Numerical Simulation of Turbulent Diffusion Flames of a Biogas Enriched with Hydrogen

Any biogas produced by the anaerobic fermentation of organic materials has the advantage of being an environmentally friendly biofuel.Nevertheless,the relatively low calorific value of such gases makes their effective utilization in practical applications relatively difficult.The present study considers the addition of hydrogen as a potential solution to mitigate this issue.In particular,the properties of turbulent diffusion jet flames and the related pollutant emissions are investigated numerically for different operating pressures.The related numerical simulations are conducted by solving the RANS equations in the frame of the Reynolds Stress Model in combination with the flamelet approach.Radiation effects are also taken into account and the combustion kinetics are described via the GRI-Mech 3.0 reaction model.The considered hydrogen fuel enrichment spans the range from 0%to 50%in terms of volume.Pressure varies between 1 and 10 atm.The results show that both hydrogen addition and pressure increase lead to an improvement in terms of mixing quality and have a significant effect on flame temperature and height.They also reduce CO_(2) emissions but increase NOx production.Prompt NO is shown to be the predominant NO formation mechanism.

IMPLEMENTATION OF THE CONDITIONAL MOMENT CLOSURE MODEL TO A NONPREMIXED METHANOLAIR FLAME STABILIZED ON A BLUFF-BODY

A turbulent nonpremixed flame of methanol air stabilized on a bluff body is simulated by the conditional moment closure(CMC) model. An elliptic, conservative formulation of the CMC is employed with the turbulent diffusion term modeled by the turbulent eddy viscosity. Results of flow, conditional temperature and species concentration profiles with respect to mixture fraction, and unconditional temperature, OH and NO fields are obtained. Reasonable agreement between experiments and predictions to show that the CMC model is capable to predicting the species concentration and temperature in turbulent nonpremixed combustion characterized by a finite reaction rate. Discrepancy on the rich side may partly be due to limitation of the singly conditional moment closure, substitution of the unconditional mean variables for the conditional ones, or lack of adequate chemical kinetics on the rich side of methanol air reaction.

Experimental Study of Electrodes Parameters Effects on Small Diffusion Combustion Flame

To study the configuration and conductivity effects on micro-scale methane-air flames by electric field and iron wind,different electric field forces and iron winds are generated by needle,circle and plate electrodes respectively in different electrodes heights under both AC and DC fields though experiments. Experimental results showed that the flame characteristics are affected by needle electrodes mainly through the action of ion wind,by plate type electrodes mainly through the action of electric field force and by annular electrodes through both the electric field force and ion wind at the same time. Under DC field 's effects of all electrodes types,the flame will consequently go down while the voltage reached to a limit value,and it will breakdown under the strong effect of the ion wind by needle electrodes. The results also showed the influence by different electrodes types to the current characteristics,resistance properties and configuration of themicro-scale flames.

Effect of Thermal Conductivity of Tube-Wall on Blow-Off Limit of a Micro-Jet Methane Diffusion Flame

The operating range of the flow rate or flow velocity for the micro-jet flame is quite wide,which can be used as the heat source.In order to optimize the micro-jet tube combustor in terms of the solid material,the present paper numerically investigates the impact of thermal conductivity(λs)on the operating limit of micro-jet flame.Unexpectedly,the non-monotonic blow-off limits with the increase ofλs is found,and the corresponding generation mechanisms are analyzed in terms of the thermal coupling effect,flow field,and strain effect.At first,the lower preheating temperature of the fuel and larger heat loss amount to the environment lead to a larger blow-off limit at a largerλs.After that,the smaller local flow velocity in the vicinity of flame root and smaller strain effect slightly increase the blow-off limit with the continuously increasingλs.Therefore,it is deduced that the applied performance of micro-jet combustor with a smaller thermal conductivity is better in terms of the blow-off limit.

