Showing papers on "Diffusion flame published in 2011"

Dynamics of lean blowout of a swirl-stabilized flame in a gas turbine model combustor

Abstract: Lean blowout (LBO) of a partially premixed swirl flame is studied using chemiluminescence imaging and simultaneous stereo-PIV and OH-PLIF measurements at repetition rates up to 5 kHz. The flame, which is operated with methane and air in a gas turbine model combustor at atmospheric pressure, features a pronounced precessing vortex core (PVC) at the inner shear layer. In the first part of the study, the stabilization mechanism of the flame close to LBO is investigated. The fields of velocity and OH show that near LBO there are essentially two regions where reaction takes place, namely the helical zone along the PVC and the flame root around the lower stagnation point. The zone along the PVC is favorable to the flame due to low strain rates in the vortex center and accelerated mixing of burned and fresh gas. The flame root, which is located close to the nozzle exit, is characterized by an opposed flow of hot burned gas and relatively fuel-rich fresh gas. Due to the presence of high strain rates, the flame root is inherently unstable near LBO, featuring frequent extinction and reignition. The blowout process, discussed in the second part of the study, starts when the extinction of the flame root persists over a critical length of time. Subsequently, the reaction in the helical zone can no longer be sustained and the flame finally blows out. The results highlight the crucial role of the flame root, and suggest that well-aimed modifications of flow field or mixture fraction in this region might shift the LBO limit to leaner conditions.

On the critical flame radius and minimum ignition energy for spherical flame initiation

Abstract: Flame initiation from an ignition kernel is studied theoretically and numerically for different fuel (hydrogen, methane, and propane)/oxygen/helium/argon mixtures by using a spherical flame. The objectives are to find the controlling length scale for flame initiation in a quiescent mixture and to reveal its relationship with the minimum ignition energy. The results show that, different from the flame thickness or flame ball radius, there is a critical flame radius that controls flame initiation. The minimum ignition energy for successful flame initiation is found to be proportional to the volume defined by the critical flame radius. Furthermore, the preferential diffusion between heat and mass (Lewis number effect) is found to play an important role in affecting the critical flame radius and flame initiation. It is shown that the critical flame radius and the minimum ignition energy increase significantly with the increase of Lewis number. Therefore, for large molecular hydrocarbon fuels with low mass diffusivity, significantly larger ignition energy is needed to initiate successfully a self-sustained propagating premixed flame.

LES flamelet modeling of a three-stream MILD combustor: Analysis of flame sensitivity to scalar inflow conditions

Abstract: Large-eddy simulations (LES) of a jet issuing into a hot and diluted coflow are performed. To model this three-stream burner configuration, which is operated in the moderate and intense low-oxygen dilution (MILD) combustion regime, a flamelet/progress variable (FPV) formulation is extended by introducing an additional conserved scalar. This additional scalar is associated with the oxidizer split, and is used to identify flamelets of different mixture composition. Due to the extended spatial structure of this jet diffusion flame, the moderate Reynolds number, and the overall lean operating condition, different flamelets interact only weakly in mixture composition space, so that the thermochemical state-space is populated from the solution of the one-dimensional flamelet equations. To account for the turbulence/chemistry interaction on numerically unresolved scales, a presumed probability density function (PDF) is used in the LES combustion model. This three-stream FPV combustion model is applied in LES of the MILD combustor, which was experimentally investigated by Dally et al. (2002) [4] . The comparison with results obtained from the single-mixture fraction FPV formulation shows that the coflow mixture composition can only inadequately be represented by a single mixture fraction, resulting in a significant overprediction of the flame temperature and CO mass fraction. The second part of this work addresses the sensitivity of the flow field and flame structure to the specification of scalar inflow conditions under kinetics-controlled, low-Damkohler number combustion conditions. To this end, LES calculations are performed that employ an increasing level of fidelity in the specification of the scalar boundary conditions, including homogeneous and intermittent turbulent scalar inflow conditions that are derived from experimental data. From this analysis, it is shown that the consideration of turbulent fluctuations in the scalar composition leads to improved predictions for temperature and mass fractions of CO and OH. Furthermore, the results from this simulation also suggest that effects of scalar inflow conditions are not only confined to the nozzle-near region but extend throughout the entire flame. It is anticipated that these findings could also be of relevance to other simulations of kinetics-controlled and low-temperature combustion systems, including autoignition, lifted flames, and premixed systems in which flames are stabilized by vitiated and hot coflows.

