Initial Equivalence Ratio Research Articles

An Experimental Investigation into Ammonia Oxidation and NOx Emission in a Packed-Bed of Quartz Particles

ABSTRACT The effect of temperature, initial NH3 concentration, equivalence ratio, and flowrate on the characteristics of ammonia (NH3) oxidation and associated NOx formation in a fixed-bed reactor packed with quartz particles has been systematically investigated with a stream of NH3 and O2 in He over a range of furnace set temperatures (900 K – 1400 K), initial NH3 concentrations (2% – 10%), equivalence ratios (ɸ = 0.8, 0.9, 1.0, 1.1), and total flowrates (145 mL/min, 289 mL/min, and 434 mL/min). Benchmark flow reactor experiments, without the quartz particles in the reactor under otherwise similar conditions, were previously performed and served as a reference case for this work. Compared to the flow reactor results, the packed-bed data indicated a lower reaction onset temperature and more gradual NH3 conversion across the temperature range. Both packed-bed and flow reactor experiments indicated that decreasing flowrate, thus increasing residence time, resulted in greater NH3 conversion. NH3 conversion in the fixed-bed reactor was similar for all equivalence ratios at temperatures ≤1200 K, above which NH3 conversion increased with decreasing equivalence ratio, a trend broadly aligned with previous flow reactor experiments. For all conditions examined, NO was the major NOx component and NO2 was insignificant. Across the range of equivalence ratios, NO emission was low, yet noticeable, from 900 K – 1100 K, before spiking at 1200 K. Above 1200 K, NO emissions decreased before drastically increasing again at 1400 K under fuel-lean conditions. At temperatures ≤1300 K, NO was generally found to decrease with decreasing equivalence ratio, while at 1400 K, NO increased with decreasing equivalence ratio. NO emissions were significantly lower in the packed-bed compared to the flow reactor scenario at temperatures ≥1300 K.

Experimental Study on Pressure Oscillation Characteristics of Di-N-Butyl Ether in a Constant Volume Vessel

ABSTRACT This paper conducted experimental research on spontaneous pressure oscillation characteristics of di-n-butyl ether (DBE). Based on a constant volume vessel (CVV) with 210 mm in length and 180 mm in diameter, the initial pressure, temperature, and equivalence ratio ranges of 0.05–0.3 MPa, 373–453 K, and 0.7–1.6. The experimental data of combustion pressure, CVV vibration and sound pressure were analyzed. The results indicate that DBE exhibits the strongest pressure oscillation at high pressures, moderate temperatures and equivalence ratios. The highest oscillation intensity is achieved at 0.3 MPa, 433 K, equivalence ratio of 1.4. The vibration and sound pressure signals exhibit the same patterns as combustion pressure signals. Spectrum analysis is conducted by Fast Fourier Transform of pressure signals, it shows that the amplitude of the characteristic frequency increases with the increase of oscillation intensity. In addition, to further reveal the influence of combustion instability on oscillation intensity, the dimensionless parameter Peclet number and critical radius are compared, it shows that the combustion instability has strong effect on pressure oscillation, and diffusive-thermal instability is the key factor that influences pressure oscillation. This study is believed to be beneficial for organizing better combustion for internal combustion engines.

Assessing turbulence–flame interaction of thermo-diffusive lean premixed H2/air flames towards distributed burning regime

