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CFD simulation of dust particle removal efficiency of a venturi scrubber in CFX
Nuclear Engineering and Design 256 (2013) 169–177
Contents lists available at SciVerse ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
CFD simulation of dust particle removal efficiency of a venturi scrubber in CFX Ali Majid ∗ , Yan Changqi, Sun Zhongning, Wang Jianjun, Gu Haifeng College of Nuclear Science and Technology, Harbin Engineering University, Harbin 150001, China
h i g h l i g h t s
Simulation of dust particle removal efficiency in venturi scrubber in ANSYS CFX. Mesh model prepared in ANSYS ICEM. Eulerian–Lagrangian approach for multiphase flow. Investigation of dust removal efficiency at different gas and liquid flow rate. Validation of simulation against throat gas velocity and dust removal efficiency.
a r t i c l e
i n f o
Article history: Received 30 May 2012 Received in revised form 8 December 2012 Accepted 11 December 2012
a b s t r a c t This research presents the computational fluid dynamics (CFD) simulation of dust particle removal efficiency of a venturi scrubber in ANSYS CFX. Venturi scrubber effectively encapsulates the dust particles from contaminated gas in petite droplets formed by disintegration of a liquid due to high kinetic energy of gas flowing into it. Eulerian–Lagrangian method is employed in ANSYS CFX to investigate the dust removal efficiency of a venturi scrubber for gas flow rate at 0.09, 0.115, and 0.14 kg s−1 and liquid flow rate at 0.1, 0.13 and 0.16 kg s−1 . The hydrophobic dust particle titanium dioxide (TiO2 ) of diameter 1 m having density 4.23 g cm−3 is used in the simulation and experimentation. Throat gas velocity, volume fraction, droplet size and removal efficiency are investigated to analyze the performance of venturi scrubber. Cascade atomization and breakup model (CAB) is used to predict the breakup of liquid in the venturi scrubber. The simulation results are verified from the experimental results of gas velocity at throat and dust removal efficiency. The simulation result predicts good with experimental results. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In Nuclear Power Plant (NPP), reactor containment is the last barrier to prevent the release of radioactive fission product into the environment due to reactor core meltdown. Reactor containment can be severely damaged due to over pressurization caused by the decay heat of molten reactor core (AREVA, 2011). To avoid the serious consequences, filtered vented containment system (FVCS) (Schlueter and Schmitz, 1990) is installed in NPP to protect the integrity of containment against the over pressurization and removal of fission products which are in aerosol, vapor and gaseous forms release from the fuel into the containment (Rust et al., 1995). FVCS mainly comprise of venturi scrubber and metallic fiber filter which is shown in Fig. 1. Venturi scrubber is one the most important type of wet scrubber. It is used to
∗ Corresponding author. Tel.: +86 451 82569655; fax: +86 451 82569655. E-mail addresses: [email protected] (M. Ali), changqi [email protected] (C. Yan), [email protected] (Z. Sun), [email protected] (J. Wang), [email protected] (H. Gu). 0029-5493/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nucengdes.2012.12.013
clean the contaminated gas via droplets within short contact time between the liquid and gas phases in it. Venturi scrubber primarily consists of three segments: first, convergent segment which accelerates the gas to its maximum velocity; second, throat segment which is between convergent and divergent segments, where gas and liquid interact with each other and the third is diffuser segment where the fluid velocities decelerate for pressure recovery (Goncalves et al., 2003). Computational fluid dynamic (CFD) method is widely used in research due to an extensive range of features in different fields of science and engineering to improve and analyze the performance of system or design. ANSYS CFX is one of the foremost tools of CFD, which uses the finite volume technique to study the complex phenomenon such as multiphase flow, chemical reactions, turbulence and heat and mass transfer, etc. in system and design. Venturi scrubber is one of the main components of FVCS to remove the aerosols or dust from contaminated gas. Therefore, it is necessary to design the venturi scrubber properly. Due to this reason, the knowledge about the fluid dynamics in venturi scrubber is very important which include three phase flow i.e. gas, liquid, and dust.
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M. Ali et al. / Nuclear Engineering and Design 256 (2013) 169–177
Nomenclature k ε F m Pk Cε1RNG Cε2RNG kRNG εRNG t r Cb CD Cf Ck C k1 Wet Wet Cd d E g Re t
v
turbulence kinetic energy (m2 s−2 ) turbulence dissipation rate (m2 s−3 ) force (N) mass (kg) shear production of turbulence (kg s−1 s−3 ) RNG k–ε turbulence model coefficient (1.42-fh ) RNG k–ε turbulence model coefficient (1.68) RNG k–ε turbulence model constant (0.7179) RNG k–ε turbulence model constant (0.7179) turbulent viscosity (kg m−1 s−1 ) radius (m) critical amplitude coefficient (0.5) damping coefficient (5.0) external force coefficient (1/3) restoring force coefficient (8.0) new droplet velocity factor (1.0) CAB breakup factor (0.05) critical Weber number for stripping breakup (80) critical Weber number for catastrophic breakup (350) drag coefficient (dimensionless) diameter (m) efficiency (dimensionless) acceleration due to gravity (m/s2 ) Reynolds number (dimensionless) time (s) velocity (m/s) inertial impaction parameter (dimensionless) viscosity (Ns m−2 ) density (kg m−3 ) surface tension (N m−1 ) Stokes number (dimensionless)
Subscript d droplet gas g p particle re removal inlet i
