Diffusion Flame Burner Research Articles

Numerical Study of Unsteady Properties of Ethylene/Air Turbulent Jet Diffusion Flame with Detached Eddy Simulation

AbstractIn this paper, unsteady process of ignition and combustion of turbulent plane-jet diffusion flame of ethylene/air is numerically simulated with detached eddy simulation (DES) and a reduced kinetic mechanism of ethylene. The kinetic mechanism consisting of 25 species and 131 steps is reduced from a 25 species/131 steps detailed mechanism via the method of error-propagation-based directed relation graph (DRGEP). The DES results of averaged temperature profiles at varied downstream locations are compared with the DNS results of Yoo et al. [

Experimental research on the diffusion flame formation and combustion performance of forced swirl combustion system for DI diesel engines

In order to optimize the fuel/air mixture formation and decrease pollutant emissions of direct injection (DI) diesel engines, a new concept of forced swirl combustion system (FSCS) included double swirl combustion system (DSCS) and lateral swirl combustion system (LSCS) was proposed and implemented by a unique design of the geometric shape of the combustion chamber. Related numerical research on combustion and emission characteristics of this new system was conducted and acceptable numerical simulation results about its feasibility were drawn initially. To make a better understanding of the mechanism of fuel/air mixture formation in FSCS, visualization of diffusive flame was conducted in a constant volume vessel. The flame images were captured by a high speed camera and the image results indicated that, compared with the traditional omega combustion system (OMECS), both of the flame spread space and spread area of FSCS were increased after the spray impingement. Then, fuel economy and emission performance of FSCS were tested in a single cylinder engine to evaluate its application in diesel combustion system. Test results shown that the LSCS could achieve a better fuel consumption and less soot emission property than DSCS. Both of the two combustion systems (LSCS and DSCS) have the promising benefit in fuel consumption and soot emission compared to OMECS. It is suggested that the fuel/air mixing and engine performance could be promoted in FSCS owing to the introduction of swirling combustion.

Studies on Characteristics and Formation of Soot Nanoparticles in an Ethylene/Air Inverse Diffusion Flame

AbstractAn ethylene/air inverse diffusion flame burner was assumed to generate a stable flame, and soot samples were captured at different locations around the outer edge along the height of the flame. The soot samples were then analyzed with transmission electron microscopy (TEM), gas chromatography-mass spectrometry (GC-MS), and other detection means, and a photoacoustic analyzer was introduced to detect the mass concentration of soot nanoparticles after inception based on their optical features. The results showed that the plot of soot concentration had the same tendency as the gas temperature’s, which increased first and then declined with the increase in flame height, but these two curves had two different turning points. As to the measurements of TEM, the morphology of soot particles exhibited diverse shapes and structures at different flame heights. Alkanes, cycloalkanes, olefins, benzene, anthracene, pyrene, and aromatic compounds were determined as the main components of soot by the detection of ...

Effect of Pressure and Type of Fuel on Laminar Diffusion Flame Height at Subatmospheric Pressures

We have measured the shape of the luminous zone in a laminar jet diffusion flame burning methane, ethylene, and propane in quiescent air in the pressure range 0.02-0.1 MPa. Soot emission by the flame was not permitted. The flame became more slender and luminous as the pressure increased. At relatively high fuel flow rates, the ethylene and propane flame heights increased with pressure. At low fuel flow rate, the flame height decreased as pressure increased for all three fuels. At high fuel flow rate, the methane flame height first decreased and then increased with pressure, due to the low tendency of methane to form soot. Finally, we observed a tendency toward decreasing – increasing – decreasing flame height at certain fuel flow rates and in certain pressure ranges.

