Flame Mode Research Articles

119 箱型フレームの変形モードに与える衝突速度の影響(材料力学III)

119 箱型フレームの変形モードに与える衝突速度の影響(材料力学III)

On the Axisymmetric Counterflow Flame Simulations: Is There an Optimal Nozzle Diameter and Separation Distance to Apply Quasi One-Dimensional Theory?

Two-dimensional axisymmetric numerical analysis of counterflow flames was employed to better understand the applicability of the quasi one-dimensional theory of Seshadri and Williams to flames produced by small diameter convergent nozzles. The computational domain considered included the convergent sections of two opposed nozzles as well as the surrounding inert annular co-flows. For computational efficiency, the fuel-oxidizer system of diluted hydrogen versus air in non-premixed flame mode, with a detailed chemical kinetic model and mixture-averaged transport property description, was considered. With an increase of nozzle diameter from 6.5 mm to larger values (with plug flow velocity profiles at the nozzle exit), the influence of the radial terms on eigenvalue and scalar variables has been compared. Error metric on nozzle diameter effects is presented with comparison to typical experimental measurement uncertainties. The analysis also showed that, for nozzle separation distances below the free-floating limit, the self-similar function in quasi one-dimensional formulation can be preserved by specifying radial velocity boundary conditions, as long as the radial gradients are negligible.

Flame and flow characteristics of an excited non-premixed swirling double-concentric flame

The flame behaviors and velocity fields of unexcited and excited swirling double-concentric jet flames were experimentally studied. Acoustic excitation was applied to the central fuel jet. The central jet Reynolds and swirl numbers were 2386 and 0.426, respectively. Three characteristic flame behaviors, wrinkled base flame, converged base flame, and diverged base flame were observed by the traditional photographic technique. Jet pulsation intensity dominated the change in the characteristic flame modes. We used a high-speed particle image velocimeter to measure the time-averaged velocity field; results of the excited swirling double-concentric jet flames showed that the streamlines that separate from the central fuel jet exit were significantly deflected toward the central jet axis, while the size of the rotating-inward single-ring vortex decreased as the jet pulsation intensity decreased. Partial flow emitted from the annular air jet flowed over the outer contour of the rotating-inward single-ring vortex and was then entrained into the central fuel jet, with the result that entrainment between the fuel and air was enhanced. The central jet region was formed by two adjacent vorticity-concentrated areas of opposite signs. As acoustic excitation was applied to the central fuel jet, these two areas expanded with increasing jet pulsation intensity. The jet pulsation induced vortical structures periodically evolving from the jet exit with the result that oscillation waveforms of the instantaneous velocities were obtained. The vortical structures entrained fresh air into the central fuel jet in radial direction, resulting in the extreme radial and axial turbulence intensities and improved mixing between fuel and air.

FLAME BEHAVIOR OF BIFURCATED JETS IN A V-SHAPED BLUFF-BODY BURNER

A plane fuel jet bifurcated into two streams of jets by a V-shaped bluff-body burner with co-flowing air streams was studied. The flame behavior, length, and temperature distributions of the bifurcated jets were experimentally examined. Flame behavior was characterized by flame visualization and instantaneous photograph. The flame length characteristics was obtained by using the long-exposure flame images. The temperatures distributions were measured by using a finewire thermocouple. Three characteristic flame modes were revealed-＂attached flame, transitional flame,＂ and ＂lifted flame＂. The flame length and width of the bifurcated jets increase with the increase of the Re_c in fixed Re_a. The length of the bifurcated flame was significantly shorter than that of the plane jet flame. The reduction was induced by the enhancement in the mixing and entrainment effects of the bifurcated flame. The maximum temperature of the bifurcated-jets flame decreases with the increase of the Re_c. The bifurcated jets enhanced the mixing of the fuel and the air and improved the combustion efficiency. The flame behavior and combustion characteristics of the bifurcated jets changed drastically due to the effects of entrainment and recirculation.

