AWWA WQTC50351

The two purposes of this research study were: to investigate the effect of Reynolds number on the length of recirculation zones; and to study the effect of residence time distribution in these zones under low Reynolds number conditions. An in situ light scattering apparatus was developed to determine concentration changes of particle suspensions in a pipe flow for two axisymmetric configurations: a step contraction; and a step expansion. Variation of recirculation zone length with Reynolds number was recorded experimentally for the expansion. The results were compared with observations reported in literature, and with theoretical predictions using a numerical fluid flow simulation package. Characteristic times for particle concentration buildup in the recirculation zones were observed and correlated with mixing and turbulence parameters. Includes 13 references, figures.