Strong Flame Research Articles

Effects of a wall on the self-ignition patterns and flame propagation of high-pressure hydrogen release through a tube

High-pressure hydrogen gas is gaining more attention as the next-generation energy carrier, but its safe handling and storage remains an important issue. The possibility of self-ignition near an obstacle is a realistic concern in practical applications such as a hydrogen car, but only a few studies related to this issue have been conducted. In this paper, experimental investigations were carried out to understand the effects of a wall on ignition patterns by high-speed imaging. This study was conducted using extension tubes of different lengths at burst pressures up to 30MPa. The wall height, distance of the wall from the tube exit and the burst pressure were considered as the main wall parameters affecting self-ignition. The results showed that the existence of a wall could not change the type of ignition patterns (i.e., the wall does not initiate or extinguish a flame) regardless of the wall height and burst pressure, but only when the tube was too short to generate a strong flame inside the tube. When the tube was long enough to induce a strong flame in the tube, however, the wall promoted flame stabilization, e.g., the flame stabilization time outside the tube was shortened by locating the wall near the tube. But its effect disappeared when the distance of the wall from the tube exceeded 10D.

Flow injection dual-syringe sorbent extraction platform for metal determination in environmental matrices utilizing a new strong cation exchange sorbent micro-cartridge and flame atomic absorption spectrometry

A novel dual-syringe flow injection (DSFI) on-line column preconcentration system coupled to flame atomic absorption spectrometry (FAAS) has been developed for automatic trace metal determination in natural waters and biological samples. The proposed method was based on the on-line retention of Cd(II), Pb(II), Cu(II), Co(II) and Ni(II) ions onto the surface of a strong cation exchanger resin named HyperSepSCX, in a readily exchangeable micro-cartridge format and subsequent elution with HCl (2 mol L−1) prior to flame atomization. The sorbent and the micro-cartridge exhibited high long term chemical and mechanical stability with fast kinetics for all analytes. All main chemical and hydrodynamic factors affecting the performance of the proposed method were studied thoroughly. For 15.0 mL sample volume, the enhancement factors were calculated as 92, 97, 93, 99 and 77 for Cd(II), Pb(II), Cu(II), Co(II) and Ni(II) respectively and the detection limits (3 s) were in the range between 0.14 and 2.1 µg L−1. The precision (RSD) obtained was lower than 3.3% for all five metal ions with a sample throughput of 12 h−1. The developed method was evaluated by analyzing certified reference materials and spiked environmental natural water samples.

Characteristics of Flame Shapes and Map For LPG and Hydrogen Inverse Confined Diffusion Flames at High Level of Fuel Excess

A combustion flame can be generated by locating the air jet supply inside the fuel jet supply. This flame is referred as an inverse diffusion flame. The size and structure of inverse diffusion flame were studied experimentally. The experiment was conducted for LPG and Hydrogen fuels. The inlet fuel and air flow rates are supplied at high level of fuel excess for its combustion reaction. These two fuels generated the flame shape having two parts. At lower part, the flame is wider and serves as a base of the flame. The upper part is longer and acts as a flame tower. The base flame was a weak flame resulted by a rich fuel-air mixture. The tower flame is formed by mixing between the entrained fuel and the air. The flame length decreases with the increase on the momentum ration between fuel and air. The flame height correlates to the fuel and air Reynold number ratio, Refu/Rea. The development of the flame shapes from continues to strong base-tower flame shape is mapped by air and fuel inlet momentum rate. Very low fuel and air momentum rates result laminar flame and continuous shapes. The turbulent flames having base-tower shape are formed at high air momentum rate. The oxygen profiles shows that the oxygen concentration decays from the burner tip, vanishes at some distance from the burner tip and increase again after this distance. The hydrogen is completely consumed before the flame tip is reached

The Thermodynamic Characteristic of Diesel with 21-30% Oxygen-Enhanced Combustion

The study was carried out on the burning test-bed which was controlled by Programmable Logic Controller (PLC) model and diesel was used as the fuel. The thermodynamic characteristic of oxygen-enhanced combustion, including fuel consumption, flue gas volume, flue gas components’ volume concentration, theoretical flame temperature, flame emissivity, energy efficiency and thermal efficiency, etc, were analyzed. The results showed that: with oxygen concentration was increased from 21% to 30%, fuel consumption was decreased by 40.6% and flue gas volume was decreased by 57.5%. Additionally, higher oxygen concentration leaded to higher theoretical temperature and stronger flame blackness because of the decreasing of N2 volume in supporting air. What’s more, the decreasing of energy brought away by flue gas and the thermal efficiency were both increased with the increasing of oxygen concentration.