Performance of Multiple-Injection Dry Low-NOx Combustors on Hydrogen-Rich Syngas Fuel in an IGCC Pilot Plant

The successful development of coal-based integrated gasification combined cycle (IGCC) technology requires gas turbines capable of achieving the dry low nitrogen oxides (NOx) combustion of hydrogen-rich syngas fuels for low emissions and high plant efficiency. Mitsubishi Hitachi Power Systems, Ltd. (MHPS) has been developing a “multiple-injection burner” to achieve the dry low-NOx (DLN) combustion of hydrogen-rich syngas fuels. The purposes of this paper are to present the test results of a multican combustor equipped with multiple-injection burners in an IGCC pilot plant, and evaluate combustor performance by focusing on the effects of flame shapes. The syngas fuel produced in the plant contained approximately 50% carbon monoxide, 20% hydrogen, and 20% nitrogen by volume. In the tests, the combustor with slenderer flames achieved lower NOx emissions of 10.9 ppm (at 15% oxygen), reduced combustor liner and burner plate metal temperatures, and lowered combustion efficiency at the maximum gas turbine load. The test results showed that the slenderer flames were more effective in reducing NOx emissions and liner/burner plate metal temperatures. A comparison with the diffusion-flame combustor demonstrated that the multiple-injection combustors achieved the dry low-NOx combustion of the syngas fuel in the plant.

Innovative Passive Flame Ameliorator for a Diffusion Flame Burner Using Chevrons

Diffusion flame burners are mainly used in industries over premixed flame burners for safety considerations. But the combustion process in a diffusion flame is not complete and the flame is usually in bright yellow in colour in contrast to the premixed flame which gives a bluish flame. To improve the combustion process in a diffusion flame burner a novel approach, using chevrons has been carried out. The chevrons are found to reduce the aero-acoustic noise in the exhaust jets of aircraft engines by allowing better mixing of the exhaust gas with the ambient air. The similar concept is used here where the tips of the burners are cut in the form of chevrons. Experimental investigations are carried out on burners with three and four chevrons in addition to a standard burner using LPG as the fuel. The results indicate that with the introduction of chevrons the diffusion flame becomes more compact. The premixed region, in the diffusion flame, where the air and fuel is mixed well is found to increase by nearly 100 % with the usage of chevrons, indicating better mixing of fuel and air. The results also indicate that increasing the number of chevrons from three to four does not show much variation. Further experiments are to be carried out to determine the improved fuel consumption with the usage of chevrons.

Performance Characteristics of a Micro-tubular Solid Oxide Fuel Cell Operated with a Fuel-rich Methane Flame

A micro-tubular solid oxide fuel cell (SOFC) is directly combined with a fuel-rich methane flame in a simple, "no-chamber" setup. The multi-element diffusion flat flame burner (MEDB) is served as a fuel reformer and implements the quick starting process for the flame fuel cell. Experiments are performed based on novel anode supported micro-tubular SOFC cells and a multi-element diffusion flame burner for various equivalence ratios and flow rates. The SOFC is directly heated up by the flame from room temperature to operating temperature in less than 120 s. The maximum power generated by the fuel cell reached 0.45 W at the equivalence ratio of 1.2. The cell was operated for 8 hours at the equivalence ratio of 1.2 at a fixed voltage of 0.45 V without significant performance degradation. A transient two-dimensional SOFC model is developed to analyze the transient temperature field and the associated thermal stresses of the SOFC when the cell is suddenly exposed to a high temperature flame. The thermal shock resistance of the direct flame micro-tubular SOFC is analyzed and the result shows the excellent performance characteristics of the micro-tubular cell in the aspect of thermal shock resistance. Figure 1

1 kHz Mid-IR Absorption Spectroscopy for CO and CO2 Concentration and Temperature Measurement

A sensor for in situ combustion gas measurements of concentration and temperature of carbon monoxide (CO) and carbon dioxide (CO2) has been developed and tested on a temperature calibrated diffusion flame burner in order to measure combustion efficiency. The fundamental bands of both molecules are exploited using distributed feedback quantum cascade lasers near 4.40 and 4.58 µm at which the absorption line-strengths are multiple orders of magnitude larger than overtone and combination bands at lower wavelengths. Spectra were taken at a rate of 1 kHz across a 50.8-mm path length of an ethylene (C2H4)-air diffusion flame at heights of 5 to 25 mm above the burner and at equivalence ratios ranging from 0.5 to 1.5. Total combustion efficiency was measured using the concentrations of CO and CO2, and temperature was obtained from relative strengths of multiple CO2 absorption lines with differing lower state energies. The results are compared to previous coherent anti-Stokes Raman spectroscopy (CARS) temperature measurements and equilibrium chemical calculations.

