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Motivated by experimental observations on premixed-gas flame propagation in Hele-Shaw cells, this work analyzes quasi-isobaric flame propagation between two adiabatic parallel plates using a simple quasi-2D formulation based on averaging the flow properties across the cell gap. Instabilities associated with thermal expansion, buoyancy, viscosity change across the front and differential diffusion of thermal energy and reactants are investigated with one-step chemistry, constant heat capacity and variable transport coefficients through time-dependent computations of the flame front evolution in large domains. These instabilities are found to induce flame wrinkling which increases flame surface area and thus propagation speeds in ways different from those associated with freely propagating flames. The simulations are compared with experiments in Hele-Shaw cells; very good qualitative and (in some cases) quantitative agreement is found.

A new type of jet-stirred chamber (JSC) incorporating multiple impinging turbulent jets is proposed as an apparatus for the study of turbulent premixed flames. ANSYS-FLUENT computations using a RANS - Reynolds Stress Model were used to simulate the flows inside the JSC and identify an optimal configuration of inlet jets and outlet ports providing the most nearly ideal flow, i.e. homogeneous and isotropic with large turbulence intensity compared to the mean velocity. A configuration of 8 jets, each surrounded by a concentric annular outlet, at the corners of an imaginary cube circumscribed by a spherical chamber, produced by far the most nearly optimal flow characteristics. The performance of this configuration, called Concentric Inlet And Outlet (CIAO), was compared quantitatively to two popular fan-stirred chamber (FSC) designs and the CIAO JSC was found to provide far more nearly ideal flow properties. The robustness of the CIAO design was demonstrated by intentionally misaligning the jets or mismatching their flow velocities. A comparison of simulated turbulent flames in CIAO and an FSC showed that CIAO enabled far more nearly spherical expanding flames with nearly the same inferred turbulent burning velocity ( S T ) regardless of the flame radius r f and the value of the mean progress variable ( c ¯ ) used to define the flame location, whereas in the FSC the flame was not nearly as spherical and there was considerable variation of S T depending on r f and c ¯ .

The propagation rates (U{sub edge}) of nonpremixed ignition (advancing) fronts and extinction (retreating) edge flames were measured as a function of global strain rate ({sigma}), jet spacing (d), mixture strength, stoichiometric mixture fraction (Z{sub st}), and Lewis number (Le) using a counterflow slot-jet burner in which edge flames propagated along its long dimension. Electrical heaters at both ends of the slot 'anchored' the flames, allowing conditions resulting in negative values of U{sub edge} to be studied by triggering local extinction of anchored flames with an N{sub 2} jet. Results are presented in terms of the effects of a dimensionless flame thickness ({epsilon}) related to {sigma} and a dimensionless heat loss (k) on a scaled U{sub edge}. Propagation rates were markedly enhanced/retarded in mixtures with low/high Le. Propagation rates and extinction conditions were highly asymmetric with respect to Z{sub st}=0.5 despite the symmetry of flame location; mixtures with Z{sub st} greater/less than 0.5 behaved like stronger/weaker mixtures, apparently due to the relative locations of the radical production zone and maximum temperature zone for varying Z{sub st}. Two extinction limits were identified, corresponding to a high-{sigma} strain-induced limit that was strongly dependent on Le but nearly independent of k and a low-{sigma}more » heat-loss-induced limit that was strongly dependent on k but not Le. Most experimental findings were in good agreement with theoretical predictions; however, unlike predictions, 'tailless' triple flames were not observed; instead standard triple flames and 'short length' edge flames were found, and only for a very narrow range of experimental conditions. This is proposed to be due to the difference between the volumetric heat loss presumed in the models and the conductive transfer to the jet exits that dominates heat loss in the experiments. (author)« less

A simple first-principles model of counter current heat-recirculating combustors is developed, including the effects of heat transfer from the product gas stream to the reactant stream, heat loss to ambient, and heat conduction in the streamwise direction through the dividing wall (and heat transfer surface) between the reactant and product streams. It is shown that streamwise conduction through the wall has a major effect on the operating limits of the combustor, especially at small dimensionless mass fluxes (M) or Reynolds numbers that would be characteristic of microscale devices. In particular, if this conduction is neglected, there is no small-M extinction limit because smaller M leads to larger heat recirculation and longer residence times that overcome heat loss if M is sufficiently small. In contrast, even a small effect of conduction along this surface leads to significantly higher minimum M. Comparison is made with an alternative configuration of a flame stabilized at the exit of a tube, where heat recirculation occurs via conduction through tube wall; it is found that the counter-current exchanger configuration provides superior performance under similar operating conditions. Implications for microscale combustion are discussed.

