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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.,
IITM 4
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.,
IITM 5
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.,
IITM 6
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.,
IITM 8
Gas-liquid mass transfer enhancement
11/22/2006Vinay Raman, Dept. of Chemical Engg.,
IITM 9
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.,
IITM 10
Homogenous Helmholtz’s equation
Wave equation:
Substituting
We get homogenous Helmholtz’s equation,
11/22/2006Vinay Raman, Dept. of Chemical Engg.,
IITM 11
Numerical Solution
Constraints:Node length
Boundary Conditions:Walls
Total reflection (soft boundary conditions)
Hard boundary conditions
11/22/2006Vinay Raman, Dept. of Chemical Engg.,
IITM 12
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.,
IITM 13
Implementation
2D plane sliced axially
Isotropic wave propagation
11/22/2006Vinay Raman, Dept. of Chemical Engg.,
IITM 14
Optimizing Geometry
11/22/2006Vinay Raman, Dept. of Chemical Engg.,
IITM 15
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.,
IITM 16
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.,
IITM 18
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