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INTRODUCTION Drinking water in Slovakia meets very high standards; the average citizen relies on the impeccable quality of the tap water. Drinking water distribution networks are expected to transport only dissolved matter rather than a few visible particles. However, it is almost impossible to make the drinking water free from suspended solid particles. The ability to determine the origins of these particles varies between different water supply systems, with possible sources coming from catchments, treatment processes, biofilm growth within the water supply pipes, and corrosion products. This paper will discuss the distribution of sediments in the drinking water distribution system in Holič in Slovakia. The main interest is the behavior (deposition and resuspension) of the particles coming from the Holič treatment plant. Sediment settling in drinking water networks is not wanted as it can lead to deteriorating water quality and so called brown- water complaints by customers. In Figure 1, the mass balance of a drinking water network can be seen, showing the incoming-and outgoing load and the different processes inside the pipe. The incoming load consists of water containing suspended solids particles, color etc., inside the pipe processes such as biofilm J. KRIŠ, GHAWI A. HADI CFD INVESTIGATION OF PARTICLE DEPOSITION AND RESUSPENSION IN A DRINKING WATER DISTRIBUTION SYSTEM KEY WORDS Computational Fluid Dynamics (CFD), pipe distribution systém, mixture model, particle deposition resuspension ABSTRACT Water distribution system models have become widely accepted within the water utility industry as a mechanism for simulating hydraulic and water quality behavior in water distribution system networks. In this paper the effect of particle size, temperature, and the velocity of fluid on the deposition and resuspension in water distribution systems of Holič in Slovakia is examined. A comprehensive computational fluid dynamics (CFD) investigation was carried out for particle deposition and suspension in a drinking water distribution system. A satisfactory concordance was established with the experimental data as validation. This was a steady and unsteady state multiphase mixture problem, which helped to understand the deposition and suspension characteristics for different particle sizes and densities. The 3D numerical multiphase mixture model solves continuity and momentum equations for the mixture and volume fraction equations for the secondary phases. The governing equations were also solved for the turbulence parameters of the particulate phases. The comparison of model predicted results with the experimental data shows good agreement. The deposition of heavier particles at the bottom of the pipe is greater at a low velocity rather than a high velocity, but light particles remain suspended in the fluid across the pipes circumference. Jozef Kriš Research field: water resources, water supplies, water treatment plant Slovak University of Technology, Faculty of Civil Engineering, Department of Sanitary and Environmental Engineering Radlinského 11, 813 68 Bratislava Slovak Republic [email protected] Ghawi A. Hadi Research field: drinking water supply, numerical modeling. Slovak University of Technology, Faculty of Civil Engineering, Department of Sanitary and Environmental Engineering, Radlinského 11, 813 68 Bratislava, Slovak Republic, [email protected] 2008/1 PAGES 1 – 7 RECEIVED 19. 7. 2007 ACCEPTED 12. 12. 2007 1 CFD INVESTIGATION OF PARTICLE DEPOSITION AND RESUSPENSION IN ... Ghawi.indd 1 5. 3. 2008 11:56:21

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Page 1: PARTICLE DEPOSITION AND RESUSPENSION

INTRODUCTION

Drinking water in Slovakia meets very high standards; the average citizen relies on the impeccable quality of the tap water. Drinking water distribution networks are expected to transport only dissolved matter rather than a few visible particles. However, it is almost impossible to make the drinking water free from suspended solid particles. The ability to determine the origins of these particles varies between different water supply systems, with possible sources coming from catchments, treatment processes, biofilm growth within the water supply pipes, and corrosion products.

This paper will discuss the distribution of sediments in the drinking water distribution system in Holič in Slovakia. The main interest is the behavior (deposition and resuspension) of the particles coming from the Holič treatment plant. Sediment settling in drinking water networks is not wanted as it can lead to deteriorating water quality and so called brown-water complaints by customers. In Figure 1, the mass balance of a drinking water network can be seen, showing the incoming-and outgoing load and the different processes inside the pipe.The incoming load consists of water containing suspended solids particles, color etc., inside the pipe processes such as biofilm

J. KRIŠ, GHAWI A. HADI

CFD INVESTIGATION OF PARTICLE DEPOSITION AND RESUSPENSION IN A DRINKING WATER DISTRIBUTION SYSTEM

KEY WORDS

• Computational Fluid Dynamics (CFD), • pipe distribution systém, • mixture model, • particle deposition • resuspension

