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Ultrasonics Sonochemistry 12 (2005) 21–27

A review and assessment of hydrodynamic cavitationas a technology for the future

Parag R. Gogate, Aniruddha B. Pandit *

Chemical Engineering Division, Institute of Chemical Technology, University of Mumbai, Matunga, Mumbai 400 019, India

Received 23 January 2004; accepted 24 March 2004

Available online 2 September 2004

Abstract

In the present work, the current status of the hydrodynamic cavitation reactors has been reviewed discussing the bubble dynamics

analysis, optimum design considerations, design correlations for cavitational intensity (in terms of collapse pressure)/cavitational

yield and different successful chemical synthesis applications clearly illustrating the utility of these types of reactors. The theoretical

discussion based on the modeling of the bubble dynamics equations aims at understanding the design information related to the

dependency of the cavitational intensity on the operating parameters and recommendations have been made for the choice of

the optimized conditions of operating parameters. The design information based on the theoretical analysis has also been supported

with some experimental illustrations concentrating on the chemical synthesis applications. Assessment of the hydrodynamic cavita-

tion reactors and comparison with the sonochemical reactors has been done by citing the different industrially important reactions

(oxidation of toluene, o-xylene, m-xylene, p-xylene, mesitylene, o-nitrotoluene, p-nitrotoluene, m-nitrotoluene, o-chlorotoluene and

p-chlorotoulene, and trans-esterification reaction i.e., synthesis of bio-diesel). Some recommendations have also been made for the

future work to be carried out as well as the choice of the operating conditions for realizing the dream of industrial scale applications

of the cavitational reactors.

� 2004 Elsevier B.V. All rights reserved.

Keywords: Hydrodynamic cavitation; Chemical processing; Sonochemistry; Cavitational yields; Scale-up

1. Introduction

Hydrodynamic cavitation can simply be generated by

the passage of the liquid through a constriction such as

throttling valve, orifice plate, venturi etc. When the liq-uid passes through the constriction, the kinetic energy/

velocity of the liquid increases at the expense of the pres-

sure. If the throttling is sufficient to cause the pressure

around the point of vena contracta to fall below the

threshold pressure for cavitation (usually vapor pressure

of the medium at the operating temperature), millions of

cavities are generated. Subsequently as the liquid jet ex-

1350-4177/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.ultsonch.2004.03.007

* Corresponding author. Tel.: +91 22 2414 5616; fax: +91 22 2414

5614.

E-mail address: abp@udct.org (A.B. Pandit).

pands and the pressure recovers, the cavities collapse.

During the passage of the liquid through the constric-

tion, boundary layer separation occurs and substantial

amount of energy is lost in the form of permanent pres-

sure drop. Very high intensity fluid turbulence is thuspresent downstream of the constriction; its intensity de-

pends on the magnitude of the pressure drop, which, in

turn, depends on the geometry of the constriction and

the flow conditions of the liquid i.e., the scale of turbu-

lence. A careful design of the system allows generating

cavity collapse conditions similar to acoustic cavitation

thereby enabling different applications requiring differ-

ent cavitational intensities, which have been successfullycarried out using acoustic cavitation phenomena but at

much lower energy inputs as compared to sonochemical

reactors. We now look into the different aspects related

22 P.R. Gogate, A.B. Pandit / Ultrasonics Sonochemistry 12 (2005) 21–27

to the hydrodynamic cavitation reactors concentrating

on the recent studies whereas the work of Gogate and

Pandit [1] can be referred for the basic concepts and

the overview of the earlier literature.

2. Engineering design of hydrodynamic cavitation reactors

2.1. Bubble dynamics studies

The bubble behavior and hence the pressure pulse

generated at the collapse of the cavity in the case of

hydrodynamic cavitation depends upon the operating

conditions as well as the geometry of the mechanicalconstriction which results in the generation of cavitating

conditions downstream of the orifice. Thus, as a first

step towards the design of hydrodynamic cavitation

reactors, it is important to understand the relationship

between the cavity behavior and the operating parame-

ters and possibly quantify the intensity of cavitation and

then the net cavitational effects as a direct function of

the operating parameters. The following sections dealwith this understanding starting from a basic single cav-

ity approach to a more realistic cluster approach (the de-

tails about the modeling aspects and the simulation

strategies have not been mentioned here but can be ob-

tained from earlier works).

