Journal of Japan Society of Civil Engineers, Ser.
B1 (Hydraulic Engineering), Vol. 68, No. 4, I_37-I_42, 2012.

Annual Journal of Hydraulic Engineering, JSCE, Vol.56, 2012, February

INFLUENCE OF AIR INJECTION ON SUCTION
POWER AND PRESSURE GRADIENT IN
DREDGER SYSTEM
Mohammad MESHKATI SHAHMIRZADI1, Tetsuya SUMI2, Sameh KANTOUSH3
and Toshiyuki TEMMYO4
1Member of JSCE, Ph.D. candidate, Disaster Prevention Research Institute, Kyoto University (Gokasho, Uji, Kyoto
611-0011, Japan)
2Member of JSCE, Professor, Disaster Prevention Research Institute, Kyoto University (Gokasho, Uji, Kyoto
611-0011, Japan)
3Member of JSCE, Associate Professor, Faculty of Engineering and Material Science, German University of Cairo(New Cairo City, Al-Tagamoa Al-Khames, Cairo, Egypt)
4Member of JSCE, Ph. D., HAZAMA Corp. (2-2-5 Toranomon Minatoku, Tokyo, Japan)

Air injection could be a possible solution to prevent blockage along the transport pipeline which
commonly occurs during the operation of dredger system. The present paper seeks to investigate
the influence of air injection on the performance of dredger system by injecting variant air
concentrations. To achieve this purpose, the experiments were conducted under various
operating condition such as: initial pump pressure (0.196, 0.294, 0.490 and 0.706 MPa),
sediment grain sizes (2, 5, 10 mm, Mixed and Large), transport sediment concentrations (0.8%
to 8.5%) and air concentrations (0, 25, 40, 60 and 100 nl/min). The experiments showed that therisk of pipeline blockage is higher in case of fine sediment compared with coarser sizes.
Furthermore, the air injection into the system is more effective for lower initial pump pressure(0.196 and 0.294 MPa). Also, the lowest pressure gradient along the transport pipeline wasobserved for the specific air concentration ranges from 40 to 60 nl/min and specific sedimentconcentration ranges from 3.5% to 6%.
Key Words: Reservoir sedimentation, ejector pump dredger system, multiphase flow, pressure gradient, 

1. INTRODUCTION㻌 㻌 Utilizing the dredger system is an effective
technique to solve the problematic of sedimentationin reservoir storage. The most sever challenge thatfrequently observed during the operation of dredger
system is the blockage of transport pipeline; defined
as an occasion in which dredger system is unable tomaintain the vacuum and transport anymore
sediment to the disposal site. In such a condition,
introducing air into the transport pipeline could be
an operative option to reduce the risk of blockage indredger system. By injection of air a complex
three-phase of air, water, and sediment flow will becreated along the transport pipeline. The
experimental studies in the area of three-phase flow

were mostly conducted in petroleum and chemicalengineering and rarely in water engineering.
In summary, the reported investigations can beclassified into three main categories: (a) Flow andsand dunes pattern study1), 2). , (b) Sand particles
motion study3), 4) (c) Pressure drop study5), 6), 7), 8).
The characteristics of sand particles (grain size,
specific weight, density, cohesiveness) prevailing on
present dredger system is slightly different fromwhat were dominant on above mentioned studies.
The sand particles used in these studies are almost
fine, light and cohesive, with high suspension rate
into the liquid phase, which named slurries. While
the present dredging system mainly aimed tovacuum the coarser sediment (gravels) rather thanfiner one.

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Annual Journal of Hydraulic Engineering, JSCE, Vol.56, 2012, February

Take into account that, the dredger system should
also be able to transport the large woods andgarbage, which already vacuumed into the system.
In this case, the proper design of dredger system isvery important to prevent the high possibility
blockage risk inside of the system.
㻌 㻌 In other hand, the experimental studies conductedin oil drilling operation and cuttings transport were
under specific hydraulic condition such as: arotation part at the beginning of transport pipeline,
extra liquid polymeric solutions, pressure of sea
water column, and temperature variation.
㻌 㻌 Although the results of these studies are relevantto the concept of three-phase flow in transport
pipeline of dredger system, their applicability for the
purpose of an efficient design are uncertain. The
main objective of this study is to experimentallyinvestigate the properties of the turbulent
three-phase flow (air-water-sediment) in transport
pipeline of dredger system. For this purpose,
extensive experiments were conducted to clarify theeffect of air injection concentration on suction
power, efficiency and pressure gradient in thedredger system.

