261596 Measurement of Particle Impact Velocities in Vibrationally Fluidized Granular Flows

Wednesday, October 31, 2012: 1:50 PM
Conference B (Omni )
Kamyar Hashemnia, Amirhossein Mohajerani and Jan. K. Spelt, Mechanical and Industrial Engineering, University of Toronto, Toronto, ON, Canada

A probe consisting of a high-speed laser displacement sensor was developed to measure directly the surface-normal impact velocities of spherical steel and porcelain particles in the vibrationally-fluidized granular flow produced by a tub vibratory finisher.  Measurements were made in various locations and directions within the flow (Fig. 1). 

Vibratory finishing is widely used to deburr, polish, burnish, harden and clean metal, ceramic and plastic parts.  In a tub vibratory finisher, a container filled with granular media is oscillated by an eccentric rotating shaft so that it develops a vibrationally-fluidized circulatory bulk flow of the media that is largely two-dimensional.  The media have both a large-scale bulk flow velocity and a much smaller-scale local impact velocity.  Therefore, work-pieces that are entrained in the flowing media are subjected to the repetitive, high-frequency impacts of the surrounding particles. 

Within a vibratory finisher, surface interactions such erosive wear and plastic deformation are affected strongly by the velocity, frequency, and direction of particle impacts.  High impact velocities can cause fracture and fragmentation while low impact velocities can make the process less efficient.  The impact velocity is also closely related to the breakage of granular materials in many processes such as drug tablet coating within rotating drums, bulk materials handling, food processing, and particle attrition in vibratory finishing.  Large particle impact velocities may also result in excessive erosion of machine components in processes such as vibratory sieving and mixing. 

Measurement of local quantities such as the impact velocity of vibrating particles in a vibrationally-fluidized granular media is challenging, because of the relatively small scale of the motion and difficulties in designing probes capable of measuring local quantities without disturbing the media.  Therefore, most existing research in the field of flowing granular media has focused on the bulk flow of the media.  In some of these studies, discrete element modeling (DEM) simulations have been used to predict the bulk flow behavior of granular media and the numerical results have been verified experimentally.  Generally, these investigations have shown that discrete element simulations give reasonable predictions of bulk flow velocity and volume fraction in both fluidized and non-fluidized flows. 

Only a few studies have focused on the local behavior of the media in granular flows.  In most of these studies, DEM predictions of collision scale quantities such as impact velocity, collision frequency or impact energy were not validated experimentally. Therefore, it is not known if the commonly used approaches and parameters in DEM simulations yield correct predictions of the impact velocity.  This remains a significant limitation since the impact forces that govern erosion, wear and fracture in granular flows are fundamentally linked to the particle impact velocity and kinetic energy. 

There are several different methods to measure particle velocity in fluidized beds: laser-Doppler velocimetry, photographic and video techniques, optical fiber probes and laser displacement sensors.  Photographic methods have been used to measure bulk flow properties, but the limited spatial resolution of these approaches makes them unsuited to the measurement of local impact velocities.  Laser-Doppler velocimetry is restricted to the low solid concentrations (loose media).  Optical fiber probes have been used to measure bulk flow velocities and the void fraction of media passing transversely across the end of the sensor.  Some designs can measure displacements along the optical probe axis using a correlation with the amount of reflected light, but such devices become inaccurate when the light reflected by the particle varies because of a significant transverse velocity across the probe tip that causes the particle to move out of the incident beam. 

Therefore, laser displacement sensors were chosen to measure the impact velocity of 6 mm diameter steel and porcelain balls inside a relatively packed granular flow produced in a tub vibratory finisher.  Laser displacement sensors measure the distance to an object using triangulation.  Laser light reflected from the object is concentrated on a linearized charge-coupled device such that its position depends on the distance to the object.  The accuracy of the impact velocity measurement using the laser displacement sensor was assessed in a drop test.  The disruption to the particle bulk flow was minimized by enclosing the sensor in a streamlined elliptical tube.

The displacement signals from the laser sensors were analyzed to obtain the probability distribution functions of the impact velocity of the particles.  The output was also interpreted to give the frequency with which media passed the sensor and hence a measure of the particle packing.  Both the impact velocity and the packing density were found to vary significantly with the orientation of the laser probe (direction in which the laser was pointing) and its location in the tub vibratory finisher.  The impact velocity was found to vary by up to 38% depending on the orientation of the laser at a single location in the tub, and by up to 24% at various locations within the tub.  The packing density varied by up to 55% with orientation at a single location and by 46% at different locations.  The average impact velocity of the lighter and stiffer porcelain balls was 15% greater than that of the steel balls for the same tub vibration.  These laser sensor impact velocity measurements compared reasonably well with those obtained in a previous study using an impact force sensor. 

The measurements can help to elucidate the distribution of impact energy within a vibratory finisher and improve the understanding of processing speed and uniformity.  The data can also serve to guide the development of DEM simulations that attempt to predict local impact velocities in vibrationally fluidized beds of granular materials.

 

Fig. 1: Schematic of the test apparatus used to measure the impact velocity of steel and porcelain balls in the vertical direction in the tub vibratory finisher.


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