DESALINATION

ELSEWIER

Investigation flow patterns model flash evaporation chamber PIV measurement

W.X. Jin”, SC Low School of ~e~~ni~~~ and Prodwtiort Engineering Naqyang TechnoIo~cffl Universi fy, Singapore 639798

email: mwxjin@ntu.edu.sg

Received 18 May 200 1; accepted 10 January 2002

Abstract

An e~r~en~ system with a assent flash elision chamber was set up in this work to simulate the single phase seawater flow in the flash stage in the multi-stage flash (MSF) deviation preps. The flash evaporation chamber is 300 mm in height x 823.8 mm in length x 40 mm in width and was designed to be able to nm under flash evaporation flow conditions. The whole field fluid flow velocity vectors in the chamber were measured using a particle image velocimetry (PIV) system to study the effects of main flow parameters such as water level and flow rate on flow patterns. The fluid flow in the chamber was also numerically simulated using a two-dimensional k-epsilon turbulent flow model. The measured f&e surface profiles and inlet gate velocity distributions were used as the boundary conditions. The streamlines were generated directly fiorn the velocity vectors using the TECPLOT software package. The simulated results were compared with the measured data. The results show that a large recirculation region with several vortices embedded would be generated at a higher water level or at a larger flow rate.

K&W&K h4SF desaliition; Flash evaporation; PIV; Numerical simulation; Flow pattern

1. Introduction principle of distillation [l]. The seawater is

The multi-stage flash (MSF) desalination process is one of the most widely used methods to produce fresh water for both industrial and domestic usage. The flashing chamber is a key element in the MSF process, and it runs on the

*Co~esponding author.

firstly heated by water vapor inside the heat recovery section and by steam in the heat-input section. Then, the seawater flows into the flash stages with temperature higher than its boiling temperature because of the lower vacuum pressure in these stages. A typical MSF flash chamber is shown in Fig. 1. In each flash stage,

001 l-9164/02/% See iiont matter 0 2002 Elsevier Science B.V. All rights reserved PII:SOO11-9164(02)00929-3

W. X. Jin. S. C. Low/Desalination 150 (2002) 51-63

Brim Fiow-out

- a

Fig. 1. A typical MSF flash chamber.

the flash evaporation process occurs. Bubbles grow and move rapidly in the brine and burst on the free surface to generate water vapor. The generated vapor flow passes the demister and condenses in the tube bundles, The left brine flows to the next flash stage. So the fluid flow inside the chamber is a very complex heat and mass transfer process involving open-channel turbulent flow, two-phase liquid-vapor flow and bubble dynamics.

The flash evaporation flow in the MSF flash stage is normally developed from the non- flashing single-ph~e flow by decreasing the pressure, and its ilow pattern will affect the thermal performance of the flash stage. When the liquid superheat is small, the effect of the bubble generation on the flashing flow pattern is limited. Many experimental and numerical studies on single-phase flow patterns inside the flash evaporation chamber have been reported. Flow visualization and measurement of temperature and pressure showed that the flow pattern inside the flash chamber is essentially a kind of an open-channel submerged hydraulic jump with a wavy free surface and recirculation region near the inlet gate [2,3]. The local velocities of the non-flashing water flow in a flash chamber were measured using a photographic technique, and the single-phase flow was simulated with a mixing length turbulence model [4]. Besides the visualization of the flow patterns, a two-phase turbulent flow model was developed to study the bubble trajectories inside the chamber [5,6].

Because of the complexities involved in the heat and mass transfer process during the flash evapo- ration, detailed measu~ment of the entire flow field, including the velocity vectors and bubble behaviors, was beyond the scope of many pre- vious works.

In this study, a model flash evaporation chamber was designed and built to simulate the typical MSF flash chamber flow. The field velo- city vectors were measured using particle image velocimetry (PIV) to study the effects of the flow parameters on the single-phase flow patterns in the chamber. The main p~~eters included the flow rate and water level. The fluid flow in the chamber was also simulated with a two-dimen- sional k-epsilon turbulent model, and the simu- lated results were compared with the measured data.

