chapter 4 - shodhganga : a reservoir of indian theses...

Chapter 4

Geometric optimization of

hydrodynamically cavitating devices

Chapter 4: Geometric optimization of hydrodynamically cavitating devices

Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants Page 112

4.1 Introduction

In the last decade many studies have indicated the use of hydrodynamic

cavitation as an energy efficient technique for the degradation of organic pollutants

(Gogate and Pandit, 2005; Sivakumar and Pandit, 2002; Wang and Zhang, 2009;

Wang et al., 2011a; Wang et al., 2011b; Sawant et al., 2008; Franke et al., 2011;

Chakinala et al., 2009; Bremner et al., 2008; Wang et al., 2009; Braeutigam et al.,

2009; Gogate, 2002). In the hydrodynamic cavitation device, the cavitational

condition and hence the efficiency can be regulated according to the application

required simply by changing the operating (inlet pressure and cavitation number) and

geometrical (size and shape of the cavitating device) parameters. Hence, there is a

need of a detailed study to understand and develop the relationship between the

cavitational yield of hydrodynamic cavitation and these parameters before its

successful implementation on an industrial scale.

The efficiency of the hydrodynamic cavitation is very much dependent on the

number of cavitational events (number of cavities) occurring inside a cavitating

device and the intensity of cavity collapse, which in turn depends on the geometry of

the cavitating device and the flow conditions of the liquid i.e., the scale of turbulence

and the rate of pressure recovery. The optimum cavitational yield of the

hydrodynamic cavitation is the result of several optimized parameters such as number

of cavitational events occurring inside a cavitating device, residence time of cavity in

the low pressure zone and the rate of pressure recovery downstream of the throat

(Sivakumar and Pandit, 2002; Bashir et al., 2011). All these parameters needs to be

clubbed and optimized together to get the enhanced cavitational yield from

hydrodynamic cavitation device because considering only one parameter in the design

of a cavitating device would not result in the possible explanation of all cavitational

conditions for the desired effects. This is due to the fact that all these parameters are

not independent. Thus, the cavitational condition inside a cavitating device can be

altered by changing the ratio of the perimeter of cavitating holes to its cross sectional

area (α), the ratio of the throat length to its diameter/height and divergent angle (in the

case of a venturi). As these parameters affects the number of cavitational events

occurring inside a cavitating device, residence time of cavity in low pressure zone and

the rate of pressure recovery downstream of the throat.



Sivakumar and Pandit (2002) have studied the effect of geometry of the

multiple hole orifice plates on the degradation of a cationic dye rhodamine B solution.

They have concluded that in hydrodynamic cavitation, altering the flow geometry and

hence the turbulent pressure fluctuation frequency (fT) could enhance the cavitational

yield. Optimum frequency of turbulence can be achieved by manipulating the flow

conditions and geometry of the cavitation device. They have observed that for the

plates having the same cross sectional flow area, it is advisable to use a plate with a

smaller hole size opening, thereby increasing the number of holes in order to achieve

a larger area occupied by the shear layer due to higher perimeter value. Because, for

smaller hole sizes, the value of fT increases, leading to a more efficient cavity collapse

and higher cavitational yield. Hence, the ratio of the perimeter of the throat to its open

area (α) is an important parameter which determines the number density of cavities

that can be generated. Also, if there is a choice on the magnitude of the flow area,

lower percentage of cross sectional flow areas should be chosen, as with a decrease in

flow area, the intensity of cavitation increases.

Bashir et al. (2011) have carried out CFD based optimization of the important

geometrical parameters of a cavitating venturi. They have found that the ability of a

cavitating device to generate cavities and the overall cavitational yield depends on the

several parameters such as ratio of the perimeter of cavitating hole to the cross

sectional flow area of its constriction (α), the ratio of the throat length to its height (in

the case of a slit venturi) and the divergence angle.

There is not much work reported in the literature on the design and

optimization of different cavitating device. Most of the studies were focused on the

degradation of different pollutants using single or multiple hole orifice plates and

synergetic effect of hydrodynamic cavitation and other additives. No studies have

been found on the comparison of venturies of different shapes and orifice plate and

the subsequent effect of the cavitating device on the degradation kinetics. As

discussed earlier the geometry of a cavitating device has a strong influence on the

entire cavitation (inception, growth and collapse) behaviors. The CFD analysis by

Bashir et al. (2011) has also theoretically indicated such a possibility and hence it is

worth validating these numerical predictions using experiments.



In the present work we have carried out degradation of orange-G dye [OG]

and optimization of three different cavitating device viz. a single hole orifice plate,

circular venturi and a slit venturi. The cavitational yield of all three cavitating devices

were compared on the basis of amount of TOC reduced per unit energy supplied.

