A radical index for the determination of the chemical kinetic contribution to diffusion flame extinction of large hydrocarbon fuels

The extinction limits of diffusion flames have been measured experimentally and computed numerically for fuels of three different molecular structures pertinent to surrogate fuel formulation: n-alkanes, alkyl benzenes, and iso-octane. The focus of this study is to isolate the thermal and mass transport effects from chemical kinetic contributions to diffusion flame extinction, allowing for a universal correlation of extinction limit to molecular structure. A scaling analysis has been performed and reveals that the thermal and mass transport effects on the extinction limit can be normalized by consideration of the enthalpy flux to the flame via the diffusion process. The transport-weighted enthalpy is defined as the product of the enthalpy of combustion per unit mole of fuel and the inverse of the square root of fuel molecular weight. The chemical kinetic contribution provided by the specific fuel chemistry has thus been elucidated for tested individual component and multi-component surrogate fuels. A chemical kinetic flux analysis for n-decane flames shows that the production/consumption rates of the hydroxyl (OH) radical govern the heat release rate in these flames and therefore play significant roles in defining the extinction limit. The rate of OH formation has been defined by considering the OH concentration, flame thickness, and flow strain rate. A fuel-specific radical index has been introduced as a concept to represent and quantify the kinetic contribution to the extinction limit owing to the fuel-specific chemistry. A relative radical index scale, centered on the radical index of a series of n-alkanes which are observed and fundamentally explained to be common, is established. A universal correlation of the observed extinction limits of all tested fuels has been obtained through a combined metric of radical index and transport-weighted enthalpy. Finally, evidence as to the validity of the fundamental arguments presented is provided by the success of the universal correlation in predicting the extinction limits of the multi-component mixtures typical of surrogate fuels.     

Methyl butanoate inhibition of n-heptane diffusion flames through an evaluation of transport and chemical kinetics

The extinction limits of methyl butanoate, n-heptane, and methyl butanoate/n-heptane diffusion flames have been measured as a function of fuel mole fraction with nitrogen dilution in counterflow with air. On a mole fraction basis, methyl butanoate diffusion flames are observed to have a much lower extinction strain rate than n-heptane diffusion flames and the extinction strain rate of n-heptane/methyl butanoate diffusion flames is observed to increase significantly as the n-heptane fraction is increased.Based on previous works, detailed chemical kinetic models to describe the high temperature oxidation of these fuel mixtures are assembled, tested and reduced. When the transport properties of ester species are re-evaluated by means of a thorough literature review, numerical computations of extinction generally reproduce experimental results for the pure fuels as well as for mixtures. An in-depth analysis of the kinetic model computations reveals that the extinction behaviour of both fuels is due to (1) fuel energy content affects and (2) the chemical kinetic potential of each fuel to produce the hydroperoxy radical. Comparatively, in n-heptane flames reactive ethyl radicals and ethylene are the major intermediates formed, but in methyl butanoate flames the major intermediates are formyl radicals and formaldehyde. In all flames studied, increased strain rates affect an increased interaction of formyl and/or vinyl radicals with molecular oxygen leading to a transition from hydrogen atom production at low strain rates, to the production of large quantities of the hydroperoxy radical at higher strain rates. The formation of the hydroperoxy radical induces extinction in each flame by directly interfering with the important radical chain branching and exothermic elementary reactions of H atoms and OH radicals that are dominant in weakly strained flames.It is postulated that the similar inhibitive effect of methyl butanoate fuelled flames will also be observed for more biodiesel like, larger n-alkyl esters when compared to equivalent n-alkanes. The diffusive extinction limits of methyl decanoate diffusion flames are also measured and show reactivity comparable to n-heptane diffusion flames by a molar comparison.     

The combustion properties of 2,6,10-trimethyl dodecane and a chemical functional group analysis

