Extended Shvab–Zel’dovich formulation for multicomponent-fuel diffusion flames

In this paper an extension of the Shvab–Zel’dovich formulation is presented. This extended formulation, based on the Burke–Schumann kinetic mechanism, describes the combustion of multicomponent fuels in a diffusion flame in terms of mixture fraction and the excess enthalpy. Under the condition of Burke–Schumann kinetic mechanism, the multicomponent fuel is burned in a single flame. The model is applied to a diffusion flame generated by the burning of mixtures of n-heptane and hydrogen diluted in nitrogen in a counterflow configuration. Due to the very small ratio of the hydrogen molecular weight to the n-heptane molecular weight, small quantities of hydrogen (in terms of mass) in the mixture does not change significantly the properties related to the mass, like as the total heat released per unit of mass at the flame. However, properties related to the hydrogen mole fraction does change expressively with small quantities, like as the radiative energy loss from the hot region around the flame. The results show the flame properties as a function of the reciprocal scalar dissipation and hydrogen quantity in the mixture. It is observed that, by reducing the reciprocal scalar dissipation, the radiative energy loss decreases and by increasing the presence of the hydrogen, the sensitivity of the flame properties with the reciprocal scalar dissipation reduces. It is also revealed by the results, the effects of the potentiated preferential hydrogen mass diffusion in compositions in which nitrogen and n-heptane are the majority species, and the potentiated preferential n-heptane thermal diffusion in compositions in which nitrogen and hydrogen are the majority species, on the flame properties. Although, this work do not treat the extinction problem, the fluid dynamical results will be properly handled to provide information about the reciprocal scalar dissipation and the Liñán’s parameter necessary for future flame stability analyses.     

Comparison of burning characteristics of live and dead chaparral fuels

Wildfire spread in living vegetation, such as chaparral in southern California, often causes significant damage to infrastructure and ecosystems. The effects of physical characteristics of fuels and fuel beds on live fuel burning and whether live fuels differ fundamentally from dead woody fuels in their burning characteristics are not well understood. Toward this end, three common chaparral fuels prevalent in southern California, chamise, manzanita, and ceanothus, were investigated by burning them in a cylindrical container. The observed fire behavior included mass loss rate, flame height, and temperature structure above the burning fuel bed. By using successive images of the temperature field, a recently developed thermal particle image velocity (TPIV) algorithm was applied to estimate flow velocities in the vicinity of the flame. A linear regression fit was used to explain the observed time difference between when maximum flame height and maximum mass loss rate occur, as a function of fuel moisture content. Two different methods were used to extract power laws for flame heights of live and dead fuels. It was observed that the parameters defined in the well-known two-fifths power law for flame height as a function of heat release rate were inadequate for live fuels. As the moisture content increases, the heat release rate in the power law needs to be calculated at the time when the maximum flame height is achieved, as opposed to the maximum mass loss rate. Dimensionless parameters were used to express local temperature and velocity structure of live and dead chaparral fuels in the form of a Gaussian profile over different regimes in a fire plume.     

Effects of differential diffusion on unsteady diffusion flames

The effects of differential diffusion on unsteady diffusion flames are considered for one-dimensional geometries. A Stefan problem is obtained for a single planar flame separating two semi-infinite regions containing fuel and oxidizer. Non-unity diffusivity ratio and the stoichiometry affect strongly the flame evolution and determine the conditions to obtain a stationary flame. A fuel strip surrounded by an oxidizer is considered and its burning can represent the burning of a gas pocket or supercritical droplet. It is demonstrated for a fuel strip that increasing O/F diffusivity ratios decrease the maximum flame expansion and decrease the burning times, while increasing initial equivalence ratios will increase the maximum flame expansion and will increase the burning times.     

Theoretical investigation of engine thermal efficiency,adiabatic flame temperature,NOx emission and combustion-related parameters for different oxygenated fuels

This paper deals with theoretical investigation of engine thermal efficiency, specific heat at constant pressure (Cp), gas temperature, molecular number, adiabatic flame temperature, oxides of nitrogen (NO_x) emission, and some fundamental combustion-related parameters such as theoretical air fuel ratio (AFR), lower calorific value (LCV) and ratio of LCV and theoretical AFR for different oxygenated fuels. The computations are carried out at standard atmospheric condition. Pentadecane (PD) was used as a base fuel in this computation.The results showed that almost all of the above combustion-related parameters are closely related to oxygen content in the fuels. The ratio of LCV and theoretical AFR is constant for all kinds of oxygenated fuels regardless of the oxygen content in their molecular structure. One of the interesting findings of this work is that with the increase in oxygen content in the fuels adiabatic flame temperature decreases linearly consequently NO_x emission decreased linearly. The engine thermal efficiency is almost unchanged below oxygen content of 30% w/w, but gradually decreased above 30% w/w. The reduction in NO_x emission is associated with the reduction in adiabatic flame temperature for the oxygenated fuels, while the reduction of engine thermal efficiency above 30% w/w of oxygen associated with the increase in gas specific heat.     

