CHAPTER that was approximately nine times that for

CHAPTER 2

LITERATURE REVIEW

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Before
undertaking any work, it is necessary to have an idea about research work that
have been carried out in that area till the commencement of present work.
Therefore, a number of previous researches are studied, and a detailed review
on the different methods to increase CHF in pool boiling is done. In the recent
years, different surface modifications such as dip coating of graphene and
graphene oxide, porous nanostructured coating, sintered copper microporous
coatings, acid etching and many more have been done to obtain pool boiling CHF
enhancement. Some of the previous investigations related to current study are
extensively summarized in this chapter.

2.1    ROLE OF SURFACE MODIFICATIONS IN POOL
BOILING CHF

Klemen
Ferjancic et al. 1 studied the effect of surface treatment, namely roughening
the surface with different sandpapers and etching in diluted acid on pool
boiling CHF in FC-72 and water. They used horizontally and vertically oriented
ribbon heaters of stainless steel 302 and steel 1010 in this experiment. It was
observed that for surfaces roughened with sandpaper the CHF in both fluids
increase with an increase in roughness. Experimental results showed that for
FC-72 and stainless steel 302 in the

 range
0.02 ?m to 1.5 ?m, the CHF increased from 87.6 kW/m2 to 115.4 kW/m2,
whereas for water in the same

 range it increased from 410.8 kW/m2
to 499.7 kW/m2. For steel 1010 in water CHF increased from 309.7
kW/m2 to 443.9 kW/m2. Another notable observation was the
increase in CHF for both stainless steel 302 and steel 1010 in water for an
etched surface by 51% with respect to a surface roughened with sandpaper.

Syed
W. Ahmad et al.2 studied the pool boiling of R-123 using five modified copper
surfaces namely an emery polished surface, a fine sandblasted surface, a rough
sandblasted surface, an electron beam enhanced surface and a sintered surface.
They observed that sintered copper surface gave the highest heat transfer
coefficient that was approximately nine times that for the emery polished
surface.

B.J.
Jones et al.3 examined the effect of surface roughness on pool boiling of
FC-77 and water at atmospheric pressure. They prepared four aluminium test
surfaces using ram-type electrical discharge machining with average surface
roughness of 1.08 ?m, 2.22 ?m, 5.89 ?m and 10 ?m. The experimental observations
showed that for FC-77, the increase in heat transfer coefficient at 100 kW/m2
heat flux was 2.4 to 3 times that for a smooth surface. The experimental
observations with water did not provide a clear relationship between heat
transfer coefficient and surface roughness and for 100 kW/m2 heat
flux, the enhancement in heat transfer coefficient was observed in between 1.5
to 1.8 times that for a smooth surface.

Arvind
Jaikumar et al.4 observed the scale effects of graphene and graphene oxide
coatings on pool boiling enhancement mechanisms. Four surfaces with nanoscale
and microscale coatings of graphene and graphene oxide were evaluated. Dip
coating was used to prepare the surfaces. The coating duration for the surfaces
were 120 s, 300 s, 600 s and 1200 s. They saw that for the lowest coating
duration of 120 s, a CHF of 182 W/cm2 was obtained which was 42%
more than the plain surface. Other CHF values obtained were 128 W/cm2,
124 W/cm2 and 124W/cm2 for coating duration of 300 s, 600
s and 1200 s respectively.

S.
Das et al.5 studied the augmentation of pool boiling heat transfer
characteristics of different surfaces with water, at atmospheric pressure. Four
surfaces were used, namely untreated (plain), treated by emery paper and
treated with silicon oxide(SiOx) TF surfaces having nano-layer
thickness of 100 nm and 200 nm. The experimental results showed that a maximum
of 45.6%, 36.8% and 10.5% enhancement in heat transfer co-efficient was
obtained for SiOx TF (200 nm), SiOx TF (100 nm), and  emery paper treated surface respectively
compared to plain surface. They further suggested that the highest enhancement
of heat transfer coefficient in thin film surfaces was due to capillary effect,
better liquid spreading and high density active nucleation site.

