Flame Formation Research Articles

Study on characterization of flame propagation of spontaneous ignition caused by high-pressure hydrogen leakage

As an ideal energy source, hydrogen is highly susceptible to spontaneous ignition once leaked, which is an urgent issue that needs to be addressed. Based on the shock tube model, this paper investigates flame propagation under various pressures, tube lengths, and diameters by employing the LES approach and a detailed hydrogen/air combustion mechanism. The results indicate that within the tube, the ignition kernels gradually evolve into tulip flames when specific conditions are satisfied. As pressure and tube length increase, the likelihood of forming a complete flame rises significantly; with the increase of tube diameter, the flame front is flatter and the flame intensity is more uniformly distributed. Furthermore, this paper develops a model to predict the formation of a complete flame: Pb/Pa=570.64(L/D)−0.6. Outside the tube, once the intact flame passes out of the tube and evolves into a jet flame, structures such as flame envelopes and jet vortices will appear. Higher release pressures make it more difficult for the flame to propagate steadily, whereas increasing tube length and diameter promotes combustion and sustains the flame outside the tube.

Resolving NOX formation of ammonia-hydrogen flame utilizing PLIF technique collaborated with flame structures and chemical kinetics analysis

Carbon-free fuels such as ammonia and hydrogen have attracted much attention in response to tackling with climate problem of global warming, but their high NOX emissions limit practical applications unavoidably. Currently, very few studies have addressed the inter-relationship between flame structures and NOX formation. In addition, few previous studies have analyzed ammonia-hydrogen combustion with low hydrogen mixing ratio through in-depth NOX formation mechanisms. In this work, NOX formation of ammonia-hydrogen swirl flame with different equivalence ratios and hydrogen mixing ratios (<30 %) has been comprehensively investigated, and the connection between flame structures and NOX has been reflected based on PLIF technique. The analytical results showed that as equivalence ratio (Φ = 0.6–1.2) increased, NOX concentration increased firstly and then decreased subsequently, and peak NOX value was observed between Φ = 0.7–0.8. Besides, NOX increased as the hydrogen mixing ratio increased from 10 % to 25 %, being capable of reaching up to 2795 ppm. Furthermore with flame structure analysis, the flame structure could be classified into single-front flame, transition flame, and double-front flame, in which transition flame featured with the largest decomposition reaction region contributing to NH3 oxidation to form NOX (intensive OH radicals propagation); while double-front flame characterized by smallest decomposition reaction region inhibiting the NOX formation via OH suppression (weak OH radicals propagation). Based on systematically flame surface density and chemical kinetics analysis, lean combustion benefited the NOX pathway, whilst rich combustion favored the N2 pathway. In addition, as the hydrogen ratio increased, and the reducibility of NH/NH2 to NOX was weakened, which ultimately promoted the production of NOX. The findings achieved suggest that future combustion techniques by the ammonia-hydrogen dual fuel should avoid the occurrence of transition flame, and prone to the generation of double-front flame, which could thus implement effective suppression on NOX formation.

The Influence of Gas Fuel Enrichment with Hydrogen on the Combustion Characteristics of Combustors: A Review

Hydrogen is a promising fuel because it has good capabilities to operate gas turbines. Due to its ignition speed, which exceeds the ignition of traditional fuel, it achieves a higher thermal efficiency while the resulting emissions are low. So, it was used as a clean and sustainable energy source. This paper reviews the most important research that was concerned with studying the characteristics of hydrogen combustion within incinerators and power generation equipment, where hydrogen was used as a fuel mixed with traditional fuel in the combustion chambers of gas turbines. It also includes an evaluation of the combustion processes and flame formation resulting from the enrichment of gaseous fuels with hydrogen and partial oxidation. A large amount of theoretical and experimental work in this field has been reviewed. This review summarizes the predictive and experimental results of various research interests in the field of hydrogen combustion and also production.

