Jet-stirred Reactor Conditions Research Articles

Exploring the Effect of Different Reactivity Promoters on the Oxidation of Ammonia in a Jet-Stirred Reactor

The low reactivity of ammonia (NH3) is the main barrier to applying neat NH3 as fuel in technical applications, such as internal combustion engines and gas turbines. Introducing combustion promoters as additives in NH3-based fuel can be a feasible solution. In this work, the oxidation of ammonia by adding different reactivity promoters, i.e., hydrogen (H2), methane (CH4), and methanol (CH3OH), was investigated in a jet-stirred reactor (JSR) at temperatures between 700 and 1200 K and at a pressure of 1 bar. The effect of ozone (O3) was also studied, starting from an extremely low temperature (450 K). Species mole fraction profiles as a function of the temperature were measured by molecular-beam mass spectrometry (MBMS). With the help of the promoters, NH3 consumption can be triggered at lower temperatures than in the neat NH3 case. CH3OH has the most prominent effect on enhancing the reactivity, followed by H2 and CH4. Furthermore, two-stage NH3 consumption was observed in NH3/CH3OH blends, whereas no such phenomenon was found by adding H2 or CH4. The mechanism constructed in this work can reasonably reproduce the promoting effect of the additives on NH3 oxidation. The cyanide chemistry is validated by the measurement of HCN and HNCO. The reaction CH2O + NH2 ⇄ HCO + NH3 is responsible for the underestimation of CH2O in NH3/CH4 fuel blends. The discrepancies observed in the modeling of NH3 fuel blends are mainly due to the deviations in the neat NH3 case. The total rate coefficient and the branching ratio of NH2 + HO2 are still controversial. The high branching fraction of the chain-propagating channel NH2 + HO2 ⇄ H2NO + OH improves the model performance under low-pressure JSR conditions for neat NH3 but overestimates the reactivity for NH3 fuel blends. Based on this mechanism, the reaction pathway and rate of production analyses were conducted. The HONO-related reaction routine was found to be activated uniquely by adding CH3OH, which enhances the reactivity most significantly. It was observed from the experiment that adding ozone to the oxidant can effectively initiate NH3 consumption at temperatures below 450 K but unexpectedly inhibit the NH3 consumption at temperatures higher than 900 K. The preliminary mechanism reveals that adding the elementary reactions between NH3-related species and O3 is effective for improving the model performance, but their rate coefficients have to be refined.

Low-temperature oxidation of a gasoline surrogate: Experimental investigation in JSR and RCM using high-resolution mass spectrometry

The oxidation of a gasoline model-fuel (2500 ppm of n-heptane and 2500 ppm of isooctane), called RON 50, was studied in a jet-stirred reactor (JSR) over the temperature range 560 ‒ 700 K, at a total pressure of 10 atm, at a residence time of 1.5 s, and an equivalence ratio of 0.5. Gas samples were collected as a function temperature. Ignition of RON 50/air mixtures was also studied in a rapid compression machine (RCM) under the same fuel-lean conditions, 20 bar, 640 K. Gas samples were collected at variable reaction time. Products of low-T oxidation formed in a jet-stirred reactor and a rapid compression machine were dissolved in acetonitrile and analyzed by high-resolution mass spectrometry. Flow injection analyses and ultrahigh-pressure liquid chromatography coupled to an OrbitrapⓇ were used to characterize a wide range of species such as hydroperoxides, diols, ketohydroperoxides, carboxylic acids, diketones, cyclic ethers (CnH2nO) formed by decomposition of alkyl hydroperoxy radicals, and highly oxidized species formed via up to six O2 additions on alkyl radicals (CnH2nO11). Mass spectrometry analyses were conducted using atmospheric pressure chemical ionization running in negative and positive ionization modes. For confirming the presence of –OH or –OOH groups in the products, we performed H/D exchange by addition of D2O to samples. Under JSR conditions, we observed a wide range of n-heptane oxidation products: C7H14Ox (x = 1‒11), C7H12Ox (x = 1‒11), C7H10Ox (x = 1‒9), C7H8Ox (x = 1‒9), C7H6Ox (x = 1‒8), and C7H4Ox (x = 1‒6). Similarly, the following products of isooctane oxidation were observed: C8H16Ox (x = 1‒12), C8H14Ox (x = 1‒11), C8H12Ox (x = 1‒12), C8H10Ox (x = 2‒10), C8H8Ox (x = 2‒8), C8H6Ox (x = 1‒7). Finally, CnH2n (n = 4‒8), CnH2n-2 (n = 4‒8), CnH2nO (n = 3‒8), CnH2n-2O (n = 3‒8), CnH2n-4O (n = 3‒8), CnH2n+2O2 (n = 3‒8), CnH2nO2 (n = 2‒8), CnH2n-2O2 (n = 3‒8), CnH2n-4O2 (n = 3‒8), and CnH2nO3 (n = 2‒8) were also observed. Most of these products were also detected in RCM samples. Products measurements indicated that RON 50 oxidation routes are similar under RCM and JSR experimental conditions. A kinetic reaction mechanism was used to compare the formation of products versus temperature in a JSR. New pathways need to be introduced in existing reaction schemes for predicting newly detected cool-flame products.

