Chemical Kinetic Mechanism Research Articles

A study of ignition and combustion of liquid hydrocarbon droplets in premixed fuel/air mixtures in a rapid compression machine

The combustion of two fuels with disparate reactivity such as natural gas and diesel in internal combustion engines has been demonstrated as a means to increase efficiency, reduce fuel costs and reduce pollutant formation in comparison to traditional diesel or spark-ignited engines. However, dual fuel engines are constrained by the onset of uncontrolled fast combustion (i.e., engine knock) as well as incomplete combustion, which can result in high unburned hydrocarbon emissions. To study the fundamental combustion processes of ignition and flame propagation in dual fuel engines, a new method has been developed to inject single isolated liquid hydrocarbon droplets into premixed methane/air mixtures at elevated temperatures and pressures. An opposed-piston rapid compression machine was used in combination with a newly developed piezoelectric droplet injection system that is capable of injecting single liquid hydrocarbon droplets along the stagnation plane of the combustion chamber. A high-speed Schlieren optical system was used for imaging the combustion process in the chamber. Experiments were conducted by injecting diesel droplet of various diameters (50 µm < do < 400 µm), into methane/air mixtures with varying equivalence ratios (0 < ϕ < 1.2) over a range of compressed temperatures (700 K < Tc < 940 K). Multiple autoignition modes was observed in the vicinity of the liquid droplets, which were followed by transition to propagating premixed flames. A computational model was developed with CONVERGE™, which uses a 141 species dual-fuel chemical kinetic mechanism for the gas phase along with a transient, analytical droplet evaporation model to define the boundary conditions at the droplet surface. The simulations capture each of the different ignition modes in the vicinity of the injected spherical diesel droplet, along with bifurcation of the ignition event into a propagating, premixed methane/air flame and a stationary diesel/air diffusion flame.

Experimental and modelling study of hydrogen ignition in CO2 bath gas

Direct-fired supercritical CO2 power cycles, operating on natural gas or syngas, have been proposed as future energy technologies with 100 % carbon capture at a price competitive with existing fossil fuel technologies. Likewise, blue or green hydrogen may be used for power generation to counter the intermittency of renewable power technologies. In this work, ignition delay times (IDTs) of hydrogen were measured in a high concentration of CO2 bath gas over 1050 – 1300 K and pressures between 20 and 40 bar. Measured datasets were compared with chemical kinetic simulations using AramcoMech 2.0 and the University of Sheffield supercritical CO2 (UoS sCO2 2.0) chemical kinetic mechanisms. The UoS sCO2 2.0 mechanism was recently developed to model IDTs of methane, hydrogen, and syngas in CO2 bath gas. Sensitivity analyses were used to identify important reactions and to illustrate the trends observed among various datasets. The performance of both mechanisms was evaluated quantitatively by comparing the average absolute error between the predicted and experimental IDTs, which showed UoS sCO2 2.0 as the superior mechanism for modelling hydrogen IDTs in CO2 bath gas. The importance of OH time-histories is identified as the most appropriate next step in further validation of the kinetic mechanism.

Elemental carbon and hydrogen concentrations as the main factors in gas-phase graphene synthesis: Quantitative fourier-transform infrared spectroscopy study

Gas-phase microwave plasma-assisted synthesis of freestanding graphene is a promising route to produce high-quality graphene flakes. However, a comprehensive understanding of the gas-phase kinetics that is required to advance production rates and yields is still lacking. Here, we use line-of-sight Fourier-transform infrared (FTIR) absorption spectroscopy as an in situ diagnostic to measure gaseous species formed during graphene synthesis, thereby elucidating the chemical kinetics mechanism. Different carbonaceous precursors that lead to the formation of either few-layer graphene flakes or soot-like particles are examined, and the results are compared with numerical simulations and mass spectrometry measurements. Quantitative FTIR measurements show a correlation between concentration of atomic carbon and hydrogen in the post-plasma region and particle morphology. Lower carbon and higher hydrogen concentrations lead to graphene formation, while higher carbon and lower hydrogen concentrations shift the reaction toward soot-like particles. We also investigate how the precursor chemical composition, precursor flowrate, and delivered microwave power affect the carbon concentration in the post-plasma region, thus enabling to control particle morphology.

