Non-premixed Combustion Model Research Articles

CFD Analysis of Scramjet Combustor with Non-Premixed Turbulence Model Using Ramp Injector

This paper presents the supersonic combustion of hydrogen using strut injector along with two-dimensional turbulent non-premixed combustion model with air inlet temperature of 750 0k and vitiated Mach number of 2. In this process, a PDF approach is created and this method needs solution to a high dimensional PDF transport equation. As the combustion of hydrogen fuel is injected from the strut injector, it is successfully used to model the turbulent reacting flow field. It is observed from the present work that, the maximum temperature of 2096 0k occurred in the recirculation area which is produced due to shock wave-expansion and the fuel jet losses concentration and after passing successively through such areas, temperature decreased slightly along the axis. From the maximum mass fraction of OH, it is observed that there is very little amount of OH around 0.0017 were found out after combustion. By providing strut injector, expansion wave is created which causes the proper mixing between the fuel and air that results in complete combustion.

Thermodynamic optimisation and computational analysis of irreversibilities in a small-scale wood-fired circulating fluidised bed adiabatic combustor

An analysis of irreversibilities generated due to combustion in an adiabatic combustor burning wood was conducted. This was done for a reactant mixture varying from a rich to a lean mixture. A non-adiabatic non-premixed combustion model of a numerical code was used to simulate the combustion process where the solid fuel was modelled by using the ultimate analysis data. The entropy generation rates due to the combustion and frictional pressure drop processes were computed to eventually arrive at the irreversibilities generated. It was found that the entropy generation rate due to frictional pressure drop was negligible when compared to that due to combustion. It was also found that a minimum in irreversibilities generated was achieved when the Air–Fuel mass ratio was 4.9, which corresponds to an equivalence ratio of 1.64, which are lower than the respective Air–Fuel mass ratio and equivalence ratio for complete combustion with theoretical amount of air of 8.02 and 1.

Analysis of Flame Characteristic of Kerosene Pool Fire

Based on the computational fluid dynamics (CFD) simulation software and its parallel techniques, the standard k-ε turbulent flow equations with additional buoyancy-modified, the PDF(Probability Density Function) non-premixed combustion model, and the discrete ordinates (DO) radiation model were used to simulate the kerosene pool fire scenarios of various pool diameters in stagnant air and in a cross-wind. Based on these simulations, the relationships between the pool size and the flame height and flame temperature were obtained, as well as the effective law of the wind speed change on the fire tilt angle. The results presented in this paper are of great significance in the hazard evaluation of the thermal radiation of the pool fire.

Computational fluid dynamics simulation of LNG pool fire radiation for hazard analysis

In this paper, a three-dimensional computational fluid dynamics (CFD) model has been conducted on liquefied natural gas (LNG) pool fire radiation. Besides the general governing equations (continuity, energy and momentum), three key components as viscous model of large eddy simulation (LES), non-premixed combustion model, and radiation model for pool fire radiation have been considered to take account of the unsteady and pulsed burning flames, which are especially important in capturing characteristics of large LNG pool fire. The experimental data from Montoir series field tests of LNG pool fire, which could demonstrate a relatively complete performance of large LNG pool fire, have been applied to validate the CFD model of fire radiation. The relative error is less than 10% and the results are in good agreement with the test data. CFD model performs better than the commonly used engineering model as Solid Flame Model. The verified CFD model is then applied to perform a hazard analysis for a LNG satellite station. The spacing distances between facilities (e.g. LNG tanks and vaporizers) and ignition source have been evaluated numerically to avoid thermal radiation damage. It is concluded that the spacing distance between AAV banks and impoundment walls should be enlarged.

Numerical Simulation of the Multi-Component Slurry Gasification with the Non-Premixed Combustion Model

Considering the volumetric reactions and particle surface reactions in an entrained flow gasifier, the non-premixed combustion model based on the wet particles combustion method is adopted to simulate the multi-component slurry gasification process with FLUENT software. The velocity, temperature and concentration profiles of the gas products in the gasifier are obtained. Simulated results agree well with the commercial data, which verified the validity and applicability of the present model. Simultaneously, the reaction subarea in the gasifier has turned out to be obvious, the water evaporation, volatiles cracking and combustion reactions mainly appear in the jet region of the gasifier, the char gasification mainly appear in the freeboard region, but few reactions appear in the recirculation region. Moreover, with the increase of the mass fraction of the multi-component slurry, the content of the syngas at the gasifier outlet increases, while with the oxygen carbon atom ratio increasing, the syngas yield is rising early but declining later.

