The Effect of Geometrical Parameters on Mixing and Parallel Jets Mixing in a Liquid Static Mixer

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    International Journal of Advanced Research in Engineering and Technology (IJARET)ISSN 0976 6480(Print), ISSN 0976 6499(Online) Volume 1, Number 1, Sep - Oct (2010), IAEME

    92

    THE EFFECT OF GEOMETRICAL PARAMETERS ONMIXING AND PARALLEL JETS MIXING IN A LIQUID

    STATIC MIXER

    D.S.Robinson SmartSchool of Mechanical Sciences, Karunya University

    Coimbatore-641 114E-Mail id: [email protected]

    ABSTRACT

    Experimental investigations and computational analysis were carried out topredict the effect of parallel, vertical liquid jets mixing and the geometrical parameters

    which are effecting the mixing in a liquid static mixer. The computer analysis was carried

    out by using commercially available CFD software package FLUENT computational

    fluid dynamics (CFD) methods [7].An experimental set up was designed and

    investigations were carried out to evaluate the parallel and vertical fluid jets mixing in a

    static liquid mixer. Conductivity probe technique was used to evaluate the mixing [3].

    The results obtained by experimental investigation and computer analysis were compared

    and discussed in detail to decide upon the effectiveness of parallel and vertical liquid jets

    mixing. The investigations and computer analysis revealed that the mixing efficiency

    increases with the opening of parallel ports and the primary fluid nozzle position reaches

    50mm with mixing inserts.

    Keywords : Parallel jets; Liquid mixing; Static mixing

    1. INTRODUCTION

    Mixing of two or more ingredients is essential in number of different process

    industries such as chemical, pharmaceutical petroleum, plastics, and food processing,

    water and waste water treatment plants. There are two major types of mixers are available

    namely dynamic and static mixers. The efficiency of mixing depends on the efficient use

    of energy to generate flow of the components .Stirred tanks perform the mixing by a

    motor driven agitator. This type of mixer is generally employed when the mixing are

    International Journal of Advanced Research in Engineeringand Technology (IJARET), ISSN 0976 6480(Print),ISSN 0976 6499(Online) Volume 1,Number 1, Sep - Oct (2010), pp. 92-111 IAEME, http://www.iaeme.com/ijaret.html

    IJARET IAEME

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    undertaken in successive batches. Static mixers are in-line mixing devices generally

    consisting of mixing elements inserted into a pipe. Mixer of this type is used in

    continuous operation, with the energy for mixing being derived from the pressure loss

    incurred in the process of fluid flow through the elements [7].Over the years there hasbeen increasing emphasis in the process industries towards continuous type of liquid

    mixing wherever practical or feasible and innovative designs for mixing became

    apparent. Hence the process industries are in need of a mixing system, which mixes the

    liquids, which are having different properties to produce various liquid products with less

    power requirement. In the present work an experimental test facility is designed,

    developed and the experimental investigations and computational analysis have been

    carried out to predict the efficiency of parallel, vertical liquid jets mixing, the effect of

    geometrical parameters such as position of driving nozzle, cone angle of divergent

    nozzle, position of mixing insert and position of secondary fluid inlet on mixing with a

    view to optimize them [10].

    2. EXPERIMENTAL SET UP

    The experimental set up consists of a centrifugal pump, reservoirs, rotameter, mixing

    nozzle, four U tube manometers, control valves and conductivity meter . The primary

    fluid is stored in a tank. A control valve is used to regulate the primary fluid discharge.

    A centrifugal pump is used to supply the primary fluid from the tank to the mixer.

    Figure 1 Experimental set up of parallel and vertical jets mixing nozzle

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    As the primary fluid passes through the driving nozzle the velocity of flow

    increases as the area of flow decreases as it passes through the driving nozzle.

