Flame Characteristics Research Articles

A Direct Numerical Simulation Study for Flame Structure and Propagation Characteristics of Multi-Jet Flames

ABSTRACT Lifted multi-jet flames of hydrogen and steam-diluted oxygen are studied using direct numerical simulations (DNS). The DNSs have been carried out for laminar and turbulent flame configurations. The simulation data are analyzed to investigate the flame and flame-base structures and the flame-base propagation in a configuration akin to a gas turbine with multiple fuel jets. The laminar flames have a tribrachial structure, although lean premixed and diffusi on flame branches collide downstream owing to preferential diffusion. The turbulent multi-jet flames have a single connected base region, whereas the laminar flames have separate bases, each of which is associated with the corresponding fuel jet. Although turbulent multi-jet flames have substantially different flame-base characteristics from laminar multi-jet flames (and conventional single-jet lifted flames), the propagation speed is similar for both conditions. The results suggest that the well-known flame-base stabilization theory can be straightforwardly applied to the multi-jet configuration, despite the turbulent flame structure’s unconventional features.

MILD combustion of a premixed NH3/air jet flame in hot coflow versus its CH4/air counterpart

Moderate or intense low-oxygen dilution (MILD) combustion is suitable for solving the problems of unstable flames and high NOx emissions (ENOx) of ammonia fuels; however, studies on this are rare. This paper numerically investigates the MILD combustion characteristics of a premixed NH3/air jet flame in hot coflow (JHC) under different jet equivalence ratios (ΦJ), coflow temperatures (TC), and oxygen levels (XO2,C). For comparison, similar CH4/air MILD-JHC flame characteristics are calculated. The results show that the NH3 flame generally has a lower temperature increase and heat release, as well as a larger reaction zone, than CH4. This suggests that the NH3 flame can develop into a MILD regime more easily than CH4. ENOx of the NH3 flame is two orders of magnitude higher than that of CH4 flame at all TC and XO2,C values for ΦJ < 1. At ΦJ > 1, ENOx of the NH3 flame rapidly decreases to nearly zero, but unburned NH3 emissions (ENH3) and H2 emissions (EH2) are significantly high (∼1000 ppm) and the combustion efficiency is low. Moreover, ENOx increases rapidly (gradually) with XO2,C (TC), whereas ENH3 and EH2 are considerably low at TC ≥ 1300 K and XO2,C ≥ 1% for the NH3 flame. ENOx primarily originates from NO; however, N2O is also important at low ΦJ and TC values. Therefore, considering ENOx, ENH3, EH2 and the burning efficiency, the optimal conditions for NH3 MILD combustion are ΦJ = 1, TC ≥ 1500 K, and XO2,C = 1%–3%. In addition, compared with conventional flames, NH3 MILD combustion can reduce ENOx by one to two orders of magnitude.

Numerical study of an industrial burner to optimise NOx emissions and to evaluate the feasibility of hydrogen-enriched fuel

Reducing NOx emissions from industrial burners is a significant concern due to their harmful environmental and human health impact. A computationally efficient numerical model was developed and validated using a detailed GRI 3.0 chemistry mechanism to simulate the combustion process and precisely predict the NOx emissions from industrial burners. The numerical model was implemented to reduce NOx emissions by varying the burner's primary to secondary air mass flow ratio. An optimum nozzle diameter was proposed to abate NOx emissions by a factor of 0.845. A feasibility study on the optimised burner was conducted by blending up to 50% hydrogen by volume with natural gas by maintaining the same burner power output. Results showed that the burner exhibited similar flame characteristics until 40% hydrogen was added to natural gas. A 41.8% increase in NO and a 76.8% decrease in CO emissions were observed by enriching natural gas with 50% hydrogen.

