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In this paper, the effects of variations in the fuel tube diameter and co-flow velocity in the combustion chamber on the non-premixed laminar flame are investigated. Methane gas, as a fuel, and the dry air, as an oxidizer. The size of the combustion chamber is constant and, by changing the fuel tube diameter and co-flow velocity, changes in the num...
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... Too close proximity of the outer boundaries to the swirling jet and the pressure specified at the outlet can influence the bottleneck position and cause a slow structural change in the flame. As it is shown in [44], variation in the co-flow velocity shifts the bottleneck and can even lead to its disappearance. All these issues will be considered in future work. ...
Swirling flames are widely used in engineering to intensify mixing and stabilize combustion in gas turbine power plants and industrial burners. Swirling induces new instability modes, leading to intensification of coherent structures, asymmetric geometry, vortex core precession, and flame oscillations. Large-Eddy Simulation (LES) has the capability to furnish more accurate and reliable results than the simulations based on Reynolds-averaged Navier–Stokes equations (RANS). Subgrid-scale models in LES need to describe the backscatter (local transfer of kinetic energy from small scales to larger scales) that is intensified in swirling flames. In this paper, the Differential Subgrid Stress Model (DSM), previously developed by the authors, is assessed using an experimental database from Sydney University on swirl-stabilized turbulent unconfined non-premixed methane-air flame. Regime without vortex precession is simulated numerically using the DSM and Smagorinsky subgrid-scale model. Experimental measurements of mean velocity, profiles of mass fractions, and temperature are used for comparison with the simulation data. The standard Smagorinsky model is considered the basic approach. Differences in the flow field statistics obtained in both subgrid-scale LES models are analyzed and discussed. The importance of taking the backscatter into account is highlighted.
... Turbulence level also explains part of the differences observed between the simulations. Indeed, the lower efficiency of the micro-scale mixing between the gas and oxygen can explain the enlargement of the temperature profile at the expense of its sharpness in Case 2 [72,73]. Similar results were reported for other feedstocks combustion in different reactors [74][75][76]. ...
Lignocellulosic biomass is an established source of energy with various applications. Yet, its diversity renders the proper combustion of its thermochemical degradation vapors challenging. In this work, the combustion of syngas obtained from biomass thermochemical conversion was numerically investigated to limit pollutant emission. The Computational Fluid Dynamics (CFD) simulation was performed using the open-source OpenFOAM. The reactor was considered in an axisymmetric configuration. The gas mixture resulting from the pyro-gasification devolatilization was composed of seven species: CO, CO2, H2O, N2, O2, light, and heavy hydrocarbon, represented by methane (CH4) and benzene (C6H6), respectively. The evolutions of mass, momentum, energy, and species' concentrations were tracked. The flow was modeled using the RANS formulation. For the chemistry, reduced kinetic schemes of three and four steps were tested. Moreover, the Eddy Dissipation Concept (EDC) model was used to account for the turbulence-chemistry interaction. The numerical prediction enabled us to describe the temperature and the species. Results show that all transported variables were closely dependent on the mass flow rate of the inflow gas, the primary and the secondary air injections. Finally, from a process perspective, the importance of the secondary air inlet to limit pollutants emissions can be concluded.
... For NO x reduction and to avoid safety risks related to the high hydrogen-air flame speed (up to 50 m/s) and low quenching diameters (up to 0.5 mm), required to ensure prevention of the flashback in case of the fully premixed burner concepts, several micro-mixers or micro-nozzle burner solutions were recently patented and results of their operation reported in the literature [1][2][3]. To optimize the flame stability and combustion properties of H 2 , a common approach is the modification of the burner or nozzle geometry [4][5][6]. In this regard, previous studies deal with the comparison of different combustion models using detailed chemistry and concluded that the reduced reaction mechanism method showed the best agreement with experiments in terms of capturing and mapping the typical flame structure [3,7]. ...
