Equivalence Ratio Dependence Research Articles

NH3/C2H6 and NH3/C2H5OH oxidation in a shock tube: Multi-speciation measurement, uncertainty analysis, and kinetic modeling

This study examines NH3, NO, and CO profiles during NH3/C2H6 and NH3/C2H5OH oxidation in a shock tube using laser absorption spectroscopy at 1317–1957 K and ∼2.8 bar, with 5–20% C2H6 or C2H5OH, equivalence ratios of 0.5/1.0/1.5, and a 0.9 argon dilution ratio. Metrological uncertainty analysis on mole fractions have been performed. A novel dynamic uncertainty methodology is proposed to time-resolved speciation profiles. From the experimental measurements, the promotional effect on ammonia ignition by C2H6 or C2H5OH is identical under investigated conditions. The multi-speciation profiles demonstrate reasonable relations among the consumption of NH3 and the formation of NO and CO, which show strong temperature and equivalence ratio dependence. An updated PTB-NH3/C2 1.1 mechanism accurately predicts ignition delay times and speciation profiles. Sensitivity analysis reveals NH3 chemistry’s significant control, with NH3+O2/H/OH and N2H2+M reactions being critical at >1300 K, and HO2 interactions being pivotal at 800–1200 K. The reactions related to N2H2 and N-C chemistries dominate for fuel-rich mixtures at high temperatures, resulting in a reverse equivalence ratio dependence on IDTs, namely fuel-rich mixtures exhibit the lowest reactivity. The N-C chemistries, particularly the pronounced reaction pathway of HCN→CN→NCO→HNCO→CO play a crucial role in maintaining the CO level constant after reaching its peak position under fuel-rich conditions·NH3 consumption pathways shift from NH2→H2NO→HNO→NO→NO2→N2O→N2 at intermediate temperatures to NH3→NH2→NH→N(HNO)→NO→(N2O)N2 at high temperatures. The C2-additives participate in the oxidation process by introducing additional OH/H/CH3 radicals, thereby facilitating the reactive radical pool and developing the N-C intersystem-crossing pathways. In summary, the extensive multi-speciation data, metrological uncertainty analysis, mechanism validation and development, together with comprehensive kinetic modelling can be valuable resources for further studies of ammonia combustion.

Insight into premixed diethoxymethane flames: Laminar burning velocities, temperatures, and emissions behaviour

Diethoxymethane ((CH3CH2O)2CH2, DEM) is a promising carbon-neutral fuel. DEM is a diether or acetal with a molecular structure similar to oxymethylene ethers (CH3O–(CH2O)n–CH3, OMEn). Thus, DEM can be expected to have a similar combustion behavior to OMEs, reducing harmful emissions such as NOx and particulate matter (PM) in internal combustion engines. From both experimental and kinetic modeling, fundamental studies on DEM are scarce in the literature. More studies are required to gain a detailed insight into the oxidation kinetics of DEM. Laminar burning velocity (LBV) is a critical property that allows a detailed assessment of the potential application of DEM in combustion devices. Unfortunately, the literature on the LBV of DEM is limited. Therefore, in this study we have investigated the LBV of DEM using two reactors for the first time, namely a heat flux burner and a combustion chamber. The experimental data is reported for equivalence ratio between 0.7 and 1.7, initial temperatures of 368–423 K, and initial pressure of 1–5 bar. In addition, we developed a detailed kinetic model extending our recent work of Shrestha et al. (Combust. Flame. 246 (2022) 112,426) to characterize the combustion behavior of DEM utilizing the new experimental data from this work and the literature data. Our model performs remarkably well in capturing the newly measured LBV experimental data over various experimental conditions. We found that DEM and dimethoxy methane (DMM) have similar values of LBVs (within ±1.5 cm/s) for a given condition, which indicates that intermediate chemistry governs the flame chemistry. Despite DEM being a larger molecule that is expected to have slightly lower LBVs than DMM, its effect on the measured values of LBVs is negligible. Finally, we experimentally measured NOx formation in DEM flame for the first time. The stochiometric flame has the highest NOx formation. The proposed model predicted the equivalence ratio dependence of NOx nicely. However, it overestimates the NOx formation for stoichiometric DEM/air mixtures by ∼30 %. The model suggests that the thermal NO formation route is favored at lean and stochiometric conditions. In contrast, the prompt NO formation route is enhanced for rich mixtures.

