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The behavior of technically premixed CO2-diluted methane oxyflames is compared to operation with methane-air flames in a labscale combustor equipped with an axial-plus-tangential swirler. It has been shown in a former study that the stabilization regimes and topologies of CO2-diluted oxy-flames can be deduced from methane-air flames despite large changes of the oxidizer and fuel flowrates. This work proceeds and focuses on differences in distributions of wall temperature and wall heat fluxes for a methane-air and a CO2-diluted methane oxyflame sharing the same thermal power, equivalence ratio, adiabatic flame temperature and swirl number. Laser-Induced Phosphorescence measurements are used to determine the temperature distributions along the combustor metallic back plane wall and along the internal and external surfaces of the quartz windows. These data are used to determine the local heat flux through the combustor windows. It is found that both the air and CO2-diluted oxy-flames feature approximately the same temperature distributions and total thermal loads along the combustion chamber walls. As high concentrations of carbon dioxide are known to significantly enhance radiative heat transfer, an analysis is carried out so as to account for the mechanisms that provide this similarity in the wall temperatures and heat fluxes between the two flames. The low order model allows for determining the origin of the heat flux and shows how convective heat transfer is supplemented by radiative heat transfer when switching from air to CO2-diluted oxy-combustion with a global thermal load that remains roughly unaltered for the studied combustion chamber.

International audience; Impact of the diverging cup angle of a swirling injector on the flow pattern and stabiliza- tion of technically premixed flames is investigated both theoretically and experimentally with the help of OH* chemiluminescence, OH laser-induced fluorescence and particle image velocimetry (PIV) measurements. Recirculation enhancement with a lower position of the internal recirculation zone (IRZ) and a flame leading edge protruding further upstream in the swirled flow are observed as the injector nozzle cup angle is increased. A theoretical analysis is carried out to examine whether this could be explained by changes of the swirl level as the diffuser cup angle is varied. It is shown that pressure effects need in this case to be taken into account in the swirl number definition and expressions for changes of the swirl level through a diffuser are derived. It is demonstrated that changes of the swirl level including or not the pressure contribution to the axial momentum flux are not at the origin of the changes observed of the flow and flame patterns in the experi- ments. The swirl number without the pressure term, designated as pressure-less swirl, is then determined experimentally with laser Doppler velocimetry (LDV) measurements at the injector outlet for a set of diffusers with increasing quarl angles under nonreacting conditions and the values found corroborate the predictions. It is finally shown that the decline of axial velocity and the rise of adverse axial pressure gradient, both due to the cross section area change through the diffuser cup, are the dominant effects that control the leading edge position of the IRZ of the swirled flow. This is used to develop a model for the displacement of the recirculation bubble as the quarl angle varies that shows very good agreement with experiments. [DOI: 10.1115/1.4041518]

An extensive experimental study is carried out to analyze scaling laws for the length of methane oxy-flames stabilized on a coaxial injector. The central methane fuel stream is diluted with N2, CO2 or He. The annular air stream is enriched with oxygen and can be impregnated with swirl. Former studies have shown that the stoichiometric mixing length of relatively short flames is controlled by the mixing process taking place in the vicinity of the injector outlet. This property has been used to derive scaling laws at large values of the stoichiometric mixture fraction. It is shown here that the same relation can be extended to methane oxy-flames characterized by small values of the stoichiometric mixture fraction. Flame lengths are determined with OH* chemiluminescence measurements over more than 1000 combinations of momentum ratio, annular swirl level and composition of the inner and outer streams of the coaxial injector. It is found that the lengths of all the flames investigated without swirl collapse on a single line, whose coefficients correspond to within 15% of flame lengths obtained for fuel and oxidizer streams at much larger stoichiometric mixture fractions. This relation is then extended to the case of swirling flames by including the contribution of the tangential velocity in the flow entrainment rate and is found to well reproduce the mixing degree of the two co-axial streams as long as the flow does not exhibit a vortex breakdown bubble. At higher swirl levels, when the flow features a central recirculation region, the flame length is found to also directly depend on the oxygen enrichment in the oxidizer stream.

