Experimental investigation into the pool boiling heat transfer of aqueous based c-alumina nanofluids
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Journal of Nanoparticle Research (2005) 7: 265–274 Springer 2005
DOI: 10.1007/s11051-005-3478-9
Technology and applications
Experimental investigation into the pool boiling heat transfer of aqueous based
c-alumina nanofluids
Dongsheng Wen and Yulong Ding
Institute of Particle Science and Engineering, University of Leeds, Leeds, LS2 9JT, UK (E-mail: y.ding@
leeds.ac.uk)
Received 22 December 2004; accepted in revised form 7 March 2005
Key words: boiling heat transfer, nanofluids, c-alumina nanoparticles, heat transfer enhancement, colloids
Abstract
This paper is concerned about pool boiling heat transfer using nanofluids, a subject of several investigations
over the past few years. The work is motivated by the controversial results reported in the literature and the
potential impact of nanofluids on heat transfer intensification. Systematic experiments are carried out to
formulate stable aqueous based nanofluids containing c-alumina nanoparticles (primary particle size
10–50 nm), and to investigate their heat transfer behaviour under nucleate pool boiling conditions. The
results show that alumina nanofluids can significantly enhance boiling heat transfer. The enhancement
increases with increasing particle concentration and reaches 40% at a particle loading of 1.25% by weight.
Discussion of the results suggests that the reported controversies in the thermal performance of nanofluids
under the nucleate pool boiling conditions be associated with the properties and behaviour of the nano-
fluids and boiling surface, as well as their interactions.
Introduction surface wettability and rheology etc.), and micro-
scopically (through changing, e.g. the number of
As one of the most effective and efficient mode of active nucleation sites, nucleation frequency, and
heat transfer, boiling occurs in a variety of engi- bubble growth and departure rates etc.). This work
neering applications. Enhancement of boiling heat is concerned about the effect of the presence of
transfer has therefore been a subject of numerous nanoparticles on the boiling heat transfer behav-
investigations in the past century; see for example iour. This category of work is referred to as boiling
Rohsenow (1952), Judd and Hwang (1976), Yang heat transfer of nanofluids, which has attracted
and Maa (1983, 2001), Carey (1992), Wu et al., considerable attention in the past few years (Das
(1995), Wasekar and Manglik (1999, 2000, 2002), et al., 2003a, b; Tsai et al., 2003; You et al., 2003;
Yang et al., (2002) and Hetsroni et al., (2004). Tu et al., 2004; Vassallo et al., 2004).
Boiling heat transfer is a very complex process Nanofluids are liquid suspensions containing
even for pure water. It is influenced by the wall particles that are significantly smaller than 100 nm
heat flux (or wall superheat), heating surface and have a bulk solids thermal conductivity of
geometry and properties, and the presence of orders of magnitudes higher than the base liquids.
additives such as surfactants, particulate solids A considerable amount of effort has been made to
and dissolved gases. The presence of additives investigate the thermal behaviour of nanofluids
affects the heat transfer behaviour both macro- since the pioneering work by Choi (1995). These
scopically (through changing e.g. surface tension, studies can be classified into three categories,266 namely, effective thermal conductivity under static boiling heat transfer. However, they observed a conditions (Masuda et al., 1993; Choi, 1995; dramatic increase in the values of the critical heat Eastman et al., 1996, 2001; Lee et al., 1999; Wang flux (CHF) in the presence of nanoparticles. At et al., 1999; Xuan & Li, 2000; Keblinski et al., about 0.198 bar, over 200% enhancement of CHF 2002; Xie et al., 2002; Wang et al., 2003; Wen & was achieved by using 5 · 10)4 wt% Al2O3–water Ding, 2004a), convective heat transfer (Pak & nanofluids and 10)3 wt% silica–water nanofluids. Cho, 1999; Xuan & Roetzel, 2000; Xuan & Li, It was also observed that the addition of nano- 2003; Wen & Ding, 2004b), and phase change heat particles in water increased the bubble size but transfer (Das et al., 2003a, b; Tsai et al., 2003; You decreased the bubble departure frequency. It is et al., 2003; Tu et al., 2004; Vassallo et al., 2004). unclear, however, how these observations are The work on the first and second categories has linked to the observed CHF enhancement. shown qualitatively consistent results despite sig- Vassallo et al., (2004) studied silica–water nificantly scattering i.e. nanofluids can enhance nanofluids boiling on a 0.4 mm diameter hori- both thermal conduction and convective heat zontal NiCr wire at the atmospheric pressure. In transfer (Ding et al., 2004). However, some con- their experiments, silica particles of 15, 50 and troversies occur in the few studies on the phase 3000 nm were tested and particle concentration change heat transfer, particularly the pool boiling was fixed at 1.3 wt%. Similar to the results of You heat transfer, which is briefly reviewed in the fol- et al., (2003), no obvious heat transfer enhance- lowing. ment was found at low and medium heat flux Das et al., (2003a) investigated the nucleate pool conditions, but about 200% enhancement of CHF boiling heat transfer on the surface of a cylindrical was observed. cartridge heater of 20 mm diameter using Al2O3– Tu et al., (2004) carried out pool boiling exper- H2O nanofluids. The presence of nanoparticles iments using Al2O3–H2O nanofluids on a was found to deteriorate boiling performance and 26 · 40 mm2 rectangular surface at the atmo- the degradation increased with increasing nano- spheric pressure and significant heat transfer particle concentration. Similar phenomenon was enhancement was obtained in both nucleate boil- also observed in a later study by Das et al., (2003b) ing and CHF. For instance, comparing with boil- using smaller heaters with 4.5 and 6 mm outside ing using pure water, the time and spatial averaged diameters. It was observed that the extent of deg- superheat was observed to drop from 27.3 to radation in heat transfer was less for the smaller 16.6 K by using a very dilute aqueous nanofluid heaters, particularly at relatively high heat fluxes. containing 37 ppm of Al2O3 nanoparticles under a The authors attributed the deterioration to chan- fixed heat flux of 212 kW/m2. This is equivalent to ges in the surface characteristics of the heaters. a significant heat transfer coefficient enhancement They argued that the surface became smoother in of about 64%. The maximum wall temperature as boiling with nanofluids due to sedimentation of recorded by an infrared camera showed a signifi- nanoparticles on the nucleate sites. The higher the cant decrease, and the active nucleation sites concentration, the smoother the surface, and hence increased by four times in comparison with pure more considerable deterioration of the heat water. Tu et al., (2004) also observed smaller transfer coefficient observed. This interpretation bubbles with no obvious changes of bubble disagrees with the observations of Bang and departure frequency in comparison with pure Chang (2004), who studied boiling of alumina– water, which is in contradiction to the visual water nanofluids on a 100 mm square surface at observations by You et al., (2003). high heat fluxes and found that the surface Witharana (2003) studied boiling heat transfer roughness after boiling increased with nanoparti- of aqueous nanofluids containing gold nanoparti- cle concentration. The increased roughness caused cles on a 100 mm diameter heater surface and a fouling effect with poor thermal conductivity. found a significant nucleate boiling heat transfer You et al., (2003) investigated pool boiling enhancement under the atmospheric pressure. For behaviour of silica–water and alumina–water example, 21% enhancement of heat transfer coef- nanofluids on a 10 mm square heater under sub- ficient was achieved with 0.001 wt% of gold atmospheric pressures. They found little effect of nanoparticles at a heat flux of 45 kW/m2. The the presence of nanoparticles on the nucleate enhancement was also observed to increase with
267
nanoparticle concentration and heat flux under the
nucleate boiling condition. Witharana (2003) also
made some efforts to investigate silica–water and
silica–ethylene glycol nanofluids. However, no
consistent results were produced.
