Investigations of selection of nanofluid applied to the ammonia absorption refrigeration system

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 5 ( 2 0 1 2 ) 2 2 4 8 e2 2 6 0

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Investigations of selection of nanofluid applied to the ammonia absorption refrigeration system Liu Yang*, Kai Du, Shuaiyang Bao, Yunlong Wu School of Energy and Environment, Southeast University, 2# SiPaiLou, Nanjing, Jiangsu 210096, China

article info

abstract

Article history:

20 types of nanoparticles mixed pairwise orthogonally with 10 types of dispersants are

Received 16 March 2012

added in ammonia-water, respectively, to observe the dispersion stability of suspension.

Received in revised form

Ratio of varying absorbency is defined to investigate the dispersion stability of nanofluids

28 June 2012

compared with the conventional methods of measuring the height of supernatant layer or

Accepted 3 August 2012

just measuring the absorbency. The dispersion stabilities of nanoparticles employed are

Available online 10 August 2012

divided into three levels and the influence of boiling on the dispersion is investigated. The results show that the nanoparticles with better dispersive characteristics in dry condition

Keywords:

can more easily disperse in ammonia-water directly or be strengthened dispersivity by

Absorption system

dispersant. Ratio of varying absorbency is more practical than the height of supernatant

Particle

layer to investigate the dispersion stabilities of some nanofluids which just fade or aren’t

Stability

stratified clearly after storage, and is more efficient to contrast the dispersion stabilities of

Refrigerant

different types of nanofluid than absorbency. ª 2012 Elsevier Ltd and IIR. All rights reserved.

Ammoniaewater

Etudes sur la se´lection des nanofluides utilise´s dans les syste`mes frigorifiques a` ammoniac a` absorption Mots cle´s : Syste`me a` absorption ; Particule ; Stabilite´ ; Frigorige`ne ; Ammoniac-eau

1.

Introduction

Ammonia/water absorption refrigerator has superiority in utilizing the waste-heat and low-grade heat source and it has been drawing renewed attention with the growing awareness of the dual threats of global warming and ozone depletion. However, the performance of the absorption system is lower than that of the compression system and it should be enhanced to be an effective alternative. Many researches therefore have been performed to improve its performance.

Generally, there are three methods to enhance the efficiency of heat and mass transfer: the mechanical treatment, the chemical treatment, and nanotechnology (Kim et al., 2006). In recent years, many experiments have been performed on the application of nanofluid in ammonia-water absorption. Cu, CuO and Al2O3 nanoparticles were added into NH3/H2O solution to make the binary nanofluids, and 2ethyl-1-hexanol, noctanol and 2-octanol are used as the surfactants were studied by Kim et al. (2007). It was found that the addition of surfactants and nanoparticles improves the

* Corresponding author. Tel.: þ86 25 83793214. E-mail address: [email protected] (L. Yang). 0140-7007/$ e see front matter ª 2012 Elsevier Ltd and IIR. All rights reserved. http://dx.doi.org/10.1016/j.ijrefrig.2012.08.003

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 5 ( 2 0 1 2 ) 2 2 4 8 e2 2 6 0

Nomenclature A Aini Aaf Ii I ε

absorbency absorbency of the initial nanofluid absorbency of the nanofluid after static storage of 24 h incident light intensity transmitted light intensity molar absorptivity (l mol1 cm1)

absorption performance up to 5.32 times and the addition of both surfactants and nanoparticles enhances significantly the absorption performance during the ammonia bubble absorption process. Besides nanoparticles, nanoemulsion was also applied to ammonia-water absorption process. Kim et al. (2011) studied the absorption characteristics of NH3 bubbles in binary nanoemulsions and to quantify the effect of oildroplet on the bubble absorption performance. C12E4 and Tween20 were used as the surfactants and n-decane oil was added into NH3/H2O solution to make the binary nanoemulsion. The results showed that the effective absorption ratio for 2.0 vol% oil and 14.3 wt% NH3/H2O becomes 17% higher than that for the base fluid. Sheng et al. (2008) found that ammonia bubble absorption effect was enhanced by adding Al2O3 nanoparticles. Stability of nanofluid and the pressure difference are the two main factors which possibly induce the enhancing absorption effect. Liu et al. (2009) found that that the absorption effect of ammonia was enhanced by the prepared FeO nanofluid. At a constant flow rate of ammonia gas, the enhanced absorption effect was not observed until several minutes later from the beginning of the absorption process. Under the condition of constant inlet pressure, the absorption effect was observed immediately at the very beginning of the absorption process. Wu et al. (2010) studied the effect of mono nano-Ag 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 1.55 when the initial ammonia concentration is 20% and the mono nano-Ag concentration is 0.02%. Yang et al. (2011a) carried out the comparative experiments on the falling film absorption between ammonia-water and ammonia-water with various kinds of nanoparticles. It was found that 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%. However, one of the biggest challenges in the application process of nanoparticles is the dispersion of the nanoparticles into liquids, because only uniformly dispersed nanoparticles can obtain stable reproduction of physical properties or superior characteristics of the nanofluids (Yang et al., 2011b). Current researches on the types of nanofluids used in ammonia absorption refrigeration system are deficient in quantity, and generally focus on the ammonia absorption performance of nanofluid (Kim et al., 2006; Liu et al., 2009; Wu et al., 2010; Yang et al., 2011a). Seldom systematic researches on the types of nanofluid appropriate for the ammonia absorption refrigeration system are found. To investigate the types of nanoparticles which are appropriate for using in ammonia absorption refrigeration system and the qualification for further

b c K

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optical path (cm) molar concentration (mol l1) ratio of varying absorbency

