Distribution Of OH Research Articles

Development of a simplified n-heptane/methane model for high-pressure direct-injection natural gas marine engines

High-pressure direct-injection (HPDI) of natural gas is one of the most promising solutions for future ship engines, in which the combustion process is mainly controlled by the chemical kinetics. However, the employment of detailed chemical models for the multi-dimensional combustion simulation is significantly expensive due to the large scale of the marine engine. In the present paper, a reduced n-heptane/methane model consisting of 35-step reactions was constructed using multiple reduction approaches. Then this model was further reduced to include only 27 reactions by utilizing the HyChem (Hybrid Chemistry) method. An overall good agreement with the experimentally measured ignition delay data of both n-heptane and methane for these two reduced models was achieved and reasonable predictions for the measured laminar flame speeds were obtained for the 35-step model. But the 27-step model cannot predict the laminar flame speed very well. In addition, these two reduced models were both able to reproduce the experimentally measured in-cylinder pressure and heat release rate profiles for a HPDI natural gas marine engine, the highest error of predicted combustion phase being 6.5%. However, the engine-out CO emission was over-predicted and the highest error of predicted NOx emission was less than 12.9%. The predicted distributions of temperature and equivalence ratio by the 35-step and 27-step models are similar to those of the 334-step model. However, the predicted distributions of OH and CH2O are significantly different from those of the 334-step model. In short, the reduced chemical kinetic models developed provide a high-efficient and dependable method to simulate the characteristics of combustion and emissions in HPDI natural gas marine engines.

Effect of the stagnation plate on PAHs, soot and OH distributions in partially premixed laminar flames fueled with a blend of n-heptane and toluene

The stagnant plate influence on spatial distributions of polycyclic aromatic hydrocarbons (PAHs), soot and hydroxyl radical (OH) in the surrogate fuel (80% n-heptane and 20% toluene in volume) partially premixed flame on a McKenna burner were originally investigated in terms of different plate heights above the burner (Hp) and plate temperatures (Tp). Both PAHs and OH distributions were qualitatively measured by laser-induced fluorescence (LIF) and soot volume fraction field was quantitatively measured by two-color laser-induced incandescence (2C-LII). It is found that the LIF intensity of 2&3-ring aromatics (A2&A3) and 4-ring aromatics (A4) and soot volume fraction firstly increase and then reduce when the Hp is set from 35 to 10 mm. The variation tendency transition happens between Hp = 20 and 25 mm. In contrary, OH-LIF intensity gradually decreases as the Hp decreases. On the other hand, the Tp has an opposite effect on the A2&A3-LIF, A4-LIF and soot volume fraction variation tendency at two Hp. At Hp = 15 mm, A2&A3-LIF, A4-LIF and soot volume fraction increase as the Tp increases. However, A2&A3-LIF, A4-LIF and soot volume fraction decrease with the rise of the Tp at Hp = 35 mm. Interestingly, OH-LIF intensity always increases as the Tp rises at any Hp. Therefore, it can be concluded that the PAHs-LIF intensity and soot volume fraction depend on the Hp and Tp, and the maximums of them would be reached at the specific Hp and Tp. While both the Hp and Tp have a monotonic effect on the OH-LIF intensity.

Combustion for aircraft propulsion: Progress in advanced laser-based diagnostics on high-pressure kerosene/air flames produced with low-NOx fuel injection systems

The challenges in designing high-performance aeronautical combustion systems have not changed significantly over the years, but the increase of stringent regulations and the need to tackle rising fuel prices require new sophisticated analysis processes. To address this concern, the theme of this study was cast in terms of the ability of improving the performances of advanced non-intrusive instrumentation in high-pressure turbulent reactive two-phase flows to enhance the combustion efficiency and reduction of pollutants of new innovative low-NOx injection systems fed with liquid kerosene. In the first part, a Lean Premixed injection system designed for helicopter engines was first studied using the Particle Image Velocimetry (PIV), Phase Doppler Anemometry (PDA) and Planar laser-induced fluorescence (PLIF) of OH, kerosene vapor and NO. These diagnostics were applied individually or in combination for custom-made solutions for accessing to detailed information on the spatial distributions of velocity, droplets size, OH, kerosene and NO molar fractions in pressure conditions ranging from 0.4 to 1.8 MPa. Complementary large-Eddy simulations of these flames were performed using the PCM-FPI tabulated chemistry approach in conjunction with a polydisperse Euler–Lagrangian approach. Comparison of simulations with experiments highlighted the importance of the size distribution of the droplets injected into the flow on the mutual predictions of the aerodynamic field, the structure of the flame and its anchorage point as well as the spatial distribution of the fuel and OH species. The second part of this work was aimed to study the advantages and limitations of the chirped probe pulse femtosecond coherent anti-Stokes Raman scattering (CPP fs-CARS) diagnostic for performing time-series of single-shot temperature measurements on an aircraft multi-point injection system developing two-phase flames under realistic pressure conditions.

