power. The oxidant (air-O2, + H2O, + CO2) is introduced in the annular part though a swirler. The fuel (CH4) is fed though the central tube and injected radially at the exit section. The study focuses on laminar burning velocity, pollutant emissions, flame stability, and flow fields measurements with different fractions of O2, H2O and CO2 in the mixture. The fraction of diluents is varied from 0 to 20%, O2 concentration from 21 to 25% (in vol.) and the swirl number from 0.8 to 1.4. Different measurements are recorded: OH* chemiluminescence to locate the flame front, Stereo-PIV to analyze the flow field, pollutant emissions analysis (NOx and CO) and temperatures in the combustion chamber. Results show that dilution significantly influences flame characteristics. Dilution increases the lift-off height and reduces flame stability especially with high fractions (16–20%), whereas O2 enrichment decreases lift-off height and enhances flame stability. The increase dilution reduces NOx and increases CO emissions. Stereo-PIV measurements reveals the turbulent coherent structure of the swirling flow as well as the effect of dilution on the corresponding axial and tangential velocities. The effect of dilution on the underlying laminar burning velocity were determined by 1D calculation using COSILAB with GRI3.0 mechanism. [ABSTRACT FROM AUTHOR]

This paper reports results on the effect of oxygen enrichment on flow and turbulent flames in a coaxial swirl burner. The investigation was performed by S-PIV in CH4-air-O2 non-reacting and reacting flows. This work is the continuation of studies carried out previously by the authors on the same burner, with different new operating conditions. The burner consists of 2 concentric tubes, with a swirler located in the annular part through which the oxidant (air-O2) is supplied. The CH4 gas is delivered into the central tube and injected radially into the combustion chamber through small holes. The burner is placed in a parallelepiped combustion chamber with a volume of 120×50×50 cm3. The paper reports detailed results on mean velocity fields, profiles of the three velocity components (U,V,W) and velocity fluctuations (U',V',W') in the case of a 150 Nl/min oxidant flow rate, a swirl number Sn = 1.4 and an equivalence ratio Φ = 1. Three cases of O2 addition were investigated, 21%, 25%, and 30% (in vol.) corresponding to 9.4 kW, 11.7 kW, and 14.5 kW of combustion power, respectively. In-depth analysis of the field velocity differences between reactive and non-reactive cases was carried out and showed a significant difference in flow velocities between reacting and non-reacting flows. The profiles of the mean velocities, fluctuations and velocity decays differed, and higher values were observed in the reacting case. The enrichment of air with oxygen affected the velocity fields and their fluctuations in both distribution and values. The addition of O2 increased the flame temperature, leading to a greater radial expansion and a reduction in the size of the recirculation zone. The maximum axial velocity increased significantly as a function of the O2 rate, whereas the swirling velocity varied only slightly. The velocity fluctuation results also revealed a difference in profiles and values depending on O2 enrichments and locations along the flow. [ABSTRACT FROM AUTHOR]

This paper presents experimental results of the effects of CO2 and H2O dilution on CH4-air flames enriched by oxygen. The study focuses on pollutant emissions, flame stability through the determination of liftoff heights and flow fields by LDV measurements. Different parameters of the burner are studied such as the swirl number, the global equivalence ratio, and the fractions of O2, H2O, and CO2 in the mixture. The fraction of diluents varies from 0% to 20%, O2 enrichment from 21% to 30% (in vol.), and the swirl number from 0.8 to 1.4. The experiments are carried out on a coaxial swirled burner placed in a vertical combustion chamber. The flame is visualized by the chemiluminescence technique on OH* in order to locate the flame front. Measurements of combustion products such as NOx, CO, and CO2 are done using a HORIBA PG250 multi-gas analyzer. Flow fields are measured by LDV measurement for the longitudinal velocity component and its fluctuations. Results show that the CO2 and H2O dilution influences significantly the flame characteristics. With dilution the liftoff height increases but the flame remains stabilized. With O2 enrichment the liftoff height decreases and flame stability is enhanced. If we therefore want to dilute more, we should enrich with oxygen. The increase of dilution rate induces a decrease in NOx emission and exhaust gas temperature and an increase in CO emissions. LDV measurements showed the longitudinal velocity distribution of flow and its fluctuations in reactive and non-reactive conditions. Note that the presence of the flame induces an increase in the longitudinal velocity downstream of the flow due to the expansion of gases. [ABSTRACT FROM AUTHOR]