Performance Characteristics of a Micro-tubular Solid Oxide Fuel Cell Operated with a Fuel-rich Methane Flame

A micro-tubular solid oxide fuel cell (SOFC) is directly combined with a fuel-rich methane flame in a simple, "no-chamber" setup. The flame serves as a fuel partial oxidation reformer and a quick starter for the SOFC in the system. Experiments were performed using an anode supported micro-tubular SOFC and a multi-element diffusion flame burner for various equivalence ratios and gas flow rates. The SOFC was directly heated up by the flame from room temperature to operating temperature in less than 120 s. The maximum power generated by the fuel cell reached 0.45 W at the equivalence ratio of 1.2. The cell was operated at a fixed voltage of 0.45 V for three thermal cycles in 18 hours without significant performance degradation when the equivalence ratio was 1.2.

Influence Of Acoustic Fluctuations On The Round Jet, Formed In Curved Channel

The results of experimental investigations on the effect of the transverse acoustic fluctuations for subsonic round jet formed in curved channel (d = 20; 9; 1.5 mm) are presented. Using laser – smoke visualization the jet instant picture sections were received, which showed the presence of two modes jets instabilities – Kelvin – Helmholtz vortices and Dean vortices and their interaction. The influence of the acoustic excitation frequencies on the jets are shown, in particular on a wavelength of Kelvin – Helmholtz instability. The picture of the diffusion combustion of propane jet with a nozzle diameter of d = 1,5 mm without acoustic excitation and with acoustic excitation are compared. It has been observed that the diffusion flame combustion of propane is subject to transformation by the action of the acoustic field and developing instabilities in the jet.

Effect of precursors and radiation on soot formation in turbulent diffusion flame

Soot formation in ‘Delft Flame III’, a pilot stabilized turbulent diffusion flame burning natural gas/air, is investigated using ANSYS FLUENT by considering two different approaches for soot inception. In the first approach soot inception is based on the formation rate of acetylene, while the second approach considers the formation rate of two and three-ringed aromatics to describe the soot inception (Hall et al., 1997). Transport equations are solved for soot mass fraction and radical nuclei concentration to describe inception, coagulation, surface growth, and oxidation processes. The turbulent–chemistry interactions and soot precursors are described by the steady laminar flamelet model (SLFM). Two chemical mechanisms GRI 3.0 (Gregory et al.) and POLIMI (Ranzi et al., 2012) are used to represent the effect of species concentration on soot formation. The radiative properties of the medium are included based on the non-gray modeling approach by considering four factious gases; the weighted sum of gray gas (WSGGM) approach is used to model the absorption coefficient. The effect of soot on radiative transfer is modeled in terms of effective absorption coefficient of the medium. A beta probability density function (β-PDF) in terms of normalized temperature is used to describe the effect of turbulence on soot formation. The results clearly elucidate the strong effect of radiation and species concentration on soot volume fraction predictions. Due to increase in radiative heat loss with soot, flame temperature decreases slightly. The inclusion of ethylene has less synergic effect than that of both benzene and ethylene. Both cases have less impact on the nucleation of soot. The increase in soot volume fraction with soot–turbulence interaction is in consistence with the DNS predictions.