Subscripts i incident ini initial af after static storage

application, in this paper, preliminary investigations on the selection process of nanoparticles are carried out. 20 types of nanoparticles mixed pairwise orthogonally with 10 types of dispersant are added in ammonia-water, respectively, to observe the dispersion stability of suspension. The types of dispersant for improving the dispersion of each type of nanoparticle in ammonia-water are investigated. A new parameter called ratio of varying absorbency are defined to investigate the dispersion stability of variation of nanofluids compared with the conventional method of measuring the height of supernatant layer or just measuring the absorbency. Some welldispersed nanofluids are selected for investigating the dispersion stabilities after treated in boiling temperature as in generator. It is expected that this study can give some basic ideas for further engineering application of nanofluid in ammonia absorption refrigeration system.

2.

Experiment

2.1.

Selection of basefluids

In the past studies of some scholars, the nanofluids applied to ammonia absorption refrigeration system were prepared by adding nanoparticles in water (Kim et al., 2007; Liu et al., 2009; Wu et al., 2010), and then the performance of absorbing ammonia were investigated. However, there is a question that may be overlooked. Can the nanofluid prepared in water still keep stable in ammonia-water for a long time? To assure the nanoparticles be stable in ammonia-water, the basefluid for preparing the nanofluid should not be water but ammoniawater. Because there may be some kinds of nanoparticles that can keep stable in water but can not keep stable in ammonia-water, and circumstances such as chemical reaction, flocculation, sedimentation, et al. may occur in ammonia-water for nanoparticles. In addition, the practical fluid in ammonia absorption refrigeration system is not water but ammonia-water at different concentrations. Therefore, for the preparation of nanofluids applied to ammonia absorption refrigeration system, using ammonia-water as basefluid is more factual than using water as basefluid. And in this study, 25% homemade ammonia-water is used as basefluid for the preparation of nanofluids.

2.2. Selection of preparation method of ammonia-water nanofluid The existing preparation methods of nanofluids are divided into one-step and two-step methods. One-step method is to

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make the nanoparticles simultaneously disperse in the basefluid during the particle synthesis procedure, while two-step method is to add the prepared nanoparticles in their required basefluid, and making the nanoparticles dispersion in the basefluid by ultrasonic vibration, adding surfactant, or changing the pH value of basefluid et al (Yang et al., 2011c). One-step method contains physical methods and chemical methods, physical methods generally include vapor deposition, laser ablation, vacuum buried arc, and chemical methods generally include coprecipitation method and precursor conversion method. Vapor deposition is not suitable to prepare ammonia-water nanofluid because of the volatility of ammonia-water. Laser ablation and vacuum buried arc methods are in high-cost and the production equipments may be corroded by ammonia. The coprecipitation method and precursor conversion method generally need a certain set of solution circumstances (such as pH value) and perhaps generates by-products in the fluid. Therefore, one-step method is difficult to handle for two component nanofluids, especially for nanofluids containing volatile gas. To the authors’ best knowledge, the current nanofluids used in ammonia absorption refrigeration system are all prepared by two-step method (Kim et al., 2006; Liu et al., 2009; Wu et al., 2010; Yang et al., 2011a,b,c). Therefore, in this study, twostep method is employed in the preparation of ammoniawater nanofluids.

2.3.

Selection of types of nanoparticles

Besides the basefluid, the selection of the type of nanoparticle is another problem need to be solved. The nanoparticle should be endowed with higher chemical durability and not react chemically with ammonia. Metal simple substance nanoparticles such as Fe, Al, Cu are easily oxidized and therefore not very appropriate for applying to ammonia absorption refrigeration system. The price of nanoparticle should be also

taken into account, the cost of nanoparticles such as gold, diamond, and some expensive nanotubes, is higher to apply to ammonia absorption refrigeration system which needs a considerable amount of ammonia-water. Therefore, in this paper, 20 types of chemically stable and low-cost nanoparticles are selected for preliminary investigations on the preparation of ammonia-water nanofluid. Table 1 shows the crystal shape, size, purity, appearance and specific surface of each type of nanoparticle. It can be seen that the types nanoparticle include non-metallic compounds, metal oxides, nitride, sulphide, carbide et al. According to the commercial product information of Nanjing AiPuRui nano-material Ltd, the mean sizes of nanoparticles range from 15 nm to 60 nm and the purities are all more than 99% through the use of ultraviolet emission spectrometer. Fig. 1 shows the SEM or TEM images of 4 couples of nanoparticles (Hydroxylapatite (HAP), TiO2, Al2O3, Fe2O3) with different crystal shapes or sizes. Fig. 2 shows the SEM or TEM images of metal oxide nanoparticles. And Fig. 3 shows the TEM images of SiO2, MoS2, SiC, TiN nanoparticles. It can be seen that the crystal shapes of nanoparticles are predominantly spherical or analogously spherical, and there are also monoclinal and acicular.

2.4.