2. Experimental set-up

This is the first part of the experimental work to investigate the fluid flow in a model flash evaporation chamber. This flash evaporation chamber was designed to experimentally simulate both the non-flashing (single-phase) water flow and the flash evaporation (multi-phase) flow in a MSF process. The experimental set-up is shown schematically in Fig. 2. It was designed such that it is able to run at a flow rate of 1 .O kg/s and at a temperature of 60°C and at a absolute pressure of 200 mbar. To allow easier optical access, the flash chamber was consorted from 1 .O”-thick, clear and ~~sparent PERSPEX sheets. The dimensions of the flash chamber are 300 mm (height) x 823.8 mm (length) x 40 mm (width). The designed test rig is operated at two different pressure levels. In the single-phase experimental test, the pressure inside the flash chamber is at atmospheric conditions. In the multi-phase experiment, the whole system is in sub-atmos- pheric pressure controlled by a vacuum pump.

W.X. Jin, S.C. Low /Desalination I50 (2002) 51-63 53

CI CyIindrical lens

Fig. 2. Schematic of the experimental set-up (including the arrangement for the particle image velocimetry).

The PIV is the Dantec Flow~ap system. It consists of four family groups: the illumination system, CCD camera, PIV 2000 processor, and FlowManager software. In the illumination sys- tem, a green laser light beam at a = 532 nm is generated by a double pulsed (at 500 mJ) Continuum Nd:YAG laser. The laser light beam is tr~sfo~ed into a laser light sheet through a cylindrical lens. The laser light path is designed to come from the top of the flash chamber through a reflective mirror.

The velocity of the seeding particle in the flow can be calculated using the following equation after PIV analyzed the two images taken by the CCD camera:

v= As MxA$ (1)

where V is the seeding particle velocity, AS is the seeding particle displacement during the time interval, At is the time intervat between two images, and Mis the magnification of the images.

So the accuracy of velocity measurement depends upon the accuracy in measuring the image dis- placement and the magnification of images by the PIV optical components and the accuracy in determining the time interval between successive images. There are three agencies that cause the magnification in an experiment to vary. The first is design imperfection in the lens. The second is refraction caused by anything, including windows and fluid interfaces, that lies between the object plane in the illuminating light sheet and the image plane. The third is misalignment of the image recording plane with respect to the object plane. The errors in measuring the magnification and determining the time interval are typically al%. The image displacement is usually computed using the cross-correlation or auto- correlation method by the PIV image analysis software. PIV evaluation errors were found to be extremely small given very high image quality.

The accuracy of the measured liquid velocity also depends on the accuracy with which particle displacements are equal to fluid displacements.

When using effective seeding particles, the measured seeding particle velocity can be assumed to be equal to the fluid flow velocity. In this measurement, a polystyrene sphere was selected as the seeding particles. The range of the seeding particle size is 125 pm to 400 pm; the material density is 1050 kg/m3.

The PIV system should be properly set in order to obtain high-resolution, high-contrast particle images and good me~u~ng results. It includes the software of system parameters and the hardware arrangement between the test rig, the illumination system and CCD camera. In this experimental measurement, the width of the laser light sheet in the water was constant by fixing the distance between the flash chamber and the PIV cylindrical lens. But the laser line mirror and the PIV cylinder lens can move along the horizontal position to illuminate the different regions. Main PIV settings were the scale factor and time between two laser pulses. The scale factor of a CCD camera is equal to object size over chip size. In this PIV measuring system, the resolution of the CCD chip is 768 by 484, and the size is 8.9 mm by 6.6 mm. So if the object height is 117 mm, the scale factor S is equal to 17.775.

The time difference between pulses is one of the most important PIV system parameters. It controls the quality of the particle images and the effectiveness of the image processing software. The image size of a particle is determined by two factors: the geometric magnification and the diffraction in the optical aperture of the camera. The effective diameter of the particle image is given by

de=/= (2)

where dg =M x dputiclc and d, = 2.44 (M+l) Fh, where M= l/S is the magnification, F = ffDA is the F-number calculated from the focal length of the camera lensfand the diameter of the camera aperture DA.

The time difference between pulses tbp should in general be long enough to let the particle move a few particle-diameter distances (or a few pixels for autocorrelation).

tbp ’ 3.5 xd,

&tide x M (3)

At the same time, the maximum particle image displacement on the CCD chip should be less than a quarter of the interrogation size Zintcrrogation (32 x 32 pixels size used in this experiment).

t bp

< o-25 x 4lwrogafi*n J&tidi? x M

(4)

For dpticlc = 250 pm, Vpticle = 1 .O m/s, M= l/18, d, is equal to 15.9 l&and the range of the time difference between pulses is from 1 .O to 1.67 ms.