There are two approaches available in CFX to simulate the multiphase flow; the Eulerian approach treats the dispersed phases as interpenetrating continua while the Lagrangian approach treats the dispersed phases as individual entities (ANSYS, 2010). The advantage of Lagrangian over the Eulerian approach is the tracking of particle (droplet or solid) injected into the domain. In Eulerian approach, the average values such as velocity, mass, and temperature of particle phase is calculated whereas Lagrangian approach calculate these values for each representative particle (Guerra et al., 2012). In the present study, the removal efficiency of dust particle in venturi scrubber is simulated and analyzed by Eulerian–Lagrangian scheme in ANSYS CFX. For this purpose, the features of venturi scrubber such as throat gas velocity, liquid fraction, droplet size, and removal efficiency are studied. Further, the simulation results are compared with experimental results for validation of model and simulation methodology. The rest of the paper is organized as follows. Section 2 discusses the previous work for the dust removal efficiencies in different software packages. Section 3 explains the facility developed for experimentation of dust removal efficiency. Section 4 describes the mathematical model for turbulence, particle transport, and dust
removal efficiency. Section 5 enlightens the numerical methodology to solve the problem. In Section 6, the results obtained from simulation are discussed. Section 7 summarizes the conclusions of this conducted research work. 2. Previous work Pak and Chang (2006) simulated the collection efficiency of dust particle in a venturi scrubber by using KIVA code. The model used in KIVA code was modified for the flow of gas, liquid, and dust flow. Eulerian method is used for gas phase, and Lagrangian method is used for droplet and dust. Navier–Stokes equation solved the equation of motion for gas, whereas Basset–Boussinesq–Oseen (B–B–O) equation for droplet and dust. Schmehl et al. correlation was recommended to determine the drag coefficient for droplet. Dust particle was captured into droplets by inertial impaction mechanism. The pressure drop was underpredicted due to inaccurate prediction of droplet and not considering the film on the walls. The experimental and simulation results for collection efficiency agree well with each other. Tao and Kuisheng (2009) studied the heat and mass transfer phenomenon between droplet and gas phase in a venturi scrubber numerically by FLUENT software. The gas phase was regarded as continuous phase with Euler’s approach, whereas droplet and dust phase was regarded as discrete phase with Lagrangian approach in the domain. RNG k–ε turbulence model was used for simulation. Water was injected in the form of spray. The single droplet collection efficiency was calculated for inertial impaction. Heat transfer coefficient was calculated from Ranz and Marshal correlation, while Nusselt correlation used for calculation of the mass transfer coefficient. The performance of a venturi scrubber is evaluated at different baffle openings and liquid to gas ratio. The parameters included to analyze the performance of a venturi scrubber were: pressure drop, collection efficiency, mole fraction, velocity, and temperature profiles. It was observed that with the decrease of liquid to gas ratio and baffle opening, the pressure drop and collection efficiency was increased. Goniva et al. (2009) simulated the capturing efficiency of a venturi scrubber with OpenFOAM package. TAB model is used for the droplet breakup. The capture efficiency of single droplet is calculated by the sum of inertial, interception, and diffusion mechanism. The effect of wall film was also investigated. The models for deposition and entrainment of droplet were used in research. The overall pressure drop and collection efficiency calculated from simulation, which predicts well with the experimental data. 3. Experimental facility The schematic diagram of an experimental facility to study the dust removal efficiency of a venturi scrubber is shown in Fig. 2. In the first stage, the highly compressed gas is generated from an air compressor. This compressed gas is further stored in an air tank. The compressed gas is cleaned with air filters before injected into the main loop. In second stage, the titanium dioxide (TiO2 ) having mean size diameter of 1 m is injected into the main loop from dust particle injection device. Third stage contains the water tank for injection of liquid into the venturi scrubber. Venturi scrubber is mounted in the venturi tank such that the direction of gas flows against the gravity. The venturi scrubber is operated at gas mass flow rate 0.09, 0.115, and 0.14 kg s−1 and liquid flow rate is 0.1, 0.13, and 0.16 kg s−1 . The concentration of dust particle is measured at sampling point at inlet Si and outlet So by filtration technique. The dust removal efficiency is calculated from the following equation: Ere =
Cin − C0 Cin
(1)
M. Ali et al. / Nuclear Engineering and Design 256 (2013) 169–177
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Fig. 1. Filtered vented containment system in Gosgen Nuclear Power Plant.
4. Mathematical modeling 4.1. Turbulence equation 4.2. Particle transport phase The RNG k–ε model has been used in simulation to model the turbulence of gas phase in the domain. The conservation equations for the turbulent kinetic energy k and the dissipation energy ε for gas phase are (ANSYS, 2010)
t ∂k + ∇ vg k − + kRNG ∂t
∂ε t + ∇ vg ε − + εRNG ∂t + Cε1RNG Pεb ]
∇ k = Pk − ε + Pkb
∇ε =
(2)
In the CFD model, droplet and dust particle are treated as a discrete particle. The force acting on each particle is the sum of the drag force and buoyancy force due to gravity on the particle. The equation of motion of particle can be written as (ANSYS, 2010) Fp = FD + FB
ε [Cε1RNG Pk − Cε2RNG ε k (3)
mP
1 dvP = CD g Ap (vg − vP )|vg − vP | + dP3 (P − g )g 2 6 dt
Fig. 2. Experimental facility for dust removal efficiency of a venturi scrubber.
(4)
(5)
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Fig. 3. (a) Mesh of venturi scrubber, (b) mesh at throat and (c) mesh near to boundary wall.
Fig. 4. Gas velocity at gas flow rate (A) mg = 0.09 kg s−1 , (B) mg = 0.115 kg s−1 , and (C) mg = 0.14 kg s−1 .
M. Ali et al. / Nuclear Engineering and Design 256 (2013) 169–177
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Fig. 5. Droplet size at mass flow rate of liquid 0.1 kg s−1 and gas mass flow rate: (A) 0.09 kg s−1 , (B) 0.115 kg s−1 , and (C) 0.14 kg s−1 .
