Actual Flame Research Articles

Modelling of opposed flame spread over thin electric wires: estimation of flame spread rates and limiting oxygen concentrations

A model of opposed flame spread along thin electric wires is developed to estimate the variation of flame spread rate ( V f ) with oxygen concentrations and oxidiser flow velocity. The length of pyrolysis zone ( L py ) and V f are treated as eigenvalues of the problem, and one-dimensional energy balance equations for the wire core and insulation are formulated to derive their solutions. To obtain a set of steady solutions for L py and V f , the macroscopic energy balance equation for the entire system is calculated from the one-dimensional energy balance equations with appropriate boundary conditions. Since the equations are nonlinear function of L py and V f , multiple solutions appear. The physical meaning of each solution is evaluated based on the deviation of energy from the equilibrium. Comparison of the temperature distribution along the wire with experimental data confirms that the model reproduces well the actual flame spread process. The variation of V f with oxygen concentration shows a C-shaped curve. The upper branch is the stable solution, and the lower branch is the unstable solution and the merging point of two branches correspond to the limit condition for flame spread. Therefore, the model can be used to estimate the limiting oxygen concentration based on the existence of a solution for V f . Furthermore, the variation of V f with oxidiser flow velocity shows O-shaped curve, and it is confirmed that radiative extinction under low flow velocity and blow-off under high flow velocity can also be estimated with the model. Mapping of variations of radiative extinction and blow-off extinction with oxygen concentration reveals the U-shaped flammability diagram. The predicted flame spread limits are in good agreement with the limiting oxygen concentrations measured in both microgravity and normal gravity, demonstrating the validity of using this model to predict flammability characteristics of electric wires under various oxidiser flow velocity.

Study on the flame radiative heat transfer and near-field radiation heat flux predictive model of vehicle fires in a tunnel

As a major form of heat transfer, flame thermal radiation has an important effect on the ignition of adjacent energy, fire spread and environmental damage. Forecasting the flame radiation heat flux can provide theoretical basis for fire hazard assessment, fire extinguishing and salvation. Meanwhile, the distinctive structure of the arc tunnel can result in significant variations in the local heat and mass transfer processes during tunnel fires compared to those in rectangular tunnels, leading to different fire characteristics parameters. In this study, experiments and theoretical analyses were conducted to investigate the flame length scale and near-field radiation heat flux on the lower surface of an arc-shaped ceiling induced by vehicle fires in a tunnel. The findings reveal that the flame length scale in the transverse direction beneath the arc-shaped ceiling decreases with an increasing burner aspect ratio and increases with the increasing heat release rate. A dimensionless model of the flame length scale under the arced ceiling with various burner aspect ratios was proposed based on the amount of remaining unburned fuel after flame impingement and the buoyancy component along the arc-shaped ceiling. Simultaneously, radiative heat fluxes on the lower surface also increase with rising heat release rate. The flame geometry beneath the arc-shaped ceiling is assumed to be a combination of a cuboid and an arc plate model based on the actual flame length scale. Subsequently, the view factor, radiative fraction, and flame surface area are analyzed to predict the radiative heat fluxes on the lower surface. The predictions of the cuboid and arc plate flame model show good agreement with the current experimental data. This work presents a dimensionless correlation of the flame length scale and calculates the view factor between the target and the arc plate to predict the near-field radiative heat flux more accurately. This approach also facilitates the understanding of flame spreading behavior and thermal hazards impacted by arced ceiling.

A machine learning-based approach for flames classification in industrial Heavy Oil-Fire Boilers

The burner combustion tuning is a complex problem that has been studied through flame monitoring and characterization. It has been observed that the flame electromagnetic spectrum and flickering contain specific flame information in combustion processes. This information is helpful for combustion stoichiometry tuning on burners. This paper described a method for selecting the best flame feature subset that can be computed from the scanner signal, in order to get the flame index and induce combustion stoichiometry on burners under specific combustion conditions. We propose a method for selecting a reduced subset with only the useful flame features for flame index classification. To extract the most relevant flame features we use a feature subset selection (FSS) algorithm and to determine the combustion state in burners, five flame indices were defined that represent the most common flame states in oil fuel-fired boilers. FSS includes complete, sequential, and random searches in order to eliminate redundant and noisy flame features to decrease the flame feature set dimension. A probabilistic neural network (PNN) algorithm was implemented for flame feature clustering. Signals from the actual flame scanner system and relevant variables from the boiler data acquisition system were used by the algorithms to calculate the burner flame index. A set of parametric tests was done in a heavy oil-fired boiler under well-known flame and index conditions to train and test the flame classifier. The results showed that only the four more relevant features are enough to classify flames with a good performance (92.3% accuracy), which is useful for burner combustion monitoring and optimization.