Equivalence ratio independence and dependence ranges of system responses for a nonlinear thermoacoustic oscillation in a Rijke tube

Thermoacoustic instabilities in a combustion system remains a challengeable problem in industry over decades. It usually involves with the nonlinear interaction between unsteady heat release and pressure fluctuations. The nonlinear oscillation excited by this interaction can be intense enough to result in combustion problems even severe damage of the engine. Therefore, it is important to understand the system response to various parameters. In this study, the system nonlinearity to equivalence ratio change was mainly focused on. A Rijke tube system with a premixed methane-air laminar flame was utilised to trigger the self-excited oscillations with varied equivalence ratios (Φ). The time-domain analysis methods including phase difference, Recurrence Plot (RP) and Recurrence quantification analysis (RQA) were applied to obtain the characteristics of system nonlinearities. Both Φ-independence and Φ-dependence ranges of system frequency responses have been discovered. The results of phase difference and recurrence analysis have also validated the finding of both ranges. The corresponding characteristics of system nonlinearity in both ranges have been investigated. The similarity among the systems in the Φ-independence range under the same burner position and methane flowrate can be highlighted from the system frequency response, phase difference and system dynamics. It is found that the phase difference analysis, RP and RQA utilised in this study are capable of investigating the nonlinearities of the self-excited thermoacoustic oscillation.

Experimental characterization of ozone-enhanced n-decane cool flames and numerical investigation of equivalence ratio dependence

Low temperature combustion behavior is critical to understand for advanced combustor design and safety, but cool flames are challenging to study under chemistry-transport coupled conditions because of the generally transient nature of these flames. A series of studies have utilized a laminar flat flame Hencken burner platform and ozone enhancement at sub-atmospheric pressures to stabilize dimethyl ether, propane and heptane cool flames and measure flame characteristics such as propagation speed, flame temperature and product species. This work builds upon those prior studies to investigate n-decane, first through experimental characterization of ozone-enhanced n-decane cool flames, including measurement of propagation speeds and flame temperatures and imaging of formaldehyde planar laser induced fluorescence (CH2O PLIF) over a range of equivalence ratios and, second, through comparison of experimental results with one-dimensional cool flame simulations. Experimental findings showed moderate equivalence ratio dependence of cool flame propagation speeds and temperatures at lean equivalence ratios, and no significant equivalence ratio dependence for richer mixtures. Experimental CH2O profiles showed equivalence ratio dependence and no downstream consumption, as is characteristic of cool flames. Simulated n-decane cool flames showed significant dependence of propagation speeds and temperatures on equivalence ratio, as well as downstream consumption of CH2O, at the experimental conditions. Experimental and numerical discrepancies were first investigated by examining the influence of different ozone enhancement strategies on equivalence ratio dependence and the results indicate a minimal influence of ozone enhancement strategy for n-decane, while also providing insights to into equivalence ratio dependencies observed in previous ozone-enhanced and ozoneless cool flame studies. The effect of ozone concentration on simulated n-decane cool flames was then investigated, revealing a strong influence of ozone enhancement on equivalence ratio dependence. Simulated propagation speed and temperature dependence on equivalence ratio were shown to develop with increase in ozone enhancement levels. A kinetics analysis revealed enhancement of low temperature peroxy chemistry and other cool flame heat releasing reactions with ozone addition, increasing simulated flame temperatures and promoting the onset of early intermediate temperature chemistry (ITC), inducing equivalence ratio dependence which was not seen in experimental results. Finally, insights from this analysis were used to investigate reconciliation of differences in experimental and simulated cool flame temperatures and identify potential kinetics model discrepancies from experimental findings.

