Enhancements of Nucleate Boiling Under Microgravity Conditions

14 Pages·2000·0.77 MB·English

by  Zhang, Nengli

_ ..r
AIAA-2000-0853
Enhancements of Nucleate Boiling
under Microgravity Conditions
Nengli Zhang and David F. Chao
NASA Glenn Research Center
Cleveland, Ohio
W. J. Yang
Department of MEAM
University of Michigan
Ann Arbor, Michigan
38th Aerospace Sciences
Meeting & Exhibit
10-13 January 2000 / Reno, NV
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AIAA-2000-0853
Enhancements of Nucleate Boiling under Microgravity Conditions
Nengli Zhang and David F. Chao
NASA Glenn Research Center, Cleveland, Ohio 44135
W. J. Yang
Department of Mechanical Engineering and Applied Mechanics,
University of Michigan, Ann Arbor, Michigan 48109-2125
Abstract Introduction
This paper presents two means for Boiling heat transfer will be a key
enhancing nucleate boiling and critical heat technology in space thermal management
flux under microgravity conditions: using systems and other future space applications.
micro-configured metal-graphite composites A two-phase liquid-loop system, utilizing
as the boiling surface and dilute aqueous boiling heat transfer, is an attractive
solutions of long-chain alcohols as the alternative to the single-phase liquid loop
working fluid. In the former, system used in the conventional Space
thermocapillary force induced by Transportation System and Spacelab[1-3]
temperature difference between the graphite- due to its much higher heat transfer
fiber tips and the metal matrix plays an efficiency.
important role in bubble detachment. Thus It is well known that surface tension
boiling-heat transfer performance does not effects, including temperature driven
deteriorate in a reduced-gravity surface- tension gradients, are dominant
environment. In the latter cases, the surface when the buoyancy force is diminished in
tension-temperature gradient of the long- microgravity conditions. Unfortunately,
chain alcohol solutions turns positive as the most previous studies have shown that,
temperature exceeds a certain value. rather than assisting in the detachment
Consequently, the Marangoni effect does not process, surface tension and its temperature
impe_te, _"but rather aids Jin bubble _d_pa_-fe dri'ven gradients tend to keep the bubbles on
from the heating surface. This feature is the Wall, and in that way impede bubble
most favorable in microgravity. As a result, detachment. This, of course, is quite
the bubble size of departure is substantially detrimental to boiling heat transfer.
reduced at higher frequencies. The dependence of gravity and surface
Based on the existing experimental data, tension on the diameter of a bubble at
and a two-tier theoretical model, correlation departure is expressed through the Fritz
formulas are derived for nucleate boiling on relation [4]:
the copper-graphite and aluminum-graphite
-C[ cr ],,2
composite surfaces, in both the isolated and
D- Lg(pl- pv)J (1)
coalesced bubble regimes. In addition,
performance equations for nucleate boiling where D is the departure diameter of the
bubble, C is a constant which is a function
and critical heat flux in dilute aqueous
solutions of long-chain alcohols are of the properties of the liquid, cr is the
obtained. surface tension, g is the gravitational
Copyright © 2000 by the American Institute
of Aeronautics and Astronautics, Inc. All
rights reserved.
1
This isa preprintor reprint of a paper intended for presentation at aconference.
Because changes may be made before formal publication, this ismade available with the
understanding that itwill not be cited orreproduced without the permission of the author.
acceleration, Pt and _ are the liquid density boiling in reduced gravity environments,
and the vapor density, respectively. three conclusions are generally agreed upon:
According to this correlation, saturated 1) bubble departure becomes more
nucleate boiling cannot occur when the difficult and the bubbles that detach
gravity approaches zero. Indeed, Zell and from a heater surface readily coalesce
Straub [5] observed that film boiling with each other,
occurred immediately without the 2) the transitions from the isolated bubble
appearance of nucleate boiling when a regime to the coalesced bubble regime
moderate heat flux (30 kW/m _) was imposed and from the coalesced bubble regime to
on a plate heater in a pool of saturated R113 the vapor slug regime take place at
during a ballistic rocket flight. Based on relatively small heat fluxes, and
this observation, Zell and Straub concluded 3) the critical heat flux (CHF) for all
that saturated nucleate boiling on a flat plate liquids in microgravity is considerably
could not be maintained under microgravity smaller than in normal gravity.
