Search Results (12)

Reasonably configuring the concentration distribution of the mixture to achieve partially premixed combustion has been proven to be an effective method for improving energy utilization efficiency. However, due to the significant influence of concentration non-uniformity and flow field disturbances, the combustion behavior and mechanisms of partially premixed combustion have not been fully understood or systematically analyzed. In this study, the partially premixed combustion characteristics of methane–hydrogen–air mixtures in a confined space were investigated, focusing on the combustion behavior and key parameter variation patterns under different equivalence ratios (0.5, 0.7, 0.9) and hydrogen contents (10%, 20%, 30%, 40%). The global equivalence ratio and degree of partial premixing of the mixture were controlled by adjusting the fuel injection pulse width and ignition timing, thereby regulating the concentration field and flow field distribution within the combustion chamber. The constant-pressure method was used to calculate the burning velocity. Results show that as the mixture formation time decreases, the degree of partial premixing increases, accelerating the heat release process, increasing burning velocity, and shortening the combustion duration. It exhibits rapid combustion characteristics, particularly during the initial combustion phase, where flame propagation speed and heat release rate increase significantly. The burning velocity demonstrates a distinct single-peak profile, with the peak burning velocity increasing and its occurrence advancing as the degree of partial premixing increases. Additionally, hydrogen’s preferential diffusion effect is enhanced with increasing mixture partial premixing, making the combustion process more efficient and concentrated. This effect is particularly pronounced under low-equivalence-ratio (lean burn) conditions, where the combustion reaction rate improves more significantly, leading to greater combustion stability. The peak of the partially premixed burning velocity occurs almost simultaneously with the peak of the second-order derivative of the combustion pressure. This phenomenon highlights the strong correlation between the combustion reaction rate and the dynamic variations in pressure. Full article

The sustainability of diesel engines has come to the forefront of research with the growing global interest in reducing greenhouse gas emissions and improving energy efficiency. The aim of this paper is to support the goal of sustainable development by improving the volatile properties of diesel fuel to promote cleaner combustion in engines. In order to study the effect of diesel fuel volatility on spraying, combustion, and emission, the tests were carried out with the help of the constant volume chamber (CVC) test rig and an engine test rig, respectively. CVC test: A high-speed video camera recorded the spray characteristics of different volatile fuels in a constant-volume combustion bomb. The effects of different rail pressures and ambient back pressures on the spray characteristics were investigated. Engine test: The combustion and emission characteristics of different volatile diesel fuels under different load conditions (25%, 50%, 75%) were investigated in a four-stroke direct-injection diesel engine with the engine speed fixed at 2000 rpm. The test results show that as the rail pressure increases and the ambient pressure decreases, the spray characteristics of the fuels tend to increase; for the more volatile fuels, although reducing the spray tip penetration, the spray projected area and spray cone angle increase, which is conducive to improving the homogeneity of the fuel and air mixing in the cylinder. The improvement of fuel volatility can form more and better-quality mixtures within the ignition delay time (ID), resulting in a 1–2% increase in peak cylinder pressure and a 2–4% increase in peak heat release. For different loads, pre-injection heat release is generated to redefine the ID and combustion duration (CD). Improved fuel volatility effectively reduces carbon monoxide (CO) emissions by about 8–10% and hydrocarbon (HC) emissions by about 13–16%, but it increases nitrogen oxide (NOx) emissions by about 8–11%. Analyzing from the perspective of particulate matter (PM), combined with the aromatic content of volatile fuels, it is recommended to use fuels with moderate volatility and aromatic content under low load conditions, and at medium to large loads, the volatility of the fuel has less weight on particulates and more weight on aromatics, so it is desirable to use the fuel with the lowest volatility and lowest aromatic content of the fuel selected. Full article

