i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 6 4 0e6 4 7

www. i ifi i r .org

ava i lab le at www.sc iencedi rec t .com

journa l homepage : www.e lsev ier . com/ loca te / i j re f r ig

Experimental study on enhancement of ammoniaewaterfalling film absorption by adding nano-particles

Liu Yang a, Kai Du a,*, Xiao Feng Niu a,b, Bo Cheng a, Yun Feng Jiang a

aSchool of Energy and Environment, Southeast University, 2# SiPaiLou, Nanjing, Jiangsu 210096, ChinabDepartment of Building Services Engineering, The Hong Kong Polytechnic University, Kowloon, Hong Kong

a r t i c l e i n f o

Article history:

Received 16 October 2010

Received in revised form

21 November 2010

Accepted 22 December 2010

Available online 30 December 2010

Keywords:

Ammoniaewater

Falling film

Absorption

Mass transfer

Heat transfer

* Corresponding author. Tel.: þ86 25 8379321E-mail address: du-kai@seu.edu.cn (K. Du

0140-7007/$ e see front matter ª 2010 Elsevdoi:10.1016/j.ijrefrig.2010.12.017

a b s t r a c t

Based on the preparation of Al2O3, Fe2O3 and ZnFe2O4 nanofluid, the comparative experi-

ments on the falling film absorption between ammoniaewater and ammoniaewater with

various kinds of nano-particles are carried out. Experimental results show that the sorts

and mass fraction of nano-particles, the viscosity and stability of nanofluid, as well as the

mass fraction of ammonia in the basefluid are considered as the key parameters. The

absorption of ammonia is weakened by only adding surfactants or adding poorly dispersed

nano-particles. The increase of mass fraction of nano-particles with matched surfactants

can improve the absorption rate of ammonia under the condition that the viscosity of

nanofluid does not increase remarkably, and there is an optimal mass fraction for each

kind of nano-particles and surfactant. With the increase in ammonia mass fraction of

initial nanofluid, the absorption potential capacity decline, but the enhancing effect

induced by the nanofluid is more obvious compared to that without nano-particles. The

effective absorption ratio can be increased by 70% and 50% with Fe2O3 and ZnFe2O4

nanofluid respectively when the initial ammonia mass fraction is 15%. The absorption

enhancement by the nanofluid is attributable to the heat transfer enhancement and the

decrease in viscosity of nanofluid, which are strongly proved by the temperature differ-

ences in cooling water and nanofluids as well as the falling film flowing time.

ª 2010 Elsevier Ltd and IIR. All rights reserved.

Etude experimentale sur l’amelioration de l’absorption d’unfilm tombant utilisant une solution d’ammoniac/eau a l’aidede l’ajout de nanoparticules

Motscles : Ammoniac-eau ; Film tombant ; Absorption ; Transfert de masse ; Transfert de chaleur

4.).ier Ltd and IIR. All rights reserved.

Nomenclature

i absorption rate, defined in Eq. (1), g s�1

mfin solution mass after absorption, g

mini solution mass before absorption, g

t absorption time, s

ieff effective absorption ratio, defined in Eq. (2)

ina absorption rate of nanofluid, g s�1

iam absorption rate of ammoniaewater solution, g s�1

uS mass fraction of surfactant, %

m viscosity, mPa s

DTna temperature difference between inlet and outlet

of nanofluid, �C

DTw temperature difference between inlet and outlet

of cooling water, �Cq heat transfer rate of cooling water, kJ s�1

Subscripts

fin finish

ini initial

eff effective

na nanofluid

am ammoniaewater solution

s surfactant

w cooling water

i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 6 4 0e6 4 7 641

1. Introduction

Ammonia/water absorption refrigerators have been widely

used from the middle of the 20th century and they have been

drawing renewed attentionwith the growing awareness of the

dual threats of global warming and ozone depletion. However,

the performance of the absorption system is worse than that

of the compression system, and it should be improved.

