Wide Range Of Equivalence Ratios Research Articles

Laminar burning velocity with oxygen-enriched air of syngas produced from biomass gasification

Several studies on the laminar burning velocity of syngas mixtures have been conducted by various researchers. However, in most of these studies, dry air was used as the oxidizer, whereas very few studies have been conducted on syngas combustion in oxygen – enriched air. In this work, a numerical and experimental study on the laminar burning velocity of a mixture of H2, CO and N2 (20:20:60 vol%) was performed using air enriched with oxygen as the oxidizer, varying the oxygen content from 21% up to 35% for different equivalence ratios. Numerical calculations were conducted using three detailed reaction mechanisms and transport properties. Flames were generated using contoured slot-type nozzle burners, and Schlieren images were used to determine the laminar burning velocity with the angle method. The experiments were performed under the conditions of Medellin (1550 m.a.s.l.), 0.838 atm and 298 K. The laminar burning velocity increases with the concentration of the oxygen in the mixture due to the increase of the reaction rate; for a stoichiometric mixture, the laminar burning velocity increases by almost 25% with an increment of 4% of oxygen in the oxidant. However, the flammability limits also increase, allowing stable flames to exist in a wider range of equivalence ratios.

Experimental and modeling study of the oxidation of n- and iso-butanal

Understanding the kinetics of large molecular weight aldehydes is essential in the context of both conventional and alternative fuels. For example, they are key intermediates formed during the low-temperature oxidation of hydrocarbons as well as during the high-temperature oxidation of oxygenated fuels such as alcohols. In this study, an experimental and kinetic modeling investigation of n-butanal (n-butyraldehyde) and iso-butanal (iso-butyraldehyde or 2-methylpropanal) oxidation kinetics was performed. Experiments were performed in a jet stirred reactor and in counterflow flames over a wide range of equivalence ratios, temperatures, and pressures. The jet stirred reactor was utilized to observe the evolution of stable intermediates and products for the oxidation of n- and iso-butanal at elevated pressures and low to intermediate temperatures. The counterflow configuration was utilized for the determination of laminar flame speeds. A detailed chemical kinetic interpretative model was developed and validated consisting of 244 species and 1198 reactions derived from a previous study of the oxidation of propanal (propionaldehyde). Extensive reaction pathway and sensitivity analysis was performed to provide detailed insight into the mechanisms governing low-, intermediate-, and high-temperature reactivity. The simulation results using the present model are in good agreement with the experimental laminar flame speeds and well within a factor of two of the speciation data obtained in the jet stirred reactor.

Experimental analysis of laser-induced spark ignition of lean turbulent premixed flames: New insight into ignition transition

Recently, Shy and his co-workers reported a turbulent ignition transition based on measurements of minimum ignition energies (MIE) of lean premixed turbulent methane combustion for a wide range of equivalence ratios. Our study used a similar approach to present an additional and complete MIE data set for laser-induced spark ignitions in a flow configuration and under turbulent conditions (i.e., for different ranges of length and time scales) that differed from those chosen by Shy and his colleagues. To extend the previous analyses to very lean premixed methane/air flames, the study examined a characteristic chemical time (τCB), which was defined as the start of the chain-branching reactions. Physically, this chemical time corresponded to the time when the initial chemical reactions of the ignition process released enough heat to compensate for heat losses and to enable a self-sustained reaction in successful ignition. Values for τCB, which were experimentally obtained from the temporal evolution of the mean hot kernel emissions (in laminar flow), strongly decreased with the equivalence ratio Φ for very lean flames and remained nearly constant and equal to 150μs for Φ>0.7. We obtained a clear turbulent ignition transition on the MIE as a function of the rms velocity (u′) and reconfirmed the two modes of ignition. Temporal kernel emission recordings served to describe the transition through the study of the interaction between the turbulence and the hot kernel at the time t=τCB. For low turbulence, when the smallest time scales of the turbulence τK were larger than the τCB values, the hot kernel/turbulence interaction occurred after the initiation of the chemical reactions. The amount of deposited energy required to initiate the reactions and to attain a self-sustained flame kernel was consequently similar to that found under laminar conditions. After the transition, the smallest time scales of the turbulence were smaller than the τCB values. Turbulence may have affected and interacted with the hot kernel before the initiation of chain-branching reactions occurred. Larger amounts of deposited energy were therefore required to compensate for this turbulent dissipation and to attain a self-sustained flame. A Peclet number (PeCB) was introduced, which is equal to the ratio of the turbulent diffusivity at the time of the initiation of the chemical reactions and the thermal diffusivity. These very scattering MIE data (dependent on Φ and u′) are plotted as a function of PeCB and collapsed into a single curve with two drastically different increasing slopes showing the ignition transition.