Performance enhancement of inverse diffusion flame burners with distributed ports

An experimental study regarding the influence of the air port design on the inverse diffusion flame characteristics was performed for two different burner configurations, wherein the combustion/heat transfer performance was optimized. In the first configuration that comprised a single air port and distributed fuel ports, decreasing the air jet diameter enhanced the combustion efficiency by 3.6% as testified by recording a maximum decrease of 81.3% in the CO emissions. As the central air jet Reynolds number thus reached 18038, the stability limit was extended by 181% in terms of the air jet velocity; while the flame length was reduced by 38.6%. Inasmuch as the flow residence time across the high-temperature zone thus decreased, NO x emissions as low as 6 ppm were obtained. In the other configuration that had multiple air ports, there was a decrease in the CO, HC, and NO x emissions respectively by 69%, 58%, and 31% in comparison to those of the single port having the same port area. It was found that the smallest jet diameter was accompanied by the highest peak temperature as well as the maximum heat flux to a plate on which the flame impinged, respectively, having the values of 1761℃ and 11.4 kW/m2. In this regard, increasing the air jet Reynolds number by 30% increased the peak flame temperature by 8.3%. In the primary equivalence ratio range between 0.8 and 2.0, the impingement plate optimum spacing for the maximum heat flux was found to correlate well with Reynolds number. Combining the nonunity Lewis number and the Gas Research Institute-3 (GRI-3) kinetics with the k − ɛ model verified the fuel jet deflection and soot development, but overpredicted the NO x emissions by 11.5%. Increasing the center-to-center distance between the central air jet and the distributed fuel ports reduced the flame length by 19%.

Numerical study of the effects of pressure on soot formation in laminar coflow n-heptane/air diffusion flames between 1 and 10 atm

Laminar nitrogen-diluted n-heptane diffusion flames burning in coflow air were numerically simulated at pressures from 1 to 10atm. Numerical simulations were performed by using a detailed reaction mechanism containing 175 species and 1086 reactions with PAH formation up to pyrene and a sectional soot model. The soot model consists of inception as a result of the collision of two pyrene molecules, heterogeneous surface growth and oxidation following the hydrogen abstraction acetylene addition (HACA) mechanism, and soot particle coagulation and PAH surface condensation. Comparisons with available experimental data indicate that the numerical model reproduces successfully the influence of pressure on the flame structure and soot production. All the soot formation processes are enhanced with increasing pressure and HACA dominates the soot mass growth over the pressure range considered. PAH condensation is the most sensitive process to pressure whereas the inception and HACA processes exhibit similar pressure dependency. These trends are directly related to the pressure dependence of the molar concentrations of the species involved in soot formation. The increase in the molar concentrations of benzene and pyrene with pressure results from both physical and chemical effects. Propargyl recombination and interconversion of phenyl to benzene dominate the formation of benzene low in the flame over the pressure range considered, though the contributions of the addition of acetylene to n-butadienyl radical increases significantly with pressure in the flame centerline region.

Numerical and experimental study of soot formation in laminar diffusion flames burning simulated biogas fuels at elevated pressures

The effects of pressure and composition on the sooting characteristics and flame structure of laminar diffusion flames were investigated. Flames with pure methane and two different methane-based, biogas-like fuels were examined using both experimental and numerical techniques over pressures ranging from 1 to 20atm. The two simulated biogases were mixtures of methane and carbon dioxide with either 20% or 40% carbon dioxide by volume. In all cases, the methane flow rate was held constant at 0.55mg/s to enable a fair comparison of sooting characteristics. Measurements for the soot volume fraction and temperature within the flame envelope were obtained using the spectral soot emission technique. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible flows, which included complex chemistry, detailed radiation heat transfer and soot formation/oxidation. Overall, the numerical simulations correctly predicted many of the observed trends with pressure and fuel composition. For all of the fuels, increasing pressure caused the flames to narrow and soot concentrations to increase while flame height remained unaltered. All fuels exhibited a similar power-law dependence of the maximum carbon conversion on pressure that weakened as pressure was increased. Adding carbon dioxide to the methane fuel stream did not significantly effect the shape of the flame at any pressure; although, dilution decreased the diameter slightly at 1atm. Dilution suppressed soot formation at all pressures considered, and this suppression effect varied linearly with CO2 concentration. The suppression effect was also larger at lower pressures. This observed linear relationship between soot suppression and the amount of CO2 dilution was largely attributed to the effects of dilution on chemical reaction rates, since the predicted maximum magnitudes of soot production and oxidation also varied linearly with dilution.

