Laminar Jet Flames Research Articles

The Effects of Cracking Ratio on Ammonia/Air Non-Premixed Flames under High-Pressure Conditions Using Large Eddy Simulations

Ammonia is a promising carbon-free fuel. However, one of the main challenges for ammonia combustion is the high level of NO emissions. In this study, simulations were conducted for ammonia/air laminar counterflow flames and turbulent non-premixed jet flames in the KAUST high-pressure combustion duct (HPCD) at a pressure of 5 bar, with two ammonia cracking ratios of 14% and 28%. The influence of ammonia cracking ratio on the flame structure and NO formation mechanism were examined. The laminar counterflow flame results showed that HNO is one of the most critical species related to NO formation and NO is mainly generated through the path of NH2→NH→HNO→NO. For the turbulent flames, the flamelet/progress variable (FPV) approach was employed in the context of large eddy simulations (LES) for high-fidelity simulations. The simulation results were compared with the measured data with promising agreements, which proves the accuracy of the FPV method for the present flames. It was shown that with increasing cracking ratio, not only the flame reactivity is enhanced, but also the generation of NO is increased. The correlation between NO and HNO is weaker when compared to that between NO and radicals such as O, H and OH in the entire flame. Through the distribution of NO source terms, it was found that the NO source term has a higher absolute value in the upstream region and the absolute value rapidly decreases with increasing streamwise distance. The total NO source term is positive in the fuel-lean zone and shows negative values in the fuel-rich zone.

Assessing requirements for modeling radiation in diffusion flames using an analytical, non-local model

An analytical approach for the calculation of radiative self-absorption in flames is presented, with the objective of providing an effective and computationally inexpensive tool for selecting radiation modeling methods for combustion applications. This one-dimensional non-local model approximates the flame structure as an infinitely long cylinder where all properties vary only along the radial direction. Flamelet solutions are then mapped to the radial direction and the radiative source terms, in particular the absorption, are computed analytically. The model is validated for a laminar jet flame and a turbulent pool fire, with results for the net radiative loss and the flame self-absorption showing good agreement to those of multidimensional, coupled simulations. To illustrate its applicability, the model is employed to study the characteristics of radiation in high-pressure combustion, in fire suppression and in hydrogen combustion. For the pressurized case, increased significance of radiation self-absorption is observed compared to an atmospheric flame, and accounting for the non-gray nature of radiation is found to be critical. In the fire suppression case, comparisons of the S-curves show that the choice of radiation model modifies the lower extinction limit, which becomes more important with reduced oxygen content. Lastly, for the hydrogen flame, the Planck-mean absorption coefficient is found to be more uniformly distributed in the mixture fraction space, and the optical thickness is smaller than a hydrocarbon flame at comparable conditions, indicating that radiation modeling requirements for that flame are likely less stringent. In general, the findings regarding radiation characteristics made on the basis of the non-local model are consistent with observations reported in the literature, but without the need for complex coupled combustion-radiation calculations. Therefore, the model provides a viable approach for selecting appropriate radiation models in combustion simulations.

Electric field assisted reduction of NOx emission: A numerical study

The paper reports simulation results on the influence of a direct-current driven radial electric field on the emission characteristics; especially NOx and CO of a premixed methane/air laminar jet flame. A multi-physics computational model is developed in the OpenFOAM framework to simulate electric-field-coupled premixed combustion process. The computational framework consists of coupled species, momentum and energy conservation together with a Poisson’s equation solver to resolve the electric field distribution. Electron and ion conservation equations are resolved to consider the ionic wind body force in the momentum conservation equation and the associated possible electric field distortion due to the space charge distribution. The simulations are conducted for a stochiometric and fuel rich condition and over a range of jet flow rates for a configuration representative of a test-scale experimental setup. The model predictions show that for an applied voltage of 50 kV, the flame structure changes significantly for both the stoichiometric and fuel rich conditions. The flame is stretched significantly by the electric field due to ionic wind. For the fuel rich condition, the ionic wind allows additional mixing of the fuel rich stream with the surrounding air and drastically altering the flame structure. The electric field was found to reduce the NOx emission significantly for both stoichiometric and rich conditions. Over the entire range of flowrate conditions, the stochiometric fuel-oxidizer mixture showed a decrease in maximum NOx by a factor of 1.6 in presence of electric field. For the fuel rich case, however as the flow rate is increased, the NOx reduction factor decreased from 12.0 to 1.6. For CO emissions, the presence of electric field reduces the concentration under fuel rich conditions and vice versa for the stoichiometric flame. The role of kinetics is analyzed and discussed.

