Fundamental Combustion Research Articles

The explosion of non-nano iron dust suspension in the 20-l spherical bomb

The understanding of dust explosion is still incomplete because of the lack of reliable data and accurate models accounting for all the physic-chemical aspects. Besides, most of the experimental data available in the current literature has been accumulated on the 20-l spherical bomb tests, which gives coarse results for the pressure history that cannot be easily converted into fundamental combustion parameters. Nevertheless, the large amount of experimental data available in the spherical bomb is attractive. In this work, the explosion of non-nano iron dust in the standard spherical vessel is analyzed, aiming at evaluating the burning velocity from the theoretical point of view and the simple experiments performed by the standard explosion tests. The choice of iron is of relevance because its adiabatic flame temperature is below the boiling temperature of both the reactants and oxidized gaseous, liquid, or solid (intermediate and final) products and for the negligible particle porosity, which instead is typical of organic dust. Therefore, a non-nano iron dust explosion can be reconducted to a reduced mechanism since heterogeneous (surface) combustion may be determinant, and the diffusion mechanism for oxygen is the only relevant. The laminar burning velocity is strongly dependant on the particle diameter, whereas little effects are due to the dust concentration. The reported final value was found in agreement with typical limiting laminar burning velocity, adopted for the estimation of flammability limits.

Prediction of Flame Length in Opposed-Flow Flame Spread: Global Similarity Analysis and Experiments

ABSTRACT Opposed-flow flame spread over solid fuels is an important case of fire safety studies. Spread rate and flame structure depend on fuel nature and burning conditions, and they can be used to study material flammability and fundamental combustion mechanisms. Previous studies provided important insights on flames developing over vertical walls and under the effect of a forced flow. However, there are limited works available in literature on the prediction of flame length of spreading flames even though it is the main driver of flame growth in concurrent-flow flame spread. In this work we consider the dependence of flame length on fuel thickness and flow velocity. Acrylic samples, with thicknesses from 0.05 to 12 mm, are burnt vertically in a wind tunnel against flow velocities between 0 and 200 cm/s. Without a forced flow, the flame length strongly depends on fuel thickness. Using scaling theory, closed-form expressions for both opposed-flow and downward spreading flames are developed and agree qualitatively with the data. For downward spread, the developed expression is shown to be quite similar to the theoretical flame length expression for gaseous diffusion flame by Roper. A least-squares fit of the developed expression with the data produces the undetermined constant of the scaled expression, producing a formula from which flame length over solid fuels (with both sides burning) can be predicted with a 90% accuracy.

Laminar burning velocities, CO, and NOx emissions of premixed polyoxymethylene dimethyl ether flames

Polyoxymethylene dimethyl ethers (OMEn) with the chemical structure of CH3O(CH2O)nCH3 are a promising synthetic e-fuel class that can be produced from renewable electricity and improves the emission characteristics of internal combustion engines. However, knowledge of their fundamental combustion properties, such as the laminar burning velocity, is limited in the literature, preventing the full assessment of their application potential in combustion devices. Thus, a heat flux burner is applied in this study to measure the laminar burning velocities of OME1 and OME2 over a wide range of equivalence ratios of 0.6–1.9 and at initial temperatures of 383–401 K under atmospheric pressure for the first time. The obtained data are compared with computed results using a recently developed reaction mechanism for OME1–4. Good agreement is observed. The experimental results are also discussed in the context of a comparison to literature data of dimethyl ether and OME3, demonstrating the minor impact of chain length of (OMEn) on their burning velocities, which is further analyzed in terms of sensitivity analysis on reaction kinetics. Furthermore, the exhaust gas composition of NOx and CO is determined experimentally for flames of OME1 and OME2 using a probe method and compared to the numerical results, observing mostly good agreement, where the remaining differences could be explained on the basis of combined experimental and model uncertainties. This provides unique knowledge on the emission performance of the combustion of (OMEn).

