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Schematic view of the nozzle for Sandia flames (a) and the isosurfaces of instantaneous temperature (b)
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In this paper, the Large Eddy Simulation (LES) together with the Conditional Moment Closure (CMC) and flamelet combustion models have been applied for modelling of methane flame Sandia F. In the case of the CMC model, both instantaneous and time averaged values predicted numerically agree well with measurements. Attention was devoted to modelling a...
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... Sandia flames ( Barlow and Frank, 1998) are commonly used test cases for non-premixed combustion models and are probably the most often computed flames over the world. Sketch of the computational configuration is shown in Fig. 1 Preliminary computations were necessary in order to set-up the computational domain and meshes ensuring a sufficient resolution. Various cuboidal or hexahedral shapes with meshes consisting of various number of nodes and stretching parameters were analysed. Finally, the computational domain had a hexahedral shape with dimensions: ...
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... solution for Q k is given the entire range 0 ξ 1; (i) at the inlet plane in the regions of the fuel jet and coflow the inert solution was assigned, i.e. it was assumed that the species and enthalpy vary linearly for 0 < ξ < 1; in the region of the pilot flame the steady flamelet corresponding to burning state was defined (shown schematically in Fig. 1); (ii) all remaining boundaries are assumed as the Neumann boundary ...
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... Sandia flame F corresponds to a fully developed turbulent flame, the complexity of the flow structure represented by isosurfaces of the temperature is shown in Fig. 1 on the right hand side. Strongly unsteady flame behaviour is manifested by the occurrence of local extictions which may be well seen in instantaneous experimental data or in numerical results provided that the combustion model predicts the extinction phenomena properly. Figure 2 presents instantaneous contours of the temperature and ...
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In this work, we use grand canonical Monte Carlo (GCMC) simulation to study methane adsorption in various clay nanopores and analyze different approaches to characterize the absolute adsorption. As an important constituent of shale, clay minerals can have significant amount of nanopores, which greatly contribute to the gas-in-place in shale. In pre...
Citations
... Most often it focuses on the use of mechanisms with different numbers of species [2,3,20,34]. The same can be said for tuning/comparison of the combustion and sub-grid models [14,25,26,46,51]. The methodology of solving ODE systems formulated based on chemical source terms and the accuracy of ISAT tables in the models involving pre-tabulated values of these terms has got also some attention [30,60]. ...
This paper presents an analysis of numerical and modelling issues based on a simulation of nitrogen-diluted hydrogen lifted flame evolving in a hot co-flow. We apply the large-eddy simulations (LES) method with the so-called ‘no combustion model’ and concentrate on the impact of chemical mechanisms and various discretisation schemes on the obtained results. The main attention is put to the latter issue in which we analyse to what extent a discretisation method of the convective terms of the scalar equations for the species and enthalpy affects the solutions. We consider eight commonly known total variation diminishing (TVD) schemes and three upwind schemes of the second order. The remaining terms in the scalar equations and all the terms in the Navier–Stokes equations are discretised applying the sixth-order compact difference method. Such an approach makes the discretisation errors of the convective terms the main factor affecting the solutions from the numerical point of view. Prior to the main analysis, the differences caused by the use of various TVD or upwind schemes are highlighted based on a single scalar transport equation with a known analytical solution. The results obtained for the flame are compared to experimental data taken from the literature. It is shown that the differences due to the application of a particular discretisation method are of similar magnitude as the differences between the simulation results and experimental data. Moreover, analysis of the impact of the chemical mechanism showed that observed differences are comparable to these originating from the use of different discretisation methods.
... Khosousi et al. (2015) carried out experiments and numerical models to study the effects of ethanol addition to soot formation in vaporized gasoline flame. Reynolds Average Navier-Stokes (RANS) approach was used in these works for turbulent flames modeling; large Eddy simulation (LES) is another approach which has been used in several validated numerical studies to model turbulent diffusion flames; and the predictions from this model seem to be more accurate than the RANS predictions (Castineira 2006;Jatale et al. 2000;Lawal et al. 2013;Smith et al. 2015Smith et al. , 2017Smith et al. , 2018Tyliszczak 2013). Smith et al. (2017) compared the results from different numerical studies to evaluate the performance of LES and RANS approaches in the modeling of various combustion applications; according to this study, the LES model is more applicable to predict the behavior of flare flame. ...
