1,721,059 research outputs found
OptiSMOKE++: A toolbox for optimization of chemical kinetic mechanisms
Full text linkAs detailed chemical mechanisms are becoming viable for large scale simulations, knowledge and control of the uncertainty correlated to the kinetic parameters are becoming crucial to ensure accurate numerical predictions. A flexible toolbox for the optimization of chemical kinetics has therefore been developed in this work. The toolbox is able to use different optimization methodologies, as well as it can handle a large amount of uncertain parameters simultaneously. It can also handle experimental targets from different sources: Batch reactors, Plug Flow Reactors, Perfectly Stirred Reactors, Rapid Compression Machines and Laminar Flame Speeds. This work presents the different features of this toolbox together with five different test cases which exemplifies these features. Program summary: Program Title: OptiSMOKE++ CPC Library link to program files: https://doi.org/10.17632/tvjky2n8md.1 Licensing provisions: GPLv3 Programming language: C++ Nature of problem: Optimization of uncertain kinetic parameters with respect to experimental data. Solution method: Using the optimization capabilities of DAKOTA [1], and solving reacting systems with OpenSMOKE++ [2], OptiSMOKE++ determines the optimal combination of specified kinetic parameters, within their uncertainty, and with respect to the experimental data. References [1] B. M. Adams, M. S. Ebeida, M. S. Eldred, G. Geraci, J. D. Jakeman, K. A. Maupin, J. A. Monoscheke, L. P. Swiler, J. A. Stephens, D. M. Vigil, T. M. Wildey, W. J. Bohno, K. R. Dalbey, J. P. Eddy, R. W. Hooper, K. T. Hu, P. D. Hough, E. M. Ridgwat, A. Rushdi, Dakota, A Multilevel Parallel Object-Oriented Framework for Design Optimization, Parameter Estimation, Uncertainty Quantification, and Sensitivity Analysis: Version 6.5 User's Manual (2014). [2] A. Cuoci, A. Frassoldati, T. Faravelli, E. Ranzi, OpenSMOKE++: An object-oriented framework for the numerical modeling of reactive systems with detailed kinetic mechanisms, Computer Physics Communications 192 (2015) 237-264. doi:10.1016/j.cpc.2015.02.014
Numerical modeling of reacting systems with detailed kinetic mechanisms
No full textRealistic numerical simulations of pyrolysis and combustion require not only accurate modeling of fluid dynamics, but also a detailed characterization of chemical reactions. In recent years, increasing efforts have been devoted to the development of more complex reaction mechanisms, with high levels of detail and comprehensiveness. Clearly, thanks to new advancements in chemical kinetics, the size of mechanisms tends to grow with time. Recent trends suggest that in the near future mechanisms with more than ~ 20,000 species will be available. While such large mechanisms may provide very detailed information about the chemistry of combustion and pyrolysis, it is very expensive to accommodate them in numerical simulations. The computational cost can be prohibitive even for ideal systems (i.e., reacting systems in which no transport phenomena are included) or one-dimensional laminar flames. Even if the size (i.e., number of species and reactions) of detailed kinetic mechanisms is the most important challenge for the numerical simulations, also the stiffness (due to the wide range of chemicals times) of the nonlinear chemical equations governing the evolution of species plays a fundamental role in controlling the performance and the robustness of numerical algorithms. The points mentioned above suggest the need of using specifically conceived numerical techniques and tools for carrying out numerical simulations of reacting systems involving detailed kinetics (thousands of species and reactions). The purpose of this contribution is to present the equations of reacting systems typically adopted for developing and validating large kinetic mechanisms and discuss the basic numerical techniques to solve them efficiently. Moreover, because of their importance in the interpretation of results of simulations involving complex chemistry, numerical tools such as sensitivity analysis, rate of production, and reaction path analysis are introduced and described
Globalizzazione, apertura dei mercati e ICT
No full textGlobalizzazione: precedenti storici di apertura agli scambi internazionali. Caratteristiche del commercio internazionale negli ultimi decenni. Investimenti diretti esteri. Paesi sviluppati, in via di sviluppo ed emergenti. Imprese ed attrattività dei paesi
Large Eddy Simulation of MILD combustion using finite rate chemistry: Effect of combustion sub-grid closure
Full text linkIn this work, we present a detailed comparison between the conventional Partially Stirred Reactor (PaSR) combustion model and two implicit combustion models, named Quasi Laminar Finite Rate (QLFR) model and Laminar Finite Rate (LFR) model, respectively. Large Eddy Simulation (LES) is employed and the Adelaide Jet in Hot Co-flow (AJHC) burner is chosen as validation case. In the implicit combustion models, the filtered source term comes directly from the chemical term, without inclusion of turbulence effects. Results demonstrate that the two implicit models behave similarly to the conventional PaSR model, for the mean and root-mean-square of the temperature and species mass fractions, and that all models provide very satisfactory predictions, especially for the mean values. This justifies the use of implicit combustion models in low Damkohler number (Da <= 1.0) systems. The QLFR model allows to reduce the computational cost of about three times, compared to the LFR model. Moreover, the comparison between two 4-step global mechanisms and the KEE58 mechanism proves the importance of finite rate chemistry in MILD combustion. (C) 2018 The Authors. Published by Elsevier Inc. on behalf of The Combustion Institute
