Chemical Kinetic Calculations Research Articles

Development of a Reduced Biodiesel Surrogate Fuel Model for Multi-Dimensional Computational Fluid Dynamic Simulations

This work reports the development of a reduced biodiesel surrogate fuel model for multi-dimensional Computational Fluid Dynamic simulations. The model is derived using an integrated kinetic mechanism reduction scheme and the final chemistry comprises only 83 species. The model is well validated in zero-dimensional chemical kinetic calculations under a wide range of auto-ignition and jet-stirred reactor conditions. The fidelity of the model is also further assessed in two-dimensional simulations of a constant-volume combustion vessel with respect to the experimental results of soy-methyl ester combustion. The results obtained reveal that ignition delay, lift-off lengths and soot volume fractions are reasonably well replicated by the reduced model. Furthermore, the compositions of the reduced model are varied according to the saturation/unsaturation levels of various biodiesel feed-stocks, such as palm-methyl ester and sunflower-methyl ester. In this work, it is demonstrated that the reduced model can potentially be used as a universal surrogate fuel model to predict the reactivity of biodiesel feed-stocks with low degree of saturation (≤30%) in both kinetic and fuel spray simulations.

Auto-ignition characteristics of end-gas in a rapid compression machine under super-knock conditions

Spark ignition induced super-knock was generated in a rapid compression machine using stoichiometric iso-octane/O2/N2 mixture. In addition to the pressure traces, the combustion processes were also recorded using high-speed photography, from which the characteristics of the end-gas auto-ignition were analyzed. The effect of negative temperature coefficient (NTC) on the end-gas auto-ignition was investigated using detailed and reduced reaction mechanisms. The end-gas auto-ignition of diluted methanol/O2/Ar mixture without NTC behavior was also tested for comparison. The results showed that the end-gas auto-ignition of iso-octane/O2/N2 mixture exhibited a two-stage ignition process. During the second ignition stage of the end-gas, two auto-ignition events with very short time interval were sequentially observed in the end-gas region. The first auto-ignition event generated a weak shock wave, and the second one initiated detonation. Both the two auto-ignition events occurred near the wall but at different sites. Due to the heat loss to the wall, the near-wall region is generally considered to be colder than the adiabatic core region, and thus the near-wall auto-ignition of end-gas was usually considered as a result of fuel's NTC behavior. However, in this study the chemical kinetic calculation showed that the evolution of the end-gas almost bypassed the NTC region in the ignition delay-temperature diagram. Furthermore, for the methanol/O2/Ar mixture the end-gas auto-ignition also started at the very near-wall region. Considering that methanol fuel does not has an NTC behavior, the near-wall auto-ignition of methanol/O2/Ar mixture should be a result of other factors than NTC. Therefore, it was concluded that NTC might not play a dominant role in the near-wall auto-ignition of the end-gas.

A diesel/natural gas dual fuel mechanism constructed to reveal combustion and emission characteristics

As natural gas is an ideal alternative fuel with low cost and clean combustion, this study constructs a mechanism of diesel/natural gas dual fuel, couples it with CFD, and then discusses the effects of natural gas replacement rates on engine combustion and emission under medium load conditions. The detailed mechanism of natural gas is simplified by two methods into a reduced mechanism, which are verified to prove that it can accurately predict ignition delay, laminar flame speed, temperature and pressure. The study also combines the reduced mechanism with the 95/5vv diesel mechanism to obtain a dual-fuel mechanism. This dual fuel mechanism is verified through chemical kinetics calculations, to predict the ignition delay, laminar flame speed, important free radicals and components. Furthermore, this mechanism is verified through coupling CFD, to predict that the combustion pressure and integrated heat release agreed with the experimental data. Results show that the dual-fuel mechanism displays its correctness, effectiveness and applicability. Calculations within the range of 0–60% natural gas replacement rates indicate a regular combustion sequence; at 40% replacement rate, NO emissions goes down by 6.8%, and soot emissions decrease by 92.9%. Yet when the replacement rate of natural gas is over 55%, combustion in cylinder deteriorates.

Influence of Ammonia and Water on the Fate of Sulfur Trioxide in the Troposphere: Theoretical Investigation of Sulfamic Acid and Sulfuric Acid Formation Pathways.