Effect of initial pressure on the critical characteristics and overpressure of hydrogen-air premixed gas combustion and explosion

In order to ensure the safe utilization of hydrogen, the flame characteristics and explosion overpressure at lean-burn, stoichiometric and rich-burn conditions are investigated systematically by varying initial pressures. Experiments were conducted in a constant-volume combustion chamber using the Schlieren system to investigate the critical characteristics of hydrogen-air premixed flames at different equivalence ratios (from 0.2 to 6.5) and initial pressures (from 50 kPa to 150 kPa). The cellular instabilities of hydrogen-air premixed flames were interpreted and evaluated from the viewpoint of thermal-diffusive and hydrodynamic instabilities. The influence of initial pressure on the cellular instability and combustion and explosion characteristics were analyzed. The results show that the variation of thermal-diffusive instability dominantly governs the cellular instability at lean-burn conditions (φ = 0.3–1.0). In contrast, the cellular instability at rich-burn conditions (φ > 1.5) is governed by the variation of hydrodynamic instability. The increase of initial pressure promotes the cellular instabilities noticeably caused by the rise of the hydrodynamic instability. With the increase of P0, the Lewis number Le and thermal expansion ratio σ almost remain the same while flame thickness δ noticeably decreases. Increasing initial pressure at lean-burn and stoichiometric conditions (φ < 2.0) facilitates cellular instability, making the explosion more intense and resulting in higher explosion overpressure. In contrast, the variation of initial pressure suppresses the laminar combustion at rich-burn conditions (φ > 3.0).

Characterization of Flame Spread Over PMMA Using a Temperature Reconstruction Method

ABSTRACT The contribution of heat transfer mechanisms for different configurations of opposed flow flame spread on cast PMMA is presented. Three opposed flow flame spread scenarios are considered, each in a quiescent atmosphere: downward spread on a vertical surface, buoyant horizontal spread (i.e. a pool fire-type spread), and lateral spread along a vertical surface. Video analysis and temperature measurements were used to determine flame spread rates; average flame spread rates were found to be 2.67, 2.54, and 3.54 mm/min for downward, horizontal, and lateral spread, respectively. These differences result from the varying magnitudes of the heat transfer mechanisms (e.g. gas-phase heat transfer and conduction through the solid) due to the changes in the flame characteristics. Past studies have investigated these regimes of flame spread, but have not directly compared experiments from the configurations used here to identify the relative importance of each heat transfer mechanism. Using temperature measurements, the thermal gradient, and hence conductive heat fluxes through the solid, were quantified. This allowed the different rates of heat transfer to be compared across each configuration. Downward and horizontal flame spread displayed similar flame spread rates and similar heat transfer contributions for both the gas phase and solid phase. However, lateral flame spread displayed characteristically higher rates of gas-phase heat transfer compared to downward and horizontal.

Heat release characteristics of ammonia flames in MILD conditions

Ammonia (NH3) combustion is an attractive energy source with zero carbon dioxide (CO2) emissions and a potential hydrogen-combustion enabler. In addition, the availability of ammonia and its amenability to transportation make it appealing to future energy supply. To provide fundamental insights into the heat release characteristics of ammonia in moderate or intense low oxygen dilution (MILD) combustion, one-dimensional laminar and two-dimensional turbulent premixed flames are studied numerically. In particular, the flame structure and heat release distribution of ammonia and ammonia-hydrogen premixed flames are examined under conventional and MILD, and preheated conditions, respectively. The results show that stoichiometric ammonia MILD flames differ from normal ammonia flames producing NH2 earlier, which results in the formation of a radical pool that shifts the chemical pathway towards hydrogen-related chemistry. The NH2 radical serves as a precursor for the ammonia flame and characterizes the reaction zone, and plays a crucial role in bringing the explosive nature into the system. For fixed ratios of the root-mean-square turbulent velocity fluctuation to the laminar flame speed and integral length scale to the laminar flame thickness, the flame surface wrinkling is comparable between the stoichiometric ammonia flame at MILD and non-MILD conditions. Furthermore, the hydrogen enrichment of the lean ammonia flame results in a significant increase in heat release rate due to the preferential diffusion effect. The study also examines chemical markers to identify heat-releasing regions of the flames and proposes the use of NH2 × O (on a mass fraction basis) as a suitable indicator of regions with high chemical activity and correlates well with the heat release for the MILD and non-MILD ammonia flames as well as ammonia-hydrogen flames at both laminar and turbulent conditions.