On the flame transfer function models for laminar premixed conical and V- flames considering the stretch effect

This paper investigates a predictive model that considers the impact of stretch on the dynamic response of laminar premixed conical and V- flames; the flame stretch consists of two components: the flame curvature and flow strain. The steady and perturbed flame fronts are determined via the linearized G-equation associated with the flame stretch model. Parameter analyses of the effects of Markstein length L, flame radius R and unstretched flame aspect ratio β are also conducted. Results show that the flame stretch reduces the steady flame height, with this effect being more significant for larger Markstein lengths and smaller flame sizes. The effects of flame stretch on perturbed flames are evaluated by comparing the flame transfer function (FTF) considering the flame stretch and not. For flames of different sizes, the impact of flame stretch on FTF gain can be divided into three regions. When both β and R are relatively small, due to the decrease in steady flame height and the impact of flow strain, the FTF gain increases. As β and R gradually increase, the FTF gain of the conical flame oscillates periodically while the FTF gain of the V-flame decreases, primarily due to the flame curvature enhancing the flame front disturbance and the wrinkle counteracting effect. When β and R are large, a disruption in wrinkle counteracting effect ensues, leading to a significant increase in FTF gain. Furthermore, as the actual flame height is reduced, the flame stretch also reduces the FTF phase lag which is related to the disturbance propagation time from the flame root to the tip.

Toward Evacuation Training in Virtual Reality: Requirements Gathering for Wildfire Experiences

With increasing wildfires in the Western U.S., preparing individuals for evacuations has become vital. However, putting people in the presence of actual flames and smoke would be dangerous. Virtual Reality training provides valuable experience without endangering safety. To be optimally effective, a training should be designed by combining best practices with user insights. This study analyzed interviews with ten evacuees to identify their regrets and needs for future evacuations, informing VR training design. In a thematic analysis, the themes of Communication, Reflection, and Item Management emerged as critical. In a follow-up survey, participants ranked medicine, documents, and a first-aid kit as the most important items to pack during evacuation. By leveraging evacuee insights, VR training can address communication, mitigate regrets, and improve item management, empowering individuals with skills to navigate evacuations confidently.

Relations Between Statistics of Three-Dimensional Flame Curvature and its Two-Dimensional Counterpart in Turbulent Premixed Flames

The relations between the actual flame curvature probability density function (PDF) evaluated in three-dimensions and its two-dimensional counterpart based on planar measurements have been analytically derived subject to the assumptions of isotropy and statistical independence of various angles and two-dimensional curvature. These relations have been assessed based on Direct Numerical Simulation (DNS) databases of turbulent premixed (a) statistically planar and (b) statistically axisymmetric Bunsen flames. It has been found that the analytically derived relation interlinking the PDFs of actual three-dimensional curvature and its two-dimensional counterpart holds reasonably well for a range of curvatures around the mean value defined by the inverse of the thermal flame thickness for different turbulence intensities across different combustion regimes. The flame surface is shown to exhibit predominantly two-dimensional cylindrical curvature but there is a significant probability of finding saddle type flame topologies and this probability increases with increasing turbulence intensity. The presence of saddle type flame topologies affects the ratios of second and third moments of two-dimensional and three-dimensional curvatures. It has been demonstrated that the ratios of second and third moments of two-dimensional and three-dimensional curvatures cannot be accurately predicted based on two-dimensional measurements. The ratio of the third moments of two-dimensional and three-dimensional curvatures remains positive and thus the qualitative nature of curvature skewness can still be obtained based on two-dimensional curvature measurements. As the curvature skewness is often taken to be a marker of the Darrius-Landau instability, the conclusion regarding the presence of this instability can potentially be taken from the two-dimensional curvature measurements.

