Constant Volume Combustion Chamber Research Articles

Experimental Combustion and Flame Characterization of a Chemical Looping-Based Oxidative Dehydrogenation Byproduct Fuel Mixture Containing High CO2 Dilution

Abstract This study investigates the combustion performance of a CO2-rich fuel mixture containing ethane and methane as active species using a constant volume combustion chamber. This fuel is obtained as a byproduct of a chemical looping-based oxidative dehydrogenation (Cl-ODH) process ethylene production. The byproduct gas mixture has 40.79% CO2, 39.49% ethane, and 4.88% methane by weight with other minor compounds. Using this fuel for energy extraction would improve the process efficiency of the ethane to ethylene conversion. After initial combustion modeling, the gas fuel mixture was reduced to just the major species: CO2, ethane, and methane. The mixture was then tested for flammability limits and combustion performance under spark-ignition conditions. Effects of ambient conditions like temperatures between 300 and 400 K with initial pressures from 1 to 10 bar were tested. The effects of stoichiometry were tested to understand flame velocities and heat release. The fuel mixture showed an overall reduced flame velocity compared to gasoline. Instability in combustion was believed to be caused by the dissociation of ethane under elevated conditions. At higher pressures, the flame produces lower cumulative heat release. Simulations were also performed using a model tuned to replicate the operations of the combustion chamber used in the experiments. Heat release and unburnt fuel mass data were calculated to identify the discrepancies in the combustion completeness at elevated pressures. The effects of CO2 quenching the flame coupled with the increased dissociation of the fuel species can lead to up to more than 75% of the fuel mixture being unburnt. Data from this study were used to modify a small-scale spark-ignition engine to use this fuel and produce usable energy.

Cellular instabilities of expanding hydrogen-propane-nitrous oxide spherical flames: experiment and theory

The cellular instabilities of expanding hydrogen-propane-nitrous oxide spherical flames were experimentally and theoretically investigated at various equivalence ratios (ϕ=0.4-1.6), hydrogen fractions (XH=0-0.8), and initial pressures (Pu=1.1-1.5 bar). The experiments were conducted in a constant volume combustion chamber (CVCC) using high-speed schlieren technology to record the flame morphology, while the theoretical analysis was based on linear stability theory. The controlling parameters, critical conditions for the onset of instability, flame front oscillation intensity, logarithmic growth rate of perturbation, etc. were studied. The results indicate that the instability of premixed hydrogen-propane-nitrous oxide flames is dominated by hydrodynamic instability at XH≤0.6, while the competitive mechanism between thermal-diffusion and hydrodynamic instabilities is considered at XH=0.8. The oscillation intensity in the horizontal direction of the flame profile is more prominent than those in other directions, and with the increasing hydrogen fraction, the equivalence ratio at which the highest oscillation strength occurs shifts from ϕ=1.0 to ϕ=0.6. The trends of the experimental critical radius and Peclet number with each initial condition are completely identical to that of the theoretical value, but the experimental value is somewhat underpredicted. The three main reasons are proposed in the paper.Novelty and Significance Statement: This work is the first study on the cellular instabilities of hydrogen-propane-nitrous oxide flames by experiment and theory. It is under a small scale and reduced initial pressure condition that the cellular destabilization of hydrogen-propane flames is realized in the nitrous oxide atmosphere, while in the air atmosphere, the initial condition at which flame cellular destabilization occurs is harsher. Flame front oscillation strength, growth rate of disturbance, etc. were used to characterize quantitatively the degree of wrinkles and instabilities. Some novel findings were presented in the study. The present work is of great significance for the combustion community to understand deeply the cellular instabilities characteristic of expanding spherical flames supported by nitrous oxide, as well as for the engineering community to optimize the engine, propulsion, and energy supply systems, etc.

