Similar Flame Research Articles

Fire-Resistant and Thermal Stability Properties of Fluorosilicone Adhesives by Incorporation of Surface-Modified Aluminum Trihydrate.

Fluorosilicone was combined with aluminum trihydrate (ATH) to induce synergistic flame-retardant and thermal-resistant properties. The surface of ATH was modified with four different silane coupling agents. The flammability and mechanical properties of the fluorosilicone/ATH composites were assessed using an UL94 vertical test and a die shear strength test. The change in shear strength was investigated under aging for 1000 h at -55 °C and 150 °C. Pure fluorosilicone had inherent fire resistance and thus achieved a V-0 rating even at 20 wt.% ATH loading. Upon addition of ATH treated with 3-glycidoxypropyl trimethoxysilane, the composites exhibited the highest shear strength of 3.9 MPa at 23 °C because of the additional crosslinking reaction of fluorosilicone resin with the epoxide functional group of the coupling agent. Regardless of the types of coupling agents, the composites exhibited similar flame retardancy at the same ATH content, with a slight reduction in shear strength at 180 °C and 250 °C. The shear strength of the adhesives gradually decreased with aging time at -55 °C, but increased noticeably from 3.9 MPa to 11.5 MPa when aged at 150 °C due to the occurrence of the additional crosslinking reaction of fluorosilicone.

Turbulent partially cracked ammonia/air premixed spherical flames

The combustion of ammonia requires, for most energy conversion systems, a combustion promoter such as hydrogen to guarantee the start-up, stability and combustion efficiency. Partially cracked ammonia (PCA) can provide sufficient hydrogen concentrations to enhance the burning velocity in comparison with pure ammonia. However, little work exists on the use of PCA blends operating under relevant turbulent conditions. To that end the outwardly propagating spherical flame configuration was employed to determine the laminar and turbulent flame propagation characteristics of PCA (NH3/(H2+N2)) and corresponding binary (NH3/H2) mixtures across various turbulent combustion regimes. First, PCA and ammonia-hydrogen blends exhibit similar flame propagation rates under various turbulent intensities, even for the laminar case. The highest turbulent burning velocity was observed at leanest conditions, as opposed to laminar flames which exhibited highest flame speed at conditions above stoichiometry. Under rich conditions, no substantial flame enhancement due to turbulence was measured irrespective of the hydrogen content. This lack of flame enhancement under turbulent conditions is attributed to the effect of preferential diffusion with good agreement observed with trends in measured Markstein numbers. The normalized turbulent flame speed is dominated by the enhanced molecular diffusivity afforded by the presence of hydrogen up to 15 % enrichment, prior to decreasing upon further hydrogen addition under lean and stoichiometric conditions. This ‘bending’ phenomenon may be the contribution of several factors including; the transitioning between combustion regimes associated with low Damköhler numbers (Da) and flame thickening; merging of flamelets due to the presence of ammonia enhancing wrinkling; and combined changes in laminar burning velocity and preferential diffusional behavior. Furthermore, good agreement for turbulent flame speed is observed with a correlation that includes the influence of turbulent stretch (Ka) and non-equidiffusion (Le), with the agreement reducing with decreasing chemical to turbulent time scale ratios (Da << 1).

Uncertainty quantification for high-temperature flame sensing using wavelength modulation spectroscopy

In this study, we explore the uncertainty of the wavelength modulation spectroscopy (WMS) technique applied to water profile measurement in a flat flame. The scanned WMS-2f/1f method is used to measure the water profile in a C2H4-O2-Ar flame, using an absorption transition line near 1391.7 nm. The Monte Carlo sampling method is used for the uncertainty quantification (UQ) analysis. The results show a non-negligible uncertainty on the water concentration with an uncertainty range from 10% to 30%, depending on the uncertainty level considered for the input parameters of the spectroscopic model. Further sensitivity analyses show that the lower state energy (E′′), the line strength, and the temperature are the most important uncertainty sources. When multiple transition lines are involved, the line strength value controls the overall influence of the other parameters associated with these specific lines. We also compared the uncertainty obtained when applying the direct absorption spectroscopy (DA) and WMS techniques for a similar flame configuration. Only a slight advantage is observed for the WMS.