Combustion characteristics of alternative gaseous fuels

Abstract: Fundamental flame properties of mixtures of air with hydrogen, carbon monoxide, and C1–C4 saturated hydrocarbons were studied both experimentally and numerically. The fuel mixtures were chosen in order to simulate alternative gaseous fuels and to gain insight into potential kinetic couplings during the oxidation of fuel mixtures. The studies included the use of the counterflow configuration for the determination of laminar flame speeds, as well as extinction and ignition limits of premixed flames. The experiments were modeled using the USC Mech II kinetic model. It was determined that when hydrocarbons are added to hydrogen flames as additives, flame ignition, propagation, and extinction are affected in a counterintuitive manner. More specifically, it was found that by substituting methane by propane or n-butane in hydrogen flames, the reactivity of the mixture is reduced both under pre-ignition and vigorous burning conditions. This behavior stems from the fact that propane and n-butane produce higher amounts of methyl radicals that can readily recombine with atomic hydrogen and reduce thus the rate of the H + O2 → O + OH branching reaction. The kinetic model predicts closely the experimental data for flame propagation and extinction for various fuel mixtures and pressures, and for various amounts of carbon dioxide in the fuel blend. On the other hand, it underpredicts, in general, the ignition temperatures.

A DNS study on the stabilization mechanism of a turbulent lifted ethylene jet flame in highly-heated coflow

Abstract: Direct numerical simulation (DNS) of the near-field of a three-dimensional spatially-developing turbulent ethylene jet flame in highly-heated coflow is performed with a reduced mechanism to determine the stabilization mechanism. The DNS was performed at a jet Reynolds number of 10,000 with over 1.29 billion grid points. The results show that auto-ignition in a fuel-lean mixture at the flame base is the main source of stabilization of the lifted jet flame. The Damko¨ hler number and chemical explosive mode (CEM) analysis also verify that auto-ignition occurs at the flame base. In addition to auto-ignition, Lagrangian tracking of the flame base reveals the passage of large-scale flow structures and their correlation with the fluctuations of the flame base similar to a previous study (Yoo et al., J. Fluid Mech. 640 (2009) 53–481) with hydrogen/air jet flames. It is also observed that the present lifted flame base exhibits a cyclic ‘saw-tooth’ shaped movement marked by rapid movement upstream and slower movement downstream. This is a consequence of the lifted flame being stabilized by a balance between consecutive auto-ignition events in hot fuel-lean mixtures and convection induced by the high-speed jet and coflow velocities. This is confirmed by Lagrangian tracking of key variables including the flame-normal velocity, displacement speed, scalar dissipation rate, and mixture fraction at the stabilization point.

Direct numerical simulations and analysis of three-dimensional n-heptane spray flames in a model swirl combustor

Abstract: Three-dimensional n -heptane spray flames in a swirl combustor are investigated by means of direct numerical simulation (DNS) to provide insight into realistic spray evaporation and combustion as well as relevant modeling issues. The variable-density, low-Mach number Navier–Stokes equations are solved using a fully conservative and kinetic energy conserving finite difference scheme in cylindrical coordinates. Dispersed droplets are tracked in a Lagrangian framework. Droplet evaporation is described by an equilibrium model. Gas combustion is represented using an adaptive one-step irreversible reaction. Two different cases are studied: a lean case that resembles a lean direct injection combustion, and a rich case that represents the primary combustion region of a rich-burn/quick-quench/lean-burn combustor. The results suggest that premixed combustion contribute more than 70% to the total heat release rate, although diffusion flame have volumetrically a higher contribution. The conditional mean scalar dissipation rate is shown to be strongly influenced, especially in the rich case. The conditional mean evaporation rate increases almost linearly with mixture fraction in the lean case, but shows a more complex behavior in the rich case. The probability density functions (PDF) of mixture fraction in spray combustion are shown to be quite complex. To model this behavior, the formulation of the PDF in a transformed mixture fraction space is proposed and demonstrated to predict the DNS data reasonably well.