A technique alternative to the direct numerical simulation of turbulent combustion of gas mixtures is proposed. It is based on the solution of the three-dimensional transport equations for species concentrations and the energy conservation equation in the “synthetic” field of constant-pressure homogeneous, isotropic and statistically stationary (forced) turbulence using the detailed reaction mechanism. The synthetic turbulence with given spatial and temporal correlation functions is generated using the Monte Carlo method, assuming that the components of the vector of fluctuation velocity obey the normal Gaussian distribution. The technique is applied to the problem of turbulent combustion of fuel-lean and stoichiometric mixtures of hydrogen and methane with air at a turbulence intensity up to 10 m/s. The calculated turbulent flame propagation velocities agree satisfactorily with the values measured in the fan-stirred bomb. The predicted volume fractions of active reaction centers H, O, and OH in a turbulent flame are shown to be less than in a laminar flame up to an order of magnitude, which also agrees with the experiment. In general, calculations indicate that the “wrinkled flame” model is applicable to fuel-lean and stoichiometric mixtures of hydrogen and methane with air at turbulence intensities up to 10 m/s Full article

In this work, an experimental study and kinetic characterization of the combustion process and a morphologic study of hydrogen/methane–air mixtures are presented. The experimental study was performed in an optical access cylindrical constant-volume combustion bomb. This bomb is equipped to register the instantaneous pressure during combustion and records the combustion images using the high-speed Schlieren optical technique. This provides straightforward information to compute the flame propagation speed and direct evidence of the apparition of cellularity on the flame front. Through the images of the combustion process, it is possible to conduct a morphological study of the process using a flame monitoring model. Simultaneously, by means of a two-zone thermodynamical model, with the temporal evolution of pressure as the main intake, significant parameters are determined during the combustion process of different fuels under premixed conditions: burning velocity, rate of combustion, burned and unburned temperature, burned mass fraction, and rate of heat release, among others. Experimental results are compared with kinetic modeling results obtained with the Cantera package using the Gri-Mech 3.0 kinetic mechanism. Results show that a greater percentage of hydrogen in the fuel mixture increases the burning velocity and the cellularity of the flame front surface. At the same time, leaner mixtures and higher equivalence ratios enhance the apparition of the cellularity onset in the flames. Burning velocity increases with the increase in the initial temperature and the fuel/air mixture equivalence ratio. All the results obtained were validated with other data from the literature. Full article

Burning woody biomass for energy is gaining attention due to the environmental issues associated with fossil fuels and carbon emissions. The carbon released from burning wood is absorbed by plants and, hence, offsets pollution. The purpose of this study was to investigate the combustion characteristics (heat calorific values and ash contents) of three timbers: Araucaria cunninghamii, Instia bijuga, and Pometia pinnata to recommend for fuelwood. The test samples were sawdust particles (treatment) and solid woods (control) extracted from the heartwoods. The sawdust particles were oven dried, sieved, and pelletized into pellets using a hand-held pelletizing device, thus, forming a cylindrical dimension (volume 1178.57 mm³, oven-dry density 0.0008 g/mm³). Meanwhile, the solid woods were cubed and oven dried (volume 1000.00 mm³, oven-dry density 0.001 g/mm³). Prior to combustion in a semi-automatic bomb calorimeter, 90 test specimens (15 replicates per treatment and control per species) were conditioned to 14% moisture content (at a temperature of 105 °C) and weighed to a constant (unit) mass (1.0 g). The heat energy outputs and ash residues (of treatments) were analyzed statistically. The results indicated variability in heat energy outputs and ash residues between the test specimens of the three species. Comparatively, the treatment specimens of A. cunninghamii produced a higher calorific value (18.546 kJ/g) than the control (18.376 kJ/g), whilst the treatment specimens of I. bijuga and P. pinnata generated lower heat calorific values (17.124 kJ/g and 18.822 kJ/g, respectively) than the control (18.415 kJ/g and 20.659 kJ/g, respectively). According to ash content analysis, A. cunninghamii generated higher residues (6.3%), followed by P. pinnata (4.5%), and I. bijuga (2.8%). The treatment specimens of the three species could not meet the standard heat energy requirement (20.0 kJ/g) and, thus, were unsuitable for fuelwood. However, the control specimens of P. pinnata generated an equivalent heat energy (20.659 kJ/g) and could be a potential fuelwood. Full article