Development of a skeletal mechanism for four-component biodiesel surrogate fuel with PAH

A four-component skeletal mechanism consisting of 1,4-hexadiene, methyl decanoate, methyl trans-3-hexenoate, n-hexadecane was developed to represent biodiesel fuel from various sources. The methyl decanoate and n-hexadecane were selected to represent saturated fatty acid methyl esters, and trans-3-hexenoate and 1,4-hexadiene were applied to adjust degree of unsaturation. A skeletal mechanism for this four-component biodiesel surrogate fuel with PAH was first formulated based on decoupling methodology, which contained a detailed C1 mechanism, a reduced C2–C3 mechanism, PAH mechanism and sub-mechanisms of four surrogate fuels, including 314 reactions and 98 species. After that, the skeletal mechanism was widely verified against various fundamental combustion experiments for each pure component and their mixtures. Furthermore, the experimental results of biodiesel soot volume fractions from biodiesel spray combustion in a constant-volume combustion vessel were used to verify the accuracy of the mechanism, and the skeletal mechanism was also coupled into the CFD-software to simulate the combustion characteristics of a diesel engine. Results showed that the calculated results agreed well with the experimental data including ignition delay times (IDTs), primary species concentrations, laminar flame speed, soot prediction and in-cylinder pressure of the engine. Overall, the developed compact skeletal mechanism was suitable for the combustion simulation of biodiesel fuel.

Effects of ammonia addition on soot formation in ethylene laminar diffusion flames

In light of the increasingly stringent pollutant emission regulation and energy crisis, ammonia (NH3) has been regarded as a highly competitive alternative fuel. Previous studies mainly focused on the fundamental combustion characteristics and NOx emission of co-firing NH3 with hydrocarbons; however, soot emission was rarely studied. In this study, effects of NH3 addition with varying ratios (between 0% and 60% by mole fraction while the remainder is argon) on soot formation in ethylene (40% by mole fraction) laminar diffusion flames were experimentally and numerically investigated. Two-color laser induced incandescence and planar laser induced fluorescence method were used to measure the spatial distributions of soot volume fraction, polycyclic aromatic hydrocarbons (PAHs), and OH radical. Temperature profiles along the flame centerline were obtained through rapid thermocouple insertion technique. Results show that with NH3 addition, the flame height increases and the temperature at the same height decreases. Peak concentration and total loading show that the inhibitory effect of NH3 addition on PAHs and soot volume fraction (SVF) is in the order: A1 < A2&A3 < A4 < SVF. The primary particle size obtained by transmission electron microscope substantially decreases with the increasing NH3. In addition, the signal intensity of OH radical was diminished, indicating that instead of promoting the soot oxidation, NH3 reduces soot formation by inhibiting PAH growth, soot inception, and surface growth. Chemical kinetic analysis suggests that NH3 addition reduces the concentrations of H, C2H2, and C3H3, thus markedly inhibiting the rate-limiting step of A1 formation and PAH growth.

An experimental study on spray auto-ignition of RP-3 jet fuel and its surrogates

Jet fuel is widely used in air transportation, and sometimes for special vehicles in ground transportation. In the latter case, fuel spray auto-ignition behavior is an important index for engine operation reliability. Surrogate fuel is usually used for fundamental combustion study due to the complex composition of practical fuels. As for jet fuels, two-component or three-component surrogate is usually selected to emulate practical fuels. The spray auto-ignition characteristics of RP-3 jet fuel and its three surrogates, the 70% mol n-decane/30% mol 1,2,4-trimethylbenzene blend (Surrogate 1), the 51% mol n-decane/49% mol 1,2,4-trimethylbenzene blend (Surrogate 2), and the 49.8% mol n-dodecane/21.6% mol iso-cetane/28.6% mol toluene blend (Surrogate 3) were studied in a heated constant volume combustion chamber. Surrogate 1 and Surrogate 2 possess the same components, but their blending percentages are different, as the two surrogates were designed to capture the H/C ratio (Surrogate 1) and DCN (Surrogate 2) of RP-3 jet fuel, respectively. Surrogate 3 could emulate more physiochemical properties of RP-3 jet fuel, including molecular weight, H/C ratio and DCN. Experimental results indicate that Surrogate 1 overestimates the auto-ignition propensity of RP-3 jet fuel, whereas Surrogates 2 and 3 show quite similar auto-ignition propensity with RP-3 jet fuel. Therefore, to capture the spray auto-ignition behaviors, DCN is the most important parameter to match when designing the surrogate formulation. However, as the ambient temperature changes, the surrogates matching DCN may still show some differences from the RP-3 jet fuel, e.g., the first-stage heat release influenced by low-temperature chemistry.