Prior Bush Fire Identification Mechanism based on Machine Learning Algorithms

Besides causing awful fatalities resulting in deaths and significant resources like many acres of timberland and dwelling places, forest fires are a significant threat to sound enormous wilderness biologically and environmentally. Consistently, a considerable number of fires around the globe reason debacles to different habitats and layouts. The stated matter has been the investigation premium for a significant length of time; there is a considerable amount of good concentrated on arrangements available for testing or perhaps ready to be utilized to determine this disadvantage. Woods and actual flames have been severe issues for quite some time. Presently, there is a wide range of answers for distinguishing woods fires. Individuals are utilizing sensors to determine the fire. However, this case isn't workable for vast sections of land woods. This paper discusses another fire-recognition methodology with incremental advancements. Specifically, we put forward a stage-Artificial Intelligence. The PC innovation strategies for acknowledgment and whereabouts of smog and fires, in light of the inert photographs or the graphics captured by the cameras. AI for tracing down the fires. The accuracy relies on the calculations that use dataset values later divided in various test and train sets, respectively.

Gabor-GLCM-Based Texture Feature Extraction Using Flame Image to Predict the O2 Content and NO x.

Flame image feature extraction is the basis for boiler combustion monitoring and control. The flame video images of recent research are mainly derived from experimental burners in the laboratory, and few pay attention to the flame images in industrial boilers. The actual industrial boiler flame images differ significantly from the laboratory flame images. Additionally, certain flame image features cannot be captured in the laboratory owing to the limitations of the camera installations. Therefore, a flame image texture feature extraction algorithm based on an industrial boiler is proposed in this paper. The texture features were enhanced using a Gabor filter for the RGB channels of the flame images, and then, the statistics of the texture features were scalarized by a gray-level co-occurrence matrix (GLCM). The data were filtered and downscaled by a data compressor consisting of Gaussian-weighted mean and principal component analysis (PCA) to obtain eight key variables. The extracted eight variables were verified to be effective in characterizing the O2 and NOx contents of flue gas using the mutual information method. The combustion process regression model was constructed using a gated recurrent unit (GRU) on the 8 h combustion data of the boiler, and the predicted mean absolute percentage error (MAPE) for O2 and NOx content in the test set reached 7.5 and 10.2%, respectively. Compared to the conventional methods of direct PCA on images and GLCM plus PCA on images, the MAPE for O2 content prediction was reduced by 12.3 and 7.3%, and the MAPE for NOx content prediction was reduced by 10.5 and 6.1%, respectively. The advantage of the new flame feature based on Gabor-GLCM is suitable for the subsequent analysis and control of an industrial combustion system.

Experimental investigation on flame morphology and improved flame radiation model of rectangular heptane pool fire

AbstractThe safety process and effectiveness of in situ burning (ISB) method dealing with spilling liquids depend on flame morphology, mass burning rate and flame radiation. In this work, eight rectangular pools with a fixed area of 400 cm2 but different aspect ratios (length–width ratio, n = 1, 2, 4, 6, 8, 10, 12, and 16, respectively) are used. As the aspect ratio increases from n = 1 to n = 16, the flame height decreases from 105.6 to 72.9 cm, whereas the incident heat flux increases from 0.283 to 0.562 kW/m2. The mass burning rate decreases initially at n ≤ 6 and increases afterwards at n > 6. The traditional flame radiation model overestimates experimental measurement because the radiation model assumes that the flame surface is rectangular, but the actual flame shape shows a trapezoidal surface due to air entrainment. The mathematical linear interpolation method is proposed to make an improvement of rectangular radiation model. The new‐proposed flame radiation model is validated by experimental measurements with the relative error < 5%. The current work provides a good method to evaluate the magnitude of loss prevention of oil leakage.

Rising From the Ashes: The Role of Fires in National Parks

Fire is a natural and healthy part of many ecosystems, and many plant species rely on it to reproduce. Some species require the actual heat and flames for their seeds to be released and sprout. Other species rely on fire to burn off dried needles and leaves on the ground as well as many of the shrubs and small trees. This opens up the forest and allows more space and light for small plants and trees. While fire is a good thing for many plants and ecosystems, this is not the case everywhere. In areas that get high amounts of rain, ecosystems are not adapted to fire. Human-caused fires in these places can be devastating to the plants there. It is the job of fire managers and ecologists to manage wildfires, make decisions about fire suppression, and decide if additional fires are needed in key ecosystems.