A numerical study on the quasi-steady spray and soot characteristics for soybean methyl ester and its blends with ethanol using CFD-reduced chemical kinetics approach

This work numerically examines the quasi-steady spray combustion and soot development for soybean methyl ester (SME) and SME-ethanol blends (S90E10, S70E30). A validated reduced ethanol mechanism (37 species and 149 reactions) was formulated using a temperature sensitivity analysis under auto-ignition and jet-stirred reactor conditions. At initial pressures between 10.0 bar and 60.0 bar, initial temperatures between 750 K and 1350 K and equivalence ratios between 0.5 and 2.0, the ignition delays and key species profiles (C2H5OH, CO2, O2) of the reduced mechanism deviated by 0.5 order and 40%, respectively as compared to the experimental measurements. Subsequently, the reduced ethanol mechanism was combined with a reduced biodiesel mechanism to form a surrogate mechanism (102 species and 446 reactions) for biodiesel-ethanol blends and integrated into OpenFOAM for spray combustion modelling. Under reacting spray conditions at a constant ambient density of 22.8 kg/m3 and ambient temperatures of 900 K and 1000 K, the spray penetrations for SME-ethanol blends were shortened by 35.5% (maximum difference). Smaller flame areas with 60 K higher local flame temperature were obtained. Due to a 20% decrease in acetylene mass fractions and soot formation rates, the peak soot volume fractions for SME-ethanol blends were 19.6% lower.

A Compact Naval Fuel Kinetic Model for Marine Diesel Engine Combustion

In this study, a compact multi-component kinetic model with 129 species is used as a surrogate model to represent the actual naval fuel, namely F-76. The surrogate fuel compositions are determined according to the actual fuel compositions of F-76. Subsequently, the model is comprehensively validated in zero-dimensional chemical kinetic simulations under a wide range of shock tube and jet-stirred reactor conditions. Good agreement between the reduced and detailed model is achieved for ignition delay and species profile predictions, with a maximum deviation maintained to within 40%. The surrogate model has demonstrated a satisfactory compromise between model accuracy and kinetic size.

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.

