B.H. Chao

Structure and propagation of premixed flame in nozzle-generated counterflow

The accuracy of the counterflow, twin-flame technique for the determination of laminar flame speeds was examined analytically, numerically and experimentally. The analysis was conducted by using multiple-expansion, large activation energy asymptotics, while the numerical simulation incorporated detailed chemistry and transport. In both approaches the solutions were obtained in a finite domain and with plug flow boundary conditions in order to better simulate the actual experiments. Results show that linear extrapolation of the minimum velocity to zero stretch overestimates the true laminar flame speed. This overestimate, however, can be reduced by using larger nozzle separation distances. The theoretical results were further confirmed by experimental measurements for methane/air flames with various stoichiometries and nozzle separation distances. The numerical and experimental results indicate that for atmospheric methane/air flames, nozzle separation distances in excess of about 2 cm yield laminar flame speeds obtained by linear extrapolation accurate to within the uncertainty range of the experiment. The results obtained herein thus provide further support for the viability of the counterflow technique, when the influence of the nozzle separation distance is properly accounted for. The viability of an alternate technique for the determination of laminar flame speeds, based on the variation of flow velocity at a constant temperature near the upstream boundary of the flame with stretch, was also theoretically investigated.

Analysis of thermal ignition in the supersonic mixing layer

Ignition in a laminar supersonic mixing layer between two parallel streams of initially separated reactants is studied both numerically and through the use of large activation energy asymptotics. The asymptotic analysis provides a description of ignition characteristics over the entire range of system parameters. In particular, it is demonstrated that, for small values of viscous heating, the ignition distance scales approximately linearly with the freestream Mach number, whereas for large viscous heating it decreases rapidly due to the temperature-sensitive nature of the reaction rate. This indicates the potential of using local flow retardation to enhance ignition rather than relying solely on external heating. The asymptotic analysis further identifies several distinct ignition situations, yielding results that compare well with those obtained from the full numerical calculation. The effects of flow nonsimilarity are also assessed and are found to be more prominent for the mixing layer flow in comparison to the flat-plate configuration studied previously.

SOOTING LIMITS OF MICROGRAVITY SPHERICAL DIFFUSION FLAMES IN OXYGEN-ENRICHED AIR AND DILUTED FUEL

Limiting conditions for soot-particle inception were observed in microgravity spherical diffusion flames burning ethylene at 0.98 bar. Nitrogen was supplied to the ethylene and/or oxygen to obtain the broadest available range of stoichiometric mixture fraction, Zst. Both normal flames (surrounded by oxidizer) and inverse flames (surrounded by fuel) were considered. Soot-free conditions were found to be favored at increased Zst and there was no observed effect of convection direction on the sooting limits. The sooting limits follow a linear relationship between adiabatic flame temperature and Zst, with Zst accounting for a variation of about 700 K in the sooting-limit adiabatic flame temperature. This relationship is in qualitative agreement with a simple theory that assumes soot inception requires the local C/O atom ratio and temperature to be above threshold values, (C/O)c and Tc, respectively. The theory indicates that different mechanisms are responsible for sooting limits at low and high Zst. When inert is added to a fuel/air flame, a sooting limit is obtained when temperature becomes so low that the kinetics of soot inception are too slow to produce soot. On the other hand, a flame with a high Zst has low C/O ratios far into the fuel side of the flame. For such a flame, soot-free conditions can be attained at much higher temperatures because there is sufficient oxygen on the fuel side to favor oxidation of light hydrocarbons over formation of soot precursors.

