Multiple Flame Research Articles

On the influence of distance between two jets on flickering diffusion flames

The interactions of multiple flames are often encountered in real-world fire and industrial burners. The distance between two jets has a strong influence on the instability mode, oscillation frequency, and mean height of flickering diffusion flames, which is experimentally studied and analyzed in this paper. Five different types of instability modes are identified as the separation distance between two nozzles is increased. When the nozzle separation distance is smaller, the flame mode is similar to that of a single-nozzle flame. In this case, the flame can switch between the merged sinuous mode and the merged varicose mode due to external disturbances on the flame. As the nozzle separation distance increases, the probability of mode switching from merged varicose to merged sinuous decreases. As the nozzle separation distance increases further, the flame mode translates into symmetric sinuous mode, alternated sinuous or independent mode. In addition, the flame height and oscillation frequency of a dual-nozzle flame have their own characteristics, which are different from that of the single-nozzle flame. The increase of the nozzle separation distance, leads to a decrease and then an increase of the flame mean height. It is interesting that the flame mean height of the alternated sinuous flame is the lowest, even lower than that of the single-nozzle flame. The oscillation frequency of the symmetrical sinuous flame is lower than that of the single-nozzle flame, and the frequency of alternated sinuous flame is higher than that of the single-nozzle flame.

Experimental study on horizontal gas temperature distribution of two propane diffusion flames impinging on an unconfined ceiling

Ceiling gas temperature is one of the most important factors for heat detection and alarm once an undesirable fire along with releasing hot and toxic smoke is erupted in a building. The impinging flame characterized by the unburnt fuel burning along the ceiling has received much attention in recent years as it poses a greater threat to the ceiling structure, devices and trapped people than the non-impinging flame. Many studies have been focused on the ceiling gas temperature induced by a single flame, while little effort has been put with respect to the multiple flames. The interaction between multiple flames might lead to flames tilt to each other and even merge together with small spacings, resulting in different ceiling gas temperature distribution from the single flame. The aim of this experimental work is to investigate the ceiling gas temperature decay profile induced by two impinging flames. Propane was used as the fuel. The heat release rate (HRR), burner edge spacing and ceiling height above the fuel were overall changed. The ceiling gas temperatures along the direction of changing spacing were measured to determine the impingement point position and temperature decay profile. The results showed that the impingement point position is dependent on the HRR and the spacing as well as the ceiling height, while the maximum gas temperature is weakly affected by the spacing. The established correlation reveals that the maximum excess temperature increases first and then maintains unchanged with increasing the HRR normalized by the ceiling height. The plume radius proposed for the single impinging flame is not enough to characterize the ceiling gas temperature of two impinging flames. A new correlation for temperature decay profile induced by two impinging flames is therefore proposed and validated using the experimental results.

Experimental Study of Multipoint Ignition in Methane–Air Mixtures by Pulsed Microwave Power

Microwave plasma offers a promising method to achieve faster combustion in internal combustion engine. In this paper, microwave multipoint ignition was confirmed via the high-speed imaging technique. The experiment was conducted with microwave resonant ignition system and optical system. A 2-ms, 3000-W, 2.45-GHz pulsed microwave power was used as energy source to produce initial flame kernel in combustion chamber. High-speed imaging was used to record the ignition characteristics via high-speed camera. Ignition experiments were conducted at high pressure of 2 bars of stoichiometric methane–air mixtures. It can be observed in the image and the pressure curves that multiple flame kernels are generated simultaneously. Faster combustion speed is achieved by pulsed microwave power ignition than traditional spark plug ignition in constant volume chamber.