Development of multi-component diesel surrogate fuel models – Part II: Validation of the integrated mechanisms in 0-D kinetic and 2-D CFD spray combustion simulations

The aim of this study is to develop compact yet comprehensive multi-component diesel surrogate fuel models for computational fluid dynamics (CFD) spray combustion modelling studies. The fuel constituent reduced mechanisms including n-hexadecane (HXN), 2,2,4,4,6,8,8-heptamethylnonane (HMN), cyclohexane (CHX) and toluene developed in Part I are applied in this work. They are combined to produce two different versions of multi-component diesel surrogate models in the form of MCDS1 (HXN+HMN) and MCDS2 (HXN+HMN+toluene+CHX). The integrated mechanisms are then comprehensively validated in zero-dimensional chemical kinetic simulations under a wide range of shock tube and jet stirred reactor conditions. Subsequently, the fidelity of the surrogate models is further evaluated in two-dimensional CFD spray combustion simulations. Simulation results show that ignition delay (ID) prediction corresponds well to the change of fuel constituent mass fraction which is calculated to match the cetane number (CN). In addition, comparisons of the simulation results to the experimental data of #2 diesel fuel (D2) in a constant volume combustion chamber show that IDs and lift-off lengths are reasonably well replicated by the models. The MCDS2 model is also found to perform better in the soot formation prediction in D2 fuel combustion as the model contains aromatic and cyclo-alkane components which provide an additional pathway to the formation of rich species such as C2H2 and C6H6. Implementation of MCDS2 predicts an increase of maximum local soot volume fraction by a factor of 2.1 when the ambient temperature increases from 900K to 1000K, while the prediction by MCDS1 is lower at 1.6. This trend qualitatively agrees with the experimental observation. This work demonstrates that MCDS1 serves as a potential surrogate fuel model for diesel fuels with CN values ranging from 15 to 100. It also shows that MCDS2 is a more appropriate surrogate model for fuels with aromatics and cyclo-paraffinic contents, particularly when soot calculation is of main interest.

Development of multi-component diesel surrogate fuel models – Part I: Validation of reduced mechanisms of diesel fuel constituents in 0-D kinetic simulations

In the present work, development and validation of reduced chemical kinetic mechanisms for several different hydrocarbons are performed. These hydrocarbons are potential representative for practical diesel fuel constituents. n-Hexadecane (HXN), 2,2,4,4,6,8,8-heptamethylnonane (HMN), cyclohexane (CHX) and toluene are selected to represent straight-alkane, branched-alkane, cyclo-alkane and aromatic compounds in the diesel fuel. A five-stage chemical kinetic mechanism reduction scheme formulated in the previous work is applied to develop the reduced HMN and CHX models based on their respective detailed mechanisms. Alongside with the development of the reduced CHX model, a skeletal toluene sub-mechanism is constructed since the elementary reactions for toluene are subset of the detailed CHX mechanism. The final reduced HMN mechanism comprises 89 species with 319 elementary reactions, while the final reduced CHX mechanism which includes the toluene sub-mechanism consists of 80 species with 287 elementary reactions. Both reduced models are approximately 92% smaller than their respective detailed models in terms of total number of species and elementary reactions. Following that, both the newly developed fuel constituent reduced mechanisms, together with the formerly derived reduced HXN mechanism are comprehensively validated in zero-dimensional chemical kinetic simulations under a wide range of shock tube and jet-stirred reactor (JSR) conditions. Well agreement between the reduced and detailed mechanisms is achieved for ignition delay (ID) and species concentration predictions under both auto-ignition and JSR conditions, with a maximum relative error of 40%. In addition, the reduced models are further validated against the JSR experimental results for each diesel fuel constituents. The surrogate models are able to reasonably reproduce the experimental species concentration profiles in view of their simplified fuel chemistries. Deviations within one order of magnitude in the absolute values are recorded between the computations and measurements. Validation of these reduced models for each diesel fuel constituents in this work serves as a prerequisite for constructing a multi-component diesel surrogate fuel model. The compact yet accurate chemical models proposed here aid to reduce the chemistry size of the final multi-component diesel surrogate model such that it remains computationally efficient when it is incorporated with multi-dimensional CFD simulations. The reduced mechanism of each fuel constituent can also be used individually for other CFD applications.