ABSTRACT

Water distribution system models have become widely accepted within the water utility industry as a mechanism for simulating hydraulic and water quality behavior in water distribution system networks. In this paper the effect of particle size, temperature, and the velocity of fluid on the deposition and resuspension in water distribution systems of Holič in Slovakia is examined. A comprehensive computational fluid dynamics (CFD) investigation was carried out for particle deposition and suspension in a drinking water distribution system. A satisfactory concordance was established with the experimental data as validation. This was a steady and unsteady state multiphase mixture problem, which helped to understand the deposition and suspension characteristics for different particle sizes and densities. The 3D numerical multiphase mixture model solves continuity and momentum equations for the mixture and volume fraction equations for the secondary phases. The governing equations were also solved for the turbulence parameters of the particulate phases. The comparison of model predicted results with the experimental data shows good agreement. The deposition of heavier particles at the bottom of the pipe is greater at a low velocity rather than a high velocity, but light particles remain suspended in the fluid across the pipes circumference.

Jozef Kriš Research field: water resources, water supplies, water treatment plantSlovak University of Technology, Faculty of Civil Engineering, Department of Sanitary and Environmental Engineering Radlinského 11, 813 68 Bratislava Slovak [email protected]

Ghawi A. HadiResearch field: drinking water supply, numerical modeling.Slovak University of Technology,Faculty of Civil Engineering, Department of Sanitary and Environmental Engineering, Radlinského 11, 813 68 Bratislava, Slovak Republic, [email protected]

2008/1 PAGES 1 – 7 RECEIVED 19. 7. 2007 ACCEPTED 12. 12. 2007

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formation and soughing, corrosion, the formation and coagulation of particles, and deposition and resuspension; theses processes affect the outgoing load. Corrosion, resuspension and biofilm soughing can lead to an increase in sediment, whereas a decrease in sediment can occur when sediment becomes trapped inside biofilm, is deposited or suspended by formation and coagulation. Many researches have attempted to understand the possible deterioration of water quality once it enters a distribution system (Anderson and Russell 1970; Laurinat, et al., 1985, David, et al., 1987, and Thomson, 2003).An improvement in our understanding of the complex hydrodynamic behavior of suspended and/or deposited particles involved in these distribution pipe networks requires mathematical and physical models. Computational Fluid Dynamics (CFD), along with an analytical turbulent model is one of the most popular mathematical techniques, as it has the ability to predict the behavior of complex flows for such multiphase flow applications. A CFD investigation was carried out to predict the hydrodynamic behavior of turbid particles flowing through a pipe network.

MATERIALS AND METHODS

The CFD model uses the characteristics of sediment as input; this means that velocities at which the sediment suspends, resuspends and/or settles have to be determined. This was performed by obtaining samples of particulates from the water distribution systems of Holič in Slovakia and of Melbourne, Adelaide, Sidney and Brisbane in Australia. The samples in Australia were analyzed using a pipe test-loop and a water tunnel at CMIT (CSIRO Manufacturing & Infrastructure Technology) (Grainger, et al., 2003). The rig consisted of a test pipe with a diameter of 100 mm; a schematic drawing of the test pipe and boundary conditions are shown in Figure 2 and Table 1 respectively. The test rig was used to model the validation and determine the flow velocity of the water when the sediment starts to settle and the sediment velocity with which the sediment settles.

GOVERNING EQUATION

The multiphase mixture model of Fluent 6.2 (Fluent Inc., 2005) used in this study solves the continuity and momentum equations for the mixture. The volume fraction equations are solved for the secondary phases. The model also solves the well-known algebraic expressions for the relative velocities for the secondary phases (Fluent Inc., 2006).

Continuity Equation for the MixtureThe term “mixture” can be defined by the combination of all the primary and secondary phases. The continuity equation for the mixture is

(1)

Fig. 1 Mass balance of a pipe in a drinking water network.

Fig. 2 Schematic drawing of pipe a test loop (Grainger, et al., 2003).

Table 1 Physical and hydraulic characteristics of the system used for CFD simulation.Pipe loop length (m) 41.0Pipe length in Holič (m) 1200Diameter of the pipe D (m) 0.1Total volume of water (m3) 0.322No. of phases 6VF of each secondary phase 342 ppmParticle density (kgm-3) 1640Particle sizes (μm) 1 – 100Average water velocities (ms-1) 0.05-0.50

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Where is the mass-averaged velocity of the mixture, which is:

(2)

And ρm is the mixture density:

(3)

αk is the volume fraction of phase k.