Reaction of ammonia with SO3 as a potential source of sulfamic acid in the troposphere has been investigated by means of electronic structure and chemical kinetic calculations. Besides, the hydrolysis reaction, which is known to be a major atmospheric decay channel of SO3, has also been investigated. The catalytic effects of ammonia and water on both the reactions have been studied. Rate coefficients for all the studied reaction channels were calculated using the transition state theory employing pre-equilibrium approximation. Calculated rate coefficients for a number of catalyzed hydrolysis and ammonolysis processes were found to be much higher (by ∼105 to ∼109 times) than the gas kinetic limit at ambient temperature. With decrease in temperature because of negative temperature dependence of rate coefficients, that difference became even larger (up to ∼1016 times). Therefore, in order to remove the discrepancies, rate coefficients for all the studied reaction channels were calculated by means of the master equation. The results showed marked improvements, with only one channel showing a slightly higher rate coefficient above the gas kinetic limit. The rate coefficients for catalyzed channels obtained from the master equation also showed negative temperature dependence, albeit to a much smaller extent. The uncatalyzed ammonolysis reaction, similar to the corresponding hydrolysis, was found to be too slow to have any practical atmospheric implication. For both reactions, ammonia-catalyzed pathways have higher rate coefficients than water-catalyzed ones. Between hydrolysis and ammonolysis, the latter showed a higher rate coefficient. In spite of that, ammonolysis is expected to have negligible contribution in the tropospheric loss process of SO3 because of large difference in concentration values between water and ammonia in the troposphere.

Ammonolysis as an important loss process of acetaldehyde in the troposphere: energetics and kinetics of water and formic acid catalyzed reactions.

The reaction of ammonia with acetaldehyde as a potential source of 1,1-aminoethanol in the troposphere has been investigated by electronic structure and chemical kinetics calculations. The reaction was found to involve very high activation energy and consequently the rate coefficient value was found to be too small to have any significant atmospheric implication. Therefore, the catalytic effects of water (monomer and dimer) and formic acid on the reaction have also been studied. The water catalyzed reaction involves significantly lower barrier height, more so for dimer than monomer, whereas formic acid makes the reaction effectively barrierless. In terms of rate coefficients too, formic acid was found to be the most efficient followed by water dimer and monomer. Further, comparative studies under various tropospheric conditions among the catalyzed reaction channels have been carried out using relative rate calculations with respect to the uncatalyzed channel. Relative rate calculations indicate that the formic acid catalyzed channel would dominate over the water catalyzed channels at 0 km altitude when the concentration of the former is high, but water catalyzed channels would dominate under hot and humid conditions if the concentration of formic acid remains low. At higher altitudes in the troposphere, 1,1-aminoethanol formation through ammonolysis of acetaldehyde would almost exclusively follow the formic acid catalyzed channel.

Energy dissipation in polymer-bonded explosives with various levels of constituent plasticity and internal friction

The ignition of energetic materials (EM) under dynamic loading is mainly controlled by localized temperature spikes known as hotspots. Hotspots occur due to several dissipation mechanisms, including viscoplasticity, viscoelasticity, and internal friction along crack surfaces. To analyze the contributions of these mechanisms, we quantify the ignition probability, energy dissipation, damage evolution, and hotspot characteristics of polymer-bonded explosives (PBXs) with various levels of constituent plasticity of the energetic phase and internal crack face friction. Using PBX9501 consisting of HMX (Octahydro-1,3,5,7-Tetranitro-1,2,3,5-Tetrazocine) and Estane as a reference material, we analyze variants of this material with several values of the yield stress of the energetic phase and coefficients of internal crack face friction, while other parameters are kept unchanged. The impact loading involves piston velocities between 200 and 1200 m/s. The analysis uses a Lagrangian cohesive finite element framework that explicitly accounts for finite-strain elastic-viscoplastic deformation of the grains, viscoelastic deformation of the binder, arbitrary crack initiation and propagation in the grains and the binder, debonding between the grains and the binder, contact between internal surfaces, friction and frictional heating along internal surfaces, heat generation resulting from inelastic bulk deformation, and heat conduction. To determine the ignition status of the material or “go” or “no-go” state, we use a criterion based on a criticality threshold obtained from chemical kinetics calculations. For PBX with various levels of HMX plasticity and friction, the probability of ignition, the evolution of dissipation caused by plasticity and friction, the density of cracks, and the locations of cracks are quantified. Results show that samples with higher levels of constituent plasticity (lower yield strengths) or lower levels of internal friction are less likely to ignite. The relative importance of plasticity and friction depends on load intensity, with frictional heating decreasing as load intensity increases. Although the overall viscoplastic heating outweighs the overall frictional heating, friction plays a very important role in hotspot development at all load intensities analyzed, owing to the fact that frictional heating is more localized than viscoplastic heating. The predicted thresholds and ignition probabilities are expressed in a load intensity-load duration relation for PBX with different constituent properties.

