Flame Shape Research Articles

FSDF: A high-performance fire detection framework

Fire detection is crucial in the protection of human life and property. Traditional methodologies and deep learning techniques have been extensively employed in this area, yet they often fall short due to the considerable variability in the shape, size, and intensity of flames. Conventional approaches lean on predefined fire characteristics, failing to adapt sufficiently across a broad spectrum of fire conditions. Simultaneously, deep learning techniques may struggle with managing diverse and unusual flame attributes. In response to these challenges, we introduce a novel framework that synergizes traditional methods with deep learning techniques—the Fire Segmentation-Detection Framework (FSDF). FSDF enhances flame feature detection by extracting color and texture information from images, utilizing Hue, Saturation, and Value (HSV), and the Complete Local Binary Pattern (CLBP). In addition, we weave YOLOv8 and Vector Quantized Variational Autoencoders (VQ-VAE) into the fabric of our framework to facilitate image segmentation and carry out unsupervised fire detection, respectively. To gauge the accuracy and robustness of our proposed method, we implemented a comprehensive assessment using a dataset constructed from real-world forest and urban fires. Experimental results unequivocally demonstrate the competitive edge of our approach over some baseline methods. For instance, in contrast to YOLOv8, our framework has bolstered precision, recall, and F-score by 19.5%, 1.2%, and 11.7% respectively. Finally, we conducted extensive field tests by deploying a robot with the relevant algorithm in an actual fire scenario, further emphasizing our dedication to real-world application. These experiments underline not only the performance of the method, but also its potential for practical deployment.

Flame and burning dynamics of pool fire inside semi-confined compartment under external cross wind

In this paper, experimental and theoretical studies of the flame shape and burning rate of pool fire inside a semi-confined compartment (with one sidewall and ceiling open) under the cross wind were investigated. The effects of fuel pool size and cross wind velocity on the flame morphological parameters and burning rate were obtained. Three typical bending behavior of the tilted flame was observed during the experiments. The dimensionless burning rate m˙″* decreased slightly when Frw<1, and increased linearly with Frw when Frw≥1. An air gap (δair) between the flame and sidewall was observed because of the restriction of the sidewall and blow-off by strong airflow recirculation. Theoretical analysis of wind-blown pool fire inside the semi-confined compartment was conducted based on the asymmetrical air entrainment and the buoyancy length scale. A dimensionless parameter (sm˙fρ∞uwlb2) considering the burning rate and cross wind velocity was proposed to predict the flame height, flame tilted angle and air gap thickness. The flame morphological characteristic parameters and the thickness of the air gap were correlated linearly well with the dimensionless parameter in exponent expression.

Design and combustion characteristics analysis of a static shaft turbofan engine

Abstract A design scheme of a static shaft turbofan engine is proposed to meet the requirements of light weight and large thrust weight ratio of small aeroengine. As the core component of stable combustion, the thermal protection problem of the mid-mounted combustion chamber is particularly prominent. This paper designs a mid-mounted combustion chamber configuration that combines gas film cooling and central combustion. The influence of structural parameters on combustion characteristics is explored by numerical simulation, and the theoretical design and numerical simulation is verified based on combustion test results. The results show that the flame shape of the mid-mounted combustion chamber conforms to the characteristics of central combustion. The combustion effect of the nozzle with spray angle of 45° and flow rate of 1.87 kg/h meets the requirements of secondary combustion of the static shaft turbofan engine, and the air inlet of the combustion liner effectively increases the thickness of the cooling gas film. The experimental results are in good agreement with the numerical simulation results, and the temperature of the combustion liner wall can be reduced effectively. The above research provides a theoretical basis for the combustion chamber design of small static shaft turbofan engines and a reference for the thermal protection methods of small aeroengine combustion chambers.

