Tubular Flame Research Articles

Composite combustion behaviors of tubular flame and central jet flame in a reduced-diameter vortex combustor

The characteristics of the tubular flame and the flow field mixing mechanism in a reduced-diameter vortex combustor are investigated by varying the fuel flow rate and equivalence ratio through experiments and numerical simulations. Single-layer tubular flame and double-layer composite tubular flame are found in the combustor under the coupling effect of the cyclone and the central jet. The composite tubular flame will be formed when the air in the combustor is more abundant (Ф>1.00), and the center flame frequency is linearly and positively correlated with the Reynolds number of the central jet air. When the Reynolds number exceeds 2000, the double-layer composite flame tends to stabilize, with the center flame frequency higher than 9 Hz. The turbulence of the central jet air promotes the mixing of reactants by enhancing the recirculation near the nozzle, guaranteeing the central flame's stable combustion. Through analysis of the burn rate of methane at the outlet and the emission species of CO and CO2, it is evident that the double-layer flame achieves the methane burn rate exceeding 95 %, while also exhibiting lower CO emissions compared to the single-layer flame.

Calibration of Raman spectroscopy signals for measurements of methane-air tubular flames

Calibration of Raman spectroscopy signals for measurements of methane-air tubular flames

Characterizing potassium release under turbulent conditions during biomass stream combustion: A comparative analysis of different pretreatments

The release rate of potassium during biomass combustion significantly influences slagging, fouling, and corrosion in industrial equipment. Given that dispersed biomass stream combustion predominantly occurs under turbulent ambient conditions, we investigate the combustion characteristics and potassium release behavior of rice husk and its derived fuels (water-washed, torrefied, and char) under high-turbulent conditions in a tubular flame burner (TFB). Tunable diode laser absorption spectroscopy (TDLAS) technology, combined with high-speed imaging, was employed to online monitor the release of atomic potassium (K(g)) and the ignition characteristics of biomass stream combustion. The results showed that the rice husk stream followed a joint hetero-homogeneous ignition mechanism, whereas the biochar stream underwent heterogeneous ignition in the TFB. The K(g) peak, occurring ahead of the maximum spontaneous radiation position, was observed at approximately 25 mm from the injectors, with a concentration of 0.067 ppmv during rice husk stream combustion. In contrast, the peak K(g) concentrations for biochar and torrefied rice husk were delayed, reaching 0.026 ppmv and 0.021 ppmv, respectively, compared to the raw biomass samples combusted in the TFB. Distinct differences in activation energy for K(g) release were identified, with pulverized rice husk exhibiting the lowest activation energy (75.53 kJ/mol) and rice husk-char showing the highest (94.20 kJ/mol).

Forced swirl tubular flame under different equivalence ratios and oxygen mole fractions

The swirl tubular flame has been widely used in combustors owing to its unique merits of high combustion efficiency, uniform flame temperature and inherent safety to avoid flashback in the rapidly mixed (RM) mode. However, it also suffers from combustion instability, which would deteriorate the system performance. To explore the mechanism behind, the influences of acoustic forcing disturbances on both the premixed (PM) and rapidly mixed flame characteristics are experimentally investigated, in which the equivalence ratio (Φ) and the oxygen mole fraction (β) are also adjusted to examine the flame responses. Firstly, the measured combustion regimes show that there is an additional flame liftoff region for the RM flames in the mid-acoustic frequency range compared with the PM flames. In addition, the forced RM swirl tubular flame present similarly low-pass filtering with the PM flame, but has a 10 Hz smaller critical frequency for the flame extinction. Finally, the forced flame is found to be lifted off when Φ or β is low. By increasing Φ or β, the lifted flame evolves to the anchored flame, and the transition process is studied via the Proper Orthogonal Decomposition analysis and the 1D opposed-flow flame analysis. The results show that, for both of PM and RM tangential tubular swirl flames, the lifted flame would become anchored when the energy fraction of the axial mode is smaller than 5% of total energy and the extinction strain rate is larger than 1000 s−1.