International audience; The stabilization of premixed flames within a swirling flow produced by an axial-plus-tangential swirler is investigated in an atmospheric test rig. In this system, flames are stabilized aerodynamically away from the solid components of the combustor without the help of any solid anchoring device. Experiments are reported for lean CH4/air mixtures, eventually also diluted with N2, with injection Reynolds numbers varying from 8500 to 25,000. Changes of the flame shape are examined with OH* chemiluminescence and OH laser-induced fluorescence measurements as a function of the operating conditions. Particle image velocimetry (PIV) measurements are used to reveal the structure of the velocity field in nonreacting and reacting conditions. It is shown that the axial-plus-tangential swirler allows to easily control the flame shape and the position of the flame leading edge with respect to the injector outlet. The ratio of the bulk injection velocity over the laminar burning velocity Ub/SL, the adiabatic flame temperature Tad, and the swirl number S0 are shown to control the flame shape and its position inside the combustion chamber. It is then shown that the axial velocity field produced by the axial-plus-tangential swirler is different from those produced by purely axial or radial devices. It takes here a W-shape profile with three local maxima and two minima. The mean turbulent flame front also takes this W-shape in an axial plane, with two lower positions located slightly off-axis and corresponding to the positions where the axial flow velocity is the lowest. It is finally shown that these positions can be inferred from axial flow velocity profiles under nonreacting conditions.

International audience; Revamp of air powered industrial boilers to oxy-combustion operation raises several issues. In this study, the stabilization of CO2-diluted premixed swirling CH4/O-2 flames is compared to operation with CH4/air flames by a set of experiments in a generic labscale combustor equipped with an axial-plus-tangential swirler. The investigated flames are stabilized aerodynamically within the swirling flow without help of any solid anchoring device. The structure of the turbulent swirling flames is examined by recording their OH* chemiluminescence. Laser induced OH fluorescence measurements are carried out to delineate the location of the flame front and burnt gases and infer the shape taken by the flame. Particle imaging velocimetry measurements reveal the corresponding velocity field. The temperature is also recorded with thermocouples in the internal and the external recirculation zones of the flow and inside the combustion chamber walls. These diagnostics reveal similarities between the topology of CH4/air and CH4/O-2/CO2 flames near their stabilization point. For a fixed swirl number, it is found that N-2- and CO2-diluted CH4/O-2 flames at the same equivalence ratio feature very similar shapes provided the adiabatic flame temperature and the ratio of bulk velocity to laminar burning velocity are kept the same although the absolute value of bulk velocity and the general velocity level inside the combustor are different. This result was found for injection Reynolds numbers varying from Re = 8500-28000 and for swirl numbers ranging from So = 0.5 to 1.2. The operability range of well stabilized CO2-diluted flames is however reduced. It is also found that the temperature of the burnt gases in the outer recirculation zone differs between the CO2- and N-2 diluted combustible mixtures sharing the same adiabatic temperature, but this does not affect the stabilization of the flames near the burner outlet, the temperature in the burnt gases and inside the combustor sidewalls. The flame leading edge is stabilized on average off-axis due to the structure of the W axial velocity profile produced by the axial-plus-tangential swirler at the burner outlet. This study indicates that CO2-diluted CH4/O-2 premixed swirling flames can be stabilized with similar shapes as CH4/air flames without design modification provided the suggested similarity is obeyed.

International audience; Stabilization of nearly stoichiometric CO2- and N-2-diluted premixed methane/oxygen swirling flames is investigated in an atmospheric test rig equipped with an axial-plus-tangential swirler exhausting in a cylindrical injection tube eventually ended by a diverging quarl. The investigated flames are stabilized aerodynamically away from the solid elements of the combustor without the help of any central bluff body in the injector. The flowrates through the axial and tangential slits of the swirler can be adjusted separately. Effects of swirl and quarl angle on the flowfield and flame shape are analyzed. Laser tomography on small oil particles reveals that fuel, oxidizer and diluents injected in separated channels are well mixed at the injector outlet. Velocimetry measurements and flame images show that the axial plus-tangential swirler allows aflexible control of the flame leading edge position with respect to the injector outlet. For a fixed injector geometry with a given quarl angle and swirl number, it is found that N-2- and CO2-diluted flames feature the same topology if the injected combustible mixtures feature the same adiabatic flame temperature, while they may feature different bulk injection velocities and laminar burning velocities. The operability range of well stabilized CO2-diluted flames is however reduced.