Thermal versus acoustic response of velocity sensitive premixed flames

Premixed flames respond to velocity perturbations with fluctuations in heat release rate (“thermal response”), which in turn generate acoustic perturbations (“acoustic response”). The latter may subsequently influence the velocity field in such a manner that feedback leads to self-excited thermoacoustic instability. The present paper investigates interrelations between the thermal and the acoustic responses of premix flames. The analysis is formulated such that it properly represents the underlying causality of acoustics–flow–flame–acoustics interactions. A flame-intrinsic feedback loop is revealed, which is quite independent of the acoustic environment of the flame, i.e. the acoustic impedances of plenum and combustor. The eigenmodes of this flame-intrinsic feedback loop coincide with poles of the acoustic scattering matrix of the flame. The corresponding frequencies, where the acoustic response is maximum, are in general quite different from frequencies where the thermal response is strong, i.e. where the flame transfer function exhibits “excess gain”. Even more remarkable, the intrinsic flame modes may result in thermoacoustic instabilities without lock-on to one of the acoustic eigenmodes of the combustor. Experimental results from two combustor test rigs with laminar conical as well as turbulent swirl flames are scrutinized and are found to confirm our analysis. In particular, unstable modes are identified that are strongly related to flame-intrinsic feedback.

Multistage oscillatory “Cool Flame” behavior for isolated alkane droplet combustion in elevated pressure microgravity condition

Recently, large diameter, isolated n-heptane droplet experiments under microgravity conditions (aboard the International Space Station) exhibited “Cool Flame” burning behavior, resulting from a heat loss mechanism that extinguishes hot combustion and a transition into a sustained, low temperature second stage combustion. In atmospheric pressure air, a single combustion mode transition to “Cool Flame” burning is followed by diffusive extinction. But with increasing pressure, multiple cycles of hot initiation followed by transition to “Cool Flame” burning are observed. This paper reports experimental observations that characterize the transition time histories of this multi-cycle, multi-stage behavior. Transient sphero-symmetric droplet combustion modeling that considers multi-stage detailed kinetics, multi-component diffusion, and spectral radiation is applied to analyze the experimental observations. The simulations indicate that as parameters change the chemical time scales dictating low temperature degenerate chain branching, multiple hot/cool flame burning transitions are induced by increasing the cool flame burning heat generation rate compared to the diffusive loss rate. The balance of these terms in the negative temperature coefficient kinetic regime defines whether reactions accelerate into re-ignition of a hot flame event, burn quasi-steadily in the cool flame mode, or diffusively extinguish. The rate of reactions controlling ketohydroperoxide formation and destruction are shown to be key re-ignition of hot combustion from the cool flame mode. Predictions are found to be in good agreement with the experimental measurements. Modeling is further applied to determine how these observations are dependent on initial experimental conditions, including pressure, and diluent species.

Modeling the optical properties of combustion-generated fractal aggregates

Combustion-generated carbonaceous aerosols are generally in the form of fractal aggregates (FA’s) with shapes that vary from long chain-like structures to much more compact, almost spherical structures, depending upon the mode or stoichiometry of the combustion process. Typically, as combustion moves from fuel-lean to fuel-rich, aggregate morphologies change from the former to the latter. Accompanying this change in morphology is a change in the chemistry of the aggregates as the percent of carbon in the aggregates also decreases. These combined changes produce radically different scattering and absorption signatures that define their radiative transfer properties. To improve our ability to predict how these optical properties change, experiments were conducted to measure both the physical and optical properties of these aggregates for both flaming and non-flaming modes of combustion. Using the aggregate property data from these experiments, numerical calculations were then performed using both the discrete dipole approximation (DDA) and the Rayleigh–Debye–Gans (RDG) approximation to generate their characteristic scattering and absorption signatures. This paper presents the experimental results, the comparison of the modeling results with the experimental results and discusses those parameters most important to obtain agreement between the modeling and the experiments.

Effect of SO2 on the chain reaction of hydrogen oxidation: Intermittent flames

The effect of SO2 on the chain reaction of hydrogen oxidation in the autoignition region (T = 470–510°C and p < 200 torr) is studied. In the absence of SO2 under flow conditions, the process occurs in a low-temperature flame mode with a characteristic pale blue glow. With the addition of sulfur dioxide, SO2 is transformed to elemental sulfur and a new phenomenon is observed. At a small contact time (less than 4–5 s), the process enters the mode of intermittent flames. Dependences of the rate and intensity of light flashes on pressure, temperature, and contact time are studied. An interpretation of the observed phenomenon is given.

On the stability of a turbulent non-premixed biogas flame: Effect of low swirl strength

Biogas like other low calorific value fuels has a very narrow stable region when operating in diffusion flame mode owing to their low burning velocity in conjunction with the unburned flow high velocity. This paper presents an experimental study on the effect of the burner geometry on the stability limits of a turbulent non-premixed biogas flame. The main focus of the study is on the role of the low swirl strength of the co-airflow, and the fuel nozzle diameter. The results revealed that the swirl plays a dominant role on the flame mode (attached or lifted) as well as on its operating/stability limits. However, the results revealed that the swirl effect prevails only at relatively moderate to high co-airflow velocity. That is, the swirl does not have an apparent effect at weak co-airflow when the flame is attached. Whereas, it becomes dominant at relatively high co-airflow velocity where the attached flame lifts off and stabilizes at a distance above the burner. Correlations were proposed to describe the lifted biogas flame blowout limits.