Dynamics of triple-flames in ignition of turbulent dual fuel mixture: A direct numerical simulation study

Pilot-ignited dual fuel combustion involves a complex transition between the pilot fuel autoignition and the premixed-like phase of combustion, which is challenging for experimental measurement and numerical modelling, and not sufficiently explored. To further understand the fundamentals of the dual fuel ignition processes, the transient ignition and subsequent flame development in a turbulent dimethyl ether (DME)/methane-air mixing layer under diesel engine-relevant conditions are studied by direct numerical simulations (DNS). Results indicate that combustion is initiated by a two-stage autoignition that involves both low-temperature and high-temperature chemistry. The first stage autoignition is initiated at the stoichiometric mixture, and then the ignition front propagates against the mixture fraction gradient into rich mixtures and eventually forms a diffusively-supported cool flame. The second stage ignition kernels are spatially distributed around the most reactive mixture fraction with a low scalar dissipation rate. Multiple triple flames are established and propagate along the stoichiometric mixture, which is proven to play an essential role in the flame developing process. The edge flames gradually get close to each other with their branches eventually connected. It is the leading lean premixed branch that initiates the steady propagating methane-air flame. The time required for the initiation of steady flame is substantially shorter than the autoignition delay time of the methane-air mixture under the same thermochemical condition. Temporal evolution of the displacement speed at the flame front is also investigated to clarify the propagation characteristics of the combustion waves. Cool flame and propagation of triple flames are also identified in this study, which are novel features of the pilot-ignited dual fuel combustion.

Computational realization of multiple flame stabilization modes in DLR strut-injection hydrogen supersonic combustor

Inspired by the existence of multiple flame stabilization modes in cavity-assisted supersonic combustor, multiple flame stabilization modes of DLR hydrogen-fueled strut injection supersonic combustor were numerically realized and analyzed for a wide ranges of inflow stagnation temperature from 607 to 2141 K and overall equivalence ratio from 0.022 to 0.110. Finite-rate chemistry large eddy simulation with detailed hydrogen mechanism was employed to capture unsteady flow characteristics and the effects of chemical kinetics. Two typical flame stabilization modes were identified and presented in a regime nomogram, which shows the dominant influence of the stagnation temperature and the secondary influence of overall equivalence ratio. At relatively low stagnation temperatures, the flame is stabilized in an “attached flame” mode, which requires a low-speed recirculation zone behind the strut for radical production and a high-speed intense combustion zone for heat release. At relatively high stagnation temperatures, the flame is stabilized in a “lifted flame” mode, in which the effect of the low-speed recirculation zone is negligible, rendering most reactions take place in supersonic flow. At intermediate stagnation temperatures, blow-out was always observed and flame cannot be stabilized in the combustor even with initially forced ignition.

An experimental study on coflow diffusion combustion in a pellet-packed bed with different bed lengths.

In experimental investigations on axial symmetry, over-ventilated CH4/air diffusion combustion in a packed bed is executed to study the height, shape and stability of the flame. The combustor is a quartz tube packed with alumina pellets in which a cylindrical fuel stream is surrounded by a coflow air nozzle. The results show that the bed length and pellet diameter have a significant influence on the flame properties. In general, the flame above the pellet surface has axial symmetry, and its shape and colour are similar to those of a conventional diffusion flame when the bed length is smaller. The colour of the flame front varies with the bed length. The changed colour indicates an increased flame front temperature and that the combustion regime above the bed surface may change from non-premixed combustion to partially premixed combustion or even premixed combustion owing to the mix and dispersion effect in the packed bed. In addition, multiple flame behaviours, such as an inclined flame front, isolated reaction zone and oscillatory motion followed by a pulsating sound with a few hertz in a packed bed, are observed experimentally. The possible reasons for these phenomena are discussed.

