Propane-air Flames Research Articles

Laminar propane-air flame in a weak electric field

Three mechanisms of the electric field influence on combustion of gases are considered: ohmic heating, effect of chemical reactions on the kinetics, and ionic wind, which is the most important factor. Experimental data presented in the paper testify to a local action of the electric field directly on the zone of chemical reactions, which leads to deformation of the flame front.

Flame-Flow Interaction in a Turbulent Premixed Flame (Fluid Mechanical Structure of the Flamelet)

An LDV system was employed to measure three components of velocity at a mid-flame point on the centerline of a propane-air flame of equivalence ratio 1.10 on a 26 mm-diameter Bunsen burner with fully developed turbulent pipe flow upstream at a Reynolds number of 6700. The LDV measurement volume was 1 mm below the leading electrode of a four-element electrostatic probe that provided the velocity and direction of motion of flamelets in the turbulent flame brush. It was found that when the coordinate system was rotated about a vertical axis to the plane of the flamelet velocity vector, most of the burnt-gas motion following flamelet passage remained largely in this plane. Moreover, within the accuracy of the LDV measurements, profiles of changes of the component of gas velocity normal to the flamelet agreed with velocity profiles calculated for steady, planar, unstrained, premixed laminar flames. These results indicate relatively little distortion of laminar flamelet structures in the turbulent flame brush at these turbulent intensities of about 5 %.

Influence of pressure distribution on flow field temperature reconstruction

This research proposes an issue that has previously been omitted in flow field temperature reconstruction by optical computerized tomography (OCT). To prove that it is not reasonable to always assume an isobaric process occurs when OCT is adopted to obtain the temperature distributions of flow fields, a propane-air flame and an argon arc plasma are chosen as two practical examples for experiment. In addition, the measurement of the refractive index is achieved by moiré deflection tomography. The results indicate that the influence of pressure distribution on temperature reconstruction is a universal phenomenon for various flow fields. Hence, the condition that can be introduced to estimate when an isobaric process can no longer be assumed is presented. In addition, an equation is offered to describe the temperature reconstruction imprecision that is caused by using the supposed pressure instead of the practical pressure.

Self-Turbulent Flame Speeds

This paper reports an experimental investigation of premixed propane and methane-air flames propagating freely in tubes 1.5 m long and with diameters ranging from 26 to 141 mm. The thermo-acoustic instability was eliminated by means of a novel acoustic absorber placed at the closed end of the tube. We first remark that the flame can adopt different shapes either quasi-axisymmetric and normal to the mean direction of propagation, or inclined with a larger propagation speed because of the increase in flame surface area. The minima of the propagation speeds, corresponding to non-tilted flame propagation, are then analyzed using analytical models for the self-turbulent flame propagation. The concept of a cut-off wavelength appears to be relevant to explain the different behaviors observed on the rich side of methane-air and propane-air flames.

Laminar premixed flame fuel consumption rate modulation by shocks and expansion waves

The main contributing effects leading to premixed laminar flame fuel consumption rate changes following variable strength shock or expansion wave passage were studied analytically and numerically. The effects were separated into two groups: gas compression, or one-dimensional effects, and flame front distortion, or two-dimensional effects. The first group was examined analytically using a one-dimensional flame model and solution of shock and expansion wave refraction. Two-dimensional effects were examined numerically using both shock and expansion wave passage through a sinusoidally perturbed propane-air laminar flame. Flame fuel consumption amplification due to the gas compression increased rapidly (from 1.3 to 24 times for Mach 1.1–1.7 shocks), while flame length amplification peaked (∼15 times) at a given shock strength (∼Mach 1.3). Thus gas compression became the dominant effect for stronger shocks. In contrast to the shocks, a slower reaction kinetic produced by the expansion wave passage allowed for a significantly longer period of flame length growth. This resulted in a temporary burning rate increase above initial values. The negative chemical kinetic effect of the weak expansion wave passage was temporarily offset by the flame length increase. The overall reaction rate amplification was up to four times stronger in fast/slow refractions. Also, the observed values of the flame stretch are reported in the paper.

