Tracing Thermal Creep and Thermophoresis in Porous Structures at Low Ambient Pressure and Low Gravity

We carried out two types of drop tower experiments to quantify gas and particle motion induced by temperature gradients inside a porous structure in a low pressure environment. In one setup 400 μm sized particles were traced inside heated channels at pressures of a few Pascal. Their motion is consistent with pure thermophoresis. In the second setup tracer particles were used to track the thermal creep gas flow through a porous dust bed. Here, the flow was traced outside of the dust bed and without thermophoretic motion. The results are consistent with a simple capillary model of the dust bed.     

Investigation of Flows of a Gas–Dust Mixture by the Methods of Molecular-Kinetic Theory

Using a direct numerical solution of the Boltzmann kinetic equation the problem of the flow of a gas–dust mixture is investigated with allowance for the motion of the dust. The qualitative analysis made has shown that in describing the flow of gas–dust mixtures it becomes possible to simplify the system of kinetic equations. The dependences of the density, the temperature, and the velocity of the gas on the coordinate have been obtained for different concentrations and velocities of the dust particles.     

Parameters of the positive column of glow discharge with dust particles

We calculate the measured nonlocal parameters of the plasma of the positive column of direct current glow discharge in the presence of dust structures with different dust particle concentrations. The calculations are performed for typical conditions of the positive column of low-pressure glow discharge in air at which a collisional regime of maintaining discharge is achieved. The discharge plasma is described using the diffusion approximation; the flows to the surface of the dust particles are described in the orbital motion limited approximation. Calculation is carried out for micron-size particles with concentrations of up to 10¹¹ m^?3. The distribution of the dust component is assumed to be independent of the discharge parameters. Radial distributions of the plasma components and of the electric field component are obtained. The charges of dust particles for various concentrations and discharge parameters are calculated. It is demonstrated that for a certain particle concentration, the absorption efficiency of plasma particles becomes comparable with diffusion losses at the tube walls. The influence of the dust cloud on the electric field configuration at different dust particle concentrations in the cloud is analyzed. The current-voltage characteristics of the positive column of glow discharge are calculated. A higher discharge stability toward the perturbative action of dust particles at high discharge current values is demonstrated.     

Study of the Mechanisms of Dust Suspension Generation in a Closed Vessel Under Microgravity Conditions

An experimental method has been developed to investigate the mechanisms of dust suspension generation in a closed vessel under microgravity conditions. The objective is to characterize the evolution as function of time of the aerodynamic flow field and the dust distribution inside the vessel, in particle concentration conditions representative of actual dust explosions. Cornstarch suspensions in air have been chosen for this purpose. Special adaptation of high frame rate video records with laser sheet tomography allowed performing PIV measurements under microgravity conditions, in the unsteady flow generated during the dispersion process. It was demonstrated that cornstarch particles, in spite of their relatively large size, were suitable to be used as tracers for PIV measurements. Video registrations, at high frame rate (5,000 fps) at discrete instants in the time interval 0–10 s, of the dispersion of cornstarch particles under microgravity conditions allowed to explore the time interval 600 ms to 10 s for which there were no experimental data until now. It appears that after 500 ms, velocity and rms velocity fields become similar for g = 0 and g = 1. For characteristic times beyond 7–8 s, these fields evolve very slowly. Recordings at slower frame rate (125 fps) continuously during the whole process, showed the tendency of particles to become uniformly distributed in the vessel. The variations of particle concentration were followed using the signal of light intensity diffused by the particles. Large fluctuations of the diffused light are detected during the dispersion process. However, for g = 0, the average value of the diffused light intensity remains quasi-constant as function of time, whereas for g = 1, it decreases quickly until approximately 4 s, and then decreases much slowly and becomes slightly smaller than in the case g = 0. To get further information about the subsequent evolution of the suspension, observation of the dispersion process beyond 10 s should be done in future experiments.     

Two-Phase Flow of Blood with Magnetic Dusty Particles in Cylindrical Region: A Caputo Fabrizio Fractional Model

The present study is focused on the unsteady two-phase flow of blood in a cylindrical region. Blood is taken as a counter-example of Brinkman type fluid containing magnetic (dust) particles. The oscillating pressure gradient has been considered because for blood flow it is necessary to investigate in the form of a diastolic and systolic pressure. The transverse magnetic field has been applied externally to the cylindrical tube to study its impact on both fluids as well as particles. The system of derived governing equations based on Navier Stoke’s, Maxwell and heat equations has been generalized using the well-known Caputo–Fabrizio (C–F) fractional derivative. The considered fractional model has been solved analytically using the joint Laplace and Hankel (L&H) transformations. The effect of various physical parameters such as fractional parameter, Gr, M and γ on blood and magnetic particles has been shown graphically using the Mathcad software. The fluid behaviour is thinner in fractional order as compared to the classical one.     