Experimental study of dynamics of flames from discrete methanol fuel wicks in ullages

The paper presents an experimental study of the burning characteristics of discrete methanol wicks in ullages, where the burning occurs at distances much lower than the lip of the burner. Burning configurations consisting of two methanol wicks burning in varying ullages (from zero to 24 cm at intervals of 6 cm) have been considered in the experiments. At zero and 6 cm ullages, flame shapes resemble that of a general buoyant pool flame. As the ullage is increased to 12 cm or higher, a plume-induced instability triggers in the ullage and the flame bends towards one side over the wicks, resembling a boundary layer type of flame. The transition from a pool flame to a boundary layer-type flame has been illustrated with direct flame photographs. At deeper ullages, as the flame leans over the fuel wicks, the mass burning rates are enhanced from 9% to 35% for different ullages. The resulting air flow inside the ullage and flame shape affects the mass burning rate and has been visualized using smoke flow in the ullage. This study opens further research on the coupling effect of burning of heterogeneous materials in ullages with the flow field induced by the buoyant plume.

Influence of vent duct bend angle on inner dust explosion

A visualized tank was applied to carry out explosion venting experiments to investigate the effect of the bend angle of the vent duct on the venting dynamics process during the dust explosion. By changing the bend angle of the duct and comparing the conventional ductless and straight duct venting, the distribution law of venting overpressure and the development process of flame shape in the duct and container were analysed. There are typically three-peaked features in the duct caused by film-breaking shock waves, secondary explosions, and continuous combustion of the flame in the container. The secondary explosion strength in the duct increases with the bend angle, which exacerbates the severity of the explosion compared to conventional ductless or straight duct explosions. In the case of bend venting, as the bend angle of the duct increases, the Pmax and the (dP/dt)max all show an increasing trend in the container, and the Pred,max difference on both sides of the bend is positively correlated with the bend angle. In addition, with an increase in the bend angle, the total reaction time in the container gradually decreased, namely, the flame duration shortened.

Simulation analysis of synthetic electric field of UHV transmission line under mountain fire condition

Factors such as high temperature, charge density and charged particles have a major impact on the insulation strength of the transmission line gap. When the DC transmission line is in normal operation, the surface of the conductor is easy to adsorb dirt due to the environmental influences, so the local electric field strength is larger than the critical field strength of the corona discharge, and the gas molecules in the air are ionized to form space charges. The electric field of the space charge is superimposed with the electric field of the wire and the earth (nominal electric field) to form a synthetic electric field. In this work, the calculation model of the synthetic electric field is created and the influence of mountain fires on the synthetic electric field of the transmission line is simulated and analyzed. The influence of flame temperature, charge density, particle type and shape on the electric field is simulated and investigated. The law of variation of the electric field of the transmission line in mountain fire under different factors is obtained. The distorting effect of particles on the electric field under the UHV DC transmission line is simulated. The distorting effect of particle size and particle chain on the synthetic electric field is obtained and the simulation results are compared to the calculation results.

Experimental and numerical investigation on the premixed methane/air flame propagation in duct with obstacle gradients

This work experimentally and numerically investigates the effect of the obstacle gradient on the characteristics of the methane/air explosion in obstructed ducts. The obstacle gradient is expressed through various blockage ratios, and three different obstacle gradients are investigated, i.e., C357, C555, and C753. Noted that C357 means that the blockage ratios of three obstacles arranged sequentially in the duct are 0.3, 0.5 and 0.7 respectively, and so do C555 and C753. A two-dimensional (2D) model is adopted and the Scale-Adaptive Simulation method with the thickened flame model is considered. Experimental results show that the obstacle gradient significantly affects the flame evolution structure, flame propagation speed and overpressure. The obstacle gradient is accountable for the “tulip flames” appearing downstream of the obstacle. For the case with a fixed methane volume fraction, the average flame front speed is fixed. However, with the increasing obstacle gradient, the maximum flame front speed increases until it achieves the maximum at C357, and so does the maximum overpressure. The numerical simulation can predict the flame evolution behaviour precisely. It is evident that the generation of different flame shapes appearing is derived from the flow field evolution of unburned gas downstream of the flame front.

