Low Temperature Combustion Research Articles

Analysis of the effects of lubricating oil viscosity and engine speed on piston-cylinder liner frictions in a single cylinder HCCI engine by GT-SUITE program

Engines with homogenous charge compression ignition (HCCI) are the low-temperature combustion models in which automatic oxidation reactions occur with the effect of in-cylinder pressure and heat at the end of preparation of homogenous air-fuel mixture and compression stroke within the cylinder by means of port injection or early direct injection. In-cylinder gas temperatures of such engines during the cycle are lower than conventional internal combustion engines. Therefore, they cause zero NOx and soot emissions. Moreover, the occurrence of combustion in very small crankshaft angles results in a decrease in heat losses observed in cylinder walls and an increase in thermal efficiency. Reducing mechanical friction is highly significant for increasing the effective productivity of HCCI engines. In internal combustion engines, 10% of total energy obtained from the fuel is spent on the heat emerging due to mechanical frictions. 20% of mechanical friction results from the friction occurring between piston ring and liners. In this article, frictional characteristics of compression ring (TOP) and piston skirt area have been examined for two different engine speeds and lubricants, by using technical properties of single-cylinder HCCI engine and in-cylinder pressure and temperature data obtained under full load by means of GT-SUITE software. As the increase in pressure, occurring inside the cylinder at the end of the exhaust stroke and at the beginning of intake stroke, increases ring pressure load in HCCI engines, higher level of piston ring frictions has been observed when compared to internal combustion engines with the same technical properties. Piston ring contact pressure force is a more effective parameter in terms of piston frictions, when compared to hydrodynamic pressure force. The use of lubricants with higher viscosity (SAE 10W-40) has enabled the piston to move more laterally. According to the analysis results, a maximum piston speed of 3.92 m/s for 800 rpm engine speed and 7.85 m/s for 1600 rpm engine speed has been obtained. Maximum friction power losses have been found as 63.84 W at 800 rpm engine speed and 85.91 W at 1600 rpm engine speed. Oil film thickness has obtained in the middle of the piston stroke in the intake, compression, power and exhaust strokes, respectively, 1.809, 1.674, 1.547 and 1.792 µm at 800 rpm engine speed and 1.101, 1.018, 0.932 and 1.119 µm at 1600 rpm engine speed.

Effects of Carbon Chain Length on N-Alkane Counterflow Cool Flames: A Kinetic Analysis

An in-depth understanding of the low-temperature reactivity of hydrocarbon fuels is of practical relevance to developing advanced low-temperature combustion techniques. The present study aims to study the low-temperature chemistry of several large n-alkanes with different carbon chain lengths in counterflow cool diffusion flames by kinetic analysis. The large n-alkanes that were chosen are n-heptane (NC7H16), n-decane (NC10H22) and n-dodecane (NC12H26), which are important components of practical fuels. Firstly, the thermochemical structure of a typical cool diffusion flame is understood through its comparison with that of a hot diffusion flame. The boundary conditions, including the ozone concentration, fuel concentration and flow velocity—where cool flames can be established—are identified with a detailed chemical mechanism that evaluates the low-temperature reactivity of the investigated n-alkanes. The results show that the n-alkane with a longer carbon chain length is more reactive than the smaller one, thereby indicating the order of NC12H26 > NC10H22 > NC7H16. This trend is qualitatively similar to the findings from non-flame reactors. The reaction pathway and sensitivity analysis are performed to understand the effects of carbon chain length on the low-temperature reactivity. The contribution of an n-alkane with a longer carbon chain to the dehydrogenation reaction, oxidation reaction and isomerization reaction is greater than that of a smaller n-alkane, and abundant O and OH radicals are generated to promote the fuel low-temperature oxidation process, thereby resulting in an enhanced low-temperature reactivity. The effects of ozone addition on the low-temperature reactivity of n-alkanes are also highlighted. It is found that the addition of ozone could provide a large number of active O radicals, which dehydrogenate with the fuels to generate OH radicals and then promote fuel low-temperature oxidation. The present results are expected to enrich the understanding of the low-temperature characteristics of large n-alkanes.

