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The stabilization mechanisms of turbulent lifted jet flames in a co-flow have been investigated at a pressure of 7 bar. The structure of the flame base was measured with combined OH and CH 2 O planar laser induced fluorescence (PLIF) and the spatial distribution of equivalence ratio was imaged, simultaneously, with CH 4 Raman scattering. The velocity field was also measured with particle imaging velocimetry (PIV). Different bulk jet velocities U j and co-flow velocities U c were examined. Data show that flames with U c = 0.6 m/s stabilize much further away from the nozzle than those with U c = 0.3 m/s and that their structure does not resemble that of the edge-flames found closer to the nozzle. In addition, for U c = 0.6 m/s, the measured lift-off height decreases with increasing bulk jet velocity, which is opposite to what is typically observed for lifted flames. Statistical examination of CH 4 Raman images shows that the flames with U c = 0.6 m/s propagate through regions of the flow where the equivalence ratio is not always stoichiometric but, instead, spans the whole flammability range. This is not consistent with edge-flames and is, instead, indicative of premixed burning. This is corroborated by PIV results which show that the flame base velocity exceeds that typically reported for edge-flames. Measurements of relevant flow properties were also conducted in non-reacting jets to predict the turbulent burning velocity of these lifted flames burning in a premixed mode. For U c = 0.6 m/s and relatively large bulk jet velocities (U j = 10 and 15 m/s), the predicted turbulent burning velocities are sufficiently high to counter the incoming flow of reactants and, in turn, allow flame stabilization. However, for a lower bulk jet velocity of U j = 5 m/s, the predicted turbulent burning velocity is much less, leading to blow-out. This explains why the lift-off height decreases with increasing jet velocity for methane at 7 bar and U c = 0.6 m/s. Data also shows that increasing pressure promotes transition from edge-flames to premixed flames due to reduced laminar burning velocity and enhanced mixing. [ABSTRACT FROM AUTHOR]

Laser-induced incandescence (LII) was used to quantify the soot volume fraction in five piloted turbulent non-premixed C 2 H 4 -N 2 jet flames at elevated pressures. In one series of flames, the bulk jet velocity was held constant as the pressure was increased from 1 bar (Re = 10,000) to 3 bar (Re = 30,000), and then 5 bar (Re = 50,000). In the other series, a Reynolds number of 10,000 was held constant at 1 bar, 3 bar, and 5 bar. LII measurements were calibrated with laminar diffusion flames at each pressure using 2D diffuse line-of-sight attenuation. Mean and RMS soot volume fractions along the flame centerline exhibit Gaussian profiles for all of the flames. The magnitude of soot volume fraction and the spatial soot distribution are enhanced by increases in pressure. In both series of flames, the peak mean soot volume fraction scales with the pressure as p 2.2. In the constant velocity series, the scaling drops to p 1.5 if the volume-integrated mean soot volume fraction on a per fuel mass basis is considered instead. At 3 bar, a threefold increase in Re leads to a decrease in the mean soot volume fraction despite the decrease in soot intermittency. At 5 bar, a fivefold increase in Re results in soot intermittency near zero at the flame's center and a slight increase in the mean soot volume fraction. [ABSTRACT FROM AUTHOR]

This study describes a technique that utilizes a single, tunable, pulsed dye laser and two intensified CCD cameras to image NH and NO simultaneously in turbulent ammonia-hydrogen-nitrogen jet flames. The NO radical is excited at 236.214 nm in its (0,1) band, while NH is excited in its A3Π-X3Σ–(1,0) band using the residual energy of the beam at 303.545 nm, necessary to yield 236.214 nm via mixing with the fundamental of the pump laser at 1064.ßnm. Data show that it is possible to image the instantaneous structure of these turbulent flames, specifically, NH delineates the reaction zone while NO also marks the location of burnt products. [ABSTRACT FROM AUTHOR]

This work presents the first quantitative Raman measurements of temperature, mass fractions, and mixture fractions in two turbulent ammonia/hydrogen/nitrogen-air diffusion jet flames simulating 14% (CAJF14) and 28% (CAJF28) partial ammonia cracking ratio with Reynolds numbers of 11,200. The precision and accuracy of the system were validated in laminar counterflow and laminar jet flames to ensure data quality. The scalar structure in turbulent flames is examined using conditional mean and rms radial profiles, scatterplots in mixture fraction space, as well as statistics conditioned on mixture fraction and physical spaces. These results are compared with opposed diffusion flow 1D simulation with multi-component transport model and Le=1 transport model to visualize the effects of differential diffusion on the scalar structure at various axial positions. Finally, localized extinction was found in CAJF14 but not for the more stable CAJF28, and the probability of localized extinction was less than 2.5%. The dataset is provided as supplemental material and can be used for the validation of numerical models. [ABSTRACT FROM AUTHOR]