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Electro-Hydro-Dynamics Enhancement of Multi-phase Heat Transfer Thai Nguyen Faculty of Engineering (Mechanical) University of Technology, Sydney

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What is EHD? The application of Electric Fields to induce the fluid motion. Hence, Enhance Heat Transfer caused by disruption of boundary layer near heat transfer surface Pumping Action

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Why is EHD? Controllable Dielectric fluid Simplified implementation Localised cooling of complex curved passages Applicable in zero gravity

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Applications Air conditioning, refrigerant systems Electronic cooling Biomedical (alternative E, natural frequency) Cryogenic processing system Thermal control system

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Electric Fields in Pool Boiling Gravitational Field Electric Field Controllable On Earth: 1D, constant g In space: Absent Heat Transfer Enhancement by Heating Surface Treatment No boiling Active Heat Transfer Enhancement Complexity!? Electrode Design High Voltage

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Interactions among the fields in EHD Electric Field Flow Field Thermo Field Joule Heating Buoyancy Forced Convection Electric Force Density f e Temperature dependence on Electrical Conductivity, Permittivity and Mobility Convection Current Dielectrophoretic force Hydro-Dynamics

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Governing Equations of EHD Phenomena Conservation Equations Conservation Equations Momentum Equation Momentum Equation Equation of Continuity Equation of Continuity Energy Equation Energy Equation Equation of State Equation of State

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Governing Equations of EHD Phenomena Maxwell Equations Maxwell Equations Poisson’s Equation Poisson’s Equation Conservation of Electric Current Conservation of Electric Current Definition of Electric Current Definition of Electric Current Definition of Electric Potential Definition of Electric Potential Electric Force Density Electric Force Density

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Governing Equations of EHD Phenomena Charge Relaxation Equation Charge Relaxation Equation where, charge relaxation time:

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Research Stages Macroscopic Approach EHD Bubble Dynamics

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Macroscopic Analysis Quantitative Analysis - Modelling Quantitative Analysis - Modelling q” = C T a n b q” = C T a n b Variation of Heat transfer coefficient ratio: h ehd /h 0 with the Parameters: Variation of Heat transfer coefficient ratio: h ehd /h 0 with the Parameters: Heat Flux Heat Flux Electrode Voltage Electrode Voltage Electric field feature Electric field feature

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Experimental apparatus

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Test Rig Features Specific design for EHD study Specific design for EHD study Computational and digital recording data (Labview) Computational and digital recording data (Labview) Multi-temperature readings at diverse circumferential locations on the heating tube Multi-temperature readings at diverse circumferential locations on the heating tube

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Effects of Nonuniformity of E on Heat Transfer Coefficient Ratio 8-wire electrode16-wire electrode Nucleate Boiling Free Conv. Bubble Initiation

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Effects of Electrode Voltages on Heat Transfer Coefficient ratio 8-wire electrode 16-wire electrode

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Bubble Behaviour under EHD effects - 16 wire electrode 0kV 6kV 9kV 12kV Refrigerant R11, at atmospheric pressure Heat flux = 14.2kW/m2

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First Approach _ Conclusions Qualitative Analysis Qualitative Analysis Bubbles behave differently at diverse locations of the heating tube: Bubbles behave differently at diverse locations of the heating tube: * Coalescing of bubbles underneath the heating tube * Suppression of nucleate sites on the sides Quantitative Analysis Quantitative Analysis Heat transfer enhancement: large in natural convection region, decrease in nucleate region Heat transfer enhancement: large in natural convection region, decrease in nucleate region

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EHD Bubble Dynamics Analysis of bubble behaviour under the influence of electric fields Analysis of bubble behaviour under the influence of electric fields Bubble parameters: Bubble parameters: Frequency Frequency Deformation Deformation Number of nucleate site Number of nucleate site Bubble diameter Bubble diameter

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Experimental apparatus

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Electric field distribution -Kauss Analysis in Homogeneous media

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Images of Bubbles as at different Electrode Voltage - V(t) = mt Heat Flux = 30kW/m 2, Fluid Temperature = 22 0 C 0kV (No EHD)2.0kV 4.5kV 6.0kV6.6kV 8.0kV

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EHD effect on Bubble Deformation

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EHD effect on Bubble Diameter

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EHD effect on Nucleate Site Density

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EHD effect on Frequency of Bubble Departure

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EHD effect on Proportion of Latent heat to Total heat flux

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Second Approach - Conclusion Bubble Behaviour Bubble Behaviour Time Dependency Time Dependency Threshold Value Threshold Value Contribution of latent heat on total heat transfer in pool boiling Contribution of latent heat on total heat transfer in pool boiling

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Future Investigation Theoretical Theoretical Hysteresis effectHysteresis effect Time DependencyTime Dependency Frequency dependency of dielectric propertiesFrequency dependency of dielectric properties Mechanical oscillation of liquid-vapour interfaceMechanical oscillation of liquid-vapour interface Line of zero forceLine of zero force Electrolysis (DC)Electrolysis (DC)

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Future Investigation Experiment Experiment Design and build of power supplier with frequency variable (pulse wave) Design and build of power supplier with frequency variable (pulse wave) Measuring temperature of the wire Measuring temperature of the wire Development the test rig compatible with R123, aerospace fuel Development the test rig compatible with R123, aerospace fuel

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Time dependency in EHD Phenomena Charge relaxation time Charge relaxation time In general, reduce of , increasing of heat transfer enhancement Bubble frequency Bubble frequency Frequency of alternating field Frequency of alternating field

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Time dependency in EHD Phenomena - Dielectric theory Complex permitivity Complex permitivity

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Heating Wire - Electrode arrangement

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