CFD study of non-premixed swirling burners: Effect of turbulence models

This research investigates a numerical simulation of swirling turbulent non-premixed combustion.The effects on the combustion characteristics are examined with three turbulence models:namely as the Reynolds stress model,spectral turbulence analysis and Re-Normalization Group.In addition,the P-1 and discrete ordinate(DO)models are used to simulate the radiative heat transfer in this model.The governing equations associated with the required boundary conditions are solved using the numerical model.The accuracy of this model is validated with the published experimental data and the comparison elucidates that there is a reasonable agreement between the obtained values from this model and the corresponding experimental quantities.Among different models proposed in this research,the Reynolds stress model with the Probability Density Function(PDF)approach is more accurate(nearly up to 50%)than other turbulent models for a swirling flow field.Regarding the effect of radiative heat transfer model,it is observed that the discrete ordinate model is more precise than the P-1 model in anticipating the experimental behavior.This model is able to simulate the subcritical nature of the isothermal flow as well as the size and shape of the internal recirculation induced by the swirl due to combustion.

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.

Analysis of Turbulence and Surface Growth Models on the Estimation of Soot Level in Ethylene Non-Premixed Flames

Soot prediction in a combustion system has become a subject of attention, as many factors influence its accuracy. An accurate temperature prediction will likely yield better soot predictions, since the inception, growth and de- struction of the soot are affected by the temperature. This paper reported the study on the influences of turbulence closure and surface growth models on the prediction of soot levels in turbulent flames. The results demonstrated that a substantial distinction was observed in terms of temperature predictions derived using the k-c and the Rey- nolds stress models, for the two ethylene flames studied here amongst the four types of surface growth rate model investigated, the assumption of the soot surface growth rate proportional to the particle number density, but inde- pendent on the surface area of soot particles,f（As） = pNs, yields in closest agreement with the radial data. Without any adjustment to the constants in the surface growth term, other approaches where the surface growth directly proportional to the surface area and square root of surface area, f （As） = As and f （A,） = √As, result in an un- der-prediction of soot volume fraction. These results suggest that predictions of soot volume fraction are sensitive to the modelling of surface growth.

Investigation of Detailed Kinetic Scheme Performance on Modelling of Turbulent Non-Premixed Sooting Flames

This paper presents the results of an application of a first-order conditional moment closure （CMC） approach coupled with a semi-empirical soot model to investigate the effect of various detailed combustion chemistry schemes on soot formation and destruction in turbulent non-premixed flames. A two-equation soot model repre- senting soot particle nucleation, growth, coagulation and oxidation, was incorporated into the CMC model. The turbulent flow-field of both flames is described using the Favre-averaged fluid-flow equations, applying a stan- dard k-c turbulence model. A number of five reaction kinetic mechanisms having 50 - 100 species and 200 - 1000 elementary reactions called ABF, Miller-Bowman, GRI-Mech3.0, Warnatz, and Qin were employed to study the effect of combustion chemistry schemes on soot predictions. The results showed that of various kinetic schemes being studied, each yields similar accuracy in temperature prediction when compared with experimental data. With respect to soot prediction, the kinetic scheme containing benzene elementary reactions tends to result in a better prediction on soot concentrations in comparison to those contain no benzene elementary reactions. Among five kinetic mechanisms being studied, the Qin combustion scheme mechanism turned to yield the best prediction on both flame temperature and soot levels.

Development of non-premixed porous inserted regenerative thermal oxidizer

In this study, a porous inserted regenerative thermal oxidizer （PRTO） system was developed for a 125 kW industrial copper-melting furnace, due to its advantages of low NOr emissions and high radiant efficiency. Zirconium dioxide （ZrOz） ce- ramic foams were placed into the combustion zone of a regenerative thermal oxidizer （RTO）. Different performance characteris- tics of the RTO and PRTO systems, including pressure drop, temperature distribution, emissions, and energy efficiency, were evaluated to study the effects of the porous inserts on non-premixed CH4 combustion. It was found that the PRTO system achieved a significant reduction in the NOx emission level and a fuel saving of approximately 30% compared to the RTO system. It is most suitable for a lean combustion process at an equivalence ratio 〈0.4 with NOx and CO emission levels within 0.002%~）.003% and 0.001%q3.002%, respectively.

Non-premixed turbulent combustion modeling based on the filtered turbulent flamelet equation

In turbulent combustion simulations, the flow structure at the unresolved scale level needs to be reasonably modeled. Following the idea of turbulent flamelet equation for the non-premixed flame case, which was derived based on the filtered governing equations(L. Wang, Combust. Flame 175, 259(2017)), the scalar dissipation term for tabulation can be directly computed from the resolved flowing quantities, instead of solving species transport equations. Therefore, the challenging source term closure for the scalar dissipation or any assumed probability density functions can be avoided;meanwhile the chemical sources are closed by scaling relations. The general principles are discussed in the context of large eddy simulation with case validation. The new model predictions of the bluff-body flame show sufficiently improved results, compared with these from the classic progress-variable approach.

