Robert W. Schefer

Hydrogen enrichment for improved lean flame stability

The stability characteristics of a premixed, swirl-stabilized flame were studied to determine the effects of hydrogen addition on flame stability under fuel-lean conditions. The burner configuration consisted of a centerbody with an annular, premixed methane/air jet introduced through five, 45° swirl vanes. Flame stability was studied over a range of operating conditions. Under fuel-rich conditions the flame was lifted from the burner surface due to the mixing with entrained ambient air that was needed to form a flammable mixture. As the fuel/air mixture ratio was decreased toward stoichiometric, the resulting increase in flame speed allowed the flame to propagate upstream through the low-velocity wake region and attach to the centerbody face. The maximum blowout velocity occurred at stoichiometric conditions, and decreased as the mixture became leaner. OH PLIF measurements were used to study the behavior of OH mole fraction as the lean stability limit was approached. Near the lean stability limit the overall OH mole fraction decreased, the flame decreased in size and the high OH region took on a more shredded appearance. The addition of up to 20% hydrogen to the methane/air mixture resulted in a significant increase in the OH concentration and extended the lean stability limits of the burner.

Combustion of hydrogen-enriched methane in a lean premixed swirl-stabilized burner

The combustion characteristics of a premixed, swirl-stabilized flame were studied to determine the effects of enriching methane with hydrogen under fuel-lean conditions. The burner consisted of a center-body with an annular, premixed fuel-air jet. Swirl was introduced to the flow using 45-degree swirl vanes. The combustion occurred within an air-cooled quartz chamber at atmospheric pressure. Flame stability and blowout maps were obtained for different amount of hydrogen addition at several fuel-air flow rates. Gas probe measurements were obtained to demonstrate reductions in CO concentration with hydrogen addition, without adversely affecting the NO x emissions. The flame structure near the lean stability limit was described by direct luminous photographs and planar laser-induced fluorescence measurements of the OH radical. Results show that the addition of a moderate amount of hydrogen to the methane/air mixture increased the peak OH concentration. Hydrogen addition resulted in a significant change in the flame structure, indicated by a shorter and more robust appearing flame. The observed trends concur with the strained opposed premixed flame analysis using RUN-1DL. The computations revealed that enriching the methane with hydrogen increased the strain resistance of the flame as well as the OH levels in the flame.

Mechanism of Flame Stabilization in Turbulent, Lifted-Jet Flames

Particle image velocimetry was used to study the velocity field in the stabilization region of lifted, turbulent CH 4 -jet flames over a range of Reynolds numbers from 7000 to 19,500. Measured velocities at the flame base are considerably below the turbulent flame speeds derived from previous studies and show a dependence on the Reynolds number. The average velocity at the stabilization point is nearly a factor of five below the premixed laminar burning velocity at the lowest Reynolds number and asymptotes to a value about 20% higher as the Reynolds number is increased. Planar images of OH show that the flame zone structure near the stabilization point is also highly dependent on the Reynolds number. Comparison of the present OH images with previous CH 4 Raman imaging results shows that the flame thickness is determined by the width of the flammable region. At a low Reynolds number, the flame is stabilized near the jet exit where the flammable layer is thin, resulting in a thin flame zone. At an increased Reynolds number, the stabilization point is located farther downstream where the flammable region is wider, resulting in a correspondingly wider flame zone. It is proposed that the lower velocities observed at the flame base are related to thinning of the flame zone at low Reynolds, which results in greater curvature of the flame base. The increased flame curvature effectively defocuses the transport of heat and flame radicals to reactants upstream of the propagating flame front, resulting in reduced burning velocities. The implications of these results for mechanisms controlling turbulent flame stabilization, with an emphasis on the applicability of triple flame concepts to turbulent flows, are discussed.

Stabilization of lifted turbulent-jet flames

Abstract Planar imaging measurements of CH, CH4, and temperature are used to evaluate current models for flame stabilization in lifted, turbulent CH4-jet flames. The experimental system consists of two cameras that simultaneously record the instantaneous distributions of species concentration and temperature. Measurements were made in two flames with fuel-jet Reynolds numbers of 7,000 and 12,100. The results show that the fuel and air are premixed and within the CH4 flammability limits at the flame stabilization point. The flame zone also falls considerably outside the region where scalar dissipation is significant (dissipation levels are well below the critical value for extinction). It is concluded that local stoichiometry, and not scalar dissipation, is the primary factor controlling flame stability. Considerable interaction exists between the large-scale structure associated with the central fuel jet and the instantaneous flame zone, which is consistent with both premixed flame propagation and the concept of large-scale motion resulting in the ignition of unreacted fuel by not combustion products.

Conserved scalar fluxes measured in a turbulent nonpremixed flame by combined laser Doppler velocimetry and laser Raman scattering

Abstract This paper presents a new combined laser Doppler velocimetry-laser Raman scattering (LDV-Raman) apparatus which simultaneously measures velocity and scalars. Measurements in a ducted nonpremixed turbulent flame using this apparatus are presented and compared with previous LDV-Mie and LDV-Rayleigh measurements. A partial equilibrium numerical model of the hydrogen flame herein investigated predicts the major species to be at their equilibrium concentration while radical species, such as hydroxyl, can have mean concentrations that are three times greater than their equilibrium concentrations.