The brief review above illustrates the inconsis-
tencies and indicates that our understanding of the
thermal behaviour of nanofluids related to the
pool boiling is still poor. As part of an ongoing
effort to systematically investigate the thermal
behaviour of nanofluids at the University of Leeds,
this work aims at providing more experimental
data and to explore possible mechanisms that
account for the different results observed. Sys-
tematic experiments were carried out on the pool Figure 1. SEM image of dispersed Al2O3 nanoparticle in
boiling behaviour of aqueous nanofluids contain- water.
ing c-Al2O3 nanoparticles. The nanofluids were
formulated by using the electrostatic stabilisation attraction between particles in both dry and wet
method to eliminate the influences of surfactants/ environments. Dry nanoparticles frequently occur
dispersants, which were believed to have a con- in the form of agglomerates. Some of the
siderable effect on the boiling process (Wu et al., agglomerates, particularly formed due to sintering,
1995; Wasekar & Manglik, 1999, 2000, 2002; Yang are difficult to break even by using prolonged ul-
& Maa, 2001; Wen & Wang, 2002; Yang et al., trasonification and magnetic stirring (Xuan & Li,
2002; Hetsroni et al., 2004). 2000; Das et al., 2003a; Wang et al., 2003). In
order to overcome the agglomeration problem
Experimental work inherent to the two-step method, except for
application of ultrasonification and magnetic stir-
The experimental work consisted of formulation ring methods, a high-speed homogeniser (Ultra-
and characterisation of nanofluids and investiga- Turrax T25, IKA) was used to assist breaking
tion of the pool boiling heat transfer behaviour nano-agglomerates. The homogeniser had a gap of
using the nanofluids. The details are described in 0.5 mm between the rotor and the stator. The
the following. rotational speed of the rotor was adjustable and
the highest speed was 24,000 rpm, which could
Formulation and characterisation of nanofluids provide a shear rate up to 40,000 s)1 and oppor-
tunities to break large nano-agglomerates. In
In this work, Al2O3 nanofluids were prepared by order to prevent formation of agglomerates, surf-
dispersing dry nanoparticles into the base liquid, actants and/or dispersants are often used. As
so called the two-step method. Distilled water was mentioned previously, surfactants may experience
used as the host liquid and Al2O3 nanoparticles changes in their properties and even fail at elevated
were purchased from a commercial company temperatures, electrostatic stabilisation method
(Nanophase Technologies Corporation, USA). was adopted. Such a method makes use of repul-
The nanoparticles were used as received, which sion due to electric double layers surrounding
were produced by a physical vapour deposition individual nanoparticles. For electrostatic stabili-
technique. Figure 1 shows an SEM image of the sation to be effective, the pH value of a suspension
alumina nanoparticles. It can be seen that the has to be away from the iso-electrical point (IEP)
primary alumina nanoparticles are spherical and of alumina, which is about 9.1. The pH value of
their size is widely distributed in a range of nanofluids formulated in this work was adjusted to
10–50 nm. This agrees with the nominal particle 7. The reasons for this are (a) it is reasonably
size provided by the manufacturer. As is well- away from the IEP, and (b) nanofluids with very
known, nanoparticles have a strong tendency to low or very high pH values may cause damages to
agglomerate due to relatively strong van der Waals the boiling surface.268
To examine the effectiveness of the method for T6
nanofluids preparation, a Malvern Nano ZS
Vent
device was used to characterise the particle size
distribution in nanofluids. It was found that par- Condenser
Insulation layer
ticle size decreased sharply with processing time in
the first 30 min after which little further decrease
Observation window
Boiling surface
was observed. It was also found that particle
concentration exerted little effect on the final size
distribution in the range studied in this work. The
processing time for all nanofluids produced for the DAQ
experiments was therefore set at 30 min. Figure 2
shows the results for a particle loading of Drain valve
0.71 wt% after 30 min processing. It is seen that
T1 T2 T3 T4 T5
particle size lies within 90–450 nm with an average Heater
diameter of 167.5 nm and a peak size around Voltmeter
190 nm. These are much larger than the size of the
primary nanoparticles as shown in Figure 1, indi-
cating that nano-agglomerates are very difficult to
break up into individual primary nanoparticles Variable Power supply
even under a very high shearing action. Surpris- Figure 3. Setup for pool boiling experiments.
