Reduced Reaction Model Research Articles

Role of temperature-gradient steepness on development of hydrogen-oxygen detonation with ozone sensitization

This work carries out one-dimensional simulations on spontaneous initiation and development of detonations from hot spots with initial temperature gradients embedded with non-autoignitive H2O2 mixtures. Role of temperature-gradient steepness on the development of detonations inside and outside hot spots and the relationship of it to ozone sensitization are examined by changing gradient slope and O3 addition. It is found that steepening temperature gradient has negative effect on initiations and a critical gradient is identified for different O3 additions, below which spontaneous initiation fails. However, the sustenance of a developing detonation is not only related to the steepness, but also to the reactivity of mixtures with O3 additions. High O3 additions permit a detonation to be more likely to sustain outside hot spots; while for low O3 additions, the sustenance depends nonmonotonically on the slope, which cannot be predicted by the idealized reactive flow system with a constant specific heat ratio and the reduced reaction model. The results clarify the suggestions of He & Clavin [1] that temperature gradient may prevent sustenance of a developing detonation and the transmission of detonation to the uniform fresh mixture.

Reduced Reaction Model for Secondary Gas Phase in Biomass Gasification

Biomass gasification is an energy-generating process widely recognized as an efficient and sustainable technology, representing a viable alternative to the burning of fossil fuels. However, the for...

A realistic skeletal mechanism for the oxidation of biodiesel surrogate composed of long carbon chain and polyunsaturated compounds

To obtain a realistic reduced reaction model for the combustion of biodiesel in compression ignition engine, the heavy methyl esters with long carbon chain and polyunsaturated degree were used to characterize real biodiesel fuel in this study. Methyl palmitate (MHD), methyl stearate (MOD), methyl linoleate (MOD9D12D), methyl–5–decenoate (MD5D) and n–decane were selected as the surrogate of biodiesel. A skeletal mechanism with 187 species and 982 reactions was developed for the oxidation of biodiesel surrogate using the decoupling methodology. The mechanism was partitioned into three, including the relatively simplified fuel–related sub-mechanism, the ester group–related sub–mechanism, and the detailed core sub–mechanism during the construction process. It was validated by zero–dimensional ignition delay for each component in biodiesel surrogate under variable conditions. The predicted outcomes are in concordance with the experimental information, which preliminarily verifies the effectiveness and accuracy of the mechanism. A three–dimensional computational fluid dynamics code coupled with the mechanism was subsequently utilized to simulate engine combustion. The findings show that the combustion characteristics of biodiesel are robustly replicated, portraying that the newly developed biodiesel mechanism can be assuredly used in the practical engine simulation.

Development and validation of a reduced chemical kinetic model for used vegetable oil biodiesel/1-Hexanol blend for engine application

In this study, a compact kinetic model consisting of 588 reactions and 139 species is proposed for 1-Hexanol and used vegetable oil (UVO) biodiesel blend for simulating internal combustion (IC) engines. Suitable modifications were made to the pre-exponential factor to ascertain predictive performance comparable to the investigational results. This reduced reaction model was validated with experimental data from the published work for laminar flames speed, ignition delay and species mole fractions. The reduced mechanism and the experimental results are comparable within a factor of 1.3–2.5. A zero-dimensional single zone model was employed for simulating internal combustion engine conditions with the reduced mechanism using Cantera software and the results are analogous and as expected from the simulation study. The reduced reaction mechanism could be utilized by the researchers working on alternative fuels to simulate and study the combustion of used vegetable oil/1-Hexanol blend in an engine using computational fluid dynamics (CFD) modeling software.

Sensitivities of direct numerical simulations to chemical kinetic uncertainties: spherical flame kernel evolution of a real jet fuel

A systematic computational study is conducted to understand the propagation of uncertain chemical kinetic parameters into turbulent premixed flame simulations. Two snapshots of the cross-section of a spherical flame kernel in homogeneous isotropic turbulence is extracted from a three dimensional direct numerical simulation (DNS) and serve as the initial conditions for a series of subsequent two-dimensional DNS that sample and quantify the impact of chemical kinetic uncertainties on the integrated heat release rates and certain species concentrations. The target configuration features real jet fuel chemistry modeled by a 31-species reduced reaction model. The importance of each reaction is first ranked, by comparing the normalized reaction flux based on a snapshot from the DNS. A two-step process is then taken where initial Monte Carlo screening is performed by perturbing 99 reaction rate constants simultaneously. Following this, a subset of 19 reactions is studied using a larger collection of 2D DNS simulations. As the major quantity of interest, the integrated heat release rate is consistently the most sensitive to reactions R8 (H+O2=O+OH) and R32 (CO+OH=CO2+H). When combined with a consideration of the reaction rate uncertainty, R71 (CH3+OH=CH2*+H2O) consistently impacts the quality of flame prediction most prominently. For the case starting with a strong burning flame, the rate limiting reactions are found to be not significantly different between the turbulent and laminar conditions, although the magnitudes of sensitivity coefficients are amplified for certain reactions under turbulent conditions due to an enlarged thermochemical space. For the case initialized with a near-extinction flame, more upstream and/or competing reactions with R8 and R32 are observed. Another manifestation of the enlarged thermochemical space in turbulent flames is the wider range of influential reactions for integrated mass of CO, in comparison with the laminar condition where the sensitivity is dominated by R32. For H and C2H2, reactions R71, R42 (HCO+O2=CO+HO2) and R40 (HCO+M=CO+H+M) are identified to impact the model predictions for such species as acetylene in the most significant manner.