4.2 Materials and Methods

4.2.1 Materials

Orange-G dye (molecular weight: 452.38 g/mol; molecular formula:

C16H10N2Na2O7S2) was purchased from S.D. fine chemicals (India). The chemical

structure of orange-G dye is shown in Figure 4.1. All the solutions were prepared with

tap water as a dissolution medium. H2SO4 was used for maintain the acidic pH, while

NaOH was used for maintaining basic pH.

NN

OH

SO3Na

SO3Na

Figure 4.1: Molecular structure of orange-G

4.2.2 Hydrodynamic cavitating device

Figure 4.2 shows three cavitating devices used in this work. The dimensions of

circular and slit venturi are given in Table 4.1 and a orifice plate (1 mm thickness)

with 2 mm hole at the centre is shown in Figure 4.2. As explained earlier (section 1)

in the case of venturi the important parameters which needs to be considered are

throat area, the ratio of the throat length to its diameter/height and divergent angle.

Bashir et al. (2011) have explained the effect of all these parameter on the cavity



dynamic and cavitational yield of a cavitating device using CFD study and proposed

optimized cavitating device for best cavitational activity. The dimensions of

cavitating devices used in our study are based on the optimized parameters found out

by Bashir et al. (2011).

Figure 4.2: Schematic diagram of cavitating devices

Orifice plate(c)

Inlet

Throat

W

H

C/S of slit venturiC/S of circular venturi

do

(a) Circular venturi(b) Slit venturi



Table 4.1: Dimension of circular and slit venturi

Dimension Circular venturi Slit venturi

Dimension of throat

Circular hole of 2

mm diameter

W = 6.0 mm; H = 1.9 mm;

L= 1.9 mm

Venturi length 87 mm 87 mm

Length of convergent section 18 mm 20 mm

Length of divergent section 67 mm 65 mm

Half angle of convergent section 22.6º 23.5º

Half angle of divergent section 6.4º 5.5º

4.2.3 Experimental and Analytical Methods

Hydrodynamic cavitation based degradation of OG dye was carried out at

different conditions using fixed solution volume of 6 liter and for a constant

circulation time of 2h. The detail description and schematic of hydrodynamic

cavitation setup has been explained in chapter 2 (section 2.2.2). The concentration of

OG was varied from 30 to 150 µM to study of the degradation kinetics. The pressure

study was done over a range of 2 to 7 bar. The pressure shown here are gauge

pressure in bar. The temperature of the solution during experiments was kept constant

in all the cases at about 32±2°C and was maintained by circulating cooling water

through the jacket provided to the holding tank. The absorbance of OG dye was

monitored using UV-Spectrophotometer (Shimadzu-1800) and then the concentration

of OG was calculated by analyzing the absorbance at the wavelength of 478.5 nm.

The absorbance spectrum of orange-G dye is shown in the Figure 4.3. It is observed

from the spectrum that the absorbance of orange-G dye is reducing with increasing

treatment time through the hydrodynamic cavitation device. The complete



mineralization was analyzed by measuring the Total Organic Carbon (TOC) content

of the dye solution using TOC analyzer (TOC-L, Shimadzu Corporation, Japan).

Figure 4.3: Absorbance spectra of orange-G dye w.r.t treatment time (Conditions: volume of solution: 6 lit, inlet pressure: 5 bar, pH of solution: 2.0,

cavitating device: orifice plate)

4.3 Result and Discussion

4.3.1 Hydraulic characteristics of cavitating devices

The hydraulic characteristics of the all the cavitating devices have been

studied first by measuring the main line flow rate at different pump discharge

pressures (inlet pressure to the cavitating device). The cavitation number (Cv) and

power dissipated (PD) into the system per unit power supplied (PI) to the system was

then calculated. The power dissipated into the system is defined as

Power dissipated into the system (PD, J/s) = Pressure drop across the cavitating

device (ΔP, Pa) × Volumetric flow rate through the cavitating device (Vo, m3/ s)



The Cavitation number is a dimensionless number used to characterize the

condition and degree of cavitation in hydraulic devices (Gogate and Pandit, 2000;

Shah et al., 1999). The cavitation number is defined as

−

=2

2

21

o

vV

v

ppC

ρ

Where, p2 is the fully recovered downstream pressure, pv is the vapor pressure of the

liquid, vo is the velocity at the throat of the cavitating constriction which can be

calculated by knowing the main line flow rate and area of the opening. A sample

calculation for the cavitation number and power dissipation is shown in Appendix 4A.