Flame synthesis of gamma-iron-oxide (γ-Fe2O3) nanocrystal films and carbon nanotubes on stainless-steel substrates

Using a rapid, open-atmosphere flame synthesis technique, iron-oxide nanoparticles (α-Fe2O3) and nanocrystals (γ-Fe2O3) and carbon nanotubes (CNTs) are grown directly on stainless steel substrates. Multiple inverse-diffusion flames provide both oxide species (H2O and CO2) and carbon species (CH4, CO, C2H2), at elevated temperature, conducive for the growths of iron-oxide nanoparticles/nanocrystals and CNTs. The growth of α-Fe2O3 nanoparticles occurs at 500 °C. At a temperature of 850 °C, the growth of CNTs occurs on type 304 stainless steel, while γ-Fe2O nanocrystal films are grown on 304 L and 316 L stainless steels with relatively lower innate carbon content. Interestingly, a temperature-step process, where 304 stainless steel substrate is initially at 500 °C and then increased to 850 °C, distinctly converts the “seed” α-Fe2O3 nanoparticles into larger γ-Fe2O3 nanocrystals, with some scattered CNTs. Strategically time-varied local conditions should produce different compositional ratios of iron-oxide/CNT hybrid films, as well as targeted functionally-graded nanostructures and nanocomposites.

Thermal expansion effect on the propagation of premixed flames in narrow channels of circular cross-section: Multiplicity of solutions, axisymmetry and non-axisymmetry

The present study examines, in presence of thermal expansion effects, the existence of the multiplicity of solutions previously reported within the context of diffusive-thermal modeling in [15], for lean premixed flames with low Lewis number (Le < 1) propagating in narrow circular adiabatic channels subject to a Poiseuille flow. For this, direct numerical simulations have been carried out within the framework of variable-density Navier–Stokes equations and zero-Mach-number approximation. The simulations, conducted for both axisymmetric and three-dimensional cylindrical geometries, confirm the coexistence of multiple steady flame structures for a given flow rate. They show that axisymmetric flames concave towards the upstream are more unstable to three-dimensional perturbations than convex (toward the upstream) flames. This result evinces earlier findings obtained from stability analysis. The non-axisymmetry property of the flame is also found to push back the critical flashback limits at larger flow rate when compared to those predicted under the assumption of flame axisymmetry.

Purtscher-like retinopathy associated with dermatomyositis and hemophagocytic lymphohistiocytosis

Abstract Case report An 11 year-old girl with progressive muscle weakness due to dermatomyositis and hemophagocytic lymphohistiocytosis in which multiple cotton exudates, venous tortuosity and flame hemorrhages are detected in the funduscopic examination, leading to the diagnosis of Purtscher-like retinopathy. Discussion Purstcher's retinopathy is a microvascular disorder with clinical signs of probable thrombotic origin. Its treatment is controversial, with systemic corticosteroids being the most widespread choice.

Formation and stabilization of multiple ball-like flames at Earth gravity

Near-limit low-Lewis-number premixed flame behavior is studied experimentally and numerically for flames of H2–CH4–air mixtures that are located in a 55 mm diameter tube and below a perforated plate in a downward mixture flow. A combustion regime diagram is experimentally identified in terms of equivalence ratio and ratio of H2 to CH4 (variation of fuel Lewis number). Planar flames, cell-like flames, distorted cap-like flames, and arrays of ball-like flames are progressively observed in the experiments as the equivalence ratio is decreased. The experimentally observed ball-like lean limit flames experience chaotic motion, which is accompanied by sporadic events of flame splitting and extinction, while the total number of simultaneously burning flamelets remains approximately the same. In separate experiments, the multiple ball-like lean limit flames are stabilized by creating a slightly non-uniform mixture flow field. The CH* chemiluminescence distributions of the lean limit flames are recorded, showing that the ball-like lean limit flame front becomes more uniform in intensity and its shape approaches a spherical one with the increase of H2 content in the fuel. Numerical simulations are performed for single representative flames of the array of stabilized flamelets observed in the experiments. The simulated ball-like lean limit flame is further contrasted with the single ball-like flame that forms in a narrow tube (13.5 mm inner diameter) with an iso-thermal wall. The numerical results show that the ball-like lean limit flames present in the array of ball-like flames are more affected by the buoyancy-induced recirculation zone, compared with that in the narrow tube, revealing why the shape of the ball-like flame in the array deviates more from a spherical one. All in all, the wall confinement is not crucial for the formation of ball-like flames at terrestrial gravity.