ingly, even with such big aggregates, the stability
of the produced nanofluids is still good, which can
stay for a few days without visually observable atmospheric operations. There was an observation
sedimentation. window on one side of the vessel for visual
observations. The boiling surface was situated at
Setup for pool boiling heat transfer experiments and the bottom of the vessel, which was the upper side
principle of measurements of a 3 mm thick polished stainless steel disc with
150 mm diameter. A ring heater with a maximum
The pool boiling system consisted of a boiling power of 2.4 kW was closely attached to the back
vessel, a heating and measuring unit and a data surface of stainless steel disc and heat flux was
acquisition unit; see Figure 3. The boiling vessel, controlled through varying the voltage. Five Type
made of adiabatic material, was cylindrical in T thermocouples were embedded on the back of
shape and had an inner diameter of 160 mm and a the boiling surface via high temperature cement
height of 300 mm. There was an aluminium lid on and another thermocouple was inserted into the
top of the vessel to condense the vapour. A venting bulk liquid through the vent to measure the bulk
hole was drilled in the middle of the lid to allow fluid temperature. All the thermocouples were
Figure 2. Size distribution of Al2O3 nanoparticles in 0.71 wt% nanofluids.269
interfaced into the data acquisition unit integrated
into a PC via a NI PCI-6052E board. A SCXI-
1102 32-channel thermocouple amplifier was used
to achieve high accuracy temperature measure-
ments during heat transfer experiments and the
Labview software was used for data acquisition
and system configuration.
In a typical experiment, nanofluid of a preset
concentration was prepared and filled into the
boiling vessel. The fluid was then preheated to the
saturated temperature, followed by measurements
in the nucleate boiling regime under the steady
state. Temperature data were recorded at each
heat flux, q, calculated by
q ¼ U2 =ðRAÞ; ð1Þ
where U is the voltage, R is the heater resistance,
and A is the surface area of the heater. The steady Figure 4. Wall temperature as a function of heat flux as
state heat diffusion equation was adopted to ob- well as nanoparticle concentration.
tain the boiling surface temperature
Tw ¼ Tm qd=kw ; ð2Þ
increases with increasing heat flux. The wall tem-
where Tm is the temperature of the back of the perature also decreases with increasing nanoparti-
stainless steel disc, d is the thickness of the disc, and cle concentration. A more traditional plot of heat
kw is the thermal conductivity of the stainless steel flux against wall superheat is shown in Figure 5,
disc. The heat transfer coefficient, h, is calculated by together with the prediction by the following clas-
h ¼ q=ðTw Ts Þ; ð3Þ sical correlation by Rohsenow (1952) for pool
boiling for comparison purpose:
with Ts the saturation temperature. rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1=3
The processing of the data and discussion of the cp ðTw Ts Þ q r
s
¼ Csf ; ð4Þ
results require the effective thermal conductivity of hfg Pr lhfg gðq qv Þ
nanofluids for which no theory is available for an
adequate prediction based on thermal conductivity where hfg is the latent heat of the fluid, r is the
of the host liquid, the bulk thermal properties of surface tension, qv is the vapour density, g is
nanoparticles, and the solids volume fraction. gravitational acceleration, and Pr is the Prandtle
Measurements were therefore carried out by using number defined as l cp/kL with kL the thermal
a KD2 thermal property meter (Labcell Ltd, UK). conductivity of the liquid. Csf and s are constants
The device is based on the hot-wire method with taken as 0.013 and 1 for pure water, respectively. It
an uncertainty 3% under the conditions of this can be seen that the Rohsenow correlation agrees
work. well with the measurements using pure water, but
deviation occurs for nanofluids and the deviation
increases with nanoparticle concentration.
Results and discussion The heat transfer coefficient and the enhance-
ment ratio compared with pure water are shown
Experimental results in Figures 6 and 7, respectively. Significant
improvement can be seen on the heat transfer
The average surface temperature at different coefficient of nucleate pool boiling due to the
nanoparticle concentrations is shown in Figure 4. presence of nanoparticles. The improvement
The presence of nanoparticles is seen to reduce the increases with nanoparticle concentration and is
surface temperature significantly, and the reduction more considerable at high heat fluxes. At the270
Figure 5. Comparison with Rohsenow pool boiling corre- Figure 7. Heat transfer enhancement ratio.
lation.