Laminar Flame Characteristics of iso-Octane/n-Butanol Blend–Air Mixtures at Elevated Temperatures

In this work, laminar flame speeds and Markstein lengths of iso-octane/n-butanol–air mixtures were measured via the outwardly expanding spherical flame method and high-speed schlieren photography over a wide range of equivalence ratios and blending ratios of n-butanol at elevated initial temperatures. Results show that laminar flame speeds of fuel blends slightly increase with increasing blending ratio of n-butanol. The effect of blending ratio on laminar flame speed of fuel blends was mechanistically interpreted through examining the thermodynamic and diffusive property, as well as the overall oxidation kinetics. Measurements on the burned gas Markstein length show a generalized behavior. There exists a minimum absolute Markstein length at the critical equivalence ratio ϕ*, under which the stretch effect on the flame propagating speed is minimized. At equivalence ratios less than ϕ*, Markstein length is decreased with increasing blending ratio of n-butanol, indicating that the addition of n-butanol reduces the diffusional thermal stability of the blend; while at equivalence ratios larger than ϕ*, Markstein length increases with increasing blending ratio of n-butanol. This experimental observation on the Markstein length is consistent with the theoretical investigation. A correlation of laminar flame speed of n-butanol/iso-octane blend as a function of equivalence ratio, temperature, and blending ratio of n-butanol is given on the basis of experimental data.

The Generation of a Reduced Mechanism for Flame Inhibition by Phosphorus Containing Compounds Based on Path Flux Analysis Method

In order to analyze the complex chemical kinetic mechanism systematically and find out the redundant species and reactions, a numerical platform for mechanism analysis and simplification is established basing on Path Flux Analysis (PFA). It is used to reduce a detailed mechanism for flame inhibited by phosphorus containing compounds, a reduced mechanism with 65 species and 335 reactions is obtained. The detailed and reduced mechanism are both used to calculate the freely-propagating premix C3H8/air flame with different dimethyl methylphosphonate doped over a wide range of equivalence ratios. The concentration distributions of free radicals and major species are compared, and the results under two different mechanisms agree well. The laminar flame speed obtained by the two mechanisms also matches well, with the maximum relative error introduces as a small value of 1.7%. On the basis of the reduced mechanism validation, the correlativity analysis is conducted between flame speed and free radical concentrations, which can provide information for target species selection in the further mechanism reduction. By analyzing the species and reactions fluxes, the species and reaction paths which contribute the flame inhibition significantly are determined.

Flame Color as a Lean Blowout Predictor

The study characterizes the behavior of a premixed swirl stabilized dump plane combustor flame near its lean blow-out (LBO) limit in terms of CH* chemiluminiscence intensity and observable flame color variations for a wide range of equivalence ratio, flow rates and degree of premixing (characterized by premixing length, L fuel). LPG and pure methane are used as fuel. We propose a novel LBO prediction strategy based solely on the flame color. It is observed that as the flame approaches LBO, its color changes from reddish to blue. This observation is found to be valid for different levels of fuel-air premixing achieved by changing the available mixing length of the air and the fuel upstream of the dump plane although the flame dynamics were significantly different. Based on this observation, the ratio of the intensities of red and blue components of the flame as captured by a color CCD camera was used as a metric for detecting the proximity of the flame to LBO. Tests were carried out for a wide range of air flow rates and using LPG and CH4 as fuel. For all the operating conditions and both fuels tested, this ratio was found to monotonically decrease as LBO was approached. Moreover, the value of this ratio was within a small range close to LBO for all the cases investigated. This makes the ratio suitable as a metric for LBO detection at all levels of premixing.