Furthermore, the absorber is so critical in the absorption

systems because its size and performance can influence the

system overall performance significantly. Therefore, the

research on the absorption enhancement has been performed

actively. Generally, there are three methods to enhance the

efficiency of heat and mass transfer: the mechanical treat-

ment, the chemical treatment, and nanotechnology (Kang

et al., 2003).

Nanofluid is defined as a fluid in which the nano-particles

below 100 nm in diameter are suspended in the basefluid. In

recent years, the enhancement of nano-particles on the

ammoniaewater bubble absorption has been widely studied.

Kim et al. (2005) defined binary nanofluid as the binary

mixture in which nano-particles were evenly distributed and

the effect of binary nanofluid on the ammoniaewater bubble

absorption performance was studied. It was found that,

compared with ammoniaewater, the absorption rate of

ammoniaewater nanofluid adding nano-particles and the

nanofluid adding both nano-particles and surfactants was

3.21 times higher and 5.32 times higher respectively.

Researchers (Kang et al., 2007) from South Korea found that

the absorption rate and heat transfer rate of ammoniaewater

nanofluid with 0.001% CNT particles were 20% and 29.4%

higher than that of the ammoniaewater without nano-parti-

cles, and the ammoniaewater nanofluid with 0.001% of CNT

particles was the optimal candidate for ammoniaewater

absorption enhancement. Al2O3 nano-particles were used to

enhance the ammonia bubble absorption by Sheng and Wu,

(2008). The stability of the nanofluid and the pressure differ-

ence between the inlet of the absorber and the gas phase

surface in the absorber were considered the two main factors

which possibly induce the enhancing absorption effect. Liu

et al. (2009) used FeO nanofluid to enhance the ammonia

absorption. The results showed that, at a constant flow rate of

ammonia gas, the enhanced absorption effect was not

observed until several minutes after the beginning of the

absorption; under the condition of constant inlet pressure, the

absorption enhancement was observed immediately at the

very beginning of the absorption process. Wu et al. (2010)

studied the effect of mono Ag nano-particles on the heat

transfer and mass transfer characteristics in NH3/H2O bubble

absorption process, it was found that the effective absorption

ratio can reach the maximum of 1.55 when the initial

ammonia concentration is 20% and the mono nano Ag

concentration is 0.02%.

Although many studies focused on bubble absorption with

nanofluids has been performed, few literatures on falling film

absorption of ammoniaewater with nano-particles were

found. According to the research results of other scholars, the

mass transfer coefficients has a more significant effect in

the bubble mode than that in the falling film mode, while the

heat transfer coefficients has a more significant effect on heat

exchanger size (absorption rate) in the falling film mode than

that in the bubblemode (Kang et al., 2000). Hence, it hasmajor

significance to carry out the experimental study on ammo-

niaewater falling film absorption with nanofluid and then

obtain the influence factors of ammoniaewater nanofluid

falling film absorption. In this paper, the comparative experi-

ments on the falling filmabsorption between ammoniaewater

and ammoniaewater with various kinds of nano-particles are

carried out. The influence factors on the efficiency of ammo-

niaewater absorption are studied in details.

2. Preparation of nanofluids

Three different types of nanofluids were obtained by mixing

sodiumdodecyl benzene sulfonate (SDBS)with ZnFe2O4, Fe2O3

and Al2O3 in the ammoniaewater basefluid, respectively. The

mass fraction of the homemade ammoniaewater basefluid

is 0%, 5%, 10%, and 15% respectively. Fig. 1 (a), (b) and (c) shows

the SEM images of Al2O3, ZnFe2O4 and Fe2O3 nano-particles

respectively. The nano-particles are spherical or analogously

spherical and the purity is higher than 99.8% through the

detection by ultraviolet emission spectrometer. Themean size

of Al2O3, ZnFe2O4 and Fe2O3 nano-particles is less than 20 nm,

30 nm and 30 nm respectively.