A review of motivations, methods, and achievements on microgravity combustion research

Microgravity combustion has always been one of the key and frontier research fields in the international combustion community. Why has microgravity combustion attracted so much attention? First of all, fire safety and combustion reliability are imminent to spacecraft. Secondly, the convection effect is weakened in the microgravity environment, and the basic combustion phenomena are more prominent, which is greatly helpful to study the fundamental laws of combustion and develop reliable combustion models for fuels. This article systematically summarized and analyzed the research focuses, namely fire safety of spacecraft, droplet combustion, soot formation and flame structures, and current research status in the field of microgravity combustion during the past decades of years. In this paper, motivations, methods, and achievements of fire safety in spacecraft and fundamental combustion characteristics in microgravity were analyzed, which provide a comprehensive review for the microgravity combustion research. Suggestions for the development direction of microgravity combustion science were also made. Improving the application of efficient and clean combustion and optimizing fire safety strategies of new generation spacecraft materials should still be regarded as two critical targets in the future.

Effects of strain rate and Lewis number on forced ignition of laminar counterflow diffusion flames

Forced ignition is one of the most fundamental and important combustion problems. It is used in advanced engines and closely related to fire safety and accidental explosion. In this study, forced ignition of laminar counterflow diffusion flames with one-step chemistry is studied through two-dimensional simulations. The initial ignition kernel is simply modelled as a hot spot of unburned gas with a specified radius and uniform high temperature inside it. We focus on the subsequent ignition kernel development, during which an expanding axisymmetric triple flame appears under certain conditions. Non-monotonic change of the flame kernel radius with time is observed near the critical ignition conditions. The effects of strain rate and Lewis number on the ignition kernel development and critical ignition conditions are assessed. For the same initial hot spot, successful ignition can be achieved at relatively low strain rate and fuel Lewis number; while the ignition kernel quenches at relatively high strain rate or fuel Lewis number. The strain rate and Lewis number are shown to have great impact on the critical hot spot radius and on the minimum ignition energy. At relatively low strain rates, the minimum ignition energy increases linearly with the strain rate. This is consistent with previous theory on flame ball in a mixing layer. Besides, the optimum ignition position and the influence of the hot spot shape are investigated. It is demonstrated that the optimum ignition position is close to the stoichiometric surface and it depends on the Lewis number and global equivalence ratio.

Theoretical Characterization of the Structure and Spectroscopy of HCNO2 Isomers and Applications.

We carried out a theoretical, fully ab initio, investigation of the stable forms of the [H,C,N,O,O] pentatomic molecular system, whose isomers are involved in fundamental combustion and atmospheric processes and are of potential interest for astrophysics. By adopting the MP2 and CCSD(T) electronic structure methods, combined with extrapolations to the complete basis set (CBS) limit, we characterized 20 low-energy isomers, excluding weak van der Waals complexes. For these molecules, we determined a set of geometrical parameters, relative energies, anharmonic vibrational frequencies, IR intensities, and fragmentation/formation energies from various atomic and/or molecular fragments. We discuss the relevance of the present findings for the search of new molecular species in astrophysical and atmospheric media and give suggestions for their possible detection in laboratory experiments. The set of data provided by the present work should facilitate the identification of these species from their gas-phase and low-temperature solid matrix spectra, whenever measured.

Development of biofuel from Nigella sativa biomass and its suitability for energy application

Nigella sativa is one such plant species indigenous to Middle-east, Eastern Europe, and the Asian Subcontinent. Black cumin seeds derived from N. sativa are one of the identified biomasses to produce biodiesels for the use in an unmodified CI engine. The current study aims to evaluate the credibility of Nigella sativa methyl ester (NSME) as a suitable blend alongside mineral diesel in a common rail direct Injection engine via fundamental combustion analysis. In the present work, 20% of Nigella sativa methyl ester has been tested in diesel engine at 600 bar injection pressure under different pilot and main injection forms. The pilot injection mass has been varied from 5–15% along with variation in main injection also. An optimized injection strategy has been found with respect to performance and emissions for effective implementation of EGR at 10% and 20% rate. At 5% pilot injection, the NSME20 fuel showed comparable brake thermal efficiency and brake-specific fuel consumption at all engine loads. Furthermore, NOx emission of NSME20 fuel has been reduced by increasing the pilot injection rate when compared to diesel and this has been further reduced with 10–20% exhaust gas recirculation. On the other hand, the combustion characteristics of NSME20 blend also shown similar pattern like diesel fuel with slight reduction in cylinder pressure and heat release rate. Finally, it has been concluded that the newly developed Nigella sativa methyl ester can be used as sustainable biofuel for various energy applications in the present scenario.

