Showing papers on "Premixed flame published in 1985"

On the determination of laminar flame speeds from stretched flames

Abstract: The effects of stretch on the determination of the laminar flame speed are experimentally studied by using the positively-stretched stagnation flame and negatively-stretched bunsen flame, and by using lean and rich mixtures of methane, propane, butane, and hydrogen with air whose effective Lewis numbers are either greater or less than unity. Results demonstrate that flame speed determination can be influenced by stretch through two factors: (1) Preferential diffusion which tends to increase or decrease the flame temperature and burning rate depending on the effective Lewis number, and (2) Flow divergence which causes the flame speed to assume higher values when evaluated at the upstream boundary of the preheat zone instead of the reaction zone. Recent data on flame speed including the present ones are then examined from the unified viewpoint of flame stretch, leading to satisfactory resolution of the discrepancies between them. The present study also proposes a methodology of determining the laminar flame speeds by using the stagnation flame and linearly extrapolating the data to zero stretch rate.

Visible structure of buoyant diffusion flames

Abstract: Natural gas diffusion flames stabilized on 0.10, 0.19 and 0.50 m. diameter porous bed burners have been studied for heat release rates ranging from 10 to 200 kW. Flame heights were measured from video tape recordings and by eye averaged techniques. The dependence of flame height on a dimensionless heat addition parameter shows a transition for values of the parameter around unity. For flames taller than three burner diameters, the initial diameter of the fire does not affect the length of the flame whereas for short flames, initial geometry becomes important. Another prominent feature of these flames is the presence of large scale axisymmetric structures which are formed close to the burner surface with more or less regular frequency and which rise through the flame region. These structures are responsible for the fluctuations of the flame top and strongly influence the geometry of the flame.

Turbulent flame propagation in obstacle-filled tubes

Abstract: Turbulent flame acceleration experiments have been carried out in steel tubes of 5 cm, 15 cm and 30 cm diameter and ranging from 11 m to 17 m in length. Circular orifice plates spaced on diameter apart were used as flow obstructions. The blockage ratios BR=1−(d/D)2 are 0.44, 0.39 and 0.28 corresponding to orifice diameters of 3.74 cm, 11.7 cm and 25.8 cm for the 5 cm, 15 cm and 30 cm diameter tubes, respectively. Mixtures of hydrogen, acetylene, ethylene, propane and methane with air were used over a range of fuel compositions. The results indicate the existence of four propagation regimes: the quenching, the choking, the quasi-detonation and the C-J detonation regimes. In the quenching regime, the flame is first found to accelerate and then extinguish itself after propagating past a certain number of orifice plates. The flame propagation process in the quenching regime can be considered as the successive ignition of a sequence of chamber separated by the orifice plates. Ignition in one chamber is due to the venting of the hot combustion products from the upstream chamber through the orifice. Quenching occurs when the jet fails to ignite the mixture due to too short a mixing time when compared to the chemical reaction time. For mixtures very near the limits, an alternative quenching mechanism due to flame stretching is proposed. When the flame is not quenched, it eventually reaches a steady state. It is found that gasdynamic choking (sonic conditions) due to friction and heat release provide the controlling mechanism for the steady state flame speed. Again, for near limit mixtures, the steady state condition may be brought about by the positive and negative effects of flame stretching in the augmentation of the burning rate. For most of the cases of interest, the choking mechanism prevails. For the more sensitive mixtures, transition to detonation is observed. The transition criterion requires that the ratio of the orifice diameter d to detonation cell size λ be in the range 1d/λ≤13. In the present study where the blockage ratios are of the order of BR≈0.4, the critical value is d/λ≈3. The detonation under these conditions is observed to travel in the obstacle-filled tube at velocities significantly below the normal C-J value in accord with the previous observations of detonation propagation in very rough tubes. Such detonations are referred to as quasi-detonations. When the mixture is sufficiently sensitive such that d/λ13, then the detonation propagation becomes insensitive to the blockage effects of the obstacles and the combustion front is observed to propagate as a normal C-J detonation wave.