Laminar flame speeds of C5 to C8 n-alkanes at elevated pressures: Experimental determination, fuel similarity, and stretch sensitivity

Abstract: Experimental data of high fidelity on the laminar flame speeds and Markstein lengths of C5–C8 n-alkane mixtures with air at elevated pressures were determined from the propagation velocities of spark-ignited, expanding flames in a newly-designed heated, high- and constant-pressure chamber, using nonlinear extrapolation. Results show that the laminar flame speeds of these fuels are basically similar, hence extending previous observations of the fuel similarity to the high-pressure range of 10–20 atm. A companion analysis of the computed flame structure reveals comparable similarity for the thermal properties as well as the key intermediates and reactions, thereby supporting the observed global flame speed similarity. The study further shows that the influence of stretch diminishes with increasing pressure because of the concomitant reduction of the flame thickness, implying not only reduced error in the determination of laminar flame speeds from stretched flames at elevated pressures, but also substantial simplification in the modeling of turbulent flames because of the diminished importance of stretch.

Turbulent flame speed for syngas at gas turbine relevant conditions

Abstract: Modifications of conventional natural-gas-fired burners for operation with syngas fuels using lean premixed combustion is challenging due to the different physicochemical properties of the two fuels. A key differentiating parameter is the turbulent flame velocity, S T , commonly expressed as its ratio to the laminar flame speed, S L . This paper reports an experimental investigation of premixed syngas combustion at gas turbine like conditions, with emphasis on the determination of S T / S L derived as global fuel consumption per unit time. Experiments at pressures up to 2.0 MPa, inlet temperatures and velocities up to 773 K and 150 m/s, respectively, and turbulence intensity to laminar flame speed ratios, u ′/ S L , exceeding 100 are presented for the first time. Comparisons between different syngas mixtures and methane clearly show much higher S T / S L for the former fuel. It is shown that S T / S L is strongly dependent on preferential diffusive-thermal (PDT) effects, co-acting with hydrodynamic effects, even for very high u ′/ S L . S T / S L increases with rising hydrogen content in the fuel mixture and with increasing pressure. A correlation for S T / S L valid for all investigated fuel mixtures, including methane, is proposed in terms of turbulence properties (turbulence intensity and integral length scale), combustion properties (laminar flame speed and laminar flame thickness) and operating conditions (pressure and inlet temperature). The correlation captures effects of preferential diffusive-thermal and hydrodynamic instabilities.

Effects of non-equilibrium plasma discharge on counterflow diffusion flame extinction

Abstract: A non-equilibrium plasma assisted combustion system was developed by integrating a counterflow burner with a nano-second pulser to study the effects of atomic oxygen production on the extinction limits of methane diffusion flames at low pressure conditions. The production of atomic oxygen from the repetitive nano-second plasma discharge was measured by using two-photon absorption laser-induced fluorescence (TALIF). The results showed that both the atomic oxygen concentration production and the oxidizer stream temperature increased with the increase of the pulse repetition frequency for a constant plasma voltage. The experimental results revealed that the plasma activated oxidizer significantly magnified the reactivity of diffusion flames and resulted in an increase of extinction strain rates through the coupling between thermal and kinetic effects. Numerical computations showed that atomic oxygen quenching strongly depends on the oxidizer stream temperature. The kinetic effect of atomic oxygen production by a non-equilibrium plasma discharge on the enhancement of flame extinction limits was demonstrated, for the first time, at high repetition frequencies with elevated oxidizer temperatures. The reaction paths for radical production and consumption were analyzed. It was concluded that in order to achieve significant kinetic enhancement from atomic oxygen production on flame stabilization, the plasma discharge temperature needs to be above the critical crossover temperature which defines the transition point from radical termination to chain-branching.

Laminar flame speeds, non-premixed stagnation ignition, and reduced mechanisms in the oxidation of iso-octane

Abstract: Experimental data on the laminar flame speeds of iso-octane/air mixtures at atmospheric and elevated pressures were acquired using the counterflow flame and the outwardly expanding flame, while the non-premixed ignition temperatures were determined for an iso-octane pool in the stagnation flow of a heated air jet at atmospheric and slightly reduced/elevated pressures. These experimental measurements were compared with calculations based on the mechanisms of Curran et al. and Chaos et al., with the former mechanism systematically and substantially reduced, using directed relation graph and computational singular perturbation, to facilitate the calculation. It was found that the Curran mechanism yielded substantially higher laminar flames speeds as compared to the present experimental results while results from the Chaos mechanism agree well with the present measurements. These trends are in agreement with previous results on the laminar flame speeds for n-heptane. Both mechanisms yield acceptable comparison with the observed non-premixed stagnation ignition temperature.