Effects of equivalence ratio and blending ratio on the ignition delays of n-pentane/hydrogen mixtures under engine relevant pressure

In this work, new ignition delay times of n-pentane/hydrogen binary mixtures were measured at a pressure of 20 atm, equivalence ratios of 0.5, 1.0, and 2.0 using a shock tube. Literature Pentane Isomer model is found to well capture the ignition delay times of n-pentane/hydrogen mixtures under the current condition. Kinetic analyses are performed to interpret the equivalence ratio dependence for n-pentane/hydrogen blends and to understand the interplay chemistry between the two fuels. It is found that the ignition delay times of all the n-pentane/hydrogen mixtures in this study show the equivalence ratio dependence similar to pure n-pentane. The reaction pathway indicates that n-pentane reacts independently from the hydrogen after blending. Hydrogen addition only gives the moderate promotion on n-pentane ignition for small hydrogen proportions since the H radicals in the dual-fuel system were mostly supplied to the n-pentane H-abstractions and the oxidation chemistry hydrogen is inhibited after blending.

Ignition delay times of iso-butane activated by DME under various equivalence ratios in the shock tube

The ignition delay time of the high cetane number dimethyl ether (DME), high octane number i-butane, and their binary blends were investigated in a shock tube over the temperature range of 1091–1797 K, pressures of 2–10 atm, and equivalence ratios of 0.5–2.0. The LLNL iso-Octane Model and the DME sub-set of the Aramco3.0 Model were assembled together to reproduce the ignition delay behavior of the binary blends. Chemical kinetic analyses were conducted to understand the interplay between DME and i-butane at high temperatures. The ignition delay of DME and i-butane show the opposite equivalence ratio dependence under the current conditions. The promoting effect of DME addition on the ignition delay of i-butane becomes more significant as the equivalence ratio increases. The research result shows that there is no significant fuel to fuel interactions between DME and i-butane in the present conditions. The high reactivity of DME and its rapid accumulation of the free radicals lead to the promoted auto-ignition of i-butane.

A kinetics and dynamics study on the auto-ignition of dimethyl ether at low temperatures and low pressures

Though the combustion chemistry of dimethyl ether (DME) has been widely investigated over the past decades, there remains a dearth of ignition data that examines the low-temperature, low-pressure chemistry of DME. In this study, DME/‘air’ mixtures at various equivalence ratios from lean (0.5) to extremely rich (5.0) were ignited behind reflected shock waves at a fixed pressure (3.0 atm) over the temperature range 625–1200 K. The ignition behavior is different from that at high-pressures, with a repeatable ignition delay time fall-off feature observed experimentally in the temperature transition zone from the negative temperature coefficient (NTC) regime to the high-temperature regime. This could not be reproduced using available kinetic mechanisms as conventionally homogeneous ignition simulations. The fall-off behavior shows strong equivalence ratio dependence and disappears completely at an equivalence ratio of 5.0. A local ignition kernel postulate was implemented numerically to quantifiably examine the inhomogeneous premature ignition. At low temperature, no pre-ignition occurs in the mixture. A conspicuous discrepancy was observed between the measurements and constrained UV simulations at temperatures beyond the NTC regime. A third O2 addition reaction sub-set was incorporated into AramcoMech 3.0, together with related species thermochemistry calculated using the G3/G4/CBS-APNO compound method, to explore the low-temperature deviation. The new reaction class does not influence the model predictions in IDTs, but the updated thermochemistry does. Sensitivity analyses indicate that the decomposition of hydroperoxy-methylformate plays a critical role in improving the low-temperature oxidation mechanism of DME but unfortunately, the thermal rate coefficient has never been previously investigated. Further experimental and theoretical endeavors are required to attain holistic quantitative chemical kinetics based on our understanding of the low-temperature chemistry of DME.

