Flame Extinction Research Articles

Experimental Study of the Influence of Even Harmonics on Flame Extinguishing by Low-Frequency Acoustic Waves with the Use of High-Power Extinguisher

The acoustic technique appears to be a novel and innovative way to extinguish flames, in which properly generated waves emitted by a high-power sound source are used for extinguishing purposes. The highest extinguishing efficiency is demonstrated by low-frequency waves. In practice, changing the parameters of the acoustic signal results in the possibility of universal and reusable use of the extinguisher, which is limited only by access to the power supply, unlike the currently known traditional methods of fighting fire (such as gases, foams, and extinguishing powders). The purpose of this paper is to analyze whether flame extinguishing by low-frequency acoustic waves is possible using signals containing higher harmonics with the use of large and very large powers delivered to the sound source, which is a scientific novelty. Analyzing the extinguishing capabilities of low-frequency acoustic waves allows one to fill the gap in the literature. This paper presents the results of research in the range of the influence of even sinusoidal harmonics on the extinguishing of flames originating from organic substances. For this purpose, in the experimental part, a high-power acoustic extinguisher and a point source of flames, i.e., a candle containing paraffin wax, were applied. The capabilities of the acoustic method in flame extinguishing have been experimentally demonstrated. The results address both the power that had to be delivered to the sound source of a high-power acoustic extinguisher to extinguish flames and the sound pressure level at which this phenomenon was observed. The added value is also to analyze how the order of even harmonics affects the process of acoustic extinguishment of flames (the order of harmonics for each fundamental frequency was varied from two to ten). Furthermore, the potential benefits and limitations of this method are explained, and future research directions are presented.

Experimental investigation on macrostructure and evolution of hydrogen-air micro-mix multi-jet flames

Considering the requirements for developing hydrogen combustion chambers and the application potential of micro-mix combustion, the flame macrostructure and evolution of hydrogen-air multi-microjet flames have been investigated to discover the relationship between flame macrostructure and thermoacoustic instability. A novel burner has been tested under different combustor liner lengths to simultaneously produce stable and unstable combustion under the same operation conditions. A compact conical flame shape without adjacent flame front interference is observed. In comparison with the combustor liner length, the flame temperature variation shows an insignificant effect on the thermoacoustic instability but triggers an oscillation mode transition from low-frequency (∼ 210-240Hz) to high-frequency (∼ 400-440Hz) under unstable combustion. Different flame evolutions are revealed for these oscillation modes. DMD analysis and LES simulation show that the high-frequency oscillation mode is mainly controlled by flame front oscillation and flame pinch-off process. However, the low-frequency oscillation mode is characterized by flame extinction on a large scale. Both equivalent ratio and velocity fluctuations contributed to flame and heat release oscillations under low and high flame temperatures. These findings help understand the mechanisms, driving hydrogen-air flame dynamics, and designing hydrogen combustors.

Insights into spatio-temporal dynamics during shock–droplet flame interaction

This study comprehensively investigates the response of a combusting droplet during its interaction with a high-speed transient flow imposed by a coaxially propagating blast wave. The blast wave is generated using a specially designed miniature shock generator that produces blast waves using the wire-explosion technique, facilitating a wide range of Mach numbers (1.03 < Ms < 1.8). The experiments are performed in two configurations: open field and focused blast wave. The charging voltage and the configuration determine the Mach number (Ms) and flow characteristics. The flame is found to exhibit two major response patterns: partial extinction followed by reignition and full extinction. Increasing the Mach number (Ms > 1.1) makes the droplet flame more vulnerable to extinction. Additionally, the flame exhibits stretching and shedding, followed by reignition at lower Mach numbers (Ms < 1.06). In all cases, the flame base lifts off in response to the imposed flow, and the advection of the flame base interacting with the flame tip results in flame extinction. The entire interaction occurs in two stages: (i) interaction with the blast wave and the decaying velocity profile associated with it, and (ii) interaction with the induced flow behind the blast wave as a result of the entrainment (delayed response). Alongside the flame's response, the droplet also interacts with the flow imposed by the blast wave, exhibiting different response modes including pure deformation, Rayleigh–Taylor piercing bag breakup and shear-induced stripping.