Nonlinear Dynamic Characteristics of Turbulent Non-Premixed Acoustically Perturbed Swirling Flames

The main objective of this article was to experimentally investigate the dynamic response of diffusion flame under acoustic excitation in a laboratory-scale burner.Two parametric variations of the burner,the burner inlet length and variation of the airflow rate,were studied.Experimental results were analyzed through nonlinear time series analysis and several resonance characteristics were obtained.Results indicate that the flame-acoustic resonance only appears under certain frequencies together with the fuel tube vibration.Resonance characteristics of the combustion chamber and air inlet in the non-premixed burner indicate quasi-periodic or limit cycle oscillations,respectively.Flame-acoustic resonance would trigger the frequency and amplitude mode-transition in burners.Moreover,the intermittency of flame heat release was observed under variation of inlet length and airflow rate in the burner;the 445 mm case shows more frequency peaks and fluctuations than the 245 mm one.Four typical flame forms were examined during the flame-acoustic resonance conditions,evolves from wrinkled flames to diverged flames,then evolves to reattached flames and finally to blow-off flames.This study proposed the practical application of nonlinear time-series analysis method as a detection tool for flame-acoustic resonance in laboratory non-premixed burners,which could contribute to the detection and prevention of potential thermoacoustic instabilities or resonance structure failures of industrial boilers.Finally,this study demonstrates an alternative to conventional linear tool for the characterization of nonlinear acoustic resonance in industrial boilers.

Investigation on C_(2)H_(4)-Air combustion mode in a non-premixed rotating detonation combustor

Based on the working characteristics of the rotating detonation combustor,the combustion mode of C_(2)H_(4)-Air under non-premixed injection conditions is experimentally studied in this paper.By changing the equivalence ratio,we observed the acoustic deflagration mode,fast deflagration mode,stable detonation mode,and weak detonation mode in the combustor.The velocity and pressure of the shock wave increase gradually as the equivalence ratio increases from 0.6 to 1.8.The stable detonation region appears near the stoichiometric ratio and the velocity of the detonation wave is relatively stable.When the equivalence ratio of the mixture is larger than 1.32,the stable detonation wave will suddenly extinguish,forming a weak detonation mode until the end of the combustor operation.The combustion mode of weak detonation is greatly affected by the fuel injection pressure ratio,and the release rate of energy is the main reason for the formation of deflagration mode or detonation mode.

Combustion of Renewable Biogas Fuels

Biogas fuel is a sustainable and renewable fuel produced from anaerobic digestion of organic matter. The biogas fuel is a flammable mixture of methane and carbon dioxide with low to medium calorific values. Biogas is an alternative to conventional fossil fuels and can be used for beating, transportation and power generation. CFD （computational fluid dynamic） analysis of the combustion performance and emissions of biogas fuel in gas turbine engines is presented in this study. The main objective of this study is to understand the impact of the variability in the biogas fuel compositions and lower heating values on the combustion process. Natural gas, biogas from anaerobic digester, landfill biogas, and natural gas/biogas mixture fuels combustion were investigated in this study. The CFD results show lower peak flame temperature and CO mole fractions inside the combustor and lower NOx emissions at the combustor exit for the biogas compared to natural gas fuel. The peak flame temperature decreases by 37% for the biogas landfill （COJCH4 = 0.89） and by 22% for the biogas anaerobic digester （CO2/CH4 = 0.54） compared to natural gas fuel combustion. The peak CO mole fraction inside the combustor decreases from 9.8 × 10-2 for natural gas fuel to 2.22 × 10-4 for biogas anaerobic digester and 1.32 × 10-7 for biogas landfill. The average NOx mole fraction at the combustor exit decreases from 1.13 × 10-5 for natural gas fuel to 0.40 × 10-6 for biogas anaerobic digester and 1.06 × 10-6 for biogas landfill. The presence of non-combustible constituents in the biogas reduces the temperature of the flame and consequently the NOx emissions.

Inhibition Effects of CH_(4)/CH_(3)OH on Dimethyl Ether Cool Flames

CH_(4)/DME mixtures can be used for engines and gas turbines,and have already been studied for many years.However,DME has a strong cool flame phenomenon,which will greatly influence the ignition and combustion characteristics of following hot flames.Therefore,the cool flame characteristics of CH_(4)/DME mixture are very important for their utilization.Recently,the inhibition effect of CH_(4)on DME cool flames has been discovered,but the mechanisms of the inhibition effects lack further verification and research.In this study,the inhibition effects were investigated via both experiments and simulations.In order to validate the inhibition effects,a comparison fuel of CH_(3)OH/DME was also used in this study.The extinction limits,flame temperatures and combustion products of the cool flames of the CH_(4)/DME and CH_(3)OH/DME mixtures were measured using a counterflow burner,and the reaction paths and heat release rate were derived from the HPMech-v3.3.The results indicate that CH_(4)and CH_(3)OH will both inhibit the cool flame of DME via competing with DME for OH and O radicals,and CH_(3)OH has stronger inhibition effects than CH_(4),because it is more competitive and produces more CH2O,which inhibits the oxidation of DME.The HPMech-v3.3 closely agrees with the experimental data,but still needs to be improved.