How ozone affects the product distribution inside cool flames of diethyl ether

Cool flames of diethyl ether were stabilized and extensively studied in a stagnation plate burner. Two conditions were selected, enabling the assessment of the impact of ozone-seeding on the cool flame: In the first case, ozone was seeded to stabilize a lean cool flame with an equivalence ratio of 0.5, and in a second case a stoichiometric flame was stabilized without O3 addition. Excited formaldehyde chemiluminescence (CH2O*) was used to measure the cool flame position in the burner. Detailed temperature and mole fraction profiles of stable intermediates of the oxidation of diethyl ether were measured for both conditions. Relevant detailed kinetic models issued from the literature, respectively developed by Tran et al. [Tran et al., Proc. Comb. Inst. 2019, 37, 511–519] and Serinyel et al. [Serinyel et al., Combust. Flame 2018, 193, 453–462] were used in order to improve the understanding of the low temperature kinetics of this ether, as well as the effect of ozone on the species distribution after the cool flame. The Tran et al. model is able to correctly predict the cool flame position as well as its heat release in the lean case, but shows high discrepancy in the prediction of products formation, while the Serinyel et al. model shows a fair prediction of the cool flame products distribution. A numerical comparison between a stoichiometric flame, with and without ozone-seeding, was also performed in order to gain some insight into the ozone influence on the low temperature products distribution. Main results show that the RO˙ radical decomposition is of particular importance in our conditions, which formation is directly linked to RO˙2 bimolecular reactions, reinforcing the link between atmospheric chemistry and low temperature combustion.

High- and low-temperature ignition delay time study and modeling efforts on vinyl acetate

Although esters in general have received much attention over the last decade of combustion research, the combustion of vinyl esters have yet not been studied in detail. Recent studies on ethyl acetate show that vinyl acetate is a major intermediate but its combustion is not well understood. This may be due to the fact that vinyl acetate itself can presumably not be used as a fuel or fuel additive, but both the fundamental understanding of vinyl ester combustion and the improvement of the ethyl ester modeling motivate the present study. Building on the work on ethyl acetate, a first kinetic model for the high- and low-temperature combustion of vinyl acetate is proposed, which includes reactions and intermediates that have not been considered before. These reactions are based on low-level quantum mechanical calculations as well as analogies drawn to mainly ethyl acetate. Seven additional species are considered compared to the ethyl acetate study for which the thermochemical data is derived by ab-initio calculations. The vinyl acetate kinetic model is validated against ignition delay times obtained in a shock tube and a rapid compression machine at pressures of 20 and 40 bar and temperatures ranging from 850 to 1250 K. Overall, a satisfactory agreement between the predictions of the kinetic model and the experimental data was found for all investigated conditions. Rate of production and brute force sensitivity analyses were performed to identify the most relevant reaction pathways, which underline the strong connection between the vinyl acetate and ethyl acetate chemistry.

Effect of the Combination of High Temperature and Fluid Fly Ash on the Compressive Strength of Concrete and its Dynamic Modulus of Elasticity

This paper deals with the effect of combining high temperature and fluidised fly ash, not only on the compressive strength of concrete but also on the determination of the dynamic modulus of concrete test bodies. The use of high-temperature fly ash in concrete mixes has become commonplace. However, the demand for this type of active admixture has increased to such an extent that it currently far exceeds supply, especially after selective non-catalytic nitrogen oxide reduction (SNCR) was introduced and the switch to low-temperature combustion units. This lack of supply makes it necessary to look for other alternative solutions to this situation. The partial replacement of high-temperature fly ash with fluidised bed fly ash appears to be a possible solution.