Laminar Flame Speed and Ignition Delay Time Data for the Kinetic Modeling of Hydrogen and Syngas Fuel Blends

Laminar flame speeds and ignition delay times have been measured for hydrogen and various compositions of H2/CO (syngas) at elevated pressures and elevated temperatures. Two constant-volume cylindrical vessels were used to visualize the spherical growth of the flame through the use of a schlieren optical setup to measure the laminar flame speed of the mixture. Hydrogen experiments were performed at initial pressures up to 10 atm and initial temperatures up to 443 K. A syngas composition of 50/50 by volume was chosen to demonstrate the effect of carbon monoxide on H2-O2 chemical kinetics at standard temperature and pressures up to 10 atm. All atmospheric mixtures were diluted with standard air, while all elevated-pressure experiments were diluted with a He:O2 ratio of 7:1 to minimize instabilities. The laminar flame speed measurements of hydrogen and syngas are compared to available literature data over a wide range of equivalence ratios, where good agreement can be seen with several data sets. Additionally, an improved chemical kinetics model is shown for all conditions within the current study. The model and the data presented herein agree well, which demonstrates the continual, improved accuracy of the chemical kinetics model. A high-pressure shock tube was used to measure ignition delay times for several baseline compositions of syngas at three pressures across a wide range of temperatures. The compositions of syngas (H2/CO) by volume presented in this study included 80/20, 50/50, 40/60, 20/80, and 10/90, all of which are compared to previously published ignition delay times from a hydrogen-oxygen mixture to demonstrate the effect of carbon monoxide addition. Generally, an increase in carbon monoxide increases the ignition delay time, but there does seem to be a pressure dependency. At low temperatures and pressures higher than about 12 atm, the ignition delay times appear to be indistinguishable with an increase in carbon monoxide. However, at high temperatures the relative composition of H2 and CO has a strong influence on ignition delay times. Model agreement is good across the range of the study, particularly at the elevated pressures.

Study of Extinction and Ignition Characteristics for Oxygenated Fuels in Nonuniform Flows

Experimental studies are carried out for oxygenated and hydrocarbon fuels flames using counterflow flame configurations. Oxygenated fuels include DME, ethanol, and butanol, and hydrocarbon fuels include propane, n-heptane, and iso-octane. A comparative study has been conducted to investigate the extinction, autoignition of all fuels. Studies on nonpremixed flames for extinction and autoignition measurements are carried out by injecting a fuel stream made up of fuel and nitrogen from one duct and an oxidizer stream made up of oxygen and nitrogen from the other duct. In the premixed flame configuration, the strain rate at extinction is measured over a wide range of equivalence ratio and oxygen concentration by injecting the premixed-reactant stream from one duct and nitrogen from the other duct. Numerical calculations are also performed for oxygenated fuels using detailed chemistry at conditions corresponding to those used in the experiments to validate chemical kinetics.

Influence of Steam Dilution on Nitrogen Oxide Formation in Premixed Methane/Hydrogen Flames

The premixed combustion of natural gas and hydrogen at atmospheric pressure is investigated over a wide range of equivalence ratios and different degrees of steam and nitrogen dilution of up to 30% in the air. Emission formation, local temperatures, OH radical concentrations, and flame shape and position are measured in gas-fired tests. The measurement results confirm that steam effectively inhibits the formation of emissions, even at constant flame temperature. For both natural gas and hydrogen, and CO emissions below 10 ppm were measured up to near-stoichiometric fuel concentrations at wet conditions. Nitrogen dilution does not lead to reduced emissions. A numerical analysis conducted with a reactor network shows that the influence of steam dilution on radical concentrations and formation depends strongly on the degree of humidity and on the equivalence ratio. The thermal, the , and the NNH pathway are significantly constrained at wet conditions mainly due to reduced O atom concentrations, whereas prompt can be increased due to higher CH radical concentrations for the natural gas flame. Thus, the inhibiting effect of steam dilution on formation is stronger at high flame temperatures.

