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Prediction of head loss for fly ash-water slurry flow through 90° bend pipe using Computational fluid dynamics

Abstract

The current work was performed to investigate the head loss characteristics in bend pipes for the transportation of solid–liquid suspension using commercial Computational fluid dynamics (CFD) tool ANSYS Fluent. Fly-ash was taken as the solid particle. The concentration was varied from 10 to 50% (by weight) along with the flow velocity ranging from 1.5 to 4.5 m/s. A number of simulations were performed by the SST k-ω turbulence model. The mean diameter of fly ash particle was taken 45 μm. According to the findings, head loss increases with the flow velocity and solid concentration. The head loss increased by 8.12, 9.91 and 11.76% as the solid concentration changed from 20 to 30, 30–40 and 40–50% respectively for flow velocity of 4.5 m/s. Head loss increases more with increase in velocity as compare to solid concentration. With the help of contours, distribution of the solid suspension, flow velocity, and the influence of turbulence inside the bend shape was also investigated.

Introduction

Slurry pipelines are widely used for transportation of solid materials across shorter and longer distances by means of carrier fluids. In industrial applications, solid materials are transported using carrier fluids like air, water, and oil. Pipelines are used to transport solid materials of various particle shapes and sizes. Particle size, concentration of solid material, flow velocity, slurry viscosity, density etc. are other elements which influence solid–liquid flow behavior. Water is mostly employed in slurry transportation since it is less expensive and more readily available .

From the last decade, researchers have worked on effective transportation of slurry with high solid concentrations and they have investigated the head loss characteristics of slurry flow in pipelines. Kumar et al. has performed the experiment by adding bottom ash with the fly ash which enhanced the head loss of the suspension.

The flow behavior of the slurry differs from the single-phase fluid flow. When compared to single phase, the existence of multiphase fluids in a pipeline causes significant fluctuations in flow behaviors. Many studies have been done to predict pressure drop in horizontal pipelines and 90° pipe bends for flow of various solid materials. Because experimental work are costly and complicated, only minor modifications are feasible in a pilot test loop that has already been built. Therefore, numerical simulations were used to improve design parameters, with the numerical findings being validated against experimental data.

Using CFD, the head loss in vertical and horizontal pipelines for flow of solid–liquid slurries was simulated by various researchers. Among them, Gopaliya et al. used CFD to predict the flow of a solid–liquid suspension via a horizontal pipe by taking sand as the solid material. Kumar et al. investigated the pressure loss characteristics of coal-water slurry suspension in 90° bend pipe varying r/D ratio in addition to different solid concentration at different flow velocity using CFD.

According to the literature, the Eulerian model provides the most logical framework for describing high concentration slurries. The interaction of solid–solid, wall-solid, and liquid–solid particles are all included in this model. The physical characteristics of the carrier fluid and solid material are the only inputs required. In previous research, RNG k-∊ and SST k-ω turbulence modelling methods are also shown to be the best suitable for finding flow behaviors at high concentration slurry flow.

The modelling of slurry transport structure depends on physical, chemical and morphological properties with surface roughness of pipe and existence of various fixtures for example elbows, tee, sudden converging or diverging sections. Only a few studies of the pressure drop behavior of slurry flow over a 90° pipe bend have been conducted so far. In industrial applications, the usage of a 90° bend is limited since it causes significant losses and needs a lot of pumping effort. Due to geometrical and space restrictions, however, the usage of 90° bends is not totally prohibited. ANSYS-Fluent was used to examine the head loss behaviors for transport of fine fly ash-water slurry. Because soft solid fly ash has a greater viscosity at smaller particle sizes. The mean diameter of fly-ash was taken 45 μm for this investigation. The bend is fixed for this investigation with a r/D ratio of 2, where r is the radius of curvature and D is pipe diameter. The solid concentration and velocity were varied from 10 to 50% and 1.5–4.5 m/s respectively.

Mathematical modelling

In the CFD study of slurry flows, the selection of an appropriate multiphase model is critical, and it is mostly determined by the range of volume fraction of solid phase under consideration. The Eulerian model with granular sub-model was utilized for these simulations according to the nature of the current problem. The granular version is capable of capturing the influence of particle friction and collusions, which is critical in higher concentration slurry flows.

Rheological characteristics of fly ash

The fly-ash specimen has taken from Tata Tinplate Company Ltd, Jamshedpur, India. Experiments were carried out on a fly ash sample. A pycnometer technique was utilized to obtain the specific gravity of the fly-ash, which was 2.515. A MCR 92 rheometer was used to evaluate the rheological behavior of the fly ash-water slurry for solid concentrations of 10–50 % by weight. At a constant temperature of 26°C, shear rates were varied from 0 to 300 s-1. The mean diameter of fly ash particle was taken

Geometry

The geometry of the bend section was designed in the ANSYS Design modeler 19.0. Fig. 1 shows the geometry that has been used for the simulation in the current study. The model is consisting of two circular pipes connected with a bend having the diameter of 100 mm. The inlet section is the upstream section having a length of 3 m and the downstream has a length of 1.5 m and the bend having a radius of curvature of 200 mm.

Meshing

In this study, the fluid domain is discretized using the ANSYS Workbench

Effect of fly-ash concentration on head-loss

The influence of solid fraction upon the head loss for the conveying of a fly-ash particle mixture was investigated using a series of numerical simulations. The head loss characteristics of a bend pipe with a r/D ratio of 2 were investigated. The solid concentration was altered between 10% and 50%, with velocity varying from 1.5 − 4.5 m/s. The variation in head loss as a consequence of flow velocity and solid fraction is shown in Fig. 5, Fig. 6.

As the slurry flow velocity increases from 1.5 to

Conclusions

The goal of this study was to minimize the head-loss features of concentrated fly-ash-water slurry flow over 90° pipe bends. Slurries with varying solid concentrations were transported through bend pipe with varying speeds. The numerical simulations for the 100 mm pipe bend geometry were carried out using a Eulerian multiphase model by means of SST k-ω approach. The present study has led to the following findings.

Head loss through pipe bends increased as the flow velocity and material