The global combustion characteristics of 2,6,10-trimethyl dodecane (trimethyl dodecane), a synthetic fuel candidate species, have been experimentally investigated by measuring extinction limits for strained laminar diffusion flames at 1 atm and reflected shock ignition delays at 20 atm. The Derived Cetane Number (DCN) of trimethyl dodecane, (59.1) and Hydrogen/Carbon (H/C) ratio (2.133) are very close to the DCN and H/C ratio of a previously studied synthetic aviation fuel, S-8 POSF 4734 (S-8) and its surrogate mixture composed of n-dodecane/iso-octane (58.9 and 2.19, respectively). Identical high temperature global kinetic reactivities are observed in all experiments involving the aforementioned compounds. However, at temperatures below ∼870 K, the S-8 surrogate mixture has ignition delay times approximately a factor of two faster. A chemical functional group analysis identifies that the methylene (CH₂) to methyl (CH₃) ratio globally correlates the low temperature alkylperoxy radical reactivity for these large paraffinic fuels. This result is further supported experimentally, by comparing observations using a surrogate fuel mixture of n-hexadecane (n-cetane) and 2,2,4,4,6,8,8-heptamethyl nonane (iso-cetane) that shares the same methylene-to-methyl ratio as trimethyl dodecane, in addition to the same DCN and H/C ratio. Measurements of both diffusion flame extinction and reflected shock ignition delays show that the n-cetane/iso-cetane model fuel has very similar combustion behavior to trimethyl dodecane at all conditions studied. A kinetic modeling analysis on the model fuel suggests the formation of alkylhydroperoxy radicals (QOOH) to be strongly influenced by the absence or presence of the methyl and methylene functional groups in the fuel chemical structure. The experimental observations and analyses suggest that paraffinic based fuels having high DCN values may be more appropriately emulated by further including the CH₂ to CH₃ ratio as an additional combustion property target, as DCN alone fails to fully distinguish the relative reaction characteristics of low temperature kinetic phenomena.     

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.     

A computational study on the combustion of hydrogen/methane blended fuels for a micro gas turbines

To understand the combustion performance of using hydrogen/methane blended fuels for a micro gas turbine that was originally designed as a natural gas fueled engine, the combustion characteristics of a can combustor has been modeled and the effects of hydrogen addition were investigated. The simulations were performed with three-dimensional compressible k-ε turbulent flow model and presumed probability density function for chemical reaction. The combustion and emission characteristics with a variable volumetric fraction of hydrogen from 0% to 90% were studied. As hydrogen is substituted for methane at a fixed fuel injection velocity, the flame temperatures become higher, but lower fuel flow rate and heat input at higher hydrogen substitution percentages cause a power shortage. To apply the blended fuels at a constant fuel flow rate, the flame temperatures are increased with increasing hydrogen percentages. This will benefit the performance of gas turbine, but the cooling and the NO_x emissions are the primary concerns. While fixing a certain heat input to the engine with blended fuels, wider but shorter flames at higher hydrogen percentages are found, but the substantial increase of CO emission indicates a decrease in combustion efficiency. Further modifications including fuel injection and cooling strategies are needed for the micro gas turbine engine with hydrogen/methane blended fuel as an alternative.     

Experimental study on the structure of opposed flow gaseous diffusion flames doped with n-decane

The destruction of n-decane is investigated with a perturbative approach by adding hundreds of ppm to the fuel stream of two gaseous counterflow diffusion flames at atmospheric pressure: a blue methane flame and an incipiently sooting ethylene flame that offer distinct reacting environments. The detailed chemical structure of the flames including the products of n-decane consumption is determined using a microprobe gas sampling technique followed by GC/MS analysis. Experimentally, principal products of n-decane destruction are C2–C9 linear alpha-olefins that are found at ever increasing concentrations with decreasing carbon number, starting with 1-nonene all the way to propene and ethylene, the most abundant products. Successive fragmentation steps of the n-decane primary products lead to the formation of C2–C5 dienes and other hydrocarbons with multiple unsaturated bonds. The consumption rate of n-decane is more abrupt in the methane flame as compared to the gentler decay observed in the ethylene flame. The addition of n-decane in the ethylene flame does not contribute to the formation of soot precursors such as aromatic compounds because the pool of C2–C4 fragments of the baseline flame, playing a key role in aromatic growth, is only marginally affected by n-decane addition. The comprehensive database of stable species of the experimental component of the study is tested by a comparison with the results of modeling the flames using two semi-detailed chemical kinetic mechanisms, Ranzi-mech and JetSurF. Shortcomings of these mechanisms are highlighted for different classes of compounds by comparison of the model results with the experimental data leaving room for future improvements in their formulation.     

