Chemical Kinetic Effects Research Articles

Numerical study and correlation development on laminar burning velocities of n-butanol, iso-octane and their blends: Focusing on diluent and blend ratio effects

Bio-n-butanol as a potential alternative bio-fuel is accepted more and more attention presently in the fields of both scientific research and commercial market; however, n-butanol is impossible to substitute conventional fuels completely in the short and medium term. Therefore, understanding the premixed flame’s characteristics of iso-octane/n-butanol mixture fuels is no doubt an important way to make better use of n-butanol as a fuel applied in conventional SI (spark-ignition) engines. On the other hand, the engines always operate with EGR (exhaust gas recirculation), so the premixed flame characteristics of fuels (pure fuels or iso-octane/n-butanol mixture fuels) are needed to be investigated under EGR operations. Driven by these facts, we performed a numerical study, which is validated by experimental data, on the laminar burning velocity of n-butanol, iso-octane and their blends. Based on this, correlations including the influences of diluent and fuel component blend ratio, respectively, are developed and their fitting parameters are resolved carefully through a wide range of dataset. From results, our correlations are sufficient in accuracy to describe the effects mentioned above. Within each effect, for deeper understanding the appearance influence on the laminar burning velocity, the deep mechanism of chemical kinetic effect is revealed and discussed. In particular, as diluent effect as concerned, each single diluent gas component, such as CO2, H2O, and N2, is analyzed separately, the combined diluent effect can then be solved by grouping contributions brought by each species. In addition, our correlations were compared to those in literature widely. Concerning the mixture fuels, a correction factor is introduced by present study, such that the accuracies of mixing rules are improved at fuel-rich equivalence ratios.

Numerical Study on the Effect of Real Syngas Compositions on Ignition Delay Times and Laminar Flame Speeds at Gas Turbine Conditions

Depending on the feedstock and the production method, the composition of syngas can include (in addition to H2 and CO) small hydrocarbons, diluents (CO2, water, and N2), and impurities (H2S, NH3, NOx, etc.). Despite this fact, most of the studies on syngas combustion do not include hydrocarbons or impurities and in some cases not even diluents in the fuel mixture composition. Hence, studies with realistic syngas composition are necessary to help in designing gas turbines. The aim of this work was to investigate numerically the effect of the variation in the syngas composition on some fundamental combustion properties of premixed systems such as laminar flame speed and ignition delay time at realistic engine operating conditions. Several pressures, temperatures, and equivalence ratios were investigated for the ignition delay times, namely 1, 10, and 35 atm, 900–1400 K, and ϕ = 0.5 and 1.0. For laminar flame speed, temperatures of 300 and 500 K were studied at pressures of 1 atm and 15 atm. Results showed that the addition of hydrocarbons generally reduces the reactivity of the mixture (longer ignition delay time, slower flame speed) due to chemical kinetic effects. The amplitude of this effect is, however, dependent on the nature and concentration of the hydrocarbon as well as the initial condition (pressure, temperature, and equivalence ratio).

Effects of buffer gas composition on autoignition

This work quantifies the chemical kinetic and thermal effects of buffer gas composition on autoignition of three fuels at conditions relevant to engines, combustors, and experimental facilities used to study ignition kinetics. Computational simulations of autoignition of iso-octane, n-heptane, and of n-butanol were used to characterize the effects of buffer gas composition on ignition delay time and heat release rate. Stoichiometric mixtures, ϕ=1.0, and a temperature range of 600–1100K were considered. Iso-octane and n-heptane were studied at initial pressures of 9.0atm and 60.0atm, and n-butanol was studied at initial pressures of 3.2atm and 60.0atm. Two dilution levels of buffer gas to O2 of 3.76:1 (mole basis) and 5.64:1 were considered (∼21% and ∼15% O2 respectively, mole basis). The fuels and simulation conditions were selected based on the relevance to engine operating conditions and previously published ignition studies. The buffer gases considered were argon, nitrogen, water, and carbon dioxide. Simulation results predicted changes of greater than a factor of 2 in ignition delay time and heat release rate as a function of buffer gas composition in the negative temperature coefficient (NTC) region for n-heptane and iso-octane. Outside the NTC region, the predicted effects of changes in buffer gas composition were small (<20%); however, experimental data for n-heptane indicate larger effects of buffer gas composition on ignition delay time at higher temperatures (>a factor of 2). The heat release rates were also sensitive to buffer gas composition, with carbon dioxide exhibiting relatively high levels of early and late heat release relative to the other buffer gases. Sensitivity analysis of the third-body collision efficiencies for the buffer gases showed the effects of uncertainties in the third body collision efficiencies on ignition delay time and heat release rate. The results highlight the significance of buffer gas composition on low-temperature combustion chemistry, particularly via H2O2 and HO2 decomposition and recombination reactions.