Selection of type of dispersion methods

In general, there are three techniques to improve the dispersion of nanoparticles in liquid based on electrostatic and steric stabilizations. The first is modifying the surface of nanoparticles, which involves adsorbing or grafting organic groups on nanoparticles by using various dispersants (Sondi et al., 2004). The second method is changing the pH values of the basefluid of nanofluids (Zhang et al., 2004). The third method is using ultrasonic waves. The current studies on the dispersion stability of nanofluids generally focus on the influences of the above three parameters (Deiss et al., 1996; Zhu et al., 2005; Piao et al., 2009;

Table 1 e Parameters of each type of nanoparticle. Nanoparticles HAP-01 HAP-02 a-TiO2 r-TiO2 a-Al2O3 g-Al2O3 a-Fe2O3 g-Fe2O3 MgO ZrO2 ZnO CuO Fe3O4 SnO2 Cr2O3 ZnFe2O4 SiO2 MoS2 SiC TiN

Crystal shape

Sizes, nm

Purity, %

Appearance

Specific surface

Acicular Acicular Anatase Rutile a-phase spheric g-phase spheric a-phase spheric g-phase magnetic Spheric Monoclinic Single crystal Spheric Analogously spheric Spheric Spheric Analogously spheric Spheric Analogously spheric Spheric Spheric

60 40 20 20 30 20 30 20 20 20 20 60 20 50 60 30 15 40 40 15

99 99 99.5 99 99.9 99.9 99.9 99 99.5 99.9 99.8 99 99 99 99 99.8 99.9 99.5 99.5 99

White White White White White White Gulf red Dull red White White White Black Black Grayish black Grayish black Dull red White fluffy Black Grayish green Black

e e 120 40 60 160 50 30 60 25 90 80 66 30 25 50 600 80 90 120

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Fig. 1 e SEM/TEM images of 4 couples of nanoparticles with different crystal shapes or sizes.

Cho et al., 2009; Yang et al., 2011c). For the nanofluids used in ammonia absorption refrigeration system, the method of changing pH value is not appropriate since the ammoniawater is within a certain pH scale (pH >7). By contrast the method of adding dispersant and ultrasonic bath, it is found

through experience and empirical inquiry that the former plays greater roles in promoting the dispersion situation than the later for the selected types of nanoparticles in this study. Because the nanoparticles that can’t disperse stably in ammonia-water directly still can not disperse stably after

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Fig. 2 e SEM/TEM images of metal oxide nanoparticles.

ultrasonic bath, while adding some kind of dispersant can greatly improve the dispersion situation of some kind of nanoparticles and the dispersion-promoted dispersant is effective within a greater range of concentration. Considering

above circumstances, the research procedure of the dispersion method is arranged as shown in Fig. 4. The first step is to investigate the effect on the dispersion stability by various kinds of dispersant selected, and obtain the types of effective

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 5 ( 2 0 1 2 ) 2 2 4 8 e2 2 6 0

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Fig. 3 e TEM images of SiO2, MoS2, SiC, TiN nanoparticles.

dispersant. The second step is to study the effect on the dispersion stability by different contents of effective dispersant obtained, and obtain the optimal content of each effective dispersant. After obtaining optimal content of the effective dispersant, the third step is to study the effect on the dispersion stability by ultrasonic bath, and then obtain the optimal ultrasonic time for improving the dispersion stability

Step 1

Study the effect of dispersant type Obtain the kinds of effective dispersant

Step 2

Study the effect of dispersant content Obtain optimal dispersant content

Step 3

Study the effect of ultrasonic bath Obtain optimal ultrasonic time Obtain optimal dipersion process

Fig. 4 e Research procedure of the dispersion method.

of ammonia-water nanofluids. Thus the optimal dispersion process is obtained. This paper mainly focuses on the first step, i.e. to obtain the type of effective dispersant for each kind of nanoparticle selected. However, the special circumstance of nanofluids used in ammonia absorption refrigeration system is that the nanofluid will boil in the generator at a higher temperature. The question is that even though the nanofluids are stable at normal temperature and can improve the absorption of ammonia, dose it can continuously keep stable after boiling in the generator? Therefore, in this paper, some well-dispersed nanofluids are selected for investigating the dispersion stabilities of nanofluids after boiling.

2.5.

Selection of dispersion stability evaluating methods

The dispersion stability is indeed directly related to the absorption capability of the absorbent solution, which has been verified in the study of Yang et al. (2011a). It was found in their experiments that only the well stabilized nanofluid can enhance ammonia absorption performance. The reasons that only the well stabilized nanofluid can enhance absorption may 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, using poorly stabilized nanofluid is exposed to many problems, such as sedimentation of particles, stoppage of pipeline, corrosion

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of transport equipment and higher pumping power requirement. Therefore, the dispersion stability of nanofluids is the principal criterion for the application of nanofluid on the ammonia absorption refrigeration system. The current evaluating methods mainly include sedimentation observation method, zeta potential method, microscopy of grain size, and light absorbency index method. The zeta potential method is based on the electrostatic stabilization mechanism and is not appropriate to evaluate the dispersion stability of nanofluid based on the volume restriction mechanism. Microscopy of grain size is more expensive to investigate numerous kinds of nanofluids which are needed for obtaining the optimal dispersing process. Therefore, sedimentation observation method and light absorbency index method are selected to evaluate the dispersion stabilities of ammonia-water nanofluid in this study. Sedimentation observation method is embodied by measuring the height of supernatant layer. Some nanofluids will be stratified clearly after static standing by the role of natural sedimentation, the nanofluids in better dispersion situation will generate shorter supernatant layer after a certain period of standing. Therefore, the height of supernatant layer can reflect the dispersion stability intuitively. Light absorbency index method is to measuring the absorbency of nanofluid, and the absorbency of the suspension is defined by following equation (Li, 2005). A ¼ lg ðIi =IÞ ¼ εbc