Experimental Study on the Interactions for Bluff-body and Swirl in Stabilized Flame Process

This paper focuses on investigating the interaction effects for swirl and bluff-body in stabilized flame process. Particle image velocimetry was used to measure velocity fields in three burners. First, the comparison of flames in bluff-body stabilized burners with and without swirl is presented. The results of the experiments present the variations of bluff-body stabilized flame when swirl is added into burner: the maximum reverse flow velocity and the maximum mean average radial velocity decrease; the maximum radial rootmean squared fluctuating (rms) velocity increases; the values of the axial velocity peak on the side of nozzle axis are lower, and the distance between the peak and centerline is bigger; the location of the maximum radial rms velocity moves to the outlet of annular air-flow from central recirculation zone (CRZ). Then, the comparison of flames in swirl burners with and without bluff-body is provided. The results of the experiments show the changes of swirling flame when bluff-body is added into swirl burner: the air vortex in the CRZ moves to the burner; the peak values of axial mean and rms velocity decrease; the distance between centerline and the mean axial and rms velocity peak increase; the peak of mean radial velocity decreases, and the peak of rms raidial velocity increase. The data from this experiment can also be established as benchmarks for the development and validation of combustion numerical simulations.

A numerical study of counterflow diffusion flames of methane/air at various pressures

A numerical study of the counterflow diffusion flames of methane/air at both subcritical and supercritical pressures,which have very important applications in the air-breathing rocket and advanced gas turbine engines,is conducted to obtain fundamental understanding of the flame characteristics.The analysis is based on a general mathematical formulation and accommodates a unified treatment of general fluids thermodynamics and accurate calculations of thermophysical properties.Results reveal that the maximum flame temperature occurs on the fuel-rich side for low-pressure conditions and shifts toward the stoichiometric position when the pressure increases.The maximum flame temperature increases with an increasing pressure,but decreases with an increasing strain rate.The flame width is inversely proportional to the square root of the product of the pressure and strain rate as■■1 p·a2/1.The total heat release rate varies with the pressure and strain rate in a relationship of Q release ■(p·a)0.518.An increased pressure leads to a slightly more complete combustion process near the stoichiometric position,but its effect on NO production is minor.Under the test conditions,variations of the strain rate have significant impacts on the formation of major pollutants.An increased strain rate leads to the decreased mole fraction of CO in the fuel-rich region and significantly reduced NO near the stoichiometric position.

Numerical Investigation of Effect of Porosity and Fuel Inlet Velocity on Diffusion Filtration Combustion

Methane-air diffusion filtration combustion in a radiative round-jet burner was numerically investigated in this work.The purpose of this study was focused on the effects of porous media porosity and gas velocity on temperature distribution and CO and NO_(x)emissions.Reduced chemical kinetics was used where air and methane were assumed to be at their stoichiometric ratio,while thermo-physical properties were varied per the solid matrix porosity variation.Combustion characteristics were evaluated based on conduction and radiation as the two primary heat transfer modes within the solid matrix.Numerical simulations were carried out based on a packed bed with 3 mm alumina pellets.Results show that combustion temperature increases while the temperature gradient decreases with the increase in porosity,yielding higher NO_(x),and lower CO emissions.Furthermore,the combustion temperature is the lowest and most uniformly distributed with 1 m/s and 3 m/s gas velocities,wherewith 3 m/s gas velocity,combustion occurs outside of the porous zone.The corresponding NO_(x)and CO emissions are the lowest with 1 m/s gas velocity and increase with the increase in gas velocity from 1 m/s to 10m/s.

A 3D Numerical Study of LO_2/GH_2 Supercritical Combustion in the ONERA-Mascotte Test-rig Configuration

Cryogenic propellants LOx/H2 are used at very high pressure in rocket engine combustion. The description of the combustion process in such application is very complex due essentially to the supercritical regime. Ideal gas law becomes invalid. In order to try to capture the average characteristics of this combustion process, numerical computations are performed using a model based on a one-phase multi-component approach. Such work requires fluid properties and a correct definition of the mixture behavior generally described by cubic equations of state with appropriated thermodynamic relations validated against the NIST data. In this study we consider an alternative way to get the effect of real gas by testing the volume-weighted-mixing-law with association of the component transport properties using directly the NIST library data fitting including the supercritical regime range. The numerical simulations are carried out using 3D RANS approach associated with two tested turbulence models, the standard k-Epsilon model and the realizable k-Epsilon one. The combustion model is also associated with two chemical reaction mechanisms. The first one is a one-step generic chemical reaction and the second one is a two-step chemical reaction. The obtained results like temperature profiles, recirculation zones, visible flame lengths and distributions of OH species are discussed.