4.3. Drag coefficient
the damping forces (ANSYS, 2010). The deformation of droplet in CAB model can be expressed as (ANSYS, 2010)
Schilller Nauman drag model is used for calculation of drag coefficient for particles (ANSYS, 2010)
CD = max
24 Re
(1 + 0.15Re0.687 ), 0.44
(6)
y¨ =
2 Cf g Vrel
Cb d rd 2
−
Ck d d rd3
y−
CD d d rd2
y˙
(7)
The rate of generation of product of child droplet is calculated from the following equation (ANSYS, 2010): dn(t) = 3Kbr n(t) dt
(8)
The size of child droplet is calculated as (ANSYS, 2010) 4.4. Droplet breakup and coalescence model The droplets are formed due to turbulence of gas and external aerodynamic forces acting on the liquid. Cascade atomization and breakup model (CAB) model treats the deformation of droplet with the analogy of a spring-mass system. Accordingly to the CAB model, the restoring force of the spring is represented by the droplet surface tension whereas the external force is replaced by aerodynamic force exerted on the droplet and liquid viscosity represents
rd,parent rd,child
= e−Kbr t
(9)
The Kbr follows the given breakup regime (ANSYS, 2010):
Kbr =
⎧ k ω ⎪ ⎨ 1 √ ⎪ ⎩
k2 ω We
k3
ωWe3/4
5 < We < 80 80 < We < 35 350 < We
Fig. 6. Droplet size at mass flow rate of liquid 0.13 kg s−1 and gas mass flow rate: (A) 0.09 kg s−1 , (B) 0.115 kg s−1 , and (C) 0.14 kg s−1 .
(10)
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Fig. 7. Droplet size at mass flow rate of liquid 0.16 kg s−1 and gas mass flow rate: (A) 0.09 kg s−1 , (B) 0.115 kg s−1 , and (C) 0.14 kg s−1 .
where k2 = k1 k3 =
1 − (1/2)(Ck Cb /Cf Wet )
a cos(1 − (C k Cb /Cf Wet )) k2
(11) (12)
1/4
Wet2
The single droplet collection efficiency is calculated from inertial impaction mechanism. Inertial impaction is a function of Stokes number, which depends on densities, relative velocity, and diameters. Inertial impaction is dominant for particle size having a diameter of 1 m (Lim et al., 2006). The inertial impaction of single droplet efficiency defined by Calvert (1970) is
2
+ 0.7
=
p dp2 (vp
− vd )
9g dd
(14)
4.6. Dust removal efficiency The dust removal efficiency is calculated from the following equation:
4.5. Single droplet collection efficiency
=
is expressed as (Pak and Chang, 2006),
where
(13)
Ere = 1 −
m (1 − ) di mdi
(15)
5. Numerical methodology 5.1. Turbulence model Eulerian–Lagrangian approach for three phase flow is employed in this research. The computations are performed in the steady
Fig. 8. Liquid fraction at mass flow rate of liquid 0.1 kg s−1 and gas mass flow rate: (A) 0.09 kg s−1 , (B) 0.115 kg s−1 , and (C) 0.14 kg s−1 .
M. Ali et al. / Nuclear Engineering and Design 256 (2013) 169–177
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Fig. 9. Liquid fraction at mass flow rate of liquid 0.13 kg s−1 and gas mass flow rate: (A) 0.09 kg s−1 , (B) 0.115 kg s−1 , and (C) 0.14 kg s−1 .
state. The gas (air) is treated as a carrier field, whereas the dust particles as transport dispersed solid and liquid (water) as transport dispersed fluid in the domain. RNG k– turbulence model for gas is used. This model performs well in bending shapes and rotating flows (ANSYS, 2010).
5.3. Solver High resolution advection scheme is selected for accurate numerical solutions. It is used due to its robustness. 5.4. Convergence criteria The convergence criteria for simulations are 10−4 RMS for residuals.
5.2. Mesh The unstructured mesh of a venturi scrubber is created in ANSYS ICEM. The domain of a 3D Venturi Scrubber model is discretized into 272,136 hexahedral elements and 284,607 nodes. The mesh quality of the venturi scrubber model is greater than 0.6 on the scale from 0 to 1 based on mesh quality criteria defined in ANSYS ICEM (ANSYS ICEM CFD, 2009). Fig. 3 (a) depicts the mesh of venturi scrubber on a plane and, (b) shows the mesh at throat on a plane whereas, (c) shows the mesh near to the boundary wall.
5.5. Boundary condition The boundary conditions for the CFD venturi model are defined as follows: at the inlet, the mass flow rate of the gas is applied, while dust is injected with particle transport solid with mean diameter of 1 m. The liquid is injected as particle transport fluid with mass flow rate boundary condition. At the outlet of a venturi scrubber,
Fig. 10. Liquid fraction at mass flow rate of liquid 0.16 kg s−1 and gas mass flow rate: (A) 0.09 kg s−1 , (B) 0.115 kg s−1 , and (C) 0.14 kg s−1 .
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the pressure boundary condition is used. The parallel and perpendicular restitution coefficients are 0.0 for droplets in diffuser wall. 6. Numerical results and discussion The simulation of three phase flow in venturi scrubber is conducted by Eulerian–Lagrangian approach. The dust and droplet particles are tracked by Lagrangian method. To study the dust removal efficiency, the hydrophobic particle titanium dioxide (TiO2 ) is injected at an inlet. The diameter of TiO2 is 1 m and density 4.23 g cm−3 . The dust removal efficiency of a venturi scrubber is calculated from the mass flow rate of dust at inlet and outlet of a venturi scrubber. The removal efficiency of a venturi scrubber depends on droplet diameter, fluid velocity, and impaction. The present simulation does not incorporate the heat and mass transfer, and liquid film on walls.
