Flow, Turbulence and Combustion最新文献

Large Eddy Simulations (LESs) use Sub-Grid Scale (SGS) models to account for the effects of the unresolved scales of turbulence. The complex processes that occur in the small scales make the development of SGS models challenging. This complexity is even compounded in the presence of multiphase physics due to the mutual interactions between the small-scale hydrodynamics and the dispersed phase distribution and behaviour. In this study, we propose to avoid using an SGS model and demonstrate a novel technique to use a Periodic Box (PB) Direct Numerical Simulation (DNS) solver to find and represent the local SGS turbulence for supplementing a LES. This technique involves matching the local characteristic strain rate in the LES with the large-scale characteristic strain rate in the PB DNS. For simplicity, we assume Homogeneous Isotropic Turbulence (HIT) to be a good representation of SGS turbulence. For a test case, viz. HIT, we compare the averaged turbulence spectra from the LES and the PB DNS with the exact solution from a full DNS simulation. The results show an almost seamless coupling between the large and small scales. As such, this model is more accurate than the common Smagorinsky model in describing the properties of small scales while working within the same assumptions. Further, the effective Smagorinsky constant predicted by our model and the DNS simulation agree. Finally, a two-way coupling is introduced where an effective viscosity is computed in the PB DNS and supplied back to the LES. The results show a definitive improvement in the LES while maintaining stability. The findings showcase the capability of a PB DNS to support a LES with a near-exact simulation of the SGS turbulence.

For the development of upper stage rocket engines with laser ignition, the transition of oxidizer and fuel from the pure cryogenic liquid streams to an ignitable mixture needs to be better understood. Due to the near vacuum conditions that are present at high altitudes and in space, the injected fuel rapidly atomizes in a so-called flash boiling process. To investigate the behavior of flashing cryogenic jets under the relevant conditions, experiments of liquid nitrogen have been performed at the DLR Lampoldshausen. The experiments are accompanied by a series of computer simulations and here we use a highly resolved LES to identify 3D effects and to better interpret results from the experiments and existing 2D RANS. It is observed that the vapor generation inside the injector and the evolution of the spray in the combustion chamber differ significantly between the two simulation types due to missing 3D effects and the difference in resolution of turbulent structures. Still, the observed 3D spray dynamics suggest a suitable location for laser ignition that could be found in regions of relative low velocity and therefore expected low strain rates. Further, measured droplet velocities are compared to the velocities of notional Lagrangian particles with similar inertia as the measured droplets. Good agreement between experiments and simulations exists and strong correlation between droplet size and velocity can be demonstrated.

In this work, we revisit the application of the compressible linear eddy model for large eddy simulation (CLEM-LES) of calorically perfect gas detonations in an attempt to clarify if the Kolmogorov number can be treated as a constant instead of a tuning parameter when no-slip boundary conditions are included in three-dimensional simulations. In its early development, the CLEM-LES with a one-step combustion chemistry model was used to simulate two-dimensional methane-oxygen detonations to gain insight on the roles and impact of turbulent mixing rates on the presence of unburned pockets of reactive gas and cellular structure. In these past simulations, special treatment of the boundary conditions was not considered, and therefore wave speeds always recovered the Chapman-Jouguet (CJ)-velocity. Moreover, tuning of the Kolmogorov number was required in order to qualitatively capture the experimentally observed flow fields. In this work we carefully perform three-dimensional simulations of detonation propagation using the CLEM-LES, and include no-slip walls as boundary conditions. Also, instead of tuning the Kolmogorov number to obtain the correct cell size, as was done in the past, we instead use a standard value of 1.5. We found that by carefully specifying the boundary conditions, and treating the Kolmogorov as a constant (thus no model calibration), both the expected propagation velocity deficit and cellular structure are recovered. Finally, upon constructing the resulting energy spectrum, we found that the kinetic energy cascade follows the well-known −5/3 power law description of incompressible turbulence in the inertial subrange, but was not symmetric nor isotropic.

The Head-on Quenching (HoQ) of laminar premixed ammonia–hydrogen-air flames under lean to stoichiometric condition is numerical investigated. Detailed chemistry including 34 reactive species and detailed multi-component transport model including thermal diffusion (Soret effect) are applied. The quenching distance is considered as a representative quantity for the HoQ process, and the influence of different system parameters on it has been investigated. These parameters involve fuel/air equivalence ratios, hydrogen content in gas mixture and pressure. It was found that an increase of quenching distance can be caused by a lower hydrogen addition and a leaner mixture condition. Furthermore, it was found that, regardless of the gas mixture, the quenching distance decreases monotonically with increasing pressure, obeying a power function with the exponent (-) 0.7. Moreover, numerical results show a relation between the quenching Peclet number and the dimensionless wall heat flux normalized by the flame power. Additionally, sensitivities of quenching distances with respect to the transport model, considering the heat loss in the wall and the chemical kinetics are studied.