A non-premixed combustion model based on flame structure analysis at supercritical pressures

This work presents a study of non-premixed flames at supercritical-pressure conditions. Emphasis is placed on flame stability in liquid rocket engines fueled with liquid oxygen and gaseous hydrogen. The flame structure sensitivity to strain, pressure, temperature and real-fluid effects was investigated in detailed opposed-jet flames calculations. It is shown that the flame is very robust to strain, that the flamelet assumption is valid for the conditions of interest, and that real-fluid phenomena can have a significant impact on flame topology. At high-pressure supercritical conditions, small pressure or temperature variations can induce strong changes of thermodynamic properties across the flame. A substantial finding was also that the presence of water from combustion significantly increases the critical pressure of the mixture, but this does not lead to a saturated state where two-phase flow may be observed. The present study then shows that a single-phase real-fluid approach is relevant for supercritical hydrogen–oxygen combustion. Resultant observations are used to develop a flamelet model framework that combines detailed real-fluid thermodynamics with a tabulated chemistry approach. The governing equation for energy contains a compressible source term that models the flame. Through this approach, the solver is capable of capturing compressibility and strain-rate effects. Good agreements have been obtained with respect to detailed computations. Heat release sensitivity to strain and pressure variations is also recovered. Consequently, this approach can be used to study combustion stability in actual burners. The approach preserves the density gradient in the high-shear region between the liquid-oxygen jet and product rich flame region. The latter is a key requirement to properly simulate dense-fluid jet destabilization and mixing in practical devices.

Investigation of Numerical Simulation in Combustion Chamber Predictability

For the purpose of increasing applicability of combustion chamber simulation, computational domain, boundary condition, simplicity of complicated structures, mesh generation and physical parameters are investigated in this paper. An annular combustion chamber of some aero-engine is studied by means of predictive numerical simulation. The computational domain includes diffuser, swirler, inner flame tube, inner ring of combustion chamber and the flow channel of all the holes on the wall of flame tube. The film cooling holes row was simplified into a slit filled with porous media. Realizable k-turbulent model and non-premixed combustion model were adopted. Model of pressure atomization nozzle were calibrated and validated through inner nozzle flow property two-phase flow VOF model and experimental data. Physical parameters are express through polynomial functions. A commercial CFD code was adopted on a high performance computing cluster with parallel algorithm and the solving method are high-order discretization scheme. The velocity, pressure, temperature, fuel spray, density of fuel and productions, etc. are calculated and validated with the experimental data.

The Comparison of Experimental and Predicted Flame Temperature of Natural Gas Combustion

This present work was focused on the application of the probability density function method in conjunction with the k–∊ turbulence model in the goal to predict temperature fields of a non-premixed confined natural gas diffusion flame. A combined experimental and computational fluid dynamics modeling study of the turbulent non-premixed natural gas on a laboratory scale has been performed. An experimental study has been carried out on a fire-tube water heater. The flame and gas temperature in the combustion chamber were measured. The real geometry of the fire-tube water heater and the burner were modeled and analyzed by FLUENT code numerically. Turbulent diffusion flames were investigated numerically using a finite volume method for the solution of the conservation and reaction equations governing the problem. The boundary conditions were specified as the same as the measured values. The elemental analysis of the natural gas was taken as the mixture of hydrocarbon, and the air was the oxidizer. The standard k–∊ model was used for the modeling of the turbulence phenomena in the combustor. The non-premixed combustion model was chosen. In the conserved scalar approach, turbulence effects are accounted for with the help of an assumed shape probability density function or probability density function. The discrete ordinates radiation model is used for modeling of the radiative heat transfer in the combustion room. The model results were compared with the experimental results, in good agreement with the measurements.