The effects of chemical kinetics and wall temperature on performance of porous media burners

This paper reports a two-dimensional numerical prediction of premixed methane-air combustion in inert porous media burner by using of four multi-step mechanisms: GRI-3.0 mechanism, GRI-2.11 mechanism and the skeletal and 17 Species mechanisms. The effects of these models on temperature, chemical species and pollutant emissions are studied. A two-dimensional axisymmetric model for premixed methane-air combustion in porous media burner has developed. The finite volume method has used to solve the governing equations of methane-air combustion in inert porous media burner. The results indicate that the present four models have the same accuracy in predicting temperature profiles and the difference between these profiles is not more than 2 %. In addition, the Gri-3.0 mechanism shows the best prediction of NO emission in comparison with experimental data. The 17 Species mechanism shows good agreement in prediction of temperature and pollutant emissions with GRI-3.0, GRI-2.11 and the skeletal mechanisms. Also the effects of wall temperature on the gas temperature and mass fraction of species such as NO and CH4 are studied.

A New Soot Emission Model for Diesel Methanol Dual Fuel Engine

The modified Hiroyasu soot model is widely used to describe the soot emission from diesel engine, but diesel methanol dual fuel engine is different from diesel one. In order to obtain quantitative comparisons with experiments, a new soot emission model was presented, which was formulated to incorporate the effects of both chemical kinetics and turbulent mixing. The model was applied to simulate soot emission from diesel methanol dual fuel engine. The calculating results indicate that the predicted soot emission from the new model agrees well with the measured data.

Simulation of biodiesel combustion in a light-duty diesel engine using integrated compact biodiesel–diesel reaction mechanism

This computational fluid dynamics (CFD) study is performed to investigate the combustion characteristics and emissions formation processes of biodiesel fuels in a light-duty diesel engine. A compact reaction mechanism with 80 species and 303 reactions is used to account for the effects of chemical kinetics. Here, the mechanism is capable of emulating biodiesel–diesel mixture of different blending levels and biodiesel produced from different feedstock. The integrated CFD-kinetic model was validated against a test matrix which covers the entire saturated–unsaturated methyl ester range typical of biodiesel fuels, as well as the biodiesel–diesel blending levels. The simulated cases were then validated for in-cylinder pressure profiles and peak pressure values/timings. Errors in the peak pressure values did not exceed 1%, while the variations in peak pressure timings were kept within 1.5 crank angle degree (CAD). For this reported work, in-cylinder formation mechanisms of nitrogen monoxide (NO) and soot from the combustion of biodiesel fuels such as coconut methyl ester (CME), palm methyl ester (PME) and soybean methyl ester (SME) were simulated and compared against that of the baseline diesel fuel. It is established here that an increase in the degree of unsaturation in biodiesel fuels can have detrimental effects on engine-out NO and soot concentrations. Biodiesel fuel with greater unsaturation level such as SME has the predisposition to produce greater thermal NO due to larger regions of high temperature achieved during combustion. Besides, the highly unsaturated SME has greater tendencies to produce acetylene which are precursors for prompt NO and soot, leading to higher engine-out emissions of these pollutants. In all, this study has demonstrated the feasibility of integrating a compact multi-component surrogate fuel mechanism with CFD to elucidate the in-cylinder combustion and emissions characteristics of biodiesel fuels. The knowledge acquired here is useful for the development of biodiesel usage strategies in light-duty diesel engines for mass adoption.

