Fuel-oxidizer Mixtures Research Articles

Long-term storage for reanalysis purposes: A stability study of extracts from post-explosion/burning residues of fuel-oxidizer explosive mixtures

Post-explosion residue analysis is vital in forensic chemistry, providing valuable insights into incidents involving explosives. One significant challenge in this field is preserving samples for long-term storage for reanalysis. Presently, many laboratories conducting post-explosion analyses worldwide do not retain samples for reanalysis purposes, since the extraction process for sample preparation often necessitates the complete consumption of the scarce material. Even in cases where a portion of the material remains, such as a swab, it is difficult to ensure its representativeness and select a specific portion for preservation. One possible solution to overcome this challenge is to consider preserving the extracts used in the analyses for potential reanalysis. However, the stability characteristics of these extracts are currently unknown. The results presented in this article provide valuable insights into the feasibility of long-term storage for samples containing target analytes in extracts from post-explosion/burning residues of frequently encountered fuel-oxidizer explosive mixtures, such as flash powder, explosive emulsion, and black powder. These results shed light on the potential viability of preserving such extracts for future reanalysis, offering promising prospects for enhancing forensic investigations in this field. Based on the current findings, it has been demonstrated that the aqueous extracts can be effectively preserved as long-term storage samples for a minimum of 24 months, encompassing all the studied explosives. Only cyanate ion showed significant degradation among the 27 compounds studied. Additionally, the organic extract, specifically in the case of emulsion explosive, can be maintained for at least 12 months.

Evaluation of canine training aids containment for homemade explosive and components by headspace analysis and canine testing.

While canines are most commonly trained to detect traditional explosives, such as nitroaromatics and smokeless powders, homemade explosives (HMEs), such as fuel-oxidizer mixtures, are arguably a greater threat. As such, it is imperative that canines are sufficiently trained in the detection of such HMEs. The training aid delivery device (TADD) is a primary containment device that has been used to house HMEs and HME components for canine detection training purposes. This research assesses the odor release from HME components, ammonium nitrate (AN), urea nitrate (UN), and potassium chlorate (PC), housed in TADDs. Canine odor recognition tests (ORTs) were used with analytical data to determine the detectability of TADDs containing AN, UN, or PC. Headspace analysis by gas chromatography/mass spectrometry (GC/MS) with solid-phase microextraction (SPME) or online cryotrapping were used to measure ammonia or chlorine, as well as other unwanted odorants, emanating from bulk AN, UN, and PC in TADDs over 28 weeks. The analytical data showed variation in the amount of ammonia and chlorine over time, with ammonia from AN and UN decreasing slowly over time and the abundance of chlorine from PC TADDs dependent on the frequency of exposure to ambient air. Even with these variations in odor abundance, canines previously trained to detect bulk explosive HME components were able to detect all three targets in glass and plastic TADDs for at least 18 months after loading. Detection proficiency ranged from 64% to 100% and was not found to be dependent on either age of material.

Critical Decay Time Model for Direct Detonation Initiation Energy in Gaseous Mixtures

A critical decay time (CDT) model is developed to predict the critical energy of direct detonation initiation in gaseous mixtures. It is based on the global initiation criterion that the energy deposit should allow the decaying shock speed to stay in a specific range below the Chapman–Jouguet (CJ) speed at least for a critical decay time. The speed range is estimated with the sub-CJ Zel’dovich–von Neumann–Döring (ZND) simulations. The critical decay time is calculated as the minimum time to reach unity Mach number in the sub-CJ ZND simulations. The lower-speed bound is taken as a characteristic extinction speed below (which means the lower-speed bound) which the direct initiation should fail. This speed is calibrated using one-dimensional simulations for hydrogen (H2)–oxygen (O2)–argon (Ar) mixtures. The calibrated CDT model is then applied to estimate the critical initiation energy with the point-blast theory. The model yields better agreement with experimental data for hydrogen-fueled mixtures such as H2─O2─Ar and H2─air mixtures than the well-known critical decay rate model. For small hydrocarbon-fueled mixtures such as acetylene (C2H2)─O2─Ar, methane (CH4)–air, ethylene (C2H4)–air, and ethane (C2H6)─air mixtures, the predicted critical energies also agree well with experimental results. The CDT model provides an efficient tool to evaluate the detonability of fuel–oxidizer mixtures, which could be beneficial for ignition initiation in propulsion and power devices such as rotating detonation engines.