Experimental investigation on the stabilization mechanism of jet diffusion flames

Abstract: Two contrary concepts have been suggested in order to explain the mechanism of flame stabilization based on premixed and diffusion flamelet combustion respectively. To contribute to the understanding of which model represents the true stabilization mechanism, two different natural gas jet diffusion flames, at exit velocities between flame detachment and blow-off, were investigated. The measured profiles of gas composition and velocity around the stabilization zone were used to derive the rate of mixing and fuel burnout. The results show that, for the flames investigated, about forty to fifty percent of the total fuel flow is already mixed at a molecular level upstream of the flame stabilization zone. This mixture then reacts over a very short distance, supporting the concept of premixed combustion in lifted jet diffusion flames.

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

Abstract: 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.

Effects of preferential diffusion on the burning intensity of curved flames

Abstract: Using the Bunsen flame as a model curved flame, the coupled influence of preferential diffusion and flame stretch on the burning intensity of lean and rich mixtures of methane, propane, butane, ethylene, and hydrogen with air has been experimentally studied. The results substantiate theoretical predictions and quantify previous experimental observations that, for mixtures whose effective Lewis numbers (Le) are less than unity, the flame temperature is less than the adiabatic flame temperature. This temperature also decreases towards the flame tip, which has the largest curvature and therefore may locally extinguish. The dominance of diffusional transport in influencing the intensity of curved flames is demonstrated by showing that the tip opening of the highly diffusive hydrogen/air flames occurs at constant hydrogen equivalence ratios of about 1.15 to 1.20, being almost independent of the flow intensity and uniformity.

On the behavior of premixed flames in a rotating flow field: Establishment of tubular flames

Abstract: The behvior of premixed flames in a rotating flow field has been investigated using a swirl type burner, in which a fuel/air mixture is ejected tangentially into a cylindrical combustion chamber. The results show that for appropriate conditions for the ejection velocity and the fuel concentration of the mixture, a tubular flame of circular cross-section is established inside the burner. By this flame front, the combustion field is separated into two regions, an outer unburned gas region and an inner burned gas region, and the burned gas flows inside the flame with rotation. This rotating hot gas core is very stable according to the Rayleigh stability criterion. However, further experiments show that this type of tubular flame can be established for lean methane/air or hydrogen/air mixtures, but not for lean propane/air mixtures. In the former case, the diameter of the flame decreases with a decrease of the fuel concentration, and the flame is eventually extinguished after the tubular flame is merged into a luminous rod, whereas in the latter case, an unsteady, corrugated flame is established around the wall of the cylindrical burner, and the flame is extinguished without forming a uniform flame front. The fuel concentration at the extinction limits is close to the lean flemmability limit in the former case, whereas very far from that in the latter case. These two distinct behaviors of flames in the rotating flow field have been discussed on the basis of the tangential velocity distribution of the Rankine's combined vortex and the flame stretch theory considering the Lewis number effect.

Simultaneous velocity and temperature measurements in a premixed flame

Abstract: Procedures which allow the correlation of velocity signals from a laser anemometer and temperature signals from a compensated, small-diameter thermocouple are described together with the error sources associated with the use of the technique in premixed flames. The digital compensation procedure includes the effect of velocity and temperature on the time constant of the thermocouple and the influence of its exposure to the solid particles required by the laser anemometer are quantified and shown to be able to cause large differences in the measured probability-density-distribution of the reaction progress variable. The technique has been used to measure the probability-density-distribution of temperatures, conditioned by the arrival of velocity signals and velocity conditioned by the temperature signal and sample results are presented to help quantify the accuracy of the measurements.

Phase conjugation in a flame

Abstract: The first observation to our knowledge of degenerate four-wave mixing in a sodium-seeded flame is reported. In addition, the theoretical dependence of the phase-conjugate output on the sodium concentration is verified experimentally, demonstrating the potential of this procedure for use as a diagnostic tool to measure species concentration in combusting or noncombusting media.