The design of a combustion chamber operated in MILD regime — Numerical modeling of hydrogen combustion in oxygen–steam mixtures

The goal of the current work is to develop a combustion chamber that can operate on recirculated steam produced by burning hydrogen in oxygen under conditions of Moderate or Intense oxygen Dilution (MILD) in atmospheric and stoichiometric conditions. The study investigates several configurations of combustor with nozzles, taking into account the overall temperature, OH radicals, and heat release rate distribution throughout the combustor’s domain. Thermal power variations of the steam generator (5 to 20 kW) were examined in conjunction with different oxygen dilutions with steam, down to 3% of O2 (by mol.). The outcomes reveal that a rise in dilution degree promotes a drop in the mean temperature across every case and reagents’ recirculation with homogeneous temperature field, suggesting the presence of MILD combustion. The highest temperature values were observed at the stoichiometric mixture fraction. Higher dilution degree revealed more efficient heat release across the domain with low fluctuations from the reference MILD combustion data. Of the two combustion models studied, the Partially Stirred Reactor model did not show flame extinction at the highest dilution degrees, unlike the Eddy Dissipation model. The selected final design of the combustion chamber was used for constructing the actual combustor dedicated for lab-scale operation.

Synthesis of phosphorus-containing modifier based on phenolic epoxy resin and its application in flexible poly(vinyl chloride)/magnesium hydroxide composites

In this study, a phosphorus-containing and polyether structure modifier, phenolic epoxy phosphate ester (PEPE), was prepared by the reaction of phenolic epoxy resin (EPN) and phosphoric acid. It was used to solve the trade-off dilemma of simultaneously improving the mechanical properties and flame resistance of flexible poly (vinyl chloride) (fPVC)/magnesium hydroxide (MH) composites. The peak heat release rate, total heat release, peak smoke production rate, and total smoke production of the fPVC/MHPEPE-5 (PEPE modified MH as filler) composite were decreased by 35.31 %, 49.2 %, 40.42 %, and 27.26 %, respectively, in comparison with the fPVC/MH composite. More importantly, the fPVC/MHPEPE-5 composite passed V-0 rating in the UL-94 test, while the fPVC and fPVC/MH composite passed V-2 and V-1 rating. The presence of phosphorus compounds in the condensed phase promoted the formation of a dense char residue of the fPVC/MHPEPE-5 composite. Therefore, the heat and flammable volatiles cannot migrate between the substrate zone and the combustion zone. In the gas phase, the dilution effect of H2O reduced the concentration of oxygen and combustible volatiles. The radicals quenching effect of the primary and secondary pyrolysis products of PEPE (such as PO· and PO2·) with ·H and ·OH. radicals played a crucial role in flame extinguishing and combustion termination. In addition, the scanning electron microscopy results showed that MHPEPE performed good compatibility with the fPVC matrix. The tensile and impact strength of the fPVC/MHPEPE-5 composite was 12.19 % and 19.26 % higher than that of the fPVC/MH composite, respectively.

Flow-field analysis and performance assessment of rotating detonation engines under different number of discrete inlet nozzles