Hydrodynamic instability of a suspension of spherical particles through a branching network of circular tubes

A numerical method is presented for computing the unsteady flow of a monodisperse suspension of spherical particles through a branching network of circular tubes. The particle motion and interparticle spacing in each tube are computed by integrating in time a one-dimensional convection equation using a finite-difference method. The particle fraction entering a descendent tube at a divergent bifurcation is related to the local and instantaneous flow rates through a partitioning law proposed by Klitzman and Johnson involving a dimensionless exponent, q ≥ 1. When q = 1, the particle stream is divided in proportion to the flow rate; as q → ∞, the particles are channeled into the tube with the highest flow rate. The simulations reveal that when the network involves two or more generations, a supercritical Hopf bifurcation occurs at a critical value of q, yielding spontaneous, self-sustained oscillations in the segment flow rates, pressure drop across the network, and particle spacing in each tube. A phase diagram is presented to establish conditions for unsteady flow. As found recently for blood flow in a capillary network, oscillations can be induced for a given network tree order by decreasing the ratio of the tube diameter from one generation to the next or by decreasing the diameter of the terminal segments. The instability is more prominent for rigid than deformable particles, such as drops, bubbles, and cells, due to strong lubrication forces between the tightly fitting particles and tube walls. Variations in the local particle spacing, therefore, have a more significant effect on the effective viscosity of the suspension in each tube and pressure drop required to drive a specified flow rate.     

The Dynamics and Heat Transfer of a Droplet in a Dusty Gas in the Presence of Phase Transformations and Dust Catching

The dynamics and heat transfer of a droplet in a mixture of gas with fine particles (dusty gas) are investigated in the presence of deposition of fine particles on the droplet (dust catching) and of phase transformations (evaporation, condensation). Separately treated is a case in which only phase transformations occur. Analytical solutions of the problem are found in the limiting cases of Stokes and Newtonian flow past a droplet (corresponding to low and high values of the Reynolds number). The combined effect of dust catching and phase transformations on the droplet motion is investigated numerically.     

Aerodynamic forces and flow structures of an airfoil in some unsteady motions at small Reynolds number

Summary The aerodynamic forces and flow structures of a NACA 0012 airfoil in some unsteady motions at small Reynolds number (Re=100) are studied by numerically solving the Navier-Stokes equations. These motions include airfoil acceleration and deceleration from one translational speed to another and rapidly pitching up in constant freestream (equivalent to pitching up during translational motion at constant speed). It is shown that at small Reynolds number (Re=100), when the airfoil is performing fast acceleration or deceleration from one speed to another, a large aerodynamic force can be generated during and for a time period after the acceleration or deceleration; a large aerodynamic force can also be generated when the airfoil is performing a fast pitching motion in a constant freestream. In these fast unsteady motions, an airfoil in low Re flow can produce a large aerodynamic force as effective as in large Re flow, or the effect of unsteady motion dominates the Reynolds number effect. During the fast unsteady motion of the airfoil, new layers of strong vorticity are formed near the upper and lower surfaces of the airfoil under the previously existing thick vorticity layers, and it is the generation and motion of the new vorticity layers that is mainly responsible for the generation of the large aerodynamic force; the large-scale structure and movement of the newly produced vorticity layers are similar to that of high Re flow.     

Characteristic Analyses of Plasma Air Cleaning Systems for the Removal of Indoor Air Pollutants

Tobacco smoke is one of the most common manmade aerosols. Yellow sand dust and pollen are the particular and regional pollutants generated by natural phenomena. These pollutants have different removal characteristics, respectively, when the air cleaning system is operated. It is well known that tobacco smoke particles are removed effectively with electrostatic precipitators. But it is necessary to evaluate whether the plasma air cleaning system has good performance of removing Yellow sand dusts and pollens simultaneously and also to establish the operation modes for efficient removal of those particular air pollutants by controlling the air flow rates passing the electrostatic precipitator and operating times of air cleaning system. In this study, the performance evaluation of plasma air cleaning systems is investigated with tobacco smoke particles, Yellow sand dusts, and pollens. For the multi-pass test in occupied spaces of 150 m³, the operation time required to reduce dust concentration from the initial concentration of 300 µg/m³ to 150 µg/m³, the criteria of indoor air quality in Korea, are 40 min for tobacco smoke, 28 min for Yellow sand dust, and 5 min for pollen when the flow rate is 17 m³/min. Also, the optimal operation modes for each pollutant are suggested for the efficient removal of indoor air pollutants. At first, most particles are removed by maximum flow operation. Second, the rest of the particles are removed by medium flow operation. Next, the plasma air cleaning systems are maintained by minimum flow for tobacco smoke mode and by repeating minimum flow and medium flow for Yellow sand and pollen modes. Edit to “Because the Yellow sand dust and the pollen flow into the room continuously and settle down … noise reduction.” Because the Yellow sand dust and the pollen flow into the room continuously and settle down. The plasma air cleaning system is suitable for the removal of the tobacco smoke, the Yellow sand dust, and the pollen for maintaining suitable indoor air quality, and, if it is operated through the suitable operation mode, energy efficiency will improve noise reduction.     