2.1.1. Single cavity approach

Gogate and Pandit [2] numerically investigated theeffect of the operating parameters such as inlet pressure

through the system of orifice, initial cavity size and the

indirect effect of the diameter of the hole on the orifice

plate (it affects the frequency of turbulence in the vicin-

ity of the orifice) on the cavity behavior. The simula-

tion of the bubble dynamics has been done in two

stages, first considering Rayleigh–Plesset equation up

to the point of bubble wall velocity=1500 m/s andthen the compressibility of medium has been consid-

ered (the details of the modeling and solution proce-

dure can be obtained directly from the work of

Gogate and Pandit [2]). Based on the results of the si-

mulations and subsequent comparison with the experi-

mental results (It has also been established with

experiments on decomposition of potassium iodide in

hydrodynamic cavitation reactors [3] that the numeri-cal trends in the magnitudes of collapse pressure match

the observed experimental trends.) following design

considerations for achieving maximum benefits have

been established:

(1) Inlet pressure into the system/rotor speed depend-

ing on the type of equipment: use increased pressures or

rotor speed but avoid super-cavitation by operating

below a certain optimum value. (Some guidelines aboutthe onset of super-cavitation phenomena and the

dependency of the critical operating pressure on the sys-

tem parameters can be obtained from the work of Yan

and Thorpe [4].) It has been observed in the case of

experiments with KI decomposition that the cavitational

yields decreases when the inlet pressure is increased be-

yond 40 psig (value is specific for the configuration

and reaction as described in the work of Vichare et al.[3]).

(2) Physicochemical properties of the liquid and ini-

tial radius of the nuclei: it is important to have lower ini-

tial sizes of the cavitation nuclei in the reactor and the

liquid phase physicochemical properties can be suitably

adjusted [5] (use liquids with low vapor pressure, viscos-

ity and higher surface tension so as to get violent col-

lapse of the cavities).(3) Diameter of the constriction used for generation

of cavities e.g., hole on the orifice plate: optimization

needs to be carried out depending on the application.

Higher diameters are recommended for applications

which require intense cavitation e.g., degradation of

complex chemicals such as Rhodamine B [6], whereas

lower diameters with large number of holes should be

selected for applications with reduced intensity e.g., KIdecomposition [3].

(4) Percentage free area offered for the flow (Ratio of

the free area available for the flow i.e., total cross-sec-

tional area of holes on the orifice plate to the total

cross-sectional area of the pipe.): lower free areas must

be used for producing high intensities as the rate and

magnitude of pressure recovery affects the dynamics

and the collapse of the cavities and hence the desiredbeneficial effects. Experiments with iodine liberation [3]

have indicated that an increase in the free area from

around 25 mm2 to 140 mm2 decreases the cavitational

yield by one order of magnitude.

An empirical correlation developed by Gogate and

Pandit [2] for the prediction of the collapse pressure

generated as a function of the above-mentioned param-

eters is worth mentioning here as it forms the basic steptowards the development of generalized design equa-

tions.

P collapse ¼ 7527ðF Þ�2:55 � fðP 1Þ2:46ðR0Þ�0:80ðdoÞ2:37g

The above correlation uses the initial cavity size in

units of mm, inlet pressure in units of atmospheres,the diameter of the hole in the orifice plate in units of

mm and free area as percentage of the total cross-

sectional area of the pipe, while the collapse pressure

is given in terms of atmospheres and is developed over

the following range of operating parameters.