• Turbulent non-premixed swirling flames with oxygen enrichment. • Stereo-PIV measurements for different planes. • Velocity fields, turbulence intensity, swirl number and entrainment rates for reacting and non reacting flows. • Effects of swirl number and global equivalence ration. • Evolution of NOx emissions throughout gas entrainment rate measurements. This paper presents an investigation of the dynamic characteristics of non-premixed turbulent swirling flames using the stereo-PIV technique. To finely characterize the flow field, experiments are performed in the longitudinal and transverse planes in non-reacting and reacting conditions. The burner configuration studied in this work is a coaxial swirl stabilized burner used in the turbulent regime. The swirler is placed in an annular part supplying the oxidant flow and the central pipe delivers radially a fuel through eight holes symmetrically distributed on the periphery of the tube. The experiments are conducted using a 25 kW parallelepiped combustion chamber, 1 m high and 0.5 m wide, using circulating water as coolant. In the reacting case, the CH 4 -air flame is oxygen enriched from 21% to 30% in volume. Effects of the swirl number (0.8–1.4) and the global equivalence ratio (0.8–1) on the flows topology and structure are analyzed. Results of the present work concern the 3D mean velocity fields, the turbulence intensities, the swirl number measurements and the entrainment rate along the flow. SPIV results show the three-dimensional aspect of the flow including the recirculation zone and the annular zone with swirling jet effects. The comparison between reacting and non-reacting flows shows that the presence of the flame induces a larger radial flow expansion, higher mean velocities and higher turbulence intensities. The results show a bifurcation of the tangential velocity towards the jet central axis by crossing the flame front. It is also shown that the fluctuations of the tangential velocity are mainly responsible for the highest turbulence kinetic energy levels at the top of the central recirculation zone. The entrainment rate of the surrounding gases is measured by stereo-PIV as a function of the swirl number and equivalence ratio. The results explain one possible mechanism for the decrease of the NO x emissions when the global equivalence ratio increases via an increase of the entrainment rate at the flame base. [ABSTRACT FROM AUTHOR]

Effects of oxygen enrichment and CO2 dilution on the characteristics of non-premixed methane-air turbulent flames in a coaxial swirl burner are investigated in this article. The work primarily focuses on pollutant emissions (NOx and CO), flame structure, and stability investigations. The experiments are conducted using a 1-m high, 0.5-m wide, 25-kW parallelepiped combustion chamber, cooled by outside water. The burner configuration consists of two concentric tubes in which a swirler is placed inside the annular part for air or air-O2-CO2 supply that can allow for the rotation of the oxidant. Fuel is injected radially from the central tube. OH* chemiluminescence measurements are performed to describe the structure and stability of the flame providing information on the flame lengths and lift-off heights. The measurements are conducted with the oxygen concentrations ranging from 21% to 35%, CO2 concentrations ranging from 0 to 20%, swirl numbers ranging from 0.8 to 1.4, and global equivalence ratios ranging from 0.8 to 1. The lift-off heights, the fluctuations of the flame base, and the flame lengths are determined as a function of these parameters. The results show that oxygen enrichment stabilizes better the flame, promotes the formation of NOX and CO2, and decreases the formation of CO. The dilution by CO2 changes significantly the flame shape and its behavior. The flame becomes longer, less intense, and unstable. CO2 dilution reduces considerably the flame temperature which obviously reduces the NOx formation, but it is observed that CO2 and CO concentrations in the flue gases are increased. [ABSTRACT FROM AUTHOR]