Thermal characteristic identification of an alcohol flame by measuring its density gradient in Shack-Hartmann optical system

Understanding the flame structure for different combustion in industries has drawn the increasing attention around the world. Particularly, for increasing the recent interest of using the hydrogen fuelled vehicles in recent world, structural analysis of flame in combustion chamber has attracted the attention of researchers. However, the high flame temperature and strong flame emissions increase the experimental difficulties, especially, in all kinds of intrusive measurement systems for determining the flame structures and flame temperatures. Therefore, a non-intrusive laser interferometer technique based on Shack-Hartmann optical system has been proposed to measure the thermal characteristic of a flame structure. In the present study, a low-stretched diffusion flame of methanol burner has been used. Shack-Hartmann optical system is a type of wave front sensor. It is commonly used in adaptive optical systems. It consists of an array of lenses to focus the image onto a photon sensor (photo-detector) at the focal plane and measures the wave front tilt. The major objective of the present study is to develop a laser interferometer measurement technique for analyzing the flame structure and its temperature propagation by measuring the density gradient of the flame. Optical interferometer technique is a potential candidate for the non-invasive measurement. In the present paper, a novel method for the measurement of density gradient in flame by using Shack-Hartmann optical system is proposed. A collimated laser beam that has been passed through the flame is tilted due to the density gradient inside the flame. A CCD camera (CCD photo sensor) has been used to observe the wave front tilts at the focal plane.

Effects of Lewis Number on Scalar Dissipation Transport and Its Modeling in Turbulent Premixed Combustion

The effects of Lewis number (Le) on scalar dissipation rate transport of the reaction progress variable have been studied using 3-dimensional DNS data of freely propagating turbulent premixed flames with global Lewis number ranging from 0.34 to 1.2. It has been found that the effects of dilatation rate become increasingly strong with decreasing Le, and is particularly strong for the Le ≪ 1 flames because of greater rate of heat release. Moreover, increasingly strong flame normal acceleration in flames with decreasing Le gives rise to countergradient transport for the turbulent flux of the scalar dissipation rate, whereas this flux exhibits gradient transport for flames with Le ≥ 1. This turbulent flux is observed to correlate with the behavior of the turbulent scalar flux and this behavior is used to propose an algebraic closure for the turbulent flux of the scalar dissipation rate. The modeling of nonunity Le effects on the unclosed terms arising from the density change across the flame front, the turbulence–scalar interaction, the correlation between scalar gradient and the gradient of reaction rate and the molecular dissipation have been addressed. The extent of reaction progress variable gradient alignment with the most extensive (compressive) principal strain rate is found to increase (decrease) with decreasing Le. This alignment behavior gives rise to a sink contribution arising from the turbulence–scalar interaction in the Le ≪ 1 flames. By contrast, the turbulence–scalar interaction acts predominantly as a source term, although this contribution becomes a sink toward the burned gas side for the Le ≥ 1 flames. It has been demonstrated that existing models for the unclosed terms originating from the density change across the flame front and the turbulence–scalar interaction cannot account for strengthening (weakening) of heat release effects when Le < 1 (Le > 1). Suitable modifications to these models have been proposed to account for nonunity Lewis number effects, and validated using direct numerical simulation data. A simple algebraic expression is suggested that successfully relates the scalar dissipation rate of normalized temperature to the scalar dissipation rate of the reaction progress variable in nonunity Lewis number turbulent flames.