Based on the previous studies of the authors (Yang et al.,

2010a,b,in press), the optimal mass fraction of SDBS for 0.1%

mass fraction of Al2O3, Fe2O3 and ZnFe2O4 nanofluid is 0.1%,

Fig. 1 e SEM images of Al2O3, ZnFe2O4 and Fe2O3 nano-particles.

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 6 4 0e6 4 7642

0.8% and 1.5% separately. Two hours of mechanical agitation

and 30 min of ultrasonic vibration are exerted on the mixing

solution sequentially to get the stable nano-particle suspen-

sion of ammoniaewater solution. And after 1 h of static

storage, when the bubbles generated by the agitation of

surfactants disappears, the viscosity of the each kind of

nanofluid was measured through the use of NDJ-1E digital

viscometer (accuracy: 0.01 mpa s) in thermostated container

with temperature of 26.5 �C at atmospheric pressure.

Fig. 2 e Schematic diagram of the experimental system for

NH3/H2O nanofluid falling film absorption.

3. Experimental system and procedures

3.1. Experimental system

Fig. 2 shows the schematic diagram of the experimental

system for NH3/H2O falling film absorption which is mainly

composed of NH3 vessel, container of solution (13 L), falling

film absorber, constant flow controller and the sub-system of

cooling water. The materials of the end cover and body of

absorber are stainless steel and plexiglass separately, thus the

process of falling film flowing is visible. There are six thermal

resistances (precision: 0.01 �C) withmaterials of Pt100 used for

the temperaturemeasurement in the system. Two of them are

set equally spaced inside the inlet and outlet of the pipeline of

cooling water. The other two are set at the inlet and outlet of

the falling film solution. The last two are hung inside the

absorber to measure the temperature of ammonia gas.

Besides, there is a pressure measurement points in the

experimental system with measure range of 0e500 kPa and

precision of 0.1% FS. All the signals of thermal resistances and

pressure transmitter are sent into a computer via real-time

data acquisition card. The signal acquisition and monitor of

temperature and the pressure are auto-completed by

computer. The falling film absorber is a cylinder with height of

1200 mm and inner diameter of 300 mm, consists of a heat

transfer tube, end cover and liquid distributor. The stainless

steel heat transfer tube has an outer diameter of 25 mm, and

the falling film height is 1000 mm.

The wettability of falling film on the tube is an important

variable. In order to ensure a good wettability, several

measures had been taken in the experiment. A liquid

distributor was designed lies at the top of the absorber, and

the structure of it is shown in Fig. 3. The liquid distributor is

a kind of reservoir tray, in which the initial liquid forms

a certain height of liquid level for the purpose of maintaining

the stable falling film. The liquid distribution oillets are

a series of oval nicks in the button of the reservoir tray around

the falling film tube, the solution will be distributed uniformly

falling film tubereservoir trayadjusting screw distribution oillets

Fig. 3 e Structure of the liquid distributor.

i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 6 4 0e6 4 7 643

at the tube surface through the nicks, and then falls in thin

film. In order to make the falling film tube located in the very

centre of the hole of liquid distributor, 3 adjusting screws are

installed, which can be adjusted out of the absorber. A great

number of experiments proved that liquid distributor with

such structure can ensure the uniform of liquid distribution

outside the falling film tube. Before the falling film tube was

installed into the absorber, the outside surface of it was

cleaned by ethanol to remove the oil stain. Moreover, sand

paper was used to grind the surface of tube and make it has

certain roughness, which is also helpful to achieve good

wettability. The conditions of solution film distribution can be

observed in real-time through the transparent shell of the

absorber all through the experiment. In each test, we verified

that the solution was distributed uniformly at the tube

surface, or the test was failed and we would redo it.

The falling film tube is a counter-flow heat exchanger, as

the direction of cooling water flow is opposite to that of the

falling film, that is, the cooling water enters from the bottom

and then flows upward inside the tube to remove the

absorption heat.