Effects of ignition position on the explosion of methane-air mixtures with concentration gradients

To investigate the effects of ignition position on the explosion of non-uniform methane-air mixtures, experiments were carried out in a closed pipe of 1 m length and 0.3m × 0.3 m cross-section. Methane is uniformly charged at the top of the pipe and then diffused freely, creating a concentration gradient. The concentration gradient of the methane-air mixture depends on the diffusion time. The effects of the three ignition positions – top (IP1), center (IP2) and bottom (IP3) - on the flame propagation at different ignition delay time (tig) were investigated. The ignition position and tig are the key factors influencing the success of ignition. For IP2 and IP3, the flame propagation velocity increases with the increase of tig, but the flame propagation velocity at IP1 does not change significantly with an increase in tig. When tig ≤ 25min, the temperature (Tmax) and maximum overpressure (Pmax) in the pipe at IP1 are significantly higher than those at IP2 and IP3. When the methane-air mixtures are nearly uniform, the Tmax and Pmax at different ignition locations are almost identical. The maximum pressure rise rate, (dP/dt)max, was affected by the flame pipe collision and different flame shapes caused by the ignition position.

Experimental study on upward flame spread over XPS under the influence of facade openings

This study experimentally investigates the upward flame spread and combustion behavior of facade XPS boards under the coupled action of continuous and discontinuous materials. The research reveals the opening law of typical building facades, and changes the opening height (h) and ratio (α) to deeply analyze flame geometrical characteristic, such as flame shape, which is 'convex' at h < 25 cm and 'concave' at h ≥ 25 cm. The functional relationship between flame height xf and pyrolysis length xp is fitted, confirming the turbulent state of flame spread under opening influence by n < 1. The mass loss rate varies periodically with time and negatively correlates with h. The effect of the competition mechanism between oxygen supply and combustibles on the flame spread rate (vf) is studied, which shows that the vf tends to increase and then decrease with h. A new model for predicting flame spread rate related to the length of preheating zone is established, and the maximum prediction error is within 10%. The study finds that the dominance of the opening three-dimensional fire gradually changes from the top opening three-dimensional fire to the side. The interior temperature field gradually decreases with the increasing h due to the increased heat transfer distance of gas phases. This research provides a better comprehension of the combustion characteristics on building facades under different opening influence and their effects on interior spaces.

OH* Chemiluminescence Characteristics of the RP-3 Fueled Dual-Swirl Direct-Mixing Combustor.

The flame structure characteristics of the RP-3 fueled dual-swirl direct-mixing combustor are studied experimentally. The flame shape is marked by the OH* radical, which is captured by a CMOS camera with an image intensifier. The flow fields and spray distributions are obtained by particle image velocimetry. The variation of pilot/main-stage flame structure with global fuel-air ratio (FAR), fuel ratio (FR), pilot/main-stage swirl numbers (Sp and Sm), and Δp/p (total pressure loss coefficient) is further investigated. The typical flame structure, consisting of two main-stage combustion zones (pilot-stage and main-stage combustion zones), is first analyzed. Then, according to the relationship between integral OH* intensity and global heat release rate, increasing swirl number will weaken the effect of strain rate on OH* chemiluminescence. The global FAR has little impact on the flame structure, while modifying the FR will alter the flame mode. The influence of the Sp on the flame structure is more significant than that of the Sm. Within the range of experimental conditions, the greater the swirl number, the smaller the Δp/p is required to obtain the maximum OH* radical intensity. The addition of non-swirl flow in the main-stage swirler can improve the stability of the pilot-stage flame. The equivalent swirl number is further evaluated by the neural network.

A Numerical Study of Flow Structures and Flame Shape Transition in Swirl-Stabilized Turbulent Premixed Flames Subject to Local Extinction

ABSTRACT Large Eddy Simulations (LES) of turbulent lean-premixed flames of V- and M-shape are presented. A simple algebraic closure with the ability to capture finite-rate chemistry effects is used for subgrid reaction rate modeling. The V-shaped flame is stabilized in the inner shear layer between a swirling annular jet and a central recirculating bubble in a sudden expansion duct. The M-shaped flame is stabilized in the inner and outer shear layer, adjoining the corner recirculation zone induced by the vertical step. The focus of the study is on the flow fields and shapes of the flames, which distinguish themselves through different heat load and sensitivity to local extinction. Good agreement with measurements is observed for the cold and the reacting flow cases. The numerical results suggest that the entrainment of hot gases into the outer recirculation zone occurs close to the impingement point of the swirling annular jet on the wall and this process is strongly dependent on intense vortical structures in the outer shear layer. The results further suggest that local extinction influences the position of the flame in the inner shear layer and, thereby, also the intensity of the local entrainment process.

