Measurement of instantaneous flow velocities in a concentric reducer using Particle Image Velocimetry : study of scale deposition

Deposition of scale (dispersed solids) on the walls of pipework of process plants is a serious maintenance issue. A study conducted at an alumina refinery has revealed a phenomenon of a substantial increase in the rate of scale deposition on the walls of a concentric reducer when compared to that for adjacent straight pipes. To explain this phenomenon the measurement of instantaneous velocity fields in a liquid passing a concentric reducer has been undertaken using Particle Image Velocimetry (PIV). The variation of both the stream-wise and cross-stream components of the fluctuating velocity (u'x and u'r respectively )in the proximity of the wall in the scale model of a concentric reducer has been measured. It has been found that the cross-stream component of the fluctuating velocity, u'r varies significantly as flow passes through the concentric reducer. At a Reynolds number of 43,740 and at the distance from the wall of 0.05R (R is the radius of the pipework) the component u'r in the reducer becomes five times greater than that at the walls of the straight pipes connected to the reducer. Such a variation in the near-wall u'r may be accountable for the increase in gibbsite scale deposition rate observed on the surface of a concentric reducer.