Hydrodynamic instability of premixed tubular flame stabilized in stretched rotating flow

The paper analyzes the hydrodynamic instability of a cylindrical flame stabilized in a converging rotating gas flow simulating the flow in a cyclone combustion chamber. The analysis was carried out within the framework of a two-dimensional model describing the gas flow with radial and tangential supply of combustible gas through the walls of a cylindrical chamber. In the stationary case, the gas flow in combustion products and the combustible mixture is described by the solution for a rotating tornado-shaped vortex. The rotation of the gas creates a centrifugal force, which has a stabilizing effect on the flame perturbations. The dependences of the perturbations growth rate on the wave vector of perturbations, the radial and tangential inlet gas velocities, and other parameters of the problem are obtained. The analytical criterion of neutral stability separating regions of stable flame with respect of any perturbations and flame instability in the plane of radial and tangential velocities is obtained. In the absence of rotation and at the large flame radii, the model converts into the Landau- Darrieus model of flame hydrodynamic instability.

Stability and emission characteristics of ammonia-air flames in a lean-lean fuel staging tangential injection combustor

The present work suggests the stability and emission characteristics of an ammonia-air tubular flame in a fuel staging tangential injection combustor. Owing to the development of a large inner recirculation zone induced by a high swirling motion, the ammonia tubular flame exhibited wide operational flexibility at an ambient temperature condition. With ammonia fuel staging, we confirmed that the Thermal DeNOx process significantly mitigates NO emissions when the exit temperature of the primary region is close to the threshold for active NO reduction. In this study, fuel multi-stage technology was used for pure ammonia-air combustion, and the result effectively challenges the existing paradigm of the NO reduction strategy: Rich-Quench-Lean staging configuration. Thus, our results demonstrate the potential use of ammonia as a fuel in a dry low-NOx system.

Topological transitions of Jet A-1 lean azimuthal flames (LEAF)

Prior studies about liquid fuel combustion in a vitiated air environment have shown increased combustion efficiency with reduced NOx, CO, and soot emissions. The concept of lean azimuthal flame (LEAF), which can be associated to the latter combustion mode, is based on opposed injections of air and liquid fuel sprays in an axisymmetric chamber with a central outlet, which can result in a highly turbulent toroidal reaction zone. The mixture of fresh air and hot combustion products of each spray provides a vitiated cross-flow configuration to the next spray distributed along the chamber circumference, leading to ignition and sequential combustion of the sprays by the others. The present paper deals with a LEAF combustor with air-assisted spray atomization, which has not been investigated so far. The combustor is fueled with Jet A-1 and operated from 15 to 25 kW with variations in the atomization-air to liquid mass flow ratio (ALR). This study focuses on the flame topology transitions as a function of atomizer ALR. Experimental results based on flame chemiluminescence and OH planar laser-induced fluorescence show two flame topologies: tubular and LEAF topology for ratio of 2 and 4, respectively (denoted ALR2 and ALR4). The spray Mie scattering indicates a significant presence of unburnt droplets for ALR2, whereas quick evaporation is observed for ALR4 cases. In this paper, we propose and validate a basic model based on the spray droplet size distribution, and the evaporation and convection timescales, which are the prominent factors governing the flame topology. Indeed, for ALR2, the evaporation timescale is longer than the convective timescale, which causes incomplete spray evaporation and insufficient vitiated environment, leading to a tubular flame topology and preventing a LEAF to develop. In contrast, for ALR4, the spray evaporation timescales are smaller than the convective timescales, which aids the LEAF topology.

H₂-CO₂-공기 관상 화염 측정을 위한 라만 분광학 데이터 정리

An approach of reducing spontaneous Raman spectroscopy data, based on the matrix inversion method which logically removes undesired sources of the Raman scattering signals including cross-talk due to the signals from other species, is discussed. The approach contains extensive calibration of temperature-dependent system response. The Raman spectra of several cold and hot calibration flames and their resulting calibration matrix are elaborated. The structure of H₂-CO₂-air tubular flames, in terms of major species (CO₂, O₂, N₂, H₂O, and H₂) concentrations and temperature, was successfully measured with the optical technique. The Raman spectra data used in this study are taken from a uniquely designed experimental apparatus – a tubular burner and Raman spectroscopy system at Vanderbilt University. The characteristic uncertainties for chemical species are approximately ±2% by mole fraction for hot products and ±0.5% for room temperature reactants. The approach described here for hydrogen/air tubular flames may be adapted to hydrocarbon/air tubular flames when other necessary major species, for example, CH₄ and CO for methane/air tubular flame, are considered in the whole process.