Propagation and extinction of benzene and alkylated benzene flames

Laminar flame speeds and extinction strain rates of benzene, n-propylbenzene, toluene, o-, m-, and p-xylene, and 1,2,4- and 1,3,5-trimethylbenzene flames were studied experimentally in the counterflow configuration under atmospheric pressure and at the elevated temperature of 353 K for the unreacted fuel-containing stream. The experimental data revealed that the aromatic fuel structure plays a critical role on flame propagation, with the laminar flame speed decreasing with an increase in methylation of benzene. Numerical simulations suggest that the aromatics flames are highly sensitive to fuel-specific chemistry and more specifically to the reaction kinetics of the first few intermediates in the oxidation process following the fuel consumption, and that the different flame propagation speeds relate strongly to radical–radical termination facilitated by benzyl or benzyl-like intermediates. The tendencies of stretch-induced extinction of non-premixed flames was found to follow a trend that is identical to the laminar flame speed, but the extinction data revealed a more discriminative effect arising from fuel-structure differences. Comparisons between kinetic model predictions and experimental data showed that there exist significant discrepancies among these models and uncertainties in the oxidation and pyrolysis kinetics of one-ring aromatics.     

The structure of toluene-doped counterflow gaseous diffusion flames

The structure of gaseous counterflow diffusion flames perturbed with the addition of hundreds of ppm of prevaporized toluene is studied in two distinct flame environments: a blue methane flame stabilized on the fuel side of the gas stagnation plane and an incipiently sooting ethylene flame stabilized on the oxidizer side. The goal is to provide a well-defined testbed in terms of temperature–time history, major species and part of the radical pool, for the examination of reference fuels that are critical components of practical fuel blends. Gas samples are extracted from the flame with fused silica microprobes for subsequent GC/MS analysis and thermocouples and thin filament pyrometry are used to characterize the temperature field. Profiles of critical toluene pyrolysis products and stable soot precursors are compared with computational models using two semi-detailed chemical mechanisms. Results show that in the methane flame some oxygen containing radicals like O and OH are contributing early on to the toluene destruction path. In the incipiently sooting ethylene flame, the primary attack is from H alone. This finding confirms the different challenges that such flames pose to the validation of a chemical kinetic mechanism. The onset of toluene decay in these flames begins at relatively modest temperatures, on the order of 800 K. This reactivity is captured reasonably well by both chemical mechanisms in the methane flame, in the absence of reactants larger than C2, but not so in the ethylene flame, in the presence of a richer, more complex mixture. The aromatic ring opening mechanisms are not adequately modeled in either case. This discrepancy has implications for the modeling of practically relevant fuel blends with both aliphatic and aromatic compounds. The dominant species larger than toluene in the doped methane flame is ethylbenzene, which at least one of the mechanisms reproduces quite well. The largest measured species in the incipiently sooting flame is indene, whose concentration increase due to toluene addition is properly captured by one of the models. The experimental dataset reported here may help identifying future improvements to chemical kinetic mechanisms and complement other reactor datasets lacking the coupling of kinetics and transport of flame environments.     

Experimental determination of some hydrogen combustion characteristics,its performances as a clean fuel compared to fossil fuel ones

The present study was undertaken to acquire an improved understanding of the mechanisms involved in the energy transfer from oxy-fuel flames to solids. Industrial fuels like hydrogen, propane and acetylene have been investigated. Three different measurement techniques were used to compare potential of these fuels. ESR spectroscopy and IR spectroscopy were used to obtain respectively H radical concentration and OH concentration and temperature. Heat fluxes were measured using a differential calorimeter. We obtained, for the three oxy-fuel flames, curves showing distributions of radicals concentrations, temperature and heat transferred from different parts of the flame. These results for H₂-O₂ flames point out: (1) that experimental values are different from the predicted values for an adiabatic flame, (2) the heat transfer efficiency is for some equivalence ratios as well as or better than for other hydrocarbon - O₂ flames. This study gives also elements to create new flames using hydrogen fuel to replace in part fossil fuels.     

Study on stretch extinction limits of CH4/CO2 versus high temperature O2/CO2 counterflow non-premixed flames

