Reactor Combustion Model Research Articles

Combustion stability analysis for non-standard low-calorific gases: Blast furnace gas and coke oven gas

The effective and efficient utilization of byproduct fuels has gained increasing attention in all industrial fields. In this study, flame stabilization mechanisms for non-standard low-calorific value (NLCV) gases of blast furnace gas and coke oven gas have been systematically analyzed under practical operating conditions of hot air heaters. The complex oscillatory dynamics has been successfully reproduced with the unsteady perfectly stirred reactor combustion model. The differences in the combustion stabilities between NLCV gases have been further revealed, with the kinetic importance of individual variable and chemical reaction for the complex dynamics being quantified with chemical explosive mode analysis. Then the effects of gas composition and fuel blending on combustion instability are investigated. The optimal fuel switching temperature, switching schemes and oscillating combustion inhibition methods are finally proposed for stable combustion of blast furnace gas and coke oven gas. The tolerance range of flow rate fluctuations are increased by 10% and 30% with selecting fuel switching scheme and initial combustion state reasonably.

A Brief Review on How to Make a Database for Flamelet Approach

The simulation of turbulent combustion has been of more importance to develop practical combustors. The flamelet (or tabulated chemistry) approach is one of the techniques to estimate combustion reaction rate and to describe turbulent combustion field. This approach can reduce computational cost in comparison with the reduced reaction schemes because the combustion reaction rate is just looked up from the pre-calculated database. The difficulty of the flamelet approach comes from the variety of how to make this pre-calculated database. In the present study, we reviewed previous researches in terms of the flamelet approach and summarized the ideas to make the database for the flamelet approach. First, the fundamental combustion reaction models such as the detailed reaction model and skeletal model were introduced. And then, how to generate the dataset for the database in flamelet, flamelet progress variable, and flamelet generated manifolds models were mentioned. Moreover, how to process the dataset into the database for the turbulent combustion simulation was pointed out. Finally, apart from making the database, the treatment of the database in the computational fluid dynamics was described briefly.

Piloted Liquid Spray Flames: A Numerical and Experimental Study

ABSTRACT Liquid-fuel spray flames are the primary mode of energy conversion in many high-power-density practical combustion devices. As compared to gaseous fuels, the phase change in liquid-fuel spray flames complicates the environment for laser-based combustion diagnostics and formulating predictive computational models. In this work, we present in-situ, non-intrusive measurement strategies and predictive models for combustion and carbon monoxide (CO) formation in piloted liquid-methanol spray flames. Resolving CO is essential to understand the incomplete oxidation of liquid hydrocarbon fuels and subsequent soot formation. A modified, flat-flame McKenna burner fitted with a direct-injection high-efficiency nebulizer (DIHEN) is used to produce piloted liquid-methanol spray flames. Hydroxyl (OH) planar laser-induced fluorescence (PLIF) is used for characterizing the reaction zones and temperature profiles of liquid-spray flames and two-dimensional (2D) images of CO are obtained via two-photon laser-induced fluorescence (TPLIF) using ultrashort, femtosecond-duration (fs) laser pulses. A three-dimensional (3D) computational model comprised of compressible continuous gas phase using unsteady Reynolds-averaged Navier-Stokes (URANS) in conjunction with two-way coupled Eulerian-Lagrangian spray modeling approach and partially-stirred reactor combustion model has been adapted to the modified McKenna burner. The computational model predicts the general trends of OH, temperature and CO profiles well at certain heights above the burner surface. Limitations of experimental measurements and strategies for improved model predictions pertaining to distributions of droplet sizes, pilot flame temperature, and the coflow temperature are discussed.

