Grid-generated turbulence in two-phase flow: The effect of parameters of the carrier medium and particles

Results are given of an experimental investigation of the initial region of decay of grid-generated turbulence in a downward two-phase flow of the “gas-solid particles” type. Optical diagnostic methods are used to obtain the distributions of the dynamic parameters of two-phase flow, namely, averaged and fluctuation velocities of gas, and the curves of decay of turbulence along the flow axis are constructed for grids with square meshes sized 4.8, 10, and 16 mm. The investigation results demonstrate that solid particles 700 μm in size have varying effect on the degree of decay of turbulence. In the case of grids with small mesh sizes of 4.8 and 10 mm, the presence of such particles leads to additional generation of turbulence; in the case of a grid with mesh size of 16 mm, vice versa, the particles suppress the turbulence. Investigations reveal that these tendencies become still more pronounced with increasing concentration of particles. In addition, the investigation of the effect of velocity phase slip reveals that the generation of turbulence increases with the difference between phase velocities. In so doing, variation of the pattern of the effect of particles on turbulence is observed for a grid with large meshes, namely, the suppression of turbulence at low values of velocity slip and generation of turbulence with increasing slip. Based on the results of analysis of experimental data, a criterional parameter is suggested, which defines the effect of particles on the turbulence of two-phase flow, i.e., the ratio of the Reynolds number of particle to the turbulence Reynolds number for gas.     

The distribution of the velocities of doubly dispersed particles in a downward turbulent flow of air in a pipe

Conclusion We have demonstrated the possibility of using laser Doppler anemometers to investigate the characteristics of turbulent heterogeneous flows which carry solid doubly dispersed particles. The distribution has been obtained for the first time of the averaged and pulsation velocities of doubly dispersed particles moving in a turbulent flow of air in a pipe.     

Wavelet multi-resolution analysis on particle dynamics in a horizontal pneumatic conveying

The particle velocities are measured by the high-speed particle image velocimetry (PIV) in the acceleration and fully developed regimes of a horizontal pneumatic conveying. Based on the measured particle fluctuation velocities, continuous wavelet transform and one-dimensional orthogonal wavelet decomposition were applied to reveal particle dynamics in terms of time frequency analysis, the contribution from wavelet level to the particle fluctuation energy, spatial correlation and probability distribution of wavelet levels. The time frequency characteristics of particle fluctuation velocity suggest that the small-scale particle motions are suppressed and tend to transfer into large scale particle motions from acceleration regime to fully developed regime. In the near bottom part of pipe, the fluctuation energy of axial particle motion is mainly contributed from the wavelet levels of relatively low frequency, however, in the near top part of pipe, wavelet levels of relatively high frequency make comparable contribution to the axial particle fluctuation energy in the suspension flow regime, and this contribution decreases as particles are accelerated along the pipe. The low frequency wavelet levels exhibit large spatial correlation, and this spatial correlation increases as the particles flow from acceleration regime to fully developed regime. The skewness factor and kurtosis factor of wavelet level suggest that the deviation of Gaussian probability distribution is associated with the central frequency of wavelet level, and the deviation from Gaussian distribution is more evident as increasing central frequency. The higher wavelet levels can be linked to small sale particle motions, which lead to irregular particle fluctuation velocity.     

Experimental analysis on particle fluctuation velocity in a horizontal air–solid two-phase pipe flow having a dune model

To further elucidate the mechanism of energy-conserving conveying in horizontal pneumatic conveying with the dune model, the high-speed particle image velocimetry is applied to measure particle fluctuation velocity near the minimum conveying velocity of the conventional pneumatic conveying. This study focuses on the effect of mounting dune models on the horizontal pneumatic conveying in terms of power spectrum, autocorrelation coefficients, two-point correlation coefficients, fluctuation intensity of particle velocity, skewness factor, and probability density function. It is found that the power spectrum peaks with the dune model are larger than those of the nondune system, suggesting the acceleration and suspending efficiency of the dune model, especially dune models mounted at the bottom of the pipe. Meanwhile, the profiles of particle fluctuation velocity intensity indicate that the large particle fluctuating energy is generated due to mounting the dune model so that the particles are more easily accelerated and suspended. This is one of the important reasons why the mounted dune model results in a low pressure drop and low minimum conveying velocity. Based on the distribution of skewness factor and probability density function, it is found that the particle fluctuation velocities of all cases follow the Gaussian distribution in the lower and middle parts of the pipe. The particle fluctuation velocities in the case of the dune models mounted at the bottom of the pipe obey the Gaussian-type fluctuation more.     