Several methods are applied for soot reduction in industrial flares. However, they usually involve other issues such as flame safety and other pollutant formations; therefore, they should be handled carefully. Splitting flare tip into multiple branches is an idea that is thought to reduce soot formation by increasing the contact surface and providing better mixing. In this study, changing a single tip of an industrial flare into multiple tips was investigated by Computational Fluid Dynamic software Ansys Fluent 18. Reynolds Average Navier–Stokes approach was used to model the fluid flow, and its turbulence was modeled by realizable k–ε model. The steady laminar flamelet model was chosen as the combustion model. Soot formation was modeled using Moss–Brookes approach, and validated by comparing the simulation results with available experimental data. Effects of tip diameter, number of branches, and distance between branches on soot and NOx yields, and flame stability were studied. According to the results, increasing the number of branches having larger diameters by taking flame stability limitations into consideration, and choosing optimum distances between branches caused significant soot reduction without noticeable changes in the NOx formation.
Graphic abstract
... The majority of LES of reactive flows [10][11][12] tend to employ a low Mach number approximation primarily to speed up the simulation with low velocity flows (no acoustic CFL constraint). Typically, in these solvers, the continuity and momentum equations are solved using traditional central schemes, but the species and energy transport equations are discretised using upwind methods (at least in the vicinity of strong scalar gradients), which, due to their larger numerical dissipation, provide increased stability [13][14][15] . ...
The capability of artificial fluid properties (AFP) to serve as a sub-grid scale (SGS) model and to provide stability in high-order accurate large-eddy simulations (LES) of shock-free compressible turbulent flows with strong temperature gradients is assessed. Predictive LES of such flows, which are directly relevant to applications such as engines, require an accurate description of turbulence, which typically makes them susceptible to issues of accuracy and numerical instabilities. In the present implementation, AFP schemes along with a high-order compact filter ensure robustness of otherwise non-dissipative high-order central difference spatial discretisation schemes. It is shown that a judicious choice of both filter and AFP parameters is required in order for AFP to be used as an accurate SGS model. Optimal parameter values for the AFP and the corresponding filter parameter are identified and validated in the framework of the current LES solver. First, an inviscid Taylor-Green vortex configuration is examined to evaluate the ideal combination of the filter parameter and the coefficients in the AFP framework. It is verified that AFP works as an alternative model for the unclosed subgrid-scale (SGS) dissipation terms. The AFP results are also compared to those from the dynamic Smagorinsky model (DSM), with good agreement. Second, the optimal parameter values of AFP obtained from the inviscid Taylor-Green vortex tests are validated on a series of test configurations with increasing complexity, that include an inviscid 1D advection problem involving a thermal contact discontinuity, an inviscid 2D Kelvin-Helmholtz instability test and 3D experimental unheated and heated spatially developing turbulent round jets. Robust and accurate predictions are shown with the LES solver employing the optimal model parameters in each of these tests. In particular, good agreement between the LES predictions and experimental measurements for both the flow and temperature statistics is achieved for the laboratory jets, demonstrating the accuracy of the LES/AFP simulations applied to practical turbulent configurations. The robustness of the solver in the presence of strong temperature gradients is also demonstrated by simulating a round jet case having very high temperature gradients similar to those occurring in reacting flows. In contrast, the LES/DSM simulation of this case is noted to be unstable, and failed to provide converged results.
... In the context of LES, the CMC equations are derived using the density-weighted conditional filtering operation to the transport equations for species mass fraction (Y k ) and total enthalpy. To date, the LES-CMC approach has been used to study lifted [20], excited [21] as well as piloted flames [22] providing detailed investigations and reliable results. It is regarded as one of the most powerful CFD tools for reacting flows. ...
... To the best authors' knowledge, there are no experimental or numerical data which could be used to validate the present numerical results. However, the fact that the LES-CMC code used in this work was thoroughly verified in previous studies [18,22,21] allowed the authors to assume that the obtained results are reliable. Geometrical complexity (e.g. ...
... The pressure at the outflow is assumed equal to zero. This formulation of the outflow boundary condition is stable and allows vortices to pass without any visible distortion [18,21,22]. On the lateral sides of the domain there is no flow through the boundaries. ...
The paper presents the results of numerical simulations of a combustion process for a pure hydrogen jet in the atmosphere of oxygen and water vapor. The research was performed using Large Eddy Simulations (LES) combined with Conditional Moment Closure (CMC) model and included the analysis of auto- and forced (spark) ignition, flame development and flame propagation. In the configuration analyzed in the study, the fuel jet issues into a hot co-flowing oxidizer stream (O2/H2O). We considered two different co-flow temperatures, 1030K and 1045K, and various oxidizer compositions with H2O mass fraction over the range of YH2O=0.1-0.9 and with YO2=1-YH2O. The analyses were carried out with assuming constant 30% excess of the mass flow rate of oxygen relative to the hydrogen. It was found that: (i) the auto-ignition process occurs only for a low content of H2O in the co-flow (YH2O<0.5), (ii) and is additionally determined by its temperature; (iii) depending on O2/H2O proportion the flames stabilizes as attached or lifted. By alteration of YH2O in the oxidizer, the maximum flame temperature and the lift-off height (Lh) can be changed in the range of 1500K-3000K and Lh=0-11 jet diameters, respectively.
