Boiling Crisis Phenomenon Part1

7/23/2019 Boiling Crisis Phenomenon Part1

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The boiling crisis phenomenonPart I: nucleation and nucleate boiling heat transfer

T.G. Theofanous *, J.P. Tu, A.T. Dinh, T.N. Dinh

Center for Risk Studies and Safety, University of California, Santa Barbara, 6740 Cortona Drive, Goleta, CA 93117, USA

Accepted 10 December 2001

Abstract

This paper (Part I) and the companion paper (Part II, Exp. Therm. Fluid. Sci. 26 (67) (2002) 793810) present results of an

experimental study on nucleate pool boiling. The experiments were conducted under highly-controlled conditions, using electrically

heated, vapor-deposited sub-micron metallic films. A high-speed, high-resolution infrared camera was used to visualize dynamic

thermal patterns on the heaters surface over a broad range of heat fluxes, starting from the onset of nucleation and up to boiling

crisis. Both fresh heaters and aged heaters were experimented with. The heaters surface nanomorphology and chemistry were

characterized with atomic force microscopy, scanning electron microscopy, and X-ray diffraction spectroscopy. First-of-a-kind

experimental data on nucleation and boiling heat transfer at high heat fluxes are presented, and a stark difference between fresh and

aged heaters is revealed. Remarkable are the origin, evolution and dynamics of the heater dryout process (leading to burnout),

identified quantitatively and captured in action for the first time.

2002 Elsevier Science Inc. All rights reserved.

Keywords: Nucleation; Nucleate boiling; Boiling heat transfer; Boiling crisis

1. Introduction

Performance of boiling equipment is limited by a

transition from nucleate to film boiling, a regime of

deficient heat transfer (see Fig. 1), characterized by a

dried out heater surface and accompanied by the ulti-

mate physical destruction of the heaterthe so-called

burnout. The phenomenon that causes this transition

is called boiling crisis, and the heat flux at which this

maximum in performance occurs is called critical heat

flux (CHF). In this two-part paper, we are interested in

the conceptual and quantitative definition of this tran-

sition for an infinite flat plate under pool boiling con-

ditions. Part I is concerned with key aspects leading up

to boiling crisis, and these include nucleation and nu-

cleate boiling heat transfer, especially under high (near

burnout) heat flux conditions.

At a very high level of generality, basic considerations

lead to an overall structure of the problem, as illustrated

in Fig. 2. Microlayers are the micrometer-scale liquid

layers left attached to the heater surface, beneath vapor

bubbles that grow on it (i.e. [2]). These microlayers oc-

cupy a central position because on the one hand, they

interface with the heater surface to effect phenomena

such as nucleation, and hydrodynamic action around

contact lines, while on the other, they interact with

the external two-phase hydrodynamics to effect

liquid supply and possibly rejection (re-entrainment)

phenomena. The net effect of all these interactions,

in principle, define the microhydrodynamics that under

appropriate conditions lead to dryout (and burnout).

As the figure illustrates, the problem is clearly of mul-

tiscale character. It is also complex, not only in its

multiphysics content (fluid, solid, thermal physics), but

also in the apparent presence of a multitude of poten-

tially highly interactive instability mechanismsnucle-

ation, contact line motion, counter-current vaporliquid

flow.

Clearly, comprehensive, first-principle simulations

are out of question, and more focused modelling ap-

proaches have been hindered by the mechanisms re-

maining effectively veiled behind the heavy multiplicity

Experimental Thermal and Fluid Science 26 (2002) 775792

www.elsevier.com/locate/etfs

* Corresponding author. Tel.: +1-805-894-4900; fax: +1-805-893-

4927.

E-mail address: [email protected] (T.G. Theofanous).

0894-1777/02/$ - see front matter 2002 Elsevier Science Inc. All rights reserved.

PII: S 0 8 9 4 - 1 7 7 7 ( 0 2 ) 0 0 1 9 2 - 9
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of chaotically undulating liquidvapor interfaces (see

insets in Fig. 1). As a result, all we have available are ad

hoc approaches, based on hypotheses that embrace one

or another mechanism, active at one or another

scale.

In particular, we have: (a) crisis due to breakdown of

stability in external hydrodynamics ([36]), (b) dryout

spreading due to attainment of a critical temperature at

the contact lines ([7]), (c) contact line instability due to

vaporization recoil ([8]), and (d) rapid vapor blanket-

ing due to heterogeneous spontaneous nucleation ([9]).

In addition, we have reductionist approaches that are

based on superposition of idealized elementssuch as

nucleation site density (NSD), bubble growth and de-

parture, vapor microjets that penetrate a liquid macro-

layer which is vaporizing (at the contact lines) around a

dried out baseand presume that burnout is reached

gradually, through a succession of states, which with

increasing heat flux yield increasing (macroscopically)

heater dried out areas ([1015]).

Current understanding is well summarized in a recent

review by Dhir [16]. For pool boiling from horizontal

flat plates, he delineates two kinds of limits to coola-

bility. One is heater-surface-controlled, and pertains to

poorly-wetted heaters. The other is (macro)hydrody-

namically-controlled, and obtains as a limit for suffi-

ciently well-wetted heaters. The surface-controlled limit,

thought to originate in merging of the dryout areas

noted above (reductionist approach) has been related to

the applicable contact angles [10], as illustrated in Fig. 3.

The data base in this regime is meager, and as Dhir

notes, no clear consensus exists in the technical com-

Nomenclature

g gravitational acceleration, m/s2

Hlv latent heat of evaporation, J/kg

N nucleation site density, cm2

q heat flux, kW/m

2

R individual gas constant, J/(kg K)

Rc cavity size, lm

t time, s

T temperature, C

Greek symbols

d capillary length, m

q density, kg/m3

r coefficient of surface tension, N/m

DT temperature difference, K

Subscripts and superscripts

a adiabaticc cooldown, cooling

cr critical

liquids superheat

sat saturation

v gas, vapor

w wall, glass substrate

Fig. 1. The pool boiling curve. The inserts illustrate the visual obscuring that builds up with heat flux due to corresponding increases of vapor

content. The left insert is an isolated bubble regime obtained at low heat fluxes ($100 kW/m2). The right insert shows a complex two-phase con-figuration at a moderate heat flux (400 kW/m2). Near crisis ($1 MW/m2) the view is completely obscured.

