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Title: Investigation of high concentration slurry disposal through pipe bend
Authors: Singh, Jatinder Pal
Supervisor: Kumar, Satish
Mohapatra, S. K.
Keywords: Head loss;90° bend;pressure drop;slurry
Issue Date: 18-Oct-2019
Abstract: The present investigation is carried out to investigate the head loss characteristics across 90˚ pipe bends for the flow of concentrated coal and coal-ash slurries. Before head loss experiments, different bench-scale tests are carried out to analyze the physical and chemical characteristic of solid particles. The specific gravity of coal, fly and bottom ash sample is determined as 1.46±2%, 2.27±0.5% and 2.09±1.6%. The maximum settled concentration for coal, fly ash and bottom slurry is observed as in range 65.37-76.55%, 59.67-73.92% and 53.79-68.58% for a solid concentration range of 30-60% respectively. The pH of coal-water slurry varies in the range of 6.38-5.93 as solid concentration is increased from 10-60% (by weight). The pH value for fly and bottom ash suspensions are lies in the range of 7.62-7.42 and 7.68-7.45 respectively for the solid concentration range of 30-60%. Extensive rheological experimentation is performed to analyze flow characteristics of coal-water slurry with and without the use of the additive. The coal-water slurry exhibits Newtonian behaviour at a solid concentration of 30% (by weight). The coal-water slurry exhibits shear thinning behaviour at higher solid concentrations (i.e. above 30%) that show its pseudo-plastic nature. The addition of a small amount of sulphonic acid resulted in an improvement in the rheological properties of the coal-water slurry suspension. Similarly, the rheological experiments are carried out for fly and bottom ash slurry with and without the use of additive (sodium triphosphate). The fly ash slurry exhibits Newtonian behaviour for a solid concentration of 30%. Fly ash slurry shows Bingham plastic behaviour for a solid concentration range of 40-60% (by weight). The bottom ash slurry exhibits Newtonian behaviour for solid suspension up to a solid concentration of 30%. The bottom ash slurry shows Bingham flow behaviour above 40% solid concentration. The addition of sodium triphosphate results in a decrease in viscosity of bottom and fly ash slurry. The maximum percentage decrease in apparent viscosity is observed with 4% addition of sodium triphosphate in fly and bottom ash slurry. After the rheological analysis of slurry, the head loss experiments are conducted with slurries of different solid concentrations, varying from 30-60% (by weight). During experimentation, the average flow velocity is kept in the range of 2-5 m/s. The r/D ratio for the pipe bend varied within the range of 1.5-2.5. Pulverized coal sample having particle size less than 53 µm is used to prepare the coal-water slurry whereas the as-collected samples fly and bottom ash are directly used to prepare the coal-ash slurries. From results, it is found that the head loss across 90° pipe bend increases with the increase in the solid concentration of coal and coal-ash slurries. The head loss across pipe bends is a function of the solid concentration and r/D ratio. A sudden drop in head loss is found as the r/D ratio is increased from 1.5 to 2.0. The optimum r/D ratio for minimum head loss in a 90˚ pipe bend is 2.0; beyond this, the increase in head loss is seen due to increase in effective length of pipe bend. The head loss across pipe bends for fly ash slurry is more than coal and bottom ash slurry. The maximum reduction in head loss is observed with a 3% (by weight) addition of sulfonic acid in coal˗water slurry and 4% addition of sodium triphosphate in coal-ash slurry. Thus, the addition of additive is one of the economical ways to convey the coal and coal˗ash slurry at higher solid concentrations. Computational fluid dynamics (CFD) simulations are also carried out to obtain the head loss, flow patterns and contours for the flow of coal and coal-ash slurry. An Eulerian multiphase model with an SST k-ω approach is used for carrying out the numerical simulations for 50 mm pipe bend geometry. The effect of flow separation and secondary flows on turbulence, flow velocity and distribution of the solid suspension inside the bend geometry is also studied with the help of contours. The r/D ratio of 90˚ and 45˚ pipe bend is varied within the range of 1.5-3.0. The minimum head loss is observed across 45˚ bend. The optimum r/D ratio for minimum head loss across 45˚ and 90˚ pipe bend is observed as 1.5 and 2.0 respectively. The detailed phenomenon of flow in pipe bends is also discussed with the help of volume fraction, turbulence intensity and velocity contours at different locations in pipe bend.
Appears in Collections:Doctoral Theses@MED

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