Experimental and numerical investigation on diluted DME flames: Thermal and chemical kinetic effects on laminar flame speeds

Experiments and computational simulations were used to study effects of diluents on laminar flame speeds of stoichiometric, laminar, premixed dimethyl ether (DME)/air flames. The experiments were conducted in a constant volume bomb under initial temperature of 298K and initial pressure of 0.1MPa. Outwardly propagating spherical flames were used to measure the laminar flame speeds of mixtures with varying concentration of Ar, N2 or CO2 addition. Laminar flame speeds were also computed using the steady, one-dimensional laminar premixed flame code PREMIX with detailed chemical kinetics. Predictions showed a good agreement with experiments. For all the three diluting agents invested in this work, laminar flame speed decreases with the increase of dilution ratio, while for a given dilution ratio, the significance of suppression effect on laminar flame speed is in the order of CO2, N2 and Ar. The suppression effects of N2 and Ar on flame propagation are mainly caused by reduced reactant concentration and modified heat capacity, both of which result in a decreased flame temperature, while besides that, CO2 further influences the chemical kinetics because it is a major product of combustion. Further experimental study on flame speed and simulation study on flame structure at fixed adiabatic flame temperature were conducted to evidence this speculation and the possible reaction pathways which contribute to the chemical retarding effect by CO2 addition were recognized through sensitivity analysis.

Some insights into the facilitated ion transfer voltammetric responses at ITIES exhibiting interfacial and bulk membrane kinetic effects

General analytical equations corresponding to the Facilitated Ion Transfer (FIT) at ITIES (Interface between Two Immiscible Electrolyte Solutions) are presented for the most frequent case in which the complexing agent is present only in the organic phase, and considering both the ion transfer and the chemical complexation kinetic effects. Under these conditions, the FIT process can be regarded as an EC mechanism. This study is of great interest to elucidate the origin of the kinetic effects which affect the electrochemical signal. Normal Pulse Voltammetry and Derivative Normal Pulse Voltammetry are chosen as representative and easy understandable voltammetric techniques. From the general equations, the expressions corresponding to some interesting particular situations in which one or the two kinetic processes (ion transfer and organic complexation) are in equilibrium are derived. Moreover, working curves of the characteristic peak parameters of the derivative voltammetric response are given, from which it is possible to determine the kinetic constants. The results obtained here are applicable to a wide range of liquid membrane systems, from traditional liquid-liquid interfaces to plasticized polymeric membranes and supported liquid-liquid interfaces.

Thermodynamic modeling based on a generalized cubic equation of state for kerosene/LOx rocket combustion

Based on a generalized cubic equation of state proposed by Cismondi and Mollerup [Fluid Phase Equilib. 232 (2005) 74–89], this study derived formulations for the thermodynamic properties of mixtures that are needed for combustion simulations of liquid rocket engines. The present model was validated thoroughly against reference data provided by NIST in order to assess its validity over a wide range of critical compressibility factors, pressures, and temperatures. The numerical results clearly indicate that the present model is superior in its handling of fluid mixtures with quite different critical compressibility factors, while maintaining the advantages of the cubic equation of state compared to those of the Soave–Redlich–Kwong and Peng–Robinson equations of state. In addition, steady flamelet analysis has been performed to investigate the effects of detailed chemical kinetics, real fluid behavior, and increased pressure on the local flame structures of the kerosene surrogate and liquid oxygen relevant to liquid rocket engines.

Numerical simulation of angular injection of hydrogen fuel in scramjet combustor

Angular injection of hydrogen fuel in a scramjet combustor is explored numerically. Three-dimensional Navier–Stokes equations with turbulence and combustion models are solved using commercial computational fluid dynamics software. Both infinitely fast kinetics and single-step finite rate H2–air kinetics are used to find out the effect of chemical kinetics in the thermochemical behaviour of the flow field. Grid independence of the results is demonstrated and gridconvergence index-based error estimate provided. k- ω turbulence model performs better, in comparison to k– ϵ and shear stress transport models, in predicting the surface pressure. Single-step finite rate chemistry (SSC) performs extremely well in predicting the flow features in the combustor. Computed temperature and species mole fraction and wall pressure distributions with SSC match better with the experimental results compared to fast chemistry calculation and detailed chemistry calculation of other workers. It has been observed that simple chemistry can describe H2–air reaction in scramjet combustor reasonably well.