Numerical simulation on flow, combustion and heat transfer of ethylene cracking furnaces

With the study object of an 100kt/a SL-II ethylene cracking furnace, this paper used Computational Fluid Dynamics (CFD) method to carry out coupled simulation studies on the flow, combustion, radiative heat transfer and thermal cracking reaction processes in the cracking furnace. The standard k– ε two-equation model was applied to turbulence simulation. The finite-rate/eddy-dissipation model was used for modeling of non-premixed combustion of the bottom burners and premixed combustion of the sidewall burners. The Discrete Ordinates (DO) model was applied to the simulation of radiative heat transfer of furnace. The simulation results show the detailed information about velocity, temperature and concentration fields in the furnace and heat flux distribution on the reactor tubes skin. This work will provide a theoretical basis for the optimization of the geometrical structure and operational parameters of the cracking furnace.

3-D Simulation of the combustion process for di-methyl ether-fueled diesel engine

The characteristics of spray, auto-ignition, and the combustion process of di-methyl ether(DME) were investigated using a 3-D simulation of a combustion vessel under various engine conditions, including high temperature and pressure. Spray impingement and nonpremixed combustion models were incorporated into the computational fluid dynamics code, called STAR-CD. A Peng-Robinson equation of state was introduced to calculate the evaporation rate of DME droplets that were exposed to high-pressure conditions. A Laminar flamelet concept was used to simulate non-premixed combustion. A skeletal chemical kinetics mechanism consisting of 28 species and 45 reactions was used as the chemical mechanism for DME. Compared with the experimental data, the flamelet concept combustion model predicted the essential features of the combustion process and auto-ignition characteristics of the DME spray reasonably well for various initial conditions. The combustion process of a direct injection engine fueled by DME was also simulated and verified through experimental data.

Numerical Simulation of Supersonic Nonpremixed Turbulent Combustion in a Scramjet Combustor Model

Nonpremixed turbulent combustion is studied in situations relevant to those encountered in advanced airbreathing engines operating at high flight Mach number values. Accurate simulations would require modeling both finite rate chemical kinetics and molecular diffusion effects (mixing) as well as the complex compressible flowfield structure associated with the presence of multiple shock and expansion waves that significantly influence combustion. The chemical kinetics are modeled by using a Lagrangian framework for turbulent combustion that has been recently extended to reactive high-speed flows. The corresponding framework incorporates both finite rate kinetics and turbulent molecular mixing rates with minimal additional parameters. An efficient anisotropic mesh adaptation strategy is used to obtain a satisfactory description of both nonreactive and reactive compressible flowfields. The present study confirmed that the proposed approach provides a well-suited framework for future developments devoted to nonpremixed turbulent combustion modeling in high-speed flows. A detailed comparison with data gathered from an experimental study of a laboratory scramjet model indicated the validity of the approach.

Experimental and numerical modeling of the effect of solid surface on NO x emission in the combustion chamber of a water heater

A combined experimental and CFD modeling study of the turbulent non-premixed natural gas on a laboratory scale has been performed. Effect of solid surface enhancement in combustion chamber on the flame temperature and NO emission was investigated. The solid surface called as filling material (FM) was cylindrical and was placed coaxially in the center of combustion chamber. The temperature and NO distribution in the combustion chamber were compared for different geometries of the filling material. The diameters of the filling materials were 25 and 30 cm with two lengths of 20 and 40 cm. Experimental study has been carried out on a fire tube water heater. The flame temperature on the center line of the combustion chamber, gas temperature and NO emission in the combustion chamber were measured. The actual geometry of the fire tube water heater and the burner were modeled and then analyzed by the FLUENT code. Turbulent diffusion flames were investigated numerically using a finite volume method for the solution of the conservation and reaction equations governing the problem. The measured values were specified as the boundary conditions. The elemental analysis of the natural gas was taken as a mixture of hydrocarbon and air was the oxidizer. The standard k– ε model was used for the modeling of the turbulence phenomena in the combustor. The non-premixed combustion model was chosen. In the conserved scalar approach, turbulence effects were accounted for with the help of an assumed shape probability density function or PDF. The discrete ordinates (DO) radiation model was used for modeling of the radiative heat transfer in the combustion room. The model results were compared with the experimental results. The model results were in good agreement with the measurements. The filling material provided the recirculation of the cooler gases into the flame. The recirculation reduced the oxygen concentration in the flame and controlled the flame temperature. It was found that the filling material with the diameter bigger than the flame diameter increased the heat transfer rate in the back flow around the flame.

