APPROVED: Huseyin Bostanci, Major Professor Seifollah Nasrazadani, Committee Member Shuping Wang, Committee Member Enrique Barbieri, Chair of the Department of

Engineering Technology Costas Tsatsoulis, Dean of the College of

Engineering Mark Wardell, Dean of the Toulouse Graduate

School

DIRECT IMMERSION COOLING VIA NUCLEATE BOILING OF HFE-7100 DIELECTRIC LIQUID

ON HYDROPHOBIC AND HYDROPHILIC SURFACES

Nihal E. Joshua

Thesis Prepared for the Degree of

MASTER OF SCIENCE

UNIVERSITY OF NORTH TEXAS

December 2014

Joshua, Nihal E. Direct Immersion Cooling via Nucleate Boiling of HFE-7100 Dielectric Liquid

on Hydrophobic and Hydrophilic Surfaces. Masters of Science (Engineering Systems - Mechanical

Systems), December 2014, 57 pp., 4 tables, 25 figures, references, 21 numbered titles.

This study experimentally investigated the effect of hydrophobic and hydrophilic surfaces

characteristics on nucleate boiling heat transfer performance for the application of direct

immersion cooling of electronics. A dielectric liquid, HFE – 7100 was used as the working fluid in

the saturated boiling tests. Twelve types of 1-cm2 copper heater samples, simulating high heat

flux components, featured reference smooth copper surface, fully and patterned hydrophobic

surface and fully and patterned hydrophilic surfaces. Hydrophobic samples were prepared by

applying a thin Teflon coating following photolithography techniques, while the hydrophilic TiO2

thin films were made through a two step approach involving layer by layer self assembly and

liquid phase deposition processes. Patterned surfaces had circular dots with sizes between 40 –

250 µm. Based on additional data, both hydrophobic and hydrophilic surfaces improved nucleate

boiling performance that is evaluated in terms of boiling incipience, heat transfer coefficient and

critical heat flux (CHF) level. The best results, considering the smooth copper surface as the

reference, were achieved by the surfaces that have a mixture of hydrophobic/hydrophilic

coatings, providing: (a) early transition to boiling regime and with eliminated temperature

overshoot phenomena at boiling incipience, (b) up to 58.5% higher heat transfer coefficients,

and (c) up to 47.4% higher CHF levels. The studied enhanced surfaces therefore demonstrated a

practical surface modification method for heat transfer enhancement in immersion cooling

applications.

Copyright 2014

by

Nihal E. Joshua

ii

ACKNOWLEDGMENTS

I would like to express my gratitude to my major professor and advisor Dr. H. Bostanci

for his immense support, and encouragement throughout this research. His continued

guidance, suggestions and inspiration have enabled me to complete this thesis. I consider

myself very fortunate to have had the opportunity to work with him.

I would like to thank Dr. S. Nasrazadani and Dr. S. Wang for serving as committee

members and for their valuable comments.

I would also like to thank Dinesh K. Ajakumar, a former student, for his help and

contribution in the fabrication of enhanced surfaces.

A special thanks to my wife Elizabeth for her support and encouragement throughout

my research. I would also like to thank my parents and in-laws for their prayers and moral

support.

iii

TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS ..................................................................................................................... iii

LIST OF TABLES ................................................................................................................................ vi

LIST OF FIGURES ............................................................................................................................. vii

LIST OF ACRONYMS/ABBREVIATIONS ............................................................................................. ix

CHAPTER 1 INTRODUCTION ............................................................................................................ 1

CHAPTER 2 LITERATURE REVIEW .................................................................................................... 6

2.1 Inception of Boiling ....................................................................................................... 8

2.2 Model of Ebullition Cycle ............................................................................................ 10

CHAPTER 3 EXPERIMENTAL SETUP AND PROCEDURE .................................................................. 17

3.1 Experimental Setup .................................................................................................... 17

3.2 Working Fluid .............................................................................................................. 19

3.3 Test Section ................................................................................................................ 20

3.4 Reference and Enhanced Boiling Surfaces ................................................................. 23

3.6 Test Conditions and Procedure .................................................................................. 27

3.7 Experimental Uncertainties ........................................................................................ 31

CHAPTER 4 RESULTS AND DISCUSSIONS ...................................................................................... 32

4.1 Reference Surface ....................................................................................................... 32

4.2 Hydrophobic Surfaces ................................................................................................. 35

4.3 Hydrophilic Surface ..................................................................................................... 41

4.4 Hydrophobic/Hydrophilic Surface .............................................................................. 46

iv

4.5 Overall Performance Comparison .............................................................................. 49

CHAPTER 5 CONCLUSIONS ............................................................................................................ 52

5.1 Concluding Remarks ................................................................................................... 52

5.2 Recommendations for Future Research ..................................................................... 53

REFERENCES .................................................................................................................................. 55

v

LIST OF TABLES

Page

Table 1: Comparison of the Common Cooling Methods ............................................................ 3

Table 2: Comparison of the Physical Properties of Various Liquids ......................................... 19

Table 3: Heater Description ...................................................................................................... 22

Table 4: Performance Summary of Heater Surfaces ................................................................ 51

vi

LIST OF FIGURES

Page

Figure 1: Growing power levels of electronic processors over time. .................................... 2

Figure 2: Graph showing no. of transmitters increasing in a chip yearly .............................. 2

Figure 3: Boiling curve ............................................................................................................ 7

Figure 4: Surface with cavities and grooves .......................................................................... 9

Figure 5: Bubble in equilibrium .............................................................................................. 9

Figure 6: Ebullition cycle ...................................................................................................... 10

Figure 7: Schematic diagram of the experimental setup. .................................................... 18

Figure 8: (a) Heater boiling surface side, (b) resistor side, (c) overall assembly in adapter plate. ........................................................................................................ 20

Figure 9: Pattern design hydrophobic and hydrophilic surface coatings. ........................... 22

Figure 10: Contact angle measurement of water on reference, plain-smooth copper surface ................................................................................................................... 25

Figure 11: LabView front end. ................................................................................................ 29

Figure 12: LabViewblock diagram. ......................................................................................... 30

Figure 13: Boiling curves of the reference surfaces. ............................................................. 34

Figure 14: HTC of reference surfaces as a function of heat flux. .......................................... 35

Figure 15: Boiling curves of reference surface and fully hydrophobic surface. .................... 36

Figure 16: Boiling curves of reference and patterned hydrophobic surfaces. ...................... 38

Figure 17: Surface superheat (a) and temperature overshoot (b) at boiling incipience of reference and patterned hydrophobic surfaces ................................................... 39

Figure 18: HTC of reference and hydrophobic surfaces as a function of heat flux. .............. 41

Figure 19: Boiling curves of reference surface and fully hydrophilic surface. ....................... 42

vii

Figure 20: Boiling curves of reference surface and patterned hydrophilic surfaces. ............ 44

Figure 21: HTC of reference and hydrophilic surfaces as a function of heat flux. ................ 45

Figure 22: Boiling curves of reference surface and patterned hydrophobic/hydrophilic surfaces. ................................................................................................................ 47

Figure 23: HTC of reference and hydrophobic/hydrophilic surfaces as a function of heat flux. ............................................................................................................... 48

Figure 24: Boiling curves of reference and enhanced surfaces as a function of heat flux. .. 49

Figure 25: HTC of reference and enhanced surfaces as a function of heat flux. .................. 50

viii

LIST OF ACRONYMS/ABBREVIATIONS

A Area, cm2

CHF Critical heat flux, W/cm2

h Heat transfer coefficient, W/m2° C

I Current, A

K Heater wall thermal conductivity, W/m°C

Psat Saturation pressure, kPa

q” Heat flux, W/cm2

Tsat Saturation temperature, °C

Tsurf Surface temperature, °C

TCavg Average thermocouple reading, °C

V Voltage, V

σ Surface tension, N/m

Tsup Superheated liquid

Pambient Ambient pressure

Rb Radius of bubbles

ix

CHAPTER 1

INTRODUCTION

Complexity of electronic components has changed dramatically over the past several

decades. An increase in complexity has caused a simultaneous increase in power dissipation

and clock speeds. It is crucial to keep the temperatures of these components within the

specifically designed ranges. High temperatures can cause thermal stresses, break the soldered

joints in the system and decrease the lifespan of the components drastically. It is thus

necessary to actively cool systems to maintain their temperatures at desired levels for reliable

and efficient operation.