Experimental Jet Flame Bioethanol Sago Pulp Fuel as an Energy Source for Isolated Areas in Papua Province, Indonesia

Papua Island is at the eastern end of Indonesia, with massive potential of natural resources. However, the human resource growth of this province is low. In 2020, the Human Development Index reached 60.62 and the poverty rate was 27.38 %. The lack of energy at isolated areas is the reason for these facts. So, there is a need of research to utilize the local potential, such as sago waste as bioethanol alternative fuel. This research consist of pre-experimental and experimental process. The pre-experimental purpose is to analyze sago pulp material using Scanning Electron Microscope (SEM), where the Carbon Value (CK) is 76 %, proximate 82.4 % carbohydrate content is very feasible in the process as a bioethanol fuel. The experimental process shows bioethanol sago pulp 70 % is very viable as a fuel, in research applications laminar jet flame bioethanol atmospheric at a fuel mass flow rate of 60 mL min-1, with a temperature of at least 327 °C up to a maximum temperature of 729 °C. The results of bioethanol diffusion laminar jet flame temperature testing are very feasible to use in household furnace combustion applications in isolated areas in the Papua region.

Pump-probe strategy for instantaneous 2D detection of CH3 in flames using a single laser

Visualization of the reaction zone of flames using CH radicals as markers is restricted by the low concentration of CH in fuel-lean conditions. To address this, methyl radicals (CH3) are employed as a substitution of CH in premixed methane/air flames. A pump-probe method was adopted with the pump laser photolyzing CH3 and the probe laser detecting the photolyzed CH (X2Π) fragments. Laser excitation scans were performed to ensure that the fluorescence detected was from CH only. Visualization of the reaction zone of flames was accomplished by a CH3 photofragmentation laser-induced fluorescence technique in fuel-lean conditions (the equivalence ratio of 0.4), where CH planar laser-induced fluorescence did not work in both laminar and turbulent jet flames. The proposed pump-probe method of CH3 can be used to visualize the reaction zone of hydrocarbon combustion under both fuel-lean and fuel-rich conditions with a superior signal-to-noise ratio.

Measurements of soot and response optimisation of laminar pool and prevaporised jet flames for various oxygenated biofuels

In this study, the generation of soot from the combustion of biodiesels and their blend with fossil diesel are investigated under laminar pool flame and prevaporised diffusion jet flame conditions. Neat biodiesels from eight feedstocks including carotene palm, palm, soy, coconut, rice bran, waste vegetable oil, duck, and goose, alongside two methyl esters of methyl laureate and methyl myristate are blended with fossil diesel at 20% volumetric intervals to form 51 different blends. The soot volume fractions for the combustion of all blends are determined using continuous-wave laser cavity extinction (CW-LCE) calibrated 2D laser-induced incandescence (2D-LII) technique. Results show that biodiesels with higher degree of saturation produce less soot, as reflected in both laminar pool and prevaroprised diffusion jet flames. The experimental data is further analysed using design of experiments (DOE) methodology. The mixture design model (MDM) following the simplex design is used to focus on the soot generating effects arising from the chemical compositions by classifying the chemical components into hydrocarbons and saturated, monounsaturated and polyunsaturated fatty acid methyl esters (FAME). The results from the Cox response trace plot associated with DOE elucidate the effects of the individual chemical group within the fuels. Both laminar flame types for the all 51 biodiesel-diesel blends exhibit the same trends with differing magnitudes. Saturated FAME has the largest singular effect on soot generation followed by hydrocarbons. However, when only neat biodiesels are considered, the laminar pool flame and laminar diffusion jet flames show differing trends for all the chemical groups. Monounsaturated FAME and saturated FAME have the greatest influence on neat biodiesel-generated soot for laminar pool flame and laminar diffusion jet flame, respectively. From the mixture design model, an optimum synthetic biodiesel mixture containing 16.1%, 34.6% and 49.3% of saturated, monounsaturated and polyunsaturated methyl esters is recommended for the lowest soot generation.

