11/22/2006Vinay Raman, Dept. of Chemical Engg.,

IITM 1

Mapping Local Cavitation Events in High Intensity Ultrasound (HIU) fields

Vinay Raman, Ali Abbas, Sunil Chandrakant Joshi

Presenter: Vinay RamanIndian Institute of Technology Madras,Dept. of Chemical Engineering

Presented at the COMSOL Users Conference 2006 Bangalore

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

IITM 2

Overview

IntroductionCavitation and acoustic streaming

Chemical effectsAugmentation of chemical reaction rates

Physical effectsEnhanced natural convection heat transferEnhanced gas-liquid mass transferCavitation Erosion

Finite Element Modeling of HIU fieldsNumerical Solution and implementationIncorporating bubble dynamics Validation with experimental observations

Conclusions

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

IITM 3

Ultrasonic Cavitation

CavitationLocal pressure becoming lesser than vapor pressure of water creating cavities

Acoustically induced cavitation

Liquid ruptured apart in the rarefaction cycle

Creating cavitiesTransient bubble collapsesStable cavitation

Cloud of transient cavitation bubbles fromPlesset and Ellis (1955)

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

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Acoustic streaming

Time independent fluid motion generated by sound fieldTwo mechanisms

Spatial attenuation of wave in free spaceFriction between medium and solid wall

Laborde et al (2000)

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

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Chemical Effects

UltrasoundInitiates reactions

Bremmer (1986)Augments chemical reaction rates

Kristol et al (1981), Cum et al (1988), Lorimer and Mason (1980), Einhorn et al (1991), Javed et al (1995)Saudagar and Samant (1995), Lie Ken Jie (1995)Low (1995), Tuulmets et al (1995), Li et al (1996)

Changes reactions pathwaysDickens and Luche (1991), Ando et al (1984)

Applications in organic synthesisHomogenous and heterogeneous reactions

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

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Physical Effects

Enhanced natural convection heat transfer Relationship between heat transfer coefficient and acoustic streamingCavitation is a reinforcing effect

Enhanced gas-liquid mass transferHigher volumetric mass transfer coefficients at higher input ultrasonic power levels and at lower frequencies of operationCavitation is a dominant mechanism whereas acoustic streaming has only a reinforcing effect

Cavitation ErosionParticle comminution (Vinay Raman and Ali Abbas, 2006)

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

IITM 7

Enhanced heat transfer

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

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Gas-liquid mass transfer enhancement

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

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Finite Element Modeling: Overview

Solving homogenous Helmholtz’s equationSpatial distribution of acoustic pressure in the sono-chemical reactor

Cavitation bubble dynamics incorporatedCluster dynamics preferredCalculation of Collapse pressures

Discussions onEffect of ultrasonic frequencyEffect of input ultrasonic power

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

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Homogenous Helmholtz’s equation

Wave equation:

Substituting

We get homogenous Helmholtz’s equation,

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

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Numerical Solution

Constraints:Node length

Boundary Conditions:Walls

Total reflection (soft boundary conditions)

Hard boundary conditions

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

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Numerical Solution

Boundary ConditionsSides of Sonicator horn

Hard boundary conditionsSonicator tip

Constant pressure (= maximum pressure amplitude of acoustic wave)

Air water interfaceTotal reflection (soft boundary conditions)

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

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Implementation

2D plane sliced axially

Isotropic wave propagation

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

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Optimizing Geometry

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

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Incorporating Cavitation Bubble Dynamics

Cluster dynamics (P. M. Kanthale et al)Rayleigh Plesset equation (growth phase)

Pressure considerations

PA is the acoustic pressure distribution obtained by FEM modeling using COMSOL Multiphysics (3.2b)

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

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Incorporating Cavitation Bubble Dynamics

Void volume of cluster

Rayleigh Plesset equation (collapse phase)

Incorporating appropriate initial conditions, equations solved in MATLAB, and collapse pressure is obtained as follows:

Termination of simulations Cluster reaches 10 % of initial sizeSurface – instability criteria (Sudarkodi and Kannan)

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

IITM 17

Validation with experimental results

0

50

100

150

200

250

300

350

400

450

0 20 40 60 80 100 120

% Power Level

Cav

itatio

n In

tens

ity (W

/sq.

in)

Local cavitation intensity using hydrophone measurementsApproximately 25% deviation from experimental and numerical solutionLimitations in the modelExperimental inaccuracies

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

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Conclusions

Homogenous Helmholtz’s equation solvedAcoustic pressure field is obtainedSinusoidal variation as opposed to exponential decay obtained by others (V. Saez et al, 2006)

This is further incorporated in bubble dynamics equation to arrive at collapse pressureExperimentally validated with hydrophone measurementsMismatch due to limitations in the cavity cluster approachFuture work

Incorporating single bubble theory with Gaussian size distribution of bubbles

11/22/2006Vinay Raman, Dept. of Chemical Engg.,

IITM 19

Self - References

“Gas-Liquid Mass Transfer Enhancement using Power Ultrasound”Proceedings - 17th International Symposium on Transport Phenomena, Toyama, Japan (September 6th, 2006)Vinay R., Vetrimurugan R., Nagarajan R“Experimental Studies on Heat Transfer Augmentation in High Intensity, Multi-frequency Ultrasonic Fields”Proceedings - 17th International Symposium on Transport Phenomena, Toyama, Japan (September 6th, 2006)Vetrimurugan R., Vinay R., Nagarajan R“Experimental Investigations on ultrasound mediated particle breakage”Ultrasonics Sonochemistry (ACCEPTED on NOVEMBER 6th, 2006, Ref No: 06J56)Vinay Raman, Ali Abbas“Particle Grinding by High Intensity Ultrasound: Kinetic modeling and identification of breakage mechanisms”(Submitted to Chemical Engineering Journal) (Review in progress, Ref No: CEJ-D-06-00635)Vinay Raman, Ali Abbas, Wentin Zhu“Finite Element Modeling studies on linear acoustic pressure fields in sonochemical reactors”(Manuscript in preparation)Vinay Raman, Ali Abbas, Sunil Chandrakant Joshi, Kwang Sang Kyu