Study of a full scale oxy-fuel cement rotary kiln

Oxy-fuel combustion has been shown to be an attractive technology to be implemented in cement production with CO2 capture from a process point of view. Due to the market situations and future perspectives it is necessary that the technology can be retrofitted into existing plants. As part of the H2020 EU project CEMCAP, the study presented here investigates the retrofitting of oxy-fuel combustion technology on a typical full-scale cement rotary kiln of 3000 ton/year clinker capacity. The flame generated by a commercial burner has been modelled by CFD with a seven steps combustion reaction model, the k-omega SST turbulence model, and the discrete ordinates radiation model with the Weighted Sum Grey Gas model. The burner is composed of an annular outlet where coal is fed and nozzles from which angled high velocity jets generate a swirling motion for flame stabilization and mixing purposes. As in all rotary cement kilns, a high temperature low velocity secondary oxidizer stream coming from the clinker cooler flows coaxially to the burner in the kiln for heat efficiency purposes. A flame developing in air has been used as a reference case and several oxy-fuel flames have been characterized and compared. The oxy-fuel inlet parameters that are varied in the study are the oxidizer composition and flow rates in both the primary and secondary streams. The work presented is part of an optimization procedure based on iterative interaction with the process modelling of the whole cement plant with full oxy-fuel capture (not presented in this article). With retrofit application in mind, the objective was to reproduce a heat radiation profile to the clinker equivalent to that obtained in the air reference case. The optimized oxy-fuel case is characterized by a primary burner oxidizer of lower flow rate, but with higher oxygen concentration of up to 52.1% to generate the necessary heat in the near burner region.

Auto-ignition of single n-heptane droplets in low-temperature and high-pressure air using Lagrangian–Eulerian approach

The objective of this research is to investigate the evaporating, mixing, and auto-ignition processes of isolated fuel droplets using Lagrangian–Eulerian numerical approach. By using Peng–Robinson equation for the phase-equilibrium, the gas solubility at the droplet surface is considered at high-pressure ambient gases. Using perfectly stirred reactor combustion model and a low-temperature detailed mechanism for the n-heptane, the auto-ignition of the premixed fuel vapor–air mixture is verified at a constant pressure. Then, the auto-ignition of a fuel droplet with 60 µm diameter is studied at various low air temperatures. The analysis indicated that the results are strongly mesh dependent in this approach. Therefore, in order to predict the correct values of auto-ignition delay time and maximum gas temperature, the computational mesh size should be chosen in a specific range of cell sizes. Also, the auto-ignition of different size fuel droplets is studied at the air initial temperature of 800 K. It was observed that for accurate results, the computational mesh size should be chosen around seven to eight times the diameter of initial fuel droplets.

A mesoscopic numerical analysis for combustion reaction of multi-component PBX explosives

A mesoscopic combustion reaction model of multi-component PBX explosives by using a coupled thermo-mechanical constitutive model is studied. Thereinto, based on the heat balance equation, the present temperature can be calculated; the changing physical state of different materials is described by establishing different equations of state, and based on specially appointed constitutive equations, the microcracks in PBXs are described. Furthermore, the concept of combustion reaction degree is established, which takes temperature as a criterion for changing physical state. The model is implemented within the framework of the material point method so that the different gradient in the governing differential equations could be discretized in a single computational domain and that continuous remeshing is not required with increasing reaction time. The proposed model-based simulation procedure is verified with the burn rate, the peak pressure, and temperature of PBX explosives under macroscopic scale.

Effect of methane-hydrogen mixtures on flow and combustion of coherent jets

Coherent jets are widely used in electric arc furnace (EAF) steelmaking to increase the oxygen utilization and chemical reaction rates. However, the influence of fuel gas combustion on jet behavior is not fully understood yet. The flow and combustion characteristics of a coherent jet were thus investigated at steelmaking temperature using Fluent software, and a detailed chemical kinetic reaction mechanism was used in the combustion reaction model. The axial velocity and total temperature of the supersonic jet were measured via hot state experiments. The simulation results were compared with the experimental data and the empirical jet model proposed by Ito and Muchi and good consistency was obtained. The research results indicated that the potential core length of the coherent jet can be prolonged by optimizing the combustion effect of the fuel gas. Besides, the behavior of the supersonic jet in the subsonic section was also investigated, as it is an important factor for controlling the position of the oxygen lance. The investigation indicated that the attenuation of the coherent jet is more notable than that of the conventional jet in the subsonic section.