    Consequently there is a decrease in pressure. This drop in pressure creates a suction

    pressure in the converging area and the secondary fluid will be drawn. The suctionpressure at the inlet ports of secondary fluid is measured using the manometers. There are

    four sets of secondary fluid ports in the mixing nozzle. The ports which are on the left

    side of the converging portion are called parallel ports. Ports on the top of the converging

    portion are called top ports and ports on the bottom are called bottom ports. Ports which

    are normal to the plane of top and bottom ports are called side ports. The position of the

    various secondary inlet ports is shown in Figure 2.Three suction nozzles (convergent) are

    fabricated with different cone angle 21deg, 23deg and 25 deg.

    Two types of inserts are made and it is braced to a long screw in order to move

    the insert to the desired location. Conductivity probes are used to measure the

    conductivity of mixed fluid .

    EXPERIMENTAL PROCEDUREThe aim of the experiment is to find out the extent of mixing of the two fluids by

    providing parallel jets, varying the geometrical parameters like, position of the driving

    nozzle, position of the insert and position of the secondary suction inlet and to evaluate

    the effect in on mixedness of the mixing nozzle.

    Figure 2 Locations of parallel, vertical andcircumference secondary fluid ports

    Top Ports TP1, TP2, TP3, TP4

    Down Ports

    Side Ports1,2,3,4

    Parallel Port P1

    Parallel Port

    Parallel Port P3Parallel Port P4

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    3.1. Experimentation and mixing efficiency

    Conductivity or specific conductance is the measure of the ability of the water to

    conduct an electric current. Conductivity depends upon the number of ions or charged

    particles in water. The specific conductance is measured by passing a current between

    two electrodes (one centimeter apart) that are placed into a sample of water. In solution,

    the current flows by ion transport. Therefore, an increasing concentration of ions in the

    solution will result in higher conductivity values. The Conductivity Probe is actually

    measuring in ohms, conductance is measured using the SI unit, siemens (formerly known

    as a mho). Since the siemens is a very large unit, aqueous samples are commonly

    measured in micro siemens, or S.

    Initially the discharge of primary liquid is kept as 2600 lit/hr by adjustingthe ball valve and the 21 convergent portion is connected with the throat. Parallel port 1

    is opened and all the other ports are closed. The secondary fluid discharge is obtained by

    noting down the time required for the suction of 500 ml of secondary fluid. The suction

    pressure is noted down from the manometer. Mixed fluid samples are collected from the

    samples points and the average electrical conductivity of the samples is measured. This

    is referred as the mixed fluid conductivity. Standard solution is prepared by taking a

    proportion of primary and secondary fluids which is having a ratio of the mixed fluid.

    This proportion of primary and secondary fluid will be well mixed by using a stirrer and

    the conductivity of mixed fluid is measured. This is referred as the standard conductivity.

    The closeness of mixed fluid conductivity with standard conductivity can be taken

    as a measure of mixing efficiency. Mixing efficiency is calculated as the ratio of mixed

    fluid conductivity and standard conductivity. The effectiveness of mixing of each port is

    obtained experimentally by finding out the mixing efficiency (mixing

    efficiency=Conductivity of mixed fluid /Standard conductivity of mixed fluid).

    The experiment is repeated by opening the parallel ports P1,P2,P3,P4individually, P1&P3 , P2&P4, P1&P2&P3&P4 combine and the down ports

    D1,D2,D3,D4 individually & D1&D2&D3&D4 combine .Samples are collected at the

    points 450mm,900mm & 1800mm from the throat entrance . The whole experiments

    were repeated by varying the discharge of secondary fluid as 3100lpm & 3600lpm and

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    the distance between the tip of the driving nozzle and the throat entrance as 10mm,

    20mm, 30mm, 40mm & 50mm.

    5. COMPUTER MODELING AND ANALYSIS

    5.1. Effect of Voticity and inserts on mixingDifferent models have been created by varying geometrical parameters such as

    secondary fluid inlet position, cone angle (convergent) of suction nozzle and driving

    nozzle position [5,6]. Similarly Each case has been analyzed by keeping port open and

    other ports have kept closed and also by varying the position of driving nozzle away from

    the throat entrance. Another set of models have been created by providing an inserts in

    the throat of the nozzle. All these models have been created by using a pre-processor

    called Gambit. The computer analysis is done by exporting the meshed or gridgenerated model form GAMBIT software to the FULENT 6.0 [7].