Surfactant-assisted synthesis of MOF-derived CeO2 for low-temperature catalytic o-xylene combustion

CeO2 applied for catalytic elimination of volatile organic compounds have received great attention for their superior oxygen storage capacity and reducibility. In this work, CeO2-C catalyst is prepared through pyrolysis of Ce-MOF precursor with assistance of cetyltrimethylammonium bromide (CTAB). The T50 and T90 of the strip-like CeO2-C are 155 and 193 °C (WHSV= 48000 mL·g−1·h−1, o-xylene concentration= 500 ppm), which take significant advantages over literature results. Compared with the catalyst prepared without CTAB (CeO2-B), the CeO2-C possesses a 1.5 times reaction rate and an apparent activation energy (Ea) as low as 39.82 kJ·mol−1. Meanwhile, CeO2-C also demonstrates good thermal stability in long-term test and water resistance. The characterization results reveal that the assistance of CTAB increases oxygen vacancy density and generates more active oxygen species, resulting in the improvement of catalytic performance. Furthermore, the reaction route, o-xylene → 2-methyl benzaldehyde (benzaldehyde) → benzoic acid → maleic anhydride → ethanol and acetone → H2O and CO2, is also probed.

Experimental and kinetic modeling study of the low- temperature and high-pressure combustion chemistry of straight chain pentanol isomers: 1-, 2- and 3-Pentanol

Pentanols have received significant attention as a potential alternative fuel or fuel additive owing to their high energy densities and low vapor pressure. The development of robust chemical kinetic models for alternative fuels which can provide accurate and efficient predictions of combustion performance across a wide range of engine relevant conditions is important in developing cleaner, more efficient combustors. Although the high temperature oxidation kinetics of pentanol isomers has been researched considerably, their low temperature combustion chemistry needs further investigation. While previously proposed low temperature mechanisms for 1-pentanol based on analogy and rate rules need further refinement, the low temperature oxidation kinetics of 2-pentanol and 3-pentanol has not been studied previously by any means, experimentally or theoretically. A newly developed kinetic mechanism is presented in this work for the three straight chain pentanol isomers: 1-, 2- and 3-pentanol. Low temperature kinetics is based on a recent study by Lockwood et al., 2022 [20] involving theoretical calculations at the CCSD(T)/cc-pV∞Z level of theory for the oxidation pathways involving alcohol peroxy radicals. Rate of production analyses performed in this study highlight the importance of the newly added pressure-dependent reactions of the α-alcohol peroxy radical forming an RȮ2 adduct. While the α-alcohol fuel radical reacts with O2 to directly decompose via a chemically activated pathway at low pressures, the formation of the RȮ2 adduct is favored at high pressures. The detailed model is comprehensively validated against new ignition experiments at low temperature and high pressure, together with the wide range of data available in the literature. Both qualitative and quantitative predictions of the experimental data using the proposed kinetic model are satisfactory for all three pentanol isomers studied here.

Emergent Behavior in Oxidation Catalysis over Single-Atom Pd on a Reducible CeO2 Support via Mixed Redox Cycles

Experimental kinetics demonstrate catalytic lean CO oxidation over single Pd atoms with a CO reaction order greater than unity. A CO order greater than unity is unique to Pd single atoms supported on CeO2 nanocubes and is not observed for Pd nanoparticles on the same support nor other supported single-atom systems. A reaction mechanism including eight elementary steps, in which Pd dynamically samples three formal oxidation states, is unable to capture a reaction order in CO greater than unity. Vibrational spectroscopy of CO adsorption ranging from lean to rich conditions reveals Pd accesses an extensive array of formal oxidation states, ranging from Pd4+ to Pd0. Density functional theory calculations demonstrate Pd atoms supported on CeO2 can access four formal oxidation states during lean CO oxidation. Two catalytic redox cycles are possible, with cross-over between them leading to a reaction mechanism yielding reaction orders in CO greater than unity. A microkinetic model, utilizing Bayesian inference to adjust elementary energetics to fit the experimentally measured activation barrier and reaction orders, confirms the reaction order in CO of >1 arises from the ability of the Pd active site to access four formal oxidation states, spanning from +4 (PdO2) to −2 (Pd adjacent to a CeO2 lattice oxygen vacancy). This work demonstrates the emergent redox behavior of Pd single atoms on CeO2, which may be important to explain their high activity for low-temperature catalytic combustion of hydrocarbons.