The original PIV measurement data are the image maps taken by CCD camera. They are saved as picture files in the database. After obtaining the image maps, the raw velocity vector maps can be produced immediately using auto- correlation or gross-~o~elation processing methods. The last step is to present the experi- mental results in a comprehensible form by selecting data analysis methods in the PIV system. The data analysis methods include the validation, filtering, and derivation for vorticity, strain, and streamlines.

3. Mathematical method

The single-phase flow patterns in the model flash evaporation chamber were also simulated with numerical methods. A two-dimensional steady state standard k-s turbulent flow model was used to calculate the distribution of fluid flow parameters [5]. The liquid parameters of two velocities, turbulence kinetic energy, dissi- pation rate of turbulence kinetic energy, temp-

W.X. Jin, S.C. Low /Desalination 150 (2002) 51-63

erature, and pressure were solved by the Table 1

following equations of continui~ equation, two Constants of the R-E mulct flow model

momentum equations, k-equation, e-equation. and an energy equation: l Continuity equation:

a@uJ = 0

ax,

l Two momentum equations:

a +- atij axj i 1 cr c”iq

, (i=irl’)

l k-equation:

l a-equation:

+, ay k t axj

i I a2_++atij e2 - -C’Pt axj axi

l energy equation:

a(puiJT) a kff aT =. i3Xj -q i i

-- at axj

I.44 1.92 0.09 1.0 0.5 0.41

(5) where

k2 P,=PC,T

Oe = (c2-&y (12) (6)

The constants of the turbulent flow model are listed in Table 1.

(7)

The effect of the static hydraulic pressure was included in the pressure term in the momentum equations. There are three main boundaries: inlet and outlet gates, free surface, and solid walls. The log-law was used for the boundary layer near the walls. The “dynamic boundary condition” and “kinematic boundary condition” were applied for the free surface boundary. The “kinematic bound- ary condition” can be explained that the vertical to horizontal liquid velocities on the free surface must be equal to the ratio of the slope of the free surface. In this study, the free surface shape obtained from the experimental measurement was introduced as a boundary condition.

(8) 4. Results and discussion

4. I. Experimental

(9)

In this experimental test rig, the water flow from the submerged inlet gate into the chamber forms a hydraulic jump near the inlet gate. This kind of hydraulic jump can be a free jump, a submerged jump, or a plane turbulent wall jet, depending on its inlet gate height, inlet velocity,

56 W.X. Jin, S.C. Low /Desalination 150 (2002) 51-43

Fig. 3. Original PIV image map (above) and velocity vectors (below).

and tailor water depth [7]. So the water level, inlet gate height and flow rate were selected as the independent variables in this project to study the velocity distributions and flow patterns of the single-phase water flow in the model flash evapo- ration chamber. Due to the limitation of the laser light sheet width and the m~imum object size that the CCD camera can capture, the flash chamber was divided into five measuring regions along its longitudinal direction. For each experi- ment, ail five regions were measured. The main direct measurement results from the PIV system are PIV image maps and velocity vectors. The free surface curves at different flow conditions were obtained from the original PIV image maps that were captured by the CCD camera. The velo- city vectors for the five regions were calculated

by the PIV system from the original image maps. The streamlines of the entire field at different flow conditions were const~cted from the measured velocity vectors. One of the original PIV image maps captured by the CCD camera and the velocity vectors are shown in Fig. 3.

Using the image processing software, the points representing the free surface can be obtained directly from the image maps. One of the measured free surface profiles of the whole flow region is shown in Fig. 4. The free surface shape is determined theoretically by the flow rate, inlet gate height and water level inside the chamber. But it can be seen from the experi- mental results that the shape of the free surface can be considered uniform when the water level is high enough or the flow rate is low enough.