ChemInform Abstract: Kinetic Modeling of Propane Oxidation and Pyrolysis

Propane oxidation in jet-stirred reactor was modeled using a comprehensive kinetic reaction mechanism including the most recent findings concerning the kinetics of the reactions involved in the oxidation of C1C4 hydrocarbons. The present detailed mechanism is able to reproduce experimental species concentration profiles obtained in our high-pressure jet-stirred reactor (900 ⩽ T/K ⩽ 1200; 1 ⩽ P/atm ⩽ 10; 0.15 ⩽ ϕ ⩽ 4) and in a turbulent flow reactor at 1 atm; ignition delay times measured in shock tube (1200 ⩽ T/K ⩽ 1700; 2 ⩽ P/atm ⩽ 15; 0.125 ⩽ ϕ ⩽ 2); H-atoms concentrations measured in shock tube during the pyrolysis of propane and burning velocities of freely propagating premixed propane-air laminar flames. The computed results are discussed in terms of pressure and equivalence ratio (ϕ) effects on propane oxidation. The same detailed kinetic reaction mechanism can also be used to model the oxidation of methane, ethylene, ethane, and propene in similar conditions. © John Wiley & Sons, Inc.

Flame dynamics of a meso-scale heat recirculating combustor

The dynamics of premixed propane–air flame in a meso-scale ceramic combustor has been examined here. The flame characteristics in the combustor were examined by measuring the acoustic emissions and preheat temperatures together with high-speed cinematography. For the small-scale combustor, the volume to surface area ratio is small and hence the walls have significant effect on the global flame structure, flame location and flame dynamics. In addition to the flame–wall thermal coupling there is a coupling between flame and acoustics in the case of confined flames. Flame–wall thermal interactions lead to low frequency flame fluctuations (∼100 Hz) depending upon the thermal response of the wall. However, the flame–acoustic interactions can result in a wide range of flame fluctuations ranging from few hundred Hz to few kHz. Wall temperature distribution is one of the factors that control the amount of reactant preheating which in turn effects the location of flame stabilization. Acoustic emission signals and high-speed flame imaging confirmed that for the present case flame–acoustic interactions have more significant effect on flame dynamics. Based on the acoustic emissions, five different flame regimes have been identified; whistling/harmonic mode, rich instability mode, lean instability mode, silent mode and pulsating flame mode.

Synergism and antagonism in the action of powder inhibitors on propane-air flames

Experimental data on the flame-extinguishing ability of multicomponent mixtures of powder inhibitors regarding propane-air flames are presented. It is demonstrated that mixtures exhibiting synergism and high flame-extinguishing ability can be used as the active component in fire-fighting powders (mixtures of potassium carbonate with sodium fluoride or sodium hydrocarbonate). The procedure for selecting the components of effective multicomponent mixtures is analyzed.

Blowoff dynamics of bluff body stabilized turbulent premixed flames

This article concerns the flame dynamics of a bluff body stabilized turbulent premixed flame as it approaches lean blowoff. Time resolved chemiluminescence imaging along with simultaneous particle image velocimetry and OH planar laser-induced fluorescence were utilized in an axisymmetric bluff body stabilized, propane-air flame to determine the sequence of events leading to blowoff and provide a quantitative analysis of the experimental results. It was found that as lean blowoff is approached by reduction of equivalence ratio, flame speed decreases and the flame shape progressively changes from a conical to a columnar shape. For a stably burning conical flame away from blowoff, the flame front envelopes the shear layer vortices. Near blowoff, the columnar flame front and shear layer vortices overlap to induce high local stretch rates that exceed the extinction stretch rates instantaneously and in the mean, resulting in local flame extinction along the shear layers. Following shear layer extinction, fresh reactants can pass through the shear layers to react within the recirculation zone with all other parts of the flame extinguished. This flame kernel within the recirculation zone may survive for a few milliseconds and can reignite the shear layers such that the entire flame is reestablished for a short period. This extinction and reignition event can happen several times before final blowoff which occurs when the flame kernel fails to reignite the shear layers and ultimately leads to total flame extinguishment.