Under ideal condition cavities are generated at a condition Cv≤1 but in many cases

cavities are known to get generated at a value of Cv greater than one due to the

presence of some dissolved gases and suspended particles which provide additional

nuclei for the cavities to form (Shah et al., 1999). The cavitation number at which first

cavity appears is called cavity inception number (Cvi). Figure 4.4 shows the effect of

the pump discharge pressure (inlet pressure to the cavitating device) on the main line

flow rate and cavitation number for all three cavitating devices. The liquid flow rate

through the main line increases with an increase in the pump discharge pressure (inlet

pressure to the cavitating device). It was found that cavitation number decreases with

an increase in inlet pressure to the cavitating device. An increase in the discharge

pressure increases the flow through the main line, the velocity at the throat of the

venturi also increases, which subsequently reduces the cavitation number as per the

definition of Cv. It was observed that in the case of slit venturi, a higher volumetric

flow rate was obtained for a given pressure drop as compared to orifice plate and

circular venturi. The cavitation number in both the venturi was less as compared to

orifice plate at a given inlet pressure because of higher volumetric flow rate obtained

in both the venturies. The number of cavities generated increases with a decrease in

cavitation number i.e. with an increase in the liquid velocity.



Figure 4.4: Effect of inlet pressure on the main line flow rate (VO) and Cavitation Number (CV).

Yan and Thorpe (1990) have reported the cavitation number for the inception

of cavity for different orifice sizes. They observed that for a given size orifice, the

cavitation inception number remains constant within an experimental error for a

specified liquid. The cavitation inception number does not change with the liquid

velocity and is a constant for a given orifice size and is found to increase with an

increasing size and dimension of the orifice. Figure 4.5a and 4.5b shows the power

dissipated (ΔP × volumetric flow rate) into the system per unit power supplied (1.1

kW in all the cases) to the system at different inlet pressures and operating cavitation

number. It can be observed that power dissipated into the system per unit power

supplied to the system is higher for the slit venturi for a given inlet pressure and

cavitation number as compared to orifice plate and circular venturi. The power

dissipated into the system for the slit venturi is almost 3 times higher than circular

venturi and 4 times higher than orifice plate.

0

200

400

600

800

1000

1200

1400

0

0.5

1

1.5

2

2.5

3

0 1 2 3 4 5 6 7 8 9 10 11

Volu

met

ric fl

ow ra

te (V

O),

LPH

Cav

itatio

n nu

mbe

r (C

v)

Inlet pessure(bar)

Cv (orifice plate)Cv (circular venturi)Cv (slit venturi)VO (circular venturi)VO (orifice plate)VO (slit venturi)



Figure 4.5a: Effect of inlet pressure on the power dissipation in a cavitating device

Figure 4.5b: Effect of cavitation number on the power dissipation in a cavitating device

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 1 2 3 4 5 6 7 8 9 10 11

P D /

P I

Inlet pressure (bar)

circular venturi

orifice plate

slit venturi

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

P D /

P I

Cavitation number (CV)

circular venturi

orifice plate

slit venturi



4.3.2 Effect of initial dye concentration

In order to investigate the kinetics of degradation, experiments were

conducted with different initial concentration ranging from 30 μM to 150 μM. The

method of initial rate was used to determine the reaction order and the specific rate

constant. The operating pressure and the pH of solution were kept constant in all the

experiments at 5 bar and 2.0 respectively and hydrodynamic cavitation was carried

out using circular venturi. Initial rates were calculated at different initial dye

concentration. In the case of degradation of dye where the concentration of dye is

very small (<150 μM) the rate can be expressed as follows:

nAoA kCR =

( ) ( ) ( )AoA CnkR lnlnln +=

Where, RA is the initial rate of degradation of orange-G in mol lit-1 min-1, CAo is the

initial concentration of orange-G in mol/lit, k is the rate constant (litn-1moln-1min-1)

and n is the order of reaction.

Figure 4.6a shows the plot of ln(RA) v/s ln(CAo), the slope of which gives the

order of reaction. From the Figure 4.6a it is clear that the degradation of orange-G

follows first order kinetics and also the plot of rate v/s concentration (Figure 4.6b) is a

straight line passing through the origin, which also confirms that the degradation of

orange-G is a first order reaction. The rate of degradation increases with an increase

in initial concentration of dye and the first order rate constant calculated was found to

be constant, irrespective of initial dye concentration, confirming the validity of the

kinetic expression.


Hydrodynamic Cavitation based degradation of Bio-Refractory pollutants 2

Figure 4.6a: Degradation kinetic of OG (Conditions: volume of solution: 6 lit, inlet pressure: 5 bar, pH of solution: 2.0, cavitating device: circular

venturi)

Figure 4.6b: Effect of initial concentration on degradation rate (Conditions: volume of solution: 6 lit, inlet pressure: 5 bar, pH of

solution: 2.0, cavitating device: circular venturi)

y = 1.004x - 3.508R² = 0.986

-14.5

-14

-13.5

-13

-12.5

-12

-11.5

-11

-10.5

-10-11 -10.5 -10 -9.5 -9 -8.5 -8

ln(R

A)

ln(CAO)

0

5

10

15

20

25

30

35

40

45

0 2 4 6 8 10 12 14 16

RA

×10

7(m

ol/li

t/min

)

CAO (µM)



4.3.3 Effect of pH

Solution pH is an important parameter in determining the efficiency of

hydrodynamic cavitation as it affects the chemical property of the solution and the

possible location of the solute (at the interface or in bulk). Many researchers have

studied the effect of solution pH on the efficacy of cavitation process in degradation

of organic pollutant (Ku et al., 1997; Kotronarou et al., 1991; Lin and Ma, 1999). In

this study, the effect of pH was investigated by carrying out HC experiments (using

circular venturi as a cavitating device) at different pH in the range 2-13. Figure 4.7

shows the effect of pH on the decolorisation rate of OG.