6.3. Liquid fraction The distribution of liquid inside the venturi scrubber highly affects the performance of a venturi scrubber. The distribution of liquid is affected by the flow rates of gas and liquid. Therefore, distribution of liquid fraction is analyzed in a venturi scrubber. The amount of gas and liquid introduced into the scrubber depends on mass flow rate. The venturi scrubber is simulated with three different gas flow rate 0.09, 0.115, and 0.14 kg s−1 whereas liquid flow rate 0.1, 0.13, and 0.16 kg s−1 . The contours of liquid fraction at different mass flow rate for gas and liquid inlet are shown in Figs. 8–10. 6.4. Validation The simulation of dust removal efficiency of a venturi scrubber is validated by comparing the throat gas velocity and dust removal efficiency with the experimental results.
6.1. Gas velocity The gas phase in a venturi scrubber is treated as a continuous field. When gas entered into the convergent segment, it accelerates and reaches to its maximum velocity at throat segment while decelerate in divergent segment. The velocity vectors of gas at mass flow rate 0.09, 0.115, and 0.14 kg s−1 illustrate the behavior in convergent, throat and diffuser segment is as shown in Fig. 4. The maximum throat gas velocity is achieved at gas mass flow rate of 0.14 kg s−1 . 6.2. Droplet size The droplet size plays a significant role in the removal process of a venturi scrubber. Liquid is introduced with mass flow rate boundary condition into the venturi scrubber domain from the liquid inlet as shown in Fig. 3. When the liquid entered into the throat segment, it interacts with the high velocity of gas. The high gas velocity atomized the liquid into tiny droplets. The droplet breakup in venturi scrubber domain is predicted by CAB model. The contour of droplet sizes at different flow rates are shown in Figs. 5–7. The size of droplet in contours range is 1–100 m. The result shows that the diameter of droplet decreases with the increase of gas flow rate.
Fig. 11. Comparison of experimental and simulation result of throat gas velocity at different gas flow rate.
6.4.1. Throat gas velocity In Fig. 11, the experimental values of gas velocity at throat are compared with simulation values. The result shows that the simulation values are very near to experimental values which validate the simulation results. 6.4.2. Dust removal efficiency The dust removal efficiency is calculated from the simulation result by the mass ratio of the removed dust particles to the total inflow mass of dust particles. The simulation results of dust removal efficiency at 0.1, 0.13, and 0.16 kg s−1 liquid flow rates are validated by comparing with experimental results as shown in Figs. 12–14. The effect of wall film is ignored, therefore parallel and perpendicular restitution coefficients are 0.0 on diffuser wall. The droplet which collided with diffuser wall was not further tracked. The CAB model is used for the droplet breakup in the domain. The dust particles introduced with the gas at inlet accelerated in the convergent segment. The throat is very short; therefore, mostly collision of dust with droplet will take place in diffuser segment. The collision of dust particle with the liquid droplets primarily depends on inertial impaction. However, impaction mechanism depends on characteristics of dust and droplet. The Stokes number is greater than 1, which specifies that the particle remains unaffected by the
Fig. 12. Comparison of experimental and simulation result of dust removal efficiency at mass flow rate of liquid 0.1 kg s−1 .
M. Ali et al. / Nuclear Engineering and Design 256 (2013) 169–177
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increases with the increase of gas mass flow rate. The simulation result of dust removal efficiency agrees well with experimental results. 7. Conclusion
Fig. 13. Comparison of experimental and simulation result of dust removal efficiency at mass flow rate of liquid 0.13 kg s−1 .
The simulation of venturi scrubber model has been carried out in ANSYS CFX to study the dust removal efficiency of venturi scrubber. Venturi Scrubber model is meshed in ANSYS ICEM. Eulerian–Lagrangian is implemented for three phase flow simulation. In venturi scrubber model, gas is regarded as an Eulerian approach while dust and droplet are tracked by Lagrangian approach. The turbulence effects are calculated by RNG k–ε model. CAB model is used for droplet breakup in venturi scrubber domain. The dust removal efficiency is calculated from the dust mass flow rate at inlet and outlet. Inertial impaction is used for capturing of dust particle. The throat gas velocities at different mass flow rate are verified with experimental results. The liquid fraction is analyzed to determine the liquid fraction distribution inside the scrubber. The dust particle removal efficiency increases with the increase in gas mass flow rate and liquid flow rate. The simulation and experimental results concur well which validates the simulation work. Acknowledgements The authors admiringly acknowledge the support of the present work from College of Nuclear Science and Technology, Harbin Engineering University. The author also gratefully acknowledges the awarded scholarship from Chinese Scholarship Council. References
Fig. 14. Comparison of experimental and simulation result of dust removal efficiency at mass flow rate of liquid 0.16 kg s−1 .