Flamelet Motion in Premixed Turbulent Axisymmetric Rod-Stabilized and Bunsen Flames

A four-element electrostatic probe was employed to examine flamelet motion in axisymmetric propane-air flames at normal atmospheric pressure and temperature. Besides a number of turbulent (rim-stabilized) Bunsen flames, a turbulent V-shaped flame, stabilized by a vertical rod placed along the axis of the Bunsen-type burner, was investigated. Measurements were conducted at three different positions in the Bunsen flames (i.e., one on the centerline and two off axis, the lowest being at half the height of the centerline position) and at two positions in the V flame (roughly corresponding to the off-axis positions of the Bunsen flames). For the Bunsen flames, measurements were made at three different propane-air equivalence ratios, two of them (one lean and the other rich) having the same laminar burning velocity. To aid in interpretation of the propane-air results for these last two measurements, Bunsen-flame measurements also were made for two (lean and rich) pairs of methane-air flames having the same laminar burning velocities. Fuel flow rates and turbulence intensities and scales were essentially the same in all experiments (fully developed turbulent pipe flow at a Reynolds number of 7000, with a ratio of mean velocity to laminar burning velocity on the order of ten). In all cases, scatterplots for flamelet velocity vectors were obtained when flamelets passed the probe in the direction of fresh-to-burnt mixtures and, separately, of burnt-to-fresh mixtures. Correlations between magnitudes and directions of flamelet velocities were measured, and probability distributions were generated for magnitudes and for directions of flamelet velocities. The results showed significant differences in distributions of magnitudes and of directions for fresh-to-burnt and for burnt-to-fresh passage. They also showed differences between the lean and rich propane flames but not between the lean and rich methane-flame pairs, the characteristics of which tended to resemble those of the lean propane flames. Finally, flamelet motions for the V flame were noticeably different in many respects from those for the Bunsen flames, as explained by the different effects of thermal expansion on the mean flow in the two configurations, the fresh mixture being radially on the inner side of the Bunsen flames and on the outer side of the V flame. The exceptional characteristics of rich propane were explained by nonlinear enhancement of the average flamelet burning velocity through preferential oxygen diffusion to turbulence-induced flamelet bulges convex toward the fresh mixture. The differences in the probability distributions for the two passage directions were thought to arise from the laminar burning velocity measured in the burnt gas being not negligible in comparison with average flamelet velocities, thereby influencing distribution shapes for burnt-to-fresh passage.

Propagation and Extinction of a Cylindrical Premixed Flame Undergoing Equivalence Ratio Fluctuation Near the Lean Limit

Experimental study was made to investigate the propagation and extinction characteristics of a stretched cylindrical flame undergoing periodic fluctuation of equivalence ratio near the lean limit. With a lean methane-air and a lean propane-air mixture, burning velocity, flame luminosity and flame stretch rate were measured or evaluated for the fluctuation frequencies of 5Hz and 20Hz. The results were summarized as follows: (1) In some part of a period, burning velocity and flame luminosity of the dynamic flame near the lean limit were possible to become lower than those at the lean flammability limit of the static flame. (2) At the high frequency of 20Hz, the burning velocity took a negative value in a certain time range. In spite of this loss of propagation ability, the flame was not extinguished but sustained, indicating the recovery of the flame intensity due to the dynamic effect of fluctuating flame. (3) Flame recovery phenomenon could occur more easily for the methane flame which was strengthened by the Lewis number effect than the propane flame which was weakened by that effect.

Radiative heat transfer during turbulent combustion process

We investigate the radiative heat transfer in a co-flowing turbulent nonpremixed propane-air flame inside a three-dimensional cylindrical combustion chamber. The radiation from the luminous flame, which is due to the appearance of soot particles in the flame, is studied here, through the balance equation of radiative transfer which is solved by the Discrete Ordinates Method (DOM) coupling with a Large Eddy Simulation (LES) of the flow, temperature, combustion species and soot formation. The effect of scattering is ignored as it is found that the absorption dominates the radiating medium. Assessments of the various orders of DOM are also made and we find that the results of the incident radiation predicted by the higher order approximations of the DOM are in good agreement.