Effects of sub-atmospheric pressure on appearance and pollutant formation of inverse diffusion flame within a confined space

Gas-fired boilers operating at high-altitude regions often suffer from inadequate output, decreased thermal efficiency, and excessive NOx emissions. The effect of sub-atmospheric pressure on flame appearance and pollutant formation is the main reason for those problems, and thus needs to be clarified particularly under furnace combustion conditions with a fixed excess air coefficient. Inverse diffusion is a widely employed fuel–air configuration in burners of gas-fired boilers, and therefore the flame appearance, CO generation, and NO generation were experimentally investigated in this paper by adopting a low-pressure quartz tube reactor. Results show that the flame is elongated from reducing pressure under fuel-lean conditions, mainly due to the reduced oxygen mass concentration and the elevated jet velocity. Under fuel-rich combustion conditions, however, the flame is shorted at sub-atmospheric pressure from the suppressed soot formation. The reduced pressure leads to an increase in the global strain rate, making the flame more prone to uplift. With decreasing pressure, the increased air–fuel mixing and flame length facilitate the gas burnout, thus decreasing CO generation. The sub-atmospheric pressure could evidently reduce the NO generation under fuel-rich conditions, but slightly increase it under fuel-lean conditions. Under fuel-lean conditions, the NO major pathways (prompt, thermal, NNH, and N2O) are promoted which leads to an increase in NO generation with decreasing pressure. Under fuel-rich conditions, however, NO formation is suppressed from the decreased rate of reaction N2+CH↔HCN+N.

Effects of internal intervention in tubes on shock wave attenuation, formation and propagation of flame during high-pressure hydrogen release

Experiments on the impacts of the combination of fin structure and expansion chamber on shock wave attenuation, hydrogen ignition, and flame characteristics during high-pressure hydrogen release were carried out. The fin is a four-edged platform hollowed out on all sides. The inflated upper and lower tube walls are located 85, 95, and 105 mm away from the nickel burst disk. With the incident shock wave traveling through the fin, the reflected shock and the superposition of multiple reflected shock waves appear, and the reflected shock wave intensity can equal that of the release pressure. The decrease of the incident shock wave intensity behind the fin leads to the decrease of the non-uniformity of the mixed layer temperature, further causing the decrease of the flame propagation velocity. The presence of fins increases the probability of ignition in the fin region and the critical release pressure for ignition is significantly lower than that in the barrier free tube, mainly because the fin inside intensifies the hydrogen/air mixture, resulting in a lower minimum ignition energy required for self-ignition. Furthermore, the decrease of flame propagation velocity leads to the increase of hydrogen consumption inside the tube and the decrease of shock overpressure outside.

Large Eddy Simulation of NO Formation in Non-Premixed Turbulent Jet Flames with Flamelet/Progress Variable Approach

Large Eddy Simulation of NO Formation in Non-Premixed Turbulent Jet Flames with Flamelet/Progress Variable Approach

Numerical investigation of premixed syngas/air flame evolution in a closed duct with tulip flame formation

In this study, the dynamic behavior of the premixed syngas/air flames in enclosed ducts with obstacles at the blockage ratio of 0.5, as well as without obstacles, was studied via large eddy simulation and the turbulent flame speed closure model. The hydrogen volume fraction (hydrogen content in fuel) was set to 50 %. Results indicate that the simulation reproduced the flame shapes, flame speed and explosion overpressure observed in the experiments. In cases without obstacles, a “tulip” flame with a tulip cusp can be seen, and secondary cusps emerge on tulip lips due to the high-pressure zone ahead of “tulip” flame lips, leading to in a distorted “tulip” flame. In cases with obstacles, the flame develops hemispherical and finger shapes upstream of the obstacles and evolves into a “tulip” flame downstream of the obstacles due to vortices and adverse pressure gradients. The alteration of the flow direction within the unburned and burned zones results in intricate flame shapes. Flame speed and overpressure show a close relationship and exhibit oscillations following the generation of the “tulip” flame. Typically, the velocity, pressure and flow fields adjacent to the flame front can affect dynamic behaviors of premixed syngas/air flames.