776 T.G. Theofanous et al. / Experimental Thermal and Fluid Science 26 (2002) 775792


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munity as to the actual mechanisms . . . involved. The

hydrodynamic limit (see Fig. 3) thought to arise due

to liquid depletion for well-wetted heater surfaces, has

been related to vapor counterflow interfering with liq-

uid supply ([3,5,17,18]). The result is

qcr CkqvHlvrgq qv

q2v

1=41

where q and qv are the liquid and vapor densities, Hlv is

the latent heat of vaporization, r is the surface tension,

and gis the acceleration due to the applicable body force

field. While apparently well-established, as discussed in

Part II, this limit turns out to be very problematic, too.

The confusion is well described by Sadasivan et al. [19],

who underscore the need for new experimental ap-

proaches, and it is rendered in philosophical and poetic

twists in a very enjoyable lecture by Lienhard [20].

In this Part I of the paper, we address three main

topicsthe experiment, nucleation, and nucleate boil-

ingSections 24, respectively. Besides the description

of the experimental apparatus (referred to as the BETA

experiment), Section 2 includes the instrumentation and

key aspects of measurement, as well as the experimental

techniques utilized, particularly in relation to establish-ing the heater aging and purity control protocols. In

Section 3, we provide data that for the first time depict

nucleation at high heat fluxes directly (past the obscur-

ing noted in Fig. 1). Thus, we make accessible, also for

the first time, such important features as NSDs, spatial

distributions, and even dynamics. A stark dependence

on heat surface aging is revealed. On the other hand, we

show that nucleation can take place on nanoscopically-

smooth surfaces, and this raises interesting new ques-

tions on the mechanism(s) of heterogeneous nucleation.

In Part II, we connect all this to boiling crisis, both

empirically, by the relation to CHF performance, as well

as conceptually, in terms of the scale separation

phenomenon and the attendant microhydrodynamics.

On the other hand, we find that the mechanistic link

between nucleation and burnout is in the form of hot

spots that appear dynamically at the center of a small

fraction of the bubble-cooled areas on the heater sur-

facethat is, in nucleate boiling. In Section 4, we ad-

dress this behavior by examining the detailed dynamics

of these hot spots, and the context in which they arise

from in bubble dynamics. We show that actually these

are dry spots; and in Part II, we follow up with data that

show how burnout originates by the sudden, irreversible

Fig. 2. Schematic of processes involved in boiling and their relations to burnout. The scale separation is explained in Section 4 of Part II [1].

Fig. 3. The limits to coolability on a horizontal, infinite flat plate

(adapted from Dhir [16]). The hydrodynamic limit is approached with

improved wetting. qcr is the CHF, and qKZ is the KutateladzeZuber

value given by Eq. (1).

T.G. Theofanous et al. / Experimental Thermal and Fluid Science 26 (2002) 775792 777


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spreading (i.e., instability) of a small (but in general not

singular) sub-set of such dry spots.

2. The BETA experiment

To obtain definitive results we chose to perform ex-

periments in the simple configuration of pool boiling on

a horizontal upward-facing (effectively) infinite flat platewith uniform heat flux. The chief idea is to maximally

reduce/eliminate extraneous factors. This requires us to

control, define, and characterize the experimental con-

ditions at a very high level of detail. The experimental

approach was designed to allow:

(a) isolation of the fluid dynamicsthis was achieved

by nano-scale film heaters that impose an essentially

constant (and uniform) heat flux;

(b) direct observation of the dynamic thermal pattern

of the heater surface, including the origin(s) and

spreading of dryoutthis was accomplished by high

speed infrared (IR) imaging of the nano-film heater

from below;

(c) direct observation of the detailed void pattern and

consequently of the underlying liquid motionsthis

was possible by quantitative flash X-ray radiogra-

phy.

2.1. Experimental arrangement

The experimental set up is schematically illustrated in

Fig. 4. The test vessel was made with optical quality

Pyrex glass, and heating was provided by passing elec-

trical current at controlled voltages through a nano-film

heater. Special techniques allow gasket-free sealing and

hence avoidance of contamination.

In Configuration A (Fig. 4), the test section is a

rectangular glass vessel, closed at the bottom by the

heater element, it occupying the entire 20 40 mm, (or26:5 40 mm) cross-section of the vessel. We wished toeliminate end effects. That our test section represents

well an infinite flat plate geometry is demonstrated bythe uniformity of the boiling process, as revealed by the

IR imaging (see Section 3). The pool above the heater

has dimensions that are 8 and 16 times of the capillary

length, d, where d fr=gq qvg1=2

[21]. It is about

2.5 mm for water boiling at atmospheric pressure. These

dimensions are much greater than the minimum (2d)

required according to Gogonin and Kutateladze [22]. 1

To confirm this further, we ran an experiment with the

test section sub-divided into 8 cells, 1-cm square each,

obtained by inserting an egg crate-like structure made

from stainless steel foil (200 lm thick). The results in

this 4d geometry were indistinguishable from those ob-

tained without the partition (8d 16d).In Configuration B, the side walls of the test section

are reduced to a height of 1 mm so as to allow direct

visual observation from above. The boiling microfilm in

this case is related to that in Configuration A through

the scale separation phenomenon discussed in Part II.

Fig. 4. Schematic of the BETA experiment setup. Temporal and spatial resolutions shown in parenthesis are upper limits possible with the available

instruments.

1 Through a systematic experimental study Gogonin and Ku-

tateladze [22] and Gogonin et al. [23] showed that the heater size effect

on CHF in pool boiling is absent as it becomes larger than 2d. For

heaters smaller than 2d, there was a pronounced effect.



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The test vessel (Configuration A) is shown in Fig. 5together with the IR camera. At right angles to the IR

camera, is a Kodak digital video camera (not shown)

with capability of up to 20,000 frames per second at

resolution of 34 128 pixels. The two cameras can besynchronized. A gold-coated mirror and special IR op-

tics are used in the arrangement. Much higher spatial

resolutions are possible with additional lenses, and a

Sapphire heater-film substrate could further enhance the

IR transmission characteristics.

2.2. The BETA heaters and surface characterization

The heater elements are manufactured by electron

beam metal vapor deposition on glass (or sapphire)

substrates. In the main series of BETA experiments re-

ported here we used titanium films of 140, 270, 300, 450,

500 and 1000 nm deposited on 130 lm borosilicate glass.