The spectral characterisation of reduced order models in chemical kinetic systems

The size and complexity of multi-scale problems such as those arising in chemical kinetics mechanisms has stimulated the search for methods that reduce the number of species and chemical reactions but retain a desired degree of accuracy. The time-scale characterisation of the multi-scale problem can be carried out on the basis of local information such as the Jacobian matrix of the model problem and its related eigen-system evaluated at one point P of the system trajectory. While the original problem is usually described by ordinary differential equations (ODEs), the reduced order model is described by a reduced number of ODEs and a number of algebraic equations (AEs), that might express one or more physical conservation laws (mass, momentum, energy), or the fact that the long-term dynamics evolves within a so-called Slow Invariant Manifold (SIM). To fully exploit the benefits offered by a reduced order model, it is required that the time scale characterisation of the n-dimensional reduced order model returns an answer consistent and coherent with the time-scale characterisation of the N-dimensional original model. This manuscript discusses a procedure for obtaining the time-scale characterisation of the reduced order model in a manner that is consistent with that of the original problem. While a standard time scale characterisation of the (original) N-dimensional original model can be carried out by evaluating the eigen-system of the () Jacobian matrix of the vector field that defines the system dynamics, the time-scale characterisation of the n-dimensional reduced order model (with n<N) can be carried out by evaluating the eigen-system of a () constrained Jacobian matrix, , of the reduced vector field that accounts for the role of the constraints.

Development of a virtual kinetics chemistry for prediction of ignition delay time

The simulation of complex reactive flows in combustion applications generally requires a large amount of computational power, and the reactive part of the solution is usually the most time consuming. Detailed chemical kinetics mechanisms can reach up to tens of thousands species, while numerical simulations of complex reactive flows can handle mechanisms, at most, with a few hundred species. Virtual kinetic mechanisms are a very effective strategy to drastically reduce the computational time spent in the reactive part of numerical simulations. They are formed by artificial species and reaction paths, that are optimized to reproduce the important characteristics of a canonical problem of interest. This work presents the development of a virtual mechanism able to reproduce the temperature evolution and ignition delay times of the thermal ignition of a homogeneous, constant mass, constant pressure, adiabatic, fuel-air mixture. Three different reaction structures are built for a methane-air mixture and their predictions are tested over a range of temperatures from 1000 K to 1500 K and pressures from 1 atm to 3 atm. The limitations observed are due to the small number of reactions and parameters of the kinetic model. However, mechanism reduction methodologies, such as DRG, may present much larger errors if used to achieve the same mechanism size of the virtual mechanisms presented in the current study.

Experimental and computational investigation of the influence of iso-butanol on autoignition of n-decane and n-heptane in non-premixed flows

Experimental and computational investigation is carried out to elucidate the influence of iso-butanol on critical conditions of autoignition of n-decane and n-heptane. The counterflow configuration is employed. In this configuration an axisymmetric stream of air is directed over the surface of an evaporating pool of a liquid fuel. A stagnation plane is established. The temperature of the air is increased until autoignition is achieved in the mixing layer in the vicinity of the stagnation plane. The temperature of the air stream at autoignition, Tig, is measured at various values of the strain rate, which is defined as the axial gradient of the axial component of the flow velocity at the stagnation plane. Fuels tested are n-decane, n-heptane, iso-butanol and various mixtures of n-decane/iso-butanol and n-heptane/iso-butanol. Kinetic modeling is carried out using the recently updated version of the comprehensive CRECK chemical-kinetic mechanism. Critical conditions of autoignition are predicted for all fuels and fuel mixtures and the results are compared with the measurements. Computations show that low-temperature chemistry plays a significant role in promoting autoignition of n-decane and n-heptane, and the influence of low-temperature chemistry decreases with increasing strain rates because there is insufficient residence for the low-temperature reactions to take place. Experimental data and numerical simulations show that addition of even small amounts of iso-butanol to n-decane or n-heptane increases the value of Tig at low strain rates indicating that iso-butanol strongly inhibits the low-temperature chemistry of n-decane and n-heptane. Furthermore, the simulations show that when iso-butanol is present in the liquid fuel, only a small amount of n-decane is available in the gas phase and its low concentration cannot effectively sustain the low temperature degenerate branching oxidation process. Predicted flame structures show that the peak values of mole fraction of ketohydroperoxide are significantly reduced when iso-butanol is added to n-decane indicating that the kinetic pathway to low temperature ignition is blocked. This observation is confirmed from sensitivity analysis.