Auto-ignition kinetics of ammonia and ammonia/hydrogen mixtures at intermediate temperatures and high pressures

Auto-ignition properties of NH3/O2 and NH3/H2/O2 mixtures have been studied in a rapid compression machine at pressures from 20 to 60 bar, temperatures from 950 to 1150 K, and equivalence ratios from 0.5 to 2. The effect of the ammonia/hydrogen ratio in the fuel mixture has been also investigated. The experiments demonstrate that a higher H2 mole fraction in the fuel mixture increases its reactivity, while the equivalence ratio shows different influence as follows. When the fuel mixture contains 20% H2, the fuel-richer mixtures have shorter ignition delay times, while for mixtures containing 1% H2 in fuel the equivalence ratio dependence is opposite. With 5% H2 in fuel, the stoichiometric mixture presents the shortest ignition delay time. In mixtures without hydrogen, i.e., pure NH3, leaner mixtures show higher reactivity. In addition, numerical simulations were performed based on the literature mechanisms of Glarborg et al. (2018), Mathieu and Petersen (2015), and Klippenstein et al. (2011). While these models can predict well the ignition delay time of NH3/O2 mixtures, none of the models can predict the behavior of NH3/H2/O2 mixtures satisfactorily. The predictions are most sensitive to the branching reactions NH2 + NO and to the reaction H2NO + O2 = HNO + HO2. Hydrogen addition enriches the O/H radical pool consuming NH3 and NH2, but it has small effect on NOx emissions.

Equivalence Ratio Dependence of Reactivity of Low and High Temperature Reactions for Ultra-Lean Gasoline Surrogate/Air Weak Flames in Micro Flow Reactor with Controlled Temperature Profile

Equivalence Ratio Dependence of Reactivity of Low and High Temperature Reactions for Ultra-Lean Gasoline Surrogate/Air Weak Flames in Micro Flow Reactor with Controlled Temperature Profile

Ignition and formaldehyde formation in dimethyl ether (DME) reacting spray under various EGR levels

As an alternative fuel to diesel in compression-ignition (CI) engines, dimethyl ether (DME) has gained interest in combustion research due to its high cetane number for fast ignition and ultra-low emission of particulate matter. In this study, ignition and important intermediate species including formaldehyde (CH2O) are experimentally investigated in a constant-volume combustion vessel facility that includes a fuel injection system for spray-like behavior of liquid DME. Experiments of different oxygen concentrations simulating various levels of exhaust-gas recirculation (EGR) were performed to study its corresponding effect on the flame structure and emissions from DME combustion. Results from the experiment were then used to validate a 3-D CFD simulation using a detailed chemistry solver. Different stages of ignition characterized by temperature profile, and certain species (e.g., CH2O for cool flame) after start of injection, are provided to conceptualize the DME combustion process under the effect of low-to-high oxygen ambient gas concentrations. Both simulations and experiments showed that there is a supporting link between CH2O formation and low-temperature combustion prior to diffusion-controlled flame uniquely for DME. By studying the temperature and equivalence ratio dependence of the reacting spray at different O2 levels, different stages of ignition along with the formation of CH2O suggested that the start of the depletion of CH2O can be used as an ignition indicator.

Effects of Continuous Volumetric Direct-Coupled Nonequilibrium Atmospheric Microwave Plasma Discharge on Swirl-Stabilized Premixed Flames

The effect of continuous volumetric direct-coupled nonequilibrium atmospheric microwave plasma discharge on swirl-stabilized premixed methane–air flames was investigated using quantitative OH planar laser-induced fluorescence and spectrally resolved emission. The plasma discharge was found to influence the dynamics of flame stabilization, i.e., plasma-assisted flames stabilized in the quiescent center body wake were relatively stable while swirl flames stabilized in the active inner shear layer were prone to local extinction due to aerodynamic shear. At coupled plasma powers corresponding to less than 3% of the thermal power output, in addition to the improved flame stability, significant improvement in the lean blow-out limit ( $\sim 43$ %) and OH number density ( $\sim 150$ %) was observed. The enhancements are shown to be nonequilibrium plasma effects and not predominantly ohmic heating as significant equivalence ratio dependence of OH number density in plasma-assisted flames was observed. Spectrographic measurements indicated nitrogen vibrational temperatures as high as 6100 K, suggesting both vibrational and electronic excitation of nitrogen molecules in the presence of a plasma discharge. The activation of highly reactive species through vibrational–vibrational relaxation and direct impact dissociation accelerates the combustion chemistry. It is demonstrated that microwave direct plasma coupling can drastically enhance the dynamic flame stability of swirl-stabilized flames, especially at very lean operating conditions.