Scalar structure in turbulent non-premixed NH3/H2/N2 jet flames at elevated pressure using Raman spectroscopy

This work presents the first quantitative Raman measurements of temperature, mass fractions, and mixture fractions in two turbulent ammonia/hydrogen/nitrogen-air diffusion jet flames simulating 14% (CAJF14) and 28% (CAJF28) partial ammonia cracking ratio with Reynolds numbers of 11,200. The precision and accuracy of the system were validated in laminar counterflow and laminar jet flames to ensure data quality. The scalar structure in turbulent flames is examined using conditional mean and rms radial profiles, scatterplots in mixture fraction space, as well as statistics conditioned on mixture fraction and physical spaces. These results are compared with opposed diffusion flow 1D simulation with multi-component transport model and Le=1 transport model to visualize the effects of differential diffusion on the scalar structure at various axial positions. Finally, localized extinction was found in CAJF14 but not for the more stable CAJF28, and the probability of localized extinction was less than 2.5%. The dataset is provided as supplemental material and can be used for the validation of numerical models.

Measurements of Soot Particulate Emissions of Ammonia-Ethylene Flames Using Laser Extinction Method

Ammonia (NH3) has emerged as an attractive carbonless fuel that can be co-fired with hydrocarbon fuel to reduce carbon dioxide emissions. To understand the influence of NH3 on soot formation when co-fired with hydrocarbons, the soot formation propensity is experimentally investigated via a laminar diffusion jet flame. A stable ethylene (C2H4) jet flame doped with NH3 at different volume percentages was established for the investigation of soot formation tendency. OH* chemiluminescence imaging revealed the change of flame structure, in which the signals emitted from the heat release region weakened with increasing NH3 addition, while the peak intensity shifted from the flame wings towards flame centerline region. The laser extinction method used to measure the soot volume fraction (SVF) at different heights above the burner, which showed the effect of NH3 on soot suppression is significant, owing to the interaction between N-containing compounds with carbon atoms that result in the reduction of key intermediate products required for the formation of benzene and polycyclic aromatic hydrocarbons (PAH). The effect of soot inhibition appears to be stronger for the low NH3 blend fraction. The chemistry effect of NH3 on soot reduction for C2H4 flame is ascertained by comparing with N2-doped C2H4 flame at the same volume percentage. This work highlights the need for improved understanding of hydrocarbon fuel with NH3 to enable detailed understanding on the soot generation and oxidation process.

Experimental and Numerical Study on the Combustion Characteristics of a Laminar Non-Premixed Methane Jet Flame in Oxygen/Carbon Dioxide Coflow

The combustion characteristics of laminar non-premixed CH4 jet flame in an O2/CO2 coflows with different oxygen mole fractions were studied experimentally. The flame heights at different oxygen concentrations and fuel jet velocity were obtained. The experimental observation shows that the luminosity of the CH4 jet flame in O2/CO2 coflow is different from that of the flame in air stream. A two-dimensional numerical study of a laminar non-premixed CH4 jet flame in the O2/CO2 coflow with the O2 mole fraction of 0.35 was conducted to analyze the effects of CO2 dilution on the flame. The distribution of OH radicals in the flame was measured experimentally using planar laser-induced fluorescence (PLIF) to validate the computational method adopted in this work, and the computational and experimental results of the OH distributions showed good consistency at various fuel flow velocities. Three artificial species were created in the numerical experiment to analyze the effects of the chemical reactions, third-body collisions, and transport properties of CO2 on the height, width, and temperature distribution of the flame. The results showed that CO2 participation in chemical reactions exerts significant effects on the flame. However, the influences of the third-body effects and transport properties of CO2 on the jet flame are unremarkable. The global reaction pathways and distributions of important species in the laminar non-premixed CH4 jet flame were analyzed in detail to investigate the influence mechanisms of CO2 on the flame height and temperature. The entire flame can be divided into two oxidation parts, which separated by the boundary of the HCCO. The H, O, and OH concentrations and distributions in different parts of the flame were influenced by CO2 dilution, resulting in different flame heights and temperature distributions.