Application of the global soot model based on smoke point in simulation of multiple laminar diffusion flames

ABSTRACTIn this study, a new conservation scalar, sooting tendency parameter, which has the same control differential equation with mixture fraction except the boundary values at fuel inlets, is introduced to evaluate the average laminar smoke point (LSP) height of multiple fuels mixed in combustion region. According to theoretical analysis, the average LSP height of the mixed fuels in combustion region will be equivalent to the ratio of mixture fraction and sooting tendency parameter. The soot oxidation model is also modified in present work to allow it to be used with mixture fraction combustion model. Then this improved global soot model is implemented in Fire Dynamics Simulator and two different laminar flames are simulated in one case to validate the predictability. Good agreements between the predicted and measured soot volume fraction demonstrate the applicability of accounting for the sooting propensity of different fuels by the introduced scalar, and the reasonability of the modification of the soot oxidation model. The difference of stoichiometric mixture fraction between the fuel used in combustion model and the realistic fuels will lead to some error in soot prediction, yet which can be reduced by multiplying the boundary value of sooting tendency parameter by a safety factor (SF) greater than 1. The introduction of the SF is also very essential in fire protection design, because it will lead to the increase of predicted soot volume fraction, making the fire risk assessment more conservative.

Impact of the pre-chamber nozzle orifice configurations on combustion and performance of a Natural Gas Engine

In this study, a pre-chamber was designed to form near stoichiometric mixture and provide multiple turbulent flame jets to ignite the lean mixture and accelerate the combustion in the main combustion chamber for a natural gas engine. A CFD simulation was employed to investigate the impact of the pre-chamber nozzle configurations on flow and combustion processes inside the engine, as well as on the performance of the engine. Various configurations were investigated, including orifice number of 4 to 8 and orifice diameter ranging from 1.6 m to 2.9 mm. A non-dimensional parameter, β, was used to characterize the relative flow area of these configurations. The numerical results indicate that, for a given nozzle flow area, among the design of different orifice numbers, the 6-orifice design can obtain the optimal combustion and engine performance. Otherwise, a design of more orifices leads to slower flame penetrating speed in the main-chamber, and the design of less orifices leads to slower circumferential flames propagations in the main-chamber. Moreover, for a 6-orifice pre-chamber, the optimal orifice diameter was found to be 2.0 mm, corresponding to a β value of 0.3. A design of larger diameters leads to slower penetrating for the flame jets and insufficient radial flame propagations in the main-chamber, while a design of relatively smaller orifice diameters leads to insufficient circumferential flames propagations in the main-chamber. Additionally, for the engine performance, all the pre-chamber designs improve the indicated efficiency and reduce the NOx emission. Especially, the design of 6-orifice with diameter of 2.0 mm achieves a 35.0% increase of indicated thermal efficiency and a 78.0% reduction of NOx emission compared to the prototype engine.

Numerical analysis for attenuation effects of perforated plates on thermoacoustic instability in the multiple flame combustor

In the present study, the FDF-based approach together with the Helmholtz solver has been applied to precisely analyze the attenuation effects of the perforated plates on thermoacoustic instability in the lab-scale multiple flame combustor. In order to effectively suppress the combustion instability, we derive the optimal damping conditions for perforated plates in terms of the porosity and cavity length. To overcome the weaknesses of the Howe model, the Luong model is utilized for an impedance boundary condition of perforated plates. After then, we validated our methodology to the impedance tube, which shows the predicted results are well agreed with the experimental data for all validation cases Another new approach for the dual perforated plates has been proposed to enlarge the absorption bandwidth in a wider frequency regime. Numerical results clearly indicate that since the dual plates are tuned at two different target frequencies, the system can have two maximum absorption points and their combined effects in a frequency range. This makes the dual-plate method capable of stabilizing the combustion instability in the much wider frequency range than does the single-plate scheme. Therefore, it is expected that the dual-plate methodology would be more effective to suppress the unexpected nonlinear behavior such as the mode change, frequency shift, and hysteresis thanks to the wide attenuation coverage.