Yu
and Lu6 investigated the heat transfer performance of rectangular fin arrays
for saturated pool boiling of FC-72 at 1 atm. The EDM process was used to
manufacture 7 × 7,
5 × 5, and 4 × 4 fin array test
surfaces from copper blocks of 10 mm ×10
mm base area, with fin spacing of 0.5 mm, 1 mm, and 2 mm, respectively. Four
different fin lengths (0.5 mm, 1 mm, 2 mm, and 4 mm) were investigated and the
thickness of the fins was fixed as 1 mm. In general, the heat transfer rate
increased as the fin length increased and the fin spacing decreased, with the maximum
value being achieved with the fin array having the narrowest fin gaps (0.5 mm)
and the highest fins (4 mm), more than five times that for the reference plain
surface.

Rainey
and You7 investigated the effect of microporous coated surfaces on pool boiling
of saturated FC-72 at atmospheric pressure. Copper test surfaces, 20 mm × 20 mm and 50 mm × 50 mm, were coated
using a mixture of diamond particles, Omegabond 101, and methyl ethyl ketone
(MEK), known as DOM, by drip-coating onto the 20-mm square surface and
spray-coating onto the 50-mm square surface. Evaporation of the MEK produced a
microporous layer on the surface, approximately 50 ?m thick and containing 8–12
?m diamond particles. Heat transfer coefficients for nucleate boiling on the microporous
coated surfaces were always augmented by more than 300% compared to those for
plain polished surfaces.

Kim
et al.8 studied the pool boiling characteristics of treated surfaces,
including the effects of sub-cooling and surface orientation, using the
dielectric liquid PF5060 and 20 mm ×
20 mm copper test surfaces. Four different surfaces were tested: a plain
surface, a sanded surface, a microfinned surface, and a microporous coated
surface. The sanded surface was prepared using grade 80 sandpaper and had an
average roughness height of 1.546 ?m. For saturated conditions and horizontal
orientation, the sanded surface achieved a wall superheat reduction of 43% at
120 kW/m2 compared to that measured for the plain surface. The
microfinned surface tested was fabricated by etching a copper test block to
produce microfins of 100 ?m × 100
?m square cross section with a height of 50 ?m. The spacing between the fins
was 200 ?m and the increase in heat transfer area was 43.6% compared to the
original plain surface. Their PF5060 pool boiling curves show that for a heat
flux of 120 kW/m2, the wall superheat for the microfinned surface was
47% lower than for a plain surface.

Yeung
Chan Kim9 investigated the effect of surface roughness on pool boiling heat
transfer in sub-cooled water-CuO nanofluid. Experiment was performed using 0.1%
volumetric water-CuO nanofluid and pure water for comparison. The heat flux tended
to increase as the liquid sub-cooling increased in the region of low wall
superheat. However, the effect of liquid sub-cooling gradually decreased as the
wall superheat increased. The heat flux of pure water and nanofluid was almost
similar in the region of low wall superheat. As the wall superheat increased,
however, the heat flux of nanofluid decreased compared to that of pure water.
It was suggested that the nanoparticles mixed with pure water reduced the heat
flux by deteriorating boiling on the heat transfer surface. The heat flux
increased as the surface roughness increased in the pure water, but the effect
of surface roughness on heat flux was unclear in the nanofluid. This was
attributed to the decreased difference of surface roughness, which was caused
by the coating or deposition of nanoparticles on the heat transfer surface
during the experiment.

Russell
P. Rioux et al.10 studied the effects of macroscale, microscale and nanoscale surface
modifications in water pool boiling heat transfer. Nanostructured surfaces are
created by acid etching, while microscale and macroscale structured surfaces
are synthesized through a sintering process. Six structures from nanoscale
through microscale to macroscale: polished plain, flat nanostructured, flat
porous, modulated porous, nanostructured flat porous and nanostructured
modulated porous were investigated. Both HTC and CHF were greatly improved for all
modified surfaces compared to the polished baseline. The CHF and HTC of the
hierarchical multiscale modulated porous surface have achieved the most
significant improvements of 350% and 200% over the polished plain surface
respectively.

Kang11
examined the surface roughness effect on saturated pool boiling heat transfer
in water over stainless steel tubes (oriented horizontally 0? , inclined 45? ,
and vertically 90? ) that were treated with sandpaper (

= 0.151 and 0.609 µm). He noticed that
increased surface roughness gives better heat transfer results than the smooth
surface at a given wall superheat due to the presence of more active nucleation
sites. In addition, he showed that the pool boiling heat transfer coefficient
(h) depended on the nucleation site density and increased up to 15% with
increasing roughness.