Experimental and kinetic study on JP-10/air autoignition and the effect of NO2 at high temperatures

Ignition delay times of JP-10 in air and the vitiation effect of NO2 on JP-10 autoignition at high temperatures were investigated behind reflected shock waves. The experiments were carried out at pressures of 2 and 10 atm, temperatures between 900 and 1460 K, fuel lean and stoichiometric conditions with and without 1.0 % NO2. The experimental results show that the ignition delay times of JP-10/air decrease with the increase of pressure or the decrease of equivalence ratio. The effect of adding 1.0 % NO2 is pressure dependence. At 10 atm, the addition of NO2 promotes the ignition of fuel, which shortens the ignition delay time. At 2 atm, the addition of 1.0 % NO2 inhibits the ignition of JP-10, which prolongs the ignition delay time. A chemical kinetic mechanism of JP-10/NO2 consisting of 290 species and 1768 reactions was developed by extending from a C0-C5 alkane and a cyclopentene sub-mechanism. The proposed mechanism has been well validated by ignition delay times, laminar flame speeds and species concentration profiles from literature. To further investigate the autoignition of JP-10 and the vitiation effect of NO2, kinetics analyses including reaction pathways and sensitivity coefficients were performed by the developed mechanism. The main consumption pathways of JP-10 in air are H-abstraction reactions from JP-10 by HO2 radicals. With 1.0 % NO2 vitiation, the addition of NO2 has a strong effect on JP-10 consumption via the H-abstraction pathway JP-10 + NO2 = C10H15 + HONO, which further affects the autoignition of JP-10.

Numerical and optical soot characterization through 2-color pyrometry technique for an innovative diesel piston bowl design

The development of innovative diesel piston bowl designs has shown significant improvement of the near-wall flame evolution, resulting in lower fuel consumption and engine-out soot emissions. With this aim, a novel hybrid piston bowl for a 1.6 L light-duty diesel engine was designed, coupling a sharp-stepped bowl and radial-bumps in the inner bowl rim. The effects of the proposed hybrid bowl were analysed through both single-cylinder optical engine and 3D-CFD models, which feature a detailed chemical kinetic mechanism and the Particulate Mimic (PM) soot model. The 2-color pyrometry optical technique was adopted to obtain the optical soot density (KL) and the temperature of the soot surface. Then, a line-of-sight integration of the numerical soot distribution was adopted to obtain a planar KL distribution, which is directly comparable with the experimental KL images. The results showed a good agreement in terms of soot distribution between 3D-CFD and experiments, confirming the high prediction capabilities of the developed numerical methodology. The synergetic application of numerical and optical techniques highlighted that the hybrid bowl strongly mitigates the flame-to-flame interaction with respect to a conventional re-entrant bowl, leading to lower soot formation in the flame collision area. Moreover, faster flame propagation toward the cylinder axis is highlighted with a consequent higher soot oxidation rate in the late combustion phase.