Polycyclic aromatic hydrocarbons in flames, in diesel fuels, and in diesel emissions

Numerous chemical analyses of gaseous and particulate samples from laboratory flames provide a library of data on the polycyclic aromatic hydrocarbon (PAH) species found in diverse flame types burning fuels consisting of pure gaseous hydrocarbons. The diesel fuels utilized by the more complex combustion in compression ignition engines are composed of thousands of hydrocarbon species. Mass spectrometry by the laser microprobe and gas chromatography were used in a complementary manner to distinguish the PAHs originating in the fuel from those produced by engine combustion. The C_xH_y PAH products of premixed and diffusion flame processes, which also occur in the unsteady diesel combustion, range in mass from 128 u (two rings, x=10, y=8) to beyond 350 u (eight rings, x=28, y=14). Graphs of the number of hydrogen atoms y vs the number of carbon atoms x for the species found by many investigators of laboratory flames show these pyrogenic PAHs to lie on or near the staircase curve that describes the most stable, pericondensed, benzenoid PAHs. In contrast, samples of diesel fuels from the United Kingdom and the United States contain petrogenic alkyl-PAHs with high hydrogen contents. Samples of diesel particulate emissions typical of the 1990s from two different sources display the full mass range of PAHs from 128 to 350 u, including both the benzenoid PAHs and the alkyl-PAHs. Thus diesel emissions, in general, may contain petrogenic fuel components ranging up to 206 u and also the combustion-generated four- to seven-ring species in the 228 to 302 u mass range that have greater carcinogenic potency. The absence of petrogenic components larger than 206 u facilitates their detection and delineation from pyrogenic PAHs by methods of chemical analysis.     

Influence of fuel properties on the burning characteristics of sour plum (Ximenia americana L.) seed oil compared with Jatropha curcas L. seed oil

The aim of the research was to determine fuel properties and burning characteristics of sour plum (Ximenia americana L.) seed oil compared with Jatropha curcas seed oil when unblended and blended with kerosene. Fossil oil fuel products have witnessed increased demand all over the world with prices reaching new peaks. Sour plum (Ximenia americana L.) seed oil as one potential biofuel was evaluated to determine its fuel properties as substitute for kerosene. The seed oil was blended with kerosene in varying ratios and the parameters: burning rate and flame height determined. The blended oil was also burned in modified kerosene stove. It was found that Density, viscosity, fire point, carbon residue and ash content influenced its burning parameters. Parameter burning characteristics and energy transfered improved with increasing blend of kerosene. In conclusion, Ximenia americana L. seed oil when blended with kerosene in ratio above 10% can supplement kerosene as biofuel.     

Hydrogen, propane and gasoline laminar flame development in a spherical vessel

A series of tests are conducted on a constant volume spherical vessel combustion bomb to study the laminar flame development and combustion characteristics of fuel/oxygen/nitrogen mixtures. The experiments are carried out on three types of fuel, namely H₂, C₃H₈ and vapor gasoline for a wide range of operating parameters. The operating parameters studied are equivalence ratio, initial pressure and temperature, and molar oxygen concentration in the supplied air. The significance of the results is discussed, and with the use of published data, a burning rates correlation, laminar burning velocity and flammability limits are presented and compared. A pressure transducer installed in the spherical vessel is used to obtain pressure vs time curves for the combustion process when centrally ignited. From these curves, mass burn rates are calculated using an energy analysis program developed for this calculation. It is proved that the relations between mass burnt ratio and pressure increment ratio can be rearranged into a curve irrespective of the differences in fuels and initial pressures. Laminar burning velocities are obtained at a wide range of operating parameters for the three fuels considered.     