A numerical study was conducted to examine the structure and dynamics of steady, source-free spherical premixed flames (“flame balls”), which have been observed in microgravity experiments. A time-dependent spherically symmetric code was used with detailed chemical, transport, and radiation submodels. Steady properties, stability limits, and dynamics of flame balls are computed for H 2 -air mixtures. The chemical and radiation models used were found to affect flame ball properties substantially. The special and unusual role of thermal radiation from the combustion products is described. In particular, the far-field radiative loss is found to affect the behavior of flame balls in a manner very different from propagating planar flames. One new feature was identified: mixtures capable of exhibiting both stable flame balls and steadily propagating flames depending on the initial condition. Numerical results are compared to theoretical predictions, prior steady-state numerical calculations, and prior experimental results on flame ball size. Additional experiments were performed to measure flame ball radiant emission. Qualitative agreement with theory and experiment is found; however, quantitative agreement with experiment is only fair, indicating the need for improved microgravity conditions, e.g., in orbiting spacecraft.

Near-infrared (IR) and ultraviolet (UV) emission profiles of flame balls at microgravity conditions in H 2 -O 2 -diluent mixtures were measured in the JAMIC 10 s drop-tower and compared to numerical simulations and supplemental KC135 aircraft μg experiments. Measured flame ball radii based on images obtained in the JAMIC, KC135, and recent space experiments (IR only) were quite consistent, indicating that radius is a rather robust property of flame balls. The predicted IR radii were always smaller than UV radii, whereas the experiments always showed the opposite behavior. Agreement between measured and predicted flame ball properties was closer for UV radii than IR radii in H 2 -air mixtures but closer for IR radii in H 2 -O 2 -CO 2 mixtures. The large experimental IR radii in H 2 -air tests is particularly difficult to interpret even when uncertainties in chemical and radiation models are considered. Experimental radii would be consistent with a chemiluminescence reaction of the form HO 2 + HO 2 → H 2 O 2 + O 2 producing an excited state of H 2 O 2 , since HO 2 is consumed at large radii through this reaction and its exothermicity is sufficient to create excited states that could emit at the observed wavelengths, however, no appropriate transition of H 2 O 2 ∗ could be identified.© 1998 by The Combustion Institute

Turbulent-flame speeds in methane-air mixtures were measured in a Taylor-Couette apparatus with counter-rotating cylinders, used to generate turbulence that is nearly homogeneous and isotropic over many integral length and time scales. While laminar-flame propagation is found to be influenced by the Darrieus-Landau instability and heat loss to the walls of the apparatus, turbulent-flame propagation in high-intensity turbulence is found to be uninfluenced by these effects. A decreasing sensitivity of the turbulent-flame speed to increases in turbulence intensity is found to occur beyond turbulence intensities of approximately 2.5 times the laminar-flame speed. This is possibly due to a transition to a nonflamelet combustion regime where flame propagation is influenced by both small-scale flame-structure modification and large-scale flame-front wrinkling. Results are compared with those obtained by earlier investigators using other experimental apparatuses and with theoretical predictions.

Nonpremixed flames subject to steady but spatially varying straining flows were studied to examine one aspect of nonpremixed flames in strongly turbulent flows or near quenching conditions (e.g., near a burner rim), where strain-rate gradients are present and local strain rates may be high enough to cause local flame-front extinguishment. The spatially varying straining flow were created using an opposed slot-jet burner with slightly nonparallel jet exits. The most significant observation was that steady flame “edges” could be created where the flame would exist in the low-strain region but would be extinguished in the high-strain region. The local strain at the location of the stationary flame edge was almost always lower than the strain required to extinguish flames in the same mixture subject to a spatially uniform strain. The strain rate at the edge-flame location was independent of the strain-rate gradient and gradual transitions from edge-flame behavior to uniformly strained flame behavior were not observed, indicating that conventional nonpremixed flames and edge flames are quite distinct structures yet each has well-defined properties. At the flame edge, interferometer images indicate a region of locally intense burning and an abrupt transition to nonburning conditions away from the edge. These observations were found to be qualitatively consistent with recent theoretical models of flame edges. These results indicate that “laminar flamelet” models of nonpremixed turbulent combustion may require reevaluation at conditions approaching those where local flame quenching occurs. It is proposed that these models could be improved by adding edge-flame libraries to existing laminar flamelet libraries.

The authors examine flame balls that are not optically thin so that radiation from the hot burned gas is not lost but is reabsorbed by the surrounding cool gas. The steady solution is shown to be multivalued in general and defines an intrinsic (apparatus independent) flammability limit for a sufficiently small ignition source. Beyond the flammability limit a steady solution exists for arbitrarily weak mixtures, but is unrealizable since it is unstable to one-dimensional perturbations. An unsteady propagating flame solution is also possible for arbitrarily weak mixtures, but can only be generated by a large ignition source. Observations of flame-strings in SF[sub 6]-diluted mixtures are reported and suggest that the peculiar nature of the radiative properties of SF[sub 6] is responsible for peculiar dynamics.