fluid velocity and does not influence it. As a result, the particle collides with the droplet rather than flowing around it. The impaction increases with the increase in relative velocity between droplet and dust, and with the decrease of diameter of droplet. With the increases of gas flow rate, the diameter decreases which is shown in Figs. 5–7. The decrease in diameter increases the surface area, and increases the chances of interaction between dust particle and droplet. The result shows that the dust removal efficiency
ANSYS ICEM CFD, 2009. User Manual. ANSYS Inc., Southpointe, 275 Technology Drive, Canonosburg, PA 15317. ANSYS, 2010. CFX-Solver Theory Guide. ANSYS Inc., Southpointe, 275 Technology Drive, Canonosburg, PA 15317. AREVA, 2011. Filtered Containment Venting System. AREVA Inc., Bethesda, Maryland. Goniva, C., et al.,2009. Simulation of offgas scrubbing by a combined Eulerian–Lagrangian model. In: iSeventh International Conference on CFD in the Minerals and Process Industries. CSIRO, Melbourne, Australia. Rust, H., et al., 1995. Pressure release of containments during severe accidents in Switzerland. Nucl. Eng. Des. 157, 337–352. Goncalves, J.A.S., et al., 2003. Atomization of liquid in a Pease–Anthony venturi scrubber. Part I. Jet dynamics. J. Hazard. Mater. 97 (B), 267–297. Lim, K.S., et al., 2006. Prediction for particle removal efficiency of a reverse jet scrubber. J. Aerosol. Sci. 37, 1826–1839. Kernkraftwerk Gosgen-Daniken, A.G., 1999. Gosgen Nuclear Power Plant Technical Information. Kernkraftwerk Gösgen-Daniken AG, Switzerland. Tao, L., Kuisheng, W., 2009. Numerical simulation of three dimensional heat and mass transfer in spray. Chin. J. Mech. Eng. 22 (5), 745–754. Schlueter, R.O., Schmitz, R.P., 1990. Filtered vented containment. Nucl. Eng. Des. 120, 93–103. Calvert, S., 1970. Venturi and other atomizing scrubbers efficiency and pressure drop. AlChE J. 16 (3), 392–396. Pak, S.I., Chang, K.S., 2006. Performance estimation of a venturi scrubber using a computational model for capturing dust particles with liquid spray. J. Hazard. Mater. 138 (B), 560–573. Guerra, V.G., et al., 2012. Pressure drop and liquid distribution in a venturi scrubber: experimental data and CFD simulation. Ind. Eng. Chem. Res. 51, 8049–8060.
Contents lists available at SciVerse ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
CFD simulation of dust particle removal efficiency of a venturi scrubber in CFX Ali Majid ∗ , Yan Changqi, Sun Zhongning, Wang Jianjun, Gu Haifeng College of Nuclear Science and Technology, Harbin Engineering University, Harbin 150001, China
h i g h l i g h t s
Simulation of dust particle removal efficiency in venturi scrubber in ANSYS CFX. Mesh model prepared in ANSYS ICEM. Eulerian–Lagrangian approach for multiphase flow. Investigation of dust removal efficiency at different gas and liquid flow rate. Validation of simulation against throat gas velocity and dust removal efficiency.
a r t i c l e
i n f o
Article history: Received 30 May 2012 Received in revised form 8 December 2012 Accepted 11 December 2012
a b s t r a c t This research presents the computational fluid dynamics (CFD) simulation of dust particle removal efficiency of a venturi scrubber in ANSYS CFX. Venturi scrubber effectively encapsulates the dust particles from contaminated gas in petite droplets formed by disintegration of a liquid due to high kinetic energy of gas flowing into it. Eulerian–Lagrangian method is employed in ANSYS CFX to investigate the dust removal efficiency of a venturi scrubber for gas flow rate at 0.09, 0.115, and 0.14 kg s−1 and liquid flow rate at 0.1, 0.13 and 0.16 kg s−1 . The hydrophobic dust particle titanium dioxide (TiO2 ) of diameter 1 m having density 4.23 g cm−3 is used in the simulation and experimentation. Throat gas velocity, volume fraction, droplet size and removal efficiency are investigated to analyze the performance of venturi scrubber. Cascade atomization and breakup model (CAB) is used to predict the breakup of liquid in the venturi scrubber. The simulation results are verified from the experimental results of gas velocity at throat and dust removal efficiency. The simulation result predicts good with experimental results. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In Nuclear Power Plant (NPP), reactor containment is the last barrier to prevent the release of radioactive fission product into the environment due to reactor core meltdown. Reactor containment can be severely damaged due to over pressurization caused by the decay heat of molten reactor core (AREVA, 2011). To avoid the serious consequences, filtered vented containment system (FVCS) (Schlueter and Schmitz, 1990) is installed in NPP to protect the integrity of containment against the over pressurization and removal of fission products which are in aerosol, vapor and gaseous forms release from the fuel into the containment (Rust et al., 1995). FVCS mainly comprise of venturi scrubber and metallic fiber filter which is shown in Fig. 1. Venturi scrubber is one the most important type of wet scrubber. It is used to
∗ Corresponding author. Tel.: +86 451 82569655; fax: +86 451 82569655. E-mail addresses: [email protected] (M. Ali), changqi [email protected] (C. Yan), [email protected] (Z. Sun), [email protected] (J. Wang), [email protected] (H. Gu). 0029-5493/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nucengdes.2012.12.013
clean the contaminated gas via droplets within short contact time between the liquid and gas phases in it. Venturi scrubber primarily consists of three segments: first, convergent segment which accelerates the gas to its maximum velocity; second, throat segment which is between convergent and divergent segments, where gas and liquid interact with each other and the third is diffuser segment where the fluid velocities decelerate for pressure recovery (Goncalves et al., 2003). Computational fluid dynamic (CFD) method is widely used in research due to an extensive range of features in different fields of science and engineering to improve and analyze the performance of system or design. ANSYS CFX is one of the foremost tools of CFD, which uses the finite volume technique to study the complex phenomenon such as multiphase flow, chemical reactions, turbulence and heat and mass transfer, etc. in system and design. Venturi scrubber is one of the main components of FVCS to remove the aerosols or dust from contaminated gas. Therefore, it is necessary to design the venturi scrubber properly. Due to this reason, the knowledge about the fluid dynamics in venturi scrubber is very important which include three phase flow i.e. gas, liquid, and dust.