    The Figure 3 shows that the vorticity magnitude reaches the maximum value of

    9.56(1/s) thus increases the mixedness, when the driving nozzle position DN is 50 mm &

    all the parallel ports are opened. The value of vorticity magnitude reduces to 8.08(1/s)

    when all the down ports are opened and leads to less mixing.

    Figure 3 Contours of vorticity when all the parallel ports are open.

    It can be observed from the vorticity contours that the vorticity is more when the

    DN=50 mm and all the down ports are opened. The increase in vorticity leads to more

    interaction of mixing fluids and increasing the mixing efficiency. However near the

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    inserts the values of vorticity is fluctuating and it is higher near the inserts and low

    without inserts .Hence the presence of inserts enhances the liquid-liquid mixing in a static

    mixing nozzle and the efficiency of mixing can be increased.

    The Figure 3 shows that the vorticity magnitude reaches the maximum value of 9.56(1/s) thus increases the mixedness, when the driving nozzle position DN is 50 mm &

    all the parallel ports are opened. The value of vorticity magnitude reduces to 8.08(1/s)

    when all the down ports are opened and leads to less mixing. Also the COV is nearing

    zero [3] due to more interaction of fluids and more mixing.

    5.2. Effect of driving nozzle position on vorticity magnitude

    Figure 4 Comparison of experimental, computational and literature results of Vorticitymagnitude when DN=50 mm.

    Figure 5 Contours of turbulent kinetic energy distribution with inserts (Lobes ).

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    5.3. Effect of Turbulent kinetic energy

    Figure 6 Contours of turbulent kinetic energy when parallel ports are opened

    Figure 7 Contours of Turbulent kinetic energy when all the down ports are open

    It can be seen that the turbulence kinetic energy is maximum in case when the

    parallel ports P1 &P2 & P3 & P4 are opened simultaneously and the driving nozzle

    position DN is 50mm as it can be observed in Figure 5&6

    From the contours of turbulent kinetic energy it is observed that the turbulent

    kinetic energy is 1.87x10 m 2 /s2 when the DN=50 mm and all the parallel ports are opened

    and 1.27x10 m 2 /s2 when DN=50 mm & down ports are opened. The turbulent kinetic

    energy is found to be still reducing when any ports is opened individually or combines

    with any other port.

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    The computational analysis of Belovich [25] also proved that ,the parallel jets

    mixing is more effective .The increase of turbulent kinetic energy and vorticty are

    responsible for good mixing of fluids. Hence the mixing efficiency increases when

    DN=50 mm and all the parallel ports are opened.5.4. The effect of DN position & LDNP on mixing efficiency when down

    ports are open.

    Figure 8 The effect of DN position & LDNP on mixing efficiency when down ports areopen

    Experiments were conducted as mentioned in the section above by opening the

    ports alternately by changing the distance between the tip of the driving nozzle to the

    entrance of the throat (DN) as 10 mm, 20 mm, 30 mm, 40 mm & 50 mm. When the DN ischanged the distance between tip of the driving nozzle to side wall entrance which is

    facing the entrance of the throat(LDNP) also changes as -40 mm(as it is behind the

    driving nozzle), -30 mm, -20 mm, -10mm and 0 respectively. Negative sign indicates that

    the corresponding port is behind the tip of the driving nozzle.

    Down Ports VS Efficiency

    60

    65

    70

    75

    80

    85

    90

    95

    100

    -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50

    LDNP ( Distance between tip of the DN to port side wall ) in mm

    M i x i n g

    E f f i c i e n c y

    %

    D1 open

    D2 open

    D3 open

    D4 open

    D1,D2,D3&D4 open

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    It is clear that the mixing efficiency increases with decrease in LDNP when the

    D1, D2 opens .Further the LDNP increases and becomes more than 20 mm the mixing

    efficiency starts reduces. The mixing efficiency is found to reduce when the D3 & D4

    opens and the LDNP becomes 31 mm ,35mm & 45mm as the chance of interaction of secondary fluid with primary fluid becomes very less (since the tip of the driving nozzle

    becomes away from the port side wall).