Momentum Equation for the MixtureThe momentum equation for the mixture can be obtained by totaling the individual momentum equations for all the phases. It can be expressed as:

(4)

Where n is the number of phases, is the body force, and μm is the viscosity of the mixture:

(5)

is the drift velocity for the secondary phase k:

(6)

Relative (Slip) Velocity and the Drift VelocityThe relative velocity (also referred to as the ”slip” velocity) is defined as the velocity of the secondary phase (p) relative to the velocity of the primary phase (q):

(7)

The drift velocity and the relative velocity ( ) are connected by the following expression:

(8)

The basic assumption of the algebraic slip mixture model is that in order to prescribe an algebraic relation for the relative velocity, a local equilibrium between the phases should be reached over short spatial length scales. The form of the relative velocity is given by

(9)

Where is the secondary-phase particle’s acceleration and τqp is the particulate relaxation time. Following Manninen, et al., 1996 τqp is of the form:

(10)

Where dp is the diameter of the particles of the secondary phases p, and the drag function fdrag is taken from Schiller and Naumann, 1935:

(11)

And the acceleration is of the form

(12)

The simplest algebraic slip formulation is the so-called drift flux model, in which the acceleration of the particle is given by gravity and/or a centrifugal force, and the particulate relaxation time is modified to take into account the presence of other particles.

Volume Fraction Equation for the Secondary PhasesFrom the continuity equation for the secondary phase p, the volume fraction equation for the secondary phase p can be obtained:

(13)

Turbulence Viscosity (The Spalart-Allmaras Model)Instead of μm (equation 5) the turbulent viscosity, μt, is computed from

(14)

Where the viscous damping function, fv1, is given by

(15)

Where Cv1 = 7.1 and

(16)

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VALIDATION OF MODEL

In this paper the results have been validated with the experimental result conducted by Grainger, et al., 2003. At CSIRO Grainger, et al., 2003, demonstrated an experiment for particle distribution and deposition in a test loop. In order to compare the experiment results (Grainger, et al., 2003), we have used the same geometric and boundary conditions. Figure 3 represents the cumulative particle volume fraction as a function of the heights across the pipe at a certain location in the loop for both the experiment and the CFD results. Nevertheless, the trend is similar, but the experimental results show a marginally lower volume fraction. This is because of the shortcomings of the measuring instruments that were used in the experiment.

RESULTS AND DISCUSSION

By determining the velocities for the typical sediment found in Australian networks, the problems with the theory of settling and resuspension are short cut. This simplification was made in order to characterize the sediment characteristics and use them in the CFD model. Deposition is the phenomenon by which particles settle under the influence of the gravity force. In water, the rate at which a particle settles is a function of both the grain and fluid properties. Resuspension is the phenomenon by which particles collected in drinking water pipes are resuspened due to hydraulic changes. Resuspension occurs when forces caused by the flow of a fluid are larger than the forces of the own weight of the particle captured inside a sediment bed under water.

Figure 4 shows the change in the settling velocity of particles with a given particle size and density in water with a fixed density for different temperatures (and changing dynamic viscosities). The settling velocity at 0 0C and at 24.5 0C are respectively 1.95*10-4 and 3.90*10-4 m/s; this is the difference in the settling velocity of 199%. This shows that the effect of the temperature on settling in laminar conditions is very large. Also the temperatures that can occur inside networks range from 0 to 22.3 0C (Vreeburg, et al., 2004). This means that the viscosity of the fluid is affected. This viscosity directly affects the settling velocity in laminar flow conditions; a difference of settling speed of 199% between 0 and 24.5 0C is possible. If these temperature differences are not considered, the settling of the sediment can be totally different.Particles will resuspend when the flow is turbulent because the profile of the flow velocity in a pipe is different for a laminar or

Fig. 3: Comparison of the CFD results and experimental data for the velocity 0.43 m.s-1 at the center of the pipe.

Fig. 4 Dynamic viscosities and settling velocities at different temperatures (20 μm, 1310 density particle kg/m3) at Holič.

Fig. 5 Particle and turbidity (NTU) measurements of the distribution network at Holič.