Transition of flat flames to turbulent motion induced by external laser irradiation

Experiments with flames premixed C2H4/CO2-O2 (Le<1) in a tube have been conducted. The mixture was ignited at the top, open end of the tube, and a flame front propagated downward toward the closed end of tube. To investigate details of motion of flame tip fluctuations at the initial moment of irradiating the CO2 laser light, a completely flat flame front was considered a default flame, corresponding to the primary acoustic instability as reported by Searby [1]. The laser exposure to the mixture induced local flame front deformation, resulting in a strong turbulent flame transforming eventually via the secondary acoustic instability. To elucidate the effect of the flame curvature, the flame velocity and curvature prior to establishment of the secondary acoustic instability were analyzed using a high speed camera. The results showed a sudden acceleration of the flame tip front during the advancing period. This was followed by a time lag between the flame velocity and curvature, indicating an increase in the flame velocity even with the decreasing flame curvature. These behaviors can be explained by selective acceleration at the center of the flame by an acoustic field and diffusive-thermal effects at a critical curvature beyond which flame extinction occurs.

Non-premixed acoustically perturbed swirling flame dynamics

An investigation into the response of non-premixed swirling flames to acoustic perturbations at various frequencies ( f p = 0–315 Hz) and swirl intensities ( S = 0.09 and 0.34) is carried out. Perturbations are generated using a loudspeaker at the base of an atmospheric co-flow burner with resulting velocity oscillation amplitudes | u′/U avg | in the 0.03–0.30 range. The dependence of flame dynamics on the relative richness of the flame is investigated by studying various constant fuel flow rate flame configurations. Flame heat release rate is quantitatively measured using a photomultiplier with a 430 nm bandpass filter for observing CH∗ chemiluminescence which is simultaneously imaged with a phase-locked CCD camera. The flame response is observed to exhibit a low-pass filter characteristic with minimal flame response beyond pulsing frequencies of 200 Hz. Flames at lower fuel flow rates are observed to remain attached to the central fuel pipe at all acoustic pulsing frequencies. PIV imaging of the associated isothermal fields show the amplification in flame aspect ratio is caused by the narrowing of the inner recirculation zone (IRZ). Good correlation is observed between the estimated flame surface area and the heat release rate signature at higher swirl intensity flame configurations. A flame response index analogous to the Rayleigh criterion in non-forced flames is used to assess the potential for a strong flame response at specific perturbation configurations and is found to be a good predictor of highly responsive modes. Phase conditioned analysis of the flame dynamics yield additional criteria in highly responsive modes to include the effective amplitude of velocity oscillations induced by the acoustic pulsing. In addition, highly responsive modes were characterized by velocity to heat release rate phase differences in the ± π/2 range. A final observed characteristic in highly responsive flames is a Strouhal number between 1 and 3.5 based on the burner co-flow annulus diameter ( St = f pU avg/d m ). Finally, wavelet analyses of heat release rate perturbations indicate highly responsive modes are characterized by sustained low frequency oscillations which accompany the high amplitude velocity perturbations at these modes. Higher intensity low frequency heat release rate oscillations are observed for lean flame/low pulsing frequency conditions.

Numerical visualization of vortical structures in a lean premixed swirl combustor using LES

Large eddy simulation was performed to visualize the three-dimensional vortical structures interacting with a turbulent premixed in a lean premixed swirl combustor with varied equivalence ratio. It was found that the fluctuation of unsteady heat release due to the deformation of flame surface was significantly decreased as the equivalence ratio increased because of the change in interaction between inner vortical structures and flames. This phenomenon was another evidence of the amplification mechanism in the combustion instabilities due to the strong flame–vortex interactions under lean premixed conditions. Graphical Abstract text

Physical Insight and Modelling for Lewis Number Effects on Turbulent Heat and Mass Transport in Turbulent Premixed Flames