Magnesium stable isotope fractionation in marine biogenic calcite and aragonite

This survey of magnesium stable isotope compositions in marine biogenic aragonite and calcite includes samples from corals, sclerosponges, benthic porcelaneous and planktonic perforate foraminifera, coccolith oozes, red algae, and an echinoid and brachiopod test. The analyses were carried out using MC-ICP-MS with an external repeatability of ±0.22‰ (2SD for δ 26Mg; n = 37), obtained from a coral reference sample (JCp-1). Magnesium isotope fractionation in calcitic corals and sclerosponges agrees with published data for calcitic speleothems with an average Δ 26Mg calcite–seawater = −2.6 ± 0.3‰ that appears to be weakly related to temperature. With one exception ( Vaceletia spp.), aragonitic corals and sclerosponges also display uniform Mg isotope fractionations relative to seawater with Δ 26Mg biogenic aragonite–seawater = −0.9 ± 0.2. Magnesium isotopes in high-Mg calcites from red algae, echinoids and perhaps some porcelaneous foraminifera as well as in all low-Mg calcites (perforate foraminifera, coccoliths and brachiopods) display significant biological influences. For planktonic foraminifera, the Mg isotope data is consistent with the fixation of Mg by organic material under equilibrium conditions, but appears to be inconsistent with Mg removal from vacuoles. Our preferred model, however, suggests that planktonic foraminifera synthesize biomolecules that increase the energetic barrier for Mg incorporation. In this model, the need to remove large quantities of Mg from vacuole solutions is avoided. For the high-Mg calcites from echinoids, the precipitation of amorphous calcium carbonate may be responsible for their weaker Mg isotope fractionation. Disregarding superimposed biological effects, it appears that cation light isotope enrichments in CaCO 3 principally result from a chemical kinetic isotope effect, related to the incorporation of cations at kink sites. In this model, the systematics of cation isotope fractionations in CaCO 3 relate to the activation energy required for cation incorporation, which probably reflects the dehydration of the cation and the crystal surface and bond formation at the incorporation site. This kinetic incorporation model predicts (i) no intrinsic dependence on growth rate, unless significant back reaction upon slow growth reduces the isotope fractionation towards that characteristic for equilibrium isotope partitioning (this may be observed for Ca isotopes in calcites), (ii) a small decrease of isotope fractionation with increasing temperature that may be amplified if higher temperatures promote back reaction and (iii) a sensitivity to changes in the activation barrier caused by additives such as anions or biomolecules or by the initial formation of amorphous CaCO 3.

CO2 Capture by Calcined Dolomite in a Fluidized Bed: Experimental Data and Numerical Simulations

The capture of CO2 by particles of calcined dolomite in a gas-fluidized bed was investigated experimentally in a laboratory-scale reactor. Step-response experiments were performed to determine CaO conversion rates in the bed as a function of time and dolomite particle diameter. It was found that the CO2 capture dynamics in the fluidized bed itself could be extracted from the overall system response curves by making use of a blank test—performed under identical flow and temperature profile conditions but with no calcined dolomite present. A simple flow-with-reaction model of the process was proposed; it provided a good description of the transient behaviour in terms of an empirical transfer coefficient, which was found to depend linearly on dolomite particle diameter. A shrinking core model was then developed and applied, to evaluate the individual effect of chemical kinetics and pore diffusion on the global reaction rate. The chemical process was found to control the smallest particle size carbonation test, in very good agreement with the kinetic constant value proposed in literature (Bhatia and Perlmutter, 1983), whereas the diffusion coefficient, needed to describe CO2 uptake as particle grows, appeared to be a function of sorbent size: this points to the need of more sophisticated SRD (simultaneous reaction and diffusion) models to describe the real behaviour of systems at hand.

