Flame Tip Research Articles

Experimental investigation on burning characteristics of diesel pool fires in naturally ventilated compartment

SummaryIn this study, experiments were performed in a 4 m × 4 m × 4 m compartment having a door size of 2 m × 1 m in the front wall. The pan diameter has been varied from 0.2 to 1.0 m in order to evaluate parameters, such as mass loss rate, CO/CO2 yield, global equivalence ratio (GER), flame height, flame pulsation frequency and total heat flux within the compartment. Experimental mass loss flux was found to be 0.23 times lesser than the mass loss flux obtained with an open fire test. A correlation was developed to estimate the flame height for compartment fire as the function of pool diameter and heat release rate. Pulsation frequency has been well correlated with previous studies for smaller pool sizes, due to no interaction between the flame tip and upper smoke layer. However, for a larger pool, this seems to be deviated due to a larger smoke layer thickness. Heat flux variations were observed to follow power law trends for all the inner surfaces of the compartment with respect to pool diameters. This investigation is also essential for better understanding of diesel (sooty) fuel burning effects with respect to pool size in a compartment having well‐ventilated door opening.

The role of the unsaturation degree on the droplet combustion characteristics of fatty acid methyl ester

Biodiesel is composed of various types of methyl ester compounds with different concentrations. The role of unsaturation degree was observed through single-molecule droplet combustion from three fatty acid methyl ester compounds. This research was conducted under normal gravity at ambient temperature and atmospheric pressure. Analysis of droplet combustion characteristics was observed along the heating, evaporation, ignition, and combustion stages. The cis configuration and allyl groups in unsaturated molecules related to different unsaturation degrees affect the droplet combustion process. Allyl and bis-allyl groups play an important role in accelerating hydrogen atom abstraction due to the weaker bond dissociation energy of the CH bond adjacent to the carbon double bond. The lower energy gap in unsaturated molecules is prone to electron excitation. Increasing the unsaturation degree resulted in shorter ignition delay and burning duration but higher droplet temperature, burning rate constant, specific power output, and flame dimensions. Soot particles formed above the flame tip on the unsaturated molecules gave a higher flame dimension. A high concentration of linoleic methyl ester in biodiesel fuel is recommended due to of high power output. However, the soot formation at a higher level of unsaturation degree should be considered.

Experimental observation of lean flammability limits using turbulent jet ignition with auxiliary hydrogen and methane in pre-chamber

The combustion characteristics of CH4-air mixture under the lean-fuel conditions ignited by different methods including SI, unfueled pre-chamber ignition and fueled pre-chamber ignition were investigated experimentally in a constant volume combustion chamber (CVCC) through high-speed schlieren photography. And a self-designed fueled pre-chamber turbulent jet ignition (TJI) combustion system with a single orifice configuration and small volume was employed. Results show that TJI could enhance combustion including the rapid increases of flame tip velocity and flame area growth rate, which also was manifested by increasing peak pressure and shortening time to peak. The pre-chamber ignition fueled with hydrogen can greatly extend the lean flammability limit in the main chamber and reduce the sensitivity of flame development with the excess air coefficient. However, the oscillations in flame propagation and pressure evolution due to the flame-shock interaction were obvious under the pre-chamber fueled with hydrogen. Overall, in the present work, the turbulent flame development in the main chamber depends on the flame consumption velocity in pre-chamber, orifice size of pre-chamber, and the mixture concentration in main chamber. These results are of great help to achieve the highly-efficient combustion and to shorten the combustion duration in practical SI engines.

Effect of Ionic Wind Induced by DC Electric Field on Biogas/Air Turbulent Premixed Flame Structure

ABSTRACT Effect of DC electric field on lean premixed biogas turbulent flame structure at low and medium turbulence intensity was studied experimentally using OH-PLIF. OH signal distribution, turbulent burning velocity and flame structure geometric parameters were derived. Results show that downward electric field could broaden the downstream burned gas and push the flame tip down. The flame base is closer to the burner rim in downward electric field for the modification of thermal diffusion and flow modification caused by ionic wind. The effect on flame tip height reduction is more obvious for biogas flame with a higher CO2 ratio. According to the flame structure analysis, the turbulent burning velocity of biogas is increased slightly due to the modification of flow field by ionic wind generated by downward electric field, and the increment decreases with the increase of turbulence intensity. In addition, there’s a decrease in mean flame surface density and an increase in centerline flame brush thickness. The flame surface curvature PDF distribution is nearly not affected by the application of DC electric field. For turbulent flames, the effect of electric field is non-uniform and inconstant, which induces extra flow perturbation. This perturbation is different from turbulence, and the combined effects lead to the variation of flame structure.