Performance optimization of a diesel engine fueled with hydrogen/biodiesel with water addition based on the response surface methodology

The purpose of this research was to achieve the best performance, combustion and emission characteristics of a marine engine fueled with hydrogen (5%, 10% and 15% energy fraction), water (2, 4 and 6 wt%) and rapeseed methyl ester (RME) blend fuel by the multi-objective optimization. Based on the experimental results of the engine at 50%, 75% and 100% loads, the computational fluid dynamics (CFD) model was developed, and an improved chemical kinetic mechanism was developed to simulate the fuel combustion process. The response surface methodology (RSM) was used to optimize the combustion and emission characteristics of the marine diesel engine. The result showed that the hydrogen and water can improve the combustion and emission characteristics of the marine engine. The best solution was to select the desirable value of 0.632, and the engine achieves the best state by fueling R + 15H + 2.5W at 74.69% load. At the point, the optimum values of brake specific fuel consumption (BSFC), brake thermal efficiency (BTE), nitrogen oxide (NOx), hydrocarbon (HC) and carbon monoxide (CO) emissions were 208.31 g/kW·h, 39.22%, 941.21 ppm, 325.86 ppm and 1073.4 ppm, respectively. For this study, adding appropriate hydrogen and water to biodiesel can achieve efficient, low carbon and clean combustion in marine engine.

Numerical study on effects of hydrothermal aging to composite regeneration in CeO2-based catalyzed diesel particulate filter

The effects of hydrothermal aging on the performance of CeO2-based catalyzed diesel particulate filter (CDPF) was numerically investigated in this study based on a zero-dimensional model using the plug flow reactor in which a chemical reaction kinetic mechanism was established and validated by the simulated gas environment experiment. The effects of regeneration temperature, O2 concentration in the ultimate emission conditions, the ratio of NO2 in NOx (α) and the ratio of NOx to soot (β) on catalyst deactivation temperature and soot oxidation rate were investigated with fresh and hydrothermal aging CeO2-based CDPF. The results show that hydrothermal aging of CeO2-based catalysts raises the regeneration temperature from 613 to 783 K and shifts the soot catalytic combustion reaction path from complete to incomplete oxidation. Soot oxidation rate of fresh catalyst first increases rapidly at 516 K and then starts to slow down gradually at 633 K, but for hydrothermal aging catalysts, are 601 K and 789 K, respectively. With O2 concentration increased from 1.5 mol/m3 to 5.5 mol/m3, the catalyst deactivation of fresh and hydrothermal aging catalyst increased from 609 K to 602 K, 791 K to 818 K, respectively. The increase in α and β leads to an increase in soot oxidation rate and a decrease in regeneration temperature. The deactivation temperature of catalyst is increased in higher α (1.0) and lower β (0.1), which the highest is 821 K. Synergistic mechanisms of NOx, regeneration temperature, and hydrothermal aging effects on soot catalytic combustion in CeO2-based CDPF are revealed deeply with the help of zero-dimensional model.

An extended FGM model with transported PDF for LES of spray combustion

An enhanced flamelet generated manifold (FGM) model for large eddy simulation (LES) of turbulent spray combustion is presented. In the enhanced FGM model, a transported probability density function (TPDF) description of the FGM variables is employed. The TPDF is represented using the Eulerian stochastic fields (ESF) approach, and the method is applied to LES of spray combustion under conditions relevant to internal combustion engines. The new ESF/FGM method achieves an improved accuracy of predictions due to the ESF modelling of the subgrid-scale turbulence-chemistry interaction. It also achieves high computational efficiency due to the FGM tabulation of the chemical kinetic mechanism. The performance of the new ESF/FGM model is assessed by simulation of the Spray-A flames from Engine Combustion Network (ECN) and comparison of the results, firstly, with experimental measurements, and secondly, with conventional FGM model simulation results. It is shown that the ESF/FGM method is capable of predicting both global and local combustion characteristics, i.e., pressure rise, ignition delay time, flame lift-off length and the thermo-chemical structure of the spray flames with improved accuracy compared to the conventional FGM model that is based on the presumed PDF description of FGM variables. The sensitivity of the predictions using ESF/FGM to the number of stochastic fields is examined by varying the number of these fields in the range of 4–128. Furthermore, the influence of different FGM reaction progress variables on the simulations is investigated, and a new reaction progress variable based on the local consumption of oxygen is proposed. The results show that the new progress variable improves predictions of spray combustion, including the prediction of the start of injection, the quasi-steady state liftoff length, the post-injection oxidation, and the pressure evolution.