Autoignition Control of Fuel and Air Mixtures Using Photochemical Isomerization

Autoignition control of fuel and air mixtures via photochemical isomerization was simulated using chemical kinetic calculations for the ignition of 1,3-cyclohexadiene diluted in n-pentane. The calculations showed that ignition in a homogeneous charge compression ignition engine may be controlled if the fuel is photochemically isomerized using light irradiation prior to the combustion process. The calculations illustrate how photochemical reactions may be applied to fuels, to control the combustion process in engines. The theoretical energy required to achieve the photochemical conversion, based on the quantum conversion, constitutes 0.3% of the fuel energy, which may be offset by increased engine efficiency resulting from ignition timing control.

Influence of Boundary Conditions on the Flame Stabilization Mechanism and on Transient Auto-ignition in the DLR Jet-in-Hot-Coflow Burner

Transient auto-ignition is a key factor for flame stabilization and flame initialization in several technical combustion systems such as internal combustion engines or gas turbine combustors. Reliable numerical simulations of auto-ignition stabilized flames are important for the development of new combustor systems. For detailed model validation, knowledge of the sensitivity of different system response quantities (SRQs) of interest to the boundary conditions in combination with the accuracy of boundary conditions is essential, especially with respect to uncertainty quantification of numerical simulations. In the current study, the flame stabilization and auto-ignition in the DLR Jet-in-Hot-Coflow burner was examined experimentally using high-speed OH* chemiluminescence. Here, methane was either injected continuously to study the flame stabilization mechanism of steady state lifted jet flames, or in a pulsed manner to study the formation of auto-ignition kernels, into the hot exhaust gas of a lean, premixed hydrogen/air flame. The flame stabilization height, and the location and time of initial auto-ignition kernels for a case with transient auto-ignition were evaluated with respect to several boundary conditions, such as coflow temperature as well as coflow- and jet-velocity. A relative sensitivity of the measured SRQs on the boundary conditions was introduced in order to quantitatively compare steady state flame to transient auto-ignition characteristics and to assess the quantitative influence of different boundary conditions. Comparison of the auto-ignition dynamics in the steady state and during transient fuel injection allowed assessing the role of auto-ignition in the flame stabilization mechanism for different boundary conditions; accompanying chemical kinetic calculations were used to quantify the influence of strain on auto-ignition and flame propagation for the current conditions, allowing further insight into the flame stabilization mechanism in Jet-in-Hot-Coflow flames.

Numerical study of non-premixed air-methane swirl combustor flows using RANS method

The simulation is based on the experimental results obtained from German Aerospace Center (DLR) using a non-premixed swirling flow gas turbine combustor. The simulation case allows for the understanding of CFD’s capabilities in modeling combustion for pre-mixed flows and adds the complexity of modeling mixing and combustion for un-mixed streams. The case is simulated in steady-state condition, with typical boundary conditions being applied. For turbulence modeling, the Reynolds Stress Model (RSM) approaches are being utilized here to study the flow and combustion processes occurring inside a combustion chamber where lean methane-air streams are burnt. For chemical kinetics calculations, the Probability Density Function (PDF) transport model and Eddy Dissipation Model (EDM) using a single-step reaction are considered. The simulation results are compared and evaluated qualitatively with the experimental data,including velocity field and temperature profile.The results of swirl flow combustor show that the adopted numerical model reproduced the trends of axial, radial and tangential velocity profiles, especially near the centerline. However, the model failed to predict the temperature and species mass fraction profiles.