3.2. Procedures

The experiments to study the influence factors of the

absorption with nanofluid including the following:

1) The comparative experiments between ammoniaewater

and ammoniaewater with different mass fractions of

surfactant.

2) The comparative experiments between ammoniaewater

and ammoniaewater with different mass fractions of

nano-particles matched with optimal mass fractions of

surfactant.

3) The comparative experiments between well stabilized

nanofluid and the nanofluid without mechanical agitation

and ultrasonic vibration.

4) The comparative experiments between different mass

fractions of ammonia in the initial basefluid.

By measuring the total mass of the detachable solution

containers before and after the absorption and the corre-

sponding absorption time, the absorption rate can be calcu-

lated as Eq. (1).

i ¼ �mfin �mini

��t (1)

The effective absorption ratio is defined to examine the

effect of the addition of nano-particles on the absorption rate.

It is defined as Eq. (2).

ieff ¼ ina=iam (2)

The test procedures are as follows:

1) Obtain the initial weight of empty container 4 and initial

weight of container 11 filled up with initial fluid (13 L) by

using electronic balance (accuracy: 1 g).

2) After the pipeline has been connected and tested, electrify

the system and remove the air of system by vacuum pump.

The method is: vacuumize the absorber until the pressure

decreases to 10 kPa, then stop vacuumization and feed NH3

to the absorber until the pressure back to 100 kPa, the purity

of NH3 in absorber can achieve 90% at this moment. Then

repeat the vacuumization and feed processes three times,

the purity of NH3 in absorber can reach to 99.9%

theoretically.

3) Keep the pressure in absorber to be 90 kPa and regulate the

cooing water flow rate to be 250 L per hour and be stable.

Then open the valve of solution container and let solution

start to absorb the NH3, keep the absorption pressure stable

by the constant pressure controller.

4) After the solution of container 11 is flow out completely,

switch off the power of system and remove the two

containers to weigh them respectively. After cooling,

charge the container 11 with other initial fluid with the

same volume and restart experiment from step 1.

4. Results and discussion

4.1. The influence of mass fraction of surfactant onammoniaewater falling film absorption

Fig. 4 shows the variation of absorption rate when the mass

fraction of surfactant varies, it can be seen that the absorption

rate sharply decreases when the mass fraction of surfactant

exceed 0.5%. And the absorption rate decreases by 30% when

the mass fraction of surfactant reaches 1.5%. This phenom-

enon can be explained by the variation of viscosity with the

increase in mass fraction of surfactant as shown in Fig. 5, the

viscosity sharply increases when the mass fraction of

surfactant exceed 0.5%. The increase in the viscosity will

1.2

0.0 0.5 1.0 1.5

0.4

0.5

0.6

0.7

0.8

Abs

orpt

ion

rate

/ gs

-1

Mass fraction of surfactant (%)

Fig. 4 e Variation of absorption rate with the increase in

mass fraction of surfactant.

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 6 4 0e6 4 7644

increase the flow time of solution. Moreover, the resistance for

the ammonia molecular passing through the falling film from

the surface also increases. Therefore, the ammonia absorp-

tion process is weakened by only adding SDBS.

4.2. The influence of mass fraction of nano-particleswith matched surfactants

In our previous studies (Yang et al., 2010a,b,in press), we used

light absorbency ratio index method to investigate the

dispersion stability of nanofluids with different mass fraction

of surfactants to find the optimal mass fraction of surfactants

for each kind of nanofluid. The absorbency of each kind of

nanofluid was measured by ultraviolet-visible spectropho-

tometer after a certain period of static storage, and the

absorbency is proportional to the current mass fraction of the

suspended nano-particles in suspension. Higher absorbency

means higher mass fraction of nano-particles in the solution,

namely, the better dispersion of nanofluid. The results

showed that, the optimal mass fraction of SDBS for 0.1%mass

0.0 0.5 1.0 1.5

0.8

0.9

1.0

1.1

Dyn

amic

vis

cosi

ty /

mpa

s

Mass fraction of SDBS (%)