Instability of burner-stabilized flames with volumetric heat loss

Behavior of the planar, burner-stabilized premixed flame that suffers volumetric heat loss, with its rate described by a linear function of temperature, is analyzed by activation energy asymptotics. Both its steady burning characteristics and its diffusional-thermal flame-front instability are investigated. The effect of volumetric heat loss is found to slow down the reaction so that the flame moves away from the burner. Consequently, the heat transfer from the flame to the burner is reduced and the flame temperature is increased. By continuously increasing the intensity of volumetric heat loss, the flame eventually is detached from the burner and becomes freely propagating. The stability analysis then reveals that the volumetric heat loss renders the flame to be less stable, similar to what is known for the freely propagating flame. The increase in the flame temperature has only a weak effect to stabilize the flame. It is primarily to compensate the decrease in the burning rate that is induced by the volumetric heat loss. In addition, the volumetric heat loss has a stronger impact on the reduction of stability for flames with a lower burner supply rate, that also is the burning rate. A critical burner supply rate exists, below which the flame turns absolutely unstable before detaching from the burner so that the freely propagating limit cannot be reached by increasing the intensity of volumetric heat loss. This study further supports the existence of the dual flame behavior experimentally observed by Spalding and Yumlu in that there exist two flame speeds for a heat transfer rate to the burner because stable flame exists, even for relatively small burner supply rates, when the volumetric heat loss is weak.

On Soot Inception in Nonpremixed Flames and the Effects of Flame Structure

A simplified three-step model of soot inception has been employed with high activation energy asymptotics to study soot inception in nonpremixed counterflow systems with emphasis on understanding the effects of hydrodynamics and transport. The resulting scheme yields three zones: (1) a fuel oxidation zone wherein the fuel and oxidizer react to form product as well as a radical R, (e.g., H), (2) a soot/precursor formation zone where the radical R reacts with fuel to form “soot/precursor” S, and (3) a soot/precursor consumption zone where S reacts with the oxidizer to form product. The kinetic scheme, although greatly simplified, allows the coupling between soot inception and flame structure to be assessed. The results yield flame temperature, flame location, and a soot/precursor index S1 as functions of Damkohler number for S formation. The soot/precursor index indicates the amount of S at the boundary of the formation region. The flame temperature indirectly indicates the total amount of S integrated over...

Numerical and experimental observations of spherical diffusion flames

Spherical diffusion flames supported on a porous sphere were studied numerically and experimentally. Experiments were performed in 2.2 s and 5.2 s microgravity facilities. Numerical results were obtained from a Chemkin-based programme. The programme simulates flow from a porous sphere into a quiescent environment, yields both steady state and transient results and accounts for optically thick gas-phase radiation. The low flow velocities and long residence times in these diffusion flames lead to enhanced radiative and diffusive effects. Despite similar adiabatic flame temperatures, the measured and predicted temperatures varied by as much as 700 K. The temperature reduction correlates with flame size but characteristic flow times and Lewis number also influence temperature. The numerical results show that the ambient gas Lewis number would have a strong effect on flame temperature if the flames were steady and nonradiating. For example, a 10% decrease in Lewis number would increase the steady state flame temperature by 200 K. However, for these transient, radiating flames the effect of Lewis number is small. It was also observed that when hydrocarbon fuel is supplied from the ambient the large diffusion distances associated with these flames can lead to unusual steady state compositions near the outer boundary because decomposition products can diffuse to the outer boundary. This results in a loss of chemical enthalpy from the system but the effect on flame temperature is small. Transient predictions of flame sizes are larger than those observed in microgravity experiments. Close agreement could not be obtained without either increasing the models thermal and mass diffusion properties by 30% or reducing mass flowrate by 25%.

A multicomponent sectional model applied to flame synthesis of nanoparticles

Sodium/halide flame synthesis and encapsulation (SFE) is a promising technique for producing nonoxide materials. The reaction produces nanoparticles of the core product and a condensable by-product (e.g., NaF). In this research, the SFE process was studied numerically for a spherically symmetric non-premixed flame in a low-pressure atmosphere of sodium vapor. Experimentally, this flame can be established in microgravity. A transient flame code incorporating detailed chemistry and transport processes was developed to perform the investigation. In addition, a two-component sectional method was developed and integrated into the flame code to simulate aerosol dynamics, including coagulation and condensation. For the simulation, the reaction of CF 4 with Na was chosen such that solid carbon was the core product and NaF was the condensable by-product. the results show that near the reaction region, a large number of core particles form, yet condensation of the condensable species does not occur because of the high temperature. Slightly away from the reaction region toward the ambient, there is a narrow region within which rapid condensation of NaF occurs and a few large, heavily coated particles are formed. Most of the small core particles are not coated in this region, but instead are being scavenged by the large, heavily coated particles. Condensation is negligible further downstream because the concentration of the condensable species becomes too low. The simulation also shows that thermophoresis is important, especially at low pressure, to the distribution of particles and that the effect of sticking coefficient on NaF(1) is minimal. The effect of pressure also has been studied, and the results show that the mass fractions of carbon and NaF(1) are insensitive to pressure.