Simultaneous laser-induced fluorescence of OH radicals and particle image velocimetry, and quasi-simultaneous 1D Raman/Rayleigh and 2D Rayleigh scattering measurements are used to investigate the effects of Karlovitz number (by varying the equivalence ratio) and residence time (by varying the axial measurement location) on the internal flame structures of lean premixed hydrogen/air turbulent jet flames. The turbulent flow fields, instantaneous macroscopic flame structures, and thermochemical states of a set of lean premixed hydrogen/air turbulent flames with varying initial equivalence ratio of 0.3, 0.4, and 0.45 at a constant bulk velocity of 100m/s are discussed. With an increasing equivalence ratio, the Karlovitz number derived from the turbulent flow fields decreases rapidly from 7690 to 260 and 100, determined at a downstream location of x/D=7. At the highest Karlovitz number (i.e., the lowest equivalence ratio), a distributed burning is observed in the jet flame as turbulent transport dominates over molecular mixing, and the effects of differential diffusion and flame curvature are suppressed. With decreasing Karlovitz number, intense burning regions characterized by elevated local equivalence ratio, high water mole fraction, and super-adiabatic flame temperatures are observed in association with positive flame curvature. The same combustion diagnostics are applied to another lean premixed hydrogen/air turbulent flame with an initial equivalence ratio of 0.4 and a bulk velocity of 200m/s at selected downstream locations of x/D=3.5, 7, 10.5, and 14 to assess the effects of developing turbulence and residence time. Corresponding Karlovitz numbers are 680, 730, 775, and 690, as the local turbulent intensity changes along downstream locations. While flame structures reveal characteristics towards distributed burning at lower x/D, a locally intense burning region appears at higher x/D together with positive curvature. This is mainly because the turbulence develops with increasing x/D and the flame surface is more disturbed and curved by turbulent eddies with increasing turbulence length scales. The highly diffusive hydrogen is locally concentrated by positively curved flame surfaces, resulting in fuel-rich burning regions at a higher temperature. This indicates that, as velocity fluctuations increase in axial direction, turbulence can also promote thermo-diffusive instabilities to a certain extent, increasing the mixture inhomogeneity in temperature space by interacting with the differential diffusion effect of hydrogen at atmospheric pressure.Novelty and significanceThe novelty of the current work is the focus on turbulence–chemistry interactions of premixed hydrogen/air jet flames at high turbulence and ultra lean conditions. Comprehensive experimental results including the turbulent flow fields (particle image velocimetry), instantaneous flame structures (laser-induced fluorescence of OH radicals) and internal thermochemical states (quasi-simultaneous 1D Raman/Rayleigh and 2D Rayleigh scattering imaging) of hydrogen/air jet flames are analyzed and discussed. The data sets with well-characterized boundary conditions and high measurement accuracy, are crucial for validation and development of numerical simulation models.

A LES study on the instantaneous H2 jet-ignited combustion characteristics of H2/NH3 mixtures

This paper presents a Large Eddy Simulation (LES) study investigating jet flame ignition characteristics to achieve stable ammonia combustion for novel applications in gaseous ammonia/hydrogen-fueled engines with an active prechamber. The study investigated three cases with initial global equivalence ratios of 0.6, 0.8, and 1.0. Two main ignition modes were proposed, and five stages of coupled flame propagation processes were identified. The oscillation of supersonic hydrogen jet flame was investigated. The modes of flame-vortex interaction including vortex ring formation, along with the mechanism of highly reactive zone formation, were proposed. The effects of reactivity and turbulence transition on piloted ammonia flame were investigated to attain stable ammonia combustion and achieve a high heat release rate. The results showed that oscillations in the jet flame were strengthened at low global equivalence ratios with high hydrogen mass ratios. Enhancing propagation of initial and secondary vortex rings broadened highly reactive zones. Vorticity dissipated more rapidly at low equivalence ratios, leading to a stratified structure and enhanced piloted spherical flame. The Unsteady Flamelet Ignition Prediction (UFIP) sub-model of combustion was verified for the piloted ammonia flame, enabling a more comprehensive discussion of flame propagation tendencies.