Theoretical modeling of a compound-drop spray in premixed flames

The influence of internal heat transfer induced by dilute compound-drop sprays on one-dimensional premixed flames is investigated using large activation energy asymptotic analysis. In this study, the compound drop is composed of a single water core encased by a shell of fuel. The gasification zones of the shell fuel and the core water affect the flow and flame characteristics. A critical completely pre-vaporized burning condition, (CPB)_c, is defined as the whole compound drops finishing vaporization right at the flame and a critical shell pre-vaporized burning condition, (SPB)_c, is defined as the shell fuel of compound drops finishing vaporization right at the flame. Under the (CPB)_c and (SPB)_c conditions of lean and rich flames, the flame propagation flux, the critical values of the shell-fuel mass fraction and the initial radius vary with the water-core radius and the liquid loading. For a lean spray flame, compound drops can provide internal heat transfer in the form of heat gain from the shell fuel and heat loss from the core water. The lean spray flame may be strengthened or weakened depending on the net heat transfer. For a rich spray flame, the compound-drop spray always weakens flame propagation. An S-shaped extinction curve occurs for a rich spray flame under the (SPB)_c condition, with a sufficiently heavy liquid loading and a sufficiently large water-core size.     

The influence of molecular structure of fatty acid monoalkyl esters on diesel combustion

The subject of this paper is a series of experiments conducted on a single-cylinder research engine investigating the influence of molecular structure on the combustion behaviour of fatty acid alcohol ester (biodiesel) molecules under diesel engine conditions. The fuels employed in these experiments comprised various samples of pure individual fatty acid alcohol ester molecules of different structure, as well as several mixtures of such molecules. The latter consisted in biodiesel fuels produced by the transesterification of naturally occurring plant oils or animal fat with a monohydric alcohol. It was observed that the molecular structure of the fuel significantly influenced the formation of NO_x and particulate matter and their respective concentration in the exhaust gas. The influence on the formation of NO_x in particular, appeared to be exerted first through the effect which the molecular structure had on the auto-ignition delay occurring after the fuel was injected into the combustion chamber, and second through the flame temperature at which the various molecules burned. The emission of particulates on the other hand showed correlation with the number of double bonds in the fuel molecules for the case of larger accumulation mode particles, and with the boiling point of the fuel samples for the case of the smaller, nucleation mode particles. The effect of ignition delay on the exhaust emissions of these pollutants was isolated by adding the ignition promoting molecule 2-ethylhexyl nitrate to some of the fuel samples in closely specified concentrations, so as to equalise the ignition delay for the relevant fuel samples. The removal of the ignition delay as a main influence on the combustion process enabled the observation of the lesser effects of adiabatic flame temperature.     

A Burke-Schumann analysis of dual-flame structure supported by a burning droplet

Droplet combustion experiments carried out onboard the International Space Station, using pure fuels and fuel mixtures, have shown that quasi-steady burning can be sustained by a non-traditional flame configuration, namely a “cool flame” burning in the “partial-burning” regime where both fuel and oxygen leak through the low-temperature-controlled flame-sheet. Recent experiments involving large, bi-component fuel (n-decane and hexanol, 50/50 by volume) droplets at elevated pressures show that the visible, hot flame becomes extremely weak while the burning rate remains relatively high, suggesting the possible simultaneous presence of “cool” and “hot” flames of roughly equal importance. The radiant output from these bi-component droplets is relatively high and cannot be accounted for only by the presence of a visible hot flame. In this analysis we explore the theoretical possibility of a dual-flame structure, where one flame lies close to the droplet surface, called the “cool flame”, and the other farther away from the droplet surface, termed the “hot flame”. A Burke-Schumann analysis of this dual structure seems to indicate that such flame structures are possible over a limited range of initial conditions. These theoretical results can be compared against available experimental data for pure and bi-component fuel droplet combustion to test how realistic the model may be.     

A new approach for simplifying the calculation of flue gas specific heat and specific exergy value depending on fuel composition

In this paper, a new approach is proposed for simplifying the calculation of flue gas specific heat and specific exergy value in one formulation depending on fuel chemical composition. Combustion products contain different gases such as CO₂, SO₂, N₂, O₂, H₂O and etc., depending on the burning process. Specific heat and exergy of the flue gas differ depending on the chemical composition of fuels, excess air ratio and gas temperature. Through this new approach, specific heat and specific exergy value of combustion products can be estimated accurately in one formulation by entering the chemical composition of fuels, excess air ratio and gas temperature. The present approach can be applied to all carbon based fuels, especially biomass, fossil fuels and fuel mixtures for co-combustion and is so suitable for practical estimation of flue gas specific heat and specific exergy values provided that the fuel chemical composition is given.     