Quasi-classical trajectory investigation of H + SO2 → OH + SO reaction on full-dimensional accurate potential energy surface

The reaction H+SO2→OH+SO is important in the combustion and atmospheric chemistry, as well as the interstellar medium. It also represents a typical complex-forming reaction with deep complexes, serving as an ideal candidate for testing various kinetics theories and providing interesting reaction dynamical phenomena. In this work, we reported a quasi-classical trajectory study of this reaction on our previously developed accurate full-dimensional potential energy surface. The experimental thermal rate coefficients over the temperature range 1400 K≤T≤2200 K were well reproduced. For the reactant SO2 being sampled at the ground ro-vibrational state, the calculated integral cross sections increased slightly along the collision energy ranging from 31.0 kcal/mol to 40.0 kcal/mol, and then became essentially flat at the collision energy within 40.0−55.0 kcal/mol. The product angular distributions are almost symmetric with nearly identical backward-forward double peak structure. The products OH and SO vibrational state distributions were also analyzed.

Lean Blow-Off Scaling of Turbulent Premixed Bluff-Body Flames of Vaporized Liquid Fuels

The lean blow-off (LBO) limits and structure of turbulent premixed flames were investigated with vaporized liquid fuels stabilized by a bluff-body burner. Ethanol, heptane, and two kerosenes were used. To facilitate comparisons to gaseous-fueled flames, results were also obtained from methane flames. The measured LBO limits indicate that, for this burner, the ethanol and heptane flames are more resilient to blow-off than the kerosene fuels. Furthermore, a correlation based on a Damköhler number (), which is proportional to the laminar flame speed, does not lead to the successful collapse of the different fuels, indicating that the correlations based on laminar flame speed are not applicable. Average OH* chemiluminescence images of the ethanol and heptane flames are qualitatively similar to that from methane: the flame brushes of both exhibit an M shape when close to blow-off. In contrast, the distribution of OH* signal in the kerosene flames is primarily concentrated in regions further downstream of the bluff body. Ultimately, the results of this effort highlight the influence fuel type has on the LBO of bluff-body stabilized flames. Moreover, this work indicates the LBO behavior of flames produced with complex hydrocarbon fuels cannot be fully understood via high-temperature chemistry concepts such as the laminar flame speed.

On the use of PIV, LII, PAH-PLIF and OH-PLIF for the study of soot formation and flame structure in a swirl stratified premixed ethylene/air flame

Soot formation and oxidation are investigated in swirl stratified premixed ethylene/air flames at atmospheric pressure. The effects of both swirl and stratification are studied to understand the relationship between the flame structure, soot precursors and soot. The topology of the flame is obtained with particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) on hydroxyl radical (OH). The production of polycyclic aromatic hydrocarbons (PAHs) is investigated using PLIF by mainly probing the aromatic compounds with two benzene rings (i.e. naphthalene) that are known to actively participate in soot nucleation and growth. Soot production is investigated using laser-induced incandescence (LII), giving quantitative data on the soot volume fraction. Extensive information on the flame structure and the mechanisms of formation/consumption of soot is gathered based on the coupling of these laser diagnostics. An image analysis of velocity, OH, PAHs and soot distributions enables us to propose a scenario that describes the link between the inception, growth, aggregation and oxidation processes. In particular, the data reveal the presence of distinct zones for these processes: a thermal decomposition region in which PAHs contributes to nascent soot formation, organised as filaments along the interface between the PAH region and the inner recirculation zone (IRZ); a mixing region controlled by large moving structures that favour the growth and aggregation of nascent soot into mature soot; and an oxidation region leading to the fast consumption of soot particles. These processes are impacted to varying extents by the intensities of swirl and stratification.

