Application of WSGSA Model in Predicting Temperature and Soot in C_(2)H_(4)/Air Turbulent Diffusion Flame

Soot,a product of insufficient combustion,is usually in the form of aggregate. The multi-scattering of soot fractal aggregates has been proved to play an important role in studying the soot radiative properties,which is rarely considered in predicting the radiative heat transfer in combustion flame. In the present study,based on the weighted sum of gray soot fractal aggregate(WSGSA) model,which is used to predict the temperature field and soot aggregates in turbulent diffusion flame,the flame temperature distribution and soot volume fraction distribution under the conditions of the model without considering radiation,the default radiation model in Fluent software and the WSGSA model are calculated respectively. The results show that the flame temperature will be seriously overestimated without considering radiation and the maximum relative discrepancy of flame centerline temperature is about 64.5%. The accuracy will be improved by the default radiation model in the Fluent software,but the flame temperature is still overestimated and the maximum relative discrepancy of flame centerline temperature is about 42.1%. However,more satisfactory results can be obtained by the WSGSA model,and the maximum relative discrepancy of flame centerline temperature is no more than 15.3%. Similar conclusions can also be obtained in studying the temperature distribution along different flame heights. Moreover,the soot volume fraction can be predicted more accurately with the application of the WSGSA model. Both without considering radiation and using the default radiation model in the Fluent software will result in the underestimating of soot volume fraction. All the results reveal that the WSGSA model can be used to predict the temperature and soot aggregates in the CH/air turbulent diffusion flame.

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.

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.

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.

Modeling of the turbulent burning velocity for planar and Bunsen flames over a wide range of conditions

We develop and assess a model of the turbulent burning velocity ST over a wide range of conditions.The aim is to obtain an explicit ST model for turbulent combustion modeling and flame analysis.The model consists of sub models of the stretch factor and the turbulent flame area.The stretch factor characterizes the flame response of turbulence stretch and incorporates detailed chemistry and transport effects with a lookup table of laminar counterflow flames.The flame area model captures the area growth based on Lagrangian statistics of propagating surfaces and considers the effects of turbulence length scales and fuel characteristics.The present model predicts sT via an algebraic expression without free parameters.We assess the model using 490 cases of the direct numerical simulation or experiment reported from various research groups on planar and Bunsen flames over a wide range of conditions,covering fuels from hydrogen to n-dodecane,pressures from 1 to 30 atm,lean and rich mixtures,turbulence intensity ratios from 0.1 to 177.6,and turbulence length ratios from 0.5 to 66.7.Despite the scattering sT data in the literature,the comprehensive comparison shows that the proposed ST model has an overall good agreement over the wide range of conditions,with the averaged modeling error of 28.1%.

RANS Simulation of Methane Diffusion Flame： Comparison of Two Chemical Kinetics Mechanisms

Turbulent non-premixed combustion of gaseous fuels is of importance for many technical applications, especially for the steel and refractory industry. Accurate turbulent flow and temperature fields are of major importance in order to predict details on the concentration fields. The performances of the GRI-Mech 3.0 and the Jones and Lindstedt mechanisms are compared. Detailed chemistry is included with the GRI-Mech 3.0 and J-L kinetic mechanisms in combination with the laminar flamelet combustion model. The combustion system selected for this comparison is a confined non-premixed methane flame surrounded by co-flowing air The simulation results are compared with experimental data of Lewis and Smoot （2001）.

NUMERICAL SIMULATION ON COMPRESSIBLE TURBULENCE BY SPECTRAL METHOD

The numerical and physical issues of simulations on compressible turbulence are reviewed in the present paper. An outline of the global spectral methods and the progress of recent local spectral methods are illustrated. Several typical subjects in this field are studied, including homogeneous isotropic turbulence, autoignition in premixed turbulence, interaction between flames and turbulence, and shock wave in turbulence. The results of the numerical simulations are discussed, enabling us to discover and to understand the physical phenomena which have not been solved by experiments.