A new method for systematic 1-step chemistry reduction applied to hydrocarbon combustion

We propose a new single-step mechanism for the combustion of arbitrary hydrocarbons and alcohols. Unlike most single-step models, no tabulation is required, as the method builds upon a new analytical description of the thermochemical equilibrium of fuel-oxidizer mixtures including dihydrogen and carbon monoxide – two species usually discarded in one-step descriptions – yielding correct adiabatic temperature. The single-step chemistry includes varying stoichiometric coefficients, ensuring a convergence towards thermochemical equilibrium regardless of the local state. The reaction rate is then carefully adjusted to reproduce accurately premixed flames. To tackle ignition simultaneously, an additional passive scalar advection-diffusion-reaction equation is introduced, with a rate fitted on ignition delays. The scalar then serves as an efficiency to modify the single-step reaction rate in autoignition configurations. The obtained scheme is then validated for a wide range of equivalence ratios on homogeneous reactors, premixed flames, a triple flame, and a counterflow diffusion flame. The new analytical thermochemical equilibrium formulation may also serve in speeding up infinitely fast chemistry calculations.

Experimental study on detonation-diffraction reflection point distances in hydrogen and gaseous hydrocarbon reactive systems

In this study, the critical conditions of diffracted detonation waves expressed in terms of reflection point distances in Kawasaki and Kasahara (2020) were investigated for a wider range of fuel-oxidizer mixtures. In H2/O2, C2H2/O2, C2H4/O2, C2H6/O2, C3H6/O2, C2H6/N2O, and C3H6/N2O mixtures, these mixtures are used to measure the reflection point distance through the modification of the initial pressure and equivalence ratio of the mixture at room temperature. Moreover, the critical ideal reflection point distance divided by the channel width, which this dimensionless parameter was identified in the critical condition region of detonation diffraction. The results revealed that this region was in the range of 3.8 ± 0.8 for all investigated mixtures even though the equivalence ratio varied. The parameter lr,ip0 is the product of the ideal reflection point distance and the initial pressure, which is proportional to the energy (work) per unit area required for re-initiation. The larger the product of the reflection point distance and the initial pressure, the more difficult the mixture is to re-initiate, and the inverse of this parameter represents the ease of re-initiation, which can be considered an index of detonability. Detonability, which is the objective of clarification in this study, was found to have the order C2H2/O2 > C2H4/O2 > C3H6/O2 > C2H6/O2 > H2/O2 in the vicinity of the stoichiometric ratio, which is similar to the case using Matsui and Lee's critical initiation energy.

Characterization of opportunity for upgrading of the system based on arc plasma torch for thermal spaying of ceramic materials, by means of use of fuel vortex intensifier. Part I: Thermodynamic modeling of the system efficiency parameters

One of the main trends in the field of improving the modern technologies of thermal spraying, including plasma one, for functional ceramic coatings formation is the reducing the energy consumption of the process. In this regard, one of the important directions for improving these technologies is the development of their new versions, using the principle of adding inexpensive fuel-oxidizer mixtures based on hydrocarbons with air. This type of plasma-fuel type of spraying will be promising for application at the present time, first of all, in order to obtain refractory functional coatings. For this purpose, we investigated the opportunity for upgrading an industrial unit/system for plasma spraying of ceramic powder materials with arc plasma torch of 25–40 kW power by the use of experimental variant of a fuel gas-vortex intensifier. The thermodynamic assessment of possible parameters of the generated mixed flow after the torch with this fuel intensifier was carried out to estimate the applicability of this system to optimize the spraying of oxide and carbide coatings (based on the examples of Al2O3, Cr3C2 and other powders). The analysis of possible parameters of the produced flow after the torch with intensifier was performed for the cases of main C–H–O–N–Ar–Me (Me = Al, Cr) systems and additional C–H–O–Al-system to assess the potential of this system to modify the technology of oxide and carbide ceramic coatings formation. New regimes, which were analyzed in our research as the simulants of Al2O3 spraying, surpass on calculated energy efficiency characteristics (by 10–20 %) one of the new prospective spraying methods with (СO2+СH4)-plasma, as well as the conventional method of powder heating during the spraying with N2-plasma. The case of our proposed fuel assisted process (FA-APS) with liquefied petroleum gas (LPG) fuel for the heating of ceramic powders (especially, Al2O3) demonstrates the advantage of the process (in particular, on the energy efficiencies and energy consumption) in a comparison with the conventional regimes of APS of the powders (in N2 plasma of the standard torch). For the variants of the FA-APS with Al2O3 and Cr3C2 feedstock powders it was established to be potentially possible to obtain (at the moderate values of total electric energy consumption for the torch and auxiliary equipment, – near 1.8 and 1.0 kWh/(kg of product)) such high level of the process productivity on the final product as approximately 17 and 28 kg/h, respectively; at the values of required power of the torch:  28.2 and  22.3 kW.