Simultaneous imaging of H and OH in flames using a single broadband femtosecond laser source

Atomic hydrogen (H) and hydroxyl radical (OH) are two key intermediate species in the combustion of hydrocarbon fuels. For example, the spatial and temporal distribution of H is useful to better understand the ignition/extinction processes, whereas OH distribution profiles can yield flame structure and reaction zone in different flames conditions. In the present study, simultaneous laser-induced fluorescence (LIF) measurements of H and OH are obtained using a single ultrashort femtosecond-duration laser pulse near λ=307.7 nm for 3pLIF of H. Measurements are performed in CH4-air and C2H4-air flames stabilized over the Hencken burner. Recently developed three-photon LIF (3pLIF) technique is used for H, whereas the same excitation laser pulse can simultaneously excite OH A2Ʃ+←X2Π (0,0) transition. Red-shifting the excitation wavelength to 307.7 nm as opposed to commonly used two-photon excitation using 205 nm deep-ultra-violet (DUV) laser pulses enables the reduction of absorption losses of excitation laser pulse energy when transmitting through thick optical windows of high-pressure vessels, for example. Furthermore, the efficiency of the fs wavelength generation scheme of commercial optical parametric devices is approximately a factor of two higher near 307.7 nm as compared to 292 nm for 3pLIF of H. In this study, H and OH number densities have been reported for a range of flame equivalence ratios in CH4/air and C2H4/air flames established over the Hencken burner. Simultaneous imaging of H and OH in the premixed Bunsen flame is presented. Prospective improvements for increasing the signal-to-noise and hence potential kHz-rate simultaneous single-shot imaging are also discussed.

Oxy-fuel combustion of heavy oil based on OH - PLIF technique

Oxy-fuel combustion of heavy oil can be applied to oil field steam injection boilers, allowing the utilization of both heavy oil and CO2 resources. This paper studied the local distribution characteristics of OH on oxy-fuel combustion of heavy oil during the ignition and stable combustion processes. During the ignition process, we observed the generation and evolution of fire kernel, and got the flame propagation velocity. During the stable combustion process, the results showed that the OH distribution and its relative signal intensity were influenced by the oxygen concentration, excess air coefficient, gas flow, reaction atmosphere, oil mist scattering, incident laser energy and laser sheet position. In the same reaction atmosphere, the ranges of OH dense distribution and the high temperature area increased as O2 concentration increased. In the same O2 concentration, both the ranges of OH dense distribution and the high temperature area in O2/N2 were larger than that in O2/CO2. In 29% O2/71% CO2, the flame shape was similar to combust in air, while the OH relative signal intensity and its volatility were much larger than that in air. In the same combustion condition, the location of high concentration of OH relative concentration field lagged behind the high temperature area. The results further reveal the differences between the conventional and oxy-fuel combustion.

Solubility of hydroxyl groups in pyroxenes: Effect of oxygen fugacity at 0.2–3 GPa and 800–1200 °C