Large eddy simulation of non-premixed flame behavior and NOx emission characteristics in a staged combustor with multi-swirling flows

This study utilizes Large Eddy Simulation (LES) to investigate flame characteristics and NOx emission generation in a concentric staged combustor with three distinct pilot swirler arrangements. We examine the effects of multi-swirling flow on the instantaneous flow field and flame characteristics using the Proper Orthogonal Decomposition (POD) method, and assess how variations in swirler structures influence thermoacoustic instability. Furthermore, a comparison of NOx emissions between two pilot swirler configurations, with similar mean flow field and flame characteristics, highlights the precessing vortex core's role in NOx emissions. Compared to a single-swirler setup, a double-swirler configuration produces a more distinct single precessing vortex core, thereby amplifying thermoacoustic instability. The extended venturi structure in the double-swirler setup mitigates the development of the swirl-induced vortex core, reducing thermoacoustic instability. However, the expansion section in this setup impedes initial fuel and air mixing, leading to an extended flame length, which in turn results in higher NOx emissions due to a more concentrated temperature distribution. With similar mean flow field and flame structure, the double-swirler combustor proves more effective in reducing NO emissions than the triple-swirler combustor.

Control of Intrinsic ThermoAcoustic instabilities for methane and hydrogen air flames: DNS and network analysis

Replacing methane by hydrogen in premixed burners is a typical decarbonation scenario. This is usually done by ensuring a constant flame speed, i.e. by using ultra-lean H2 mixtures. This change can modify the thermoacoustic properties of burners. This study focuses on one class of thermoacoustic instabilities, Intrinsic ThermoAcoustic (ITA) modes, for flames stabilized on a diaphragm. An acoustic network approach is built to construct an ITA stability criterion (called ITAS criteria) for a three-duct configuration and a fully analytical expression is obtained in the limit of thin diaphragms. It shows that instability will grow when the Flame Transfer Function (FTF) gain n−π, located at the frequency at which u′-q′ phase ϕ=−π, is greater than a critical gain nc expressed by the ITAS criteria. Two-dimensional DNS, with anechoic terminations, of methane–air and hydrogen–air flames stabilized on a diaphragm are simulated with similar laminar flame speed sL0 and theoretical flame length, with an equivalence ratio of 0.73 and 0.4 respectively. The theory developed allows to analyze the differences with these two strong unstable ITA modes. For ultra-lean hydrogen flame, preferential diffusion near the diaphragm lips and tip opening reduces the flame time delay and shifts the ITA frequency towards higher values compared to methane. The assessment of two new control strategies is possible with the help of the analytical network model by increasing nc. A first strategy involves the use of preheated leaner mixtures: increasing Tu and reducing ϕ while keeping the flame speed sL0 constant. A second strategy consists of modifying the combustor geometry by changing the section ratio S2/S0, between plenum and combustion chamber. Both strategies successfully mitigate the hydrogen–air ITA modes and seems promising strategies for methane cases despite the fact that instabilities are not fully mitigated for this fuel at the conditions of interest in this study.

Laminar burning velocities of rich NH3+N2+O2 flames: Comparing the effects of elevated temperatures and oxygen ratios on model validation

Ammonia (NH3) is a promising carbon-free alternative fuel. One of its fundamental combustion properties, the laminar burning velocity (SL), is widely used to validate the combustion kinetic models. This study investigates the SL across varying unburned mixture temperature Tu and oxygen ratio xO2, particularly regarding their impact on the model validation. The focus is on the fuel-rich conditions that exhibit noticeable discrepancies among different kinetic model predictions. To provide accurate experimental data for the analyses, atmospheric measurements were carried out using the heat flux method, covering equivalence ratios ranging from 1.2 to 1.9, Tu from 298 K to 448 K, and xO2 from 0.30 to 0.38. Notably, many of these conditions have not been thoroughly explored in existing literature. Seven kinetic models were used to compare with the experimental data and assess the simulation discrepancies, where the increment parameters were used to compare the effect of SL vs. Tu and SL vs. xO2 dependences on the model validations. Through these analyses, the α coefficient, describing the SL vs. Tu dependence, is found too difficult to distinguish the performances of the various models and serve as validation targets. Sensitivity analyses reveal that the reactions governing the dependences of SL on both Tu and xO2 are fundamentally the same. Similar flame conditions with nearly identical sensitivity values were identified, suggesting that a model accurately reproducing the SL vs. Tu trend is promised to replicate the SL vs. xO2 trend, and vice versa. Consequently, the SL measurement with elevated Tu, which is difficult in experiments and in distinguishing the performance of different models, is suggested to be substituted by the much easier measurement with elevated xO2. This conclusion, alongside the methodology, holds potential for application in other flame types and help reduce the experimental efforts for model updating.