Interpretable data-driven methods for subgrid-scale closure in LES for transcritical LOX/GCH4 combustion

Although data-driven methods have shown high accuracy as closure models for simulating turbulent flames, these models are often criticized for lack of physical interpretability, wherein they provide answers but no insight into their underlying rationale. In this work, we show that two interpretable machine learning algorithms, namely the random forest regressor and the sparse symbolic regression, can offer opportunities for discovering analytic expressions for subgrid-scale (SGS) terms of a turbulent transcritical flame. These transcritical conditions are found in various combustion systems that operate under high pressures that surpass the thermodynamic critical limit of fuel-oxidizer mixtures, and require the consideration of complex fluid behaviors that can cast doubts on the validity of existing subgrid-scale (SGS) models in large-eddy simulations. To this end, direct numerical simulations (DNS) of transcritical liquid-oxygen/gaseous-methane (LOX/GCH4) inert and reacting flows are performed. Using this data, a priori analysis is performed on the Favre-filtered DNS data to compare the accuracy of random forest SGS-models with conventional physics-based SGS-models. SGS stresses calculated with the gradient model are shown to have good agreement with the exact terms extracted from filtered DNS. Results demonstrate that random forests can perform as effectively as algebraic models when modeling subgrid stresses, when trained on a sufficiently representative database and with a suitable choice of the feature set. The employment of the random forest feature importance score is shown to enable the discovery of a analytic model for subgrid-scale stresses through sparse symbolic regression. The generalizability of random forest and sparse symbolic regression is demonstrated by modeling the subgrid-scale temperature, a term that arises from filtering the non-linear real-fluid equation-of-state, with good accuracy.

Inorganic oxidizer detection from propellants, pyrotechnics, and homemade explosive powders using gradient elution moving boundary electrophoresis

Advancement in rapid targeted chemical analysis of homemade and improvised explosive devices is critical for the identification of explosives-based hazards and threats. Gradient elution moving boundary electrophoresis (GEMBE), a robust electrokinetic separation technique, was employed for the separation and detection of common inorganic oxidizers from frequently encountered fuel-oxidizer mixtures. The GEMBE system incorporated sample and run buffer reservoirs, a short capillary (5cm), an applied electric field, and a pressure-driven counterflow. GEMBE provided a separation format that allowed for continuous injection of sample, selectivity of analytes, and no sample cleanup or filtration prior to analysis. Nitrate, chlorate, and perchlorate oxidizers were successfully detected from low explosive propellants (e.g., black powders and black powder substitutes), pyrotechnics (e.g., flash powder), and tertiary explosive mixtures (e.g., ammonium nitrate- and potassium chlorate-based fuel-oxidizer mixtures). Separation of these mixtures exhibited detection without interference from a plethora of additional organic and inorganic fuels, enabled single particle analysis, and demonstrated semiquantitative capabilities. The bulk counterflow successfully excluded difficult components from fouling the capillary, yielding estimated limits of detection down to approximately 10μmol/L. Finally, nitrate was separated and detected from postblast debris collected and directly analyzed from two nitrate-based charges.