CFD based combustion model for sewage sludge gasification in a fluidized bed

Gasification is one potential way to use sewage sludge as renewable energy and solve the environmental problems caused by the huge amount of sewage sludge. In this paper, a three-dimensional Computational Fluid Dynamics (CFD) model has been developed to simulate the sewage sludge gasification process in a fluidized bed. The model describes the complex physical and chemical phenomena in the gasifier including turbulent flow, heat and mass transfer, and chemical reactions. The model is based on the Eulerian-Lagrangian concept using the nonpremixed combustion modeling approach. In terms of the CFD software FLUENT, which represents a powerful tool for gasifier analysis, the simulations provide detailed information on the gas products and temperature distribution in the gasifier. The model sensitivity is analyzed by performing the model in a laboratory-scale fluidized bed in the literature, and the model validation is carried out by comparing with experimental data from the literature. Results show that reasonably good agreement was achieved. Effects of temperature and Equivalence Ratio (ER) on the quality of product syngas (H2 + CO) are also studied.

CFD modeling of a fluidized bed sewage sludge gasifier for syngas

AbstractA three‐dimensional model of a fluidized bed for sewage sludge gasification is presented. The model describes the complex physical and chemical phenomena in the gasifier, including turbulent flow, heat and mass transfer, and chemical reactions. The simulation employs the standard κ− ε turbulence model for the gas phase in an Eulerian framework, and the discrete phase model for the sludge particles in a Lagrangian framework, coupled with the nonpremixed combustion model for chemical reactions. In terms of the Computational Fluid Dynamics (CFD) software Fluent 6.1, the simulations provide detailed information on the gas products' composition and temperature distribution inside the gasifier and at the outlet. The results are compared with the experimental data in literature (Manya et al., 2005). Effects of temperature and equivalence ratio (ER) on the product syngas (H2 + CO) quality are studied. The model shows good agreement with the experimental data. Copyright © 2008 Curtin University of Technology and John Wiley & Sons, Ltd.

Non-premixed Combustion Model of Fluidized Bed Biomass Gasifier for Hydrogen-Rich Gas

Fluidized bed biomass gasifiers can be employed to produce hydrogen-rich gas. A non-premixed combustion model is used for biomass air-steam gasification in the gasifier, and the simulations were carried out by using the FLUENT 6.0 software. The simulation results are compared with the experimental data. The effects of the steam to biomass ratio (S/B), the equivalence ratio (ER), and the size of biomass particles on the hydrogen yield were studied. Meanwhile, the distributions of hydrogen inside the gasifier at different conditions are also described.

Modelling of burner stabilised oxy-fuel flames by using non-premixed and partially-premixed turbulent combustion models

The turbulent flow and combustion process of oxy-fuel flames are studied in the wake behind a burner rim separating fuel and oxidiser inlet channels. A range of conditions was considered, from stably attached flames, via lifted flames to blow-off. It was found that the chemical equilibrium model did not predict the chemical composition well. The laminar flamelet model performed better. It appeared that extinction is not the mechanism that leads to a lifted flame and therefore partial premixing was considered. It appeared that lifted oxy-fuel flames can be predicted fairly well by adjusting a turbulent burning velocity constant.

Conditional moment closure modeling of turbulent nonpremixed combustion in diluted hot coflow

The conditional moment closure (CMC) model is applied to predict flame structures and NO formation in the moderate and intense low oxygen dilution combustion mode. The effects of oxygen concentration in a hot diluted oxidant stream are investigated in the experimental condition of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154]. The GRI 2.11 Mech is used for description of chemical reaction including NO x chemistry. The conditional scalar dissipation rate, which describes the effect of turbulent mixing on finite chemistry, is calculated by integrating the transport equation for probability density function (PDF). A new PDF is proposed to describe three stream mixing in terms of a single mixture fraction. The conditional mean predictions of temperature, and CO, OH, and NO mass fractions are in good agreement with measurements. The unconditional Favre mean predictions of CO and NO mass fractions are also in reasonable agreement. Upstream underprediction of OH and NO in the low oxygen concentration case may be attributed to uncertainty in low temperature reaction mechanism and mixing prediction. Differential diffusion effects are shown to be nonnegligible in the present flames. The CMC model is an attractive choice for simulation of MILD combustion in which conditional fluctuations of reactive scalars are small enough for first-order closure of conditional mean reaction rates to remain valid.