In today’s technological trend, devices continue to evolve with greater power and

denser volume as illustrated in Figure 1. Higher levels of integration in semiconductors, along

with smaller devices and faster speeds have led to higher heat generation and higher heat

densities. Intel co-founder Gordon Moore famously stated, what is known today as Moore's

Law, that the “number of transistors on a chip will double approximately every 2 years.” Due to

such rapid increase in complexity, functionality, higher operating speeds and reduction in

overall size, thermal management plays a vital role in the designing of components from circuits

to systems in achieving high reliability and performance. In certain critical applications, thermal

management also affects the size and weight of the overall system.

1

Figure 1: Growing power levels of electronic processors over time. [1]

Figure 2: Graph showing no. of transmitters increasing in a chip yearly. [2]

2

Thermal management technologies are divided into two distinct groups:

• Passive thermal management - Cooling technologies that depend on the

thermodynamics of conduction, convection and radiation to transfer heat. Thermal

interface materials, heat spreaders and heat sinks are a few passive technologies.

• Active thermal management - Cooling technologies that use external energy to enhance

heat transfer. Forced air cooling with fans, thermoelectric coolers and spray cooling are

some of the active technologies.

Among these various technologies, forced air cooling, generally implemented with heat

sinks and fans, is an active thermal management technique that has been a predominant

method of cooling for a long time. However due to rising heat flux levels (heat load per unit

area), it is getting rather difficult for this conventional method to efficiently cool the new

systems and maintain temperature levels for reliable operation. An alternative method to

conventional air cooling is the liquid cooling that provides higher heat transfer performance. As

shown in table 1, liquid cooling with water attains up to 63.4% higher heat transfer coefficient,

h, compared to forced air cooling and is capable of removing much higher heat loads.

Table 1

Comparison of the Common Cooling Methods [3]

Air Cooling - Natural Convection

Air Cooling - Forced Convection

Liquid Cooling with Water

Immersion Cooling

h = 5 - 30 W/m2 K h = 20 - 400 W/m2 K h = 100-1600 W/m2 K h=800-10000 W/m2 K Q = 2 - 9 W Q = 6 - 120 W Q = 30 - 480 W Q = 240 - 3000 W

Free convection Free + Forced convection

Forced convection Free convection

3

Immersion cooling is another promising cooling method that achieves even better heat

transfer capabilities. In immersion cooling the major heat transfer mode is, nucleate boiling

that utilizes liquid-vapor phase change mechanism. Phase change techniques continue to be of

interest in the area of thermal management due to their inherent advantages such as providing

high heat flux removal capability and nearly uniform temperature distribution on cooled

surfaces. In immersion cooling the heat generating device is immersed in a stagnant pool of

working fluid, and large amounts of heat are absorbed as the liquid changes phase to vapor

with a relatively low temperature difference between the device and the pool. Selection of the

appropriate coolant is critical for direct immersion cooling, which requires chemical inertness

and compatibility, high dielectric strength, safe handling, and low environmental impact. When

a dielectric liquid is used as the working fluid, it can come into direct contact with every surface

of the component. This method allows much higher heat flux dissipation than air cooling (more

than 50% as indicated in Table 1) and is a good choice for high performance computers, super

computers and compact devices generating large amounts of heat.

Many nucleate boiling studies have been conducted with dielectric liquids, such as 3M™

Fluorinert™ Electronic Liquids FC-72, FC-86 and FC–87, targeted for electronics cooling

applications [4]. Current research efforts have been focused on the recently developed

dielectric liquids, such as 3M’s Novec Engineered Fluids HFE-7100 and HFE-7200. The Novec

fluids still share the same desired properties of Fluorinerts, and additionally offer two improved

properties that are important in terms of their environmental impact, namely, low global

warming potential (GWP), and zero ozone depletion potential (ODP). The Novec dielectric fluids

provide dependable performance and require little maintenance. Since they possess high

4

resistivity, they do not damage electronic equipment or integrated circuits in the event of leak

or failure. The chemical inertness and non corrosivity of the fluids make them safe for workers

to handle.

An important thrust in the current nucleate boiling research in the context of thermal

management is the enhanced heat transfer surfaces. Such surfaces improve the heat transfer

performance measured by earlier transition to boiling, higher heat transfer coefficient, and

higher critical heat flux that defines the practical heat flux limit. The goal of the current study

was to experimentally investigate the effect of hydrophobic and hydrophilic surfaces in the

nucleate boiling performance. The HFE-7100 dielectric liquid was selected as the working fluid,

and a 1-cm2 copper test section, simulating a high power electronic device, was tested under

saturation boiling conditions at near atmospheric pressures. The performance of the carefully

prepared sample surfaces was compared to that of a reference, untreated smooth surface to

determine their effect on nucleate boiling heat transfer, and their potential use in direct

immersion cooling applications.

5

CHAPTER 2

LITERATURE REVIEW

The previous section discussed how conventional methods were not capable of keeping

up with the emerging trends in technology. Thus, there was a need for a greatly improved and

more efficient means of cooling. Boiling is a promising mode of heat transfer and could be one

of the most effective solutions for the future. This chapter explains the boiling process and the

many studies that have been conducted thus far in this field of research.

Boiling is a phase change process wherein vapor bubbles are formed on the surface that

is being heated. Nucleate boiling is an efficient method used for transferring heat and can be

used in many highly dense electronic components. Since it is a very complex process and

because of the number of variables involved, studies are presently being conducted to better

understand the boiling process. Studies for pool boiling go as far back as 1934 when Nukiyama

ran an experiment and plotted the boiling curve for the very first time. He boiled water to its

saturation temperature over Nichrome wire. Then, by calibrating the resistance of the wire as a

function of temperature, he obtained heat flux and temperature from the observed voltage and

current. At a very high heat flux, the wire melted since it surpassed the Critical heat flux (CHF).

He then ran a second experiment with a platinum wire and successfully plotted a boiling curve

with ∆T vs. q”. In 1937, Drew and Mueller ran a successful experiment in boiling water at

saturated temperature and at atmospheric pressure and were able to plot the boiling curve, the

only change being that the curve was rotated to q" vs. ∆T. Pool boiling is divided into five

regimes which are natural convection, nucleate boiling, critical heat flux, transitional boiling

regime and film boiling.

6

Figure 3: Boiling curve. [6]

The boiling curve in Figure 3shows the heat flux input against the wall superheat for a

test piece (heater) submerged in a pool of liquid. The wall superheat ∆T is the difference

between the heater temperature and the saturated temperature of the liquid at pressure of the

existing system. The curve that is being plotted is for the heater wherein the heat input to the

heater is being controlled. As the heat flux to the heater is increased, natural convection is the

first mode of heat transfer. Natural convection is a mechanism where the motion of the fluid is

7

not generated by an external source, but by differences in the density of the fluid. When the

fluid surrounding the heat source receives heat, the fluid rises and is replaced by cooler fluid

from the surrounding area.

Natural convection follows the principle of buoyancy as a result of the differences

caused in the fluid density. At a certain input of heat flux, bubbles develop on the heater

surface. This is the starting point for nucleate boiling. Nucleate boiling regime embraces the

two distinct regimes. The first is the region of isolated bubbles where bubbles rise from isolated

nucleation sites and as q and ∆T are increased, more and more sites are activated. The second

is the region of slugs and columns where when the active sites become very numerous, the

bubbles merge into one another and a totally different vapor path is developed. The vapors

formed at the surface merge into jets that feed overhead bubbles or slugs of vapor. Critical heat

flux is the point up to which the system can be operated safely and wherein nucleate boiling is

fully developed. After the point of critical heat flux, most of the heater surface is rapidly

covered with vapor. The surface is insulated and the surface temperature rises rapidly [6]. The

region where the heater surface is insulated with vapor is known as the transition boiling. In

this region, heat flux decreases drastically since the liquid takes in huge portions of heat,

causing a vapor blanket. Due to this blanket, the surrounding liquid finds it difficult to reach the

heater, causing a decrease in the heat transfer rate.

2.1 Inception of Boiling

Surfaces are not smooth as they seem to a human eye. There are always tiny grooves on

the surfaces created by chattering and bouncing during machining.