Effect of H[formula omitted] addition on NH[formula omitted] flame structure and dynamics in a mixing layer

Hydrogen addition has been widely used to improve the combustion performance of low-reactivity fuels such as NH3. In this study, a series of two-dimensional NH3/H2 flames in mixing layers were investigated numerically. The aim is to assess and interpret the effects of H2 addition on the structure and dynamics of ammonia flames. Detailed chemistry and transport models were considered in the simulations. It was shown that for pure hydrogen or ammonia flames, a classical tribrachial structure was observed. The non-premixed branch in the tribrachial flame of NH3 combustion features high heat release rate (HRR), while the two premixed branches are weak. The mixture fraction corresponding to the maximum HRR of NH3 combustion is significantly lower than the stoichiometric mixture fraction. When blending ammonia with hydrogen, a tetrabrachial flame structure consisting of two premixed branches and two non-premixed branches was observed, where one non-premixed branch corresponds to the reaction of NH3, while the other is formed due to the reaction of H2. It was suggested that the formation of the tetrabrachial flame is related to the preferential diffusion of hydrogen. After decreasing artificially the diffusivity of hydrogen to suppress the preferential diffusion effect, the flame exhibits only three branches with the merging of the two non-premixed branches. By analyzing the flame dynamics at the leading edge, it was found that the flame speed (Sd∗) is negatively correlated with the flame stretch (κ) and scalar dissipation rate (χ), with Sd∗ decreasing almost linearly with κ or χ, consistent with previous studies of methane and hydrogen flames. As the H2 concentration is increased, the values of Sd∗ increase due to the enhanced reactivity.

Effect of strain rate on nanoparticles and soot in counterflow flames of ethylene/ethanol and ethylene/OME3

This study delves into the impact of strain rate on soot and nanoparticle formation in counterflow diffusion flames (CDFs). While strain rate is generally known to reduce soot, understanding of nanoparticle formation remains limited. Strain rate (up to 207 s−1) effects in ethylene flames are here investigated along with ethanol and for the first time oxymethylene ether-3 (OME3). Ethanol and OME3 are well known to reduce particulate emission in several combustion conditions, however, especially for OME3, their effects at different strain rates lack comprehensive information. In soot-forming CDFs, two distinct zones emerge: a pyrolytic zone from the fuel nozzle to the stagnation plane (S.P.), and an oxidative zone from the oxidizer nozzle to the S.P., analogous to the centerline and wings of a coflow diffusion flame. The study aims to unravel the intricate relationship between strain rate, oxygenated fuel, and soot and nanoparticle formation in these two zones. Laser-induced fluorescence, attributed to nanoparticles, laser-induced incandescence, attributed to soot, and scattering were used to track particle formation. Both nanoparticles and soot are formed in both pyrolytic and oxidative zones, with smaller sizes in the former, emphasizing dominant nucleation and coagulation processes. Findings indicate a general particle reduction with strain rate for all the fuel blends, suggesting an inhibition effect on particle nucleation and growth. The effect of strain rate differs for pyrolytic and oxidative zones, with particle reduction in the pyrolytic zone slightly more sensitive to strain rate due to lower kinetic timescales present in this zone of the flame. Oxygenated fuels generally decrease particle formation, however, in pyrolytic zones a slight increase of nanoparticles can be detected. This effect is more evident at low strain rates, and it disappears as strain rate increases. OME3 exhibits higher reactivity and lower sensitivity to strain rate. The insights from this study contribute to developing cleaner combustion technologies, especially in blending oxygenated fuels with traditional ones. Further research is recommended for broader applicability, considering high strain rates and a wider range of dilution conditions.

Challenges and solutions for local flame development during spark ignition in a kerosene-fueled scramjet combustor

This study experimentally investigates the challenges and solutions related to the development of local flame into global flame during kerosene spark ignition in a scramjet combustor operating at Mach 4 flight conditions. The ignition and intensity of local flame are explored with different injection pressures. Two potential solutions have been proposed to facilitate the development. The results show that injection pressure plays a critical role in controlling fuel transport into the ignition cavity T1, affecting the local equivalence ratio and local flame formation. Higher injection pressures lead to less fuel transported into cavity T1, resulting in fuel-lean local equivalence ratios and potential ignition failure. Extending the duration of ignition and injection improves ignition reliability. The suppressive effect of dense spray on local flame is the main cause of the local flame development problem. A higher injection pressure can reduce the suppressive effect and increase the intensity of downstream cavity flames. When the downstream cavity flames reach a critical intensity, the flashback of downstream cavity flame will occur, achieving global flames. The dense spray can be thinned out by very low upstream injection pressure, which can also result in global flames.