Effects of mixture initial conditions on deflagration to detonation transition enhanced by transverse jets

To explore the effects of mixture initial conditions on the deflagration to detonation transition (DDT) enhanced by transverse jets, numerical studies were conducted on the flame acceleration of H2/Air mixture under different initial conditions, including initial pressure, temperature, and equivalence ratio. The results indicate significant differences in flame acceleration processes under different initial pressures (0.05, 0.1, 0.15, and 0.2 MPa). Changes in jet intensity and combustion rate induced by variations in mixture initial pressure collectively influence the flame acceleration process, but the impact of jet intensity is more significant. However, effective coupling between transverse wave propagation and flame acceleration is also crucial for the DDT process. Changes in mixture initial temperature (300, 350, 400, and 450 K) and equivalence ratio (0.8, 1.0, 1.2, and 1.4) have negligible impact on jet intensity. The variation trend of flame velocity under different initial temperatures or equivalence ratios is consistent, but the detonation initiation performance is slightly different. As the initial temperature increases, the DDT distance and DDT time are shortened. With an increasing equivalence ratio, the DDT distance and DDT time initially decrease and then increase, reaching the shortest at slightly rich fuel conditions.

Effect of initial pressure on hydrogen/propane/air flames in a closed duct

To investigate the flame propagation characteristics and chemical kinetics of premixed propane/hydrogen/air mixtures at elevated pressure, a series of experiments on propane/hydrogen/air mixtures with varying initial pressures (P0 = 100–300 kPa) and equivalence ratios (φ = 0.6–1.4) at a hydrogen fraction of 0.6 and an ambient temperature of 298 K were carried out in a closed duct with an aspect ratio of 15. The chemical kinetics of the elementary reaction were analyzed numerically. The results revealed that the combustion-induced rapid phase transition (cRPT) occurred in the premixed C3H8/H2/air flames. Both the combustion pressure and flame propagation velocity increase with initial pressure except at P0 = 300 kPa, where the flame propagation velocity decreases. Moreover, the linear dependence between the laminar burning velocity and peak mole fraction [H + O + OH]max was found by the effect of initial pressure on the peak mole fraction (H, O, OH). The explosion pressure Prea and the maximum rate of production of free radical H/O show excellent correlations. The maximum rate of pressure rise can be predicted by the maximum heat release rate (HHR). These results provide essential data on elevated-pressure duct accidents and have important implications for explosion prevention.

Laminar flame speed measurement and combustion mechanism optimization of methanol–air mixtures

AbstractLaminar flame speeds of methanol/air mixtures at 338–398 K are measured by the heat flux method, extending the range of equivalence ratio up to 2.1. And a new optimized methanol mechanism with 94 reactions is proposed by using the particle swarm algorithm, adjusting 20 Arrhenius pre‐exponential factors in their uncertainty domains. The optimized model is compared with eight methanol combustion mechanisms and experimental data published in recent years, covering a wide range of initial temperatures (298–1537 K), pressures (0.04–50 atm) and equivalence ratios (0.5–2.1). The results show that the optimized mechanism not only improves the accuracy of ignition delay time with rapid compression machine at low temperature but also moderately improve the description of laminar flame speed in lean and stoichiometric conditions. Meanwhile, the optimized model significantly enhances the prediction accuracy of CH3 and CH2O radical, and perfectly captures the evolution trend of HCO radical in laminar flat flame. Overall, the optimized mechanism provides the best overall description of the currently available measurements, leading to more accurate and comprehensive prediction of ignition delay time, laminar flame speed and species concentration.

Reduced Chemical Kinetic Models of DME Based on Variance Filtering Method

Based on the variance value of each species concentration during the combustion process, a new chemical mechanism reduction method is proposed. The data sets which are the molar concentration of each species are generated based on numerical results of zero-dimensional homogeneous ignition process using detailed chemical kinetic mechanisms under various operating conditions. By calculating and analyzing the variance value of each species concentration during the combustion process to determine the contribution of each species to the combustion process, and by selecting a suitable threshold value to determine whether the species and elementary reactions are removed or not. The skeletal mechanisms of DME generated by using the present VFM method are compared to those generated by the path flux analysis (PFA) method and the detailed mechanism. The comparisons of the temperature and the concentration of important species showed that with either the same or significantly smaller number of species, the skeletal mechanisms generated by the present VFM method are more accurate than that of PFA in a broad range of initial pressures, temperatures and equivalence ratios. In addition, the reduction speed of VFM is one orders of magnitude faster than that of PFA.