Flame dynamics in lean premixed [formula omitted]/air combustion in a mesoscale channel

The dynamics and stabilization of fuel lean premixed CO/H2/air atmospheric pressure flames in meso-scale channels were investigated numerically, using detailed gas phase chemistry and transport. Experiments in a channel flow reactor by means of chemiluminescence detection of the excited OH radical allowed for model validation at steady conditions and identification of the conditions at which unsteady flame dynamics were present. A detailed parametric study of the influence of wall temperature and CO:H2 ratio on the ensuing flame dynamics was performed. The numerical results revealed different flame modes which included oscillatory ignition, random ignition spots, as well as steady weak and V-shaped flames. The wall temperature stability intervals of these modes changed with the CO:H2 ratio. The richest variety was found for molar CO:H2 ratios between 4 and 10, while at lower ratios the random and the weak modes were absent. At higher ratios all the dynamic modes were suppressed. The Computational Singular Perturbation (CSP) method was used to obtain insights into the physicochemical processes responsible for the weak flames, which were found at relatively high inflow velocities compared to previous studies, and V-shaped flames. A kinetic explanation of the phenomena was supported by the CSP analysis.

In Vitro Estimation of Lead Content in Nepalese Traditional Ayurvedic Medicines Commonly Used in Children

Introduction: Ayurvedic remedies are popularly used in practice for long time in Nepal. It is regarded as safe and free from side effects. However there are published reports of the high content of heavy metals like lead in such preparations. No such study has been done in Nepal looking at the lead content in ayurvedic preparations. The aim of this study was to detect the level of lead in commonly used ayurvedic remedies used in paediatric population if any. Materials and Methods: Seventeen samples were selected for lead estimation based on frequency of prescription and over the counter dispense. All of them were analyzed using Atomic Absorption Spectrophotometer (AAS) 6300 using flame mode. Results: None of the samples had detectable level of lead in parts per billion. Conclusion: Though this study did not detect lead in the seventeen samples of ayurvedic medicine, a larger study is needed involving large samples of these medicines with use of more sensitive equipment for testing. DOI: http://dx.doi.org/10.3126/jnps.v33i2.8314 J Nepal Paediatr Soc. 2013; 33(2):103-105

Characteristics of Flame Modes for a Conical Bluff Body Burner With a Central Fuel Jet

Bluff body stabilized nonpremixed flames are usually used as pilot flames in lean-premixed combustors. Experiments are conducted to investigate the characteristics of the flame. Typical flame modes are investigated in both stable and unstable conditions. The flow structures, the reaction zone, and the dynamics of unstable flames are measured with particle image velocimetry (PIV), intensified charge-coupled device (ICCD) and a high-speed camera, respectively, based on which the inherent mechanisms that influence the configuration and stabilization of the flame are analyzed. Stable flames are apparently influenced by the mixing characteristics in the recirculation zone. Flame detachment, a typical phenomenon of stable flames in a turbulent air flow, can be explained by the distribution of fuel concentration in the recirculation zone. The Reynolds number of air has different effects on different parts of the flame, which results in three unstable flame modes at different Reynolds numbers of air. These results could be helpful for the design of stable burners in practice.

Environmental Protection and Fuel Consumption Reduction by Flameless Combustion Technology: A Review

In recent years global fuel consumption has increased in the world due to modernization and progress in the standard of living. The conspicuous rate of carbon dioxide and nitrogen oxide released to the environment and fuel resources are depleted day by day due to inconsiderate fuel consumption. Requirement for efficient use of any kinds of fuel has become the other concern due to the oil crisis and limitation of fuel resources. In combustion process, the abatement of pollutants often associates with efficiency loss. In the other word, high efficiency and low pollutant which are the main requirements of combustion are not fulfilled by the existing combustion. Today, flameless combustion has received more attention because of its low NOx emission and significant energy saving. Generally, compatibility between high performance and low NOx emission has been observed by preheated air application and changing the combustion characteristics from traditional flame to flameless mode. This paper aims to review the concepts and the applications of flameless combustion and gathers useful information to understand the necessity of transient from traditional flame mode to flameless combustion.