A skeletal mechanism for gasoline surrogates: Development, validation, and CFD application

In this study, a skeletal chemical kinetic mechanism for gasoline surrogates is developed from a detailed mechanism by applying several reduction techniques. Directed relation graph (DRG) routines and DRG-aided sensitivity analysis methods are applied with worst-case error tolerances equal to 30% and 40%, respectively. Issues with reaction dead-ends in the oxidation paths are resolved by adding adequate intermediate species and reactions. Sensitivity analyses are conducted to identify the most impactful fuel-dependent reactions to ignition delay, and the specific reaction rates of such reactions are optimized to replicate the ignition behavior of detailed mechanism. The final version of the skeletal mechanism consists of 152 species and 563 reactions including NOx chemistry. Experiments are carried out in Sandia's low-temperature gasoline combustion research engine with an E10 regular-grade gasoline, termed RD5-87, and the results are replicated in 0D simulations with good accuracies using both detailed mechanism and the skeletal mechanism although a slight over-estimation of the low-temperature heat release is observed. Finally, the performance of the skeletal mechanism is also tested in 3D CFD simulation using LES. Simulations are able to predict the pressure evolution and heat release rate, proving that the proposed skeletal mechanism is adequate for simulations.

NUMERICAL STUDY OF WATER ADDITION ON THE COMBUSTION CHARACTERISTICS IN AN HCCI ETHANOL ENGINE

Low-temperature combustion modes have attracted global attention in recent years, especially the homogeneous charge compression ignition (HCCI) mode. The HCCI combustion produces high-speed combustion, achieving relatively high thermal efficiencies while emitting low NOX and soot. However, due to the lack of direct control to start the combustion, one of the main challenges in this type of engine is to ensure that auto-ignition occurs close to the top dead center for a wide operation range. There are a few combustion phase control techniques to solve this limitation, and one of them is water injection. In this regard, this paper explores the influence of water concentration in an ethanol HCCI engine as a form of combustion phasing control. The work was conducted numerically using a zero-dimensional single-zone model with a chemical kinetic mechanism containing 56 species and 383 reactions. The numerical model results were well agreed with the reported experimental data. The water fraction ranged from 0 to 18% by mass at 3% intervals. The results demonstrate that water injection is an efficient strategy to control the combustion phasing by adjusting the charge reactivity. The increase in water fraction reduced in-cylinder pressure and heat release rates. Furthermore, the combustion phase was advanced, the combustion duration was extended, and thermal efficiency was about 44.3% with the increase in water concentration.

Replicating HCCI-like autoignition behavior: What gasoline surrogate fidelity is needed?