A Combustion Regime-Based Model for Large Eddy Simulation

The aim of this work is to propose a unified (generalized) closure of the chemical source term in the context of Large Eddy Simulation able to cover all the regimes of turbulent premixed combustion. Turbulence/combustion scale interaction is firstly analyzed: a new perspective to look at commonly accepted combustion diagrams is provided based on the evidence that actual turbulent flames can experience locally several combustion regimes although global non-dimensional numbers would locate such flames in a single specific operating point of the standard combustion diagram. The deliverable is a LES subgrid scale model for turbulent premixed flames named Localized Turbulent Scales Model (LTSM). This is founded on the estimation of the local reacting volume fraction of a computational cell that is related to the local turbulent and laminar flame speeds and to the local flame thickness.

Study on threshold selection method of continuous flame images of spray combustion in the low-pressure chamber

The study of combustion in the plateau environment has gradually aroused the interest of scholars. Due to the particularity of the plateau environment, these plateau combustion experiments are usually carried out in artificial low-pressure chambers. The image processing of continuous flames has recently been widely used in scientific research and detection applications for various thermal and power devices. In this paper, three methods for selecting the optimal threshold for flame binarization images were investigated by using a flame video of horizontal diesel spray combustion in a low-pressure chamber. The results show that the binarization image with the threshold value selected by the two-peak method came closer to the actual flame's morphology. The maximum class difference method greatly increases the binarization area at the bottom of the flame, while the minimum error method significantly reduce the binarization area of the flame, especially with regard to the flame length. Finally, the mechanism of influence of the threshold selection method was analyzed from the perspective of the flame luminance and buoyancy of the horizontal jet flame. This work provides a better method of threshold selection for scientific research into simulating plateau combustion flames in low-pressure chambers, which has important engineering value and significance.

Effect of flame surface area of downward propagating flames induced by single and double laser irradiation on transition to parametric instability

The single laser irradiation ( SLI ) method was adopted to develop a double laser irradiation ( DLI ) method to investigate the effects of laser-induced structures of downward-propagating flames in a tube on the transition from primary acoustic instability to parametric instability . Previously, the SLI method was effectively used to study the interaction between acoustic oscillation and flame structure by controlling the shape of the flame front. The deformed cellular structure (either concave or convex) of the SLI -induced flame can transition from primary acoustic instability to parametric instability under certain conditions. We conducted experiments in the same combustion tube with C 2 H 4 /O 2 /CO 2 mixtures using both SLI and DLI for comparison. The DLI method forms double cellular structures, while SLI only forms single cellular structures on the flames. It was found that the DLI method was more effective than SLI in generating transition to parametric instability under same total laser power. Furthermore, a linear relationship was found between the area of the deformed structure (irrespective of its dimension) and the corresponding growth rate of acoustic pressure fluctuation during the propagation of the deformed flame. SLI - and DLI -induced deformed structures having the same deformed surface area demonstrate the same growth rate of thermo-acoustic instability regardless of the difference in ( ak ) 2 , where a is the amplitude and k is the wavenumber of the deformed structures, respectively. This factor described in the velocity coupling mechanism is important to reveal the growth rate of thermo-acoustic instability due to the variation in the flame surface area. The results revealed that the actual flame surface area, rather than ( ak ) 2 of the laser-induced downward-propagating flame structure, determines the transition in the presence of nonhomogeneous cell distribution induced by SLI and DLI . The total deformed surface area is a more comprehensive criterion for transition to parametric instability.