The velocity vectors and streamlines of the whole flow field at three different water levels are shown in Figs. 5-7. In each water level, the velocity vectors and streamlines of the entire flow field at three different flow rates were also measured and compared. With the same flow rate and inlet gate height, the water level is clearly shown to have an effect on the size and the shape of the recircuIation region near the inlet gate. The recirculation length for the water level of 117 mm in Fig. 7(a) is about 0.65 m, for the water level of 97 mm in Fig. 6(a) it is about 0.4 m, and for the water level of 75 mm in Fig. 5(a) it is about 0.2 m. It can be seen that the higher the water level, the larger the recirculation region with more vortices.

Numerical simulations were performed to study the effect of flow parameters such as the flow rate and water level on the flow patterns in the model flash evaporation chamber.

4.2.X. Comparison with experimental data

In order to compare with the experimental results, the water flow was simulated using the

57

Fig. 4. Measured free surface profile. 400 8OOmm

(a) 70

0 ^__._ -. 400 800 mm

@I 70

--- 400 800 mm

(cl 70

0 400 800 mm

Fig. 5. Whole field velocity vectors and streamlines {inlet gate height: 50 mm, outlet gate height: 50 mm, averaged water level: 75 mm). Flow rates, kg/s: (a) 0.99; (b) 0.88; (c) 0.72.

100

(4

0 400 800 mm

100 09

0 m

100

Cc)

0 400 800 mm

Fig. 6. Whole field velocity vectors and streamlines (inlet gate height: 50 mm, outlet gate height: 50 mm, averaged water level: 97 mm). Flow rates as in Fig. 5.

”

400 800 mm

0 400 ‘800 mm

01, .-- --/“i’--. 3 400 800 mm

Fig. 7. Whole field velocity vectors and streamlines (inlet gate height: 50 mm, outlet gate height: 50 mm, averaged water level: 117 mm). Flow rates as in Fig. 5.

70

400

Fig. 8. Velocity vectors and streamlines with measured inlet velocity (water level: 75 mm; flow rate: 0.99 k&s).

measured data such as the free surface profile and the inlet velocity distribution. The measured free surface profile, such as shown in Fig. 4, was fur- ther approximated using a ninth-order polynomial before it cau be applied as one of the boundary conditions in the numerical simulation. The inlet velocity distribution is mainly determined by the inlet concretion of the chamber. The velocity dis~ibution function at the inlet gate was ob- tained by fitting the experimental data using a

third-order polynomial. In this numerical simu- lation, the velocity at the solid wall was set to zero because of use of the laws of the wall (such as no slip and no penetration).

The water flow with measured inlet velocity distributions at the water level of 75 mm, flow rate of 0.99 kg/s and inlet gate height of 50 mm was firstly simulated. All the field numerical simulated velocity vectors with measured inlet velocity distribution are shown in Fig. 8. Com-

WX. Jin, SC. Low /~e~~Ii~ation 150 (2002) 51-63 59

paring all the field velocity vectors measured, we can see that both have larger velocities near the inlet gate and bottom wall, and the velocities of the flow at the recirculating region are actually quite small compared to the averaged inlet velocity. However, the velocity vectors change regularly and can form clear vortices for the computed one at the recirculating region. For the measured one, the velocity vectors at the recir- culating region change irregularly and the formed vortices are not clear.

It also can be seen from Fig. 8 that the length of the recirculating region was about 0.26 m, which was very close to the one for the measured data. It also can be seen that there were three main vortices in the recirculation flow region. This is identical with the observations obtained from the experimental studies. The simulated flow pattern is similar to the measured one. Using the measured velocity distribution function at the inlet gate is a good method to predict the single- phase flow pattern inside the flash chamber.

The comparison of the measured and simu- lated velocity data at the middle of the chamber length is shown in Fig. 9. The measured velo- cities at n = 0.399 m (near the middle point of the chamber length) were selected to compare with the velocities obtained through the numerical simulation. The comparison shows that the two velocity profiles are identical to each other and the m~imum difference is below lo%, except the region near the flow surface. In this experi- ment, it is very difficult to measure the velocity vectors near the boundaries using the PIV system because of the limitation of the PIV scale factor. The velocity distribution at the inlet gate is one of the main error sources because it is one of the four boundary comers. Another one is the effect of the wave on the free surface. Actually, the numerical simulated flow is the steady flow and the measured flow is not the steady flow. The methods to improve the measurement accuracy include creating a more steady flow at the inlet

10

00 -+-Simulated -Measured

50

40

3a

20

10

_ . a 0.1 02 0.3 0.4 0.5 0.6 0.7

lJ fmw

Fig. 9. Comparison of the measured and simulated velocities at x = 0.399 m.

gate and using a larger PIV magni~cation factor to get a more detailed velocity dis~ibution.