An Experimental and Kinetic Modeling Study of Methyl Decanoate Combustion

Abstract: Biodiesel is typically a mixture of long chain fatty acid methyl esters for use in compression ignition engines. Improving biofuel engine performance requires understanding its fundamental combustion properties and the pathways of combustion. This research study presents new combustion data for methyl decanoate in an opposed-flow diffusion flame. An improved detailed chemical kinetic model for methyl decanoate combustion is developed, which serves as the basis for deriving a skeletal mechanism via the direct relation graph method. The novel skeletal mechanism consists of 648 species and 2998 reactions. The skeletal mechanism reproduces the behavior of the fully detailed mechanism in plug flow and stirred reactors for temperatures of 900–1800 K, equivalence ratios of 0.25–2.0, and pressures of 101 and 1013 kPa. This mechanism well predicts the methyl decanoate opposed-flow diffusion flame data. The results from the flame simulations indicate that methyl decanoate is consumed via abstraction of hydrogen atoms to produce fuel radicals, which lead to the production of alkenes. The ester moiety in methyl decanoate leads to the formation of low molecular weight oxygenated compounds such as carbon monoxide, formaldehyde, and ketene.

Modeling Diesel engine combustion using pressure dependent Flamelet Generated Manifolds

Abstract: Flamelet Generated Manifolds (FGMs) are constructed and applied to simulations of a conventional compression ignition engine cycle. To study the influence of pressure and temperature variations on the ignition process after the compression stroke, FGMs with several pressure levels are created. These pressure levels, and the corresponding average temperatures are obtained from experimental curves. Auto-ignition is taken into account by tabulating igniting laminar diffusion flames, with n -heptane as a surrogate for diesel. The chemistry is parameterized as a function of the mixture fraction and a reaction progress variable, and pressure variations are accounted for by using the pressure as an extra degree of freedom. General steady combustion phase characteristics are compared to experimental observations. They show a correct phenomenological spray flame picture with a flame lift-off, fuel rich inner region and a diffusion flame at the periphery. The resolution effect of pressure discretization in the database to ignition prediction is studied by performing simulations with 1, 3, 5 and 7 pressure levels in the FGM. The results show that not more than 5 pressure levels are needed to represent the chemistry evolution during an engine cycle. All tested reaction mechanisms for n -heptane give a short burn duration, especially the reduced ones. As a result, at the moment of ignition the pressure rise rate is over-predicted.

Visualization of blow-off events in bluff-body stabilized turbulent premixed flames

Abstract: Confined and unconfined turbulent methane-air lean premixed flames stabilized on an axisymmetric bluff-body of diameter d = 25 mm have been examined close to the blow-off limit and during the extinction transient, with OH∗ chemiluminescence, flame tomography and OH-PLIF operated at 5 kHz, allowing a quantification of the duration of the blow-off event. Blow-off was approached by increasing the bulk velocity U b or decreasing the equivalence ratio and the flame shape changed from conical to cylindrical with decreasing length. Close to blow-off, the flame closed on the axis and was about 2 d long, and just before the blow-off condition it took an “M” shape with reaction fronts inside the recirculation zone (RZ). During the blow-off event, fresh reactants entered the RZ from the forward stagnation region and significant fragmentation of the flame occurred, with branches of the flame remaining anchored on the bluff-body edge and separate flame pockets moving randomly inside the RZ. Overall blow-off occurred with the gradual elimination of these flame fragments. The integrated OH∗ emission decreased slowly to zero as the flame surface decreased over a period of about 15 d / U b . The results suggest that the blow-off event in recirculating flames lasts long compared to the residence time in the RZ and the structure of the flame close to extinction supports the underlying assumptions behind well-stirred reactor concepts of blow-off.

