Non-premixed Jet Flames Research Articles

Lift-off characteristics of non-premixed jet flames in laminar/turbulent transition

The stabilization mechanism of a laminar lifted flame has been explained theoretically. However, variation of the stabilization mechanism of the lifted flame in the transitional regime between laminar and turbulent flames has not been clarified. In this study, lift-off heights were estimated in relation to variation of the fuel tube diameter, fuel jet velocity, and proportions of methane and propane. With increase of the fuel jet velocity, stable lifted flames could be obtained under various conditions and the soot emission reduced. The stabilization mechanisms were explained based on the concepts of laminar and turbulent mixing cores. These were classified into five regimes: laminar similarity, laminar near-core, external mixing, transitional, and fully turbulent. Improved relationships between the flow rate and the lift-off height were proposed in two regimes (external mixing regime and the fully turbulent one). In addition, the blowout mechanism was explained using the turbulent intensity. Finally, an overall stabilization diagram showing the various stabilization mechanisms of the lifted flames was presented.

Experimental study on pressure and flow characteristics of self-ignition hydrogen flowing into the unconfined space

Accidental release of high-pressure hydrogen can result in self-ignition, non-premixed jet flame and high overpressure, which will create potential security risks to people, buildings and equipment. In this study, pressure dynamics and flame induced by the self-combustible hydrogen flowing into the unconfined space were experimentally studied. The entire process was characterized by pressure sensors and cameras. Results show that the velocity of the hydrogen jet increases first and then decreases after it flows out of the tube. Its overpressure decays rapidly and stabilizes quickly. The dramatic changes for the flow field parameters in the near-field region can cause the self-ignition region to extinguish first and then reignite. And the overpressure in the near-field region, caused by the self-ignition jet flowing out of the tube, is lower in the unconfined space than that in the semi-confined space. In addition, axial and radial variations for the jet flame are characterized by phased development, which can be divided into three stages based on features of flame morphology and temperature distribution. Among them, a typical tadpole-like flame is formed. Its head with large size shows asymmetry, develops downward deflection and eventually separates from the flame body which are all affected by asymmetric large-scale vortices. Besides, the re-ignition of the jet can be induced during the nitrogen purge process, which is much more dangerous in real accident scenarios.

Implementation of the Scalar Dissipation Rate in the REDIM Concept and its Validation for the Piloted Non-Premixed Turbulent Jet Flames

In order to address the impact of the concentration gradients on the chemistry – turbulence interaction in turbulent flames, the REDIM reduced chemistry is constructed incorporating the scalar dissipation rate, which is a key quantity describing the turbulent mixing process. This is achieved by providing a variable gradient estimate in the REDIM evolution equation. In such case, the REDIM reduced chemistry is tabulated as a function of the reduced coordinates and the scalar dissipation rate as an additional progress variable. The constructed REDIM is based on a detailed transport model including the differential diffusion, and is validated for a piloted non-premixed turbulent jet flames (Sandia Flame D and E). The results show that the newly generated REDIM can reproduce the thermo-kinetic quantities very well, and the differential molecular diffusion effect can also be well captured.

Conditional moment closure method with tabulated chemistry in adiabatic turbulent nonpremixed jet flames

The conditional moment closure (CMC) method with tabulated chemistry is proposed to reduce the computational load for integration of stiff reaction steps in the conventional CMC methods. It is based on the assumption that temperature and all species except CO and CO2 adjust fast enough so that they can be uniquely determined by the single reaction progress variable given as the sum of CO and CO2 mass fractions. The CMC equations are solved for the conditional reaction progress variable, Qpv, whereas the other species mass fractions and the reaction rates including that for Qpv are retrieved from lookup tables. A steady homogeneous form of the conditional variance and covariance equations are solved to construct the second-order closure table to be consistent with Steady Laminar Flamelet Model (SLFM)-based first-order closure. Good agreement is shown between the results by integrated and tabulated chemistry in both the first-order and the second-order CMC for Sandia Flames D, E and F. The overall computational time is reduced to about 0.5% of that by the conventional CMC method involving integration of the detailed reaction mechanism for all species and temperature.