The effects of global Lewis number on the behavior of Reynolds heat and mass fluxes in turbulent premixed flames are studied based on three-dimensional direct numerical simulation (DNS) of a number of statistically planar turbulent premixed flames with a global Lewis number ranging from Le = 0.34 to 1.2. For the same values of initial turbulent flow field parameters and duration of flame-turbulence interaction, it has been found that both Reynolds heat and mass fluxes may exhibit countergradient transport for flames with a Lewis number significantly smaller than unity; whereas predominantly gradient-type transport is obtained for flames with a Lewis number closer to unity. It is demonstrated that strong flame normal acceleration due to greater heat release in the low Lewis number flames acts to promote countergradient transport, and that the magnitude of the flame normal acceleration decreases with increasing Lewis number. Algebraic models for Reynolds heat and mass fluxes are proposed in which the effects of the Lewis number on flame normal acceleration are explicitly taken into account. The predictions of the new models are compared with DNS data, and the models are found to capture the influence of the Lewis number on turbulent scalar flux in a satisfactory manner for all the flames considered in this study.

Obtaining of Boron as an Alternative Fuel from Borax with Various Methods

This study examined how to obtain elemental boron from borax. For this purpose, first boric acid (H3BO3) was obtained from borax decahydrate with using HCl and H2SO4. The boric was then converted to boron oxide using a dehydration process. It has been studied to reduce the boric acid and the boron oxide into elemental boron with using the elemental magnesium (Mg) and butyl nitrite (C5H9NO2), and also with using sawdust, charcoal, and cellulose. Then, the obtained elemental boron was subject to the burning process to prove its purity. The pure-elemental boron gave a strong green-colored flame when it was burned. Among these, the methods using the elemental magnesium (Mg) and butyl nitrite (C5H9NO2) in order to obtain elemental boron are more reasonable than those of the others.

Analysis of a strong mass-based flame stretch model for turbulent premixed combustion

In the present paper a theory describing effects of strong flame stretch on turbulent flame propagation [L. P. H. de Goey and J. H. M. ten Thije Boonkkamp, “A flamelet description of premixed laminar flames and the relation with flame stretch,” Combust. Flame 119, 253 (1999)] is extended to volume averaged quantities and validated with direct numerical simulation (DNS). The extended theory describes the fuel consumption rate in terms of subgrid scale contributions connected to propagation effects including strong flame stretch. In case there is no preferential diffusion present, it is predicted that the total consumption rate is not affected by local stretch at all. Then the total consumption is described by the unstretched mass burning rate multiplied with the flame surface density. DNSs of turbulent flame kernels have been carried out in order to support the results from the theory. The chemistry is described by application of the flamelet generated manifold technique. The strong stretch theory is shown to be valid up to realizations in the thin reaction zone regime by three independent methods. The local effects of stretch are described, evaluated, and interpreted. Locally the mass burning rate changes by fuel leakage tangential to the flame, but this has no integral effect. The method can be used for subgrid scale modeling of turbulent flame propagation.

CONTAMINATION OF CARBON MONOXIDE WITH METAL CARBONYLS: IMPLICATIONS FOR COMBUSTION RESEARCH

In a study on syngas combustion in a swirl-stabilized combustor, red deposits quickly formed on the quartz combustor walls, preventing laser diagnostic measurements. Analysis of the deposit showed that is was composed of iron and nickel. A literature review revealed that typical pressurized cylinders of CO, even at the highest CO purity level, contain up to 100 ppmv of iron pentacarbonyl and somewhat smaller levels of nickel tetracarbonyl. Only semiconductor grade CO stored in aluminum cylinders has sub-ppm levels of carbonyls. The demonstrated strong flame inhibition effect of carbonyls suggests that the use of CO from steel cylinders in combustion experiments may affect studies of flame ignition or extinction, in addition to the effect of particle formation and deposition observed here.

Scalar mixing in direct numerical simulations of temporally evolving plane jet flames with skeletal CO/H 2 kinetics

Direct numerical simulations of three-dimensional turbulent temporally evolving plane CO/H 2 jet flames are performed with detailed chemistry at Reynolds numbers of up to 9000 and with up to 500 million grid points. The effect of Reynolds number on turbulent mixing properties and flame structure is quantified for low Damköhler number flames. These flames exhibit strong flame–turbulence interactions resulting in local extinction followed by re-ignition. The probability density of the stoichiometric scalar dissipation rate is found to be nearly log-normal with some negative skewness. Conditional statistics of the hydroxyl radical reveal increasing levels of extinction and longer re-ignition times with increasing Reynolds number. The mechanical-to-scalar mixing time scale ratio, a key quantity in transported probability density function (pdf) modeling, is investigated for both conserved and reacting scalars. The conserved scalar timescale ratio is found to be consistent with prior studies. For reacting scalars, the effects of molecular diffusivity and chemical reaction on the timescale ratio are quantified.

