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Nanouids in thermosyphons and heat pipes: Overview of recentexperiments and modelling approaches

Matthias H. Buschmann

Institut fr Luft-und Kltetechnik Dresden, Bertolt-Brecht Allee 22, 01309 Dresden, Germany

a r t i c l e i n f o

Article history:

Received 23 November 2012

Received in revised form

24 April 2013

Accepted 24 April 2013

Available online 24 June 2013

Keywords:

Nanouids

Thermosyphon

Heat pipe

Thermal performance

a b s t r a c t

Confronted with limited energy and material resources and undesirable manmade climate changes,

science is searching for new and innovative strategies to save, transfer and store thermal energy.

Currently, one of the most intensively discussed options are the so-called nanouids. Nanouids are

suspensions consisting of a liquid baseuid and solid particles of sizes ranging from 10 nm to 200 nm.

The higher thermal conductivity of these nanoparticles leads to an increased effective thermal con-

ductivity of the uid which, the general expectation is, should enhance the heat transfer of the device.

This overview aims to compile results of the application of nanouids in thermosyphons, heat pipes,

and oscillating heat pipes. The general goal is to draw conclusions with respect to the potentials for

improvement of the thermal performance of these gadgets. Additionally, possible mechanisms which

may generate these improvements are discussed. All together 38 experimental studies and 4 modelling

approaches are analyzed. While most investigations recognize nanouids as an advantageous working

uid, some others report negative effects.

Performance effects which are related to lling ratio, inclination angle, and operation temperature

seem to be similar to those for classical workinguids. Several authors report a decrease of the thermal

resistance or an increase of the efciency with increasing concentration, but also a reversing of this trend

if a certain optimal concentration is exceeded. This observation mainly follows with a signicant increase

of the evaporator heat transfer coefcient. The condenser heat transfer coefcient seems to be notor onlyweakly affected. Baseuid, nanoparticle material, size and shape, and the stabilization of the suspension

have an inuence on the thermal performance. However, the limited number of experiments does not

allow drawing rm conclusions. The main mechanism responsible for the improved thermal perfor-

mance seems to be a porous layer built from nanoparticles on the evaporator surface. Additional positive

effects may follow from the changed thermophysical properties of the workinguid.

2013 Elsevier Masson SAS. All rights reserved.

1. Introduction

Confronted with limited energy and material resources and

undesirable manmade climate changes, science is searching for

new and innovative strategies to save, transfer, and store thermal

energy. Currently, one of the most intensively discussed options arethe so-called nanouids. Rapid advances in manufacturing

methods allow the production of nanoparticles of various sizes,

shapes, and materials. Nanouids are created by suspending

nanoparticles of size 10 nme200 nm in varying baseuids. Fig. 1

compares the normalized numbers of publications in the elds of

nanouids, heat transfer, turbulence, and turbulent boundary layer.

Normalization is carried out with the 2011 values which are 425 for

the keyword nanouids, 8729 for heat transfer, 5736 for turbulence

and 896 for turbulent boundary layer. Clearly the exponential in-

crease of publications for nanouids is visible.

The motivation for this new step in heat and mass transfer can

be found in upward trends in energy density of electronic devices,

increasing packing density of heat transfer equipment in general,

miniaturization of heat exchangers, and other advanced heattransfer concepts. The general expectation is that the higher ther-

mal conductivity of the nanoparticle materialsleads to an increased

effective thermal conductivity of the uid which in turn should

enhance the heat transfer of the device. The minuteness of the

nanoparticles provides hope that these advantages are not coun-

teracted by clogging, sedimentation, and abrasion, issues known for

larger particles. Several recent overviews analyzing the current

nanouid research (e.g. Sergis and Hardalupas [1], Thomas and

Sobhan [2]) support this assumption. The darker facet of nanouids

is their extreme complexity, preventing rst-principle solutions

and customary physical models from describing uid mechanic andE-mail address:Matthias.Buschmann@ilkdresden.de.

