Flame Flicker Research Articles

Flame Detection Using Generic Color Model and Improved Block-Based PCA in Active Infrared Camera

In this paper, we proposed an all-weather flame detection algorithm which could make full use of active infrared cameras presently installed in many public places for surveillance purposes. Firstly, according to the different spectral imaging results in day and night, we propose a video type classification algorithm (VTCA) via imaging clues. VTCA could help us select different flame visual features in color image and infrared image. Secondly, we use a generic YCbCr-color-space-based chrominance model to extract regions of interest (ROI) of flame. Thirdly, two flame dynamic features are used to verify the candidate ROIs, which are common flame flicker feature and an improved block-based PCA in consecutive frames. The experimental results show that the proposed flame detection model has been successfully applied to various situations, including day and night, indoor and outdoor on our test video datasets, and it gives a better performance compared with other state-of-the-art methods.

Effect of Gravity on Premixed Methane–Air Flames

Premixed flames under different levels of gravity were studied experimentally and numerically. The experiments were carried out in the Bremen Drop tower. The object of investigation were conical premixed rich, lean, and stoichiometric CH4–air flames with wide range of flow regimes, at Reynolds numbers of 600–2000, which were generated on specially designed cone nozzle and premixed cylinder chamber with grids and beads. Planar laser-induced fluorescence OH radicals and high-speed video recording was performed under microgravity, terrestrial conditions, and inverted gravity. All the experiments were performed at atmospheric pressure. Our experiments confirm that gravity has a complex influence on laminar and weakly turbulent premixed flames. Gravity causes flame flickering, while under reduced gravity flames are stable and almost not flicker. Based on the experimental data and numerical simulation, a correlation of flickering frequency with different mixture equivalence ratio and gravity is formed.

Observation of large-scale structure in flickering diffusion flame by time-resolved particle image velocimetry and shadowgraph imaging

In order to understand the flow structure of a flickering flame, time-resolved observations are carried out of the velocity and density fields in a diffusion flame in co-flow using particle image velocimetry (PIV) and shadowgraph imaging, respectively. The PIV measurement combined with invalid velocity vector analysis using proper orthogonal decomposition (POD) allows for the high spatial resolution measurement of the velocity field in the flame, while the shadowgraph images show the high-temperature contour and soot formation of the flame. These experimental results indicate a periodic variation in the flow field and soot structure of the flickering flame. The observation of the velocity field shows that the periodic inflow and outflow are generated in the outer layer of the flame near the burner, which promotes the flow acceleration in the vertical direction leading to clip-off behavior. It is found that the vorticity contour evaluated from the velocity field matches well with the high-temperature contour observed in the shadowgraph images. Further examination of the flow structure is carried out using the first two POD modes, which demonstrate the presence of counter-rotating vortices in the outer flame contributing to stretching of the flame due to the clip-off motion in the flame.