Temporally resolving premixed turbulent flame structures using self-supervised adversarial reconstruction of CH-PLIF

Understanding the turbulence-flame interaction is crucial to model the low-emission combustors developed for energy and propulsion applications. To this end, a novel frame interpolation (FI) method is proposed to better resolve the spatiotemporal evolution of premixed turbulent flame structures. The framework is completely selfsupervised, agnostic to optical flow, and driven by leveraging transferrable feature knowledge at lower speeds and adversarial learning to statistically map the flame dynamics across frames. The method is successfully applied on a 10 kHz CH planar laser-induced fluorescence (PLIF) dataset of highly wrinkled premixed flames with turbulent Reynolds numbers (ReT ) of 1100, 1400, and 7900, by down-sampling the image sequence to 5 kHz and restoring the sequence back to 10 kHz via FI. All reconstructions recovered important flame events and displayed excellent resemblance of the corrugated CH-layer geometries to that of the ground truths, with average intersection over union (IoU) and structural similarity index (SSIM) scores of 0.49 and 0.82, which are above the high-similarity baselines of 0.36 and 0.75, respectively. The wrinkling parameters (WP) of the flames also matched the ground truths, wherein R2 was roughly 0.95 for ReT = 1100 and 1400 and 0.85 for ReT = 7900 (lower due to the turbulence-induced uncertainties). The FI is further iteratively repeated to 40 kHz on the ReT = 7900 flames to facilitate pocket analysis by confidently linking their origin of formation, thus, enabling distinction from 3D tunnels, and improving statistical characterization of their consumption speeds. Given that the object features do not exhibit highly turbulent motions with regard to the initial time step, the proposed FI method is shown to be highly accurate and useful to analyzing finite-resolution experimental image sets including, but not restricted to, CH-PLIF.

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.

Review of Lagrangian stochastic models for turbulent combustion

Predictive simulation of the combustion process in engine is crucial to understand the complex underlying physicochemical processes, improve engine performance, and reduce pollutant emissions. Key issues such as the physical modeling of the interaction between turbulence, chemistry and droplets, and the incorporation of the detailed chemistry in high-fidelity simulations of complex flows remain essential though challenging. This paper reviews the transported probability density function method for turbulent dilute spray flames in the dual-Lagrangian framework that shows potential to address some critical modeling issues. An overview is presented for the contributions made within the last decade or so for the three key ingredients for modeling the interaction between turbulence, chemistry and droplets, i.e., micro-mixing, subgrid dispersion and two-phase coupling. Then, various methods for detailed chemistry acceleration are reviewed to address the issue of high computational cost for its use in multidimensional simulations. Finally, some applications of the dual-Lagrangian method in both laboratory-scale and device-scale configurations are provided to demonstrate its capability as well as deficiency at the current stage. Some open modeling challenges are raised and recommended for further investigation.

Synthesis of TiO_2 nanoparticles by propane/air turbulent flame CVD process

Synthesis of TiO2 nanoparticles by the oxidation of titanium tetrachloride （TiCI4） in high-strength propane/air turbulent flame is investigated tentatively for mass production of TiO2 nanoparticles. Effects of reactor heat flux varying from 247 to 627 kJ/m^2 s, initial TiO2 number density from 2 × 10^20 to 1 × 10^21 m^ 3, and apparent residence time of TiO2 nanoparticles in reactor from 0.06 to 0.9 s, on particle morphology, phase composition, UV absorption and photoluminescence （PL） spectra are studied. The TiO2 nanoparticles synthesized, with mean size of 30-80 nm and futile mass fraction from 0.155 up to 0.575, exhibited a strong PL signal at the wavelength of 370-450 nm, with a wide peak signal at 400-420 nm, reflecting significant oxygen vacancies on the surface of the TiO2 nanoparticles.

Effects of Lewis and Karlovitz numbers on transport equations for turbulent kinetic energy and enstrophy

A three-dimensional Direct numerical simulation(DNS)with complex chemistry was employed to examine the statistical behavior of turbulent kinetic energy(TKE)and enstrophy transport equations in hydrogen(Lewis number(Le)≈0.4)and dodecane(Le≈4.2)flames.The Karlovitz(Ka)numbers ranged from 4 to 150,involving both the thin and broken reaction zones.Budget analyses of TKE and enstrophy transport equations are performed,and scaling terms in the literature are re-examined.Similar to thin reaction zone flames,viscous dissipation term appears to be the most important term in the TKE balance,while viscous dissipation and vortex-stretching terms are the dominant terms in the enstrophy transport equation at high Ka number.The velocity-pressure gradient and the mean velocity dilatation in the TKE transport equation and the dilatation term in enstrophy budget are found to be affected by the Le.Modified scaling estimations for those terms affected by Le are proposed in this work to account for the Le effects spanning different combustion regimes.This work confirmed that Kolmogorov’s first hypothesis is not valid for low Ka number flames investigated in this study,where the vortex stretching and viscous dissipation terms cannot be scaled with local dissipation and viscosity.At sufficiently high Ka number flames,the vorticity can be scaled with the Kolmogorov time scale,and the mean enstrophy value approaches homogeneous,isotropic,non-reacting turbulence flow,but lower Le fuels require much higher Ka number to achieve that.