Chemical kinetics investigation of toluene combustion in a shock tube using spectroscopic CO and H2O laser absorption

Toluene oxidation was investigated behind reflected shock waves using spectroscopic laser diagnostics to simultaneously measure carbon monoxide (CO) and water (H2O) time-history profiles. CO and H2O time histories are unique in the available database for toluene speciation as only a total of five H-atom profiles can be found in the literature and were carried out for pyrolysis instead. These new, high fidelity measurements can help in validating detailed chemical kinetics mechanisms over highly dilute conditions. The experiments cover three equivalence ratios (ɸ = 0.5, 1.0, and 2.0) with temperatures ranging from 1433 to 1921 K near atmospheric pressure. Comparisons with six models were conducted for a performance review: the Yuan et al. model shows the most accurate results and was selected to further understand toluene chemistry and identify the reaction pathways. Rate-of-production and sensitivity analyses support that toluene (A1CH3 in the model) decomposes via A1CH3 (+M) ⇄A1CH2 + H (+M) and mostly produces CO and H2O with the following sequences: A1CH3→ A1CH2→ A1CH2O → CH2O → HCO → CO and A1CH3→ A1CH2→ CH3→ H2O, respectively.

Chemical structure of premixed ammonia/hydrogen flames at elevated pressures

Because it is a carbon free fuel with high volumetric and gravimetric hydrogen density, ammonia is considered to be a promising hydrogen carrier molecule; its combustion chemistry, consisting of ammonia and ammonia/hydrogen blends, are of great importance in engine and gas turbine systems. This paper presents experimental data and kinetic modeling of the structure of NH3/H2/O2/Ar premixed flames at elevated pressures. Equivalence ratios were maintained at 0.8, 1.0 and 1.2, and the NH3/H2 ratio was 1:1 (molar ratio). Experiments were performed at pressures of 4 and 6 atm. Eight recently published chemical-kinetic mechanisms of ammonia combustion and oxidation were applied to numerically simulate flame structure. Experimental and numerical data showed that the main nitrogen containing compounds in the post flame zone were N2 and NO, while the concentrations of N2O and NO2 were negligible. In terms of NO emissions reduction, it was revealed that rich conditions were more effective. At the same time, pressure increases resulted in decreasing NO concentration in the post flame zone, as well as lower maximum concentration of NO and N2O. Numerical analysis showed that N2O and NO2 were formed mainly from NO. To improve the agreement between experimental and numerical data, rate kinetic parameters of these reactions should be refined.

Shock-tube study on the influence of oxygenated co-reactants on ethylene decomposition under pyrolytic conditions

The influence of various oxygenated co-reactants on soot formation was addressed by studying the pyrolysis of gas mixtures using ethylene as a base fuel in the absence of molecular oxygen. Alcohols (methanol, ethanol, and n-butanol) and ethers (diethyl ether, dimethoxymethane, furan, and tetrahydrofuran) were selected as oxygenated co-reactants. The pyrolysis process was studied behind reflected shock waves at around 3 bar. Laser extinction at 633 nm was used to determine soot-inception times and the related optical densities as a function of reaction time. The temporal variation in temperature was measured via time-resolved two-color CO absorption using two quantum-cascade lasers at 4.73 and 4.56 µm. Soot particle sizes were determined by time-resolved laser-induced incandescence using a Nd:YAG laser at a wavelength of 1064 nm. The major finding is that based on C2H4 as soot precursor, only CH3OH does not have a soot-promoting effect, whereas the addition of all other oxygenated hydrocarbons results in increased soot yields. The measured temperatures were compared with simulations based on a detailed chemical kinetics mechanism from the CRECK Modeling Group (Pejpichestakul et al. Proc. Combust. Inst. (2019)). In addition, kinetics modeling of hydrocarbon pyrolysis, PAH formation and soot growth was performed in OpenSMOKE++ software to give qualitative insight in the experimental results using the CRECK mechanism and an assembled mechanism composed of the CRECK and the recent PAH sub-mechanism (Sun et al. Proc. Combust. Inst. (2021)). The simulation reveals that the observed promotion of soot formation in presence of oxygenated co-reactants is associated with the release of CH3 and C3H3 during the thermal decomposition resulting in acceleration of C6H6 ring formation.

