Longitudinal dispersion in sewer pipes with sediments

Data from the laboratory experiments, studying the longitudinal dispersion in a pipe with sediments, simulating the real conditions in a sewer pipe. Experiments were performed in a laboratory flume with circular cross-section. In aim to simulate the real hydraulic condition in sewer pipe, sand with fraction 0.6-1.2 mm was placed on the bottom of the pipe. In total, 23 experiments, with 4 different layer thickness were performed: layer thickness of 0 mm (no sediments), 8.5 mm (3.4% of the pipe diameter), 25 mm (10%) and 35 mm (14%). For each thickness of the sediment layer a set of tracer experiment with different discharges was performed. The discharge ranges were from 0.14 up 2.5 l.s-1, this corresponds to the range of Reynolds number 500-18 000. The inner diameter of the pipe was 250 mm, length was 12 m, slope of the pipe was 5 ‰. Material of the pipe was smooth transparent plastic. Results of the tracer experiments show, that the value of the longitudinal dispersion coefficient Dx in the hydraulic conditions of circular sewer pipe with sediment and deposits decreases almost linearly to a certain limit (inflexion point), which is individual for each particular sediment thickness. Below this limit the value of the dispersion coefficient starts to rise again, together with increasing asymmetricity of the concentration time course, caused by transient (dead) storage zones.