Influences of a square and circular vent on gasoline vapor explosions and its propagation: Comparative experimental study

Although vents' shapes influence the discharge coefficients, there is a lack of discussions of the vent shapes in vent design guides and explosion-venting investigation. This work carried out detail experiments in a 10 L cuboid vented vessel containing a circular or a square vent to investigate the influence of vent shapes on explosions where the dimensionless vent coefficients, initial gasoline volumes and vent locations are different. A discussion of the various characteristics of the explosion was presented and compared. Results show that the inner flame will continue to propagate and oscillate after flame ejected from the vent in the vessel, the oscillation of flame tip propagation under circular vent is larger than the square one. As gasoline volumes increase or vent coefficients decrease, the amplitude of oscillation enhancement increases. The Pmax and KG under the circular vents are little different from the square one under different dimensionless vent coefficients, initial gasoline volumes, and vent locations, indicating that the circular vents have almost the same venting effects as the square vents with the same vent area. The results may be meaningful for vapor explosion-venting and serving as a reference for the optimal vent design in practical applications.

Modeling soot formation in laminar coflow ethylene inverse diffusion flames

A numerical study is conducted using the CoFlame code to investigate the sooting characteristics of ethylene-fueled laminar coflow inverse diffusion flames (IDFs), aiming to gain insights into the different flame structure and soot formation features between an IDF and a normal diffusion flame (NDF). The effects of oxygen mole fraction in the oxidizer stream, denoted as Oxygen Index (OI), on flame structure and soot production are studied over the OI range of 21% to 33%. The soot model parameters were chosen to produce overall reasonably good agreement between the predicted and measured in terms of soot volume fraction, mean primary particle size, and the mean number of primary particles per aggregate in both the normal and inverse configurations. A sensitivity study showed that soot nucleation and soot surface growth by PAH adsorption modeling should be considered simultaneously as a strongly coupled process through competing for polycyclic aromatic hydrocarbons (PAHs). The IDF configuration displays distinct characteristics from the NDF one in terms of flame structure as well as soot production. In IDFs soot is formed along the outer edge of reaction front, and does not cross the reaction front to the oxidizer, decoupling completely soot oxidation which leads to soot emission from the open flame tip. Modeling results indicate that in the IDF configuration the hydrogen abstraction acetylene addition (HACA) process is severely suppressed, while the adsorption of PAH molecules is dominant to soot surface growth at relatively low temperatures. Nucleation of new particles is significant all along the flame height, causing emission of young soot. IDF represents an ideal configuration to study soot surface growth by PAH adsorption. Increasing OI enhances all the soot processes, especially surface growth by PAH adsorption. Although soot oxidation rate by OH radicals is also strongly enhanced, it remains about two orders of magnitude lower than soot surface growth rate by PAH adsorption.

Experimental study on the explosion behaviors of premixed syngas-air mixtures in ducts

Explosion characteristics of premixed syngas-air mixtures at room temperature and atmospheric pressure were experimentally reported when the explosion flame propagates in ducts with various heights (H) and lengths (L). The discussion was based on flame morphology and pressure dynamics. The ratio of L/H and the ratio of H2/CO had a significant effect on the explosion flame behaviors as the explosion occurred in ducts. The structure of the explosion flame changes more drastically, as both the L/H ratio is large. The ratio of L/H affected the flame tip dynamics after the flame reached the duct wall, and the time of flame reaching the duct walls is divinable. For a given duct height, the shorter the duct length is, the faster flame propagates, and the maximum flame tip speed was higher as the duct length was small. For a given duct length, flame tip dynamics showed a nearly same development tendency, but the shorter the duct height, the faster the flame propagated. The venting pressure affected the overpressure dynamics, and the venting pressure increased with the increase of the L/H ratio and the H2/CO. For a given duct height, the overpressure reached the maximum value almost at the same time, and the longer duct length resulted in the greater maximum overpressure. Finally, for a given duct length, the higher duct height caused the higher maximum overpressure.