Effects of dimethoxymethane and isopropanol blending on soot formation in ethylene and propane counterflow diffusion flames

Introducing low-carbon oxygenated fuels into the current transport sector provides an effective pathway for mitigating the emissions of greenhouse gases and harmful pollutants such as soot. Previous studies have revealed that oxygenated fuels can reduce soot formation, but the soot-reduction potential is closely related to the chemical interaction between the oxygenates and the baseline hydrocarbons. This work is devoted to study the effects of blending dimethoxymethane (DMM) and isopropanol (IPA) on soot formation in ethylene-based and propane-based counterflow diffusion flames. Soot formation in the target flames was experimentally characterized using a planar light extinction technique, accompanied by numerical analysis to provide complementary insights. The results confirmed that the effects of blending oxygenates on soot formation are sensitive to the fuel-specific molecular structure of the oxygenates and hydrocarbons. For the C2H4-based flames, blending DMM and IPA could lead to a synergistic effect on soot formation due to chemical fuel interaction, with stronger synergy observed with IPA blending. In contrast, no evident synergistic effects on soot formation were observed in the C3H8-based flames, for which a notable soot reduction was observed with DMM blending. Reaction pathway analysis suggested that the occurrence of soot synergy in the C2H4-based flames is mainly due to the chemical interaction between the methyl radicals generated from DMM/IPA and the C2 species from C2H4. This study is expected to deepen our understanding of the soot formation behavior of DMM- and IPA-blended flames, thus contributing to their successful usage as clean alternative fuels.

An experimental study on the flame behaviors of H2/CO/Air mixtures in closed tube with varying number of obstacles

In order to investigate the flame behavior of syngas/air mixtures after leakage in industrial scenarios. A series of fence-type obstacles with a blockage ratio of 0.3 were installed in a closed tube to simulate common blockage scenarios. The stoichiometric mixtures of H2/CO/Air with hydrogen fractions ranging from 10 % to 50 % were used to simulate typical syngas compositions. Results indicate that the increase in hydrogen fractions and the number of obstacles can enhance flame acceleration and oscillation downstream of obstacles. The flame inverts into the tulip shape after it passes the obstacle area, and then the distorted “tulip” flame appears. The obstacles number has no influence on the instant of the planar flame formation, because the flow competition initiates when the flame passes the first obstacle channel. However, the initial instant of the first “tulip” distortion decrease with the increasing obstacle number. The “tulip” flame is a hydrodynamic phenomenon, while the distortion of the “tulip” flame is related to pressure waves. At higher hydrogen fractions, the obstacle number increases the maximum overpressure more significantly. In practical scenarios, it is advisable to limit the number of obstacles as much as possible and prevent syngas leaks with higher hydrogen fractions.

Effects of Oxygen Concentration on Soot Formation in Ethylene and Ethane Fuel Laminar Diffusion Flames

In studying the effects of oxygen concentration and molecular structure on the morphologies of the soot particles produced by hydrocarbon fuels, ethylene and ethane were chosen as experimental fuels. With a Gülde laminar coaxial diffusion flame device, a soot particle device was used to sample soot particles at different oxygen concentrations (21%, 24%, 26%, 28%, and 31%) and at different heights above a burner (HABs = 10 mm, 20 mm, 30 mm, 40 mm, and 50 mm). High-resolution transmission electron microscopy (HRTEM) was used to scrutinize and analyze the soot particles at varying oxygen concentrations. The findings suggest that at the same oxygen concentration, ethylene produces brighter and taller flames. With an increase in the oxygen concentration, ethylene flames and ethane flames gradually decrease in height and become brighter. With an increase in the HAB, the average primary soot particle diameter (Dp) increases initially and then decreases, the fractal dimension (Df) increases, and the aggregates transition from strips and chains to clusters. At the same flame height (HAB = 30 mm), the Dp decreases, the Df increases, the carbon layer torsion resistance (Tf) and the carbon layer spacing (Ds) increase, and the carbon layer changes from a parallel arrangement to a curved arrangement to form denser network aggregations.