Combustion dynamics of low vapour pressure nanofuel droplets

Multiscale combustion dynamics, shape oscillations, secondary atomization, and precipitate formation have been elucidated for low vapour pressure nanofuel [n-dodecane seeded with alumina nanoparticles (NPs)] droplets. Dilute nanoparticle loading rates (0.1%–1%) have been considered. Contrary to our previous studies of ethanol-water blend (high vapour pressure fuel), pure dodecane droplets do not exhibit internal boiling after ignition. However, variation in surface tension due to temperature causes shape deformations for pure dodecane droplets. In the case of nanofuels, intense heat release from the enveloping flame leads to the formation of micron-size aggregates (of alumina NPS) which serve as nucleation sites promoting heterogeneous boiling. Three boiling regimes (A, B, and C) have been identified with varying bubble dynamics. We have deciphered key mechanisms responsible for the growth, transport, and rupture of the bubbles. Bubble rupture causes ejections of liquid droplets termed as secondary atomization. Ejection of small bubbles (mode 1) resembles the classical vapour bubble collapse mechanism near a flat free surface. However, large bubbles induce severe shape deformations as well as bulk oscillations. Rupture of large bubbles results in high speed liquid jet formation which undergoes Rayleigh-Plateau tip break-up. Both modes contribute towards direct fuel transfer from the droplet surface to flame envelope bypassing diffusion limitations. Combustion lifetime of nanofuel droplets consequently has two stages: stage I (where bubble dynamics are dominant) and stage II (formation of gelatinous mass due to continuous fuel depletion; NP agglomeration). In the present work, variation of flame dynamics and spatio-temporal heat release (HR) have been analysed using high speed OH* chemiluminescence imaging. Fluctuations in droplet shape and flame heat release are found to be well correlated. Droplet flame is bifurcated in two zones (I and II). Flame response is manifested in two frequency ranges: (i) buoyant flame flickering and (ii) auxiliary frequencies arising from high intensity secondary ejections due to bubble ruptures. Addition of alumina NPs enhances the heat absorption rate and ensures the rapid transfer of fuel parcels (detached daughter droplets) from droplet surface to flame front through secondary ejections. Therefore, average HR shows an increasing trend with particle loading rate (PLR). The perikinetic agglomeration model is used to explain the formation of gelatinous sheath during the last phase of droplet burning. Gelatinous mass formed results in bubble entrapment. SEM images of combustion precipitates show entrapped bubble cavities along with surface and sub-surface blowholes. Morphology of combustion precipitate shows a strong variation with PLRs. We have established the coupling mechanisms among heat release, shape oscillations, and secondary atomizations that underline the combustion behaviour of such low vapour pressure nanofuels.

Fanning and refuelling the flickering flame of faculty development

The challenges of healthcare provision in South Africa have led to initiatives to strengthen the public sector, increase the number of healthcare workers, improve the relevance of training programmes, and develop leadership capacity to enable more positive health outcomes in communities. These initiatives in health have been implemented concurrently with developments in education with the hope to improve the quality of, and to transform, learning for the very diverse student population enrolled at institutions of higher learning. In this context, faculty development is offered to staff to stay abreast of pedagogical and disciplinary developments. Faculty development essentially includes efforts at individual, institutional or system level to capacitate staff with knowledge and skills in areas considered essential for their function as faculty members.

Acoustic control of the structure of the gas torch

Diffusion torches in the transverse acoustic field studied experimentally. The structure and the luminosity of the torch were recorded by a high-speed camera with shadow device and photodetectors. The dependence of the length of a laminar methane plume on the average velocity and volumetric flow rate was measured. The frequency spectra of the luminosity of the flame at different flow rates of methane are obtained. Flame flicker frequency depending on the volume flow of methane had defined according to the obtained spectra. It was found that the flame flicker frequency is not dependent on the diameter of the burner. It is shown that the flickers are caused by changes in the geometric dimensions of the flame due to the convective instability of the column of hot gas surrounding the torch. The repetition frequency of the vortices in the column of hot gas surrounding the torch coincides with the frequency of the flicker of the flame. It is shown that the limits of flame detachment from burner and flame quenching does not change at the acoustic impact on the torch, the biggest acoustic impact has on the contrary attaching detached torch. Fuel speed range, in which there is detached steady torch, increases.

Synchronization in flickering of three-coupled candle flames

When two or more candle flames are fused by approaching them together, the resulting large flame often exhibits flickering, i.e., prolonged high-frequency oscillation in its size and luminance. In the present work, we investigate the collective behaviour of three-coupled candle flame oscillators in a triangular arrangement. The system showed four distinct types of syncronised modes as a consequence of spontaneous symmetry breaking. The modes obtained include the in-phase mode, the partial in-phase mode, the rotation mode, and an anomalous one called the “death” mode that causes a sudden stop of the flame oscillation followed by self-sustained stable combustion. We also clarified the correlation between the inter-flame distance and the frequency with which the modes occur.