This work seeks to characterize the fidelity needed in a gasoline surrogate with the intent to replicate the complex autoignition behavior exhibited within advanced combustion engines, and specifically Homogeneous Charge Compression Ignition (HCCI). A low-temperature gasoline combustion (LGTC) engine operating in HCCI mode and a rapid compression machine (RCM) are utilized to experimentally quantify fuel reactivity, through autoignition and preliminary heat release characteristics. Fuels considered include a research grade E10 U.S. gasoline (RD5-87), three multi-component surrogates (PACE-1, PACE-8, PACE-20), and a binary surrogate (PRF88.4). Each fuel was studied at lean/HCCI-like conditions covering a wide range of temperatures and pressures that are representative of naturally aspirated to high boost engine operation. Detailed chemical kinetic modeling is also undertaken using a recently updated gasoline surrogate kinetic model to simulate the RCM experiments and to provide chemical insight into surrogate-to-surrogate differences.The LGTC engine experiments demonstrate nearly identical reactivity between PACE-20 and RD5-87 across studied conditions, while faster phasing is seen for both PACE-1 and PACE-8 due to their stronger intermediate- and low-temperature heat release (ITHR/LTHR) at naturally aspirated and boosted conditions, respectively. The RCM experiments reveal typical low-temperature, negative temperature coefficient (NTC) and intermediate-temperature autoignition behaviors at all pressure conditions for RD5-87, which are qualitatively reproduced by all surrogates. Quantitative discrepancies in both autoignition and preliminary heat release are observed for all surrogates, while their ability to replicate RD5-87 autoignition behavior follows the order of PACE-20 > PACE-1 > PACE-8 > PRF88.4. Excellent mapping is obtained between the LGTC engine and the RCM, where the engine pressure-time trajectories can be characterized by the regimes represented by the RCM autoignition isopleths. The kinetic model performs commendably when simulating both autoignition and preliminary heat release of PACE-20, while typically overpredicting ignition delay times for PACE-1, PACE-8 and PRF88.4 at high-pressure and low-temperature/NTC conditions. Sensitivity and rate of production (ROP) analyses highlight surrogate-to-surrogate differences in the governing chemical kinetics where n-pentane initiates rapid OH branching at a faster rate and an earlier timing for PACE-20 than iso-pentane does for PACE-1 and PACE-8, making it computationally more reactive than the other surrogates. The current study highlights the need to include non-standardized properties, such as the lean/HCCI-like autoignition characteristics, in addition to ASTM properties (e.g., RON, MON) as metrics of fuel reactivity and targets to be matched when formulating high-fidelity surrogates that fully capture gasoline advanced combustion behavior such as HCCI-like autoignition.

Fluidized bed combustion of high water content alcohol extracted herb residue and the impacts of blending wasted activated coke

Combustion of herb residues (HRs) for heat recovery is a good way for their utilization, but there exists such a problem as high concentration NOx emission in flue gas. The alcohol extracted herb residue (AEHR), one special type of HRs, was chosen as the object and was subjected to immediate combustion in a fluidized bed reactor to investigate the characteristics of its resulting NOx emission. The results showed that, most of the NOx in the flue gas was produced from the char nitrogen (CN); as the fuel water content increased, the NOx emission concentration exhibited a trend of first decreasing and then increasing; and a properly low combustion temperature inhibited the NOx emission upon the premise of ensuring full combustion. Air staging combustion was adopted to effectively control NOx: the NOx emission concentration in the flue gas was reduced to 296 mg·m−3 and the NOx emission reduction rate reached 46.51 % compared to conventional combustion. Co-combustion by blending wasted activated coke (WAC) into the AEHR helped both stabilize the combustion state and reduce further the NOx emission. When the blending ratio of WAC fell within a proper range of 20–30 %, the NOx emission concentration in the flue gas was as low as 231.4 mg·m−3. In addition to the dilution effect of the combustion flue gas of the blended WAC, the local reducing atmosphere caused by its incomplete combustion as well as its strong absorbability and catalytical effect was accountable for the further decrease of the NOx emission concentration.

A combustion chemistry study of tetramethylethylene in a laminar premixed low-pressure hydrogen flame

The combustion chemistry of tetramethylethylene (TME) was studied in a premixed laminar low-pressure hydrogen flame by combined photoionization molecular-beam mass spectrometry (PI-MBMS) and photoelectron photoion coincidence (PEPICO) spectroscopy at the Swiss Light Source (SLS) of the Paul Scherrer Institute in Villigen, Switzerland. This hexene isomer with the chemical formula C6H12 has a special structure with only allylic CH bonds. Several combustion intermediate species were identified by their photoionization and threshold photoelectron spectra, respectively. The experimental mole fraction profiles were compared to modeling results from a recently published kinetic reaction mechanism that includes a TME sub-mechanism to describe the TME/H2 flame structure. The first stable intermediate species formed early in the flame front during the combustion of TME are 2-methyl-2-butene (C5H10) at a mass-to-charge ratio (m/z) of 70, 2,3-dimethylbutane (C6H14) at m/z 86, and 3-methyl-1,2-butadiene (C5H8) at m/z 68. Isobutene (C4H8) is also a dominant intermediate in the combustion of TME and results from consumption of 2-methyl-2-butene. In addition to these hydrocarbons, some oxygenated species are formed due to low-temperature combustion chemistry in the consumption pathway of TME under the investigated flame conditions.