LES Investigation of Kerosene Spray Flame Emission Characteristics in a Staged Combustor

ABSTRACT Staged combustion technology is widely applied in modern low-emission energy and power systems. This study employs large eddy simulations (LES) to investigate the flame dynamics and emission characteristics in a staged two-phase partially premixed model combustor. To improve the suitability of the thickened flame model (TFM) for turbulent flames in the thin-reaction-zone regime, a new model for efficiency function is formulated such that the effects of small-scale turbulence on flame propagation are taken into account. The enhanced LES-TFM approach is then integrated with an efficient NOx model, in which an additional NOx transport equation is solved with pre-tabulated NOx formation source terms. Five LES runs are performed to investigate the evolution of flame dynamics and emission characteristics with staging ratios at the scaled cruise operating conditions, in which the optimal staging ratio is critical for NOx reduction. Results show that the trend of flame morphology and NOx emission predictions with staging ratios are consistent with the experimental results. The flame zones in the staged model combustor shift from a pilot-stage-only flame to a pilot-primary-stage flame, then to a primary-stage-only flame with decreasing fuel staging ratio. Due to the redistribution of the equivalence ratio and the NOx emission characteristics trade-off between the two-stage flows, there is an optimal staging ratio for NOx reduction in the staged model combustor. The proposed LES-TFM-NOx approach, which has both the benefits of efficient flame dynamics modeling with the global reaction mechanism and the advantages of prediction accuracy with the detailed mechanism for NOx emission, demonstrates its advantages in simulating flame and emission characteristics in low-emission combustion devices.

Anchored and Lifted Diffusion Flames Supported by Symmetric and Asymmetric Edge Flames

Numerous combustion applications are concerned with the stabilization of diffusion flames formed by injecting gaseous fuels into a co-flowing stream containing an oxidizer. The smooth operation of these devices depends on the attachment and lift-off characteristics of the edge flame at the base of the diffusion flame. In this paper, we address fundamental issues pertinent to the structure and dynamics of edge flames, which have attributes of both premixed and diffusion flames. The adopted configuration is the mixing layer established in the wake of a splitter plate where two streams, one containing fuel and the other oxidizer, merge. The analysis employs a diffusive-thermal model which, although it excludes effects of gas expansion, systematically includes the influences of the overall flow rate, unequal strain rates in the incoming streams, stoichiometry, differential and preferential diffusion, heat loss and gas–solid thermal interaction, and their effect on the edge structure, speed, and temperature. Conditions when the edge flame is anchored to the plate, lifted-off and stabilized in the flow, or blown-off, are identified. Two stable modes of stabilization are observed for lifted flames; the edge flame either remains stationary at a specified location or undergoes spontaneous oscillations along a direction that coincides with the trailing diffusion flame.

Failure analysis and correction models for upward flame characteristics subjected to shipping container fire

Failure and release of shipping chemical drums in dangerous goods container can result in spontaneous combustion, with a potential risk to escalate accidental events to catastrophes. The fire source and geometry of container openings are crucial factors deciding the potential of the ejected fires to trigger a series of domino effects. In this research, full scale simulations on ejected fires inside containers with changed container openings are carried out using FLACS, and the models are validated with previous experiment data. Six dimensions of container openings are established in comparative conditions between 1.2 × 2.6 m to 2.2 × 2.6 m, and four intensities of fire source are arranged in different accident scenarios. The purpose of this research is to reveal the effect of container openings on upward flame spread characteristics and high temperature profile in adjacent space between container groups. The results indicate that the maximum width of upward flame has a negative correlation with corresponding height, while there is a positive correlation between flame width and heat release rate. Meanwhile, the spread thickness of upward flame decreases with container opening widened, and the maximum shows an uptrend with fire source’s heat release rate enhancement. The current research can be further extended to propose a correlation relationship to determine the maximum width and thickness of upward flame with their positions in shipping dangerous goods cargo fire involving container openings.