Investigation of Reduced Kinetics Mechanisms for Accurate LES and Scaling of the Dynamics of Premixed Swirling Oxy-Fuel Combustion

ABSTRACT In this paper, we examine the role of reduced chemical kinetics mechanisms in predicting the flow structure using multi-dimensional large-eddy simulations in complex geometries. We investigate the attributes of the kinetics mechanisms required to predict flow structures such as recirculation zones. Premixed methane-oxy combustion is modeled in a swirl-stabilized combustor. Results show that kinetic mechanisms that accurately predict the extinction strain rate of the underlying flames were able to predict the evolution of the flame macrostructure with equivalence ratio and the associated velocity profiles. The recirculation zone length was found to linearly scale with the extinction strain rate, in agreement with previous findings in different fuel-oxidizer mixtures and combustor geometry. The scaling holds regardless of fuel or oxidizer type, Reynold’s number or inlet temperature. Surprisingly the scaling also held well for two different combustor geometries.

Fuel-oxidizer mixtures: a lab and field study

ABSTRACTImprovised explosive may be as primitive as a fuel and oxidizer mixture; yet not all fuel-oxidizer mixtures are explosive. Predicting explosive potential from laboratory-scale tests is desirable to screen a large matrix of potential threats in varying concentrations. Herein the properties of various fuel/oxidizer mixtures were measured at small scale (2 g) with bomb calorimetry and on large scale (5 kg) with high speed photography for detonation velocity and with piezoelectric pressure probes for TNT air blast equivalence. Potassium nitrate (KN), potassium chlorate (KC), potassium permanganate (KMnO4), potassium iodate (KIO3), ammonium nitrate (AN), and ammonium perchlorate (AP) were prepared with sucrose (Su) and aluminum (Al) fuel. Results were compared to each other as well as predictions from Cheetah thermochemical code.

Forensic Analysis and Differentiation of Black Powder and Black Powder Substitute Chemical Signatures by Infrared Thermal Desorption-DART-MS.

The trace detection and forensic analysis of black powders and black powder substitutes, directly from wipe-based sample collections, was demonstrated using infrared thermal desorption (IRTD) coupled with direct analysis in real time mass spectrometry (DART-MS). Discrete 15 s heating ramps were generated, creating a thermal desorption profile that desorbed more volatile species (e.g., organic and semivolatile inorganic compounds) at lower temperatures (250-400 °C) and nonvolatile inorganic oxidizers at high temperatures (450-550 °C). Common inorganic components of black powders (e.g., sulfur and potassium nitrate) as well as the alternative and additional organic and inorganic components of common black powder substitutes (e.g., dicyandiamide, ascorbic acid, sodium benzoate, guanidine nitrate, and potassium perchlorate) were detected from polytetrafluoroethylene-coated fiberglass collection wipes with no additional sample preparation. IRTD-DART-MS enabled the direct detection of intact inorganic salt species as nitrate adducts (e.g., [KClO4+NO3]-) and larger clusters. The larger ion distributions generated by these complex mixtures were differentiated using principal component analysis (PCA) of the mass spectra generated at two points during the thermal desorption profile (low and high temperatures), as well as at high in-source collision-induced dissociation. The PCA framework generated by the analysis of the two black powders and five black powder substitutes was used to classify samples collected from a commercial firecracker containing both flash powder and black powder. The coupling of IRTD-DART-MS and multivariate statistics demonstrated the powerful utility for detection and discrimination of trace fuel-oxidizer mixtures.

Low Cost Image Processing of Bunsen Flame Photography for Estimation of Flame Speeds

ABSTRACTThe need for measuring laminar flame speeds for various fuels and blends is crucial for determining flame stabilization characteristics and design parameters for practical burner configurations. Various methodologies have been used over the years to estimate laminar flame speeds with axisymmetric Bunsen flames being one of the oldest and “simplest” techniques. Nonetheless, the cost of the instrumentation to measure laminar flame speeds with minimum experimental uncertainties remains noticeably high. The present work describes a low-cost flame speed estimation methodology and investigates its efficacy for methane and propane-air mixtures at different equivalence ratios. Flame propagation speeds with respect to the unburned gas were evaluated using both the cone angle and the surface area measurement methods. The experimental approach involves an automated image processing procedure that numerically processes plain photographic images of the flames and estimates the Bunsen cone borders, the cone angle, and the surface area in a straightforward manner. Those values are then used to extract the flame speed. Thus, it can be considered as an integrated experimental and numerical processing methodology that affords flame speed estimation, with reasonable accuracy, for a variety of gaseous fuel-oxidizer mixtures, relevant to industrial burner applications.