Extinction limits of counterflow non-premixed flames with normal and high temperature oxidizers were studied experimentally and numerically for development of new-type oxygen-enriched mild combustion furnace. Extinction stretch rates of CH₄/CO₂ (at 300 K) versus O₂/CO₂ flames at oxygen mole fractions of 0.35 and 0.40 and oxidizer temperatures of 300 K, 500 K, 700 K and 1000 K were obtained. Investigation was also conducted for CH₄/N₂ (at 300 K) versus air (O₂/N₂) flames at the same oxidizer temperatures. An effect of radiative heat loss on stretch extinction limits of oxygen-enriched flames and air flames was investigated by computations with optical thin model (OTM) and adiabatic flame model (ADI). The results show influence of radiative heat loss on stretch extinction limits was not significant in relative high fuel mole fraction regions. The extinction curve of the oxygen-enriched flames with oxygen mole fraction of 0.35 was close to that of the air flames at the oxidizer temperature of 300 K. However, the extinction curve of air flames with high temperature oxidizer was comparable with that of oxygen-enriched flames with oxygen mole fraction of 0.40. Scaling analysis based on asymptotic solution of stretch extinction was applied and it was found that stretch extinction limits can be expressed by two terms. The first term is total enthalpy flux of fuel stream based on thermo-physical parameters. The second term is a kinetic term which reflects an effect of the chemical reaction rate on stretch extinction limits. OH radicals which play important roles in chain propagating and main endothermic reactions were used to represent the kinetic term of both oxygen-enriched and air flames. The global rates of OH formation in these two cases were compared to understand the contribution of kinetic term to stretch extinction limits. Variation of extinction curves of oxygen-enriched flames and air flames was well explained by the present scaling analysis. This offers an effective approach to estimate stretch extinction limits of oxygen-enriched flames based on those of air flames at the same oxidizer temperature.     

Soot formation in flames of model biodiesel fuels

The sooting propensities of non-premixed flames of a class of model biodiesel fuels, namely fatty acid esters, were studied systematically. Soot volume fractions were measured using the laser extinction method in the counter-flow configuration, for different fuel/N₂ molar ratios and atmospheric pressure. The experimental data were compared against those obtained in flames of n-alkanes with similar carbon numbers and a flame of a surrogate diesel fuel. For all cases considered, it was determined that the soot volume fraction increases with the fuel concentration, as expected. Furthermore, the model biodiesel fuels were shown to produce significantly less soot compared to the corresponding n-alkanes. Additional experimental studies were carried as well, in order to assess the effects of carbon number, type of ester group (methyl or ethyl), and extent of saturation on the sooting propensity of flames of these model biodiesel fuels. Three recently developed chemical kinetic models were utilized to model the flames and thus investigate the kinetic pathways controlling the formation of C₂H₄ and two key soot precursors, namely C₂H₂ and C₃H₃, aiming to provide insight into the experimentally observed differences in the sooting propensity among the flames of the various fuels that were considered.     

HCCI experiments with toluene reference fuels modeled by a semidetailed chemical kinetic model

A semidetailed mechanism (137 species and 633 reactions) and new experiments in a homogeneous charge compression ignition (HCCI) engine on the autoignition of toluene reference fuels are presented. Skeletal mechanisms for isooctane and n-heptane were added to a detailed toluene submechanism. The model shows generally good agreement with ignition delay times measured in a shock tube and a rapid compression machine and is sensitive to changes in temperature, pressure, and mixture strength. The addition of reactions involving the formation and destruction of benzylperoxide radical was crucial to modeling toluene shock tube data. Laminar burning velocities for benzene and toluene were well predicted by the model after some revision of the high-temperature chemistry. Moreover, laminar burning velocities of a real gasoline at 353 and 500 K could be predicted by the model using a toluene reference fuel as a surrogate. The model also captures the experimentally observed differences in combustion phasing of toluene/n-heptane mixtures, compared to a primary reference fuel of the same research octane number, in HCCI engines as the intake pressure and temperature are changed. For high intake pressures and low intake temperatures, a sensitivity analysis at the moment of maximum heat release rate shows that the consumption of phenoxy radicals is rate-limiting when a toluene/n-heptane fuel is used, which makes this fuel more resistant to autoignition than the primary reference fuel. Typical CPU times encountered in zero-dimensional calculations were on the order of seconds and minutes in laminar flame speed calculations. Cross reactions between benzylperoxy radicals and n-heptane improved the model predictions of shock tube experiments for ?=1.0 and temperatures lower than 800 K for an n-heptane/toluene fuel mixture, but cross reactions had no influence on HCCI simulations.     