(1)

Eq. (1) indicates that when the molar absorptivity and optical path are fixed, the absorbency is proportional to the content of the particles in suspension. Higher absorbency means higher mass fraction of nanoparticles suspended in the solution, which signifies the better dispersion of nanofluid. Therefore, the dispersion stability of nanofluid can be evaluated by measuring the absorbency of nanofluid after a given period of static standing. However, sedimentation observation method is not suitable to investigate the dispersion stability of some welldispersed nanofluids without stratification or some nanofluids which just fades and are not stratified clearly after a period of storage. As a result of different nanofluids have different specific molar absorptivities which induce different initial absorbencies according to Eq. (1), light absorbency index method is only applicable to investigate the influences of variation of dispersants on the dispersion of a certain kind of nanoparticle. Therefore, in this paper, a new parameter called ratio of varying absorbency is defined to investigate the dispersion stability of different nanofluids compared with the conventional method of measuring the height of supernatant layer or just measuring the absorbency. The ratio of varying absorbency is defined as the following equation, K ¼ Aaf =Aini

(2)

As a result of the absorbency is proportional to the content of the particles in suspension, the ratio of varying absorbency provides an expression of the ratio of content of nanoparticles suspended in solution after a certain period of static standing to the whole content of nanoparticles added in solution. This

index has a similar physical significance with the sedimentation ratio. The conventional method to obtain the sedimentation ratio is to remove the sedimentation and dry it, and then weigh the ratio of sedimentation to the incipient nanoparticles added. It is apparent that the ratio of varying absorbency is more convenient to obtain than the sedimentation ratio.

2.6.

Experimental procedure

The apparatus used in the experiments include: JA1003 electronic analytical balance (accuracy: 0.001 g), 98-2 magnetic stirrer (power: 90 W), UV-1100 ultravioletevisible spectrophotometer (accuracy: 0.001 Abs), thermostatic oven microinjector (measuring range: 100 mL), et al. The test procedures are as follows: 1) 20 types of nanoparticles mixed pairwise orthogonally with 10 types of dispersants were added in 25% ammonia-water, respectively. The mass fractions of nanoparticles were 0.2%. The dispersants included sodium dodecyl benzene sulfonate (SDBS), cetyltrimethylammonium bromide (CTAB), sodium hexametaphosphate (SHMP), polyethylene glycol 1000 (PEG 1000), PEG 10000, isopropanolamine (IPA), emulsifier OP-10 (OP-10), ammonium citrate (AC), polyacrylic acid (PAA). The mass fractions of dispersants were also preliminarily determined as 0.2%. 2) After each type of mixture (nanoparticle and dispersant) was added in ammonia-water, the nano-suspension was mechanical agitated by 98-2 magnetic stirrer (power: 90 W) for intensive mixing. And then, the absorbency of each type of suspension was tested just after the preparation. When testing the absorbency of nanofluids containing nanoparticles and dispersants, the solutions only containing dispersants were prepared as reference samples. 3) After standing for a certain period of time, the height of supernatant layer and the absorbency of each kind of nanofluid were measured. When measuring the absorbency, the depth of the sampling location was unified as 50 mm. Thus the ratio of varying absorbency can also be obtained. 4) After obtaining the dispersion condition of each type of nanofluid, some well-dispersed nanofluids were selected to study the influence of boiling on the dispersion stability at higher temperature. The nanofluids were put into a thermostatic oven at a temperature of 120  C to boil for 2 h, and then their dispersion stability were tested with the same method as mentioned above. And the influences of dispersants on the dispersion stabilities of nanofluids without boiling treated were also investigated.

3.

Results and discussions

3.1. Dispersion stability of nanofluids investigated by measuring the height of supernatant layer Table 2 shows the sedimentation situation of each kind of nanofluid after standing for 12 h. The smaller the height of supernatant layer, the better the dispersion stability of

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Table 2 e The height of supernatant layer of each kind of nanofluid after standing for 12 h (F: Faded but not stratified; D: dissolved; “d”: deposited absolutely on the bottom of test tube). Dispersant

None

SDBS

CTAB

Nanoparticle HAP 40 nm HAP 60 nm a-TiO2 r-TiO2 a-Al2O3 g-Al2O3 a-Fe2O3 g-Fe2O3 ZnO ZrO2 MgO CuO Fe3O4 SnO2 Cr2O3 ZnFe2O4 SiO2 MoS2 SiC TiN

SHMP

PEG 1000

PEG 10000

IPA

OP-10

AC

PAA

OA

d d d d d d d d D d d D d d d d d d F 0

d d 5 0 5 5 d d D F d D d d d d d d 0 0

d d d d d d d d D d d D d d d d d d 0 0

Height of supernatant layer after standing for 12 h (mm) d d 5 d d d d d D d d D d d d d d d 0 0