Aero-engine combustors may experience extreme low pressures in the case of an in-flight shutdown, which makes the study of aviation kerosene flame propagation characteristics at low pressures important. The present work examined flame propagation during the combustion of aviation kerosene over the pressure range from 25 to 100 kPa using a constant-volume bomb apparatus. The laminar burning speeds at different initial pressures, temperatures and equivalence ratios were measured and compared. In addition, numerical simulations were used to examine the reaction sensitivity of the laminar burning speed at low pressure. In trials at the lean flammability limit, the data indicated that it was more difficult to ignite the fuel under a lower pressure condition of 25 kPa and a lower temperature condition of 420 K. The experimental results of laminar burning speed were fitted to an equation providing the laminar burning speeds expected at different pressures (25–100 kPa), temperatures (400–480 K) and equivalence ratios (0.8–1.5). The temperature index ($\mathsf{\alpha }=1.76$) and pressure index ($\mathsf{\beta }=-0.15$) of the fitting equation were obtained. Both hydrodynamic and diffusional thermal flame instabilities were found to be suppressed at low pressures. The negative effects of two specific reactions on laminar burning speed were greatly reduced at these same low pressures of 25 kPa. Full article

Natural gas (NG) direct injection (DI) technology benefits the engine with high efficiency and clean emissions, and the high-pressure gas fuel injection process causes crucial effects on the combustion. This study presents an optical experimental investigation on the high-pressure methane single-hole direct injection and premixed ignition combustion based on a visualization cuboid constant volume bomb (CVB) test rig. The experimental results show that the methane jet process is divided into two stages. The methane gas jet travels at a faster speed during the unstable stage I than that during the stable stage II. The injection pressure causes more influence on both the jet penetration distance and the jet cone area during stage II. The methane jet premixed flame is a stable flame with a nearly spherical shape, and its equivalent radius linearly increases. The methane jet premixed flame area also increases while the flame stretch rate declines. The methane jet premixed flame velocity rises as both the standing time and equivalent ratio increase. The methane jet premixed flame is a partial premixed flame, and the peak of the methane jet premixed flame occurs at greater equivalence ratio ϕ, i.e., ϕ > 2. As the injection pressure rises, the jet premixed flame equivalent radius increases, and the flame velocity linearly increases. The higher the methane injection pressure, the faster the jet premixed flame velocity. Full article

The explosion-suppression effects of NSSs on overpressures, flame propagation and flame tip velocities were explored under the initial pressures of 0.2 MPa, 0.3 MPa and 0.4 MPa. All experiments tested in a constant volume combustion bomb (CVCB). Explosion reaction of premixed propane–air gas in a new designed CVCB filled with nonmetallic spherical spacers (NSSs) was analyzed. The results showed that overpressures decreased under the different initial pressures. With the increase of filling density, the overpressure decreased, the time to reach explosion overpressure decreased, and the decay rate of explosion overpressure increased. It was also found that the explosion-suppression effects of NSSs on pressures. Flame front could be captured by high-speed schlieren photography. Combustion phenomena were captured including flame propagation, corrugated laminar flame, jet flame, corrugated turbulent flame as well as tulip flame under different initial pressures. Flame tip velocities also were captured. The results demonstrate that flame tip velocities decreased with the increase of filling densities. However, compared with unfilled CVCB, flame tip velocities increased after filling NSSs in CVCB under different initial pressures. NSSs suppressed the explosion overpressure in the cylinder, and promoted the flame propagation. In both cases, NSSs played a dual role. The suppression effect of NSSs was affected by both its suppression and promotion effect on the explosion. This work provides a scientific basis for the effective prevention of explosion accidents with propane–air premixtures and the development of explosion-suppression products. Full article

The chemical kinetics mechanism is an important factor to accurately predict the combustion characteristics of constant-volume bomb (CVB). In this study, an n-heptane oxidation mechanism constructed by Wang et al. is introduced to study the correlation of the ignition behaviors with the mechanism constructed by Chang et al. The effects of the similarity factor method in the analysis of ignition behaviors of fuel in CVB were repeatedly verified by changing the important spraying parameters: injection pressure and hole diameter. Through further verification, it was found that the combustion process was controlled at approximately 850 K and stoichiometric ratio mixture of fuel/air in CVB, which corresponds to the negative temperature coefficient region at stoichiometric ratio mixture in shock tube (ST). The mechanism verified by the experiment under the condition in ST can reflect the chemical ignition in CVB. In addition, the similarity factor method was less dependent on the chemical reaction mechanism and boundary conditions. Full article

