Flame Behavior Research Articles

Impact of P-1 radiation model on simulated free jet flame characteristics of gaseous fuels: CFD with PDF approach

Simulation and analysis of a turbulent free jet flame erupting into still air are done using CFD. Using 2-D axisymmetric numerical modelling in ANSYS-FLUENT 14.5. Three distinct kinds of gaseous fuels are used: CH4, CO, and biogas (50% CH4 and 50% CO2). The effects of thermal radiation modelling utilizing the P-1 radiation model on the behavior of a free jet flame are investigated, and the impacts of air temperature and fuel velocity on the flame length are also provided. The findings demonstrated that the radiation modelling did not affect the temperature distribution and flame length for CO and biogas (i.e., lower heating value fuels). Nevertheless, the air temperature and fuel kind considerably impact the flame behavior. While the fuel inlet velocity (i.e., burner power) does not affect the flame length. Additionally, free jet flame velocity and length numerical correlations considering radiation modelling are predicted and presented with allowable errors. A comparison with earlier experimental correlation proved successful, with a maximum error of ?9.4%.

Experimental Studies on the Fire Flame Behavior of Reinforced Concrete Beams with Construction Joints

This study was experimentally investigated experimentally, investigating the effect of such mechanical properties and deflection behavior on the performance of reinforced concrete beams with construction joints exposed to fire flame. These beams were built using a slanted connection at a 45° angle in the middle of the beams. Four beams were for a wide temperature range (25–800°C) for the fire flames used on the concrete beams. Two temperature levels (600°C and 800°C) were chosen, with (1 and 2) hrs. period time, respectively. One beam wasn't burnt (Reference specimen). When put through a fire, flexural strength drastically decreased. The experimental program's results suggested the worst effect on construction joint beam for temperature 800°C with 2 hrs. period time because that heating reduces the bonds between two surfaces in the joint and makes like slip and disconnect between the joint after exposure to load. After 1 and 2 hours in a fire at 600°C, the residual flexural strength was 85% and 72%, respectively, whereas, after 1 and 2 hours in a fire at 800°C, it was 41% and 28%, this is showed that the stiffness of beam its decrease when exposed to fire flam with raising temperature and time and increase the deformation by cracks that appear on the beams. It was noticed that the load-deflection relation to beams exposed to fire flame is flat, representing softer load-deflection behavior than that of the control beam.

A comparative study of the combustion dynamics and flame properties of dead Mediterranean plants

Background The physical processes governing flame behaviour are key elements for a better understanding of forest fires.Aims To study the combustion properties of several dead Mediterranean forest fuels.Method Samples of straw, eucalyptus, shrubs and Pinus Pinaster with the same load were placed in circular containers of the same size, and ignited in the absence of wind.Key results The combustion parameters (burning rate, flame height, temperature and gas velocity) evolved according to the same trend regardless of the fuel type. A new law is proposed to account for the anomalous relaxation processes occurring in the growth and decay phases of the flame. The dynamic exponent depends on the vegetation type only in the growth phase (highest for Pinus Pinaster and lowest for straw). The relaxation times are shortest for shrubs and largest for straw. The maximum flame height and burning rate are largest for shrubs and lowest for straw. Froude modelling suggests that the scaling behaviour of the flame may depend on the fuel type.Conclusions The observed relaxation parameters driving fire dynamics and the combustion characteristics depend on the nature of the fuel.Implications Further investigation of the vegetation region’s influence on these properties is necessary.

Experimental study of the burning characteristics of dead forest fuels

Background A deeper physical understanding of flame behaviour is necessary to make more reliable predictions about forest fire dynamics. Aims To study the container size effect on the combustion characteristics of herbaceous fuels. Methods Dead samples were put in cylindrical containers of different sizes, and were ignited at the lowest circumference of the basket in the absence of wind. Key results In the growth phase, there is an anomalously fast relaxation of the fuel mass accompanied by a super-diffusion of the emitted gas species, whereas in the decay phase, there is a stretched exponential relaxation and the gas species sub-diffuse through the porous fuel. The crossover between these two anomalous processes occurs when the flame is fully developed. Thomas’s correlation between flame height and fuel bed size, and the two-third power law dependence of the normalised flame height on the Froude number, fit the experimental data well. The flame height variation with burning rate exhibits a hysteresis cycle, implying the existence of memory effects on flame formation. Conclusions The observed relaxation regimes and hysteresis cycle seem to drive fire dynamics and behaviour. Implications Further investigation of the influence of the fuel geometry and porosity on these properties is necessary.