Combustion performance of eccentrically rotated flames

Experimental/computational work highlighted the combustion performance with eccentric blockage rotation in the reactive flow passage to augment the turbulence production rates in the flame for acquiring high power to weight ratios and effective heat transfer in industrial applications. The temporal/spatial velocity gradients stimulated around the reactive stream boundaries enhanced the turbulence development in both premixed and non-premixed flames. The eccentric shapes included circular, elliptical, squared and triangular shafts as well as a triple/straight blade rotor. The design was further developed to incorporate simultaneous rotation of the eccentric blockage around its axis via a planetary gear assembly to duplicate the vortical structure. While the premixed flames responded to such cyclic action by having a mixture velocity of 16.8 m/s, the non-premixed flames acquired an increase of 341% in the largest eddy size and a flame length reduction by 42%. Increasing the Strouhal number to 0.94 and the swirl ratio to 0.78 increased the turbulent kinetic energy to 9.1 m2/s2. By controlling the drag coefficient and wake vorticity for the solid shapes, the triangular rotor enhanced the combustor outward heat flux to exhibit a heater efficiency of 36.8% in the premixed flame mode and reduced the HC and CO emissions, respectively, to 0.1% and 513 ppm for non-premixed flames. Decreasing the reactive stream cross-section favorably increased the flow shearing effects, while the elliptical shape showed the highest sensitivity to axes’ orientation with respect to the direction of rotation. Due to the reduction in peak temperatures via increasing the turbulence intensity and heat transfer rate from the flame, the NO x exhaust concentrations decreased to a minimum value around 10 ppm. The combustion efficiency of diffusion flames was optimized with the fuel port central positioning in the burner, where the planetary gear design provided a turbulence intensity of 13.7% by the triple blade rotor.

저온연소조건에서 급속압축기를 이용한 n-heptane/n-butanol 혼합연료의 착화지연에 관한 연구

This study presents both experimental and numerical investigation of ignition delay time of n-heptane and n-butanol binary fuel. The O2 concentration in the mixture was set to 9-10% to make high exhaust gas re- circulation(EGR) rate condition which leads low NOx and soot emission. Experiments were performed using a rapid compression machine(RCM) at compressed pressure 20bar, several compressed temperature and three equivalence ratios(0.4, 1.0, 1.5). In addition, a numerical study on the ignition delay time was performed using CHEMKIN codes to validate experimental results and predict chemical species in the combustion process. The results showed that the ignition delay time increased with increasing the n-butanol fraction due to a decrease of oxidation of n-heptane at the low temperature. Moreover, all of the binary fuel mixtures showed the combustion characteristics of n-heptane such as cool flame mode at low temperature and negative-tem- perature-coefficient(NTC) behavior. Due to the effect of high EGR rate condition, the operating region is reduced at lean condition and the ignition delay time sharply increased compared with no EGR condition.

Flame Structure and Combustion Capability of Non-Premixed Rifled Nozzles

The target of this study is to promote combustion capability using a novel rifled nozzle which was set at the outlet of a conventional (unrifled) combustor. The rifled nozzle was utilized to adjust the flow swirling intensity behind the traditional combustor by changing the number of rifles. The rifle mechanism enhances the turbulence intensity and increases the mixing efficiency between the central-fuel jet and the annular swirled air-jet by modifying the momentum transmission. Specifically, direct photography, Schlieren photography, thermocouples, and a gas analyzer were utilized to document the flame behavior, peak temperature, temperature distribution, combustion capability, and gas-concentration distribution. The experimental results confirm that increasing the number of rifles and the annular swirling air-jet velocity (ua) improves the combustion capability. Five characteristic flame modes—jet-flame, flickering-flame, recirculated-flame, ring-flame and lifted-flame—were obtained using various annular air-jet and central fuel-jet velocities. The total combustion capability (Qtot) increases with the number of rifles and with increasing ua. The Qtot of a 12-rifled nozzle (swirling number (S) = 0.5119) is about 33% higher than that of an unrifled nozzle. In addition, the high swirling intensity induces the low nitric oxide (NO) concentration, and the maximum concentration of NO behind the 12-rifled nozzle (S = 0.5119) is 49% lower than that behind the unrifled nozzle.

