LES and RANS calculations of particle dispersion behind a wall-mounted cubic obstacle

In the present paper, we evaluate the performances of three stochastic models for particle dispersion in the case of a three-dimensional turbulent flow. We consider the flow in a channel with a cubic wall-mounted obstacle, and perform large-eddy simulations (LESs) including passive particles injected behind the obstacle, for cases of low and strong inertial effects. We also perform Reynolds-averaged simulations of the same case, using standard turbulence models, and employ the two discrete stochastic models for particle dispersion implemented in the open-source code OpenFOAM and the continuous Lagrangian stochastic model proposed by Minier et al. (2004). The Lagrangian model is consistent with a Probability Density Function (PDF) model of the exact particle equations, and is based on the modelling of the fluid velocity seen by particles. This approach allows a consistent formulation which eliminates the spurious drifts flawing discrete models and to have the drag force in a closed form. The LES results are used as reference data both for the fluid RANS simulations and particle simulations with dispersion models. The present test case allows to evaluate the performance of dispersion models in highly non-homogeneous flow, and it used in this context for the first time. The continuous stochastic model generally shows a better agreement with the LES than the discrete stochastic models, in particular in the case of particles with higher inertia

Combustion characteristics of steam-diluted decomposed ammonia in multiple-nozzle direct injection burner

In line with the decarbonisation of power sector, carbon-free fuels are currently being investigated. In particular, green ammonia or e-ammonia is a candidate fuel which will be playing a key role in many energy-intensive industries. It calls for an in-depth understanding of eFuels combustion characteristics in the fuel flexible combustors. Therefore, the present work for the first time numerically investigates the combustion regimes of steam-diluted, decomposed eNH3 in a novel multi-nozzle direct injection (MDI) burner. Although the MDI burner is not equipped with a conventional swirler, strong flow-flame interaction is observed. The two-layer, angled channels create swirling flows featuring swirl numbers larger than 0.9 in general. The centre recirculation region can help stabilise highly steam-diluted decomposed ammonia with a maximum steam-to-air ratio of 74%. This highest H2% containing, wettest ammonia flame case is found to emit the lowest total emission (NH3+NO + NO2+N2O) of ∼400ppmvd@15%O2 at stoichiometric conditions. The wall heat loss is confirmed responsible for the formation of N2O in distributed flame, suggesting the need of reducing pollution through good chamber wall insulation. However, for flames sitting in the conventional regimes, the impact of wall heat loss is found insignificant. Further, extensive data and flame regime analyses show that NNH can always accurately mark the high heat release region of all types of flames, while OH is only an effective marker for thin flames

A joint numerical study of multi-regime turbulent combustion

This article presents a joint numerical study on the Multi Regime Burner configuration. The burner design consists of three concentric inlet streams, which can be operated independently with different equivalence ratios, allowing the operation of stratified flames characterized by different combustion regimes, including premixed, non-premixed, and multi-regime flame zones. Simulations were performed on three LES solvers based on different numerical methods. Combustion kinetics were simplified by using tabulated or reduced chemistry methods. Finally, different turbulent combustion modeling strategies were employed, covering geometrical, statistical, and reactor based approaches. Due to this significant scattering of simulation parameters, a conclusion on specific combustion model performance is impossible. However, with ten numerical groups involved in the numerical simulations, a rough statistical analysis is conducted: the average and the standard deviation of the numerical simulation are computed and compared against experiments. This joint numerical study is therefore a partial illustration of the community's ability to model turbulent combustion. This exercise gives the average performance of current simulations and identifies physical phenomena not well captured today by most modeling strategies. Detailed comparisons between experimental and numerical data along radial profiles taken at different axial positions showed that the temperature field is fairly well captured up to 60 mm from the burner exit. The comparison reveals, however, significant discrepancies regarding CO mass fraction prediction. Three causes may explain this phenomenon. The first reason is the higher sensitivity of carbon monoxide to the simplification of detailed chemistry, especially when multiple combustion regimes are encountered. The second is the bias introduced by artificial thickening, which overestimates the species’ mass production rate. This behavior has been illustrated by manufacturing mean thickened turbulent flame brush from a random displacement of 1-D laminar flame solutions. The last one is the influence of the subgrid-scale flame wrinkling on the filtered chemical flame structure, which may be challenging to model.</p

A joint numerical study of multi-regime turbulent combustion

This article presents a joint numerical study on the Multi Regime Burner configuration. The burner design consists of three concentric inlet streams, which can be operated independently with different equivalence ratios, allowing the operation of stratified flames characterized by different combustion regimes, including premixed, non-premixed, and multi-regime flame zones. Simulations were performed on three LES solvers based on different numerical methods. Combustion kinetics were simplified by using tabulated or reduced chemistry methods. Finally, different turbulent combustion modeling strategies were employed, covering geometrical, statistical, and reactor based approaches. Due to this significant scattering of simulation parameters, a conclusion on specific combustion model performance is impossible. However, with ten numerical groups involved in the numerical simulations, a rough statistical analysis is conducted: the average and the standard deviation of the numerical simulation are computed and compared against experiments. This joint numerical study is therefore a partial illustration of the community's ability to model turbulent combustion. This exercise gives the average performance of current simulations and identifies physical phenomena not well captured today by most modeling strategies. Detailed comparisons between experimental and numerical data along radial profiles taken at different axial positions showed that the temperature field is fairly well captured up to 60 mm from the burner exit. The comparison reveals, however, significant discrepancies regarding CO mass fraction prediction. Three causes may explain this phenomenon. The first reason is the higher sensitivity of carbon monoxide to the simplification of detailed chemistry, especially when multiple combustion regimes are encountered. The second is the bias introduced by artificial thickening, which overestimates the species’ mass production rate. This behavior has been illustrated by manufacturing mean thickened turbulent flame brush from a random displacement of 1-D laminar flame solutions. The last one is the influence of the subgrid-scale flame wrinkling on the filtered chemical flame structure, which may be challenging to model.</p

Large eddy simulation of unsteady lean stratified premixed combustion

Premixed turbulent flame-based technologies are rapidly growing in importance, with applications to modern clean combustion devices for both power generation and aeropropulsion. However, the gain in decreasing harmful emissions might be canceled by rising combustion instabilities. Unwanted unsteady flame phenomena that might even destroy the whole device have been widely reported and are subject to intensive studies. In the present paper, we use unsteady numerical tools for simulating an unsteady and well-documented flame. Computations were performed for nonreacting, perfectly premixed and stratified premixed cases using two different numerical codes and different large-eddy-simulation-based flamelet models. Nonreacting simulations are shown to agree well with experimental data, with the LES results capturing the mean features (symmetry breaking) as well as the fluctuation level of the turbulent flow. For reacting cases, the uncertainty induced by the time-averaging technique limited the comparisons. Given an estimate of the uncertainty, the numerical results were found to reproduce well the experimental data in terms both of mean flow field and of fluctuation levels. In addition, it was found that despite relying on different assumptions/simplifications, both numerical tools lead to similar predictions, giving confidence in the results. Moreover, we studied the flame dynamics and particularly the response to a periodic pulsation. We found that above a certain excitation level, the flame dynamic changes and becomes rather insensitive to the excitation/instability amplitude. Conclusions regarding the self-growth of thermoacoustic waves were drawn. (c) 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved

Estimation of nitrate retention in Ferralsol by a transient-flow method.

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