Validation of a mixture-averaged thermal diffusion model for premixed lean hydrogen flames

The mixture-averaged thermal diffusion model originally proposed by Chapman and Cowling is validated using multiple flame configurations. Simulations using detailed hydrogen chemistry are done on one-, two-, and three-dimensional flames. The analysis spans flat and stretched, steady and unsteady, and laminar and turbulent flames. Quantitative and qualitative results using the thermal diffusion model compare very well with the more complex multicomponent diffusion model. Comparisons are made using flame speeds, surface areas, species profiles, and chemical source terms. Once validated, this model is applied to three-dimensional laminar and turbulent flames. For these cases, thermal diffusion causes an increase in the propagation speed of the flames as well as increased product chemical source terms in regions of high positive curvature. The results illustrate the necessity for including thermal diffusion, and the accuracy and computational efficiency of the mixture-averaged thermal diffusion model.

Hyperspectral tomography based on multi-mode absorption spectroscopy (MUMAS)

This paper demonstrates a hyperspectral tomographic technique that can recover the temperature and concentration field of gas flows based on multi-mode absorption spectroscopy (MUMAS). This method relies on the recently proposed concept of nonlinear tomography, which can take full advantage of the nonlinear dependency of MUMAS signals on temperature and enables 2D spatial resolution of MUMAS which is naturally a line-of-sight technique. The principles of MUMAS and nonlinear tomography, as well as the mathematical formulation of the inversion problem, are introduced. Proof-of-concept numerical demonstrations are presented using representative flame phantoms and assuming typical laser parameters. The results show that faithful reconstruction of temperature distribution is achievable when a signal-to-noise ratio of 20 is assumed. This method can potentially be extended to simultaneously reconstructing distributions of temperature and the concentration of multiple flame species.

Propagation speeds for interacting triple flames

Triple (or tribrachial) flames propagate through mixtures faster than the premixed laminar flame speed due to streamline divergence ahead of the flame base that decelerates the flow into the leading edge of the flame. When multiple triple flames are in close proximity, the bulk propagation speed of the structure can be even faster due to additional streamline divergence. Turbulent flames in partially-premixed conditions can encounter these situations, where multiple stoichiometric crossing are in close proximity, leading to multiple interacting triple flames being formed. Propagation speeds of the flame structure with respect the bulk flow for individual triple flames have been well characterized in previous studies, and the local flame speed of interacting triple flames have been reported; however, characterization of the propagation speed for the overall flame structure of interacting triple flame speeds has not been reported. The present work utilizes a laminar five slot burner, which allows both the concentration gradients and stoichiometric separation distance of two interacting triple flames to be varied. The bulk propagation speed of the multiple edge flames has been characterized as a function of the distance between the flame bases (or stoichiometric locations) and the local flame curvatures in order to better understand the conditions which lead to larger streamline divergence and faster propagation speeds. Interaction between multiple edge flames has been found to play an essential role in this propagation speed. Interacting edge flame speeds were modeled by modifying the relationship for single triple flame propagation speed with an added term for the interaction between the two flames to account for the increased effective divergence.

Biobased polyelectrolyte multilayer-coated hollow mesoporous silica as a green flame retardant for epoxy resin

Here, we describe a multifunctional biobased polyelectrolyte multilayer-coated hollow mesoporous silica (HM-SiO2@CS@PCL) as a green flame retardant through layer-by-layer assembly using hollow mesoporous silica (HM-SiO2), chitosan (CS) and phosphorylated cellulose (PCL). The electrostatic interactions deposited the CS/PCL coating on the surface of HM-SiO2. Subsequently, this multifunctional flame retardant was used to enhance thermal properties and flame retardancy of epoxy resin. The addition of HM-SiO2@CS@PCL to the epoxy resin thermally destabilized the epoxy resin composite, but generated a higher char yield. Furthermore, HM-SiO2 played a critical role and generated synergies with CS and PCL to improve fire safety of the epoxy resin due to the multiple flame retardancy elements (P, N and Si). This multi-element, synergistic, flame-retardant system resulted in a remarkable reduction (51%) of peak heat release rate and a considerable removal of flammable decomposed products. Additionally, the incorporation of HM-SiO2@CS@PCL can sustainably recycle the epoxy resin into high value-added hollow carbon spheres during combustion. Therefore, the HM-SiO2@CS@PCL system provides a practical possibility for preparing recyclable polymer materials with multi-functions and high performances.

