N. M. Nouri *           A. Sarreshtehdari **          E. Maghsoudi ***
                                                    Department of Mechanical Engineering
                                                  Iran University of Science and Technology
                                                        Narmak, Tehran 16844, Iran

             In present research, improvement of a microbubble generator’s performance via reliance on fluid dy-
          namics characteristics is studied numerically, and then some experiments are executed. In an elementary
          cylindrical microbubble generator, water flow enters the device via six diagonal nozzles, and passes a ro-
          tational path around a central motionless hub. This flow breaks the big bubbles entering the device by
          gas injection from air nozzles. The high-intensity turbulence and shear flow in this device is the cause of
          the air bubble breaking process. These small bubbles can be used to reduce frictional drag on the contact
          surface of moving solid bodies in water flow. To improve the operation of the apparatus, some suggested
          geometrical shapes were investigated numerically and were optimized based on the bubbles’ effective
          breaking-up parameters. The experimental results illustrated good performance of the recent apparatus
          for generating smaller bubbles.

          Keywords : Microbubble, Drag reduction, Microbubble generator.

                                                                                   comprising porous materials with tiny holes, play a vital
                       1.     INTRODUCTION                                         role.
                                                                                      Fujikawa et al. [9] introduced a new device for the
   Microbubbles are used for various industrial applica-                           generation of micro air bubbles based on dissolution
tions, such as water treatment and fisheries cultivated                            and separation processes of air in water. In the disso-
shells [1]. They also have good properties for physio-                             lution process, air is dissolved in water in such a way
logical and physiochemical purposes [2]. The reduc-                                that compressed air is introduced into water in the form
tion of skin friction of ships is one of the most important                        of numerous small bubbles from a rotating circular po-
applications of microbubbles with diameters smaller                                rous plate. The mixture of water and generated air
than several tens of microns. This type of drag reduc-                             bubbles is stirred in the following mixing box where air
tion is particularly important for maritime transportation                         bubbles dissolve into the surrounding water. The di-
applications, since 80% of total drag in a large ship is                           ameter and number of separated micro air bubbles are
due to skin friction [3]. Moreover, this technique of                              controlled by adjusting both the rotational frequency of
reducing skin friction is an environmentally friendly                              the porous plate and the flow rate of air introduced into
method, compared to some another methods, such as                                  water. Sadatomi [10] invented a new microbubble
polymer drag reduction [4]. McCormick and Bhat-                                    generator that had different mechanisms for producing
tacharyya [5] towed a 1.22-m long, fully submerged hull                            microbubbles. In this design, pressurized water is in-
in a towing tank. They created small bubbles around                                troduced into a pipe with a spherical body in the core.
the hull using electrolysis and found that drag can be                             From conservation equations of mass and energy, the
reduced significantly using this technique. Some re-                               water velocity around the body in a downstream region
searchers use electrolysis for microbubble generation,                             becomes higher than the inlet velocity, and thus the
especially for laboratory purposes (e.g. [6]), but this                            pressure decreases. In this device, if the pressure be-
technique to generate microbubbles is not useful for                               comes less than atmospheric pressure, air is automati-
industrial researches and applications [7,8]. Some                                 cally sucked into the water stream through a number of
microbubble generators use a combination of methods,                               small holes drilled on the pipe wall in the lower pres-
such as depressurization of air-saturated water, bubble                            sure region. Since the water flow is highly turbulent,
break-up by shear force, and cavitation. The genera-                               the air sucked in is well broken into a great number of
tion of a homogeneous distribution of small air bubbles                            microbubbles. The main advantage of this generator is
for microbubble drag reduction experiments is indis-                               that it is independent of an air injection system.
pensable. For this purpose, sintered metal filters,                                   In the present research, optimization of a primary
                                                                                   cylindrical microbubble generator is described [11]. In
*                       **                                       ***
    Graduate student         Professor, corresponding Author           Professor