Hydrogen-air explosion with concentration gradients in a cubic enclosure

An inhomogeneous hydrogen-air mixture is induced once hydrogen leakage occurs, and an explosion may be triggered due to an accidental ignition with sufficient strength. The effects of different hydrogen concentration gradients on flame behavior and overpressure were experimentally investigated in a closed cubic vessel with a volume of 0.125 m3. The transverse concentration gradients of hydrogen injected from the top face of the vessel were recorded using five oxygen sensors. A high-speed photography was employed to capture the flame shape evolution and the flame tip velocity was derived. Three piezoelectric pressure transducers were mounted on the top and side walls to measure the pressure-time profiles. The results show that the maximum overpressure in homogeneous mixtures is larger than that in inhomogeneous mixtures for fuel-lean hydrogen-air mixtures. However, the maximum overpressure in inhomogeneous mixtures is equal to or larger than that in homogeneous mixtures for fuel-rich hydrogen-air mixtures. Due to a larger difference between the upward and downward flame speeds, a mushroom-like shape flame was recorded in inhomogeneous hydrogen-air mixtures in fuel-lean to slightly fuel-rich hydrogen-air mixtures. The flame speed in two directions has the almost same value in inhomogeneous fuel-richer mixtures. The average flame speed in inhomogeneous mixtures is significantly larger than that in homogeneous mixtures for fuel-rich hydrogen air mixtures.

Identification of the extinction mechanism of lean limit hydrogen flames based on Lewis number effect

The conventional flame stretch analysis showed some technical limitations in describing the observed extinction mechanism without considering the heat transfer characteristics related to flame flow patterns. To improve the flame stretch analysis, this study investigated the extinction mechanism of lean limit hydrogen flames with the stabilized flame method. Multidimensional lean limit hydrogen flames stabilized in a tube of inner diameter of 25 mm were simulated numerically. Unlike the methane flames, the positive effect of a stretched hydrogen flame (Le≪1) on the flame tip dominated the negative stretch effect by increasing the preferential diffusion. Hence the primary extinction of hydrogen flames occurred at the trailing edge by heat loss mechanism. The simulation confirmed that the radiation heat loss rate did not attain 5% of the conduction heat loss rate at the trailing edge. When the flame skirt length was shortened enough by the primary extinction, an enhanced recirculation flow was generated. The combined effect of conduction and recirculation flow associated with the Lewis number effect was the key to understand the extinction mechanism of lean limit hydrogen flames. Finally, generation of recirculating flow causing the stretch extinction plays a dominant role in final flame extinction.

Efficient simulation of flame acceleration and deflagration-to-detonation transition in smooth pipes

Evaluation of accident scenarios including flame acceleration and deflagration-to-detonation transition (DDT) in chemical plant piping systems increases the need for an efficient numerical simulation tool capable of dealing with this phenomenon. In this work, a hybrid pressure-density-based solver including deflagrative flame propagation as well as detonation propagation is presented. The initial incompressible acceleration stage is covered by the pressure-based solver until the flame velocity reaches the fast flame regime and transition to the density-based solver is done. The deflagration source term is formulated in terms of a turbulent flame speed closure model incorporating various physical effects crucial for flame acceleration at low turbulence conditions (Katzy and Sattelmayer, 2018). Modelling of the detonation source term is based on a quadratic heat release function (Hasslberger, 2017). The presented numerical approach is validated in terms of DDT locations and pressure data from Schildberg (2015) as well as recently completed flame tip position measurements. For this purpose, H2/O2/N2 mixtures ranging from 25.6 vol-% H2 to 29.56 vol-% H2 in two different pipe geometries are considered. The focus of the current work is on predicting the DDT location correctly and good agreement is observed for the investigated cases.