Clinopyroxene and orthopyroxene are important constituents of the upper mantle, and determine largely the water budget. Despite considerable efforts on water storage in pyroxenes, however, less attention has been paid to the effect of oxygen fugacity, a key thermodynamic factor in Earth sciences. We have systematically assessed the effect of oxygen fugacity on OH dissolution in pyroxenes, by conducting high-pressure and temperature experiments over a wide range of conditions. The runs were carried out at 0.2–3 GPa, 800–1200 °C and peridotite- and H2O-saturated conditions, with gem-quality natural diopside and enstatite single crystals and a fresh peridotite xenolith as the starting materials. The oxygen fugacities were buffered by Fe-FeO, Ni-NiO, Re-ReO2 and Fe3O4-Fe2O3 pairs, by using a modified double-capsule technique. The water contents of the recovered pyroxenes were determined by polarized analyses with a Fourier-transform infrared spectroscopy.The H-annealed pyroxenes show typical OH-related bands. The samples show no zoned OH distribution at sub-grain scales, and the OH contents are independent of run durations, suggesting the attainments of equilibrium OH incorporation. For each starting mineral, the spectral shapes and OH frequency positions are similar throughout the run conditions. The measured H2O contents are ∼14–394 and 34–227 ppm in the H-annealed clinopyroxene and orthopyroxene, respectively. For each mineral under otherwise identical conditions, the OH content increases with increasing pressure or temperature but decreases with increasing oxygen fugacity. In clinopyroxene, the ratio of either the peak heights or integrated absorbances between the bands at ∼3645 and 3435 cm−1 is less sensitive to a change of pressure, particularly above 1 GPa, but is more sensitive to a change of temperature, if the redox state is maintained as in the upper mantle. The OH groups in the spectra of clinopyroxene can be used to estimate the redox state in the source regions where the sample was finally equilibrated, if the equilibrium temperature is determined by other approach(s). On average, the OH content is enhanced by ∼40% and 60% in Fe-poor clinopyroxene and orthopyroxene, respectively, at Fe-FeO buffered reducing than at Ni-NiO buffered oxidizing conditions. This enhancement appears greater if the pyroxenes are more Fe-enriched, likely due to the enhanced reduction of Fe3+ to Fe2+ and the coupled incorporation of H in the structure toward more reducing conditions. The quantitative dependence of OH solubility on oxygen fugacity differs between pyroxenes and olivine (and other nominally anhydrous minerals), and the partitioning of OH between pyroxenes and coexisting olivine and the inventory of OH in the upper mantle are influenced by oxygen fugacity. The OH storage capacity of the shallow mantle is probably greater in Mars than in the Earth, because of the relatively more reduced state and more Fe-enriched pyroxenes in the former. Finally, the enhanced solubility of OH in pyroxenes under very reducing conditions could be important for insights into the early differentiation of Earth’s shallow mantle, in that hydrous melting may have been induced by redox-involved dehydration of pyroxenes when the shallow mantle evolved into its modern oxidizing state.

Distributed combustion mode in a can-type gas turbine combustor – A numerical and experimental study

A novel combustor design assisted with increased hot air jet momentum is used to enhance the internal recirculation. Geometric modifications are incorporated in terms of altered air injection scheme to improve the mixing and internal recirculation of hot combustion products. The combustor operates with liquefied petroleum gas in a distributed combustion mode, and thermal intensities vary in the range of 20–40 MWm−3atm−1. Numerical calculations of the reacting flow field show the presence of large recirculation zones, and they are quantified using reactant dilution ratio, Rdil. It is greater than 2.5 for the most part of the combustor, thereby creating a sufficiently heated and diluted environment for sustaining the distributed combustion at high thermal intensities. OH* chemiluminescence studies are carried out for the first time to observe the combustion zone structure for the conventional and distributed combustion regime in a gas turbine combustor. The distribution of OH* becomes increasingly uniform with reduced maximum OH* intensity in this combustion mode. The measured NOx is less than 8 ppm, and CO emission is below 25 ppm for all the operating conditions (ϕ = 0.2–1). Acoustic emissions drop significantly, as the combustor switches its operation to a distributed combustion mode. The novelty of this study lies in the application of this combustion mode in a gas turbine engine. This article also sheds light on the importance of the internal recirculation of combustion products, to achieve a distributed combustion mode in a generic can combustor at gas turbine relevant conditions.

Quantitative evaluation of wall chemical effect in hydrogen flame using two-photon absorption LIF

A premixed H2/air flame with N2 dilution (Uin = 533 cm/s, ϕ = 1) was formed in a quartz micro flow reactor with/without a 100-nm thick Inconel coating for the investigation of wall chemical effect of the metal surface. Two-dimensional distributions of OH radical, O atom and H atom in the hydrogen flame were measured via the planar laser induced fluorescence (PLIF) and two-photon absorption laser induced fluorescence (TALIF), respectively. It is found that the distributions of all these three main species in the hydrogen flame are significantly affected by the wall chemical effect. OH, O and H shift downstream in the Inconel-coated channel, and also their concentration becomes lower than those in the less-reactive quartz channel. Based on the measured distributions of OH, O and H over the Inconel surface, the initial sticking coefficients (S0) of the radical quenching model are optimized. It is found that S0 for Inconel are 0.4–0.5, 0.1–0.2 and < 0.05 for OH, O and H, respectively, showing different sticking coefficients for different species for the first time.