Differential diffusion effects in the near field of non-premixed NH3/H2/N2-air jet flames at elevated pressure

Differential diffusion (diff-diff) effects are analyzed in both non-premixed laminar and turbulent NH3-H2-N2 jet flames at 5 bar using 1D quantitative Raman scattering measurements. The two target flames feature fuel compositions simulating a 14% and a 28% NH3 cracking ratio. A diff-diff parameter is introduced for ammonia combustion, analogous to the definition used for hydrocarbon flames. Measurements in laminar flames, compared to Chemkin simulation featuring multi-component transport show that the instrument is capable of measuring the mean diff-diff parameter, but that single-shot measurements are compromised by the large sensitivity of the parameter to errors in N2 and NH3 mass fractions. The analysis of the diff-diff parameter in turbulent flames shows that both flames have strong differential diffusion in the near field up to an axial distance of 10 diameters (Z/D = 10), then the turbulent mixing becomes dominant further downstream. The flame with a 28% NH3 cracking ratio exhibits a higher diff-diff parameter especially in the low-temperature region (<1000 K) due to the higher H2 content. The competition between differential diffusion and turbulent mixing in the near field is also reflected in the NH3, H2, and NH3/H2 profiles, falling in between prediction for Le = 1 and multi-component transport. Interestingly the NH3/H2 profiles in the far field converge to a profile matching that of Le = 1, but with an increased NH3/H2 ratio, showing the persistence of the differential diffusion effects in the region dominated by turbulent mixing.

Auto-Ignition and Flame Characteristics of Ammonia Reactivity-Controlled Compression Ignition Combustion in Comparison with Methane

Auto-Ignition and Flame Characteristics of Ammonia Reactivity-Controlled Compression Ignition Combustion in Comparison with Methane

Velocity nonuniformity and wall heat loss coupling effect on supersonic mixing layer flames

With urgent demand for reliable flame stabilization in air-breathing high-speed aircrafts' combustors under a wide range of operational scenarios, understanding flame characteristics under various inflow and wall thermal conditions is crucial for mixing layer-stabilized flames. In this paper, the effect of inflow velocity nonuniformity, wall heat loss and their coupling on flame structures, flame lift-off and stabilization mechanisms in the supersonic mixing layer are analyzed. An approach based on the Beta distribution model is innovatively proposed to generate the pseudo nonuniform inflow velocity profiles. Different flame modes, the regime diagram and key controlling parameters are quantitatively analyzed via the active subspace method. The results show that the coupling of velocity nonuniformity and wall heat loss can lead to three flame modes, i.e., the heat loss-dominant flame, the intermediate flame, and the nonuniformity-dominant flame, as the flame stabilizes from upstream to downstream regions with the normalized flame lift-off length Lnorm in the range of [0,0.50], (0.50,0.58], and (0.58,1.0], respectively. This finding is consistent with the autoignition flame stabilization mechanism. In addition, the fuel burnt-out ratio of the flame is found to be nonlinearly correlated with flame lift-off length and peaks approximately with Lnorm=0.8, owing to the subtle interactions of mixing and chemical reactions in the mixing layer. The viscous heating in the boundary layer may have a pronounced impact on the flame lift-off depending on the level of wall heat loss, and the predicted Lnorm variation could be approximately 26 percent of the combustor length with/without viscous effect under the isothermal wall condition.

Assessment of burner structure for methane-oxygen inverse diffusion flame based on laser detection: Reaction characteristics and soot analysis

The structure of a burner is a key issue of boilers and gasifiers in several industries. It significantly influences the flame formation and combustion reaction determining the mixing degree of fuel and oxidant. This study aims to explore the characteristics of flames at different angles and evaluate their advantages in specific applications. The present work explores the correlation between laser-induced breakdown threshold energy and flame temperature based on laser-induced breakdown spectroscopy (LIBS). Subsequently, the local structure and reaction characteristics of the flame at different burner angles (45, 60 and 90°) were investigated with simulation. The local equivalence ratio of the laminar flame was used to determine the distribution of reactants and components in the flame. The results indicated that the breakdown threshold energy was positively correlated with the temperature of the flame, and it was dependent on the gas concentration. With increased laser energy, the O/H ratio could be used as a function of the local equivalence ratio of the flame. In addition, the study of flame images and the spontaneous radiation of soot revealed that the C/H ratio in the flame reached the maximum value when the burner incident angle was 45°. Through simulation and experimental research, we found that the flame at 60° angle is the most stable, produces the least soot, has the highest combustion efficiency, and has better thermal radiation intensity.