Visualization study the cross spray and combustion characteristics of diesel and methanol in a constant volume combustion chamber at cold and flare flash boiling regions

To optimize the performance of diesel/methanol dual fuel engine, fundamental research on the spray and combustion characteristics of diesel/methanol dual direct injection was conducted in a constant volume combustion chamber. The injection pressures were set as at 60 MPa, 80 MPa, 100 MPa, while the ambient pressures were set as 4 MPa and 0.01 MPa, corresponding to cold and flare flash boiling regions, respectively. The results showed that diesel/methanol cross-collision accelerated the droplet interaction, promoting more uniform mixing. Compared to the cold region, the diesel/methanol collision spray had a larger axial and radial diffusion rate and evaporation rate in the flare flash boiling region. In addition, in the flare flash boiling region, the collision length, collision width, and spray projected area of both the liquid and gas phases were significantly greater than those in the cold region. With the increase in injection pressure, the spray obtained more kinetic energy, leading to a more intense collision process and more homogeneous fuel-gas mixing. Besides, a higher injection pressure shortened the mixing time of the fuel-air mixture before collision, resulting in a smaller equivalent ratio, a smaller ignition delay, a larger flame lift-off length, and a lower soot formation.

Prediction of emission characteristics of diesel/n-hexanol/graphene oxide blended fuels based on fast outlier detection-sparrow search algorithm-bidirectional recurrent neural network

This study aims to utilize deep learning (DL) technology to address the issue of carbon soot emission in traditional internal combustion engines. Spraying and combustion experiments were conducted in a constant volume combustion (CVC) chamber using n-hexanol and graphene oxide nanoparticles as partial substitutes for diesel, with the goal of predicting their emission characteristics based on experimental data. Initially, spray and combustion data under different operating conditions were collected using the CVC experimental setup, and outliers were removed using the fast outlier detection (FOD) algorithm. Subsequently, the learning rate and the number of hidden layer neurons in the Bidirectional Recurrent Neural Network (BiRNN) model were optimized using the Sparrow Search Algorithm (SSA) to enhance the model's accuracy and generalization ability. Unlike previous research methods, the novel FOD-coupled SSA optimization method used in this study ensures the optimal parameter configuration of the speed prediction model while accelerating the convergence speed, which ultimately improves the stability of the model. In addition, the optimal output variables were determined based on the Spearman and Pearson correlation coefficient rules. Finally, the effectiveness of the predictive model was validated using evaluation metrics such as root mean square error (RMSE) and R-squared (R2). The results indicate that the KL coefficient for D80H20GO20 fuel in the CVC device is minimal. The FOD-SSA-BiRNN model achieved R2 values of 0.99409, 0.99529, and 0.99692 for D100, D80H20, and D80H20GO20 fuels, respectively. Therefore, this study provides an effective method for the online accurate prediction of carbon soot emissions in mixed fuels.

Effects of ignition methods and control parameters on the lean combustion performance and flame geometry of jet ignition based on constant-volume combustion chamber

Pre-chamber jet ignition provides one of the most reliable thermal efficiency improvement technologies for future gasoline engines, especially hybrid gasoline engines. The effects of jet ignition control parameters on the jet flame morphology and combustion characteristics are still unclear, for example, the fuel injection mass in the pre-chamber and the time interval between low-flow injection and ignition. The paper explored the effects of jet ignition stoichiometric and lean combustion performance on the flame propagation process of high-energy spark ignition, passive and active pre-chamber, using direct flame imaging in a constant-volume combustion chamber. The jet ignition performance under lean combustion conditions is further enhanced by optimizing the jet ignition control parameters. Based on the luminance characterization of the image, the active pre-chamber with additional low-flow injection can provide 281% of the ignition intensity equivalent to high-energy spark plug ignition. Increasing the fuel injection mass in the pre-chamber can improve heat release and enhance ignition performance. However, the jet ignition performance will drop sharply due to the too-rich pre-chamber gas mixture. Extending the mixing time interval after ignition will enrich the lean ambient mixture outside the pre-chamber and increase the ignition performance. Over-mixing will occur if the mixing time interval is too long, especially under a few injected fuel mass situations. The radial stretch coefficient of the jet flame is defined to characterize the flame geometry during the jet flame propagation process. The jet ignition performance reaches its best when the jet flame propagates at high speed in both radial and circumferential directions, namely a wide jet flame.