Effect of the composition of the hot product stream in the quasi-steady extinction of strained premixed flames

The extinction of premixed CH₄/O₂/N₂ flames counterflowing against a jet of combustion products in chemical equilibrium was investigated numerically using detailed chemistry and transport mechanisms. Such a problem is of relevance to combustion systems with non-homogeneous air/fuel mixtures or recirculation of the burnt gases. Contrary to similar studies that were focused on heat loss/gain, depending on the degree of non-adiabaticity of the system, the emphasis here was on the yet unexplored role of the composition of counterflowing burnt gases in the extinction of lean-to-stoichiometric premixed flames. For a given temperature of the counterflowing products of combustion, it was found that the decrease of heat release with increase in strain rate could be either monotonic or non-monotonic, depending on the equivalence ratio φ_b of the flame feeding the hot combustion product stream. Two distinct extinction modes were observed: an abrupt one, when the hot counterflowing stream consists of either inert gas or equilibrium products of a stoichiometric premixed flame, and a smooth extinction, when there is an excess of oxidizing species in the combustion product stream. In the latter case four burning regimes can be distinguished as the strain rate is progressively increased while the heat release decreases smoothly: an adiabatic propagating flame regime, a non-adiabatic propagating flame regime, the so-called partially-extinguished flame regime, in which the location of the peak of heat release crosses the stagnation plane, and a frozen flow regime. The flame structure was analyzed in detail in the different burning regimes. Abrupt extinction was attributed to the quenching of the oxidation layer with the entire H-OH-O radical pool being comparably reduced. Under conditions of smooth extinction, the behavior is different and the concentration of the H radical decreases the most with increasing strain rate, whereas OH and O remain comparatively abundant in the oxidation layer. As the profile of the heat release rate thickens, the oxidation layer is quenched and the attack of the fuel relies more heavily on the OH radicals.     

Flame chemiluminescence and OH LIF imaging in a hydrogen-fuelled spark-ignition engine

Research into novel internal combustion engines requires consideration of the diversity in future fuels in an attempt to reduce drastically CO₂ emissions from vehicles and promote energy sustainability. Hydrogen has been proposed as a possible fuel for future internal combustion engines. Hydrogen’s wide flammability range allows higher engine efficiency with much leaner operation than conventional fuels, for both reduced toxic emissions and no CO₂ gases. This paper presents results from an optical study of combustion in a spark-ignition research engine running with direct injection and port injection of hydrogen. Crank-angle resolved flame chemiluminescence images were acquired and post-processed for a series of consecutive cycles in order to calculate in-cylinder rates of flame growth. Laser induced fluorescence of OH was also applied on an in-cylinder plane below the spark plug to record detailed features of the flame front for a series of engine cycles. The tests were performed at various air-to-fuel ratios, typically in a range of φ = 0.50–0.83 at 1000 RPM with 0.5 bar intake pressure. The engine was also run with gasoline in direct-injection and port-injection modes to compare with the operation on hydrogen. The observed combustion characteristics were analysed with respect to laminar and turbulent burning velocities, as well as flame stretch. An attempt was also made to review relevant hydrogen work from the limited literature on the subject and make comparisons were appropriate.     

Burning velocity and flame structure of CH4/NH3/air turbulent premixed flames at high pressure

Ammonia is one of the most promising alternative fuels. In particular, ammonia combustion for gas turbine combustors for power generation is expected. To shift the fuel for a gas turbine combustor to ammonia step-by-step, the partial replacement of natural gas by ammonia is considered. To reveal the turbulent combustion characteristics, CH₄/NH₃/air turbulent premixed flame at 0.5 MPa was experimentally investigated. The ammonia ratio based on the mole fraction and lower heating value was varied from 0 to 0.2. The results showed that the ratio of the turbulent burning velocity and unstretched laminar burning velocity decreased with an increase in the ammonia ratio. The reason for this variation is that the flame area decreased with an increase in the ammonia ratio as the flame surface density decreased and the fractal inner cutoff increased. The volume fractions in the turbulent flame region were almost the same with ammonia addition, indicating that combustion oscillation can be handled in a manner similar to that for the case of natural gas for CH₄/NH₃/air flames.     

Investigation of C2H6, C2H4, CO and H2 on the explosion pressure behavior of methane/blended fuels

