Numerical Simulation of Erosion-Corrosion in the Liquid-Solid Two-Phase Flow

Erosion-corrosion of liquid-solid two-phase flow occurring in a pipe with sudden expansion in cross-section is numerically simulated in this paper. The global model for erosion-corrosion process includes three main components: the liquid-solid two-phase flow model, erosion model and corrosion model. The Eulerian-Lagrangian approach is used to simulate liquid-solid two-phase flow, while the stochastic trajectory model was adopted to obtain properties of particle phase. Two-way coupling effect between the fluid and the particle phase is considered in the model. The accuracy of the models is tested by the data in the reference. The comparison shows that the model is basically correct and feasible.

Numerical simulation of predicting and reducing solid particle erosion of solid-liquid two-phase flow in a choke

Chokes are one of the most important components of downhole flow-control equipment. The particle erosion mathematical model, which considers particle-particle interaction, was established and used to simulate solid particle movement as well as particle erosion characteristics of the solid-liquid two-phase flow in a choke. The corresponding erosion reduction approach by setting ribs on the inner wall of the choke was advanced. This mathematical model includes three parts： the flow field simulation of the continuous carrier fluid by an Eulerian approach, the particle interaction simulation using the discrete particle hard sphere model by a Lagrangian approach and calculation of erosion rate using semiempirical correlations. The results show that particles accumulated in a narrow region from inlet to outlet of the choke and the dominating factor affecting particle motion is the fluid drag force. As a result, the optimization of rib geometrical parameters indicates that good anti-erosion performance can be achieved by four ribs, each of them with a height （H） of 3 mm and a width （B） of 5 mm equaling the interval between ribs （L）.

Critical Stokes Number for Gas-Solid Flow Erosion of Wind Turbine Airfoil

Wind turbine blades are inevitable to be eroded in wind-sand environment,so it is crucial to identify the flow conditions under which the erosion happens.Here,the effect of the sand diameter on wind turbine airfoil is first investigated.When the sand diameter is less than 3μm,the sands will bypass the airfoil and no erosion occurs.When the sand diameter is larger than 4μm,the sand grains collide with the airfoil and the erosion happens.Thus,there must be a critical sand diameter between 3μm and 4μm,at which the erosion is initiated on the airfoil surface.To find out this critical value,aparticle Stokes number is introduced here.According to the range of the critical sand diameter mentioned above,the critical value of particle Stokes number is reasonably assumed to be between 0.007 8and 0.014.The assumption is subsequently validated by other four factors influecing the erosion,i.e.,the angle of attack,relative thickness of the airfoil,different series airfoil,and inflow velocity.Therefore,the critical range of Stokes number has been confirmed.

Pressure Drop of Liquid–Solid Two-Phase Flow in the Vertical Tube Bundle of a Cold-Model Circulating Fluidized Bed Evaporator

A cold-model vertical multi-tube circulating fluidized bed evaporator was designed and built to conduct a visualization study on the pressure drop of a liquid–solid two-phase flow and the corresponding particle distribution.Water and polyformaldehyde particle(POM)were used as the liquid and solid phases,respectively.The effects of operating parameters such as the amount of added particles,circulating flow rate,and particle size were systematically investigated.The results showed that the addition of the particles increased the pressure drop in the vertical tube bundle.The maximum pressure drop ratios were 18.65%,21.15%,18.00%,and 21.15%within the experimental range of the amount of added particles for POM1,POM2,POM3,and POM4,respectively.The pressure drop ratio basically decreased with the increase in the circulating flow rate but fluctuated with the increase in the amount of added particles and particle size.The difference in pressure drop ratio decreased with the increase in the circulating flow rate.As the amount of added particles increased,the difference in pressure drop ratio fluctuated at low circulating flow rate but basically decreased at high circulating flow rate.The pressure drop in the vertical tube bundle accounted for about 70%of the overall pressure drop in the up-flow heating chamber and was the main component of the overall pressure within the experimental range.Three-dimensional phase diagrams were established to display the variation ranges of the pressure drop and pressure drop ratio in the vertical tube bundle corresponding to the operating parameters.The research results can provide some reference for the application of the fluidized bed heat transfer technology in the industry.

Investigation on effect of drag models on flow behavior of power-law fluid–solid two-phase flow in fluidized bed

In this study,a Eulerian-Eulerian two-fluid model combined with the kinetic theory of granular flow is adopted to simulate power-law fluid–solid two-phase flow in the fluidized bed.Two new power-law liquid–solid drag models are proposed based on the rheological equation of power-law fluid and pressure drop.One called model A is a modified drag model considering tortuosity of flow channel and ratio of the throat to pore,and the other called model B is a blending drag model combining drag coefficients of high and low particle concentrations.Predictions are compared with experimental data measured by Lali et al.,where the computed porosities from model B are closer to the measured data than other models.Furthermore,the predicted pressure drop rises as liquid velocity increases,while it decreases with the increase of particle size.Simulation results indicate that the increases of consistency coefficient and flow behavior index lead to the decrease of drag coefficient,and particle concentration,granular temperature,granular pressure,and granular viscosity go down accordingly.