15 F 6 0 d 0 5 40 D d 10 D F F 15 10 d d 0 F

8 d d d 12 d 9 d D d d D d d F F F F F 0

d d d d d d d d D F d D d F d d d d 0 d

nanofluids. It can be seen obviously that most kinds of combinations of nanoparticles and dispersant are deposited on the bottom of test tube absolutely after storage. For each kind of nanofluid, there are one or more kinds of dispersants which can improve the dispersion situation. While each kind of dispersant can also improves the dispersion of one or more kinds of nanoparticles in ammonia-water. Therefore, the appropriate selection of dispersant is a key step to prepare stable nanofluid by two steps method. However, there are also a considerable number of combinations (nanoparticles and dispersants) which just faded but were not stratified clearly after standing. The observation method is inappropriate for those kinds of nanofluid because different kinds of nanofluids have different colors and the variations of the shading of the nanofluids’ color are difficult to quantitatively analyze by observation. Besides, there are some well-dispersed nanofluids (a-TiO2, SiC, and TiN nanofluids) in which the supernatant layer doesn’t occur. For TiO2, SiC, and TiN nanofluids, several combinations (nanoparticles and dispersant) is not stratified clearly at all after standing and the heights of supernatant layers are zero. Therefore, the influence of some kinds of dispersants on the dispersion of nanofluid can not be compared intuitively by measuring the height of supernatant layer. Thus it is difficult to evaluate the dispersion stability of nanofluid just by observation method, and actually, the circumstances of fading or no stratification occurred in more than half of nanofluids. When CuO and ZnO nanoparticles are added in ammoniawater, the nanoparticles dissolved in the solution after agitated and storage. This is because there were chemical reactions between the ammonia-water with the two kinds of nanoparticles, and the water-soluble complex compounds were produced. Therefore, those two kinds of nanoparticles are not appropriate to be applied to ammonia absorption refrigeration system. This circumstance further demonstrates the importance

d F 0 10 d 6 d d D d d D d d d d d d 0 F

8 11 6 d d 0 12 d D F d D d F d d d d 0 F

6 F 0 5 10 d 4 d D F d D d d d d d F 0 0

d 10 0 5 d F d d D d d D d d d d d d 0 F

of selecting ammonia-water as basefluid for the preparation of nanofluids applied to ammonia absorption refrigeration system.

3.2. Dispersion stability of nanofluids investigated by measuring absorbency Table 3 shows the dilution ratio and the initial absorbency of each nanofluid just after the preparation. The measuring range of absorbency of UV-1100 ultravioletevisible spectrophotometer is 0e3. As a result of each kind of nanoparticle has proprietary molar absorptivity, according to Eq. (1), the initial absorbency of each nanofluids is of considerable variation even at different orders of magnitude. To obtain higher measuring accuracy, the different dilution ratios were adopted for each kind of nanofluid to assure the initial absorbency reaches a comparatively higher value within the measuring range. Table 4 shows the absorbency of each kind of nanofluid after standing for 24 h. It can be seen obviously that for each kind of nanoparticles, the influence of each kind of dispersant on the dispersion stability of nanofluid is quantitatively demonstrated. The dispersion situations of nanofluids investigated by measuring absorbency are in accordance with that investigated by measuring the height of supernatant layer as shown in Table 3. Whereas for the nanofluids which just faded but were not stratified clearly after standing and those in better dispersion without supernatant layer at all, the method of measuring absorbency is more effective to contrast the influence of dispersant on the dispersion of nanofluid than the method of measuring the height of supernatant layer. For example, it can be seen from Table 4 that the optimal dispersant for a-TiO2, SiC, and TiN nanofluids is IPA, SDBS and PAA, respectively, which is unprocurable by measuring the height of supernatant layer. However, although absorbency of nanofluid is effective to obtain the effective dispersant for a certain kind of nanofluid,

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Table 3 e Dilution ratio and the initial absorbency of each nanofluid just after the preparation. Nanoparticle

Dilution ratio

Initial absorbency

1:50 1:50 1:100 1:100 1:50 1:50 1:60 1:50 1:60 1:15 1:50 1:100 1:100 1:50 1:50 1:60 1:30 1:50 1:30 1:100

2.880 2.088 2.448 2.109 2.411 2.538 2.585 2.874 2.347 2.905 2.496 2.839 2.667 2.748 2.136 2.735 2.901 2.340 2.550 2.502

HAP-01 HAP-02 a-TiO2 r-TiO2 a-Al2O3 g-Al2O3 a-Fe2O3 g-Fe2O3 ZnO ZrO2 MgO CuO Fe3O4 SnO2 Cr2O3 ZnFe2O4 SiO2 MoS2 SiC TiN

the contrast of dispersion stabilities of different kinds of nanofluid are hardly achieved because different nanofluids have different dilution ratios and initial absorbencies. It is difficult to select the excellent types of nanoparticles with best dispersion stabilities in ammonia-water by just measuring absorbency.

3.3. Dispersion stability of nanofluids embodied by ratio of varying absorbency Table 5 shows the ratio of varying absorbency of each kind of nanofluid after standing for 24 h. It can be concluded from Eq.