Oscillating two-phase flow in a prestressed thick elastic tube

Summary The pulsating flow of a fluid with dusty particles in a prestressed thick walled elastic tube has been studied. The tube, subjected to a static inner pressure P_i and an axial stretch λ, is taken to be an incompressible, isotropic, elastic material. The fluid with particles is treated as incompressible Newtonian. Employing the theory of small deformation superimposed on large initial deformations, for an axially symmetric perturbed motion the governing equations are obtained in cylindrical polar coordinates. The analytical solutions of the equations of motion for the dust and the fluid have been obtained. Because of the variable character of the coefficients of the resulting equations for the solid body they are solved numerically. The dispersion relation is obtained as a function of the stretch, the thickness ratio and the parameters for dusty particles.     

水预湿被爆体降低爆破粉尘机理研究

通过对气流中单个尘粒的运动分析,指出降低爆尘的有效方法是增大粒径.通过对尘流中尘粒受力的分析,得出在干燥环境下作用在尘粒上的主作用力是范德华力;在湿润情况下作用在尘粒上的主作用力是液桥力的结论.指出水预湿被爆体降尘法的机理是:尘粒间的液桥力促使润湿尘流中的尘粒变大,变大的尘粒迅速下降,从而实现降低爆尘目的.     

Unsteady axisymmetric creeping flow from an orifice

Summary The slow unsteady motion of a viscous incompressible fluid which issues from a finite orifice into the half-space,x>0 is considered. By slow it is meant that the convective acceleration (i.e. nonlinear) terms in the Navier-Stokes equations are of negligible magnitude in comparison with terms attributable to viscosity. Only axisymmetric motions will be considered. The assumed nature of the motion along with the resultant linearization of the Navier-Stokes equations allows the construction of the Stokes stream function for the flow. Others have discussed this problem when the motion is steady. A general representation for the flow is given when the motion is unsteady and numerical results are presented for the resulting evolutionary motion of specific flows issuing from the orifice.     

Dusty Plasma Studies in the Gaseous Electronics Conference Reference Cell

Particle “dust” in processing plasmas is of critical concern to the semiconductor industry because of the threat particles pose to device yield. A number of important investigations into the formation, growth, charging, transport and consequences of particulate dust in plasmas have been made using the Gaseous Electronics Conference Reference Cell as the reactor test-bed. The greatest amount of work to date has been directed toward a better understanding of the role that electrostatic, ion drag, neutral fluid drag and gravitational forces play in governing the dynamic behavior of particle cloud motion. This has been accomplished by using laser light scattering (LLS) techniques to track the motion of suspended particle clouds in rf discharges. Also, statistical correlation’s in the fluctuation of scattered laser light intensity [dynamic laser light scattering (DLLS)] can be used to determine information about particle size, motion, and growth dynamics. These results are reviewed, along with recent work demonstrating that charged dust particles in a plasma can form a strongly coupled Coulomb liquid or solid. New results from DLSS experiments performed in the Reference Cell are presented that show process-induced dust particles confined in an electrostatic trap exhibit low-frequency oscillatory motion consistent with charge density wave (CDW) motion predicted for strongly coupled Coulomb liquids.     

Entropy generation on MHD flow and convective heat transfer in a porous medium of exponentially stretching surface saturated by nanofluids

This paper examines the unsteady boundary layer magnetohydrodynamic flow and convective heat transfer of an exponentially stretching surface saturated by nanofluids in the presence of thermal radiation. The combined effect of stratifications (thermal and concentration) in the unsteady boundary layer flow past over a stretching surface embedded in a porous medium is analyzed. The system of coupled nonlinear differential equations are solved numerically by developing finite difference scheme together with the Newton’s linearization technique, which allows us to control nonlinear terms smoothly. The study shows that the thermal boundary layer thickness significantly increases with the increase of Brownian motion, thermophoresis number and magnetic field strength. The unsteadiness behavior of the flow of nanofluid has reducing effect on both momentum and thermal boundary layer thickness. The Brownian motion has controlling effect on nanoparticle migration. The entropy generation by means of Bejan number has strong impact on the applied magnetic field, dissipation of energy, thermal radiation and Biot number.     