conditions. However, other experiments, Although the acceleration of gravity
including a later one by Zell et al. [6], appears as a parameter in almost all the
reported that saturated nucleate boiling was correlations used to predict boiling
established on a fiat plate heater as well as performance, the fundamental question
on a thin wire. concerning the effect of gravity on boiling
Generally, the experimental results on has yet to be satisfactorily answered. Most
boiling heat transfer under microgravity correlations are based on experiments
conditions are contradictory. In some conducted on earth, in which several
experiments the boiling heat transfer adjustable parameters are involved. Dhir
coefficient was independent of gravity; in [11] pointed out that the usefulness of the
others it decreased or increased as gravity correlations diminished very rapidly as the
was reduced [7, 8]. The results of pool parameters of interest started to fall outside
boiling experiments under reduced gravity the range of physical parameters for which
(~ 10" g) conducted by Abe [9] and Oke et the correlations are developed. The
al. [10] show that the boiling behaviors of correlations involving several empirical
organic fluids, and water, under reduced constants tend to cloud the physics.
gravity are considerably different. In the Actually, due to its complex nature, a
boiling of organic fluids most of the bubbles comprehensive understanding of the
attached to the heater surface with a large mechanisms of boiling has not been
contact area, and some bubbles coalesced available. Also, the forces acting on the
With each other on the heater surface even at bubble_ have not been properly taken into
low heat fluxesl However, bubbie-s account. Buoyancy dominates in nucleate
generated in water were spherical in shape, boiling on the earth and masks other forces,
making only slight contact with the heater such as the Marangoni force induced by the
surface or being lifted from the surface. The surface tension gradient, the inertia force
heat transfer deterioration due to gravity produced by the bubble growth, and others.
reduction for the organic fluids was These forces would play significant roles in
bubble detachment and therefore in the
appreciable only athigh hea _ fluxes. On the
other hand, for water the deterioration nucleate bo_ng heat transfer when gravity
occurred throughout the entire nucleate is greatly reduced. Effects of some physical
boiling regime. In spite of the complexity of properties of the working fluid on the
boiling behavior could also be unmasked in
microgravity.It is the effectsof the fluid
The performance of pool boiling heat
properties that could account for the transfer on a composite surface was studied
differencein the gravity dependencyof the
experimentally and numerically by
boiling heattransferperformancebetween
Blagojevic et al. [20]. They found a plateau
organicfluids andwaterthatoccursin the in the boiling curve in the CHF regime
reducedgravity[9,10]. accompanied by a reduction in the peak heat
Therearetwo fundamentalapproaches flux. Yang and Zhang [21] divided
to enhancingboiling heattransfer:develop
composite enhanced surfaces into two
enhanced"boiling surfacesand/ordevelop
categories: discrete insert/matrix type
newworkingfluids. composites, and micro-configured insert into
A). EnhancedBoiling SurfacesI:t iswell matrix types. They presented a hypothesis to
known that surfacegeometryplaysa key explain the formation of the plateau of the
role in nucleate boiling heat transfer. boiling curve in the CHF region. This
Therefore,asahigh-technologyareaof heat explanation may provide a guideline for
transfer, many enhancedboiling surfaces searching for the proper construction of
havebeendevelopedandstudied. Only a enhanced-boiling surfaces with a wider
few of these have become available
safety margin in the CHF regime.