Optical diagnostics of hydrogen-air diffusion flame

Work motivation – adaptation of optical Hilbert diagnostic methods for visualization and study of optical density and phase temperature fields in the structure of an axisymmetric diffusion hydrogen-air flame. The diagnostic complex is implemented on the basis of the IAB-451 device with modified blocks of optical filtering, information source and processing. A laminar jet flame H2/N2 in still air is considered. The investigated torch is oriented vertically. Visualization of phase disturbances induced by the medium under study in a multi-wavelength probing (λ1 = 636 nm, λ2 = 537 nm and λ3 = 466 nm) light field is performed using polychromatic Hilbert and Foucault-Hilbert transformations in combination with registration and pixel-by-pixel processing of the dynamic RGB image structure. The dynamic phase structure of the diffusion flame is visualized. The initial temperature approximation, based on the assumption of an air mixture, is corrected so that the calculated hilbertogram matches the measured one as closely as possible. The data obtained are in good agreement with the results of thermocouple measurements. The temperature was recorded by thermocouples at reference points. The phase function is reconstructed in axisymmetric sections from RGB-hilbertograms. The reliability of the results is confirmed by comparing the experimentally obtained hilbertograms and hilbertograms reconstructed from phase structures using the Abel transform.

Stabilization Mechanism of Burner-attached Flames in Laminar Non-premixed Jets

ABSTRACT The stabilization mechanism of nozzle-attached flames in laminar non-premixed jets was studied experimentally and numerically. The local gas flow velocity and the flame propagation speed within the mixing layer at flame base were experimentally obtained before the blowout limit was reached. The experimental results show that, for burner-attached flames, the local flow velocity is much less than the burning speed, suggesting that the propagation of partially premixed flame within the mixing layer at flame base is stopped. The thickness of the fuel/air mixing layer at the attached flame base is quite narrow (1.1–1.5 mm) and less than the minimum quenching distance caused by conductive heat loss to the tube (or slit) wall in laminar premixed flame. Heat losses by the conduction from the flame base to the burner wall, and the convection between the flame base and the incoming flow (hereafter referred to as local convection) were studied through simulation. The significant roles of these effects in the stabilization of the attached flame were analyzed. While the heat loss by conduction to the fuel tube is neglected, the partially premixed flame at flame base can propagate along a much narrower mixing layer. On the contrary, a critical thickness of the mixing layer (in which the local partially premixed flame can sustain) exists near the flame base with considering the conductive heat loss. The conductive heat loss to the burner wall plays a relatively important role to flame stabilization without (or with a small amount of) coflow air. As the velocity of coflow air increases, the heat loss due to local convection gradually exhibits significant effects on flame stabilization. In addition, a simple theoretical model is provided based on the balance between the heat released by chemical reaction and the heat lost by conduction and local convection, suggesting clear dependence of the thickness of the fuel/air mixing layer on the local gas flow velocity and the laminar flame speed. The stability behaviors of flame base can be reasonably explained and predicted through such relation.

10 kHz 2D thermometry in turbulent reacting flows using two-color OH planar laser-induced fluorescence.

10 kHz two-color OH planar laser-induced fluorescence (PLIF) thermometry was demonstrated in both laminar Hencken flames and turbulent premixed jet flames using two injection-seeded optical parametric oscillators (OPOs) pumped by a high-speed three-legged burst-mode laser. The two burst-mode OPOs generate ∼5mJ/pulse at 282 nm and 286 nm to excite the Q1(5) and Q1(14) transitions of the A2Σ+←X2Π (1,0) system of OH, respectively. PLIF images were collected simultaneously from each of the two transitions and ratios of intensities from the two images were used to determine local temperatures. Analyses of flame curvature, temperature, and the correlation in time of these two quantities are also discussed. The results from this work are promising for the use of this technique in more complex flow environments and at, potentially, even higher repetition rate.

