arXiv subjects

Dominique Thévenin

Publications and source records attributed to Dominique Thévenin.

4 recordsLinked to original sources

arXiv2026

Stabilization of premixed NH3/H2/air flames via bluff-body flame holders

The stabilization mechanisms of fully premixed NH3/H2/air flames anchored behind a bluff body are investigated using combined experiments and direct numerical simulations. Particular attention is given to the interplay between preferential diffusion, heat release, flow recirculation, and turbulence-flame interaction. Comparison between non-reactive and reactive cases shows that thermal expansion strongly alters the flow field, increasing the recirculation zone length by about 40% and the shear layer width by roughly 50% near the end of the recirculation region. Excellent agreement between measurements and simulations for mean and fluctuating axial velocities validates the numerical approach. Analysis of the flame structure reveals a distinctive stabilization mechanism at the flame root: preferential hydrogen diffusion generates a localized diffusion flame branch that enhances radical production and promotes robust anchoring. Combustion proceeds sequentially, with hydrogen mainly consumed in the shear layer, followed by ammonia cracking and the main heat-release region. Near the bluff body, heat release is concentrated within the recirculation zone, while downstream regions are increasingly influenced by turbulence and velocity fluctuations. The roles of curvature and strain are quantified to assess stretch effects along the flame front. Convex curvature near the flame root enhances hydrogen enrichment and locally increases burning rates, reinforcing stabilization. In contrast, concave curvature and higher stretch near the end of the recirculation zone weaken the flame and mark a transition toward a turbulence-dominated regime. Overall, stabilization results from a coupled feedback between recirculation-driven heat exchange and rapid hydrogen oxidation, sustaining an intermediate ammonia reaction zone and enabling robust anchoring of carbon-free NH3/H2 flames.

physics.flu-dyn↗

arXiv2023

Modeling gas flows in packed beds with the lattice Boltzmann method: validation against experiments

This study aims to validate the lattice Boltzmann method and assess its ability to accurately describe the behavior of gaseous flows in packed beds. To that end, simulations of a model packed bed reactor, corresponding to an experimental bench, are conducted, and the results are directly compared with experimental data obtained by Particle Image Velocimetry measurements. It is found that the lattice Boltzmann solver exhibits very good agreement with experimental measurements. Then, the numerical solver is further used to analyze the effect of the number of packing layers on the flow structure and to determine the minimum bed height above which the changes in flow structure become insignificant. Finally, flow fluctuations in time are discussed. The findings of this study provide valuable insights into the behavior of the gas flow in packed bed reactors, opening the door for further investigations involving additionally chemical reactions, as found in many practical applications.

physics.flu-dyn↗

arXiv2023

Comparative study of flow fluctuations in ruptured and unruptured intracranial aneurysms: A lattice Boltzmann study

Flow fluctuations have recently emerged as a promising hemodynamic metric for understanding the rupture risk of intracranial aneurysms. Several investigations have reported in the literature corresponding flow instabilities using established computational fluid dynamics tools. In this study, the occurrence of flow fluctuations is investigated using either Newtonian or non-Newtonian fluid models in patient-specific intracranial aneurysms using high-resolution lattice Boltzmann method simulations. Flow instabilities are quantified by computing power spectral density, proper orthogonal decomposition and spectral entropy, and fluctuating kinetic energy of velocity fluctuations. Furthermore, these hemodynamic parameters are compared between the ruptured and unruptured aneurysms. Our simulations reveal that the pulsatile inflow through the neck in a ruptured aneurysm is subject to a hydrodynamic instability leading to high-frequency fluctuations around the rupture position throughout the entire cardiac cycle. At other locations, the flow instability is only observed during the deceleration phase; typically, the fluctuations begin there just after peak systole, gradually decay, and the flow returns to its original, laminar pulsatile state during diastole. In the unruptured aneurysm, there is only minimal difference between Newtonian and non-Newtonian results. In the ruptured case, using the non-Newtonian model leads to a considerable increase of the fluctuations within the aneurysm sac.

physics.flu-dyn↗

arXiv2020

Settling of spherical particles in the transitional regime

The settling process and wall impact of large spherical particles in a stagnant, highly viscous fluid has been observed by means of high-speed shadow imaging. The particles included in this study vary in size and material properties: steel, polytetrafluorethylen (PTFE), polyoxymethylen (POM), or rubber. The corresponding terminal Reynolds numbers range from 333 to 4012, covering in principle the transitional and Newton regime for drag forces. For collision studies, the Stokes number just before impact is also relevant, and lies in the range 50 < St < 2250. The settling curves obtained experimentally are compared with numerical and analytical solutions. The latter has been derived on the basis of nominal terminal velocity and relaxation time for the Stokes and Newton regimes. A deviation between experimental results and analytical solution was observed in all cases where the terminal Reynolds number is larger than 300 and smaller than 1100. In this flow regime, the settling process of the spheres is already affected at a long distance from the impact object, leading to an early but significant deceleration. Moreover, a reduced settling velocity was observed along the whole trajectory for the PTFE particles with the lowest terminal Reynolds number. All processed datasets are available via the Mendeley Data repository. Two additional effects have been observed during this study. First, a bright region was detected around all PTFE spheres. It finally was found to be due to total light reflection around the sphere, but, to the best of our knowledge, this peculiarity has never been reported before. Being purely optical, this does not effect the settling behavior for PTFE. On the other hand, partial absorption of liquid at the particle surface was observed for rubber, leading to a reduced sedimentation velocity.

physics.flu-dyn↗