Effects of jet interaction angle on the ignition and combustion characteristics of hydrogen-diesel dual-fuel direct injection

This study investigates the ignition and combustion characteristics of intersecting diesel surrogate (pilot) and hydrogen (H2, main) jets under engine-relevant conditions. The experiments, performed in an optically accessible constant-volume combustion chamber (CVCC), utilised two converging single-hole injectors, with the pilot fuel accounting for 12% of the total injected fuel energy. This study investigated the effects of two key parameters on the ignition process: jet interaction angle (12° to 19°) and ambient O2 concentration (10 to 21 vol.%). The results show that the presence of H2 either advances or delays pilot ignition depending on whether the pilot n-heptane jet ignites before or after interacting with the H2 jet, respectively. The pilot-main ignition transition period is influenced by both jet interaction angle and ambient O2 concentration. Under identical ambient conditions, a smaller jet interaction angle results in a longer transition, while for a constant angle, lower ambient O2 leads to a more prolonged transition. Under 10 vol.% O2 conditions, flame kernels emerge upstream of the main flame body, before eventually merging with the reacting jet downstream, with this phenomenon observed to induce variation in heat and flame stabilisation characteristics. An explanation for the upstream kernel formation is offered based on the entrainment of residual pilot n-heptane-jet fuel into the upstream region of the still-injecting main jet, with the relative jet momentum a likely key contributor influencing this entrainment that impacts kernel formation.

Experimental Observation of the Turbulent Jet Ignition for Extending the Lean Combustion Tolerance with Carbon Dioxide Diluted Mixture

ABSTRACT Exhaust Gas Recirculation (EGR) technology is widely employed to reduce NOx emissions from engine combustion, and a major challenge of this technology is to achieve stable ignition and high burning rate at higher EGR dilution ratios. Turbulent jet ignition (TJI) is a reliable ignition resource for improving ignition stability and burning rate. Therefore, in the present work, the ability of pre-chamber turbulent jet ignition for extending the lean combustion tolerance at different EGR dilution ratios was investigated in a constant volume combustion chamber. The experimental results indicate that the methane-fueled pre-chamber and the hydrogen-fueled pre-chamber cannot improve the combustion conditions as expected. On the other hand, when an appropriate quantity of air or oxygen is injected, the EGR tolerance limit can be extended due to a decrease in the EGR concentration and an increase in the mixture reactivity in the pre-chamber. However, at higher EGR dilution ratios, more air or oxygen is required to be injected to increase the mixture reactivity in the pre-chamber, which decreases the equivalence ratio of the mixture in the pre-chamber and limits further extension of the EGR tolerance. For the mixture in the active pre-chamber under EGR dilution, the competition mechanism between EGR concentration and equivalence ratio is the key that affects turbulence intensity and chemical reactivity, thereby determining the ignition and combustion results. During the extension of the EGR tolerance limit, the variations in turbulence and chemistry are accompanied by a series of ignition modes, involving jet re-ignition, re-ignition failure, and misfire. Compared to the passive pre-chamber with an EGR tolerance limit of 18%, the air-assisted pre-chamber extended the limit to 23% and the oxygen-assisted pre-chamber to 30%.

Study on ammonia/methanol blends with ammonia cracking for low-carbon combustion and NO reduction

This work aims to study the ammonia/methanol blended fuel with ammonia cracking in lean-burn condition. Ammonia on-line cracking to produce hydrogen production has the potential to realize the high-efficiency combustion. A new ammonia/methanol/hydrogen mechanism was updated and simplified using several reduction methods to optimize the computation time in 3D simulation. Visualization measurements in constant volume combustion chamber (CVCC) for flame propagation and NO generation were conducted, and 3D computational fluid dynamics (CFD) modeling was also established with the simplified mechanism simultaneously to investigate the impact of methanol addition and ammonia cracking in the lean-burn mode. The findings showed that methanol addition and ammonia cracking can significantly improve the combustion rate, and 60%ammonia/40%methanol with ammonia cracking ratio of 90% (N60R90) at equivalence ratio (ER) of 0.7 yields the similar LBV as pure methanol at ER of 1.0 in laminar burning condition. Its ER can be extended to 0.4 to obtain the similar level of LBV as pure ammonia at 1.0. The experiments and 3D simulations revealed that N60R90 at 0.4 yields the equivalent level of combustion rate as N60R0 at 0.8, while obtaining much lower NO emission. In addition, N60R100 at 0.4 without fuel-NO formation during the combustion can get ultra-low NO emission. This study provides the insights on low-carbon combustion strategy for ammonia cracking with low NO.