... This code was used previously in various studies including near-wall flows, 22 free and excited jet flows, 15,17,23-28 and flames. [29][30][31] The SAILOR code is based on the low Mach number approximation 32 which is suitable for solving a wide range of flows, varying from isothermal and constant density conditions to flows with large density and temperature variations. A solution algorithm is based on a projection method for the pressure-velocity coupling. ...
The paper presents a novel view on the absolute instability phenomenon in heated variable density round jets. As known from literature the global instability mechanism in low density jets is released when the density ratio is lower than a certain critical value. The existence of the global modes was confirmed by an experimental evidence in both hot and air-helium jets. However, some differences in both globally unstable flows were observed concerning, among others, a level of the critical density ratio. The research is performed using the Large Eddy Simulation(LES) method with a high-order numerical code. An analysis of the LES results revealed that the inlet conditions for the velocity and density distributions at the nozzle exit influence significantly the critical density ratio and the global mode frequency. Two inlet velocity profiles were analyzed, i.e., the hyperbolic tangent and the Blasius profiles. It was shown that using the Blasius velocity profile and the uniform density distribution led to a significantly better agreement with the universal scaling law for global mode frequency.
... The solution algorithm used in the SAILOR code has been verified and validated in LES studies concerning combustion problems [43,60] and also excited bifurcating and multi-armed jets [61][62][63][64]. In all these cases, the SAILOR code turned out to be computationally very efficient and accurate. ...
... In all these cases, the SAILOR code turned out to be computationally very efficient and accurate. This is expected to carry over to the simulations performed in this paper as complexity of the combustion process in the flames analysed here will be similar to the complexity occurring in the flames studied in [43,60]. Moreover, compared to the excited jets analysed in [61,62], a dynamic range of scales in the present work will not be essentially different. ...
... The pressure at the outflow is assumed constant and equal to zero. This formulation of the outflow boundary condition is stable and allow to pass vortices without any visible distortion [43,60,62]. The test computations for the longer domain showed that the applied outflow condition slightly affects the upstream region up to 1 − 2D maximally. ...
The large eddy simulation/conditional moment closure approach has been used to study excited, attached and lifted flames in a stream of hot co-flow air. The excitation is obtained by adding a flapping forcing term with amplitude of 15% of fuel velocity and frequency corresponding to Strouhal numbers St=0.175, 0.25, 0.325 to an inlet velocity profile. It is shown that such a type of forcing dramatically changes the global flame shape, resulting in a flame occupying very large volume or in bifurcating flames or even triple flames. We observed that the flame which is initially circular may transform downstream into a quasi-plane flame. The analysed cases include configurations with a small and large co-flow velocity. In the former case, the flame shape strongly depends on forcing frequency while in the latter substantial differences are seen only in an internal part of the flame. Both the lift-off heights of the flames and their spreading angles were found to be a function of the forcing frequency. The computational results are validated based on the solutions obtained for non-excited flames for which experimental data are available. The results obtained for various co-flow temperatures and velocities show that both auto-ignition times and lift-off heights are predicted correctly and exhibit trends observed experimentally.
... The SAILOR code was used previously in various studies, including laminar/turbulent transition in free jet flows [41][42][43], near wall flows [44], multi-phase flows [45] and flames [46,47]. Recently, the SAILOR code has been used in parametric studies of excited circular jets [48]. ...
... The pressure at the outflow is assumed constant and equal to zero. This formulation of the outflow boundary conditions is stable and allow to pass turbulent flow structures without any visible distortion [46][47][48]. ...