The motivation of flaming perpetrators as cyberbullying behavior in social media

The most troubling behavior of cyberbullying is flaming on social media. Technological sophistication makes it easy for everyone to create fake accounts for negative comments to bullying activities on social media. The impact is very detrimental, especially for victims or targets. The most frequent consequences are deep trauma, fear, anxiety, lack of confidence, mental illness, and depression, and they can even motivate the victim to commit suicide. This impact is considered quite dangerous for the victim. Therefore, concrete handling and solutions are needed that can be implemented according to the motivation of the perpetrators to do hate speech and cyberbullying. This study aimed to identify the motives of flaming perpetrators through social media and formulate strategic and practical recommendations for preventing flaming behavior. By identifying through a qualitative approach, phenomenological research strategies, and in-depth interviews with 10 (ten) research informants, the research aims to obtain a comprehensive picture of the motivations of flaming actors in social media. The results of this study indicate some of the motivations of flaming actors in social media, namely excessive fanaticism, personal interests, spontaneous emotions and anxiety, low digital literacy, the anonymity of netizens on social media through second accounts, and lack of empathy. The motivations found in this study are the basis for formulating strategic and practical recommendations in preventing flaming behavior in social media through the KIFE Model (Collaboration, Innovation, News Framing, and Peer group Education). It is practical and effective for Media Literacy and Healthy Digital.

Effect of slit pattern on the structure of premixed flames issuing from perforated burners in domestic condensing boilers

Domestic condensing boilers are equipped with perforated burners ensuring short-length premixed flames issues from a series of circular holes and slits. Despite some efforts that have been devoted to understanding the effect of hole diameter, pattern, and hole-to-hole distance on the resulting flames, very little is known about the flames from a series of slits. In this work 3-dimensional numerical simulations with skeletal kinetic mechanisms are performed to determine the structure of premixed methane-air flames issuing from two-slits patterns, often recurring in practical burner designs, i.e. several equally-spaced slits and a group of four slits. The arrangement of the slits greatly influences the resulting flames. A significant change in the flame behaviour occurs depending on the inlet velocity; at low speeds, the flames issuing from the slits present a flat region, while increasing the speed they assume a conical shape along the slit length. Neighbour flames are distinct from each other at low speeds, while they interact strongly with increasing the velocity. Interestingly, a series of several slits produce a single long wedge-shaped flame, while the group of four slits generates a single conical flame.

Thermal radiation distribution of low-momentum jet diffusion flame under the effect of cross airflow

Radiation from a fire usually plays a dominant role in its spread and may pose a significant threat to the surrounding environment. In this paper, flame evolution characteristics and the radiation behavior of low-momentum jet flames affected by cross airflow is investigated by varying the cross-airflow velocity and jet exit velocity. As the results show, the flame length increases at first, and then decreases with the increase of cross airflow. In addition, increased cross airflow causes the flame's inclined angle to gradually increase, while its width decreases. A model considering cross airflow velocity and fuel jet velocity is proposed for the prediction of radiation distribution. Furthermore, it has been found that the radiation flux downstream of the fire source increases at first, and then begins to decrease with the gradual increase in cross airflow velocity. Finally, an inclined cylindrical flame model is proposed to predict the radiation flux of a low-momentum jet flame under the effect of cross airflow, and a further model of flame radiation fraction considering cross airflow velocity and jet momentum is developed. These findings may provide some reference for the design of safer gas storage and transportation facilities.