An accelerated computation of combustion with finite-rate chemistry using LES and an open source library for In-Situ-Adaptive Tabulation

Abstract In this paper, a modified version of the latest implementation of In-Situ-Adaptive-Tabulation (ISAT) algorithm, ISAT-CK7, is introduced and linked to OpenFOAM for accelerating chemical computation in simulating combustion with the finite chemistry (FRC) approach. In this new version, ISAT-CK7-Cantera, which is an open-source and free code, replaces Chemkin II in ISAT-CK7 as the thermo-chemical library. This new library facilitates using LES with FRC and detailed chemistry which has not been commonly done. In addition to successful validation of ISAT-CK7-Cantera in the simple 0D and 2D geometries, the resulting package, consisting of OpenFOAM, ISAT and Cantera, is used to evaluate the performance of LES-FRC with a partially stirred reactor combustion model and DRM19 skeletal mechanism against experimental data of the Darmstadt turbulent stratified flame. LES results show good to excellent agreement with the measured data.

Stabilization of numerical simulation by modification of combustion reaction models for heavy oil recovery with <i>in-situ</i> combustion method

Stabilization of numerical simulation by modification of combustion reaction models for heavy oil recovery with <i>in-situ</i> combustion method

Large-eddy simulations of gas- and liquid-fueled combustion instabilities in back-step flows

LES of gas and spray combustion in back-step flows are performed to investigate the mechanism underlying combustion instability and the effect of initial droplet diameter on combustion instability. Methane and kerosene are used as fuel for gas and spray combustion, respectively, and two-step global reaction models are used for the reactions. A dynamic thickened flame model is employed as the turbulent combustion model. The motions of evaporating droplets are tracked using the Lagrangian manner. The results show that in gas combustion, the pressure, heat release rate and streamwise velocity oscillate with the same frequency but different phases, and that the inlet velocity oscillation periodically generates a large vortex near the dump plane that drives combustion instability. In spray combustion, an oscillation of droplet evaporation rate is also observed. Whereas the difference in the initial droplet diameter slightly affects the mode of the pressure oscillation, it strongly affects the intensity of the pressure oscillation and causes a maximum value for a specific initial droplet diameter. This arises from the difference in the initial droplet diameter causing a difference in the droplet evaporation rate, which alters the relationship between the oscillations of the heat release rate and pressure near the dump plane.

Large Eddy Simulation of a Swirl-Stabilized Pilot Combustor from Conventional to Flameless Mode

This paper investigates flame and flow structure of a swirl-stabilized pilot combustor in conventional, high temperature, and flameless modes by means of a partially stirred reactor combustion model to provide a better insight into designing lean premixed combustion devices with preheating system. Finite rate chemistry combustion model with one step tuned mechanism and large eddy simulation is used to numerically simulate six cases in these modes. Results show that moving towards high temperature mode by increasing the preheating level, the combustor is prone to formation of thermalNOxwith higher risks of flashback. In addition, the flame becomes shorter and thinner with higher turbulent kinetic energies. On the other hand, towards the flameless mode, leaning the preheated mixture leads to almost thermalNOx-free combustion with lower risk of flashback and thicker and longer flames. Simulations also show qualitative agreements with available experiments, indicating that the current combustion model with one step tuned mechanisms is capable of capturing main features of the turbulent flame in a wide range of mixture temperature and equivalence ratios.