Plasma-based multi-reforming for Gas-To-Liquid: tuning the plasma chemistry towards methanol

Because of its unique properties, plasma technology has gained much prominence in the microelectronics industry. Recently, environmental and energy applications of plasmas have gained a lot of attention. In this area, the focus is on converting CO2 and reforming hydrocarbons, with the goal of developing an efficient single-step ‘gas-to-liquid’ (GTL) process. Here we show that applying tri-reforming principles to plasma—further called ‘plasma-based multi-reforming’—allows us to better control the plasma chemistry and thus the formed products. To demonstrate this, we used chemical kinetics calculations supported by experiments and reveal that better control of the plasma chemistry can be achieved by adding O2 or H2O to a mixture containing CH4 and CO2 (diluted in N2). Moreover, by adding O2 and H2O simultaneously, we can tune the plasma chemistry even further, improving the conversions, thermal efficiency and methanol yield. Unlike thermocatalytic reforming, plasma-based reforming is capable of producing methanol in a single step; and compared with traditional plasma-based dry reforming, plasma-based multi-reforming increases the methanol yield by more than seven times and the thermal efficiency by 49%, as revealed by our model calculations. Thus, we believe that by using plasma-based multi-reforming, ‘gas-to-liquid’ conversion may be made efficient and scalable.

An experimental and chemical kinetic modeling study of 1,3-butadiene combustion: Ignition delay time and laminar flame speed measurements

Ignition delay times for 1,3-butadiene oxidation were measured in five different shock tubes and in a rapid compression machine (RCM) at thermodynamic conditions relevant to practical combustors. The ignition delay times were measured at equivalence ratios of 0.5, 1.0, and 2.0 in ‘air’ at pressures of 10, 20 and 40 atm in both the shock tubes and in the RCM. Additional measurements were made at equivalence ratios of 0.3, 0.5, 1.0 and 2.0 in argon, at pressures of 1, 2 and 4 atm in a number of different shock tubes. Laminar flame speeds were measured at unburnt temperatures of 295 K, 359 K and 399 K at atmospheric pressure in the equivalence ratio range of 0.6–1.7, and at a pressure of 5 atm at equivalence ratios in the range 0.6–1.4. These experimental data were then used as validation targets for a newly developed detailed chemical kinetic mechanism for 1,3-butadiene oxidation.A detailed chemical kinetic mechanism (AramcoMech 3.0) has been developed to describe the combustion of 1,3-butadiene and is validated by a comparison of simulation results to the new experimental measurements. Important reaction classes highlighted via sensitivity analyses at different temperatures include: (a) ȮH radical addition to the double bonds on 1,3-butadiene and their subsequent reactions. The branching ratio for addition to the terminal and central double bonds is important in determining the reactivity at low-temperatures. The alcohol-alkene radical adducts that are subsequently formed can either react with HȮ2 radicals in the case of the resonantly stabilized radicals or O2 for other radicals. (b) HȮ2 radical addition to the double bonds in 1,3-butadiene and their subsequent reactions. This reaction class is very important in determining the fuel reactivity at low and intermediate temperatures (600–900 K). Four possible addition reactions have been considered. (c) 3Ö atom addition to the double bonds in 1,3-butadiene is very important in determining fuel reactivity at intermediate to high temperatures (> 800 K). In this reaction class, the formation of two stable molecules, namely CH2O + allene, inhibits reactivity whereas the formation of two radicals, namely Ċ2H3 and ĊH2CHO, promotes reactivity. (d) Ḣ atom addition to the double bonds in 1,3-butadiene is very important in the prediction of laminar flame speeds. The formation of ethylene and a vinyl radical promotes reactivity and it is competitive with H-atom abstraction by Ḣ atoms from 1,3-butadiene to form the resonantly stabilized Ċ4H5-i radical and H2 which inhibits reactivity. Ab initio chemical kinetics calculations were carried out to determine the thermochemistry properties and rate constants for some of the important species and reactions involved in the model development. The present model is a decent first model that captures most of the high-temperature IDTs and flame speeds quite well, but there is room for considerable improvement especially for the lower temperature chemistry before a robust model is developed.