Contents lists available atSciVerse ScienceDirect

International Journal of Thermal Sciences

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . co m / l o c a t e / i j t s

1290-0729/$ e see front matter 2013 Elsevier Masson SAS. All rights reserved.

http://dx.doi.org/10.1016/j.ijthermalsci.2013.04.024

International Journal of Thermal Sciences 72 (2013) 1e17

mailto:Matthias.Buschmann@ilkdresden.dehttp://www.sciencedirect.com/science/journal/12900729http://www.elsevier.com/locate/ijtshttp://dx.doi.org/10.1016/j.ijthermalsci.2013.04.024http://dx.doi.org/10.1016/j.ijthermalsci.2013.04.024http://dx.doi.org/10.1016/j.ijthermalsci.2013.04.024http://dx.doi.org/10.1016/j.ijthermalsci.2013.04.024http://dx.doi.org/10.1016/j.ijthermalsci.2013.04.024http://dx.doi.org/10.1016/j.ijthermalsci.2013.04.024http://www.elsevier.com/locate/ijtshttp://www.sciencedirect.com/science/journal/12900729http://crossmark.dyndns.org/dialog/?doi=10.1016/j.ijthermalsci.2013.04.024&domain=pdfmailto:Matthias.Buschmann@ilkdresden.de

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thermodynamic behaviour (Feja and Buschmann [3]). Therefore,

most nanouid investigations are carried out experimentally and so

far in rather simple geometries such as straight pipes (Prabhat et al.

[4], Buschmann[5]). New challenges occur with the application ofnanouids in thermosyphons, heat pipes, and oscillating heat pipes

(Liu and Li[6]).

Thermosyphons and heat pipes are well understood and wide-

spread applied heat transfer devices. They are highly efcient due

to the utilized phase change. Design, operation principles and

thermal performance are discussed in detail in several textbooks

(e.g. Reay and Kew [7], Faghri [8]). Schematics of the operation

principles of these three types of apparatus are shown in Fig. 2.

Thermosyphons are devices of passive heat exchange employing

natural convection. Fig. 2a shows the main components of ther-

mosyphons, the ways of rising vapour, and down owing conden-

sate lm. Thermosyphons are either open-loop ore closed. In the

latter case the working uid returns to the original evaporator via

the down running condensate lm. The main parts of a heat pipe

(Fig. 2b) are the evaporator were heating takes place, the adiabatic

section, and the condenser were cooling takes place. In a standard

heat pipe condensate return is ensured via capillary force. There-

fore, the inner wall is lined with a wick which has a capillary

structure. Usually screen wicks, sintered powder wicks, axial or

helical grooves are employed. Alternatively the condensate can be

returned by centripetal, electrokinetic, magnetic, osmotic forces or

other strategies[7]. In case that a heat pipe is inclined against the

horizontal axis its operation is supported by gravity. Forlargelling

ratios and inclination angles the effect of capillary structure be-

comes insignicant. In a standard heat pipe the lling ratio is just

sufcient to saturate the capillary structure.1

When employing nano

uids as working

uids, a puzzlingmanifold of additional parameters e nanoparticle size, shape and

material, baseuid characteristics etc. e inuences the thermal

performance of these gadgets. The general goal is to lower the

thermal resistance, dened as the transported heat divided by the

temperature difference between evaporator and condenser. This

target seems to be not only reached by the enhanced thermal

conductivity of the workinguid as it is the case in e.g. laminar pipe

ow. It is rather a complex interplay of nanouid thermal proper-

ties, nanoparticle interaction with evaporator surface and wick, and

changed vapour bubble generation due to porous layers formed

from nanoparticles.

This overview aims to compile recent experimental results of

thermosyphons, heat pipes, and oscillating heat pipes operated

with nanouids. The survey is supplemented by several modelling

approaches. Possible physical mechanisms acting in gadgets oper-

ated with nanouids are discussed. The general goal is to draw

conclusions with respect to the potentials for improvement of the

thermal performance. Focus is therewith on the differences be-

tween reference uid and nanouid. Additionally suggestions for

future research are given. However, the application of nanouids

and especially in heat pipes and related devices is still associated

with severe and unresolved questions. Many issues are open or

even not commenced to investigate. Therefore, parts of the

following text and the conclusions are formulated with reserve.

