Detonation cellular structure in NO2/N2O4-fuel gaseous mixtures

Experimental and numerical studies of the detonation in NO₂-N₂O₄/fuel (H₂, CH₄, and C₂H₆) gaseous mixtures show that for equivalence ratio Φ>0.8-1, (1) the detonation has a double cellular structure, the ratio between the cell size of each net being at least one order of magnitude; (2) inside the detonation reaction zone the chemical energy is released in two successive exothermic steps. Their chemical induction lengths, defined between the leading shock front and each local maximum heat release rate associated with each step, differ by at least one order of magnitude. The chemical reaction NO₂ + H → NO + OH is mainly responsible for the first exothermic step (fast kinetics), NO being the oxidizer on the second one (slow kinetics). Existence of correlations between calculated induction lengths and corresponding cell sizes strengthen the assumption that the cellular structure originates from local strong gradients of chemical heat release inside the detonation reaction zone.     

Modeling of NOx formation in diesel engines using finite-rate chemical kinetics

A fast, physics-based model to predict the temporal evolution of NO_x in diesel engines is investigated using finite-rate chemical kinetics. The temporal variation of temperature required for the computation of the reaction rate constants is obtained from the solution of the energy equation. NO_x formation is modeled by using a six step mechanism with eight species instead of the traditional equilibrium calculations based on the Zeldovich mechanism. Fuel combustion chemistry is modeled by a single-step global reaction. Effects of various stages of combustion on NO_x formation is included using a phenomenological burning rate model. The effects of composition and temperature on the thermophysical properties of the working fluid are included in the computations. Comparison with measured single-cylinder engine-out NO shows good agreement with experimental data. The validated model is then used to demonstrate the impact of various operating parameters such as injection timing and EGR on engine-out NO_x. This fast, robust model has potential applications in model-based real-time control strategies seeking to reduce feed gas NO_x emissions from diesel engines.     

Kinetics of iron ore pellets reduced by H2N2 under non-isothermal condition

Direct reduction experiments under non-isothermal conditions are induced to simulate reaction in the actual hydrogen shaft furnace. Morphology of metalized pellets is analyzed through optical microscope. Weight loss during reduction is recorded and the model of un-reacted core is adopted for dynamic analysis in sections. Compressive strengths of products are also detected. Results show that reduction rates under heating conditions are lower compared with the isothermal situation. The un-reacted core still exists in products. It is found through kinetics analysis that the reaction is firstly mix controlled by the interfacial chemical reaction and internal diffusion, and then controlled dominantly by the interfacial chemical reaction as the temperature and efficient reduction gas content increase gradually with reaction going on. The compressive strengths under heating condition are also lower than the value obtained at constant temperature of 900 °C.This phenomenon may be caused by the crystalline transformation and volume expansion during the dominating Fe₂O₃ to Fe₃O₄ reduction at lower temperature. This study can provide scientific guide for rational utilization of hydrogen energy in iron making.     

Modeling hydrolysis kinetics of dual phase α-Mg/LPSO alloys

This work proposes a new modeling of hydrolysis reaction in simulated seawater solution (35 g/L NaCl) with alloys containing α-Mg and Long Period Stacking Ordered (LPSO) phases. Alloys with different chemical compositions or thermal treatments were synthesized, leading to different (i) microstructures, (ii) α-Mg over LPSO phase fractions or (iii) LPSO type (18R or 14H). Classical nucleation and growth equation is used as an indicator of the reaction mechanisms but a new model is proposed to describe the complex kinetics curves (Vol (H₂) = f(t)) obtained. This new model has several advantages: (i) it allows to discriminate the contribution of each phase, (ii) it could be applied to any alloy with phases reacting and (iii) it applies to the whole kinetics curves (i.e. from 0 to 100% yield). It is supported by the comparisons between the different alloys and SEM observations of their microstructure before and after exposure to the seawater solution.     