Measurements and kinetic modeling of atomic species in fuel-oxidizer mixtures excited by a repetitive nanosecond pulse discharge

This work presents the results of number density measurements of metastable Ar atoms and ground state H atoms in diluted mixtures of H2 and O2 with Ar, as well as ground state O atoms in diluted H2–O2–Ar, CH4–O2–Ar, C3H8–O2–Ar, and C2H4–O2–Ar mixtures excited by a repetitive nanosecond pulse discharge. The measurements have been made in a nanosecond pulse, double dielectric barrier discharge plasma sustained in a flow reactor between two plane electrodes encapsulated within dielectric material, at an initial temperature of 500 K and pressures ranging from 300 Torr to 700 Torr. Metastable Ar atom number density distribution in the afterglow is measured by tunable diode laser absorption spectroscopy, and used to characterize plasma uniformity. Temperature rise in the reacting flow is measured by Rayleigh scattering. H atom and O atom number densities are measured by two-photon absorption laser induced fluorescence. The results are compared with kinetic model predictions, showing good agreement, with the exception of extremely lean mixtures. O atoms and H atoms in the plasma are produced mainly during quenching of electronically excited Ar atoms generated by electron impact. In H2–Ar and O2–Ar mixtures, the atoms decay by three-body recombination. In H2–O2–Ar, CH4–O2–Ar, and C3H8–O2–Ar mixtures, O atoms decay in a reaction with OH, generated during H atom reaction with HO2, with the latter produced by three-body H atom recombination with O2. The net process of O atom decay is O + H → OH, such that the decay rate is controlled by the amount of H atoms produced in the discharge. In extra lean mixtures of propane and ethylene with O2–Ar the model underpredicts the O atom decay rate. At these conditions, when fuel is completely oxidized by the end of the discharge burst, the net process of O atom decay, O + O → O2, becomes nearly independent of H atom number density. Lack of agreement with the data at these conditions is likely due to diffusion of H atoms from the partially oxidized regions near the side walls of the reactor into the plasma. Although significant fractions of hydrogen and hydrocarbon fuels are oxidized by O atoms produced in the plasma, chain branching remains a minor effect at these relatively low temperature conditions.

Methane Ignition in a Shock Tube with High Levels of CO2 Dilution: Consideration of the Reflected-Shock Bifurcation

Experiments were performed in a shock-tube facility to examine experimentally the kinetic effect, if any, of excess amounts of CO2 as part of natural-gas-based fuel–oxidizer mixtures. An important aspect of these experiments was to also observe the role excess amounts of CO2 play in causing nonidealities, particularly shock bifurcation, in shock-tube experiments using real (nondilute) fuel–air mixtures. Mixtures were composed of methane fuel at an equivalence ratio of 0.5 to represent a typical natural gas in a modified “air” mixture designed to study the effect of large levels of CO2 dilution. These oxidizer compositions maintained constant levels of O2 while exchanging N2 for CO2 in stages to give oxidizer mixture concentrations ranging from (0.21O2 + 0.79N2) to (0.21O2 + 0.79CO2). Low-pressure and high-pressure (near 1 and 10 atm, respectively) experiments were conducted over an approximate temperature range of 1450 to 1900 K. Results showed that the observed effect of CO2 relating to reflected-shock b...

In-source collision induced dissociation of inorganic explosives for mass spectrometric signature detection and chemical imaging

The trace detection, bulk quantification, and chemical imaging of inorganic explosives and components was demonstrated utilizing in-source collision induced dissociation (CID) coupled with laser desorption/ionization mass spectrometry (LDI-MS). The incorporation of in-source CID provided direct control over the extent of adduct and cluster fragmentation as well as organic noise reduction for the enhanced detection of both the elemental and molecular ion signatures of fuel-oxidizer mixtures and other inorganic components of explosive devices. Investigation of oxidizer molecular anions, specifically, nitrates, chlorates, and perchlorates, identified that the optimal in-source CID existed at the transition between fragmentation of the ionic salt bonds and molecular anion bonds. The chemical imaging of oxidizer particles from latent fingerprints was demonstrated, including both cation and anion components in positive and negative mode mass spectrometry, respectively. This investigation demonstrated LDI-MS with in-source CID as a versatile tool for security fields, as well as environmental monitoring and nuclear safeguards, facilitating the detection of elemental and molecular inorganic compounds at nanogram levels.