2. Organization of paper

The study focuses on recent advances in the application ofnanouids in closed two-phase and open thermosyphons, heat

pipes, and oscillating heat pipes. The main focus of the study is

directed towards the effects caused when nanoparticles are added

to the working uids of these gadgets. A general rule proposed by

Fernholz and Finley [9]in their 1996 review paper on turbulent

boundary layers: In the absence of any complete or convenient

theoretical approach, our primary function is to describe what we

see is applied. In our case this means that the results seen by the

different investigators are compiled according to their logical

connectedness. Based on that ordering, the four main parts of the

survey discuss physical effects which are related to operating

Nomenclature

di inner diameter, mm

dnf diameter of nanoparticle, nm

Lc length of condenser section, mmLe length of evaporator section, mm

Lhp

geometric length of heat pipe, mm

Greek symbols

a angle of inclination against horizontal axis,

4 concentration, vol. %, wt. %, ppm

Abbreviations

Cd diamond

CNT carbon nanotubes

DI deionized

EA electromagnetic vibration

EG ethylene glycol

ESS electrostatic stabilization

FR lling ratio

gHP grooved heat pipe

HP heat pipe

MWCNT multiwalled carbon nanotubes

OHP oscillating heat pipe

oTS open two-phase thermosyphon

sHP heat pipe with sintered powder wick

ST surfactant

TS closed two-phase thermosyphon

US ultrasonic vibration

FP-OHP at plate oscillating heat pipe

Fig. 1. Normalized number of publications in different elds of uid mechanics and

heat transfer. Normalization is carried out with 2011-values. Data taken 2013-04-23

from ISI WEB of KNOWLEDGE.

1

The author is grateful to an anonymous reviewer for pointing this out to him.

M.H. Buschmann / International Journal of Thermal Sciences 72 (2013) 1e172

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parameters of gadgets, effects which are related to nanouidcharacteristics, the general thermal performance of the gadgets,

andmodelling approaches. Some of thendings cannotbe assigned

to one of these rubrics alone and appear therefore in multiple

contexts.

Based on the compiled experimental results and physical ar-

guments of technically closely related investigations, statements

with respect to the different physical mechanisms acting in gadgets

operated with nanouids are discussed.

Table 1 compiles all experimental studies discussed. The

different gadgets are denoted as done by the original authors. This

is also true for some of the so-called heat pipes which are strictly

thermosyphons. The serpentines of the oscillating heat pipes are

called turns. Their number is identical with the half of the number

of straight tubes. In cases where the original authors provide someinformation with respect to the wick this is mentioned in Table 1. In

some occasions gures of the original publications are quoted to

draw the readers attention to results which are verbally difcult to

describe.

3. Some statistics

With this study the results of 38 experimental and 4 modelling

approaches are compiled (Table 1). Among the experiments 11 turn

their attention to closed two-phase thermosyphons and one to

open thermosyphon, 18 to wicked, or grooved heat pipes, and 8 to

oscillating heat pipes. 51 nanouids have been tested all together

with these experiments. A majority, 41, are water-based nanouids.

Other basefluids are the refrigerant R11 (trichlorouoro-methane,CCl3F), ethyleneeglycol mixtures (EG), and acetone ((CH3)2CO).

Nanoparticles employed are metals, namely silver (Ag), gold (Au)

and copper (Cu), oxides (Al2O3, TiO2, SiO2, CuO, ZnO, Fe2O3) or

variations of carbon (diamond, carbon nanotubes). The number of

experiments carried out with silver nanoparticles, 13, exceeds by

far the other materials. Alumina (Al2O3) follows with 11, CuO with

5, pure copper and titanium (TiO2) with 4, silica (SiO2) and iron (II,

III)-oxide (Fe2O3) with 2, and zinc oxide (ZnO) with one experi-

ment. The size of the nanoparticles ranges from 2 nm to about

100 nm. Note that most publications provide only the size of the

primary nanoparticles. Due to agglomeration, the actual size within

the nanouid might be much larger. Fig. 3provides an overview of

the frequency of the nanoparticle size employed. A maximum ex-

ists between 20 nm and 40 nm.

Most authors provide only short comments with respect to thepreparation of their nanouids. Four papers give even no infor-

mation. The overwhelming majority (approx. 91%) employs two-

step methods where purchased or at least separately produced

nanoparticles are dispersed in the baseuid. Only three experi-

ments (Tsai et al. [10], Hajian et al. [27], Manimaran et al. [28])

were carried out with nanouids produced by one-step methods.

Here the nanoparticles were directly created within the baseuid

by chemical processes. Most two-step methods employ ultra-

sonication for dispersing the nanoparticles. The sonication time

varies between 1 h and 20 h ( Fig. 4). A maximum is found around

4 he6 h. About one third of the experimental groups state that they

have not employed any stabilization or surfactant.