Thermal quenching and re-ignition of mixed pockets of reactants and products in gas explosions

An approximate solution of the problem of quenching and re-ignition of products/reactants pockets mixed by turbulence is presented. The approach is based on the analysis of thermal regimes of the pocket, but not on the concept of flame stretch. Critical conditions for quenching and re-ignition of the mixed pockets are obtained in a simplified analytical model, and applied to the problem of the sharp difference between the cases of weak and strong flame acceleration in gas explosions. The critical conditions of the mixed eddies in the turbulent flow are shown to depend on the size of the mixed eddies, mixture properties and turbulence intensity. The critical Karlovitz number, Ka, for thermal quenching is shown to increase with the ratio of densities between reactants and products, σ, and with the overall reaction order, n, and to decrease with the Zeldovich number β (dimensionless activation energy) and Lewis number, Le. For the smallest mixed pockets, which are about the size of the laminar flame thickness, the critical Ka-number defines the boundary of the domain of broken flamelets and distributed reaction zones. This number is shown to be of the same order of magnitude as that found from experiments and from direct numerical simulations. The critical conditions for thermal quenching of the largest pockets that can be mixed by turbulence are shown to be independent of the turbulent intensity and could be expressed as a function of σ on β, n, and Le. The mixture properties, thus, may prescribe certain types of flame behavior in turbulent flows. The corresponding critical conditions are linked to the sharp boundary between the cases of weak and strong flame acceleration, which was not satisfactorily explained in previous studies. These critical conditions are shown to have similar critical σ-values and general trends as the experimental data.

LAMINAR DIFFUSION FLAMES IN A MULTIPORT BURNER

ABSTRACT The influence of stream concentrations and velocities on the flame shape in a multiple-port burner is theoretically and experimentally studied in this work. A general solution of normal and inverse diffusion flame configurations in a multiple-port burner is obtained with the inclusion of the effects of axial diffusion and unequal stream velocities. The theoretical results show that not only the flame height but also the flame shape is affected by the Peclet number. In the theoretical analysis, the fuel and oxidizer concentrations, stream temperature, and stream velocity are three important parameters. Flame configurations can be predicted well by including the temperature effect in calculating the Peclet number. For relatively weak (or strong) flame intensity, the prediction of flame configurations agrees well with the experimental results if a lower (or higher) temperature is used. In the experiment, different flame configurations are observed and discussed for various stream velocities and concentrations. The transition of inverse diffusion flame from a single cone-shaped flame to a double cone-shaped flame to an envelop flame occurs when the inner stream velocity is adjusted for fixed middle and outer stream velocities. It is of interest to note that under the same operating conditions the flame has a history-dependent configuration decided by increasing or decreasing the inner stream velocity.

B214 水素火炎ジェット点火法による燃焼制御 : ノズル部の熱流束(OS-4 内燃機関の燃焼I)

A jet ignition method by hydrogen flame jet is proposed to control the combustion of lean mixtures. Hydrogen flame jet is supplied from the cavity whose volume is less than one percent of the main chamber volume through the nozzle. It is worried that the heat loss of the jet ignition method becomes bigger than that of conventional ignition, because this method creates strong and high temperature flame jet. Especially at the nozzle, the heat flux might become bigger than other place. So the heat flux at the nozzle were measured and the relation between the jet velocity and heat flux were shown. The faster the flame jet is, the bigger the heat flux at the nozzle is. And the heat flux increases with the temperature of the jet. The maximum heat flux is proportional to the velocity and the temperature of the flame jet.