(Invited) Detailed Chemistry Modeling via Cantera: A Pathway to Understanding Battery Degradation Mechanisms

In lithium-ion batteries (LIBs), the solid electrolyte interphase (SEI) is a passivation layer that grows at the anode-electrolyte interface. While this layer reduces battery performance, it is a necessary feature of most LIBs, because of electrolyte instability. However, engineering a better SEI is challenging, due to the difficulty in characterizing and modeling its chemical composition and microstructure. The SEI chemistry is incredibly complex, but to date there are essentially no general purpose modeling capabilities which facilitate this electrochemical complexity to support SEI optimization. Herein, we will use the SEI as a framework to discuss Cantera software as a means to model electrochemical complexity. Cantera is an open-source, object oriented software package which supports thermodynamic, chemical kinetic, and transport calculations in a generalize manner. We will discuss the use of Cantera as part of a computational-experimental framework to understand the SEI, and also its application to electrochemical systems, in general.

Toward Consistent Modeling of Atmospheric Chemistry and Dynamics in Exoplanets: Validation and Generalization of the Chemical Relaxation Method

Abstract Motivated by the work of Cooper & Showman, we revisit the chemical relaxation method, which seeks to enhance the computational efficiency of chemical kinetics calculations by replacing the chemical network with a handful of independent source/sink terms. Chemical relaxation solves the evolution of the system and can treat disequilibrium chemistry, as the source/sink terms are driven toward chemical equilibrium on a prescribed chemical timescale, but it has surprisingly never been validated. First, we generalize the treatment by forgoing the use of a single chemical timescale, instead developing a pathway analysis tool that allows us to identify the rate-limiting reaction as a function of temperature and pressure. For the interconversion between methane and carbon monoxide, and between ammonia and molecular nitrogen, we identify the key rate-limiting reactions for conditions relevant to currently characterizable exo-atmospheres (500–3000 K, 0.1 mbar to 1 kbar). Second, we extend chemical relaxation to include carbon dioxide and water. Third, we examine the role of metallicity and the carbon-to-oxygen ratio in chemical relaxation. Fourth, we apply our pathway analysis tool to diagnose the differences between our chemical network and that of Moses and Venot. Finally, we validate the chemical relaxation method against full chemical kinetics calculations in one dimension. For WASP-18b-, HD 189733b-, and GJ 1214-b-like atmospheres, we show that chemical relaxation is mostly accurate to within an order of magnitude, a factor of 2, and ∼10%, respectively. The level of accuracy attained allows for the chemical relaxation method to be included in three-dimensional general circulation models.

Numerical study of auto-ignition propagation modes in toluene reference fuel–air mixtures: Toward a better understanding of abnormal combustion in spark-ignition engines

Two main abnormal combustions are observed in spark-ignition engines: knock and low-speed pre-ignition. Controlling these abnormal processes requires understanding how auto-ignition is triggered at the “hot spot” but also how it propagates inside the combustion chamber. The original theory regarding the auto-ignition propagation modes was defined by Zeldovich and developed by Bradley who highlighted different modes by considering various hot spot characteristics and thermodynamic conditions around the hot spot. Two dimensionless parameters ( ε, ξ) were then defined to classify these modes and a so-called detonation peninsula was obtained for H2–CO–air mixtures. Similar simulations as those performed by Bradley et al. are undertaken to check the relevancy of the original detonation peninsula when considering realistic fuels used in modern gasoline engines. First, chemical kinetics calculations in homogeneous reactor are performed to determine the auto-ignition delay time τi, and the excitation time τe of E10–air mixtures in various conditions. These calculations are performed for a Research Octane Number (RON 95) toluene reference fuel surrogate with 42.8% isooctane, 13.7% n-heptane, 43.5% toluene, and using the Lawrence Livermore National Laboratory (LLNL) kinetic mechanism considering 1388 species and 5935 reactions. Results point out that H2–CO–air mixtures are much more reactive than E10–air mixtures featuring much lower excitation times τe. The resulting maximal hot spot reactivity ε is thus limited which also restrains the use of the detonation peninsula for the analysis of practical occurrences of auto-ignition in gasoline engines. The tabulated ( τi, τe) values are then used to perform one-dimensional Large Eddy Simulations (LES) of auto-ignition propagation considering different hot spots and thermodynamic conditions around them. The detailed analysis of the coupling conditions between the reaction and pressure waves shows thus that the different propagation modes can appear with gasoline, and that the original detonation peninsula can be reproduced, confirming for the first time that the propagation mode can be well defined by the two non-dimensional parameters for more realistic fuels.