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turbulent flow. The velocity in turbulent flows changes little over the pipe diameter compared to the laminar flow. The turbulence causes a lateral mixing of the fluid, leading to flows that reach the pipe wall. As we have seen, water flowing in drinking water networks is usually turbulent.The sizes of particles coming from a treatment plant and in distribution networks usually range from 1 to 100 μm and have density that is smaller than 2000 kg/m3. Figure 5 shows a picture of the continued measurement of the turbidity in a distribution network. A pattern can be seen, thus identifying the extra contribution to the sediment caused by corrosion. At night when stagnant water occurs, the cast iron pipes corrode, leading to a deterioration of the water quality.A large contribution to the total sediment load originates from the treatment plant. Elements like Fe, Mn and Al are present in treated drinking water. These elements are mainly present as oxides: Fe as Fe(OH)3, Mn as MnO2 and Al as Al(OH)3. This assumption does not necessarily hold, because Al could also come from the leaching of asbestos-cement pipes and Fe from corroding cast-iron pipes.A lot of research has been performed concerning the growth and origin of biofilm (Boe Hansen, et al., 2003; and Van de Kooji, et al., 1995). Van de Kooji suggests for instance that iron and manganese are entrained in the biofilm, leading to a smaller iron and manganese concentration in the water. Biofilm is a small layer of organic material that is formed in drinking water pipes. Biofilm grows on pipe walls; nutrients that are present in the water are converted into a biomass.Figure 6 shows the relative concentration plotted as a function of the different heights of 0.125D-1D from the bottom of the pipe and time for various particle diameters. The relative concentration is a dimensionless parameter, which represents the ratio of the local particle concentration to that of the bottom of the pipe. Particles

of a size of 1-5 μm are evenly distributed throughout the drinking water distribution system. The concentration of 10 μm size particles shows a gradual increase towards the bottom. The concentration of 20 μm size particles is localized near the bottom. The larger size particles 50-100 μm are all localized at the bottom of the pipe.According to a test carried out (Grainger, et al., 2003) using a test pipe loop, particles disappeared from the suspension in a wide range of velocities up to 0.3 m/s or more. The particles resuspended again from the pipe bottom in a range of 0.15-0.25 m/s; this is also found on tests that were performed by Lut (2004). The velocity at which the water flows is u; the velocity at which it resuspends is called urs and the velocity at which all particles will suspend is called ud. Figure 7 shows the possible behavior of the sediment and the corresponding velocities.

There are three situations that can occur, depending on the mixture velocity vm: (1) vm >urs the flow velocity is more than the resuspension velocity, so the resuspension of all the sediments occurs. urs is the critical velocity beyond which the particles are resuspended; urs is a function of the particle diameter, density and packing of the sediment, (2) ud<u< urs the particle mass is transported through the pipe with no settling/resuspension, because the mixture velocity vm is between the velocity at which the sediment suspends (urs) and the velocity at which it settles (ud), and (3) vm <ud all the particles will settle, because the velocity of the water is so low that all the sediment will suspend.Now we can test if the distribution of the sediment change a lot if these parameters (us, ud, urs) are slightly changed.(1) us is the velocity with which the sediment deposits. If us is lowered, this will lead to a slower settling of particles and a different kind of distribution of the sediment over the network, the sediment will settle further away in the network. If us is raised, this will lead to a quicker settling of sediment (Table 2).(2) If the parameter ud is changed, this means that the flow velocity of the water at which the sediment starts to deposit is changed. When this velocity is lowered, it will lead to more sediment that suspends. This would lead to a different kind of distribution of the

Fig. 6 Relative concentration of particles plotted as a function of different heights and times for various particle diameters at Holič.

Fig. 7 Cross section of a pipe illustrating suspension, resuspension and settling velocity.

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sediment over the network. If the velocity ud is increased, this will lead to less material settling (Table 3). (3) If the velocity at which the sediment resuspends is lowered, the sediment that was previously suspended will resuspend. This is because during the day, the flow velocity in the pipes will increase because of the larger water demand. This flow velocity then exceeds the resuspension velocity of the sediment (Table 4).