P

G

AC

V

J

EH
SP
Nozzle
T1

Pt1
T2
TP
DG

Pt2
CCD

Pt3

T3

TP

Fig. 1 The schematic view of model setup.

pattern in TP, respectively. The locations of eachpiezometer points were selected on straight pipeline
section how the distance between each point were
3.5 m, as illustrated in Fig. 1. In order to inspect thetrustworthiness of the data acquired by dynamic
pressure gage (DG), some preliminary experiments
were conducted aim to measure the pressure
gradient along the pipeline in laminar steady state;
which indicated that the data acquisition is reliable.

2. MATERIALS AND METHODS(1) Experimental apparatus and instrumentation
The experimental model used in this study isshown in Fig. 1. This model consists of three parts:(1) Initial Force Section: includes a high pressure
pump (P) and its water storage tank (T1). When thepump is operating and valve (V) is opened, clear
water can be withdrawn from T1 and discharged outof the nozzle. The pump pressure (PP) was
measured by a pressure gauge (G), and then therelative initial water flow rate (Qw1) was calculated.
(2) Suction Section: includes ejector house (EH),suction pipeline (SP) with 3m length and innerdiameter of 25mm, and second storage tank (T2).
Due to the low (negative) pressure domain just after
the nozzle section, a pressure gradient was createdbetween entrance of suction pipeline and inside of
the system, which it caused mixed of water andsediment vacuumed (sucked up) from T2 into EH.
(3) Transport Section: includes transparent transport
pipeline (TP) with 10m length and inner diameter of
36mm, and external storage tank (T3).
Transport pipeline conveyed the sediment to thedisposal site, and fluids were released into theexternal tanks (T3). Air, also, was injected into thetransport pipeline by a compressor (AC) and a
dynamic pressure gauge (DG) with threepiezometers and a high speed camera (CCD) wereincorporated to measure the differential pressure
between locations Pt1 to Pt3 and monitor the flow(2) Definition of main parameters
The suction power (Qsuc), transport efficiency ofthe system (E) and pressure gradient (ᇞp) wereconsidered as three main criteria to evaluate theperformance of dredger system. The Qsuc is definedas Eq. (1):Qsuc

Qs  Qw 2(1)

where Qs and Qw2 are sediment and water removal
rate from T2, respectively. Suction power (Qsuc) canbe acquired by Eq. (2):

Qsuc

Vsw
't

(2)

where Vsw is the volume of water and sedimentvacuumed into the system and ᇞt is the total timeneeded to evacuate all the deposit sediment from
T2 . Vsw was obtained by deferential of water level inT2 before and after each test. Moreover, thetransport efficiency of system (E) can be simply
deliberated from Eq. (3):

E

Qs
Qw1

(3)

where Qw1 is the initial water flow rate that isentered to the EH through the nozzle and Qs is thesediment removal rate from T2, which can be simply

2

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Annual Journal of Hydraulic Engineering, JSCE, Vol.56, 2012, February

obtained by Eq. (4):

Qs

Vs
't

60

Qair=0 nl/min

pt1  pt 3'x

4VXF OPLQ

where Vs is the initial volume of deposit sedimentat the bottom of T2 and ᇞt is the total time neededfor each experiment to vacuum specific amount of
sediment (Vs= 2 liter). Lastly, the ratio of average
differential pressure between first piezometer (Pt1)
and third piezometer (Pt3) over the length distance(ᇞx) between these two points were considered aspressure gradient (ᇞp), as Eq. (5):

'p

Cop

40

20

0
0

(5)

The length distance (ᇞx) between the first and third
point was 7 meter. Moreover, the sedimentconcentration (Cop) was defined as below:Qs
Qw1  Qw 2

Without air

(4)

0.2

0.4

33 03D)

0.6

0.8

Fig. 2 Relationships between suction power (Qsuc) and pumppressure (Pp) for no air injected to the system, Qair=0 nl/min
and different grain size, data points are: , Large; Ⴃ , 10mm; რ ,Mixed; ‫ڧ‬, 5mm;i , 2mm.