Experimental and numerical analyses of the combustion characteristics of Mg/PTFE/Viton fuel-rich pyrolants in the atmospheric environment

Magnesium/polytetrafluoroethylene/Viton (MTV) fuel-rich pyrolants use the atmospheric oxygen as a complementary oxidizer to sustain and alter the performance of the combustion reaction. The flame diffusion characteristics of MTV fuel-rich pyrolants in the atmospheric environment are studied by a high-speed camera (HSC). The flame temperature and combustion components are measured by using remote sensing Fourier Transform Infrared Spectrometer (FTIR). In order to obtain the combustion component distribution more accurately, an aerobic combustion model containing the oxidation reaction of the excess reactant Mg and carbonaceous species with O2 is established in this study. Eddy dissipation concept (EDC) model is applied to the numerical simulation of the three dimensional anaerobic and aerobic combustion field coupled with Realizable k-ε two-equation turbulence model. The research results show that flame temperature is mainly contributed by anaerobic combustion reaction. The flame structure obtained by the aerobic combustion model is closer to the experimental results, and the result of component distribution calculated by aerobic combustion is more consistent with characteristic spectrum. Therefore, the aerobic combustion model is more suitable for describing the actual MTV flame, and the combustion field can be divided into an anaerobic core zone and an aerobic diffusion zone.

Joint method for reconstructing three-dimensional temperature of flame using Lucy-Richardson and nearest neighbor filtering using light-field imaging

The real-time measurement and reconstruction of three-dimensional temperature are essential when studying the combustion process inside a flame. In the proposed sectioned flame light field imaging model, flame sections are reconstructed based on the characteristics identified using the Lucy-Richardson or nearest neighbor filtering methods, or a joint method involving a combination of both, to improve on the accuracy of two-dimensional temperature reconstruction. Compared to the results of a previous work, the proposed method increases the resolution in the depth of field direction and the extraction of the flame temperature of all sections from the same height ensures the accuracy of the reconstructed temperature distribution across the cross-section of the flame. The relative error when compared to the actual flame is less than 10%. Thus, this approach allows the accurate estimation of the three-dimensional temperature distribution inside a flame, and enables the rapid and effective reconstruction of the three-dimensional temperature field.

Effect of nonuniform radiation properties on flame temperature reconstruction based on light field imaging

Temperature detection is important in flame structure analysis. However, there are few studies focusing on the effect of the distribution of radiative properties on the accuracy of flame temperature reconstruction. Based on an established temperature reconstruction system using sectioned flame imaging, this study adjusts the microlens parameters of a light field camera, combining the nearest neighbor filtering method with the light field refocusing technique, and then accurately reconstructed flame images are obtained. In addition, predicted attenuation coefficient is used to reconstruct flame sections in uniform and nonuniform participating media, to investigate comparatively the effect of nonuniform radiative properties on flame temperature reconstruction. The results show that, the temperature reconstruction results for more than 94% of pixels of the flame imaging are accurate (within 10%), and only few parts at the boundary of the flame have a relative error larger than 10%, and less than 20%. Though the difference of the attenuation coefficient between that estimated for temperature reconstruction and that of the actual flame medium is an important factor influencing the temperature reconstruction accuracy, the effect is investigated in a range using the presented method. Thus, it provides a practical method to reconstruct the flame temperature accurately in unknown radiative properties.

A Model Considering the Flame Volume for Prediction of Thermal Radiation from Pool Fire

Thermal radiation is an important parameter to evaluate the hazard of pool fires. In the conventional solid flame model for the radiant heat flux calculation, the burning flame is often assumed to be of a cylinder or a cylinder–cone combined shape whose height and diameter equal the flame height and pool diameter, respectively. Obviously, the conventional model does not take into account the actual flame volume. Thus, this paper presents a revised solid flame model by correcting the flame diameter with the flame height and flame volume. The heat release rate per unit flame volume is approximately 1100 kW/m3 for hydrocarbon pool fires, by which the flame volume can be calculated with the known heat release rate. By theoretical analysis on experimental data in literature, correlations of centerline temperature and flame emissivity are proposed to calculate the flame surface emissive power of heptane pool fires. The obstruction effect of smoke around the burning flame on the thermal radiation is also considered by correcting the transmissivity of thermal radiation in the surrounding medium. Comparisons in prediction between the new and conventional models are conducted against the experimental measurements of 0.3 m, 0.5 m, 0.7 m, 1 m, 6 m heptane pool fires. It is found that the new model with the cylinder–cone combined shape assumption can better predict the thermal radiation from heptane pool fires in both near and far fields.