8

Figure 4: Surface with cavities and grooves. [12]

Even holes are formed on surfaces due to such actions. When a liquid wets the surface it

is prevented from entering these holes and grooves due to surface tension. In these grooves

and holes, vapor pockets are formed. At these so called vapor pockets, bubble nucleation

occurs. In their book Lienhard and Lienhard [12], explained the above theory in detail. The

authors assume that a bubble at pure saturated steam is in equilibrium with an infinite

superheated liquid. The size of the bubble is determined by considering two conditions: one,

that the bubble is in thermal equilibrium, and two, that the bubble is in mechanical equilibrium.

Figure 5: Bubble in equilibrium. [12]

The bubble stays in mechanical equilibrium when the inside and the outside of the bubble are

balanced by forces of surface tension σ. For the bubble to be in thermal equilibrium, the

temperature inside and outside the bubble must be equal. Since vapor is in contact with the

liquid, its temperature is Tsup. The force balance takes the form:

9

𝑅𝑅𝑏𝑏 = 2𝜎𝜎𝑃𝑃𝑠𝑠𝑠𝑠𝑠𝑠𝑎𝑎𝑎𝑎𝑇𝑇𝑠𝑠𝑠𝑠𝑠𝑠−𝑃𝑃𝑠𝑠𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑠𝑠

(1)

If the radius of the bubble is smaller than the value calculated in the force balance

equation mentioned above, the surface tension overbalances and thus the vapor condenses

inside at high pressure and the bubble collapses. If the radius is larger than the equation

specified, liquid at the interface evaporates and the bubble begins to grow. Research in the

field of nucleate boiling has shown that as heater surface temperature increases the

equilibrium radius decreases and thus small nucleation sites are activated for bubble growth.

2.2 Model of Ebullition Cycle

An important characteristic that influences liquid vapor phase change phenomena is

surface wet ability. The wet ability of the surface is measured by contact angle. The contact

angle is the balance force acting on the interfacial line between vapor, liquid and solid. The

forces are surface tension, interfacial tension between liquid and solid, and the tension

Figure 6: Ebullition cycle. [5]

10

between vapor and solid [5]. In recent years many boiling heat transfer enhancement studies

have been made. One of the boiling enhancement methods of great interest is to increase the

number of small scale cavities on the surface. Techniques for increasing small scale cavities

include the application of porous coatings, mechanical grooves or dendrites to the desired

surface. The micro geometries of these enhanced surfaces serve to increase vapor/gas

entrapment volume and active nucleation site density [7].

Dielectric liquids have poor thermo physical properties and are highly wetting compared

to non-dielectric liquids such as water. When used in conjunction with structured, porous or

micro porous surfaces, the power removed by nucleate boiling of these liquids could be high

enough for cooling high power CPU chips and microprocessors. Liquids such as FC - 72 are

highly wetting (surface tension of 0.008N/m as compared to water 0.072 N/m) for promoting

bubble nucleation. Air is entrapped only in the tiny pores and cavities on the surface. As a

result, boiling incipience is delayed [8].

Parker et al.[8] ran experiments for porous graphite and plain copper in a pool of FC -

72. They found that heat transfer coefficient (HTC) and critical heat flux (CHF) were much

higher in porous graphite than in plain copper. They also noted that boiling incipience was

much earlier for porous graphite (<1K) as compared to plain copper (5-9K)Chang and You [9]

conducted an experiment earlier than Parker's with FC - 72, where the surfaces were diamond

particle coated. Tests were conducted on a set of thin coated surfaces known as micro porous

surface and on a set of thicker coated surfaces known as porous surface. Porous surfaces

showed higher enhancement, at heat fluxes lower than 2.5 W/cm² but the enhancement

decreased at 2.5 W/cm² and beyond, due to the thickness associated with higher impedance

11

and higher thermal resistance. Significant increase in CHF was found for the micro porous

region, but that seemed to be the upper limit for CHF and no further increase in CHF was found

in porous surfaces.

A study by El-Genk and Amir[10] consisted of nucleate boiling of micro porous layers

deposited on copper substrates in a pool of PF 5060 dielectric liquid. The basic microstructure

with patterned open macro pores surrounded by fine and dense Cu dendrites forms during the

current density (3A/cm²) electrochemical deposition stage. The thicker the deposition layer, the

larger the macro pores and denser the surrounding copper dendrites. The study showed that

these enhanced copper substrates show high surface superheats when they are run for the very

first time. As tests were repeated there was little or no temperature excursion and data was

repeatable. It was observed that PF-5060 dielectric liquid used on copper micro porous copper

surface showed 40 to 70% more improvement in CHF and maximum nucleate boiling over FC -

72 on plain copper surface.

There have been quite a few studies conducted on roughened and smooth, porous and

micro porous coated surfaces along with FC – 72, but little research has been done with HFE-

7100. El-Genk and Parker[11] conducted a study on porous graphite with HFE-7100 as the

dielectric liquid. The study showed that CHF values for HFE-7100 on porous graphite were 60%

higher and the corresponding surface superheats were 25% lower than those on smooth

copper. The results demonstrated that the relative enhancements in CHF and nucleate boiling

heat flux of HFE-7100 on porous graphite are similar to earlier studies on FC-72, except that the

actual values for the nucleate boiling heat flux for the latter are lower. The study showed that

12

graphite significantly enhances nucleate boiling and the CHF of the highly wetting HFE-7100

dielectric liquid and lowers the surface superheat compared to smooth copper.

However, a study done by El-Genk et al. [4] on a smooth copper surface with HFE 7100

showed that despite the large difference between CHF values for the FC - 72 and HFE 7100

dielectric liquids, the normalized CHF values to that in the upward facing position 0° are almost

identical. Results also demonstrated that up to 24.5W/cm² could be removed from a computer

chip with smooth copper surface by saturation nucleate boiling of HFE-7100 at 0.085 MPa,

which is 57% higher than reported earlier for FC-72 at a slightly higher pressure at 0.1 MPa. In

another study, El Genk and Parker[13] investigated saturation and 10K, 20 K and 30 K sub

cooled boiling of FC-72 and HFE-7100 dielectric liquid on porous graphite at inclination angles

0° to 180°. It was observed that at low superheats (<15K), increasing surface inclination θ

increases the nucleate boiling heat flux; however, at higher superheats, nucleate boiling heat

flux decreases with increased θ. Although increasing liquid sub cooling increases the nucleate

boiling heat flux and CHF as well as the nucleate boiling heat transfer coefficients, it decreases

the total boiling heat transfer coefficients. Therefore, for electronics cooling application

saturation boiling is far superior to sub cooled boiling because of the higher heat transfer

coefficient along with the lower surface superheat. It was also observed that CHF for FC - 72

was much higher than that for HFE-7100 and their values increase with increasing liquid sub

cooling. But at saturation temperature CHF of HFE-7100 was found to be much higher than FC-

72.

Supporting El Genk and Parker's view [13] on HFE 7100 fairing over FC-72 on porous

graphite was a study done by Priarone [14] ran an experiment for FC-72 and HFE-7100 dielectric

13

liquids but on smooth copper surfaces. Heat transfer coefficients in the nucleate boiling region

and critical heat flux values proved to be respectively 25% and 40% higher for HFE-7100 than

for FC-72 in the same operating conditions. The studies above have shown excellent results

with dielectric liquids with porous graphite and micro porous surfaces deposited on copper

substrates. The deposition of such different materials on the heater surfaces may disrupt the

boiling process by changing the micro cavities density on the heater. Today the progress in

nano coating allows modifying surface topography and chemistry at nano scale. Particles of

very small size can be deposited on the heater surface. By changing the deposited particles

material, it is possible to vary the water contact angle within 0° and 180°, hence controlling wet

ability. As scale of the nucleation sites is micrometric, only the contact angle is changed while

all other boiling parameters remain constant [15]. In the past decade or so, it has become

possible to control surface wet ability to some extent by enhancing the surface. Surfaces

developed with Hydrophobic and Hydrophilic patterns have shown enhancement in boiling.