Journal of Mechanics, Vol. 25, No. 2, June 2009                                                                                         189
lindrical microbubble generator is described [11]. In
this apparatus, water enters the device via six diagonal
nozzles and mixes with air coming from small holes at
the bottom of the device. In the primary Model, air
bubbles passing a spiral trace around a central mo-
tionless hub were broken up into tiny bubbles. Ex-
periments show the existence of bubbles smaller than 1
mm in diameter in the primary Model that have im-
proved now. In the new system, the internal flow
passes a longer spiral trajectory by geometrical shape
improvement and provides the ability of central hub
rotation. Smaller bubbles of sizes less than 100μm are
observed in trial runs of recent devices.


   The idea of microbubble generation based on rota-
tional flow without any porous media was examined by
a primary apparatus (Fig. 1). As demonstrated in Fig.
1 the apparatus consists of a cylindrical, transparent
shell with an internal diameter of 10cm, which sur-                Fig. 1     Primary bubble generator device
rounds a motionless hub with a diameter of 3cm, ad-
justed in the center of the device. Water flow enters
the device via six diagonal nozzles and mixes with air
coming from small holes in the bottom of a central hub.
Pressurized water flow injection creates a rotational
flow, which is highly turbulent, and shear flow, which
break large bubbles into smaller ones. The total height
of device is 50cm. The size of the bubbles generated
by the primary device was measured by image process-
ing [11]. According to the results, the sizes of the
generated bubbles are less than 1mm in diameter.
These results confirm that using high turbulence inten-
sity flow is an efficient tool for generating microbub-
                                                              Fig. 2     Section of improved microbubble generator

                                                            with each other. On the other side, the turbulent shear
                                                            stress breaks them continuously. In the last stage,
   Generation of a flow field with high turbulence in-      large shear stresses caused by the narrow outlet break
tensity was used as the main idea for designing the new     the bubbles for the final step.
apparatus. The improved microbubble generator de-
vice is a symmetric cylindrical device that includes a
rotary hub and an external shell. Figure 2 illustrates a
section of this device. Liquid and gas are injected into            4.    OPTIMIZATION PROCEDURES
the device and exit circumferentially through an outlet.    4.1   Effective Parameters
The geometrical shape of the region through which the
liquid-gas mixture passes and the angle of the diagonal        Experiments have shown that the size of bubbles is a
water holes are deliberately selected to maximize rota-     major parameter in microbubble drag reduction (e.g. [12
tional velocity of the liquid-gas mixture. Furthermore,     to 15]). Hence, the main target of the present research
two bearings have been used to support the central hub      has focused on generated bubble diameter in various
and facilitate the tendency of rotation. In this Model,     quantities of air to liquid volume fractions. Therefore,
bubbles are broken into three different stages. At the      the bubble size was considered as an effective parameter
first stage, they are broken due to injection through the   in operation of the apparatus in constant air/liquid vol-
holes. During the second stage, rotational flow makes       ume fractions. For regulation of the void fraction in-
the bubbles pass a longer trajectory. At this stage, high   side the apparatus, the gas flow rate was measured by a
velocity flow and long distance of movement separate        gas flow meter. The average void fraction, with re-
the bubbles far away and prevent them from merging          spect to similar studies (e.g. [3]) is estimated by Eq. (1):

