Large Eddy simulation of a pulverised coal jet flame Franchetti, B.M.; Cavallo Marincola, F.; Navarro-Martinez, S. ...
Proceedings of the Combustion Institute,
2013, 2013-00-00, Letnik:
34, Številka:
2
Journal Article
Recenzirano
Large Eddy simulation (LES) has been applied to the pulverised coal jet flame studied at the Japanese Central Research Institute of Electric Power (CRIEPI). A working set of models to represent coal ...combustion, Lagrangian particle transport and radiative heat transfer in an LES framework has been implemented and tested. The simulation results of the flow field were compared to experimental data for both a reactive and non-reactive case, and an overall good agreement emerged. A simple method for replicating pyrometer measurements was developed for the LES and results obtained from the method were compared to the experimental data. Finally the species concentrations were compared to experimental results for CO2, O2 and N2. The results show the potentials of using LES for pulverised coal combustion and open the way for further developments on the coal combustion models and the applications to more complex burners.
•Three modifications of the Eddy Break-Up combustion model are considered: basic with Said-Borghi correction, extended to quasi-laminar flame propagation, and augmented with weighted laminar flame ...speed.•The models are assessed by simulating two upward flame propagation combustion experiments, performed in large-scale vessels of different volumes.•Differences between results, obtained with different models, are much larger in the wider vessel than in the narrower one.•The quasi-laminar term in the progress variable transport equation source term may exert considerable influence when simulating deflagration in large vessels.
Hydrogen deflagration in experimental containment vessels is simulated by introducing a modified Eddy Break-Up model, with the aim to apply it for hydrogen combustion in containment vessels. First, an additional term is introduced in the source term of the progress variable (non-dimensional fuel concentration) transport equation to extend its applicability from the turbulent to the quasi-laminar flame propagation region. The model is then modified further by introducing the weighted laminar flame speed, which decreases the influence of this quasi-laminar term in the same equation. The models are assessed by simulating two experiments, performed in different large-scale facilities. Simulated axial and radial flame propagation, pressure and pressure increase rate, and flame shapes are compared to experimental results. The comparisons reveal that the model differences affect mostly the simulations in the wider vessel.
Large eddy simulations (LES) based on turbulent combustion models aid the design and optimization of combustors. Of the various combustion models available, the eddy break up (EBU) model is widely ...used because it assumes an infinitely fast chemistry. However, omitting the actual chemical kinetics can cause unexpected behavior, and the characteristics of the combustion models need to be elucidated. Here, the effects of an infinitely fast chemistry on the combustion behavior of a coaxial diffusion flame as predicted by an LES were analyzed. Although the EBU model captured the overall behavior of the chemical species as well as the flow field, the gas temperature and mass fractions of the combustion products in the mixing region of the fuel and oxidizer streams were overestimated. In contrast, the flamelet/progress variable (FPV) model yielded results that were in better agreement with the experimental data, because while the EBU model assumes an infinitely fast chemistry, the look-up tables used in the FPV model are based on the actual chemical kinetics. As these models can be used for the CFD simulations of coal and spray combustion, the results of this study should be useful for efficiently simulating practical combustion systems.
•Large eddy simulation of coaxial diffusion flame is performed.•Eddy break up model overestimates mass fractions of chemical products.•It also overestimates gas temperature.•Flamelet/progress-variable model captures slow progress of chemical reactions.•Flame index suggests coexistence of premixed and nonpremixed modes.
A three-dimensional mathematical model has been developed for the simulation of flow, temperature and concentration fields in the radiation section of industrial scale steam cracking units. The model ...takes into account turbulence–chemistry interactions through the Eddy Dissipation Concept (EDC) model and makes use of Detailed Reaction Kinetics (DRK), which allows the detailed investigation of the flame structure. Furthermore, simulation results obtained with the EDC-DRK model are compared with simulation results obtained with a simplified model combining the Eddy Break Up (EBU)/finite rate formulation with Simplified Reaction Kinetics (SRK). When the EBU-SRK model is used, much faster fuel oxidation and products formation is predicted. The location of the peak temperature is shifted towards the burner, resulting in a smaller flame and the confinement of the combustion process into a smaller area. This is most likely because of the inherent deficiency of the simplified model to correctly predict the overall (effective) burning rate when the turbulent mixing rate and the reaction rate are comparable. It is shown that when neither the “fast-chemistry” nor the “slow-chemistry” approximation is satisfied, the overall burning rate is overpredicted. The smaller flame volumes obtained with the EBU-SRK model have important effects on the predicted temperature distribution in the furnace as well as on other significant design parameters like the refractory wall and tube skin temperatures. It is suggested that more sophisticated turbulence–chemistry interaction models like the EDC model and more Detailed Reaction Kinetics should be used for combustion modeling in steam cracking furnaces under normal firing conditions.
An experimental investigation has been performed to study the effect of combined artificially roughened (ribs) with and without single Large Eddy Break-Up Devices, on flow and heat transfer ...characteristic of fully developed turbulent flow in rectangular duct. The aspect ratio of rectangular duct is 10, hydraulic diameter 72.72 mm, relative roughness pitch (P/e) 10 and relative roughness height (e/Dh) 0.05. The rib was in the form of circular shape with diameter of (4mm) which was mounted on heated wall of duct at spanwise direction. The experiments have been conducted by varying airflow rate in terms of Reynolds number ranging from 3.2x104 to 6.2x104 and constant heat flux of 600W/m2. The heat transfer and friction factor of the flow for rib and combined method were compared with those of a smooth duct under similar experimental conditions. It has been found that the combined method (rib with single Large Eddy Break-Up Devices) has significant effect on the friction factor and heat transfer with decreasing in friction factor with percent(1.2) and increasing Nusselt number with (4.1). Correlations for Nusselt number and friction factor in terms of (Reynolds number and Large Eddy Break-Up Devices) parameters are found which reasonably correlate the experimental data.