An experimental investigation on flame interaction and the existence of negative flame speeds

Abstract: Downstream interaction between two counterflow premixed flames of different stoichiometries are investigated. Various flame configurations are observed and quantified; these include the binary system of two lean or rich flames, the triplet system of a lean and a rich flame separated by a diffusion flame, and single diffusion flames with some degree of premixedness. Extinction limits are determined for methane/air and butane/air mixtures over the entire range of mixture concentrations. Results show that these extinction limits can be significantly modified in the presence of interaction such that a mixture much beyond the flammability limit can still burn if it is supported by a stronger flame. The experiment also demonstrates the existence of negative flames whose propagation velocity is in the same general direction as that of the bulk convective flow. Implications of the present results on the flammability of stratified mixtures and on the modeling of turbulent flames are discussed.

Partially premixed diffusion flamelets in non-premixed turbulent combustion

Abstract: The structure and extinction of partially premixed diffusion flamelets in a turbulent flow field is analyzed. It is shown that diffusion flamelets are disconnected if the variation of the mixture fraction around stoichiometric is large enough for the corresponding adiabatic flame temperature to be less than a characteristic freezing temperature. At the freezing temperature, a premixed flame structure develops. If stretch (expressed in terms of the instantaneous scalar dissipation rate) is increased, the premixed flame merges into the non-equilibrium diffusion flame layer that exists around the maximum temperature. Quenching of the partially premixed diffusion flamelet occurs in the merged flame layer. The inner structure of the merged flame layer is identical to that for unpremixed diffusion flamelets with, however, modified boundary conditions. A numerical analysis of the merged flame structure is performed and extinction conditions are derived. It is shown that local premixing of an initially unpremixed flow leads to lower values of the scalar dissipation rate at quenching. The analysis of the flame structure is general and may also be applied to initially partially premixed systems.

Determination of burning velocity using counterflow flames

Abstract: A technique has been developed to measure burning velocity using counterflow double flames established in the forward stagnation region of the porous cylinder. In this system, two different regimes occur for the inner premixed flame depending on whether the mixtureejection velocity is more or less than the burning velocity. A plot of the flame position versus the mixture-ejection velocity is linear, in the absence of interaction between the flame and the cylinder surface, and this linearity is destroyed suddenly and discontinuously at the critical ejection velocity, i.e., the burning velocity. A major advantage of this new method is that, unlike in burners using non-divergent flows, the flame does not blow off or change its geometry radically when the flow velocity reaches the burning velocity. The method has been used to determine the burning velocity of methane-air mixture and propane-air mixture over a wide range of equivalence ratios. Results are in good agreement with those of other accurate methods using burners. This method appears to offer a wider range, in terms of equivalence ratio, than that available with any other burner.

Lifting mechanism of free jet diffusion flames

Abstract: The lifting phenomena of free jet diffusion flames of pure hydrogen and hydrogen-inert gas mixtures stabilized on a thin-walled burner tube were studied in unconfined, quiescent, oxygen-inert gas atmospheres. Nitrogen, argon, and helium were used as the inert gases. A laminar flame base, which exists even in a fully-developed turbulent jet diffusion flame, controls the flame stability. A simplified flame stability model is presented based on the concept that the flame base is the very end of the diffusion flame and it provides a continuous ignition source to propagate towards the combustible zone between the flame base and the burner tip. The local burning velocity parallel to the diffusion flame zone is considered at the flame base to be equilibrated with the entrained stream velocity component opposite to the direction of the burning velocity. Velocity measurements were made in the flame stabilizing region under near-limit conditions by means of the laser-Doppler technique. The measured velocity component parallel to the flame zone at the flame base was found to be directly proportional to the maximum laminar burning velocity obtainable by mixing the jet and external fluids. The result is independent of whether the mixture strength was obtained by varying the hydrogen content of the jet fluid or the oxygen content of the external fluid.