Particle concentrations are either given in volume or in weight

percentage or in parts per million (ppm). Nanoparticle concen-trations ranging from 0.003 to 5.3 vol. %, from 0.1 to 0.5 wt. %, and

from 1 to 104 ppm are utilized in the experiments (Table 1). Due to

the fact that it is rather difcult to value how the baseuid density

is changed by adding stabilizers or surfactants (Feja and Busch-

mann[3]), no attempts have been made to convert these different

data.

While 16 experiments are conducted only in vertically orien-

tated gadgets, 9 studies investigate only horizontally positioned

apparatus. In seven experiments the inclination angle is varied.

Only one publication (Riehl and dos Santos[54]) reports inclination

angles between 90 (evaporator on top) and 90 (evaporator at

bottom) for an oscillating heat pipe.

4. Effects with respect to gadget parameters

4.1. Filling ratio

It is known from classical workinguids that the lling ratio has

a non-negligible inuence on the maximum heat throughput of

heat pipes (Reay and Kew [7]). The lling ratio of thermosyphons

and gravity supported heat pipes operated in thermosyphon mode

is dened as the ratio of working uid volume to internal evapo-

rator volume. Differently for oscillating heat pipes the lling ratio is

specied as ratio of working uid volume divided to total internal

volume. Experiments by Khandekar et al.[56]employing classical

working uids in an OHP showed that the maximum of the heat

throughput dependence on the lling ratio is rather at. For water

and ethanol the optimal region stretches from 10% to 80% and for

(c)

evaporator

condenser

liquid

slug

vapor

bubble

Q

selfsustained

thermally driven

bubble / slug

oscillation

(a)

working

fluidheating

down flowing

condensate

film

cooling

rising

vapor

Q

(b)

working

fluid

upward

flowing

condensate

vapor

heating

cooling

wick wick

Q

Fig. 2. Schematics of operation principles of thermosyphon (a), heat pipe (b) and oscillating heat pipe (c) adapted from Refs. [7] and[56].

M.H. Buschmann / International Journal of Thermal Sciences 72 (2013) 1e17 3

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R-123 from 35% to 80%. This knowledge motivated several nano-

uid experiments with varying lling ratios.

Lin etal. [47], investigating an OHP with 5 turns, found an optimal

lling ratio of 60% for DI-water and for Ag-nanouids (100 ppm and450 ppm). Similarly, Paramatthanuwat et al. [41,42] showed a

maximum of heattransferat FR50%for silvernanouidin a circular

thermosyphon.Butthe samewastruefor thereferenceuid DI-water.

Mousa[25] found that the thermal resistance of a vertical cir-

cular heat pipe was minimal for a lling ratio of 40e50% for both

DI-water and Al2O3-nanouid. The lling ratio was dened as the

percentage of the evaporator lled with workinguid. Thisnding

was conrmed for a vertical thermosyphon by Mousa [37]. The

optimal lling ratio here was 48%. Manimaran et al. [28](H2O/CuO

in wire meshed HP) showed an increase of thermal efciency when

the lling ratio was increased (similar as for water). The maximum

was reached for an inclination angle of 30 and FR 75%. Even with

an inclination angle of 0 maximal efciency was reached with

FR 75%. Investigating an OHP with H2O/Al2O3, Quet al. [50] foundthat a lling ratio of 70% gives the largest decrease of the thermal

resistance at a power input of 58.8 W.

An exception was the experiment by Teng et al. [19]. These au-

thors indicated that at an optimal tilt angle of 60 the thermal ef-

ciency was higher the lower the FR was. Maximal values were

achieved at FR 20% withH2O/Al2O3. Conversely, DI-water showed

a maximum at 60 when FR was 60%.

To summarize, an optimal lling ratio seems to exist between

45% and 70% depending on the design of the gadget. Clear trends

with respect to different nanouids cannot be derived. In the most

Fig. 3. Frequency of the nanoparticle size employed in the experiments compiled in

Table 1.

Fig. 4. Sonication time of the experiments compiled inTable 1.

Wannapakheetal.[

48]

B/OHP40turns

2

150

300

450

50

100

150

50

100

150

90

50

H2

O/Ag