Search

In the study of gas-particulate multiphase systems, the flow of high-speed gas through a distribution of solid particulates is of utmost importance. While these aerodynamically interacting systems have been extensively studied for low-speed gas flows in the gas continuum regime, less attention has been given to high-speed systems where non-continuum effects are significant due to the high flow gradients. To address this, the flow of rarefied gas through an aerodynamically interacting monodisperse spherical particle system is studied using the Direct Simulation Monte Carlo (DSMC) gas-kinetic approach. Since the method provides the best resolution of shocks at supersonic Mach numbers it is used to classify the weak separated shocks and strong collective shocks in these systems based on particle spacing in a two-particulate system at different orientation angles. The study used the two-particle system to help analyze more complex particle distributions of volume fractions, 1%, 5%, and 15%, exposed to gas flows in the slip and transitional gas regime for a free-stream Mach number range of 0. 2 < M a ∞ < 2. 0. We observe that the weak separated shocks in the 1% distribution allow a higher degree of gas penetration and shock-particle interactions or " hypersonic-surfing ", exposing a major fraction of the particulates to higher force magnitudes. In contrast, the strong collective shock in the 5% and 15% distributions only generates high particulate forces on the flow-facing particles. Finally, a simple stochastic model is proposed for use in large-scale Eulerian–Lagrangian simulations that captures the non-monotonic behavior of average drag and force variability generated by the complicated gas particulate interactions in the compressible gas regime. [Display omitted] • Study on high-speed gas flow through particulates, emphasizing non-continuum effects. • DSMC classifies shocks in particle systems, varying particle spacing and orientation. • ' Hypersonic surfing " in 1%, strong collective shocks in 5%, 15% distributions. • Model for simulations to capture gas-particulate interactions in compressible regime. [ABSTRACT FROM AUTHOR]

Highlights • Predicted gas transport properties using an integrated kinetic computational framework. • Analyzed the effect of material anisotropy on the permeability and pore-scale velocity. • Pore-diameter computed in this work agreed well with tomography derived data. • Determined representative elementary volume (REV) required for material characterization studies. Abstract We use the Cuda-based Hybrid Approach for Octree Simulations (CHAOS) DSMC solver to predict gas transport coefficients of Morgan felt and FiberForm TPS materials with sample size of (1 × 1 × 1) mm3. The detailed velocity flow-field of the pressure-driven flow through these materials is studied to compare the effect of material microstructures on gas transport. It is found that the effective flow path traversed by the gas is more circuitous and longer for FiberForm compared to the more porous felt. The obstruction offered by the material and the circuitous flow path is quantified by the Klinkenberg-derived permeability and hydraulic tortuosity factor, which are key material properties that govern the momentum transport through porous media. We also compute the hydraulic pore diameter of these materials and find that the through-thickness and in-plane pore diameter is equal to 86.94 and 98.7 μ m for felt and 36.25 and 60.9 μ m for Fiberform, which is within 5–6% of the average pore-size obtained from the tomography images. [ABSTRACT FROM AUTHOR]

A collision-limiter method, designated as equilibrium direct simulation Monte Carlo (eDSMC), is proposed to extend the DSMC technique to high pressure flows. The method is similar to collision-limiter schemes considered in the past with the important distinction that for inviscid flows, equilibrium is enforced in the entire flow by providing a sufficient number of collisions, based on pre-simulation testing. To test the method with standard DSMC and Navier-Stokes (NS) methods, axi-symmetric nozzle and embedded-channel flows are simulated and compared with experimental temperature data and pre-existing calculations, respectively. The method is shown to agree with third-order Eulerian nozzle flows and first-order channel flows. Chapman-Enskog theory is utilized to predict the range of initial conditions where eDSMC is potentially useful for modeling flows that contain viscous boundary layer regions. Comparison with supersonic nozzle data suggests that the eDSMC method is not adequate for capturing the large variation in flow length scales occurring in supersonic expansions into a vacuum. However, when eDSMC is used in combination with the baseline-DSMC method a near-exact solution is obtained with a considerable computational savings compared to the exact DSMC solution. Viscous flow channel calculations are found to agree well with an exact Navier-Stokes (NS) calculation for a small Knudsen number case as predicted by Chapman-Enskog theory. [ABSTRACT FROM AUTHOR]

Highlights • DSMC study of the effect of AVCOAT-like microstructure on internal gas transport. • Permeability and tortuosity computation for AVCOAT and comparison with fibrous TPS. • Coupled convection, conduction, and radiation using a random walk model. • Study of AVCOAT with spatially varying thermophysical properties at high temperatures. • Comparison of thermal response predicted by stochastic versus finite-volume approaches. Abstract A multi-scale modeling approach based on the stochastic Direct Simulation Monte Carlo (DSMC) and walker methods is developed to understand the complex flows and thermophysical phenomena through a syntactic foam TPS, similar to AVCOAT. Using novel, unstructured adaptive mesh refinement/Octree grids and newly developed subsonic boundary conditions, the counter-flow transport of boundary layer and pyrolysis gases through the porous microstructure is modeled for the first time. Permeability of the microstructure models having porosities of 0.71 and 0.86 is computed in the DSMC simulations and compared to a fibrous TPS material. The rigorous development of a stochastic based thermal response model that can couple convective, conductive, and radiative heat transfer through a porous material having non-uniform thermophysical properties and high temperature gradients is presented and compared with a one-dimensional finite-volume approach. The material thermal response is found to be dominated by conduction, yet, the interactions between boundary layer and pyrolysis species on the actual 3-D geometry, which cannot be considered in traditional material response solvers, may in fact cause them to underpredict the TPS material temperature. [ABSTRACT FROM AUTHOR]

A numerical model is developed to consider for the first time droplet coalescence along with transport, heating and desolvation in an argon inductively coupled plasma (Ar ICP). The direct simulation Monte Carlo (DSMC) method and the Ashgriz–Poo model are used, respectively, to compute droplet–droplet interactions and to determine the outcome of droplet collisions. Molecular dynamics (MD) simulations support the use of the Ashgriz–Poo coalescence model for small droplet coalescence. Simulations predict spatial maps of droplet number and mass densities within an Ar ICP for a conventional nebulizer-spray chamber arrangement, a direct injection high efficiency nebulizer (DIHEN), and a large bore DIHEN (LB-DIHEN). The primary findings are: (1) even at 1500 W, the collisions of the droplets in the plasma lead primarily to coalescence, particularly for direct aerosol injection; (2) the importance of coalescence in a spray simulation exhibits a complex relationship with the gas temperature and droplet size; (3) DIHEN droplets penetrate further into the Ar ICP when coalescence is considered; and (4) droplets from a spray chamber or the LB-DIHEN coalesce less frequently than those from a DIHEN. The implications of these predictions in spectrochemical analysis in ICP spectrometry are discussed. [Copyright &y& Elsevier]