Fuel–oxidizer mixtures: their stabilities and burn characteristics

A survey of the stability and performance of eleven solid oxidizers and thirteen fuels was performed by differential scanning calorimetry, simultaneous differential thermolysis, and hot-wire ignition. Sugars, alcohols, hydrocarbons, benzoic acid, sulfur, charcoal, and aluminum were used as fuels; all fuels except charcoal and aluminum melted at or below 200 °C. It was found that the reaction between the oxidizer and the fuel was usually triggered by a thermal event, i.e., melt, phase change, or decomposition. Although the fuel usually underwent such a transition at a lower temperature than the oxidizer, the phase change of the fuel was not always the triggering event. When sugars or sulfur were the fuels, their phase change usually triggered their oxidation. However, three oxidizers, KNO3, KClO4, and NH4ClO4, tended to react only after they underwent a phase change or began to decompose, which meant that their oxidization reaction, regardless of the fuel, was usually above 400 °C. KClO4–fuel mixtures decomposed at the highest temperatures, often over 500 °C, with the ammonium salt decomposing almost 100 °C lower. Mixtures with ammonium nitrate (AN) also decomposed at much lower temperatures than those with the corresponding potassium salt. With the exception of the oxidizers triggered to react by the phase changes of the polyols and sulfur, the oxidizer–fuel mixtures generally decomposed between 230 and 300 °C, with AN formulations generally decomposing at the lowest temperature. In terms of heat release, potassium dichromate–fuel mixtures were the least energetic, generally releasing less than 200 J g−1. Most of the mixtures released 1000–1500 J g−1, with potassium chlorate, ammonium perchlorate, and AN releasing significantly more heat, around 2000 J g−1. When the fuel was aluminum, most of the oxidizers decomposed below 500 °C leaving the aluminum to oxidize at over 800 °C. Only two oxidizers reduced the temperature of the aluminum exotherm—chlorate and potassium nitrite. To go to temperatures above 500 °C, unsealed crucibles were necessary, and with these containers, the endothermic volatilization of reactants and products effectively competed against the exothermic decomposition so that heat release values were artificially low.

Prediction of flammability limits of fuel-air and fuel-air-inert mixtures from explosivity parameters in closed vessels

Flammability limits of fuel-air and fuel-air-inert gaseous mixtures, especially at non-atmospheric conditions, are essential properties required for establishing safety operating conditions for handling and processing flammable gases. For pure fuels, an important data pool exists, formed by the flammability limits of fuel-air and fuel-air-inert gaseous mixtures at ambient initial conditions measured by standard methods. Such methods can be used for experimental determination of flammability limits for multi-fuels mixed with air, with or without additives, under non-atmospheric conditions. Their use is however a time- and material-consuming process; in addition, the flammability limits obtained by various standard methods may be scattered as a result of different choices in the operating parameters, for each standard method. It appears that a preliminary estimation of the flammability limits for fuel-air and fuel-air-inert gaseous mixtures can minimize the effort of measuring them in specific initial conditions.The present paper describes a new method for estimating the flammability range of fuel-oxidizer gaseous mixtures based on measurements of explosivity properties e.g. the peak explosion pressure and maximum rate of pressure rise recorded during closed vessel laminar explosions of fuel-oxidizer mixtures far from limits. Data obtained for several hydrocarbon-air gaseous mixtures with or without inert gas addition are used to examine the accuracy of estimated flammability limits (LFL – the lower and UFL – the upper flammability limit) as well as of the Limiting Oxygen Concentration (LOC) and the Minimum Inert Concentration (MIC). The predictive ability of the proposed method is examined against the predictive ability of other recently described methods.