(2) that ratio of varying absorbency provides an expression of the ratio of content of nanoparticles suspended in solution after a certain period of static standing to the whole content of nanoparticles added in solution. Therefore, this index can be used to contrast the dispersion stabilities of different types of nanofluids. It can be seen from Table 5 that the ratio of varying absorbency can indirectly express the dispersion situation of each kind of combination (nanoparticles and dispersants). On the whole, the parameter of ratio of varying absorbency is more practical than the height of supernatant layer to investigate the dispersion stabilities of some nanofluids in which the supernatant layer doesn’t occur, and is more efficient to contrast the dispersion stabilities of different types of nanofluids than just measuring the absorbency. By investigating the ratio of varying absorbency, following dispersion situations of nanofluids can be found: When the nanoparticles were added in ammonia-water basefluid without dispersant, the dispersion situation of each kind of nanoparticle was different. The anatase-TiO2, SiC, and TiN nanoparticles can disperse in ammonia-water relatively stably and uniformly, while other kinds of nanoparticles were poor in dispersion in ammonia-water and the nanoparticles were dropped on the bottom of the test tube after standing for 24 h by the role of nature sedimentation. Therefore, it can be concluded that the type of nanoparticle is an important factor that influence the dispersion stability of nanofluid. By referring the TEM images of anatase-TiO2, SiC, and TiN nanoparticles as shown in Figs. 1(c) and 3(c) and (d), it can be found that the dispersive characteristics of the three kinds of dry nanoparticles are preferable which probably induce the better dispersion stability in solution condition. However, not all nanoparticles with better dry dispersive characteristics can easily disperse in ammonia-water directly without dispersant, because HAP-01, g-Al2O3, a-Fe2O3, Cr2O3, SnO2 nanoparticles can not disperse stably in ammonia-water

Table 4 e Absorbency of each kind of nanofluid after standing for 24 h (“d” chemical reaction or flocculation). Dispersant

None

SDBS

CTAB

SHMP

Nanoparticle HAP-01 HAP-02 a-TiO2 r-TiO2 a-Al2O3 g-Al2O3 a-Fe2O3 g-Fe2O3 ZnO ZrO2 MgO CuO Fe3O4 SnO2 Cr2O3 ZnFe2O4 SiO2 MoS2 SiC TiN

PEG 1000

PEG 10000

IPA

OP-10

AC

PAA

OA

0.174 0.752 1.737 0.883 0.018 0.528 0.425 0.048 d 0.180 0.03 d 0.027 0.097 0.408 0.045 0.079 0.048 1.974 0.339

0.666 0.336 0.096 0.129 0.031 0.066 0.324 0.054 d 0.283 0.036 d 0.045 0.096 0.156 0.045 0.092 0.066 0.755 0.177

0.456 0.324 1.074 1.539 1.404 1.558 0.315 0.108 d 0.782 0.066 d 0.135 0.258 0.21 0.423 0.112 0.318 1.361 2.433

0.126 0.186 0.447 0.123 0.342 0.054 0.276 0.332 d 0.125 0.006 d 0.162 0.048 0.126 0.045 0.031 0.036 1.493 0.345

Absorbency after standing for 24 h 0.297 0.117 1.062 0.375 0.054 0.096 0.335 0.012 d 0.313 0.012 d 0.009 0.078 0.162 0.065 0.014 0.186 2.141 1.645

0.822 0.348 0.961 1.329 0.138 1.383 1.807 1.122 d 0.262 1.284 d 0.708 1.056 1.332 1.413 0.066 0.258 2.493 0.522

1.680 0.156 0.485 0.096 0.552 0.245 1.085 0.048 d 0.183 d d 0.168 0.324 0.378 0.251 0.158 0.576 0.663 0.153

0.204 0.162 0.455 0.078 0.078 0.108 0.297 0.036 d 1.147 0.204 d 0.102 0.858 0.204 0.325 0.066 0.186 0.884 0.237

0.774 0.354 1.539 0.593 0.096 1.112 0.324 0.048 d 0.282 0.048 d 0.075 0.678 0.216 0.115 0.043 0.151 1.398 0.435

1.524 0.726 1.118 0.288 0.054 1.638 0.895 0.054 d 1.290 0.036 d 0.063 0.894 0.234 0.123 0.031 0.090 1.360 0.606

1.890 0.348 2.415 0.693 0.612 0.096 1.884 0.042 d 0.725 0.048 d 0.195 0.138 0.156 0.085 0.064 0.504 1.852 2.217

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Table 5 e Ratio of varying absorbency of each kind of nanofluid after standing for 24 h (“d” chemical reaction or flocculation). Dispersant

None

SDBS

CTAB

Nanoparticle HAP-01 HAP-02 a-TiO2 r-TiO2 a-Al2O3 g-Al2O3 a-Fe2O3 g-Fe2O3 ZnO ZrO2 MgO CuO Fe3O4 SnO2 Cr2O3 ZnFe2O4 SiO2 MoS2 SiC TiN

SHMP

PEG 1000

PEG 10000

IPA

OP-10

AC

PAA

OA

0.231 0.161 0.039 0.061 0.013 0.026 0.125 0.019 d 0.097 0.014 d 0.017 0.035 0.073 0.016 0.032 0.028 0.296 0.071

0.158 0.155 0.439 0.730 0.582 0.614 0.122 0.038 d 0.269 0.026 d 0.051 0.094 0.098 0.155 0.039 0.136 0.534 0.972

0.044 0.089 0.183 0.058 0.142 0.021 0.107 0.116 d 0.043 0.002 d 0.061 0.017 0.059 0.016 0.011 0.015 0.585 0.138

Ratio of varying absorbency after standing for 24 h 0.103 0.056 0.434 0.178 0.022 0.038 0.130 0.004 d 0.108 0.005 d 0.003 0.028 0.076 0.024 0.005 0.079 0.840 0.653