Studies of autoignition-assisted nonpremixed cool flames

Autoignition-assisted nonpremixed cool flames of diethyl ether (DEE) are investigated in both laminar counterflow and turbulent jet flame configurations. First, the ignition and extinction limits of laminar nonpremixed cool flames of diluted DEE are measured and simulated using detailed kinetic models. The laminar flame measurements are used to validate the kinetic models and guide the turbulent flame measurements. The results show that, below a critical mixture condition, for elevated temperature and dilute mixtures, the cool flame extinction limit and the low-temperature ignition limit merge, leading to autoignition-assisted cool flame stabilization without hysteresis. Based on the findings from the laminar flame experiments, autoignition-assisted turbulent lifted cool flames are established using a Co-flow Axisymmetric Reactor-Assisted Turbulent (CARAT) burner. The lift-off heights of the turbulent cool flames are quantified using formaldehyde planar laser-induced fluorescence. Based on an analogy with autoignition-assisted lifted hot flames, a correlation is proposed such that the autoignition-assisted cool flame lift-off height scales with the product of the flow velocity and the square of the first-stage ignition delay time. Using this scaling, we demonstrate that the kinetic mechanism that most accurately predicts the laminar flame ignition and extinction limits also best predicts the turbulent cool flame lift-off height.

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.

Transient dynamics of radiative extinction in low-momentum microgravity diffusion flames

Transient dynamics of growing diffusion flames produced by a low-momentum fuel flow emanating at the flat surface has not yet been properly addressed. The flames of interest resemble properties of laminar jet flames and the candle or droplet flames produced by a point fuel source. However, due to the considerable size of the fuel source, these flames appear to be dissimilar to either of the two cases. The practical importance of studying the stability and dynamics of these flames stems from the need to scrutinize the flammability of condensed fuels in normal and zero gravity. This paper presents comprehensive 3D simulations of the low-momentum zero- and normal gravity diffusion flames generated by the BRE (Burning Rate Emulator) burner. The scenarios of interest represent the subset of the ongoing orbital experiments in which flame development in the normal atmosphere is considered. The experimental observations have shown that these flames are prone to the local extinction followed by fluctuating instability and complete quenching. Such behavior was numerically predicted in the earlier “blind” simulations and is now replicated for the orbital test conditions. Time to flame extinction predicted in this work agrees with the experimental observations. Extinction occurs due to the excessive radiative losses when the instantaneous radiative fraction grows up to 60–70% of the current heat release rate. The excessively high level of the radiative losses is facilitated by the long residence time in the low-momentum microgravity flame. The transient dynamics of flame disintegration after the local extinction onset is analyzed. The hook-like (bibrachial) structure of flame edge is predicted after the extinction onset. When the re-ignition occurs, a classical triple (tribrachial) flame develops and propagates along the stoichiometric surface.

On the use of oscillating jet flames in a coflow to develop soot models for practical applications

Oscillating jet flames in a coflow mimic certain features of turbulent flows in a simplified and controllable way. Our study aims to identify the potential of the HMOM soot model, which is validated in steady laboratory flames to be applied in practical, mostly turbulent flames. The model is found to accurately predict all the measurement parameters in a steady laminar sooting jet flame of ethylene/nitrogen mixture. When applied to an oscillating flame with the same fuel mixture, it was found that, while velocity, temperature, and OH fields are well predicted, the peak soot volume fraction is over predicted and occurs away from the experimentally measured location. The potential for soot model improvement is analysed by correlating the transient gas-phase species, polycyclic aromatic hydrocarbon, and soot formation or destruction behaviour. A need for improved soot nucleation, condensation, and oxidation formulations is identified. It is further found that the soot number density distribution in mixture fraction space is comparable to the transitional turbulent flame regime dominated by Kelvin-Helmholtz rollers.