Kinetic Modeling of Nitric Oxide Sensitization of n-heptane Auto-ignition and Combustion

This article develops a new kinetic model for better modeling auto-ignition and combustion of n-heptane, which are sensitized by the presence of nitric oxides. The mechanism has been validated for temperatures between 550 and 1100 K and pressure of 10 atm. Experimental data used for validation include jet stirred reactor conditions for species evolution as a function of temperature, as well as homogeneous charge compression ignition engine experiments for auto-ignition delay time measurements. Rate-of-production analysis and sensitivity coefficient analysis of nitrogen species are performed in order to explain the impact of NOx on n-heptane oxidation.

Detailed chemical kinetic reaction mechanisms for primary reference fuels for diesel cetane number and spark-ignition octane number

A detailed chemical kinetic reaction mechanism is developed for primary reference fuel mixtures of n-hexadecane and 2,2,4,4,6,8,8-heptamethyl nonane for diesel cetane ratings. The mechanisms are constructed using existing rules for reaction pathways and rate expressions developed previously for the primary reference fuels for gasoline octane ratings, n-heptane and iso-octane. These reaction mechanisms are validated by comparisons between computed and experimental results for shock tube ignition and for oxidation under jet-stirred reactor conditions. The combined kinetic reaction mechanism contains the submechanisms for the primary reference fuels for diesel cetane ratings and submechanisms for the primary reference fuels for gasoline octane ratings, all in one integrated large kinetic reaction mechanism. Representative applications of this mechanism to several test problems are presented, describing fuel/air autoignition variations with changes in fuel cetane and octane numbers, and others describing fuel combustion in a jet-stirred reactor environment with the fuel varying from pure 2,2,4,4,6,8,8-heptamethyl nonane (cetane number of 15) to pure n-hexadecane (cetane number of 100).

Chemical Kinetic Study of the Oxidation of Isocetane (2,2,4,4,6,8,8-Heptamethylnonane) in a Jet-stirred Reactor: Experimental and Modeling

The kinetics of oxidation of isocetane (2,2,4,4,6,8,8-heptamethylnonane), a cetane number reference fuel, was studied experimentally in a jet-stirred reactor (JSR) at 10 atm and a constant residence time of 1 s, over the temperature range 770−1070 K, and for variable equivalence ratios (0.5, 1, and 2). Concentration profiles of reactants, stable intermediates, and final products were obtained by probe sampling followed by online and offline gas chromatography analyses and online Fourier transformed infrared spectrometry. The oxidation of this fuel in these conditions was modeled using computer-generated semidetailed kinetic reaction mechanisms. A reasonable representation of the kinetics of oxidation of isocetane and the formation of the main products under the present JSR conditions could be obtained. However, the prediction of most of the intermediates was poor as a result of the use of too simplified lumped chemistry in these schemes. This indicates needs for additional model development that could ben...

Experimental and Modeling Study of the Kinetics of Oxidation of Methanol−Gasoline Surrogate Mixtures (M85 Surrogate) in a Jet-Stirred Reactor

A fused silica jet-stirred reactor (JSR) was used to study the kinetics of oxidation of M85 surrogate mixtures, i.e., methanol/surrogate gasoline (85/15 vol %). One representative of each chemical class constituting M85 was selected: iso-octane, toluene, 1-hexene, and methanol. The experiments were performed in the temperature range of 770−1140 K, at 10 atm, at four equivalence ratios covering fuel-lean to fuel-rich conditions (0.35, 0.6, 1, and 2) and with an initial fuel concentration of 0.4 mol %. Mole fraction profiles of reactants, stable intermediates, and final products were measured via sonic probe sampling followed by Fourier transform infrared spectrometry (FTIR) and gas chromatography (GC) analyses. A detailed chemical kinetic reaction mechanism resulting from the merging of validated kinetic schemes for the oxidation of the components of the present M85 surrogate (gasoline surrogate and methanol) was used. Good agreement between the experimental results and the computations was observed under the present JSR conditions.