Experimental study of fluctuations of particle velocity in a turbulent flow of air in a pipe

The distribution of longitudinal and normal components of the fluctuation velocity of particles in their motion in a downward turbulent airflow in a pipe is obtained. The effect of the concentration of particles on the intensity of fluctuations of their velocity is studied. A high rise of the longitudinal fluctuation of the velocity of particles in the pipe wall region with an increase in their concentration is revealed.     

The impact of vertical internals array on the key hydrodynamic parameters in a gas-solid fluidized bed using an advance optical fiber probe

The effect of a circular configuration of intense vertical immersed tubes on the hydrodynamic parameters has been investigated in a gas-solid fluidized bed of 0.14?m inside diameter. The experiments were performed using glass beads solid particles of 365?μm average particle size, with a solid density of 2500?kg/m³ (Geldart B). An advanced optical fiber probe technique was used to study the behavior of six essential local hydrodynamic parameters (i.e., local solids holdup, particles velocity, bubble rise velocity, bubble frequency, and bubble mean chord length) in the presence of vertical immersed tubes. The experimental measurements were carried out at six radial positions and three axial heights, which represent the three key zones of the bed: near the distributor plate, the middle of the fluidizing bed, and near the freeboard of the column. Furthermore, four superficial gas velocities (u/u_mf?=?1.6, 1.76, 1.96, and 2.14) were employed to study the effect of operating conditions. The experimental results demonstrated that the vertical internals had a significant effect on all the studied local hydrodynamic characteristics such that when using internals, both the solids holdup and bubble mean chord length decreased, while the particles velocity, bubble rise velocity, and bubble frequency increased. The measured values of averaged bubble rise velocities and averaged bubble chord lengths at different axial heights and superficial gas velocities have been compared with most used correlations available in the literature. It was found that the measured values are in good agreement with values calculated using predicted correlation for the case without vertical internals. While, the absolute percentage relative error between the measured and calculated values of these two hydrodynamic parameters indicate large differences for the case of vertical internals.     

Numerical study of the motion behaviour of three-dimensional cubic particle in a thin drum

The motion of three-dimensional cubic particles in a thin rotating drum is simulated by the SIPHPM method. The drums with frictional or smooth front and rear walls, and the particles of cubic and spherical shapes, and different particle numbers are considered to study the effect of cubic particle shape, end-wall frictions and filling levels. Different flow patterns of cubic particles are observed, which are significantly dominated by the friction from the end-walls. The probability density function of velocity components, the flatness factors are used to analyze the motion behaviour of cubic particle. The Froude number, ensemble mean and time averaged particle velocities are also analyzed. A primary and secondary mode of driving from the end-wall frictions are indicated and the mechanisms on the influences of wall friction, particle shape and filling levels are fully explained.     

On interfacial velocities during abnormal grain growth at ultra-high driving forces

Interfacial velocities during grain growth studies of nanocrystalline materials have been investigated. Two types of interfacial velocity parameters were developed in Ni and Ni–Co alloys. The first was a transformation-averaged parameter based on the time to consume the nanocrystalline matrix by abnormal grain growth. The second was a time-averaged parameter based on the rate of size increase of the largest growing grains. Despite the ultra-high driving force and rapid loss of nanostructure during annealing, the averaged grain boundary velocities are considerably lower than reported velocities during recrystallization in high purity systems for the same homologous temperature. It was found that the time-averaged abnormal growth front velocity decreased with increasing migration distance, which was interpreted in terms of a dynamic sulfur segregation model.     

A transient Eulerian-Eulerian simulation of bubbling regime hydrodynamics of coal ash particles in fluidized bed using different drag models

The transient multiphase model with the Eulerian-Eulerian approach based on the Two-Fluid Model (TFM) was executed to simulate the bubbling regime’s hydrodynamics of bed material in the fluidized bed using three different drag models. Coal ash particles having three different sizes were taken in bed for fluidization under cold conditions. The bubbling regime's superficial velocities were acquired from experimentations and used as inlet velocities during Computational Fluid Dynamics (CFD) simulation of a 2-Dimensional fluidized bed. The Syamlal-O'Brien, Gidaspow and Wen-Yu drag models were considered in this study, and their effects on the bed hydrodynamics were discussed. The study emphasized the suitability of drag models for the coal ash particles. The drag force was not adequate and showed a negligible effect on particles irrespective of the high inlet velocity displayed by the Gidaspow model. The other two drag models predicted sufficient drag, but there was more intensity in Syamlal-O'Brien than in the Wen-Yu model. The Syamlal-O'Brien model resembled more physical fluidization occurrences for smaller and larger sized coal ash particles. This study also supports the hydrodynamics of the Geldart-D type particles.     