The films are heated by passing electrical current (DC or

AC with controlled waveform), and power generation is

uniform under boiling conditions. Each burnout exper-

iment leads to heater failure, thus it was very important

to ensure reproducibility in manufacturing and han-

dling these heaters. Further, we needed to be sure that

such failures were not premature under the intense heat-

ing conditions necessary to reach burnout in water

(well in excess of 1 MW/m2), and this required signifi-

cant developments in choice of materials and manufac-

turing procedures.

Power generation in the nanofilms depends on the

local electrical resistance which varies with temperature

in the manner shown in Fig. 6 (typical). Accordingly, in

the nucleate boiling relevant range (100150 C), local

heat flux variations are limited to under $5%. Beyond150 C, as we enter the formation of dry spots and

temperatures increase rapidly, we lose locally some

power, and this reaches up to $30% at $380 C. At this

point a phase transformation occurs 2 which sends the

resistance plummeting by a factor of$3, but soon after,at 400 C, we also lose integrity of the heater due to

failure of the borosilicate glass. As we see in Part II,

progression towards this state of burnout is recognized

well before this occurs, and well before significant loss of

power uniformity comes into play.

The surface of the as-manufactured (fresh) nano-

films exhibited such a high regularity as to allow

unprecedented (for boiling work) in detail character-

ization. Typical atomic force microscope (AFM) andscanning electron microscope (SEM) images are shown

in Fig. 7. Note the similar appearance (density) of

roughness. The amplitudes, as shown in the AFM image

are relatively uniform, with an rms value of 4 nm.X-ray diffraction spectroscopy (XDS) measurements

showed a primary peak at 38.5 and minor sub-peaks at

angles 53, 70, 83. These data indicate a base (hexago-

nal) crystal orientation of (0 0 2), together with a small

amount of randomly distributed (1 0 2), (1 0 3), (1 0 4).

Fig. 5. The setup of the BETA experiment. Not shown are the video

camera, associated lighting and the condenser on top of the vessel.

Fig. 6. Temperature dependence of a BETA heaters electrical resis-

tance (titanium film of 450 nm thickness). Phase transformation in the

titanium thin film is observed at 380 C.

2

It is known that vapor-deposited titanium thin films experience aphase transformation, from the so-called C49 phase to the C54 phase,

when heated [24,25]. Nucleation of C54 depends on the Ti film

thickness, impurity, and conditions of vapor deposition. Previous data

on phase transformation in titanium thin films relate to much thinner

films (10 nm) than that used in the BETA experiments. Nonetheless,

for the BETA 450 or 500 nm Ti film, we detected that a heating up to

400 C leads to phase transformations with a drastic reduction (2.5

times) of the films electrical resistance. The change in the resistance is

associated with the extent of phase transformation and some of it

remains after temperature recovery (cooling). We found a decrease in

the resistance change in repeated heat treatments. Effect of annealing

on electrical resistance change upon C49C54 phase transformation in

TiSi2 thin films was reported also by Lin and Lee [26]. Such a repeated-

heating procedure helps to relieve stresses in the thin film.



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The heaters were aged by pulse heating in air or steam

environment. Additional, but slow, aging occurs during

boiling in water. By aging we mean that CHF per-formance improves, and as we see later this is clearly

associated with increases in bubble nucleation density.

The aging protocol is described in Section 2.4 on the

experimental procedure. Examination of heaters sur-

faces after aging shows islands of inhomogeneity. The

SEM indicates low conductivity of these islands mate-

rials. Most likely, these islands are oxides formed as a

result of the heating process. Vaquila et al. [27] reported

that for temperature below 200 C the oxidation is

characterized by the presence of only one oxide phase:

TiO2. At higher temperatures, they found the pres-

ence of Ti2O3. The oxidation is confirmed by our XDSresults. Compared to that of fresh heaters, the XDS

spectra of aged heaters measured with a small angle

of incidence revealed an additional substantial peak at

46.5a characteristic of titanium oxide. SEM images

of an area containing one such island are shown on

Fig. 8. Heavy aging requires also a prolonged period of

boiling, leading to further oxidation, and perhaps de-

posits of minute impurities present inevitably even in

HPLC-class water utilized in many of our experiments.

The static and advancing contact angles were mea-sured for both fresh and aged heaters were essentially

indistinguishable at 6075. None of the above charac-

teristics were found to change after the handling neces-

sary for installation and running a heater to failure.

The density of inhomogeneities increases with the

number of repeated pulse heating cycles applied and

their duration. It is also known from the literature, and

confirmed in the BETA testing, that the steam envi-

ronment significantly accelerates the oxidation rate of

titanium thin films [28,29] and induces formation of

islands made of oxide and hydroxyl groups. Rigorously

speaking, even pulse heating in air is affected by thehumidity level present [30]. Repeated boiling and pulse

heating were found to bring the heater to a state defined

as heavily aged, with a very high density of surface

inhomogeneities. SEM images for such heaters are

shown in Figs. 9 and 10. These heaters resulted in the

highest values of CHF reached in this work (1.51.6

MW/m2).

Fig. 7. AFM (a) and SEM (b) images of the surface of a fresh nanofilm. (a) 3 3 lm2 area, vertical scale 15 nm/division and (b) 2:3 1:8 lm2,50,000 magnification.

Fig. 8. SEM images at two different magnifications, 10,000 (left) and 50,000 (right), of an area of a pulse-heated-in-air nanofilm with an oxide

island.



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This nano-scale view of a heater surface contrast with

previous characterizations, based on profilometetry of

micron-scale roughness (e.g., Benjamin and Bala-

krishnan [31]). It also contrasts, in implication, with the

cavity-entrapment-focused models of nucleation [32,33].

Here we have nano-scale smoothness to begin with, and

we are able to induce a whole wide range of nucleation

and CHF behavior with sub-micron scale deposits,

that are visually imperceptible, and only partially con-

trolled (the pulse-heating part) even under extremely

protective measures against contamination (HPLC-class

water, boiling under cover, well-qualified heater han-

dling procedures, etc.). The obvious question is whether

in all previous experiments roughness was anything

more than an innocent bystander.

In addition to the nanofilms, we utilized a 5-cm thick

copper block heater, powered with embedded electri-

cally-heated cartridges and fitted to a similarly-sized test

section (Configuration A). Heavy aging of this copper

surface was obtained by rubbing aluminum oxide par-

ticles onto the highly-polished copper surface. We fol-

lowed the method of Wang and Dhir [33], except for

using somewhat smaller particles1 lm rubbed-in with

a cotton cloth, and 37 nm rubbed-in with velvet. The

surface was found to be well-wetting with static contact

angle of 1215.

