Droplet Agglomeration in Rocket Nozzles Caused by Particle Slip and Collision

Droplet Agglomeration in Rocket Nozzles Caused by Particle Slip and Collision. The development of the particle mass spectrum in a rocket nozzle is investigated under the assumption that droplet growth by collision and agglomeration is the dominant mechanism subsequent to initial appearance of particles in the rocket chamber. Collisions are calculated on the basis oflinearized particle slip theory and a spectral integral equation is derived describing the development of particle mass spectrum during the flow process along the nozzle. This agglomeration process continues until the droplet temperature falls below the freezing point of the material. A solution is obtained for the approximate growth in the average particle size during the expansion process. The results show that, according to this model, the particle size is strongly dependent on the initial pressure in the rocket chamber and is independent of nozzle geometry. These results suggest that the collision-agglomeration process is at least one of the critical factors that accounts for the size of solid particles in rocket exhausts

Discrete element modelling of fluidised bed spray granulation

A novel discrete element spray granulation model capturing the key features of fluidised bed hydrodynamics, liquid-solid contacting and agglomeration is presented. The model computes the motion of every individual particle and droplet in the system, considering the gas phase as a continuum. Micro scale processes such as particle-particle collisions, droplet-particle coalescence and agglomeration are directly taken into account by simple closure models. Simulations of the hydrodynamic behaviour of a batch granulation process are presented to demonstrate the potential of the model for creating\ud insight into the influence of several key process conditions such as fluidisation velocity, spray rate and spray pattern on powder product characteristics

Collision of a sphere onto a wall coated with a liquid film

Particle-particle and particle-wall collisions occur in many natural and industrial applications such as sedimentation, agglomeration, and granular flows. To accurately predict the behavior of particulate flows, fundamental knowledge of the mechanisms of a single collision is required. In this fluid dynamics video, particle-wall collisions onto a wall coated with 1.5% poly(ethylene-oxide) (PEO) (viscoelastic liquid) and 80% Glycerol and water (Newtonian liquid) are shown.Comment: 1 page, no figure

Simulation of deterministic energy-balance particle agglomeration in turbulent liquid-solid flows

An efficient technique to simulate turbulent particle-laden flow at high mass loadings within the four-way coupled simulation regime is presented. The technique implements large-eddy simulation, discrete particle simulation, a deterministic treatment of inter-particle collisions, and an energy-balanced particle agglomeration model. The algorithm to detect inter-particle collisions is such that the computational costs scale linearly with the number of particles present in the computational domain. On detection of a collision, particle agglomeration is tested based on the pre-collision kinetic energy, restitution coefficient, and van der Waals’ interactions. The performance of the technique developed is tested by performing parametric studies on the influence of the restitution coefficient (en = 0.2, 0.4, 0.6, and 0.8), particle size (dp = 60, 120, 200, and 316 μm), Reynolds number (Reτ = 150, 300, and 590), and particle concentration (αp = 5.0 × 10−4, 1.0 × 10−3, and 5.0 × 10−3) on particle-particle interaction events (collision and agglomeration). The results demonstrate that the collision frequency shows a linear dependency on the restitution coefficient, while the agglomeration rate shows an inverse dependence. Collisions among smaller particles are more frequent and efficient in forming agglomerates than those of coarser particles. The particle-particle interaction events show a strong dependency on the shear Reynolds number Reτ, while increasing the particle concentration effectively enhances particle collision and agglomeration whilst having only a minor influence on the agglomeration rate. Overall, the sensitivity of the particle-particle interaction events to the selected simulation parameters is found to influence the population and distribution of the primary particles and agglomerates formed

Particle-Interaction Effects in Turbulent Channel Flow

Large eddy simulation and a discrete element method are applied to study the flow, particle dispersion and agglomeration in a horizontal channel. The particle-particle interaction model is based on the Hertz-Mindlin approach with Johnson-Kendall-Roberts cohesion to allow the simulation of Van der Waals forces in a dry air flow. The influence of different particle surface energies on agglomeration, and the impact of fluid turbulence, are investigated. The agglomeration rate is found to be strongly influenced by the particle surface energy, with most of the particle-particle interactions taking place at locations close to the channel walls, aided by the higher concentration of particles in these regions