Fig. 5 e Variation of viscosity with the increase in mass

fraction of surfactant.

fraction of Al2O3, Fe2O3 and ZnFe2O4 nanofluid is 0.1%, 0.8%

and 1.5% respectively, and the optimal mass fraction of

surfactant increases approximately linearly with the increase

of themass fraction of nano-particles. Fig. 6 demonstrates the

effect on the variation of absorption rate by the mass fraction

of three kinds of nano-particles, each kind of them are mixed

with their optimal mass fractions of surfactant. The absorp-

tion rate increases with the increase of mass fraction of nano-

particles firstly, and then decreases later. Particularly, this

trend is more obvious for the nanofluid of Fe2O3 and ZnFe2O4.

It can be concluded that there is an optimummass fraction in

each kind of nano-particles for the absorption enhancement

with ammonia falling film. This finding is different from the

experimental results of Kim et al. (2004), in which the effective

absorption rate increases linearly with the mass fraction of

nano-particles increasing. The main reason is also the varia-

tion of viscosity shown in Fig. 7. The viscosity increase sharply

when the mass fractions of nano-particles exceed certain

values for Fe2O3 and ZnFe2O4 nanofluid, because that their

optimal mass fractions of surfactant increase sharply, which

induce the decrease of absorption rate. It can be concluded

that the increase of mass fraction of nano-particles can

enhance the absorption of ammonia under the condition that

the viscosity of nanofluid does not increase greatly.

The absorption rate decreases when only adding surfactant,

but increases when adding proper nano-particles with surfac-

tants together. The mechanisms of the absorption enhance-

ment by nanofluid have the following possible factors.

The mass fraction of “free” surfactant molecular will

decline because of the adsorptions of nano-particles, which

induces the decline on the viscosity of nanofluid. Conse-

quently, the lower viscosity of nanofluid is beneficial to

decrease the resistance for the ammonia molecular passing

through the falling film. In addition, the flow time of falling

film will shorten as the result of the decrease in viscosity,

which causes the increase in the flow rate in unit time.

Although the viscosities of nanofluid with 0.3% Fe2O3 and

nanofluid with 0.2% ZnFe2O4 are higher than the fluid with

0.0 0.1 0.2 0.3

0.6

0.7

0.8

0.9

1.0

1.1

Abs

orpt

ion

rate

/ gs

-1

Mass fraction of nano-particles(%)

Fe2O

3

ZnFe2O

4

Al2O

3

Fig. 6 e Variation of absorption rate with the increase in

mass fraction of nano-particles mixed with optimal

surfactants (Fe2O3: uS [ 0.8%, 1.5%, 2.2%; ZnFe2O4:

uS [ 1.5%, 3%, 4.5%; Al2O3: uS [ 0.1%, 0.2%, 0.3%).

0 5 10 15

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2 Fe2O3 ZnFe2O4 Al2O3 No nano-particle

Abs

orpt

ion

rati

o /g

s-1

Mass fraction of ammonia in initial nanofluid (%)

Fig. 9 e Variation of absorption rate of optimal nanofluid

when mass fraction of ammonia in initial solution varies.

0.0 0.1 0.2 0.3

0.75

0.80

0.85

0.90

0.95

1.00

1.05

1.10

Dyn

amic

vis

cosi

tym

pas

Mass fraction of nano-particles(%)

B ZnFe2O4

B Fe2O3

B Al2O3

Fig. 7 e Variation of viscosity with the increase in mass

fraction of nano-particles mixed with optimal surfactants

(Fe2O3: uS [ 0.8%, 1.5%, 2.2%; ZnFe2O4: uS [ 1.5%, 3%, 4.5%;

Al2O3: uS [ 0.1%, 0.2%, 0.3%).

i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 6 4 0e6 4 7 645

1.5% SDBS, the absorption rate of the former two are higher

than the last. This is because the viscosity is not the single

influence factor for the falling film absorption process.