2.3. Instrumentation

In this subsection, we take a closer look at the two

key diagnostics employed in this studyIR thermome-

try and X-ray radiography (X-R). The development and

use of the latter arose as a consequence of our inter-

pretation of certain IR results and a compelling need

for independent verification.

2.3.1. Infrared thermometry

IR thermographic techniques have been used for a

long time in heat transfer and fluid dynamics research

[35]. The main element of the IR thermometry is a de-

tector sensitive in the 812 lm IR wavelength range.

Modern applications of the high-speed high-resolution

IR thermographic technique include applications to

basic heat transfer [36] and in the environmental, in-

dustrial, and medical areas; see e.g. [37,38].

For boiling heat transfer, due the high frequency of

physical processes of interest, it is essential to have high-

speed imaging and storing capability. A first attempt in

Fig. 9. SEM images of an aged heater at two different magnifications: 5000 (left) and 50,000 (right).

Fig. 10. SEM images of a heavily aged heater at two different magnifications: 2000 (left) and 80,000 (right).



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using this method in boiling research was made by

Myerss group at UCSB in the early 70s. They used thin

heater plates covered with liquid crystals [39] and sep-

arately a high-speed IR camera [40] to study nucleation

in pool boiling. Technological limitations at the time,

however, allowed only qualitative results. Later, Ken-

ning [41] developed further the liquid crystal thermo-graphic technique and obtained interesting results for

low heat flux nucleate boiling [42]. However, the low

space, and time resolution prevented them too from

going to the heat fluxes (and phenomena) of interest

here (only 200 kW/m2 as compared to the well over 1

MW/m2 needed).

Key features of our (state-of-the-art) camera (Focal

Plane, Model ImagIR) can be summarized as follows.

The camera optical plane is made up of an array of

320 256 pixels, each covering an area of 30 30 lm2.Since each pixel measures a single temperature (related

to the average IR radiation flux received over its surface

area), this dimension (30 lm) defines the spatial reso-

lution limit. Over our normally-employed calibration

range (373473 K), the measurement itself is accurate

to within 0.5 K. The pixel frequency response is inthe MHz range, so the temporal resolution is limited

only by the software/hardware responsible for grabbing,

transferring, and storing data. Starting from the basic

320 256 full-window framing rate of 0.4 kHz,higher rates can be obtained by reducing the window

size, up to a maximum of 10 kHz with a 8 128 win-dow. Within this range of windows (pixel arrays), and

framing rates, actual resolutions can be further modified

by appropriate selections of optics (magnified view ofportions of the heater), and in general multiple/com-

plementary views are possible. The results presented

here were obtained with a 128 128 array viewing thewhole heater (resolution of$250 lm/pixel), at a framingrate of 1 kHz.

Crucial to achieving the needed high resolutions is

also the extremely low heat capacity and (in-plane)

thermal conductivity of the nano-film glass assembly.

By means of simple conduction calculations, we estab-

lished that the frequency response of the nanofilm and

the glass substrate subject to an oscillatory heat flux

over a typical bubble-cooled area (1 mm2) was adequate

up to $1 kHz. Similarly, for excursive transients, such aslocal dryouts, we found that a spot as small as 200 lm

could be detected on a 10-ms time scale. The back trig-

gering capability allows us to reliably capture the burn-

out sequences in action, and the data give a first glimpse

into the dryout phenomenon itself.

2.3.2. X-ray Radiography

In addition to the IR camera, we also make use of

X-ray attenuation to obtain complementary important

information about two-phase flow patterns in pool

boiling. Namely, we use flash X-ray imaging for the

measurement of void fraction to aid understanding

(through observed two-phase flow structures) the extent

to which the liquid has access to the heat transfer

surface. This exercise is extremely useful, especially in

conjunction with the IR thermometry.

Previously, only few attempts were made to measure

void fractions in pool boiling. Ida and Kobayasi [43],among others, used conductivity probes to measure the

local void fraction above the heated surface of 29-mm

diameter disk heater. To avoid the intrusive measure-

ment, Liaw and Dhir [44] used a densitometer to detect

attenuation of a gamma beam traversing a 63 mm water

pool, in parallel with a vertical heated surface. The

common characteristic of all previous measurements is

their limitations to time averaged, local void fractions

(point or line averages). This contrasts with the present

effort, whose interest is in identification of two-phase

flow patterns.

Measurement principles and procedures of the

quantitative radiography method employed in the pre-

sent work were discussed by Theofanous et al. [45]. A

detailed account of the technique and results of X-ray

imaging of pool boiling will be presented elsewhere.

Here we note that through detailed calibrations and

tests we can assert that we can approach the heater

surface to within 500 lm with an accuracy of5% inabsolute void fraction over the whole range (01).

2.4. Experimental procedure

Starting from a well-characterized initial condition

(fresh heater) the nanofilms were aged to variousdegrees and by different methods. As noted above the

intent of aging was to improve burnout performance.

The most effective way for aging we found was to pulse-

heat repeatedly a dry nano-film reaching 350400 C,

within 2 s, allowing it to cool rapidly, and then to let it

stay for a period of time before the test section is filled

with water. This aging procedure is monitored by mea-

surement of electrical resistance, to control the degree

of aging.

The experiments were carried out with the highest-

purity water availableHPLC class, used in liquid

chromatography. The water was degassed by boiling for

10 min. We used the IR camera to monitor the presence

and detachment of gas bubbles, by their footprints on

the heaters, easily identified as bright (hot) spots. After

degassing, the heater is fully wetted and the test is run at

different heat fluxes by applying corresponding voltages

to the heater. After each heat flux level has been stabi-

lized for a period of time, a IR record (4 s) is obtained

(using back triggering at high fluxes to ensure capturing

the burnout event). The test run is designed to be short

so that the nominal degree of aging is not materially

affected, and to minimize chance for contamination.

Detailed examination of surface nanomorphologies



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before and after a test demonstrated that our handling

procedures and the contamination-free test section do

not induce any alterations.

Power input to the heater was measured by online

recording of applied voltage and resulting current. The

data acquisition system (DAS) is set up so as to record

power transients as well. Power is controlled manuallyvia a control signal that is subsequently amplified in the

power supply. The control signal and the recording

signal are filtered to remove high-frequency noise. The

voltage and current are fed to the DAS to record the

power. Each instrument features an error that is esti-

mated within 2%, adding up to about 4% error in thepower measurement.