Diffusion-flame flickering as a hydrodynamic global mode

The present study employs a linear global stability analysis to investigate buoyancy-induced flickering of axisymmetric laminar jet diffusion flames as a hydrodynamic global mode. The instability-driving interactions of the buoyancy force with the density differences induced by the chemical heat release are described in the infinitely fast reaction limit for unity Lewis numbers of the reactants. The analysis determines the critical conditions at the onset of the linear global instability as well as the Strouhal number of the associated oscillations in terms of the governing parameters of the problem. Marginal instability boundaries are delineated in the Froude number/Reynolds number plane for different fuel jet dilutions. The results of the global stability analysis are compared with direct numerical simulations of time-dependent axisymmetric jet flames and also with results of a local spatio-temporal stability analysis.

Influence of Co-flow on Flickering Diffusion Flame

The influence of air co-flow on flickering methane diffusion flame was studied experimentally using the image processing technique and the proper orthogonal decomposition (POD) analysis. The flickering of the flame is characterized by the mean height, the oscillation amplitude and the Strouhal number, which are measured by the digital image analysis of the diffusion flame. The experiments are carried out for various combinations of burner diameters, fuel velocities and co-flow velocities. With increasing the velocity ratio of the co-flow to the fuel flow, the oscillation amplitude is decreased and the Strouhal number is increased slightly in proportional to the inverse Froude number, while the frequency jump occurs in the low co-flow velocity ratio. These results are commonly observed in all the burners of different diameters, while the critical co-flow velocity ratio to suppress the flickering is found to be increased with increasing the burner diameters due to the influence of Froude number. The POD analysis of the flickering flame shows that the flickering energy is dominant in the first two POD modes and they are axisymmetric except for the zero co-flow velocity case and fully suppressed case. The correlation of POD coefficients in the first two fluctuating POD modes suggests the suppression of large-scale structure of flickering due to the influence of co-flow.

Radiation measurement based on spectral emissions in industrial flames

A novel combustion diagnostics system based on flame radiation measurement and spectral emissions analysis is presented. The key aspect of this study is the proper estimation of the continuous component or baseline of spectral signals containing spurious discontinuities. We compare both, a PCA based method against an alternative method based on an Artificial Neural Networks model. The accuracy and computational cost for baseline estimation in practical industrial applications is analyzed, in order to incorpore the radiation measurement as a variable suitable for control decisions. Several experiments in industrial facilities demonstrate the suitability of radiation measurement via flame spectrum analysis, in particular the relation between the proposed optical variable and the process operational conditions in the case of an oil boiler for steam production and a ladle furnace preheating process. As results, the methodology based on principal components analysis (PCA) provides high accuracy for baseline estimation in flame spectra, with metrics of RMSE≈0.1·10-4 and GFC>0.99 with fast computation times, smaller than typical flame flickering.

Effects of Burner Configurations on the Natural Oscillation Characteristics of Laminar Jet Diffusion Flames

In this work, effects of burner configurations on the natural oscillations of methane laminar diffusion flames under atmospheric pressure and normal gravity conditions have been studied experimentally. Three regimes of laminar diffusion flames, namely, steady, intermittent flickering and continuous flickering have been investigated. Burner configurations such as straight pipe, contoured nozzle and that having an orifice plate at the exit have been considered. All burners have the same area of cross section at the exit and same burner lip thickness. Flame height data has been extracted from direct flame video using MATLAB. Shadowgraph videos have been captured to analyze the plume width characteristics. Results show that, the oscillation characteristics of the orifice burner is significantly different from the other two burners; orifice burner produces a shorter flame and wider thermal plume width in the steady flame regime and the onset of the oscillation/flickering regimes for the orifice burner occurs at a higher fuel flow rate. In the natural flickering regime, the dominating frequency of flame flickering remains within a small range, 12.5 Hz to 15 Hz, for all the burners and for all fuel flow rates. The time-averaged flame length-scale parameters, such as the maximum and the minimum flame heights, increase with respect to the fuel flow rate, however, the difference in the maximum and the minimum flame heights remains almost constant.