Another reason for the absorption enhancement by nanofluid

may be that the nano-particles can arouse the micro-

convection (Krishnamurthy et al., 2006), the grazing effect

(Alper et al., 1980), and then enhance the thermal conductivity

(Wang andWei, 2009) and the mass transfer in the absorption

process. Absorption process is a combined heat and mass

transfer process. The improvement of heat transfer can

decrease the temperature at the gaseliquid interface,

heighten the absorption potential of aqueous ammonia and

enhance the absorption rate of the ammonia vapor.

4.3. The influence of stability of nanofluid

As shown in Fig. 6, the optimal components of each kind of

nanofluid in absorption are 0.2% Al2O3 with 0.2% SDBS, 0.1%

0 30 600.5

0.6

0.7

0.8

0.9

1.0

1.1

1.2

Fe2O3

ZnFe2O4

Al2O3

Abs

orpt

ion

rati

o /g

s-1

Ultrasonic vibration time (min)

Fig. 8 e Variation of absorption rate when the ultrasonic

time of the optimal nanofluid varies.

ZnFe2O4 with 1.5% SDBS and 0.2% Fe2O3 with 1.5% SDBS

respectively. The following experiments are performed based

on these optimal fractions of nanofluids.

The results of the comparative experiments between well

stabilized nanofluid and the nanofluid without mechanical

agitation and ultrasonic vibration are shown in Fig. 8. It can be

found that the performance of absorption is the best when the

nanofluid was ultrasonic vibrated for 30 min. The nanofluid

without any mechanical agitation and ultrasonic vibration

has restraining effect on the ammonia absorption. However,

the absorption enhancement by the nanofluid with ultrasonic

vibration does not strengthen ormaintainwith the increase in

vibration time, it can be seen that the absorption rate for

ZnFe2O4 and Al2O3 nanofluid decreases when the vibration

time is longer than 30 min.

The reason of the absorption rate decreases when the

vibration time is longer than 30 min can be attributed to the

influence of supersonic vibration on the stability of nanofluid.

Thenanofluidwas impactedby theeffectof strongercavitation

0 5 10 151.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2.0Fe2O3

ZnFe2O4Al2O3

Eff

icti

ve a

bsor

ptio

n ra

tio

Mass fraction of ammonia in initial fluid (%)

Fig. 10 e Variation of effective absorption ratio of optimal

nanofluid when mass fraction of ammonia in initial

solution varies.

Table 1 e Related parameters of ammoniaewater falling film absorption with different kinds of nanofluid.

Parameter Fluid

No additives 1.5% SDBS 0.2% Al2O3 and 0.2% SDBS 0.1% ZnFe2O3 and 1.5% SDBS 0.2% Fe2O3 and 1.5% SDBS

m(mpa s) 0.85 0.98 0.84 0.81 0.79

DTna (�C) 8.03 7.51 9.58 13.03 14.7

DTw (�C) 2.08 1.69 2.21 2.51 2.83

t (s) 980 1070 965 925 905

q (kJ s�1) 0.607 0.493 0.645 0.732 0.825

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 6 4 0e6 4 7646

of supersonic vibration and the reunion of nano-particles will

be dispersed, so the nanofluid will more stable after proper

supersonic vibration. But if the time of supersonic vibration

exceeds the optimal supersonic time, with the increase of

solution temperature, the nano-particles were accelerated by

the resonance vibration induced by ultrasonic wave, which

induces the collision of nano-particles. In previous studies

(Yang et al., 2010a,b,in press), it was found that for Al2O3 and

ZnFe2O4 nanofluid, 30 min is the optimal supersonic vibration

time used light absorbency ratio index method.

The reasons that only the well stabilized nanofluid can

enhance absorptionmay lies in the following two factors. First,

some superior properties of the nanofluid, such as the micro-

convection and high heat and mass transfer coefficient, can

not be fully functioned in the nanofluid of poorly stabilized.