Experimental studies of the effect of burner location on the development of building fires

Building-related safety design and assessment requires precise and quantifiable fire laws. However, the effect of fire source location on flame behavior and combustion products has not been fully elaborated leading to a lack of precision in the corresponding risk assessment. The experiments were carried out in a 1/2-scale compartment to investigate the effect of burner location on the self-extinguishing behavior. Six burner positions were used and combustion conditions were varied by adjusting the opening factor and total heat release rate (HRR). Flame behavior, temperature, critical HRR, oxygen and carbon monoxide concentrations were recorded and analyzed. Results showed that the burner location could change the flame behavior and its critical conditions. The effect of burner position on the roof temperature could be quantified by the newly proposed position factor. The oxygen and carbon monoxide concentrations in the smoke layer were significantly higher when the burner was near a wall or corner than in other positions. The critical HRR for flame extinction varied with burner position because the process by which the flame entraining air and air into the opening was changed. Experimental results in this study provide accurate data support and physical mechanisms for predicting building fire development.

Dynamic stabilization of a hydrogen premixed flame in a narrow channel

Combustion of hydrogen can help in reducing carbon-based emissions but it also poses unique challenges related to the high flame speed and Lewis number effects of the hydrogen flame. When operated with conventional burners, a hydrogen flame can flashback at higher volumetric flow rates than a methane flame due to the difference in stabilization mechanisms of the two fuels. Due to these differences, conventional burners cannot offer similar operational ranges for hydrogen than that for hydrocarbon flames. An exploration into the unique stabilization behaviour of hydrogen flames is required which could help in envisioning non-conventional burner concepts for keeping hydrogen flames stable. Stability conditions, which describe the kinematics of premixed flames with spatially and temporally changing flow parameters, are crucial for such an exploration. Stability conditions are usually hypothesized for stable flames, where a flame upon perturbation is assumed to return to its original position. Alternatively, in the case of flashback/blow-off, it refers to a flame moving upstream of the burner or being convected out of the domain. However, it is also of interest to understand how and why a flame could move to a new location when the velocity and strain fields are varying with time and space at the original and the new location. In this paper, we investigate the flame stabilization by 1) observing the hydrogen flame’s upstream movement in a multi-slit configuration when a geometrical change is made, and 2) changing strain and velocity fields in a dynamic and periodic manner using numerical tools such that the unique behaviour of a hydrogen flame can be captured. We vary the location of high flow strain periodically in a channel by manipulating the boundary condition along a wall. It is found that a hydrogen flame follows this point in a periodic manner, also propagating against the inflow which is considerably faster than its unstretched burning velocity. Spatial and temporal stability conditions, that explain the mechanism behind the flame’s movement from its original position to a new position, are analyzed from the simulation data, advancing our knowledge on the flame movement in an unsteady setting and providing important insights into the stabilization mechanism of hydrogen flames.

Experimental and numerical study of the fuel effect on flame propagation in long open tubes

Previous works (Daubech et al., 2019) were dedicated to gaseous flame acceleration along long pipes with a set of cases studied both experimentally and numerically. In these cases, the flammable mixture was initially quiescent and homogenously distributed. The impact of the tube diameter and material were studied trough both approaches for rather slow flames, the fuel being methane. While main features of the real flame were recovered by the chosen CFD method, some limits remained.A new experimental dataset is detailed and analyzed with a quicker flame, the fuel being hydrogen and the same experimental set-up as the one used for measuring slow flames. Thus, the fuel effect on the flame dynamics can be directly highlighted.A simple CFD approach is tested for recovering two distinct flame behaviors: a deflagration flame and another undergoing deflagration-to-detonation transition. Furthermore, the modelling results are used to propose elements of interpretation for flame acceleration.

Temperature evolution, flame behavior and extinction inside compartment with a ceiling vent under the ambient wind