Initial cavity size (R0)=0.01–0.1 mm (The initial cav-

ity size formed (maximum) can be estimated from the

critical Weber number concept at the operating orificejet velocities. The range of liquid velocities up to a limit

of super-cavitation suggest this range of the initial cavity

sizes.)

P.R. Gogate, A.B. Pandit / Ultrasonics Sonochemistry 12 (2005) 21–27 23

Inlet pressure (PI)=1–8 atmospheres.

Diameter of the orifice (do)=1–3 mm.

Percentage free area of the holes (F)=1–20% of the pipe

cross-sectional area.

2.1.2. Improvement in the bubble dynamics model consid-

ering the bubble/bubble and bubble/flow interactions

The earlier model based on a single cavity does not

consider the interactions of the cavitating bubble with

the neighboring bubbles as well as the changing nature

of the liquid flow. To attain a more realistic picture of

the cavitating conditions, Moholkar and Pandit [7]

modified the model to consider these interactions for aventuri type reactor and the effect of operating parame-

ters such as downstream pressure recovery, venturi to

pipe area ratio, initial bubble void fraction in the liquid

and the initial bubble size in the flow on the dynamics of

the flow has been studied. Based on the results obtained

in the simulations following recommendations for the

efficient design of hydrodynamic cavitation reactor

(where mechanical constriction is venturi type) can bemade:

(1) A rise in the discharge (inlet) pressure and hence

the final recovery pressure results in reduced bubble

cluster life and hence a decrease in the cavitationally ac-

tive volume downstream of the throat of the venturi

where the cavitational effects can be observed though

the cavitation intensity in this reduced volume repre-

sented by the magnitude of the collapse pressure pulseis higher. This phenomenon can be used to facilitate

more stubborn chemical reactions.

(2) Manipulation of the venturi to pipe area ratio

offers a good control over the cavitational activity. An

increase in the venturi to pipe area ratio increases the ac-

tive volume of the hydrodynamic cavitation reactor.

(3) The contribution from the bubbles with smaller

initial sizes to the overall cavitational effect is higherthan the contribution from the bubbles with higher ini-

tial sizes. Although there is a wide variation in the initial

sizes of the bubbles that are generated downstream of

the throat of the venturi, a partial control over the initial

bubble size to produce lower initial size of the bubbles

can be obtained by the exposure of the flow at the throat

of the venturi to ultrasonic irradiation.

2.1.3. Simulations on the basis of liquid continuum

mixture model

It is also important to make some recommendations

for the selection of particular type of reactor desirable

for the specific application. Moholkar and Pandit [8]

have tried to address this situation and investigated

the comparative effect of several operating parameters

on the bubble motion in the cavitating flow in two differ-ent flow geometries: a venturi tube and an orifice plate.

In the case of a venturi tube, a stable oscillatory radial

bubble motion is obtained due to a linear pressure

recovery gradient whereas due to an additional oscillat-

ing pressure gradient due to turbulent velocity fluctua-

tion, the radial bubble motion in the case of an orifice

flow results in a combination of both stable and oscilla-

tory type behaviour. Thus the intensity of cavitation willbe higher in the case of an orifice system as compared to

the classical venturi tube. The simulations have enabled

us to establish definite trends in the cavitation intensity

produced in the hydrodynamic cavitation reactor with

operating/design parameters, which can form a basis

for the optimization of the hydrodynamic cavitation

reactors. The model also enables the quantification of

the magnitudes of the temperature and pressure pulsesfor a given set of design parameters. Following impor-

tant strategies for the design of hydrodynamic cavitation

reactors have been established:

(1) An orifice flow configuration is necessary only for

intense chemical reactions whereas for milder processes(requiring pressure typically between 15 and 20 bar)

and for physical effects, a venturi configuration is more

suitable and energy efficient.

(2) In the case of a venturi flow, the most economical

technique for increasing the cavitation intensity would

be to reduce the length of venturi but for higher volu-

metric liquid flow rates there could be a limitation due

to the possibility of flow instability and super-cavitation.A similar argument can be given for the enhancement in

the cavitation intensity by reducing the venturi throat to

pipe diameter ratio.