Figure 4.7: Effect of solution pH on decolorisation rate of OG (Conditions: volume of solution: 6 lit, initial concentration: 50 μM , inlet

pressure: 5bar, cavitating device: circular venturi)

The results indicate that the rate of decolorisation increases with a decrease in

solution pH i.e. acidic medium (lower pH) is more favorable for the degradation of

OG using HC. Much lower decolorisation rate was observed at pH 9.0. About 75.72

% decolorisation was obtained at pH 2.0 using HC with circular venturi as a

cavitating device. However no decolorisation was observed at pH 11.0 and 13.0. The

two main mechanisms for the degradation of pollutants using hydrodynamic

0

5

10

15

20

25

30

35

2 3 4 6 7.3 9

K ×

103

(1/m

in)

pH of solution



cavitation are the thermal decomposition/pyrolysis of the pollutant molecules

entrapped inside the cavity and near to the cavity surface during the collapse of the

cavity and secondly, the reaction of OH• radicals with the pollutant occurring at the

cavity-water interface and in the bulk medium. In the case of non volatile pollutant

the main mechanism for the degradation of pollutants will be the attack of hydroxyl

radicals on the pollutant molecules at the cavity-water interface and in the bulk fluid

medium. The mechanical effects are also significant. In some cases the intensity of

shockwaves generated by the collapsing cavity can break molecular bonds, especially

the complex large molecular weight compounds. Therefore the orientation of the

pollutants molecule in a solution (especially near or at the cavity- water interface) is

very important in getting the maximum effect. The orientation of pollutant molecule

is very much dependent on the state of the molecule, whether molecular or ionic. The

enhancement in the decolorisation/degradation rate at lower pH can be attributed to

the fact that dye molecule is present in the molecular state at lower pH, hence can

easily enter the region of gas–water interface of cavities due to hydrophobic nature

and thus, is more readily subjected to the OH radical attack and also to the thermal

decomposition. Thus, the overall decomposition of OG is attributed to the pyrolysis

and free radical attack occurred at both the cavity-water interface and in the bulk

liquid medium. Whereas, in the basic medium the dye molecules gets ionized and

becomes hydrophilic in nature thereby remaining in the bulk liquid. In the alkaline

medium, the ionic species of OG predominated and therefore it cannot evaporate into

the gaseous region or enter the region of the cavity-water interface leaving the

decomposition of OG to occur only in the bulk liquid medium by the attack of OH•

radicals, and therefore reducing the decolorisation rate.

Ku et al. (1997) have studied the effect of pH on the degradation of 2-

chlorophenol using ultrasound and showed that the 2-chlorophenol decomposes faster

at pH 3 as compared to pH 11. They have stated that the decomposition is rapid in

acidic solutions where molecular form of 2-chlorophenol dominates, part of the

molecular species may evaporate into the gaseous region, therefore the overall

decomposition of 2-chlorophenol in acidic solutions is considered to take place in

both the gaseous and film regions. For alkaline solutions, the non-evaporative ionic

species of 2-chlorophenol is the predominant species and the decomposition of 2-

chlorophenol is assumed to occur only in the film and bulk region.



4.3.4 Optimization of cavitating devices (Effect of inlet pressure and cavitation

number on degradation rate)

The inlet pressures to the cavitating device and cavitation number are the two

important parameters which affects the cavitational condition inside a cavitating

device. The number of cavities being generated and the cavitational intensity (collapse

pressure magnitude) depends very much on the inlet pressure and cavitation number.

In this study, the optimization of three cavitating devices was carried out by studying

the decolorisation rate of OG at different inlet pressure and cavitation number. The

optimized condition for a cavitating device is the inlet pressure and cavitation number

at which maximum decolorisation rate is achieved. Experiments were carried out at

different inlet pressure ranging from 3 to 7 bar. The initial concentration of dye and

pH was adjusted at 50 μM and 2.0 respectively. Figure 4.8 shows the effect of inlet

pressure and cavitation number on the decolorisation rate of OG.