Numerical simulation of counter-current flow field in the downcomer of a liquid-solid circulating fluidized bed

A comprehensive study on the hydrodynamics in the downcomer of a liquid-solid circulating fluidized bed （LSCFB） is crucial in the control and optimization of the extraction process using an ion exchange LSCFB. A computational fluid dynamics model is proposed in this study to simulate the counter-current two-phase flow in the downcomer of the LSCFB. The model is based on the Eulerian-Eulerian approach incorporating the kinetic theory of granular flow. The predicted results agree well with our earlier experimental data. Furthermore, it is shown that the bed expansion of the particles in the downcomer is directly affected by the superfcial liquid velocity in downcomer and solids circulation rate. The model also predicts the residence time of solid particles in the downcomer using a pulse technique. It is demonstrated that the increase in the superficial liquid velocity decreases the solids dispersion in the downcomer of the LSCFB,

Numerical Case Study on the LDIE Degradation Occurred in the Vent Header of a High Pressure Feedwater Heater

Piping installed in nuclear power plants is affected by various types of degradation mechanisms and may be ruptured due to gradual thinning. The degradation mechanisms such as flow-accelerated corrosion (FAC), cavitation, liquid droplet impingement erosion (LDIE), etc., can lead to costly outages and repairs and possibly affect plant reliability. In August 2008, the header pipe in the high pressure feedwater heater vent system leaked at a Korean nuclear power plant. After cutting the pipe during refueling outage, it was identified that the leak was due to LDIE. This paper presents the numerical analysis results, using various multi-phase models of ANSYS FLUENT for the purpose of identifying the cause of the LDIE. The numerical analysis methods which are most similar to the damage of the pipe are proposed for the comparison of analysis results with each multi-phase model.

Investigation of Hydroabrasion in Slurry Pipeline Elbows and T-junctions

The present study demonstrates the comparison of erosion rate of critical pipeline parts, namely elbow and T-junction which face the maximum erosion in a pipeline and may cause an early damage and failure of the system. CFD （computational fluid dynamics） with an Eulerian-Lagrangian approach coupled with an approved erosion model is applied to visualize the 3-D flow behavior of slurry flow in both parts and to predict the erosion rate and the location of erosion at the internal surfaces. The analysis of slurry erosion is performed in five steps; geometry and grid generation, grid study/refinement, fluid flow solution, solid particles tracking and finally, the erosion calculation. In previous publications in literature considering transportation of gas-solid flows in pipe parts, the application ofT-junctions instead of elbows for specified conditions in order to reduce the erosion is recommended. In this article, it is approved that for liquid-solid flows, the Stokes number is reasonably smaller than the values for gas-solid flows. This causes the solid particles tightly couple to the fluid phase and to travel more closely with the fluid streamlines. The effects of important influencing parameters such as feed flow velocity, solid concentration, particle size and shape are investigated in detail in current work. It was found that for liquid-solid flows, the erosion of T-junction for all of the mentioned influencing parameters, due to its geometrical specifications and Stokes number variation in comparison with gas-solid flows, is reasonably higher than erosion of elbow. Due to these findings, in contrary to the gas-solid mixture flows, application of T-junction instead of elbow for liquid-solid flow transportation is not recommended.

Hyperconcentrated flows as influenced by coupled wind-water processes

Using data from more than 40 rivers in the middle Yellow River basin, a study has been made of the influence of coupled wind-water processes on hyperconcentrated flows. A simple “vehicle” model has been proposed to describe hyperconcentrated flows. The liquid phase of two-phase flows is a “vehicle”, in which coarse sediment particles are carried as solid-phase. The formation and characteristics of hyperconcentrated flows are closely related with the forma-tion and characteristics of this liquid-phase and solid-phase. Surface materials and geomorphic agents of the middle Yellow River basin form some patterns of combination, which have deep influence on the formation and characteristics of liquid- and solid-phases of hyperconcentrated flows. The combination of high percentages of relatively coarse material with low percentages of fine material appears in the area predominated by the wind process, where the supply of rela-tively coarse sediment is sufficient, but the supply of relatively coarse sediment is not. The com-bination of low percentages of relatively coarse material with high percentages of fine material appears in the area predominated by the water process, where the supply of fine sediment is sufficient, but the supply of fine sediment is not. In the area predominated by coupled wind-water processes appears the combination of medium percentages of coarse and fine materials, and thus both coarse and fine sediments are in relatively sufficient supply. The manner in which the mean annual sediment concentrations of liquid- and solid-phases vary with total suspended sediment concentration is different. With the increased total suspended sediment concentration, mean annual sediment concentration of liquid-phase increased to a limit and then remained constant; however, mean annual sediment concentrations of solid-phase in-creased continuously. Thus, the magnitude of total suspended sediment concentration depends on the supply conditions of relatively coarse sediment and the ability of the flow to carry these relatively coarse sediment particles. In the area predominated by wind process, both the liquid- and the solid-phases cannot develop well, and their concentrations are low. In the area pre-dominated by the water process, the mean annual sediment concentrations of liquid- and solid-phases are also low. Only in the area predominated by coupled wind-water processes, can the conditions most favor the development of both the liquid- and solid-phases, and then the peaks of mean annual sediment concentrations of liquid- and solid-phases appear. Low values of suspended sediment concentrations appear in the areas predominated by the wind process or by the water process, a fact indicating that the predominating wind process or water process does not favor the development of hyperconcentrated flows. Peak values appear in the area where the coupled wind-water processes are predominated, indicating that the cou-pled wind-water processes most favor the development of hyperconcentrated flows.

