In this article, I will discuss the fundamental properties of particleswith particular attention to the recent development of measuring methods. Thediscussion is focused on the particle size and size distribution, mechanical properties, wettability, adhesive characteristics, and electrostatic properties. Other propertiessuch as specific surface area and zeta potential are also included.
To detect the flow rate of powder and bulk solids in pneumatic-conveying line, installing a solids flow meter at the inlet or outlet of the line is usual. If youput a gas-solids-2-phase flow meter instead, this can save installing space and cost, and make a completely enclosed system. As a matter of fact, usage of pneumaticflow meter which we developed, brought us such advantages. For yours reference, I would like to introduce you the flow meter, although its applications are not somany and its performance is not so perfect.
This review introduces the laser based measuring technique for both velocity of gas phaseand solid particle with particle sizing. Laser Doppler Velocimetry (LDV) has been the instrument ofchoice for measuring dispersed-and gas-phase velocities in particle-laden flows. LDV was used forsimultaneous measurements of continuous-and dispersed-phase velocities, which permits the particlesize discrimination based on light scattered intensity. Phase Doppler Anemometry (PDA) has beendeveloped for more precise size measurement for spherical particles. For non-spherical particles, diffraction intensity method for the equivalent diameter based on the projection area has been summa-rized. For the detection of particle shape, Shadow Doppler technique has been developed by the exten-sion of the optical system of Laser Doppler Velocimetry. Digital Particle Image Velocimetry (DPIV) is increasingly considered a proven technique for detecting both phases simultaneously. The author's group developed a measurement system to distinguish a dispersed-phase particle from a tracer in fluidflow. Several experimental results are presented in this review for gas-solid two-phase flow, such asjet, mixing layer and wall jet. Mean and Fluctuation velocities of both phases and volume concentra-tion of solids phase are given to make clear the turbulent structure of gas-solid two-phase flow.
Flow pattern of gas-solid two-phase flow is explained in horizontal and verticalpipelines based on a phase diagram where the pressure drop per unit length along the pipe isplotted against the superficial gas velocity with the mass flow rate of particle as a parameter. Conveyance limits are discussed for the horizontal and vertical upward flows. Dispersed flow, stratified flow, slug flow, plug flow and packed bed flow are qualitatively explained. As tothe method of flow visualization of gas-solid two-phase flow, we briefly review recent progressin laser sheet technique, and in tomographic methods by electrical capacitance and magneticresonance.
Bubbles of a mean diameter over 0.5×10-3m to 20×10-3m weregenerated in a water bath using a perforated nozzle. Attachment of the bubbles to acircular cylinder placed horizontally above the nozzle was observed with a stillcamera and a high-speed video camera. When the wettability of the cylinder wasgood, no bubbles attached to it. On the other hand, when the wettability waspoor, many bubbles attached to the cylinder and some of them were trapped on therear surface of the cylinder. The shape and size of the trapped bubbles just beforedetachment were determined as functions of the surface tension, contact angle, densities of liquid and gas based on an energy equation.
An air-water bubbling jet was generated along a vertical flat plate.Bubble characteristics represented by bubble frequency, void fraction, mean bubblerising velocity, and mean bubble chord length were measured with a two-needleelectro-resistivity probe. The horizontal distributions of the bubble frequency andvoid fraction followed their respective similar distributions in the vertical regionwhere the buoyancy force acting on bubbles governs the flow field. Empiricalequations were proposed for the horizontal distributions of the bubble frequency andvoid fraction both in the directions perpendicular to and parallel to the vertical flatplate. In the same vertical region the horizontal distributions of the mean bubblerising velocity and mean bubble chord length remained almost unchanged in thevertical direction except in the close vicinity of the wall.
An experimental investigation on splashing from the liquid surface inthe gas wiping process for continuous hot-dip coating was carried out by employinga cold and a hot model. A critical nozzle pressure for the beginning of splashing ismeasured for different gas wiping conditions and different liquids including moltenzinc. The impinging velocity for the beginning of splashing take the minimum valueat a certain half width of the plane impinging wall jet. The relative depth of thecavity formed on the liquid surface becomes shallow with increasing the half width, so that the swell of the liquid surface is suppressed. On the other hand, the gas jetpenetrates the liquid surface in the case of small half width. Consequently, splashing from the liquid surface is restrained. The beginning of splashing isexplained by the force balance of lift caused by gas flow, gravity and surfacetension acting on a convex portion of the liquid surface. A linear correlation wasfound between the critical Weber number for the beginning of splashing and Bondnumber based on the maximum velocity and the half width of the impinging walljet. This correlation is applicable to the beginning of splashing by an axisymmetricgas jet impinging normally on a liquid surface.
An experimental investigation was performed on splashing from thestrip edge in the gas wiping process for continuous hot-dip coating. Critical linespeeds and critical nozzle pressures for the beginning of splashing are measured byuse of a cold model under different gas wiping conditions. Though splashingbecomes violent with increase in the line speed and the nozzle pressure, theintroduction of offset and/or downward wiping reduces splashing. The critical linespeed at the same nozzle pressure increases with nozzle-strip distance and decreaseswith nozzle slit gap, so that placing the narrow slit nozzle close to the strip andlow pressure wiping is effective to prevent splashing. The mechanism of splashingfrom the strip edge is clarified by the force balance of lift caused by gas flow andsurface tension acting on a convex portion of the liquid film surface. A linearcorrelation on the log-log scale was found between critical Weber number for thebeginning of splashing and the product of liquid film thickness ratio and flow downliquid film Reynolds number.