Conversion of jet fuel and butanol to syngas by filtration combustion

Replacing batteries with fuel cells is a promising approach for powering portable devices; however, hydrogen fuel generation and storage are challenges to the acceptance of this technology. A potential solution to this problem is on-site fuel reforming, in which a rich fuel/air mixture is converted to a hydrogen-rich syngas. In this paper, we present experimental results of the conversion of jet fuel (Jet-A) and butanol to syngas by non-catalytic filtration combustion in a porous media reactor operating over a wide range of equivalence ratios and inlet velocities. Since the focus of this study is the production of syngas, our primary results are the hydrogen yield, the carbon monoxide yield, and the energy conversion efficiency. In addition, the production of soot that occurred during testing is discussed for both fuels. Finally, an analysis of the potential for these fuels and others to be converted to syngas based on the present experiments and data available in the literature is presented. This study is intended to increase the understanding of filtration combustion for syngas production and to illuminate the potential of these fuels for conversion to syngas by non-catalytic methods.

Combustion characteristic using O2-pilot strut in a liquid-kerosene-fueled strut-based dual-mode scramjet

A set of exploratory experiments are conducted to test a newly designed strut for fuel injection and flame holding in a liquid-kerosene-fueled dual-mode scramjet combustor. The thickness of the strut is 8 mm and the front blockage is about 8%. To organize stable combustion in a Mach number equal to 2.6 air flow under this thin strut using room-temperature liquid kerosene in a flash wall combustor without any cavity and other flame holders, some oxygen is injected through a set of orifices at the back of the strut, based on which a stable center local flame is generated at the back of the strut and the main flow combustion can be organized around this local flame. Experimental results show that stable combustion can be achieved at the center of the combustor with a wide range of equivalence ratio from 0.19 to 1 based on this center flame strut strategy. Through the analysis of the pressure distribution along the combustor, different combustor modes appear with different equivalence ratio. The article also gives some discussions about different influence of the oxygen to the combustion process under different equivalence ratio.

Suppression of hydrogen–oxygen–nitrogen explosions by fine water mist: Part 1. Burning velocity

A work programme has been undertaken to investigate the practical viability of using fine water mists to mitigate or suppress hydrogen explosions during nuclear decommissioning operations. In the first part of this study measurements of burning velocity, required primarily for the development of explosion models, are presented. Burning velocity measurements were made with the introduction of ultrasonically generated fine water mists. With water mist, the burning velocity was reduced over a wide range of equivalence ratios for fuel-free oxygen fractions of 0.1–0.21. Flame instability increased substantially with increasing water mist density especially with lean hydrogen mixtures. The experimental results obtained for the reduction of burning velocity with entrained water mist in hydrogen–air systems are compared to computer simulated values reported earlier in the literature. An important overall objective of the work programme was to provide information to plant engineers to assess the effectiveness of using water mists for the suppression or mitigation of explosion.

Influence of steam dilution on the combustion of natural gas and hydrogen in premixed and rich-quench-lean combustors

The combustion of different natural gas–hydrogen mixtures at dry and steam-diluted conditions is investigated for a rich-quench-lean (RQL) and for a premixed combustor. Atmospheric experiments are conducted over a wide range of equivalence ratios and degrees of steam dilution up to a steam-to-air mass ratio of 30%. Emission formation, flame shape and position, and flame stability are measured in combustion tests. An investigation of the influence of steam on the NOx formation is conducted using reactor networks.Steam effectively inhibits the formation of NOx emissions in both combustion systems. For wet conditions, NOx emissions below 10ppm are achieved for operation on natural gas as well as hydrogen up to high equivalence ratios and temperatures. Increasing the hydrogen content leads to increasing NOx emissions for dry conditions, whereas at wet conditions, NOx is lower for hydrogen-rich fuels. The numerical analysis reveals that in both combustion systems, the thermal, the N2O, and the NNH pathways are significantly constrained at wet conditions. The prompt NOx can be increased due to higher CH radical concentrations for the natural gas flame. CO emissions remain low up to moderate degrees of steam dilution. For a steam content of 20%, the RQL combustor shows increasing CO emissions for low equivalence ratios, whereas they remain low for the premixed combustor.Steam dilution is very effective for NOx reduction in natural gas and in hydrogen flames. Additionally, it lowers the reactivity of hydrogen and already a relatively low steam content prevents flashback. The combination of steam injection and hydrogen combustion provides an efficient and clean combustion mode.