Stabilization mechanisms of an ammonia/methane non-premixed jet flame up to liftoff

Ammonia is a promising alternative fuel for CO2 emission mitigation. The use of ammonia blends allows for more flexibility compared to pure ammonia fuel and is often considered for immediate CO2 emission reduction in existing facilities running on natural gas. However, fundamental studies on these fuel mixtures remain scarce. This study thus focuses on ammonia/methane blend fuels. The effect of ammonia on methane jet flame stabilization is investigated using a non-premixed jet flame configuration to observe the flame stabilization mechanisms, the flame-burner interactions and how ammonia addition affects the attached flame stabilization up to liftoff. The flame tip position was observed using CH* chemiluminescence. Heat transfer to the burner was monitored by temperature measurement at the burner lip and inside the burner to observe the impact of ammonia addition on thermal interactions. The main stabilization regimes described for the methane non-premixed jet flame are still observed in the case of ammonia addition. However, the transition between those regimes appeared to be shifted toward larger velocities relative to the methane case due to ammonia addition. Those changes could be related to the change in the mixture combustion properties which affects both flame position, heat transfer to the burner and in turn the transition between the identified stabilization regimes. The dynamic leading to liftoff was further analyzed to highlight how ammonia addition perturbated the stabilization balance up to liftoff.

Effects of pressure on soot production in piloted turbulent non-premixed jet flames

Laser-induced incandescence (LII) was used to quantify the soot volume fraction in five piloted turbulent non-premixed C2H4-N2 jet flames at elevated pressures. In one series of flames, the bulk jet velocity was held constant as the pressure was increased from 1 bar (Re = 10,000) to 3 bar (Re = 30,000), and then 5 bar (Re = 50,000). In the other series, a Reynolds number of 10,000 was held constant at 1 bar, 3 bar, and 5 bar. LII measurements were calibrated with laminar diffusion flames at each pressure using 2D diffuse line-of-sight attenuation. Mean and RMS soot volume fractions along the flame centerline exhibit Gaussian profiles for all of the flames. The magnitude of soot volume fraction and the spatial soot distribution are enhanced by increases in pressure. In both series of flames, the peak mean soot volume fraction scales with the pressure as p2.2. In the constant velocity series, the scaling drops to p1.5 if the volume-integrated mean soot volume fraction on a per fuel mass basis is considered instead. At 3 bar, a threefold increase in Re leads to a decrease in the mean soot volume fraction despite the decrease in soot intermittency. At 5 bar, a fivefold increase in Re results in soot intermittency near zero at the flame’s center and a slight increase in the mean soot volume fraction.

Soot formation in turbulent flames of ethylene/hydrogen/ammonia

This paper presents an experimental study of turbulent non-premixed jet flames of ethylene/nitrogen where the nitrogen is substituted with different proportions of hydrogen and/or ammonia. The focus is largely on the effects of hydrogen and ammonia on soot production in turbulent flames. A combination of pointwise, laser-induced fluorescence in the visible and UV bands (LIF-UV–visible), and laser-induced incandescence (LII) is used to measure soot precursors and soot along the flame centerline. All signals are collected at a repetition rate of 10 Hz with fast photomultiplier tubes to resolve the time decay. In a separate experiment, joint imaging of LIF-OH CH is also performed at a repetition rate of 10 kHz. Hydrogen substitution is found to increase the production of soot, whereas ammonia substitution inhibits soot formation. The peak mean soot volume fraction is almost a factor of 3 lower in the 25% ammonia case in comparison to the 25% nitrogen case. The mean signal decay time constant decreases with ammonia substitution, implying the formation of smaller soot nanoparticles. The mean signal decay time constant remains unaffected with hydrogen substitution. Measured peaks in LIF-CH and LIF-OH are reduced with ammonia substitution but only in regions upstream of where soot is formed. Further downstream in the sooting region, neither OH nor CH appear to be affected by the substitution of N 2 with H 2 or NH 3 .