Phan et al. [16] ran sub cooled pool boiling experiments for hydrophobic and hydrophilic

coated surfaces. In hydrophobic surfaces they found that bubbles appear at the surface for a

lower heat flux compared to standard surfaces which are usually wetted. Bubbles cannot

detach from the surface and coalesce with bubbles formed at the neighboring sites. In

hydrophilic surfaces with the enhancement of surface wet ability the bubble departure

diameter increases and the bubble emission frequency decreases. Heat transfer improves with

increased wet ability and the best heat transfer is obtained either at a contact angle close to

90° or at a low contact angle close to 0°.E. Forrest et al [17] ran an experiment for hydrophobic

and hydrophilic coated surfaces with a layer by layer assembly method. They found that thin

14

film coatings that displayed hydrophobicity were found to enhance HTC by 100%. Their studies

also showed CHF enhancement in all thin film coatings to be in the range of 44% to 101%.

Amy et al.[18] ran experiments for smooth and flat surfaces with hydrophobic and

hydrophilic patterns which improve pool boiling performance in water. The enhanced surfaces

greatly improve the heat transfer coefficient (HTC) and Critical Heat Flux (CHF). Their results

showed that hydrophobic surfaces tend to increase HTC but did not enhance CHF. A hydrophilic

surface, on the other hand, enhances both HTC as well as CHF. They found out that mixing the

two surfaces that means a hydrophilic surface with hydrophobic dots known as hydrophilic

network gave the highest enhancement and enhanced CHF and HTC by 65% and 100%

respectively over a plain hydrophilic surface. On the lines of studies shown by Amy et al [18], in

their experiment Takata et al. [19]found that surface coated with (PTFE) Teflon coating gave the

best HTC in the low flux region and the overshoot of superheating was very small. In a surface

having a mixture of Teflon and TiO₂ small overshoot of superheating on the surface in boiling

onset, better heat transfer performance in nucleate boiling region and larger CHF. The earlier

paper had shown boiling enhancement for water with TiO₂ coating. Studies with TiO₂ mostly

included water as the working fluid. Wu et al. [20]performed a study to investigate the nucleate

boiling and CHF of water and FC - 72 on TiO₂ surfaces. The test piece consisted of a 1cm² copper

heater with 1 µm TiO₂ coated surface which was used in saturated and pool boiling tests of

previously mentioned working fluids. Results showed that TiO₂ coated surface increased CHF

by 50.4% and 38.2% for water and FC-72 respectively. It was hence indicated that boiling

performance depended on level of wet ability improvement.

15

Earlier research and studies have shown how dielectric liquid are efficient and more so

the much recent Novec fluid HFE-7100. Along with type of working fluid, studies have also

indicated that types of enhanced surfaces contribute to the overall heat transfer coefficient.

Additional work needs to be done in nano scale coating of surfaces along with working fluids to

quantify the cooling potential at saturation condition and develop useful correlations to better

design and implementations.

16

CHAPTER 3

EXPERIMENTAL SETUP AND PROCEDURE

This chapter describes the experimental setup and the procedure used in the current

research. The separate sections on the test setup, working fluid, heater fabrication, and test

conditions and procedures provide detailed descriptions. The chapter concludes with the

uncertainty analysis of the measurements.

3.1 Experimental Setup

The major components of the set up include a boiling chamber, a test section,

immersion heaters, a condenser, temperature and pressure sensors, a DC power supply, and a

computer controlled data acquisition (DAQ) system, as illustrated in Figure 7. The boiling

chamber contains the HFE-7100 pool and consists of two polycarbonate shells,(the outer shell

has 152mm height, 127mm outer diameter, and 6mm thick wall while the inner shell has the

same height and wall thickness, and 114mm outer diameter) and top and bottom PEEK

lids(both have 152mm diameter and 12mm thickness). The transparent, concentric

polycarbonate shells enable visual observation during the tests and also provide an insulating

air layer for the heated pool. The shells are held tightly between the two lids with the help of 5

sets of stainless steel bolts, nuts, and washers. Gaskets are placed between the lids and the

shells on both the top and bottom ends to prevent the liquid and vapor leakages of HFE-7100

through the chamber. Three compact cartridge-style immersion heaters on the bottom lid

heats the liquid pool to desired test condition through a temperature controller. The chamber

also accommodates a condenser assembly to convert the HFE-7100 vapor back into the liquid

17

and maintain the chamber pressure. This assembly involves a copper disc with extruded fins

that are mounted on to the center opening on the top lid with the help of screws. A small

integrated pump circulates water between the condenser and an air-cooled heat exchanger

fitted with two, 120mm fans to transport and finally reject the heat to the ambient. Pool

temperature is monitored using a thermocouple (TC) probe placed near the test section while

the vapor pressure is monitored using a pressure transducer on the top lid. A programmable DC

power supply provides the heat input to the test section.

Figure 7: Schematic diagram of the experimental setup.

18

3.2 Working Fluid

The working fluid selected for this study was HFE-7100 (C4F9OCH3). HFE-7100 is a

dielectric liquid manufactured by 3M Company as part for their NOVEC Engineered Fluids. Table

2 lists relevant physical properties of HFE-7100 along with another common dielectric liquid FC-

72(C6F14), and water (H2O) for comparison purposes. Local atmospheric pressure at the test

location in Denton, TX is 98.4 kPa due to its altitude of 200 m, and the thermo physical

properties are slightly different than the listed properties at 101.3 kPa. As an example, boiling

point at 98.4kPa atmospheric pressure is 60.6oC. As indicated in Table 2, HFE-7100 possesses

much lower global warming potential, and atmospheric lifetime compared to FC-72.

Table 2

Comparison of the Physical Properties of Various Liquids

Saturation physical properties (101.3kPa)

HFE - 7100 (C₄F₉OCH₃)

FC - 72 (C₆F₁₄)

Water (H₂O)

Boiling Point (° C) 61 56 99.98 Freeze Point (° C) -135 -90 0 Ave. molecular weight (g/mole) 250 338 18.0156

Liquid density (kg/m3) 1370.2 1602.2 958.3

Vapor density (kg/m3) 9.87 13.21 0.597 Liquid viscosity (kg/ms) 3.70 x 10⁻⁴ 4.33 X 10⁻⁴ 8.9 X 10⁻⁴ Liquid specific heat(J/kg K) 1255 1101 4187 Latent heat of vaporization (kJ/kg) 111.6 88 2256.7 Liquid thermal conductivity (W/mK) 0.062 0.054 0.609 Liquid surface tension (N/m) 1.019 x 10⁻² 7.93 x 10⁻³ 58.91x10⁻³ Environmental Properties Ozone Depletion Potential - ODP 0 0 Global warming Potential - GWP 320 7400 Atmospheric lifetime - ALT (yrs) 4.1 3200 Electrical Properties @ 25 ° C Dielectric strength 28 Dielectric constant 7.39 (0.1 - 10 MHz) Volume resistivity (Ω cm) 3.3 x 10⁹

19

3.3 Test Section

The test section (heater) involves a 1-cm2 oxygen-free copper piece (10mm x10mm

x2mm) simulating a high heat flux generating device, as shown in Figure 8. The heater is

soldered on to one side of a 0.5mm thick copper interface disk. A matching size, 1-cm2 thick film

resistor (10 Ω) is soldered onto the opposite side of the copper disk which generates heat when

voltage is applied across it. Once soldering is complete, hydrophobic and/or hydrophilic surface

coatings are applied onto the copper heater surface following the procedures explained in the

following section. Finally, the whole soldered assembly is placed in a Teflon adapter that

attaches centrally into the bottom lid of the boiling chamber and the cavity around the copper

piece is carefully filled with a translucent two part epoxy adhesive to isolate the top section as

the boiling surface.

Figure 8: (a) Heater boiling surface side, (b) resistor side, (c) overall assembly in adapter plate.

20

Heat flux is determined from the total power supplied into the thick film resistor per

unit resistor base area as shown below.