Study on nitrogen diluted propane–air premixed flames at elevated pressures and temperatures

Using a high pressure constant volume combustion vessel, the propagation and morphology of spark-ignited outwardly expanding nitrogen diluted propane–air flames were imaged and recorded by schlieren photography and high-speed digital camera. The unstretched laminar burning velocities and Markstein lengths were subsequently determined over wide range of initial temperatures, initial pressures and nitrogen dilution ratios. Two recently developed mechanisms were used to predict the reference laminar burning velocity. The results show that the measured unstretched laminar burning velocities agree well with those in the literature and the computationally predicted results. The flame images show that the diffusional–thermal instability is promoted as the mixture becomes richer, and the hydrodynamic instability is increased with the increase of the initial pressure and it is decreased with the increase of dilution ratio. The normalized laminar burning velocities show a linear correlation with respect to the dilution ratio, indicating that the effect of nitrogen dilution is more obvious at higher pressures.

The impact of equivalence ratio oscillations on combustion dynamics in a backward-facing step combustor

The combustion dynamics of propane–air flames are investigated in an atmospheric pressure, atmospheric inlet temperature, lean, premixed backward-facing step combustor. We modify the location of the fuel injector to examine the impact of equivalence ratio oscillations arriving at the flame on the combustion dynamics. Simultaneous pressure, velocity, heat-release rate and equivalence ratio measurements and high-speed video from the experiments are used to identify and characterize several distinct operating modes. When the fuel is injected far upstream from the step, the equivalence ratio arriving at the flame is steady and the combustion dynamics are controlled only by flame–vortex interactions. In this case, different dynamic regimes are observed depending on the operating parameters. When the fuel is injected close to the step, the equivalence ratio arriving at the flame exhibits oscillations. In the presence of equivalence ratio oscillations, the measured sound pressure level is significant across the entire range of lean mean equivalence ratios even if the equivalence ratio oscillations arriving at the flame are out-of-phase with the pressure oscillations. The combustion dynamics are governed primarily by the flame–vortex interactions, while the equivalence ratio oscillations have secondary effects. The equivalence ratio oscillations could generate variations in the combustion dynamics in each cycle under some operating conditions, destabilize the flame at the entire range of the lean equivalence ratios, and increase the value of the mean equivalence ratio at the lean blowout limit.

Universal calibration of quantitative OH-LIPF measurements in hydrocarbon flames at elevated pressures

Quantitative measurements of radical concentrations are important in studies of the structure of flames. In this research, a quantitative analysis was performed using laser-induced predissociative fluorescence of OH radicals (OH-LIPF) in high-pressure (1–5 atm) premixed methane-air and propane-air flat flames (ϕ = 0.7–1.3). OH was excited (A2Σ+, ν′ = 3 − X2Π, ν″=0, and P28) using a KrF excimer laser and (3,2) band fluorescence was observed. OH fluorescence intensities were calibrated against the OH concentrations calculated from flame simulation results in the postflame zone using the CHEMKIN premix flame code in conjunction with the GRI-Mech 3.0 reaction mechanism. The accuracy and spatial resolution of temperature measurements are important factors for the correctness of the corresponding flame simulations, especially in the reaction zone near the burner surface. In this work, a carefully constructed thermocouple (R-type, 50 µm) positioning system was used to identify the temperatures above the burner surface. With careful evaluations of quenching rates, Voigt profiles, and normalization against room-air N2 Raman scattering intensity, a universal calibration constant [CT = (1.076 ± 0.174) · 1016 molecules/cm3] was determined. The OH concentrations obtained by flame simulations showed good agreement with the quantitative OH-LIPF measurements in all methane-air flames and fuel lean (ϕ = 0.7−0.8) propane-air flames. However, a 2-fold to 5-fold discrepancy was obtained in propane flames at ϕ > 0.9. This may be caused by the lack of C3 reaction paths in the GRI mechanism and/or the inaccuracy of the thermochemical data for large molecules.