190                                                                        Journal of Mechanics, Vol. 25, No. 2, June 2009
                            Qa                                 very sensitive to temperature; therefore, all the numeri-
                    α=                                  (1)    cal simulations and experimental studies have been
                          Qa + Qw
                                                               conducted in a constant temperature to eliminate the
where Qa and Qw are air flow rate and water flow rate          temperature effects. In order to generate microbubbles
respectively. The uncertainty of air flow rate meas-           as much as possible, break-up frequency should be in-
urement is ±0.001 according to the air flow meter, and         creased by some means. Thus, turbulent kinetic en-
that of the water flow rate is ±0.1 in relative error.         ergy ε and the initial diameter of the bubbles, which is
Therefore, the measurement uncertainty of the average          regulated by the nozzles’ diameter, are two effective
void fraction is ±0.01. In all types of bubble genera-         parameters of the present design. The increase of rota-
tors, the size of generated bubbles is increased by in-        tional speed leads to an increase in kinetic energy, while
creasing the void fraction. Therefore, generating small        the initial diameter of the bubbles is regulated by the
bubbles at high average void fractions is another target       nozzles’ diameters.
in the present research. Average void fraction is used
as a parameter for experimental control of the present
setup. This parameter does not represent the local void                 5.   GEOMETRY IMPROVEMENT
fraction at the exit of the bubble generator. The ad-
justment of the bubbles size in various average void
fraction at the exit of bubble generator is the major             In order to obtain an improved geometry for the re-
purpose of experiments. Moreover, the local void               gion the liquid-gas mixture passes, the finite element
fraction in special applications, which is completely,         method is used to simulate single-phase flow for four
affected by free stream pattern, pressure gradient, grav-      different geometries. Inlet velocity components in
ity effects and the surface curvature is not reported in       cylindrical coordinates as input boundary conditions for
present work.                                                  all Models are constant (Vr = 5m/s, Vθ = 15m/s, Vz =
     When the air is injected into the water, the turbulence   2m/s). These velocity components were calculated
shearing stress due to velocity fluctuations results in        according to pumping unit characteristics to adjust a
                                                               water flow rate equal to 4lit/sec. The external shell
some deformation forces on the bubbles, which are much
                                                               velocity is set to zero, but the central hub rotational ve-
greater than necessary failure forces, due to surface ten-     locity changes from zero to 6rev/sec in a different
sion, and then the bubbles will collapse. The amount of        simulations. In the numerical simulation, the k−ε
breakup frequency is proportional to the difference be-        Model is used. The flow rate of water is set to 4lit/sec,
tween the amount of confinement forces and the amount          the pressure at the outlet to zero, and the temperature to
of deformation forces excreted on the bubbles’ surface         293k.
[16]. The more difference there is between turbulent              Tangential velocity, turbulent kinetic energy, and the
shearing stresses caused by the velocity fluctuations          trajectories of each fluid element are considered effec-
 τt ( D) = ρΔu 2 ( D) / 2 and surface-restoring pressure       tive parameters for improving the geometry. The
                                                               higher tangential velocity, higher turbulent kinetic en-
 τs ( D) = 6σ / D the greater would be the probability of
                                                               ergy, and longer spiral trajectory of the water elements
bubble breakup in definite time. After a while, the            reduce the size of the generated bubbles. The manu-
amount of difference reduces gradually and breakup fre-        facturability of the geometry is another subject that
quency decreases to a finite value.                            needs to be considered. Figure 3 shows the qualitative
     The probability of breakup depends on the character-      distribution of tangential velocity in a section of the
istic bubble size D and the turbulent kinetic energy ε of      device in the mono phase water flow, and the four dif-
the flow field. For each value of ε, a critical capillary      ferent geometries with a motionless central hub. In all
length Dc exists, such that the turbulent shear stresses       the examined Models, a venturi-shaped region exists to
are in equilibrium with the surface tension forces.            increase rotational velocity.
This critical capillary diameter Dc is given by Eq. (2):          In Models 1, 2, and 3 the outlet is turned inversely to
                                  3           2                intensify shear stress, which increases the break up of
               Dc = 1.26(σ / ρ) 5 (ε)         5
                                                        (2)    bubbles. On the contrary, the outlet of Model 4 is
                                                               straight. The main difference between Model 1 and
The breakup frequency is zero for bubbles of size D ≤          the others is related to the water inlet nozzle angles
Dc and it increases rapidly for bubbles larger than the        which are parallel to the axis of the central hub, while
critical one D > Dc [16]. After reaching a maximum at          the others are turned aside and make an angle of 61 de-
Dgmax = 1.63Dc, breakup frequency decreases mono-              grees with to the central hub axis. By revising the
tonically with bubble size. The maximum breakup                profile of the inlet in Model 1 and Model 2, a more
frequency achieved at Dgmax is given by Eq. (3):               uniform distribution of rotational velocity is obtained.
                                                               This correction improves the performance of Models 3
                                      2       3                and 4, while Models 2 and 3 are more difficult to manu-
               g max (ε) ∝ (σ / ρ) 5 (ε) 5              (3)    facture.
The critical capillary length and the maximum breakup             Table 1 shows the maximum tangential velocity and
frequency are functions of surface tension, which is           mean value of turbulent kinetic energy in all four