Experimental and numerical investigation on flame pattern formations of non-premixed CH4 and air in a planar micro-combustor

Non-premixed CH4/air flames in a rectangular micro-combustor of 2.0 mm (width) × 10 mm (height) × 31 mm (length) were experimentally investigated. The results demonstrated that flame cannot occur within the micro-combustor if nominal equivalence ratio ϕ ≤ 1.0 for any average velocity (V). Five flame patterns appeared and were termed as “single internal C-shaped flame”, “dual internal C-shaped flames”, “single ε-shaped flame”, “dual internal and external flames” and “single external flame”. Cold-state numerical simulations unraveled that fuel stratification occurred in the height direction due to the buoyancy effect, which is especially pronounced at ϕ = 1.3–1.5. Consequently, “dual internal C-shaped flames” is formed, consisting of an upper flame cell and a lower flame cell. At ϕ = 1.1–1.3, fuel stratification phenomenon grew weakened, which led to “single ε-shaped flame” at ϕ = 1.2–1.3 and “single internal C-shaped flame” at ϕ = 1.1–1.2. At sufficient high average velocities, one or both flame cells will be pushed out of the combustor. In conclusion, the present work verified the possibilities of formation of stable flames inside the micro-combustor with a width less than the quenching distance of stoichiometric mixture. Moreover, this study revealed the important role of buoyancy effect in flame pattern formations within micro-combustors with a large enough channel height.

Suppression effects of different extinguishing agents on vent gases fires from lithium-ion batteries after thermal runaway: A comprehensive experimental and numerical study

During the thermal runaway process of lithium-ion batteries, the release of vaporized electrolyte and combustible gases can lead to the formation of a jet flame, posing a significant fire or explosion risk. In order to deal with the threat of lithium-ion battery vent gas fires to the safety of energy storage power stations, it's crucial to identify effective fire extinguishing agents for lithium-ion battery systems. This study employs a combination of experimental and numerical simulation methods to assess the suppression capabilities of CO2, N2, and HFC-227ea on vent gas/air premixed flames originating from lithium-ion batteries with various cathode materials. Laminar flame speed of vent gas/air/extinguishing agent premixed flames at specific equivalence ratios were measured using a Bunsen burner device under ambient temperature and atmospheric pressure conditions. Additionally, numerical calculations of laminar flame speed and adiabatic flame temperature were conducted using CHEMKIN-Pro, accompanied by an analysis of the chemical inhibition mechanism of HFC-227ea. The findings reveal that although HFC-227ea may slightly elevate the adiabatic flame temperature at lower equivalence ratios, its overall fire extinguishing efficacy surpasses that of CO2 and N2. These results offer valuable insights for selecting appropriate fire extinguishing agents for energy storage power stations, thereby enhancing the safety standards of energy storage systems.

Numerical Simulation on Combustion Characteristics of Methane Air Premixed Flame Impacted by Hydrogen jet

Abstract Low-concentration coalbed methane is an efficient and clean unconventional natural gas with abundant reserves. It can greatly lessen the problem of energy scarcity when used to produce combustion power. Nevertheless, the engine finds it challenging to burn the CH4 directly due to its low and fluctuating concentration. This study suggests using a hydrogen jet to ignite low-concentration coalbed methane. The simulation method is used in this paper. To investigate the effects of various ignition injection strategies on the combustion characteristics of low concentration coalbed methane ignited by a hydrogen jet, a constant volume bomb model was developed. The results show that when the ignition and hydrogen injection interval is 2.0 ms, the cold jet of hydrogen does not burn immediately when it reaches the premixed flame, and there is a transition process from the premixed flame to the jet flame. The larger the interval between ignition and hydrogen injection, the more waste gas is produced after the premixed flame combustion, which has a certain inhibition effect on the formation of the jet flame. With the decrease in the interval between ignition and hydrogen injection, the combustion duration is obviously shortened. Therefore, the earlier hydrogen is involved in the ignition, the faster the combustion speed.

Mystical alkali metal flame: Sodiophilic insights into rapid preparation of the Na metal battery anode

Extensive research has explored the utilization of tin oxide (SnO2) as a sodiophilic reagent in sodium metal batteries. However, achieving rapid encapsulation of sodium has remained elusive, despite efforts such as converting a side reaction yielding a sodiophobic Cu-Sn alloy into sodiophilic SnO2 on the Cu current collector's surface. Notably, the presence of preserved crystal water in SnO2 has proven pivotal, as it initiates the formation of a plasma-like alkali metal flame, facilitating swift encapsulation of metallic sodium at 400 °C. Analysis through XPS and TEM tests unveiled the interaction between crystal water and liquid sodium metal, leading to rapid heat release. This phenomenon results in the generation of sodium metal vapor (>882.9 °C) and the extraction of oxygen atoms from SnO2, forming Na2O within an argon environment. Consequently, Na2O, produced by the reaction of sodium and SnO2 with crystal water as the inducer, plays a critical role in the rapid encapsulation of sodium metal. Furthermore, the synthesized sodium metal electrode exhibited exceptional stability in cycles and resistance to dendrites. This discovery provides valuable insights into the reaction mechanisms of sodiophilic sites in sodium metals, serving as a crucial guide for further research in the field.