This study explores in depth rotating detonation engines (RDEs) fueled by premixed stoichiometric hydrogen/air mixtures through two-dimensional numerical simulations including a detailed chemical kinetic mechanism. To model the spatial reactant non-uniformities observed in practical RDE combustors, the referred simulations incorporate different numbers of discrete inlet nozzles. The primary focus here is to analyze the influence of reactant non-uniformities on detonation combustion dynamics in RDEs. By systematically varying the number of reactant injection nozzles (from 15 to 240), while maintaining a constant total injection area, the study delves into how this variation influences the behavior of rotating detonation waves (RDWs) and the associated overall flow field structure. The numerical results obtained here reveal significant effects of the number of inlets employed on both RDE stability (self-sustaining detonation wave) and performance. RDE configurations with a lower number of inlets exhibit a detonation front with chaotic behavior (pressure oscillations) due to an increased amount of unburned gas ahead of the detonation wave. This chaotic behavior can lead to the flame extinguishing or decreasing in intensity, ultimately diminishing the engine's overall performance. Conversely, RDE configurations with a higher number of inlets feature smoother detonation propagations without chaotic transients, leading to more stable and reliable performance metrics. This study uses high-fidelity numerical techniques such as adaptive mesh refinement (AMR) and the PeleC compressible reacting flow solver. This comprehensive approach enables a thorough evaluation of critical RDE characteristics including detonation velocity, fuel mass flow rate, impulse, thrust, and reverse pressure waves under varying reactant injection conditions. The insights derived from the numerical simulations carried out here enhance the understanding of the fundamental processes governing the performance of RDE concepts.

DNS of laboratory-scale turbulent premixed counterflow flames under elevated gravity conditions

In the present work, direct numerical simulations (DNSs) of laboratory-scale turbulent premixed counterflow flames are performed to understand the effect of elevated gravity on flame structures. In the DNS, a turbulent jet of CH4/N2/O2 mixture is injected in opposition to a stream of combustion product. Three cases were considered, i.e., one case with normal gravity and two cases with elevated gravity in different directions. The DNS results of the case under normal gravity conditions were found to be in good agreement with the experimental data. The turbulent flame speed is the highest in the case with elevated gravity aligned with the mean flame propagating direction, which is due to the increase in flame surface area and stretch factor. The gravity levels have a substantial influence on the flame extinction characteristics. It was observed that the flame front is pushed toward the direction of the gravity. When the elevated gravity antialigns with the mean flame propagating direction, the flame front travels across the stagnation plane toward the combustion product stream, leading to significant reactant dilution and flame extinction. The flame normal vector preferentially aligns with the most compressive strain rate for various gravity conditions. The tangential strain rate is the greatest (smallest) when elevated gravity antialigns (aligns) with the mean flame propagating direction. The impact of elevated gravity on the tangential strain rate at the flame fronts ultimately leads to a displacement in the flame front location relative to the stagnation plane, thereby influencing flame extinction.

Burning and flame extinction patterns of PMMA under external heating and oxygen deficiency

The paper presents an experimental study investigating the impact of under-oxygenated conditions on the burning characteristics and flame extinction of horizontal transparent poly(methyl-methacrylate) (PMMA) slabs subjected to external heating. Small-sized solids are exposed to various constant levels of radiant heat flux in a nitrogen-diluted environment at ambient pressure. The study aims to understand the multivariable dependence of PMMA combustion on oxygen and heating levels. The analysis goes beyond the study of global and local variables to provide an understanding of the mechanisms involved, revealing a notable effect on burning characteristics and flame extinction mechanisms by blow-off. The results provide a detailed assessment of the surface energy balance and flame bulk properties. The results demonstrate good repeatability up to a certain oxygen limit, beyond which the flame combustion regime exhibits stochastic behaviour. The study of heat transfer mechanisms has revealed that flame radiation and external heating play a predominant role in the heating of horizontal solid fuels, In contrast, within the flame, convection is the main contributor to solid heating, rather than flame radiation. Solid characteristics show linear trends proportional to oxygen depletion, while gas phase characteristics show monotonic trends strongly influenced by the changing combustion regime. The surface temperature of the solid is highly sensitive to the test conditions that affect the energy balance. Dimensionless parameters are developed and data are correlated with existing literature. The analysis demonstrates that the linear or monotonic trends are maintained regardless of external heating. Finally, models are proposed and validated to predict both the extinction limit and the burning rate independently of irradiance and oxygen levels.

