Voidage waves in hydraulic conveying through narrow pipes

The nature of voidage waves in hydraulic conveying systems has been investigated computationally in this study. Voidage waves consisting of alternating regions of high and low solid fractions were observed to form spontaneously in hydraulic conveying through vertical pipes. The wavelengths of these voidage waves were found to be inversely related to mean solid fraction but independent of liquid conveying velocities for elastic solid particles. The wave structure in terms of the thickness and shape of the solid fraction profile over each dense phase was dependent on the mean solid fraction used. Pressure distributions along the longitudinal direction of the conveying pipe exhibited an interesting ‘step’ profile due to unequal pressure drops across the dense and dilute regions. With inelastic solid particles, a lower tendency for voidage wave formation was observed and a phase diagram was constructed to show combinations of operating conditions under which voidage waves would form. Finally, the study of voidage waves in hydraulic conveying systems was extended to the case of hydraulic conveying through horizontal channels. In addition to the various system parameters, voidage wave formation in this case was also affected by gravitational settling of the solid particles.