The effects of diesel-OME1 blended fuel on combustion characteristics of a heavy-duty compression ignition engine by a numerical study

Diesel engines are playing an important role in the transportation system and regardless to their acceptable efficiency (near 40 %), the main problems are the amounts of CO2 and soot production of these engines. In this research, Oxymethylene ether is used as a blend with diesel by a numerical study, and engine performance, fuel economy, and emission analyses have been investigated. The OM355 engine with Euro II emission standard is selected as the base engine and the percentages of Soot, NOx, CO2, flame speed, flame surface density, brake specific fuel consumption, specific power, and specific torque has been studied from 0 to 60% of diesel-OME1 compounds. Overall results of this numerical investigation indicate that OME1 can be a suitable component due to positive effects on the exhaust emissions. Initial estimates show that engine soot and NOx emissions can be reduced more than 80% and CO2 also near to 66% reduced compared to pure paraffinic diesel. In fact, due to a lower combustion temperature of OME1, soot and NOx emissions would barely produce due to their formation limit temperature, hence the engine operates much cleaner. The main disadvantage of this synthetic fuel is the low amount of heating value and lower cetane number in comparison to diesel, which in turn leads to a performance loss and a higher amount of fuel consumption for producing a specific amount of power and torque output.

Theoretical investigation on the reaction kinetics of NO2 with CH3OH and HCHO under combustion conditions

Methanol (CH3OH) and formaldehyde (HCHO) reacting with nitrogen dioxide (NO2) contribute to the largest uncertainty for the CH3OH/NOx low temperature combustion mechanism. CH3OH and NO2 only undergo H-abstraction reactions, while HCHO + NO2 involves multiple reaction channels, among which H-abstraction dominates. In the present work, a high level quantum chemical method, CCSD(T)/aug-cc-pVQZ//M06–2X-D3/6-311++G(d,p), was employed to investigate the reaction pathways. The reaction kinetics were explored by RRKM/master equation simulations with multidimensional small-curvature tunneling (SCT) corrections and hindered rotor approximations. The H-abstraction reactions with barriers higher than 20 kcal/mol indicate a nonnegligible quantum tunneling effect even under combustion conditions. Our computations predict the tunneling factors to be 3–4 for the studied reactions at 500 K. A significant tunneling effect is also expected for H-abstraction of large alcohols and aldehydes by NO2. The computed total rate coefficients show good agreement with previous experimental measurements over narrow ranges of temperature and pressure, ensuring the accuracy of the reported branching ratios covering a wide T, P range for the two reactions. The results of CH3OH + NO2 reveal the dominant role of HONOcis + CH2OH. It's also uncovered the dominance of HONOcis + CHO pathway in HCHO + NO2 under the studied conditions. The detailed reaction kinetics information reported in this work is useful for building rate rules for the mechanisms of other nitrogen-containing alcohol-based fuels.

Development of a phenomenological model for the description of RCCI combustion in a dual-fuel marine internal combustion engine

