Determination of complete mixing length in a rectangular flume

Transverse mixing coefficient is one of the hydraulic parameters of flow by which the amount of self-refinement and dilution of river water can be estimated. In this research, the effect of variations of width to depth ratio on the transverse mixing coefficient was studied. Experiments were carried out in a straight rectangular flume of 7 m length, 0.32m width and 0.35m height. The maximum flow rate was 16.4 lit/s. Water was circulated in the channel, the flow was uniform and the depth of water was controlled with a gate way. A tracer was continuously injected with a constant head into the center of the flume. Dye concentration was measured at 4 different downstream locations from the injection point; Twenty seven points were measured and dimensionless transverse mixing coefficient was determined. The results showed that increasing the width to depth ratio from 2.8 to 5.69, the transition by secondary flow increased. Accordingly, in constant velocity of 0.33m2/s, transverse mixing coefficient decreased from 0.049 to 0.012 by increasing the depth from 5.6 to 11.4 cm.