STUDIES OF SOLID－LIQUID TWO－PHASE TURBULENT FLOWS AND WEAR IN HYDRAULIC TURBOMACHINERY

The Lagrangian equation of motion for solid particles in an arbitrary flow field is derived. The linear differential equation form and the general solution of this equation are obtained. Motion of solid particles in dilute solid-liquid turbulent flows is numerically solved and analysed. The K-εtwo-equation turbulence model, the volume fraction turbulence model, the mixed Eulerian-Lagrangian turbulence model, and the dense mixture turbulence model as well as the erosive wear model are developed. Using these models, the turbulent flows and the erosive wear in some hydraulic turbomachinery ducts are numerically predicted. The numerical results show good agreement with the experiments.

Wall-scaling prevention in cryogenic external cooler of ammonium chloride solution by liquid-solid fluidization

A double-tube cooler with liquid-solid circulating fluidization operation and corresponding parameter measuring system are developed to avoid fouling of inner walls of heat exchange tubes in a cryogenic temperature external cooler of ammonium chloride solution in soda ash production.Wall-scaling prevention performance of the cooling process is experimentally evaluated using convection and overall coefficients,enhancement factor,wall temperature and fouling resistance.Effects of different volume fractions of added particles,particle size,superficial liquid velocity,and cooling medium temperature on heat transfer are examined.Under present conditions,convection coefficient of liquid-solid flow inside the tube of external cooler is higher than that of the liquid phase flow,increased by 0.7–2.8 times,enhancing cooling performance obviously.Convection coefficient initially increases and then decreases as the volume fraction of added particles increases,reaching its maximum value at a volume fraction of 2.0%.The wall-scaling prevention effect of glass beads mainly depends on the volume fraction of added particles;optimal anti-fouling effects are achieved when adding particles at a volume fraction of 2.0%,regardless of changes in superficial liquid velocity or cooling medium temperature.This study lays a foundation for industrial applications of this new technique of fluidized bed external coolers.

Numerical simulation and experimental study on eggshell membrane separation device

In order to provide theoretical guidance for separating egg membrane from eggshell by using mechanical agitation,CFD was used to explore the flow characteristics in stirred tank,using the Sliding Grid method to deal with the impeller rotational velocity zone in flow field,and using the Euler model to deal with liquid-solid two-phase flow.This study explored the influence of dish-shape bottom or flat-shape bottom,the clearance size between baffle and the side wall,and the axial height of impeller to bottom on suspension state of particles,solids holdup distribution,solid phase velocity and power number by CFD.Simulation results showed that better particles suspension effect in dish-shape tank can reduce particles accumulation at the bottom and power consumption.If there was a small clearance size(S)between the baffle and the side wall of the stirred tank,it would reduce particles accumulation at the bottom,and reduce the power consumption.However,too large S would decrease the suspension height of particles,not only cannot strengthen the main flow,but also lead to most fluid through clearance forming tangential flow,simulation results showed that S=6 mm was perfect.While decreased axial height of impeller(C)to bottom,particles accumulation at the bottom was decreased,but power consumption would increase,simulation results showed that C=H/5(H is height of liquid surface)was perfect.According to the simulation results,the structure of the stirring tank was optimized.At the same time,the influences of stirring rotational velocity,stirring time,solid-liquid ratio and separating medium temperature on egg membrane recovery were also studied by experiment,and optimal parameter combination of factors was obtained.The experiment results showed while the stirring time was 17.1 min,stirring rotational velocity was 350 r/min,solid-liquid ratio was 1:17 g/mL,the separating medium temperature was 32℃,the membrane recovery rate can reach above 89%.The device improves the recovery and utilization of discarded eggshell,and provides a reference for the solid-liquid two-phase flow and related study.