Building a research model for the combustion process of fuel in a constant volume combustion chamber

ABSTRACT Blending biodiesel at different ratios leads to fluctuations in properties such as oxygen content, cetane number, speed, density, and calorific value, affecting performance and emissions. Therefore, studying the combustion process of biodiesel is necessary. This study describes the construction of a CVCC (Constant volume combustion chamber) model to study the fuel combustion process and the impact of temperature on the characteristic parameters of the combustion process. Fuels used for simulations included diesel fuel and B10 biodiesel derived from palm oil. The calculated results from the model are compared with experimental data to verify, analyse, and evaluate the combustion process in CVCC. Analysis of the influence of combustion chamber temperature shows that increasing the temperature from 300K to 450K leads to a decrease in the mixing time of the mixture, a decrease in the spray length, an increase in the fuel evaporation rate, and the combustion process occurs. Faster, the peak pressure appears earlier and higher, and the heat transfer rate is faster. Pressure increases faster during the delayed combustion phase. At the same time, increasing the oxygen concentration from 10% to 20% also leads to an increase in pressure rate, reaching an earlier peak, and the combustion process occurs faster.

Study of CH4-H2-CO-Air premixed combustion characteristics based on fractal dimension and flame crack method

The laminar combustion characteristics of CH4-H2-CO-Air are studied in a constant volume combustion chamber with different hydrogen content ratios and the initial conditions of 300 K and 0.2 MPa by schlieren technology. The influence laws of flame instability are analyzed through the influence laws of hydrogen content ratio and equivalence ratio (Φ) on the fractal dimensions of the flame contour and the ratio of the total flame face cracks' lengths to the flame radius (L/R), as well as the influence laws of hydrodynamic instability and thermal-diffusive instabilities on the flames. The results show that L/R is more sensitive to flame instabilities than flame front fractal dimensions due to a wider sampling range. After R exceeds a certain value, for lean and rich combustion conditions with the same distance from the stoichiometric ratio, the L/R is greater during lean combustion, and as the equivalence ratio gradually increases from 0.6 to 1.4, the effect of increasing hydrogen content (Hydrogen mole fraction ranges from 60% to 66.67%) on flame instability gradually shifts from promotion to suppression. This study has certain reference value for the combustion application of biomass synthesis gas.

Investigation on the intrinsic instabilities of ethyl methyl carbonate flames

The intrinsic instabilities of ethyl methyl carbonate flames are investigated at equivalence ratios (0.6–1.8) and initial pressures (70–160 kPa) by using a 36-liter constant volume combustion chamber. The unstable laminar premixed flame is evaluated by the laminar burning velocity, explosion characteristics, flame cellularity, and flame self-acceleration. The experimental laminar burning velocity is well reproduced by the chemical kinetic mechanism. The extent of cellularization characterized by the crack length and average cell area non-monotonically varies with increasing equivalence ratio but monotonically increases with increasing initial pressure. Meanwhile, the critical flame radius for the onset of cellularity exhibits non-monotonic variation with the equivalence ratio, while it decreases monotonically with increasing initial pressure. The experimental critical Peclet number and critical flame radius are well captured by flame stability theory. The linear correlation between the critical Peclet number and the Markstein number is Pecl = 26.46 Ma + 122.22. In addition, the dependence of the critical Karlovitz number on the Markstein number is predicted as Kacl = 0.0226e-0.0880Ma. The flame self-acceleration increases as the flame instabilities increase. The acceleration exponent and fractal dimension are smaller than 1.5 and 2.33, respectively, which are the suggested values for flame self-turbulization.

Effects of ambient pressures on cool flames in n-dodecane spray studied with laser diagnostics and large-eddy simulations

Cool flames, as a typical phenomenon in low-temperature combustion, have garnered wide interests owing to their crucial role in engine performance and emissions. In the present study, the auto-ignition process of n-dodecane spray injection into a constant volume combustion chamber was experimentally and numerically investigated. The focus is on providing insights into the underlying effects of ambient pressures on cool flames in n-dodecane spray combustion from the viewpoint of low-temperature chemistry. Laser diagnostics reveal the initial detection of formaldehyde (CH2O) at the spray periphery, followed by downstream progression toward the spray head. In addition, the production of CH2O is noted to occur earlier at higher ambient pressures. Subsequent large-eddy simulations illustrate that both CH2O and hydrogen peroxide (H2O2) shift from fuel-lean to fuel-rich regions, and this process is promoted at higher ambient pressures. Moreover, the primary distribution regions of CH2O exhibit confinement to a broader temperature span of 800–1400 K, in contrast to H2O2. Lastly, detailed chemical kinetic analyses show that the high sensitivity of R134 to the variation of ambient pressures explains for the observations in laser diagnostics. Furthermore, elevated ambient pressures are proved to accelerate low-temperature combustion and advance high-temperature combustion within the reaction system.