Numerical study on the effects of co-firing ratio and stoichiometric ratio on biomass/ammonia co-firing

Co-firing of biomass and NH3 can generate positive effect on the energy and environment. However, there is limited research in this area. In this work, Ansys Fluent, a computational fluid dynamics software, was utilized to simulate the two-dimensional combustion process of mixed wood (MW) particles with NH3 in a 15 KW furnace under various operating conditions. Various models such as turbulence, species transport, particle phase, radiation, and pollutant were reasonably considered, and the reliability of these selected models was also evaluated by comparing with the experimental results obtained from the 15 KW furnace. The four combustion cases with different NH3 ratios (NRs) of 0%, 40%, 50%, and 60% were investigated (partial MW replaced by NH3 with equivalent calorific value). Moreover, the effects of stoichiometric ratios (SRs) of 0.8, 0.9, 1.1, and 1.3 on the combustion behavior was also explored at a fixed NR of 60%. The velocity, flame shape, furnace temperature, and emissions of pollutants (CO2 and NO) were described and analyzed. The findings show that the temperature at the center of outlet can reach 1209.0 K when the SR and NR are 0.9 and 60%, respectively, which exhibits the highest value among all operating conditions. The average value of CO2 concentration is 3.7% at the outlet under the condition of SR = 1.3 and NR = 60%, which presents the lowest level among all cases, while the average value of NO concentration presents a lowest value of 111 ppm under the condition of SR = 0.8 and NR = 60% at the outlet. In summary, these numerical simulation results can provide important support for further studies of the co-firing of biomass/NH3.

Experimental investigation on the electrospray counterflow flame in a small combustor with a porous media as the grounding electrode

An experimental study on electrospray counterflow combustion of ethanol in a new type combustor with a porous media as the grounding electrode is carried out. The electrospray process of ethanol is separated from the combustion system for visualization and four typical spraying modes are identified. The characteristics of electrospray counterflow combustion are discussed. Four flame shapes are classified and the distribution region is determined. The results demonstrate that the double-layer flame and dromedary flame are easier to be observed. The change of fuel mass fraction can significantly affect the flame diameter and temperature. When the fuel mass fraction YF > 0.075, the increase of carrier gas flow rate promotes the fuel evaporation. Ethanol vapor is carried into the porous media, and the flame diameter and temperature are incrensed. When YF < 0.075, the dilution of ethanol vapor by carrier gas is dominant, resulting in the decrease of fuel concentration. The combustion efficiency and ratio of CO2/CO is highest at the dromedary flame, and the maximum combustion efficiency and ratio of CO2/CO are 95.25% and 7.78 for Φ = 0.76 respectively, which means a very efficient conversation of fuel. The results gained insights for the design and operation of electrospray counterflow combustors.

Impacts of Flow Swirl on Stability and Flow/Flame Interactions of Premixed Oxy-Methane Swirl Flames

Abstract Effects of flow swirl on stability and flow/flame interactions of premixed oxy-methane flames (CH4/O2/CO2) are investigated experimentally and numerically in a premixed model gas turbine combustor. Two swirlers of 55-deg and 45-deg swirl angles were considered to perform this study over a range of combustor operating equivalence ratio (Φ = 0.1–1.0) and oxygen fraction (OF = 21%–70%) at a constant inlet flow velocity of 5.2 m/s. Combustor stability maps (representing flashback and blowout bounds) were identified experimentally in the Φ-OF space for the two swirlers, and the results were plotted over the calculated contours of adiabatic flame temperature (AFT). Specific flames were photographed using a camera to investigate the impact of flow swirl on flame macrostructure. Also, the shapes of the selected flames were calculated numerically using the contours of OH radicals, and the results showed good agreement with the photographed flame shapes. Contours of temperature and flow streamlines were plotted based on numerical calculations to figure out the influence of flow swirl on flame/flow interactions. The results showed that CH4/O2/CO2 swirl flames blow out at fixed AFT of ∼1600 K with no effect of swirl on flame stability near the blowout. Flow/flame interactions significantly affect flame stability near the flashback limit. Flame speed (FS) and AFT correlate with one another as log(FS) ∝ 1/AFT. The 45-deg swirler resulted in a wider stable combustion zone than that of the 55-deg swirler.

