LES of Particle-Laden Flow in Sharp Pipe Bends with Data-Driven Predictions of Agglomerate Breakage by Wall Impacts

The breakage of agglomerates due to wall impact within a turbulent two-phase flow is studied based on a recently developed model which relies on two artificial neural networks (ANNs). The breakup model is intended for the application within an Euler-Lagrange approach using the point-particle assumption. The ANNs were trained based on comprehensive DEM simulations. In the present study the entire simulation methodology is applied to the flow through two sharp pipe bends considering two different Reynolds numbers. In a first step, the flow structures of the continuous flow arising in both bend configurations are analyzed in detail. In a second step, the breakage behavior of agglomerates consisting of spherical, dry and cohesive silica particles is predicted based on the newly established simulation methodology taking agglomeration, fluid-induced breakage and breakage due to wall impact into account. The latter is found to be the dominant mechanism determining the resulting size distribution at the bend outlet. Since the setups are generic geometries found in dry powder inhalers, important knowledge concerning the effect of the Reynolds number as well as the design type (one-step vs. two-step deflection) can be gained.

Influence of Particle Mass Fraction over the Turbulent Behaviour of an Incompressible Particle-Laden Flow

The presence of spherical solid particles immersed in an incompressible turbulent flow was numerically investigated from the perspective of the particle mass fraction (PMF or ϕm), a measure of the particle-to-fluid mass ratio. Although a number of different changes have been reported to be obtained by the presence of solid particles in incompressible turbulent flows, the present study reports the findings of varying ϕm in the the turbulent behaviour of the flow, including aspects such as: turbulent statistics, skin-friction coefficient, and the general dynamics of a particle-laden flow. For this purpose, a particle-laden turbulent channel flow transporting solid particles at three different friction Reynolds numbers, namely Reτ=180, 365, and 950, with a fixed particle volume fraction of ϕv=10−3, was employed as conceptual flow model and simulated using large eddy simulations. The value adopted for ϕv allowed the use of a two-way coupling approach between the particles and the flow or carrier phase. Three different values of ϕm were explored in this work ϕm≈1,2.96, and 12.4. Assessment of the effect of ϕm was performed by examining changes of mean velocity profiles, velocity fluctuation profiles, and a number of other relevant turbulence statistics. Our results show that attenuation of turbulence activity of the carrier phase is attained, and that such attenuation increases with ϕm at fixed Reynolds numbers and ϕv. For the smallest Reynolds number case considered, flows carrying particles with higher ϕm exhibited lower energy requirements to sustain constant fluid mass flow rate conditions. By examining the flow velocity field, as well as instantaneous velocity components contours, it is shown that the attenuation acts even on the largest scales of the flow dynamics, and not only at the smaller levels. These findings reinforce the concept of a selective stabilising effect induced by the solid particles, particularly enhanced by high values of ϕm, which could eventually be exploited for improvement of energetic efficiency of piping or equivalent particles transport systems.

A Numerical Investigation Into the Heat Transfer Performance and Particle Dynamics of a Compressible, Highly Mass Loaded, High Reynolds Number, Particle Laden Flow

In this work, we study the heat transfer performance and particle dynamics of a highly mass loaded, compressible, particle-laden flow in a horizontally-oriented pipe using an Eulerian-Eulerian (two-fluid) computational model. An attendant experimental configuration [1] provides the basis for the study. Specifically, a 17 bar co-flow of nitrogen gas and copper powder are modeled with inlet Reynolds numbers of 3×104, 4.5×104, and 6×104 and mass loadings of 0, 0.5, and 1.0. Eight binned particle sizes were modeled to represent the known powder properties. Significant settling of all particle groups are observed leading to asymmetric temperature distributions. Wall and core flow temperature distributions are observed to agree well with measurements. In high Reynolds number cases, the predictions of the multiphase computational model were satisfactorily aligned with the experimental results. Low Reynolds number model predictions were not as consistent with the experimental measurements.

Simultaneous two-phase flow measurements in a high-speed particle-laden under-expanded jet

The objective of this study is to understand the dynamics of a high-speed particle-laden under-expanded jet, motivated by landings on extraterrestrial bodies. In this setup, inertial particles are entrained and accelerated by an under-expanded jet. But, due to their inertia, the particle velocity is significantly lower than that of the surrounding gas close to the nozzle, so the two phases are coupled through aerodynamic drag. Sub-micron oil droplets are dispensed upstream to serve as tracers, whose velocity is determined through a PIV system; inertial particles, after image segmenting is performed to separate them from PIV data, will be tracked over time with a PTV system. This was accomplished with a single laser pulse and the camera straddle time to produce image pairs and shorten the pulse width. The results will help to understand particle-laden flow in a new regime where the background flow is compressible and the Mach number based on the slip velocity is not negligible, which may help to pave a foundation for future studies in compressible multiphase flows.