(3) In the case of an orifice flow configuration, the

most convenient way of controlling the cavitation inten-

sity will be to control the orifice to pipe diameter ratio

and to control the cross-sectional flow area through

the manipulation of the number and diameter of thehole on the orifice plate but indiscriminate growth of

the bubbles downstream of the orifice can lead to splash-

ing and vaporization (super-cavitation) of the flow.

(4) Increasing the pipe size, downstream of the orifice

(which offers a faster pressure recovery) is another op-

tion to intensify the cavitation effects but using pipes

of higher size would mean handling higher volumetric

liquid flow rates (in order to carry out operation at samecavitation number) and this may increase the fixed cost

of the processing.

2.1.4. Cluster approach

Recent studies [9] have indicated that a single cavity is

also influenced by the oscillations of the surroundingcavities and the observed effects are probably due to

the combined effect of the collapse of the several cavities

i.e., as a cluster. Thus, consideration of a cluster where

the collapse of a cavity is influenced by the dynamics

of the neighboring cavities with some transfer of energy,

possibly gives a more realistic picture. Based on the

24 P.R. Gogate, A.B. Pandit / Ultrasonics Sonochemistry 12 (2005) 21–27

numerical simulations of the cluster model developed in

their work, following design correlations for the cavita-

tional intensity in terms of the collapse pressure as well

as for the active zone of cavitation (over which the cav-

ity collapse pressure is greater than or equal to the

threshold pressure i.e., capable of inducing a chemicalreaction) can be given:

PC ¼ 0:3023ðP 2Þ0:972ðr0Þ�0:714ðdoÞ0:539ðcÞ0:9316ðr=r0Þ�2:604

V W ¼ 2:0395P�1:2929s ðP 2Þ1:2195 � ðr0Þ2:2229 � ðdoÞ0:5925

� ðc0:1251Þwhere, Ps is the starting pressure (threshold pressure) of

the active volume of cavitation depending on the chem-

ical transformation to be carried out hydrodynamically.

The above correlations are developed over the followingrange of parameters (observed generally in the litera-

ture):

recovered pressure (P2)=1–5 atm

initial cluster radius (r0)=0.001–0.02 m

diameter of an orifice (do)=0.015–0.03 m

fraction of energy transfer (c)=0.25–0.50

initial individual cavity size within the cluster (r)=1 lmr/r0=0.1–0.9

The approach of cluster dynamics appears to be more

logical as compared to the single cavity approach

though the exact quantitative comparison between the

predictions of the cluster dynamics and single cavity

(in this case, the total cavitational intensity will be num-

ber of cavities multiplied by the pressure pulse generateddue to the collapse of a single cavity) approach needs to

be done so as to make firm recommendations. A draw-

back of the cluster approach, however, is that there is no

information on the number of clusters generated in the

system, which will be needed to quantify the total cavi-

tational intensity in the reactor.

It should be noted that the developed correlations,

unique of its kind are only trend setting and by nomeans, generalized one. There is ample scope for further

work, which may include establishing the validity of the

equations over a wider range of operating parameters

(with experiments in different reactors with varying

power dissipation and/or frequency of fluid turbulence),

exactly quantifying the number of free radicals gener-

ated during the collapse (more importantly estimating

the actual number taking part in the reaction, by consid-ering the number of free radicals generated, time of col-

lapse of the cavities and the lifetime of the free radical;

also the relative rates of reaction of free radicals with

the reactants and recombination reaction of free radi-

cals), estimating the size of the nuclei/cavity which is

dependent on the type of equipment used for the gener-

ation of cavitation (using techniques such as phase Dop-

pler as well as theoretical approach based on the

thermodynamic analysis considering the lattice structure

and the intermolecular distance of the cavitating

medium), measuring the total bubble activity/transient

bubble activity in different grades of violence i.e., cavita-

tional activity (cavitation indicator IC-3) [10], combin-

ing the effects of collapse pressure generated and themaximum bubble size reached etc. There is indeed a

need for the development of these engineering equations

for different systems and the present work proposes a

methodology in that direction.