The effect of flexible obstacles with different thicknesses on explosion propagation of premixed methane-air in a confined duct

The effect of flexible obstacles with varying thicknesses on the explosion characteristics of combustible gas in a simulated confined duct (cross section 80 mm × 80 mm, length 3 m) was experimentally investigated, aiming to reduce the huge losses caused by gas explosion accidents in the process industries and mining industries. In this paper, plant fiber membranes with an opening area of 0 and thicknesses of 0.105 mm, 0.210 mm, 0.315 mm, 0.420 mm, 0.525 mm, and 0.630 mm were selected as flexible obstacles. The thickness of the flexible obstacle determines the strength of its compressive resistance. The characteristics of overpressure and flame during methane explosions are analyzed and conclusions are drawn. Results indicate that several shock wave reflection processes occur before the diaphragm ruptures, resulting in turbulent flames. In addition, the explosion wave generated numerous shock reflections during the rupture process of the diaphragm, which was gradually discharged downstream of the pipe by ejection as the pressure wave accumulated in front of the diaphragm. It should be noted that the thickness of the flexible obstacle determines the pressure accumulation in front of the membrane. Generally, the thinner the flexible obstacle, the less intense the turbulent flame is induced by the flexible obstacle, decreasing the contact area between the unignited gas downstream of the pipeline and the turbulent flame area. In conclusion, with an increase in the thickness of the flexible barrier, it exhibits a mechanism of initially suppressing and subsequently enhancing the impact on methane explosions. The increase of the thickness of the flexible obstacle motivates the flame propagation speed, which leads to an increase of turbulence intensity and explosion intensity.

Flame propagation patterns and local flame features of an annular combustor with multiple centrally staged swirling burners

In this paper, the transient ignition process of an annular combustor with 16 centrally staged swirling burners is experimentally investigated to study the mechanism of burner–burner flame propagation. The flame propagation patterns are studied by high-speed imaging. Three typical patterns of the burner–burner flame propagation are identified: the kindled-swirling pattern, entrained-swirling pattern, and sweeping pattern. The patterns are featured with different flame paths of motion. For fixed flow rates, the paths of motion are mainly determined by the overall equivalence ratio Φ. Furthermore, during the burner–burner flame propagation, the effect of the flow field on the local flame fronts is analyzed by Mie scattering and particle image velocimetry (PIV) methods. The PIV results show that the flame paths of motion are greatly influenced by the flow structure of the annular combustor. The optical diagnosis of the flame–flow interaction provides new insights into the ignition dynamics of the centrally staged annular combustor.

Numerical investigation on dynamic behavior of premixed hydrogen/air flame propagation in a closed tube

In this paper, the large eddy simulation (LES) model of OpenFOAM was used to study the flame propagation characteristics of hydrogen-air premixed flame in a closed tube with an aspect ratio of 12. The numerical simulation reproduces the characteristic flame shape evolution observed in the experiment, such as classical tulip flame, T-shaped flame, and distorted tulip deformation. The predicted flame front velocity, overpressure, and its fluctuation are all in good agreement with the measurements. The simulation demonstrates that after the classic tulip flame is formed, a series of waves cause an obvious adverse pressure gradient at the front of the flame. This adverse pressure gradient drives reverse flow which pulls the backward tip further into the combustion zone, forming an elongated tulip flame. A small amount of unburnt gas trapped in the cusp rapidly consumes to form a featured T-shaped flame. Then the T-shaped flame transforms into the second finger-shaped flame, and finally the tulip distortion occurs. The vortex and the reverse flow near the flame front generated by the interaction between the pressure field and the flow field accounts primarily for the formation of distorted tulip flame.

Effects of impinging object on geometrical flame features of horizontal jet flames

Accidental release of hydrocarbons can result in jet fires which have the potential to escalate accidental events to catastrophes. Jet fire can impinge on nearby equipment surface or a pasive firewall to make the fire plume temperature and the radiant heat flux complex. The present work experimentally investigated the flame characteristic of free horizontal jet fires and the impingement effect using propane as fuel, with different nozzle diameter, and exit velocity. To analyse the effect of flowrate to jet flame condition, the flow rates range between 30 and 600 g/min were employed with horizontal separation distance from the main source to target was at 0.8-m and 1.2-m. The results show that, the incremenet of flame features are faster for free release than impinging release. The presence of object downstream affect the flame profiles at some point. Thus, new correlations were proposed for the prediction of projected flame length and trajcetory length.