RMG Database for Chemical Property Prediction.

The Reaction Mechanism Generator (RMG) database for chemical property prediction is presented. The RMG database consists of curated datasets and estimators for accurately predicting the parameters necessary for constructing a wide variety of chemical kinetic mechanisms. These datasets and estimators are mostly published and enable prediction of thermodynamics, kinetics, solvation effects, and transport properties. For thermochemistry prediction, the RMG database contains 45 libraries of thermochemical parameters with a combination of 4564 entries and a group additivity scheme with 9 types of corrections including radical, polycyclic, and surface absorption corrections with 1580 total curated groups and parameters for a graph convolutional neural network trained using transfer learning from a set of &gt;130 000 DFT calculations to 10000 high-quality values. Correction schemes for solvent-solute effects, important for thermochemistry in the liquid phase, are available. They include tabulated values for 195 pure solvents and 152 common solutes and a group additivity scheme for predicting the properties of arbitrary solutes. For kinetics estimation, the database contains 92 libraries of kinetic parameters containing a combined 21 000 reactions and contains rate rule schemes for 87 reaction classes trained on 8655 curated training reactions. Additional libraries and estimators are available for transport properties. All of this information is easily accessible through the graphical user interface at https://rmg.mit.edu. Bulk or on-the-fly use can be facilitated by interfacing directly with the RMG Python package which can be installed from Anaconda. The RMG database provides kineticists with easy access to estimates of the many parameters they need to model and analyze kinetic systems. This helps to speed up and facilitate kinetic analysis by enabling easy hypothesis testing on pathways, by providing parameters for model construction, and by providing checks on kinetic parameters from other sources.

A skeletal mechanism for gasoline surrogates: Development, validation, and CFD application

In this study, a skeletal chemical kinetic mechanism for gasoline surrogates is developed from a detailed mechanism by applying several reduction techniques. Directed relation graph (DRG) routines and DRG-aided sensitivity analysis methods are applied with worst-case error tolerances equal to 30% and 40%, respectively. Issues with reaction dead-ends in the oxidation paths are resolved by adding adequate intermediate species and reactions. Sensitivity analyses are conducted to identify the most impactful fuel-dependent reactions to ignition delay, and the specific reaction rates of such reactions are optimized to replicate the ignition behavior of detailed mechanism. The final version of the skeletal mechanism consists of 152 species and 563 reactions including NOx chemistry. Experiments are carried out in Sandia's low-temperature gasoline combustion research engine with an E10 regular-grade gasoline, termed RD5-87, and the results are replicated in 0D simulations with good accuracies using both detailed mechanism and the skeletal mechanism although a slight over-estimation of the low-temperature heat release is observed. Finally, the performance of the skeletal mechanism is also tested in 3D CFD simulation using LES. Simulations are able to predict the pressure evolution and heat release rate, proving that the proposed skeletal mechanism is adequate for simulations.

NUMERICAL STUDY OF WATER ADDITION ON THE COMBUSTION CHARACTERISTICS IN AN HCCI ETHANOL ENGINE

Low-temperature combustion modes have attracted global attention in recent years, especially the homogeneous charge compression ignition (HCCI) mode. The HCCI combustion produces high-speed combustion, achieving relatively high thermal efficiencies while emitting low NOX and soot. However, due to the lack of direct control to start the combustion, one of the main challenges in this type of engine is to ensure that auto-ignition occurs close to the top dead center for a wide operation range. There are a few combustion phase control techniques to solve this limitation, and one of them is water injection. In this regard, this paper explores the influence of water concentration in an ethanol HCCI engine as a form of combustion phasing control. The work was conducted numerically using a zero-dimensional single-zone model with a chemical kinetic mechanism containing 56 species and 383 reactions. The numerical model results were well agreed with the reported experimental data. The water fraction ranged from 0 to 18% by mass at 3% intervals. The results demonstrate that water injection is an efficient strategy to control the combustion phasing by adjusting the charge reactivity. The increase in water fraction reduced in-cylinder pressure and heat release rates. Furthermore, the combustion phase was advanced, the combustion duration was extended, and thermal efficiency was about 44.3% with the increase in water concentration.