𝑞𝑞" = 𝑉𝑉. 𝐼𝐼 𝐴𝐴

(2)

Heater temperature is monitored with two type-T thermocouples (36 AWG) embedded

halfway in the heater wall and spaced symmetrically across the 1-cm2 area as illustrated in the

figure below. Temperature spread resulted from the two thermocouples does not exceed ±

0.2°C throughout the tests indicating uniform cooling across the surface. Boiling surface

temperature is calculated as shown in equation 3 below, by extrapolating the average of two TC

readings through the known distance (half of heater wall thickness) to the surface and

assuming steady 1-D conduction through the heater wall

𝑇𝑇𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 = 𝑇𝑇𝑇𝑇𝑠𝑠𝑎𝑎𝑎𝑎−(𝑞𝑞".𝑥𝑥)𝑘𝑘

(3)

There are twelve types of heaters tested in this study, featuring reference, hydrophobic,

hydrophilic, and hydrophobic/hydrophilic surface characteristics. The Table 3 describes all the

tested heater types, and Table 4 shows the circular pattern design for hydrophobic and

hydrophilic surface coatings.

21

Table 3

Heater Description

No. Heater ID Heater Description (surface condition, pattern sizeD* x P* in µm) 1

2

3

4

5

6

7

8

9

10

Ref # ( )* P‡* R*

HPho # ( )

HPho_40 # ( )

HPho_100 # ( )

HPhi # ( )

HPhi_40 # ( )

HPhi_100 # ( )

HPhi _250 # ( )

HPho/HPhi_40 # ( )

HPho/HPhi_250 # ( )

Reference, smooth, no patterns

Fully hydrophobic, no patterns

Patterned hydrophobic, 40 D x 80 P

Patterned hydrophobic, 100 D x 200 P

Fully hydrophilic, no patterns

Patterned hydrophilic, 40 D x 80 P

Patterned hydrophilic, 100 D x 200 P

Patterned hydrophilic, 250 D x 500 P

Patterned hydrophobic/hydrophilic, 40 D x 80 P

Patterned hydrophobic/hydrophilic, 250 D x 500 P

* # ( ): Test heater number, P: Polished, R: Retest, D: Diameter, P: Pitch

Figure 9: Pattern design hydrophobic and hydrophilic surface coatings.

D

P

22

3.4 Reference and Enhanced Boiling Surfaces

This section details the step-by-step procedures in the preparation of test surfaces on 1-

cm2 copper substrates.

3.4.1 Reference Surfaces:

Step 1: The sample is held down and sanded on a 1200 grit sandpaper from left to right

applying medium pressure on the sample while sanding for 45 to 50 seconds.

Step 2: A polishing pad is mounted on the rotating drum at high speed and a slow and steady

jet of water keeps the drum well lubricated.

Step 3: Aluminum oxide (Al2O3) solution of 5 µm is applied on the entire pad and the copper

sample is held down on the polishing pad at medium pressure for about 30 seconds.

Step 4: Aluminum oxide (Al2O3) solution of 1 µm is applied on the entire pad, and the sample is

rotated 90oin the clockwise direction to polish the sample for about 30 seconds.

Step 5: Aluminum oxide (Al2O3) solution of 0.05 µm is applied on the entire pad, and the sample

is rotated 90oin the clockwise direction to polish the sample for about 30 seconds.

Step 6: The copper sample is held under running water to remove the particles of the

Aluminum oxide (Al2O3) solution from the surface thoroughly.

3.4.2 Fully and Patterned Hydrophobic Surfaces:

Step 1: The oven is preheated to a temperature of 90oC.

Step 2: The spin coater is set to the value Q - (2500 rpm and 30 seconds) the pump is turned on

and the heater is held in the place by vacuum.

Step 3: DuPont A1400 (Teflon) is poured on the entire heater surface with the help of a

dropper and the spin coater is turned on to complete the process of coating the surface.

23

Step 4: The sample is placed in the preheated oven and baked for 20 minutes at 90oC.

Step 5: Once the sample is baked, it undergoes a process known as the oxygen plasma for 5

cycles. (1 cycle = left to right and right to left continuous)

The next steps of the process need to be done in a dark room

Step 6: The spin coater is set to the value Q - (2000 rpm and 30 seconds) the pump is turned on

and the heater is held in the place by vacuum.

Step 7: Shipley S1818 photo resist is poured on the entire heater surface with the help of a

dropper and the spin coater is turned on to complete the process of coating the surface.

Step8: The sample is placed in the preheated oven and baked for 30 minutes at 90oC.

Step 9: Once the sample is baked the sample is put in a silver foil covered on all sides properly

to shield it from the light.

Step 10: The sample is placed on the exposure machine with the photo mask of the desired size

(250 x 500 / 100 x 200 / 40 x 80) on top of the sample.

Step 11: The vacuum holds the sample and the photo mask in place and the sample is exposed

to the UV light lamp of the exposure system for roughly 26 seconds.

Step 12: The sample is placed in a clean dried beaker filled with 300 MIF developer and rinsed

thoroughly for 5 minutes in the developer to etch out the photo resist.

Step 13: The sample is then cleaned with deionized water.

Step 14: The sample is then mounted on the spin coater, which is set to a value of 2000 rpm at

30 seconds. The vacuum holds the sample in place, deionized water is poured on the sample

and the spin coater is turned on.

Step 15: The sample undergoes a process known as oxygen plasma for 5 minutes.

24

Step 16: The copper sample is then placed in another clean dried beaker filled with 20 ml of

acetone where the sample is rinsed thoroughly for 5 minutes to remove any remaining photo

resist layer on teflon patterns. With the above step complete a hydrophobic layer is created on

the copper sample surface.

Wetting characteristics of the sample hydrophobic surfaces are evaluated by measuring

the contact angles. Due to its relatively low boiling point at atmospheric pressure, liquid droplets

of HFE-7100 on the test surface quickly evaporate and make measurements very difficult.

However, deionized water droplets can be utilized to illustrate the wetting characteristics of the

samples. Such data is taken with a contact angle meter (ChemInstruments CAM-Plus) using the

“half angle” measurement method, as shown in the figure below with a sample photograph. The

contact angle of deionized water on plain smooth copper samples, and fully hydrophobic samples

was found to be 70-74° and 108-112°, respectively. These contact angles are in the same order

of the reported values in the literature.

Figure 10: Contact angle measurement of water on reference, plain-smooth copper surface indicating 70°.

25

3.4.3 Fully and Patterned Hydrophilic Surfaces:

The procedure for preparing the hydrophilic surface coatings was adopted from a recent

study, Cui et al. [21], that described combining layer-by-layer self-assembly process and liquid

phase deposition process to obtain these coatings on copper based heat transfer surfaces.

(A) Layer-by-layer self-assembly process

Step 1: The test piece is cleaned with acetone, isopropyl alcohol, and water and then dried with

compressed air.

Step 2: The copper sample is then immersed in a PDDA solution for 10 minutes. After 10

minutes the sample is removed from PDDA solution and rinsed thoroughly in a beaker filled

with deionized water. It is then removed from the deionized water and dried thoroughly all

over with compressed air.

Step 3: After step 2, the copper sample is immersed in PSS solution for 10 minutes. After 10

minutes the sample is removed from PSS solution and rinsed thoroughly in a beaker filled with

deionized water. It is then removed from the deionized water and dried thoroughly all over

with compressed air.

Step 4: Step 2 and step 3 are repeated in the exact same order 3 more times.

With completion of step 4, the layer by layer self-assembly process is complete.

(B) Liquid phase deposition process

Step 5: Two separate solutions of ammonium hexafluorotitanate and boric acid solutions are

heated to 70° C and maintained at that temperature for an hour.

26

Step 6: The two solutions are then combined into a large beaker. Once the combined solution

turns cloudy, the copper heater sample (that underwent layer by layer deposition) are

immersed in this solution for 6 hours at 70° C.

Step 7: After the copper heater sample has been immersed for 6 hours, it is removed and rinsed

thoroughly in deionized water and dried thoroughly in compressed air. With the above step

complete a hydrophilic layer is created on the copper sample surface. The copper heater

sample is then preserved in HFE-7100 dielectric liquid.

Wetting characteristics of the sample hydrophilic surfaces are evaluated by measuring

the contact angles. Cui et al. [21] reported that the TiO2 thin film reduced the water contact

angle of the copper samples from approximately 90o to 0o. Drops of deionized water that were

placed on the coated copper surface fell to 0° within no time before spreading out across the

surface rapidly in all directions. Such a characteristic allows the liquid to flow over the copper

quickly, thus making it a better dissipater of heat in cooling applications.