Effect of Chemical Kinetics on Concurrent-Flow Flame Spread over Solids: A Comparison Between Buoyant Flow and Forced Flow Cases

The authors employed detailed numerical models with a one-step finite-rate chemical reaction to investigate the kinetic rate effect (through the variation of the pre-exponential factor) on concurrent flame spread rates over thin solids. It is found that flames in forced flow are less sensitive to the change of kinetics than flames in buoyant flow, and narrow samples are more sensitive to the change of kinetics compared with wide samples. The rate of chemical kinetics affects the flame spread rates primarily through two mechanisms: the amount of unburnt fuel vapors escaping from the reaction zone and the variation of induced flow velocity through flame temperature change in the case of the buoyant flames. Detailed analyses of the flame structure and flow pattern are presented for the forced- and buoyant-flow cases to explain their different degrees of sensitivity to the combustion reaction rate.

Steady-State Thermochemical Model of a Wood-Burning Cook-Stove

In naturally aspirated wood-burning stoves, the air-flow is driven by buoyancy forces which overcome the flow resistances inside the stove. This air flow, in turn, determines the wood-burning rate as well as the overall thermal and combustion efficiencies. The flow and associated heat/mass transfer and combustion phenomena are 3-dimensional and time dependent. In this paper, a simplified 4-zone well-stirred thermochemical model is developed by capturing steady-state effects of chemical reaction kinetics of char and volatile burning. The model predicts performance parameters such as thermal efficiency, composition of combustion products, and excess-air factors for an experimental stove. Effects of stove geometry, wood fuel characteristics, cooking vessel dimensions, and other ambient conditions are investigated.

Laminar premixed flame fuel consumption rate modulation by shocks and expansion waves

The main contributing effects leading to premixed laminar flame fuel consumption rate changes following variable strength shock or expansion wave passage were studied analytically and numerically. The effects were separated into two groups: gas compression, or one-dimensional effects, and flame front distortion, or two-dimensional effects. The first group was examined analytically using a one-dimensional flame model and solution of shock and expansion wave refraction. Two-dimensional effects were examined numerically using both shock and expansion wave passage through a sinusoidally perturbed propane-air laminar flame. Flame fuel consumption amplification due to the gas compression increased rapidly (from 1.3 to 24 times for Mach 1.1–1.7 shocks), while flame length amplification peaked (∼15 times) at a given shock strength (∼Mach 1.3). Thus gas compression became the dominant effect for stronger shocks. In contrast to the shocks, a slower reaction kinetic produced by the expansion wave passage allowed for a significantly longer period of flame length growth. This resulted in a temporary burning rate increase above initial values. The negative chemical kinetic effect of the weak expansion wave passage was temporarily offset by the flame length increase. The overall reaction rate amplification was up to four times stronger in fast/slow refractions. Also, the observed values of the flame stretch are reported in the paper.

Thermoreversible associating polymer networks. I. Interplay of thermodynamics, chemical kinetics, and polymer physics

Hybrid molecular dynamics/Monte Carlo simulations are used to study melts of unentangled, thermoreversibly associating supramolecular polymers. In this first of a series of papers, we describe and validate a model that is effective in separating the effects of thermodynamics and chemical kinetics on the dynamics and mechanics of these systems, and is extensible to arbitrarily nonequilibrium situations and nonlinear mechanical properties. We examine the model's quiescent (and heterogeneous) dynamics, nonequilibrium chemical dynamics, and mechanical properties. Many of our results may be understood in terms of the crossover from diffusion-limited to kinetically limited sticky bond recombination, which both influences and is influenced by polymer physics, i.e., the connectivity of the parent chains.