    When the down ports D1, D2, D3 & D4 are opened simultaneously as the area of

    contact of the secondary with primary fluid becomes more, the mixing efficiency is found

    to be more than the efficiency when individual ports are opened. When the driving nozzle

    position (DN) is adjusted to at 10mm, only port D4 is partially open and exposed to the

    main stream of primary fluid, hence the efficiency is found to be low. As the DN is

    adjusted to 20mm, port D4 is fully exposed to the primary fluid stream and there is an

    increase of efficiency. Further there is an increase of mixing efficiency when the DN

    becomes 30mm, and the ports D3 and D4 are fully exposed to the primary fluid stream

    .When the DN is changed to 40mm, efficiency has increased more than above said three

    conditions, as the ports D3&D4 are exposed fully and D2 is partially exposed to the

    stream of primary fluid.

    The mixing efficiency has reached to 95.4% when the ports D2, D3, D4 are fully

    exposed and D1 is partially exposed the stream of the primary fluid and the DN is

    adjusted to 50mm.From the above analysis it is clear that the mixing efficiency is

    increasing when the LDNP is between 0-20mm.

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    5.5. The effect of driving nozzle position (DN) & LDNP on Mixing

    Efficiency when Parallel ports are open

    Figure 9 Effect of DN position & LDNP on mixing efficiency when parallel ports are

    openParallel ports discharges the secondary fluid, parallel to the primary fluid stream.

    When the parallel ports P1, P2, P3 & P4 are opened alternately one by one, it was

    observed that the efficiency is all most same.

    When the distance between the tip of the driving nozzle to exit of the secondary

    fluid parallel ports(LDNP) increases the mixing efficiency reduces and it is increasing

    with the decrease of LDNP .The increase of efficiency occurring due to the more contact

    of secondary fluid with the primary fluid in all the four direction when the LDNP

    decreases.

    The mixing efficiency decreases with increase in LDNP as the contact between

    the primary and secondary fluids getting reduces due the increase of distance between the

    tip of the driving nozzle to the exit of secondary fluid outlet. Hence the mixing

    efficiency is inversely proportional to the LDNP.

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    Table.1. Parallel Port 1(PP1) , 2(PP2), 3(PP3) & 4(PP4)are Open &Driving Nozzleposition, D N=50mm.

    Table.1 shows that the conductivity of mixed fluid nearing the conductivity of

    standard mixed fluid and which leads to the maximum efficiency when the parallel ports

    P1, P2, P3&P4 are opened simultaneously when the driving nozzle position DN is

    50mm.Figure 9 shows that, when the LDNP reduces from 60mm to 12mm the mixing

    efficiency reaches 96.7 at DN is 50mm.

    5.6. Effect of driving nozzle position (DN) & LDNP on Mixing

    Efficiency when the down ports, side ports & upper ports are open.

    Figure 10 Effect of driving nozzle position (DN) & LDNP on Mixing Efficiency when the downports, side ports & upper ports are open.

    The Figure 10 shows that the mixing efficiency reduces to 94.3% when all the

    down ports and the circumference ports are opened. But the efficiency is increasing to

    DN (mm)

    DischargeQ1

    (lph)Mixed fluid conductivity( mS/cm) StdConductivity

    (mS/cm)

    Mixing efficiency

    m [%]

    50 2600 5.45 6.1 89.4

    50 3100 6.89 7.3 94.5

    50 3600 9.1 9.4 96.7

    Mixing efficiency VS Circumference ports

    70

    75

    80

    85

    90

    95

    -50 -40 -30 -20 -10 0 10 20 30 40 50

    LDNP(Distance between tip of the dr iving nozzle to side w all of the

    ports) in mm

    M i x i n g

    e f f i c i e n c y

    %

    D1,SF1,UP1& SB1 ports

    open

    D2,SF2,UP2& SB2 portsopen

    D3,SF3,UP3& SB3 portsopen

    D4,SF4,UP4& SB4 portsopen

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    95.4% when all the parallel ports open. Hence it is clear that the parallel jets jets mixing

    improve the performance of the static liquid mixer.