Experimental study on the transitional behavior of the room fire under sub-atmospheric pressures

Fuel diffusion combustion and flame behavior within a confined space is a fundamental scientific problem in the energy and combustion field, it involves complicated chemical reaction, fire behavior, thermal dynamics, heat transfer, room-opening flowing and sub-processes of over-ventilated and under-ventilated room fire. However, previous classical knowledges and models about it are usually at standard atmospheric pressure (100 kPa), no work considers the fire evolution and flame behavior within the room for various sub-atmospheric pressures. In this paper, the experiment is carried by utilizing a 0.3 m × 0.3 m × 0.3 m reduced-scale cubic room with an opening of various dimensions, and using propane as fuel to simulate fuel combustion inside, which was placed at the Low Pressure Chamber creating various atmospheric pressures. The flame behavior and temperature evolution within the room was recorded and measured with 693 experimental conditions. The experimental results show that, (1) the air inflowing rate through opening into room and heat release rate within the room increase with atmospheric pressure; (2) the flame extinction or reaching well-mixed condition are more easily occurred for lower atmospheric pressure and opening dimension. These characteristic parameters and fire behaviors could be well described by new corrected opening ventilation factor involving effect of atmospheric pressure and the opening dimension. The research work fills the gap in basic scientific research of building fire at sub-atmospheric pressures, which also has important practical applicability of urban building fire protection design code (such as the materials in wall) in different altitudes, room fire suppression, and protecting urban safety.

Predicting extinction limits of concurrent smoldering spread by a reduced analytical model

Smoldering is a flameless combustion mode occurring on the surface of charring fuels, such as wood and cigarettes. Although the smoldering process is slow and has a low temperature compared to flaming, it is easy to be initiated by a weak heat source and persists under poor oxygen conditions. Extensive work has been done for flame extinction to develop scaling models to predict the limiting oxygen concentration (LOC), but limited work is available for the smoldering extinction. This study develops a reduced analytical model to predict the extinction limits of smoldering. The model simultaneously solves smoldering propagation rate, surface temperature, and surface oxygen mass fraction as part of the solutions. The extinction limit is determined as the critical condition where solutions satisfying all governing equations cease to exist. The model provides a qualitative description and captures the essential characteristics of a previous experiment. The smoldering rate decreases with increasing fuel diameter, and a larger-diameter fuel is easier to extinguish. The mechanisms of the extinction process are investigated, showing the dominant role of radiative heat loss in the smothering limit at low airflow velocities and convective heat loss near the blowoff limit at high airflow velocities. Further analysis of the effect of oxygen concentration shows an increasing trend of LOC with fuel diameter, and the smothering branch cannot be predicted without considering the heat loss through radiation from the solid surface. Novelty and Significance StatementThe novelty of this research is the prediction of smoldering propagation rates and extinction limits across various fuel diameters using a reduced analytical model. The model is modified to accommodate a thin fuel configuration and high airflow velocity condition, where convective heat and mass transfer play a more important role. The model's significance is that it not only provides essential insights into the mechanisms but also has the capability to simultaneously determine key parameters such as spread rate, reaction temperatures, and surface oxygen concentration as part of the solutions. Additionally, this study demonstrates the model's ability to reproduce the limit conditions, including smothering/blowoff limits and limiting oxygen concentration (LOC).

Optical study on spray mixing, flame propagation and jets evolution within visible methanol active pre-chamber for turbulent jet ignition

Methanol spray impingement, mixing, flame propagation and jet evolution within the narrow-confined active pre-chamber (APC) with complex walls were experimentally observed by optical method for the first time. The effects of injection pressure and duration was investigated on an actual-geometry optical APC assembled in constant volume combustion chamber (CVC). Exploration on APC internal processes within static environments forms the basis for optimization of APC and analysis of jet formation. Mie scattering coupled with flame natural luminosity imaging were used to record spray mixing and combustion. The results show that in the APC with narrow-confined space, methanol spray undergoes three wall impingements, inducing a tumble that promotes mixing. High injection pressure enhances rapid spray tip penetration, while long injection duration slows methanol diffusion. In longer injection duration cases, the spray tip after the third impingement interferes with subsequent sprays. For higher injection pressure, this interference occurs in the APC cone section due to a larger spray cone angle, whereas for lower pressure, it happens in the APC duct section. Higher injection pressure also causes more spray to impact the APC bottom after the first wall impingement, hindering flame acceleration in the duct section. Irregular flame propagation is observed in the APC cone section due to wider space and uneven mixture conditions, while the convergent geometry from the APC throat to the duct section significantly accelerates flame propagation. The APC jet appears due to the pressure difference between the APC and the main chamber, with the brightness and length of the visible flame jet increasing with APC flame luminosity and pressure difference. A Mach ring structure is observed initially, but the jet velocity decreases as the Mach ring disappears during the APC inner flame extinguishing process. Through this study, the actual spraying and combustion process in APC is experimentally captured and revealed, which are an important contribution for APC optimization and development.