4.2.2. Eflectof inletgate velocity distributions

on thejow patterns

The effect of the inlet velocity dis~ibution on the flow pattern was studied. The water flow with uniform and measured inlet velocity distributions at the water level of 75 mm, flow rate of 0.99 kg/s and inlet gate height of 50 mm were compared. The streamlines with uniform inlet velocity dis~ibution are shown in Fig. 10.

Comparing the streamlines in Fig. 8, it can be seen that the length of the recirculating region for the uniform inlet velocity distribution was less than 0.1 m, and there was only one vortex in the recirculation flow region. So the inlet velocity distribution has a large effect on the whole field velocity dis~ibution, and the assumption of the uniform velocity distribution at the inlet gate in the numerical simulation is unacceptable for the inlet gate design in this experiment.

4.2.3. E%fect of inlet gate heights on flow patterns

The flow patterns under two different inlet gate heights (i.e., 50 mm, and 20 mm) were measured by the experimental to study the effect

60 W.X. Jin, SC. Low /Desalination 150 (2002) 51-63

400 - ” 800 mm

Fig. 10. Streamlines with uniform inlet gate velocity (flow rate: 0.99 kg/s, averaged water level: 75 mm).

0 400 800 mm

Fig. 11. Effect of inlet velocity distributions on fluid flow pattern (inlet height: 20 mm, outlet height: 50 mm; flow rate: 0.99 kg/s; averaged level: 75 mm).

of the inlet gate height. In the numerical simu- lation, the measured free surface profile was used as the free surface boundary and the measured velocity profile was used in the inlet gate. The simulated velocity vectors and streamlines for the inlet gate height of 20 mm are shown in Fig. 11.

For the flow conditions in Figs. 8 and 11, all of the main flow parameters were set to be same except the inlet gate heights. They had the same flow rate of 0.99 kg/s and average water level of 75 mm.

It can be seen from the two figures that the free surface profile for the inlet gate height of 20 mm is wavier and the recirculating region size is larger. This is because a lower inlet gate has a higher inlet velocity when they have the same water leve1 and flow rate. In Fig. 11, the recir- culating region length is about 0.46 m.

For the inlet gate height of 20 mm, the wavier free surface was also observed in the experi- mental measurement. When decreasing the water level, the free surface became more turbulent and finally a water jet would be formed.

4.2.4. E#ect of water levels on fIow patterns

The effects ofthree different water levels (Le., 75 mm, 97 mm, and 117 mm} on the single-phase flow patterns were also studied. All the other flow conditions such as the flow rate and inlet gate height were kept constant. The flow rate was 0.99 kg/s and the inlet gate height was 50 mm.

AAer being fitted with ninth-order poly- nomials, the measured free surface profiles for water levels of 75 mm and 97 mm were used as the free surface boundaries in the computational domain. For a water level of 117 mm, the free surface was considered to be a plane. Using the boundary conditions described, the water flows were simulated to study the effects of the inlet velocity distribution on the water flow patterns. The simulated velocity vectors and streamlines for the water levels of 97 mm and 117 mm are shown in Fig. 12. The estimated recirculating region lengths are listed in Table 2.

The measured recirculating lengths in Table 2 were estimated from Figs. 5-7. The results show that the simulated recirculating lengths were

W.X. Jin, XC. Low ~Des~~~~~io~ 150 (2002) 51-63 61

Fig. 12. Effect of water levels on the water flow patterns (inlet gate height: 50 mm; flow rate: 0.99 kg/s). Streamlines (using measured inlet velocity, averaged water level: (a) 97 mm; (b) 117 mm).

Table 2 Comparison of recirculating region lengths for three different water levels

Water level, mm

Recirculating length, m

7.5 97 117

._ ._ __.._ ._-._-_..-_. _ - - _ _, ._ - “..__._ .__. __~

Measured 0.25 0.40 0.65 Simulated 0.26 0.44 0.60

close to the measured recirculating lengths. Both the simuIated and measured results show that the recirculating region increases when the water level is increased. It may be caused by an increase of the ratio of the water level to the inlet gate height. The higher the ratio of the water level to the inlet gate height, the more the water will be recirculated above the inlet gate flow. The larger recirculating region means a larger amount of water recirculating in the flash chamber.