Laminar jet methane/air diffusion flame shapes and radiation of low air velocity coflow in microgravity

Observations based on short-duration experiments under microgravity of the characteristics of laminar jet diffusion flames burning in coflowing air conditions are described. Experimental conditions were such to establish a flow with Reynolds number of 140 and low air flow velocities of 0–0.5m/s to produce steady laminar flames. Previous studies ignored effects of air flow velocity with small air stream Froude number, indicating that flame length and diameter were independent of air flow velocity. Distinct from previous studies, we introduced integrated flame shape (maximum flame diameter and ratio of flame length to maximum flame diameter) and residence time formulas, the coflowing air effects on the flame length and maximum diameter together with the flame oscillation, flame color and radiation were investigated both in microgravity and normal gravity. The experimental results obtained showed that the characteristics of microgravity laminar jet diffusion flames were significantly affected by air-flow velocities for the absence of buoyancy. Under microgravity, with increasing coflowing air velocity, the mixing rate of fuel and air increased and the flame sheet moved closer towards the nozzle, maximum flame diameter decreased with air flow velocity increasing; in contrast, the air velocity had little effect on flame shapes in normal gravity. The microgravity flame exhibited no oscillations with the absence of buoyancy, whereas in normal gravity, flame oscillation frequency increased accompanied by a smaller amplitude with air velocity increasing. In microgravity, the residence time for soot convected by the local flow also decreased with air velocity increasing; the radiative heat loss flux of the flame then decreased, but changed little if residence time was sufficiently large. Compared with microgravity flame, the flame radiation heat loss flux of normal gravity flame was smaller with a much shorter residence time.

Experimental and kinetic modeling study of pyrolysis and oxidation of n-decane

The pyrolysis of n-decane was investigated in a flow reactor at 5, 30, 150 and 760Torr, and the oxidation of n-decane at equivalence ratios of 0.7, 1.0 and 1.8 was studied in laminar premixed flames at 30Torr. In both experiments, synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) was used to identify combustion species and measure their mole fraction profiles. A new detailed kinetic model of n-decane with 234 species and 1452 reactions was developed for applications in intermediate and high temperature regions, and was validated against the experimental results in the present work. The model was also validated against previous experimental data on n-decane combustion, including species profiles in pyrolysis and oxidation in high pressure shock tube and atmospheric pressure flow reactor, jet stirred reactor oxidation, atmospheric pressure laminar premixed flame, counterflow diffusion flame and global combustion parameters such as laminar flame speeds and ignition delay times. In general, the performance of the present model in reproducing these experimental data is reasonably good. Sensitivity analysis and rate of production analysis were conducted to understand the decomposition processes of n-decane.

Sooting tendencies of primary reference fuels in atmospheric laminar diffusion flames burning into vitiated air

In this study, the sooting tendencies of primary reference fuels (PRFs) are measured in term of yield sooting indices (YSIs) in methane diffusion flames doped with the vapors of PRFs. The present paper represents an incremental advance complementing the original methodology prescribed by McEnally and Pfefferle. The influence of both PRF formulation and CO2 dilution of the coflowing air on the YSIs is also assessed. The diffusion flames burning in a coflowing oxidizer stream are established over the Santoro’s burner and vapor of the liquid fuel to be investigated is injected into the fuel stream. Laser extinction measurements are performed to map the two-dimensional field of soot volume fraction in the flame. For the pure liquid hydrocarbons investigated, i.e., n-hexane, n-heptane, isooctane, and benzene, the YSI reported in the original paper by McEnally and Pfefferle quantitatively predict the sooting propensities, measured here at much higher dopant concentrations. The present study therefore extends the consistency of the YSI methodology on the Santoro’s burner. For blends of n-heptane and isooctane, the sooting tendency of doped flames exhibits regular and monotonic trends and decreases with increasing n-heptane mole fraction or CO2 dilution. Interestingly, the evolution of YSI with the isooctane mole fraction exhibits a strong similarity for varying CO2 mole fraction. A quadratic least-squares fit is then derived, providing a phenomenological model of YSI as a function of both isooctane mole fraction in the fuel stream and CO2 mole fraction in the oxidizer. A non-negligible cross effect of PRF formulation and CO2 dilution on YSI is revealed. The method elaborated within the framework of the present paper could be extended to surrogate fuels. This would help develop a comprehensive database and empirical correlations that could predict the sooting propensities of different surrogate fuels, therefore their potentially mitigationed soot production through control of fuel composition and/or exhaust gas recirculation. This database would also be useful for the validation of CFD simulations incorporating sophisticated model of soot production.