Experimental and machine learning studies of thermal impinging flow under ceiling induced by hydrogen-blended methane jet fire: Temperature distribution and flame extension characteristics

Hydrogen-blended natural gas has a winged used in such thermal and combustible systems, the ceiling thermal characteristics have important parameters of impinging jet configurations. In this paper, a series of experiments were carried out to study the ceiling flame extension characteristics and temperature profile in a thermal impinging flow of methane jet diffusion flame with hydrogen addition. It was found that: for the given volume flow rate of methane fuel, the ceiling temperature increases although ceiling flame extension area (length) decreases, with the volume flow of hydrogen mixing ratios increasing. Then, a physical and mathematical model, combined influence of flame shape hypothesis based on physically the unburnt fuel, hydrogen addition, heat release rate, the effects of source-ceiling height and nozzle diameter is estimated to predict the flame extension area of methane jet diffusion flames with different hydrogen mixing ratios. Moreover, the fire plume temperature between buoyant plume and intermittent flame increased with the increase in hydrogen addition due to the fire plume temperature increased. The temperature profile models of the ceiling jet are proposed for methane diffusion flames with different hydrogen mixing ratios. Furthermore, to perform the calculation quickly and facilitate rapid thermal engineering design, this paper used machine learning methods (GABP neural network) to derive the volume flow rate of gas leak from the temperature data of the ceiling detected by thermocouples. The results show that the AI prediction for the heat release rates has a good correlation with the verification experimental value. The results of this paper may demonstrate the potential for industrial combustion systems and fire safety engineering applications of methane jet diffusion flame with hydrogen addition.

Effect of copper foam location on syngas-air explosion in a closed duct

Experiments were conducted in a closed duct filled with copper foam to study the premixed syngas-air flame propagation behaviors. A copper foam with a thickness of 20 mm is installed at different distance from the ignition end (CFLs) respectively. The stoichiometric syngas-air with different hydrogen volume fractions in syngas (α(H2)s) is employed. Results show that the setting of copper foam does not affect the flame evolution process before the flame tip reaches the copper foam. The flame shape observed when the flame tip reaches the copper foam is closely related to the distance between the copper foam and igniter. The presence of copper foam can contribute to aggravate the chaos of the flame front. In most cases, the flame accelerates after passes through the copper foam only except for α(H2) = 10%, along with rise and oscillation of the flame velocity and overpressure. The flame acceleration is enhanced with the increase of α(H2) but decrease of CFL. In general, the copper foam may be more dangerous when it is closer to the ignition end and the α(H2) is high.

Numerical investigation on dynamic behavior of premixed hydrogen/air flame propagation in a closed tube

In this paper, the large eddy simulation (LES) model of OpenFOAM was used to study the flame propagation characteristics of hydrogen-air premixed flame in a closed tube with an aspect ratio of 12. The numerical simulation reproduces the characteristic flame shape evolution observed in the experiment, such as classical tulip flame, T-shaped flame, and distorted tulip deformation. The predicted flame front velocity, overpressure, and its fluctuation are all in good agreement with the measurements. The simulation demonstrates that after the classic tulip flame is formed, a series of waves cause an obvious adverse pressure gradient at the front of the flame. This adverse pressure gradient drives reverse flow which pulls the backward tip further into the combustion zone, forming an elongated tulip flame. A small amount of unburnt gas trapped in the cusp rapidly consumes to form a featured T-shaped flame. Then the T-shaped flame transforms into the second finger-shaped flame, and finally the tulip distortion occurs. The vortex and the reverse flow near the flame front generated by the interaction between the pressure field and the flow field accounts primarily for the formation of distorted tulip flame.