Kinetic models of concentrated NaBH4 hydrolysis

The hydrolysis of NaBH₄ in liquid solution has been extensively studied in the past few years; however, data on the kinetics of self-hydrolysis in concentrated solution are few. This work reports the kinetic modeling of self-hydrolysis of 10–20 wt.% NaBH₄ at 25–80 °C. Also, pH data were obtained independently of the reaction kinetics data. The data obtained from Boron-11 NMR measurements and pH are used to determine kinetic parameters. An empirical power law model is evaluated over a wide pH range. The effects of temperature, pH and initial sodium borohyride concentration are reported. The power law model reproduced the trends of the kinetics of the hydrolysis reaction. In addition, a pseudo first order model derived from a proposed reaction mechanism is evaluated. The behavior of the pseudo first order rate constant k is interpreted in terms of the effect of pH.     

Two-step water splitting thermochemical cycle based on iron oxide redox pair for solar hydrogen production

This study deals with solar hydrogen production from the two-step iron oxide thermochemical cycle (Fe₃O₄/FeO). This cycle involves the endothermic solar-driven reduction of the metal oxide (magnetite) at high temperature followed by the exothermic steam hydrolysis of the reduced metal oxide (wustite) for hydrogen generation. Thermodynamic and experimental investigations have been performed to quantify the performances of this cycle for hydrogen production. High-temperature decomposition reaction (metal oxide reduction) was performed in a solar reactor set at the focus of a laboratory-scale solar furnace. The operating conditions for obtaining the complete reduction of magnetite into wustite were defined. An inert atmosphere is required to prevent re-oxidation of Fe(II) oxide during quenching. The water-splitting reaction with iron(II) oxide producing hydrogen was studied to determine the chemical kinetics, and the influence of temperature and particles size on the chemical conversion. A conversion of 83% was obtained for the hydrolysis reaction of non-stoichiometric solar wustite Fe_(1−y)O at 575 °C.     

Studies on hydriding kinetics of some La-based metal hydride alloys

In this paper, the hydriding kinetics of LaNi₅, LaNi_4.7Al_0.3 and LmNi_4.91Sn_0.15 is presented. Experiments were carried out by maintaining the pressure ratio (supply pressure to equilibrium pressure at the mid-point of the pressure–concentration–isotherm) equal to 2 and by maintaining nearly isothermal reaction conditions. Two widely used reaction kinetics models, namely Johnson–Mehl–Avrami (JMA) model and Jander diffusion model (JDM) are considered for the analysis. Two JMA models are considered; in the first model, the order of the reaction is assumed as unit and in the second model, the rate constant is calculated by estimating the order by fitting the reaction kinetics data with a reaction kinetics equation. The activation energy and pre-exponential constants of the above-mentioned alloys are estimated by constructing the Arrhenius plot. Activation energies estimated from the different models are compared and the accurate values of activation energy for the different alloys are determined by comparing the reaction kinetics data obtained from the models with the experimental data. The rate-controlling step of the hydriding reaction is obtained for all the alloys investigated.     

Isolated n-heptane droplet combustion in microgravity: “Cool Flames” – Two-stage combustion

Recent experimentally observed two stage combustion of n-heptane droplets in microgravity is numerically studied. The simulations are conducted with detailed chemistry and transport in order to obtain insight into the features controlling the low temperature second stage burn. Predictions show that the second stage combustion occurs as a result of chemical kinetics associated with classical premixed “Cool Flame” phenomena. In contrast to the kinetic interactions responsible for premixed cool flame properties, those important to cool flame droplet burning are characteristically associated with the temperature range between the turnover temperature and the hot ignition. Initiation of and continuing second stage combustion involves a dynamic balance of heat generation from diffusively controlled chemical reaction and heat loss from radiation and diffusion. Within the noted temperature range, increasing reaction temperature leads to decreased chemical reaction rate and vice versa. As a result, changes of heat loss rate are dynamically balanced by heat release from chemical reaction rate as the droplet continues to burn and regress in size. At reaction temperatures below the turnover, heat loss over takes the heat release rate and extinction occurs. Should heat release exceed heat loss as the temperature increases to that for hot ignition, initiation of a high temperature burning phase may be possible. Parametric study on factors leading to initiation of the second stage burning phenomena are studied. Results show that both carbon dioxide and helium diluents can promote initiation of low temperature burning at smaller initial drop diameters than found with nitrogen as diluent. Small amounts of carbon dioxide and helium in the ambient is sufficient to activate the phenomena. The chemical kinetics dictating the second stage combustion and extinction process is also discussed.     