A technique to establish liquid ethanol tubular combustion by dual swirl

This study proposes a compact dual swirl combustor to establish a steady liquid ethanol tubular flame, in which tangential air is injected through two stages. The outer intense swirl induces reversed flame on the surface wall of a concentric copper tube, and the central liquid fuel is heated to vaporize through the inner swirling air inside the copper tube. By enhancing the heat transfer process and promoting the mixing of the reactant, the overall heat output approaches 10 kW, much higher than the original combustor. With temperature measurement and heat transfer analysis, the flame stabilization mechanism has been elucidated. Two kinds of typical flame structures, a tubular flame and an anchored conical flame, have been obtained under various operating conditions. As the flow rate of inner air gradually increases, plenty of the heat aiming to vaporize the liquid fuel is taken off by the air flow, and small droplets occasionally appear downstream the nozzle; furthermore, the global equivalence ratio (Φgl) at the nozzle exit gradually decreases, and an anchored flame appears, which interrupts the fuel mixing with outer swirling air, resulting in soot formation in some cases. Plenty of tests illustrates that as Φgl is below 11, the flame anchoring will appear.

Ethanol spray tubular flame established in a swirling air flow

This study proposes an efficient technique to burn ethanol spray in an intense swirling air flow and investigates the mechanism of flame lift-off. On the basis of typical tubular burners for gaseous fuels, ethanol spray was axially induced and mixed with the tangentially injected air under room temperature, yielding the ethanol spray tubular flames under various operating conditions. The results show that from an ultra-lean condition of global equivalence ratio of 0.1 to the rich condition, two typical flames, namely attached and lifted tubular flames, were established. Then, the structure of the ethanol spray tubular flame was specified by temperature measurements, OH-PLIF imaging, indicating its overwhelming aerodynamic and thermal stability with low pollutant emissions. Meanwhile, a flue gas analyzer was used to characterize the gas composition in the hot exhaust. It is found that the burner can achieve very low emissions of both CO and NOx, illustrating high combustion completeness under both attached and lifted flame conditions. To give a better understanding and make fully utilization of spray tubular flames, a parametric study was carried out to investigate the effects of the tangentially swirling air flow, the flow rate and oxygen concentration of atomizing gas, and the flow rate of liquid ethanol. Generally, an attached flame is lifted under a higher tangential velocity of air flow; increasing oxygen concentration of the atomizing gas flow leads to the height decrease of the flame lift-off, even the reattachment of the lifted flame; by raising the ethanol flow rate to exceed a critical value, the flame also lifts off. Furthermore, the evaporation time of droplets, residence time and chemical reaction time were calculated to quantify the flame lift-off behavior. The lifted flame can be established only when the evaporation time is larger than both the reaction time and the residence time.

Quantifying the impact of heat loss, stretch and preferential diffusion effects to the anchoring of bluff body stabilized premixed flames

The response of a premixed flame subjected to either flame stretch (and associated Lewis number effects) or heat loss has been well documented in the literature and has enabled a good understanding of canonical configurations such as flat burner-stabilized, counter flow and tubular flames. However, in practical burners, flames are simultaneously subjected to stretch, heat transfer with the flame holder and preferential diffusion effects. For such flames, usually the collective effect of underlying contributions is studied and individual effects are only treated in a qualitative manner. In this paper, our objective is to use flame stretch theory to separate and quantify the underlying contributions from flame stretch, preferential diffusion and heat transfer with the flame holder to the flame speed of bluff body stabilized flames. It is shown that the theory adequately predicts the flame displacement speed in comparison to the results from the numerical simulations. Using the quantification of contributions, an overall stabilization mechanism for H2 enriched CH4-air mixtures is discussed. The role of competing contributions from preferential diffusion and heat loss is highlighted especially near the flame base region where the flame speed is heavily impacted by all the effects. Insights are also given for low Lewis number flashback prone flames.

Raman-corrected two-photon absorption laser induced fluorescence of atomic oxygen in premixed hydrogen, cellular tubular flames

Femtosecond, Two-photon Absorption Laser Induced Fluorescence (fs-TALIF) corrected for collisional quenching with Raman scattering is used to capture spatially resolved atomic oxygen profiles in lean premixed, hydrogen cellular tubular flames. This method has allowed comparisons of number density and O-atom concentration distributions in flames of variable stretch rates in a manner similar to that previously performed on the minor flame species H and OH. As stretch rate increases, the radii of peak O-atom in the cells decrease while O-atom concentrations remain relatively unaffected. This differs from non-cellular flame data where increasing stretch rate increases minor species number densities. Three chemical mechanisms are employed to perform direct numerical simulations of the O-atom profiles in the tubular flames and are found to be in close agreement with one another. For N2-diluted flames, the simulations predict O-atom number densities within the uncertainty of the data for the cellular region but over-predict the O-atom number densities in the dearth region of the 2D flames. Additionally, simulated O-atom concentrations contradict the trend of the data and increase with stretch rate. Changing the diluent from N2 to CO2 lowers the peak concentrations of atomic oxygen as CO2 becomes reactive at flame temperatures. This allows the CO+O(+M)⇌CO2(+M) reaction to consume atomic oxygen. Flames diluted with carbon dioxide caused the model to over-predict the O-atom concentration in these flames. This discrepancy is similar to past minor species measurements in cellular tubular flames though it does not occur in minor species profiles of non-cellular (1D), CO2-diluted tubular flames. The discrepancy could be caused by the simplifying relationships employed to convert the 3D geometry to 2D in the simulations.