3. Nucleation

In this section we address the nucleation character-

istics of our heaters, as determined through our IR

datadisc-shaped cool (dark) spots that grow from a

point (the nucleation site) reach a maximum size, and

disappear. The key parameter is NSD; that is, the

number of bubbles that are found (at any time) per unit

area of the heater surface. In our experiment we can find

this directly by counting in any given IR frame. Here,

we concentrate on averages over the whole heater area,

and over time (typically counting $1000 frames).NSD is an important integral characteristic of the

boiling process, whether pool or flow boiling as recog-

nized long ago. As we will see in Part II, the NSD plays

an important role in defining the topology and micro-hydrodynamics, including rupture, of the liquid micro-

layer. Besides NSD, of interest are the size of the bubble

base, the dynamics of bubble growth, and the thermal

characteristics of the heater surface in the absence and/

or in between nucleation events. These aspects are dis-

cussed in the next section.

3.1. Previous work

For the reasons already discussed, measurement of

NSDs in high heat flux boiling presents a very special

challenge. All few attempts made have been intrusive,

and the results are highly variable. Conceptual under-

standing and model/correlation development have been

guided by data obtained in low heat flux boiling (typi-

cally under 100 kW/m2), and low surface superheats,

which, not unexpectedly, are highly variable, too. Since

some of this work has been used nevertheless in building

models of boiling crisis (along the lines of the reduc-

tionist approach discussed in Section 1), we thought that

the brief account given below is necessary for complet-

ing the treatment that is undertaken here. A broader

exposition, consistent with the basic nature of the sub-

ject, that takes advantage of the new window of op-

portunity that has been created by BETA, will be

presented elsewhere.

Relevant for present purposes are the works of Ga-

ertner and Westwater [46], Wang and Dhir [47], Benja-

min and Balakrishnan [31] and Kocamustafaogullari

and Ishii [34]. Gaertner and Westwater used nickel salts

dissolved in water, and counted nucleation sites from thenumber of holes found on the electrochemically-depos-

ited nickel. They reached fluxes of up to 1 MW/m2 and

nucleation densities of up to 200 per cm2, and found the

density to be proportional to q2. Wang and Dhir gained

visual access to the heater surface by dousing it with

degassed, sub-cooled (515 K) water, and recorded (in

magnification) a 1 cm2 portion of the area by still

photography. The heater surface was mirror finished (to

0.02 lm) copper, oxidized by heating in the oven to

various degrees, so as to yield a range of contact angles

(18, 38 and 90). They found CHF to increase and

NSD to decrease with wetting, as follows: 570, 770 and

1100 KW/m2 and 700, 400 and 200 per cm2 for 90, 38

and 18, respectively. The nucleation site density was

related to contact angle (h), by

N$ 1 cos hD6c 2

where Dc, the diameter of activated wall cavity is a

function of the wall superheat ($1=DTs). Benjamin andBalakrishnan [31] attempted to gain optical access to the

heater surface by limiting the liquid quantity so as to

obtain a thin filmthey did not mention how thin the

film was, and it is not clear at all from the photos in the

paper how all nucleation events could be identified.

They used emery-paper polished stainless steel (rms

$0.2 lm) and aluminum (rms 0.52, 0.89 and 1.17 lm) asheating surfaces. With stainless steel they reached 670

kW/m2 and N 11 cm2, while with aluminum they hadup to 1 MW/m2 and N 8 cm2. They did not know oraccount for contact angle, and they ascribe a principal

significance to roughness. Their correlation for NSD

shows a gradual decrease with roughness, a minimum

at $0.6 lm, and then a steep increase with furtherdecreasing roughness down to 0.2 lm.

Kocamustafaogullari and Ishiis [34] work was based

(primarily) on the GaertnerWestwater data at high

heat flux, and pre-existing nuclei theory (PEN) as de-veloped by Corry and Faust [48], Bankoff [32], Griffith

and Wallis [49] and Cornwell [50]. Simply expressed, the

idea is that the density, N$ 1=Rmc , where Rc, the mini-mum cavity mouth radius required for activation, is

given by [49].

Rc 2rTsat

qvHlvDTs3

Thus, for a certain fluid and pressure, N$ DTms , wherethe proportionality constant and the m are supposed to

depend on surface characteristics. Kocamustafaogullari



10/18

and Ishii used the boiling heat transfer data of Bori-

shanskii et al. [51], which included a wide range of

pressure variation, together with a composite model of

nucleate boiling heat transfer [52] that incorporated

bubble dynamics and heat transfer in the region between

nucleation sites, towards a self-consistent interpretation.

The cavity size used in Kocamustafaogullari and Ishiisanalysis is

Rc 2r1 qv=q

P exp HlvDTsRTwTsat

1

h i 4

which for HlvDTs=RTwTsat ( 1 reduces approximatelyto Eq. (3). Still, the superheats needed for activation,

according to this model, are unrealistically high, as the

maximum cavity radius on the heater reduces to below

100 nm; i.e., 30, 100 and 300 K for 700, 90 and 4 nm,

respectively. This does not conform to the above ex-

perimental results, and evidently these experimental

results do not conform to each other.

3.2. The BETA results and discussion

A sampling of the IR records obtained in BETA tests

is given in Figs. 11 and 12. These are complete, original

data, in that the reading from every pixel has been

converted to temperature, using the calibration scale,

and in that the whole heater surface is imaged. These

temperatures in the figures are represented by a gray

scale, that varies in each case so that the light end

corresponds to the highest temperature in the image,

and the dark end to the lowest one. These scales arenot important for the present discussion, and they are

not shown for economy of space, but generally the light

end is at $150 C and the dark end at $100 C. Thedark circular spots (or discs) are a visualization of the

cooled areas under bubbles growing on the heater. In

the motion pictures that result by rapidly projecting the

successive such frames, one can see the complete dy-

namics of such spots, from their inception (nucleation),

through their growth, and disappearance following de-

parture. For NSD, the counting of these dark areas is

done automatically by image analysis software that were

developed in this research (counting error within 15%).

Turning back again to Figs. 11 and 12, we can see

how the NSD increases with heat flux, and also the

much higher density found on aged heaters as compared

to fresh heaters. Quantitatively, these results are de-

picted in Fig. 13 for three fresh heaters (F1, F4, F9) and

Fig. 14 for three aged heaters (A1, A3, A4). 3 The dif-

ference in NSD between fresh and aged heaters is by

about one order of magnitude. We also see a linear

dependance on heat flux, the effect of water quality (Fig.