Correspondence Between “Stable” Flame Macrostructure and Thermo-acoustic Instability in Premixed Swirl-Stabilized Turbulent Combustion

In this paper, we conduct an experimental investigation to study the link between the flame macroscale structure—or flame brush spatial distribution—and thermo-acoustic instabilities, in a premixed swirl-stabilized dump combustor. We operate the combustor with premixed methane–air in the range of equivalence ratio (φ) from the lean blowout limit to φ=0.75. First, we observe the different dynamic modes in this lean range as φ is raised. We also document the effect of φ on the flame macrostructure. Next, we examine the correspondence between dynamic mode transitions and changes in flame macrostructure. To do so, we modify the combustor length—by downstream truncation—without changing the underlying flow upstream. Thus, the resonant frequencies of the geometry are altered allowing for decoupling the heat release rate fluctuations and the acoustic feedback. Mean flame configurations in the modified combustor and for the same range of equivalence ratio are examined, following the same experimental protocol. It is found that not only the same sequence of flame macrostructures is observed in both combustors but also that the transitions occur at a similar set of equivalence ratio. In particular, the appearance of the flame in the outside recirculation zone (ORZ) in the long combustor—which occurs simultaneously with the onset of instability at the fundamental frequency—happens at similar φ when compared to the short combustor, but without being in latter case accompanied by a transition to thermo-acoustic instability. Then, we interrogate the flow field by analyzing the streamlines, mean, and rms velocities for the nonreacting flow and the different flame types. Finally, we focus on the transition of the flame to the ORZ in the acoustically decoupled case. Our analysis of this transition shows that it occurs gradually with an intermittent appearance of a flame in the ORZ and an increasing probability with φ. The spectral analysis of this phenomenon—we refer to as “ORZ flame flickering”—shows the presence of unsteady events occurring at two distinct low frequency ranges. A broad band at very low frequency in the range ∼(1 Hz–10 Hz) associated with the expansion and contraction of the inner recirculation zone (IRZ) and a narrow band centered around 28 Hz which is the frequency of rotation of the flame as it is advected by the ORZ flow.

On-line flame signal time series analysis for oil-fired burner optimization

Flame monitoring and characterization have been recognized as an important aspect for burner combustion tuning. Flame flickering and its electromagnetic flame spectrum have been extensively demonstrated to provide characteristic information of flames in the combustion processes. This information can be used for combustion and emissions optimization and control. A system based on flame characterization would comprise of an on-line monitoring instrumentation, a time series processing algorithm and a control scheme. This paper reports the development of a smart software for flame data processing, to provide flame indexing and infer combustion stoichiometry under a range of combustion conditions. The software processes standard flame scanner data in the time and frequency domain, and employs standard statistical measures, shape factor, information entropy analysis and plant data for flame classification. Artificial probabilistic neural networks algorithms were used for clustering the flame spectral data. Probabilistic neural network is a supervised learning algorithm but includes no weights in its hidden layer. It is often an excellent pattern classifier that outperforms other classifiers, including back-propagation. Data from the scanner flame monitoring system, the plant data acquisition system and the stack emissions monitoring system are processed with proprietary algorithms to provide burner flame classification. Additionally, an expert system utilizes the flame classification information and interfaces with the plant operators for open-loop control of the burners and to achieve optimal scheduling of burner maintenance. The software was deployed at the Mexican Commission of Electricity’s CTPALM Unit 1. This unit consists of a multi-burner unit that fires Bunker C oil. The results demonstrated the benefits of the software.

Research on the Flame Simulation Method Based on the Particle System

In reality, flame is a common fluid scene. In this paper, we simulated flame by using improved particle system and detail the properties and changes of particles in the model. We realized a realistic color change and dynamic flickering flame characteristics. We used texture mapping method to render the flame and used the hierarchical programming structure to provide a simple and practical interactive flame simulation. We also designed and implemented demonstration system of flame simulation. Experiments show that using the flame image generated in this paper can not only satisfy the visual effect but also meet the real-time requirements.