Experimental study on the explosion characteristics of ammonia-hydrogen-air mixtures

Ammonia-hydrogen blended fuel has drawn substantial attention as an environmentally friendly, carbon-free fuel because it effectively overcomes the inherent disadvantages of pure ammonia in combustion performance, such as a high ignition energy, slow combustion rate, and low combustion efficiency. While recent research on the combustion performance of hydrogen-ammonia blends has advanced, there remains a scarcity of studies addressing the explosion characteristics of these fuels. In this paper, a series of experiments conducted within a standard 20-L spherical combustion chamber to investigate the explosion characteristics of ammonia-hydrogen-air premixed gases under diverse conditions, spanning from low to high pressures (0.02–0.3 MPa) and from fuel-lean to fuel-rich scenarios (0.7–1.5), are discussed. The results reveal that the explosion characteristics are pronouncedly enhanced by hydrogen addition. The maximum explosion pressure (Pmax) and maximum pressure rise rate ((dP/dt)max) increase, while the explosion time (texp) decreases with increasing hydrogen concentration. Pmax and (dP/dt)max exhibit linear increases as the initial pressure rises and reach their peaks at an equivalence ratio of 1.1; texp displays nonmonotonic variations with increasing initial pressure and equivalence ratio. Hydrogen addition remarkably increases the hydrodynamic instability and thermal-diffusion instability of the reactive mixture, enhances the thermal-mass diffusion on the flame front, and promotes flame deformation and cellular structure formation, resulting in an improvement of the flame propagation and compression effect on the unburnt reactant before the flame. Research into the explosion characteristics provides valuable insights for the safe storage and transportation of ammonia-hydrogen blended fuels. These results indicate avenues for further enhancing the detonation characteristics and explosion performance of ammonia-hydrogen blended fuels.

Internal flame structures of thermo-diffusive lean premixed H2/air flames with increasing turbulence

Turbulent flow fields, instantaneous flame structures, and internal thermo-chemical states of lean premixed hydrogen/air jet flames at an initial equivalence ratio of 0.4 are experimentally investigated by simultaneous laser-induced fluorescence of hydroxyl radicals (OH-LIF) and particle image velocimetry (PIV), and quasi-simultaneous 1D Raman/Rayleigh and 2D Rayleigh scattering measurements over a range of Karlovitz numbers (Ka) from 50 to 730. At low Ka, intense burning characterized by elevated local equivalence ratio, high water mole fraction (XH2O), and super-adiabatic flame temperature is mainly observed in post-flame regions surrounded by positively curved flame surfaces, where the fast diffusive hydrogen is locally focused. The flame features stronger differential diffusion and curvature effects than that in the planar laminar flame, which indicates that both molecular and turbulent mixing play significant roles, and thermo-diffusive instabilities have synergistic interactions with turbulence at low turbulence level. With increasing Ka, the burning intensity in corresponding regions is weakened, even though the flame surface is more disturbed by the turbulence. At the highest Ka, no intense burning region is observed in the jet flame as the turbulent transport dominates over the molecular diffusion. In the temperature domain, the conditional means of hydrogen mole fraction (XH2) and local equivalence ratio feature effects of diffusive instabilities with broad distributions at low-Ka conditions. Elevated XH2O and local equivalence ratios with super-adiabatic flame temperatures are observed, which is attributed to differential diffusion in hydrogen-containing mixtures and the fuel focusing effect near positively curved flame surfaces. At high Ka, the XH2 shows a more linear decreasing trend and the local equivalence ratio profile becomes flatter over temperature with a narrower distribution, indicating the dominance of turbulent mixing.

