Flow of Fine Powders

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Author Last Name: 
Dr R M Nedderman, Dr T M Verghese
Report Type: 
Research Area: 
Powder Flow
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United Kingdom

This report covers the contract year 1985/86 and describes the work performed to investigate the flow of fine materials from conical hoppers. Six materials - Kale seed and five different grades of sand ranging in size from 2,28mm to 150pm - are used in the experiments, Mass flow rates and interstitial pressure profiles adjacent to the orifice are measured simultaneously. Results have been obtained for four orifice diameters on a cone of half angle 9.5’ and for six orifice diameters on a cone of half angle 15”.

Previous work in the field has been extensively reviewed. Experiments on the kale seed and the coarsest sand (d = 2.28mm) provide further evidence for the applicability of the Beverloo (196l)correlation in describing the flow of coarse granular materials. For conical hoppers, the form of the correction factor proposed by Hose and Tanaka has been verified but the value of the exponent is still in doubt. They proposed a value of -0.35 which compares with the value of n m -0.2 indicated by the results obtained in this work.

Experiments on the different size ranges indicate that for particles in the range 50 to 600pm the measured flow rates are less than those predicted by the Beverloo correlation. It is shown that this retarded flow is caused by self-generated interstitial pressure gradients which arise as the material dilates on approaching the orifice. The Crewdson equation (1977), originally developed for air-augmented flows is found to be adequate in describing these retarded flows. Below 50um cohesive arching becomes an important factor and it may even prevent flow.

Crewdson et al. investigated a theory of stress induced dilation but their efforts proved inconclusive because of the difficulty in obtaining an accurate voidage-stress relationship at the low stress levels prevailing near the orifice. This line of thought is revived as the commercially available consolidometer can be used to obtain a voidage- stress relationship for stresses less than 10kN/ms. However, experimentally determined voidage changes exceed those predicted from the theory by a factor of almost 100. This suggests that it is necessary to look elsewhere for a suitable mechanism to explain the cause of the dilation.

Possible avenues of further work, both experimental and theoretical, are presented. An important element in the future is to be the direct determination of the voidage profile, which will be attempted using the r-ray tomography method developed by Seville et al. (1986) at the University of Surrey.

It is hoped to be able to develop a correlation for the flow of fine powders in the near term using the fractional retardation W/Ws. Meanwhile in the absence of any suitable alternatives, Carleton’s (1972) correlation can be used to predict the flow of fine powders from orifices greater than 20mm. The results presented in this work suggest that an overprediction in the range 20% to 50% can be expected.