Electric fields effect on liftoff and blowoff of nonpremixed laminar jet flames in a coflow

The stabilization characteristics of liftoff and blowoff in nonpremixed laminar jet flames in a coflow have been investigated experimentally for propane fuel by applying AC and DC electric fields to the fuel nozzle with a single-electrode configuration. The liftoff and blowoff velocities have been measured by varying the applied voltage and frequency of AC and the voltage and the polarity of DC. The result showed that the AC electric fields extended the stabilization regime of nozzle-attached flame in terms of jet velocity. As the applied AC voltage increased, the nozzle-attached flame was maintained even over the blowout velocity without having electric fields. In such a case, a blowoff occurred directly without experiencing a lifted flame. While for the DC cases, the influence on liftoff was minimal. There existed three different regimes depending on the applied AC voltage. In the low voltage regime, the nozzle-detachment velocity of either liftoff or blowoff increased linearly with the applied voltage, while nonlinearly with the AC frequency. In the intermediate voltage regime, the detachment velocity decreased with the applied voltage and reasonably independent of the AC frequency. At the high voltage regime, the detachment was significantly influenced by the generation of discharges.     

Observation of multi-scale oscillation of laminar lifted flames with low-frequency AC electric fields

The oscillation behavior of laminar lifted flames under the influence of low-frequency AC has been investigated experimentally in coflow jets. Various oscillation modes were existed depending on jet velocity and the voltage and frequency of AC, especially when the AC frequency was typically smaller than 30 Hz. Three different oscillation modes were observed: (1) large-scale oscillation with the oscillation frequency of about 0.1 Hz, which was independent of the applied AC frequency, (2) small-scale oscillation synchronized to the applied AC frequency, and (3) doubly-periodic oscillation with small-scale oscillation embedded in large-scale oscillation. As the AC frequency decreased from 30 Hz, the oscillation modes were in the order of the large-scale oscillation, doubly-periodic oscillation, and small-scale oscillation. The onset of the oscillation for the AC frequency smaller than 30 Hz was in close agreement with the delay time scale for the ionic wind effect to occur, that is, the collision response time. Frequency-doubling behavior for the small-scale oscillation has also been observed. Possible mechanisms for the large-scale oscillation and the frequency-doubling behavior have been discussed, although the detailed understanding of the underlying mechanisms will be a future study.     

The non-linear thermo-acoustic response of a small swirl burner

The non-linear response of a swirl stabilised, lean premixed flame (CH₄/air) was determined by forcing the flame acoustically at frequencies between 40 and 200 Hz with increasing amplitude. Measuring the chemiluminescent emission from OH¹ with a photodiode sensor and calculating the flame transfer function, a linear response to increasing amplitude was observed at 40 and 60 Hz for all amplitudes with an equivalence ratio ? = 0.56. However, between 80 and 200 Hz the flame response exhibited non-linear characteristics for r.m.s velocity fluctuations greater than 20% of the mean flow velocity. With ? = 0.48, even 60 Hz became non-linear. Phase-locked Particle Image Velocimetry and Intensified CCD imaging were deployed at three amplitudes for detailed study of the flame and flow field response to forcing. At low frequencies the flow field was characterised by a pulsating inner recirculation zone, whilst at all frequencies the outer recirculation zone was modified by vortices rolling up the annular jet. As the forcing amplitude was increased, the effect on the flame shape became more pronounced, with large variations in flame volume at low frequencies and flame extinction due to stretching of the flame around the roll-up vortices at the higher frequencies. The results indicate different driving mechanisms behind the flame response at low and high frequencies. At low frequencies the flame response is governed by equivalence ratio fluctuations due to the ‘stiff’ fuel system and the volumetric fluctuations of the input air. At the higher frequencies the response is governed by flow field features such as vortex roll-up.     

