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

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

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

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

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

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

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

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

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

Research Stages Macroscopic Approach EHD Bubble Dynamics

Macroscopic Analysis Quantitative Analysis - Modelling q” = CDTanb Variation of Heat transfer coefficient ratio: hehd/h0 with the Parameters: Heat Flux Electrode Voltage Electric field feature

Experimental apparatus

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

Effects of Nonuniformity of E on Heat Transfer Coefficient Ratio 8-wire electrode 16-wire electrode Nucleate Boiling Nucleate Boiling Free Conv. Bubble Initiation

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

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

First Approach _ Conclusions Qualitative Analysis 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 Heat transfer enhancement: large in natural convection region, decrease in nucleate region

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

Experimental apparatus

Electric field distribution -Kauss Analysis in Homogeneous media

Images of Bubbles as at different Electrode Voltage - V(t) = mt Heat Flux = 30kW/m2, Fluid Temperature = 220C 0kV (No EHD) 2.0kV 4.5kV 8.0kV 6.0kV 6.6kV

EHD effect on Bubble Deformation

EHD effect on Bubble Diameter

EHD effect on Nucleate Site Density

EHD effect on Frequency of Bubble Departure

EHD effect on Proportion of Latent heat to Total heat flux

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

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

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

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

Time dependency in EHD Phenomena - Dielectric theory Complex permitivity

Heating Wire - Electrode arrangement