3.6 Test Conditions and Procedure

Once the heater is installed, the chamber is partially filled with HFE-7100 (~400ml) and

the temperature controller that controls the immersion heaters is set to the saturation

temperature at atmospheric pressure. The liquid pool is then de-aerated in order to remove

dissolved non-condensable gases from the chamber. During this process the pool is rigorously

boiled for one hour that leads to the accumulation of vapor and non-condensable gases in the

upper section of the chamber. The vapor is recovered by condensation and the non-

condensable gases are purged to the ambient through the relief valve on the top lid every 5-10

27

minutes. Chamber pressure throughout the tests was maintained at around atmospheric

pressure by controlling the fan speeds in the condenser assembly. Heat flux was then increased

gradually in steps of 0.5-2 W/cm2 (with smaller steps around the boiling incipience and near

CHF), and the corresponding heater temperatures were recorded once they reach steady state

in about a minute. A LabVIEW® program in connection with a DAQ unit displays and records all

temperature, pressure and heat flux data. Figures 10 shows the front panel (user interface),

and figure 11 shows block diagram of the developed LabVIEW® program.

28

Figure 11: LabView front end.

29

Figure 12: LabViewblock diagram.

30

When CHF is detected with a sudden temperature rise, heat flux is immediately lowered

to a safe level (80% of CHF) and then gradually decreased back to 0 W/cm2. Data collected

during the tests were used to generate boiling curves, in the form of surface superheat

(∆Tsat= Tsurf- Tsat) vs. heat flux (q"), for each heater sample.

3.7 Experimental Uncertainties

Experimental uncertainties are estimated mainly for heat flux and temperature

measurements that are critical in performance evaluation. Error involved in heat flux

measurement (considering variations in voltage, current, and area) was ± 3.0%. The estimated

uncertainty in the temperature measurements was ± 0.2°C.

31

CHAPTER 4

RESULTS AND DISCUSSIONS

This chapter provides the experimental results, and discusses the findings based on

nucleate boiling performance. Test data are organized in four sections, referring to various

types of surfaces, namely, reference, hydrophobic, hydrophilic, and hydrophobic/hydrophilic

surfaces. Another section was included at the end to summarize and compare the results from

the tested surfaces. The performance of the enhanced surfaces was evaluated considering their

boiling incipience, heat transfer coefficient, and critical heat flux.

4.1 Reference Surface

Testing effort started with smooth, plain surfaces to obtain baseline data for all other

surfaces. In these tests three sets of data were taken and processed from the same heater. First

test was conducted with the heater Ref #4. The second test was done after re-polishing the

same heater’s surface, and denoted as Ref # 4 P. The third attempt simply involved retesting

the Ref # 4 P, and thus denoted as Ref # 4 P R. Results from the three tests are included in

Figure 12 in the form of boiling curve (i.e., ∆Tsat vs. q”), and they all followed a similar trend for

the entire testing range, involving natural convection, boiling incipience, nucleate boiling, and

CHF phases.

The initial natural convection mode, at heat fluxes of ≤3 W/cm2, was extended up to a

surface superheat range of 18°C to 23°C. Results from the three tests in this part were very

consistent as indicated by the overlapped data.

32

At boiling incipience, heaters experienced a temperature overshoot. The surface

superheat and the temperature overshoot at boiling incipience were recorded as 18.0oC and

2.9oC for the Ref # 4, 19.5oC and 5.1oC for the Ref # 4 R, and 23.5°C to 6.5°C for the Ref # 4 R P.

Temperature overshoot is a common phenomenon that is observed at the boiling incipience of

highly wetting liquids, such as HFE -7100 dielectric liquid. It is marked by a sudden temperature

decrease on the boiling surface upon the start of nucleation at numerous sites simultaneously.

Since highly wetting liquids tend to fill even the small surface cavities, that might contain

air/vapor and potentially serve as active nucleation sites, the boiling on such a surface requires

a larger surface superheat in order to grow and activate the smaller air/vapor pockets

remaining on the surface cavities. When the nucleation eventually starts, and as the bubbles

depart from the surface, they are replaced by fresh, colder incoming liquid, and the surface

experiences a sudden temperature drop although the heat flux remains the same. Temperature

overshoot is not a desired aspect since nucleation starts at larger surface superheats resulting

in higher device temperatures, and it is followed by a sudden temperature drop causing

thermally induced stresses on the device. Moreover, for the case of immersion cooling with

highly wetting liquids, the surface superheat and the magnitude of the following temperature

overshoot at boiling incipience a requite unpredictable.

In the nucleate boiling mode, three tests showed similar trends, but their heat transfer

performance slightly varied. The heater Ref # 4attained CHF of 19 W/cm2ata surface superheat

of 30.5°C, which represents the practical upper limit for immersion cooling with this particular

surface. The heater Ref # 4 Preached CHF at 19 W/cm2 at a surface superheat of 33°C. The

heater Ref # 4 P R had the same CHF at 19 W/cm2 at a surface superheat of 34°C.

33

Figure 13: Boiling curves of the reference surfaces.

In an effort to better represent the performance variation of the reference surface, and

simplify the comparison with other surfaces, the data from the three tests were averaged (by

calculating the arithmetic mean of surface superheats from the three tests at a given heat flux

level)to get a single baseline data, represented as the heater Ref (black solid line in Figure 13)

throughout this study.

As a common way to define the heat transfer performance, heat transfer coefficient

(HTC, or h) values are also calculated and included in Figure 14 as a function of heat flux. As can

be seen in this plot, HTCs increase as heat flux increases due to more rigorous boiling (or

34

increased frequency of nucleation), and reaches the highest value at 19 W/cm2near CHF. The

highest HTC value for the baseline data (Ref) is 6,100 W/m2oC at a heat flux of 19 W/cm2.

Figure 14: HTC of reference surfaces as a function of heat flux.

4.2 Hydrophobic Surfaces

Once the baseline data set from the reference surface (Ref) was obtained, investigation

of the hydrophobic surfaces was started. This group of heaters involved a fully hydrophobic

surface, as well as two patterned hydrophobic surfaces that feature circular patterns of 40 and

100 µm in size.

35

The data from fully hydrophobic surface, HPho#17, is plotted in Figure 15 against the

reference surface Ref. Initially, at very low heat fluxes (≤2 W/cm2), the surface HPho #17 had a

similar boiling curve, but it did not experience any temperature overshoot at the boiling

incipience at 3 W/cm2 heat flux and approximately 15oC surface superheat. The surface HPho

#17 smoothly transitioned to nucleate boiling regime and provided a higher heat transfer rate

in the low-to-mid heat flux range as indicated by a steeper curve. However, the surface HPho

#17 reached CHF at a much lower level at 14 W/cm2with a surface superheat of 22.7°C.The low

CHF from the fully hydrophobic surface can be explained by the inherent characteristic of this

coating that would considerably reduce the working fluid’s wetting ability; in other words,

facilitate the formation of vapor layer on the surface that leads to CHF.

Figure 15: Boiling curves of reference surface and fully hydrophobic surface.

36

Results from the patterned hydrophobic surfaces HPho_40 #6 and HPho_100 #7 are

shown in Figure 16. When compared to the surface Ref, nucleate boiling for HPho_40 #6

started much earlier at 2.0W/cm2and at a surface superheat of 9.5°C, with no distinct

temperature overshoot at the boiling incipience. This surface superheat level for HPho_40 #6

was around 7.5°C lower than that of the surface Ref. In addition to the early transition to

nucleate boiling, HPho_40 #6 provided a better performance, or higher HTC, throughout, as

indicated by the boiling curve shifted to left of the surface Ref. The HPho_40 #6 attained a CHF

of 22.5 W/cm2 at 27.6°C surface superheat. Therefore, patterned hydrophobic surface also

improved CHF over the surface Ref, although such improvement was not expected initially. The

other patterned hydrophobic surface, HPho_100 #7, experienced earlier nucleation compared

to HPho_40 #6, starting at 1.0 W/cm2 heat flux at surface superheat of 2.8°C with no obvious

temperature overshoot at boiling incipience. Compared to the surface Ref, the surface

superheat at boiling incipience was around 7°C lower. With the help of active nucleation at low

surface superheats, performance of HPho_100 #7 was even better than the surface HPho_40#6

throughout the nucleate boiling regime. HPho_100 #7 also attained a higher CHF of 25.5 W/cm2

at a surface superheat of 25.7°C.