increases approximately linear with particle
nanoparticle concentration of 1.25 wt%, approxi- volume fraction under the conditions of this work,
mately 40% enhancement has been achieved. and an enhancement of 10% is achieved at a
It is interesting to compare the enhancement of particle concentration of 1.6 vol%. Also shown
the heat transfer coefficient with that of the effec- in Figure 8 are predictions by the MG (Choy,
tive thermal conductivity. A typical set of thermal 1999), the Bruggeman (Choy, 1999), the DEMT
conductivity results is shown in Figure 8 for (Garboczi & Berryman, 2000) and the HC
measurements at 22C, where k is the thermal (Hamilton & Crosser, 1962) models developed for
conductivity of nanofluids. It can be seen that the macroscopic systems, which obviously fail to pre-
effective thermal conductivity of nanofluids dict the measured results. A comparison between
the results shown in Figure 8 and those in Figure 7
Figure 8. Effective thermal conductivity of Al2O3–H2O
Figure 6. Heat transfer coefficient comparison. nanofluids at 22C.271
suggests that the enhancement of the pool boiling boiling heat transfer behaviour observed by Das
heat transfer coefficient is far greater than that of et al., (2003a, b) and Bang and Chang (2004). This
the thermal conductivity. is indeed reflected by the images of the boiling
surface after experiments (Bang and Chang, 2004),
Discussion of the results and directly observed by Das et al., (2003a).
Observations made in this work showed no parti-
The experimental observations described in cle deposition, an indication that stable nanofluids
‘Experimental results’ agree with those by You could promote boiling heat transfer.
et al., (2003) and Tu et al., (2004), but are in
contradiction to those by Das et al., (2003a, Effect of surfactant/dispersant
2003b) and Bang and Chang (2004) who observed As mentioned previously, dispersants and/or surf-
deterioration of the boiling heat transfer due to the actants are often used to stabilise nanoparticle
presence of nanoparticles. As the boiling heat suspensions, which could exert a significant influ-
transfer behaviour is expected to depend on the ence on both the rheological behaviour of the fluids
properties and behaviour of nanofluids and the and the boiling heat transfer (Wasekar & Manglik,
boiling surface, as well as interaction between 2000; Yang & Maa, 2001; Wen & Wang, 2002; Yang
the two, the following discussion will be done et al., 2002; Hetsroni et al., 2004). However, most
according to these aspects. published studies on pool boiling of nanofluids did
not clearly specify if surfactants or dispersants were
Effect of nanofluids properties used. For example, the nanofluids used by Vassello
Macroscopically speaking, the properties of a et al., (2004) were prepared through diluting a
homogenous nanofluid that affect its thermal commercial concentrated dispersion, which very
behaviour include heat capacity, thermal conduc- likely contained surfactants/dispersants. Another
tivity, density and viscosity. The effect of thermal concern is the possible failure of surfactants under
conductivity seems to be important as discussed boiling conditions. Wen and Ding (2004b) studied
above, whereas the effects of heat capacity, density the thermal behaviour of suspensions of carbon
and viscosity of nanofluids are small due to very nanotubes using sodium dodecyl benzene sulfonate
small solids concentrations. From a microscopic (SDBS) as the surfactant and found the surfactant
viewpoint, however, nanoparticle migration may failed at a temperature of 69C.