Figure 4.8: Effect of inlet pressure and cavitation number (CV) on decolorisation rate of OG (Conditions: volume of solution: 6 lit, initial

concentration: 50 μM , pH of solution: 2.0)

It has been observed that the decolorisation rate increases with an increase in

the inlet pressure reaching a maximum and then drops for all the cavitating devices

studied in this work. It can be observed that there is an optimum inlet pressure and

0

10

20

30

40

50

60

1 2 3 4 5 6 7 8

K ×

103

(1/m

in)

Inlet pressure (bar)

circular venturi

slit venturi

orifice plate

CV = 0.29

CV = 0.45

CV = 0.22

CV = 0.15

CV = 0.18

CV = 0.21

CV = 0.18CV = 0.21CV = 0.24CV = 0.29

CV = 0.38

CV = 0.11CV = 0.13



cavitation number at which decolorisation rate is maximum for all three cavitating

device. The optimized inlet pressure is 3 bar (CV = 0.29) for the slit venturi and 5 bar

for the circular venturi (CV = 0.15) and orifice plate (CV = 0.24) respectively. The

maximum decolorisation rate was obtained in the case of slit venturi, whereas lowest

decolorisation rate was obtained in the case of orifice plate. About 92 %

decolorisation was obtained in the case of slit venturi (almost 88% decolorisation

takes place in the first 45 min of circulation through the cavitating device), whereas

76 and 45 % decolorisation was obtained with circular venturi and orifice plate

respectively in 2 h at optimized inlet pressure of all three cavitating devices.

Figure 4.9 shows the change in % decolorisation with number of passes for

three different cavitating devices at their optimized inlet pressure. Where, the number

of passes can be defined as follows.

Number of passes = (Volumetric flow rate (VO)/ Total volume of solution in the

holding tank) x Time of operation

Figure 4.9: Effect of number of passes on decolorisation rate of OG (Conditions: volume of solution: 6 lit, initial concentration: 50 μM , pH of solution: 2.0)

0

10

20

30

40

50

60

70

80

90

100

0 50 100 150 200 250 300 350 400

% D

ecol

oris

atio

n

Number of passes

circular venturi at 5 bar inet pressure

slit venturi at 3 bar inlet pressure

orifice plate at 5 bar inlet pressure



It was observed that slit venturi gives higher % decolorisation as compared to

circular venturi and orifice plate for the same number of passes through the cavitating

devices indicating higher energy efficiency in the case of slit venturi as compared to

other two cavitating devices.

As the pressure increases, main line flow rate housing cavitating device

increases, so the velocity at the throat of cavitating device increases, which

subsequently reduces the cavitation number as per the definition of Cv. As the

cavitation number decreases more number of cavities are formed which results into

higher cavitational yield, hence higher decolorisation rate was obtained at lower

cavitation number. The quantum of the total collapse pressure is the result of collapse

pressure due to a single cavity and number of cavities being generated and thus the

cavitational intensity due to cavity collapse will be higher with more cavities. On the

other hand at a very low cavitation number (higher inlet pressure) the number density

of cavity becomes so high that the condition of choked cavitation occurs as discussed

earlier. Once the cavitation device is completely filled with a large number of cavities

(choked cavitation) these cavities start coalescing to form a larger cavitational bubble

(cavity cloud). These larger bubbles escape the liquid without collapsing or result into

an incomplete and/or cushioned collapse, thus reducing the cavitational yield and

thereby reducing the decolorisation rate after the optimum reached in all the cavitating

devices. This has also been explained by photographic study that once the cavitation

inception occurs, further decrease in cavitation number results into generation of more

number of cavities (Chapter 2; Section 2.3.2) and a condition of choked condition

occurs due to lager number of cavities present downstream of the cavity generator.

The first clear cavity cloud was observed at 6 bar pressure (Cv=0.13) for the circular

venturi studied and with further increase in inlet pressure almost entire downstream

area is filled with cavity cloud. Similarly, it is possible that the condition of choked

cavitation occurs in different cavitating devices at different operating conditions due

to their different geometrical design and hence different optimum cavitation number

is obtained in this study for different cavitating devices.

SenthilKumar et al. (2000) have also reported the decomposition of aqueous

KI solution using hydrodynamic cavitation. They have shown that iodine liberation

increased with a decrease in cavitation number, reaches a maxima and then drops. The



optimum cavitation number found in their work was again in the range of 0.15 to 0.25

for all the orifice plates studied in their work, which is similar to the optimum

reported in this work.

4.3.5 Comparison of cavitating devices in terms of energy efficiency

The energy efficiency of three cavitating devices was compared on the basis of

cavitational yield. The cavitational yield is defined as the total quantum (mg) of TOC

reduced per unit of energy supplied. The calculation for the cavitational yield is

shown in Appendix A. The experiments were conducted at optimized condition (inlet

pressure and cavitation number) with different cavitating devices and TOC was

measured. The concentration of OG and the pH of the solution were kept constant in

all the experiments at 50 μM and 2.0 respectively. Figure 4.10 shows the cavitational

yield for different cavitating devices.