An Investigation of Combustion Properties of Butanol and Its Potential for Power Generation

Over the last years, global concerns about energy security and climate change have resulted in many efforts focusing on the potential utilization of nonpetroleum-based, i.e., bioderived, fuels. In this context, n-butanol has recently received high attention because it can be produced sustainably. A comprehensive knowledge about its combustion properties is inevitable to ensure an efficient and smart use of n-butanol if selected as a future energy carrier. In the present work, two major combustion characteristics, here laminar flame speeds applying the cone-angle method and ignition delay times applying the shock tube technique, have been studied, experimentally, and by modeling exploiting detailed chemical kinetic reaction models, at ambient and elevated pressures. The in-house reaction model was constructed applying the reaction model generation (RMG)-method. A linear transformation method recently developed, linTM, was exploited to generate a reduced reaction model needed for an efficient, comprehensive parametric study of the combustion behavior of n-butanol-hydrocarbon mixtures. All experimental data were found to agree with the model predictions of the in-house reaction model, for all temperatures, pressures, and fuel-air ratios. On the other hand, calculations using reaction models from the open literature mostly overpredict the measured ignition delay times by about a factor of two. The results are compared to those of ethanol, with ignition delay times very similar and laminar flame speeds of n-butanol slightly lower, at atmospheric pressure.

Soot Formation Calculation by Reduced Reaction Model for 4 Ring PAH and Moment Method

Soot Formation Calculation by Reduced Reaction Model for 4 Ring PAH and Moment Method

Effect of heating rate on the kinetics and mechanism of crystallization in amorphous Se85Te10Pb5 glasses

The kinetic parameters of amorphous Se85Te10Pb5 glasses were studied for nonisothermal and predicted isothermal conditions. Differential scanning calorimetry (DSC) was used to study the kinetics of the amorphous-to- crystalline transformation at heating rates in three regimes (3–15, 20–50 and 60–99Kmin−1). Analysis of the obtained data shows that the activation energy of crystallization is not constant but varies with the degree of conversion and, hence, with temperature; thus, the variation depends on the heating rate. The Avrami exponents indicate that the mechanism of crystallization varies from one- to two- to three-dimensional growth at low, medium and high heating rates, respectively. The reduced reaction model was observed to follow the Avrami–Erofeev mechanism, but varies from A1.5 to A2 to A3 for the different heating-rate regimes. A good fit was achieved between experimental and predicted sigmoidal curves for the crystallization fraction with increasing temperature.

Numerical simulation of shock initiation of Ni/Al multilayered composites

The initiation of chemical reaction in cold-rolled Ni/Al multilayered composites by shock compression is investigated numerically. A simplified approach is adopted that exploits the disparity between the reaction and shock loading timescales. The impact of shock compression is modeled using CTH simulations that yield pressure, strain, and temperature distributions within the composites due to the shock propagation. The resulting temperature distribution is then used as initial condition to simulate the evolution of the subsequent shock-induced mixing and chemical reaction. To this end, a reduced reaction model is used that expresses the local atomic mixing and heat release rates in terms of an evolution equation for a dimensionless time scale reflecting the age of the mixed layer. The computations are used to assess the effect of bilayer thickness on the reaction, as well as the impact of shock velocity and orientation with respect to the layering. Computed results indicate that initiation and evolution of the reaction are substantially affected by both the shock velocity and the bilayer thickness. In particular, at low impact velocity, Ni/Al multilayered composites with thick bilayers react completely in 100 ms while at high impact velocity and thin bilayers, reaction time was less than 100 μs. Quantitative trends for the dependence of the reaction time on the shock velocity are also determined, for different bilayer thickness and shock orientation.