Prandtl number of optimum biodiesel from food industrial waste oil and diesel fuel blend for diesel engine

Unconventional biodiesel characterization techniques using thermophysical and transport properties have been receiving increasing attention due to its advantages over fundamental combustion and simulation of heat transfer in solving heat transfer, chemical, and bioenergy characteristics of biodiesel combustion. In this study, the optimum production yield of Food Industrial Waste Oil Methyl Ester (FIWOME, B100, FIWOB) was modelled using Response Surface Methodology (RSM) and Artificial Neural Network (ANN) techniques. The basic properties of the fuel types were determined using ASTM test methods, while specific heat capacity (Cp), thermal diffusivity (TD), thermal conductivity (TC) and Prandtl number (Pr) were determined using standard methods. Diesel engine performance indicators such as Engine Torque (ET), Brake Power (BP), Brake Specific Fuel Consumption (BSFC) and Brake Thermal Efficiency (BTE) were determined for different fuel types using a Perkins diesel engine. The estimated Coefficient of Determination (R2) of 0.9820, Root Mean-Square-Error (RMSE) of 1.7403, Standard Error of Prediction (SEP) of 0.0215, Mean Average Error (MAE) of 1.3790, and Average Absolute Deviation (AAD) of 1.6389 for RSM compared to those of R2 (0.9847), RMSE (1.6071), SEP (0.0199), MAE (1.1425), and AAD (1.2583) for ANN exhibited the robustness of the ANN tool over the RSM technique. Optimal biodiesel-- diesel fuel (B0) blend was 22.5% volume ratio called as B22.5. The optimum yield of FIWOME of 92.5% was achieved at the methanol/oil molar ratio of 5.99, KOH of 1.1 wt.%, and a reaction time of 77.6 min. The basic properties of FIWOME determined complied with both ASTM D6751 and EN 14214 specifications. The values of Cp and TD increased and decreased respectively with biodiesel percent in fuel in a quadratic manner. The TC and Pr were correlated with the biodiesel fraction through linear regressions. The brake power, brake torque, and brake specific fuel consumption were observed to increase with an increase in biodiesel percentage level. The determined performance characteristics of B22.5, which is above B20 as recommended for powered diesel engine gave a good improvement in terms of BTE and Pr making FIWOME based biodiesel a viable substitute for fossil diesel.

Two-stage autoignition and combustion mode evolution in boundary layer flows above a cold flat plate

Boundary layers are omnipresent in fundamental kinetic experimental facilities and practical combustion engines, which can cause ambiguity and misleading results in kinetic target acquisition and even abnormal engine combustion. In this paper, using n-heptane as a representative large hydrocarbon fuel exhibiting pronounced low-temperature chemistry (LTC), two-dimensional numerical simulation is conducted to resolve the transient autoignition phenomena affected by a boundary layer. We focus on the ignition characteristics and the subsequent combustion mode evolution of a hot combustible mixture flowing over a colder flat plate in an isobaric environment. For cases with autoignition occurring within the boundary layer, similarity is observed in the first-stage ignition as manifested by a constant temperature at all locations. The first-stage ignition is found to be rarely affected by heat and radical loss within the boundary layer. While for the main ignition event, an obvious dependence of ignition process on boundary layer thickness is identified, where the thermal-chemical process exhibits similarity at locations with similar boundary layer thickness, and the main ignition tends to first occur within the boundary layer at the domain end and generates a C-shape reaction front. It is found that sequential spontaneous autoignition is the dominant subsequent combustion mode at high-pressure conditions. At low to intermediate pressures, auto-ignition assisted flame propagation is nevertheless the dominant mode for combustion evolution. This research identifies novel features of autoignition and the subsequent combustion mode evolution affected by a cold, fully developed boundary layer, and provides useful guidance to the interpretation of abnormal combustion and combustion mode evolution in boundary layer flows.