    5.7. Effect of sample location and l/d ratio on mixing.

    Samples were collected at l/d = 18, l/d = 36 and l/d = 72 i.e. .450mm, 900mm &

    1800mm from the entrance of the throat during the experiments. The Conductivity of

    mixed fluid was found out and the mixing efficiency calculated. Figure 11 shows the

    results.

    Figure 11 Effect of mixing length (l/d ratio or sample point) on mixing efficiency

    It can be observed that there is only a slight increase as l/d ratio changes [10] from

    35 to 72 and there is an increase of efficiency only 5% as there is no mechanism available

    to increase the energy for mixing or to add the energy for mixing.

    5.8. Effect of discharge of primary fluid (Q1) on mixing.

    Figure 12 Effect of primary fluid discharge on mixing efficiency when parallel ports areopen

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    Figure 13 Effect of primary fluid discharge on mixing efficiency when the down ports areopen

    Mixing experiments were conducted by varying primary fluid discharge as

    2600 lph ,3100 & 3600 lph for various conditions . From the Fig.12 & 13 it is clear that

    the mixing efficiency increases with increase in secondary fluid and primary fluid

    discharge (Q1&Q2) as the velocity increases more energy being added to the mixed

    stream and leads to more mixing and the mixing of fluids take place with greater impact.

    The experimental analysis of Ahmed [17] also proved that the velocity and discharge

    influences the mixing of coaxial and parallel liquid jets.

    5.9. Influence of primary fluid discharge Q1 on Coefficient of variation-

    Experimentation

    The mean value and standard deviations are calculated for every set of mixed

    fluid density values. And the COV calculated (COV=standard deviation of concentration

    measurements/mean concentration). This is also called the intensity of mixing or degree

    of segregation

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    Figure 14 Influence of primary fluid discharge Q1 on Coefficient of variation-

    Experimentation

    At least three samples of mixed fluid were collected by changing the primary

    fluid discharge Q1, driving nozzle position DN and opening the various ports during the

    experiments. Densities of samples were measured. The Figure 14 shows that the mixing

    efficiency increasing gradually as the COV reducing when the DN=40mm and D3

    opened=50mm and P2 and P4 are open, all the down ports are opened simultaneously and

    DN=50mm and opening all the parallel ports.From the experimental result shown in Figure 14 it is clear that COV is a function

    of primary fluid discharge Q1[1] and driving nozzle position DN. When the Q1 increase

    from 2600 lph to 3600 lph, DN is 50mm and all the parallel ports are opened, COV

    decreases from 0.001169 to 0.000441 as the fluids interacts more and increase in

    efficiency. Similarly the density distribution found to be more uniform and the COV is

    nearing zero when the DN=50 mm & all the parallel ports are opened. Hence there is an

    increase of mixing efficiency.

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    Figure 15 Influence of primary fluid discharge Q1 on Coefficient of variation &

    Comparison between experimental and computational results

    Table 2 Coefficient of variation- COV by computational

    Figure 15 and Table 2 shows the comparison of COV obtain by experiment and

    computational .In both the cases it is clear that the COV approaches zero hence increase

    in mixing efficiency when the parallel ports are opened and parallel jets are getting

    mixed. There is a good agreement between COV obtained from the computational and

    experimental results.

    5.10.Effect of mixing insert on mixing efficiency

    To evaluate the influence and effect of mixing insert on mixing efficiency, helical

    and plate type of mixing inserts have been provided at 900 mm (l/d=36 mm) away from

    Opened port DN, mm COV

    P2 & P4

    open

    50 0.0017661

    D1, D 2, D 3,& D 4, open

    50 0.0008814

    P1, P2, P3 & P4 open

    50 0.0004417

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    the entrance of throat and the experiments were repeated for the few best conditions

    which were obtained during the experiments.

    Figure 16 Effect of mixing insert on mixing efficiencyThe samples are collected at the outlet and whose conductivity was measured.