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M. Ali et al. / Nuclear Engineering and Design 256 (2013) 169–177
Nomenclature k ε F m Pk Cε1RNG Cε2RNG kRNG εRNG t r Cb CD Cf Ck C k1 Wet Wet Cd d E g Re t
v
turbulence kinetic energy (m2 s−2 ) turbulence dissipation rate (m2 s−3 ) force (N) mass (kg) shear production of turbulence (kg s−1 s−3 ) RNG k–ε turbulence model coefficient (1.42-fh ) RNG k–ε turbulence model coefficient (1.68) RNG k–ε turbulence model constant (0.7179) RNG k–ε turbulence model constant (0.7179) turbulent viscosity (kg m−1 s−1 ) radius (m) critical amplitude coefficient (0.5) damping coefficient (5.0) external force coefficient (1/3) restoring force coefficient (8.0) new droplet velocity factor (1.0) CAB breakup factor (0.05) critical Weber number for stripping breakup (80) critical Weber number for catastrophic breakup (350) drag coefficient (dimensionless) diameter (m) efficiency (dimensionless) acceleration due to gravity (m/s2 ) Reynolds number (dimensionless) time (s) velocity (m/s) inertial impaction parameter (dimensionless) viscosity (Ns m−2 ) density (kg m−3 ) surface tension (N m−1 ) Stokes number (dimensionless)
Subscript d droplet gas g p particle re removal inlet i
There are two approaches available in CFX to simulate the multiphase flow; the Eulerian approach treats the dispersed phases as interpenetrating continua while the Lagrangian approach treats the dispersed phases as individual entities (ANSYS, 2010). The advantage of Lagrangian over the Eulerian approach is the tracking of particle (droplet or solid) injected into the domain. In Eulerian approach, the average values such as velocity, mass, and temperature of particle phase is calculated whereas Lagrangian approach calculate these values for each representative particle (Guerra et al., 2012). In the present study, the removal efficiency of dust particle in venturi scrubber is simulated and analyzed by Eulerian–Lagrangian scheme in ANSYS CFX. For this purpose, the features of venturi scrubber such as throat gas velocity, liquid fraction, droplet size, and removal efficiency are studied. Further, the simulation results are compared with experimental results for validation of model and simulation methodology. The rest of the paper is organized as follows. Section 2 discusses the previous work for the dust removal efficiencies in different software packages. Section 3 explains the facility developed for experimentation of dust removal efficiency. Section 4 describes the mathematical model for turbulence, particle transport, and dust
removal efficiency. Section 5 enlightens the numerical methodology to solve the problem. In Section 6, the results obtained from simulation are discussed. Section 7 summarizes the conclusions of this conducted research work. 2. Previous work Pak and Chang (2006) simulated the collection efficiency of dust particle in a venturi scrubber by using KIVA code. The model used in KIVA code was modified for the flow of gas, liquid, and dust flow. Eulerian method is used for gas phase, and Lagrangian method is used for droplet and dust. Navier–Stokes equation solved the equation of motion for gas, whereas Basset–Boussinesq–Oseen (B–B–O) equation for droplet and dust. Schmehl et al. correlation was recommended to determine the drag coefficient for droplet. Dust particle was captured into droplets by inertial impaction mechanism. The pressure drop was underpredicted due to inaccurate prediction of droplet and not considering the film on the walls. The experimental and simulation results for collection efficiency agree well with each other. Tao and Kuisheng (2009) studied the heat and mass transfer phenomenon between droplet and gas phase in a venturi scrubber numerically by FLUENT software. The gas phase was regarded as continuous phase with Euler’s approach, whereas droplet and dust phase was regarded as discrete phase with Lagrangian approach in the domain. RNG k–ε turbulence model was used for simulation. Water was injected in the form of spray. The single droplet collection efficiency was calculated for inertial impaction. Heat transfer coefficient was calculated from Ranz and Marshal correlation, while Nusselt correlation used for calculation of the mass transfer coefficient. The performance of a venturi scrubber is evaluated at different baffle openings and liquid to gas ratio. The parameters included to analyze the performance of a venturi scrubber were: pressure drop, collection efficiency, mole fraction, velocity, and temperature profiles. It was observed that with the decrease of liquid to gas ratio and baffle opening, the pressure drop and collection efficiency was increased. Goniva et al. (2009) simulated the capturing efficiency of a venturi scrubber with OpenFOAM package. TAB model is used for the droplet breakup. The capture efficiency of single droplet is calculated by the sum of inertial, interception, and diffusion mechanism. The effect of wall film was also investigated. The models for deposition and entrainment of droplet were used in research. The overall pressure drop and collection efficiency calculated from simulation, which predicts well with the experimental data. 3. Experimental facility The schematic diagram of an experimental facility to study the dust removal efficiency of a venturi scrubber is shown in Fig. 2. In the first stage, the highly compressed gas is generated from an air compressor. This compressed gas is further stored in an air tank. The compressed gas is cleaned with air filters before injected into the main loop. In second stage, the titanium dioxide (TiO2 ) having mean size diameter of 1 m is injected into the main loop from dust particle injection device. Third stage contains the water tank for injection of liquid into the venturi scrubber. Venturi scrubber is mounted in the venturi tank such that the direction of gas flows against the gravity. The venturi scrubber is operated at gas mass flow rate 0.09, 0.115, and 0.14 kg s−1 and liquid flow rate is 0.1, 0.13, and 0.16 kg s−1 . The concentration of dust particle is measured at sampling point at inlet Si and outlet So by filtration technique. The dust removal efficiency is calculated from the following equation: Ere =
Cin − C0 Cin
(1)
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Fig. 1. Filtered vented containment system in Gosgen Nuclear Power Plant.
4. Mathematical modeling 4.1. Turbulence equation 4.2. Particle transport phase The RNG k–ε model has been used in simulation to model the turbulence of gas phase in the domain. The conservation equations for the turbulent kinetic energy k and the dissipation energy ε for gas phase are (ANSYS, 2010)
t ∂k + ∇ vg k − + kRNG ∂t
∂ε t + ∇ vg ε − + εRNG ∂t + Cε1RNG Pεb ]
∇ k = Pk − ε + Pkb
∇ε =
(2)
In the CFD model, droplet and dust particle are treated as a discrete particle. The force acting on each particle is the sum of the drag force and buoyancy force due to gravity on the particle. The equation of motion of particle can be written as (ANSYS, 2010) Fp = FD + FB
ε [Cε1RNG Pk − Cε2RNG ε k (3)
mP
1 dvP = CD g Ap (vg − vP )|vg − vP | + dP3 (P − g )g 2 6 dt
Fig. 2. Experimental facility for dust removal efficiency of a venturi scrubber.