13) and the effect of heavy aging (Fig. 14).

Other noteworthy features of Figs. 11 and 12 are that

(a) uniformity of nucleation increases with heat flux, as

superheated areas get activethis is seen especially well

in the lower portion in the succession of frames a, b and

c of Fig. 11; (b) even at very high fluxes, near CHF,

there are sporatically-distributed areas that are highly

superheated in the absence of nucleation; (c) at anygiven frame there is a widespread spectrum of bubble-

cooled areas, but these are overall much larger on the

fresh heater. Overall, aged heaters run much cooler and

much more uniform than fresh heaters.

The same nucleation data expressed in terms of sur-

face-average superheat are shown in Figs. 15 and 16.

For aged heaters, the relationship is approximately lin-

ear, and with about the same slope, but there is a sig-

nificant variation with the degree of aging. For fresh

Fig. 11. IR thermometry images of a fresh heater (F1) at three dif-

ferent heat fluxes, q 406, 536, and 807 kW/m2.

Fig. 12. IR thermometry images of an aged heater (A1) at three dif-

ferent heat fluxes, q 348, 1051 and 1517 kW/m2.

3 Details about these tests are given in Part II (Table 1).



11/18

heaters, on the other hand, after a gradual early part, the

trend is almost vertical (Fig. 15); that is, the major nu-

cleation being activated within an extremely narrow

window of superheatsfrom a fraction of one degree

(F1, F9), to just a few degrees (F4) Kelvin. Remarkably,

however, the temperature level at which this nucleation

activation occurs is different for the three, nominally

same heaters.

These data, and trends, contradict current under-

standing of nucleation, as distilled in Section 3.1. Most

remarkable is the discrepancy with the PEN idea. Our

fresh heaters, with a mean roughness of 4 nm, nucleate

at a few tens of degrees in superheat, rather than the

hundreds required by Eq. (4), or the thousands required

by Eq. (3). If nanoscopic features of the surface can

effect and control heterogeneous nucleation, we then

have to ask what is the role, if any, of micron-scale, and

macroscopic roughness. Also remarkable is the dis-

crepancy with nucleation densities found in other works.

As noted in Section 3.1, both Gaertner and Westwater

[46] and Wang and Dhir [33], reported hundreds of

nuclei per cm2, while even on aged heaters, with CHF

performance that is well past the hydrodynamic limit

(see Part II), we find nucleation densities that are well

under 100 cm2. On the other hand, while the Benjamin

Fig. 13. NSD (n=A) as a function of the heaters heat flux, q, on freshheaters as found in the BETA experiment. Test F9 was run with clean

distilled water. Tests F1 and F4, with HPLC class water.

Fig. 14. NSD (n=A) as a function of the heaters heat flux, q, on agedheaters as found in the BETA experiment. Heater A1 was aged by

pulse heating in air. Heaters A3 and A4 was heavily aged by repeated

pulse heating and boiling in water.

Fig. 15. NSD (n=A) as a function of the surface average superheat,DT

s, on aged heaters as measured in the BETA experiment.

Fig. 16. NSD (n=A) as a function of the surface average superheat,DTs, on aged heaters as measured in the BETA experiment.



12/18

and Balakrishnan, [31] data at first appear to be in

line with what we found, on a closer examination they

appear to be too low, by a factor of fourfor example,

we can expect aluminum to be well-aged (it oxidizes

rapidly even in air), so their N$ 8 cm2 at $1 MW/m2

is to be compared with the N$ 30 cm2 for our heater

A3 (Fig. 14). Finally, the previously deduced trends withsuperheat and heat flux seem to be off. For example,

compare Gaertner and Westwaters [46] N$ q2 with ourN$ q in Figs. 13 and 14. Also, while the Wang and Dhir[33] and Kocamustafaogullari and Ishii [34] dependen-

cies N$ DTms with m 6 and 4.4, respectively, are quitesteep, they still do not capture the behaviors in Figs. 15

and 16, nor the differences between them. Our data seem

to indicate that parameters such as cavity size, or

wetting angle, are of derivative significance in hetero-

geneous nucleation, and that the principal cause/mech-

anism remains to be found.

4. Nucleate boiling heat transfer

4.1. Boiling curves

Heat transfer in nucleate pool boiling has been

studied and measured extensively in the past. It is

characterized by the dependence of surface superheat,

DTs Tw Tsat, on input heat flux, qthe so-calledboiling curves. The heaters surface temperature often

has been deduced from measurements by thermocouples

embedded in several locations in the heater block. The

heat flux was determined from the heaters power input(current and voltage). The BETA experiment offers a

direct measurement of the heater surface temperature

over the whole (macroscopic) area and a uniform heat

flux over the whole heater area. As the IR thermal im-

ages revealed (Figs. 11 and 12), the surface temperature

varies continuously with both space and time. In the

region of active nucleating sites, the maximum superheat

is generally low, 1520 K, whereas in the surrounding

fluid region not populated by active bubble sites the

surface superheat may reach 3540 K. Recognizing this

non-uniformity, it becomes clear that local temperature

measurements (surface microthermocouples) used in pre-

vious experiments are difficult to interpret in terms of

the bubble dynamics that drive the whole process. In the

BETA experiment, the surface-average temperature used

in boiling curves is determined by processing 1000 frames

of the whole digitized temperature map of the heater

surface.

Figs. 17 and 18 show the boiling curves obtained in

several BETA experiments with fresh and aged heaters.

We chose to use linear scale instead of commonly used

loglog scale in the presentation of boiling curves. This

allows us to distinguish important details of boiling

curves from 100 kW/m2 on to CHF. In general, we

observe steep boiling curves on both fresh and aged

heaters, i.e. while the surface heat flux changes an order

of magnitude, the wall surface-average superheat chan-

ges by only few degrees Celsius. As was the case for

NSD (Figs. 15 and 16), here too the fresh heaters exhibit

much steeper behavior.

The variability of boiling curves is remarkable, taking

into account the quality control in experimental proce-

dures employed in the BETA experiments. The surface-

average wall superheat at the critical heat flux varies

from 22 to 32 K on fresh heaters and from 18 to 25 K on

aged heaters. At this condition the corresponding peak

superheats are 60 and 40 K for fresh and aged heaters

respectively.