Formation of Lean Premixed Surface Flame Using Porous Baffle Plate and Flame Holder

A lean premixed surface flame has many advantages including low CO(Carbon Monoxide) and NOx(Nitrogen Oxide) emission and applicability of a small combustion volume leading to compact design. These advantages make it applicable to burner for condensing boilers with high thermal efficiency. Moreover recent severe regulation of global warming gas favored a lean premixed surface flame in development of a condensing boiler burner. This study focused on emission characteristics of lean premixed flame and the effect of flow distribution on flame stability of a surface flame in a cylindrical porous metal plate burner. For conceptual design of surface flame burner, the numerical calculation of a flow pattern inside the burner was performed and the calculated data were used for design of the burner system including the baffle plate and flame holder. The results show that the surface and stable premixed flame can be generated by implementing the proper baffle plate and flame holder. The surface cylindrical flame mode is changed into green flame, yellow radiation flame, blue flame and blow off with decreasing equivalence ratio. The blue flame has a wide stability region in the stability curve and showed the lowest CO and NOx emission at low equivalence ratio. And CO decreased as the mixture ratio became leaner but NOx showed almost the same emission level. For stability of a surface cylindrical flame, it was found to be very important to select the proper distribution of holes in a baffle plate and install the flame holder to prevent blow off at the rim of the cylindrical burner. NOx was measured below 6 ppm (0% oxygen base) from equivalence ratios 0.706 to 0.769 through the proper design of baffle plate and flame holder. CO which is a very important emission index in residential gas boiler was observed below 49.1ppm under the same equivalence ratio range.

Improving Combustion Intensity and Modulating Flame Behaviors Using Helical-Grooved Cones

AbstractFour helical-grooved cones were installed behind an unconfined combustion nozzle to increase the bluff-body effect and turbulence intensity (T.I.). The cone configurations included a smooth cone and the other three cones cut with 1, 2 and 3 helical v-grooves. Experimental results showed that the helical v-grooves transformed the axial momentum (or the axial velocity) to the angular momentum (or the angular velocity). TheT.I.was enhanced by increasing the tangential component of fuel-jet momentum. The direct photography and thermocouple were utilized to observe the flame structures and to delineate the characteristic flame modes, flame length, temperature distribution, and combustion intensity. The flame modes were classified as jet flame, flickering flame, bubble flame, recirculation flame, lifted flame and ring flame. The flame length decreases as the groove number increases. The increasedT.I.and groove number (or bluff-body effect) improve the fuel-air mixing. The total combustion intensity increases with annular-air jet and with the groove number.

Thermal performance of a micro combustor with heat recirculation

In this paper, numerical and experimental investigations in a three‐step microcombustor have been carried out to understand the effect of heat recirculation on flame stabilization behavior with premixed fuel–air mixtures. External heating cup is utilized to enhance the heat recirculation. From numerical simulations, it was observed that the extent of heat recirculation is a function of cup dimensions, cup material, and flow velocity and mixture equivalence ratio. Heat recirculation has been observed to significantly enhance the flame stability limits and upper flame stability limits were observed for the range of flow rates investigated during the present work. Stable flames exist for smaller flow rates with a minimum thermal input of ~2.2W at Φ=0.5. X‐shaped spinning flames exist for inter‐mediate flow rate conditions with and without heat recirculation for a broad range of equivalence ratios. These X‐shaped flames rotate around the axis at ~100–150Hz frequency. The average combustor wall temperature increases with the flow velocity for the stable flame mode and remain mostly uniform and well distributed for the X‐shaped spinning flame mode. Heat recirculation helps in increasing the mean wall temperature of the combustor by ~100–400K.

The Characteristic Modes and Structures of Bluff-Body Stabilized Flames in Supersonic Coflow Air

The stability and structure of bluff-body stabilized hydrogen flames were investigated numerically and experimentally. The velocity of coflowing air was varied from subsonic velocity to a supersonic velocity of Mach 1.8. OH PLIF images and Schlieren images were used for analysis. Flame regimes were used to classify the characteristic flame modes according to the variation of the fuel-air velocity ratio, into jet-like flame, central-jet-dominated flame, and recirculation zone flame. Stability curves were drawn to find the blowout regimes and to show the improvement in flame stability with increasing lip thickness of the fuel tube, which acts as a bluff-body. These curves collapse to a single line when the blowout curves are normalized by the size of the bluff-body. The variation of flame length with the increase in air flow rate was also investigated. In the subsonic coflow condition, the flame length decreased significantly, but in the supersonic coflow condition, the flame length increased slowly and finally reached a near-constant value. This phenomenon is attributed to the air-entrainment of subsonic flow and the compressibility effect of supersonic flow. The closed-tip recirculation zone flames in supersonic coflow had a reacting core in the partially premixed zone, where the fuel jet lost its momentum due to the high-pressure zone and followed the recirculation zone; this behavior resulted in the long characteristic time for the fuel-air mixing.