Role of porosity on the equilibration kinetics in electrical conductivity relaxation experiments

The role of porosity on the equilibration kinetics in electrical conductivity relaxation (ECR) experiments is highlighted. Both porous and dense conductivity bars are used to determine the chemical oxygen surface exchange coefficient (k_chem) of neodymium nickelate Nd₂NiO_4+δ (NNO) from 600 to 800 °C. Using porous bars allows for quicker ECR experiments during which the conductivity transient is rate limited only by oxygen surface exchange which enables the use of a simple transient model. Additionally, porous bars have similar microstructures to porous electrodes which means they are critical for determining the oxygen exchange kinetics of realistic electrodes. ECR results on dense bars with porous coatings are also presented. The conductivity transients of dense and porous bars both show similar trends with oxygen partial pressure. Additionally, both porous and dense bars show a difference between oxidation and reduction transients with oxidation transients being faster than reduction transients.     

Conjugate mass transfer to a spherical drop accompanied by a second-order chemical reaction inside the drop

Conjugate mass transfer between a drop and a surrounding fluid flow with second-order (inclusive the particular case - pseudo-first-order), irreversible chemical reaction in the dispersed phase has been analyzed. The dispersed phase reactant is insoluble in the continuous phase and its complete depletion is allowed. Two sphere models were considered: the rigid sphere and the fluid sphere with internal circulation. For each sphere model two hydrodynamic regimes were employed: creeping flow and moderate Re numbers. Slow and fast chemical reactions were analyzed. A single, constant value was considered for Pe, Pe=100. The influence of the diffusivity ratio on the particle average concentrations, total mass transferred and enhancement factor is studied. The values obtained for the enhancement factor of the pseudo-first-order chemical reaction are compared with solutions provided by published predictive equations. The chemical reaction enhances the mass transfer rate even for values of the modified Hatta modulus smaller or considerably smaller than one. For the flow patterns and sphere models considered in this work, the enhancement factor is independent on hydrodynamics.     

Modeling of autoignition and NO sensitization for the oxidation of IC engine surrogate fuels

This paper presents an approach for modeling with one single kinetic mechanism the chemistry of the autoignition and combustion processes inside an internal combustion engine, as well as the chemical kinetics governing the postoxidation of unburned hydrocarbons in engine exhaust gases. Therefore a new kinetic model was developed, valid over a wide range of temperatures including the negative temperature coefficient regime. The model simulates the autoignition and the oxidation of engine surrogate fuels composed of n-heptane, iso-octane, and toluene, which are sensitized by the presence of nitric oxides. The new model was obtained from previously published mechanisms for the oxidation of alkanes and toluene where the coupling reactions describing interactions between hydrocarbons and NO_x were added. The mechanism was validated against a wide range of experimental data obtained in jet-stirred reactors, rapid compression machines, shock tubes, and homogeneous charge compression ignition engines. Flow rate and sensitivity analysis were performed in order to explain the low temperature chemical kinetics, especially the impact of NO_x on hydrocarbon oxidation.     