Quantitative Measurement of OH* and CH* Chemiluminescence in Jet Diffusion Flames.

Quantitative measurement of chemiluminescence is a challenging work that limits the development of combustion diagnostics based on chemiluminescence. Here, we present a feasible method to obtain effective quantitative chemiluminescence data with an integrating sphere uniform light source. Spatial distribution images of OH* and CH* radiation from methane laminar diffusion flames were acquired using intensified charge-coupled device (CCD) cameras coupled with multiple lenses and narrow-band-pass filters. After the process of eliminating background emissions by three filters and the Abel inverse transformation, the chemiluminescence intensity was converted to a radiating rate based on the uniform light source. The simulated distributions of OH* and CH* agree well with the experimental results. It has also been found that the distribution of OH* is more extensive and closer to the flame front than that of CH*, demonstrating that OH* is more representative of the flame structure. Based on the change in the reaction rate of different formation reactions, OH* distributions can be divided into three regions: intense section near the nozzle, transition section in the middle of the flame, and secondary section downstream the flame, whereas CH* only exists in the first two regions. In addition, as the velocity ratio of methane and co-flowing air increases, the main reactions become more intense, while the secondary reaction of OH* becomes weaker.

Experimental studies of the effects of global equivalence ratio and CO2 dilution level on the OH* and CH* chemiluminescence in CH4/O2 diffusion flames

Focusing on the chemiluminescent properties of the flame is an effective way to study the flame structure and reaction mechanism. In this paper, based on the CCD camera imaging system, the chemiluminescent distributions of OH* and CH* in CH4/O2 diffusion flames with different global equivalence ratios and CO2 dilution levels were obtained. The changes in the reaction zones of OH* layer and CH* layer were further compared. The peak intensity ratios of OH* and CH* were used to quantitatively characterize the global equivalence ratio and the CO2 dilution level. The results show that variations in the global equivalence ratio and the CO2 dilution level do not obviously alter the shape and thickness of the CH* layer. The distribution and intensity changes of the OH* layer are much more significant than those of the CH* layer when the experimental conditions are varied. For the flame with a low dilution level, the CH* layer is located on the inner rim of the OH* layer, and the radial positions of two peaks are not much different. The increase of the CO2 dilution level leads to the displacement of the OH* peak, which becomes closer to the flame axis. In addition, due to the narrowing of the OH* layer, the CH* layer eventually becomes at the outer rim of the OH* layer. The peak intensity ratio of OH* and CH* decreases linearly with the rise of the global equivalence ratio, and changes as a power function with the increasing CO2 dilution level.

Two-Dimensional Tomographic Simultaneous Multi-Species Visualization—Part I: Experimental Methodology and Application to Laminar and Turbulent Flames

In recent years, the tomographic visualization of laminar and turbulent flames has received much attention due to the possibility of observing combustion processes on-line and with high temporal resolution. In most cases, either the spectrally non-resolved flame luminescence or the chemiluminescence of a single species is detected and used for the tomographic reconstruction. In this work, we present a novel 2D emission tomographic setup that allows for the simultaneous detection of multiple species (e.g., OH*, CH* and soot but not limited to these) using a single image intensified CCD camera. We demonstrate the simultaneous detection of OH* (310 nm), CH* (430 nm) and soot (750 nm) in laminar methane/air, as well as turbulent methane/air and ethylene/air diffusion flames. As expected, the reconstructed distributions of OH* and CH* in laminar and turbulent flames are highly correlated, which supports the feasibility of tomographic measurements on these kinds of flames and at timescales down to about 1 ms. In addition, the possibilities and limitations of the tomographic approach to distinguish between locally premixed, partially premixed and non-premixed conditions, based on evaluating the local intensity ratio of OH* and CH* is investigated. While the tomographic measurements allow a qualitative classification of the combustion conditions, a quantitative interpretation of instantaneous reconstructed intensities (single shot results) has a much greater uncertainty.