0.285 0.167 0.393 0.630 0.057 0.545 0.699 0.390 d 0.090 0.514 d 0.265 0.384 0.624 0.517 0.023 0.110 0.978 0.209

0.583 0.075 0.198 0.046 0.229 0.097 0.420 0.017 d 0.063 d d 0.063 0.118 0.177 0.092 0.054 0.246 0.260 0.061

0.071 0.078 0.186 0.037 0.032 0.043 0.115 0.013 d 0.395 0.082 d 0.038 0.312 0.096 0.119 0.023 0.079 0.347 0.095

0.269 0.170 0.629 0.281 0.040 0.438 0.125 0.017 d 0.097 0.019 d 0.028 0.247 0.101 0.042 0.015 0.065 0.548 0.174

directly though they obtain better dry dispersive characteristics. The dispersion situation in solution is comprehensive function of dispersive characteristic of dry nanoparticles, hydrophilicity, pH value et al. However, the antecedent condition of nanoparticles directly dispersing in ammoniawater is that they should have better dispersive characteristics in dry condition for the types of nanoparticles selected in this study. Although some kinds of nanoparticles with better dry dispersive characteristics can not disperse in ammonia-water directly, the dispersive characteristics of dry nanoparticles are still significative for the dispersion process of those kinds of nanoparticles. By contrasting the dispersion situations in solution and the TEM images of the 4 couples of nanoparticles: HAP-01 and HAP-02, a-TiO2 and r-TiO2, a-Al2O3 and g-Al2O3, aFe2O3 and g-Fe2O3, it can be found that the dispersion of nanoparticles with better dispersive characteristics in dry condition are more easily strengthened by dispersant than those with poor dispersive characteristics in dry condition. The HAP-01 and HAP-02 nanoparticles can not disperse in ammonia-water directly. And when adding dispersant, it can be seen that the dispersion condition of HAP-01 nanofluid can be greatly improved by adding CTAB or PEG10000 or IPA. However, the influences of dispersants on the dispersion conditions of HAP-02 nanofluid are not as significant as that of HAP-01 nanofluid. Only PEG 10000 and OP-10 provide minor help to the dispersion stability of HAP-02 nanofluid. It can be concluded that for the same crystal shape of nanoparticles, the type and the effect of dispersant are different for different size and dispersive characteristics of dry nanoparticles. For TiO2 nanofluids, the dry dispersive characteristics of aTiO2 are better than that of r-TiO2, which probably induces the dispersion stability of a-TiO2 in ammonia-water is better than that of r-TiO2 without dispersant. And when adding dispersant, it can be seen that the dispersion condition of a-TiO2

0.529 0.348 0.457 0.137 0.022 0.445 0.346 0.019 d 0.444 0.014 d 0.024 0.325 0.110 0.045 0.011 0.038 0.533 0.242

0.656 0.167 0.987 0.329 0.254 0.038 0.729 0.015 d 0.250 0.019 d 0.073 0.050 0.073 0.031 0.022 0.215 0.726 0.886

0.060 0.360 0.710 0.419 0.007 0.208 0.164 0.017 d 0.062 0.012 d 0.010 0.035 0.191 0.016 0.027 0.021 0.774 0.135

nanofluid can be greatly improved by adding IPA and OP-10. While the best candidates for improving the dispersion of rTiO2 are SDBS and PAA. Therefore, the phase and crystal shape influence the type and the effect of dispersant for improving the dispersion stability of nanofluid. However, it can be seen that the ratio of varying absorbency of optimal combination for a-TiO2 can reach 0.987, which is obviously higher than that of r-TiO2 nanofluid. The same circumstance applies to the dispersion situation of Al2O3 nanofluids. The optimal candidates for the improvement of dispersion of aAl2O3 are PAA, IPA and CTAB, while that for g-Al2O3 is SDBS, PEG 10000 and PAA. Therefore, the crystalling phase influences the type and the effect of improvement of dispersant on dispersion of nanoparticles. This founding can be further demonstrated by comparing the dispersion situation of aFe2O3 to that of g-Fe2O3. The dispersion of a-Fe2O3 can be greatly improved by adding IPA or SDBS, while that of g-Fe2O3 is hard to be improved by adding dispersant, only SDBS has little enhancement on the dispersion situation. This is because g-Fe2O3 is a kind of magnetic nanoparticles, the magnetic force cause serious reunions in dry powder as shown in Fig. 1(h). The wet dispersion in solution is difficult to be improved, and once again this appearance shows the significance of dispersive characteristic of dry powder on the dispersion in solution. Therefore, it can be found that the dispersion of nanoparticles with better dispersive characteristics in dry condition are more easily strengthened by dispersant than those with poor dispersive characteristics in dry condition. Other dispersion situations of nanofluids have arisen as follows: the dispersion of ZnFe2O4, Cr2O3 and MgO nanoparticles can be greatly improved by adding SDBS. The dispersion of ZrO2, Fe3O4, SnO2, MoS2 nanoparticles improved a little by adding relevant dispersant, the ratios of varying absorbency after standing for 24 h are below 0.5, which means