Increasingly stringent pollutant and CO2 emission standards require engine manufacturers to investigate innovative solutions. Among these techniques, low-temperature combustion (LTC) concepts have a large potential to simultaneously reduce NOx emissions and fuel consumption. A promising manner to realize LTC consists of adopting ultra-lean mixtures, where the combustion evolution is controlled by a proper spatial distribution of fuels with different chemical reactivities. In this context, depending on the proportion and stratification of the fuels, the heat release can primarily depend on chemistry progression, leading to a Reactivity Controlled Compression Ignition (RCCI) mode, or on flame propagation, locally initiated by a high reactivity fuel.In this work, the combustion characteristics of a large-bore research engine are experimentally investigated. Natural gas is supplied into the intake port, while light fuel oil (LFO) is directly injected in the cylinder. An experimental campaign is carried out including sweeps of engine load, air/fuel proportion, LFO amount, valve timing, and intake air temperature. Global engine operating parameters as well as cylinder pressure traces are recorded and analyzed.Based on the available experimental data, a phenomenological model handling both chemistries of fuels with different reactivities and flame propagation is developed and validated. The model is based on a multi-zone approach, where auto-ignition chemistry is solved by a tabulated method to preserve the computational effort. The proposed numerical approach shows the ability to simulate the experimental data with good accuracy, using a fixed tuning constant set. A dedicated correlation is built to reproduce the expected in-cylinder distribution of the directly injected liquid fuel. Global performance and combustion parameters are predicted with an average error below 5%. The model demonstrates to correctly describe the behavior of the tested engine under different operating conditions and to capture the physics behind such advanced combustion concepts.

Extinction characteristics of isolated n-alkane fuel droplets during low temperature cool flame burning in air

Large carbon number n-alkanes are a notable component in all real transportation fuels, and their chemical structure fosters substantial low temperature kinetic reactivity. Normal alkanes have been studied in various canonical configurations but rarely in systems with strong coupling between low temperature chemistry and transport for pure as well as for multi-component n-alkane mixtures. The Flame Extinguishment (FLEX) experiments onboard the International Space Station provided a unique platform for investigating low temperature multi-phase n-alkane and iso-alkane combustion. Among the many interesting phenomena experimentally observed, cool flame extinction can occur, accompanied by the concurrent formation of a surrounding cloud of condensed vapor. In this work we conduct numerical simulations of high and low temperature combustion of large, initially single-component n-heptane, n-decane and n-dodecane droplets. The role of initial droplet diameter, operating pressure, and n-alkyl carbon number on the extinction of hot and low temperature flames is investigated and compared against the available experimental data. While all three fuels exhibit similar hot flame behavior, cool flame activity increases with the carbon number, resulting in an increased cool flame temperature and decreased extinction diameter. Multi-cyclic “hot/cool flame transitions” are found in air as pressure is slightly increased above one atmosphere. The cyclic behaviors correspond to continuously varying hot and cool flame transitions across the high, low, and negative temperature coefficient (NTC) kinetic regimes. Further increase in pressure results in a second stage steady “Warm flame” transition. The extinction of hot and cool flame has a strong non-linear dependence on ambient pressure but as the hot flame extinction diameter increases with pressure the extinction diameter of the cool flame decreases. The computational results are compared with a recent asymptotic analysis of FLEX n-alkane cool flames.

Reaction Model and Mechanism of Preparing (Al2O3 + C) Precursor for Carbothermal Synthesis of AlN by a Modified Low Temperature Combustion Synthesis Method.

The preparation of a homogeneous mixture of (Al2O3 + C) precursor is the key step for the successful synthesis of AlN powders by the carbothermal reduction and nitridation method. In the present work, the homogeneous (Al2O3 + C) precursor prepared by a modified low temperature combustion synthesis (MLCS) method by using aluminum nitrate, glucose, and urea as materials exhibited high reaction activity. Furthermore, in order to absolutely control the MLCS process and continuously improve the properties of (Al2O3 + C) precursor, the reaction model of preparing precursors from various molar ratios of urea to aluminum nitrate (U/Al) was investigated by carrying out thermodynamic calculation and by performing experiments in the present work. The whole process was found to involve various phenomena. First, the type and amount of various generated nitrogen-containing gases (N2, NO, N2O, N2O3, N2O4, and NO2) vary with the change of U/Al during combustion process. Second, under the present experimental condition of ignition temperature, the decomposition reaction of aluminum nitrate is more prone to occur than the combustion reaction of urea. Third, the real reaction system with U/Al = 2.5 reaches the highest combustion temperature which is well consistent with the propellant chemical theory. The occurrence of above phenomena was discussed in detail. Moreover, the reaction mechanism of synthesizing precursor from U/Al = 1 with high reaction activity was investigated by using various techniques such as FTIR, XRD, and DTA.

