Electro-Hydro-Electro-Hydro-Dynamics Dynamics

Enhancement of Enhancement of Multi-phase Heat Multi-phase Heat

TransferTransferThai NguyenThai Nguyen

Faculty of Engineering Faculty of Engineering (Mechanical)(Mechanical)

University of Technology, University of Technology, SydneySydney

What is EHD?What is EHD?

The application of Electric Fields to The application of Electric Fields to induce the fluid motion. Hence,induce the fluid motion. Hence, Enhance Heat Transfer caused by Enhance Heat Transfer caused by

disruption of boundary layer near disruption of boundary layer near heat transfer surfaceheat transfer surface

Pumping ActionPumping Action

Why is EHD?Why is EHD?

ControllableControllable Dielectric fluidDielectric fluid Simplified implementationSimplified implementation Localised cooling of complex curved Localised cooling of complex curved

passagespassages Applicable in zero gravityApplicable in zero gravity

ApplicationsApplications

Air conditioning, refrigerant systemsAir conditioning, refrigerant systems Electronic coolingElectronic cooling Biomedical (alternative E, natural Biomedical (alternative E, natural

frequency)frequency) Cryogenic processing systemCryogenic processing system Thermal control systemThermal control system

Electric Fields in Pool Electric Fields in Pool BoilingBoiling

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

Interactions among the Interactions among the fields in EHDfields in EHD

Electric Field

Flow Field Thermo Field

Joule Heating

Buoyancy

Forced Convection

Elec

tric

Forc

e D

ensi

ty f e

Temperature dependence on Electrical Conductivity, Permittivity and Mobility

Con

vect

ion

Cur

rent

Dielectrophoretic force

Hydro-Dynamics

Governing Equations of Governing Equations of EHD Phenomena EHD Phenomena

Conservation EquationsConservation Equations

Momentum EquationMomentum Equation

Equation of ContinuityEquation of Continuity

Energy EquationEnergy Equation

Equation of StateEquation of State

0. u

ufgpuut

ue

2)(

TkTuCt

TC pp

2).(

M

RTp

Governing Equations of Governing Equations of EHD PhenomenaEHD Phenomena

Maxwell EquationsMaxwell Equations

Poisson’s EquationPoisson’s Equation

Conservation of Electric CurrentConservation of Electric Current

Definition of Electric CurrentDefinition of Electric Current

Definition of Electric PotentialDefinition of Electric Potential

Electric Force DensityElectric Force Density

eE

.

0.

ite

Eui ee

E

22

2

1

2

1EEEf ee

Governing Equations of Governing Equations of EHD PhenomenaEHD Phenomena

Charge Relaxation EquationCharge Relaxation Equation

0

e

evt

where, charge relaxation time:

f

1

Research StagesResearch Stages

Macroscopic ApproachMacroscopic Approach EHD Bubble DynamicsEHD Bubble Dynamics

Macroscopic AnalysisMacroscopic Analysis

Quantitative Analysis - ModellingQuantitative Analysis - Modelling q” = Cq” = CTTaannbb

Variation of Heat transfer coefficient Variation of Heat transfer coefficient ratio: hratio: hehdehd/h/h0 0 withwith the Parameters: the Parameters: Heat FluxHeat Flux Electrode VoltageElectrode Voltage Electric field featureElectric field feature

Experimental apparatusExperimental apparatus

Test Rig FeaturesTest Rig Features

Specific design for EHD studySpecific design for EHD study Computational and digital recording Computational and digital recording

data (Labview)data (Labview) Multi-temperature readings at Multi-temperature readings at

diverse circumferential locations on diverse circumferential locations on the heating tubethe heating tube

Effects of Nonuniformity of E on Heat Effects of Nonuniformity of E on Heat Transfer Coefficient RatioTransfer Coefficient Ratio

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

0 10000 20000 30000 40000 50000 60000

Heat Flux (W/m2)

he/h

0

3kV

6kV

9kV

12kV

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

0 10000 20000 30000 40000 50000 60000Heat Flux (W/m

2)

he/h

0

3kV

6kV

9kV

12kV

8-wire electrode 16-wire electrode

Nucleate Boiling

Nucleate Boiling

Fre

e C

onv.

Bubble Initiation

Effects of Electrode Voltages on Heat Effects of Electrode Voltages on Heat Transfer Coefficient ratioTransfer Coefficient ratio

G = 0.0717Ve + 1

G = 0.0062Ve + 1

G = 0.0019Ve + 10.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

0 2 4 6 8 10 12 14

Electrode Voltage (kV)

he/h

0

Free Convection,

Isolated Boiling,

Jet Boiling,

0.6kW/m2

5.1kW/m2

27.9kW/m2

G = 0.1514Ve + 1

G = 0.0062Ve + 1

G = 0.0019Ve + 1

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

0 2 4 6 8 10 12 14

Electrode Voltage (kV)

he/

h0

Free Convection,

Isolated Boiling,

Jet Boiling, q" =

0.6kW/m2

5.1kW/m2

27.9kW/m2

8-wire electrode 16-wire electrode

Bubble Behaviour under EHD effects - Bubble Behaviour under EHD effects - 16 wire electrode16 wire electrode