Effect of piston bowl geometry and compression ratio on in-cylinder combustion and engine performance in a gasoline direct-injection compression ignition engine under different injection conditions

Low temperature combustion (LTC) of high-octane number fuels in compression ignition engines offers an opportunity to simultaneously achieve high engine thermal efficiency and low emissions of NOx and particulate matter without using expensive after-treatment technologies. LTC engines are known to be sensitive to the operation conditions and combustor geometry. It is important to understand the fundamental flow and combustion physics in order to develop the technology further for commercial application. A joint numerical and experimental investigation was conducted in a heavy-duty compression ignition engine using a primary reference fuel with an octane number of 81 to investigate the effects of injection timing, piston geometry, and compression ratio (CR) on the fuel/air mixing and combustion covering different regimes of LTC engines, homogeneous charge compression ignition (HCCI), partially premixed combustion (PPC), and the transition regime from HCCI to PPC. The results show that with the same combustion timing, a higher CR leads to a lower NOx, but a higher emission of UHC and CO. The piston geometry shows a significant impact on the combustion and emission process in the transition regime while it has minor influence in the HCCI and PPC regimes. It is found that high engine efficiency and low emissions of NOx, CO and UHC can be achieved in the earlier PPC regime and later transition regime. The fundamental reason behind this is the stratification of the mixture in composition, temperature and reactivity, which is dictated by the interaction between the spray and the cylinder/piston walls.

Surrogate-based modeling for emulation of supercritical injector flow and combustion

High-fidelity simulations play an increasingly important role in understanding fundamental turbulence-chemistry interactions and combustion dynamics in practical propulsion and power-generation systems. These simulations are, however, too computationally expensive and unwieldy for the purposes of design and optimization, given the large group of design parameters and wide design space. In this paper, we present an efficient surrogate-based modeling strategy to emulate spatiotemporal flows and combustion at supercritical pressures with accuracy similar to that of large-eddy simulations (LES). A common kernel-smoothed proper orthogonal decomposition (CKSPOD)-based surrogate model is developed, incorporating computer experiments, projection-based model reduction, kriging, and uncertainty quantification. The surrogate model (emulator) is carefully trained using a database drawn from a set of LES-based simulations that are conducted at designated sampling points in a given design space. A common Gram matrix is built using a Hadamard product to transform reduced spatial basis functions to remedy phase deviations among different design settings. Kriging is then used to obtain temporal spatial functions and associated coefficients for flowfield reconstruction at a new design setting.The framework is examined with two case studies: an emulation of flow dynamics in a simplex swirl injector, and an emulation of mixing and combustion in a gas-centered liquid-swirl coaxial injector. The surrogate model not only faithfully captures the salient features of its LES counterpart, but also shortens the computation time dramatically, by up to five orders of magnitude. The developed surrogate model can be applied to a broad range of engineering systems and will provide support for future engineering innovation and design.

Low- and high-temperature study of n-heptane combustion chemistry

n-Heptane has been used extensively in various fundamental combustion experiments as a prototypical hydrocarbon fuel. While the formation of polycyclic aromatic hydrocarbon (PAH) in n-heptane combustion has been studied preferably in premixed flames, this study aims to investigate the combustion chemistry of n-heptane in less-studied diffusion flame and highly rich high-temperature homogeneous oxidation configurations by using a counterflow burner and a flow reactor, respectively. This work addresses the formation of higher-molecular species in the mass range up to about 160 u in both configurations. Samples are analyzed by time-of-flight (TOF) molecular beam mass spectrometry (MBMS) using electron-impact (EI) and single-photon ionization (PI). Highly resolved speciation data are reported. Laminar flow reactor experiments cover a wide temperature range. Especially the measurements at low temperatures provide speciation data of large oxygenates produced in the low-temperature oxidation of n-heptane, which are scarce in the literature. Important precursor molecules for PAH and soot formation, such as C9H8, C10H8, C11H10, and C12H8, are formed during the high-temperature combustion process in the counterflow flame, while oxygenated growth species are observed under low-temperature conditions, even at the fuel-rich equivalence ratio of ϕ=4.00.Numerical modeling for both conditions is performed by using a newly developed kinetic model of n-heptane, which includes the n-heptane and PAH formation chemistry with state-of-the-art kinetic knowledge. Good agreement between model predictions and experimental data of counterflow flame and flow reactor is observed for the major species and some intermediates of n-heptane oxidation. While the concentrations of benzene and toluene measured in the counterflow burner are well-reproduced, the numerical results for flow reactor data are not satisfactory. Differences are found between the formation pathways of fulvene, from whose isomerization benzene is produced in diffusion flame and flow reactor.