(6)60(3) Experiment procedure
To carry out the experiments, the high pressure
pump (P) was set to supply four different pumppressures:0196, 0.294, 0.490 and 0.706MPa
resulting in four different initial flow rates of 17.2,
25, 32 and 36 l/min. At first, T2 was filled with clear
water and a certain amount of sediment (2 liter).
When dredger system started to operate, sedimentand water were vacuumed into the EH and pushedforward through the transport pipeline by the force
of initial water flow rate. The three phases were
injected into the transport pipeline at different
combination of volumetric flow rates including: airconcentration (0, 25, 60 and 100 nl/min), sedimentgrain size (2, 5, 10 mm, Mixed and Large) andsediment concentrations (0.89% to 8.5%). Before
any data recording actions, each test was allowed torun 2 to 5 minutes for being stabilize along the TP
and reach to the equilibrium condition. Then thetime needed to vacuum the all sediment (2 liter) that
already have stored on the bottom of T2 wasrecorded. To specify the volume of sediment andwater removed from T2, the water elevation inside of
T2 at the beginning and at the end of eachexperiment was measured.

With air
Qair=40 nl/min

4VXF OPLQ

40

20

0

0

0.2

0.4

0.6

0.8

33 03D
Fig. 3 Relationships between suction power (Qsuc) and pumppressure (Pp) for 40 nl/min air injected to the system, Qair=40
nl/min and different grain size, data points are: , Large; Ⴃ , 10mm; რ , Mixed; ‫ڧ‬, 5mm;i , 2mm.

illustrated in Fig. 3, in addition to the different
pump pressure, 40 nl/min air was introduced into the
TP. Comparing these two figures revealed that when
the pump pressure was relatively low (Pp=0.196
Mpa) and no air injected into the system, sedimenttransport capacity was almost zero, specifically forfine sediment (ds=2, 5mm and Mixed). In otherwords, pipeline blockage was occurred (Qsuc=0l/min) and no sediment had the chance to betransported in the pipeline.
On the contrary, by injecting 40 nl/min air into theTP, the pipeline blockage solved and suction power(Qsuc) drastically improved. Indeed, injection of air
into the system increased the superficial velocity offlow in TP, and then the sediment had less chancefor deposition. Comparing Fig. 2 and 3 indicate that,at low initial pressure pump (Pp =0.196 MPa) andfine sediment (ds= 2, 5mm and Mixed), the risk of

3. RESULTS AND DISCUSSION(1) Effect of air injection on suction power
Fig. 2 and 3 show the variation of suction power(Qsuc) versus the pump pressure (Pp) for different
grain size of sediment. The cases shown in Fig. 2
are related to the sediment transport in pipeline justby the force of pump pressure without any air
injection into the dredger system (experiment withwater and sediment). However, for the cases

3

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Annual Journal of Hydraulic Engineering, JSCE, Vol.56, 2012, February

blockage in system is higher comparing with coarse
sediment (ds= 10mm and Large). Inspection of data
revealed that fine and coarse sediment has
characteristically different behavior versus air
injection into the transport pipeline. When no air
introduced in TP, more commonly fine sedimenttends to form sand dunes along the transport
pipeline, whereas coarse sediments were transported
more individually. Therefore, fine sediment may
create higher roughness against the flow andconsequently greater pressure gradient.
In other side, when air supplied into the system theintermittent wavy flow (slug flow) was createdalong the TP and fine sediment easily suspended in
the body of each coming wave and flushed out.
Indeed, with changing the rate of injected air orwater various flow patterns could be identified
along the horizontal pipeline such as: stratified flow,plug flow, slug flow, dispersed flow and etc. Slugflow is an intermittent wavy flow that mostly forms
in two-phase and three-phase flow. The
characteristics of slug flow (intermittent wavy) werealready well described by Tippetts & Priestman1).
Moreover, introducing air into the system in case of
relatively high-pressure pump has not any positiveeffect on the suction power (Qsuc) of system.
Comparing Fig. 2 and 3 indicate that for high initialpressure pump (0.490 and 0.706Mpa), by injecting
air into the transport pipeline the suction power(Qsuc) slightly reduced. At the low pump pressure(i.e.17.2 Mpa) the sediment transport pattern along
the transport pipeline is either stationary bed or
movable bed; in which the effect of surface friction
between the flow and pipe wall is considerably high.
Air injection can reduce the surface friction between
the flow and pipe wall by suspending the sediment
into the slug flow body.
In case of fine sediment due to the less load andhigh potential to be fully suspended, the air injection
was operative. In other hand, at high-pressure pump
the sediment transport pattern is either moving bed
flow or asymmetric suspension flow. In this case air
injection increases the turbulent of flow and creates
numbers of eddies, resulting in loss of energy andreduction of suction power (Qsuc).
Therefore, a critical velocity can be obtained forthree-phase flow inside of horizontal pipeline forwhich the injection of air and its subsequently
increasing the velocity until it reaches to criticalvelocity can increase the suction power; while thevelocity over the critical velocity can decrease thesuction power.