On lumped-reduced reaction model for combustion of liquid fuels

A systematic approach to developing compact reduced reaction models is proposed for liquid hydrocarbon fuels using n-dodecane and n-butane as the model fuels. The approach has three elements. Fast fuel cracking reactions are treated by the quasi-steady state approximation (QSSA) and lumped into semi-global reactions to yield key cracking products that are C1–C4 in size. Directed relation graph (DRG) and sensitivity analysis reduce the foundational fuel chemistry model to a skeletal model describing the oxidation of the C1–C4 compounds. Timescale-based reduction using, e.g., QSSA, is then employed to produce the final reduced model. For n-dodecane, a 24-species reduced model is derived from JetSurF and tested against the detailed model for auto-ignition, perfectly stirred reactors (PSR), premixed flame propagation, and extinction of premixed and non-premixed counterflow flames. It is shown that the QSSA of fuel cracking reactions is valid and robust under high-temperature conditions from laminar flames, where mixing is controlled by molecular diffusion, to perfectly stirred reactors, which correspond to the limit of fast turbulent mixing. Bifurcation analysis identifies the controlling processes of ignition and extinction and shows that these phenomena are insensitive to the details of fuel cracking. To verify the applicability of the above finding to turbulent flames, 2-D direct numerical simulation (DNS) of a lean turbulent premixed flame of n-butane/air with Karlovitz number of 250 was carried out using a reduced model developed from USC-Mech II. The results show that QSSA for fuel cracking remains valid even under intense turbulence conditions. Statistical analysis of the DNS data shows that fuel cracking is complete before the flame zone, and for the conditions tested, turbulent transport does not bring any significant fuel molecules into the flame zones, thus further substantiating the validity of the approach proposed.

Large-eddy simulation of flame-turbulence interaction in a spark ignition engine fueled with methane/hydrogen/carbon dioxide

A three dimensional large-eddy simulation of in-cylinder charge and combustion process of a spark ignition engine was conducted. Partially stirred reactor combustion model and Arc and kernel tracking ignition model were implemented and applied to capture in-cylinder ignition and turbulent combustion. Interaction between flame and in-cylinder background turbulence for different mixtures of methane, hydrogen and carbon dioxide were studied. When a spark ignition engine is fueled with methane and carbon dioxide, its found that gas heating effect plays a dominant role in flame-induced turbulence. Increase of volume fraction of carbon dioxide has little effect on turbulent flame speed in each combustion phase defined by mean combustion progress variable, but it weakens gas heating effect owing to reduction in burnt temperature. Flame-induced turbulence produces small-scale vortices with high vorticity within gas preheating zone. When engine is fueled with methane, hydrogen and carbon dioxide, large volume fraction of hydrogen leads to high turbulent flame speed, which plays a dominant role in flame-induced turbulence. If volume fraction of hydrogen is small, flame-induced turbulence mainly is determined by gas heating effect due to large volume fraction of carbon dioxide. Flame-induced turbulence produces many vortex-pairs, which are the major sources of wrinkles of flame front. Rotation direction of vortex-pair determines whether the flame front bulges towards unburnt area or burnt area. Level of vorticity value and space size of vortex-pair affect sizes of wrinkles on the flame front.

The Effects of Nucleation on Soot Dynamic Evolution Based on the Method of Moments

The mathematical model of soot dynamical evolution process is built based on the discrete particles population balance theory, including particle nucleation, collision coagulation, oxidation and surface growth processes; a corresponding numerical model is established using Lagrange interpolation method of moments. Based on the perfectly-stirred reactor combustion model, coupled the numerical model with detailed chemical kinetic model. The computing platform of soot particle growth evolution is established. Meanwhile, three nucleation models are built for the soot nucleation in this paper. Two chemical reaction mechanisms are used to study different fuels combustion process. The related information of soot particle growth evolution is obtained, the effect of different nucleation model on other dynamical events such as coagulation, oxidation and surface growth processes is discussed.

저온 탄화한 바이오매스와 석탄 그리고 혼합물의 연소특성 평가

The following are the results from an evaluation of the combustion characteristics of biomass processed with lowtemperature carbonization and coal, and those of a blend of both. Differential thermo-gravimetric (DTG) analysis has revealed that the number of curves was reduced as a result of carbonization and that the fuel quality was improved due to the increase of initial temperature (IT). It was also confirmed that the carbonized samples consisting only of the biomass required less combustion time (t ), while samples blended with coal burned longer than the weighted average value. The combustion time of a blended sample was shorter at an carbonization temperature of 400 C than at 300 C, and the combustion stability was achieved due to a narrow range of change in the combustion characteristics. The reaction rate constant (k) of the samples blended with coal was found to be smaller for all blend ratios, when compared with that of the unblended samples (raw, carbonized biomass). The combustion reaction models that were applicable for the devolatilization-combustion zone were diffusion (D1, D3) and Reaction order (O3) models; diffusion (D1-D4) model was primarily employed in the char combustion zone. In summary, low-temperature carbonization contributed to minimizing the change in the combustion characteristics of the biomass/coal blend.