Effect of titanium surface topography on plasma deposition of antibacterial polymer coatings

Plasma processing, e.g., functionalisation and deposition of antibacterial coatings, is often used to enhance surface properties of biomaterials. Plasma is, however, a non-uniform active medium, and the result of processing depends on the nature of both the plasma and the substratum. Here we show that when an antibacterial coating (i.e., polyterpenol) is plasma polymerised onto four types of titanium substrata that differ in their micro- and nano-scale topography (but not the bulk chemistry), the distribution of functional groups, e.g., OH and CO, in the polymer across the surface differs sufficiently, and so does the antibacterial activity of the resulting material system. While the addition of a coating hinders biofilm formation by Staphylococcus aureus and Pseudomonas aeruginosa, the bactericidal effect is significantly stronger in polymers deposited onto surfaces possessing lower degrees of nanoscale roughness, e.g., substrata after mechanical and chemical polishing. The reduced antibacterial efficacy of polymers on substrata with greater surface roughness (e.g., on mechanically polished or lotus leaf-like surfaces) is attributed to a greater extent of thickness non-uniformity and heterogeneity in the functional group distribution across the surface. These findings suggest that the magnitude and distribution of topographical features of the substratum should be considered when designing plasma-enabled surface modification strategies.

Efficient degradation of Fipronil in water by microwave-induced argon plasma: Mechanism and degradation pathways

Fipronil and its metabolites are potentially harmful to the ecological environment and have chronic neurotoxic effects, which makes it to be classified as class C carcinogens. Fipronil has been banned from agricultural use in China since 2009, but its residue remains in the environment. Therefore, an efficient and economical method is urgently needed to degrade fipronil residues in the environment. Herein, the degradation of fipronil in water solution using argon microwave-induced plasma (MIP) system was studied and a plausible reaction pathway was proposed in combination with Density Functional Theory (DFT) calculations. The degradation of fipronil by MIP system was optimized in terms of input power, plasma-sample distance, initial concentration and gas flow rate. After short time MIP treatment with an input power of 150 W, as high as 85.62% degradation efficiency was achieved for the fipronil at concentration of 20 mg·L‐−1 under the optimized conditions, and the corresponding energy efficiency was 1334.8 mg·kwh−1. Optical emission spectrometry (OES) was employed to characterize the distribution and intensity of OH, H and O species which play key roles in the degradation of fipronil by plasma, and it revealed that the degradation reaction mainly occurs at gas-liquid interface where the highest intensity of OH, H and O species was observed. High resolution mass spectrometric analysis in combination quantum chemical calculations indicate that a wide diversity of reaction processes occurred for fipronil degradation under MIP treatment, involving oxidation or reduction, nitro reduction, oxidative dichlorination, reductive dichlorination, hydration, dehydration and thiourea to urea. The possible degradation mechanism and pathways were proposed based on the degrading species identified by high resolution Mass Spectrometry (HRMS) and the thermodynamic profiles.

Kinetics of the OH + NO2 reaction: effect of water vapour and new parameterization for global modelling

Abstract. The effect of water vapour on the rate coefficient for the atmospherically important, termolecular reaction between OH and NO2 was determined in He–H2O (277, 291, and 332 K) and N2–H2O bath gases (292 K). Combining pulsed-laser photolytic generation of OH and its detection by laser-induced fluorescence (PLP-LIF) with in situ, optical measurement of both NO2 and H2O, we were able to show that (in contrast to previous investigations) the presence of H2O increases the rate coefficient significantly. We derive a rate coefficient for H2O bath gas at the low-pressure limit (k0H2O) of 15.9×10-30 cm6 molecule−2 s−1. This indicates that H2O is a more efficient collisional quencher (by a factor of ≈6) of the initially formed HO–NO2 association complex than N2, and it is a factor of ≈8 more efficient than O2. Ignoring the effect of water vapour will lead to an underestimation of the rate coefficient by up to 15 %, e.g. in the tropical boundary layer. Combining the new experimental results from this study with those from our previous paper in which we report rate coefficients obtained in N2 and O2 bath gases (Amedro et al., 2019), we derive a new parameterization for atmospheric modelling of the OH + NO2 reaction and use this in a chemical transport model (EMAC) to examine the impact of the new data on the global distribution of NO2, HNO3, and OH. Use of the new parameters (rather than those given in the IUPAC and NASA evaluations) results in significant changes in the HNO3∕NO2 ratio and NOx concentrations (the sign of which depends on which evaluation is used as reference). The model predicts the presence of HOONO (formed along with HNO3 in the title reaction) in concentrations similar to those of HO2NO2 at the tropical tropopause.

