Diffusion characteristics of the industrial submicron particle under Brownian motion and turbulent diffusion

The industrial release of submicron aerosol particles at workplace could cause undue health effect on workers. To effectively capture and remove airborne particles, we need to study the characteristics of various interactive particle motion forces (drag force, Brownian force, Saffman lift force, etc.) and the dispersion of these aerosol particles in indoor air. In this study, the dominant force of submicron particles was determined by calculating the acting forces with different particle sizes. Then, a Discrete Particle Model (DPM) was used to calculate the trajectory of particle movement in turbulent thermal plume flow. Horizontal dispersity ( DH) was defined to evaluate the horizontal diffusion of the particulate matter. The impact of different particle diameters, heat source temperatures and initial relative velocities on DH was investigated. This study showed that the main acting forces for submicron aerosol particles were drag force, Brownian force, Saffman lift force and thermophoresis force. Brownian force cannot be ignored when the particle diameter was below 0.3 µm, which would promote the irregular movement of particles in space and enhance their diffusion ability. The smaller the particle size, the higher the heat source temperature and the lower the particles' initial velocity would lead to the increase of DH.

Assessment of Mechanisms for Particle Migration in Semi-Solid High Pressure Die Cast Aluminium-Silicon Alloys

In semi-solid metal high pressure die casting and in conventional high pressure die casting, it is common to find a defect band just below the surface of the component. The formation of these bands is not fully understood. However, there are several theories as how they occur, and it has been suggested that segregation is caused by the migration of aluminium-rich externally solidified crystals. In the present work the formation of these bands is investigated theoretically by reviewing suitable potential mechanisms for the migration of such crystals. Two mechanisms are identified as the most probable: Saffman lift force and the Mukai-Lin-Laplace effect. However, it was not possible to identify which of these two mechanisms acted in the case studies. Further testing is required to identify the mechanism that is causing the migration of the aluminium globules and suitable tests are proposed.

Effects of properties of silt particles on cavitating flow characteristics in a nozzle

To research the effects of properties of silt particles on cavitating flow, silt-laden cavitation flow in one 3D nozzle is simulated. Silt mean diameters are 0.005 mm, 0.007 mm, 0.009 mm, 0.010 mm, 0.012 mm, 0.013 mm, 0.015 mm, 0.020 mm, 0.026 mm, 0.030 mm, 0.035 mm, 0.040 mm, 0.046 mm, 0.050 mm and 0.056 mm. Silt concentrations vary from 1.0% to 10%. To measure the effects of silt particles, vapor contents under pure water cavitation flow and silt-laden cavitation flow conditions are calculated and compared. Results show that silt particles first promote the development of cavitation then inhibit the evolution of cavitation with the increase of silt concentration. Silt particles promotion scope decreases gradually and inhibition span increases constantly with the increase of silt mean diameter. Cavitation nuclei, vortices, slip velocity, virtual mass force and Saffman lift force have a closed relation with the promotion of silt particles. Vortices, viscosity and silt abrasion deeply influence the inhibition of cavitation.

Influencing Factors on Submicron Particle Movement Patterns in Supersonic Laminar Boundary Layers

The process of submicron particle movement in laminar boundary layers is present in many practical applications such as the particles depositing on the turbine blade and mist droplets depositing on the surface of aircrafts. Although great progress has been made on this issue during the last decades, many underlying mechanisms still remain unclear. Here, we developed a theoretical model to understand how submicron particles will behave when they enter a supersonic laminar boundary layer above an adiabatic plate along with the main stream. In this model, we used the Lagrangian method to track the particles and calculate their trajectories, and the Eulerian method was used to calculate the flow field. Because of the large velocity and temperature gradient near the wall and the small size of the particle in this question, four forces (e.g., drag force, Saffman lift force, thermophoretic force and Brownian force) acting on the particle are considered. The effects of entering position, Mach number, the size and density of particles are investigated. We discovered that there are three particle movement patterns when they enter the supersonic boundary layer, and that the drag force and Saffman lift force play dominating roles on which pattern will happen in this process. Moreover, the results also reveal that the particle tends to move towards the wall as the diameter and the density of the particle and the Mach number of main flow increases. Finally, we suggested a dimensionless number to describe the three patterns of particle motion. This research provides a better understanding of the particle movement process in the supersonic laminar boundary layer, which can be a useful guidance for the industrial processes involving this phenomenon.

Particle Contamination on a Thermal Flying-Height Control Slider

Particle contamination on a slider in a hard disk drive (HDD) affects the HDD’s reliability. With the introduction of the thermal flying-height control (TFC) slider, the temperature in the head-disk interface (HDI) becomes non-uniform, which induces a temperature-gradient dependent force on particles moving in the HDI. This paper investigates the effect of this force, the so called thermophoretic force, on a particle’s motion in the HDI as well as its effect on particle contamination on the TFC slider. By numerical simulation of the particle’s trajectory together with an analytical analysis, we show that the thermophoretic force is always negligible compared to the Saffman lift force, which points to a direction parallel to the thermophoretic force. We conclude that the current particle contamination simulator without any thermophoretic forces included would not be significantly altered by the inclusion of these forces.

Two Phase Analysis of Heat Transfer and Dispersion of Nano Particles in a Microchannel

The effect of different parameters on dispersion of nanoparticles in a microchannel in slip flow regime is studied. The equations of particle motion and energy balance are solved numerically and the effect of particle diameter, starting position of particles in microchannel, and slip coefficient on dispersion of particles is discussed. Radiative heat flux in energy equation and drag force, Saffman lift force, Brownian force and gravitational force in momentum equation are included. The results show that the Brownian force has considerable effect on particle motion in microchannel. Particles temperature at the outlet can be controlled by variation of their diameter and starting position in microchannel.

A theory of particle deposition in turbulent pipe flow

The paper describes a theory of particle deposition based formally on the conservation equations of particle mass and momentum. These equations are formulated in an Eulerian coordinate system and are then Reynolds averaged, a procedure which generates a number of turbulence correlations, two of which are of prime importance. One represents ‘turbulent diffusion’ and the other ‘turbophoresis’, a convective drift of particles down gradients of mean-square fluctuating velocity. Turbophoresis is not a small correction; it dominates the particle dynamic behaviour in the diffusion-impaction and inertia-moderated regimes.Adopting a simple model for the turbophoretic force, the theory is used to calculate deposition from fully developed turbulent pipe flow. Agreement with experimental measurements is good. It is found that the Saffman lift force plays an important role in the inertia-moderated regime but that the effect of gravity on deposition from vertical flows is negligible. The model also predicts an increase in particle concentration close to the wall in the diffusion-impaction regime, a result which is partially corroborated by an independent ‘direct numerical simulation’ study.The new deposition theory represents a considerable advance in physical understanding over previous free-flight theories. It also offers many avenues for future development, particularly in the simultaneous calculation of laminar (pure inertial) and turbulent particle transport in more complex two- and three-dimensional geometries.