CONCLUSION

The effect of velocities on the deposition of particles has been investigated numerically. This paper investigated the effect of particle size, temperature, and the velocity of fluid on deposition and resuspension in the Holič drinking water distribution system. This

CFD results have also been validated with the experimental results (Grainger, et al., 2003). In these numerical simulation six different flow profiles and particle-load profiles were used to compute the particles’ deposition and re-entrainment into the systems and to identify the conditions of the deposition and suspension mechanisms. The velocity at which particles start to deposit was found to be at least 0.15 m/s; the velocity at which particles start to resuspend was found to be 0.25 m/s. These values are used to make predictions for the whole network of a drinking water distribution system. A reasonably good concordance between the simulation and experiment results has been established.

ACKNOWLEDGEMENT

Thanks are due to Ing. Ales Prechazkza for his help in the laboratory in Holič water treatment plant and to the numerous collaborators involved in the sampling at the treatment plant. This study was supported by Grant No. 1/3313/06 and projected APVT 20-031804 solved at the Department of Sanitary and Environmental Engineering, Faculty of Civil Engineering, Slovak University of Technology, Bratislava.

Table 2 Settled and suspended mass compared to the initial situation

us m/sDeposited per

meter pipe kg/mIn suspension per meter

pipe kg/m5.45*10-6 0.08312541 0.000461261.06*10-7 0.00286004 0.000561351.06*10-5 0.12845790 0.00050561

Table 3 Settled and suspended mass compared to the initial situation.

ud m/sDeposited per

meter pipe kg/mIn suspension per meter

pipe kg/m0.14 0.08012501 0.000474120.11 0.06756893 0.000456130.26 0.10845790 0.00040561

Table 4 Settled and suspended mass compared to the initial situation.

urs m/sDeposited per meter

pipe kg/mIn suspension per meter

pipe kg/m0.26 0.07801250 0.000434120.11 0.906756893 0.050045610.51 0.09645790 0.00042356

Notationρm Mixture density (kg.m-3)u Average velocity (m.s-1)αk Particle volume fractionμm Dynamic mixture viscosity (Pa-s)q Primary phasep Secondary phase

τqp Particulate relaxation time (s)dp Diameter of the particles (μm)

fdrag Drag functionRe Reynolds numberD Pipe diameter (m)

Acceleration (m.s-2)

Drift velocity (m.s-1)

Relative velocity (m.s-1)

Mass-averaged velocity (m.s-1)

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REFERENCES

• Anderson, R. J. and Russell, T. W. F. (1970): Film formation in two-phase annular flows, AIChE Jl, 14, 626-633.

• Boe Hansen, R. , A. C. Martiny, E., Arvin, E., and Albrechtsen. H. J. (2003): Monitoring biofilm formation and activity in drinking water distribution networks under oligotrophic conditions. Water Science and Technology,Vol. 47, No. 5.

• David, Y. H. P., Francisco Romay-Novas, and Benjamin Y. H. Liu (1987): Experimental study of particle deposition in bends of circular cross sections, Aerosol Science and Technology, 7, 301-315.

• Fluent INC., Fluent, Ver. 6.1.22, Rel. 2005. USA.• Fluent INC. Fluent Manual. 2006. USA.• Grainger, C., Wu, J., Nguyen, B. V., Ryan, G., Jayanratne,

A., and Mathes, P., (2003): Part 1: Settling, Re-Suspension and Transport, CRC, CFC, Melbourne, Australia, Apr. 2003.

• Kooji van der, D., H. S. Vrouwerivelder, and H. R. Veenendaal (1995): Kinetic aspects of biofilm formation on surfaces exposed to drinking water. Water Science and Technology, Vol. 32, No. 8.

• Laurinat, J. E., Hanratty, T. J., and Jepson, W. P. (1985): Film thickness distribution for gas-liquid annular flows in a horizontal pipe, Phys. Chem. Hydrodynam., 6, 179-195.

• Lut M. C. (2004): Hydraulic behavior of particles in a drinking water distribution system. M.Sc. Report TU Delft 2004.

• Manninen, M., Taivassalo, V., and Kallio, S. (1996): On a mixture model for multiphase flows, VTT Publications 288, Technical Research Center of Finland.

• Schiller, L. and Nuamann, (1935): A Drag Coefficient Correlation.” Z. Ver. Deutsch. Ing., 77:318.

• Thomson, D. J. (2003): Dispersion of particle pairs and decay of scalar fields in isotropic turbulence. Physics of Fluids, 15, 801-813.

• Vreeburg, j. H. G., P. G. Schaap, M. Den Boer and J. C. Van Dijk (2004): Particles in a drinking water system: from source to discoloration. Proceedings of the 4th IWA World Water Congress.

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