A Numerical and Experimental Study on the Effects of CO2 on Laminar Diffusion Methane/Air Flames

AbstractFlue gas recirculation (FGR) is an effective method to reduce NOx emissions from the combustion of fuels, such as natural gas. Nitrogen, carbon dioxide, and water are the main components of flue gas. Nitrogen is an inert gas, and water can be condensed out of the effluent before FGR. However, recycled CO2 can alter the physical and chemical combustion characteristics of a fuel. This research investigated the effects of CO2 on CH4/air laminar diffusion flames, both experimentally and numerically. Experiments used laser-induced fluorescence to measure OH and CH distributions in the resulting flames, at different CO2 concentrations. Numerical methods were used to investigate the reaction mechanism and predict temperature and species concentration fields, as well as the NOx formation. Experiments showed that the CH fluorescence intensities decreased with the addition of CO2, while the OH fluorescence intensities increased. Both the directed relation graph method and the sensitivity analysis method were used to reduce the GRI-mech 3.0 mechanism. The chemical kinetics of methane combustion were analyzed using the reduced mechanism with the diffusion opposed-flow flame model in the chemkin 4.1 software package to determine the main reactions among the major species. Numerical simulations showed that as the amount of CO2 in the fuel increased, the concentration of CH decreased. These CFD simulations using the reduced mechanism were in agreement with the experimental data. Thus, the reduced mechanism was then used to predict NO concentrations. Numerical simulations showed that as the amount of CO2 in the fuel increased, the concentration of CH decreased, and, as a result, lower amounts of NO were predicted.

Simulation-guided nanofabrication of high-quality practical tungsten probes.

Micro/nanoscale tungsten probes are widely utilized in the fields of surface analysis, biological engineering, etc. amongst several others. This work performs comprehensive dynamic simulations on the influences of electric field distribution, surface tension and the bubbling situation on electrochemical etching behaviors, and then the tip dimension. Results show that the etching rate is reliant on the electric field distribution determined by the cathode dimension. The necking position lies in the meniscus rather than at the bottom of the meniscus. A bubble-free condition is mandatory to stabilize the distribution of OH− and WO42− ions for a smooth tungsten probe surface. Such simulation-guidance enables the nanofabrication of probes with a high aspect ratio (10 : 1), ultra-sharp tip apex (40 nm) and ultra-smooth surface. These probes have been successfully developed for high-performance application with Scanning Tunneling Microscopy (STM). The acquired decent atomic resolution images of epitaxial bilayer graphene robustly verify the feasibility of the practical level application of these nanoscale probes. Therefore, these nanoscale probes would be of great benefit to the development of advanced analytical science and nano-to-atomic scale experimental science and technology.

Numerical Study on the Morphology of a Re-Ignited Laminar Partially Premixed Flame with a Co-Axial Pilot Flame

Re-ignited partially premixed flame (PPF) is a quite extensive flame type in real applications, which is directly relevant to the local and global extinction and re-ignition phenomenon. The authors designed a model burner to establish laminar re-ignited PPFs. Numerical simulations were carried out to reveal the morphology of laminar re-ignited PPF. Based on the distributions of temperature, heat release and radicals, the morphologies of re-ignited flames were explored. W-shaped flames were formed under pilot-lean conditions. Line-shaped and y-shaped flames were formed under pilot-rich conditions. Both w-shaped and y-shaped flames had a triple-flame structure. The re-ignited flames can stand beyond the rich flammability limit. Additionally, OH distributions indicated both pilot flame and re-ignited flame well as it rapidly increased near the flame front. OH concentration did not increase visibly while CH2O concentration mildly increased during the mild re-ignition process in the pre-zone of the re-ignited PPF. According to the results of 0-D simulations using closed homogeneous reactor, both OH and CH2O reduced ignition time significantly. The results of this work are helpful for understanding re-ignited PPF more closely.