Experimental and Numerical Investigation of LPG-Fueled Inverse Diffusion Flame in a Coaxial Burner

An experimental and numerical investigation of unconfined liquefied petroleum gas (LPG) fuelled inverse diffusion flame (IDF) is presented. The effect of air-fuel velocity ratio on the appearance of the LPG IDF are investigated. A fixed air jet velocity of Va = 11.31 m/s is chosen for the analysis, while the fuel jet velocity is varied from 0.014 m/s to 0.083 m/s. The observations indicate that the air-fuel jet velocity ratio has significant effect on the flame appearance of LPG IDF. It is found that the flame length increases with increase in fuel jet flow. Actual flame in burner is highly complex in nature as it involves momentum, mass and energy transfer in highly turbulent flow regime. To extract detailed information of flow physics, numerical analysis of LPG IDF is performed. It is observed that temperature along the centerline of IDF increases as distance from burner exit (Z) increases. This nature of temperature distribution indicates that the central cold air in the air jet is gradually heated toward the downstream of the IDF. The highest flame temperature is noticed at Z = 30 mm. The highest flame temperature predicted numerically at axial height of 30 mm is central portion of the blue zone of the flame which is main reaction zone.

Can a spreading flame over electric wire insulation in concurrent flow achieve steady propagation in microgravity?

Concurrent flame spread over electric wire insulation was studied experimentally in microgravity conditions during parabolic flights. Polyethylene insulated Nickel-Chrome wires and Copper wires were examined for external flow velocities ranging from 50 mm/s to 200 mm/s. The experimental results showed that steady state flame spread over wire insulation in microgravity could be achieved, even for concurrent flow. A theoretical analysis on the balance of heat supply from the flame to the unburned region, radiation heat loss from the surface to the ambient and required energy to sustain the flame propagation was carried out to explain the presence of steady spread over insulated wire under concurrent flow. Based on the theory, the change in heat input (defined by the balance between heat supply from flame and radiation heat loss) was drawn as a function of the flame spread rate. The curve intersected the linear line of the required energy to sustain the flame. This balance point evidences the existence of steady propagation in concurrent flow. Moreover, the estimated steady spread rate (1.2 mm/s) was consistent with the experimental result by considering the ratio of the actual flame length to the theoretical to be 0.5. Further experimental results showed that the concurrent flame spread rate increased with the external flow velocity. In addition, the steady spread rate was found to be faster for Copper wires than for Nickel-Chrome wires. The experimental results for upward spreading (concurrent spreading) in normal gravity were compared with the microgravity results. In normal gravity, the flame did not reach a steady state within the investigated parameter range. This is due to the fact that the fairly large flame spread rate prevented the aforementioned heat balance to be reached, which meant that such a spread rate could not be attained within the length of the tested sample.

Characterization of the thermal structure of six clustered microflames seeded with TaN particles through emission spectroscopy

The temperatures of TaN seed particles and electronically excited CH radicals (i.e. CH∗) emanating from a methane-air microflame burner were characterized using VIS-NIR emission spectra while varying fuel-air flow rates. The burner consisted of six micro-nozzles arranged in a circle surrounding a central nozzle through which air and TaN seed particles with sizes between 0.3 and 3 μm were injected. The temperature profiles of the particles at various heights above the base of the central nozzle, obtained by their VIS-NIR continuum emission, showed a well-defined constant temperature region that extended well beyond the actual flame front and changed as fuel and oxidizer flow rates were varied. High spectral resolution measurements were conducted to obtain CH∗ A2Δ-X2Π emission spectra which were then compared with theoretical CH∗ emission spectra from Lifbase simulations to determine rotational and vibrational temperatures. The CH∗ vibrational temperatures were clearly greater than the rotational temperatures, the difference of which was attributed to equilibration of the rotational temperature with the gas dynamic temperature and the excitation of vibrational states through chemical reactions. Analyses of the TaN particles before and after seeding using scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy with energy dispersive X-ray analysis (XPS/EDX) showed surface oxidation occurred in the flame. Comparing the shapes of the spectral emission with theoretical Planck emission corrected for TaN emissivity and Ta2O5 transmittance, the oxide layer was found to be clearly thinner than 100 nm in the high temperature regions. Our results show an ability to control the duration to which seed particles are subjected to high temperature reactions by adjusting fuel and oxidizer flow rates.