Experimental study on effects of gas flow rate on soot characteristics in diffusion flames coupled with plasma

This study examined the evolution of morphology and nanostructure of soot particles from the plasma-flame interaction for various gas flow rates.The current study used both optical diagnostic and sampling methods to explore the soot production and combustion characteristics.Soot particles were characterized at the same positions downstream from the flame zone by thermophoretic sampling and transmission electron microscopy.Moreover,X-ray diffraction analysis,and thermogravimetric analysis were performed to study the nanostructure and oxidation reactivity of soot.A reduction in soot concentration was found with the plasma addition,which illustrated an inhibition effect of plasma on soot emission.The increased gas flow rate promoted soot concentration since a growing number of carbons participated in the combustion process.Depending on the gas flow rate(carbon content)variation and plasma activation,either liquid-like soot material with irregularly shaped protrusions or chain-like structure,or a mixture of both,were generated from the diffusion flames.The soot produced by plasma-flame interaction also demonstrated a high correlation between nanostructure and reactivity.The soot from lower carbon content with plasma activation had a shorter fringe length and larger fringe tortuosity related to higher oxidation reactivity.On the contrary,soot from the highest carbon content without plasma-flame interaction exhibited prevalent fullerene-like nanostructures with evident large or small shells and also had a higher carbonization degree resulting in lower oxidation reactivity.

Sensitivity analysis of modeling parameters to soot and PAHs prediction in ethylene inverse diffusion flame

The soot formation model based on inverse ethylene diffusion flames was performed to study the sensitivity of the soot formation process to the prediction results.The effects of efficiency parameters such as soot inception,surface growth and coagulation on the simulation results were studied by using the adjustable efficiency model.In addition,the reversible soot model and conjugate heat transfer(CHT)model were also introduced to explore their advantages.Results indicated that,among adjustable efficiency parameters,the nucleation efficiency had the greatest influence on the predicted soot and PAHs distributions,while the Habstraction-C2H2-addition(HACA)process and PAH adsorption surface growth efficiencies impacted little.The adjustable efficiency parameters had a significant effect on the concentration of soot gaseous precursors and soot particles,but their effects on temperature,gas phase molecules,and intermediate species were not obvious.When the nucleation efficiency increased from 2×10^(-6)to 1×10^(-4),the predicted value of the integrated soot was increased by nearly 50%,and the maximum primary particle number density and the number of aggregates were increased by an order of magnitude.The maximum concentration of BAPYR was doubled.However,the peak temperature along the axial direction increased by only 3.5 K.Using the reversible soot model,the approximation results of the adjustable efficiency parameters could be modified,which showed the feasibility of the model.The use of the CHT model promoted pyrolysis of the fuel below the outlet of the fuel tube,with high-temperature zones,soot zones,and PAHs zones moving towards higher flame heights.Besides,when using the reversible model and the CHT model,the maximum soot volume fraction decreased by 39%compared with the basic efficiency parameters,while the concentration of BAPYR increased by 162%,and the concentrations of gas phase species were decreased.

Effects of dimethyl ether and ethanol additions on soot transition in ethylene counterflow diffusion flames

This paper investigates the effect of blending dimethyl ether(DME)and ethanol on the soot transition periods in ethylene counterflow diffusion flames by using a novel optical diagnostic method.The soot critical transition point in different conditions is identified experimentally and numerically.Two kinds of flames are carried out to gain the soot critical transition point in counterflow diffusion flames by changing oxygen fraction(Xo)and changing volume flow rates of fuel and oxidizer(Qv).The red-green-blue(RGB)ratio method is used to precisely identify the soot critical transition point,and chemical kinetic simulations are performed to analyze the detailed reaction paths.The results show that compared to the ethylene flame,the soot critical transition point occurs at a higher Xoand a lower Qvwhen DME or ethanol is blended.The addition of DME and ethanol can inhibit soot formation,due to the degree of soot formation reaction being lower than the degree of the oxidation reaction in the blending flames.

Experimental Study on Soot Suppression of Acetylene Diffusion Flame by Acoustic-Excited Oscillation in Rijke-Type Burner

This work reports that the flame oscillation induced by acoustic excitation can effectively suppress soot generation in Rijke-type burners.When the acoustic frequency is close to the natural frequency of the burner system,it can produce resonance resulting in intense oscillation of the flame.The relationship between the soot suppression efficiency and the acoustic field of standing wave at different flame positions is discussed.Compared with that under self-excited oscillation,when there is external forced acoustic force introduced to the flame,oscillation combustion occurred in a lager zone in the glass tube.The fundamental cause of different soot suppression efficiency at different positions is that the standing wave acoustic field causes the particles to move at different speeds in different positions of the glass tube.The axial particle velocity difference results in the formation of acoustic vortexes and the change of the flame shape.The high particle velocity causes the air in the glass tube to turn into the turbulent condition and make the flame temperature rise.Simulation results show that the surface growth rate of soot is reduced,while the oxidation rate of soot is enhanced,which result in the soot suppression under acoustic oscillation.This study can provide some reference for the practical application of oscillate combustion in soot suppression.