Experimental research on combustion dynamics of medium-scale fire whirl

Abstract: The medium-scale fire whirl was extensively investigated by experimental means, in order to establish correlations of the burning rate, flame height and flame temperature of fire whirl, and to clarify the difference between fire whirls and general pool fires. Experimental observations and data confirmed that a free burning fire whirl is a highly stable burning phenomenon with large quasi-steady periods. Burning rates of fire whirls depend on pool diameter similarly to those of general pool fires; however the transition turbulent burning occurs sooner as the pool diameter increases. The lip height seems to have little effect on the burning rate of fire whirls. The correlation H ∗ = K · ( Q ˙ ∗ · Γ ∗ 2 ) m was proposed to couple the height of fire whirl to the fire release rate and ambient circulation. It correlates the data from both this work and the literature. Radial temperature profiles in the continuous region of the fire whirl were confirmed to be hump-type, implying the existence of fuel-rich inner core. The pool diameter and heat release rate do not significantly affect the radial temperature profiles in non-dimensional radial coordinates. It was found that the fire plume of fire whirl involves three distinct zones just like that of pool fire, but with different normalized ranges. Fire whirls maintain a higher ratio of continuous flame height to the overall flame height, and also higher maximum centerline excess temperature in continuous flame region, as compared to general pool fires. It was further demonstrated that the fire whirl plume at its origin behaves like a turbulent jet with moderate swirling, and then tends to become buoyancy dominated downstream, with slight swirling. With an increase in dimensionless height adjusted by the plume origin, the plume centerline excess temperature decays rapidly and approaches the theoretical value of −5/3 for free buoyancy plume.

Combustion characteristics of n-heptane at high altitudes

Abstract: Presented in this paper is part of an experimental series conducted at different altitudes to investigate the influence of ambient pressure on buoyancy driven diffusion combustion of a liquid fuel. A set of n-heptane pool fire experiments were conducted at two geographic locations on the Tibetan plateau with altitudes greater than 3600 m. In addition to the measurements of fuel mass loss, flame temperature and irradiance, transmittance through smoke and flame height were also measured. The experimental results confirm the findings of previous studies at relatively lower altitudes and show that the burning rate per unit area, radiation heat flux and average flame axis temperature decrease when the altitude is increased. Direct evidence was also obtained to show that the radial average extinction coefficient at a given height above the fuel surface is inversely proportional to the altitude. A preliminary discussion is made on the mechanism of pressure influence on buoyancy driven fire behavior.

Kinetic effects of aromatic molecular structures on diffusion flame extinction

Abstract: Kinetic effects of aromatic molecular structures for jet fuel surrogates on the extinction of diffusion flames have been investigated experimentally and numerically in the counterflow configuration for toluene, n -propylbenzene, 1,2,4-trimethylbenzene, and 1,3,5-trimethylbenzene. Quantitative measurement of OH concentration for aromatic fuels was conducted by directly measuring the quenching rate from the emission lifetimes of OH planar laser induced fluorescence (LIF). The kinetic models for toluene and 1,2,4-trimethylbenzene were validated against the measurements of extinction strain rates and LIF measurements. A semi-detailed n -propylbenzene kinetic model was developed and tested. The experimental results showed that the extinction limits are ranked from highest to lowest as n -propylbenzene, toluene, 1,2,4-trimethylbenzene, and 1,3,5-trimethylbenzene. The present models for toluene and n -propylbenzene agree reasonably well with the measurements, whereas the model for 1,2,4-trimethylbenzene under-estimates extinction limits. Kinetic pathways of OH production and consumption were analyzed to investigate the impact of fuel fragmentation on OH formation. It was found that, for fuels with different molecular structures, the fuel decomposition pathways and their propagation into the formation of radical pool play an important role to determine the extinction limits of diffusion flames. Furthermore, OH concentrations were found to be representative of the entire radical pool concentration, the balance between chain branching and propagation/termination reactions and the balance between heat production from the reaction zone and heat losses to the fuel and oxidizer sides. Finally, a proposed “OH index,” was defined to demonstrate a linear correlation between extinction strain rate and OH index and fuel mole fraction, suggesting that the diffusion flame extinctions for the tested aromatic fuels can be determined by the capability of a fuel to establish a radical pool in a manner largely governed by molecular structure.