Experimental analysis of biomass co-firing flames in a pulverized fuel swirl burner using a CCD based visualization system

Biomass co-firing with pulverized coal in conventional, large-scale power plants is considered a low-cost strategy to reduce CO2 emissions and fossil-fuel dependence. This work analyzes the effects of biomass co-firing in the physical characteristics and oscillation patterns of the flame, by means of a digital imaging system installed in a 500kWth semi-industrial scale swirl burner. The system is based on a monochrome CCD (Charged Couple Device) camera of high-speed frame rate (120 frames per second). Processing stage comprises the digital analysis in the spatial and spectral domain of recorded videos. Several co-firing flames with two biomass fuels (Cynara cardunculus and Populus sp.) were investigated, at substitution percentages ranging from 0% to 15%, energy basis. Spatial analysis of swirling flames highlights the effects of biomass addition in terms of luminous fluctuation rates and symmetry properties. With the joint analysis of flame derived parameters and CO emissions, flame flicker has been proved as a potential indicator of combustion performance. Additionally, a deeper investigation of luminous flame signal throughout its resulting histogram, have found new outcomes in the identification of instabilities of combustion process. Kurtosis and skewness parameters have shown an interesting response when CO emissions increase resulting in a promising option to develop monitoring and control systems based in non-intrusive measurements.

Measurement of Three-Dimensional Temperature Field of Flickering Premixed Flame with and without Coflow

The three-dimensional (3D) temperature field of the flickering flame with and without coflow can be measured using the flame reaction technique combined with tomographic reconstruction. This combined experimental technique facilitates the non-intrusive measurement of the unsteady 3D temperature field of a premixed methane/air flame. The target flame visualization, which was achieved by the flame reaction of sodium in the supplied mists of sodium chloride solution and line-of-sight intensity images of the flame, was transformed into the temperature field using calibration with the sodium D-line reversal method combined with imaging from six CCD cameras located around the flame. The uncertainty in tomographic temperature measurement was confirmed for the steady axisymmetric flame under the influence of strong coflow. Tomographic temperature measurements were applied to the flickering flame with and without coflow, and the results were analyzed using proper orthogonal decomposition (POD) to understand the unsteady behavior of the temperature field of the flickering flame. The flickering energy was found to be dominant in the first two POD modes. Flame flickering with and without coflow was found to be dominant in the axisymmetric and non-axisymmetric modes, respectively. The characteristics of the flickering flame with and without coflow are discussed in this paper, based on spectrum analysis. The results suggest that the structure of the flickering flame is highly modified by the presence of even a small magnitude of coflow.

Thermo-acoustic instabilities in lean premixed swirl-stabilized combustion and their link to acoustically coupled and decoupled flame macrostructures

We investigate the onset of thermo-acoustic instabilities and their link to the mean flame configurations – or macrostructures – under acoustically coupled and decoupled conditions. Methane–hydrogen mixtures are used to explore the role of the fuel in changing the flame macrostructure, as determined by chemiluminescence, as the equivalence ratio (ϕ) varies. We observe four different configurations: a columnar flame (I); a bubble-columnar flame (II); a single conical flame (III); and a double conical flame (IV). We also observe different thermo-acoustic modes in the lean regime investigated, ϕ∈[0.5-0.75], that correspond to different flame configurations. By changing the combustor length without affecting the underlying flow, the resonant modes of the combustor are shifted to higher frequencies allowing for the decoupling of heat release fluctuations and the acoustic field over a range of equivalence ratio. We find that the same flame macrostructures observed in the long, acoustically coupled combustor arise in the short, acoustically decoupled combustor and transition at similar equivalence ratios in both combustors. The onset of the first fully unstable mode in the long combustor occurs at similar equivalence ratio as the flame transition from configuration III to IV. In the acoustically decoupled case, this transition occurs gradually starting with the intermittent appearance of a flame in the outer recirculation zone (ORZ). Spectral analysis of this phenomenon, referred to as “ORZ flame flickering” shows the existence of an unsteady event occurring over a narrow frequency band centered around 28Hz along with a weaker broadband region at lower frequency in the range [1–10]Hz. The tone at 28Hz is shown to be associated with the azimuthal advection of the flame by the outer recirculation zone flow. Changes in the fuel composition, by adding hydrogen (up to 20%), do not affect the correspondence between the thermo-acoustic modes and the flame macrostructures, but shift the transition points to lower equivalence ratio.