On the combined effect of internal and external intermittency in turbulent non-premixed jet flames

This paper analyzes the combined effect of internal intermittency and external intermittency on the dynamics of small-scale turbulent mixing in a turbulent non-premixed jet flame. The phenomenon of external intermittency in turbulent jet flames originates from a very thin layer, known as turbulent/non-turbulent interface, that separates the inner turbulent core from the outer irrotational surrounding fluid. The impact of external intermittency on turbulence is evaluated across the jet flame by the self-similarity scaling of higher-order structure functions of the mixture fraction. It is shown that the scaling of structure functions exhibits a growing departure from the prediction of classical scaling laws toward the edge of the flame. Empirical evidence is provided that this departure is primarily due to external intermittency and the associated break down of self-similarity. External intermittency creates local gradients that are significantly more intense compared to those created by internal intermittency alone. As chemical reactions usually take place in thin layers in the vicinity of the turbulent/non-turbulent interface, an accurate statistical description of these intense events is necessary to predict the turbulence-chemistry interaction. The study is based on data from a highly-resolved direct numerical simulation of a temporally evolving planar jet flame.

Dynamics of laminar ethylene lifted flame with ozone addition

The effect of ozone (O3) addition on ethylene (C2H4) non-premixed jet flames is investigated. Stable C2H4 lifted flames are established with oxygen/nitrogen co-flow at reduced oxygen content conditions. It is observed in the experiments that after O3 addition, the flame liftoff height could either increase or decrease, depending on the initial value of the flame liftoff height before O3 is added. Formaldehyde (CH2O) planar laser-induced fluorescence (PLIF) measurement shows that prompt ozonolysis reaction between C2H4 and O3 produces large amounts of CH2O upstream of the flame. In contrast to previous studies of O3 addition on lifted flames—with saturated hydrocarbon fuels in which O3 decomposition dominates—the ozonolysis reaction between C2H4 and O3 changes the chemical composition of flow even at room temperature. Such chemical reaction causes the simultaneous increase of both the triple flame propagation speed of lifted flame and the axial jet velocity along the stoichiometric contour, which also therefore changes the dynamic balance between these two values to stabilize the flame. While the increase of triple flame propagation speed tends to decrease the flame liftoff height, the increase of axial jet velocity along the stoichiometric contour tends to have the opposite effect. A competing kinetic-dynamic process forms, and the final location of the flame depends on the degree of ozonolysis reaction, which is determined by the initial flame liftoff height before O3 is added.

A-Priori Validation of Scalar Dissipation Rate Models for Turbulent Non-Premixed Flames

The modelling of scalar dissipation rate in conditional methods for large-eddy simulations is investigated based on a priori direct numerical simulation analysis using a dataset representing an igniting non-premixed planar jet flame. The main objective is to provide a comprehensive assessment of models typically used for large-eddy simulations of non-premixed turbulent flames with the Conditional Moment Closure combustion model. The linear relaxation model gives a good estimate of the Favre-filtered scalar dissipation rate throughout the ignition with a value of the related constant close to the one deduced from theoretical arguments. Such value of the constant is one order of magnitude higher than typical values used in Reynolds-averaged approaches. The amplitude mapping closure model provides a satisfactory estimate of the conditionally filtered scalar dissipation rate even in flows characterised by shear driven turbulence and strong density variation.

Study of the Combined Effect of Ammonia Addition and Air Coflow Velocity on a Non-premixed Methane Jet Flame Stabilization

ABSTRACT Ammonia is a promising carbon-free fuel that can be used for CO2 reduction. However, ammonia industrial use presents challenges, including flame stabilization. In this study, a non-premixed methane-ammonia jet flame in an air coflow was considered to observe the effects of ammonia addition on conventional fuels flame stabilization. First, the effects of the introduction of ammonia on the stabilization regimes of the methane jet flame were studied. Then the coupled effects of the variation in ammonia concentration and air coflow velocity on flame stabilization were investigated. In the present jet configuration, a sharp reduction of the stabilization domain was observed with ammonia addition: the liftoff and re-attachment velocities were obtained for mixtures of up to 50% of ammonia in the fuel, a ratio above which the flame could not be stabilized. When increasing the coflow velocity, a sudden drop in the re-attachment velocity occurred for methane/ammonia flames. This re-attachment drop was associated with an increase in the height of the lifted flame when the jet velocity decreases before re-attachment, for large enough coflow velocity and ammonia concentration. A critical height above which the lifted flames all present the same ascending behavior could be defined and characterizes this peculiar phenomenon.