A simulation of the combustion of hydrogen in HCCI engines using a 3D model with detailed chemical kinetics

The influence of changes in the swirl velocity of the intake mixture on the combustion processes within a homogeneous charge compression ignition (HCCI) engine fueled with hydrogen were investigated analytically. A turbulent transient 3D predictive computational model which was developed and applied to the HCCI engine combustion system, incorporated detailed chemical kinetics for the oxidation of hydrogen. The effects of changes in the initial intake swirl, temperature and pressure, engine speed and compression and equivalence ratios on the combustion characteristics of a hydrogen fuelled HCCI engine were also examined. It is shown that an increase in the initial flow swirl ratio or speed lengthens the delay period for autoignition and extends the combustion period while reducing NO_x emissions. There are optimum values of the initial swirl ratio and engine speed for a certain mixture intake temperature, pressure, compression and equivalence ratios operational conditions that can achieve high thermal efficiencies and low NO_x emissions while reducing the tendency to knock     

Optimization of TiH2 content for fast and efficient hydrogen cycling of MgH2-TiH2 nanocomposites

Magnesium hydride is extensively examined as a hydrogen store due to its high hydrogen content and low cost. However, high thermodynamic stability and sluggish kinetics hinder its practical application. To overcome this last drawback, different Ti amounts (y = 0, 0.025, 0.05, 0.1, 0.2 and 0.3) were added to magnesium to form (1-y)MgH₂+yTiH₂ nanocomposites (NC) by reactive ball milling under hydrogen gas. Thermodynamic stability of the MgH₂ phase in NCs was determined using a manometric Sieverts rig. Reversible hydrogen capacity and reaction kinetics were determined at 573 K over 20 sorption cycles under a limited reaction time of 15 min. On increasing Ti amount, reaction kinetics are enhanced both in absorption and desorption leading to a higher reversibility for hydrogen storage with the MgH₂ phase. However, titanium increases the molar weight of NCs and forms irreversible titanium hydride. The highest reversible capacity (4.9 wt% H) was obtained for the lowest here studied TiH₂ content (y = 0.025).     

Steam-iron process kinetic model using integral data regression

A kinetic model describing the gas–solid non-catalytic reaction between iron oxides and hydrogen/methane gas mixtures has been proposed. This steam-iron process constitutes an interesting alternative in order to produce hydrogen without CO₂ generation, purifying streams of thermocatalytically decomposed natural gas. The study departed from a kinetic model obtained from differential regression of data acquired by thermogravimetry. This differential model (Avrami type) did not take into account some effects regarding the chemical equilibrium between reactants and products, neither those provided by the solid bed. To cope with this problem, some parameters were introduced in the kinetic model and experiments were performed in order to test the validity of the changes. These consisted of reduction steps with hydrogen and oxidations with steam along five alternated cycles in a fixed bed reactor. The refurbished reactor model (including kinetic model) consisted of a mono-dimensional fixed bed reactor working in non-stationary state. Initial parameter values were taken from the former kinetic model and later optimized with the aid of a Levenberg–Marquardt algorithm. The new model is able to predict with great accuracy the behaviour of the fixed bed reactor and represents an interesting tool for scale-up and process design.     

Assessment of integration of methane-reduced ceria chemical looping CO2/H2O splitting cycle to an oxy-fired power plant

In this paper, we investigated the effect of reaction kinetics and moving bed reactors for chemical looping (CO₂/H₂O) splitting unit (CL) that produces syngas and fed back to the power plant to gain the efficiency loss due to carbon capture. The reduction reactor (RED) produces methane is partially oxidized to make syngas and reducing the non-stoichiometric ceria which is transported to oxidation reactor (OXI) where the flue gases (CO₂ and H₂O) split to produce syngas. We developed the kinetics for methane reduced ceria and CO₂/H₂O splitting in a tubular reactor for an operating temperature range of (900–1100 °C) for different methane concentration which yielded to Avrami-Erofeev (AE3) model fits well for both redox reaction with different reaction constants. A moving bed reactors system is developed representing RED and OXI reactors of CL unit with kinetics hooked to the model in Aspen Plus with FORTRAN code. The effect of thermodynamics and the kinetics of redox reaction was investigated in the proposed integrated plant. The CL unit efficiency obtained is 42.8% for kinetic-based CL unit compares to 64% for thermodynamic based CL unit. However, the maximum available efficiency of the proposed layout lowered as 50.9% for kinetic-based CL unit plant compare to than 61.5% for thermodynamic based CL unit. However, the proposed plant shows an improvement in the energy efficiency penalty from 11.3% to 3.8% after CCS.     