Journal of Mechanics, Vol. 25, No. 2, June 2009                                                                      191
                                                                        Fig. 4 Fluid particles’ trajectories from inlet to outlet
                                                                               in different geometries with a fixed central hub
Fig. 3 Qualitative distribution of tangential velocity in                      (Models 1a, 2a, 3a and 4a respectively, from
       a section of the device in the mono phase water                         left to right)
       flow for different geometries with a fixed cen-
       tral hub (Models 1a, 2a, 3a, 4a respectively from
       left to right)

mentioned Models. In this table, Model 4 provides
maximum turbulent kinetic energy and tangential veloc-
ity in comparison with other Models for a fixed central
   Figures 4 and 5 show the trajectory of some particles
of fluid from inlet to outlet in four different geometries
with a fixed central hub and a rotary central hub respec-
tively. As illustrated in Fig. 4, in Model 4 each parti-
cle of fluid travels a very long trajectory, in spite of
non-attendance of central hub excitation. In Fig. 5,
rotational excitation of the central hub has a desired
effect on Model 1 and makes trajectories longer espe-
cially in Model 4.                                                      Fig. 5 Fluid particles’ trajectories from inlet to outlet
                                                                               in different geometries with a rotary central hub
   The turbulent kinetic energy, tangential velocity, and                      (Model 1b [with 4rev/s], 2b [with 4rev/s], 3b
length of trajectories of fluid particles in Model 4 are                       [with 6rev/s], 4b [with 6rev/s] respectively,
much more than the others. Also, simple manufactur-                            from left to right)
ing and assembling of this Model was interesting.

                                                                        5.1   Manufacturing Consideration
Table 1 Comparison of deferent numerical analysis for                      The rotary hub and external shell’s profiles play an
        various models                                                  important part in the functionality of the apparatus op-
                                                                        eration, and they were performed by CNC machine. In
              Central hub       Mean Value of         Maximum           addition, the water nozzles’ holes were performed by
  Model     angular velocity   turbulent kinetic tangential velocity    special fixtures to achieve enough accuracy. The angle
               (rev/sec)            energy       in the section (m/s)   of the nozzles was adjusted upon the numerical analysis
 Model 1a          0                1.63                 15.8           to prepare maximum inlet rotational velocity. Two
 Model 1b          1                5.57                 18.5           cylindrical collectors were used to conduct water and air
 Model 1c          4               28.24                 36.4           separately to related nozzles. The final apparatus ex-
 Model 1d          6               437.41                62.9           amined for bubble generation in a symmetric cylindrical
 Model 2a          0                1.57                 15.8           device is shown in Fig. 7. In this experimental setup,
 Model 3a          0                4.67                 15.8           water enters the device via six diagonal nozzles with a
 Model 4a          0               60.18                 26.5           diameter of 10mm and an angle of 61 degrees, and
                                                                        mixes with air coming from 24 vertical straight holes
                                                                        with a diameter of 2mm at the bottom (Fig. 6).