Combustion characteristics of non-premixed CH4/CO2 jet flames in coflow air at normal and elevated temperatures

Experimental study on the combustion characteristics of CH4/CO2 non-premixed jet flames in a coflow air at normal and elevated temperatures was conducted for the development of biogas-fueled regenerative burner. The CH4/N2 non-premixed jet flame was also investigated for comparison. The laminar flame height, lift-off height and blow-out limit were obtained and compared. The experimental results show that the laminar flame height of CH4/CO2 flame is lower than that of CH4/N2 flame in the same condition. The reason was clarified by theoretical analysis and two-dimensional numerical computation. For the turbulent flame, the CO2 dilution in the fuel steam leads to a higher liftoff height and a lower blowout limit. The fuel velocity range of stable lifted flame and the blowout limit are significantly increased at elevated air temperature. The premixed flame model was employed to analyze the liftoff heights and blowout velocities. The non-dimensional liftoff height increases linearly versus the non-dimensional fuel jet velocity. And linear relationship between non-dimensional blowout velocity and Reynolds number was obtained. The influences of laminar flame speed, fuel kinematic viscosity and the density ratio of fuel to coflow air on liftoff height and blowout limit were investigated by using the premixed flame model and sensitivity analysis.

Soot emission reduction in oxygenated co-flow jet flames

A computational study was performed for ethylene/air non-premixed laminar co-flow jet flames using an axisymmetric CFD code to explore the effect of oxygenation on PAH and soot emissions. Oxygenated flames were established using N2 diluted fuel stream along with O2 enriched air stream such that the stoichiometric mixture fraction (Ζst) is varied but the adiabatic flame temperature is not materially changed. Simulations were carried out using a spatially and temporally accurate algorithm with detailed chemistry and transport. A detailed kinetic model involving 111 species and 784 reactions and a fairly detailed soot model were incorporated into the code. Two different approaches, one with constant flame height and other with constant inlet velocity are comprehensively examined to bring out the effects of changes in flame structure and residence time on soot emissions with respect to Zst. With increase in Ζst, a drastic reduction in the formation of soot precursors (acetylene and benzene) and thus in soot emissions are observed. In the present study, oxygenated flames with Ζst ≥ 0.424 are considered as blue flames or completely soot free. For various oxygenated flames a C/O ratio between 0.45 and 0.6 is found to be most favorable for soot formation.

The effect of fuel composition and Reynolds number on soot formation processes in turbulent non-premixed toluene jet flames

The soot formation processes in three different turbulent prevaporized non-premixed toluene jet flames stabilized on a jet-in-hot-coflow (JHC) burner were investigated in this study. The jet Reynolds number and the stoichiometric mixture fraction were varied in order to manipulate the flow time scales and the chemistry, respectively. Time-resolved laser-induced incandescence (TiRe-LII), non-linear two-line atomic fluorescence of indium (nTLAF), and OH planar laser induced fluorescence (PLIF) were simultaneously applied to yield spatially resolved and instantaneous fields of soot volume fraction, primary particle size, temperature, and OH. The mean distributions of the detected quantities are used to identify major differences among the flames. The highest soot loading is observed for the low Reynolds number and low stoichiometric mixture fraction flame. However, this flame features also the lowest temperature and primary particle size. Based on these observations, the simultaneously detected data sets and flamelet computations are employed to elucidate differences in the soot formation pathways in the flames. The analyses reveal that the high soot loading causes greater heat losses in the low Reynolds number and low stoichiometric mixture fraction flame. This has a significant impact on the soot formation pathways and causes a reduction in the particle size.

Physical models of flame height and air entrainment of two adjacent buoyant turbulent jet non-premixed flames with different heat release rates