40

High Pump
Pressure

32

( 

24

Medium Pump
Pressure

16

Low Pump
Pressure

8
0
0

20

40

60

4DLU QOPLQ

80

100

Fig. 4 Relationships between efficiency (E) and air
concentration (Qair), data points are: ႑ , ds= 2mm, PP=0.196
MPa; რ , ds= Mixed, PP =0.196 MPa;‫ڹ‬, ds= 2mm, PP =0.294 MPa; , ds= 2mm, PP =0.490 MPa.
40
32

( 

24

Medium Pump
Pressure

16
8

Low Pump
Pressure

0
0

20

40

4DLU QOPLQ

60

80

Fig. 5 Relationships between efficiency (E) and air
concentration (Qair), data points are: x, ds= Large,
PP =0.196 MPa; ႒ , ds= 10mm, PP =0.196 MPa; Ⴃ , ds= 10mm,
PP =0.294 MPa.

for group of fine (ds=2mm and Mixed) and coarse
sediment (ds=10mm and Large), respectively. As
shown in Fig. 4, for fine sediment, increasing theconcentration of air (Qair) continually improved theefficiency of dredger system (E). However, the same
overall trend was not observed with respect to thecoarse sediment (Fig. 5). Indeed, the higher
concentration of air (Qair) positively intensified thesuspension of fine sediment into the slug flow body.
Therefore, fine sediment could be transported inlonger distance by slug flow. While for coarse
sediment, each particle of sediment makes its own
friction against the driven force of flow and hardly
suspend in slug flow body.

(2)Effect of air injection on efficiency of system
Fig. 4 and 5 depict the relationships between
efficiency of system (E) and air concentration (Qair)(3)Effect of air injection on pressure gradientFig. 6 to 8 illustrates the variation of pressure
gradient (ᇞp) versus the sediment concentration
4

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Annual Journal of Hydraulic Engineering, JSCE, Vol.56, 2012, February

0.001‫ڹ‬3‫[ڹ‬

03DP(Cop). As can be seen in these figures, the pressure
gradient (ᇞp) was relatively high when no air(Qair=0 nl/min) was introduced into the system.
While by injecting an intermediate concentration ofair (Qair=60 nl/min), the pressure gradient (ᇞp)
considerably reduced and that is why the air
injection into the system could solve the pipeline
blockage for fine sediment, as shown in Fig. 2 and3. Adversely, introducing a high concentration of air(Qair=100 nl/min) into the TP, showed a negative
impact that caused the increase of pressure gradient(ᇞp) in system. Furthermore, as exhibited in thesefigures, the pressure gradient (ᇞp) became
proportionally greater with increasing the transport
sediment concentration (Cop). Particularly, whenthe experiments were carried out without any air
injection (Qair=0 nl/min) sediment particles settled at
the bottom of TP and created a stationary bed insideof the pipeline. Resulting in a hydrodynamic radiusof TP became smaller and accordingly the frictionof pipeline and the pressure gradient (ᇞp) wereincreased. Nevertheless, as can be seen in Fig. 6 to8, for a specific range of sediment concentration(Cop=3.5% to 6%) and particular air concentration(Qair=60 nl/min), as sediment concentration (Cop)increased the pressure gradient (ᇞp) reduced. This
result is generally in good agreement with the result
obtained by Okayama5), where they explained the
reduction of pressure loss for soft mud slurry with
air in case of high solid concentration. And it is incontradiction with the result reported by Bjerkholt7)
who noted that no significant reduction in pressure
gradient will be occurred as a result of air addition
into the transport pipeline. Indeed, increase of
sediment concentration means the growth of theparticles quantity inside of the transport pipeline. As
a result of exist turbulent in the pipeline, the tiny air
bubbles consistently can enter to the slug flow bodyand exit again. When the quantity of sedimentparticles increased, more tiny air bubbles can beabsorbed between the sediment particles. These tiny
air bubbles positively can reduce the settling
velocity of those sediments, which alreadysuspended in the slug flow body. Hence, sedimentcan be transported in longer distance by a wave. It
noteworthy to mention that the air in TP, generally
has an inherent tendency to scape and gather at theupper part of pipeline and make a bigger bubble. A
bigger bubble means the greater gap between eachslug flow. Then by injecting too much air the
distance between each slug flow will be increased
and thus sediment particles have enough time toredeposit on the bottom of TP. While injecting an
optimal concentration of air could adjust a
proportion gap between each slug flow how the next
coming slug flow will arrive and flushed sediment