The properties of sustainable aviation fuel II: Laminar flame speed

This work reports on the study of flame characteristics and laminar flame speeds (LFS) of sustainable aviation fuel (SAF) and conventional fuels (Jet-A1 and JP-5) premixed with air at elevated pressures. The experimental facilities included the constant volume combustion chamber (CVCC) and used a shadowgraph system to quantify the LFS. The effects of equivalence ratio and ambient pressure on the flame speeds were examined by varying the initial pressure 1, 2, and 3 bar (in addition to 0.1 and 0.5 bar for flame morphology) and the equivalence ratio from 0.8 to 1.8 at 423 K of temperature. The results indicated that JP-5 had the highest LFS value (90.1 cm/s) compared to the other two fuels. Jet-A1 and SAF have the same LFS result, which is less than a 15% difference, excluding the high equivalence ratio and initial pressure. The higher the initial pressure, the lower the LFS for all conditions. The equivalence ratio of 1.2 resulted in the highest LFSs. In addition, in order to understand the laminar flame speed of SAF and its combustion chemical kinetics. Therefore, the sensitivity analysis of SAF was performed. Results indicated that the chain branching reaction (O2 + HOH + O) positively enhanced flame speed as equivalence ratios (φ) increased, while chain propagation reactions, notably O2 + HHO2 (+M), had a negative effect on flame speed, particularly diminishing their influence at higher φ due to limited oxygen availability.

Impact of flash boiling spray on soot generation of a rich fuel–air mixture under various ambient pressures

Particulate emission is one of the critical challenges for the modern gasoine direct injection (GDI) engines. Especially under cold start conditions, the global equivalence ratio inside the cylinder is often set quite rich, leading to the generation of a large amount of soot. The fuel distribution state, such as homogeneous mixing and heterogeneous mixing with local rich pockets prior to the combustion is the key factor affecting soot generation under cold start conditions in GDI engines. Flash boiling injection, as a method to promote fuel air mixing, could be a potential solution to sooting problems of rich mixture combustion under cold start conditions. However, the unclear influences of flash boiling spray to fuel–air mixing state and subsequent soot generation pose a challenge to soot reduction in GDI engines. This work aims to study the effect of various flash boiling degrees on fuel–air mixture formation and subsequent soot generation utilizing a customized constant volume combustion chamber (CVCC) incorporated with the gasoline direct injection scheme under moderately cold start ambient temperature of 30 °C. A global high equivalence ratio of 1.7 representing extremely rich combustions under cold start conditions is selected for strong sooting propensity. By setting different fuel temperatures (Tf= 30 °C and 120 °C) and various ambient pressures (Pa= 40 kPa, 100 kPa and 160 kPa), various spray flash boiling states (subcooled, transitional flash boiling and flare flash boiling sprays) are achieved, and finally various fuel distribution states of rich mixtures prior to combustion are obtained. High speed color camera and soot sampling TEM grids were used to capture and evaluate the characteristics of spray atomization, combustion and soot aggregate structures. Based on this investigation, the effect of flash boiling degree on fuel–air mixing and subsequent soot generation is analyzed. The results show that flash boiling spray enhances fuel atomization and produces much more homogeneous rich mixture with smaller local rich pockets and richer global premixed gaseous portion. Over-rich global premixed mixture combustion is prone to nuclei inception and agglomeration, producing soot aggregates with small primary particles. While the local rich pocket combustion is prone to surface growth, producing larger primary particles. Flash boiling spray could reduce the size of soot particles under cold start conditions by generating smaller local rich pockets.