This paper investigated the flame height and air entrainment of two adjacent buoyant turbulent jet non-premixed flames with the same base dimension whilst having different heat release rates (HRRs). Propane was used as fuel. The HRR of one of the burners was fixed at 8, 16, 24, 32 and 40 kW, respectively. By increasing the HRR of the other burner, the interaction behaviors of two jet flames were studied under various burner spacings. A total of 15 categories of HRRs combinations of two jet flames were conducted in this work. Two flames heights are different to lead the heat pressure on both sides of the flame to be non-axisymmetric. And the interaction law, air entrainment mechanism etc. become more complicated than two same HRRs flames. Results showed that with increasing burner spacing, the flame merging behaviors can be divided into three stages. The flame height firstly deceases to a minimum value, then slightly rises again and finally maintains stable due to the weak interaction of non-merging flames. Two effective weighting factors are supposed to quantize the maximum and minimum flame heights to derive a flame height model which then is validated using the experimental data in literatures. The merging point height increases with increasing HRR and burner spacing. Based on the air entrainment mechanism of two interaction flames, a general method for calculating the global volume air entrainment is presented to build a physical entrainment model. Above flame height and physical entrainment models are suitable for the calculation with two flames combinations with the same and different HRRs, which greatly extends the application scope of interaction fires. In addition, two creative methods of calculating flame height and global air entrainment volume have been provided a reference for the research on the combustion behaviors of multiple flames with different HRRs combination.

A new modeling approach for mixture fraction statistics based on dissipation elements

The probabilty density function (PDF) of the mixture fraction is of integral importance to a large number of combustion models. Here, a novel modelling approach for the PDF of the mixture fraction is proposed which employs dissipation elements. While being restricted to the commonly used mean and variance of the mixture fraction, this model approach individually considers contributions of the laminar regions as well as the turbulent core and the turbulent/non-turbulent interface region. The later region poses a highly intermittent part of the flow which is of high relevance to the non-premixed combustion of pure hydrocarbon fuels. The model assumptions are justified by means of the gradient trajectory based analysis of high fidelity direct numerical simulation (DNS) datasets of two turbulent inert configurations and a turbulent non-premixed jet flame. The new dissipation element based model is validated against the DNS datasets and a comparison with the beta PDF is presented.

Single-shot imaging of major species and OH mole fractions and temperature in non-premixed H2/N2 flames at elevated pressure

Simultaneous, single-shot imaging of all major species (N2, O2, H2, and H2O), OH, temperature, and mixture fraction is demonstrated for the first time in H2N2 non-premixed jet flames at 12 bar. The spatial distribution of mole fraction is obtained for the four major species by recording images of Raman scattering on four separate back-illuminated CCD cameras. The available field-of-view is 25(H) × 8(V) mm2. Temperature and mixture fraction are derived from Raman scattering images. Images of OH fluorescence intensity are recorded using OH-PLIF and are converted into OH mole fraction by calculating the Boltzmann population distribution and the quenching rate accurately for each pixel. Precision and accuracy are assessed by comparing 2-D Raman/OH-PLIF measurements in laminar flames to 1-D flame computations accounting for differential diffusion. Single-shot precision on N2, O2, H2, and temperature is better than 5%. It is better than 6% and 10% for H2O and OH, respectively. Accuracy lies within these values, except for H2O with 10%. Such good performance of Raman imaging is attributed to (a) the use of non-intensified CCD cameras, (b) wavelet adaptive thresholding (WATR) image denoising, (c) rejection of flame luminosity by a Pockels cell electro-optical shutter with 500 ns gating, and (d) elevated pressure that boosts the Raman signal intensity. Measurements in a turbulent flame (3.6N2:H2 by vol. and Re = 29,000) show that most of the flame's thermochemical structure is accurately captured by unity Lewis number computations, suggesting that effects of differential diffusion are less important above some Reynolds number. This is consistent with expectations and it engenders confidence in the Raman imaging technique. Because images of both mixture fraction and OH mole fraction are available, it is also possible to reconstruct a more accurate scalar dissipation rate by projection onto the 2-D flame front normal.

In-Situ Adaptive Manifolds: Enabling computationally efficient simulations of complex turbulent reacting flows