2.2. Development of correlations for cavitational yield

A second step in the engineering design protocol is todevelop design equations for the quantification of the net

cavitational effects.We now highlight a simple methodol-

ogy in terms of cavitational yield (quantification of chem-

ical effects) for a model reaction. Gogate et al. [11] have

used Weissler reaction for the estimation of the cavita-

tional yield in different hydrodynamic cavitation reactors

(orifice plate setup with plates of different geometries,

time of operation=60 min, KI concentration=1%, oper-ating temperature of 30±2 �C, Cv in the range of 0.156–

0.87 depending on the plate) as well as for the high-speed

homogeniser (operating speed of rotation=12,000 rpm).

These operating conditions were then used in the simula-

tions of bubble dynamics equations andmagnitude of the

collapse pressure pulse was predicted using a single cavity

approach. The developed equation for the prediction of

cavitational yield can be given as

Cavitational yield ¼ 8:834� 10�11ðP collapseÞ1:1633

The above equation requires the collapse pressure

values in terms of atmospheres whereas the cavitational

yield value is given in units of g/(J/ml). It should be

noted here that the equation is only a starting point inestablishing the design strategies for hydrodynamic cav-

itation reactors and valid for the specific reaction con-

sidered in the work. Similar equations need to be

developed for different class of chemical reactions based

on the methodology depicted above.

These design equations (for cavitational yield as well

as collapse pressure) helps in the selection of set of oper-

ating parameters for achieving the desired objectives in agiven hydrodynamic cavitation setup.

2.3. Optimization of hydrodynamic cavitation reactor

based on the experimental studies

It is always necessary to verify the results obtained

with the theoretical approach using experimental inves-

tigations. We now highlight the results obtained for theoptimization studies [12] for the reaction of oxidation of

toluene using aqueous KMnO4. The hydrodynamic cav-

itation reactor used in the work (Fig. 1) consists of a

Fig. 1. Schematic representation for experimental setup for the orifice

plate hydrodynamic cavitation reactor.

P.R. Gogate, A.B. Pandit / Ultrasonics Sonochemistry 12 (2005) 21–27 25

reservoir or a collecting tank with 10 l capacity that is

connected to the pump (rating of 1.5 kW) which allowsre-circulation of the contents through a mainline hous-

ing an orifice plate (different configurations of the holes

or the orifice plates can be inserted in the main line) and

a bypass line (for controlling the inlet pressure and the

flow rate into the cavitation chamber). A cooling water

jacket is also provided to the reservoir for controlling

the temperature of the system as the temperature rises

due to the mechanical heat dissipation into the systemand higher operating temperature are detrimental for

Table 1

Comparative results for different industrially important reactions in hydrody

No. Reactants Productc C

ca

1 Toluene Benzoic acid 3.

2 p-Xylene Terephthalic acid 2.

3 o-Xylene Phthalic acid 1.

4 m-Xylened Isophthalic acid 1.

5 Mesitylene Trimesic acid 7

6 o-Nitrotoluene o-Nitrobenzoic acid 1.

7 m-Nitrotoluene m-Nitrobenzoic acid 1.

8 p-Nitrotoluenee p-Nitrobenzoic acid –

9 o-Chlorotoluene o-Chlorobenzoic acid 1.