Numerical Study on Chemical Kinetic Characteristics of Counterflow Diffusion Flame Extinction of Methane/Ammonia/Air Flame under High Pressure or Air Preheating Temperature.

Green ammonia has become an increasingly popular fuel in recent years because of its combustion process without carbon oxide release. Adding ammonia to methane fuel for co-combustion has become one of the important research topics in the current combustion field. In the present study, the CH4/NH3/Air counterflow diffusion flame was taken as the research object, and Chemkin-2019 R3 software was used to explore and analyze the flame extinction limit and chemical kinetics characteristics under different ammonia mixing ratios, initial pressures, and air preheating temperatures. It was obtained that the flame extinction stretch rate was decreased by increasing the NH3 mole fraction in the CH4/NH3 mixed fuel. The increase in pressure or air preheating temperature would accelerate the chemical reaction rate of each component in the combustion process, increase the flame extinction limit, and counteract the "stretching effect" of the flame, thus restraining the flame extinguishing phenomenon. The results of a path analysis show that the formation and consumption of OH had an important influence on flame extinction in the chain reaction. The net reaction rate of OH increases with increasing the initial pressure or air preheating temperature, which leads to an increase in flame intensity, combustion stability, and the extinction limit. Furthermore, the function curve between the reaction influences the RIF factor and the stretch rate of the first-to-ten reactions, affected by the heat release of flame combustion, was drawn and quantitatively analyzed. Eventually, a sensitivity analysis of the flame under different working conditions was completed, which found that promoting the forward reaction R39 H + O2<=>O + OH also promotes the positive combustion as a whole when the flame was near extinction. The sensitivity coefficient of R39 in the CH4/NH3/Air flame increases with the growing initial pressure. The increasing air preheating temperature was capable of switching the reaction of R248 NH2 + OH<=>NH + H2O in the CH4/NH3/Air flame from an inhibiting reaction to a promoting reaction, while decreasing the sensitivity coefficient of inhibiting the forward reaction R10 O + CH3<=>H + CH2O, R88 OH + HO2<=>O2 + H2O, and R271 H + NO + M<=>HNO + M. Thus, the inhibition effect of flame extinction was weakened, and the positive progress of combustion was promoted.

Reduced scale test bench for investigating the upward flame heat impact on external thermal insulation composite system facades

The project aimed to develop a reduced scale test protocol designed to be repeatable and capable of providing the upward fire evaluation for assessing fire risk on external thermal insulation composite system facade samples. The variation of heat flux is simulated by radiant panel. This experimental setup permits the recording of dynamic variables in flame propagation, such as ignition time, flame extinguishing time, and variations in both surface and back temperatures of the insulation panel. Notably, the measurement of back temperature provides a means to assess the thermal insulation efficiency of the external thermal insulation composite system facade. Furthermore, the fire spreading temperature is collected during the test could also be useful for larger-scale testing.