Kinetic modelling of extinction and autoignition of condensed hydrocarbon fuels in non-premixed flows with comparison to experiment

A detailed chemical–kinetic mechanism is used to predict critical conditions of extinction and autoignition of condensed hydrocarbon fuels in non-premixed flows. The mechanism includes reactions that describe “low temperature chemistry” as well as “high temperature chemistry” for many high molecular weight fuels. The fuels considered here are n-heptane, n-decane, n-dodecane, n-hexadecane, and iso-octane. The kinetic model is validated by comparing its predictions of critical conditions of extinction and autoignition for these fuels with experimental data obtained in a counterflow configuration where a steady laminar flow of an oxidizer is directed over the vaporizing surface of a condensed fuel. The residence time in this configuration is given by the strain rate. The kinetic model predicts that n-heptane is most difficult to extinguish followed by n-decane, n-dodecane, and n-hexadecane. This is in agreement with experimental data. Computations show that the influence of low temperature chemistry on critical conditions of extinction is small. The kinetic model predicts that at low strain rates n-hexadecane is most easy to ignite followed by n-dodecane, n-decane, and n-heptane. At high values of strain rate, n-heptane is more easy to ignite in comparison to n-decane. This is again in agreement with experimental data, including the “cross-over” in relative reactivities of n-heptane and n-decane. Sensitivity analysis shows that at low strain rates autoignition is promoted by low temperature chemistry for all fuels. At high strain rates, autoignition for n-heptane is promoted by high temperature chemistry, whilst low temperature chemistry continues to play a significant role in promoting autoignition for the other straight-chain hydrocarbon fuels. The “cross-over” in relative reactivities of n-heptane and n-decane is attributed to competition between the rates of low temperature chemistry, rates of high temperature chemistry, and rates of molecular transport for these fuels.     

An experimental and kinetic modeling study of the autoignition of α-methylnaphthalene/air and α-methylnaphthalene/n-decane/air mixtures at elevated pressures

The autoignition of α-methylnaphthalene (AMN), the bicyclic aromatic reference compound for the cetane number (CN), and AMN/n-decane blends, potential diesel surrogate mixtures, was studied at elevated pressures for fuel/air mixtures in a heated high-pressure shock tube. Additionally, a comprehensive kinetic mechanism was developed to describe the oxidation of AMN and AMN/n-decane blends. Ignition delay times were measured in reflected shock experiments for Φ = 0.5, 1.0, and 1.5 AMN/air mixtures (CN = 0) for 1032-1445 K and 8-45 bar and for Φ = 1.0 30%-molar AMN/70%-molar n-decane/air (CN = 58) and 70%-molar AMN/30%-molar n-decane/air mixtures (CN = 28) for 848-1349 K and 14-62 bar. Kinetic simulations, based on the comprehensive AMN/n-decane mechanism, are in good agreement with measured ignition times, illustrating the emerging capability of comprehensive mechanisms for describing high molecular weight transportation fuels. Sensitivity and reaction flux analysis indicate the importance of reactions involving resonance stabilized phenylbenzyl radicals, the formation of which by H-atom abstractions with OH radicals has an important inhibiting effect on ignition.     

Effects of ammonia substitution on extinction limits and structure of counterflow nonpremixed hydrogen/air flames

The potential of partial ammonia substitution to improve the safety of hydrogen use was evaluated computationally, using counterflow nonpremixed ammonia/hydrogen/air flames at normal temperature and pressure. The ammonia-substituted hydrogen/air flames were considered using a recent kinetic mechanism and a statistical narrow-band radiation model for a wide range of flame strain rates and the extent of ammonia substitution. The effects of ammonia substitution on the extinction limits and structure, including nitrogen oxide (NO_x) and nitrous oxide (N₂O) emissions, of nonpremixed hydrogen/air flames were investigated. Results show reduction of the high-stretch extinction (i.e., blow-off) limits, the maximum flame temperature and the concentration of light radicals (e.g., H and OH) with ammonia substitution in hydrogen/air flames, supporting the potential of ammonia as a carbon-free, clean additive for improving the safety of hydrogen use in nonpremixed hydrogen/air flames. For high-stretched flames, however, NO_x and N₂O emissions substantially increase with ammonia substitution even though ammonia substitution reduces flame temperature, implying that chemical effects (rather than thermal effects) of ammonia substitution on flame structure are dominant. Radiation effects on the extinction limits and flame structure are not remarkable particularly for high-stretched flames.     

Effects of hydrogen addition on combustion characteristics of n-decane/air mixtures

To explore the possibility of simultaneously extending the lean extinction limit and reducing the emission levels with hydrogen addition, a computational study is performed to investigate the effects of hydrogen addition on the fundamental combustion characteristics of n-decane/air mixtures. It is found that a small amount of hydrogen addition can significantly promote the reactivity of n-decane/air mixtures, leading to shortened ignition delays at high temperatures, increased laminar flame speeds, and reduced extinction residence times. The results on emissions show that the addition of hydrogen leads to a reduction in CO emission index under fuel rich conditions, while NO emission index increases with increasing hydrogen addition for all the conditions examined. The extent of the hydrogen addition effects on different combustion responses at varying pressures has also been investigated. In addition, sensitivity analysis has been conducted to identify the key reactions that are responsible for the enhanced reactivity associated with hydrogen addition. The present results further demonstrate that with the aid of hydrogen addition, leaner and hence cleaner combustion can be achieved without compromising static flame stability.     