This work presents a Computational Fluid Dynamics (CFD) study of the non-premixed combustion of natural gas with air in an axisymmetric cylindrical chamber, focusing on the contribution of the ...chemical reaction modeling on the temperature and the chemical species concentration fields. Simulations are based on the solution of mass, momentum, energy and chemical species conservation equations. Thermal radiation heat transfer in the combustion chamber is computed through the Discrete Transfer Radiation Method, and the Weighted-Sum-of-Gray-Gases model solves the dependence of gas absorption coefficient on the wavelength. Turbulence is modeled by the standard k-ε model. Regarding the combustion modeling, it is performed a comparison of solutions obtained with the combined Eddy Break-Up/Arrhenius (EBU/Arrhenius) and the Steady Laminar Diffusion Flamelet (SLDF) models. The finite volume method is employed to treat the differential equations. Among other results, the solution of the governing equations allows for the determination of the region where combustion takes place, the distribution of the chemical species and the velocity fields. The numerical results are compared to experimental measurements, showing varied agreements. Results indicate that, in this case, the EBU/Arrhenius model can predict the flame temperature and the concentration of the most important species with better accuracy than the more sophisticated SLDF model.
A well-resolved large eddy simulation (LES) of a large-eddy break-up (LEBU) device in a spatially evolving turbulent boundary layer is performed with, Reynolds number, based on free-stream velocity ...and momentum-loss thickness, of
R
e
θ
≈ 4300. The implementation of the LEBU is via an immersed boundary method. The LEBU is positioned at a wall-normal distance of 0.8
δ
(
δ
denoting the local boundary layer thickness at the location of the LEBU) from the wall. The LEBU acts to delay the growth of the turbulent boundary layer and produces global skin friction reduction beyond 180
δ
downstream of the LEBU, with a peak local skin friction reduction of approximately 12 %. However, no net drag reduction is found when accounting for the device drag of the LEBU in accordance with the towing tank experiments by Sahlin et al. (Phys. Fluids
31
, 2814,
1988
). Further investigation is performed on the interactions of high and low momentum bulges with the LEBU and the corresponding output is analysed, showing a ‘break-up’ of these large momentum bulges downstream of the LEBU. In addition, results from the spanwise energy spectra show consistent reduction in energy at spanwise length scales for
λ
z
+
>
1000
independent of streamwise and wall-normal location when compared to the corresponding turbulent boundary layer without LEBU.
The effects of implementing a large-eddy break-up device (LEBU) in a turbulent boundary layer on the interaction with the boundary layer is investigated with particular emphasis on the ...turbulent/non-turbulent interface (TNTI). The simulation data is taken from a recent well-resolved large eddy simulation (Chin et al. Flow Turb. Combust.
98
, 445–460
2017
), where the LEBU was implemented at a wall-normal distance of 0.8
δ
(local boundary layer thickness) from the wall. A comparison of the TNTI statistics is performed between a zero-pressure-gradient boundary layer with and without the LEBU. The LEBU is found to delay the growth of the turbulent boundary layer and also attenuates the fluctuations of the TNTI. The LEBU appears to alter the structure size at the interface, resulting in a narrower and shorter dominant structure (in an average sense). Further analysis beneath the TNTI using two-point correlations shows that the LEBU affects the turbulent structures in excess of 100
δ
downstream of the LEBU.
Turbulent boundary layers create aerodynamic noise inside all vehicles, and especially inside jetliners. The objective of this project is to modify the turbulence in an attached boundary layer, not ...to reduce skin friction, but to weaken the wall pressure fluctuations, or to shift them to less damaging frequencies and wavelengths. In order to benefit an entire airliner windshield, the effect would be sustained over about 25 boundary-layer thicknesses,
δ, which far exceeds the common rule that the relaxation of the turbulence takes about 10
δ. Various flow-control devices are studied by Direct Numerical Simulation, which is free of modeling and provides the full details of the pressure field. The Reynolds number is far lower than in the real flow, but this limitation of DNS is tolerable, because the focus is on the larger eddies. The multi-block implicit numerical method can represent fairly complex devices at a manageable cost. Inflow turbulence is provided by a recycling procedure derived from that of Lund, Wu and Squires, but much simpler. It occupies less than 5
δ in the streamwise direction. Flow visualizations, Reynolds stresses, and spectra are shown; the baseline spectra are within the experimental scatter. Co-rotating vortex generators are tried first. They reduce the turbulence intensity away from the wall, as hoped, but actually intensify the wall pressure fluctuations and were therefore abandoned. Large-eddy-break-up devices resembling a highway bridge are tried next, and succeed in reducing the wall fluctuations, but only over about 6
δ. Thus, the technology is not successful yet for a windshield, but it might be applied to other windows or other vehicles, and the simulation methodology appears to be well developed and of some interest particularly regarding inflow conditions.
The numerical modeling of High Temperature Air Combustion (HTAC) could correctly predict the flame and emission characteristics so far, however, the precision is not satisfactory. In this paper, a ...parameter-modified Eddy-Break-Up (EBU) model with a three-step reaction scheme was used to simulate the HTAC process in a furnace of 2m ×2m×6.25m. Reynolds stress model (RSM) is used for the computational fluid dynamics (CFD) simulation. Comparison between experimental data from published papers, the original and modified EBU combustion models demonstrates that a smaller coefficient ‘A’ in the volumetric fuel consumption rate of the EBU model could improve the modelling precision. When ‘A’ is modified to be 1 other than its original value, 4, in the EBU model, the modeling could give improved results of the local distribution of CH4, temperature and NO emission.
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