(4)
(5)
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Fig. 3. (a) Mesh of venturi scrubber, (b) mesh at throat and (c) mesh near to boundary wall.
Fig. 4. Gas velocity at gas flow rate (A) mg = 0.09 kg s−1 , (B) mg = 0.115 kg s−1 , and (C) mg = 0.14 kg s−1 .
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Fig. 5. Droplet size at mass flow rate of liquid 0.1 kg s−1 and gas mass flow rate: (A) 0.09 kg s−1 , (B) 0.115 kg s−1 , and (C) 0.14 kg s−1 .
4.3. Drag coefficient
the damping forces (ANSYS, 2010). The deformation of droplet in CAB model can be expressed as (ANSYS, 2010)
Schilller Nauman drag model is used for calculation of drag coefficient for particles (ANSYS, 2010)
CD = max
24 Re
(1 + 0.15Re0.687 ), 0.44
(6)
y¨ =
2 Cf g Vrel
Cb d rd 2
−
Ck d d rd3
y−
CD d d rd2
y˙
(7)
The rate of generation of product of child droplet is calculated from the following equation (ANSYS, 2010): dn(t) = 3Kbr n(t) dt
(8)
The size of child droplet is calculated as (ANSYS, 2010) 4.4. Droplet breakup and coalescence model The droplets are formed due to turbulence of gas and external aerodynamic forces acting on the liquid. Cascade atomization and breakup model (CAB) model treats the deformation of droplet with the analogy of a spring-mass system. Accordingly to the CAB model, the restoring force of the spring is represented by the droplet surface tension whereas the external force is replaced by aerodynamic force exerted on the droplet and liquid viscosity represents
rd,parent rd,child
= e−Kbr t
(9)
The Kbr follows the given breakup regime (ANSYS, 2010):
Kbr =
⎧ k ω ⎪ ⎨ 1 √ ⎪ ⎩
k2 ω We
k3
ωWe3/4
5 < We < 80 80 < We < 35 350 < We
Fig. 6. Droplet size at mass flow rate of liquid 0.13 kg s−1 and gas mass flow rate: (A) 0.09 kg s−1 , (B) 0.115 kg s−1 , and (C) 0.14 kg s−1 .
(10)
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Fig. 7. Droplet size at mass flow rate of liquid 0.16 kg s−1 and gas mass flow rate: (A) 0.09 kg s−1 , (B) 0.115 kg s−1 , and (C) 0.14 kg s−1 .
where k2 = k1 k3 =
1 − (1/2)(Ck Cb /Cf Wet )
a cos(1 − (C k Cb /Cf Wet )) k2
(11) (12)
1/4
Wet2
The single droplet collection efficiency is calculated from inertial impaction mechanism. Inertial impaction is a function of Stokes number, which depends on densities, relative velocity, and diameters. Inertial impaction is dominant for particle size having a diameter of 1 m (Lim et al., 2006). The inertial impaction of single droplet efficiency defined by Calvert (1970) is
2
+ 0.7
=
p dp2 (vp
− vd )
9g dd
(14)
4.6. Dust removal efficiency The dust removal efficiency is calculated from the following equation:
4.5. Single droplet collection efficiency
=
is expressed as (Pak and Chang, 2006),
where
(13)
Ere = 1 −
m (1 − ) di mdi
(15)
5. Numerical methodology 5.1. Turbulence model Eulerian–Lagrangian approach for three phase flow is employed in this research. The computations are performed in the steady
Fig. 8. Liquid fraction at mass flow rate of liquid 0.1 kg s−1 and gas mass flow rate: (A) 0.09 kg s−1 , (B) 0.115 kg s−1 , and (C) 0.14 kg s−1 .
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Fig. 9. Liquid fraction at mass flow rate of liquid 0.13 kg s−1 and gas mass flow rate: (A) 0.09 kg s−1 , (B) 0.115 kg s−1 , and (C) 0.14 kg s−1 .
state. The gas (air) is treated as a carrier field, whereas the dust particles as transport dispersed solid and liquid (water) as transport dispersed fluid in the domain. RNG k– turbulence model for gas is used. This model performs well in bending shapes and rotating flows (ANSYS, 2010).
5.3. Solver High resolution advection scheme is selected for accurate numerical solutions. It is used due to its robustness. 5.4. Convergence criteria The convergence criteria for simulations are 10−4 RMS for residuals.
5.2. Mesh The unstructured mesh of a venturi scrubber is created in ANSYS ICEM. The domain of a 3D Venturi Scrubber model is discretized into 272,136 hexahedral elements and 284,607 nodes. The mesh quality of the venturi scrubber model is greater than 0.6 on the scale from 0 to 1 based on mesh quality criteria defined in ANSYS ICEM (ANSYS ICEM CFD, 2009). Fig. 3 (a) depicts the mesh of venturi scrubber on a plane and, (b) shows the mesh at throat on a plane whereas, (c) shows the mesh near to the boundary wall.
5.5. Boundary condition The boundary conditions for the CFD venturi model are defined as follows: at the inlet, the mass flow rate of the gas is applied, while dust is injected with particle transport solid with mean diameter of 1 m. The liquid is injected as particle transport fluid with mass flow rate boundary condition. At the outlet of a venturi scrubber,
Fig. 10. Liquid fraction at mass flow rate of liquid 0.16 kg s−1 and gas mass flow rate: (A) 0.09 kg s−1 , (B) 0.115 kg s−1 , and (C) 0.14 kg s−1 .