Experimental study on suppression of methane-air explosion with different surface dispersants combined with modified attapulgite powder

This paper investigated the Inhibitory effect of attapulgite powder, modified attapulgite powder, and modified attapulgite powder after compounding by adding different types and different quality of surface dispersants to the explosion of methane-air premixed gas. A self-made semi-closed transparent pipe is the leading experimental platform in the research process. The maximum explosion overpressure, average pressure increase rate, and explosion power index under different working conditions are calculated by analyzing the pipeline's flame shape, pressure, and flame spread speed. Comparing methane explosion and adding attapulgite powder and modified attapulgite powder as explosion suppression materials, experiments were carried out, and the experimental results were compared and analyzed, which proved the inhibitory effect of modified attapulgite powder on methane explosion. Adding different qualities of sodium polyacrylate, ammonium hexametaphosphate, and sodium dodecyl benzene sulfonate as surface dispersants to the modified attapulgite powder. The effect of adding different dispersants on the ability of modified attapulgite powder to suppress the explosion of methane premixed gas was explored. Compared with the experiments of methane explosion, attapulgite powder, and modified attapulgite powder to suppress explosion, it is proved that after adding the above three surface dispersants, the modified attapulgite powder has better inhibition on methane explosion. The effect and the influence law and the corresponding inhibition mechanism are analyzed. The experimental results show that the modified attapulgite powder added with 120 mg of sodium dodecyl benzene sulfonate has the best suppression effect on the explosion of 9.5% pure methane-air premixed gas.

Spherical Diffusion Flames of Ethylene in Microgravity: Multidimensional Effects

The joint American–Russian Space Experiment Flame Design (Adamant) was implemented on the International Space Station (ISS) in the period from 2019 to 2022. The objectives of the experiment were to study the radiative extinction of spherical diffusion flames (SDF) around a porous burner (PB) under microgravity conditions, as well as the mechanisms of control of soot formation in the SDF. The objects of the study were the normal and inverse SDFs of gaseous ethylene in an oxygen atmosphere with nitrogen dilution at room temperature and pressures ranging from 0.5 to 2 atm. The paper presents the results of transient 1D and 2D calculations of 24 normal and 13 inverse SDFs with and without radiative extinction. The 1D calculations revealed some generalities in the evolution of SDFs with different values of the stoichiometric mixture fraction. The unambiguous dependences of the ratio of flame radius to fluid mass flow rate through the PB on the stoichiometric mixture fraction were shown to exist for normal and inverse SDFs. These dependences allowed important conclusions to be made on the comparative flame growth rates, flame lifetime, and flame radius at extinction for normal and inverse SDFs. The 2D calculations were performed for a better understanding of the various observed non-1D effects like flame asymmetry with respect to the center of the PB, flame quenching near the gas supply tube, asymmetrical flame luminosity, etc. The local mass flow rate of fluid through the PB was shown to be nonuniform with the maximum flow rate attained in the PB hemisphere with the attached fluid supply tube, which could be a reason for the flame asymmetry observed in the space experiment. The evolution of 2D ethylene SDFs at zero gravity was shown to be oscillatory with slow alterations in flame shape and temperature caused by the incepience of torroidal vortices in the surrounding gas. Introduction of the directional microgravity, on the level of 0.01g , led to the complete suppression of flame oscillations.

Investigation of the effects of flame surface development on the ignition mechanism of aero-engine combustors

Large eddy simulation was employed to simulate the effects of different vane angles of the primary and pilot stages on the ignition process of a combustor under normal temperature and pressure conditions. The simulation results of different vane angles of the pilot stage were then verified experimentally. Kernel initiation and flame propagation in the cases of ignition success and failure were analysed. It was found that in the ignition failure case, the flame kernels are confined to the downstream zones of the venturi and cannot propagate radially, and almost no negative displacement exists in the axial direction. However, in the ignition success case, sub-fire kernels are always present outside the radial boundary of the primary stage. Five ignition modes were finally proposed: (I1) typical ignition, (I2) relatively high-speed ignition, (I3) single-kernel alternating dominant, (I4) main kernel dominant, and (I5) dual-kernel dominant modes. Using high-speed photography, the flame shape changes and ignition characteristics in the cases of successful and failed ignition with different pilot-stage vane angles were investigated.