37

Figure 16: Boiling curves of reference and patterned hydrophobic surfaces.

Figure 17 illustrates the surface superheat and temperature overshoot data at boiling

incipience from this study along with relevant data from an earlier study [3]. Figure 17a shows

the high surface superheats from the reference surfaces ranging from 20 to 24°C, along with

those from patterned hydrophobic surfaces at 9.5°C and 9°C. The temperature overshoots

observed in Figure 17b for reference surfaces are in the range of 5°C to 11°C. The patterned

hydrophobic surfaces HPho_40 #6 and HPho_100 #7showed negligible temperature overshoots

of 0.15°C and 0°C, respectively.

38

Figure 17: Surface superheat (a) and temperature overshoot (b) at boiling incipience of reference and patterned hydrophobic surfaces

The observed performance enhancement with the surfaces HPho_40 #6 and HPho_100

#7 can be attributed to the hydrophobic patterns that promote bubble nucleation starting at

lower surface superheats. Hydrophobic patterns on a surface would help capture vapor/gas

embryos around the pattern border during the initial filling process, and therefore increase the

potential number of active nucleation sites, eventually enhancing HTC. The visual observations

with patterned hydrophobic surfaces in this study include bubbles of very similar sizes

0

5

10

15

20

25

Ref HPho_40 # 6 HPho_100 # 7

∆T s

at (o

C)

Current study

El-Genk and Bostanci [11]

0

3

6

9

12

15

Ref HPho_40 # 6 HPho_100 # 7

∆T o

ver (

o C)

39

nucleating from uniformly distributed sites, which support remarks from earlier research by

Takata et al. [19].

Figure 18 shows change of HTC as a function of heat flux for the surfaces Ref, HPho #17,

HPho_40 #6, and HPho_100 #7. Initially at low heat fluxes(<3 W/cm2), HTCs were low and

relatively steady in the single-phase natural convection dominated mode. Upon transition to

nucleate boiling mode, HTCs increased with increasing heat flux (or temperature) due to

stronger contribution of active nucleation sites. Highest HTCs (up to 9,900 W/m2oC) were

reached towards the end of fully-developed nucleate boiling regime, with corresponding

surface superheats of 22-28°C. The HTC enhancement provided by the patterned hydrophobic

surfaces over the smooth, plain surface in the mid-to-high heat flux region (>12W/cm2) were up

to 40%, and 64.3% for HPho_40 #6 and HPho_100 #7 surfaces, respectively. Moreover,

hydrophobic patterned surfaces provided higher CHF values compared to the surface Ref;

specifically 18.4% and 31.5% with HPho_40 #6 and HPho_100 #7, respectively. Previous studies

[17-18] that evaluated similar surface characteristics with water, also showed an increase in

CHF which was not necessarily expected, since a hydrophilic surface condition would mainly be

responsible for such improvement. Most likely the hydrophobic patterns control the size and

departure of the vapor bubbles and offer an extended CHF.

40

Figure 18: HTC of reference and hydrophobic surfaces as a function of heat flux.

4.3 Hydrophilic Surface

This group of heaters involved a fully hydrophilic surface, as well as three patterned

hydrophilic surfaces that feature circular patterns of 40, 100, and 250 µm in size. The data from

fully hydrophilic surface, HPho#18, is plotted in Figure 19 against the reference surface Ref.

Initially, at very low heat fluxes (≤2 W/cm2), the surface HPho #18 had a similar boiling curve,

but it did not experience any temperature overshoot at the boiling incipience at 2.5 W/cm2

heat flux and approximately 16.5oC surface superheat. The surface HPho #18 smoothly

transitioned to nucleate boiling regime and provided a higher heat transfer rate throughout this

regime as indicated by a steeper curve. Moreover, the surface HPho #18 reached CHF at a

41

higher level at 27.5 W/cm2with a surface superheat of 30°C.Thehigh CHF from the fully

hydrophilic surface can be explained by the inherent characteristic of this coating that would

considerably help the working fluid’s wetting ability by wetting the surface sufficiently and

efficiently and thus delaying the formation of vapor layer on the surface that leads to CHF.

Figure 19: Boiling curves of reference surface and fully hydrophilic surface.

Results from the patterned hydrophilicsurfacesHPhi_40 #12,HPhi_100 #13 and

HPhi_250 # 11 are shown in Figure 20. When compared to the surface Ref, nucleate boiling for

HPhi_40 #12 started much earlier at 2.0W/cm2 and at a surface superheat of 13.5°C, with no

distinct temperature overshoot at the boiling incipience. This surface superheat level for

HPhi_40 #12 was around 3.5°C lower than that of the surface Ref. In addition to the early

42

transition to nucleate boiling, HPhi_40 #12did not provide a greater performance, but it did

provide a higher HTC, throughout, as indicated by the boiling curve. The HPhi_40 #12 attained a

CHF of 26W/cm2 at 36.5°C surface superheat. Therefore, the patterned hydrophilic surface

mainly improved the CHF over the surface Ref, as expected. The other patterned hydrophilic

surface, HPhi_100 #13, experienced a later nucleation compared to HPhi_40 #12, starting at 3.0

W/cm2 heat flux at surface superheat of 14.5°C with no obvious temperature overshoot at

boiling incipience. Compared to the surface Ref, the surface superheat at boiling incipience was

around 2.5°C lower. The performance of HPhi_100 #13 was at par with the surfaceHPhi_40#12

throughout the nucleate boiling regime. HPho_100 #13 also attained a high CHF of 24W/cm2 at

a surface superheat of 35°C. The other patterned hydrophilic surface, HPhi_250 #11,

experienced an earlier nucleation compared to HPhi_100 #13, starting at 2.0 W/cm2 heat flux at

surface superheat of 13°C with no obvious temperature overshoot at boiling incipience.

Compared to the surface Ref, the surface superheat at boiling incipience was around 4°C lower.

The performance of HPhi_250 #13 overlapped the data of the surfaces HPhi_40#12 and

HPhi_100 #13 throughout the nucleate boiling regime. HPho_250 #13 also attained a high CHF

of 25W/cm2 at a surface superheat of 35.5°C.

43

Figure 20: Boiling curves of reference surface and patterned hydrophilic surfaces.

Figure21 shows change of HTC as a function of heat flux for the surfaces Ref, HPhi #18,

HPhi_40#12, HPhi_100 #13, and HPhi_250 #11. Initially at low heat fluxes(<3 W/cm2), HTCs

were low and relatively steady in the single-phase natural convection dominated mode. Upon

transition to nucleate boiling mode, HTCs increased only for the fully hydrophilic surface with

increasing heat flux (or temperature). Highest HTCs (up to 9,800 W/m2°C) were reached

towards the end of fully-developed nucleate boiling regime, with corresponding surface

superheats of 25 - 30°C for the fully hydrophilic surface. The HTC enhancement provided by the

patterned hydrophilic surfaces over the smooth, plain surface in the mid-to-high heat flux

region (>12W/cm2) were low, at 13%, 7% and 9% for HPhi_40 #12, HPhi_100 #13 and HPhi_250

44

#11 surfaces, respectively. However, hydrophilic patterned surfaces provided higher CHF values

compared to the surface Ref; specifically 36.8%, 26.3% and 31.5% with HPhi_40 #12, HPhi_100

#13 and HPhi_250 #11, respectively. Based on the previous studies [17-18] that evaluated

similar surface characteristics with water, an increase in CHF was expected, since a hydrophilic

surface condition would mainly be responsible for such improvement due to better wettability

and delayed occurrence of vapor coalescence near CHF.

Figure 21: HTC of reference and hydrophilic surfaces as a function of heat flux.

45

4.4 Hydrophobic/Hydrophilic Surface

This final group of heaters involves a combination of hydrophobic and hydrophilic

surfaces that feature circular patterns of 40and 250 µm in size.

