Pipe bends are an integral part of any pipeline network system as they provide the flexibility for pipeline routing. In the case of single-phase fluid flows, secondary flows are generated in bends as a result of forces caused by change in flow direction, which result in additional pressure losses across them. For solid-liquid flows, the secondary flows generated in bends also affect the solid distribution in the downstream side in addition to the increase in pressure loss. Limited studies have been carried out for solid-liquid flows to establish the pressure drop across the bend and hardly any study is reported on the effect of bend on the redistribution of solids downstream of the conventional 90° horizontal circular bend.

It is expected that any data provided on these two aspects will be of great relevance to the slurry pipeline designers for optimizing the life of the pipe bends. Hence, in the present study an attempt has been made to establish the effect of a conventional 90° horizontal circular bend with radius ratio of 5.6 on the re-distribution of narrow-sized as well as bi-modal solid particles downstream of the bend for fully developed flow at the inlet. The effect of addition of fines on the pressure drop has also been established.

The pilot plant test loop used for the present study is a re-circulating loop of 53mm diameter pipe and consists of mixing chamber, pump with motor, pipe loop and measuring tank. The total test loop length is 30m. The test bend is located in the loop downstream of one bend with upstream length being 6m. This ensured a fully developed flow at the inlet of the test bend. The straight length downstream of the test bend is 7.6m. The solid concentration is measured at 6D downstream of the test bend in two orthogonal planes namely mid-horizontal plane and mid-vertical plane for narrow-sized as well as bi-modal slurry with average afflux concentration of 20.4% (by weight) at three average flow velocities [1.78, 2.56 and 356 m/s]. The bi-modal slurry is prepared by mixing water to narrow-sized coarse silica sand particles [mean diameter < 448.5 mm and specific gravity of 2.65] and the fine fly ash particles [mean diameter < 75 mm and specific gravity of 2.17] in different proportions. The ratios in which the particles are mixed to coarser particles are approximately 10, 20, 30 and 40% for 20% average efflux concentration of bi-modal slurry. The concentration is measured near iso-kinetic conditions using a sampling tube. The sampling tube is traversed either from bottom of the pipe or from inner wall of the pipe depending on the plane of measurement. The pressure drop across the bend is measured between different pairs of pressure taps located across the test bend for the above said values of efflux concentrations and flow velocities.

Based on the present study, the following major conclusions are drawn:

1. In bi-modal slurry, at any given Reynolds number, the bend loss coefficient decreases with increase in percentage of the fines mixed.
2. The re-distribution of solid particles takes place downstream of the bend due to the interaction of centrifugal force and pressure driven secondary flows.
3. The concentration distribution is more uniform just downstream of the bend.
4. The re-distribution of solid particles is also seen in the mid-horizontal plane for both mixture as well as individual coarse particles.
5. The finer fraction of the particles is distributed uniformly across the pipe cross section at all velocities of the flow. This uniform suspension of finer particles provides better suspension to coarse particles.