commercially. Commercially enhanced Most of the current enhanced-boiling
boiling surfacescan be arrangedinto two surfaces, either commercially available or
basiccategories:integralroughnesssurfaces under development, generally increase heat
ORS) and porous boiling surfaces (PBS). transfer performance in the nucleate boiling
Webb [12] and Thome [13, 14] gave general regime but do not have both a higher value
reviews of the enhancement of the boiling of CHF and a uniform CHF operating
surfaces. Some of the surfaces can both region. The boiling performance on the
enhance nucleate boiling heat transfer and enhanced surfaces is expected to deteriorate
increase the CHF, but some only enhance sharply with reduced gravity. The reason for
the nucleate boiling without any increase or this is that the complex construction of the
even with much of a decrease in the CI-IF surfaces makes the bubble detachment
value [15]. Recently, O'Connor and You crucially dependent upon buoyancy. These
[16] investigated a painting technique to enhanced surfaces usually also require
create a surface microstructure for enhanced higher primary and maintenance costs or
boiling heat transfer. They reported an have disadvantages of introducing additional
increase in boiling heat transfer three times pressure drops, and gradual loss of their
greater than the unpainted surface, and a enhancement effects on the nucleate boiling
CHF increase of 109 percent. They pointed due to fouling. New enhanced-boiling
out that the CHF increase is due to surface surfaces, metal-graphite micro-configured
microstructure and its influence on the composite surfaces, may provide the
boiling heat transfer characteristics. Wright solution. The pool boiling experimental
and Gebhart [17, 18] investigated enhanced results show that the average boiling heat
boiling performance on microconfigured transfer coefficient of R113 in the nucleate
surfaces and suggested a two-bubble model boiling regime on the copper-graphite (Cu-
to explain the enhancement of the boiling Gr) composite surfaces, with up to 35°C
heat transfer. A two-tier model was wall superheat, is 3.0 to 4.6 times that for
proposed by Zhang et al. [19] to explain the the pure copper heater surface [22].
nucleate boiling process and performance Compared to other enhanced boiling
enhancement on microconfigured surfaces. surfaces, these types of composite surfaces
haveunique attributessincethey do not 40% from the terrestrial level.
incurextrapressuredrops,havenofouling,
Unfortunately, the boiling heat transfer
and offer low primary and maintenance performance of the mixtures at normal
costs.It hasbeenalsofoundthatthe CHF
gravity is much worse than that of pure
for Cu-Gr compositeswas extendedto
water, and, although enhanced under
highervalues comparedto a purecooper microgravity, it still cannot reach the level
surface. The metal-graphite micro- of pure water at normal gravity. Therefore,
configuredcompositesurfaceswere found
the water-ethanol mixtures are unacceptable
to have nonisothermal surfaces under
for space applications. Ahmed and Carey
boiling conditions [22, 23]. Basedon the [29] conducted an experiment with water-2-
nonisothermal surface result, a reduced
propanol mixtures under reduced gravity.
sensitivityoftheCHFtosuperheavtariation
They concluded that the Marangoni effect
for thesurfaceswaspredictedby Yangand
arising from the surface tension gradients
Zhang[21]. Mostrecently,theauthorshave due to concentration gradients is an active
suggesteda model to predictthe CHF on mechanism in the boiling of binary mixtures
metal-graphitecomposites[24]. However, and that the boiling mechanism in these
the nucleateboiling mechanismson the
mixtures is nearly independent of gravity.
compositesurfaces,especiallyin the CHF Additionally, they found that in
regime,havenot beenadequatelystudied.
microgravity the larger the surface tension
B). New Working Fluids: Recently, gradient with the concentration of 2-
many efforts have been madeto try to propanol in the mixture, the higher the CHF.