Results from the patterned hydrophobic/hydrophilic surfaces HPho/HPhi_40 #20 and

HPho/HPhi_250 #19 are shown in Figure 22. When compared to the surface Ref, nucleate

boiling for HPho/HPhi_40 #20 started much earlier at 1.5 W/cm2and at a surface superheat of

8.5°C, with no distinct temperature overshoot at the boiling incipience. This surface superheat

level for HPho/HPhi_40 #20 was around 8.5°C lower than that of the surface Ref. In addition to

the early transition to nucleate boiling, HPho/HPhi_40 #20 provided a better performance, or

higher HTC, throughout, as indicated by the boiling curve shifted to left of the surface Ref. The

HPho/HPhi_40 #20 attained a high CHF of 28W/cm2at 33.5°Csurfacesuperheat. Therefore,

patterned hydrophobic/hydrophilic surface also improved CHF over the surface Ref, as

expected. The other patterned hydrophobic surface, HPho/HPhi_250 #19, experienced a little

delayed nucleation compared to HPho/HPhi_250 #19, starting at 2W/cm2 heat flux at surface

superheat of 8.5°C with no obvious temperature overshoot at boiling incipience. Compared to

the surface Ref, the surface superheat at boiling incipience was around 8.5°C lower. With the

help of active nucleation at low surface superheats, performance of HPho/HPhi_250 #19 was

comparable with the surface HPho/HPhi_40#20 throughout the nucleate boiling regime.

HPho/HPhi_250 #19 also attained a higher CHF of 27W/cm2 at a surface superheat of 29.5°C.

46

Figure 22: Boiling curves of reference surface and patterned hydrophobic/hydrophilic surfaces.

Figure23 shows change of HTC as a function of heat flux for the surfaces Ref,

HPho/HPhi_40 #20 and HPho/HPhi_250 #19. Initially at low heat fluxes(<3 W/cm2), HTCs were

low and relatively steady in the single-phase natural convection dominated mode. Upon

transition to nucleate boiling mode, HTCs increased with increasing heat flux (or temperature)

due to stronger contribution of active nucleation sites. Highest HTCs (up to 10,000 W/m2oC)

were reached towards the end of fully-developed nucleate boiling regime, with corresponding

surface superheats of 25-33°C. The HTC enhancement provided by the patterned

hydrophobic/hydrophilic surfaces over the smooth, plain surface in the mid-to-high heat flux

region (>12W/cm2)wereupto49%,and59% for HPho/HPhi_40 #20 and HPho/HPhi_250 #19

surfaces, respectively. Moreover, hydrophobic/hydrophilic patterned surfaces provided higher

47

CHF values compared to the surface Ref; specifically 47.3% and 42.1% with HPho/HPhi_40 #20

and HPho/HPhi_250 #19, respectively. Previous studies and earlier results that evaluated

similar surface characteristics showed an increase in CHF for a hydrophilic surface due to its

high wetting characteristic which delays the formation of the vapor layer thus delaying CHF and

that of a hydrophobic surface wherein the surface controls the size and departure of the vapor

bubbles causing an increase in HTC. Therefore both these characteristics of HTC and CHF were

achieved in the mixed pattern since they combine the benefits of hydrophobic and hydrophilic

surfaces.

Figure 23: HTC of reference and hydrophobic/hydrophilic surfaces as a function of heat flux.

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4.5 Overall Performance Comparison

Based on the obtained results from the four types of surfaces, this section aims to

summarize the performance trends and improvement levels by comparing selected data side by

side.

Figure 24: Boiling curves of reference and enhanced surfaces as a function of heat flux.

Data in Figure 24 indicates that all the surfaces eliminate temperature overshoot

effectively as compared to reference surface Ref. The nucleate boiling for HPho_40 and

HPho/HPhi_40 surfaces start much earlier compared to the Ref and the other surfaces, at 2.0

W/cm2at surface superheats of 9.5°C and 8.7°C, respectively.

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Figure 25: HTC of reference and enhanced surfaces as a function of heat flux.

The highest HTC enhancement is provided by the HPho_40 and HPho/HPhi_40 surfaces

as well, in low-to-mid heat flux region, reaching 40% and 49% enhancements, respectively,

compared to the surface Ref. In high heat flux region, the surface HPhi provided comparable,

even slightly better performance in terms of HTC.

The significant improvements in CHF, as the other important performance criteria, were

achieved by the surfaces HPho/HPhi_40 and HPhi, at28 W/cm2, and 27.5 W/cm2, respectively,

compared to the Ref.

In summary, this study aimed to improve nucleate boiling heat transfer performance by

(a) eliminating temperature overshoot phenomena at boiling incipience, (b) increasing HTC, and

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(c) increasing CHF level. Figures 24-25 clearly illustrates what hydrophobic and hydrophilic

surface characteristics can provide towards these goals. Based on the obtained experimental

data we can conclude that, all three performance aspects can be achieved by utilizing a mixed

patterned hydrophobic/hydrophilic surface that promotes early nucleation, effectively

eliminates temperature overshoot at boiling incipience, and significantly increases both HTC

and CHF. Table 4 illustrates how different types of tested surfaces performed based on the

three aspects of nucleate boiling heat transfer.

Table 4

Performance Summary of Tested Surfaces

Surface Temperature overshoot

HTC Enhancement

CHF Enhancement

Reference Present - - Hydrophobic Eliminated Low Very low Patterned hydrophobic Eliminated Medium Medium Hydrophilic Eliminated Medium High Patterned hydrophilic Eliminated Medium High Hydrophobic/Hydrophilic Eliminated High High

HTC : < 6000 W/m²°C - very low; < 8000 W/m²°C - low; < 10000 W/m²°C - medium; > 10000 W/m²°C - high

CHF: < 15 W/cm² - very low; < 20 W/cm² - low; < 25 W/cm² - medium; > 25 W/cm² - high

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CHAPTER 5

CONCLUSIONS

The last chapter presents concluding remarks, highlights important contributions, and

outlines some recommendations that can be considered for future related research.

5.1 Concluding Remarks

This study experimentally investigated the effect of hydrophobic, hydrophilic and

hydrophobic/hydrophilic patterned surfaces in saturated nucleate boiling of HFE-7100 dielectric

liquid. Based on the results obtained from a plain, smooth surface(Ref), two hydrophobic

patterned surfaces (HPho_40,HPho_100), three hydrophilic patterned surfaces (HPho_40,

HPho_100, and HPho_250), and two hydrophobic/hydrophilic surfaces (HPho/HPhi_40,

HPho/HPhi_250) the following conclusions can be drawn:

• Hydrophobic patterned surfaces would help capturing vapor/gas around the pattern

border, increase number of active nucleation sites, and promote nucleation, resulting in

(a) an early transition to nucleate boiling, and (b) effective elimination of the

temperature overshoot at boiling incipience.

• Hydrophobic patterned surfaces enhance boiling performance throughout, with

up to 64.3% higher HTC, and up to 22.5% increase in CHF over the smooth, plain surface.

• Hydrophilic patterned surfaces would help wet the surface better because of their

highly wetting characteristic and thus delay occurrence of vapor coalescence near CHF,

and hence would increase the CHF.

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• Hydrophilic patterned surfaces do not substantially enhance HTC but increase CHF up to

36.8% over the smooth, plain surface.

• Hydrophobic/hydrophilic mixed patterned surfaces provide the optimum enhanced

surfaces since they would combine the benefits of hydrophobic and hydrophilic surfaces.

• Hydrophobic/hydrophilic mixed patterned surfaces thus achieve enhanced boiling

performance throughout, with up to 58.5% higher HTC, and up to 47.4% increase in CHF

over the smooth, plain surface.

The present study through experimental investigation emphasized that hydrophobic and

hydrophilic surface characteristics can substantially enhance the nucleate boiling heat transfer

performance of highly wetting liquids, such as the dielectric liquid HFE-7100, as well.

5.2 Recommendations for Future Research

Based on the experience obtained from this study, the following recommendations can

be made for future nucleate boiling studies that might consider to further evaluate the effect of

hydrophobic and hydrophilic surface characteristics on performance.

• The current study should be proceeded by identifying the effect of pattern size, as well

as the percentage coverage area of the hydrophobic and hydrophilic surfaces on the

nucleate boiling performance through additional experimental data and theoretical

analysis.

• Additional testing with hydrophobic and hydrophilic surfaces to evaluate their

consistency and durability would be an important step for their implementation in

practical applications.

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• Investigation of the similar surfaces with alternative working fluids such as water and

ethanol would provide useful database to better understand and predict the heat

transfer enhancement.

54

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