Disaggregation of a tracer cloud in accelerating transitional pipe flow

Previous studies on longitudinal solute transport in a pipe during acceleration from laminar to turbulent flow highlighted a disaggregation of the tracer cloud resulting in two peaks occurring downstream from a single pulse injection. It was hypothesized that the disaggregation was caused by the spatially nonuniform acceleration of flow at different radial locations. This study improves the previous understanding of solute transport during accelerating flow using novel laboratory measurements of pipe cross-sectional tracer distribution from planar laser-induced fluorescence and radial velocity profiles from an ultrasonic velocity profiler. Disaggregation of the tracer cloud was observed, with the tracer not being uniformly distributed within the cross-section at the first peak but cross-sectionally well-mixed at the second peak. The relative magnitude of the first peak, compared with the second peak, decreased with the time of injection (and therefore with increased Reynolds number) after the start of the acceleration. Radial velocity profiles showed that the central core of the flow exhibited a smooth, linear increase in velocity compared with the flow closer to the pipe boundary. The data also revealed, for the first time to our knowledge, that the transition time to different flow regimes increased with distance downstream. A model based on the 2D advection-dispersion equation, parameterized using experimental data, was employed to describe the flow and mixing processes during the acceleration. In addition to the combined effects of the radial velocity profile and radial diffusion coefficient, this study confirms that it is essential to also include the delay in the transition time at downstream locations to create the observed disaggregation.