Flickering characteristics and temperature field of premixed methane/air flame under the influence of co-flow

The flickering characteristics and the temperature field of an axisymmetric premixed methane/air flame under the influence of co-flow are studied experimentally using the image analysis and the flame reaction technique. The premixed flame is visualized by the alkali metal solution of sodium (Na) for characterizing the flickering characteristics. The temperature measurement of the flame is carried out using the flame reaction technique combined with the relationship between the local intensity of the flame and the temperature from the sodium D-line reversal method, and the influence of the number density distribution of Na on the measured temperature field is corrected by the measurement integrated analysis of the flame with the iterative procedure. This technique is validated by the local temperature measurement in the steady flame under the influence of co-flow using the thermocouple calibrated by the sodium D-line reversal method. The flame visualization and temperature measurement in the flickering flame of the premixed methane/air flame indicates that the flame contour and the temperature field oscillate periodically with the flickering frequency due to the Kelvin–Helmholtz instability of the flame. The oscillation amplitude decreases and the frequency increases gradually with the co-flow velocity increases similar to the observation in the diffusion flame in literature, while the oscillation amplitude grows with the equivalence ratio increases. These changes in the flickering characteristics of the flame are caused by the variations of the temperature field in the premixed flame.

Characterization of PF flames under different swirl conditions based on visualization systems

The complex phenomena underlying reacting flows are at present an active research topic for improving efficiency and control of industrial combustion processes. The present study assesses the use of a reliable and low cost visualization system based on CCD (charged couple device) technology for the identification of different flame states as a function of swirl number. The experiments were performed in a 500kWth pulverized coal (PF) swirl burner. The visualization system was previously used for similar analysis of an atmospheric gas swirl burner of 50kWth. Both flames were analyzed under low and high swirl conditions in order to identify possible common features concerning swirl flames. The analysis procedure proposed in this work extracts different statistical and spectral parameters from the intensity of radiation stored by each CCD element. The representation of flame features as two-dimensional distributions has led to the identification of typical structures in swirl flows and has allowed us to detect changes in flame structure as swirl number increases. Besides, the quantification of aforementioned parameters has led to relevant findings concerning frequency analysis and flame stability as a function of swirl. Specifically, results show that the characteristic frequency in terms of flame flicker is sensible to changes in swirl conditions.

Lewis-number-induced and buoyancy-driven self-excitations in nitrogen-diluted laminar non-premixed free-jet flames

Experiments in horizontally and vertically injected laminar free-jet flames with applying a DC electric field of −10kV were conducted to explore distinct differences between Lewis-number-induced and buoyancy-driven self-excitations, both of which have been known to be of O(1.0) Hz and thereby indiscernible. A horizontal injection method with the DC electric field, with which one could eliminate accumulated, preheat, partially premixed mixture in front of edge flame and also attach lifted flame to nozzle, was adopted to suppress heat-loss-induced self-excitation and buoyancy effects, thereby identifying Lewis-number-induced self-excitation. Each self-excitation was also discernible in the phase diagram of displacement speed versus stand-off distance. Based on these, flame stability maps as functions of initial fuel mole fraction and nozzle exit velocity were presented in horizontally and vertically injected free-jet flame. The results showed that the Strouhal number for Lewis-number-induced self-excitation increased and then decreased in increase (decrease) of Lewis number (normalized nozzle exit velocity). The mechanism of Lewis-number-induced self-excitation in free-jet flame, formed near nozzle exit, was also discussed. The buoyancy-driven self-excitation due to the accumulation of partially premixed preheated mixture in front of edge flame was also characterized well by the function dependency of a Strouhal number upon a Richardson number, the ratio of molecular weight of corresponding fuel to air, and the ratio of flame front width to mean stand-off distance; meanwhile the buoyancy-induced self-excitation due to a flame flicker was well described by the function of Strouhal number on a Richardson number. The results also showed that the buoyancy-driven self-excitation was originated from buoyancy-induced self-excitation due to a flame flicker. Once a buoyancy-driven self-excitation was launched in a situation without having a heat-loss-induced self-excitation, it overwhelmed the other self-excitations (Lewis-number-induced and buoyancy-induced self-excitations).