Characteristics of the Spray and Combustion Process of DME under Engine Conditions(CNG and Alternative Fuels, Oxygenated Fuels)

The characteristics of spray, auto-ignition and combustion process of DME (Dimetyl ether) were investigated by 3-D simulation in a combustion vessel under high temperature and pressure conditions and engine conditions. Spray impingement and non-premixed combustion model have been developed and incorporated into the computational fluid dynamics code, STAR-CD. Laminar flamelet concept was used to simulate non-premixed combustion. In order to reduce the enormous computational time, fitted function of the integration of pre-assumed PDF (Probability density function) was introduced. For the chemical mechanism of DME, a skeletal chemical kinetics mechanism which consists of 28 species and 45 reactions was deduced by approximation of detailed mechanisms by means of sensitivity analysis and steady-state with addition of generic species or global reactions. To verify this combustion model, autoignition and combustion processes were investigated in a pre-combusted vessel. The simulation results were well matched with experimental results of the spray tip penetration of liquid and vapor phase, and ignition delay time of DME spray. In comparison with the trends observed experimentally, the flamelet-concepted combustion model predicted the essential feature of combustion processes and the autoignition characteristics of DME spray reasonably well on the various initial conditions. The combustion process of DI engine fueled by DME was simulated with moving grid of a commercial diesel engine.

A robust and accurate algorithm of the β-pdf integration and its application to turbulent methane–air diffusion combustion in a gas turbine combustor simulator

The β-pdf has been widely assumed for the probability distribution of the mixture fraction in many turbulent mixing and turbulent non-premixed combustion models in the literature. The numerical integration of the β-pdf often encounters the singularity difficulties and only few publications have addressed this issue. An efficient, accurate and robust numerical treatment of the β-pdf integration was proposed. The present treatment of the β-pdf integration was implemented into a flamelet model to calculate turbulent methane–air combustion in a model gas turbine combustor. Numerical results obtained using the present β-pdf integration method and those based on the properties of the beta and gamma functions were compared to illustrate the accuracy of the present method. Effect of assuming the β-pdf to the mass-weighted pdf and unweighted pdf of the mixture fraction on the calculated density field was also investigated.

The effect of turbulence on mixing in prototype reaction‐diffusion systems

The effect of turbulence on mixing in prototype reaction-diffusion systems is analyzed here in the special situation where the turbulence is modeled ideally with two separated scales consisting of a large-scale mean flow plus a small-scale spatiotemporal periodic flow. In the limit of fast reaction and slow diffusion, it is rigorously proved that the turbulence does not contribute to the location of the mixing zone in the limit and that this mixing zone location is determined solely by advection of the large-scale velocity field. This surprising result contrasts strongly with earlier work of the authors that always yields a large-scale propagation speed enhanced by small-scale turbulence for propagating fronts. The mathematical reasons for these differences are pointed out. This main theorem rigorously justifies the limit equilibrium approximations utilized in non-premixed turbulent diffusion flames and condensation-evaporation modeling in cloud physics in the fast reaction limit. The subtle nature of this result is emphasized by explicit examples presented in the fast reaction and zero-diffusion limit with a nontrivial effect of turbulence on mixing in the limit. The situation with slow reaction and slow diffusion is also studied in the present work. Here the strong stirring by turbulence before significant reaction occurs necessarily leads to a homogenized limit with the strong mixing effects of turbulence expressed by a rigorous turbulent diffusivity modifying the reaction-diffusion equations. Physical examples from non-premixed turbulent combustion and cloud microphysics modeling are utilized throughout the paper to motivate and interpret the mathematical results. © 2000 John Wiley & Sons, Inc.