This work deals with the numerical investigation of a three-dimensional, laminar hydrogen-air diffusion flame in which a cylindrical fuel jet is surrounded by in-flowing air. To calculate the distribution of gas molecules, the model solves the species conservation equation for N-1 components, using infinity fast chemistry and irreversible chemical reaction. The consideration of the component-specific diffusion has a strong influence on the position of the high-temperature zone as well as on the concentration distribution of the individual gas molecules. The calculations of the developed model predict the radial and axial species and temperature distribution in the combustion chamber comparable to those from previous publications. Deviations due to a changed burner geometry and air supply narrow the flame structure by up to 50% and the high-temperature zones merge toward the central axis. Due to the reduced inflow velocity of the hydrogen, the high-temperature zones develop closer to the nozzle inlet of the combustion chamber. As the power increases, the length of the cold hydrogen jet increases. Furthermore, the results show that the axial profiles of temperature and mass fractions scale quantitatively with the power input by the fuel.
Combustion phenomenon in combustion chambers used in gas turbines is a complex process in which various factors are
involved. Energy loss in these combustion chambers due to chemical reaction factors, internal heat transfer, mass transfer
and viscous losses reduces the efficiency of these units and ultimately greatly reduces the overall efficiency of a gas turbine
unit. Therefore, it will be very useful to provide a method by which the combustion process and the type of flame can be
modelled. Since the fuel used in combustion chambers as an energy carrier may change, it is both time-consuming and costly
to perform testing processes under the conditions of using new fuels to determine operating point parameters, so the novelty
of this paper presents a general approach. It can be used for any type of fuel only by changing the environmental parameters.
The β-PDF approach is proposed to model the thermochemical scalars (i.e. temperature, species mass fractions, and density)
as functions of mixture fraction by assuming the fast chemistry. Next, the entropy generation analysis is applied to quantify
the contribution of each irreversible process (i.e. heat transfer, mass transfer, chemical reaction, and viscous dissipation) to
the total exergy destruction, and theoretical findings are finally presented to relate the exergy losses to the design parameters
of the combustor. In this regard, a well-known non-premixed jet flame is considered as the case study in this paper,
named the Sandia/ETH H2/He flame. A comparison of the obtained results based on the proposed modelling method and
the experimental results confirms the effectiveness of the proposed estimation strategy. Also, entropy generation analysis
of the flame demonstrates that the chemical reaction is the dominant irreversible process in the total exergy destruction in
turbulent non-premixed flames (85.13%), followed by mass transfer (8.05%) and heat transfer (6.80%). By modelling any
desired flame based on the proposed PDF method, two main goals are achieved: first, all the necessary parameters to determine
the physical model of the flame can be estimated, thus avoiding multiple experimental tests. Secondly, it is possible to
identify the factors affecting the entropy production and ultimately the effect on heat loss by estimating the model for each
flame, and by optimizing these factors, heat loss in combustion chambers can be prevented.
Laminar diffusion-controlled jet flames are found in a wide range of applications and also used for fundamental studies on reactive flows. Diffusion flames are classically studied considering infinitely fast chemistry and equal mass and heat diffusivities. Such assumptions can be unrealistic for many practical situations. The present work evaluates the effects of preferential diffusion in an axisymmetric confined reactive flow using an extension of the classic Shvab-Zel’dovich formulation solely in terms of the Lewis numbers of the reacting species. The governing equations are solved using the finite volume method and a structured mesh. The WUDS (Weighted Upstream Differencing Scheme) is used for the discretization of the convective terms, and the SIMPLEC (Semi-Implicit Method for Pressure Linked Equations-Consistent) method is employed for
pressure–velocity coupling in solving for the velocity field. Initially, results obtained for the limiting case of unity Lewis numbers are verified and validated using numerical and experimental data available in the literature. Results are then obtained for different fuel and oxidant Lewis numbers, allowing for analysis of off-adiabatic flame temperatures and of changes in flame height and width. Besides, results also show that for over-ventilated flames, the oxidant Lewis number has a more pronounced influence on the combustion processes than the fuel Lewis number.