Experimental Study and Kinetic Modeling for the Laminar Flame Speed of Methyl Octanoate and n-Nonane

ABSTRACT Biodiesel is one of the most important alternative fuels due to limited conventional resources. Methyl octanoate with long alkyl chain and mono-alkyl ester is one of the surrogate fuels of actual biodiesel. In order to clarify distinctive combustion characteristics of methyl octanoate, comparison with alkane in traditional diesel is necessary. Therefore, this work chose n-nonane with same carbon number as methyl octanoate. The laminar flame speeds of two fuels were measured at different conditions (T = 423 K, P = 1 and 3 atm, and φ = 0.7–1.5) in a constant volume combustion bomb. Results show that laminar flame speed of n-nonane is higher than that of methyl octanoate in most conditions, but lower in fuel-rich conditions. The combined chemical kinetic model of methyl octanoate and n-nonane was developed through eliminating unnecessary reactions, which can well predict the experimental results of laminar flame speed and ignition delay time. The adiabatic flame temperature results from the updated kinetic model show that the temperatures of n-nonane are higher than methyl octanoate in whole conditions, which is unexpectedly opposite to their laminar flame speeds order in fuel-rich conditions. In overall conditions, the intermediates of methyl octanoate tend to produce CH3, which consumes H radicals, thereby reducing the laminar flame speed, which can explain the higher laminar flame speed of n-nonane in most conditions. Meanwhile, the oxidation reactions with oxygen molecules of intermediates of n-nonane are suppressed due to low combustion temperature at φ = 1.5 unlike at φ = 0.7 and 1.0. On the contrary, those of methyl octanoate have similar reactions between φ = 0.7, 1.0, and φ = 1.5. It means that the laminar flame speed of n-nonane decreases more significantly than methyl octanoate as the equivalence ratio increases from φ = 1.0 to φ = 1.5.

Review of morphological and chemical characteristics of particulates from compression ignition engines

Particulates from compression ignition (CI) engines have received serious attention in the last two decades. CI engines emit higher particulate matter (PM) and nitrogen oxides (NOx) than spark ignition (SI) engines. Both these species are harmful to human health and the environment. Compared to NOx emissions, PM constitutes many more chemical species in solid and liquid phases. This review paper focuses on soot morphology and chemical characterization of PM emissions from CI engines. Effects of different fuels, lubricating oil, and engine operating conditions on particulate characteristics are analyzed exhaustively. The first part of this paper focuses on the effects of particulates on living organisms, the consequences of exposure to diesel particulates, and the composition of diesel particulates. In recent decades, micro and nano-scale characteristics of PM have been exhaustively investigated to understand its structure, formation, and chemical functionalities. Typically, particulates comprise of elemental carbon (EC), organic carbon (OC), polycyclic aromatic hydrocarbons (PAHs), the soluble organic fraction (SOF), and trace metals. This paper summarizes most aspects of diesel particulate emissions for the benefit of active researchers in the field and underlines the importance of particulate emission reduction from the CI engines. Diesel combustion generates particles with enormous long-chain aggregates of smaller sizes and immature soot particles. Low-temperature combustion (LTC) modes and oxygenated fuels reduce the soot emissions and generate compact/clustered aggregates. Oxygenated fuels in CI engines produce more nucleation mode particles (NMPs) and high-reactivity soot aggregates. Higher trace metal concentrations were observed in diesel origin particulates than biofuel origin particulates. Biodiesel origin particulates possess higher mutagenicity and carcinogenicity because of nitro-PAHs. Transient engine operations cause higher particulates than steady-state engine operations.