Enhancement of critical heat flux in nucleate boiling of nanofluids: a state-of-art review
Hyungdae Kim
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Department of Nuclear Engineering, Kyung Hee University
, Yongin, Gyunggi 446-701,
Republic of Korea
Nanofluids (suspensions of nanometer-sized particles in base fluids) have recently been shown to have nucleate boiling critical heat flux (CHF) far superior to that of the pure base fluid. Over the past decade, numerous experimental and analytical studies on the nucleate boiling CHF of nanofluids have been conducted. The purpose of this article is to provide an exhaustive review of these studies. The characteristics of CHF enhancement in nanofluids are systemically presented according to the effects of the primary boiling parameters. Research efforts to identify the effects of nanoparticles underlying irregular enhancement phenomena of CHF in nanofluids are then presented. Also, attempts to explain the physical mechanism based on available CHF theories are described. Finally, future research needs are identified.
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Introduction
Nanofluids are a new class of nanotechnology-based
heat-transfer fluids, engineered by dispersing and
stably suspending nanoparticles (with dimensions on the
order of 1-50 nm) in traditional heat-transfer fluids.
The base fluids include water, ethylene, oil, bio-fluids,
and polymer solutions. A variety of materials are
commonly used as nanoparticles, including chemically
stable metals (e.g., copper, gold, silver), metal oxides
(e.g., alumina, bismuth oxide, silica, titania, zirconia),
several allotropes of carbon (e.g., diamond,
singlewalled and multi-walled carbon nanotubes, fullerence),
and functionalized nanoparticles.
Nanofluids originally attracted great interest because
of their abnormally enhanced thermal conductivity [1].
However, recent experiments have revealed additional
desirable features for thermal transfer. Key features of
nanofluids that have thus far been discovered include
anomalously high thermal conductivity at low
nanoparticle concentrations [2,3], a nonlinear relationship
between thermal conductivity and concentration for
nanofluids containing carbon nanotubes [3], strongly
temperature-dependent thermal conductivity [4], and a
significant increase in nucleate boiling critical heat flux
(CHF) at low concentrations [5,6]. State-of-the-art
reviews of major advances on the synthesis,
characterization, thermal conductivity, and single-phase
and two-phase heat transfer applications of nanofluids
can be found in [7-17]. However, the available reviews
have paid much more attention to thermal properties
and single-phase convective heat transfer than to
twophase heat transfer, and even reviews including
twophase heat transfer have only briefly touched upon
important new research on the significant increase of
CHF in nanofluids.
This paper presents an exhaustive review and analysis
of CHF studies of nanofluids over the past decade. The
characteristics of CHF enhancement in nanofluids are
systemically reviewed according to the effects of boiling
parameters. Efforts to reveal the key factors leading to
nanofluid CHF enhancement are summarized. Attempts
to understand the precise mechanism of the
phenomenon on the basis of existing CHF theories are also
presented. Finally, future research needs are identified in
the concluding remark.
CHF enhancement in nanofluids
You et al. [5] first demonstrated that when a nanofluid
is used instead of pure water as a coolant, CHF can be
significantly enhanced. Their test results for pool boiling
of alumina-water nanofluid showed that the CHF
increased dramatically (approximately 200% increase) at
low concentrations (less than 0.01 vol.%) compared with
pure water. Significant enhancement of CHF was further
Table 1 Summary of studies on CHF of nanofluids in pool boiling
Reference Nanofluids
Concentration Test heater
0.001-0.025 g/l Cu plate (10 10 mm2)
SS wire (j = 0.381 mm)
Cu disk (j = 10 and 15 mm)
NiCr wire (j = 0.64 mm)
NiCr wire (j = 0.32 mm)
Cu plate (9.5 9.5 mm2)
Ti wire (j = 0.25 mm)
confirmed for SiO2 particles in water by Vassallo et al.
[6]. However, the causes of CHF increases in nanofluids
could not be explained using traditional CHF
correlations. Since the publication of these pioneering works,
extensive experimental studies have been conducted in
this area over the past decade. Studies of CHF increase
in nanofluids are summarized in Tables 1 and 2
according to pool and flow conditions, respectively.
In this section, characteristics of CHF enhancement in
nanofluids that have been identified from an exhaustive
review of published studies over the past decade will be
summarized in terms of the effects of primary
Cu (10-20 nm) in water
Table 2 Summary of studies on CHF of nanofluids in flow boiling
Al2O3 (47 nm) in water
Al2O3 (25 nm) in water
parameters as follows:
Influence of nanoparticle concentration
CHF enhancement in nanofluids is strongly dependent
on nanoparticle concentration. Figure 1 shows the
experimental results of You et al. [5] and Kim et al. [18]
for the CHF of nanofluids in pool (...truncated)