0.000750.00050.00025

ds= 2mm0024&RS 

6

8

Fig. 6 Relationships between pressure gradient (ᇞp) andconcentration of sediment (Cop) in transport pipeline for given
PP =0.196 MPa and ds= 2mm and different concentrations ofair, data points are: , 0 nl/min; რ , 25 nl/min; ሁ, 60 nl/min;ᇞ, 100 nl/min.

‫ڹ‬3‫ [ڹ‬03DP

0.001
0.000750.00050.00025
ds= 5mm0024&RS 

6

8

Fig. 7 Relationships between pressure gradient (ᇞp) andconcentration of sediment (Cop) in transport pipeline for given
PP =0.196 MPa and ds= 5 mm and different concentrations ofair, data points are: , 0 nl/min; რ , 25 nl/min; ሁ, 60 nl/min;ᇞ, 100 nl/min.

‫ڹ‬3‫ [ڹ‬03DP

0.001
0.000750.00050.00025

ds= Mixed

0
0

2

4
&RS 

6

8

Fig. 8 Relationships between pressure gradient (ᇞp) andconcentration of sediment (Cop) in transport pipeline for given
PP =0.196 MPa and ds= Mixed and different concentrations ofair, data points are: , 0 nl/min; ሁ, 60 nl/min.

toward the downstream area before re-deposition of
sediment particles.

5

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Annual Journal of Hydraulic Engineering, JSCE, Vol.56, 2012, February(4) Velocity distribution by using LSPIV

4. CONCLUSION

Large-scale particle image velocimetry (LSPIV)
was used to evaluate the variation of flow velocities
in TP. This technique is now widespread andextensive researches exist on this topic9). Fig. 9
presents the velocity distribution of two-phase flow
(water-sediment) in TP when no air was added into
the system.
As shown on this treated image, almost all velocity
vectors confined to the upper half of TP and wereordered into the direction of stream-wise flow andcreate a laminar velocity profile inside of TP. In thiscase, no velocity vectors visualized for the sedimentdeposited at the bottom of TP. However, when 40
nl/min air injected into the system (Fig. 10), thevelocity distribution was significantly affected bypresence of air. In fact, by injecting air into the TP
an intermittent wavy flow pattern (slug flow) was
created and the velocity vectors periodicallyoriented towards the bottom of TP. Resulting in thesediment deposited on the bottom of TP was
effectively slid forward then held up and lastly
suspended into the slug flow body. The maximum
velocity magnitude in this case was observed at thebottom of TP.
Likewise, based on the LSPIV analysis mainly for
low initial pump pressure, the relationships between
average velocity of flow and air concentration wasinvestigated. The results proof that an optimum
concentration of air should be injected into thesystem to reach a maximum velocity in the TP (inrange 40 to 60 nl/min), depend on the sedimentconcentration and initial pump pressure.

The following conclusions were drawn in thispaper: (a) The risk of pipeline blockage by finesediment was substantially more than coarse
sediment and it more often happened in case of low
initial pump pressure. (b) Air injection into thetransport pipeline could prevent the pipeline
blockage. (c) Injection of air has no significant
impact on performance of dredger system in case ofhigh initial pressure pump. (d) To achieve themaximum performance of dredger system, it isrecommended
toutilizeanintermediateconcentration of air and relatively highconcentration of sediment. (e) A tendency for
reduction of pressure gradient was observed for an
optimal fraction of air, water and sediment in thepipeline.
REFERENCES
1)

2)

3)

4)

Qair= 0 nl/min
Y (mm)

5)

6)

X (mm)
Fig. 9 The average velocity distribution of the multiphase flowin transport pipeline, ds=2 mm, PP=0.196 MPa, Qair= 0 nl/min.

7)
Y (mm)

Qair= 40 nl/min

8)

X (mm)

9)

Fig. 10 The average velocity distribution of the multiphase flowin transport pipeline, ds=2 mm, PP=0.196 MPa, Qair= 40 nl/min.

6

I_42

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