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Resumo: Este trabalho apresenta um modelo de fluidodinâmica computacional da combustão com mecanismo de cinética química detalhada para escoamento incompressível laminar. Na modelagem CFD, o campo de velocidade é obtido pela solução das equações de Navier-Stokes, o campo de temperatura é obtido pela solução de uma equação de conservação da energia e a composição da mistura de gases é obtida pela solução de equações de conservação de espécies químicas individuais e o mecanismo de reação GRI-MECH 3.0 é utilizado para o cálculo da cinética química. O método numérico para discretização de uma equação genérica de conservação e a técnica de divisão de operadores usada para a avaliação dos termos de fonte químicos são apresentados em detalhes. Um código computacional programado na linguagem Matlab é utilizado para simulação de um caso de teste e dados experimentais de uma chama de difusão laminar de metano são utilizados para validação da modelagem CFD proposta. Palavras-chave: fluidodinâmica computacional, combustão de metano, mecanismo de cinética química detalhada. Abstract: This work presents a computational fluid dynamics modeling of combustion with detailed chemical kinetics mechanism for incompressible laminar flow. In CFD modeling, the velocity field is obtained by solving the Navier-Stokes equations, the temperature field is obtained by solving an energy conservation equation, and the gas mixture composition is obtained by solving the conservation equations of individual chemical species and the GRI-MECH 3.0 reaction mechanism is used in the calculation of chemical kinetics. The numerical method for discretization of a generic conservation equation and the operator splitting technique used for the evaluation of chemical source terms are presented in detail. A computer code programmed in the Matlab language is used to simulate a test case and experimental data from a laminar diffusion flame of methane are used to validate the proposed CFD modeling of combustion. A melhoria do projeto de máquinas térmicas pode minimizar a emissão de poluentes, o que é uma tarefa complexa e por isso necessita de uma boa descrição da combustão, que pode ser obtida por uma combinação de experimentação em laboratório e cálculos teóricos. Muito esforço foi realizado em décadas passadas para a descrição da combustão considerando somente a cinética química, mas tais estudos não conseguiram descrever detalhadamente a combustão em equipamentos reais. Uma boa descrição da combustão também requer a determinação espacial da distribuição de espécies químicas, com a zona onde o combustível e oxidante estão em proporção estequiométrica, a chama se estabelece e ocorre a temperatura máxima da reação (Westbrook e Dryer, 1981). Na maioria das máquinas industriais a combustão ocorre em escoamento turbulento, ainda assim, Mitchell,
It is difficult to choose the appropriate numerical simulation methods to model the character of non-premixed diffusion combustion in particle packed bed among so many various methods. Four different numerical methods were used to simplify the gas flow through the packed beds, the non-premixed diffusion combustion of methane in a plane-parallel packed bed was simulated considering the chemical equilibrium and thermal radiation. The gas flow field, temperature and flame characteristics were analyzed using computational fluid dynamics coupled with discrete element method (CFD-DEM) which calculates the fluid-particle interaction force considering the drag force and the pressure gradient force, the gas flow velocity magnitudes in the particle packed bed simulated by CFD-DEM are smaller than those by porous media and Ergun equation. The maximum velocity of gas varies from 0.22 m/s to 0.80 m/s using four different methods. Compared the flame shape and height with experimental test values, the flame heights simulated by M2 and M3 are both 10% smaller than that of the experimental result, which is reasonable because of carbon black. The maximum combustion temperature simulated by M4 is more 400 K higher than those by M2 and M3. This study shows that only using CFD-DEM, the flame height of non-premixed diffusion combustion in a packed bed can be reasonably simulated. For industrial engineering packed bed, CFD-DEM is the priority choice to calculate the fluid-particle interact force.
