Distribution of Homogeneous Particles on the Bottom of a Desilting Basin Due To Settling In a Nonuniform Low-Velocity Turbulent Flow

Abstract

Translated from Gidrotekhnicheskoe Stroitel 'stvo, No. 2, February 2005, pp. 30 -35. The chambers of intermittent flushing desilting basins with hydraulic removal of the sediment have a constant width, and their bottom is usually made sloped. The surface of silt particles differing in hydraulic size and deposited in these chambers is also not horizontal. For this reason, the motion of the flow in a desilting basin during the settling process is nonuniform. Many works have been devoted to studying the laws of scattering of settled homogeneous particles [1 -4]. All such experiments have been performed in troughs with a horizontal bottom at uniform motion of the flow.The present work is devoted to the regular features and characteristics of the distribution of uniform particles settling in a low-velocity nonuniform turbulent flow. The experiments were performed [5] for two models with different sizes, i.e., a small trough with bottom slope i = 0.01, 0.02, and 0.03 and a large trough with i = 0.01 and 0.02 (Fig. 1). The small glass trough had a width of 18.5 cm, a depth of 31 cm, and a functional length of 5 m, and the large glass trough had a width of 0.6 m, a depth of 0.75 m, and a functional length of 12 m.We used strictly homogeneous elongated polystyrene particles with hydraulic sizes w = 4.09, 3.08, and 2.2 cm/sec. The ratio of w to the mean velocity v in in the initial cross section was 0.4, 0.3, 0.2, and 0.15 in the small trough and 0.19, 0.14, and 0.1 in the large trough. The total number of homogeneous particles participating in one test was 260 in the small trough and 250 in the large trough, which ensured reliable characteristics of their distribution on the bottom after settling.The tests were performed at various depths and velocities of flow in the initial (inlet) cross section. The particles were introduced into the flow with the help of a special device (particle pusher) at different relative depths (Fig. 1) h in /H in = 0.806, 0.531, and 0.376 in the small trough and h in /H in = 0.803, 0.592, 0.381, and 0.803, 0.591, and 0.379 in the large trough. What is important, the particle pusher introduced the particles into the flow individually and without contact with air at a velocity virtually equal to that of the flow in the initial section. In such experiments it is very important to ensure reliable determination of the length of the settling path of the particles without perturbing the flow. For this purpose we developed a special particle arrester (Fig. 1) and organized headrace and tailrace regions in front of and behind the functional part of the model, which were carefully lubricated with plasticine over the perimeter of their contact with the walls and the bottom of the trough.Visual observation and test data show that the measures taken have fully eliminated the appearance of secondary flows in the cells of the particle arrester, additional perturbation of the flow, and distortion of the process of particle settling and of the distribution of particles on the bottom.Altogether we performed 5...