Propagation and extinction of cyclopentadiene flames

Laminar flame speeds and extinction strain rates of cyclopentadiene/air mixture were determined in the counterflow configuration at atmospheric pressure, unburned mixture temperature of 353K, and for a wide range of equivalence ratios. The experiments were modeled using recently developed kinetic models. Sensitivity analyses showed that both flame propagation and extinction of cyclopentadiene/air mixtures flames depend notably on the fuel kinetics and subsequent intermediates such as cyclopentadienyl, cyclopentadienone, and cyclopentadienoxy. Analyses of the computed flame structures revealed that the high temperature oxidation of cyclopentadiene depends in general on the kinetics of first few intermediates in the oxidation process following the fuel consumption. The potential reaction pathways of the consumption of cyclopentadienyl radicals were discussed and further investigation and validation is recommended for two relevant reactions that could improve the high temperature oxidation kinetic model of cyclopentadiene. The experimental flame data of this study are the first ones to be reported.

Coagulation of combustion generated nanoparticles in low and intermediate temperature regimes: An experimental study

Coagulation of combustion-generated particles has been investigated in low and intermediate temperature regimes in a tubular reactor with a residence time of 1.65s. Particles, generated by premixed ethylene/air flames with equivalence ratios above the soot threshold limit, are fed to a tubular reactor, which can be operated at temperatures up to 650K. A wide range of equivalence ratios are used to generate particles with different characteristics. The evolution of the particle size distributions has been evaluated by a differential mobility analyzer with high sensitivity in the 2–100nm size range. The effect of the reactor temperature on coagulation has been systematically studied. Particles exhibit different coagulation efficiencies at the different temperatures. At room temperature, 2–4nm particles fed to the reactor coagulate forming particles as large as 10–20nm, whereas at higher temperatures the size distribution of the particles does not change with respect to that measured at the inlet of the reactor. This behavior suggests a very ineffective coagulation efficiency at higher temperatures for small nanoparticles. Larger particles do not exhibit this high sensitivity to temperature, substantially maintaining very high coagulation efficiencies. These considerations have been confirmed by numerical simulations conducted both with constant and size-dependent coagulation efficiency. The numerical results confirm that also at low and intermediate temperature regimes, the use of a size-dependent coagulation efficiency is mandatory to match the evolution of the particles during coagulation. On the other hand, the simple model of coagulation based on the van der Waals interactions between particles in the framework of gas kinetic collision theory is in slightly disagreement with the experimental results for very small particles, suggesting that more advanced modeling based on quantum mechanism and molecular dynamics are necessary to correctly reproduce the experimental data.

Jet-stirred reactor and flame studies of propanal oxidation

There is a strong drive towards utilizing oxygenated biofuels in blends with existing fossil fuels. Improving the kinetic modeling of the oxidation of these bio-derived oxygenates requires further investigation of their key stable intermediates such as the aldehydes. In this study, an experimental and chemical kinetic modeling investigation of propanal oxidation was carried out. Experiments were conducted in a jet stirred reactor and in counterflow flames over a wide range of equivalence ratios, temperatures, and ambient pressures. Stable species concentration profiles were measured in the jet stirred reactor and laminar flame speeds were measured. A detailed chemical kinetic reaction model was validated using the present experimental results and existing literature data. The model was used also to provide insight into the controlling reaction pathways for propanal oxidation in both the low- and high-temperature kinetic regimes.