The paper investigates the dynamics of methane – oxygen enriched air turbulent non-premixed swirled flames and their emission characteristics. The burner configuration consists of two concentric tubes with a swirler placed in the annular part supplying the oxidant. Fuel injection is radial through holes in the central tube at the burner exit. This allows enhancing air-fuel mixing and eases flame stabilisation. Fuel is injected into the highest turbulence level regions where the local mixing is enhanced, and generating a shift from a non-premixed flame towards a partially premixed one. The idea of oxygen enrichment of the air is related to the augmentation of CO 2 concentration in the flue gases to improve the CO 2 capture efficiency by membranes. Stereo-Particle Image Velocimetry is used to analyse the velocity flow fields. The measurements are performed for oxygen concentrations ranging from 21% to 30%, with swirl numbers from 0.8 to 1.4 and global equivalence ratio from 0.8 to 1. Results show that combustion noticeably affects the swirling motion and that tangential velocities rapidly decrease along the axial axis. The recirculation mass flow ratio dramatically increases compared to non-reactive cases. Increasing global equivalence ratios increases the recirculation mass flow ratio contrary to oxygen enrichment effects. The exhaust gas compositions are measured using gas analysers. It is observed that NO x emissions decrease when the global equivalence ratio increases. It suggests that CO 2 in the recirculated burned gases could be responsible for NO x destruction at relatively high oxygen enrichment rates. Globally, the results show that, by combining enhanced mixing and moderate oxygen enrichment of the air, the developed and characterized new burner configuration has a good potential to be used to stabilize non-premixed fuel and oxidant streams as partially premixed low emission flames with an increased concentration of CO 2 in the flue gases. [ABSTRACT FROM AUTHOR]

• Turbulent diffusion swirling flames are experimentally investigated upon the oxygen enrichment. • Stereo-PIV and OH* Chemiluminescence measurement was used to analyze the velocity flow fields and flame stability. • Experiments were conducted both at constant power (9.41 kW) and variable power of the flame (from 9.41 to 22.42 kW). • Temperature evolutions and pollutant emissions (NOx, CO) were examined using several parameters. • Effects of the injection type of fuel (axial or radial) are analyzed. This work investigates the combustion characteristics of CH 4 -air/O 2 turbulent swirling flames in a co-axial burner with radial fuel injection. The burner configuration consists of two coaxial tubes with a swirler positioned within the annular space for the oxidizer flow. The central tube delivers the fuel through eight holes placed that are uniformly distributed along the circumference of the inner tube nearby the burner exit. The experiments were carried out in a 25-kW combustion chamber of dimensions 50x50x120 cm3. Stereo Particle Image Velocimetry (SPIV) was used to characterize the different velocity fields, whereas the OH* Chemiluminescence is employed to examine the flame structure and stability. The exhaust gas compositions were measured using multi-gas analyzers. The structure and stability of flames, temperature evolutions, CO 2 , and pollutant emissions (NOx, CO) were examined under different parameters, such as: oxygen enrichment (21–50 %), global equivalence ratio (0.4–1), swirl number (0.5 and 1.4), injection type of fuel (axial or radial). Experiments were conducted at a constant power of 9.41 kW and variable powers in the range from 9.41 to 22.42 kW. The present results show that the oxygen enrichment decreases the lift-off height and enhances the flame stability using constant flame power. Increasing the swirl number improves the flame stability and produces a better attached flame. At constant flame power and variable oxidizer flow rate, a high oxygen concentration yields lower NOx emissions. For lean combustion with excessive oxygen enrichment over 35 %, retaining the same flame power leads to a significant decrease in NOx emissions. The radial injection of CH 4 does not affects the NOx emissions, however, the axial injection is shown to be advantageous in terms of CO emissions in the present studied configurations. [ABSTRACT FROM AUTHOR]