Experimental Determination of Solute Mixing in Pipe Joints

Solute mixing within water distribution systems is, in general, controlled by turbulent mixing within straight lengths of pipes as well as partitioning of the solute into different outlets at pipe junctions. Detailed experimental examination of solute mixing at pipe junctions conducted over the past year shows that solute mixing at these junctions does not follow the often assumed rule of complete mixing and flow-weighted outlet mass fractions. Experiments were conducted using a salt solution and in-line conductivity and flow measurements to determine the normalized mass fraction of the solute exiting the outlet pip that is adjacent to the inlet pipe carrying the salt solution. Here, "Cross" junctions with two inlet and two outlet pipes meeting at a single point are examined. Parameters varied in these experiments include pipe diameter, Reynolds numbers of inputs and outputs, and proportions of tracer and ambient water flowing into the joint. Results show that the largest deviations from the flow-weighted mixing assumption occur when the proportions of the inlet and outlet flows are nearly equal. Changing pipe diameter as well as increasing or decreasing all Reynolds numbers in concert has relatively little effect on the mixing results. Mass-balance calculations of the solute result in generally less than 3% error. When the proportions of flows in the tracer and ambient inlets are quite different, the mass fraction results tend toward those predicted with the complete mixing model.