Using Hydrogen as Gas Turbine Fuel: Premixed Versus Diffusive Flame Combustors

This work aims at estimating the efficiency gain resulting from using lean premixed combustors in hydrogen-fired combined cycles with respect to diffusive flame combustors with significant inert dilution to limit NOx emissions. The analysis is carried out by considering a hydrogen-fired, specifically tailored gas turbine whose features are representative of a state-of-the-art natural gas–fired F-class gas turbine. The comparison between diffusion flame and lean premixed combustion is carried out considering nitrogen and steam as diluents, as well as different stoichiometric flame temperatures and pressure drops. Results show that the adoption of lean premixed combustors allows us to significantly reduce the efficiency decay resulting from inert dilution. Combined cycle efficiency slightly reduces from 58.5%–57.9% when combustor pressure drops vary in the range 3%–10%. Such efficiency values are comparatively higher than those achieved by diffusive flame combustor with inert dilution. Finally, the study investigated the effects of decreasing the maximum operating blade temperature so as to cope with possible degradation mechanisms induced by hydrogen combustion.

Experimental Characterization of a Direct Methane Flame Solid Oxide Fuel Cell Power Generation Unit

A direct flame solid oxide fuel cell unit was designed and built in this work for potential combined heat and power generation. The direct combination of a methane flat flame and a solid oxide fuel cell (SOFC) in a simple, “no-chamber” setup was implemented. Direct flame fuel cell experiments were performed using an anode-supported planar SOFC with 50-mm-long square anode and 30-mm-long square cathode. At the SOFC anode, a multi-element diffusion flame burner with an outlet of 55 mm by 55 mm square provided methane/air rich flames for various equivalence ratios and flow rates. The maximum power generated by the direct flame fuel cell unit reached 0.35 W with the power density of 388 W/m2 at the equivalence ratio of 1.2. The unit was operated for 3 hours at the equivalence ratio of 1.1 at a fixed voltage of 0.4 V with the current deteriorating from 0.29 A to 0.25 A. The system efficiency of the power generation unit was analyzed by considering the combustion efficiency, the fuel utilization efficiency and the SOFC electrical efficiency. The fuel utilization efficiency was found to be the dominant factor of the total efficiency.

Effect of hydrogen percentage and air jet Reynolds number on fuel lean flame stability of LPG-fired inverse diffusion flame with hydrogen enrichment

The flame type studied in this paper is a circumferential-fuel – jet inverse diffusion flame, and the fuel is liquefied petroleum gas enriched with hydrogen gas. Fuel lean flame stability limit regarding to the volumetric percentage of hydrogen and the air jet Reynolds number was investigated. There were three flame stable-related limits examined: local extinction limit, restore limit, and complete extinction limit. Global Energy Consumption Rate of fuel, fuel jet velocity, and overall equivalence ratio of the air/fuel mixture at the three stable-related limits were presented. Experimental results indicate that with hydrogen addition, the inverse diffusion flame can sustain burning with a lower global energy than without it. The most significant stabilization effect was obtained with 30% hydrogen addition for complete extinction limit and 30%–90% for local extinction limit. The corresponding fuel jet velocity at complete extinction limit also decreases with hydrogen addition. However, fuel jet velocities at local extinction limit and restore limit increase significantly, when hydrogen percentage is larger than 70%. Air jet Reynolds number does not show notable influence on Global Energy Consumption Rate or fuel jet velocity at the three stability limits. In addition, overall equivalence ratio, which is an important parameter of inverse diffusion flame combustion dropping dramatically with air jet Reynolds number when it is less than 2000.