Flame Stabilization and Blow-Off of Ultra-Lean H2-Air Premixed Flames

The manner in which an ultra-lean hydrogen flame stabilizes and blows off is crucial for the understanding and design of safe and efficient combustion devices. In this study, we use experiments and numerical simulations for pure H2-air flames stabilized behind a cylindrical bluff body to reveal the underlying physics that make such flames stable and eventually blow-off. Results from CFD simulations are used to investigate the role of stretch and preferential diffusion after a qualitative validation with experiments. It is found that the flame displacement speed of flames stabilized beyond the lean flammability limit of a flat stretchless flame (ϕ=0.3) can be scaled with a relevant tubular flame displacement speed. This result is crucial as no scaling reference is available for such flames. We also confirm our previous hypothesis regarding lean limit blow-off for flames with a neck formation that such flames are quenched due to excessive local stretching. After extinction at the flame neck, flames with closed flame fronts are found to be stabilized inside a recirculation zone.

Direct spray combustion in a tubular flame burner toward fine particle synthesis

Direct spray combustion in a tubular flame burner toward fine particle synthesis

Ignition Characteristics of Ammonia/Air Mixture with Tubular Flame Burner

Ignition Characteristics of Ammonia/Air Mixture with Tubular Flame Burner

Quantitative oxygen atom measurements in lean, premixed, H2 tubular flames

Quantitative O-atom profiles are measured for the first time in tubular flames and used to assess the performance of chemical kinetic mechanisms in tubular flame simulations. Atomic oxygen is measured via femtosecond, Two-Photon Laser Induced Fluorescence (fs-TPLIF) to avoid photolytic interference. The fs-TPLIF signal is corrected for collisional quenching from major species concentrations measured by Raman scattering. Temperature-dependent quenching rate in the form of T−0.5 for H2O is applied to better represent the actual physics, and all simulations are found to agree with this method. Atomic oxygen is reported in H2/O2 flames diluted with N2 or CO2 at 200 and 400 s−1 stretch rates. The oxygen radical data is compared to predictions using three different, detailed chemical kinetic mechanisms. Predictions of profile shape vary slightly, but the peak O-atom number density is calculated within experimental uncertainty by eachmechanism.

An Experimental Investigation on Flame Stability and Lean Extinction Limit in Tubular Flame Burners Operating on Jet Fuel

ABSTRACT To solve problems of the low burn-off rate and the flame instability in a conventional rapidly mixed tubular flame (RMTF) burner and problems of the flashback and the narrow extinction limits in a premixed tubular flame burner, a new fuel/air mixing mode with multi-staged inlets was proposed and applied to a tubular flame burner, called the multi-staged tubular flame (MSTF) burner. To understand the mechanism of the fuel/air mixing mode of MSTF burner and their effects on the combustion, a series of experiments and simulations were carried out in comparison with the RMTF burner, operating on RP-3 jet fuel in this work. High-speed photography was applied to examine the flame characteristics of the two burners based on the flame irradiance intensity and flame morphology analysis combined with statistic methods. The results show that the tubular flames can be established stably without flash-back for both two burners in a wide range of equivalence ratios. The lean extinction limits can be extended to a global equivalence ratio as low as Фg = 0.26 for MSTF burner, while Фg = 0.40 for RMTF burner. Planar laser induced fluorescence (PLIF) measurements and 3D CFD simulations were also conducted to see the difference in fuel/air distributions for the two mixing modes of MSTF and RMTF burners at cold conditions. One of the findings is that the MSTF burner has better flame stability and wider extinction limits than the RMTF burner. The physics is that there stably forms a fuel-rich zone in the upstream of the inlet section which increase the resident time of rich fuel/air mixtures. The reveal of this mechanism for the multi-staged mixing mode is important as it has swept the main hurdles for the burner to find more applications to the industries.