Impact of co-flow air on buoyant diffusion flames flicker

This paper describes experimental investigation of co-flow air velocity effects on the flickering behaviour of laminar non-lifted methane diffusion flames. Chemiluminescence, high-speed photography, schlieren and Particle Imaging Velocimetry (PIV), have been used to study the changes in the flame/vortex interactions as well as the flame flickering frequency and magnitude by the co-flow air. Four cases of methane flow rates at different co-flow air velocities are investigated. It has been observed that the flame dynamics and stability of co-flow diffusion flames are strongly affected by the co-flow air velocity. When the co-flow velocity has reached a certain value the buoyancy driven flame oscillation was completely suppressed. The schlieren and PIV imaging have revealed that the co-flow of air is able to push the initiation point of the outer toroidal vortices beyond the visible flame to create a very steady laminar flow region in the reaction zone. Then the buoyancy driven instability is only effective in the plume of hot gases above the visible flame. It is observed that a higher co-flow rate is needed in order to suppress the flame flickering at a higher fuel flow rate. Therefore the ratio of the air velocity to the fuel velocity, γ, is a stability controlling parameter. The velocity ratio, γ, was found to be 0.72 for the range of tested flow rates. The dominant flickering frequency was observed to increase linearly with the co-flow rate (a) as; f = 0.33a + 11. The frequency amplitudes, however, were observed to continuously decrease as the co-flow air was increasing.     

Burner platform for sub-atmospheric pressure flame studies

Hencken burner flames at sub-atmospheric pressure were characterized experimentally to show their unique structure for detailed flame studies. Methane–air flames at 16.7 kPa were shown to be lifted and stably anchored at significant distances (up to 18 mm) above the burner surface, while maintaining a flat and one-dimensional laminar structure and near adiabatic conditions. Particle image velocimetry was used to identify the weakly stretched regime (strain rate = 20–70 s^?1) of the flames, as well as the flame speeds, while OH number densities were measured through laser-induced fluorescence and calibrated through absorption. The flame speeds and quantitative OH profiles were compared to one-dimensional and two-dimensional flame simulations using the chemical kinetic mechanisms of USC Mech II and GRI-3.0 and showed good agreement. Flames produced by a Hencken burner at sub-atmospheric pressure were shown to accurately represent a steady, laminar, nearly one-dimensional, minimally curved, weakly stretched, and near adiabatic flame, which could be compared to one-dimensional freely-propagating flame simulations with minimal corrections and extrapolations.     

Flame-flow interactions and flow reversal

The interaction of a premixed methane/air flame with flow unsteadiness is studied computationally using a stagnation-point flow configuration. The problem is of fundamental interest and also relevant for turbulent combustion in the laminar flamelet regime. In the present study, of particular interest is the flame residing in a weakly strained flow field such that the flame is stabilized away from the viscous boundary layer adjacent to the stagnation plane and is free to move in response to flow perturbations. An unsteady sinusoidal strain rate field is imposed on the flame, and an extensive parametric study is conducted by varying the frequency and amplitude of strain rate fluctuation. It is found that for high frequencies and large amplitudes, flow direction reverses upstream of the flame, thereby establishing a new stagnation plane in the preheat zone ahead of the flame. This observation indicates that the flame strongly affects the upstream flow field and could also possibly explain the reported occurence of flow reversal in experimental studies of turbulent jet flames. Effects of other key parameters such as the Lewis number, mean flame speed, and gas expansion ratio on flame-flow interaction and flow reversal are studied by investigating highly lean (Le < 1) and rich (Le > 1) hydrogen/air flames. The physical mechanism responsible for flow reversal phenomenon is explained.     

Structure of laminar premixed flames of methane near the auto-ignition limit

Auto-ignition and flame propagation are the two different controlling mechanisms for stabilizing the flame in secondary stage combustion in hot vitiated air environment and at elevated pressure. The present work aims at the investigation of the flame stabilization mechanism of flames developing in such an environment. In order to better understand the structure of turbulent flames at inlet temperature well above the auto-ignition temperature, the behavior of laminar flames at those conditions needs to be analyzed. As an alternative to challenging and expensive measurements at high temperature and pressure, the behavior of laminar flames at such conditions can be predicted from theory using mathematical simulation. In the present work, the laminar burning velocities and flame structures of premixed stoichiometric methane/air mixtures for inlet temperatures from 300 to 1450 K and absolute pressures from 1 to 8 bar have been calculated using a freely propagating laminar, one dimensional, planar flame model. The prediction shows that at inlet temperatures below the auto-ignition temperature, the predicted laminar burning velocity which corresponds to the unburned mixture velocity in order to create a steady laminar flame decreases with increase in pressure. When the inlet temperature of the mixture goes well beyond the auto-ignition temperature of the mixture, however, the unburned mixture velocity increases steeply at higher pressure level, because of a complete transition of the flame structure.     