Heater

Electrode wires

Bubbles

Bubble growth on electrode

Bubble coalescence

Bubble growth on wires

Bubbles

Electrode wires

Heater

0kV 6kV

9kV 12kV

Refrigerant R11, at atmospheric pressureHeat flux = 14.2kW/m2

First Approach _ First Approach _ ConclusionsConclusions

Qualitative AnalysisQualitative Analysis Bubbles behave differently at diverse Bubbles behave differently at diverse

locations of the heating tube:locations of the heating tube:* Coalescing of bubbles underneath the heating Coalescing of bubbles underneath the heating

tubetube

* Suppression of nucleate sites on the sidesSuppression of nucleate sites on the sides

Quantitative AnalysisQuantitative Analysis Heat transfer enhancement: large in Heat transfer enhancement: large in

natural convection region, decrease natural convection region, decrease in nucleate regionin nucleate region

EHD Bubble DynamicsEHD Bubble Dynamics

Analysis of bubble behaviour Analysis of bubble behaviour under the influence of electric under the influence of electric fieldsfields

Bubble parameters:Bubble parameters: FrequencyFrequency DeformationDeformation Number of nucleate siteNumber of nucleate site Bubble diameterBubble diameter

Experimental apparatusExperimental apparatus

Electric field distribution Electric field distribution -Kauss Analysis in -Kauss Analysis in

Homogeneous mediaHomogeneous media

Images of Bubbles as at different Images of Bubbles as at different Electrode Voltage - V(t) = mtElectrode Voltage - V(t) = mt

Heat Flux = 30kW/m2, Fluid Temperature = 220C

0kV (No EHD) 2.0kV 4.5kV

6.0kV 6.6kV 8.0kV

EHD effect on Bubble DeformationEHD effect on Bubble Deformation

, q" = 35.4kW/m2

q" = 30.0 kW/m2

1.00

1.05

1.10

1.15

1.20

1.25

1.30

0 200 400 600 800 1000 1200 1400 1600 1800

Electric Field Strength (kV/m)

dx/d

y

Time Dependend,

Time Independent,

EHD effect on Bubble DiameterEHD effect on Bubble Diameter

q" = 30kW/m2

q" = 35.4kW/m2

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

0 200 400 600 800 1000 1200 1400 1600 1800

Electric Field Strength (kV/m)

Eq

uiv

ale

nt

Bu

bb

le D

iam

ete

r (m

m)

Time Dependent,

Time Independent,

EHD effect on Nucleate Site DensityEHD effect on Nucleate Site Density

q" = 40.0kW/m2

q" = 30.0kW/m2

q" = 35.4 kW/m2

0.0E+00

5.0E+04

1.0E+05

1.5E+05

2.0E+05

2.5E+05

3.0E+05

3.5E+05

4.0E+05

4.5E+05

0 200 400 600 800 1000 1200 1400 1600 1800

Electric Field Strength (kV/m)

Nu

cle

ate

Site

Den

sity

(1/

m2 )

Time Independent,

Time Independent,

Time Dependent,

EHD effect on Frequency of Bubble EHD effect on Frequency of Bubble DepartureDeparture

0

20

40

60

80

100

120

140

160

180

1200 1300 1400 1500 1600 1700 1800

Electric Field Strength (kV/m)

Fre

qu

ency

of B

ub

ble

Dep

artu

re (1

/sec

) Nucleate Site 1

Nucleate Site 2

Nucleate Site 3

Nucleate Site 4

EHD effect on Proportion of Latent heat EHD effect on Proportion of Latent heat to Total heat fluxto Total heat flux

q" = 35.4kW/m2

q" = 30.0 kW/m2

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

1200 1300 1400 1500 1600 1700 1800Electric Field Strength (kV/m)

Pro

po

rtio

n o

f L

ate

nt

He

at

to T

ota

l He

at (

%)

Time Dependent,

Time Independent,

Second Approach - Second Approach - ConclusionConclusion

Bubble BehaviourBubble Behaviour Time DependencyTime Dependency Threshold ValueThreshold Value Contribution of latent heat on total Contribution of latent heat on total

heat transfer in pool boilingheat transfer in pool boiling

Future InvestigationFuture Investigation

TheoreticalTheoretical• Hysteresis effectHysteresis effect• Time DependencyTime Dependency

• Frequency dependency of dielectric Frequency dependency of dielectric propertiesproperties

• Mechanical oscillation of liquid-vapour Mechanical oscillation of liquid-vapour interfaceinterface

• Line of zero forceLine of zero force• Electrolysis (DC)Electrolysis (DC)

Future InvestigationFuture Investigation

ExperimentExperiment• Design and build of power supplier Design and build of power supplier

with frequency variable (pulse with frequency variable (pulse wave)wave)

• Measuring temperature of the wireMeasuring temperature of the wire• Development the test rig Development the test rig

compatible with R123, aerospace compatible with R123, aerospace fuelfuel

constdfb

tieEE 0

f

1

Time dependency in Time dependency in EHD PhenomenaEHD Phenomena

Charge relaxation timeCharge relaxation time

In general, reduce of In general, reduce of , increasing of , increasing of heat transfer enhancementheat transfer enhancement

Bubble frequencyBubble frequency Frequency of alternating fieldFrequency of alternating field

j

Time dependency in Time dependency in EHD Phenomena - EHD Phenomena - Dielectric theoryDielectric theory

Complex permitivityComplex permitivity

Heating Wire - Electrode Heating Wire - Electrode arrangementarrangement