Morphology and agglomeration control of LiMnPO4_4 micro- and nanocrystals

Microwave-assisted hydrothermal synthesis was used to grow LiMnPO$_4$ micro- and nanocrystals from acetate precursors. By appropriate adjustment of the precursor concentration and the pH-value of the reactant, the product composition and purity along with the crystal size can be manipulated, resulting in particle-dimensions from around 10 mm down to a few 100 nm. Prisms and plates with hexagonal basal face as well as cuboid and rod-like particles were produced. The effects on the crystal morphology as well as on the materials texture and agglomeration tendency are discussed and a comprehensive agglomeration phase diagram is constructed

Reynolds number effects on particle agglomeration in turbulent channel flow

The work described in this paper employs large eddy simulation and a discrete element method to study particle-laden flows, including particle dispersion and agglomeration, in a horizontal channel. The particle-particle interaction model is based on the Hertz- Mindlin approach with Johnson-Kendall-Roberts cohesion to allow the simulation of Van der Waals forces in a dry air flow. The influence of different flow Reynolds numbers, and therefore the impact of turbulence, on particle agglomeration is investigated. The agglomeration rate is found to be strongly influenced by the flow Reynolds number, with most of the particle-particle interactions taking place at locations close to the channel walls, aided by the higher turbulence and concentration of particles in these regions

Grinding kinetics and equilibrium states

The temporary and permanent equilibrium occurring during the initial stage of cement grinding does not indicate the end of comminution, but rather an increased energy consumption during grinding. The constant dynamic equilibrium occurs after a long grinding period indicating the end of comminution for a given particle size. Grinding equilibrium curves can be constructed to show the stages of comminution and agglomeration for certain particle sizes

The Influence of Gravity on Particle Collision and Agglomeration in Turbulent Channel Flows

The study described in this paper concerns the simulation of a particle-laden turbulent channel flow at high mass loadings, with and without the presence of gravity. Large eddy simulation (LES) is used to simulate the fluid phase, with solutions combined with a Lagrangian particle tracker to model the particle phase. Particle-particle interactions are detected using an algorithm based on a deterministic collision treatment (hard-sphere collision model), and particle agglomeration is based on the use of a particle restitution coefficient, energy balance and the sum of the van der Waals’ force on each colliding particle. In order to establish the validity of the treatment, results are compared with those based on a DNS, with good agreement being found. Subsequent runs for colliding and agglomerating particles in a channel flow demonstrate that the rate of particle agglomeration peaks towards the channel walls due to increased particle concentrations and turbulence levels in these regions. Agglomeration is also greatly influenced by the presence of gravity, with this effect accentuated on the lower wall of the channel

Large Eddy Simulation of Particle-Particle Interactions in Turbulent Flow: Collision, Agglomeration and Break-Up Events

A numerical study of particle-particle interactions in a turbulent flow is performed using an Eulerian-Lagrangian particle tracking code with a hard-sphere collision model extended to take into account coalescence between the colliding particles and break-up of agglomerates. The effect of the agglomerate fractal dimension on the break-up events, and eventually on collision and agglomeration, is presented. The computational domain was seeded with primary particles (calcite, a nuclear waste simulant) of size 60 micron and allowed to run until steady state before the particle-particle interactions were activated. Break-up events reduce as the agglomerate fractal dimension (df = 2.0, 2.5, 2.8 and 3.0) increases, and with no break-up event as the control, the effect of break-up on particle-particle interactions is presented. The results show an increase in the number of collisions, and the number of collisions leading to agglomeration, with a decrease in the agglomeration rate and agglomerate size with increasing hydrodynamic stress, as a consequence of break-up