Decrease In Flame Speed Research Articles

Optical investigation of the influence of high-reactivity iso-octane turbulent jet ignition on the combustion characteristics of ammonia/air mixtures

Ammonia has attracted considerable attention as a zero-carbon fuel; however, challenges such as ignition difficulty and slow flame propagation persist. These challenges can be addressed by employing a highly reactive fuel in the pre-chamber to initiate the ignition of ammonia. This work aims to experimentally investigate the influence of pre-chamber fuel composition and concentration on ammonia combustion characteristics across various equivalence ratios. The results are as follows: elevating iso-octane equivalence ratios in the pre-chamber enhances stoichiometric ammonia combustion characteristics, reducing ignition delay and burn duration. However, excessive values result in significant delays and a decrease in flame speed. At an iso-octane equivalence ratio of 0.8, the main chamber exhibited the shortest combustion duration. By utilizing the optimal pre-chamber total equivalence ratios of 2.0 and 2.3, the ammonia lean combustion limit was achieved at an excess air ratio of 1.89 under experimental conditions (0.8 MPa, 440 K). As main chamber equivalence ratios decreased, ignition delay shortened. The pre-chamber total equivalence ratios exert a more pronounced impact on the main chamber ignition delay. In contrast, the main chamber equivalence ratios substantially influence the middle and later stages of combustion.

Laminar flame speeds of nano-aluminum/methane hybrid mixtures

An existing flame speed bomb was utilized to study the fundamental phenomena of flame propagation through a uniformly dispersed aerosol with a relatively small mass of metal nanoparticles. The aerosol was dispersed with a burst of gas and then allowed to settle for at least 45s, to ensure that the conditions inside the test chamber were quiescent and uniform. Extinction of the light from a HeNe laser was used so that the mass of suspended nano-particles with a fundamental size of 100nm could be determined as a function of time prior to combustion, and the suspended mass of aluminum (up to 90mg) was measured in situ during an experiment. A particle size distribution was measured as well, resulting in an average pre-combustion agglomeration size of 446.1nm. Two series of experiments were performed with CH4 fuel, both at stoichiometric conditions: one with the mixture in air and the second with the mixture in a 70/30 N2/O2 mix. The results herein show a maximum decrease in flame speed, 5–7% for the baseline mixture, when nano-aluminum was introduced at the suspended masses utilized in the present study. For the 70/30 N2/O2 mixture, the aluminum resulted in a maximum decrease of 5cm/s from the baseline value of 80.5cm/s; and in the air mixture, a 2cm/s maximum decrease from 35.3cm/s was observed. Simple modeling of the process extremes indicates that the observed decrease in flame speed is somewhere between the limits of simple particle heating with no particle reaction and the kinetics limit where the Al reacts in the gas phase. Interestingly, the trends qualitatively matched those of the kinetics model applied herein. It was also found that the addition of nanoparticles caused the flame to become unstable much sooner when compared to the baseline mixture of CH4/air.

Stabilized flames in hybrid aluminum-methane-air mixtures

A premixed methane–air bunsen-type flame is seeded with micron-sized (d32=5.6μm) atomized aluminum powder over a wide range of solid fuel concentrations. The burning velocities of the resulting two-phase hybrid flame are determined using the total surface area of the inner flame cone and the known volumetric flow rate, and spatially resolved flame spectra are obtained with a spectral scanning system. Flame temperatures are derived through polychromatic fitting of Planck’s law to the continuous part of the spectrum. It is found that an increase in the solid fuel concentration changes the aluminum combustion regime from low temperature oxidation to full-fledged flame front propagation. For stoichiometric methane–air mixtures, the transition occurs in the aluminum concentration range of 140–220g/m3 and is manifested by the appearance of AlO sub-oxide bands and an increase in the flame temperature to 2500K. The flame burning velocity is found to decrease only slightly with an increase in aluminum concentration, in contrast to the rapid decrease in flame speed, followed by quenching, that is observed for flames seeded with inert SiC particles. The observed behavior of the burning velocity and flame temperature leads to the conclusion that intense aluminum combustion in a hybrid flame only occurs when the flame front propagating through the aluminum suspension is coupled to the methane–air flame.

An experimental and kinetic study of syngas/air combustion at elevated temperatures and the effect of water addition

Laminar flame speeds of premixed syngas/air mixtures were measured at various fuel equivalence ratios (0.6–3.0), H2 content of the fuel, and preheat temperatures (298–500K) using a spherically expanding flame configuration. The measured laminar flame speeds were compared with simulations using with three existing chemical kinetic models – GRI Mech 3.0, H2/CO Davis Mechanism and San Diego mechanism. Reasonable agreement between computations and measurements was achieved at room temperature that validated the new experimental configuration. However, at higher preheat temperatures discrepancies between computed and measured values were large, especially for fuel rich mixtures. Addition of H2O to two fuels (H2/CO=5/95 and 50/50) up to 40% in the fuel–air mixture was studied to understand the effect of moisture in coal derived syngas. For the H2/CO=5/95 fuel, flame speed was observed to increase with up to 20% H2O addition and then to decrease with any further water addition. However, the higher H2 content fuel (H2/CO=50/50) only showed a decrease in flame speed with an increasing water concentration in the fuel–air mixture. The different trends have been explained as a result of the competing chemical and physical (dilution and thermal) effects of H2O addition on the syngas flames using sensitivity analyses and by analyzing reaction rates and radical concentrations.