Evolution of size distribution of nascent soot in n- and i-butanol flames

The impact of fuel bound oxygen on the sooting behavior of butanol fuels was examined by following the evolution of the particle size distribution function (PSDF) of nascent soot produced in atmospheric pressure burner stabilized stagnation (BSS) flames of n-butanol and i-butanol. Similar experiments were carried out for i-butane and n-butane flames to better understand the influence of fuel structure and the presence of the alcohol group on detailed processes of soot nucleation and growth. In terms of fuel structure, the branched chain functionality has the most observable effect on soot formation. The onset of soot nucleation is faster in the branched fuels in comparison to the straight-chain counterparts. Under the same C/O ratio, however, the butanol flames were found to nucleate soot earlier and have higher soot volume fraction than the butane flames. A combustion reaction model for i-butanol and n-butanol was used to explore the precursor chemistry. Similar to the measured PSDF, benzene is computed to rise earlier in flames of the branched fuels than the straight-chain fuels.

Effect of Connected Pipe Test Conditions on Scramjet Combustor Performance

Scramjet engine combustion chamber is normally ground tested in connected pipe mode and has limitations in simulating all combustor entry parameters as in flight engine. Effect of variation in combustor entry pressure, test gas medium due to vitiation and combustor entry flow profile on combustor performance has been brought out through CFD simulations. Computations are carried out using in-house CFD software “PARAS-3D” which solves the Reynolds Averaged Navier-Stokes equations and uses 7 species 7 reaction model for combustion. Entry pressure is having substantial influence on combustor performance at pressures lower than a certain value and this pressure limit reduces for higher fuel equivalence ratios. In the case of vitiated medium, presence of CO2 and the excess H2O result in lower total temperatures leading to reduced pressures in the combustion chamber. Non-uniform flow profile at combustor entry is also found to lower the scramjet combustor performance due to reduced mixing.

Effect of Radiation on a Spray Jet Flame

Two-dimensional direct numerical simulations (DNS) with and without a radiation model are applied to a spray jet flame, and the effect of the radiation on the spray flame behavior is studied in detail. N-decane (C10H22) is used as a liquid spray fuel, and the droplet motion is calculated by the Lagrangian method. As the combustion reaction model, Arrhenius formulation (ARRS) and enthalpy-based flamelet/progress-variable approach (FPVA-E) for a one-step global reaction are employed. Radiation is taken into account using the discrete ordinate method. The validity of the FPVA-E coupled with the radiation model is assessed by comparing with the reference solution obtained by the ARRS with the radiation model. The results by the ARRS with the radiation model show that the radiation acts to enhance the thermal diffusion and greatly affects the spray flame structure. The effect of the radiation on the spray jet flame can be well captured by the FPVA-E coupled with the radiation model.

Effects of radiation on spray flame characteristics and soot formation

Two-dimensional numerical simulations are applied to spray flames formed in a laminar counterflow and the effects of radiation on spray flame characteristics and soot formation are studied. N-Decane (C 10H 22) is used as the liquid fuel, and the droplet motion is calculated by the Lagrangian method. A single-step global reaction is employed for the combustion reaction model. A kinetically based soot model with a flamelet model is used to predict soot formation. Radiation is taken into account using the discrete ordinate method. The results show that radiation strongly affects the spray flame behavior and soot formation. Without the radiation model, flame temperature and soot volume fraction are greatly overestimated. The soot is formed in the diffusion flame regime, and its radiation emission increases with the increase in the equivalence ratio of the droplet fuel. This trend is in good agreement with that of the luminous flame behavior observed in the experiments.

Travelling wavefronts in combustion and chemical reaction models

Travelling wavefronts for a system of two reaction–diffusion equations are studied. The existence of a family of wavefronts (one for each wave speed) as well as the existence of the minimal speed (in the case that the Lewis number is greater than $1$ ) are proved. Asymptotic formulae for the wavefronts are established. New results are obtained when applying the main theorems to an isothermal autocatalytic chemical reaction system.