Mathematical Model of Combustion in Blunt Annular Ceramic Burner

The computer simulation of the combustion process in blast furnace(BF) stove has been studied by using the k-ε-g turbulent diffusion flame model.The combustion process in blunt annular ceramic burner was calculated by using the software.The profiles of gas and air velocity,temperature of the combustion products,concentration of the components,and the shape and length of the flame during combustion have been researched.Compared with the original annular ceramic burner,the new design of the blunt one improves the mixing of the gas and the air significantly,and shortened the length of the flame.

The effect of ammonia on soot formation in ethylene diffusion flames

This paper originally investigates the effect of NH_(3) dilution on soot formation when NH_(3) is gradually added into the fuel stream in an ethylene laminar diffusion flame stabilized on a Santoro burner.The variations of flame diameter and two flame heights,i.e.,mixture-strength flame height and visible flame height are carefully documented and analyzed.Moreover,local soot volume fraction(SVF)and soot temperature fields are simultaneously measured by compact-modulated absorption and emission technique,and the corresponding measurement random errors are also provided by the error propagation calculations for the first time.All the reported measurement random errors of SVF and soot temperature fields are estimated within the range of±0.07–±0.08 ppm and±40–±91 K,respectively.As an original database,the concomitantly measured SVF and soot temperature distributions are provided as high-fidelity datasets for refining soot formation model that is overrode by NH_(3).In addition,the flame cross-section average SVF F_(soot)(z)is calculated for every NH_(3) diluted flame,and the relative contributions of NH_(3) dilution and chemical effect are quantitatively assessed in terms of F_(max)-X_(NH_(3))plotting.It is found that when X_(NH_(3))<30%,the chemical effect of ammonia is about twice that of the dilution effect.While X_(NH_(3))>30%,the chemical effect and dilution effect of ammonia are gradually equal.Eventually,through modeling of the soot formation rate V in the flames,the relative contributions of chemical effect,dilution effect and thermal effect of NH_(3)are further novelty discriminated within the X_(NH_(3))from 0 to 46%and it is shown that NH_(3) chemical effect plays the dominate role in soot suppression,then the dilution effect and the thermal one at the least.

Combining flamelet-generated manifold and machine learning models in simulation of a non-premixed diffusion flame

Flamelet Generated Manifold(FGM)is an example of a chemistry tabulation or a flamelet method that is under attention because of its accuracy and speed in predicting combustion characteristics.However,the main problem in applying the model is a large amount of memory required.One way to solve this problem is to apply machine learning(ML)to replace the stored tabulated data.Four different machine learning methods,including two Artificial Neural Networks(ANNs),a Random Forest(RF),and a Gradient Boosted Trees(GBT),are trained,validated,and compared in terms of various performance measures.The progress variable source term and transport properties are replaced with the ML models.Particular attention was paid to the progress variable source term due to its high gradient and wide range of its value in the control variables space.Data preprocessing is shown to play an essential role in improving the performance of the models.Two ensemble models,namely RF and GBT,exhibit high training efficiency and acceptable accuracy.On the other hand,the ANN models have lower training errors and take longer to train.The four models are then combined with a one-dimensional combustion code to simulate a counterflow non-premixed diffusion flame in engine-relevant conditions.The predictions of the ML-FGM models are compared with detailed chemical simulations and the original FGM model for key combustion properties and representative species profiles.