Aged heaters show a linear relation between q and

DTs, which is in contrast to deductions from previous

work that the dependence may be as high as to the third

power. On the other hand, the behavior on fresh heaters

consists of two parts: an early slowly increasing part and

a nearly vertical branch at high heat fluxes. Such an

increase of NSD under an essentially similar wall su-

perheat 4 indicates that activation of nucleation sites is

a critical phenomenon that responds to a very slight

increase of the fluid superheat.

4.2. Bubble heat transfer. Cold spots

Active bubble sites are very effective heat sinks. The

bubble bases correspond to dark spots on the heaters

thermal pattern. The IR images show a highly dynamic

thermal response in the vicinity of the nucleation sites

over the full range of heat fluxes. Apparently, the nu-

Fig. 17. Boiling curves from the BETA experiments. Fresh heaters.

4 Note that even at CHF, only 20% of the heaters area is covered by

bubble bases. Consequently, the wall superheat in regions between

active bubble sites is close to the surface-average superheat.



13/18

cleated bubbles continuously detach from the heater

surface to allow other bubbles to nucleate and grow.

This dynamic behavior contrasts with the physical pic-

ture postulated in thermal (static) models (e.g., [10]),

in which heat is removed by evaporation at the wedge

of static vapor stems surrounded by a relatively thick

(macro)layer. More generally this also contrasts with

previous concepts that emphasize strongly the contact

line region as dominating heat transfer [53,54]. Fur-

thermore, this static vapor-stem approach requires an

extremely high nucleation site density, whereas the

BETA experiments show a relatively small fraction of

the heater surface covered by bubble bases.

Figs. 1922 show temperature transients in the center

of a cold spot on a fresh heater at different heat fluxes.

Fig. 18. Boiling curves from the BETA experiments. Aged heaters.

Fig. 19. Heater surface temperature as measured in the center of a cold

spot at q 90 kW/m2.


spot at q 200 kW/m2.







14/18

A distinct bubble life cycle begins with a rapid cooling

(510 K in $110 ms) during bubble nucleation andgrowth. As the bubble detaches from the heaters sur-

face, the cold spot gradually heats up until another

bubble nucleates and grows. The bubble nucleation

generally starts at a heater surface (local) superheat of

about 1015 K at lower heat fluxes (Figs. 1921), and of$2022 K at higher heat fluxes (Fig. 22). Similarly, theminimum temperature in the cold spot center was found

to increase with the increasing heat flux.

Activation of nucleation sites can be either regular

or irregular. Correspondingly, they form what we call

regular and irregular bubbles. At regular sites, the nu-

cleation is more-or-less periodic, while the irregular

activation has a silent period between aperiodic bub-

bling cycles. With the increase of heat flux, irregular

bubbles become more regular. Typical regular bubbling

cycles are shown in Figs. 19 and 20, with time period of

about 100 ms (10 Hz) for q 90 kW/m2 and about 25ms (40 Hz) for q 200 kW/m2. At q 400 kW/m2 andq 900 kW/m2, the bubbling frequency increases (toover 50 Hz), while the temperature transient loses in

significant part its periodic character. The heatup rate

between two nucleation events is in the range from 50

100, $250 and $400 K/s for heat fluxes of 90, 200 and400 kW/m2, respectively.

The spatial variation of heater temperature across a

cold spot at different moments during a bubbling cycle is

shown in Fig. 23. At 200 kW/m2 the bubble base is large,

about 3 mm in diameter. During the bubbling cycle the

heater surface under the bubble and a liquid sub-layer

attached to it remain cold. The wall superheat variesfrom 6 K at the cold spot center to about 20 K in the

peripheral ring. This variation corresponds to a decrease

in heat transfer associated with the gradual thickening

of the liquid meniscus beneath the bubble. Based on

such records we will be able to reconstruct the complete

bubble (and microlayer) dynamics analytically, and this

is the necessary next step for understanding.

4.3. Hot spots and dry spots

As the surface heat flux increases, the BETA IR im-

ages show bright spots appearing within the bubble

bases. These bright spots, typically 12 mm in diameter,

represent overheating of the heater surface with tem-

peratures of up to 170 C. They are easily identifiable

within a surrounding darker peripheral region. Such

bright spots, called here hot spots, were observed in

the BETA experiments with both fresh and aged heaters.

In fresh heaters, the hot spots start to appear at q 200kW/m2 and they become more frequent at q 400 kW/m2. In fact, we observe 1015% of the bubble bases si-

multaneously generating hot spots within them under

400500 kW/m2. In aged heaters such hot spots first

appear at higher fluxes, above 600 kW/m2. The hot spot

formation follows a bubble nucleation event and is

therefore more evident on fresh heaters with the larger-

size bubble bases and even more so in irregular bubbles.

At low heat fluxes, life duration of hot spots is con-

trolled by regular bubbling cycles and are generally of

short duration, 1020 ms. The hot spot temperature is

peaked only 510 K higher than the surrounding fluid,

and they are periodically replaced by cool areas under a

newly growing bubble. As heat flux increases, the hot

spot maximum superheat may reach 5060 K. Such ahot spot can be seen in Figs. 25 and 26.

Fig. 25 shows a fresh heater surface temperature

transient measured at the center of a hot spot. At time

t 0 ms on the scale shown in Fig. 25, the surface su-perheat is about 30 C. A short period of rapid cooling

was observed between t$ 7 ms and t$ 9 ms, indicatinga bubble nucleation and growth. At t$ 9 ms the surfacebegins a rapid heat-up and reaches its maximum su-

perheat of$55 C at t$ 38 ms. Our analysis of a largenumber of hot spots, formed in the bubble base of either

regular or irregular bubbles, both in fresh and aged

heaters, shows that the heating rate of the heater sur-

face, dTw=dt, measured in the center of hot spots, isabout half ($4050%) of the adiabatic heating ratedTa=dt (dTa=dt q=qwCpwdw).

In general, irregular bubbles often nucleate in regions

with high superheat and previously not populated with

regular active bubble sites. Due to the higher liquid

superheat, the irregular bubbles are often larger in size

(35 mm), as are the hot spots formed within them (23

mm in diameter). While the life time span of regular hot

spots (1020 ms) is limited by the bubble nucleation

frequency, the time life span of irregular hot spots is

significantly longer (50100 ms). Fig. 26 depicts tem-

Fig. 23. Temperature profiles across a cold spot of regular bubble on a

fresh heater at five time moments covered in the scale of Fig. 24.

Heater F1 at q 200 kW/m2.