Acceleration characteristics of composite flame in the initial stage of gas/coal dust explosion and analysis of disaster intensification

To clarify the strengthening mechanism of gas/coal dust coupling explosion disasters, a 20 L spherical explosion test system, high-speed schlieren, and PIV analysis were used to study the acceleration characteristics and pressure change of composite flames in the initial stage of gas/coal dust explosion, and the flow field characteristics were also analyzed. The results showed that methane concentration had the most important influence on the formation and development of composite flame. When the methane concentration was 7.5% and 9%, the composite flame propagation showed obvious acceleration characteristics and developed spherically, the flame boundary was clear and smooth, with the whole flame being continuous and bright white. Meanwhile, the Markstein length of composite flame is positive, and increased with the increase of methane concentration and dust particle size but decreased with the increase of coal dust mass concentration, the Markstein length for 9% gas concentration was 7 times larger than 6% gas concentration, indicating the increase of gas concentration can obviously inhibit the instability of composite flame, which prolonged the residence time of coal dust particles in the composite flame, resulting in the enhancement of the pyrolysis of coal dust and improvement of explosion intensity. The PIV test results of 9% gas/coal dust explosion showed that due to the high initial explosion intensity, the negative pressure in the center of the composite flame made most of the coal dust particles undergo centripetal movement, and a large number of coal dust particles gathered around the front of the flame. At the same time, a large number of large vortices in opposite directions were formed around the front of the flame, which promoted rapid contact between the coal dust surface and oxygen and intensified the combustion reaction. The research results can provide guidance for the prevention and control of gas/coal dust composite explosion disasters.

Numerical investigation on the effect of porosity distribution on the flame characteristics

The low velocity filtration combustion of a lean methane/air mixture in the fixed bed reactors is numerically investigated using the system scale method. The effect of porosity on the flame characteristics, such as temperature profile, flame displacement speed and flame shape is examined in this study. For comparison, porosity is considered in two forms, i.e., volumetric average porosity (VAP) and porosity spatial distribution function (PSDF). The simulation results show that, local distribution of porosity primarily affects the flame shape, while having minimal impact on the peak flame temperature and mean flame displacement velocity. Quantitative analysis indicates that the flame displacement velocity obtained using VAP is closer to the experimental data compared to the PSDF. However, the simulation with PSDF explicitly captures the flame front bending caused by local porosity variation. Velocity and temperature fields suggests that flame front bending predominantly depends on the local flame displacement speed (LFDS), which is the vector sum of local interstitial airflow speed and the laminar flame speed of the combustible gas. The radial difference in LFDS, which is associated with uniformity in velocity and temperature fields, serves as the main driving force for flame front instability. The inclusion of PSDF enables a more accurate prediction of flame front development by capturing the preferential flow near the wall.

Comparative study of NH3 and H2 impacts on combustion and emission characteristics of n-Hexadecane under different flame conditions

The addition of renewable fuel to hydrocarbon fuel mitigated the carbon monoxide, carbon dioxide and soot particle emissions of internal combustion engines. A comparative study of the ammonia and hydrogen effect on combustion and emission characteristics of n-hexadecane flames was executed at various flame conditions. The results showed that alternative fuels had a stronger impact on peak temperature at diffusion flame conditions rather than at premixed flame conditions. The premixed flame released carbon monoxide emission lower than the diffusion flames with/without additional fuels. The carbon monoxide emissions decreased highly with ammonia enrichment instead of hydrogen. At premixed and coflow diffusion flame conditions, the ammonia highly reduced carbon dioxide emission compared to hydrogen. Hydrogen addition reduced the nitrogen oxide emission, whereas it was increased by ammonia. The soot particle number density was reduced highly by hydrogen addition instead of ammonia at the premixed flow flame and diffusion coflow flame conditions. The impact of hydrogen on the reduction of soot mass concentration emissions was greater than that of ammonia at all combustion conditions. It is suggested that H2 ≥ 70% should be mixed in n-hexadecane fuel because a higher burning velocity can be achieved with a higher emission reduction. A reduction in the emission of carbon-related species can be realized with a small reduction in burning velocity at NH3 < 70%.