An experimental study the cross spray and combustion characteristics diesel and ammonia in a constant volume combustion chamber

Recently, the use of ammonia, a carbon-free fuel, in internal combustion engines, particularly in dual fuel dual direct injection engines, has attracted wide attention. However, the spray and combustion characteristics of diesel and ammonia dual direct injection especially on cross injections remain unclear. An optical study of diesel and ammonia spray and combustion was conducted using a constant-volume combustion chamber. The results revealed that spray tip penetration (STP) and spray projected area (SPA) of ammonia are lower than those of diesel, indicating that ammonia has a higher evaporation rate and a smaller diffusion rate than diesel. Besides, diesel and ammonia atomization was promoted through cross injection and collisions. As the cross injection interval increased, the liquid-phase STP of the collision spray increased, and its liquid-phase SPA decreased, while its vapor-phase SPA increased, promoting the diffusion and evaporation rates of the collision spray. In addition, increasing the interval of diesel and ammonia injection can shorten the ignition delay, and increase the flame lift-off length, leading to a more intense combustion process and less soot emissions. Therefore, the combustion and emission characteristics can be effectively improved by adjusting the injection timing of the two fuels reasonably.

An optical study on spray and combustion characteristics of ethanol/hydrogenated catalytic biodiesel blends in a constant volume combustion chamber

Ethanol fuel holds significant potential for emissions reduction, but its poor ignition capability poses challenges in achieving stable compression ignition within internal combustion engines. To enhance the ignition properties of ethanol fuel, this study blended with another highly reactive and renewable fuel, hydrogenated catalytic biodiesel, conducted an investigation on spray and combustion characteristics of two different blending ratios of ethanol/hydrogenated catalytic biodiesel/n-octanol (co-solvent) blends in a constant volume combustion chamber. The results show that the influence mechanism of cetane number and latent heat of evaporation on ignition delay varies depending on ambient temperature and oxygen concentration. At low temperatures and low oxygen conditions, E15 has a longer ignition delay than D100 due to its higher latent heat of evaporation. However, its larger cetane number makes its ignition delay less than D100 under high temperature and high oxygen conditions. When the ambient temperature exceeds 825 K and the oxygen concentration is 21 %, E15 has better ignition and combustion characteristics compared to diesel on ignition delay and lift-off length, whereas E30 exhibits inferior performance under various conditions. To further explore the quantitative effects of ambient variables on the ignition of E15, when the ambient temperature increases from 750 K to 900 K, the ignition delay decreases by 81.5 %. When the oxygen concentration increases from 15 % to 21 %, the ignition delay decreases by 50.6 %. Therefore, the research findings demonstrate the better potential of E15 under high temperature and high oxygen conditions, offering theoretical guidance for the practical engineering application of ethanol fuel.

Effect of Machine Learning Algorithms on Prediction of In-Cylinder Combustion Pressure of Ammonia–Oxygen in a Constant-Volume Combustion Chamber

Road vehicles, particularly cars, are one of the primary sources of CO2 emissions in the transport sector. Shifting to unconventional energy sources such as solar and wind power may reduce their carbon footprints considerably. Consequently, using ammonia as a fuel due to its potential benefits, such as its high energy density, being a carbon-free fuel, and its versatility during storage and transportation, has now grabbed the attention of researchers. However, its slow combustion speed, larger combustion chamber requirements, ignition difficulties, and limited combustion stability are still major challenges. Therefore, authors tried to analyze the combustion pressure of ammonia in a constant-volume combustion chamber across different equivalence ratios by adopting a machine learning approach. While conducting the analysis, the experimental values were assessed and subsequently utilized to predict the induced combustion pressure in a constant-volume combustion chamber across various equivalence ratios. In this research, a two-step prediction process was employed. In the initial step, the Random Forest algorithm was applied to assess the combustion pressure. Subsequently, in the second step, artificial neural network machine learning algorithms were employed to pinpoint the most effective algorithm with a lower root-mean-square error and R2. Finally, Linear Regression illustrated the lowest error in both steps with a value of 1.0, followed by Random Forest.

Role of Altitude in Influencing the Spray Combustion Characteristics of a Heavy-Duty Diesel Engine in a Constant Volume Combustion Chamber. Part II: Impinging Diesel Jet.