Effects of NH3 and H2 addition on morphology, nanostructure and oxidation of soot in n-decane diffusion flames

The effects of ammonia and hydrogen addition on soot formation characteristics in n-decane laminar diffusion flames were studied. The addition of argon was specifically considered to identify the chemical effect of ammonia. The flame temperature, particle morphology, and oxidation characteristics were analyzed. Results showed that both ammonia and hydrogen addition increase the flame height and suppress soot formation, while the suppression effect of ammonia is more pronounced. The addition of argon or ammonia reduces the temperature of flame upstream and yields the similar flame temperature, while hydrogen addition increases the flame temperature. Compared to the flame with argon, the addition of ammonia delays particle nucleation, indicating that the soot nucleation and growth process are chemically inhibited. In contrast, doping hydrogen leads to an earlier nucleation process and aggregation phenomenon. Further analysis on particle nanostructure indicated that the addition of ammonia yields shorter lattice fringes, larger interlayer spacings and amorphous carbon contents, demonstrating a looser and higher disorder internal structure. The addition of hydrogen yields longer fringes and smaller interlayer spacings, showing the particles have a more compact internal structure and higher degree of graphitization. This indicates hydrogen addition reduces the oxidation activity of the particles, which is confirmed by thermogravimetric analysis.

Characteristics of OH formation during single coal particle ignition and volatile combustion in O2/N2 and O2/CO2 atmospheres

The behaviors of volatile combustion and ignition of lignite coal were assessed on a concentrating light heating platform using hydroxyl-Planar laser-induced fluorescence (OH-PLIF) over a range of 21–60% O2 in both N2 and CO2 diluent gases. The results indicated that when the O2 concentration was below 40%, substituting CO2 for N2 delayed the ignition delay time (tign). This was evident as no ignition occurred in the 21% O2/CO2 mixture, and a nearly 1.0 s difference in tign emerged when substituting CO2 for N2 in 30% and 40% air atmospheres. Additionally, the weaker flame luminosity and the shorter volatile flame duration time (tvol) were also observed in O2/CO2 compared with that in O2/N2. Correspondingly, a significantly lower OH signal intensity was observed in O2/CO2 compared to O2/N2, which was primarily due to physical and chemical effects of CO2. Firstly, the higher heat capacity and lower diffusion rate of O2 in O2/CO2 leads to a lower volatile release rate and decreasing flame temperature, as evidenced by the closer proximity of the flame front to the coal particle surface. The lower flame temperature and volatile release rate would reduce the number of hydrocarbon molecule (RH) and the reaction rate of RH→hydrocarbon radical (R) + H, leading to a decrease in the concentration of OH radical from the reaction: H + O2→OH + O. Secondly, CO2 participates directly in H + OH + three-body (M)→H2O + M reaction, competing with the primary OH formation pathway, H + O2→OH + O, for H radicals, thus suppressing the OH generation. However, when O2 concentration exceeded 40%, the tign was shorter in O2/CO2 by about 1.2–1.4 s than in O2/N2 with the same O2 concentration. Simultaneously, compared to the O2/N2, similar flame luminosity and tvol were observed in the O2/CO2. An obvious increase in OH intensity was also observed in O2/CO2. With higher O2 concentrations in the O2/CO2, the absorption and emission radiation characteristics of CO2, increased O2 diffusivity and the decreased specific heat capacity in the O2/CO2 gas mixture, synergistically facilitate temperature rise and volatile release, which promotes the generation of RH, and then increases the H concentration due to the reaction RH→R + H. This would lead to a substantial increase in concentration of OH produced by the reaction H + O2→OH + O, which was detected by OH-PLIF.