This article presents an experimental and CFD simulation investigation and analysis on temperature evolution and flame behavior inside compartment with a ceiling vent under ambient wind. Experiments were conducted employing a reduced-scale model containing a cubic fire compartment with a ceiling vent under the external wind generated by a wind tunnel. The temperature of windward and leeward inside compartment as well as the flame behavior were recorded for various vent dimensions, heat release rates and wind speeds for a total of 720 test conditions. Results show that there are two types of fire behavior regimes inside the compartment for relatively small- (Bernoulli flow regime) or large vents (oscillatory exchange flow regime). With a relatively small vent, the flame is located at the center of compartment and the temperatures of windward and leeward are almost the same with or without the ambient wind. Then the flame turns to extinct due to lack of oxygen with increasing of fuel supply. However, with a relatively large vent, there is a flame transition from windward to leeward accompanied by the transition of temperature distribution with increasing of fuel supply when subject to ambient wind. Such extinction and transition mechanisms are interpreted by the aid of CFD simulation of global equivalence ratio and flow/oxygen field inside compartment. The critical heat release rate for the occurrence of flame transition decreases with raising wind speed while increases with vent size. Their complex dependence is found to be well represented in terms of a non-dimensional heat release rate as a function of the wind Froude number, employing the vent area-equivalent characteristic diameter as length scale. These new findings facilitate the understanding of the compartment fire evolution with a ceiling vent subject to ambient wind.

A NUMERICAL STUDY ON THE TRANSITION CHARACTERISTICS OF AN INVERSE MICRO-SCALE DIFFUSION FLAME TO MICRO-SCALE PREMIXED FLAME UNDER HEAT RECIRCULATION CONDITIONS

In this investigation, the gaseous hydrogen peroxide (H2O2) is issued at the inner port, and methane (CH4) is injected at the outer port of the sub-millimeter scale burner in order to establish an inverse CH4/H2O2 micro-scale diffusion flame numerically under heat recirculation conditions. The velocity of gaseous H2O2 is considered only as a numerical parameter in this study and the velocity of CH4 is kept constant to examine the flame structure and the transition behavior of an inverse micro-scale diffusion flame. The numerical simulation has been conducted using ANSYS Fluent 14.5 based on Finite Volume Method (FVM) under normal gravity (1G) conditions with low thermally conductive burners. The semi-detailed reaction model, which consists of 58 chemical reactions and 17 chemical species, has been applied in this study. It is found that when the momentum of the gaseous H2O2 is high, then a CH4 /H2O2 inverse micro-scale diffusion flame is formed, and this flame is attached just on the top of the burner by which one can easily analysis an active flame-wall interaction phenomenon. On the other hand, when the momentum of the gaseous H2O2 has been reduced significantly, then H2 /O2 micro-scale premixed flame is established entirely inside the micro-burner. Furthermore, it is found that when the momentum of H2O2 reduces remarkably, then CH4 does not diffuse inside the micro-burner at the flame zone, and consequently only H2 /O2 micro-scale premixed flame is established there as H2O2 is playing the role of a monopropellant, where no extra oxidizer is needed for the survival of flames. This type of flame mode transition phenomenon under micro-burner systems is the first ever reported event in the literature of reacting fluid dynamics (i.e., combustion dynamics). Besides, this is the primary findings of our ongoing research, and hence, the further numerical calculation adopting different pertinent parameters is now under progress with the aim of extracting more profound physical and chemical insights that are associated with the aforesaid transition phenomena.

Lift-off behaviors of the partially-premixed jet flame in a supersonic vitiated coflow

Achieving fast and efficient combustion is considered as the primary difficulty in overcoming the high-Mach-number scramjet. A novel injection strategy with coflows has been proposed to improve the combustion efficiency, however, the inherent dynamics that drives the partially-premixed combustion is still not very clear. Numerical simulation is adopted to investigate the supersonic partially-premixed flame formed by the coflows of inner jet and annular vitiated air. Validation of numerical results has been conducted by comparing with the experimental data, and the predicted values show a reasonably good agreement with that in the experiments. Numerical results indicate that the supersonic jet flame is stabilized with a certain of lift-off height, with the dynamics balance of flow transport and flame propagation in the autoignition-dominated flame base. According to the flame index, the supersonic reactive flow fields are divided into three stages, including the no reaction region, premixed flame region and diffusion flame region. On this basis, the influence of equivalence ratio and injecting velocity of inner jet on the lift-off behaviors of jet flame, is explored by a series of contrastive analysis.