A theoretical study of the reaction mechanism and rate constant of C4H ( $$ {{\tilde{\text{X}}}}^{2} {\varSigma^{ + }} $$ X ~ 2 Σ + ) + C2H6

Theoretical investigations have been carried out on the mechanisms and kinetics of the reaction of linear butadiynyl radical with ethane at the CCSD(T)/aug-cc-pVTZ//ωB97X-D/6-311++G(3df,2p) level. Four hydrogen abstraction channels (M1a, M1b, M2a and M2b) were investigated. The calculated results indicate that two competitive channels M1a and M1b are the predominant mechanisms, while M2a and M2b are unfavorable due to the higher barriers. The canonical variational transition state theory (CVT) with the small-curvature tunneling correction (SCT) was utilized to calculate the rate constants for M1a and M1b. The reactant side wells along the two reaction paths (M1a and M2b) were found and considered in chemical kinetic calculations. The three-parameter rate constant expressions are fitted over a wide temperature range of 145–1000 K.

Isotopic constraints on heterogeneous sulfate production in Beijing haze

Abstract. Discerning mechanisms of sulfate formation during fine-particle pollution (referred to as haze hereafter) in Beijing is important for understanding the rapid evolution of haze and for developing cost-effective air pollution mitigation strategies. Here we present observations of the oxygen-17 excess of PM2.5 sulfate (Δ17O(SO42−)) collected in Beijing haze from October 2014 to January 2015 to constrain possible sulfate formation pathways. Throughout the sampling campaign, the 12-hourly averaged PM2.5 concentrations ranged from 16 to 323 µg m−3 with a mean of (141 ± 88 (1σ)) µg m−3, with SO42− representing 8–25 % of PM2.5 mass. The observed Δ17O(SO42−) varied from 0.1 to 1.6 ‰ with a mean of (0.9 ± 0.3) ‰. Δ17O(SO42−) increased with PM2.5 levels in October 2014 while the opposite trend was observed from November 2014 to January 2015. Our estimate suggested that in-cloud reactions dominated sulfate production on polluted days (PDs, PM2.5 ≥ 75 µg m−3) of Case II in October 2014 due to the relatively high cloud liquid water content, with a fractional contribution of up to 68 %. During PDs of Cases I and III–V, heterogeneous sulfate production (Phet) was estimated to contribute 41–54 % to total sulfate formation with a mean of (48 ± 5) %. For the specific mechanisms of heterogeneous oxidation of SO2, chemical reaction kinetics calculations suggested S(IV) ( = SO2 ⚫ H2O + HSO3− + SO32−) oxidation by H2O2 in aerosol water accounted for 5–13 % of Phet. The relative importance of heterogeneous sulfate production by other mechanisms was constrained by our observed Δ17O(SO42−). Heterogeneous sulfate production via S(IV) oxidation by O3 was estimated to contribute 21–22 % of Phet on average. Heterogeneous sulfate production pathways that result in zero-Δ17O(SO42−), such as S(IV) oxidation by NO2 in aerosol water and/or by O2 via a radical chain mechanism, contributed the remaining 66–73 % of Phet. The assumption about the thermodynamic state of aerosols (stable or metastable) was found to significantly influence the calculated aerosol pH (7.6 ± 0.1 or 4.7 ± 1.1, respectively), and thus influence the relative importance of heterogeneous sulfate production via S(IV) oxidation by NO2 and by O2. Our local atmospheric conditions-based calculations suggest sulfate formation via NO2 oxidation can be the dominant pathway in aerosols at high-pH conditions calculated assuming stable state while S(IV) oxidation by O2 can be the dominant pathway providing that highly acidic aerosols (pH ≤ 3) exist. Our local atmospheric-conditions-based calculations illustrate the utility of Δ17O(SO42−) for quantifying sulfate formation pathways, but this estimate may be further improved with future regional modeling work.

Calculated Impacts of Diluents on Flame Temperature, Ignition Delay, and Flame Speed of Methane–Oxygen Mixtures at High Pressure and Low to Moderate Temperatures

Oxy-fuel combustion is one method to produce concentrated streams of carbon dioxide for subsequent sequestration. An additional benefit of oxy-firing is a reduction in NOx formation. The high combustion temperatures resulting from oxy-firing are typically controlled by exhaust gas recirculation. In this work, we performed chemical kinetic (CHEMKIN) calculations using a mechanism validated for these conditions to study the effects of dilution by either carbon dioxide or water vapor on methane oxy-combustion, and to compare the results with methane air-combustion (N2 as the diluent). The study was performed under adiabatic conditions at a pressure of P = 30 atm, an equivalence ratio of φ = 1, and initial temperatures of T = 800–1200 K, which mimic the inlet conditions of many gas turbines and flameless combustors. The calculations show that H2O addition at low initial temperatures and high pressure leads to considerable reduction in the ignition delay time. This result is mainly due to changes in the radica...