Cellularization and intrinsic instability characteristics of 2-methylfuran spherical expanding flame

ABSTRACT The flame morphology, flame instability and thermodynamic parameters of 2-methylfuran (2-MF) spherically expanding flame were studied at different initial temperatures, initial pressures, and equivalence ratios. Quantitative analysis of 2-MF flame surface cracks and cell structures was performed. The radius for the crack to enter the accelerated growth stage is reduced by 52.9% as the initial pressure increases from 2 bar to 4 bar. Also, the radius at which the crack enters the accelerated growth stage is reduced by 16.4% when the equivalence ratio is increased by 0.1. The radius required for the average cell area to stabilize is reduced by 31.5% when the initial pressure increases from 2 bar to 4 bar. Additionally, instability parameters were studied, and the results revealed that hydrodynamic instability and thermal-diffusion stability increase with increasing Peclet number. The relationship between the local instability and stability curves was discussed. The average wavenumber fitting line is closer to the upper limit of the instability curve when the equivalence ratio is 1.4, which is 8.5% higher than that of the equivalence ratio of 1.3. Next, the flame self-acceleration was studied, the initial pressure and equivalence ratio greatly promote the self-acceleration of the flame, but the temperature has no obvious effect, and the self-acceleration exponent of 2-MF flame is less than 1.5. Finally, the flame image information was reconstructed by a reconstruction algorithm and the cellularity factor was formulated to measure the degree of flame cellular. The cellularity factor is affected by initial pressure and equivalence ratio. Comparing and analyzing the evolution of flame cracks, flame self-acceleration, and cellularity factor, it was found that flame cracks promote flame self-acceleration and exacerbate flame cellularization and instability.

Confined deflagration of propanal/air in the presence of intrinsic instability at elevated pressures

The partial oxidation process is a chemical technique with a high risk of deflagration, which involves directly mixing of fuels with oxidizers at elevated conditions. Under non-standard conditions, the deflagration behaviors of fuels are often unavailable to the general public, which hampers progress in further basic research. In this study, the deflagration behaviors of propanal-air mixtures at 75 °C and 0.1–1.0 MPa were investigated, and the effect of initial pressure (P0) and equivalence ratio (φ) on the deflagration characteristics of propanal was discussed. To explain the similarities and differences in the deflagration characteristics under various conditions, the buoyancy instability, hydrodynamic instability and thermal-diffusion instability suffered by the flames were also analyzed. Results show that the pressure generated by the burned mixture is a cubic function of time, and the state of flame propagation can be determined by the monotonicity of the second-order derivative of pressure. Moreover, the peak deflagration pressure (Pm) is more sensitive to changes in φ on the fuel-lean side, but Pm/P0 at the fuel-rich condition is markedly higher than Pm/P0 at the fuel-lean condition. At all φ, an increase in P0 effectively shortens the ignition time and reduces the minimum ignition energy. Nevertheless, there are discernible differences in the variation patterns of the deflagration time (θm) with P0 at various φ. Specifically, θm gradually prolongs with increasing P0 at fuel-lean conditions, whereas it tends to be the same at different P0 and φ = 1. Furthermore, under fuel-rich conditions, θm progressively shortens as P0 increases. The decrease in θm is attributed to the increasing flame speeds in the presence of intrinsic instability, and the self-accelerating flame yields pressure waves, which travel faster than the flame. Then a series of reflected pressure waves are formed after the pressure waves strike the vessel wall, interacting with the flame front and resulting in the pressure oscillation. Moreover, increasing P0 advances the pressure oscillation and augments the intensity of pressure oscillation.

Investigations on methyl pentanoate-air mixtures confined explosion and cellularity