Accumulation of solid particles in convective flows

Accumulation of solid particles suspended in unsteady convective flows is under theoretical investigation. The principal goal is to understand and interpret recent experiments by D. Schwabe [1,2]. Providing that volume particle concentration, nonisothermality, and relative size of particle are small, an effective single-fluid theoretical model is developed. The peculiarity of the obtained model is taking into account the distinction between fluid and particle inertia. This model is further applied to study particle accumulation in different flow setups: in a model oscillatory flow in a canal heated from below and subjected to the modulated gravity and in the Marangoni flow in a half-zone under microgravity conditions. These problems are investigated numerically by means of finite difference technique. We demonstrate, that the developed theoretical model properly describes generic features of particle accumulation in unsteady flows. Particularly, heavy particles tend to leave the centers of vortices, where the flow vorticity is maximal, and accumulate at their periphery. From numerical simulations in a floating zone, we try to clarify particle dynamics in Schwabe’s setup.     

Weak discontinuities in a non-equilibrium flow of a dusty gas

Summary The behavior of a discontinuity in flow gradients at the head of a disturbance propagating through a homogeneous mixture of gas and small dust particles has been studied. It is shown that the presence of dust particles results in increasing the shock formation time as compared with a similar pure-gas case. When the disturbance is arbitrarily small in amplitude, the solution to the first order in the whole disturbed domain is constructed and analysed. It is found that the concentration of solid particles has a decaying, effect on the shock strength as one, might expect.     

The investigation of particle flow mechanisms of bulk materials in dustiness testers

ABSTRACT

The generation of dust occurs in many bulk materials handling applications, including during free-fall, material impact on conveyor transfers, or impact with other materials. Dust has potentially serious consequences to the surrounding environment as well as workers and nearby communities. Companies need to identify and quantify the dust being generated so they can find ways to reduce or eliminate this dust generation. Dustiness testers are one method which can be used to quantify dust generation. This paper investigates the experimental material flow and the subsequent discrete element method (DEM) simulation in the rotating drums of two dustiness testers: the European Standard dustiness tester and the Australian Standard dustiness tester. Preliminary comparisons of the rotating drum designs were undertaken using particle/bulk parameters of polyethylene pellets, a granular “non-dusty” material to investigate the flow behavior, to provide a reference base to compare equivalent simulations and subsequent analysis. A calibrated DEM material model for polyethylene pellets was generated via experimental comparison. Investigations of the rotational speed, volume, and initial loading location of product sample have been performed. The motion of particles in the simulated rotating drums has been compared to visual observation from experimental testing.     

Analytical determination of the time dependence of the concentration of equally charged monoparticles

For a flow of equally charged particles with identical mass, the time dependence of their concentration has been derived in the general form based on the continuum-mechanics equations. It is shown that the concentration depends on only the charge and mass of individual particles and their motion time; electric fields from external sources do not affect the volume occupied by the particles. The results obtained are applicable in cases in which magnetic fields can be neglected and electric field can be assumed to be quasi-steadystate.     

Some simple unsteady unidirectional flows of a binary mixture of incompressible Newtonian fluids

The problems dealing with some simple unsteady unidirectional flows of a mixture of two incompressible Newtonian fluids are investigated. By using the constitutive equations appeared in the literature for binary mixtures of chemically inert incompressible Newtonian fluids, the equations governing the motion of the binary mixture are reduced to a system of coupled partial differential equations. By means of integral transforms, the exact solutions of these equations are obtained for the following three problems: (i) unsteady Couette flow, (ii) unsteady plane Poiseuille flow, (iii) unsteady axisymmetric Poiseuille flow.     

Unsteady flows of a binary mixture of incompressible Newtonian fluids in an annulus

This paper is concerned with applying the mixture theory of two chemically inert incompressible Newtonian fluids to some simple unsteady flows in the annular region between two infinitely long coaxial cylinders. The equations governing the motion of the binary mixture under discussion are reduced to a system of coupled partial differential equations. With the help of finite Hankel transforms, the exact solutions of these equations are obtained in series form for the following three problems: (i) unsteady axial Couette flow in an annulus, (ii) unsteady Poiseuille flow in an annulus, (iii) unsteady circular Couette flow in an annulus.