The propagation rates and thermal characteristics of flames spreading downward over thin solid fuel samples are measured. Effects of diluent type, oxygen concentration, pressure, and fuel bed thickness are examined. Pressure and fuel bed thickness effects showed the trends expected based on existing flame spread theory. However, comparison of observed and predicted spread rates in various O2diluent atmospheres indicate a Lewis number (Le) effect which has not been considered previously. An approximate extension of previous flame spread theory to include Le effects is proposed and found to compare favorably with the experimental results. A new phenomenon is observed in O2SF6 and O2CO2 atmospheres near the limit: cellular flame front propagation. An explanation based on Le effects and finite-rate chemistry is proposed. Application of these results to the assessment of fire hazards and fire suppression in air and nonstandard atmospheres is discussed.

A theory of spherical premixed flames in which the only transport processes are diffusion and radiation (“flame balls”) is extended to include the effects of heat loss from the far-field (unburned gas). Using matched asymptotic expansions for large activation energy, stationary solutions are constructed and an evolution equation for the radial motion of the flame is derived. Linear stability analyses are performed for both one-dimensional and three-dimensional perturbations. It is shown that when far-fieldlosses are included, the stability properties are qualitatively changed. A smaller range of conditions produces flames that are stable to one-dimensional disturbances and, in some cases, the linear growth rates are found to have imaginary components. Numerical intergation of the evolution equation reveals oscillations (but not limit cycles), in agreement with a bifurcation analysis. The minimum flame-ball radius for which three-dimensional instabilities will occur is shown to depend only on the near-field losses, and becomes arbitrarily large as these losses are reduced.

The characteristics of premixed gas flames in mixtures with low Lewis numbers near flammability limits were studied experimentally using a low-gravity environment to reduce buoyant convection. The behavior of such flames was found to be dominated by diffusive-thermal instabilities. For sufficiently reactive mixtures, cellular structures resulting from these instabilities were observed and found to spawn new cells in regular patterns. For less reactive mixtures, cells formed shortly after ignition but did not spawn new cells; instead these cells evolved into a flame structure composed of stationary, apparently stable spherical flamelets. Experimental observations are found to be in qualitative agreement with elementary analytical models based on the interaction of heat release due to chemical reaction, differential diffusion of thermal energy and mass, flame front curvature, and volumetric heat losses due to gas and/or soot radiation.

Recent experiments in microgravity suggest the possibility of stationary spherical premixed flames (flame balls) in which the only fluxes are diffusional. We construct stationary solutions of this nature, starting with simple model equations and using activation energy asymptotics. Sufficiently large volumetric heat losses quench the flame, and for heat losses less than the quenching value there are two possible solutions, a small flame, and a large flame. For vanishing heat loss the small solution is identical to one constructed by Zeldovich, and is known to be unstable, whereas the large solution is characterized by a flame of infinite radius. We examine the linear stability of these stationary solutions, and show that all small flames are unstable to one-dimensional (radial) perturbations. Large flames are unstable to three-dimensional perturbations, but only if they have a radius greater than some critical value. Thus there is a band of large flames, lying between the quenching point and unstable flames, that are stable.

Fuel-lean flammability limits and burning velocities in a closed vessel were measured for methaneair mixtures burning at earth gravity (one-g) and zero-gravity (zero-g) at initial pressures of 50–1500 Torr. The zero-g flammability limit was found to be between the one-g upward and one-g downward flammability limits. For sublimit mixtures burning at zero-g, an extinguishment phenomenon unlike any found at one-g was observed. For fast burning mixtures (Su > 15 cm/s), burning velocities were identical at one-g and zero-g. For slower burning but still flammable mixtures, only the zero-g observations could be interpreted to obtain burning velocity data because at one-g natural convection caused severe flame front distortion. Zero-g burning velocities for these mixtures were in good agreement with existing models of laminar flame propagation and with extrapolation of current and prior one-g data. The main conclusions are that the one-g upward flammability limit occurs at a mixture which has a burning velocity which is so low that flame propagation is impractical, that the one-g downward flammability limit is related to the inability of the flame front to propagate downward against buoyant forces, and that near-limit flame propagation at zero-g is mostly independent of the experimental apparatus. Because of the unusual nature of the extinguishment process for sublimit mixtures burning at zero-g, further experiments are required to determine the cause of the zero-g flammability limit.

Minimum ignition energies and flame radii as a function of time were measured for near-limit, limit, and sublimit fuel-lean methaneair mixtures burning at one-g and zero-g. Minimum ignition energy values were the same at one-g and zero-g except for mixtures very near the zero-g flammability limit and leaner, where the zero-g values were much higher than the one-g values. For sublimit mixtures at zero-g a previously unreported mode of unstable flame propagation was observed; this mode was characterized by a flame radius increasing in proportion to the square root of the time lapse from ignition, an energy release often orders of magnitude greater than the spark energy input, and sudden extinction. This mode of flame propagation was observed at all gas pressures tested but was more pronounced at higher pressures. All zero-g propagation was spherically symmetric except for a few unusual flame extinguishments at high pressures. The principal conclusions are that flame extinguishment at zero-g is caused by a flame-front instability and that gravitational forces have a stabilizing effect on upward flame propagation. The cause of the instability could not be determined; further experiments which might aid in determining the cause are suggested.