Figure 4.10: Cavitational yield of different cavitating devices at optimized conditions (operating pressures: slit venturi, 3 bar; circular venturi, 5

bar; orifice plate, 5 bar)

It has been observed that the slit venturi gives higher cavitational yield as

compared to circular venturi and orifice plate. As discussed in the previous section the

decolorisation rate and the % decolorisation per pass is also higher in the case of slit

venturi as compared to other two cavitating device (Figure 4.8 and 4.9). About 37 %

0.00E+00

1.00E-06

2.00E-06

3.00E-06

4.00E-06

5.00E-06

6.00E-06

slit venturi circular venturi orifice plate

mg

of T

OC

redu

ced/

Ene

rgy

supp

lied



reduction in TOC was obtained with slit venturi, whereas 28 and 14 % reduction in

TOC was obtained with circular venturi and orifice plate respectively in 2 h. As

discussed earlier (section 4.3.1) in the case of slit venturi, power dissipated into the

system is higher as compared to circular venturi and orifice plate for all the inlet

pressure and resulting cavitation numbers. The higher dissipated power in the case of

slit venturi enhances the number of cavitational events occurring inside the cavitating

device and hence the cavitational yield.

As discussed earlier, the final collapse pressure and the intensity of cavity

collapse and hence the effect of HC on degradation of organic pollutants depends on

the number of cavities being generated, the maximum size attained by a cavity before

its collapse and the rate of pressure recovery. In the case of HC, all these parameters

depends on the geometry of the cavitating device such as throat size, the ratio of

throat perimeter to its cross sectional flow area and the angle of divergent section in

the case of venturi. In case of both the venturies the pressure recovers smoothly due to

divergent angle and cavities get enough time to grow to maximum size before

collapsing, whereas in the case of orifice plate the pressure recover immediately and

cavities collapses before attaining a maximum size, thereby reducing the intensity of

cavity collapse or cavitational yield for the orifice plate. In the case of venturies the

cavitational zone spreads to most of the divergence section as compared to orifice

plate. Therefore pollutant molecule will experience cavitational condition for a longer

time in the case of both the venturies as compared to orifice plate and hence higher

cavitation yield (more reduction in color and TOC) is obtained in the case of both the

venturies as compared to orifice plate.

Bashir et al. have (2011) carried out CFD based optimization of the important

geometrical parameters of a cavitating venturi (circular and slit venturi). They have

found that the ratio of the perimeter of the venturi to the cross sectional flow area of

its constriction quantifies the number of cavities being generated. The ratio of the

throat length to its height (in the case of a slit venturi) controls the maximum size of

the cavity and the angle of the divergence section controls the rate of collapse of a

cavity. Based on the numerical study, they have concluded that a slit venturi (α = 2.7)

with the slit length equal to its height (1:1) and a half angle of divergence section of

5.5 degrees (as shown in Table 4.1) is an optimum geometry for the best cavitational



activity. They have observed that the length of cavitational zone and the number of

cavitational events is higher in the case of slit venturi as compared to the circular

venturi and hence the maximum cavitational yield is expected to occur in the case of

slit venturi. The higher cavitational yield obtained in our study for the slit venturi as

compared to circular venturi and orifice plate thus, is consistent with the CFD result

found out by Bashir et al. (2011). Hence, it can be concluded that throat geometry,

throat size (area and perimeter) and divergent angles are the important parameters

which affects the cavitational condition inside a cavitating device and hence should be

optimized to obtain highest cavitational yield.

4.4 Conclusions

The efficiency of the hydrodynamic cavitation was found to be dependent on

the geometry of the cavitating device and operating parameters (inlet pressure and

cavitation number). By manipulating the operating conditions such as inlet pressure

and cavitation number, the intensity of cavitation and hence the chemical effect

associated with it can be controlled. The number of cavitational events occurring

inside a cavitating device and intensity of cavity collapse was found to be dependent

on the geometry of cavitating device. The study indicates that the maximum

cavitational yield through the cavitating device can only be obtained by considering

the effect of all these parameters collectively. By considering only one parameter in

the design of a cavitating device would not result in the possible optimization of all

cavitational conditions for the desired effects. The study reveals that the effects of the

physical and chemical properties of the solution are also significant. The pH of the

solution and state of the molecule whether in ionic or molecular state plays an

important role in getting the maximum degradation rate using hydrodynamic

cavitation as it affects the location of the pollutant molecule in the solution. The CFD

based simulation as a qualitative design tool also has a future in the designing of the

optimized hydrodynamic cavitating device.