enhance heat transfer as indicated by some con-
vective heat transfer studies (Xuan & Li, 2003; Effect of the boiling surface and its interaction
Wen & Ding, 2004a). These arguments also seem with nanofluids
to support the enhanced boiling heat transfer Boiling heat transfer consists of a number of sub-
observed in this work as well as that by You et al., processes in parallel and/or series, including
(2003). For non-homogenous nanofluids, instabil- unsteady-state heat conduction, growth and
ity due to sedimentation is expected to affect the departure of bubbles, and convection due to
heat transfer behaviour in a negative way. As bubble motion and liquid re-filling. Apart from the
mentioned in the introduction section, commer- properties of nanofluids that have been discussed
cially available nanoparticles were used in most in ‘Effect of nanofluids properties’, these
reported studies for nanofluids preparation with sub-processes are affected by parameters such as
an aid of either or both of ultrasonification and heater geometry, properties of the boiling surface,
magnetic stirring methods. Nanofluids formulated orientation of the heater, liquid sub-cooling, sys-
in such a way are normally unstable even in the tem pressure, and the mode in which the system is
presence of surfactant/dispersant due to large operated. Among these, the boiling surface prop-
agglomerates (Xuan & Li, 2000; Xie et al., 2002; erties are among the key factors that influence the
Das et al., 2003a, b; Wang et al., 2003). The boiling heat transfer. The surface properties
agglomerates are likely to deposit on the heating include surface finish (roughness), surface wetta-
surface, generating an extra thermal resistance to bility, and surface contamination as they all
the boiling surface and hence preventing the direct influence the number and distribution of active
contact of liquid with the boiling surface. This may nucleation sites for bubbles and their subsequent
be one of the reasons for the deterioration of growth (Hsu, 1962). In the published studies,272
however, only the surface roughness is the most nucleate on the welded positions and the measured
often used parameter, and interpretations of the temperature may not be representative of the boiling
effect of surface roughness on the boiling heat surface. Vassalao et al., (2004) used fine resistance
transfer have been based on the relative size of the wires for temperature measurements. Large uncer-
suspended particles to the surface roughness. For tainties are expected for this sort of method as tem-
example, Bang and Chang (2004) used a boiling perature is converted from the measured resistance
surface of nanometre scale roughness hence sedi- of the heating wire against the standard temperature-
mentation of particles was regarded to effectively resistance curve. Indeed, for boiling with pure water,
increase the roughness of the surface, whereas a more than 10C deviance of superheat was observed
commercial cartridge heater with a micron scale under a fixed heat flux condition in different runs; see
surface roughness was employed by Das et al., Figure 1 of Vassallo et al., (2004). It may be sensible
(2003a, b) onto which sedimentation of nanopar- for a qualitative comparison of the critical heat flux
ticles was thought to decrease the effective surface (CHF), but it is less meaningful for a quantitative
roughness. comparison of nucleate boiling heat transfer.
No systematic studies have been carried out on
the interaction between the surface and nanofluids,
e.g. wettability, and little consideration has been
given to the surface contamination. As mentioned Concluding remarks
in ‘Effect of surfactant/dispersant’, surfactant
could fail at elevated temperatures. The failed Pool boiling heat transfer experiments using stable
surfactant could stick on and hence contaminate/ c-Al2O3–H2O nanofluids produced through elec-
modify the boiling surface. These factors can be trostatic stabilisation method with the aid of a
significant as indicated by the bubble nucleation high shear homogeniser have been conducted. The
theory, and some recent studies on spreading and results show that alumina nanofluids can signifi-
wetting of nanofluids (Wasan & Nikolove, 2003; cantly enhance the boiling heat transfer. The
Chengara et al., 2004). In this work, no surfactant enhancement increases with increasing particle
was used. The boiling surface was of micron size concentration and reaches 40% at a particle
roughness and was cleaned before experiments by loading of 1.25% by weight. Discussion of the
de-ionised water before each run. These measures results suggests that the reported controversies in
minimised the effects of boiling surface properties the pool boiling heat transfer behaviour be asso-
and contamination. As the work was only on ciated with the properties and behaviour of both
alumina nanofluids boiling on one surface, gener- nanofluids and boiling surface, as well as their
alisation of the results requires more systematic interactions. Future work should be focused on
work by considering all factors discussed above. the effect of the interaction between nanofluids
and the boiling surface on the thermal behaviour.
Other factors The primary particles used in this work lies within
Other factors that may influence pool boiling are 10–50 nm but the measured particle size is consid-
the measurement techniques and the characteristic erably larger, in the range of 90–450 nm. This
size of the system. The effect of the characteristic indicates the presence of aggregates, which may
size of the system is expected to be important when affect the performance of nanofluids. Further
the system is very small (Guo & Li, 2003; Celata quantitative studies should also be carried out on
et al., 2004). This, however, is out of the scope of the effect of aggregation on nanofluids heat transfer.
this work and the following discussion will only be
on the measurement techniques.
Different temperature measurement methods may
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