10 p-Chlorotoluenef p-Chlorobenzoic acid 2

11 Sunflower oil Bio-diesel (methyl ester of sunflower oil) 2.

*In moles/J.a Toluene (1 mol), (o-/p-/m)-xylene (0.5 mol), mesitylene (0.4 mol), (o-/m

(1 mol) with excess of methanol, KMnO4 for reactions 1–10 (2 mol), and f

1 time=5 h, except for oxidation of toluene where it is 3 h and trans-esterifib Toluene (10 mmol), (o-/p)-xylene (5 mmol), mesitylene (4 mmol), (o-/m-/p

(1 mol) with excess of methanol, KMnO4 for reactions 1–10 (20 mmol), and

toluene where it is 3 h and trans-esterification where it is 15 min.c Identification of compounds was done by TLC and melting point.d Not used in the acoustic cavitation.e This compound is not used in hydrodynamic cavitation.f Not used in the acoustic cavitation.

the cavitational reactions (for reaction of synthesis of

bio-diesel, however, lower viscosity of the oil has been

found to be beneficial). The operating parameters used

in the experimental work for the optimization exercise

include inlet pressure (1–4 kg/cm2), concentration of

the controlling reactant species (oxidant i.e., KMnO4

in this case with concentration varied in the range 0.2–

0.5 mol/l per mol of the toluene), type of the orifice plate

(without orifice plate, two different geometries). The op-

timized conditions as obtained in the study are inlet

pressure of 3 kg/cm2, 0.4 mol/l of the oxidant (beyond

these values, the increase in the cavitational yield is only

marginal) and orifice plate with more number of holes

(cavitational yield is maximum in all the orifice geome-tries considered in the work). The results regarding the

variation of the inlet pressure and the geometry of the

orifice plate indeed confirm the earlier findings obtained

using the theoretical analysis.

3. Comparison of cavitational yields in acoustic and

hydrodynamic cavitation

Different chemical reactions (oxidation of toluene,

(o-/p-/m)-xylenes, mesitylene, (o-/m)-nitrotoluenes and

(o-/p)-chlorotoluenes and trans-esterification reaction)

have been carried out in 10 l capacity orifice plate hydro-

dynamic cavitation reactor (under optimized conditions

as illustrated earlier; the details of the experimental

procedure and the reactor details have been already dis-cussed in the earlier work [13]) and also in conventional

sonochemical reactor (ultrasonic bath, reaction volume

namica and acousticb cavitation reactors

avitational yield in hydrodynamic

vitation reactor (gm/J)

Cavitational yield in acoustic

cavitation reactor (gm/J)

3·10�6 5.6·10�7

1·10�6 3·10�7

9·10�6 3·10�7

9·10�6 –

·10�6 1·10�7

9·10�6 1·10�7

3·10�6 1·10�7

3·10�7

1·10�6 1·10�7

·10�6 –

1·10�6* 5.1·10�7*

)-nitrotoluene (1 mol) and (o-/p)-chlorotoluene (1 mol), sunflower oil

or all above reactions, water (5 l), pressure 3 kg/cm2, orifice plate no.

cation where it is 30 min.

)-nitrotoluene (10 mmol) and o-chlorotoluene (10 mmol), sunflower oil

for all above reactions, water (50 ml) time=5 h except for oxidation of

26 P.R. Gogate, A.B. Pandit / Ultrasonics Sonochemistry 12 (2005) 21–27

of 55 ml, power dissipation of 120 W and operating

frequency of 20 kHz). For the comparison purpose, cav-

itational yield has been used which is defined as the

quantity of product formed per unit of supplied energy.

Table 1 shows the values of cavitational yields obtained

for all the reactions in hydrodynamic and acoustic cav-itation reactors (the specific operating conditions are

mentioned in the footnote for the table). It can be clearly

seen from the table that the cavitational yield values in

the hydrodynamic cavitation reactors are order of mag-

nitude higher for all the reactions considered in the work

and also the processing volume is about 100 times more

as compared to the conventional sonochemical reactor.

The results have conclusively proved the better efficacyof the hydrodynamic cavitation reactors as compared

to the ultrasonic bath reactor considered in the work.

Similar results have been also obtained in some of the

earlier literature reports (degradation of potassium io-

dide [11], destruction of p-nitrophenol [14] and micro-

bial cell disruption [15]).