enhance boiling heat transfer through
The experimental results obtained by Abe et
Marangoni effects in fluid mixtures at
al. and by Ahmed and Carey clearly show
normal gravity [25-27] as well as in that for so-called positive mixtures, in which
microgravity [28, 29].It wasfoundthat a
the more volatile component has a lower
small amount of surface-activeadditive
value of surface tension, the Marangoni
considerablyincreasesthe nucleateboiling mechanism is strong enough in the mixtures
heattransfercoefficientof wateratnormal
to sustain stable nucleate boiling under
gravity [25, 26]. However,no testhasyet
microgravity conditions. The basic physical
been conducted under microgravity explanation of the Marangoni effect is as
conditions.A seriousproblemwith usinga follows: The preferential evaporation of the
surfactant is its foaming in the vapor.
more volatile component from the liquid-
McGillis and Carey[27] foundthat small vapor interface occurs more intensively at
additionsof alcoholto waterincreasedthe
positions closer to the heater surface. The
CHFconditionabovethatof thepurewater
variation in concentration along the bubble
and higher concentrationsof the alcohol surface results in a surface tension gradient,
begandecreasingtheCHFconditiontonear and therefore, a Marangoni flow directed
thatof thepurealcohol. On the other hand, from the top of the bubble to the bottom of
for water-ethylene glycol mixtures, addition the bubble must be produced. The liquid
of the latter decreased the CHF condition adjacent to the bubble surface is dragged
relative to that of pure water. Abe et al. [28] down towards the bottom of the bubble and
tested water-ethanol mixtures of 11.3 wt% thereby lifts it from the heater surface.
and 27.3 wt% of ethanol and found that heat However, for so-called negative mixtures, in
transfer is enhanced by reductions in gravity which the more volatile component has a
over the major portion of the nucleate higher surface tension, such as water-
boiling regime, but the CHF decreases 20- ethylene glycol mixture, the surface tension
gradient induced by the variation in
mainly contributed by micro bubbles, with
concentrationis in the oppositedirectionto
negligible heat conduction across the
thatforpositivemixtures.Consequentlyth, e
microlayer, and can be expressed as
Marangoniflow tendsto keepthe bubbles
,eD 3
pressedto the heatersurfaceandimpedes
q, = pvh:,C,(aT,o,)" (2)
bubbledetachment.This iswhytheCHFof
where D,, is the maximum micro bubble
a water-ethyleneglycol mixturedecreased
when comparingwith that of pure water diameter, p_ is the density of saturated vapor
[28].It is clearthat onlypositive mixtures at the boiling point, hfg is the latent heat of
vaporization at the boiling point, ATsat is the
have the potential to be a boiling working
superheat, and C, and m are constants
fluid for space applications. However, the
determined by the experimental data. For
proper operating regimes of these binary
the composite surfaces, D,, is related to the
mixtures are too narrow to use in practice,
fiber diameter, d, and the area fraction of the
especially for space applications.
fibers in the base material, a; by
Metal-Graphite Composite Surfaces
D. =7 (3)
Experimental studies were performed on
In the case of a= 0.5 and d = 8 l.tm, the
nucleate pool boiling of pentane on cooper-
value of Din is calculated at 10.03 _tm.
graphite (Cu-Gr) and aluminum-graphite
In the high heat flux boiling region
(AI-Gr) composite surfaces with various
(coalesced bubble regime), the boiling heat
fiber volume concentrations for heat fluxes
flux consists of two parts: latent heat
up to 35 W/cm 2. It is revealed that the onset
transport by micro bubbles under the vapor
of nucleate boiling (the isolated bubble
stems and by evaporation on the interface
regime) occurs at wall superheat of about 10
along the macrolayer, which equals heat
°C for the Cu-Gr surface and 15 °C for the
conduction across the macrolayer. The total
AI-Gr surface, much lower than their
heat flux is
respective pure metal surfaces. A significant
qh = q,.,. + k,Ct (AT,,, - ATt) (4)
enhancement in boiling heat transfer
performance on the composite surfaces is where, qt.,, is the maximum value of the heat
achieved [30], as shown in Fig.l, due to the flux in the isolated bubble regime, kt is heat
presence of micro-graphite fibers embedded conductivity of the working fluid, ATt is
in the matrix. Transition from an isolated superheat at the transition from the isolated
bubble regime to a coalesced bubble regime bubble regime to the coalesced bubble
in boiling occurs at a superheat of about 14 regime, Ct is a constant which can be
°C on Cu-Gr surface and 19 °C on AI-Gr determined by experiments. The properties
surface. of pentane saturated vapor and liquid at the
According to a two-tier configuration boiling point can be found in reference [31]
and the relevant data are listed in Table 1.