Deep Neural Network-Based Generation of Planar CH Distribution through Flame Chemiluminescence in Premixed Turbulent Flame

Flame front structure is one of the most fundamental characteristics and, hence, vital for understanding combustion processes. Measuring flame front structure in turbulent flames usually needs laser-based diagnostic techniques, mostly planar laser-induced fluorescence (PLIF). The equipment of PLIF, burdened with lasers, is often too sophisticated to be configured in harsh environments. Here, to shed the burden, we propose a deep neural network-based method to generate the structures of flame fronts using line-of-sight CH* chemiluminescence that can be obtained without the use of lasers. A conditional generative adversarial network (CGAN) was trained by simultaneously recording CH-PLIF and chemiluminescence images of turbulent premixed methane/air flames. Two distinct generators of the C-GAN, namely Resnet and U-net, were evaluated. The former net performs better in this study in terms of both generating snap-shot images and statistics over multiple images. For chemiluminescence imaging, the selection of the camera’s gate width produces a trade-off between the signal-to-noise (SNR) ratio and the temporal resolution. The trained C-GAN model can generate CH-PLIF images from the chemiluminescence images with an accuracy of over 91% at a Reynolds number of 5000, and the flame surface density at a higher Reynolds number of 10,000 can also be effectively estimated by the model. This new method has the potential to achieve the flame characteristics without the use of laser and significantly simplify the diagnosing system, also with the potential for high-speed flame diagnostics.

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.

Development of a fan-stirred constant volume combustion chamber and turbulence measurement with PIV

A fan-stirred combustion chamber is developed for spherically expanding flames,with P and T up to 10 bar and 473 K,respectively.Turbulence characteristics are estimated using particle image velocimetry(PIV)at different initial pressures(P=0.5-5 bar),fan frequencies(ω=0-2000 r/min),and impeller diameters(D=100 and 114 mm).The flame propagation of methanol/air is investigated at different turbulence intensities(u′=0-1.77 m/s)and equivalence ratios(f=0.7-1.5).The results show that u′is independent of P and proportional toω,which can be up to 3.5 m/s at 2000 r/min.L_(T)is independent of P and performs a power regression withωapproximately.The turbulent field is homogeneous and isotropic in the central region of the chamber while the inertial subrange of spatial energy spectrum is more collapsed to-5/3 law at a high Re_(T).Compared to laminar expanding flames,the morphology of turbulent expanding flames is wrinkled and the wrinkles will be finer with the growth of turbulence intensity,consistent with the decline of the Taylor scale and the Kolmogorov scale.The determined S_(L)in the present study is in good agreement with that of previous literature.The S_(L)and S_(T)of methanol/air have a non-monotonic trend with f while peak S_(T)is shifted to the richer side compared to S_(L).This indicates that the newly built turbulent combustion chamber is reliable for further experimental study.

Laser-induced spark ignition of H_2/O_2/Ar mixtures

Laser-induced spark ignition of hydrogen-oxygen-argon mixtures was experimen- tally investigated using a Q-swiched Nd:YAG laser to break down the gas at 532 nm. The laser-based high-speed schlieren system was employed to record flame front evolution for the gas mixtures with different initial pressure or laser output energy or argon dilution. The results show that the breakdown of the gas leads to the generation of ellipsoidal plasma. The rarefaction waves create the toroidal rings at the leading and trailing edges of the plasma, which provides a reasonable explana- tion for inward wrinkle of the plasma and the resultant flame. The toroidal rings at leading edge decays more rapidly and a gas lobe is generated that moves towards the laser. The hot gas in the plasma induces the generation of the spark kernel. Affected by the very weak shock wave or compression waves reflected off the wall, the initial laminar flame decelerates. The arc flame front interactions with the wall, reversed shock wave or compression waves, rarefaction waves, etc. induce the transition from laminar flame to turbulent one. These induce the transition from laminar flame to turbulent flame. For stoichiometric hydrogen-oxygen mixtures diluted by 76.92% argon at an initial pressure of 53.33 kPa, the minimum output energy of the laser is 15 mJ for successful laser-induced spark ignition. With in- creasing initial pressure or the output energy of the laser, or decreasing argon di- lution, the speed of the flame front increases.