Experimental and Kinetic Study on Ignition Delay Times of Dimethyl Ether at High Temperatures

An experimental investigation was performed on the effects of the temperature, pressure, equivalence ratio, and fuel concentration on ignition delay times of dimethyl ether (DME)/O2/Ar mixtures behind the reflected shock wave. Experimental conditions used temperatures over 1000–1600 K, pressures of 1.2–20 atm, and equivalence ratios of 0.5–2.0, with fuel concentrations of 0.5–2.457%. The measurements showed that the DME mixtures have different global activation energies under different equivalence ratios. Thus, correlations were derived under different equivalence ratios based on all experimental data, which fits fairly well with the experimental data. Four recently developed models were compared to the measurements, and their predictabilities were thoroughly discussed. Finally, a systematic kinetic chemical analysis was performed to chemically interpret the observed equivalence ratio dependence and to ascertain the key reactions that control ignition of DME, which are the potential candidates for improvement of LLNL DME Mech.

Effect of pressure and equivalence ratio on the ignition characteristics of dimethyl ether-hydrogen mixtures

Experimental and numerical study on the effect of pressure and equivalence ratio on the ignition delay times of the DME/H2/O2 mixtures diluted in argon were conducted using a shock tube and CHEMKIN II package at equivalence ratios of 0.5–2.0, pressures of 1.2–10 atm and hydrogen fractions of 0–100%. It was found that the measured ignition delay times of the DME/H2 mixtures demonstrate three ignition regimes. For the DME/H2 mixture at XH2 ≤80%, the ignition is controlled by the DME chemistry and ignition delay times present a typical Arrhenius pressure dependence and weak equivalence ratio dependence. For the DME/H2 mixture at 80% < XH2 < 98%, the ignition is controlled by the combined chemistries of DME and hydrogen, and the ignition delay times give higher ignition activation energy at higher pressures and a typical Arrhenius equivalence ratio dependence. However, for the DME/H2 mixture at XH2≥98%, the ignition is controlled by the hydrogen chemistry and ignition delay time shows complex pressure dependence and weak equivalence ratio dependence. Comparison of the measurements of neat DME and neat hydrogen with the calculations using three generally accepted mechanisms, NUIG Aramco Mech 1.3 [1], LLNL DME Mech [2–4] and Princeton-Zhao Mech [5], shows that NUIG Aramco Mech 1.3 gives the best predictions and can well capture the pressure and equivalence ratio dependence at various hydrogen fractions. The sensitivity and normalized H-radicals consumption analysis were performed using NUIG Aramco Mech 1.3 and the key reactions that control the ignition characteristics of DME/H2 mixtures were revealed. Further chemical kinetic analysis was made to interpret the ignition delay time dependence on pressure and equivalence ratio at varied hydrogen fractions.

Effect of radical quenching on CH4/air flames in a micro flow reactor with a controlled temperature profile

The effect of radical quenching on CH4/air flames was investigated using a micro flow reactor with a controlled temperature profile experimentally and numerically. To evaluate the radical quenching, two methods were employed in this study; changing the inner diameter of the reactor at atmospheric pressure, d; and changing pressure in the reactor, P. The stabilized flame locations were measured and gas sampling analysis was conducted. The results were compared with two-dimensional computation including detailed gas-phase kinetics with/without a radical quenching surface reaction mechanism. At atmospheric pressure, there was no significant difference in the flame locations between the inert and quench wall cases at d=2.0 and 1.5mm. The measured flame location agreed with the computational flame locations. Thus the effect of radical quenching on the flame locations were negligible. The measured CO mole fraction agreed with the computed CO mole fraction for the inert wall, whereas significant amount of CO was predicted for the quench wall. At reduced pressures (0.5, 0.1 and 0.05atm), the flame locations in computations for the inert wall fairly agreed with those in experiments. Flames blew out in computations for the quench wall with the sticking coefficient, S, of unity. The present surface reaction mechanism overestimated the effect of the radical quenching on flames. The equivalence ratio dependence of the CO mole fraction of burned gas was computed for the quench wall with S=0.0005 at all pressures. When ϕ>1, the effect of the radical quenching on the CO mole fraction was not significant at P=0.5atm but some effects were observed at P=0.1 and 0.05atm. When ϕ<1, the enhancement of CO oxidation was numerically observed for the quench wall with S=0.0005. Reaction pathways of OH production by the interaction between gas-phase reactions and radical quenching surface reactions were identified for interpreting this CO oxidation enhancement.