Analysis of flows by deposition of diamond-like structures

The Direct Simulation Monte Carlo (DSMC) method is used to simulate the hydrogen–methane mixtures flowing through a heated cylindrical tungsten tube and expanding into a low-pressure chamber in the substrate holder direction. The DSMC method takes into account heterogeneous reactions in the tube and on the substrate surface. The results of DSMC simulation are used for the chemical kinetics calculations, i.e., axial distributions of species concentrations in various H/C mixtures. The effects of various parameters (reactor configuration, flow rate, initial concentration of methane in the mixture with hydrogen, and pressure in the chamber) on species fluxes to the substrate, the degree of hydrogen dissociation, the degree of methane decomposition, and further conversion of CxHy components up to atomic carbon C are numerically studied. The developed method provides a possibility of solving similar problems for nonequilibrium flows.

Impact of non-ideal behavior on ignition delay and chemical kinetics in high-pressure shock tube reactors

Here, we study real gas effects on high-pressure combustion by comparing simulated and experimentally-measured shock tube ignition delay measurements for n-dodecane/O2/N2 mixtures. Experiments and simulations occur at conditions relevant to diesel engines: 40–80 atm, 774–1163 K, equivalence ratios of 1.0 and 2.0, and O2 concentrations of 13–21%. At these conditions the fuel, oxidizer and intermediate species may exist in a supercritical state during combustion, requiring a real gas equation of state to incorporate non-ideal effects on thermodynamics, chemical kinetics, and the resulting ignition characteristics. A constant-volume, adiabatic reactor model is developed to simulate the reflected shock tube experiments, and simulations compare results for real and ideal gas equations of state, with different expressions for reacting species’ activity concentrations. This paper focuses particularly on the cubic Redlich–Kwong equation of state and thermodynamically consistent chemical kinetic rate calculations based on it. Results demonstrate that the equation of state can have considerable influence on ignition delay times with increasing pressure, particularly in the negative temperature coefficient region. Additionally, the results establish important practices for incorporating real gas effects, namely that (i) the compressibilities of key species (i.e. those participating in rate-limiting reactions) are the appropriate way to screen for real gas effects, rather than the average mixture compressibility; and (ii) incorporating a real gas equation of state without also incorporating thermodynamically consistent chemical kinetics significantly under-predicts the magnitude of real gas effects.

Impact of Vitiation on a Swirl-Stabilized and Premixed Methane Flame

Vitiation refers to the condition where the oxygen concentration in the air is reduced due to the mix of dilution gas. The vitiation effects on a premixed methane flame were investigated on a swirl-stabilized gas turbine model combustor under atmospheric pressure. The main purpose is to analyze the combustion stability and CO emission performance in vitiated air and compare the results with the flame without vitiation. The N2, CO2, and H2O (steam) were used as the dilution gas. Measurements were conducted in a combustor inlet temperature of 384 K and 484 K. The equivalence ratio was varied from stoichiometric conditions to the LBO (Lean Blowout) limits where the flame was physically blown out from the combustor. The chemical kinetics calculation was performed with Chemkin software to analyze the vitiation effects on the flame reaction zone. Based on the calculation results, the changes in the temperature gradient, CO concentration, and active radicals across the flame reaction zone were identified. The time-averaged CH chemiluminescence images were recorded and the results indicated the features of the flame shape and location. The CH signal intensity provided the information about the heat-release zone in the combustor. The combustion LBO limits were measured and the vitiation of CO2 and H2O were found to have a stronger impact to elevate the LBO limits than N2. Near the LBO limits, the instability of the flame reaction was revealed by the high-speed chemiluminescence imaging and the results were analyzed by FFT (Fast Fourier Transfer). CO emission was measured with a water-cooled probe which is located at the exit of the combustor. The combustion vitiation has been found to have the compression effect on the operation range for low CO emission. However, this compression effect could be compensated by improving the combustor inlet temperature.