Three-dimensional temperature reconstruction of diffusion flame from the light-field convolution imaging by the focused plenoptic camera

The plenoptic imaging technique provides a promising approach to the non-invasive three-dimensional measurement, especially for the high-temperature combustion diagnosis. We establish a light-field convolution imaging model for diffusion flame in this work, considering the radiation transfer process inside the diffusion flame and the light transfer process inside the focused plenoptic camera together. The radiation transfer process is described by the radiation transfer equation and solved by the generalized source multi-flux method. Wave optics theory is adopted to describe the light transfer process, combining Fresnel diffraction and the phase conversion of the lens. The flame light-field image is obtained by the light-field convolution imaging model and adopted as the measurement signal to reconstruct three-dimensional temperature field. The inverse problem of temperature reconstruction is solved by the least square QR decomposition method. The simulative temperature reconstruction work is conducted, including the inverse analysis, the uncertainty analysis, and the measurement noise influence. All the results show that the proposed measurement method is available to reconstruct three-dimensional temperature with satisfactory accuracy and acceptable uncertainty. Both symmetric and asymmetric distributed temperature fields are investigated, and the reconstructed results prove the validity and universality of the measurement method.

Comparative Study on Soot Reduction, Soot Nanostructure and Oxidation Reactivity of n-heptane/DMC and Isooctane/DMC Inverse Diffusion Flames

Dimethyl carbonate(DMC)is an environmentally oxygenated compound which can be used efficiently for soot reduction.This paper compared the soot reduction,soot nanostructure and oxidation reactivity from inverse diffusion flames(IDFs)of the hydrocarbon fuels,namely n-heptane and isooctane doped with DMC.Effects of DMC additions on soot reduction were discussed.DMC addition is more effective for the soot reduction of n-heptane/DMC IDF than isooctane/DMC IDF.The morphology and nanostructures of soot particles were investigated by Transmission Electron Microscopy(TEM)and High Resolution TEM(HRTEM),and the soot graphitization and oxidation reactivity were analyzed by X-ray Diflfraction(XRD)and Thermogravimetric Analyzer(TGA),respectively.The results of HRTEM images showed that many larger aggregates were observed for the structures of soot particles from IDFs with DMC additions.The soot particles exhibited more liquid-like material,more amorphous,higher disorganized layers,and less graphitic than that of IDFs without DMC additions.With increasing of DMC blending rate,soot particles changed younger to have shorter fringe length,higher tortuosity,and greater fringe separation.Based on the XRD and TGA results,the degree of the soot graphitization level decreased;the soot mass lost significantly faster,and the soot become more reactive.

Transient process of methane-oxygen diffusion flame-street establishment in a microchannel

“Flame-street”is an interesting diffusion flame behavior in which a series of flame-segments is separately distributed along the mixing layer in a narrow channel.This experimental phenomenon was experimentally and numerically investigated with the focus on the steady-state,thermo-chemical flame structures in previous literature.In the present paper,the dynamic formation process of a methane-oxygen diffusion flame-street structure was simulated with a reacting flow solver developed based on the open-source framework OpenFOAM.By imposing a certain amount of ignition-energy near the channel outlet,a reaction-kernel was formed and bifurcated.Subsequently,three separate flames were consecutively generated from this kernel and propagated within the channel.The whole process was completed within 15 ms and all the discrete flames were eventually in a steady-state.Interestingly,different propagation features were observed for the three flame segments:The leading flame experienced a flame shape/type change from a tribrachial structure in its fastpropagating phase to a long,trailing diffusion tail after being anchored to the inlet.The successive flame had a much lower propagation speed,keeping its two wing-like(fuel-lean premixed and fuel-rich premixed)structure while moving toward its stabilization location,which was approximately in the middle of the channel.The last flame,after the ignition source was turned-off,was immediately convected a bit downstream,and eventually featured a similar two-branch-like structure as the second one.Moreover,chemical insights for the premixed and diffusion branches of the leading flame were also provided with the change of significance of some key elementary reactions focused on,in order to attain a detailed profiling of the flame-type transition.This paper is a first-ever one discussing the transient formation of flame-streets in literature and is believed to be useful for obtaining a comprehensive understanding of this unique flame characteristics from a dynamic point of view.

Numerical Simulation Model of Laminar Hydrogen/Air Diffusion Flame

A numerical simulation model is developed for a laminar hydrogen/air diffusion flame. Nineteen species and twenty chemical reactions are considered. The chemical kinetics package (CHEMKIN) subroutines are employed to calculate species thermodynamic properties and chemical reaction rate constants. The flow field is calculated by simultaneously solving a continuity equation, an axial momentum equation and an energy equation in a cylindrical coordinate system. Thermal diffusion and Brownian diffusion are considered in the radial direction while they are neglected in the axial direction. The results suggest that the main flame is buoyancy-controlled.