Characterization of Lean Misfire Limits of Mixture Alternative Gaseous Fuels Used for Spark Ignition Engines

Increasing on gaseous fuels as clean, economical and abundant fuels encourages the search for optimum conditions of gas-fueled internal combustion engines. This paper presents the experimental results on the lean operational limits of Recardo E6 engine using gasoline, LPG, NG and hydrogen as fuels. The first appearance of almost motoring cycle was used to define the engine lean limit after the fuel flow was reduced gradually. The effects of compression ratio, engine speed and spark timing on the engine operational limits are presented and discussed in detailed. Increasing compression ratio (CR) extend the lean limits, this appears obviously with hydrogen, which has a wide range of equivalence ratios, while for hydrocarbon fuel octane number affect gasoline, so it can' t work above CR=9:1, and for LPG it reaches CR=12:1, NG reaches CR=15:1 at lean limit operation. Movement from low speeds to medium speeds extended lean misfire limits, while moving from medium to high speeds contracted the lean misfiring limits. NOx, CO and UBHC concentrations increased with CR increase for all fuels, while CO 2 concentrations reduced with this increment. NOx concentration increased for medium speeds and reduced for high speeds, but the resulted concentrations were inconcedrable for these lean limits. CO and CO 2 increased with engine speed increase, while UBHC reduced with this increment. The hydrogen engine runs with zero CO, CO 2 and UNHC concentrations, and altra low levels of NOx concentrations at studied lean misfire limits

Characterization of Lean Misfire Limits of Mixture Alternative Gaseous Fuels Used for Spark Ignition Engines

Increasing on gaseous fuels as clean, economical and abundant fuels encourages the search for optimum conditions of gas-fueled internal combustion engines. This paper presents the experimental results on the lean operational limits of Recardo E6 engine using gasoline, LPG, NG and hydrogen as fuels. The first appearance of almost motoring cycle was used to define the engine lean limit after the fuel flow was reduced gradually. The effects of compression ratio, engine speed and spark timing on the engine operational limits are presented and discussed in detailed. Increasing compression ratio (CR) extend the lean limits, this appears obviously with hydrogen, which has a wide range of equivalence ratios, while for hydrocarbon fuel octane number affect gasoline, so it can' t work above CR=9:1, and for LPG it reaches CR=12:1, NG reaches CR=15:1 at lean limit operation. Movement from low speeds to medium speeds extended lean misfire limits, while moving from medium to high speeds contracted the lean misfiring limits. NOx, CO and UBHC concentrations increased with CR increase for all fuels, while CO2 concentrations reduced with this increment. NOx concentration increased for medium speeds and reduced for high speeds, but the resulted concentrations were inconcedrable for these lean limits. CO and CO2 increased with engine speed increase, while UBHC reduced with this increment. The hydrogen engine runs with zero CO, CO2 and UNHC concentrations, and altra low levels of NOx concentrations at studied lean misfire limits.

Propane Oxidation in a Plasma Torch of a Low-Current Nonsteady-State Plasmatron

This paper describes the plasma-assisted combustion system intended to generate a torch flame with a high power density per unit area. In the system, a kind of hybrid concept is proposed. A primary unit for combustion sustaining is a low-current nonsteady-state plasmatron with a low level of electric power. The plasmatron activates an air/hydrocarbon mixture and sustains the oxidation processes in the plasma torch. In turn, the heat power of the torch sustains the main burning process in the torch flame. The results of experiments on propane oxidation in the plasma torch of plasmatron in a wide range of equivalence ratio are presented. As applied to the combustion system design, the plasma torch can provide both the complete and the partial propane oxidation with syngas generation.

Combustion Characteristics of Ethanol and PRF Laminar Flames

In this study, laminar burning velocities of ethanol, iso-octane and n-heptane-air mixtures were determined experimentally over a wide range of equivalence ratio under the various pressures up to 0.5MPa, employing spherically expanding flames. Considering non-linear extrapolations, the effects of stretch was minimized by extrapolating the reference flame speed to vanishing stretch. The laminar burning velocities of iso-octane were found to be lower than those of ethanol and n-heptane throughout the range of experimental equivalence ratios. For all fuels, laminar burning velocities were found to decrease with increase initial pressure. From schlieren images, ethanol, iso-octane and n-heptane-air mixtures on the fuel rich region were found to become unstable at 0.1MPa. For all fuels, increasing initial pressure was found to get flame more unstable and narrow the stable flame region.