Effects of Sonic Vibrations on Packed Granular Materials
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GILFORD G. QUARLES SCHOOL OF ENGINEERING, THE PENNSYLVANIA STATE COLLEGE, STATE COLLEGE, PA.
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N EXPERIMENTAL study has been made of the effects of sonic vibrations on packed granular materials. This program was prompted by the observation that when wet sand was packed loosely and subjected to vibrations from a 70-kc. transducer, it tended to flow like a puddling mixture of water and sand. It was thought that this might have some practical application for such purposes as accelerating the discharge of granular materiale from hoppers-for example, the removal of coal, sand, flour, or other materials from railway gondolas, storage bins, gravity-feed hoppers, or similar containers. It wae originally planned to study the effects of high-frequency or ultrasonic vibrations only, refining the original crude experimental setup, consisting of a thin metal plate and a loosely contacting transducer; to study the various aspects of the phenomenon for the possible existence of an optimum frequency; and t o determine whether enough high-frequency energy could be applied to be of any commercial value. Early experiments directed attention to vibrations in the low audio-frequency range and most of the work was confined to frequencies below 500 cpe. Packed wet sand of various grain sizes was used in the preliminary experiments, which are described in this paper. Two methods of study were used-the crumbling of molded briquets and the discharge from small hoppers. The effects of grain size, moisture content, and frequency of vibration were studied over limited ranges. A review of the literature revealed nothing of direct importance. Experimental work has apparently been confined to microscopic particles with large separations. A small amount of theoretical work on the response of macroscopic particles to acoustic vibration was located (1). Wolf derived expressions for the motion of a sphere in terms of the ratio of its velocity to that of the surrounding medium as a function of the ratio of the circumference of the sphere to the wavelength of thesoundin themedium, with the ratio of the mass of the sphere to the mass of the displaced medium as a parameter. The results indicated that the relative motion of the sphere in a fluid would increase with decreasing size to wave length ratio, reaching a maximum when this ratio was equal to zero. For the case of the sphere embedded in a solid, the response would reach a masimum when the size t o wave length ratio was slightly above zero. Consideration was given to the possibility of extending this work to apply to the case of a large number of particles in close proximity, but this led to no worthwhile results. The only apparent value of this work to the present investigation was to indicate, qualitatively a t least, that the response of the granular materials to vibrations might be expected to increase with decreasing frequency. The first preliminary experiments tended to confirm thii.
EXPERIMENTAL PROGRAM
The program as originally considered involved a study of the relation between the viscosity of the wet granular materials and the various factors that were thought possibly to influence it. The factors considered, though not all thought to be independent, were as follows: frequency of the vibratione; amplitude of the vibrations used; size and shape of the particles; amount of moisture; the particular material used; size and shape of the entire volume; nature of the lubricating and/or binding fluid; uniformity of size and shape of particles; mass of the particles; and packinp fraction. Only the first four of these have been investigated. For a purely academic first approach to the problem it would seem that the best experimental materials would be granular crystalline substances of regular shape, simple form, and uniform size and composition. Such materials as carefully screened table salt or sugar should meet these requirements. Their simple form should make them simpler to analyze than more complicated forms. However, their solubility in water and their consequent tendency to cake when left exposed to the atmosphere made their use somewhat unattractive. In view of this and in order to obtain a closer approach to the possible ultimate practical application, it was decided to use sand in the first studies. As a simple experimental attack on the problem of measuring the viscosity of the wet granular materials, it was decided to study first the time of crumbling of molded briquets. These were molded by packing sand into a brass cylinder 1 inoh in diameter and I'/z inches long and tapping the sides to loosen the briquet. The first experimental setup consisted of nothing more than a thin metal plate clamped in a vise with the vibrations applied by means of a small magnetostrictive transducer operating a t approximately 70 kc. An illustration of the experimental arrangement used is shown in Figure 1. In order to obtain a good impedance match between the transducer and the plate, the face of the transducer was wet with water. It was observed in the first experiment that unless cavitation occurred in the water between the stack and the plate, there was no observable effect on the sand brir This was mterpreted to mean that it was not the high-:,. - ,ency vibrations that were producing the o~servedphenomenon, but rather low-frequency vibrations resulting from the collapse of the cavitation bubbles. A few simple experiments in which cavitation was prevented by the use of oil instead of water between the transducer and the plate quickly indicated that this was true. The next step was to set up an experimental arrangement which would permit the application of audio frequencies to the briquets. The first arrangement that was used consisted of a large dynamic loud-speaker, over the face of which was placed an aluminum
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 44, No. 1
plate. This arrangement, as was expected, was of value only for qualitative experiment, but it did definitely indicate that the response of the sand to vibrations in the audio-frequency range was exactly that which had been observed to occur with the higher frequency when cavitation ocourred. Briquet Meial Plate
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