Numerical predictions of flashback limits of H2-enriched methane/air premixed laminar flames
Full text linkHydrogen is considered as a promising resource for decarbonizing not just the industrial sector but also domestic heating systems. By partially substituting natural gas with hydrogen, domestic combustion-based conversion systems have the potential to enhance efficiency, decrease carbon emissions, and achieve cleaner combustion, specifically reducing levels of particulate matter. Nevertheless, hydrogen possesses properties that differ significantly from natural gas. In particular, due to its higher laminar flame speed, hydrogen has a much higher propensity to flashback than natural gas, raising notable safety concerns. This study aims to examine the impact of H2 addition (up to 100%) to natural gas on the combustion process in domestic condensing boilers. To achieve this objective, 3D numerical simulations are conducted, modeling the multi-hole geometry that emulate perforated burners commonly found in these appliances. The simulations incorporate detailed kinetics and conjugate heat transfer with the burner plate and consider various hole-to-hole distances for a more comprehensive analysis. Flashback limits are found for a wide range of operating conditions of interest for domestic applications, with equivalence ratios from 0.5 to 1 and hydrogen fractions from 0 (pure methane) to 1 (pure hydrogen). The results confirm the observations of previous works on planar, multi-slit configurations. More specifically, the results shows that the conventional flashback correlation based on the concept of critical velocity gradient becomes inaccurate for H2 fractions larger than 0.50 as it does not take into account stretch induced preferential diffusion effects, which are especially large in the multi-hole configuration here investigated
Feature extraction and artificial neural networks for the on-the-fly classification of high-dimensional thermochemical spaces in adaptive-chemistry simulations
Full text linkThe integration of Artificial Neural Networks (ANNs) and Feature Extraction (FE) in the context of the Sample- Partitioning Adaptive Reduced Chemistry approach was investigated in this work, to increase the on-the-fly classification accuracy for very large thermochemical states. The proposed methodology was firstly compared with an on-the-fly classifier based on the Principal Component Analysis reconstruction error, as well as with a standard ANN (s-ANN) classifier, operating on the full thermochemical space, for the adaptive simulation of a steady laminar flame fed with a nitrogen-diluted stream of n-heptane in air. The numerical simulations were carried out with a kinetic mechanism accounting for 172 species and 6,067 reactions, which includes the chemistry of Polycyclic Aromatic Hydrocarbons (PAHs) up to C. Among all the aforementioned classifiers, the one exploiting the combination of an FE step with ANN proved to be more efficient for the classification of high-dimensional spaces, leading to a higher speed-up factor and a higher accuracy of the adaptive simulation in the description of the PAH and soot-precursor chemistry. Finally, the investigation of the classifier's performances was also extended to flames with different boundary conditions with respect to the training one, obtained imposing a higher Reynolds number or time-dependent sinusoidal perturbations. Satisfying results were observed on all the test flames
Flame-controlling continuation method for extinction of counterflow sooting flames with detailed chemistry
Full text linkThe generation of S-curves for the extinction of counterflow sooting flames has been accomplished by implementing a flame-controlling continuation method inclusive of soot model. The code can generate solutions for augmented flamelets databases, including soot scalars, useful for Flamelet Progress Variable (FPV) tabulations for sooting turbulent simulations. Indeed, the inclusion of all S-curve’s branches brings substantial improvements in the reproduction of extinction/re-ignition regimes or flame/acoustic interactions. In this context, developing a reliable tool for S-curve generation, with coupled reproduction of gas-phase and soot characteristics, is of great importance. The algorithm calculates the flamelet states through a 2-point flame-controlling continuation method with control on species mass fractions. Soot calculation is coupled with gas kinetics at every continuation so that flamelet states are inclusive of soot formation effects on precursors’ consumption and flame temperature. The flame and soot features can be correctly predicted along the whole curve with smooth transitions between branches. A brief introduction on general S-curve properties is given, using the implementation on hydrogen flames with different oxidizer’s inlet temperatures. Besides, soot characteristics are thoroughly investigated on ethylene flames at different pressures