Flammability limits for exhaust gas diluted flames

Flammability limits of propane/air (=1.0) flames diluted by exhaust gases have been determined by extrapolating the calculated laminar flame speeds to a minimum flame speed limit as a function of pressure, dilution and unburned gas temperature. The flame speed calculations show an approximately linear decrease in flame speed with dilution. This result compares well with the two available experimental determinations of the functional behavior of the flame speed with dilution. The flammability limits for these diluted flames decrease monotonically, but not linearly with pressure. The limits do, however, increase with unburned gas temperature. A typical dilution limit is 38% by volume of exhaust gases for an unburned mixture temperature of 500 K and a pressure of 4 atmospheres. Length scales based on these laminar flame calculations were also determined. These length scales are an indicator of the turbulent flame structure. The results presented here show that dilution causes a large increase in the flame widths. This large increase in the flame width is sufficient to change the turbulent flame structure from a well defined “wrinkled” flame to a regime which is somewhere between the extremes of a wrinkled flame and the distributed reaction. This means that the turbulent flame structure in a homogeneous-charge, S.I. engine operating with a sufficiently diluted mixture may be much different than that which has been previously assumed.

The interaction of flame curvature and stretch, part 1: The concave premixed flame

Premixed flames with radii of curvature on the order of the preheat zone thickness are studied with activation energy asymptotics. The changes in flame speed caused by varying the flame stretch as the flame radius is held fixed are calculated. The result of this calculation is that the decrease in flame speed due to flame stretch is amplified by more highly curved flames. When there is no flame stretch, the flame speed is independent of the flame radius, and so flame curvature in and of itself does not alter flame speed, but is important in determining the changes in flame speed when stretch is present.

The role of centrifugal force fields in the stabilization of swirling flames

The importance of the centrifugal force G in the stabilization of flames in swirling flows is difficult to access under normal operating conditions owing to the presence of other stabilizing factors. However, at conditions near the blowoff limit the effect of centrifugal force tends to be predominant. Results calculated from several experimental sources have shown there is little effect on flame propagation for G less than or equal to 200 g. For G greater than or equal to 600g, good agreement has been found with the ''bubble transport'' correlation: S /SUB B/ ..cap alpha..G /SUP 1/2/ No abrupt decrease in flame speed is obtained for centrifugal force G greater than or equal to 3500g as experienced by Lewis. The effect of G on flame speed is likely to be beneficial to small highly loaded combustors, where it is possible to generate high values of G in an operating range where other flame stabilization factors are also present.

Computation of time-dependent laminar flame structure

The time-dependent approach for computing the speed of plane laminar flames in one dimension has been extended by providing a more general formulation which allows the computation of cylindrical and spherical flames in addition to planar flames. A rational approach for obtaining minimum ignition energies of flammable mixtures based on multireaction kinetics has been presented. Examples of flame speed as well as ignition energy for both planar and spherical flames were calculated using a computer program based on the formulation. An appreciable decrease in flame speed was computed when the flame radius was of the same order as the flame thickness. Good agreement has been obtained with (1) Spalding's computation of the hydrazine decomposition flame and (2) experlmental results for the ozone decomposition flame.

The effect of isotopic substitution on the decomposition flame of hydrazine

The mode of decomposition of hydrazine in the low-pressure flame has been investigated by a comparison of flame speeds for the N2D4 system with those of the previously investigated N2H4 system. Measurements were made using a closed-vessel technique, whereby flame speeds relative to the burned gas were determined by schlieren photography. For pure N2D4, flame speeds were measured at an initial temperature of 62 C in the pressure range 20–70 torr. In this region flame speed is virtually pressure independent, indicating an overall reaction order of 2. The comparison was extended by measuring flame speeds for binary mixtures containing inert diluents. Increasing amounts of two diluents, argon and nitrogen, were added to a fixed partial pressure (50.2 torr) of N2D4, and flame speed was measured over the entire flammability range. The effect of the added diluents is to decrease the flame speed with increasing partial pressure of diluent, and to extinguish the flame when the concentration of diluent exceeds ca. 50%. The effective activation energy was estimated from the dependence of flame speed on temperature for diluted mixtures. It is estimated to be 35 ± 2 kcal mole−1, based on calculated values of adiabatic flame temperature and thermal conductivity, assuming a stoichiometry identical to that for N2H4 decomposition. A large isotope effect of flame speed is apparent when the present results are compared directly with those of a previous investigation of the N2H4 flame, obtained in the same apparatus and under the same initial conditions. The speed of the hydrazine decomposition flame is halved when deuterium is substituted for hydrogen. This decrease in flame speed is explained in terms of transport properties, equilibrium thermodynamics, and kinetic factors, Isotopic changes in thermal conductivity contribute a factor of 1.1 to the total effect: isotopic changes in adiabatic flame temperature contribute a factor of 1.3–1.5; and kinetic isotope effects (kH/kD) contribute a factor of 1.5. The results have been applied to test the validity of two possible modes of hydrazine decomposition: by a chain reaction and by a unimolecular reaction in the falloff region. Both mechanisms show inconsistencies with the present measurements, and it is impossible to eliminate unambiguously either possibility. The present evidence, however, favors the chain reaction.