Buoyancy effect on heat transfer and surface coking of hydrocarbon fuel in horizontal square channel at supercritical pressures

Heat and mass transfer of pyrolytic hydrocarbon fuels at supercritical pressure has important implications for regenerative cooling technology in advanced engines. Based on the detailed chemical kinetic mechanism of China No. 3 jet fuel (a.k.a. RP-3), the heat and mass transfer of RP-3 in a horizontal square cooling channel under asymmetric heating and buoyancy at supercritical pressure is investigated numerically. The results show that buoyancy induces secondary flow. The secondary flow enhances the heat and mass transfer between the near wall region and the core flow field, which can effectively improve the utilization level of fuel heat absorption capacity and suppress the carbon deposition on the surface of the cooling channel. In the case of bottom heating, the dramatic change in fuel density with fuel pyrolysis induces the strongest secondary flow, resulting in the highest fuel heat absorption capacity and the least carbon deposition. In addition, as the operating pressure increases, the buoyancy effect becomes stronger duo to fuel pyrolysis, which is significantly different from the buoyancy effect in the non-cracking zone. Under the buoyancy effect, the increased operating pressure enhances the secondary flow intensity, which reduces the heated wall temperature and suppresses the carbon deposition on the cooling channel surface. The results of this paper are expected to provide further insight into the design and optimization of regenerative cooling technologies.

Laminar burning velocities and nitric oxide formation in premixed dimethyl ether/air flames: Experiments and kinetic modeling

Adiabatic laminar burning velocities and post-flame NO mole fractions for flat, non-stretched, premixed dimethyl ether/air flames were experimentally determined with a heat flux burner combined with laser-induced fluorescence diagnostics, over equivalence ratios ranging from 0.7 to 1.6, at atmospheric pressure and initial temperatures from 298 to 338 K. The present burning velocity measurements were then compared with selected data available in the literature obtained with different techniques. The comparison showed reasonably good agreement with recent datasets obtained at different temperatures, as well as possible outliers not suitable for the validation of kinetic models. The detailed kinetic mechanism of the authors was extended by the reactions of dimethyl ether. A comparison of experimental and computational results using two contemporary detailed chemical kinetic mechanisms, along with the one from the authors presented in this work, was also conducted and discussed. Discrepancies between experiments and model predictions and among models themselves were observed, especially under rich conditions. Further numerical analyzes were performed to identify the main causes of the observed differences. Notwithstanding, the present model showed overall good performances in reproducing both laminar burning velocities and nitric oxide mole fractions. Kinetic modeling was also performed to enhance the understanding of the intrinsic NOx emission characteristics of dimethyl ether by comparing the present measurements with those obtained for ethanol/air flames under identical conditions.

Using Ammonia as Future Energy: Modelling of Reaction Mechanism for Ammonia/Hydrogen Blends

To utilize ammonia-based fuels, it is fundamental to understand chemical mechanisms of combustion process, in which reaction characteristics of such a chemical are described in detail. Detailed chemical-kinetics mechanism of ammonia was modelled by an automatic reaction mechanism generation program to investigate characteristics of premixed combustion for ammonia/hydrogen fuel mixture. To develop an accurate model for practical combustion applications, validation of the reaction mechanism was carried out in terms of laminar flame speed under different conditions. Results suggested that the established mechanism model has satisfying performance under different ammonia/hydrogen ratio conditions. Moreover, comparison with other mechanism models demonstrated that the developed model can be used to describe flame propagation of ammonia/hydrogen fuels. Then characteristics of laminar flame speed were predicted under various ammonia concentration and equivalence ratio conditions. Sensitivity analyses showed that ammonia mole fraction has a prominent impact on kinetics of flame speed for ammonia/hydrogen blends. Flame structure analyses showed that hydrogen can enhance ammonia flames with higher light radical concentrations whilst deteriorate NOx emission in exhaust gases.