Numerical Simulations of Flat Laminar Premixed Methane-Air Flames at Elevated Pressure

Two-dimensional axisymmetric simulation of stoichiometric methane-air flames stabilized on flat burners at elevated pressure is reported in the present work. Such flames, in practice, are experimentally obtained using the heat flux method for measurement of laminar burning velocity of fuel-oxidizer mixtures (Bosschaart and de Goey, 2004; Goswami et al., 2013). The method makes use of a burner with a perforated brass burner plate. The dimensions of such a plate play an important role in creating flat flames. The present investigation is focused on studying laminar premixed flame structure numerically at elevated pressure up to 15 bar using a one-step and a detailed chemical reaction mechanism. Three burner plate models (of varying hole diameter and porosity) are used in the simulations for pressures up to 7 bar with a one-step mechanism. The surface area increase of the flame was evaluated based on an isotherm at 900 K and the net reaction rate of methane compared to a flat flame. The comparison of these models shows that the surface area increase can significantly be reduced by choosing a smaller hole diameter and larger porosity. The results of the detailed simulations using an appropriate chemical reaction mechanism up to 15 bar using a burner plate model, which is similar to the ones used in experiments (mentioned above), show a nonlinear increase of the flame curvature with elevating pressure. A hole diameter of 0.25 mm and a pitch of 0.29 mm is suggested for a burner plate in such experiments. Flame structure at elevated pressure is also analyzed further based on species profiles obtained.

Laminar burning velocity of lean H2–CO mixtures at elevated pressure using the heat flux method

Laminar burning velocity measurements of 50:50 and 85:15% (by volume) H2–CO mixtures with O2–N2 and O2–He oxidizers were performed at lean conditions (equivalence ratio from 0.5 to 1) and elevated pressures (1 atm–9 atm). The heat flux method (HFM) is employed for determining the laminar burning velocity of the fuel–oxidizer mixtures. HFM creates a one-dimensional adiabatic stretchless flame which is an important prerequisite in defining the laminar burning velocity. This technique is based on balancing the heat loss from the flame to the burner with heat gain to the unburnt gas mixture, in a very simple way, such that no net heat loss to the burner is obtained. Instabilities are observed in lean H2–CO flames with nitrogen as the bath gas for pressures above 4 atm. Stable flames are obtained with helium as the bath gas for the entire pressure range. With the aim to cater stringent conditions for combustion systems such as gas turbines, an updated H2–CO kinetic mechanism is proposed and validated against experimental results. The scheme was updated with recent rate constants proposed in literature to suit both atmospheric and elevated pressures. The proposed kinetic model agrees with new experimental results. At conditions of high pressure and lean combustion, reactions H + O2 = OH + O and H + O2 (+M) = H2 (+M) compete the most when compared to other reactions. Reaction H + HO2 = OH + OH contributes to OH production, however, less at high-pressure conditions. At higher CO concentrations and leaner mixtures an important role of reaction CO + OH = CO2 + H is observed in the oxidation of CO.

Simulations of Nanosecond Pulsed Plasmas in Supersonic Flows for Combustion Applications

Nanosecond pulse plasma discharges have demonstrated the ability to significantly reduce ignition delay time in the combustion of supersonic fuel–oxidizer mixtures. Although it is generally believed that such plasmas enhance combustion through fast production of radicals, there is still uncertainty over the detailed kinetics that take place during the pulse, as well as the importance of other mechanisms such as gas heating. In this work, we develop a computational model for study of nanosecond pulse discharges interacting with supersonic oxygen/hydrogen (O2/H2) flows and pure-argon flows. Both positive (anodic) and negative (cathodic) pulse polarities are investigated at voltages ranging from4 to 8 kV.Results indicate that the plasma develops asfilamentary streamers, withO radical densities of 10 m 3 ( 0:5% by volume of the mixture) in addition to other important radicals such as H, OH, O D , O2 a1 g , and O2 b1 g . Gas heating is most intense near the electrode edges, due to ion Joule heating densities of 10 W=m, with a temperature increase of approximately hundreds of kelvins. Gas heating due to quenching of ion andmetastable species also occurswithin the bulk of the plasma, but is about an order ofmagnitude less than ion Joule heating.Although gas heating is observedwithin thebulk plasma for allO2/H2 pulses, heating only takes place at the electrode edges for the argon anodic pulse and no heating is observed in the argon cathodic pulse. Increasing voltages increases plasma volume, peak species densities, and peak gas temperatures. The anodic pulse streamers produce radicals over a greater volume and deeper into the flow than cathodic pulses, which suggests that anodic pulses are better suited to combustion applications.