Pyrolysis and oxidation of decalin at elevated pressures: A shock-tube study

Ignition delay times and ethylene concentration time-histories were measured behind reflected shock waves during decalin oxidation and pyrolysis. Ignition delay measurements were conducted for gas-phase decalin/air mixtures over temperatures of 769–1202K, pressures of 11.7–51.2atm, and equivalence ratios of 0.5, 1.0, and 2.0. Negative-temperature-coefficient (NTC) behavior of decalin autoignition was observed, for the first time, at temperatures below 920K. Current ignition delay data are in good agreement with past shock tube data in terms of pressure dependence but not equivalence ratio dependence. Ethylene mole fraction and fuel absorbance time-histories were acquired using laser absorption at 10.6 and 3.39μm during decalin pyrolysis for mixtures of 2200–3586ppm decalin/argon at pressures of 18.2–20.2atm and temperatures of 1197–1511K. Detailed comparisons of these ignition delay and species time-history data with predictions based on currently available decalin reaction mechanisms are presented, and preliminary suggestions for the adjustment of some key rate parameters are made.

1-Butanol ignition delay times at low temperatures: An application of the constrained-reaction-volume strategy

Ignition delay times behind reflected shock waves are strongly sensitive to variations in temperature and pressure, yet most current models of reaction kinetics do not properly account for the variations that are often present in shock tube experiments. Particularly at low reaction temperatures with relatively long ignition delay times, substantial increases in pressure and temperature can occur behind the reflected shock even before the main ignition event, and these changes in thermodynamic conditions of the ignition process have proved difficult to interpret and model. To obviate such pressure increases, we applied a new driven-gas loading method that constrains the volume of reactive gases, thereby producing near-constant-pressure test conditions for reflected shock measurements. Using both conventional operation and this new constrained-reaction-volume (CRV) method, we have collected ignition delay times for 1-butanol/O2/N2 mixtures over temperatures between 716 and 1121K and nominal pressures of 20 and 40atm for equivalence ratios of 0.5, 1.0, and 2.0. The equivalence ratio dependence of 1-butanol ignition delay time was found to be negative when the oxygen concentration was fixed, but positive when the fuel concentration was held constant. Ignition delay times with strong pre-ignition pressure increases in conventional-filling experiments were found to be significantly shorter than those where these pressure increases were mitigated using the CRV strategy. The near-constant-pressure ignition delay times provide a new database for low-temperature 1-butanol mechanism development independent of non-idealities caused by either shock attenuation or pre-ignition perturbations. Comparisons of these near-constant-pressure measurements with predictions using several reaction mechanisms available in the literature were performed. To our knowledge this work is first of its kind that systematically provides accurate near-constant-enthalpy and -pressure target data for chemical kinetic modeling of undiluted fuel/air mixtures at engine relevant conditions.

Radical Luminescence from Premixed Flame. Characteristics of Radical Luminescence Spectrum from Flames of Gasoline, Methanol and Methane, and Equivalence Ratio Dependence.

The possibility of combustion diagnostics through the measurement of radical luminescence is discussed in this paper. Radical species in premixed flames of gasoline, methanol and methane are explored using spectroscopic measurements. It was found that major species having strong radical luminescence are OH, CH and C2 for gasoline and methane, and OH and CH for methanol. The effects of the equivalence ratio and premixed gas temperature on spectral intensities are discussed. We found that the correlation between the equivalence ratio and spectrum intensity ratio of two radical species is linear. There is a possibility of predicting the equivalence ratio from the measurement of the radical emission intensity of a flame.