    The Figure 15 shows the trend of mixing efficiency with and without inserts. The

    mixing efficiency is found to be increased by 2 to 3 % by addition of helical type of

    mixing insert. Hence it can be concluded that the addition of mixing insert improves the

    mixing efficiency. The sample points can be changed as l/d=18 mm, l/d=36 mm & l/d=72

    mm (mixing length as 450mm, 900mm & 1800mm). The absence of mixing insert does

    not have much influence on mixing efficiency even though there is an increase of mixing

    length (l/d ratio or sample point). By introduction of mixing insert the mixing efficiency

    is found to be increase as it adds more energy for mixing when fluid flow through the

    helical path of insert. Hui Hu [24] has studied the effect of mixing insert on mixing

    experimentally and proved that ,mixing inserts improves the mixing.

    5.11.Effect of driving nozzle position on vorticity magnitude

    The Figure 6.10 shows the comparison between the vorticity magnitude obtained

    by the computation and literature datas. The vorticity magnitude reaches the maximum

    value of 9.56(1/s) thus increases the mixedness, when the driving nozzle position DN is

    50 mm & all the parallel ports are opened and due to the inserts.

    Driving Nozzle position(DN) VS Mixing efficiency with & without insert

    70

    75

    80

    85

    90

    95

    100

    0 10 20 30 40 50 60DN position in mm

    M i x i n g e f f i c i e n c y

    %

    D1 to D4open &withoutinsert

    D1 to D4open &withinsert

    P1 to P4open &with outinsert

    P1 to P4open &withinsert

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    Figure 17 Vorticity magnitude when DN=50 mmAlso the COV is nearing zero due to more interaction of fluids and more mixing

    The value of vorticity magnitude reduces to 8.08(1/s) when all the down ports are openedand leads to less mixing. The results were found agreeing with the literature data.

    6. FINDINGS AND CONCLUSIONS

    In the present work a mixing nozzle was designed, fabricated and its performance

    was evaluated experimentally. Theoretical analysis is also carried out by using CFD

    method. The influencet of geometrical parameters on mixing and the parallel jets mixing

    were evaluated. The mixing efficiency was evaluated by using conductivity which is

    simple and reliable technique to evaluate the mixing efficiency of the mixing nozzle. The

    effect parallel jets mixing in a static mixing nozzle on various conditions have been

    analyzed and the results are reported.

    An experimental set up was fabricated and experiments were carried out to predict

    the performance on the mixing by varying the locations of secondary fluid inlet to 5mm,

    15mm, 20mm&40mm, driving nozzle position 10mm, 20mm, 30mm, 40mm&50mm,

    cone angle of the suction nozzle to 21deg, 23deg & 25deg and the location of the insert to

    50mm, 100mm&150mm from the entrance of the throat.

    The investigations revealed that the change in sample point (l/d) does not have

    much effect on mixing efficiency without adding mixing insert. The addition of mixing

    insert improves the mixer performance. The mixing efficiency depends on the direction

    of fluids entry. The increase of primary fluid discharge Q1 influences the suction of

    secondary fluid which in turn has an effect on mixing efficiency. When the driving

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    nozzle was kept at 50mm and the all the parallel ports are opened and the parallel jets

    mixing taking place the mixing efficiency was increasing as vorticity magnitude and the

    turbulent kinetic energy are increasing and the fluids interaction becomes more which

    intern increases the mixedness.Computational modeling and the analysis shows that COV is found to be

    minimum and gives more effective mixing when all the parallel ports ie., P1, P2, P3 & P4

    are opened at DN = 50 mm. The COV obtained by the experimentation and computation

    were compared and found to be in good agreement.

    7. SCOPE OF FURTHER WORK

    Further this study can be extended by studying the effect of temperature, viscosity

    of fluids and twisting angle of inserts on mixing. Mapping methods can be used to studythe distributive mixing processes. Further the standard models can be developed to

    predict the drop size evolution during the flow in the static mixer.

    REFERENCES

    [1] Hiroshige Kumamaru, Takashi Kanada, Kenji Fujith and Naoyuki Sawada,

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