A comparison of bluff-body and swirl-stabilized flames

Abstract Bluff-body and swirl-stabilized flames are similar in that they represent, in simplest terms, the fundamental interaction between a fuel jet and a surrounding toroidal vortex. The vortex in this case is the recirculation vortex which affects the properties of the flames. It is found, not surprisingly, that the two most important fundamental parameters that govern both types of flames are (1) the vortex circulation (Γ), and (2) the fuel jet momentum. Comparisons are made of the properties of the two types of flames using the proper nondimensional parameters, including the fuel-to-air momentum flux ratio and the properly nondimensionalized vortex strength. Such comparisons can help to illustrate the tradeoffs between the degree of swirl and the choice of bluff-body size in devices such as industrial burners, gas turbines, and ramjets. The data also show how one can control flame properties by controlling the vortex strength Γ and fuel momentum and thus gain a degree of control that is not provided ...

Error estimates for Rayleigh scattering density and temperature measurements in premixed flames

Rayleigh scattering has become an accepted technique for the determination of total number density during the combustion process. The interpretation of the ratio of total Rayleigh scattering signal as a ratio of densities or temperatures is hampered by the changing composition through a flame, since the average Rayleigh scattering cross-section depends on the gas composition. Typical correction factors as a function of degree of reaction, fuel and equivalence ratio were calculated. The fuels considered were H2, CH4, C2H4, C2H6 and C3H8. Factors as low as 0.7 and 0.56 were found for the heaviest hydrocarbon fuel at large equivalence ratio for interpreting the Rayleigh scattering intensity as gas density and inverse temperature, respectively. This is primarily due to the presence of CO and H2 as intermediates. As CO and H2 are subsequently oxidized to CO2 and H2O, these factors approach 1.0. Conversely, the worst case, when using H2 as a fuel, occurs in the post flame zone. However, the correction factors for H2 are near 1.0 and the errors involved will, in general, remain within the expected experimental accuracy of a typical Rayleigh scattering system. Linear correlations of correction factors with equivalene ratio and with the product of equivalence ratio and fuel molecular weight were found and presented. The interpretation of Rayleigh scattering as temperature was found to have larger errors than the interpretation as density. Corrections for changes in gas composition were applied to Rayleigh scattering temperature measurements in the post flame region of CH4 and C3H8 flames with equivalence ratios of 0.75 and 1.0. The corrected temperatures were in excellent agreement with thermocouple measurements.

Nonreacting turbulent mixing flows

Abstract This paper provides a comprehensive review of available data on turbulent nonreacting flows which could be of use in the evaluation of turbulence modeling schemes. The review is limited to parabolic, stationary shear flows with well-defined boundary conditions; these flows have many of the characteristics of flows found in combustion devices and are typical of laboratory flames for which model evaluation data are most likely to be available. This report is an outgrowth of a study conducted under the direction of the Air Force Office of Scientific Research. A bound volume containing, in detail, the results of that study will be available in the near future.

Conditional sampling of velocity and scalars in turbulent flames using simultaneous LDV-Raman scattering

The laser Doppler velocimeter (LDV) measures the velocity distribution of particles which is often an acceptable representation of the distribution of gas velocities. However, in turbulent two stream mixing flows, the particle velocity distribution will differ from the gas velocity distribution when the particle densities in the two streams are unequal. This bias is explored in a reacting and nonreacting turbulent jet which is surrounded by coflowing air. By adding seed particles to only the coflow air and then to only the jet fluid, the limits of this bias are established. Additional measurements with an LDV triggered laser Raman scattering system demonstrate that the bias in the LDV sampling is propagated to the Raman measurements. An analytical equation is presented which will generate unbiased velocity and scalar distributions from measurements obtained from seeding only one stream at a time.

EFFECT OF SYNGAS COMPOSITION AND CO2-DILUTED OXYGEN ON PERFORMANCE OF A PREMIXED SWIRL-STABILIZED COMBUSTOR

Future energy systems based on gasification of coal or biomass for co-production of electrical power and fuels may require gas turbine operation on unusual gaseous fuel mixtures. In addition, global climate change concerns may dictate the generation of a CO2 product stream for end-use or sequestration, with potential impacts on the oxidizer used in the gas turbine. In this study the operation at atmospheric pressure of a small, optically accessible swirl-stabilized premixed combustor, burning fuels ranging from pure methane to conventional and H2-rich and H2-lean syngas mixtures is investigated. Both air and CO2-diluted oxygen are used as oxidizers. CO and NOx emissions for these flames have been determined from the lean blowout limit to slightly rich conditions (ϕ ∼ 1.03). In practice, CO2-diluted oxygen systems will likely be operated close to stoichiometric conditions to minimize oxygen consumption while achieving acceptable NOx performance. The presence of hydrogen in the syngas fuel mixtures results in more compact, higher temperature flames, resulting in increased flame stability and higher NOx emissions. Consistent with previous experience, the stoichiometry of lean blowout decreases with increasing H2 content in the syngas. Similarly, the lean stoichiometry at which CO emissions become significant decreases with increasing H2 content. For the mixtures investigated, CO emissions near the stoichiometric point do not become significant until ϕ > 0.95. At this stoichiometric limit, CO emissions rise more rapidly for combustion in O2–CO2 mixtures than for combustion in air.