Effect of high hydrogen enrichment on the outer-shear-layer flame of confined lean premixed CH4/H2/air swirl flames

In this study, we investigated the H2-induced transition of confined swirl flames from the “V” to “M” shape. H2-enriched lean premixed CH4/H2/air flames with H2 fractions up to 80% were conducted. The flame structure was obtained with Planar Laser-Induced Fluorescence (PLIF) of the OH radical. Flow fields were measured with Particle Image Velocimetry (PIV). It was observed that the flame tip in the outer shear layer gradually propagated upstream and finally anchored to the injector with the hydrogen fractions increase, yielding the transition from the “V” to “M” flame. We examined the flame structures and the flame flow dynamics during the transition. The shape transition was directly related to the evolution of the corner flame along the outer shear layer. With H2 addition, the outer recirculation zone first appeared downstream where the corner flame started to propagate upstream; then, the recirculation zone expanded upward to form a stable “M” flame gradually. The flow straining was observed to influence the stabilization of the outer shear layer flame significantly. This study can be useful for the understanding of recirculation-stabilized swirling flames with strong confinement. The flame structure and the flow characteristics of flames with a high H2 content are also valuable for model validation.

Experimental study on flame propagation through stratified crude oil vapor in a horizontal duct

Stratified vapor–air mixtures are formed owing to evaporation after crude oil leaks in confined spaces. The concentration distribution, ignitability of the stratified vapor, flame propagation in it, and pressure buildup owing to combustion must be investigated to obtain knowledge for explosion protection and mitigation. In this study, the concentration distribution of the vapor above a 1.0-mm-thick crude oil layer in a horizontal duct was measured, and the effects of the ignition delay (tig) on the ignitability, flame behavior, and combustion overpressure were investigated. The experimental results revealed that the average concentration gradient of vapor decreased with the evaporation of the crude oil layer. The stratified vapor could be ignited from 10 to 50 min, after fuel supply, by an electric spark 150 mm above the oil surface. A two-zone flame structure including a blue premixed flame and yellow diffusion flame was observed, and the flame was farther from the oil surface as tig increased. The maximum horizontal speed of the blue flame tip and overpressure owing to combustion initially increased and then decreased as tig increased from 10 to 50 min. The yellow diffusion flame had an important function in the maximum explosion overpressure. A maximum overpressure of 530 kPa was measured at tig = 25 min, which was 80 kPa higher than that of stratified gasoline vapor obtained under nearly identical conditions in previous research.

Dynamics of flame extinction in narrow channels with cold walls: Heat loss vs acceleration

Propagation of a premixed flame from a closed to an open end in micro-channels with smooth non-slip isothermal walls is considered in the context of flame extinction dynamics. Powerful exponential flame acceleration in micro-channels with adiabatic walls has been demonstrated at the initial quasi-isobaric stage of the process [Bychkov et al., Phys. Rev. E 72, 046307 (2005)]. In contrast to the previous studies, here we investigate flame propagation in channels with isothermal walls. The problem is solved by means of high-fidelity laminar numerical simulations of the complete set of the Navier–Stokes combustion equations. For most of the problem parameter sets chosen, we obtain initial flame acceleration after ignition at the closed channel end. This acceleration resembles qualitatively the adiabatic case, but it develops noticeably slower, in an approximately linear regime instead of the exponential one and persists only for a limited time interval. Subsequently, heat loss to the walls reduces the temperature and hence the volume of the burnt gas behind the flame front, which produces a reverse flow in the direction of the closed channel end. When the amount of the burnt gas becomes sufficiently large, the reverse flow stops the acceleration process and drives the flame backwards with modifications of the flame front shape from convex to concave. Eventually, the flame extinguishes. Qualitatively, the process obtained reproduces a possible combustion failure during deflagration-to-detonation transition observed in previous experiments. We investigate the key characteristics of initial flame acceleration such as the acceleration rate and the maximum speed of the flame tip.

Numerical modelling to study the effect of DC electric field on a laminar ethylene diffusion flame

An applied DC electric field was experimentally demonstrated to modify the flame structure and gas dynamic in an ethylene diffusion flame. The aim of this paper is to investigate the influence of the electric field on the flow field and its impacts on the flame behavior. A numerical study has been performed to elucidate the experimental observations and to monitor the effect of electric body force on the flame. The numerical model was validated by comparing the computed results to experimental measurements from the literature. The resulting computed flame shape was compared to a visible image taken during the experiment. The simulated OH mole fraction, the burning rate and the computed velocity and temperature are presented. The developed model proved the ability to reproduce qualitatively the experimental flame behavior when submitted to the electric field. The electric field is shown to modify the flame shape (flame tip, flame shortness and flame deformation), to promote the burning process and to improve the ion production. Results show that the modifications are due to an air entrainment acting specifically near the burner zone enhancing the mixture and changing the fluid dynamic in this region. The ionic wind is demonstrated to increase the maximum burning rate and promoting ions' formation mostly near the burner. A more detailed model (detailed ions' chemistry and soot model with charged particles, detailed electric diffusion) is necessary to gain a better understanding of the influence of electric field on diffusion combustion and soot formation.