Diffusional–thermal instability of cylindrical burner-stabilized premixed flames

Abstract Steady burning behavior and diffusional–thermal instability of cylindrical burner-stabilized premixed flames are studied via activation energy asymptotics. For the steadily burning flame, the analysis yields a solution of the flame temperature and standoff distance as functions of the mass flow rate supplied by the burner. The result shows that the flame may be stabilized by either heat loss to the burner or flow divergence, in agreement with an earlier investigation by Eng et al. In addition, the flame exhibits two dual flame behaviors; namely, there exist two flame speeds either for the same heat loss rate to the burner or at the same standoff distance, which is consistent with earlier investigations on the one-dimensional, planar burner-stabilized flame. The linear stability analysis reveals that there exists a critical wave number on both the angular and axial directions for which the flame is the most unstable, that the critical wave number on the angular direction increases with increasing flow supply rate from the burner, that the effect of flow divergence–induced flame curvature is to stabilize the flame so that the cylindrical burner-stabilized flame is more stable than its one-dimensional counterpart, that the stability boundaries of a cylindrical flame approach those of the adiabatic, freely propagating planar flame by continuously increasing the burner supply rate, and that a flame stabilized by a larger burner is less stable than that supported by a smaller burner.

A theoretical study of spontaneous ignition of fuel jets in an oxidizing ambient with emphasis on hydrogen jets

An analysis was performed for the spontaneous ignition of a hydrogen (or other gaseous fuel) jet emanating from a slot into an oxidizing ambient (e.g., air). A similarity solution of the flow field was obtained. This was combined with the species and energy conservation equations, which were solved using activation energy asymptotics. Limits of spontaneous ignition were identified as functions of slot width, flow rate, and temperatures of the hydrogen jet and ambient gas. Two scenarios are examined: a cool jet flowing into a hot ambient and a hot jet flowing into a cool ambient. For both scenarios, ignition is favored with an increase of either the ambient temperature or the hydrogen supply temperature. Moreover, for the hot ambient scenario, a decrease in fuel Lewis number also promotes ignition. The Lewis number of the oxidizer only has a weak effect on ignition. Because spontaneous ignition is very sensitive to temperature, ignition is expected to occur near the edge of the jet if the hydrogen is cooler than the ambient gas and near the centerline if the hydrogen is hotter than the ambient gas.

Triaxial Burke-Schumann Flames with Applications to Flame Synthesis

The problem of a flame generated by three coaxial flows is solved by extending the Burke-Schumann methodology to include a third stream. The solution is particularly relevant to flame synthesis wherein multiple tubes are often employed either to introduce inert as a diffusion barrier or to introduce more than two reactants. The general problem is solved where the inner and outer tubes contain reactants and the middle tube contains either an inert or a third reactant. Relevant examples are considered and the results show that the triaxial Burke-Schumann flame can be substantially more complicated than the traditional Burke-Schumann flame. When the middle flow is inert the flame temperature is no longer constant but increases axially, reaching a maximum at the flame centerline. At the exit the flame does not sit on the tube exit but instead resides between the inner and outer tubes, resulting in an effective barrier for particle build-up on the burner rim. For the case of a third reactant in the middle flow, synthesis chemistry where the inner reaction is endothermic and the outer reaction is exothermic is considered. In addition to showing the flame temperature and flame shape, the results identify conditions wherein reaction is not possible due to insufficient heat transfer from the outer flame to support the inner flame reaction.