An excess enthalpy flame combustor for extended flow ranges

Abstract: An experimental study was conducted on the stability and combustion characteristics of the combustor system, based on the excess enthalpy flame, for extended flow rate ranges using a city gas (natural gas) and air mixture, and an entirely new combustor. The combustor was designed in light of previous studies, and the inner tube for combustion was surrounded by a heat exchanger to facilitate external heat recirculation, while a bundle of narrow ceramic tubes was used to produce internal heat recirculation. The stability limits in flow rate and equivalence ratio of the three flames stabilized ahead of, in, and behind the tubes were extended remarkably as compared to those of the previous studies. It became possible to burn stably mixtures as lean as equivalence ratio of 0.151 for large flow rate of 10.0 L/s. The measured temperature distributions showed that the combustion proceeded through the one-dimensional laminar flame, and that the combined external and internal heat recirculation produced very high flame temperature of more than twice the adiabatic flame temperature. The emission characteristics of the combustor were found excellent, while the pumping loss to force the flow through the narrow pores remained in the tolerable range.

Temperature and fuel effects in sooting diffusion flames

Abstract: An investigation was carried out into the differing sooting tendencies of various fuels in free round laminar diffusion flames. Measurements included soot concentrations, soot temperatures and flame reaction zone and centreline temperatures. It is shown that flames emit smoke when the soot temperature in the oxidation zone falls below about 1300 K. This temperature is controlled by heat losses through radiation from the soot formed. Measured flame temperatures are well below adiabatic temperatures in the sooting flames and are determined by fuel flow rate and radiation losses. Thus acetylene and butene produced the highest soot yields at the lowest flame temperatures for the fuels tested. Fuel dilution with N2 reduced soot yield more than could be explained by variation of the flame temperature. Carbon to hydrogen ratio of the fuel is not itself a parameter determining soot yeild. Soot yield profiles were normalized according to the stoichiometric flame length. Under these conditions the profiles from differing fuels showed good axial similarity except for C2H2.

A study of the controlling mechanisms of flow assisted flame spread

Abstract: An experimental study has been performed of the spread of flames over the surface of thick PMMA sheets in a forced gaseous flow of varied oxygen concentration moving in the direction of flame spread. It is shown that the rate of spread of the pyrolysis front is time independent, linearly dependent on the gaseous flow velocity, and approximately square power dependent on the oxygen concentration of the gas. The spread rate data can be correlated very well in terms of parameters deduced from heat transfer models of the flame spread process indicating that in the flow assisted mode of flame spread, heat transfer from the flame to the condensed fuel is the primary mechanism controlling the spread of the flame. Finite rate chemical kinetic effects have apparently a small influence on the flame spread process itself. Extinction processes are limited primarily to the upstream leading edge of the flame and to the flame tip.

Flame Propagation in Channels: Secondary Bifurcation to Quasi-Periodic Pulsations

Abstract: We consider premixed flame propagation in long rectangular channels. A steadily propagating planar flame is stable for Lewis numbers less than a critical value. For Lewis numbers exceeding this critical value a sequence of primary bifurcation states, corresponding to time-periodic pulsating cellular flames, emanates from the steadily propagating solution. We analyze the problem in a neighborhood of a multiple primary bifurcation point. By varying the channel dimensions, we split the multiple bifurcation point and show that a stable quasi-periodic pulsating flame can arise as a secondary bifurcation from one of the primary bifurcation states. We also exhibit the phenomenon of mode-jumping, in which there is an exchange of stability between two primary states.

Substantiation of the probe mass-spectrometric method for studying the structure of flames with narrow combustion zones

Abstract: This work is devoted to the substantiation of the probe method for the case when the ratio of the width of the combustion in the flame to the outer diameter of the sampler tip is close to unity. It is precisely this value of the ratio that was used in the probe mass-spectrometric study of the structure of an ammonium perchlorate flame. A flat flame of the mixture 8% methane + 80% air + 12% argon was stabilized at atmospheric pressure in a flat burner. The authors measured the temperature distribution in the flame by a microthermocouple method. The measurement of the distribution of the methane concentration in the flame was done by mass-spectrometry. The authors measured the distributions of the absolute methane and nitrogen concentrations in the flame using a spectrometer with intracavity spontaneous Raman light scattering. The authors conclude that the data (v=0.3 and 1.2 mm/sec) were obtained under quasistatic conditions and the conclusions drawn regarding the heat flows from the probe to the combustion surface, obtained from analysis of the stationary methane flame, can be applied to them.