Local reignition scenarios in H2/N2 turbulent diffusion flames are investigated using a one-dimensional turbulence (ODT) model and chemical explosive mode analysis (CEMA). Through analogy with the flame fronts in homogeneous ignition and laminar premixed flames, four reignition scenarios are distinguished and analyzed by CEMA. Results show that the reignition scenario via premixed flame propagation corresponds to a high-temperature explosion index and the reignition process is segmentally dominated by the reactions related to the consumption and production of the hydrogen radical. While reignition mode through an independent flamelet corresponds to a high radical explosion index, the whole reignition process is dominated by the production reaction of the hydrogen radical. When these two reignition processes are terminated by turbulent eddies, a hybrid reignition process or reignition scenario through turbulent flame folding may occur, and the turbulent folding mode corresponds to a high dissipation rate and obvious temperature jump near the end of the reignition process. In addition, statistical analysis shows that the premixed flame propagation mode is more effective to ignite the fluid parcel with a low temperature and the reignition scenario via an independent flamelet is a quicker process.

This paper investigated the flame projection distance (defined as the distance from the nozzle exit to the furthest point of the flame in the horizontal direction) of horizontally oriented buoyant turbulent rectangular jet fires. The experiments as well as the correlations reported previously were limited for axi-symmetrical fire sources that are not applicable for non-axi-symmetrical sources. In this work, experiments were conducted using horizontally oriented rectangular nozzles with aspect ratio, n (nozzle length to nozzle width: n = L/W), varied from 1:1 to 71:1 covering the axi-symmetrical, rectangular and linear sources, employing propane as fuel. Results showed that the flame projection distance increased with the heat release rate growth. At the same time, the flame projection distance decreased with the increase of the aspect ratio n for a given nozzle exit area. A non-dimensional function was then derived for the projection distance, in which a characteristic length scale was found in relation to the nozzle length and width, based on the balance of the momentum flux to the buoyancy flux of the projected flame. A new non-dimensional heat release rate was defined based on the proposed characteristic length scale. The data obtained in this work for different aspect ratios, as well as those reported previously for axi-symmetrical sources, were shown to be well correlated by the derived function in two regimes: (1) for relative small non-dimensional heat release rates, the flame projection distance has a 2/3 power dependency on the heat release rate as for a 2-D trend fire; (2) for relative large non-dimensional heat release rates, the flame projection distance has a 2/5 power dependency on the heat release rate as for a 3-D trend fire. The proposed new correlation provides a more general and practical base for estimating the projection distance of horizontally oriented buoyant turbulent jet diffusion flames.

The influence of some halon replacements as the fire suppressants on premixed and non-premixed hydrocarbon flames was experimentally and numerically investigated. Unstretched flame speeds and extinction stretch rates of methane/air and propane/air flames with the addition of different loadings of C2HF5 (HFC-125), C2HF3Cl2 (HCFC-123), and C3H2F3Br (2-BTP) were measured for a range of fuel/air equivalence ratios. Experiments were conducted using the counterflow technique at ambient temperature and pressure. A newly-developed kinetic model, which was based on a C1‒C4 hydrocarbon model and agent-inhibition sub-models, was used to compare with the measured flame speeds and extinction stretch rates. Good agreement was obtained regarding the laminar flame speeds. For the extinction rate of the non-premixed flames, the absolute experimental data were over-predicted uniformly for the entire range of conditions examined, although the model correctly predicted the trends of increasing extinction stretch rates with the increasing fuel/(N2+agent) molar ratio. Since this new kinetic model was tested against the extinction limits for the first time, the optimization of the rate constants of the reaction(s) regarding the flame extinction is expected. Significant thermal expansion was detected in larger and brighter flames when the different agents were added to the neat mixture. This observation provided new experimental evidence of the fuel-like properties of the inhibitor. Despite the difference in the fuel type, C2HF5 and C3H2F3Br were found to be less effective at reducing the flame speeds of the lean flames as compared to that of the rich flames. However, C2HF3Cl2 reduced the flame speeds for all of the investigated mixture conditions. Thermodynamic equilibrium calculations were performed to interpret the behavior of these fire suppressants in hydrocarbon flames. The extinction data for non-premixed flames revealed that the resistance of the agents to extinction decreases in the order of C2HF5, C2HF3Cl2, and C3H2F3Br, which is consistent with the suppression effect on flame speed.