Second, the surfactant molecular can not be adsorbed by the

nano-particles without mechanical agitation and ultrasonic

vibration, thus the remaining “free” surfactant molecular

results in the increase of the viscosity of nanofluid, which

eventually leads to the weakening in the solution absorption

capacity.

4.4. The influence of mass fraction of ammonia in initialnanofluid

Fig. 9 shows the variation of absorption rate of optimal

nanofluid when the ammonia mass fraction in initial solution

varies. With the increase of mass fraction of ammonia in

initial solution, the absorption potential capacity declined,

and the absorption rates of all kinds of nanofluid decrease.

However, it can be concluded from the variation of effective

absorption ratio of optimal nanofluid under the same varied

condition (Fig. 10) that, with the increase of mass fraction of

ammonia in initial solution, the enhancing effect induced by

the nanofluid ismore obvious compared to that without nano-

particles, namely, the effective absorption ratio increases. It

can be also found that the effective absorption ratio, for

ZnFe2O4 and Fe2O3 nanofluid, increases by 50% and 70%

respectively when the initial ammonia mass fraction is 15%.

An approximate analysis for this phenomenon might be as

follows, the pH value of solutions increases with the increase

in mass fraction of ammonia, as the pH values of the three

kinds of nano-particles corresponding to the iso-electric point

are below 7 (Hou et al., 1998; Garcell and Morales, 1998; Pan

and Somasundaran, 2004), the increase in pH value means it

is farther away from the iso-electric point and the nano-

particles in higher zata potential will be dispersedmore stable

and uniformly, hence the superiority of the nanofluid will be

more effective.

4.5. The heat transfer enhancement of nanofluid

Table. 1 shows the values of viscosity, the temperature

difference in cooling water and nanofluid, the flowing time

of each kind of nanofluid, as well as the heat transfer rate of

cooling water. It can be found that the heat transfer rate of

cooling water is in accordance with the mass transfer of

ammonia for each kind of nanofluid. Fe2O3 nanofluid has

the best absorption performance, which is reflected by the

largest temperature difference in cooling water and nano-

fluid as well as the shortest flowing time among the several

kinds of nanofluid. In addition, the fluid only added SDBS

has the worst absorption performance, because that the

temperature difference in cooling water and nanofluid of it

is the smallest, moreover, its flowing time is the longest.

The results well prove that the enhancement in absorption

attributes to the heat transfer strengthening and the flowing

time shortening, which originates from the decrease in the

viscosity.

When proper nano-particles and surfactants are added in

the ammoniaewater solution, a virtuous cycle will be gener-

ated in the ammonia absorption process. The absorption can

be enhanced as the result of the heat transfer strengthening

and the decrease in viscosity, and the nanofluid temperature

rise in the absorption process will decrease the viscosity

further, and vice versa. When SDBS is added only or poorly

stabilized nano-particles are added, a vicious cycle will be

generated.

5. Conclusions

1) The mass fraction and sorts of nano-particles, the

viscosity and stability of nanofluid, as well as the mass

fraction of ammonia in the basefluid are considered as the

key factors for ammoniaewater falling film absorption

with nanofluid.

2) The absorption effect of ammonia is weakened by only

adding surfactants or adding poorly dispersed nano-parti-

cles. The increase of mass fraction of nano-particles with

matched surfactants can improve the absorption perfor-

mance under the condition that the viscosity of nanofluid

does not increase greatly, and there is an optimal mass

fraction for each kind of nano-particles and surfactant.

3) With the increase of mass fraction of ammonia solution,

the absorption capacity declines, but the enhancing effect

induced by the nanofluid is more obvious compared to that

without nano-particles. The effective absorption ratio can

be increased by 70% and 50% with Fe2O3 and ZnFe2O4

i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 3 4 ( 2 0 1 1 ) 6 4 0e6 4 7 647

nanofluid respectively when the initial ammonia mass

fraction is 15%.