Combustible fuels like methyl pentanoate (MPe) could cause accidental explosions and fire. Therefore, MPe explosion characteristics have to be well characterized and understood for its safe and reliable use in industries and combustion devices. Considering this, MPe’s explosion pressure, maximum explosion pressure, rate of pressure rise, maximum rate of pressure rise, heat release rate, etc., were investigated in this study at the initial conditions of 1, 2, and 4 bar and 423 K, 453 K, and 483 K using the equivalence ratios of 0.7–1.4. The maximum explosion pressures and maximum rate of pressure rise of MPe increased as the initial pressures increased but the maximum rate of pressure rise was insignificantly affected by the initial temperature. The explosion duration decreased with a decrease in the initial pressure. The maximum heat release rate and deflagration index increased with increasing initial pressure. The MPe maximum deflagration index was 224.8 bar•m/s, which occurred at a higher initial pressure and fuel-rich mixture. Therefore, the explosion severity of MPe was enhanced at higher initial pressures and fuel-rich mixtures. Moreover, the flame cellularity study showed that MPe unstable flame crack length and cell number were more affected by increments in the initial pressure and equivalence ratio. Therefore, flame instability and cellularity would significantly influence explosion severity at higher pressures and equivalence ratios. This study’s findings provide first-hand data that could be used to further investigate and characterize MPe’s explosion risk.

Manipulating hydrogen oxy-combustion through carbon dioxide addition

Hydrogen (H2) is increasingly viewed as an attractive carbon-free fuel due to its potential compatibility with the existing transportation and conversion infrastructure. However, one of the major challenges facing large-scale deployment is the fundamentally different combustion properties it has in comparison to commonplace hydrocarbon fuels such as natural gas. For example, the laminar burning velocity (LBV) of a combustible mixture has a direct impact on how it can be used in internal combustion engine or gas turbines; the LBV of hydrogen is over 7 times greater than that of natural gas when combusted in air. Carbon dioxide (CO2) can be used as a working fluid, as opposed to nitrogen (in air), to reduce the flame speed of hydrogen combustion. A mixture of hydrogen, oxygen, and carbon dioxide as a working fluid can provide LBVs comparable to natural gas in air, which potentially enables existing conversion architectures. Furthermore, by replacing nitrogen (N2) with CO2 in the mixture, NOx emissions are avoided and opportunities for carbon sequestration or closed-cycle processes are possible. This study experimentally explores fundamental premixed oxy-combustion properties of H2-CO2 mixtures in a constant volume combustion chamber (CVCC) across a range of initial pressures (1, 1.5, and 2bar) and equivalence ratios (0.4, 0.6, 0.8, and 1). The spherically expanding flames are examined to determine the flame speed, LBV, and lower flammability limits (LFL) with respect to different CO2 concentrations (40%, 60%, 65%). Furthermore, at an initial pressure of 1bar and stoichiometric conditions, it was found that the flame speed of the 65% CO2 case was lower than that of CH4-air combustion.

Numerical study on spherical flame propagation in dispersed liquid ammonia droplets

Directly using liquid ammonia in combustion devices can simplify the fuel supply system and reduce equipment costs. However, the detailed ignition and propagation mechanisms of liquid ammonia flames have not been fully understood. This study aims to investigate the ignition and propagation of liquid ammonia flames numerically under various conditions. Spherically expanding liquid ammonia flames are studied using detailed chemistry. The effects of initial temperatures, droplet diameters, equivalence ratios, and heat fraction are investigated. The results indicate that the rapid flash boiling of liquid ammonia droplets leads to a strong local heat loss, hindering the ignition and propagation of the flame. Preheating can significantly increase the reaction rate, which compensates for the heat loss caused by phase change, allowing the flame kernel to ignite and propagate outwards. While droplet-flame interaction is present but not significant due to the rapid evaporation of liquid ammonia. The effect of droplet on the flame surface curvature is only distinguishable at conditions of large diameter. Equivalence ratio and ammonia heat fraction ratio change the amount of liquid ammonia. The more liquid ammonia, the more heat is absorbed by the phase change process, making the flame more difficult to be ignited.