A weakly nonlinear analytical model for the transversely forced flame describing function of a slit flame

In this work, a weakly nonlinear analytical model of the flame describing function (FDF) of a two-dimensional laminar premixed slit flame subjected to both transverse perturbations and two-dimensional mean flows has been proposed. Two oncoming velocity convection models are used to investigate their effects on the flame tip movements and the nonlinear FDFs. The perturbation method is then used to solve the G-equation. As the flame tip movement plays an important role in the transverse perturbed FDF, its analytical solution is also derived. Validations of these weakly nonlinear solutions are then conducted by comparing them to the previous linear flame transfer function for weak perturbations and results obtained numerically solving the G-equation model. Results show that the transverse oncoming velocity convection models have a significant effect on the nonlinear flame tip dynamics and the nonlinear FDFs, which can also be found in the previous linear model. The transverse nonlinear effects are not so large, as the effect of the transverse perturbation is typically smaller than that of the longitudinal perturbation, which still contributes to reveal the physical mechanisms of the transverse combustion instabilities. For strong transverse perturbations, the analytical solutions of the FDFs derived by the nonlinear model are different from those derived by the linear model both using the uniform model, especially at low frequencies, which indicates that the nonlinear effect is more significant at low frequencies. Transverse mean flow velocity and the amplitude of the transverse disturbed flow velocity can enhance the nonlinearities at low frequencies.

Numerical investigation of a novel micro combustor with a central and bilateral slotted blunt body

To improve the combustion stability at micro scale, the micro combustor with a central and bilateral slotted blunt body (MCSB) exhibiting significant combustion performance improvement is designed. The new one can not only effectively prevent the flame tip opening, but also exhibit higher combustion efficiency and blown-off limit. Its blown-off limit can reach 756 cm3/s at the flow rate ratio of 0.2 and the bluff body angle of 90°, which is 61.5% higher than that of the conventional one with the blown-off limit of 468 cm3/s. The combustion efficiency improves with the growth of the flow rate ratio, while the blown-off limit of MCSB increases first and then decreases. The blown-off limit of MCSB with the bluff body angle of 90° reaches to the peak value of 792 cm3/s at the flow rate ratio of 0.15. Moreover, the increase of the bluff body angle provides better combustion efficiency and stability.

Experimental study on the premixed syngas-air explosion in duct with both ends open

Premixed flame of stoichiometric syngas-air mixture with various hydrogen volume fractions, 10% ≤ X (H2) ≤ 90%, propagating in a duct with both ends open is experimentally investigated in this study. Two representative ignition locations, i.e., Ig-1, locating at the center of the duct, and Ig-2, locating at the right open end, are considered. Results show that the tulip flame is first attained in the duct with both ends open at 10% ≤ X (H2) ≤ 50% as the flame is ignited at Ig-1. However, the flame maintains the convex shape with the cellular structure on the flame surface as the flame is ignited at Ig-2. The cellular structure results from Darrieus-Landau instability, but the Darrieus-Landau instability cannot invert the convex flame front. The flame tip and pressure dynamics have been examined. When the flame is ignited at Ig-1, the flame oscillates violently, and the overpressure profiles oscillate as a Helmholtz-type. When the flame is ignited at Ig-2, the left flame front propagates in an atmospheric pressure with a nearly constant speed. The prominent flame acceleration and oscillation are not observed at Ig-2 because of lacking flame acoustic interaction. What's more, the characteristic time of flame propagation has been compared. The time tw is shorter while the time tp is longer than the calculated value, and the time te has been delayed by both open ends. The flame propagation process is moderated as the flame propagates in the duct with both ends open.