Shock induced variable density flows in the vacuum microchannel: I. medium laser fluence

Laser-matter interactions with metal target cause plasma explosion and shock accelerated variable density flow instabilities in the Semiconfined Configuration (SCC). Their study gives deeper insight into the flow instabilities present in all microchannel devices. Blast wave motion along the SCC microchanel causes the Kelvin–Helmholtz (KH) instability and formation of vortex filaments for the critical Reynolds number. Appearing in all shear layers—it affects the fluid transport efficiency. Shear layer acceleration causes a Raleigh-Taylor instability (RTI). Oriented bubble growth by discrete merging indicates anisotropic RTI mixing. Similar RTI flame instability appears in the conversion of chemical energy into electricity affecting microcombustion efficiency. Another case of anisotropic RTI is the flow boiling for cooling of chips and microelectronic devices. The RTI boiling which appears for the critical heat flux is based on rising surface vapor columns (oriented bubble growth) with liquid counterflow (spike prominences) for the critical wavelength at density interface. The RT bubble merging graph trees determine turbulent mixing which affects the heat transfer rates. Bottom-wall turbulent flow in the SCC microchannel causes streaks of the low momentum fluid and formation of hairpin vortex packets with lattice organization. This makes possible to quantify parameters responsible for the evolution of hairpin vortex packets in the microchannel devices. Appearing from the low to the high Reynolds numbers they affect the transport properties, control of the fluid motion, enhancement of mixing, or the separation of fluids. Fluid particle ejecta—thin supersonic jets - evolve into long needle-like jets which start spiraling, helical pairing and swirling in the field of thermal gradients. Such instabilities appear in the microcombustion flame instability and in the space micropropulsion systems. Oscillating and spiral flames appear in the presence of thermal gradient in the microchannel, due to the combined effects of thermal gradient fields and the mixture flow rates.

Numerical study of an industrial burner to optimise NOx emissions and to evaluate the feasibility of hydrogen-enriched fuel

Reducing NOx emissions from industrial burners is a significant concern due to their harmful environmental and human health impact. A computationally efficient numerical model was developed and validated using a detailed GRI 3.0 chemistry mechanism to simulate the combustion process and precisely predict the NOx emissions from industrial burners. The numerical model was implemented to reduce NOx emissions by varying the burner's primary to secondary air mass flow ratio. An optimum nozzle diameter was proposed to abate NOx emissions by a factor of 0.845. A feasibility study on the optimised burner was conducted by blending up to 50% hydrogen by volume with natural gas by maintaining the same burner power output. Results showed that the burner exhibited similar flame characteristics until 40% hydrogen was added to natural gas. A 41.8% increase in NO and a 76.8% decrease in CO emissions were observed by enriching natural gas with 50% hydrogen.

Characterization of Flame Spread Over PMMA Using a Temperature Reconstruction Method

ABSTRACT The contribution of heat transfer mechanisms for different configurations of opposed flow flame spread on cast PMMA is presented. Three opposed flow flame spread scenarios are considered, each in a quiescent atmosphere: downward spread on a vertical surface, buoyant horizontal spread (i.e. a pool fire-type spread), and lateral spread along a vertical surface. Video analysis and temperature measurements were used to determine flame spread rates; average flame spread rates were found to be 2.67, 2.54, and 3.54 mm/min for downward, horizontal, and lateral spread, respectively. These differences result from the varying magnitudes of the heat transfer mechanisms (e.g. gas-phase heat transfer and conduction through the solid) due to the changes in the flame characteristics. Past studies have investigated these regimes of flame spread, but have not directly compared experiments from the configurations used here to identify the relative importance of each heat transfer mechanism. Using temperature measurements, the thermal gradient, and hence conductive heat fluxes through the solid, were quantified. This allowed the different rates of heat transfer to be compared across each configuration. Downward and horizontal flame spread displayed similar flame spread rates and similar heat transfer contributions for both the gas phase and solid phase. However, lateral flame spread displayed characteristically higher rates of gas-phase heat transfer compared to downward and horizontal.