Jabardo
et al.12 investigated the effect of roughness of copper and brass surfaces on
pool boiling heat transfer with R-134a and R-123. The surfaces were treated by
several processes in order to obtain different degrees of roughness from 0.07
?m to 10.5 µm (polishing, sandpaper, shot peening with glass beads, and
sandblasting). These authors verified what other researchers had already
observed in the distant past when examining roughness values greater than 1 µm;
that HTC increased with surface roughness up to a certain value and then started
to decrease with further increase in roughness. In this experiment the surface
roughness for maximum HTC was 3 ?m. This was attributed again to the density of
the active cavities. The number of cavities available for nucleation increases
with the increment of surface roughness, which in turn allows enhancement of
heat transfer.

Guan
et al.13 worked with brass surfaces randomly roughened with values varying
from 0.15 ?m to 5 µm and employed pentane, hexane, and FC-72 as the working
fluids. During pool boiling experiments they observed that the CHF increased
with increasing surface roughness and they attributed it to the wicking liquid
flow to the heating surface. They also developed an equation for predicting CHF
enhancement by characterizing these wicking velocities and modifying the pool
boiling CHF lift-off model for smooth surfaces that had been previously
developed by other researchers.

Hosseini
et al.14 computed the value of HTC during R-113 pool boiling on copper
surfaces treated with different grit sizes of sandpaper (Ra = 0.09 ?m–0.901
µm). Their results showed significant improvement in the value of HTC by 38.5%
as the surface roughness increased from 0.09 ?m to 0.901 µm.

Sudev
Das and Swapan Bhaumik15 studied nucleate pool boiling heat transfer using
water on thin film surface. Electron beam evaporation method, was
used for fabrication of nanoparticle-coated thin-film surface. The nucleate
pool boiling heat transfer performance of untreated, treated, treated with
titanium oxide and silicon oxide thin-film surfaces was experimentally
investigated at atmospheric pressure. After analyzing the experimental results,
they found that thin-film surface is superior from the point of boiling heat
transfer coefficient than other surfaces. The results showed a maximum of 45%
and 60 % enhancement in heat transfer coefficient for higher thickness of
silicon oxide and titanium oxide thin-film surface as compared to untreated
surface. They suggested that the highest enhancement of heat transfer
coefficient in thin-film surfaces was due to improvement in level of
wettability,  enhanced surface roughness
and creating high-density active nucleate site on the surface.

M.
Dharmendra et al.16 investigated pool boiling heat transfer enhancement using
vertically aligned carbon nanotube coatings on a copper substrate. Experiments
were conducted on bare, sandblasted and vertically aligned CNT coated copper
substrate of dimension 14 mm×14 mm using de-mineralized water as working fluid.
A remarkable enhancement of 38% in the CHF was observed on the CNT coated
surface. The sandblasted also showed enhancement in CHF by 8.5% with respect to
the base copper surface.

Hee
Seok Ahn et al.17 investigated the effect of nano-structured surface on pool
boiling. Horizontal heater surface coated with vertically aligned multiwalled
carbon nanotubes of different heights( Type A: 9 ?m height and Type B: 25 ?m
height) were used in the experiment. The test fluid used was PF-5060. Observations
showed that Type-B MWCNTs yield higher heat fluxes under sub-cooled and
saturated conditions for nucleate boiling. The critical heat flux in case of
Type B MWCNTs increased by 40%. In contrast, Type-A MWCNTs provided only
marginal enhancement of 10% in pool boiling CHF compared to plain silicon
surface.

2.2   
INFERENCE FROM LITERATURE REVIEW

From
the above mentioned papers, it has become clear that surface modifications will
enhance the critical heat flux in pool boiling. Significant amount of work has
been done in the area of surface modification for heat transfer enhancement in
pool boiling but most of these works are concentrated to using different type
of surface coatings, mostly of the nano-scale level on the substrate.
Relatively few works are available on surface modification using chemical roughening,
that is acid etching that too on copper substrate and also very limited
literature are available which have combined both mechanical roughening and
chemical roughening as techniques to increase CHF in pool boiling. The present
work is an attempt to combine both the areas of mechanical roughening
technique, that is roughening with sandpaper and chemical roughening technique
on copper substrate and study the enhancement in pool boiling CHF.

 

 

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