We present numerical studies of active flow control applied to jet flow. We focus on rectangular jets, which are more unstable than their circular counter parts. The higher level of instability is expressed mainly by an increased intensity of mixing of the main flow with its surroundings. We analyze jets with aspect ratio Ar=1, Ar=2 and Ar=3 at Re=10,000. It is shown that the application of control with a suitable excitation (forcing) at the jet nozzle can amplify the mixing and qualitatively alter the character of the flow. This can result in an increased spreading rate of the jet or even splitting into nearly separate streams. The excitations studied are obtained from a super position of axial and flapping forcing terms. We consider the effect of varying parameters such as the frequency of the excitations and phase shift between forcing components. The amplitude of the forcing is 10% of the inlet centreline jet velocity and the forcing frequencies cor respond to Strouhal numbers in a range St= 0.3–0.7. It is shown that qualitatively different flow regimes and a rich variety of possible flow behaviours can be achieved simply by changing aspect ratio and forcing parameters. The numerical results are obtained applying large eddy simulation in combination with a high-order compact difference code for incompressible flows. The solutions are validated based on experimental data from literature for non-excited jets for Ar=1 at Re = 1.84×10^5 and Ar=2 at Re = 1.28×10^5. Both the mean velocities as well as their fluctuations are predicted with good accuracy.
... The PDF flamelet model using mixture fraction theory and (CH 4 -Air) GRI reaction mechanism 2.11 were used to simulate the Sandia flame (Nik et al., 2010). Tyliszczak (2013) used an unsteady-state LES model along with the PDF turbulence-chemistry model coupled with GRI 2.11 reaction mechanism to simulate the Sandia flame. The diameter of the lab-scale burner used for Sandia F flame was 18.9 mm. ...
A computational fluid dynamics (CFD) methodology for simulating the combustion process has been validated with experimental results. Three different types of experimental setups were used to validate the CFD model. These setups include an industrial-scale flare setups and two lab-scale flames. The CFD study also involved three different fuels: C3H6/CH/Air/N2, C2H4/O2/Ar and CH4/Air. In the first setup, flare efficiency data from the Texas Commission on Environmental Quality (TCEQ) 2010 field tests were used to validate the CFD model. In the second setup, a McKenna burner with flat flames was simulated. Temperature and mass fractions of important species were compared with the experimental data. Finally, results of an experimental study done at Sandia National Laboratories to generate a lifted jet flame were used for the purpose of validation. The reduced 50 species mechanism, LU 1.1, the realizable k-epsilon turbulence model, and the EDC turbulence-chemistry interaction model were usedfor this work. Flare efficiency, axial profiles of temperature, and mass fractions of various intermediate species obtained in the simulation were compared with experimental data and a good agreement between the profiles was clearly observed. In particular the simulation match with the TCEQ 2010 flare tests has been significantly improved (within 5% of the data) compared to the results reported by Singh et al. in 2012. Validation of the speciated flat flame data supports the view that flares can be a primary source offormaldehyde emission.
... The SAILOR code was used previously in various studies including laminar/turbulent transition in near-wall flows [22], free jet flows [23,24], multi-phase flows [25] and flames [26,27]. The SAILOR code is based on a projection method with time integration performed by a predictor-corrector (Adams-Bashforth / Adams-Moulton) method [28]. ...
A computational analysis of excited round jets is presented with emphasis on jet bifurcation phenomenon due to superposition of axial and flapping forcing terms. Various excitation parameters are examined including the amplitudes of the forcing, their frequencies and phase shift. It is shown that alteration of these parameters significantly influences the spatial jet evolution. This dependence may be used to control the jet behaviour in a wide range of qualitatively different flow structures, starting from a modification of the spreading rate of a single connected jet, through large scale deformation of an asymmetric jet, onto jet bifurcation leading to a doubly and even triply split time-averaged jet, displaying different strengths and locations of the branches. We establish that: (i) jet splitting is possible only when the amplitudes of the forcing terms are comparable to or larger than the level of natural turbulence; (ii) the angle between the developing jet branches can be directly controlled by the frequency of the axial forcing and the phase shift between axial and flapping forcing. An optimum forcing frequency is determined, leading to the largest spreading rate.
... This code may be used for solving a wide range of flows under various conditions, varying from isothermal and constant density to situations with considerable density and temperature variations. The SAILOR code was used previously in various studies including laminar/turbulent transition in near-wall flows [10] free jet flows [11,12], multi-phase flows [13] and flames [14,15]. The solution algorithm used in the SAILOR code for the case constant density flows is presented in the next subsection. ...
This paper presents the results of computations of incompressible flows performed with a high-order compact scheme and the immersed boundary method. The solution algorithm is based on the projection method implemented using the half-staggered grid arrangement in which the velocity components are stored in the same locations while the pressure nodes are shifted half a cell size. The time discretization is performed using the predictor-corrector method in which the forcing terms used in the immersed boundary method acts in both steps. The solution algorithm is verified based on 2D flow problems (flow in a lid-driven skewed cavity, flow over a backward facing step) and turns out to be very accurate on computational meshes comparable with ones used in the classical approaches, i.e. not based on the immersed boundary method.