Stabilization of an inverted propane-air flame on a string stretched along the flow

The stabilization conditions for an inverted flame on a long thin string stretched along the cylindrical burner axis were studied experimentally. Gas temperature distributions in the inverted flame are obtained. The boundaries of stable combustion that are simultaneously the condition of excitation of acoustic self-oscillations of the flame are found. Inverted flame blow-off velocities are measured, and the variation of its geometrical characteristics are studied. It is found that during overturning of the flame relative to the vector of the acceleration due to gravity, stabilization of the inverted flame in the open atmosphere is impossible. The physical mechanism involved in the formation of the inverted flame in flow parallel to the stabilizer surface is considered. The role of the hydrodynamic stretching of the flame in flame blow-off and extinction is determined.

Effects of hydrogen addition on cellular instabilities of the spherically expanding propane flames

Abstract An experimental study on the effects of hydrogen addition on the instabilities of spherically expanding propane–air flames was conducted in a constant volume combustion vessel over a wide range of mixture compositions and initial temperatures and pressures. The measured laminar burning velocities were compared with those calculated values by using one dimensional freely propagating flames and a recently developed detailed kinetic mechanism. Good agreement was obtained between the experiment and calculation. The schlieren images show that for lean mixture combustion, hydrogen addition will increase the hydrodynamic instability due to the decreased flame thickness and increase the diffusional-thermal instability due to the decreased Lewis number. While for rich mixture combustion, the flame front is initially destabilized and later tends to the stabilized with the increase of hydrogen fraction. This is due to the competing effects of the hydrodynamic instability and the diffusional-thermal instability.

乱流予混合火炎で観察される選択拡散の痕跡(熱工学,内燃機関,動力など)

Effect of the preferential diffusion between oxygen and fuel molecules is observed as the cellular flame in laminar premixed flames. When the diffusion coefficient of the deficient species is larger, the deficient species diffuses preferentially to the reaction zone to cause the local stratification of the mixture ratio along the flame. Therefore, the cellular flame is established. If the preferential diffusion should occur in propane-air turbulent premixed flames, the wrinkle scale of rich flames would be smaller than that of lean flames, consequently, the turbulent burning velocity and/or the heat release rate of the former would be larger than those of the latter. Based on these considerations, traces of the preferential diffusion are sought from experimental results of propane-air turbulent premixed flames. Comparing the radius of curvature of the flamelet of rich and lean propane-air flames with the same laminar burning velocity, the radius of curvature of the flamelet of the former are smaller than those of the latter. The gas expansion ratio across the flamelet of the former is larger than that in the latter. The magnitudes of the flamelet velocity vector of the former are larger than the latter.

Conditional moment closure modelling of soot formation in turbulent, non-premixed methane and propane flames

Presented are results obtained from the incorporation of a semi-empirical soot model into a first-order conditional moment closure (CMC) approach to modelling turbulent, non-premixed methane–air and propane–air flames. Soot formation is determined via the solution of two transport equations for soot mass fraction and particle number density, with acetylene and benzene employed as the incipient species responsible for soot nucleation, and the concentrations of these calculated using a detailed gas-phase kinetic scheme involving 70 species. The study focuses on the influence of differential diffusion of soot particles on soot volume fraction predictions. The results of calculations are compared with experimental data for atmospheric and 3 atm methane flames, and propane flames with air preheated to 323 K and 773 K. Overall, the study demonstrates that the model, when used in conjunction with a representation of differential diffusion effects, is capable of accurately predicting soot formation in the turbulent non-premixed flames considered.

Some Features of Propane-Air Flames Under Quenching Conditions in Narrow Channels

Knowledge of flame–wall interaction helps in the understanding of flame phenomena near a wall and of flame extinction. In the present work different flame characteristics were measured in square, narrow quenching channels. A propane/air mixture was employed and direct visualization was used to observe flame behavior under quenching conditions. Flame propagation velocity was determined during downward propagation. As expected, close to quenching condition it was found to be lower than that under adiabatic conditions and close to the values predicted by the quenching theory. The dead space appeared to be larger for rich mixtures than that found with lean ones. Flame curvature reached its maximum value for stoichiometric mixtures decreased for both leaner and richer mixtures. Numerical simulation revealed the structure of limit flames during their propagation in quenching channels. It was confirmed that flame quenching depends on the relation between heat release rate and heat loss rate.