Interseismic pore compaction suppresses earthquake occurrence and causes faster apparent fault loading

[1] Physical and chemical processes operating on faults during interseismic periods are important for earthquake generation. In this study, we focus on pore compaction within fault zones driven by chemical kinetic effects. One established model of compaction causes an increase of pore fluid pressure almost linearly with time, if fluid diffusion is neglected. We introduced it into a simple numerical model for earthquake cycles and found that this effect can drastically change the recurrence of earthquakes: (1) It gradually stabilizes the fault and eventually suppresses earthquake occurrence (2) It surprisingly causes faster “apparent” fault loading. Furthermore, we developed an expression for the “apparent” loading velocity and checked that it is consistent with the results of numerical calculations. We also extended the expression for the “apparent” loading velocity to include the effect of pore dilatancy. This “apparent” loading would not only appear in a simple elastic system with a single degree of freedom, but also in complicated systems that involve simulations of realistic earthquake cycles.

Computed extinction limits and flame structures of H2/O2 counterflow diffusion flames with CO2 dilution

A narrowband radiation model is coupled to the OPPDIF program, which uses detailed chemical kinetics and thermal and transport properties to enable the study of one-dimensional counterflow H2/O2 diffusion flames with CO2 as dilution gas over the entire range of flammable strain rates. The effects of carbon dioxide dilution, ambient pressure and inlet temperature of opposed jets on the extinction limits and flame structures are compared and discussed. The extinction limits are presented using maximum flame temperature and strain rate as coordinates. Both high-stretch blowoff and the low-stretch quenching limits are computed. When the CO2 dilution percentage is higher, the flame is thinner and flame temperature is lower. The combustible range of strain rates is decreased with increasing CO2 percentage due to the effects of CO2 dilution, which is categorized as dilute effect, chemical effect and radiation effect. In addition, the flame temperature of low-stretch diffusion flame with radiation loss is substantially lower than that computed with the non-radiation model. This large temperature drop results from the combined effect of flame radiation and chemical kinetics. The extinction limits and flame temperature are increasing with increasing atmospheric pressure and temperature, but the flame thickness is decreased with the pressure. At higher pressure and temperature, the extinction limits are extended more on the high-stretch blowoff limits, indicating the influence of the ambient pressure and temperature on the chemical reaction.

Multi-dimensional Numerical Simulation of Operating Process of CNG Engine Using Detailed Chemical Kinetics with CFD

Numerical simulation of engine combustion using CFD and detailed chemical kinetics is accomplished by integrating CFD code KIVA3V with chemical kinetics code CHEMKINII. To reduce the computation time, the coupled software is reprogrammed to have the function of parallel computing and the revised software is calculated in a massively parallel processor. In order to have a better prediction of in-cylinder combustion and intermediate involving process, the coupled simulation takes the effects of turbulence, thermodynamic and chemistry into account and a turbulent combustion model which balances the effect of turbulence and detailed chemical kinetics is applied. The model is validated by using the experiment data from modified diesel engine-WT615. The results show that the simulation result has a good consistency with experiment data, the simulation provides the information of in-cylinder distribution of intermediates and computation has good parallel efficiency and acceleration ratio.

Effect of particle size on combustion of aluminum particle dust in air

The combustion of aluminum particle dust in a laminar air flow is theoretically studied under fuel-lean conditions. A wide range of particle sizes at nano and micron scales is explored. The flame speed and temperature distribution are obtained by numerically solving the energy equation in the flame zone, with the particle burning rate modeled as a function of particle diameter and ambient temperature. The model allows for investigation into the effects of particle size, equivalence ratio, and chemical kinetics on the burning characteristics and flame structures of aluminum-particle/air mixtures. In addition, the flame behavior with ultra-fine particles in the sub-nanometer range is examined by asymptotically treating particles as large molecules. Calculated flame speeds show reasonable agreement with experimental data. As the particle diameter decreases from the micron to the nano range, the flame speed increases and the combustion transits from a diffusion-controlled to a kinetically controlled mode. For micron-sized and larger particles, the flame speed can be correlated with the particle size according to a d − m relationship, with m being 0.92. For nano-particles, a d −0.52 or d −0.13 dependence is obtained, depending on whether the d 1.0 - or d 0.3 -law of particle burning time is implemented in the flame model, respectively. No universal law of flame speed exists for the entire range of particle sizes.