Influence of fuel fraction gradient on triple flame velocity in plain and axis-symmetrical channels

Dynamics of laminar triple flame investigated numerically for the different mixture degrees. One-step methane–air chemistry adequate to reach and lean mixture combustion was accepted. Velocity of triple flame is determined as a function of methane concentration logarithm gradients μ = d(ln Y₁)/dx (characterizing mixing degree). It is found that maximum velocity of the triple flames correspond to the value of the methane concentration logarithm gradients μ ≈ 1000 m^?1 for plain and μ ≈ 2000 m^?1 for axis-symmetrical channels. The maximum velocity of triple flame in plain and axis-symmetrical channels in the case of non-gradient incoming gas flow is about twice bigger than normal laminar flame velocity S_f ≈ 2.1S_l.     

Computed NOx emission characteristics of opposed-jet syngas diffusion flames

This paper reported a numerical study on the NO_x emission characteristics of opposed-jet syngas diffusion flames. A narrowband radiation model was coupled to the OPPDIF program, which used detailed chemical kinetics and thermal and transport properties to enable the study of 1-D counterflow syngas diffusion flames with flame radiation. The effects of syngas composition, pressure and dilution gases on the NO_x emission of H₂/CO synthetic mixture flames were examined. The analyses of detailed flame structures, chemical kinetics, and nitrogen reaction pathways indicate NO_x are formed through Zeldovich (or thermal), NNH and N₂O routes both in the hydrogen-lean and hydrogen-rich syngas flames at normal pressure. Zeldovich route is the main NO formation route. Therefore, the hydrogen-rich syngas flames produce more NO due to higher flame temperatures compared to that for hydrogen-lean syngas flames. Although NNH and N₂O routes also are the primary NO formation paths, a large amount of N₂ will be reformed from NNH and N₂O species. For hydrogen-rich syngas flames, the NO formation from NNH and N₂O routes are lesser, where NO can be dissipated through the reactions of NH + NO → N₂ + OH and NH + NO → N₂O + H more actively. At a rather low pressure (0.01 atm), NNH-intermediate route is the only formation path of NO. Increasing pressure then enhances NO formation reactions, especially through Zeldovich mechanisms. However, at higher pressures (5–10 atm), NO is then converted back to N₂ through reversed N₂O route for hydrogen-lean syngas flames, and through NNH as well for hydrogen-rich syngas flames. In addition, the dilution effects from CO₂, H₂O, and N₂ on NO emissions for H₂/CO syngas flames were studied. The hydrogen-lean syngas flames with H₂O dilution have the lowest NO production rate among them, due to a reduced reaction rate of NNH + O → NH + NO. But for hydrogen-rich syngas flames with CO₂ dilution, the flame temperatures decrease significantly, which leads to a reduction of NO formation from Zeldovich route.     

Swirl effects on harmonically excited,premixed flame kinematics

This paper describes the response of a swirling premixed flame with constant burning velocity to non-axisymmetric harmonic excitation. This work extends prior studies of axisymmetric forcing, which have shown that wrinkles are excited on the flame that propagate downstream along the mean flame surface at a speed given by U_o cos ψ, where U_o is the mean flow velocity and ψ is the flame angle. The swirl component in the flow field introduces an azimuthal transport mechanism for disturbances on the flame. As such, the flame response at any given position is a superposition of flame wrinkles excited at earlier times, upstream axial locations, and different azimuthal positions. These swirl transport effects do not arise in problems where axisymmetric flames are subjected to axisymmetric excitation, but enter quite prominently in the presence of non-axisymmetries, such as when the flame is subjected to transverse excitation. The solution characteristics are strongly dependent upon the ratio of angular rotation rate to excitation frequency, denoted by σ = Ω/ω, which describes the fraction of azimuthal rotation a disturbance makes in one acoustic period. When σ ? 1 and σ ? 1, the axial wavelength of flame wrinkles scales with the convective wavelength, λ_c, but becomes much longer for σ ～ O(1). The spatial variation in phase of flame wrinkling is also strongly dependent upon σ. Regardless of swirl number, flame wrinkles propagate in helical spirals along the solution characteristics at a phase speed equal to the local tangential velocity. The axial phase characteristics of flame wrinkling at a fixed azimuthal location, as would be measured by laser sheet imaging, are much more complex. For σ < 1, the wrinkles exhibit the familiar negative roll-off character for the phase with axial downstream distance, indicative of an axially convecting disturbance. The slope of this phase roll-off decreases with increasing σ, however, and becomes zero at σ = 1 for a compact flame. For σ > 1, the wrinkles actually have a positive roll-off character for the phase with axial downstream distance, indicating a flame wrinkle with a negative trace velocity, but whose actual propagation velocity is positive. Finally, these results show that while the flame response to transverse acoustic excitation is quite strong locally, its spatially integrated effect is much smaller for acoustically compact flames. This suggests that the dominant mechanism through which the flame responds globally to transverse excitation is the induced vortical and longitudinal acoustic fluctuations.     