Data-driven simulation of ammonia combustion using neural ordinary differential equations (NODE)

The direct use of detailed chemical kinetics in combustion simulations is limited by the extremely high computational costs. Recently, Owoyele and Pal (Energy and AI, 2022), proposed the neural ordinary differential equations (NODE) method to accelerate calculations of chemical kinetics and proved its effectiveness in zero-dimensional calculations of hydrogen combustion considering 9 species and 21 reactions. However, its performance for more realistic high-dimensional calculations and more complex kinetic systems remains unexplored. Therefore, this study further applies the method for more complex chemical kinetics of ammonia combustion, especially with optimizations in the data sampling, model training strategies, and model application methods that remedy the problems of versatility and application to more practical simulations. The newly developed NODE models are comprehensively validated in the zero-dimensional calculations of ammonia auto-ignition, one-dimensional calculations of laminar freely-propagating ammonia-premixed flames, and two-dimensional direct numerical simulation (DNS) of ammonia-premixed flames in a temporally evolving jet. Present NODE models focus on seven chemical species, namely NH3, O2, H2, OH, H2O, N2, and NO, and the results show that, compared with the results obtained by using detailed chemical kinetics, this method is able to reduce the computational costs of the zero-dimensional auto-ignition reaction to 1/24 while reproducing the ignition delay time for a wide range of initial temperatures and equivalence ratios with relatively good accuracy. Additionally, the method is able to reduce the computational costs of the one-dimensional freely propagating flame and two-dimensional jet flame to 1/4 and 1/38 respectively, while acceptable reproduction of the laminar flame speed and temporal evolution of the gas temperature and mass fractions of the interested species can be achieved.

Determination of 12 Combustion Products, Flame Temperature and Laminar Burning Velocity of Saudi LPG Using Numerical Methods Coded in a MATLAB Application

The characterization of a specific fuel has always been an important point for developing and designing new components or systems with the maximum efficient possible. Studying the laminar burning velocity can lay a necessary prerequisite for the accurate poststudy of the turbulent range and to understand how the combustion process takes place. The study of the combustion products from a specific reaction is a requisite for any system in order to understand the elements that are taken in the process and if it is possible to improve it. In this study, a new open code methodology was developed for the determination of combustion products, flame temperature and laminar burning velocity using numerical methods (Newton–Raphson, Taylor series and Gaussian elimination) in an application codified in MATLAB. The MATLAB application was applied for the study of Saudi LPG setting parameters such as initial temperature, pressure and equivalence ratio that are meaningful because they have a great effect on the results. In addition, simulation in Ansys Chemkin using San Diego and RedSD mechanisms was carried out. The results from the MATLAB application were compared with other experimental research and Ansys Chemkin simulation. These are presented in different plots and it is shown that: (1) For the laminar burning velocity results, the numerical method agrees with the experimental results for ratios (0.6–1.2) by other authors and the simulation in Ansys Chemkin. (2) For the highest studied equivalence ratios (1.3–1.7) the laminar burning velocity results between all the resources have more difference. (3) The combustion products calculated by the MATLAB application agree with those simulated in Ansys Chemkin except N and NO. (4) The MATLAB application gives a maximum value of 40.35 cm/s, that is greater than 35 ± 0.91, the one determined by Bader A. Alfarraj. (5) The flame temperature calculated by the MATLAB application overestimates that simulated in Ansys Chemkin but has the same behavior for all the calculated ratios. (6) The MATLAB application has also been developed for the study and analysis of other fuels.

Analysis of explosion and laminar combustion characteristics of premixed ammonia-air/oxygen mixtures