Soot nucleation in diffusion flames and the role of aromatic hydrocarbons

We perform spatially resolved measurements of light scattering of soot in atmospheric pressure counterflow diffusion flames to complement previously reported data on soot pyrometry, temperature and gaseous species up to three-ring polycyclic aromatic hydrocarbons (PAHs). We compare two flames: a baseline ethylene flame and a toluene-seeded flame in which an aliquot of ethylene in the feed stream is replaced with 3500 ppm of pre-vaporized toluene. The goal is twofold: directly adding an aromatic fuel to bypass the formation of the first aromatic ring, widely regarded as the main bottleneck to soot formation from aliphatic fuels, and assessing the impact of a common component of surrogates of transportation fuels on soot formation. The composition of the fuel and oxidizer streams are adjusted to ensure invariance of the temperature-time history, thereby decoupling the chemical effects of the fuel substitution from other factors. The doping approach enables the comparison of very similar flames with respect to combustion products, radicals and critical precursors to aromatic formation (C2–C5 species), in addition to the temperature-time history. Doping with toluene boosts the aromatic content and soot volume fraction relative to the baseline ethylene flame, but, surprisingly, the soot number density and nucleation rate are affected modestly. As a result, the observed difference in volume fraction in the toluene-doped flame is reflective of larger initial particles at the onset of soot nucleation. The nucleation rate when soot first appears near the flame is of the same order as the dimerization rate of single-ring aromatics, in contrast with the expectation that the dimerization of larger PAHs initiates the process. Even though in and of itself nucleation contributes modestly to the overall soot loading, nucleation conditions the overall soot loading by affecting the size of the initial particle, which ultimately affects subsequent growth.

Ignition detection with the breakdown voltage measurement during nanosecond repetitively pulsed discharges

Nanosecond Repetitively Pulsed Discharge (NRPD) is a promising ignition concept for introducing diesel-like process parameters for hard-to-ignite renewable fuels in Spark Ignition (SI) engines. Knowing whether an ignition event initiated by a series of nanosecond electrical discharges was successful or not gives the possibility of using this information for closed-loop ignition control. This paper presents a methodology for detecting successful ignition under NRPD ignition.After a nanosecond discharge, the heat loss from the particles (plasma-gas) between the electrodes and the surrounding gas is different if a robust flame kernel is established. If a flame kernel is present, the heat losses are lower, resulting in a lower local density of the gas between the electrodes. The breakdown voltage value of a nanosecond pulse is proportional to the local density. A control pulse is applied after the main ignition sequence to detect successful ignition. Lower breakdown voltages of the control pulse are present if a robust early flame kernel is present. The control pulse is applied before the pressure rises due to the presence of fast combustion, allowing ignition to be detected during the inflammation phase, thus allowing the possibility to place additional ignition events, if necessary.This technique was experimentally analyzed in a Constant Volume ignition Cell (CVC) and in a Rapid Compression Expansion Machine (RCEM). In the CVC at the ignitability limit, lower breakdown voltages of the control pulse are mostly measured when no pressure rise is measured. In the RCEM, the heat release rate is analyzed with a two-zone thermodynamic model, and the early flame kernel formation is monitored with Schlieren imaging. Some overlap exists in the control pulses' breakdown voltages for the ignition and quenching experiments; nevertheless, the Schlieren videos outline that the overlapping cases have a similar flame kernel formation, and the difference arises thereafter.

Large eddy simulation of OME3 and OME4 spray combustion under heavy-duty conditions

The synthetic diesel fuel (Poly-)oxymethylene ether (OME) has various interesting properties for the transport sector. OME reduces emissions in direct injection (DI) diesel engines, e.g., soot and NOx, and is considered CO2 neutral. While most studies have examined a mixture of OMEs with different chain lengths, recent studies have focused on neat OME3 and OME4. In the literature similar chemical ignition delay times were shown under homogeneous conditions and significant differences were reported in their thermophysical properties. Furthermore, differences between OME3 and OME4 in terms of the mixture formation were shown on a heavy-duty injector under non-reactive conditions. The question of how this mixture formation affects the ignition and flame characteristics of neat OME3 and OME4 spray flames remains unresolved. To address this topic, this study conducts a combined experimental and numerical investigation under inert and reactive spray conditions for OME3 and OME4 on a heavy-duty injector. Large Eddy Simulations (LESs) and high-pressure, high-temperature spray chamber measurements are conducted. The distinct differences between neat OME3 and OME4 are analyzed along the cause–effect chain comprising the mixture formation, ignition behavior and its effects on the overall heat release rate. Based on the differences in the thermophysical properties, e.g., density and vapor pressure, the investigations in inert conditions reveal differences in the liquid penetration and mixture formation. The reactive spray flame measurements and simulations show a significant offset in the ignition delay time for OME3 and OME4. An explanation is found in the significantly different distribution of the scalar dissipation rate due to the differences in the mixture formation. Despite the differing ignition delay times, similar local flame structures are found for both fuels.

