EXECUTIVE SUMMARY
The flow of coarse materials from hoppers is well correlated by the Beverloo correlation. However this over-predicts the flow rate of particles of size less than about 5:00 pm. The purpose of this work was to investigate the hypothesis that the retarded flow is caused by air drag resulting from the dilation of the material as it approaches the orifice.
The work was carried out in Cambridge by T.M.Verghese under the supervision of R.M.Nedderman. This report is a summary of the work presented in Verghese’s Ph.D. Dissertation.
Experimental work was carried out on kale seed and 6 closely-graded sands with mean sixes ranging from 2280 pm to 150 pm. Measurement of the flow rates confnmcd the validity of the Beverloo correlation for coarse materials and the existence of related flow for finer materials. Direct measurement of the interstitial pressure profile showed that the immediate cause of retardation was the existence of adverse pressure gradients near the orifice. From this it can be inferred that dilation wcurs and direct confmnation of this was obtained using gamma-ray tomography.
We conclude therefore that the work has confirmed the starting hypothesis and this has lead to a correlation for the discharge rate of fine sands which shows good agreement with experiment.
A theoretical analysis for the discharge rate of coarse materials from bunkers and hoppers has been undertaken. This analysis differs from all previous analyses in that allowance has been made for the non-uniform velocity distribution in the hopper, For mass flow hoppers the predictions do not differ greatly from those of earlier work. However, the theory also predicts the discharge rate from core flow bunkers and to the best of our knowledge this is the first time that a theoretical prediction has been made for this situation.