4.5. Treatment of industrial wastewater using hydrodynamic cavitation

4.5.1 Introduction to the problem

In order to analyze the efficacy of hydrodynamic cavitation in treating the

industrial wastewater, the water effluent from a common effluent treatment plant

(CETP, patalganga, Maharashtra, India) was taken and subjected to the hydrodynamic

cavitation reactor for the possible reduction in pollutant level. The retention of high

COD (≈ 600 ppm) even after the UASB anaerobic treatment was the major challenge

faced by CETP unit, because wastewater stream containing such a high COD

(approximate 600 to 1000 ppm, day by day variation due to inefluent variation) after

the secondary treatment cannot be discharded direclty into the waterbody. Hence

advanced treatment is required for the further reduction in COD level. Therefore, the

aim of this study was to check the efficacy of hydrodynamic cavitation in bringing

down this level of COD to an acceptable range so that it can be dischared or reused

for other utilities into the plant. The sample was collected after the anaerobic digester

(Upflow anaerobic sludge blanket (UASB)), i.e. at the UASB Outlet. The

characteristic of wastewater effluent (UASB O/L) is shown in Table 4.2. In this study

three different cavitating devices: circular venturi, slit ventur and orifice plate were

tried to generate the cavitaional condition and to analyze their possible effect on the

reduction of pollutant level.

Table 4.2: Characteristics of UASB O/L effluent

Parameter Unit Value

pH

7.3

Color

Colorless

TSS ppm 196

TS ppm 3108

TDS ppm 2912

COD ppm 628

TOC ppm 116



4.5.2 Experimental procedure

Hydrodynamic cavitation based degradation of UASB O/L effluent was

carried out using different cavitating devices (circular venturi, slit venturi, orifice

plate) at their optimized parameters (inlet pressure and cavitation number) using fixed

solution volume of 6 liter and for different number of passes through the HC reactor.

The water effluent was first mixed thoroughly by circulating water effluent through

the bypass line for 1 minute and then the initial sample (250 mL) was taken. The

water effluent was then allowed to pass through the cavitating device and samples

(250 mL) were collected after each 10 passes through the cavitating device. The

samples were then analyzed for COD (chemical oxygen demand) and TOC (total

organic carbon). The operating inlet pressure was kept at 5 bar for circular venturi and

orifice plate and 3 bar for slit venturi. Here, the selection of operating pressure is

based on the optimization of these devices in terms of maximum cavitational effect

for the degradation of orange-G dye (section 4.3.4). The operating inlet pressure to the

cavitating device and corresponding volumetric flow rate through the cavitating

device and cavitation number are shown in Table 4.3.

Table 4.3: Operating parameters for different cavitating devices

Cavitating device

Operating pressure

Volumetric flow rate

Cavitation number (CV)

Circular venturi 5 410 0.15

Slit venturi 3 1060 0.29

Orifice plate 5 320 0.24

4.5.3 Result and discussion

The UASB O/L effluent was treated using three different cavitating namely,

circular venturi, slit venturi, and orifice plate. Table 4.4 shows the COD and TOC of

the UASB O/L sample values at different number of passes. It has been found that

around 47 % COD can be reduced using slit venturi as a cavitating device, whereas 25

and 8 % reduction in COD was achieved using circular venturi and orifice plate



respectively. It was also observed that most of the COD reduction was achieved in

first 10 passes only and afterward only slight changes in COD reduction was

observed. Similarly higher TOC reduction was obtained in the case of slit venturi.

Around 22 % reduction in TOC was obtained using slit venturi and almost 19 and 7 %

reduction in TOC was obtained using the circular venturi and orifice plate

respectively.

Table 4.4: COD and TOC reduction w.r.t number of passes for the UASB O/L effluent treated using HC

Cavitating device

Number of passes

COD (mg/L)

% Reduction in COD

TOC (mg/L)

% Reduction

in TOC

Slit venturi

Initial 628 – 116.22 10 390 37.89 101.08 13.02 20 345 45.06 – – 30 334 46.81 91.87 20.95 40 331 47.29 90.81 21.86

Circular venturi

Initial 628 – 116.22 – 10 507 19.27 110.24 5.14 20 495 21.18 – – 30 480 23.57 94.72 18.5 40 473 24.68 94.72 18.5

Orifice plate

Initial 628 – 116.22 – 10 628 0.00 116.22 0.0 20 593 5.57 – – 30 589 6.2 107.78 7.26 40 576 8.3 107.78 7.26

Figure 4.11 shows the cavitational yield (mg of COD reduced per unit energy

supplied) of different cavitating devices. The sample calculation is shown in the

appendix 4B. It was found that slit venturi gives higher cavitational yield as compared

to circular venturi and orifice plate. Cavitational yield for slit venturi is almost 6 times

higher than circular venturi and 20 times higher than orifice plate. These results are in

accordance with the previous results found out through the orange-G dye degradation

(section 4.3.5). Here, comparatively higher cavitational yield is obtained for the slit

venturi as against the cavitational yield obtained in the case of orange-G dye

degradation may be due to the fact that the effectiveness of the hydrodynamic



cavitation in degrading the organic compound also depends on the initial effluent

composition and pollutant concentration.