Role of interphase interactions during gas detonation in inert porous medium

A mathematical model of detonation in a two-phase mixture consisting of a gaseous monofuel and closely packed noninflammable solid particles is proposed. The structure of detonation waves in a pure gas is compared to that in monodisperse mixtures with various diameters of particles. Two special regimes of detonation are separated, in which (i) gas is immediately inflamed due to shock compression and (ii) ignition starts at the surface of particles, upon reflection of the chock wave front. It is shown that inertial effects during the flow past particles can both increase and decrease the detonation velocities. The calculated detonation velocities well agree with experimental data.     

The effect of oscillating flow on a horizontal dilute-phase pneumatic conveying

ABSTRACT

A horizontal dilute-phase pneumatic conveying system using vertically oscillating soft fins at the inlet of the gas–particle mixture was studied to reduce the power consumption and conveying velocity in the conveying process. The effect of different fin lengths on horizontal pneumatic conveying was studied in terms of the pressure drop, conveying velocity, power consumption, particle velocity, and intensity of particle fluctuation velocity for the case of a low solid mass flow rate. The conveying pipeline consisted of a horizontal smooth acrylic tube with an inner diameter of 80 mm and a length of approximately 5 m. Two types of polyethylene particles with diameters of 2.3 and 3.3 mm were used as conveying materials. The superficial air velocity was varied from 10 to 17 m/s, and the solid mass flow rates were 0.25 and 0.20 kg/s. Compared with conventional pneumatic conveying, the pressure drop, MPD (minimum pressure drop), critical velocities, and power consumption can be reduced by using soft fins in a lower air velocity range, and the efficiency of fins becomes more evident when increasing the length of fins or touching particles stream by the long fins. The maximum reduction rates of the MPD velocity and power consumption when using soft fins are approximately 15% and 26%, respectively. The magnitude of the vertical particle velocity for different lengths of fins is clearly lower than that of the vertical particle velocity for a non-fin conveying system near the bottom of the pipeline, indicating that the particles are easily suspended. The intensities of particle fluctuation velocity of using fins are larger than that of non-fin. The high particle fluctuation energy implies that particles are easily suspended and are easily conveyed and accelerated.     

Relative magnitudes of the effects of electrostatic image and thermophoretic forces on particles in the respiratory tract

The National Radiological Protection Board's Advisory Group on Non-ionising Radiation has recommended research into the deposition, in the lung, of charged particles in the size range 0.005-1 microm. In vivo measurements of the temperature distribution in the respiratory tract have been used to estimate the temperature gradients in the generations up to the segmental bronchus. These gradients define the thermophoretic velocities, which oppose deposition during inhalation and assist it during exhalation. The thermophoretic forces are effective over a longer range than those due to the electrostatic image of a single charge; and, at distances greater than a few microns from the airway wall, the thermophoretic velocities of 0.02 and 0.1 microm particles are greater than those due to electrostatic drift. It is concluded that models describing the effects of electric charge on the deposition of particles with diameters of order 0.1 microm need to take account of the thermal conditions in the respiratory tract.     

Effect of intense turbulence on turbulent premixed V-flame

The influence of free stream turbulence on the statistics of turbulent premixed V-flame is numerically investigated in this paper. The flame front is tracked using the level-set algorithm with the effect of exothermicity and baroclinicity. Results indicate that free stream turbulence affects the statistics greatly including the conditioned and unconditioned mean axial and transverse velocities, fluctuation velocities and Reynolds stresses. The unconditioned and conditioned fluctuating velocities, unconditioned mean axial velocities and the flame brush thickness increase with increased free stream turbulence. The maximum unconditioned mean axial velocity along the centre line increases linearly with turbulence. The peaks of unconditioned transverse fluctuation velocity smoothens with the increase of free stream turbulence level, indicating that the effect of intermittency decreases with the increase of free stream turbulence. These results show that free stream turbulence is a major influence on turbulent statistics in premixed V-flame.     

EFFECT OF MASS FLOWRATE ON POWDER FLOW VELOCITIES

The effect of mass flowrate on powder velocities during flow down a chute and in free fall from a hopper was studied using a microwave Doppler technique. The velocities on an inclined chute, which was either smooth or had its surface roughened by coating with a layer of the sample, increased as the mass flowrate increased; this effect was most significant at large angles of inclination. Free fall from a hopper also led to higher velocities being monitored as the mass flowrate increased.