Waste heat recovery optimization in ammonia-based gas turbine applications

E-fuels are promising alternatives to fossil fuels in the transition towards zero-emission energy system. In this study, a novel chemical-recuperated and humidified gas turbine concept aiming at the application of e-fuel ammonia is proposed to overcome the technical hitches and exploit the waste heat to enhance the cycle performance. The thermodynamic analysis shows that the highest net electrical efficiency (56.7%) under the design conditions exceeds that of the ammonia-fueled Brayton cycle by 20.6%-points. The chemical recuperation of fuel contributes to the efficiency improvement by 7%-points under dry condition, while steam injection provides 8%-points to 12%-points efficiency increase corresponding to the ammonia decomposition ratio of 5%–96%. A non-monotonic relation between the net electrical efficiency and steam-to-air ratio is found to be the result of the competition between the enthalpy change from to the varied steam and air mass flow rates. Analyzing the flame characteristics in the combustor under the design conditions, we found that conditions with high decomposition ratio (>88%) and high steam-to-air ratio (>0.3) exhibit similar flame speed with that of methane fueled combustor and thus existing designs can be reused. The prediction shows the NOx emission can be restricted when the steam-to-air ratio exceeds 0.4.

Thermodiffusively-unstable lean premixed hydrogen flames: Phenomenology, empirical modelling, and thermal leading points

Thermodiffusively-unstable lean premixed hydrogen flames are investigated using three-dimensional direct numerical simulation with finite-rate chemical kinetics. A large number of simulations have been performed to investigate the influence of reactant conditions (pressure, temperature, and equivalence ratio) on thermodiffusive response in freely-propagating and turbulent flames. Reactant conditions are characterised using an instability parameter ω2, which was recently shown to characterise freely-propagating flames well in 2D. Freely-propagating flame speeds and thicknesses are found to correlate with ω2 using the same functional form found in 2D (with larger model constants), and different correlations are required either side of the most-unstable surface in ω2-space. The freely-propagating values are demonstrated to be more appropriate for characterising turbulent flames than the corresponding 1D laminar flame values. Both turbulence and thermodiffusive instability bring about a similar flame response (increasing curvature, reaction rates and temperature), and so starting from strongly thermodiffusively-unstable conditions limits the potential turbulent response; an empirical model for local flame speed and thickness is provided incorporating this dependence. Joint probability density functions are used to correlate local consumption-based flame speed with curvature, strain-rate and stretch with single and independent Markstein numbers. A simple curvature-based model coupled with the empirical flame speed model is found to yield reasonable results. Principal curvatures are used to partition the flame surface into six classifications, allowing for conditional analysis in different parts of the flame surface. Fractional contributions show that the bulk of the fuel consumption occurs in flat regions and leading edges, shifting from the former to the latter with increasing instability and/or turbulence. Furthermore, the flat flame regions experience speeds in excess of the reference value despite the lack of focussing of fuel by preferential diffusion in these regions, contrary to the conventional expectations of thermodiffusive instability. We propose a thermal leading point interpretation of these phenomena: as expected, strong positive curvature in the leading points/edges result in diffusive focussing of fuel, increasing the reaction rates, resulting in superadiabatic temperatures; these high temperatures left behind the leading points/edges then support higher-than-expected reaction rates in regions where the flame surface is relatively-flat.

The impact of hydrogen substitution by ammonia on low- and high-temperature combustion

The combustion behaviour of ammonia has attracted intermittent interest with the original patent by Lyon (US3900554A) relating to its use for nitric oxide reduction through selective non-catalytic reduction (SNCR) a focal point. The recent interest in ammonia stems from its use as a hydrogen rich energy carrier with practical use requiring a much wider parameter space. The corresponding challenges (e.g. Kobayashi et al., Proc. Combust. Inst. 37 (2019) 109–133) include different flame dynamics and high emissions of oxides of nitrogen. The current paper explores the complex nature of ammonia oxidation and provides a reduced size reaction mechanism that enables application, without approximation, to the computation of turbulent flames through a joint-scalar transported probability density function (JPDF) method. Comprehensive validation suggests similar accuracy to a reference mechanism (Glarborg et al., Prog. Energy Combust. Sci. 67 (2018) 31–68) and highlights some uncertainties. The selected turbulent flame configuration features auto-ignition stabilised flames supported by a coflow of hot combustion products. The base case features a H2/N2 fuel jet that permits flame stabilisation at 1045 K corresponding to the onset of the SNCR temperature window. The impact of a gradual substitution of hydrogen by ammonia on flame stabilisation, emissions of oxides of nitrogen and the flame structure is quantified. It is shown that ammonia substitution leads to more prevalent local extinction, a more distributed flame structure and requires substantially increased coflow temperatures to achieve a similar flame stabilisation point. A lowering of the coflow temperature to operate within the SNCR regime substantially reduces NOx and leads towards a homogeneous/distributed reaction mode. The reduced fuel reactivity highlights the importance of turbulence-chemistry interactions leading to complexities in the design of practical devices.