Exploration on laminar flame propagation of ammonia and syngas mixtures up to 10 atm

Reactivity enhancement is crucial for the potential applications of ammonia (NH3) as a gas turbine fuel. Doping more reactive fuels like H2, methane and syngas is a widely adopted strategy in this field, however fundamental combustion studies of NH3/reactive fuel mixtures under gas turbine-relevant high pressure conditions are still very limited. This work reports an effort to study laminar flame propagation of NH3/syngas mixtures up to 10 atm. Laminar burning velocities (LBVs) of NH3/syngas/air mixtures were measured at 298 K, various syngas contents in fuel mixtures (α) and H2 contents in syngas (β), and equivalence ratios of 0.7–1.5 in a high-pressure constant-volume cylindrical combustion vessel. A kinetic model was developed for NH3/syngas combustion based on our recent NH3 model and a recent syngas model in literature. It shows reasonable predictions on the present NH3/syngas LBVs at 1–10 atm, as well as previous data including NH3/syngas LBVs at 1 atm, NH3/syngas ignition delay times at various pressures and pure NH3 LBVs at various pressures. Modeling analysis including the sensitivity analysis and rate of production analysis provides kinetic interpretation for the effects of fuel composition (α and β), equivalence ratio and pressure on NH3/syngas LBVs. It is found that the addition of syngas shifts the chemistry from NH3 sub-mechanism to syngas sub-mechanism. A modified fictitious diluent gas method was proposed to separate the chemical and thermal effects of syngas addition, which shows that the chemical effect is more responsible for the enhanced laminar flame propagation. NH3/syngas/air flames have similar reaction networks but different preferred pathways under lean and rich conditions. Compared with pure NH3 flames, the addition of syngas also improves the importance of H + O2 (+ M) = HO2 (+ M) and consequently leads to strong pressure dependency of NH3/syngas/air flames.

Autoignition of CRC diesel surrogates at low temperature combustion conditions: Rapid compression machine experiments and modeling

As federal programs require increasingly stringent engine emissions and fuel economy standards, these ambitions can only be met if next-generation combustion technology is developed focusing on high-efficiency and low-emissions engines. Recent research has indicated the need to operate engines at higher compression ratios and with low temperature combustion (LTC) to achieve the needed gains in engine efficiency and reductions in emissions. Because there is a lack of understanding of the chemistry of diesel fuel components and their mixtures at these LTC conditions, this limits the ability to develop predictive chemical kinetic models that can be used to optimize engine combustion. The current study aims to fill in gaps in fundamental combustion data on surrogate fuel mixtures relevant to diesel fuels. Specifically, four multicomponent diesel surrogates formulated by the Coordinating Research Council (CRC) to emulate an ultra-low-sulfur research-grade #2 certification diesel fuel (CFA), namely V0a (4 components), V0b (5 components), V1 (8 components), and V2 (9 components), have been investigated in a rapid compression machine (RCM) through determination of total and first-stage ignition delay times. Autoignition characteristics of lean to rich fuel/O2/N2 mixtures, for the four CRC surrogates and CFA, have been measured using an RCM at LTC relevant pressures and temperatures, in the ranges of 10–20 bar and 650–1000 K, respectively. The equivalence ratios have been varied by independently changing the oxygen mole fraction and the fuel mole fraction in the test mixtures, thereby illustrating the individual effects of oxygen concentration and fuel loading on diesel autoignition. Autoignition results of these four CRC surrogates are compared among them and with those of CFA. Some degree of agreement in autoignition response between each CRC surrogate and CFA is observed, while discrepancies are also identified and discussed. In addition, a detailed chemical kinetic model for diesel surrogates has been developed and validated against these newly-acquired RCM data. This model shows reasonable agreement with the overall ignition delay time results of the current RCM experiments. Chemical kinetic analyses of the developed model were further conducted to help identify the reactions controlling the autoignition processes and the consumption of fuel components in CRC surrogates.

