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.
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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).
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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
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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).
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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.
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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.
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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.
Fig. 20. Heater surface temperature as measured in the center of a cold
spot at q 200 kW/m2.
Fig. 21. Heater surface temperature as measured in the center of a cold
spot at q 400 kW/m2.
Fig. 22. Heater surface temperature as measured in the center of a cold
spot at q 900 kW/m2.
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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.
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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.
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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.
References
[1] T.G. Theofanous, T.N. Dinh, J.P. Tu, A.T. Dinh, The boiling
crisis phenomenon. Part II: Critical heat flux and burnout, Exp.Therm. Fluid Sci. 26 (67) (2002) 793810.
[2] T.G. Theofanous, T.H. Bohrer, M.C. Chang, P.D. Patel, Exper-
iments and universal growth relations for vapor bubbles with
microlayers, J. Heat Transfer (ASME) 100 (1) (1978) 4148.
[3] S.S. Kutateladze, On the transition to film boiling under natural
convection, Kotloturbostrenie 3 (1948) 10.
[4] S.S. Kutateladze, Hydrodynamic model of heat transfer crisis
in free-convection boiling, J. Tech. Phys. 20 (11) (1950) 1389
1392.
[5] N. Zuber, On the stability of boiling heat transfer, ASME J. Heat
Transfer 80 (2) (1958) 711720.
[6] J.H. Lienhard, V.K. Dhir, Hydrodynamic prediction of peak
pool-boiling heat fluxes from finite bodies, Trans. ASME, J. Heat
Transfer 95 (2) (1973) 153158.
5 Note that during the same cooling period, temperature in the hot
spots peripheral regions remains either unchanged or even increases.
This indicates a significant reduction of the role of lateral heat
conduction on hot spots energy balance, compared to the conduction
contribution during the heating up period.
790 T.G. Theofanous et al. / Experimental Thermal and Fluid Science 26 (2002) 775792
-
7/23/2019 Boiling Crisis Phenomenon Part1
17/18
[7] R. Reyes, P.S. Wayner, An adsorption model for the superheat at
the critical heat flux, Trans. ASME J. Heat Transfer 117 (3) (1995)
779782.
[8] K. Sefiane, D. Benielli, A. Steinchen, A new mechanism for pool
boiling crisis, recoil instability and contact angle influence,
J. Colloids Surf. A: Phsyicochem. Eng. Aspects 142 (1998) 361
373;
A. Steinchen, K. Sefiane, Stability analysis of the pool-boiling
crisis, J. Phys.: Condens. Matter 8 (47) (1996) 95659568.
[9] A. Sakurai, Mechanisms of transitions to film boiling at CHFs in
subcooled and pressurized liquids due to steady and increasing
heat inputs, Nucl. Eng. Design 197 (3) (2000) 301356.
[10] V.K. Dhir, S.P. Liaw, Framework for a unified model for nucleate
and transition pool boiling, J. Heat Transfer 111 (1989) 739
745.
[11] C. Unal, P. Sadasivan, R. Nelson, On the hot-spot-controlled
critical heat flux mechanism in pool boiling of saturated fluids, in:
V.K. Dhir, A.E. Bergles (Eds.), Proceedings of Engineering
Foundation Conference on Pool and External Flow Boiling,
ASME, New York, 1992, pp. 193201.
[12] N.I. Kolev, How accurately can we predict nucleate boiling, Exp.
Therm. Fluid Sci. 10 (1995) 370378.
[13] Yu.A. Buyevich, Towards a unified theory of pool boiling-the case
of ideally smooth heated wall, Int. J. Fluid Mech. Res., (Begell
House) 26 (2) (1999) 189223.
[14] S.J. Ha, H.C. No, A dry-spot model of critical heat flux applicable
to both pool boiling and subcooled forced convection boiling,
Int. J. Heat Mass Transfer 43 (2) (2000) 241250.
[15] V.V. Yagov, Heat transfer with developed nucleate boiling of
liquids, Thermal Eng. 35 (2) (1988) 6570.
[16] V.K. Dhir, Boiling heat transfer, Annu. Rev. Fluid Mech. 30
(1998) 365401.