The role of DME addition on the evolution of soot and soot precursors in laminar ethylene jet flames

Dimethyl ether (DME) has received considerable attention as a fuel additive to reduce the emission of particulate matter (PM) due to its low-temperature chemistry, molecularly bound oxygen atom and the absence of CC bonds. However, the effect of DME addition on the evolution of soot and particularly soot precursors is not entirely understood. This study aims to shed light on this issue by blending different proportions of DME with diffusion, E60, and partially premixed, PP12, base cases of laminar ethylene flames using the Yale benchmark burner. Laser-induced fluorescence (LIF) intensity and decay time are used to characterize the structure and evolution of soot precursors, while laser-induced incandescence (LII) is utilized to determine the soot volume fraction (SVF) and the effective primary particle diameter (Dp). For the diffusion flames, the addition of 10% DME increases the concentrations of both soot and soot precursors. With the further addition of DME to 20%, the SVF decreases to levels similar to those of E60 and then decreases further with 30% DME addition. All diffusion flames with DME addition exhibit higher concentrations of soot precursors than those of the reference E60 case. For PP12, the addition of 10% DME shows similar concentrations of soot precursors and a slight reduction in the SVF which continues to decrease with further increases in DME additions to the PP12 flame. The addition of DME seems to have little effect on the soot particle diameters for all the studied flames. Overall, the PP flames result in smaller mean particle diameters than the diffusion flame counterparts.

Experimental limit on the validity of Markstein relation

A PIV-based system has been set-up for the simultaneous measurement of the local propagation velocity and the flame stretch of laminar jet premixed flames. The contributions of the flame front curvature and the strain rate of the fresh gas flow to the flame stretch are separately discussed. Laminar jet flames of methane-air lean mixtures have been studied with a secondary flame in a coflow configuration to avoid blow-off. In weakly curved flame tips (low inlet gas velocity), these measurements confirm the prediction of linear velocity-stretch dependence and allow the indirect determination of the Markstein length and planar flame propagation velocity, by extrapolating the experimental results to the zero-stretch condition. In moderately curved flame tips (large inlet gas velocity), experiments show a faster than linear increase of the flame velocity with the stretch. The transition between these two regimes occurs suddenly when a threshold value of the inlet gas velocity is exceeded.

Reduced Kinetic Schemes for Complex Reacting Flow Computations of Propane–Air Combustion

This paper describes the development of two simplified reduced kinetic models for high-temperature oxidation of propane, which can be incorporated into complex turbulent flame simulations. Equilibrium, 0D or 1D propagating premixed flames and 2D co-flowing laminar jet flames, with or without preheating, attached or lifted, are computed during the iterative optimization process. Accompanying computations with the USC-II mechanism, as well as available experimental data are exploited for validation. Comparisons demonstrate that these reduced kinetic models ensure satisfactory agreement with data over the investigated parameter space.

Experimental study of premixed hydrogen-air flame quenching in a channel with the perforated plate

The effect of the perforated plate on the premixed hydrogen-air flame quenching was experimentally investigated with various perforated plate lengths and initial pressures in a channel. The perforated plate has the same cross-sectional area and five lengths in the range of 20 mm, 40 mm, 60 mm, 80 mm and 100 mm. High-speed Schlieren photography was used to track the flame evolution, and correspondingly the flame front speed was calculated. Two piezoelectric pressure sensors were mounted upstream and downstream of the perforated plate to detect the local pressure. The results show that three kinds of flame phenomena were observed: “Pass”, “Quench” and “Near limit”. The “Pass” mode is characterized by successful flame propagation, and involving laminar flame, jet flame, and turbulent flame. Three pressure peaks can be observed on the pressure curve of the sensor in the upstream region. The first peak pressure is equal to two times the initial pressure. The flame tip speed in the downstream region decreases with the increase of the perforated plate length, and two pressure peaks were observed. In the “Quench” model, the flame does not successfully pass through the perforated plate. Furthermore, there are only two pressure peaks on the pressure curve in the upstream region and one in the downstream region. With the increase in the perforated plate length, the critical initial pressure for flame quenching also increases.