and its mathematical model [19], and based
on the existing experimental data, Based on the experimental data shown in
correlations for the boiling heat transfer Fig. I, the values of the constants C,, m, and
Ct, then can be estimated at 2.828x107,
performance in the isolated bubble regime
and in the coalesced bubble regime are 2.443, and 2.389x103, respectively for the
obtained as follows: Cu-Gr composite surface, and 2.544xi0 s,
The boiling heat flux in the low heat flux 3.805, and 3.39x103, respectively for the AI-
boiling region (isolated bubble regime) is Gr composite surface. The results calculated
Table I. Properties of saturated pentane vapor and liquid at boiling t'°int
Mol. weight tb, °C pv, kg/m 3 kl, W/m K hrv, kJ/mol
,m
72.146 36.06 0.003002 0.1086 25.79
4O
-""' .... I .... I .... i .... ' .... ' ....
35 I • _omioum l--i...... !)......i......
/ • AI-Grcomposite I I _ i
.I o copp_ L-;......-,...!.. L....
3O / o Cu-Gco, ,,,piteo,/1 ,"i g
L "'" Eqs:(2)and(4) I ] ; i ]
=" 25
2O .....F...._. ..i.-.k'-,m--r--._..i.-.o---.
1 i lYl, i ° i :
15 L . . • ...... I__-__ ............ .°....
..........I.......-T- _ ,, _ d #!
i _ 8;" /, I ,t i
10 .... _........ L..... ,.._...... .,-[.............._........ L.j ....
t } -= #" = "
..........!_._ _ .............. [............_..............-...............
5
0
0 5 10 15 20 25 30 35
ATsat °C
Fig. 1. Pool boiling performances of pentane on different surfaces
by Eqs. (2) and (4) are shown in Fig. 1 in New Working Fluids
dash lines. The two-tier configuration
explains the constants Cs and m as the Water is an ideal working fluid for
parameters related to the product of the boiling heat transfer in both terrestrial and
active site density and the frequency of microgravity environments by virtue of its
micro bubbles emitted from the composite availability, cost and safety. However, its
surface; the constant G, is the parameter major shortcoming under microgravity is its
related to macrolayerl significant deterioration in nucleate boiling
It is obvious that both qi and qh are performance and CHF with a decrease in
independent from gravity, and therefore, the gravity. This is due to lack of a driving force
boiling performance on the metal-graphite for vapor detachment from the heating
composite surfaces would not deteriorate surface. Furthermore, the Marangoni flow
with reduced gravity. Undoubtedly, this kind around bubbles induced by a negative
of material is suitable to the space surface tension-temperature gradient presses
applications. the bubbles onto the heating surface,
SO
4O
I0 20 30 40 _k) _ 70 aO_ "0 20 20 tO SO (SO"tO *C
,¢
(a) for• serieso[n-_kohds atconccntradons (b)_'orn-_'Ixa.olco._.tradons[mo1_]:
withanequib"oriumsurfacetensionof (1)0(pure wa_), (2)6.31x10"_,
40dynecm4at15"C (3)&OOx104.(4) l.OOxlO4, (5) 1.30xlO4,
(6) 1.59x104.(7)2.00xlO4,(8) &OOxlO"z,
(9) 7.00xlO4
Fig. 2. Variation of the surface tension with temperature for aqueous solutions with long
chain alcohols
resulting in an unfavorable situation for the order of 10.3 mole per liter, to alter
boiling performance. surface tension characteristics of water,
In contrast, aqueous solutions of without affecting other bulk properties [33].