Chemical Kinetic Study of the Effect of a Biofuel Additive on Jet-A1 Combustion

The kinetics of oxidation of kerosene Jet A-1 and a kerosene/rapeseed oil methyl ester (RME) mixture (80/20, mol/mol) (biokerosene) was studied experimentally in a jet-stirred reactor at 10 atm and constant residence time, over the temperature range 740-1200 K, and for variable equivalence ratios (0.5-1.5). Concentration profiles of the reactants, stable intermediates, and final products were obtained by probe sampling followed by on-line and off-line gas chromatography analyses. The oxidation of these fuels in these conditions was modeled using a detailed kinetic reaction mechanism consisting of 2027 reversible reactions and 263 species. The surrogate biokerosene model fuel used here consisted of a mixture of n-hexadecane, n-propylcyclohexane, n-propylbenzene, and n-decane, where the long-chain methyl ester fraction was simply represented by n-hexadecane. The proposed kinetic reaction mechanism used in the modeling yielded a good representation of the kinetics of oxidation of kerosene and biokerosene under jet-stirred reactor conditions and of kerosene in a premixed flame. The data and the model showed the biokerosene (Jet A-1/RME mixture) has a slightly higher reactivity than Jet A-1, whereas no major modification of the product distribution was observed besides the formation of small unsaturated methyl esters produced from RME's oxidation. The model predicts no difference in the ignition delays of kerosene and biokerosene. Using the proposed kinetic scheme, the formation of potential soot precursors was studied with particular attention.

The oxidation of a diesel fuel at 1–10 atm: Experimental study in a JSR and detailed chemical kinetic modeling

A good knowledge of the kinetics of the fuel combustion under high pressure is necessary for predicting the performances of modern diesel and HCCI engines and pollutants emissions.The kinetics of oxidation of a synthetic diesel fuel was studied in a jet-stirred reactor (JSR) at pressures extending from 1 to 10 atm, in the temperature range 800–1400 K, and for equivalence ratios in the range 0.5–2.0. The concentration profiles of molecular species, reactants, stable intermediates, and products, were measured by sonic probe sampling followed by on-line GC-MS-FID analyses, and off-line GC-TCD-FID and GC-MS analyses. The experiments were modeled using a detailed kinetic reaction mechanism consisting of 2755 reversible reactions and 377 species. The chemical surrogate model-fuel used consisted of a mixture of n-hexadecane (36.1% by weight, 23.5% vol.), n-propylcyclohexane (23.1% w, 26.9% vol.), n-propylbenzene (18.7% w, 22.9% vol.), iso-octane (14.7% w, 19% vol.), and 1-methylnaphthalene (7.4% w, 7.7% vol.). The proposed kinetic reaction mechanism used in the modeling yielded a good representation of the kinetics of oxidation of a synthetic diesel fuel under JSR conditions. Reaction path analyses were used to delineate the most important reaction paths during the oxidation of the fuel.

An experimental and kinetic modeling study of propyne and allene oxidation

New experimental data for propyne and allene oxidation were obtained in a jet-stirred reactor (JSR) in the temperature range 800–1200 K at 1–10 atm for different fuel/oxygen equivalent ratios (0.2–2.0). Experimental data clearly indicate the different oxidation behavior of the two isomers and provide valuable information for extending the simulation capabilities of an already existing comprehensive kinetic scheine. Critical reactions are presented and discussed together with extended comparisons of model predictions with experimental data obtained in the JSR conditions, in the Princeton turbulen flow reactor (PTFR). and in a shock tube at higher temperatures. Finally, the effects on reaction paths of allene addition to a fuel-rich acetylene premixed flame, are also discussed. Isomerization reactions proceeding via direct and H addition routes are significant in the oxidation. As a result of H-abstraction reactions, both propyne and allen form the resonantly stabilized propargyl radical. These species are important intermediates in all the combustion processes, and their successive reactions are relevant candidates in explaining the formation of aromatic and polyaromatic species, possible presursors of particulate and soot. The analysis of the mild combustion conditions of the JSR reactor also allows the reliability of the overall kinetic scheme to be extended to the so-called flameless conditions.