•First study about the pyrolysis characteristics and kinetics of rape straw.•Pyrolysis mechanism was f(α) = (3/2)[(1 − α)2/3]/[1 − (1 − α)1/3] for α greater than 0.4.•Particle Swarm Optimization was adopted to optimize kinetic parameters.•The simulated results can capture well the characteristic regions of DTG curves.•Practicability of optimized parameters is cross-validated for wider heating rates.The aim of this work is to implement an efficient and robust optimization approach for parameter estimation of pyrolysis kinetic model to study the pyrolysis of a typical agricultural residue. Thermogravimetric profiles of rape straw were obtained at a wide heating range of 10, 20, 30 and 40 °C/min in an inert atmosphere of nitrogen and the thermal decomposition process was studied in detail. First, three different kinetic methods (Friedman, KAS and OFW) were applied for activation energy determination. The activation energies ranged from 191.16 kJ/mol to 264.31 kJ/mol. Then, the reaction mechanism and pre-exponential factor were analyzed by using the generalized master-plots method. It is found that at conversion level higher than 0.4 the rape straw decomposition was governed by 3-D diffusion model and it tended to high order reaction model at lower conversion. Finally, a multi-component parallel reactions scheme incorporated into the Particle Swarm Optimization (PSO) technique was presented to determine kinetic parameters. The kinetic triplets and other stoichiometric parameters were optimized against profiles for heating rates of 20 and 30 °C/min. The optimized activation energy value is 156.41 kJ/mol, 211.26 kJ/mol and 57.84 kJ/mol for hemicellulose, cellulose and lignin, respectively. These results are in conformity with results reported in previous literatures. Meanwhile, the obtained pre-exponential factors values also lie in the reasonable range of lnAi = 25.32–39.14 ln/s according to intrinsic transition-state theory. Furthermore, cross-validated results show that the optimized parameters can be applied not only to conditions where they were obtained, but also to conditions beyond (10 and 40 °C/min), indicating that the derived results are appropriate to simulate and predict rape straw pyrolysis under various heating rates, which will be useful for further design and sizing of biomass thermochemical process reactors.Download high-res image (151KB)Download full-size image

•ODT model is performed in H2/CO/N2 syngas non-premixed flame.•Damköhler number based on chemical explosive mode analysis is used to identify flame extinction.•Diagnostic tools are defined to distinguish the different scenarios of local extinction and re-ignition.An investigation of the local flame extinction and re-ignition of hydrogen syngas non-premixed flame are carried out using one-dimensional turbulence (ODT) model with detailed chemistry. Based on the Lagrangian tracer method and chemical explosive mode analysis (CEMA), diagnostic tools have been defined to distinguish the different scenarios of local extinction and re-ignition. The results show that the extinction regime can be characterized by a large negative Damköhler number based on CEMA. From Lagrangian investigation, two scenarios of local extinction, i.e., severe extinction with lower temperature and moderate extinction with higher temperature, are shown. Meanwhile, two re-ignition mechanisms are distinguished by the explosion index of the process prior to re-ignition. It is found that re-ignition mechanism through premixed flame propagation corresponds to a rather long preheat period with thermal runaway and high temperature explosion index, while re-ignition mechanism via independent flamelet corresponds to a high radical explosion index and small temperature gradient.

•The turbulence-chemistry interactions were investigated with ODT model.•The function route of fire extinguishment agent in diffusion flame was analyzed.•The agent can reduce the chemical explosive mode and lead to flame extinction.•The acting areas for control variables and reactions are clearly visualized.An idealized extinguishing agent with two main inhibition mechanisms was designed to test the chemistry of hydrogen flame extinguishment. The One-Dimensional Turbulence (ODT) model and chemical explosive mode (CEM) analysis (CEMA) were used to perform an in-depth analysis on the role of turbulence-chemistry interaction in diffusion hydrogen/air flame. The function route of the fire extinguishment agent in diffusion flame has been analyzed. The results show that the agent can significantly reduce the CEM. The decrease of CEM will decrease the Da number, and lead to flame extinction. The CEMA of ODT results present that the extinction of flame is controlled by the CEM, the mixing rates and the eddy events. The results also illustrated the main inhibition mechanism in which catalytic cycles lead to the destruction of major chain carriers, H. The acting areas of variables and reactions are consistent and clearly visualized with CEMA, which proves that the CEMA is a very efficient method to identify the control variable during the combustion process.

A reduced mechanism for propane/air combustion and its flame inhibition by phosphorus-containing compounds (PCCs) is constructed with the level of importance (LOI) method. The analysis is performed on solutions of freely propagating premixed flames with detailed chemical kinetics involving 121 species and 682 reactions proposed by Jayaweera et al. For the non-homogeneous reaction-diffusion system, the chemical lifetime of each species is weighted by its diffusion timescale, and the characteristic flame timescale is used to normalize the chemical lifetime. The definition of sensitivity in LOI is extended so that multi-parameters can be used as sensitivity targets. Propane, oxygen, dimethyl methylphosphonate (DMMP), and flame speed are selected to be perturbed for sensitivity analysis, the species with low LOI index are removed, and reactions involving the redundant species are excluded from the mechanism. A skeletal mechanism is obtained, which consists of 57 species and 268 elementary reactions. Calculations for laminar flame speeds, key flame radicals and catalytic cycles using the skeletal mechanism are in good agreement with those by using the detailed mechanism over a wide range of equivalence ratio undoped and doped with DMMP.