Direct Current Forcing of an Atmospheric Multiburner Flame for Rocket Combustor Emulation

This study investigated the effect of direct current electric fields on the response of an atmospheric, premixed methane-air flame using electrode geometries intended to emulate a simplified liquid rocket engine combustion chamber. The burners consisted of single- and multielement configurations to determine the difference between element radial location and flame-to-flame interaction as an atmospheric analogy to the functionality of a multiport liquid-propellant rocket engine injector. The results showed that electric field–induced ionic winds were capable of improving flame stability by extension of the lean flammability limit and increased blowoff velocity. Single-burner configurations closer to the anode wall had more significant effects on the flame response due to stronger electric fields generated at those locations. The cylindrical anode was more effective in changing the flame response due to larger surface area, leading to higher field strengths. It has been concluded that a system with multiple flames enhances the effects of applied electric fields, which seems to indicate the potential to effectively modify the flame response in rocket engines with multiple injection ports.

FDF-based combustion instability analysis for stabilization effects of a slotted plate in a multiple flame combustor

In the present study, new promising methods are suggested to analyze thermoacoustic instability and the stabilization effects in the multiple flame combustor with a slotted plate. Using the generalized regression neural network (GRNN), the flame describing function (FDF) is effectively modeled from a limited number of experimental data. This neural-network based FDF method is able to generate more refined FDF data in an extended range. These refined FDF data are utilized in a Helmholtz solver for thermoacoustic instability analysis. According to the velocity perturbation ratio, eigenfrequencies are investigated to know the unstable regimes of the combustor. To take account of the effects of plate thickness, the present approach has slightly modified the Dowling method for modeling the impedance of a slotted plate. To find the effective damping conditions of a slotted plate, parametric studies have been carried out with the help of simulated annealing (SA) algorithm in wide-range operating conditions. It is identified that the absorption bandwidth becomes wider by decreasing slit width, and narrower width yields the higher average absorption coefficient. All the numerical results confirm that these new methodologies are quite reliable and widely applicable for the analysis of combustion instability encountered in many practical combustion systems.

Flame spread over inclined electrical wires with AC electric fields

Flame spread over polyethylene-insulated electrical wires was studied experimentally with applied alternating current (AC) by varying the inclination angle (θ), applied voltage (VAC), and frequency (fAC). For the baseline case with no electric field applied, the flame spread rate and the flame width of downwardly spreading flames (DSFs) decreased from the horizontal case for −20° ≤ θ < 0° and maintained near constant values for −90° ≤ θ < −20°, while the flame spread rate increased appreciably as the inclination angle of upwardly spreading flames (USFs) increased. When an AC electric field was applied, the behavior of flame spread rate in DSFs (USFs) could be classified into two (three) sub-regimes characterized by various functional dependences on VAC, fAC, and θ. In nearly all cases of DSFs, a globular molten polyethylene formed ahead of the spreading flame edge, occasionally dripping onto the ground. In these cases, an effective flame spread rate was defined to represent the burning rate by measuring the mass loss due to dripping. This effective spread rate was independent of AC frequency, while it decreased linearly with voltage and was independent of the inclination angle. In DSFs, when excessively high voltage and frequency were applied, the dripping led to flame extinction during propagation and the extinction frequency correlated well with applied voltage. In USFs, when high voltage and frequency were applied, multiple globular molten PEs formed at several locations, leading to ejections of multiple small flame segments from the main flame, thereby reducing the flame spread rate, which could be attributed to the electrospray phenomenon.