Unsupervised Data Analysis of Direct Numerical Simulation of a Turbulent Flame via Local Principal Component Analysis and Procustes Analysis
Full text linkDirect Numerical Simulations (DNS) of reacting flows provide high-fidelity data for combustion model reduction and validation, although their interpretation is not always straightforward because of the massive amount of information and the data high-dimensionality. In this work, a completely unsupervised algorithm for data analysis is investigated on a data-set obtained from a temporally-evolving DNS simulation of a reacting n-heptane jet in air. The proposed algorithm combines the Local Principal Component Analysis (LPCA) clustering algorithm with a variables selection algorithm via dimensionality reduction and Procustes Analysis. Unlike other data-analysis algorithms, it requires null or limited user expertise as all of its steps are unsupervised and solely entrusted to mathematical objective functions, without any hyperparameter tuning step required
A virtual chemical mechanism for prediction of NO emissions from flames
Full text linkA reduced order kinetic model for NO (nitric oxide) prediction, based on the virtual chemistry methodology [M. Cailler, N. Darabiha, and B. Fiorina, Development of a virtual optimized chemistry method. Application to hydrocarbon/air combustion, Combust. Flame 211 (2020), pp. 281–302], is developed and applied. Virtual chemistry aims to optimise thermochemical properties and kinetic rate parameters of a network of virtual species and reactions. A virtual main chemical mechanism is dedicated to temperature and heat release prediction and is coupled with the flow governing equations, whereas satellite sub-mechanisms are designed to predict pollutants formation. Two virtual chemistry mechanisms are here employed: a main mechanism for calculating the temperature and heat release rate and a second mechanism dedicated to NO prediction. To recover the chemical structure of multi-mode combustion, both premixed and non-premixed flamelets are included in the learning database used to optimise the virtual NO mechanism. A multi-zone optimisation procedure is developed to accurately capture both fast and slow NO chemistry that include prompt, thermal and reburning pathways. The proposed NO sub-mechanism and optimisation methodology are applied to CH (Formula presented.) /air combustion. Laminar 1-D premixed and non-premixed flamelet configurations are first tested. The approach is then further assessed in 2-D CFD laminar flame simulations, by providing a direct comparison against detailed chemistry. 2-D premixed, non-premixed and partially premixed flame configurations are numerically investigated. For all cases, the virtual mechanism fairly captures temperature and (Formula presented.) chemistry with only 12 virtual species and 8 virtual reactions with a drastic CPU time reduction compared to detailed chemistry
Experimental and numerical study of pollutant emissions from a domestic condensing boiler fed with natural gas enriched with H2
No full textHydrogen is recognized as a promising resource for decarbonizing not only the industrial sector, but also the domestic heating systems. Through the partial substitution of natural gas with hydrogen, domestic combustion- based conversion systems can potentially offer improved efficiency, reduced carbon emissions, and cleaner combustion, i.e., lower levels of particulate matter. However, hydrogen exhibits properties that are significantly different from natural gas: (i) because of its higher laminar flame speed, hydrogen is more susceptible to flashback, which may pose significant concerns from the safety point of view; (ii) because of its higher adiabatic temperature, NOx emissions are expected to increase. Thus, experimental and numerical investigations are needed to better understand how the addition of hydrogen to the fuel mixture modifies the combustion process and how to mitigate/control the higher propensity to flashback and NOx formation within domestic devices. In this study, we investigated experimentally and numerically the performances and the emissions of a domestic condensing boiler with a stainless steel coil heat exchanger equipped with a perforated cylindrical burner fed with mixtures of H2 2-enriched natural gas and air, at several power levels (15, 24, and 30 kW), in a wide range of dilution ratios (from 1.16 to 1.4). 3D numerical simulations, including a detailed kinetic mechanism and conjugate heat transfer between the gaseous phase and the burner plate, were carried out with satisfactory agreement with the experimental data. The experimental results demonstrated the ability of the investigated device to properly work with fuel mixtures including up to 35% (molar basis) of hydrogen. The numerical simulations were repeated by considering pure hydrogen as a fuel in more diluted conditions (with dilution ratios from 1.4 to 2) and the same heat exchanger with a modified perforated burner to prevent the occurrence of flashback phenomena. The numerical results suggested the possibility to (partially) replace natural gas with hydrogen in domestic boilers with minimal modifications to existing perforated burners
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