Flame behaviors and explosion characteristics of two‐phase propylene oxide/air mixture under different ambient pressures

AbstractPropylene oxide (PO) is widely used in fuel‐air explosives and pulse detonation engines. Based on computational fluid dynamics, the effects of ignition position, initial pressure, and mass concentration on the explosion characteristics of the PO/air mixture were studied by using a 20 L spherical container. The results showed that the ignition position had a significant effect on the flame structure. When the fuel cloud was ignited at the center, the flame structure experienced spherical, elliptical, and tulip shapes. The explosion time first decreased and then increased with the increase of ignition position, while the maximum explosion pressure decreased correspondingly. The maximum explosion pressure and the temperature had an inverted “U” correlation with concentration at normal pressure. However, the overpressure was insensitive to concentration in the negative pressure condition. The maximum flame temperature showed different flame behaviors under different initial negative pressures and cloud concentrations. This study provides a theoretical basis for understanding the explosion behaviors of PO/air mixture with different ignition positions and initial vacuum pressures, which is of great significance in preventing explosion accidents.

Experimental studies on flame propagation and detonation characteristics of premixed nitrous oxide, ammonia, and propane in cylindrical channel

Nitrous oxide fuel blends have been identifiedas a promising propellant to be used as a hydrazine replacement. In this study, the flame propagation and detonation characteristics of nitrous oxide, ammonia, and propane mixtures were investigated experimentally in a cylindrical channel (length, 2000 mm; inner diameter, 15 mm). Tests were performed for five propane mass fractions (0, 2.0, 3.8, 5.7, and 7.4 wt%) to investigate flame behavior and determine the suitablepropellant formulation. The results demonstrated that the flame acceleration rate first increased and then decreased with additional increases of propane. The overpressures and shock velocities exhibited a similar trend and achieved the steady detonation stage except for the mixture with propane fraction of 7.4 wt%. Propane outperformed ammonia as an additive to nitrous oxide, and the addition of 2.0 wt% propane had a positive influence on the flame acceleration process, pressure, and shock buildup for premixed nitrous oxide and ammonia. However, for lower concentrations of nitrous oxide with the addition of excessive propane, a decrease in the aforementioned parameters was observed in the doped flames.

Toward a Better Air-Assisted Flare Design for Safe and Efficient Operation during Purge Flow Conditions: Designing and Performance Testing.

In this study, a new air-assisted flare tip was designed, built, and tested under different operating conditions. Lacking sufficient energy to mix with air, low waste gas flow rates will lead to incomplete combustion of these gases. This increases pollutant emissions and soot formation which leads to a decline in flare performance. This flare tip design enhances the waste gas mixing energy through implementation of an air jet in a crossflow orientation. This is done by adjusting the exit area of the waste gas exit by injecting a radial jet of air from an inclined slot jet located around the flare tip. This flare tip design also provides protection for the flare tip from high flame temperatures that can damage through convective cooling. Several tests were conducted to assess the new flare tip design with varying waste gas flow rates of 5, 10, 25, and 120 standard liters per minute (SLPM). These tests also considered varying assistant air flow rates. In addition, test results showed high combustion efficiency of the flaring process and significant soot formation suppression. The new flare tip design yielded better flame behavior with respect to the flare tip, caused by the flame stability that prevented the flame from attaching to the flare tip.

High-precision 2D surface phosphor thermometry at kHz-rates during flame-wall interaction in narrow passages

This work demonstrates high-speed 2D wall temperature measurements occurring during flame-wall interaction (FWI) within a narrow channel. Such measurements are essential to understand transient wall heat transfer and flame behavior occurring within micro-combustors and designated engine crevices. Wall temperature is measured using the phosphor Bismuth-doped Scandium vanadate (ScVO4:Bi3+). ScVO4:Bi3+ exhibits a short phosphorescence lifetime (∼ 2 µs at room temperature), enabling kHz measurement rates. ScVO4:Bi3+ also exhibits a high temperature sensitivity, which yields single-shot precision < 0.5 K within the temperature range of 295 – 335 K. A frequency-doubled Ti:Sapphire laser emitting light at 400 nm is used to excite ScVO4:Bi3+, and wall temperature is measured within a ∼ 22 × 22 mm2 region with 380 µm spatial resolution. Phosphor thermometry and CH* imaging are combined at 1 kHz to measure the spatiotemporal dynamics of the flame and wall temperature (Twall) within a 2 mm crevice passage in a fixed volume chamber designed for heat transfer studies. Measurements describe Twall signatures associated with transient FWI events, including unique Twall features associated with wrinkled flame fronts. For our operating conditions, Twall associated with flame cusps consistently exhibit temperatures 5 – 20 K lower than flame crest regions. The most extreme difference is seen for large cusp formation, where local wall cooling is noticeable at the trough of the flame cusp. This cooling feature may be caused by intrinsic flame/flow instabilities, which locally and temporally cool the wall.