A general model for air-side proton exchange membrane fuel cell contamination

This paper presents a general model for air-side feed stream contamination that has the capability of simulating both transient and steady-state performance of a PEM fuel cell in the presence of air-side feed stream impurities. The model is developed based on the oxygen reduction reaction mechanism, contaminant surface adsorption/desorption, and electrochemical reaction kinetics. The model is then applied to the study of air-side toluene contamination. Experimental data for toluene contamination at four current densities (0.2, 0.5, 0.75 and 1.0 A cm⁻²) and three contamination levels (1, 5 and 10 ppm) were used to validate the model. In addition, it is expected that, with parameter adjustment, this model can also be used to predict performance degradation caused by other air impurities such as nitrogen oxides (NO_x) and sulfur oxides (SO_x).     

An Exact-Steady-state Adaptive Chemistry method for combustion simulations: Combining the efficiency of reduced models and the accuracy of the full model

Many reduced-model methods have been developed to alleviate the computational expense of simulating chemically reacting flows with detailed kinetics. However, it is still impossible to determine exactly the loss in accuracy relative to the full model when reduced kinetic models are used for predicting quantities of interest (typically state variables). Ideally, one wishes to obtain the predictions of the full chemistry model at the fast speed of the simplified model(s). This paper describes a technique for achieving this goal for steady-state simulations. The new method, called Exact-Steady-state Adaptive Chemistry (ESAC), performs multiple fast reduced-model simulations of the steady-state problem, each time refining the accuracy of the solution by using increasingly accurate reduced models. Smaller (less accurate, but faster) reduced models are used when the simulation is far from (the full-model) steady-state; and more accurate (larger, slower) models are used as the simulation approaches the final steady-state solution. The simulation is completed by applying the trusted full kinetic model, guaranteeing the accuracy of the steady-state solution obtained using ESAC. We have developed a basic algorithm that applies this method and we present results from 2-D CFD simulations of steady-state methane and ethylene flames. ESAC simulations yielded the full-model solution (as guaranteed by the method) and were generally a factor of 3–4 times faster than the equivalent standard full-model-everywhere simulations. Future refinement of the basic implementations described here can further increase the speedup obtained when using ESAC. In applications where computational time rather than computer memory availability is the limiting factor, this technique enables efficient computation of the steady-state predicted by the full, detailed chemical kinetics model.     

Compaction of LiBH4-MgH2 doped with MWCNTs-TiO2 for reversible hydrogen storage

According to catalytic effects of TiO₂ on kinetic properties of hydrides and thermal conductivity of multiwall carbon nanotubes (MWCNTs) favoring heat transfer during de/rehydrogenation, improvement of dehydrogenation kinetics of compacted 2LiBH₄-MgH₂ by doping with MWCNTs decorated with TiO₂ (MWCNTs-TiO₂) is proposed. Via solution impregnation of Ti-isopropoxide on MWCNTs and hydrothermal reaction to produce TiO₂, high surface area and good dispersion of TiO₂ on MWCNTs surface are obtained. Composite of 2LiBH₄-MgH₂ is doped with 5–15 wt. % MWCNTs-TiO₂ and compacted into the pellet shape (diameter and thickness of 8 and 1.00–1.22 mm, respectively). By doping with 15 wt. % MWCNTs-TiO₂, not only fast dehydrogenation kinetics is obtained, but also reduction of onset dehydrogenation temperature (ΔT = 25 °C). Besides, gravimetric and volumetric hydrogen storage capacities of compacted 2LiBH₄-MgH₂ increase to 6.8 wt. % and 68 gH₂/L, respectively, by doping with 15 wt. % MWCNTs-TiO₂ (～twice as high as undoped sample). The more the MWCNTs-TiO₂ contents, the higher the apparent density (up to ～1.0 g/cm³ by doping with 15 wt. % MWCNTs-TiO₂). The latter implies good compaction, resulting in the development of volumetric hydrogen capacity. In the case of mechanical stability during cycling, compacted 2LiBH₄-MgH₂ doped with at least 10 wt. % MWCNTs-TiO₂ maintains the pellet shape after rehydrogenation. Although increase of porosity (up to 30%), leading to the reduction of thermal conductivity, is detected after rehydrogenation of compacted 2LiBH₄-MgH₂ doped with 15 wt. % MWCNTs-TiO₂, comparable kinetics during cycling is obtained. This benefit can be achieved from thermal conductivity of MWCNTs.     