In this work are presented experimental values of the burning velocity of iso-octane/air, n-heptane/air and n-heptane/toluene/air mixtures, gasoline surrogates valid over a range of pressures and temperatures similar to those obtained in internal combustion engines. The present work is based on a method to determine the burning velocities of liquid fuels in a spherical constant volume combustion bomb, in which the initial conditions of pressure, temperature and fuel/air equivalence ratios can be accurately established. A two-zone thermodynamic diagnostic model was used to analyze the combustion pressure trace and calculate thermodynamic variables that cannot be directly measured: the burning velocity and mass burning rate. This experimental facility has been used and validated before for the determination of the burning velocity of gaseous fuels and it is validated in this work for liquid fuels. The values obtained for the burning velocity are expressed as power laws of the pressure, temperature and equivalence ratio. Iso-octane, n-heptane and mixtures of n-heptane/toluene have been used as surrogates, with toluene accounting for the aromatic part of the fuel. Initially, the method is validated for liquid fuels by determining the burning velocity of iso-octane and then comparing the results with those corresponding in the literature. Following, the burning velocity of n-heptane and a blend of 50% n-heptane and 50% toluene are determined. Results of the burning velocities of iso-octane have been obtained for pressures between 0.1 and 0.5 MPa and temperatures between 360 and 450 K, for n-heptane 0.1–1.2 MPa and 370–650 K, and for the mixture of 50% n-heptane/50% toluene 0.2–1.0 MPa and 360–700 K. The power law correlations obtained with the results for the three different fuels show a positive dependence with the initial temperature and the equivalence ratio, and an inverse dependence with the initial pressure. Finally, the comparison of the burning velocity results of iso-octane and n-heptane with those obtained in the literature show a good agreement, validating the method used. Analytical expressions of burning velocity as power laws of pressure and unburned temperature are presented for each fuel and equivalence ratio. Full article

In this article, the critical spontaneous ignition temperature of both hydrocarbon and alcohol fuel was acquired on a constant volume combustion bomb platform by slowly heating the inner charges, and then followed by using the CHEMKIN-PRO software to simulate the auto-ignition-dominated characteristic and parameter sensitivity of the two kinds of fuels. Results revealed that in different conditions, the critical spontaneous ignition temperature of methanol changed dramatically, with a maximum temperature of 50 K, while the counterpart temperature of n-heptane remained an invariable value of 553 K within a large changeable scope of temperature, and only a maximum temperature of 10 K was observed. The maximum difference of spontaneous ignition temperature between methanol and n-heptane reached 270 K. At the same time, a minimum difference of 170 K was obtained as well. The complete reaction of methanol requires 5 steps, involving 6 components and 11 elementary reactions. However, for the comparative part-n-heptane, more than 20 main self-ignition reactions were involved, which indicated that the whole reaction process of n-heptane has more reaction pathway branches and it was much more complicated compared to methanol. The differences of the reaction pathways triggered a considerable distinction of critical self-ignition temperature between the two charges, making a “step-by-step” spontaneous ignition combustion mode possible. In this way, a further high-efficient and clean combustion can be available to cater to much more stringent emission regulations in the future. Full article

Experimental characterization of the burning behavior of gaseous mixtures has been carried out, analyzing spherical expanding flames. Tests were performed in the Device for Hydrogen-Air Reaction Mode Analysis (DHARMA) laboratory of Istituto Motori—CNR. Based on a high-pressure, constant-volume bomb, the activity is aimed at populating a systematic database on the burning properties of CH₄, H₂ and other species of interest, in conditions typical of internal combustion (i.c.) engines and gas turbines. High-speed shadowgraph is used to record the flame growth, allowing to infer the laminar burning parameters and the flame stability properties. Mixtures of CH₄, H₂ and air have been analyzed at initial temperature 293÷305 K, initial pressure 3÷18 bar and equivalence ratio = 1.0. The amount of H₂ in the mixture was 0%, 20% and 30% (vol.). The effect of the initial pressure and of the Hydrogen content on the laminar burning velocity and the Markstein length has been evaluated: the relative weight and mutual interaction has been assessed of the two controlling parameters. Analysis has been carried out of the flame instability, expressed in terms of the critical radius for the onset of cellularity, as a function of the operating conditions. Full article