alcohols with a chain length longer than four Another important feature of these aqueous
carbon atoms have a positive surface solutions is a very high value of the positive
tension-temperature gradient when the fluid surface tension-temperature gradient when
temperature exceeds a certain value, as the fluid temperature is near its saturation
shown in Fig. 2 [32]. This surface tension point, thus inducing considerable driving
force is assisted by the buoyant force in force for bubble departure.
nucleate boiling on earth, while it becomes
the principal driving force for bubble (i) Nucleate Pool Boiling
departure from the heating surface in Experimental observations of nucleate
microgravity environments. Consequently, pool boiling under microgravity conditions
bubble departure size is smaller in these revealed that vapor bubbles generated in
aqueous solutions than in water, with a water tend to stay near the heating surface
higher departure frequency. and to slide or oscillate on the surface being
Note that it requires addition of only a induced by coalescence. Thus, one may
small quantity of the long-chain alcohols, on interpret the agitation of the thermal
boundarylayer as servingastheprincipal
microgravity environments using the bubble
mechanismof heat transfer. It is justifiable departure diameter predicted by Zhang and
to apply Rohsenow's model for nucleate Chao [34]:
boiling under normal gravity to that in
1 do" z D
_1p, gD _- /at_C. 2/. [T3C.n 2+ n(n - I)](D) m" -I)ln T¢--d-_-AT.,D - C 2'.n2r2--_- (.-_.)P_'"-"'" =
(5)
where C,, _, and n are constants which (ii) Critical Heat Flux
should be experimentally determined, ATwb Following McGillis and Carey [27], the
is the temperature difference between the hydrodynamic model of Zuber [36] is
wall and the bulk liquid, and r is the vapor employed to derive a correlation formula for
bubble contact radius on the heating surface. the maximum pool boiling heat flux. The
As a first approximation, take _=1, n = 1/2 CHF is postulated to occur when the vapor
and Cn = 2bJa(m/zJ n, where b is an column rising from a heating surface is
empirical constant which is about 0.86 for distorted and blocks the liquid flowing down
toward the surface, often referred to as
water under atmospheric conditions, x is the
Kelvin-Helmholtz instability [37]. At any
liquid thermal diffusivity, Ja is the Jakob
instant, the gravitational energy resulting
number, which equals _Cpl(Tl-Tsad//z, hfg, Cpl
from the body force to return the liquid to
is the liquid specific heat, Tt is the
the heating surface per unit cell (i.e., divided
temperature in the liquid surrounding the
by the spacing of vapor columns) is given by
departure bubble, and Ts_t is the liquid
2 2
saturation temperature.
Rohsenow's model [35] proposes the
relationship between the Nusselt number, where 2t_ is the spacing between two
Nut,, the Reynolds, Ret,, and Prandtl number, adjacent Taylor wave nodes defined as
Prt, as 2z[3o'/(pt-/_)g] l/e. The surface tension force
exerting on the liquid-vapor interface
Nu b = _hD = A Re _,_p) Pr/' _') (6)
kt induced by a surface tension-temperature
where h is the heat transfer coefficient gradient provides additional energy to return
which is defined as the liquid to the heating surface per unit cell
of
h = q (7)
AT,., (p,)
Ag o = 8/, AT,,b(z )2 D (I1)
Here q is the heat flux while the superheat
AT,,,t is a function of the liquid pressure, pt. where _" is a correction factor to be
determined by experimental data. An
The Reynolds number is defined as
addition of Eqs. (10) and (11) yields the
Re b= P"UbD (8)
total available energy as
where #t is the liquid viscosity. The A z + zSo" At.. 2o2
superficial velocity Uo is defined as
(12)
U b = q (9) Note that the second term in Eq. (12) was
Pvh :g dropped in Zuber's original model. This is
The quantities A, p, and s in Eq. (6) are justified in pure liquids because of the low
empirical constants. surface tension-temperature gradient.