Flame stabilization between two beds of alumina balls in a porous burner

Conditions for flame stabilization in a porous media combustor formed by two beds of different sizes of alumina balls were studied. Premixed combustion of lean methane–air mixtures were used as variables. Measurements performed included temperature profiles and chemical products compositions. Stabilized flames were observed in the range of volumetric flow rate from 7.01 l/min to 19.00 l/min at equivalence ratio of ? = 0.6 and ? = 0.7. Low pollutants emissions were found in the entire operation range.     

Effect of pulsed,sub-breakdown applied electric field on propane/air flame through simultaneous OH/acetone PLIF

The effects of chemi-ion current induced flow perturbations in a premixed, laminar propane/air flame at atmospheric pressure have been measured with 30 ms-wide applied pulsed voltages. Single-shot OH and acetone planar laser-induced fluorescence (PLIF) images have been collected to measure the spatio-temporal structural changes to a laminar flame with incoming flow speed of 2 m/s in response to positive polarity voltage pulses of 2.8 kV over a 20 mm electrode gap. OH and acetone PLIF are specifically chosen to measure reaction zone modification as the flame undergoes large-scale, stochastic changes. These large-scale changes of flame structure are observed after the flame becomes fully crushed and unstable behavior occurs lasting until the end of the applied voltage pulse. The experimental results of combined OH and acetone PLIF presented in this paper show a significant widening of the reaction zone observed during this unstable behavior. This widening of the reaction zone is indicative of a flame brush normally observed in turbulent flames, demonstrating the ability of the sub-breakdown applied voltage to cause a laminar flame to a transitioning-to-turbulent behavior.     

Analysis of off-grid hybrid wind turbine/solar PV water pumping systems

While many remote water pumping systems exist (e.g. mechanical windmills, solar photovoltaic, wind-electric, diesel powered), few combine both the wind and solar energy resources to possibly improve the reliability and the performance of the system. In this paper, off-grid wind turbine (WT) and solar photovoltaic (PV) array water pumping systems were analyzed individually and combined as a hybrid system. The objectives were to determine: (1) advantages or disadvantages of using a hybrid system over using a WT or a solar PV array alone; (2) if the WT or solar PV array interfered with the output of the other; and (3) which hybrid system was the most efficient for the location. The WT used in the analysis was rated at 900 W alternating current (AC). There were three different solar PV arrays analyzed, and they were rated at 320, 480, and 640 W direct current (DC). A rectifier converted the 3-phase variable voltage AC output from the WT to DC before combining it with the solar PV array DC output. The combined renewable energies powered a single helical pump. The independent variable used in the hybrid WT/PV array analysis was in units of W/m². The peak pump efficiency of the hybrid systems at Bushland, TX occurred for the 900 W WT combined with the 640 W PV array. The peak pump efficiencies at a 75 m pumping depth of the hybrid systems were: 47% (WT/320 W PV array), 51% (WT/480 W PV array), and 55% (WT/640 W PV array). Interference occurred between the WT and the different PV arrays (likely due to voltage mismatch between WT and PV array), but the least interference occurred for the WT/320 W PV array. This hybrid system pumped 28% more water during the greatest water demand month than the WT and PV systems would have pumped individually. An additional controller with a buck/boost converter is discussed at end of paper for improvement of the hybrid WT/PV array water pumping system.     