Combustion and extinction characteristics of an ethanol pool fire perturbed by low–frequency acoustic waves

To study combustion and extinction characteristics of flames perturbed by acoustic waves, the evolution of the shape of the pool fire, mass loss rate (ma′) of fuel, and mechanisms of extinction of the flames under acoustic perturbations were studied. Moreover, based on parameters of combustion characteristics of flames, parametric models of ma′ and flame extinction were developed. The research results demonstrate that the shapes of unperturbed flames change during combustion, however, when perturbed by acoustic waves, the flame shapes become irregular and the flame fronts become disorderly. Compared with the combustion of unperturbed flames, ma′ is increased under acoustic perturbation. There are upper and lower limits for the change of ma′, so the models of upper and lower limits of ma′ in the concentrated area were established. Low–frequency acoustic waves tend to extinguish the pool fire, and the increased distance of acoustic response and acoustic source power can promote flame extinction. A linear relationship between flame extinction and acoustic frequency was semi-quantitatively characterized by a modified Damköhler (Da*) number. Based on the critical oscillation–induced displacement for extinguishing a pool fire, local acoustic parameters were characterized, and a critical model for local displacement during the extinction of the pool fire was established.

Ignition and combustion characteristics of magnesium-based nanofluid fuel

Mg-based nanofluid fuel is a promising fuel for propulsion system in Mars exploration. In this study, Mg-based nanofluid fuel were prepared by one-step method, and the effect of Mg(magnesium) content, Mg size(68 μm,110 μm,175 μm), and oleic acid (OA) on droplet ignition and combustion under the atmosphere of 90 % CO2 and 10 % O2 were investigated. According to the TG-DSC results, 68 m Mg particles under CO2 had the highest exothermic efficiency of 4472 J/g. The combustion of Mg/OA/kerosene droplets consists of five stages: ignition, d2-law combustion, micro-explosion, vapor flame extinguishment, and agglomerate explosion. The ignition delay time of Mg (68 μm, 5 %)/kerosene is 0.045s, which is half that of kerosene (0.0890s). The agglomerates explode violently and emit a brilliant white light at the end of the Mg-based nanofluid fuel combustion. The explosive effect of Mg agglomerates diminishes as the Mg content increases. The rate of combustion decreases with increasing particle size. In the sample with added oleic acid, Mg (68 μm, 5 %)/OA/kerosene shows the best combustion rate of 0.6320 mm2/s. Finally, a numerical model for the combustion of Mg-based nanofluid fuel droplets is proposed. The predictions are within 2 % of the experimental results. The findings are expected to explore future in-situ utilization of CO2 on the Mars surface.

Experimental Attempts of Using Modulated and Unmodulated Waves in Low-Frequency Acoustic Wave Flame Extinguishing Technology: A Review of Selected Cases

In practice, some undesired signals perceived as noise (because of the sound sensation) can effectively extinguish flames. This article provides information on the applicability of acoustic waves (a case of using sine waves and sine waves amplitude modulated by a square waveform) in low-frequency acoustic wave flame extinguishing technology based on experimental results obtained from a laboratory stand, which is a scientific novelty. The benefits of using deep neural networks for flame detection based on dedicated solutions developed through international cooperation are also recognized. In addition, the potential advantages of combining both technologies (use in fully automatic systems), limitations, open problems, and plans for further research are identified. The paper concludes that it is possible to extinguish flames as soon as they appear (firebreaks) and prevent large fires from breaking out. This feature may effectively reduce material losses and increase widespread safety.

The Suppression Effect of Water Mist Released at Different Stages on Lithium-Ion Battery Flame Temperature, Heat Release, and Heat Radiation

Thermal runaway (TR) is a serious thermal disaster that occurs in lithium-ion batteries (LIBs) under extreme conditions and has long been an obstacle to their further development. Water mist (WM) is considered to have excellent cooling capacity and is widely used in the field of fire protection. When used in TR suppression, WM also exhibits strong fire-extinguishing and anti-re-ignition abilities. Therefore, it has received widespread attention and research interest among scholars. However, most studies have focused on the cooling rate and suppression effect of TR propagation, and few have mentioned the effect of WM on flame heat transfer, which is a significant index in TR propagation suppression. This study has explored the suppression effect of WM released at different TR stages and has analyzed flame temperature, heat release, and heat radiation under WM conditions. Results show that the flame extinguishing duration for WM under different TR stages was different. WM could directly put out the flame within several seconds of being released when SV opened, 3 min after SV opening and when TR ended, and 3 min for WM when TR was triggered. Moreover, the heat radiation of the flame in relation to the battery QE could be calculated, and the case of WM released 3 min after SV opening exhibited the greatest proportion of heat radiation cooling η (with a value of 88.4%), which was same for the specific cooling capacity of WM Qm with a value of 1.7 × 10−3 kJ/kg. This is expected to provide a novel focus for TR suppression in LIBs.