Reduced-order manifold approaches to turbulent combustion modeling traditionally involve precomputation of manifold solutions and pretabulation of the thermochemical database versus a small number of manifold variables. However, additional manifold variables are required as the complexity of turbulent combustion processes increases through consideration of, for example, multi-modal, non-adiabatic, or non-isobaric combustion, or combustion featuring multiple and/or inhomogeneous inlets. This increase in the number of manifold variables comes with an increase in the computational cost of precomputing a greater number of manifold solutions, most of which are never actually utilized in a CFD calculation. The memory required to store the pretabulated high-dimensional thermochemical database also increases, practically limiting the complexity of manifold-based combustion models. In this work, a new In-Situ Adaptive Manifolds (ISAM) approach is developed that overcomes this limitation by combining ‘on-the-fly’ calculation of manifold solutions with In-Situ Adaptive Tabulation (ISAT), enabling the use of more complex manifold-based turbulent combustion models. The performance of ISAM is evaluated via LES of turbulent nonpremixed jet flames with both hydrogen and hydrocarbon fuels. A performance assessment indicates that the computational overhead associated with ISAM compared to pretabulation ranges from negligible up to a factor of two, with most of this overhead associated with convolution of the thermochemical state against a presumed subfilter PDF. In addition, the memory requirements of ISAM are more than two orders of magnitude less than conventional tabulation. These results demonstrate the potential for ISAM to accommodate significantly more complex manifold-based combustion models.

Hierarchical model form uncertainty quantification for turbulent combustion modeling

All models invoke assumptions that result in model errors. The objective of model form uncertainty quantification is to translate these assumptions into mathematical statements of uncertainty. If each assumption in a model can be isolated, then the uncertainties associated with each assumption can be independently assessed. In situations where a series of assumptions leads to a hierarchy of models with nested assumptions, physical principles from a higher-fidelity model can be used to directly estimate a physics-based uncertainty in a lower-fidelity model. Turbulent nonpremixed combustion models fall into a natural hierarchy from the full governing equations to Conditional Moment Closure to flamelet-like models to thermodynamic equilibrium, and, at each stage of the hierarchy, a single assumption can be isolated. In this work, estimates are developed for the uncertainties associated with each assumption in the hierarchy. The general method identifies a trigger parameter that can be obtained with information only from the lower-fidelity model that is used to estimate the error in the lower-fidelity model using only physical principles from the higher-fidelity model. Starting from the lowest fidelity model, the trigger parameters identified in this work are the product of a chemical time scale and the scalar dissipation rate for the equilibrium chemistry model, which characterizes the errors associated with neglecting finite-rate chemistry and transport; the reciprocal of the product of a generalized Lagrangian flow time and the scalar dissipation rate for the steady flamelet model, which characterizes the errors associated with neglecting flow history effects; and the relative magnitude of the conditional fluctuations for Conditional Moment Closure. The approach is applied in LES to a turbulent nonpremixed simple jet flame to quantify the errors associated with the equilibrium chemistry model and the steady flamelet model. The results indicate that errors associated with neglecting finite-rate chemistry and transport in the equilibrium chemistry model are dominant upstream while errors associated with neglecting flow history in the steady flamelet model become more important downstream.

Fully coupled sectional modelling of soot particle dynamics in a turbulent diffusion flame

Soot particle dynamics, including particle size distributions (PSDs) and related statistics, are of increasing practical significance due to evolving regulatory demands. The combination of a mass and number density preserving sectional model with a transported joint probability density function (JPDF) method ensures a full coupling of the joint scalar space, e.g. soot and gas phase reactions and radiative heat losses, within a method that can represent ignition/extinction phenomena as well as the slow (low Damköhler number) soot inception and oxidation chemistry in turbulent flames. This approach is here applied to the sooting non-premixed Sandia ethylene jet flame via a 78-dimensional joint-scalar space, including enthalpy, gas phase species and 62 soot sections. Soot nucleation is treated as a global step from acetylene to pyrene with the rate fitted using comparisons with full detailed chemistry. Soot surface growth is treated via a PAH analogy and soot oxidation is considered via O, OH and O2 using a Hertz–Knudsen approach. Comparisons with measured temperature, gas phase species and the mean soot volume fraction show good agreement while the introduction of zero soot diffusivity leads to substantially improved predictions of the RMS of the soot volume fraction. The calculated PSDs at the burner centreline show a transition from one to two-peaks along the axial direction with the mode of the second peak increasing from 14 to 32 nm. Scatter plots, joint statistics of soot parameters and temperature, and the chemical source terms across soot sections suggest that surface growth is dominant when PSDs are unimodal and that the competition of oxidation, coagulation/aggregation and surface growth leads to a PSD shape transition. It is also shown that local extinction events lead to the presence of soot in cool fuel lean mixtures.