HEV battery heating using AC currents

A unique method has been developed for internally heating hybrid electric vehicle (HEV) batteries at cold temperatures using alternating current (AC). The poor performance of these batteries in cold climates is of major concern because they suffer a huge loss in capacity. Another symptom of this low performance is a dramatic increase in the series resistance of the battery, R_B, as the temperature drops. Experiments were performed with both low and high frequency heaters, and several tests were conducted on both lead acid and nickel metal hydride (NiMH) batteries at different AC amplitudes, states of charge (SOCs) and cold temperatures. Low frequency 60 Hz heating was first tested on several different 38 Ah lead acid batteries. The feasibility of using high frequency heating was then tested using a 10–20 kHz inverter on a pack of 6.5 Ah nickel metal hydride (NiMH) batteries. A technique also was developed to estimate the internal battery temperature, T_bat, by measuring the battery source resistance, R_B.     

Flame structure and kinetic studies of carbon dioxide-diluted dimethyl ether flames at reduced and elevated pressures

The flame structure and kinetics of dimethyl ether (DME) flames with and without CO₂ dilution at reduced and elevated pressures were studied experimentally and computationally. The species distributions of DME oxidation in low-pressure premixed flat flames were measured by using electron-ionization molecular-beam mass spectrometry (EI-MBMS) at an equivalence ratio of 1.63 and 50 mbar. High-pressure flame speeds of lean and rich DME flames with and without CO₂ dilution were measured in a nearly-constant-pressure vessel between about 1 and 20 bar. The experimental results were compared with predictions from four kinetic models: the first was published by Zhao et al. (2008) [9], the second developed by the Lawrence Livermore National Laboratory (LLNL) (Kaiser et al., 2000) [13], and the third has been made available to us as the Aramco mechanism (Metcalfe et al., 2013) [14]; as the fourth, we have used an updated model developed in this study. Good agreement was found between measurements and predictions from all four models for all major and most typical intermediate species with and without CO₂ addition in low-pressure flat flame experiments. However, none of the models was able to reliably predict high-pressure flame speeds. Although the updated model improved the prediction of flame speeds for lean mixtures, errors remained for rich conditions at elevated pressure, likely due to uncertainty in the rates of CH₃ + H(+M) = CH₄(+M) and the branching and termination reaction pair of CH₃ + HO₂ = CH₃O + OH and CH₃ + HO₂ = CH₄ + O₂. CO₂ addition considerably decreased the flame speed. Kinetic comparisons between inert and chemically active CO₂ in DME flames showed that CO₂ addition affects rich and lean DME flame kinetics differently. For lean flames, both the inert third-body effect and the kinetic effect of CO₂ reduce H-atom production. However, for rich flames, the inert third-body effect increases H-atom production via HCO(+M) = H + CO(+M) and suppression of the kinetic effect of CO₂ by shifting the equilibrium of CO + OH = CO₂ + H.     

Transition to detonation of an expanding flame ring in a sub-millimeter gap

Deflagration-to-detonation transition of a flame ring circularly expanding in a 260 μm gap filled with stoichiometric ethylene/oxygen mixtures initially at atmospheric pressure and temperature has been experimentally visualized. The results show that DDT can occur under the influence of wall confinement even for an expanding flame. DDT could be observed at a distance as short as ～70 mm from the ignition spot, which corresponds to ～130 μs after the ignition spark voltage breakdown. Velocity overshoot of reaction front velocity exceeding Chapman–Jouguet velocity was characterized. Cell structures were observed on the reaction fronts after DDT occurred. The visualizations also showed that smooth circular flame developed right after ignition quickly evolved into wrinkled flame as the flame ring propagated outwards. Flame propagating velocity was accelerated from ～600 m/s to ～1000 m/s during the wrinkled flame stage. A series of local explosion on the flame ring was observed during the DDT process, and resulted in an abrupt surge on reaction front propagation velocity.     

Negative pressure dependence of mass burning rates of H2/CO/O2/diluent flames at low flame temperatures