Ammonia is suggested to be a potential carbon-free energy source because of its high volumetric energy density and has attracted attention recently. However, systematic investigations into the explosion characteristics and flame instability of ammonia are still lacking, particularly for low-pressure ammonia-oxygen mixture. The use of low-pressure ammonia-oxygen mixtures could not only benefit for increasing engine's efficiency but also enhance its operational safety. In this study, explosion experiments of premixed ammonia-air/oxygen mixtures are carried out in a 20-L spherical chamber. By adjusting the initial pressure (P0) and equivalence ratio (ϕ), the maximum explosion pressure (Pmax) and maximum rate of pressure rise ((dP/dt)max) are measured. The laminar flame characteristics are analysed simultaneously. The results show that for the ammonia-air mixture, Pmax increases as P0 increases and reaches the peak at ϕ = 1.1; the unstretched laminar burning velocity (SL), Markstein length (Lb), and laminar flame thickness (δ) decrease as the initial pressure increases and reach the peak at ϕ = 1.1. For the ammonia-oxygen mixture, the explosion parameters linearly increase as P0 increases, while the maximum values of the parameters are obtained at ϕ = 1.0; the unstretched laminar burning velocity increases first and then gradually decreases as P0 increases; the maximum SL is 1.14 m/s at P0 = 0.1 MPa and ϕ = 1.0, which is 18 times that of the premixed ammonia-air mixture under the same condition. The schlieren images demonstrate that the buoyancy instability has significant impacts on the premixed ammonia-air flame propagation, while it has negligible impacts on the premixed ammonia-oxygen flame propagation. The effect of hydrodynamic instability on flame propagation could be observed in both the premixed ammonia-air flame at ϕ > 1.0 and the premixed ammonia-oxygen flame at P0 > 0.1 MPa and ϕ < 1.0. In addition, it is proposed that for the premixed ammonia-air flame, the buoyancy instability would remarkably influence the flame propagation when the unstretched laminar burning velocity is < 5 cm/s. The fundamental research on the combustion characteristics of ammonia-air and ammonia-oxygen in this study can provide an experimental reference for the later development of chemical reaction mechanisms, especially for the mechanism suitable for ammonia combustion in oxygen-rich environments. Meanwhile, the combustion characteristics of the ammonia-oxygen mixture at ultra-low pressure also provide a data basis for the possibility of using ammonia fuel as a propellant for hypersonic vehicles in the future.

Effects of turbulence–heat loss interactions on detonation development in end gas and its resulting knock intensity

The main objective of the present work is to investigate the end-gas autoignition and detonation development in a confined space with the presence of wall heat loss by two-dimensional numerical simulations with a hydrogen/air mixture. The effects of turbulence–heat loss interactions, initial temperature, equivalence ratio, and wall temperature on end-gas combustion modes are analyzed in detail. The results show that with the presence of wall heat loss, end-gas autoignition takes place in the hot core regions away from the walls, and the autoignition fronts touching the wall can lead to a much larger wall heat flux than that induced by main flame–wall interactions. In the base cases, increasing the turbulence intensity promotes the end-gas autoignition mode transition from thermal explosion-detonation to thermal explosion-deflagration and finally to no-autoignition, whereas detonation takes place in all cases regardless of the turbulence intensity after the initial temperature or equivalence ratio is raised. However, in these cases with a low equivalence ratio, the detonation propagation is unstable, which can be easily decoupled spontaneously after it encounters the cold flow. It is further found that for the cases with unstable detonation propagation, the burned mass fraction (BMF) dominates the knock intensity, whereas for the cases with stable detonation propagation, the maximum pressure in a chamber will extremely depend on the local and instantaneous interactions between the pressure/shock waves, but the effect of BMF becomes minor.

Extensive study on the detonation cell size of biogas-oxygen mixtures

Detonation cell size is one of the most important and most widely used parameters to describe the detonation process of a given mixture. It is used in both safety analyses and the design process of experiments and engines utilizing detonative combustion. Although much research has been carried out to collect data on the detonation cell of a wide range of combustible mixtures, few studies have focused on biogas. The presented study aims to fill this gap. Several biogas compositions, initial pressures p0 and equivalence ratios Φ were tested, totaling 200 different cases. Both a decrease of initial pressure and a decrease of methane content in the biogas result in an increase in the detonation cell width. When the equivalence ratio is moved away from stoichiometric (Φ = 1) towards either lean or rich mixtures it is accompanied by an increase in detonation cell size. Additionally, ZND induction length and instability parameter χ were calculated. Induction length correlates linearly with the cell size and the χ shows that the cellular structure is expected to be very irregular. Because of that it does not suffice to report merely the average cell size. Thus, a more detailed statistical description is provided.