Combustion mechanism of fluorinated organic compound-modified nano-aluminum composite particles: Towards experimental and theoretical investigations

Fluorinated Organic Compounds (FOCs) are commonly used as modifiers for Aluminum (Al) powder to improve its ignition, combustion, and agglomeration characteristics. However, the effects of FOCs on combustion and inhibition mechanisms of agglomeration of Al powder are not well understood. In this paper, based on the experimental study of Fluorinated Graphite (FG)-modified Al matrix composite particles, the combustion and aggregation inhibition mechanisms of FOCs on Al particles were studied by the quantum chemical calculation at B3LYP/6-311+G(d,P) and G3//B3LYP/6-311+G(d,p) levels. The flame behavior and single particle burning behavior of FG-modified samples were compared through ignition experiments, and the characteristic spectra of Al related oxides of different samples in the initial ignition stage were captured. It is found that FG increases the burning intensity of Al composite samples significantly, while it decreases the emission intensity of Al secondary oxides. Quantum chemical calculation results show that the thermal decomposition intermediates of FOCs, namely C2F4, can react with AlO and Al2O, which weakens the characteristic emission intensity of AlO and Al2O in the sample, and thus inhibits the formation of Al2O3 in the combustion process. These results contribute to enriching the combustion dynamics model of Al-FOCs reaction system.

Electric-field effects on methane coflow flames aboard the international space station (ISS): ACME E-FIELD flames

This work describes the preparatory and initial measurements of diffusion flames under the influence of an electric field aboard the International Space Station (ISS), as part of the Advanced Combustion via Microgravity Experiments (ACME) project. Intended as the foundation publication for the experimental effort, the work comprehensively includes the space experiment methods, the capabilities of the ACME insert, experiment procedures, data, and limitations and constraints. The measurements presented include images and ion currents of small diffusion flames of methane (in air), subjected to a ramping electric field in microgravity. While there have been prior microgravity studies of Electric Field Effects on Laminar Diffusion Flames (E-FIELD Flames) using a drop tower, the current measurements represent the first mapping of the effects of an ion-driven wind on flame behavior under an electric field in the absence of the confounding influences of buoyancy. The paper describes the challenges of remote measurement and manipulation of flames on the ISS and presents preliminary results from the first set of coflow flames. The on-orbit tests began in March 2018 using the modular ACME hardware within the ISS’ Combustion Integrated Rack (CIR) and were completed in the same year November. The results show that the flame is most compact at saturation while the measured voltage-current characteristic (VCC) curve demonstrates parabolic behavior after saturation which differs from observations in 1 g on Earth.

Experimental and numerical study on flame fusion behavior of premixed hydrogen/methane explosion with two-channel obstacles

Methane blending with hydrogen has become an effective method to utilize clean energy, the safety of which has attracted much attention. However, there is still lack of studies on the propagation characteristics of premixed flame in split conditions. Therefore, experimental and numerical studies have been conducted to investigate the explosion flame behavior of hydrogen/methane with two-channel obstacle. The effects of the position and barrier ratio of the two-channel obstacle were analyzed based on nine distinguished explosion experiments with different setups. Experimental results show that the explosion flame will generate split and fusion, during its propagation through the two-channel obstacle. The fusion behavior of the split flame changes with the position and barrier ratio of the obstacle, i.e., either complementary or merging. As the obstacle moving further away from the ignition, the maximum speed increases and then decreases. The maximum speed can be increased by more than 5 times. Additionally, the dominance of the cumulative effect of pressure at increasing leads to the monotonicity of pressure. Finally, the flame propagation process, the mechanism of flame fusion, and the turbulence intensity distribution were predicted and visualized by computational fluid dynamics (CFD) based numerical simulations.