4) The temperature differences in cooling water and nano-

fluid as well as the falling film flowing time well prove that

the absorption enhancement is attributable to the heat

transfer strengthening and the decrease in the nanofluid

viscosity.

Acknowledgments

This research is supported by the National Natural Science

Foundation of China under the contract No. 50876020. The

support is gratefully acknowledged.

r e f e r e n c e s

Alper, E., Wichtendahl, B., Deckwer, W.D., 1980. Gas absorptionmechanism in catalytic slurry reactors. Chem. Eng. Sci. 35,217e222.

Garcell, L., Morales, M.P., 1998. Interfacial and rheologicalcharacteristics of maghemite aqueous suspensions. J. ColloidInterface Sci. 502, 470e475.

Hou, Y.Y., Li, L., Yang, J.Y., 1998. Preparation of highly dispersedand stabilized a-Al2O3 and nano-SiC single-phase and mixedaqueous suspension. J. Chin. Ceram. Soc. 26 (2), 171e177.

Kang, Y.T., Akisawa, A., Kashiwagi, T., 2000. Analyticalinvestigation of two different absorption modes: falling filmand bubble types. Int. J. Refrigeration 23, 430e443.

Kang, Y.T., Kim, J., Kim, J.-K., 2003. Comparisons of mechanical,chemical and nano technologies for absorption applications.

In: Proceedings of International Seminar on ThermallyPowered Sorption Technology, pp. 69e77. Fukuoka, Japan.

Kang, Y.T., Lee, J.-K., Kim, B.-C., 2007. Absorption heat transferenhancement in binarynanofluids. Int. Congr. RefrigerationBeijing. ICR07-B2-371.

Kim, J., Kang, Y.T., Choi, C.K., 2004. Analysis of convectiveinstability and heat transfer characteristics of nanofluids.Phys. Fluids 16, 2395e2401.

Kim, J.K., Jung, J.Y., Kim, J.H., 2005. The effect of chemicalsurfactants on the absorption performance during NH3/H2Obubble absorption process. Int. J. Refrigeration 29, 170e177.

Krishnamurthy, S., Bhattacharya, P., Phelan, P.E., 2006. Enhancedmass transport in nanofluids. Nano Letter 6 (3), 419e423.

Liu, H., Wu, W.D., Sheng, W., 2009. Experimental study onenhancing ammonia bubble absorption by FeO nanofluid.Chem. Ind. Eng. Progr. (China) 28 (7), 1138e1141.

Pan, Z., Somasundaran, P., 2004. Interactions of cationicdendrimers with hematite mineral. Colloids Surf. APhysicochem. Eng. Aspects 238, 123e126.

Sheng, W., Wu, W.D., Zhang, H., 2008. Enhancing influence ofAl2O3 nano-particles on ammina bubble absorbtion process. J.Chem. Ind. Eng. (China) 59 (11), 2762e2767.

Wang, L.Q., Wei, X.H., 2009. Heat conduction in nanofluids.Chaos, Solitons & Fractals 39 (5), 2211e2215.

Wu, W.D., Pang, C.W., Sheng, W., 2010. Enhancement on NH3/H2Obubble absorption in binary nanofluids by mono nano Ag [J].J. Chem. Ind. Eng. (China) 65 (1), 1112e1117.

Yang, L., Du, K., Cheng, B., 2010a. The influence of Al2O3 nanofluidon the falling film absorption with ammoniaewater. In: Asia-Pacific Power and Energy Engineering Conference Chengdu,China, 5449136.

Yang, L., Du, K., Zhang, X.S., 2010b. Study on preparation andstability of zinc ferrite nano-particle suspension of ammoniasolution. J. Southeast Univ. (English edition) 26 (2), 368e371.

Yang, L., Du, K., Zhang, X.S. Dispersing of Fe2O3 nano-particles inammoniaewater suspension. China Society of EngineeringThermophysics, Nan Jing, in press.