15/18

perature profiles across the hot spot location at different

time moments. It can be seen that the hot spot center

coincides with the bubble base center, which features

both the areas lowest ($124 C) and highest ($158 C)temperatures during the 70-ms transient (Fig. 25). The

heatup rate of the hot spot area is $1250 K/s, which is

$40% the adiabatic heatup rate ($3000 K/s) for the heatflux of 900 kW/m2 applied.Now, we can argue that such a high heatup rate is

possible only because the hot spots are effectively dry.

To have a reference point, we revisit the heatup process

in bubble bases (cold spots) without hot spot (see Figs.

1924). First of all, the heatup rate in the bubbles cold

spots is 510 times smaller than the adiabatic heating

rate. This indicates that, in addition to the glass sub-

strate, the heat removal process should involve heating-

up of a sub-millimeter liquid sub-layer adjacent to the

heater surface. More interestingly, the IR thermometry

data show the local temperature within the cold spots

being affected by the dynamics of the surrounding

liquid, e.g. due to nucleation and growth of vapor

bubbles in the proximity. Namely, during the heatup

period temperature fluctuations, with amplitude of up to

$1 K and frequency up to $0.5 Hz, were measured evenat the center of the cold spot (Fig. 24). The heating

period is followed by a short period (12 ms) of rapid

cooling associated with new bubble nucleation and

growth.

Now turning to hot spots within bubble bases, if they

were covered by a liquid layer, such a liquid layer would

be metastably overheated with DTw $ 4060 K and

hence be extremely prone to nucleation. In fact, the onlyway for such a highly superheated liquid layer to cool

down is to allow nucleation and bubble growth within it.

However, the IR data shows that the hot spots heatup

period is typically ended by a temperature turnaround

rather than terminated by a nucleation-induced rapid

cooling (see Fig. 25 vs. Fig. 24). In addition, the heating

transient in the hot spots central area (Fig. 25) shows no

sign of temperature fluctuations characteristic for cold

spots (as discussed above). This indicates a separability

between heat transfer processes in the hot spot and fluid

dynamics outside it. The above evidences lead naturally

to suggesting that the hot spot is dry.

In fact, we note that the heatup rate is maximum at

the hot spot center while the heatup rate is much lower

in the hot spots peripheral region. The heatup ratedifference of about three times can be deduced from data

shown on Fig. 26, which depicts temperature profiles

across a hot spot at different time moments. Apparently,

Fig. 26. Temperature profiles across of a hot spot (900 kW/m2) at five

different time moments in the scale of Fig. 25.

Fig. 24. Temperature history at the center of the cold spot shown in

Fig. 23.

Fig. 25. Temperature history at the center of a hot spot shown in

Fig. 26.



16/18

the lateral conduction plays an essential role in the hot

spots peripheral region due to highly efficient heat re-

moval at the edge of evaporating meniscus. However,

since the hot spots characteristic dimension is macro-

scopic (23 mm in diameter), contribution of lateral heat

conduction to the energy balance is significantly reduced

in the central area of a hot spot.That the hot spots identified within bubble bases are

dry is further confirmed by our detailed observations of

their cooldown behavior. During the cooldown period

that lasts as long as 20 ms (see e.g., Fig. 25), we see a

rapid cooling of the hot spot, at a rate, dTc=dt, that issimilar or even faster, (jdTc=dtj $ 0:5 . . . 1dTa=dt), thanthe hot spots heatup rate, dTw=dt(dTw=dt$ 0:4 . . . 0:5dTa=dt). Taking into account heat generation in theheater, q, and sensible heat released during the cooling

of the heater-glass assembly, the total heat removal rate

in the hot spot during the cooldown period is estimated

at the level of (1:5 . . . 2:0)q. Such a high heat removalrate cannot be accommodated by either lateral heat

conduction 5 nor heat conduction to an adjacent liquid

layer if such were to station on the heater surface at the

hot spot location. This is particularly true under con-

ditions, typically with heat flux higher than 300 kW/m2,

when we detect the formation of hot spots within bubble

bases. Furthermore, the relatively long cooling period

(20 ms) indicates that the cooldown is unlikely caused by

a nucleation event. More likely, the rapid cooling ob-

served with hot spots is associated with the advancement

of evaporating meniscus front that effectively rewets

the hot spots dry area.

Finally, we note that the above-analyzed hot spotsare prone to overheating at heat fluxes near and at

CHF level. Called dry spots, these high-temperature

(Tw > 170 C) spots can first be reversible and then be-come irreversible leading to the burnout. A detailed

analysis of the dryout dynamics in reversible spots and

the heater burnout in irreversible spots is given in the

companion paper.

5. Concluding remarks

This is Part I of a two-part paper in which we de-scribe a new experimental approach that was devel-

oped and employed to study the physics of nucleate

boiling heat transfer and pool boiling crisis. The most

important element of the present experimental ap-

proach is that it allows direct visualization of the heat

transfer patterns on the heated wall, and thus the

quantitative characterization of the key processes

that underlie the boiling phenomenon all the way to

the occurrence of crisis. This is achieved by means

of a high-speed, high-resolution IR thermometry on

a nano-scale heater. The other key element is the abil-

ity to control and characterize experimental con-ditions, through the use of high-purity water, the

contamination-free test section, and a protocol for

the heater aging, and the heaters pre- and post-test

micro- (and nano-) scopic examination.

In this paper we present first-of-a-kind, quantitative

information on NSD and nucleate boiling heat trans-

fer over a broad range of heat fluxes, from the onset

of nucleate boiling to the occurrence of crisis. This

knowledge is an essential step toward the understand-

ing of the boiling crisis phenomenon, whose detailed

examination in presented in the companion paper

(Part II).

In particular, the BETA experiments conducted on

fresh, nanoscopically smooth heaters (free of mi-

cron-scale cavities) show that nucleate boiling can

start under a wall superheat of as little as $10 K. Thiscontrasts with the cavity theory of heterogeneous

nucleation that requires a presence of gas/vapor bub-

bles entrapped in the heater surface microscopic cav-

ities. Apparently, nano-scale imperfections and defects

present on the heater surface are sufficient to initiate

the heterogeneous nucleation.

The BETA experiments show a stark difference in nu-

cleation patterns between the fresh and aged heaters.

The NSD was found to increase with the degree ofheater aging.

Significant new insights were gained from direct ob-

servations and quantification of the origin and dy-

namics of hot spots. The hot spots formed within

bubble base were identified as dry spots, which serve

as precursors of burnout at high heat fluxes.

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Boiling Crisis Phenomenon Part1

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