Effect of preheating on extending lean extinction limit of ammonia/air combustion in a two-section porous burner

Ammonia utilization is promising way to reduce greenhouse gases emissions. We propose the concept of reactants preheating to achieve stable combustion of lean NH3 in two-layer porous burner, this concept is validated by numerical study with full kinetics. Calculations are conducted over wide range of preheating temperature (Tpre) and mass flow rate (m″) to gain insight in stable map, with special emphasis on effect of Tpre on extending lean extinction limit (LEL). It is shown that Tpre has vital effect in expending LEL. LEL is continually extended for different m″ with preheating, the LEL for NH3/air is successfully extended from equivalence ratio (ϕ) of 0.54 to 0.08 with increasing Tpre from 400 K to 1000 K. It is found that there exist three distinct mechanisms (Ⅰ, Ⅱ and Ⅲ) with respect to ϕ. When ϕ is close to one, self-sustaining combustion is realized without preheating (Ⅲ). The combustion is becoming unstable when ϕ is decreased, stable combustion can only be achieved with preheating (Ⅰ). Flame extinction takes place even with the preheating (Ⅱ) when the Tpre is lower than critical one for the given ϕ. The results present guidelines for burner operating with lean NH3 mixture over wide parameters range.

Response Behavior of Inverse Diffusion Flame to Low Frequency Acoustic Field

ABSTRACT Conducting the investigation of flame dynamics perturbed by acoustic waves had guiding significance for flame extinguishing technology. In this paper, the behavior of inverse diffusion flame under the influence of 200–300 Hz sound waves was reported for the first time. Detailed analysis of the entire process of flame from steady state to extinction, including flame structure, periodic characteristics, temperature and KL factor distribution. The relationship between flame morphology parameters and acoustic parameters was established through dimensionless analysis. In addition, the flame extinguishing performance of sound waves in this frequency band was measured and analyzed. The results indicated that the flame could still maintain the typical inverse diffusion flame shape under low sound pressure, and the pulsation frequency was stable. However, when the sound pressure rose to a certain level, the flame began to pulsation in a disorderly manner. In addition, with the increase of sound pressure, the flame morphology would undergo significant changes, and the temperature and KL factor showed a trend of first increasing and then decreasing. The mechanism of sound wave suppression of flames was related to the unsteady flow generated by sound waves. Through dimensionless analysis, it was found that the intensity of this disturbance was directly proportional to the arithmetic square root of sound pressure and inversely proportional to frequency.

Physics-guided fuel-switching neural networks for stable combustion of low calorific industrial gas

Dual fuel combustion of low calorific industrial gas has been widely used in various types of burners for industrial production needs. Fuel switching rule is purely empirical lacking theoretic basis, potentially leading to flame extinction or excessive waste of ignition fuel. This study focuses on exploring physics-guided fuel-switching strategy for stable combustion of coke oven gas (COG) and blast furnace gas (BFG) in representative swirl burner. Stable operating regimes for COG and BFG are firstly identified with chemical eigen-analysis. The intrinsic propensity of flame extinction during fuel switching is revealed by positive chemical eigenvalues, and the key species and elementary reactions are identified. A clustered neural network with flag controller (CNNF) is then constructed to achieve stable combustion during fuel switching from COG to BFG as early as possible to reduce the use of COG. Results show that COG must be used to ignite and increase the ambient temperature to at least 673 K to avoid flame extinction. The chemical eigen-guided CNNF controller can adjust the switch speed to advance the fuel switching time by 14 % and can be further expanded to other fuel switching applications.