4. Conclusions and recommendations for future work

The present work based on the theoretical bubble

dynamics analysis as well as experimental analysis

regarding the effect of different operating parameters

has enabled us to make the following useful guidelines/

recommendations for the design of different hydrody-

namic cavitation reactors viz. orifice plate setup, venturi,high speed and high pressure homogenizers:

(1) An orifice flow configuration is necessary only for

intense chemical reactions whereas for milder processes

(typically between 15 and 20 bar) and physical transfor-

mations, a venturi configuration is recommended.(2) Select higher operating inlet pressures upstream of

the mechanical constriction but just below the onset of

super-cavitation.

(3) Select optimum combination of liquid physical

properties so as to enable easy generation of lower initial

size nuclei as discussed earlier.

(4) For orifice plate setup, optimize the number and

diameter of the holes for same free flow area based onthe type of applications; smaller number of large dia-

meter for applications which require higher cavitation

intensity e.g., destruction of complex chemicals and

higher number of smaller diameter for applications

requiring relatively lower intensities e.g., cell disruption.

(5) Design the hydrodynamic cavitation reactor with

lower free area for the flow.

(6) Choose speed of rotation in the case of high speedhomogenizers much above the critical speed for the cav-

itation inception and below the speed where air induc-

tion takes place.

The present work has also enabled us to conclusively

establish the efficacy of the hydrodynamic cavitation

reactors as compared to the acoustic counterparts at

least for the different chemical processing applications

discussed in this work. It can be said that the hydrody-

namic cavitation reactors offer immediate and realistic

potential for industrial scale applications as compared

to the sonochemical reactors and the scale up of thesereactors is comparatively easier as vast amount of infor-

mation about the fluid dynamics downstream of the

constriction is readily available and the operating effi-

ciency of the circulating pumps which is the only energy

dissipating device in the system is always higher at large

scales of operation. The following studies are required to

achieve this goal:

(1) Realistic modeling of the turbulence phenomenawhich then can be used to model the cavity/bubble

dynamics either in isolation or in the form of cavity

clusters in high velocity flows. The modern sophisti-

cated CFD codes can be employed to get the flow field

information i.e., mean and fluctuating velocity compo-

nents, Reynolds stresses, turbulent pressure fluctua-

tions, which can then be used to understand the

role of these flow field parameters in altering cavitydynamics.

(2) Development of generalized design equations also

provide a link between the bubble dynamics and net

cavitational effects. The design correlations for cavita-

tional yield and cavitational intensity should be made

generalized and applicable for a variety of applications

as well as reactors.

(3) It is necessary to develop user friendly computercodes (similar to modern CFD codes) for the use of

Engineers, which will allow them to change the geomet-

rical and operating parameters of the hydrodynamic

cavitation set-up, define physico-chemical properties

of the chemical system under consideration. These

codes, with the help of bubble/cavity dynamics and

the equilibrium chemistry at cavity collapse conditions,

will then predict the expected chemical effects avoidingtrial and error type of experimentation for the engi-

neers.

(4) Design and fabrication of different types of hydro-

dynamic cavitation set-ups differing in flow field, turbu-

lence characteristics and geometry to study the effect of

these on cavity/bubble/cluster dynamics. Sophisticated

measuring techniques such as Laser Doppler Anemo-

metry, hydrophones, Cavity luminescence spectralmeasurement need to be adapted for fast flowing cavi-

ties/clusters to be able to measure the magnitudes of cav-

ity/cluster oscillations/collapse pressure pulse and

temperature generated and the identification of the

intermediate chemical species along with their concen-

trations.

(5) Experiments are needed on different scales of

operation to understand and address the scale up issues,such as alteration in the flow field and turbulence char-

acteristics due to the scale of operation.

P.R. Gogate, A.B. Pandit / Ultrasonics Sonochemistry 12 (2005) 21–27 27

Acknowledgment

Authors would like to acknowledge the funding of

the Indo-French Center for Promotion of Advanced Re-

search (Centre Franco-Indien Pour La Promotion de La

Recherche Avancee), New Delhi, India for the collabo-rative research work.

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