The purpose of this study is to investigate the characteristics of soot particles in C2H4/CO2/O2/N2 combustion at equivalence ratio of 3.0–5.0. As the oxidant is switched from conventional air to CO2/O2/N2 mixture, the key species C2H2, C3H3 responsible for formation of first aromatic ring, the typical aromatics and 4-ring aromatics total production rate all decrease greatly. In addition, with CO2 mole fraction from 0.2 to 0.5 in the mixture, the soot particle number density, volume fraction, surface area density, which are three most important parameters to soot particle property, are suppressed obviously. Furthermore, the increasing content of CO2 in the oxidizer influences mostly H, OH radical concentrations by two reactions: COOHCO2H and HO2OOH, and the production rate of H, OH from the two reactions declined, which revealed that CO2 in mixture has an inhibiting effect on soot particle generation.

An idealized extinguishing agent with two main inhibition mechanisms including recombination of radicals and HO2 destruction was designed to test the chemistry of hydrogen flame extinguishment. The One-Dimensional Turbulence (ODT) model was used to perform an in-depth analysis on the role of turbulence–chemistry interaction in flame suppression. The results show that fire suppression effect is generally significant under a higher Reynolds number due to the increasing global mixing rate. The Reynolds-averaged mean and rms of the chain-carrying radical are very sensitive to the agents. Furthermore, it is shown that the differential diffusion effects are strong in the near field. The numerical results also show that a higher global mixing rate would lead to the high discrepancy of flame temperature in the near-field, and flame temperature appeared to be unaffected by the different mixing rate in the far-field. Increasing the global mixing rate will lead to larger extinguished regions even without addition, and extinction mainly occurs in the near-field, the local extinction is a rapid process while the reignition of the extinguished mixture is slow.Highlights► The turbulence–chemistry interactions were investigated with ODT model. ► An idealized extinguishing agent was designed to test the extinguishing mechanism. ► The fire suppression effect is more significant under a higher Reynolds number. ► The chain-carrying radical are very sensitive to the agents and decrease fast. ► There are strong differential diffusion effects in the near field.

In chemical processes, fault diagnosis is relatively difficult due to the incomplete prior-knowledge and unpredictable production changes. To solve the problem, a case-based extension fault diagnosis (CEFD) method is proposed combining with extension theory, in which the basic-element model is used for the unified and deep fault description, the distance concept is applied to quantify the correlation degree between the new fault and the original fault cases, and the extension transformation is used to expand and obtain the solution of unknown faults. With the application in Tennessee Eastman process, the result indicates that CEFD method has a flexible fault representation, objective fault retrieve performance and good ability for fault study, providing a new way for diagnosing production faults accurately.

In order to analyze the complex chemical kinetic mechanism systematically and find out the redundant species and reactions, a numerical platform for mechanism analysis and simplification is established basing on Path Flux Analysis (PFA). It is used to reduce a detailed mechanism for flame inhibited by phosphorus containing compounds, a reduced mechanism with 65 species and 335 reactions is obtained. The detailed and reduced mechanism are both used to calculate the freely-propagating premix C3H8/air flame with different dimethyl methylphosphonate doped over a wide range of equivalence ratios. The concentration distributions of free radicals and major species are compared, and the results under two different mechanisms agree well. The laminar flame speed obtained by the two mechanisms also matches well, with the maximum relative error introduces as a small value of 1.7%. On the basis of the reduced mechanism validation, the correlativity analysis is conducted between flame speed and free radical concentrations, which can provide information for target species selection in the further mechanism reduction. By analyzing the species and reactions fluxes, the species and reaction paths which contribute the flame inhibition significantly are determined.

Large hydrocarbons, such as n-heptane and iso-octane, are always the main fuels of fire experiments. The detailed mechanisms of these large hydrocarbons provide a powerful tool for the numerical simulation to study complex turbulent reacting flows. But it is necessary to reduce the mechanisms because of the huge computational cost for detailed mechanisms. The directed relation graphs (DRG) and DRG with error propagation (DRGEP) methods are almost the most efficient models to reduce detailed mechanisms. The differences between these two methods have been analyzed in this paper. The results show that the numbers of species in the skeletal mechanisms obtained by DRG have a significant drop when the threshold limits are between 10-4 and 10-2, both for n-heptane and iso-octane. Main products are essential to be included in the target species when the DRGEP method is used, while the main reactants are enough as the target species for the DRG method. Validation of the skeletal mechanisms shows good accuracy for both DRG and DRGEP methods over wide parameter ranges when the species numbers of the skeletal mechanisms are more than 87. The results also indicate that the skeletal mechanisms obtained by DRG method have a smaller ratio of reactions numbers to species numbers, and the skeletal mechanisms obtained by the DRG method have a better performance than that of DRGEP when the species numbers of the mechanisms are of the same scale.