Effect of Hydrogen Addition on Soot Reduction in Diffusion Flames

This study explores the effects of hydrogen addition on soot reduction in methane-air diffusion flames, employing computational analysis coupled with chemical equilibrium and k-epsilon combustion turbulence models. By varying the fuel mixture with hydrogen mass percentages ranging from 0% to 11%, alterations in flame behaviour were observed. Results revealed a notable increase in maximum flame temperature at 1.3% hydrogen addition, attributed to enhance combustion efficiency, while subsequent additions led to temperature decrements due to dilution effects. Concurrently, soot production exhibited a general decrease, with reductions quantified up to 96.7% compared to baseline conditions, primarily due to improved oxidation and decreased carbon content. Additionally, the position of maximum flame temperature and soot mass fraction shifted along the axial direction owing to hydrogen's higher flammability compared to methane-air diffusion. These findings offer insights into optimizing flame characteristics for reduced particulate matter emissions, emphasizing the potential of hydrogen addition in mitigating soot formation in diffusion flames.

Experimental and numerical investigations of soot formation in propane laminar coflow flames in O2/N2 and O2/CO2 atmospheres

Oxygen-rich combustion is a new type of clean combustion technology with important application prospects. At present, there are few detailed analyses on the mechanism changes of soot formation under oxygen enrichment of propane. In this paper, the effects of oxygen-rich (O2/N2, O2/CO2) combustion on soot formation in the propane laminar flow coaxial jet diffusion flame were investigated by using the experiment and numerical simulation. The flame image was taken by a color (CCD) camera in the visible band, and the two-dimensional distribution of temperature and soot volume fraction in the flame was reconstructed. In numerical simulation, the soot production model considers a detailed description of nucleation via collisions among heavy polycyclic aromatic hydrocarbons, particle aggregation, polycyclic aromatic hydrocarbons condensation, surface growth and oxidation through the hydrogen abstraction acetylene addition mechanism. The results show that the experimental results were compared with the numerical simulation results, and the numerical simulation results predict the temperature and soot change of oxygen-rich flame well. With the increase in oxygen concentration, the peaks of soot and temperature in the two oxygen-rich atmospheres gradually increased. With the increase in oxygen concentration, the peaks of soot and temperature in the two oxygen-rich atmospheres gradually increased. Comparing the combustion mechanism changes of the two oxygen-rich combustion systems, it was found that the hydrogen abstraction acetylene addition mechanism was the main cause of soot generation. The OH oxidation is dominated near the fuel nozzle side, and O2 oxidation is dominant downstream. With the substitution of CO2 for N2, the chemical effect of CO2 reduces the flame temperature and inhibits the formation mechanism and oxidation mechanism of soot to some extent. This in turn leaded to a reduction in the amount of soot produced.

Coupling effect of electric field and corona discharge on combustion and soot formation of ethylene jet flame

The coupling of electric field (E-field) and corona discharge is designed to adjust the appearance and soot formation of the ethylene jet flame. The temperature and content of soot are measured by the improved two-color pyrometry (TCP). Experiments indicate that the flame is stretched and deflected laterally under the coupling field. The maximum temperature of soot and the proportion of high temperature zone in the flame increase, and oxidation rate of soot increases. The flame is more stable under the coupling of reverse E-field and corona discharge. From the non-coupled field to the coupling field of 100 kV/m and −100 kV/m, the temperature proportion of the flame in the range of 1700 K–1800 K increases from 0 to 44.2 % and 54.8 % respectively, and the peak oxidation rates of soot also increase by 204 % and 219 % respectively. The E-field in the coupling field accounts for the main influence. The high voltage side of the flame is affected by the ionic wind generated by corona discharge, and the amount of soot generated under the high coupling field is less than that on the left side.