Stability and emissions control using air injection and H 2 addition in premixed combustion

We experimentally study lean premixed combustion stabilized behind a backward-facing step. For a propane–air mixture, the lean blowout limit is associated with strong pressure fluctuation arising simultaneously with strong flame–vortex interactions, which have been shown to constitute the mechanism of heat release dynamics in this flow. A high-speed air jet, issuing from a small slot and injected perpendicular to the main flow near the step, is used to disrupt this mechanism. For momentum ratio of jet to main flow below unity, the jet dilutes the mixture, further destabilizing the flame or leading to complete blowout. Above unity, the flame becomes more stable, and the pressure oscillations are suppressed. Flow visualization and OH*/CH* chemiluminescence measurements show that a strong jet produces a more compact flame that is less driven by the wake vortex, anchored closer to the step, and deflected upwards away from the lower wall of the channel. This renders the flame less vulnerable to heat loss and strong strains, which improves its stability and extends the flammability limit. Adding hydrogen to the main fuel improves the flame stability over the entire range of the air jet mass flow, with better results for momentum ratio larger than 1; H 2 pulls the flame further upstream, away from the shear zone and the unsteady vortex. NO x emission benefits from the air jet, while, with H 2 addition, NO x concentration is higher in the products as the overall burning temperature rises. However, hydrogen addition enables extending the flammability limit further by increasing air supply in the primary stream, hence achieving lower NO x . The study suggests a simpler, almost passive, multi-objective combustion control technique and indicates that hydrogen addition can be a successful in situ approach for NO x reduction.

Temperature and CO 2 concentration measurements in the exhaust stream of a liquid-fueled combustor using dual-pump coherent anti-Stokes Raman scattering (CARS) spectroscopy

Single-shot, dual-pump coherent anti-Stokes Raman scattering (CARS) measurements of N 2 and CO 2 were performed in the exhaust stream of a swirl-stabilized JP-8-fueled combustor under sooting conditions. The combustor is designed to study particulate formation and particle-size distributions for different flame conditions and therefore is operated at near-stoichiometric overall fuel–air ratios. Various jet fuels and additive concentrations were studied. These conditions pose a significant challenge for temperature measurements using standard N 2 CARS due to strong flame emission and absorption of the CARS signal by the C 2 Swan band. With the dual-pump CARS technique employed in this study, the N 2 CARS signal is generated at a wavelength (496 nm) that is not absorbed by C 2, and concentration measurements of CO 2 can be performed. The standard deviations of the single-shot temperature measurements were approximately 3–4% of the mean values for equivalence ratios ranging from 0.4 to 1.1, whereas those of the single-shot CO 2 concentration measurements were between 9 and 20% of the mean values. Previous single-shot temperature and CO 2 concentration measurements using dual-pump CARS in this liquid-fueled combustor were limited to an equivalence ratio of 0.45, with standard deviations in temperature of about 5–6% of the mean value of 1143 K (Lucht et al., AIAA J. 41 (4) (2003) 679–686). The current study demonstrates a significant improvement in the applicability of single-shot CARS temperature and CO 2 concentration measurements to practical, swirl-stabilized combustors under sooting conditions.

Radiative combustion of pyrolyzing fuel in a cylindrical combustor

Investigation on ignition and flame propagation of pyrolyzing fuel in a cylindrical combustor is accomplished. The pyrolyzing fuel of cylindrical shape is concentrically located in a combustor sustained at high temperature. Due to gravity, the buoyancy motion is inevitably incurred in the combustor and this affects the flame initiation and propagation behavior. The radiative heat transfer due to absorption gas plays an important role since it absorbs and emits radiative energy. Numerical studies have been performed over various parameters relevant to gas radiation as well as overheat ratios. The ignition criterion is decided on experimental basis. For the case of relatively small overheat ratio, the gas absorption reduces the flow intensity by the far-reaching effect of radiation. Visible blue flame hence does not evolve. For a high overheat ratio, the strong visible blue flame is generated and self-propagates. The gas radiation makes the considerable effect on ignition delay and flame thickness as well as thermal and reactive nature of visible blue flame.