Sooting tendencies of primary reference fuels in atmospheric laminar diffusion flames burning into vitiated air

In this study, the sooting tendencies of primary reference fuels (PRFs) are measured in term of yield sooting indices (YSIs) in methane diffusion flames doped with the vapors of PRFs. The present paper represents an incremental advance complementing the original methodology prescribed by McEnally and Pfefferle. The influence of both PRF formulation and CO₂ dilution of the coflowing air on the YSIs is also assessed. The diffusion flames burning in a coflowing oxidizer stream are established over the Santoro’s burner and vapor of the liquid fuel to be investigated is injected into the fuel stream. Laser extinction measurements are performed to map the two-dimensional field of soot volume fraction in the flame. For the pure liquid hydrocarbons investigated, i.e., n-hexane, n-heptane, isooctane, and benzene, the YSI reported in the original paper by McEnally and Pfefferle quantitatively predict the sooting propensities, measured here at much higher dopant concentrations. The present study therefore extends the consistency of the YSI methodology on the Santoro’s burner. For blends of n-heptane and isooctane, the sooting tendency of doped flames exhibits regular and monotonic trends and decreases with increasing n-heptane mole fraction or CO₂ dilution. Interestingly, the evolution of YSI with the isooctane mole fraction exhibits a strong similarity for varying CO₂ mole fraction. A quadratic least-squares fit is then derived, providing a phenomenological model of YSI as a function of both isooctane mole fraction in the fuel stream and CO₂ mole fraction in the oxidizer. A non-negligible cross effect of PRF formulation and CO₂ dilution on YSI is revealed. The method elaborated within the framework of the present paper could be extended to surrogate fuels. This would help develop a comprehensive database and empirical correlations that could predict the sooting propensities of different surrogate fuels, therefore their potentially mitigationed soot production through control of fuel composition and/or exhaust gas recirculation. This database would also be useful for the validation of CFD simulations incorporating sophisticated model of soot production.     

HCCI experiments with gasoline surrogate fuels modeled by a semidetailed chemical kinetic model

Experiments in a homogeneous charge compression ignition (HCCI) engine have been conducted with four gasoline surrogate fuel blends. The pure components in the surrogate fuels consisted of n-heptane, isooctane, toluene, ethanol and diisobutylene and fuel sensitivities (RON-MON) in the fuel blends ranged from two to nine. The operating conditions for the engine were pin=0.1 and 0.2 MPa, Tin=80 and 250 °C, ?=0.25 in air and engine speed 1200 rpm. A semidetailed chemical kinetic model (142 species and 672 reactions) for gasoline surrogate fuels, validated against ignition data from experiments conducted in shock tubes for gasoline surrogate fuel blends at 1.0?p?5.0 MPa, 700?T?1200 K and ?=1.0, was successfully used to qualitatively predict the HCCI experiments using a single zone modeling approach. The fuel blends that had higher fuel sensitivity were more resistant to autoignition for low intake temperature and high intake pressure and less resistant to autoignition for high intake temperature and low intake pressure. A sensitivity analysis shows that at high intake temperature the chemistry of the fuels ethanol, toluene and diisobutylene helps to advance ignition. This is consistent with the trend that fuels with the least Negative Temperature Coefficient (NTC) behavior show the highest octane sensitivity, and become less resistant to autoignition at high intake temperatures. For high intake pressure the sensitivity analysis shows that fuels in the fuel blend with no NTC behavior consume OH radicals and acts as a radical scavenger for the fuels with NTC behavior. This is consistent with the observed trend of an increase in RON and fuel sensitivity. With data from shock tube experiments in the literature and HCCI modeling in this work, a correlation between the reciprocal pressure exponent on the ignition delay to the fuel sensitivity and volume percentage of single-stage ignition fuel in the fuel blend was found. Higher fuel sensitivity and single-stage fuel content generally gives a lower value of the pressure exponent. This helps to explain the results obtained while boosting the intake pressure in the HCCI engine.