Experimental measurements of burning rates, analysis of the key reactions and kinetic pathways, and modeling studies were performed for H₂/CO/O₂/diluent flames spanning a wide range of conditions: equivalence ratios from 0.85 to 2.5, flame temperatures from 1500 to 1800 K, pressures from 1 to 25 atm, CO fuel fractions from 0 to 0.9, and dilution concentrations of He up to 0.8, Ar up to 0.6, and CO₂ up to 0.4. The experimental data show negative pressure dependence of burning rate at high pressure, low flame temperature conditions for all equivalence ratios and CO fractions as high as 0.5. Dilution with CO₂ was observed to strengthen the pressure and temperature dependence compared to Ar-diluted flames of the same flame temperature. Simulations were performed to extend the experimentally studied conditions to conditions typical of gas turbine combustion in Integrated Gasification Combined Cycle processes, including preheated mixtures and other diluents such as N₂ and H₂O.Substantial differences are observed between literature model predictions and the experimental data as well as among model predictions themselves – up to a factor of three at high pressures. The present findings suggest the need for several rate constant modifications of reactions in the current hydrogen models and raise questions about the sufficiency of the set of hydrogen reactions in most recent hydrogen models to predict high pressure flame conditions relevant to controlling NO_x emissions in gas turbine combustion. For example, the reaction O + OH + M = HO₂ + M is not included in most hydrogen models but is demonstrated here to significantly impact predictions of lean high pressure flames using rates within its uncertainty limits. Further studies are required to reduce uncertainties in third body collision efficiencies for and fall-off behavior of H + O₂(+M) = HO₂(+M) in both pure and mixed bath gases, in rate constants for HO₂ reactions with other radical species at higher temperatures, and in rate constants for reactions such as O + OH + M that become important under the present conditions in order to properly characterize the kinetics and predict global behavior of high-pressure H₂ or H₂/CO flames.     

Analysis and forecasting of wind velocity in chetumal,quintana roo,using the single exponential smoothing method

In this paper the analysis and forecasting of wind velocities in Chetumal, Quintana Roo, Mexico is presented. Measurements were made by the Instituto de Investigaciones Eléctricas (IIE) during two years, from 2004 to 2005. This location exemplifies the wind energy generation potential in the Caribbean coast of Mexico that could be employed in the hotel industry in the next decade. The wind speed and wind direction were measured at 10 m above ground level. Sensors with high accuracy and a low starting threshold were used. The wind velocity was recorded using a data acquisition system supplied by a 10 W photovoltaic panel. The wind speed values were measured with a frequency of 1 Hz and the average wind speed was recorded considering regular intervals of 10 min. First a statistical analysis of the time series was made in the first part of the paper through conventional and robust measures. Also the forecasting of the last day of measurements was made utilizing the single exponential smoothing method (SES). The results showed a very good accuracy of the data with this technique for an α value of 0.9. Finally the SES method was compared with the artificial neural network (ANN) method showing the former better results.     

Soot formation and temperature structure in small methane–oxygen diffusion flames at subcritical and supercritical pressures

An experimental study was conducted to examine the characteristics of laminar methane–oxygen diffusion flames up to 100 atmospheres. The influence of pressure on soot formation and on the structure of the temperature field was investigated over the pressure range of 10–90 atmospheres in a high-pressure combustion chamber using a non-intrusive, line-of-sight spectral soot emission diagnostic technique. Two distinct zones characterized the appearance of a methane and pure oxygen diffusion flame: an inner luminous zone similar to the methane–air diffusion flames, and an outer diffusion flame zone which is mostly blue. The flame height, marked by the visible soot radiation emission, was reduced by over 50% over the pressure range of 10–100 atmospheres. Between 10 and 40 atmospheres, the soot levels increased with increasing pressure; however, above 40 atmospheres the soot concentrations decreased with increasing pressure.     

Laminar flame speeds of cyclohexane and mono-alkylated cyclohexanes at elevated pressures

The propagation speeds of expanding spherical flames of cyclohexane, methylcyclohexane and ethylcyclohexane in mixtures of oxygen/inert were measured in a heated, dual-chamber vessel, with the corresponding laminar flame speeds extracted from them through nonlinear extrapolation. Measurements were conducted at atmospheric and elevated pressures up to 20 atm. Computational simulations were conducted using the JetSurF 2.0 mechanism, yielding satisfactory agreement with the present measurements at all pressures, with a slight over-prediction at 1 atm. Measurements reveal the following trend for the flame speeds: cyclohexane > n-hexane > methylcyclohexane ≈ ethylcyclohexane at all pressures, with the maximum difference being approximately 5% at 1 atm and 13% at 10 atm. Examination of the computed flame structure shows that owing to its symmetric ring structure, decomposition of cyclohexane produces more chain-branching 1,3-butadiene and less chain-terminating propene. On the contrary, a more balanced distribution of intermediates is present in the flames of methylcyclohexane and ethylcyclohexane due to substitution of the alkyl group for H.