Reciprocating propagation of premixed flame using time-variable-angle swirl vanes

Lean premixed combustion can be expected to reduce the high-temperature area and NOx emission. However, it has a risk of flashback due to flame propagation. It can be thought to modulate the gas swirl intensity to prevent flashback and to stabilize combustion. In this research, we developed time-variable-angle swirl vanes which had 56 mm of the inner diameter, 70 mm of the outer diameter and consisted of 36 vanes, each vane was directly connected to a stepping motor, and the swirl intensity could be changed periodically by keeping the constant air ratio and flow rate. We confirmed the periodical movement of the swirl vane angle using a high-speed camera. The flame position could be made reciprocating move upstream and downstream by the periodic change of the swirl vane angle. The upstream direction flow at the tip of the flame appeared when the flame moved in the upstream direction with increasing vane angle and the downstream direction flow appeared when that moved in the downstream direction with decreasing the vane angle. When the flame propagation was changed from upstream direction to downstream, the upstream direction flow near the flame tip was weakened. In advance of changing the flame propagation from upstream direction to downstream, the axial velocity near the flame tip changed from decreasing to increasing.

Experimental investigation and modeling of boundary layer flashback for non-swirling premixed hydrogen/ammonia/air flames

Carbon free fuels such as hydrogen/ammonia blends show a promising potential to become sustainable and renewable fuels for gas turbines and other combustion systems. One interesting aspect about these blends is the possibility to adjust different combustion properties like the laminar burning velocity or ignition delay time by changing the ratio between H2 and NH3. Such fuel blends can be produced via partial catalytic decomposition of NH3. However, such mixtures can lead to flame instabilities such as flashback, especially if the hydrogen content is high. In the present study, the boundary layer flashback of premixed hydrogen/ammonia/air mixtures is investigated experimentally for non-swirling flows at normal temperature (293 K) and normal pressure (101 kPa). A new experimental setup for boundary layer flashback investigation with a fully automated measurement procedure is introduced. With preliminary studies, the influence of various measurement procedures on the flashback limits is firstly investigated. For a broad flashback study, the data of 351 flashback experiments are collected. The ammonia content in H2/NH3 fuel mixtures is varied from 0 vol% to 50 vol% in 10 vol% steps. The fuel–air equivalence ratio is ranging from 0.38 to 1.17. As the ammonia content is increasing, the mean flow velocities at flashback are exponentially decreasing. Additionally, theoretical modeling is performed. A model is derived based on the concept of the critical velocity gradient which is able to predict the measured data with high accuracy. For two exemplary cases with H2/air and 80% H2/20% NH3/air mixtures, the process of boundary layer flashback is investigated in detail with low and high speed direct imaging and image post-processing. During the flashback onset of H2/NH3/air flames a separate reaction of H2 followed by the reaction of NH3 can be observed. Also, a flame-oscillation between fused silica tube and burner head with approximately 10 Hz was observed. Furthermore, indications for an adverse pressure gradient based on the flame propagation speed is seen. Details about the flame structure during the flashback process of H2/NH3/air flames are shown. During the upstream flame propagation of H2/NH3/air flames, high frequency oscillations with about 830 Hz of the leading flame tip are observed, which are assumed to be related to thermoacoustic instabilities.

Effects of backwall on inner thermal structure in opposed-flow horizontal flame spread of thick PMMA panel

Flame spread on solid fuel is an important aspect of fire safety engineering. In this work, an unsteady numerical model was developed to investigate the opposed-flow horizontal flame spread mechanism of thick polymethyl methacrylate (PMMA) on backwall with different thermal properties. The temperature fields in both the condensed fuel and backwall material, the fuel gas distribution and the pyrolysis profiles along the fuel bed were analysed in detail. The results showed that the thermal properties of the backwall play an important role in the structure of the melting interface of the PMMA flame front. The molten layer will not penetrate the sample obviously for the case of gypsum backwall. However, the molten liquid penetrates the PMMA sample with more molten liquid generated on the aluminium backwall, thus resulting the higher possibility of molten droplets. It is also indicated that the heat accumulation at the preheated region was enhanced by the heat transfer through the aluminium backwall from the flame and pyrolysis region. On the other hand, the aluminium wall behind the flame tip normally remain at relatively high temperatures, which in turn improved the pyrolysis rate, and lead to higher gas temperature. Therefore, the flame spread of polymer would be accelerated obviously for the case of metal backwall. This work provides important information for the acceleration mechanism of metal backwall on the opposed-flow horizontal flame spread of polymer and the design of polymer components, and also provides an important basis for engineering application to evaluate the fire risk of materials by considering the thermal performance of the backwall.