Wall impingement, particularly liquid-wall impingement, has been demonstrated to be one of the critical causes of combustion deterioration in plateau diesel engines. Obviously, the complexity of wall impingement is exacerbated by the plateau scenario. However, fundamental studies specifically dedicated to this phenomenon are still inconclusive and insufficiently detailed, obviating the feasibility of the targeted design and optimization of diesel engines operating in regions with different altitudes. Consequently, the second part of this investigation, presented in this work, focused on the detailed physical and chemical processes of impinging spray combustion under different altitude conditions. A wall impingement system was designed to generate an impinging spray flame. The impingement distance was varied from 77 to 37 mm to cover different situations of wall impingement. The liquid spray, ignition, and combustion processes were visualized in detail by using different optical diagnostics. The results showed that the variation of the liquid length with the impingement distance was mainly dependent on the liquid impingement under the same altitude condition. The effect of the impingement distance on the ignition distance was more sensitive to the altitude. The quantitative analysis of the flame natural luminosity confirmed the decisive effect of the impinging flame morphology on the ambient entrainment and fuel-air mixing under different altitude conditions, and it also revealed that there was an optimal impingement distance under identical altitude conditions to achieve minimum soot emissions. And interestingly, the optimal impingement distance increased with altitude. Finally, the spray combustion processes of an impinging diesel jet were determined to occur in four typical regions, upon which a schematic diagram depicting the flame structure of an impinging diesel jet was proposed to phenomenologically describe the role of altitude in impinging spray combustion processes. Based on this, an attempt was made to explore some new perspectives beyond the popular solutions to recover and improve the performance of plateau diesel engines.

Laminar burning velocity of Ammonia/Air mixtures at high pressures

Ammonia (NH3) is one solution for decarbonizing the economy. This study used high-speed schlieren imaging to investigate the NH3/air flame morphology and propagation characteristics (laminar burning velocity (LBV) and flame thickness) inside a constant volume combustion chamber (CVCC), at conditions relevant to different types of combustors used in power generation. Experiments were carried out for equivalence ratios (ϕ) from 0.7 to 1.3, initial pressures from 1 atm to 10 atm, and an initial temperature of 298 K using a variable gap ignition system. A novel passivation method that minimized the effect of NH3 adsorption on stainless steel surfaces was used during the CVCC filling to reduce the uncertainty in the actual ϕ before ignition. Results showed that the low speed of NH3/air flames increased the effect of buoyancy on the flame shape, especially at high initial pressures or near the flammability limits when flames adopted a bean-shaped structure. A comparison of Rayleigh and Peclet numbers (indicators of buoyancy and diffusion phenomena in flames, respectively) supported these findings. Maximum LBV was measured at ϕ = 1.1 and decreased non-linearly with the initial pressure. Similarly, flame thickness decreased with pressure, and it was thinnest at ϕ = 1.1 for all the examined conditions. Numerical simulations using Cantera produced similar LBV values as the experiment. A sensitivity analysis indicated that reactions involving NH2 and NO have an important role on LBV. Reactions leading to H, O, and OH radicals generation had the strongest effects on LBV enhancement, while LBV-inhibiting reactions had more accentuated effects at high pressures at which LBV is lower.

An optical study on the cross-spray characteristics and combustion flames of automobile engine fueled with diesel/methanol under various injection timings

The fundamental research on the spray and combustion characteristics of diesel/methanol dual injection was explored in this study. A constant-volume combustion chamber was modified to accommodate cross-fuel injection by adding another fuel supply and an injection system. The effects of diesel/methanol injection timings on cross-spray and flame development were analysed for various injection intervals and pressures. The results showed that as the injection pressure increased, the high atomisation of the diesel–methanol mixed spray plume promoted its combustion. After diesel–methanol impingement at injection pressure of 100 MPa, a significant increase in the spray area occurred up to 62.5 % compared to its value at 60 MPa, resulting in a shortened combustion ignition delay, increased flame lift-off length and reduced soot generation. In addition, when methanol was used prior to diesel injection, the combustion ignition delay and flame lift-off length increased and soot generation decreased, as the diesel injection timings delay increased. Soot generation can be reduced by up to 33.3 % compared to that generated via the simultaneous injection of diesel/methanol at 60 MPa, by utilising a high injection pressure and proper injection timing. The study findings will provide a theoretical basis for the development of diesel/methanol dual-fuel direct-injection engines.