Figure 4.11: Cavitational yield of different cavitating devices for the UASB O/L effluent

In overall, it can be concluded that hydrodynamic cavitation can be effectively

utilized for the degradation of refractory pollutant molecule, which are hard to

degrade by conventional process and hence can be served as a treatment method for

such effluent stream on industrial scale and also as a pretreatment tool prior to the

treatment through conventional process so that the efficiency of treatment plant can be

improved.

4.5.4 Scaleup aspect

The scaleup of hydrodynamic cavitation reactor is purely based on the volume

of effluent to be treated. Whereas the selection of HC reactor of required cavitational

intensity depends on the cavity dynamics which in turn depends on many factor such

as cavity inception, cavity growth and collapse of cavity. Further, the cavitational

behaviour depends on the many geometrical parameters of a cavitating device such as

throat area, number of throat or ratio of throat perimeter to throat cross sectional area

0

5

10

15

20

25

30

35

40

45

slit venturi circular venturi orifice plate

Cav

itatio

nal y

ield

×10

4



and divergance and convergence angle of venturies and physico-chemical properties

of fluid to be treated. The cavitational phenomena inside a hydrodynamic cavitation

can be understood using the computational fluid dynamic code (CFD) depending

upon the geometrical and operating parameters of the hydrodynamic cavitation set-up

and physico–chemical properties of the chemical system under consideration. Once

the cavitating device of required intensity is decided based on the simulation and CFD

study, the size of the cavitating device can be constructed using the required

cavitation number for the desired physico-chemical transformation and volume of

effluent to be treated.



Appendix 4A

Sample calculation of cavitation number and cavitational yield for circular venturi at

optimized condition of 5 bar inlet pressure

Inlet fluid pressure = 601325 Pa

Downstream pressure (p2) = 101325 Pa

Vapor pressure of water at 30°C (pv) = 4242.14 Pa

Volumetric flow rate at 5 bar pressure (VO) = 410 LPH

VO = 1.14 × 10-4 m3/s

Area of flow (ao) = (π/4) × do2 (m2)

Where do is the diameter of the throat

Area of flow (ao) = (π/4) × (2 × 10-3) 2

= 3.14 × 10-6 m2

Velocity at the throat (vo) = VO/ao (m/s)

= (1.14 × 10-4) / (3.14 × 10-6)

= 36.30 m/s

Cavitation number (CV) = (P2 - Pv) / ( 1/2 ρvo2)

= (101325 - 4242.14) / (0.5 x 1000 x (36.3)2) = 0.15



Number of passes in 15 min = ((1.14 x 10-4) / (6 x 10-3)) x (15 x 60) ≈ 17

Power dissipated into the system (PD) = ΔP × VO (J/s)

= 5 × 105 × 1.14 × 10-4 = 57 J/s

Power input to the system (PI) = 1.1 kW = 1100 J/s

Power dissipated into the system / Power input to the system (PD / PI) = 57/1100

= 0.051

Amount of TOC reduced in 2 h = (mg of TOC reduced (mg/lit) ×

volume of solution (lit))

= 5 × 6 = 30 mg

Energy input to the system in 2 h = 1100 (J/s) × (2 ×3600) (s)

= 7.92 × 106 J

Cavitational yield = mg of TOC reduced / energy supplied

Cavitational yield = 30 / 7.92 × 106 = 3.78 × 10-6



Appendix 4B

Cavitating device = circular venturi

Operating gauge pressure = 5 bar

Inlet fluid pressure = 601325 Pa

Downstream pressure (p2) = 101325 Pa

Vapor pressure of water at 30°C (pv) = 4242.14 Pa

Volumetric flow rate at 5 bar pressure (VO) = 410 LPH

VO = 1.14 × 10-4 m3/s



Time for 20 passes = ((20 × (5.75 ×10-3)) / (1.14 × 10-4)

= 1008.77 sec = 16.81 min

Energy dissipated into the system (PD, J) = Pressure drop across the cavitating device

(ΔP) × Volumetric flow rate through the

cavitating device (VO) × Circulation

time through the device

= 5 × 105 × 1.14 × 10-4 × (16.81 ×60)

= 57490.2 J

Energy delivered to the system in 20 passes time (PI, J)

= 1.1 (kW) × 1000 × (16.81 ×60)

= 1109460 J

Cavitational yield = (mg of COD reduced per unit energy supplied)

Mg of COD reduced = (628 – 495) (mg/L) × 6 (L) = 798 mg in 20 passes

Cavitational yield = (798/1082400) = 7.19 × 10-4 (mg of COD reduced/J)



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