Combustion of liquid ethanol in an innovatory vortex-tube combustor with Self-evaporating and edge-like flame properties

The combustion performance of a novel self-evaporating vortex-tube combustor for liquid fuel proposed in this study was experimentally explored by taking liquid ethanol as fuel. The new combustor was developed via installing a double-layer self-evaporating tube made of stainless steel and copper into the center of a localized stratified vortex-tube combustor. The evaporation temperature and rate of liquid fuel can be controlled by adjusting the flow rate of cooling air flowing through the tube interlayer. The effects of global equivalence ratio and fuel flow rate on the stability limit, pressure fluctuation, flame configuration, and NOx and CO emissions were investigated. The results corroborate that the novel vortex-tube combustor can achieve full evaporation of ethanol up to 30 mL/min, corresponding to a heat output of 10.57 kW. The localized stratified distribution of species formed in the combustor results in an edge-like tubular flame structure and correspondingly a wide stability limit, which enables stable ultra-lean combustion with the global equivalence ratio as low as 0.22. The pressure fluctuation amplitudes are always less than 800 Pa throughout the entire operating map. The combustor can achieve low NOx and CO emissions below 18 ppm under appropriate conditions. The high-steady combustion procedure makes the ultra-lean combustion conducted, whilst the evaporating of liquid fuel can also decrease the temperature of the post-flame zone, then reducing the formation of thermal NOx. Furthermore, the high-temperature post-flame zone locates in the fuel-rich region with high concentrations of CO and H2, which can further inhibit the production of NOx.

Effects of swirl on the heating process of a central gas stream in a tubular flame

This study experimentally investigated the heating process of an axially induced cold gas stream in a swirl tubular flame established through tangential fuel/air pre-mixture injection, aiming to clarify the effects of swirl motion on the heat and mass transfer enhancement. Tubular flame burners with geometry swirl numbers (S) of 0.35 to 1.4 were employed in addition to a conventional slit-type burner to systematically study the flame structure, stability, temperature distribution and flow field. The results show that in comparison to the slit burner without rotation, the swirl tubular flame burner provides a much wider range of stable flame from lean to rich limits even under large flow rates. The heating rate of the inner cold stream by the outer burned gas stream increases as the swirl number raises. Cold flow visualization illustrates that the interface between the two streams is rolled up and instability appears at the interface for the larger swirl numbers of S = 0.7 and 1.4, resulting in enhancement of mixing rate. At large flow rates the two streams are well mixed under cold flow condition, however, they are separated and ordered under combustion condition in the upstream, probably resulted from the burned gas expansion outside the central gas stream. While their mixing rapidly enhances as S raises to beyond 0.7. A simple theoretical analysis illustrates that the enhancement of the heating rate with a rotational motion can be quantitatively evaluated by replacing the numeric unity in Eq. (1) with the value of S + 1. The enhancement seems to be through an increase of heat transfer area at low rotational velocities, or increases of heat and mass transfer rates at high rotational velocities.

Response of lean premixed swirl tubular flame to acoustic perturbations

This study experimentally examined the response of a lean premixed methane/air swirl tubular flame to the longitudinal acoustic perturbations. Experiments were conducted by varying the acoustic frequency (f0), acoustic power (P0) and inlet air flow rate (QA) to systematically study the acoustic-excited flame dynamics, including the flame stability, flame structure, heat release rate and acoustic pressure fluctuations. The results demonstrated that the lean tubular flame was much sensitive to longitudinal perturbations below 210 Hz. Even at low power of P0, the flame was forced to oscillate intensively when the perturbation was operated in two discontinuous frequency ranges of f0 = 150–200 and f0 = 50–100 Hz, while a weakened oscillation regime was observed in the middle range of f0 = 100–150 Hz; a lower frequency (less than 50 Hz) would extinguish the flame. An increase in P0 intensified the flame oscillation under the perturbations of 50–100 Hz, and even resulted in flame lift-off and blow off with f0 between 150 and 200 Hz. In comparison to f0 and P0, the effects of air flow rate (QA) seemed to be weaker. The proper orthogonal decomposition (POD) analysis was performed to interpret the contributions from the mean mode and fluctuating axial and circumferential modes that led to combustion instability. It was found that as the energy fraction of the axial mode exceeded 5.0%, the flame lift-off appeared in the lean swirl tubular configuration.