192                                                                                  Journal of Mechanics, Vol. 25, No. 2, June 2009
                                                             formed for various void fractions and various flow out-
                                                             let gap sizes respectively, and the results are presented
                                                             in Table 2. The first series of tests was performed in
                                                             constant water flow rate. By changing the air flow rate
                                                             from minimum to maximum, photographs were taken to
                                                             investigate the void fraction effects on bubbles’ size.
                                                             Flow rate of water was adjusted at Ql = 3.5l/sec and
                                                             flow rate of air was changed from Qa = 0.5l/sec to Qa
                                                             = 5l/sec, so that the void fraction varied from %0.15 to
                                                             %0.60 which bubbles’ diameter increases in higher void
                                                                In the second series of experiments, the effect of
                                                             outlet gap on bubbles’ size was investigated. Three
               Fig. 6   Water and air nozzles                sets of experiments with three different outlet gap sizes
                                                             20mm, 8mm, and 3mm were examined. Obtained
                                                             bubble sizes were presented in Table 2.
                                                                In this device, the total flow head loss is proportional
                                                             to the gap size; therefore the required pumping power
                                                             increased by gap size reduction. Hence, it is essential
                                                             to consider required power in special application.

                                                                               7.   CONCLUSIONS

                                                                 In the present paper a new microbubble generator
                                                             device has been developed based on increasing the tur-
                                                             bulence intensity of the flow field. This idea is espe-
        Fig. 7    Schematic of experimental setup            cially useful for high void fractions. Improved ge-
                                                             ometry for a microbubble generator was obtained nu-
                                                             merically according to the distribution of turbulent ki-
          6.     EXPERIMENTAL RESULTS                        netic energy and the order of rotational velocity. The
                                                             prototype of this improved microbubble generator has
    Figure 7 shows the experimental set up. Microbub-        been examined and the results obtained demonstrate the
ble generator (D) was installed at the bottom of a cylin-    ability of microbubble generation with less than 100
drical tank (C) with height of 1.5m. The tank has four       microns in diameter in a void fraction of 15%.
transparent windows for lighting and image processing.           The size of microbubbles is adjustable by changing
Pressurized water enters the water nozzles via a cen-        the outlet gap size, and microbubbles have been pro-
trifugal pump (P). Water and air flow rates are meas-        duced in high void fraction up to 60%, hence this device
ured by flow meters (A) and (F), and are adjusted using      is firmly recommended for industrial applications.
valves (B) and (G). At the top of the tank, a membrane
(E) was used to separate air and water in order to re-
circulate the water. A compressor with a maximum                               NOMENCLATURE
pressure of 8 bars is used to inject pressurized air into
the air nozzles. The temperature of the water during           D     Characteristic size of the bubble
the tests was constant around 22°C.                            Dc    Critical capillary length
    To measure the bubbles’ diameter, a ruler was in-          σ     Surface tension force
stalled near the outlet of the device. This ruler was
used as a reference scale for measurement calibration in       τ     Turbulent stress
image processing. Each image with a special size and           ε     Turbulent kinetic energy
resolution was scaled by this reference scale, afterwards      ρ     Density
the size of bubbles identified. Images were taken by a         g     Breakup frequency
CCD digital camera and shutter speed was adjusted to
1/1000 sec. During the manipulation, a 400 watt so-           Δu 2   Velocity root mean square (rms)
dium-vapor lamp focused on the micro bubble generator         Vr     Radial Velocity
devise.     A sample of the images, for an outlet gap of      Vθ     Angular Velocity
8mm and void fraction of 25% is presented in Fig. 8.
                                                              Vz     Vertical Velocity
The maximum measurement error is about two pixels in
images, and it is less than 0.001mm in all cases.             QL     Flow rate of water
    The central hub starts suddenly to rotate by inlet wa-    Qa     Flow rate of air
ter excitation. Two series of experiments were per-            α     Void Fraction

Journal of Mechanics, Vol. 25, No. 2, June 2009                                                                     193
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                                                                         Drag Reduction Mechanism by Microbubble Injection
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194                                                                               Journal of Mechanics, Vol. 25, No. 2, June 2009

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