Linear eddy mixing based tabulation and artificial neural networks for large eddy simulations of turbulent flames

A large eddy simulation (LES) sub-grid model is developed based on the artificial neural network (ANN) approach to calculate the species instantaneous reaction rates for multi-step, multi-species chemical kinetics mechanisms. The proposed methodology depends on training the ANNs off-line on a thermo-chemical database representative of the actual composition and turbulence (but not the actual geometrical problem) of interest, and later using them to replace the stiff ODE solver (direct integration (DI)) to calculate the reaction rates in the sub-grid. The thermo-chemical database is tabulated with respect to the thermodynamic state vector without any reduction in the number of state variables. The thermo-chemistry is evolved by stand-alone linear eddy mixing (LEM) model simulations under both premixed and non-premixed conditions, where the unsteady interaction of turbulence with chemical kinetics is included as a part of the training database. The proposed methodology is tested in LES and in stand-alone LEM studies of three distinct test cases with different reduced mechanisms and conditions. LES of premixed flame–turbulence–vortex interaction provides direct comparison of the proposed ANN method against DI and ANNs trained on thermo-chemical database created using another type of tabulation method. It is shown that the ANN trained on the LEM database can capture the correct flame physics with accuracy comparable to DI, which cannot be achieved by ANN trained on a laminar premix flame database. A priori evaluation of the ANN generality within and outside its training domain is carried out using stand-alone LEM simulations as well. Results in general are satisfactory, and it is shown that the ANN provides considerable amount of memory saving and speed-up with reasonable and reliable accuracy. The speed-up is strongly affected by the stiffness of the reduced mechanism used for the computations, whereas the memory saving is considerable regardless.     

Experimental and numerical low-temperature oxidation study of ethanol and dimethyl ether

Low-temperature combustion (LTC) receives increasing attention because of its potential to reduce NO_x and soot emissions. For the application of this strategy in practical systems such as internal combustion engines and gas turbines, the fundamental chemical reactions involved must be understood in detail. To this end, reliable experimental data are needed including quantitative speciation to assist further development of reaction mechanisms and their reduction for practical applications.The present study focuses on the investigation of low-temperature oxidation of ethanol and dimethyl ether (DME) under identical conditions in an atmospheric-pressure laminar flow reactor. The gas composition was analyzed by time-of-flight (TOF) mass spectrometry. This technique allows detection of all species simultaneously within the investigated temperature regime. Three different equivalence ratios of ϕ = 0.8, 1.0, and 1.2 were studied in a wide, highly-resolved temperature range from 400 to 1200 K, and quantitative species mole fraction profiles have been determined.The experiments were accompanied by numerical simulations. Their results clearly show the expected different low-temperature oxidation behavior of both fuels, with a distinct negative temperature coefficient (NTC) region only observable for DME. With detailed species information including intermediates, differences of the kinetics for both fuels are discussed. Small modifications of the mechanisms served to identify sensitivities in the model. The experimental results may assist in the improvement of kinetic schemes and their reduction.