The effect of pressure on lean premixed hydrogen-air flames

The effect of pressure on the propagation of lean premixed hydrogen-air flames is investigated numerically with a hierarchy of canonical configurations including direct numerical simulations of turbulent flames in homogeneous isotropic turbulence. Thermo-diffusive instability and the ratio of turbulent to laminar burning velocity are found to be clearly amplified for increasing pressure. The observed amplification results from the combined effects of multi-dimensional geometrical features of the reaction front induced by stretch, chemical kinetics and flame speed / thickness sensitivity to preferential diffusion induced by the flame geometry. High-pressure flames exhibit stronger sensitivity to fast molecular diffusion of hydrogen into the reaction front of a reference freely-propagating flame that is nominally weaker at high pressure due to the greater relative importance of chain-termination. In this context, two-dimensional flames subjected to monochromatic velocity perturbations demonstrate locally enhanced flame speeds and reduced flame thickness at high pressure in regions of curvature that are convex towards the reactants. Furthermore, it is found that a Peclet number definition, based on the ratio of diffusive to convective time scales, provides a satisfactory global representation of pressure effects upon thermo-diffusive instabilities of freely-propagating flames. Finally, employing a series of carefully designed direct numerical simulations, it is empirically demonstrated that very similar flame responses can be obtained at atmospheric and pressurized conditions by approximately matching a selection of four key parameters (Leeff, Ze, Ma, Pe). This key finding demonstrates that the flame speed enhancement often observed at elevated pressure is not a pressure effect per se, but rather determined by the sensitivity of the flame to preferential diffusion (Pe) versus its own reactivity (Ze), with nominally ‘weaker’ flames showing stronger sensitivity.

Analysis and improvement of hot work management in China

AbstractHot work, work involving electric or gas welding, cutting, brazing, or similar flame or spark‐producing operations, is a major industrial hazard. Many countries have formulated relevant regulations and standards for hot work to regulate their management procedures and operational requirements. This article compares and analyses the safety‐related regulations and standards for hot work in China and the United States. The article puts forward relevant suggestions for the revision and improvement of China's hot work regulations and standards in comparison with the US regulations and standards. In terms of details, the US regulations and standards are more scientific, stringent, and guiding. China's standards are more closely related to China's national conditions, but some provisions lack scientific basis and are poorly operable. Further enhancement of the scientific, guidance, and operationality of China's hot work‐related regulations and standards is recommended.

Flame Transfer Functions for Turbulent, Premixed, Ammonia-Hydrogen-Nitrogen-Air Flames

Abstract Ammonia is a promising hydrogen and energy carrier but also a challenging fuel to use in gas turbines, due to its low flame speed, limited flammability range, and the production of NOx from fuel-bound nitrogen. Previous experimental and theoretical work have demonstrated that partially dissociated ammonia (NH3/H2/N2 mixtures) can match many of the laminar flame properties of methane flames. Among the remaining concerns pertaining to the use of NH3/H2/N2 blends in gas turbines is their thermoacoustic behavior. This paper presents the first measurements of flame transfer functions (FTFs) for turbulent, premixed, and NH3/H2/N2-air flames and compares them to CH4-air flames that have a similar unstretched laminar flame speed and adiabatic flame temperature. FTFs for NH3/H2/N2 blends were found to have a lower gain than CH4 FTFs at low frequencies. However, the cutoff frequency was found to be greater, due to a shorter flame length. For both CH4 flames and NH3/H2/N2 flames, the confinement diameter was found to have a strong influence on peak gain values. Chemiluminescence resolved along the longitudinal direction shows a suppression of fluctuations when the flame first interacts with the wall followed by a subsequent recovery, but with a significant phase shift. Nevertheless, simple Strouhal number scalings based on the flame length and reactant bulk velocity at the dump plane result in a reasonable collapse of the FTF cutoff frequency and phase curves.