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the pressure boundary condition is used. The parallel and perpendicular restitution coefficients are 0.0 for droplets in diffuser wall. 6. Numerical results and discussion The simulation of three phase flow in venturi scrubber is conducted by Eulerian–Lagrangian approach. The dust and droplet particles are tracked by Lagrangian method. To study the dust removal efficiency, the hydrophobic particle titanium dioxide (TiO2 ) is injected at an inlet. The diameter of TiO2 is 1 m and density 4.23 g cm−3 . The dust removal efficiency of a venturi scrubber is calculated from the mass flow rate of dust at inlet and outlet of a venturi scrubber. The removal efficiency of a venturi scrubber depends on droplet diameter, fluid velocity, and impaction. The present simulation does not incorporate the heat and mass transfer, and liquid film on walls.
6.3. Liquid fraction The distribution of liquid inside the venturi scrubber highly affects the performance of a venturi scrubber. The distribution of liquid is affected by the flow rates of gas and liquid. Therefore, distribution of liquid fraction is analyzed in a venturi scrubber. The amount of gas and liquid introduced into the scrubber depends on mass flow rate. The venturi scrubber is simulated with three different gas flow rate 0.09, 0.115, and 0.14 kg s−1 whereas liquid flow rate 0.1, 0.13, and 0.16 kg s−1 . The contours of liquid fraction at different mass flow rate for gas and liquid inlet are shown in Figs. 8–10. 6.4. Validation The simulation of dust removal efficiency of a venturi scrubber is validated by comparing the throat gas velocity and dust removal efficiency with the experimental results.
6.1. Gas velocity The gas phase in a venturi scrubber is treated as a continuous field. When gas entered into the convergent segment, it accelerates and reaches to its maximum velocity at throat segment while decelerate in divergent segment. The velocity vectors of gas at mass flow rate 0.09, 0.115, and 0.14 kg s−1 illustrate the behavior in convergent, throat and diffuser segment is as shown in Fig. 4. The maximum throat gas velocity is achieved at gas mass flow rate of 0.14 kg s−1 . 6.2. Droplet size The droplet size plays a significant role in the removal process of a venturi scrubber. Liquid is introduced with mass flow rate boundary condition into the venturi scrubber domain from the liquid inlet as shown in Fig. 3. When the liquid entered into the throat segment, it interacts with the high velocity of gas. The high gas velocity atomized the liquid into tiny droplets. The droplet breakup in venturi scrubber domain is predicted by CAB model. The contour of droplet sizes at different flow rates are shown in Figs. 5–7. The size of droplet in contours range is 1–100 m. The result shows that the diameter of droplet decreases with the increase of gas flow rate.
Fig. 11. Comparison of experimental and simulation result of throat gas velocity at different gas flow rate.
6.4.1. Throat gas velocity In Fig. 11, the experimental values of gas velocity at throat are compared with simulation values. The result shows that the simulation values are very near to experimental values which validate the simulation results. 6.4.2. Dust removal efficiency The dust removal efficiency is calculated from the simulation result by the mass ratio of the removed dust particles to the total inflow mass of dust particles. The simulation results of dust removal efficiency at 0.1, 0.13, and 0.16 kg s−1 liquid flow rates are validated by comparing with experimental results as shown in Figs. 12–14. The effect of wall film is ignored, therefore parallel and perpendicular restitution coefficients are 0.0 on diffuser wall. The droplet which collided with diffuser wall was not further tracked. The CAB model is used for the droplet breakup in the domain. The dust particles introduced with the gas at inlet accelerated in the convergent segment. The throat is very short; therefore, mostly collision of dust with droplet will take place in diffuser segment. The collision of dust particle with the liquid droplets primarily depends on inertial impaction. However, impaction mechanism depends on characteristics of dust and droplet. The Stokes number is greater than 1, which specifies that the particle remains unaffected by the
Fig. 12. Comparison of experimental and simulation result of dust removal efficiency at mass flow rate of liquid 0.1 kg s−1 .
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increases with the increase of gas mass flow rate. The simulation result of dust removal efficiency agrees well with experimental results. 7. Conclusion
Fig. 13. Comparison of experimental and simulation result of dust removal efficiency at mass flow rate of liquid 0.13 kg s−1 .
The simulation of venturi scrubber model has been carried out in ANSYS CFX to study the dust removal efficiency of venturi scrubber. Venturi Scrubber model is meshed in ANSYS ICEM. Eulerian–Lagrangian is implemented for three phase flow simulation. In venturi scrubber model, gas is regarded as an Eulerian approach while dust and droplet are tracked by Lagrangian approach. The turbulence effects are calculated by RNG k–ε model. CAB model is used for droplet breakup in venturi scrubber domain. The dust removal efficiency is calculated from the dust mass flow rate at inlet and outlet. Inertial impaction is used for capturing of dust particle. The throat gas velocities at different mass flow rate are verified with experimental results. The liquid fraction is analyzed to determine the liquid fraction distribution inside the scrubber. The dust particle removal efficiency increases with the increase in gas mass flow rate and liquid flow rate. The simulation and experimental results concur well which validates the simulation work. Acknowledgements The authors admiringly acknowledge the support of the present work from College of Nuclear Science and Technology, Harbin Engineering University. The author also gratefully acknowledges the awarded scholarship from Chinese Scholarship Council. References
Fig. 14. Comparison of experimental and simulation result of dust removal efficiency at mass flow rate of liquid 0.16 kg s−1 .
fluid velocity and does not influence it. As a result, the particle collides with the droplet rather than flowing around it. The impaction increases with the increase in relative velocity between droplet and dust, and with the decrease of diameter of droplet. With the increases of gas flow rate, the diameter decreases which is shown in Figs. 5–7. The decrease in diameter increases the surface area, and increases the chances of interaction between dust particle and droplet. The result shows that the dust removal efficiency
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