Fuel Effects in Turbulent Premixed Pre-vaporised Alcohol/Air Jet Flames

To study combustion fundamentals of complex fuels under well-defined boundary conditions, a novel Temperature Controlled Jet Burner (TCJB) system is designed that can stabilise both gaseous or pre-vaporised liquid fuels. In a first experimental exploratory study, piloted turbulent jet flames of pre-vaporised methanol, ethanol, 2-propanol and 2-butanol mixtures are compared to methane/air as a reference fuel. Complementary one-dimensional laminar flame calculations are used to provide flame parameters for comparison. Blow-off and flame length as global flame characteristics are measured over a wide range of equivalence ratios. For fuel rich conditions, blow-off limits correlate well with extinction strain rate calculations. Differing flame lengths from lean to rich conditions are explained partly by different flame wrinkling that is assessed using planar laser-induced fluorescence imaging of the hydroxyl radical (OH-PLIF). A study of Lewis-number effects indicates that they have substantial influence on flame wrinkling. Lean alcohol/air flames, opposed to methane/air, have a Lewis-number greater than unity. This impedes curvature development, which promotes relatively large flame lengths. In contrast, across stoichiometric conditions, all alcohol/air mixture Lewis-numbers decrease significantly. At such conditions, alcohol/air flames show alike or even larger wrinkling compared to methane/air flames. However, quantitatively, the differences in flame length and wrinkling observed among the flames can neither be explained alone by Lewis-number differences, nor other global mixture parameters available from 1D laminar flame calculations. This study shall therefore emphasise the need for more detailed experimental analyses of the full thermochemical state of laminar and turbulent flames fuelled with complex fuels.

The effect of ammonia addition on the low-temperature autoignition of n-heptane: An experimental and modeling study

Ammonia (NH3) is receiving increasing attention as an alternative engine fuel due to its carbon-free nature. However, its fundamental combustion characteristics, such as higher autoignition temperature and lower burning velocity compared to conventional hydrocarbons, limit its direct use in traditional engines. To confront this dilemma, hydrocarbon/NH3 blending fuels are seen as one of the most appropriate ways to exploit its advantage and offset its weaknesses. Using n-heptane as a representative of hydrocarbons, this study investigated the effect of NH3 addition on the low-temperature autoignition of n-heptane. The ignition delay times of five n-heptane/NH3 mixtures with NH3 fractions of 0%, 20%, and 40% were measured in a rapid compression machine at temperatures of 635–945 K, pressures of 10 and 15 bar, and equivalence ratios of 1.0 and 2.0. Experimental results show that the n-heptane/NH3 blending fuels exhibit pronounced low-temperature reactivity, and both the total and the first-stage ignition delay times increase with the increase of NH3 fraction. A blending mechanism of n-heptane/NH3 was compiled based on the existing n-heptane mechanism and NH3 mechanism. It is found that the blending mechanism is capable to predict the inhibition effect of NH3 addition on n-heptane autoignition and qualitatively capture the dependence of ignition delay time on equivalence ratio and oxygen mole fraction. Nevertheless, there are significant discrepancies between the experiments and the simulation. Furthermore, kinetic analyses, including species evolution, rate of production and sensitivity of OH radical were conducted sequentially to reveal the autoignition kinetics of NH3/n-heptane blends and the interaction between n-heptane and NH3. Suggestions are provided for the further development of the blending mechanism.

Effects of alcohol addition to traditional fuels on soot formation: A review

Soot is one of the main pollutants produced by incomplete combustion of hydrocarbon fuels, which poses a serious threat to human health and the urban environment. Alcohols have been generally considered to be a potential clean fuel to reduce soot emissions. At present, alcohol fuels represented by methanol, ethanol, butanol and pentanol have been extensively studied and have made remarkable progress. Therefore, this article is written as a broad overview of the subject. The effects of methanol, ethanol, butanol and pentanol additions on the formations of soot and polycyclic aromatic hydrocarbons were systematically summarized from the two aspects of the internal combustion engine bench and the fundamental combustion system. These studies have shown a decreasing or synergistic effect of alcohols addition on soot formation and initially revealed the mechanism of soot formation. However, the formation mechanism of soot in complex environments and the mechanism of the effects of alcohols on soot still need to be studied in depth, which can provide more opportunities for humans to reduce soot production during the combustion of hydrocarbon fuels. Finally, some challenges and research suggestions are outlined in the last section of this review.