The distribution of particle velocities was also estimated; during flow on a chute at high angles of inclination the distribution of velocities was greater for low flowrates while at low angles, where the powder only just managed to flow, the trend was reversed. There was no difference in the distribution of velocities measured for the free fall of particles as the mass flowrate varied.     

A terminal-velocity model for super-ellipsoidal particles

The prediction of the terminal velocity of non-spherical particles, such as sediments and microplastics, is essential for understanding their transport processes in rivers or marine environments. However, most of the existing models have been proposed based on specific particle materials, and there is a lack of systematic research on the effects of different shape factors on terminal velocity. In this study, super-ellipsoidal particles were selected as test particles for settling experiments, and a particle–velocity tracking code was developed to measure their terminal velocities during falling through glycerin–water mixtures. A terminal–velocity model for super-ellipsoidal particles was proposed based on the measured data. Owing to the new model, multivalued predictions of the terminal velocity based on a single shape factor, such as sphericity and Corey shape factor, were disclosed, and the prediction errors were evaluated. The results of this study can provide a basis for establishing a general terminal–velocity model that considers the influence of particle shape.     

Numerical study of the mixing process of binary-density particles in a bladed mixer

Discrete element method (DEM) simulations of binary mixing of particles with different densities were conducted to study the influence of density ratio, blade speed, and filling level on the particle dynamics and mixing performance in a bladed mixer. Four particles with different densities at different locations were tagged to discuss the influence of three factors on the particle trajectory and velocity field in the mixer. A method based on cubic polynomial fitting of relative standard deviation was used to determine the critical revolution during the mixing process. It was found that the non-dimensional tangential velocity decreases with the increase of the blade speed and filling level, the fluctuation of vertical velocity increases with the radial location, blade speed, and filling level, and it is more pronounced than the fluctuation of tangential and radial velocity during the mixing process. Results obtained indicate that the mixing performance of particles with different density increases with the decrease of density ratio and filling level, while it increases with the increase of blade speed.     

Laser Doppler anemometer measurement of the velocity pulsations of large particles

A procedure has been developed for estimating the velocity pulsations of polydisperse particle velocities. The mean velocities of glass particles as well as their root-mean-square deviations were measured in trials for various sensitivities of the laser Doppler anemometer system used. Translated from Izmeritel'naya Tekhnika, No. 6, pp. 35–39, June, 1999.     

Circulation of the solid phase through a vertical gas jet injected into a layer of granular material

A method is developed for determining the mass of particles circulating through a vertical gas jet. On the basis of experimental data dependences are obtained for determining the circulation of particles and the velocities of their motion in the jet.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 37, No. 4, pp. 635–640, October, 1979.     

Single particle impact damage of fused silica

Single-impact damage of fused silica by spherical and conical tungsten carbide (WC) projectiles of velocities up to 200 m sec⁻¹ has been investigated with a high-speed framing camera photographing at a rate of up to 1.7×10⁶ frames per second. For spherical particles the Hertzian cone cracks, which for higher impact velocities are accompanied by median and radial cracks, form during the loading part of the impact; some growth of all these cracks also occurs during unloading. With the conical particles the Hertzian cone cracks do not form; only radial and median cracks form during the loading; in this case both radial and median cracks grow during the unloading. In both cases “lateral” cracks form during unloading. From these experiments values of the static equivalent of the dynamic stress-intensity factor for high-velocity cracks are also obtained; these are found to be considerably lower than those obtained from quasi-static indentation experiments. Finally, the extent of the damage produced by a single impact has been discussed.     

Investigation on dynamic movement of cylindrical biomass particles in a fast fluidized bed

The processing of biomass particles is complex in a fluidized bed due to their heterogeneous characteristics. To further understand biomass particles, the dynamic movement of cylindrical biomass particles were investigated in a fast fluidized bed. Cylindrical particles were studied via the impulse momentum theorem and the Discrete Element Method (DEM). Meanwhile, the contact action, the drag force and other conventional forces were considered. To validate the present method, the predicted orientation and the minimum fluidization velocities of cylindrical particles were compared with the relative results, and validate the present approach. Then the characteristics of biomass particle flow dynamics were analyzed in terms of the particle concentration, the orientation distribution and the residence time distribution (RTD). It was found that most particles present as the horizontal or nearly horizontal states (0°) during the fluidization, and the percentage can reach and exceed 16%. High concentration occurs near the wall due to the back flow. The lower fluidization velocity corresponds to a wider RTD. The particles with the same size leave the riser with various residence times. The applied method and the obtained results provide helpful consults to study the cylindrical and other non-spherical biomass particles in an extensive way.