[17] Y. Haramura, Y. Katto, A new hydrodynamic model of critical
heat flux, applicable widely to both pool and forced convection
boiling on submerged bodies in saturated liquids, Int. J. Heat
Mass Transfer 26 (3) (1983) 389399.
[18] S.S. Kutateladze, I.G. Malenkov, Hydrodynamic analogy be-
tween heat transfer and nucleate boiling crisis in boiling andbubbling, Exp. data. Heat TransferSov. Res. 16 (4) (1984) 146.
[19] P. Sadasivan, C. Unal, R. Nelson, Perspective: issues in CHF
modelingthe need for new experiments, Trans. ASME, J. Heat
Transfer 117 (1995) 558567.
[20] J.H. Lienhard, Snares of pool boiling research: putting our history
to use, in: G.F. Hewitt (Ed.), Proceedings of the Tenth Interna-
tional Heat Transfer Conference, Brighton, UK, vol. 1, Chame-
leon Press Ltd, London, 1994, pp. GK11, Keynote Papers.
[21] M. Jacob, W. Linke, Der warmeubergang beim verdampfen von
flussigkeiten an senkrecten und waagerechten flachenm, Phys. Z 8
(1935) 267280.
[22] I.I. Gogonin, S.S. Kutateladze, Critical heat flux as a function
of heater size for a liquid boiling in a large enclosure, J. Eng. Phys.
33 (5) (1977) 12861289.
[23] I.I. Gogonin, I.A. Shemagin, V.M. Budov, A.R. Dorokhov, Heattransfer in film condensation and boiling in nuclear equipment,
Energoatomizdat, Moscow, 1993, p. 208.
[24] M.G. Grimaldi, F. La Via, V. Raineri, S. Bocelli, M. Galli,
F. Marabelli, F. Bonoli, M. Iannuzzi, L. Miglio, Kinetics of the
C49C54 phase transition in TiSi2: new indications from sheet
resistance, infrared spectroscopy and molecular dynamics simula-
tions, Microelectron. Eng. 3738 (1997) 441448.
[25] L. Miglio, M. Iannuzzi, M.G. Grimaldi, F. La Via, F. Marabelli,
S. Bocelli, S. Santucci, A.R. Phani, Texturing, surface energetics
and morphology in the C49C54 transformation of TiSi2, Solid-
State Electron. 43 (6) (1999) 10691074.
[26] Z. Lin, Y.K. Lee, Effect of multi-cycle annealing on the C49C54
phase transformation in TiSi2 thin film, Mater. Sci. Semicond.
Process. 3 (3) (2000) 215219.
[27] I. Vaquila, L.I. Vergara, M.C.G. Passeggi, R.A. Vidal, J. Ferron,
Chemical reactions at surfaces: titanium oxidation, Surf. Coat.
Technol. 122 (1) (1999) 6771.
[28] Y. Wouters, A. Galerie, J.-P. Petit, Thermal oxidation of titanium
by water vapour, Solid State Ionics, Diffusion Reactions 104 (12)
(1997) 8996.
[29] G. Lu, S.L. Bernasek, J. Schwartz, Oxidation of a polycrystalline
titanium surface by oxygen and water, Surf. Sci. 458 (13) (2000)
8090.
[30] E. McCafferty, J.P. Wightman, T.F. Cromer, Surface properties
of hydroxyl groups in the air-formed oxide film on titanium,
J. Electrochem. Soc. 146 (8) (1999) 28492852.
[31] R.J. Benjamin, A.R. Balakrishnan, Nucleation site density in
pool boiling of saturated pure liquids: effect of surface micro-
roughness and surface and liquid physical properties, Exp. Therm.
Fluid Sci. 15 (1) (1997) 3242.
[32] S.G. Bankoff, Entrapment of gas in the spreading of liquid over
a rough surface, AIChE J. 4 (1958) 2426.
[33] C.H. Wang, V.K. Dhir, On the gas entrapment and nucleation
site density during pool boiling of saturated water, Trans. ASME.
J. Heat Transfer 115 (3) (1993).
[34] G. Kocamustafaogullari, M. Ishii, Interfacial area and nucleation
site density in boiling systems, Int. J. Heat Mass Transfer 26
(1983) 13771389, 670679.