The introduction of other combustible gases to methane could change accident consequences. This work aimed to experimentally compare the effects of blended fuels (C₂H₆, C₂H₄, CO, and H₂ mix in different proportions) on the explosion pressure behavior of methane and blended fuels in air. The explosion pressure properties of the mixtures were tested. Principal component analysis was employed, and multiple regression models were developed to predict the influences of principal components on the maximum explosion pressure of CH₄. In addition, the software was used to analyze and compare the sensitivity analysis. For the fuel-lean CH₄-air mixture(φ = 0.72), the results demonstrated that the value of P_max and (dP/dt)_max of the mixed gases increased, as the volume fraction of the blended fuels increased. The effects of other gases followed the order C₂H₆>C₂H₄>H₂>CO. For stoichiometric concentrations (φ = 1), the order of the influence degree was H₂>CO > C₂H₆>C₂H₄, and the value of P_max and (dP/dt)_max of the mixed gases tended to decrease slightly. As the volume fraction of the blended fuels was increased to 2%, the P_max and (dP/dt)_max of CH₄ showed a decreasing trend for the fuel-rich CH₄-air mixture. The effects of blended fuels on the equivalence ratio of φ = 1.1 followed the order C₂H₄>H₂>CO > C₂H₆. The sensitivity variation analysis and normalized sensitivity variation analysis indicated that the chain branching reaction 2CH₃(+M)<=>C₂H6(+M) was the most sensitive reaction under various conditions. However, it was not very susceptible to changes in the blended fuel ratio. Most reactions were susceptible to changes in the blended fuel ratio. The 10 most sensitive reactions were generally much more susceptible to changes in the blended fuel ratio in the 2% blended fuels than they were in the 0.4% blended fuels.     

Measurement of laminar burning velocity and Markstein length relative to fresh gases using a new postprocessing procedure: Application to laminar spherical flames for methane,ethanol and isooctane/air mixtures

The purpose of this study is to present a new tool for extracting the laminar burning velocity in the case of spherically outward expanding flames. This new procedure makes it possible to determine the laminar burning velocity directly based on the flame displacement speed and the global fresh gas velocity near the preheat zone of the flame front. It therefore presents a very interesting alternative to the standard method (commonly used in the literature), which is based on the flame front displacement and the ratio of unburned and burned gas densities. The influence of external flame stretching on the burning velocity can be characterized and the Markstein length relative to the unburned gases (i.e., fresh gases) can be deduced by using this new tool. Contrary to the standard procedure, the unstretched laminar burning velocity is determined directly without using the fuel mixture properties. The temporal evolution of the flame front is visualized by high-speed laser tomography and the algorithm, based on a tomographic image correlation method, makes it possible to accurately measure the fresh gas velocity near the preheat zone of the flame front. The measurements of laminar flame speeds are carried out in a high-pressure and high-temperature constant-volume vessel over a wide range of equivalence ratios for methane, ethanol, and isooctane/air mixtures. To validate the experimental facility and the postprocessing of the flame images, fresh gas velocities and unstretched laminar burning velocities, as well as Markstein lengths relative to burned and unburned gases, are presented and compared with experimental and numerical results of the literature for methane/air flames. New results concerning ethanol/air and isooctane/air flames are presented for various experimental conditions (373 K, equivalence ratios range 0.7–1.5, pressure range 0.1–5 MPa).     

Sooting behavior in temperature-controlled laminar diffusion flames

The sooting tendency of gaseous and liquid hydrocarbon fuels has been determined systematically in an axisymmetric laminar diffusion flame whose temperature was controlled by nitrogen dilution. Sooting tendency was measured by the minimum mass flow rate of fuel (FFM) at the smoke height. Result, plotted as log 1/FFM versus $\text{1}\text{T}$, where T is a calculated adiabatic flame temperature, show that fuel structure plays a significant role in diffusion flames. Comparison of these flame results with basic pyrolysis studies in the literature supports the concept that pyrolysis of the fuel molecule is a controlling factor in determining the overall tendency to soot, even though such tendency results from the competition of pyrolysis of the fuel and heterogeneous oxidation of the soot particles. The pyrolysis characteristics affecting the sooting process are rate, sequence and nature of products, and pyrolysis mode (pure or oxidative). The aromatics show a temperature sensitivity with respect to sooting tendency significantly lower than the other fuels. Conjugation of the initial fuel molecule and pyrolysis intermediates enhances sooting propensity.     

Experimental investigation of burning rates of pure ethanol and ethanol blended fuels

A fundamental experimental study to determine the burning rates of ethanol and ethanol-blended fossil fuels is presented. Pure liquid ethanol or its blends with liquid fossil fuels such as gasoline or diesel, has been transpired to the surface a porous sphere using an infusion pump. Burning of the fuel takes place on the surface of the porous sphere, which is placed in an air stream blowing upwards with a uniform velocity at atmospheric pressure and temperature under normal gravity conditions. At low air velocities, when ignited, a flame envelopes the sphere. For each sphere size, air stream velocity and fuel type, the fuel feed rate will vary and the same is recorded as the burning rate for that configuration. The flame stand-off distances from the sphere surface are measured by post-processing the digital image of the flame photograph using suitable imaging software. The transition velocity at which the flame moves and establishes itself at the wake region of the sphere has been determined for different diameters and fuel types. Correlations of these parameters are also presented.