Fig. 8 shows the whole field velocity vectors and streamlines for a water level of 75 mm using the measured velocity distribution at the inlet gate. The vortices were found in the recirculation

region. For the water levels of 97 mm and 117 mm as shown in the simulated streamlines in Fig. 12(a) and 12(b), only one vortex in each recirculating region was predicted. But for the measuredstreamlines inFigs. 6(a) and 7(a), more vortices inside the recirculating region were detected by the PIV measuring system. This may be caused by the non-~ifo~~ow distribution in the chamber width direction in the experimental measurements. More vortices at higher water levels may be predicted when the mathematical models have been improved to be able to simulate the three-dimensional problem.

4.2.5. Eflecct offlow rates o~~owpatter~

Water flow rate is one of the main par~ete~ used to determine the flow pattern in the flash chamber. The effect of flow rate on flow pattern was studied through numerical simulation and compared with the experimental data. Three flow rates of 0.99,0.88 and 0.72 kg/s were tested with a water level of 75 mm and inlet gate height of 50 mm. Using the same data processing method, the velocity distributions at the inlet gate were determined. The free surface profiles are fitted

W.X. An, S.C. Low /Desalination 150 (2002) 51-63

Fig. 13. Effect of the flow rate on the flow pattern (inlet gate height: 50 mm; averaged water levef: 75 mm). Streamlines (flow rates: (a) 0.88 kg/s; (b) 0.72 kg/s).

Table 3 Comparison of recirculating region lengths for three different flow rates

also can be seen that the effect of the water level on the pattern is larger than the effect of the flow rate.

Water flow rate, kg/s

Recirculating length, m

0.99 0.88 0.72

Measured 0.25 0.23 0.21 Simulated 0.26 0.24 0.20

with ninth-order polynomials. The measured coefficients of the inlet velocity distribution functions and the fitted free surface profiles were used as input data to simulate the effect of the water flow rates on the single-phase flow patterns. The simulated velocity vectors and streamlines for the flow rates of 0.88 kg/s and 0.72 kg/s are shown in Fig. 13. The estimated lengths of the recirculating region for those three flow conditions are listed in Table 3.

It was found from Figs. 9, 13(a), 13(b) and from Table 3 that the recirculating length was decreased when reducing the inlet flow rate. It

5. Conclusions

The velocity distributions of the single-phase water flow inside a model flash evaporation chamber was measured using PIV and simulated with a k-c turbulent flow model to determine the effect of the flow conditions on the flow patterns. PIV measurement provides a useful method to obtain whole field velocity vectors and to study the flow patterns in a large flow field. Its accuracy depends on the CCD chip solution and the size of the flow field to be measured. A higher CCD chip solution with a greater magnifi- cation factor can obtain more accurate and detailed measurement results.

Both the measured and numerical simulated results for the single-phase flow in the flash chamber show that there is a recirculating region near the inlet gate with vortices. The size of the recirculating region and the vortex number varied with flow conditions. The flow parameters such

W.X. Jin, S.C. Low /Desalination I50 (2002) 5163 63

as the flow rate, inlet velocity distribution, inlet gate height, and the water level exhibited signi- ficant effects on the single-phase flow patterns in the flash evaporation chamber. Besides the above flow parameters, the configurations of the flash chamber such as the inlet gate profile and using baffle near the inlet gate also have large effects on the flow patterns, which will be studied in the future.

6. Symbols

G, G,

CP - 4 -

4 -

Turbulence model constants Effective diameter of the particle image Geometric diameter of the particle image

dptic,c - Particle diameter

DA - Diameter of the camera aperture F - F-number of the camera k - Turbulence kinetic energy P - Pressure AS - Seeding particle displacement At- Time interval

tbp -

T - u,u - y,v - x,x -

Y,Y -

GrC?#?k

& -

Time between of two laser pulses Temperature Velocity in x direction Velocity in y direction Longitude coordinate Vertical coordinate

Dissipation rate of turbulence kinetic energy

c1 - hII - Pr - v - K -

P - 0 -

Ok, 0, - I. -

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VI

PI

E31

r41

PI

I61

[71

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