[35] H. Thomann, B. Frisk, Measurement of heat transfer with an
infrared camera, Int. J. Heat Mass Transfer 11 (5) (1968) 819
826.
[36] G. Hetsroni, I.A. Kowalewski, B. Hu, A. Mosyak, Tracking of
coherent thermal structures on a heated wall by means of infrared
thermography, Exp. Fluids 30 (3) (2001) 286294.
[37] M. Schulz, W. Gross, H. Scheuerpflug, High-resolution thermo-
physical measurements using staring infrared detector arrays,
High Temp. -High Pressure 32 (5) (2000) 547556.
[38] A. Mori, Y. Oguchi, Y. Okusawa, M. Ono, H. Fujishima, K.
Tsubota, Use of high-speed, high-resolution thermography to
evaluate the tear film layer, Am. J. Ophthalmol. 124 (6) (1997)
729735.
[39] T. Raad, J.E. Myers, Nucleation studies in pool boiling on thinplates using liquid crystals, AIChE J. 17 (5) (1971) 12601261.
[40] J.E. Sgheiza, J.E. Myers, Behavior of nucleation sites in pool
boiling, AIChE J. 31 (10) (1985) 16051613.
[41] D.B.R. Kenning, Wall temperature patterns in nucleate boiling,
Int. J. Heat Mass Transfer 35 (1) (1992) 7386.
[42] D.B.R. Kenning, N. Youyou, Pool boiling heat transfer on a thin
plate: features revealed by liquid crystal thermography, Int. J.
Heat Mass Transfer 39 (15) (1996) 31173137.
[43] Y. Iida, K. Kobayasi, Distribution of void fraction above a
horizontal heating surface in pool boiling, Bull. JSME 12 (1969)
283290.
[44] S.P. Liaw, V.K. Dhir, Void fraction measurements during
saturated pool boiling of water on partially wetted vertical
surfaces, Trans. ASME, J Heat Transfer 111 (3) (1989) 731738.
[45] T.G. Theofanous, S. Angelini, X. Chen, R. Luo, W.W. Yuen,Quantitative radiography for transient multidimensional, multi-
phase flows, Nucl. Eng. Design 184 (23) (1998) 163181.
[46] R.F. Gaertner, J.W. Westwater, Population of active sites in
nucleate boiling heat transfer, Chem. Eng. Prof. Symp. Ser. 56
(30) (1960) 39048.
[47] C.H. Wang, V.K. Dhir, Effect of surface wettability on active
nucleation site density during pool boiling of water on a vertical
surface, J. Heat Trans. 115 (3) (1993b) 659669.
[48] C. Corry, A. Foust, Surface variables in nucleate boiling, Chem.
Eng. Prog. Symp. Ser. 51 (17) (1955) 112.
[49] P. Griffith, J.D. Wallis, The role of surface conditions in nucleate
boiling, Chem. Eng. Prog. Symp. Ser. 56 (1960) 4962.
[50] K. Cornwell, Naturally formed boiling site cavities, Lett. Heat
Mass Transfer 4 (1977) 6372.
T.G. Theofanous et al. / Experimental Thermal and Fluid Science 26 (2002) 775792 791
-
7/23/2019 Boiling Crisis Phenomenon Part1
18/18
[51] V. Borishanskii, G. Bobrovich, F. Minchenko, Heat transfer from
a tube to water and to ethanol in nucleate pool boiling, in: S.S.
Kutateladze (Ed.), Symposium of Heat Transfer and Hydraulics
in Two-Phase Media, Gosenergoizdat, Moscow, 1961.
[52] N. Zuber, Nucleate boiling. The region of isolated bubbles and the
similarity with natural convection, Int. J. Heat Mass Transfer
6 (1963) 5378.
[53] P.C. Wayner Jr., Evaporation and stress in the contact line region,
in: V.K. Dhir, A.E. Bergles (Eds.), Proceedings of Engineering
Foundation Conference on Pool and External Flow Boiling,
ASME, New York, 1992, pp. 251256.
[54] V.V. Yagov, A physical model and calculation formula for critical
heat fluxes with nucleate pool boiling of liquids, Thermal Eng.
35 (6) (1988) 333339.
792 T.G. Theofanous et al. / Experimental Thermal and Fluid Science 26 (2002) 775792