Explicit reduced reaction models for ignition, flame propagation, and extinction of C 2H 4/CH 4/H 2 and air systems

Large-scale simulations of multidimensional unsteady reacting flows with detailed chemistry and transport can be computationally extremely intensive even on distributed computing architectures. With the development of computationally efficient reduced chemical kinetic models, the smaller number of scalar variables to be integrated can lead to a significant reduction in the computational time required for the simulation with limited loss of accuracy in the results. A general MATLAB-based automated procedure for the development of reduced reaction models is presented. Based on the application of the quasi-steady-state (QSS) approximation for certain chemical species and on the elimination of selected fast elementary reactions, any complex starting reaction mechanism (detailed or skeletal) can be reduced with minimal human intervention. A key feature of the reduction procedure is the decoupling of the QSS species appearing in the QSS algebraic relations, enabling the explicit solution of the QSS species concentrations, which are needed for the evaluation of the elementary reaction rates. In contrast, previous approaches mainly relied on an implicit solution, requiring computationally intensive inner iterations. The automated procedure is first tested with the generation of an implicit 5-step reduced reaction model for CH 4/air flame propagation. Next, two explicit robust reduced reaction models based on ignition data (18-step) and on flame propagation data (15-step) are systematically developed and extensively validated for ignition delay time, flame propagation, and extinction predictions of C 2H 4/air, CH 4/air, and H 2/air systems over a wide range of equivalence ratios, initial temperatures, pressures, and strain rates. In order to assess the computational advantages of the explicit reduced reaction models, comparisons of the computational time required to evaluate the chemical source terms as well as for the integration of unsteady nonpremixed flames for each model are also presented.

Development of an adaptive chemistry model considering micromixing effects

In numerical simulation of combustion models, solution of the chemical kinetics is often the most expensive part of the calculation, since accurate description of kinetic mechanism involves large number of species and reactions, leading to a large set of coupled ODEs, often too complex to be considered in their entirety along with a detailed flow simulation. Hence the need for representing the complex chemical reactions by simple reduced models, which can retain considerable accuracy while rendering computational feasibility. Realistically, under different conditions and at different points in time, different reactions become important, which has been exploited to develop an adaptive reduction scheme such that the reduced reaction model adapts itself to the changing reactor conditions.A methodology is developed in this paper to construct reduced mechanisms by solving an optimization problem, where the objective is to determine the range of conditions along the reaction trajectory over which a prespecified number of reactions can predict the actual profile within an allowable tolerance. Such an adaptive reduced mechanism is then coupled with the reactive flow algorithm, which selects an appropriate mechanism depending on reactor condition and integrate the corresponding ODEs for the specified valid range. These ideas are demonstrated using the mechanism of CO/H 2 combustion in air.

Chemical kinetic study of HAN decomposition

A numerical analysis was conducted to deduce the Arrhenius-type reaction rates of a reduced reaction model for HAN. The reaction rates were obtained by an inverse-based analysis in a way that minimized an objective function, which consisted of the difference between the calculated concentrations from the numerical model and experimental data as well as the uncertainties in the experimental data. The experimental decomposition process was modeled by applying the species conservation equations to the condensed-phase and the gas-phase regions separately. The experimental data that were used to deduce the rate coefficient parameters were the gas-phase species concentrations observed during thermal decomposition of 13M HAN including HNO 3, N 2O, NO, and NO 2. The energy equation was not considered in the numerical model since measured condensed-phase reaction temperatures were used as input data. The uncertainties in the deduced reaction rates were calculated using the standard deviations of the experimental data. With the best-fit rate constants of the reaction model, the species evolution profiles of 13M HAN were reasonably recovered, and the condensed-phase mass fractions were predicted. The reaction model was applied to the simulation of the species evolutions for solid HAN and HAN-water solutions including 10.7 and 9M HAN. The simulated gas-phase concentrations coincided well with the experimental data, indicating that the proposed global reaction mechanism captured most of the key reactions of HAN.

A reduced reaction model for carbon CVD/CVI processes

Carbon–carbon composites are produced by chemical vapor deposition/chemical vapor infiltration (CVD/CVI) processes. Models of carbon–carbon composite production processes will help reduce production costs. Reliable process models must, however, include details of the gas phase kinetics in order to identify optimal conditions. We have combined detailed gas phase kinetics, surface kinetics, and a pore closure model to predict pore geometry changes with respect to time. To determine the dominant gas phase kinetics, we reduced a large set of reactions to a minimal set using a sensitivity, rate, and dimensional analysis approach. These robust and relatively fast techniques can be used under a variety of conditions, including those within the pores of the composite. The process model shows that the deposition profile depends on the kinetic model chosen. Using the more realistic reaction model, conditions for uniform, or inside-out, densification can be suggested.