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more than half of the nanoparticles deposited on the bottom of test tube. However, as a result of the contents of relevant dispersants also affect the dispersions of nanofluids, there are additional potentials on the enhancement of the dispersion situations of those kinds of nanofluids. However, for SiO2 nanofluids, almost all kinds of dispersant can not improve the dispersion, and the most attractive of them is CTAB which can only obtain a ratio of varying absorbency of 0.054. By the preliminary investigations on the selection of nanofluid applied to the ammonia absorption refrigeration system, three levels of dispersion stability of nanofluid are obtained in this study. The fist level is a-TiO2, TiN, SiC nanoparticles which have nice dispersion stabilities in ammoniawater and also can be improved further by adding related dispersants. The second level includes DAP-01, r-TiO2, Al2O3, a-Fe2O3, ZnFe2O4, Cr2O3, MgO nanoparticles whose dispersion stabilities can be greatly improved by adding dispersant but can not disperse in ammonia-water directly. Others nanoparticles are divided into the third levels, the ratios of varying absorbency of those kinds of nanofluids after standing for 24 h are below 0.5, and those kinds of nanoparticles are not employed initially in the study of applying nanotechnology to ammonia absorption refrigeration system. The nanoparticles of first and second levels are the candidates to be applied to ammonia absorption refrigeration system. And the dispersion stability of nanofluid can be further improved by obtaining the optimal content of dispersant for each kind of nanofluid as shown in Fig. 4. The ratios of varying absorbency of nanofluids after standing for 7 days are shown in Table 6. It can be seen that after 7 days’ standing, the dispersions of all kinds of nanofluids including those with optimal dispersants were exacerbated. However, the dispersions of nanofluids with optimal dispersants were still better than those without dispersants.

And it can be found that after 7 days’ standing, there was a new flocculation in the combination of ZnFe2O4 nanoparticles and CTAB. It should be mentioned that the nanofluids have not achieve the optimal dispersion condition when just obtain effective surfactant type, and it can be further improved by obtaining the optimal surfactant content, ultrasonic bath time, and ammonia concentration in base fluids. And the selection of dispersant types will lay the foundation for the further improvement on the dispersion stability of ammonia-water nanofluids.

3.5. Influence of boiling on dispersion stability of nanofluids To apply the techniques that nanoparticles enhance the heat and mass transfer to the ammonia-water absorption refrigeration, the dispersion stability of nanofluid after boiling treated should be studied. Some nanoparticles in first and second levels’ dispersion stability are selected for investigating the influence of boiling on dispersion stability of nanofluids. Fig. 5 shows the comparative ratio of varying absorbency of nanofluid without dispersant after standing for 24 h between after and without boiling. It can be seen that for SiC and TiN nanofluids, which achieve the two of the best dispersions, the ratios of varying absorbency of nanofluid without dispersant are improved after boiling. While for the three other kinds of nanofluids, the dispersion stabilities are weakened after boiling for 2 h. It can be concluded that when adding no dispersant, 2 h’ boiling can make the nanofluids in good dispersion disperse better, and make the nanofluids in poor dispersion disperse more poorly. This circumstance reflects that the dispersion stability of nanofluid is even more important when considering the effect of boiling.

Table 6 e Ratio of varying absorbency of each kind of nanofluid after standing for 7 days (“d” chemical reaction or flocculation). Dispersant

None

SDBS

CTAB

Nanoparticle HAP-01 HAP-02 a-TiO2 r-TiO2 a-Al2O3 g-Al2O3 a-Fe2O3 g-Fe2O3 ZnO ZrO2 MgO CuO Fe3O4 SnO2 Cr2O3 ZnFe2O4 SiO2 MoS2 SiC TiN

SHMP

PEG 1000

PEG 10000

IPA

OP-10

AC

PAA

OA

0.021 0.031 0 0.001 0 0 0.005 0 d 0.004 0.006 d 0 0 0 0 0 0 0.058 0.001

0.011 0.019 0.063 0.158 0.295 0.308 0002 0.001 d 0.028 0.003 d 0.001 0.002 0 0.024 0 0.003 0.114 0.287

0 0 0.004 0.002 0.001 0.001 0 0 d 0.004 0.003 d 0 0 0 0 0 0 0.136 0.006

Ratio of varying absorbency after standing for 7 days 0.007 0.002 0.106 0.024 0.001 0.003 0.001 0 d 0.001 0.002 d 0 0 0.001 0 0 0.001 0.204 0.105

0.052 0.015 0.059 0.116 0.002 0.281 0.314 0.021 d 0 0.216 d 0.015 0.081 0.115 0.218 0 0.005 0.358 0.018

0.136 0 0.006 0.001 0.017 0.001 0.085 0.001 d 0.002 d d 0.001 0 0.026 d 0.001 0.061 0.031 0.008

0 0 0.014 0 0.001 0 0 0 d 0.131 0.001 d 0 0.048 0.006 0.014 0 0.001 0.058 0.003

0.059 0.011 0.153 0.034 0.004 0.115 0 0 d 0.005 0.001 d 0 0.032 0.002 0.001 0 0.001 0.143 0.018

0.141 0.086 0.114 0.025 0.001 0.154 0.057 0 d 0.126 0.002 d 0 0.018 0 0 0 0 0.114 0.038

0.189 0.024 0.254 0.031 0.014 0.005 0.413 0 d 0.034 0.003 d 0 0 0 0 0 0.008 0.158 0.325

0 0.064 0.210 0.064 0.006 0.015 0.014 0 d 0.003 0.004 d 0 0 0.002 0.001 0 0 0.326 0.005

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 5 ( 2 0 1 2 ) 2 2 4 8 e2 2 6 0

1.0

Ratio of varying absorbency

0.9

without boiling after boiling

0.8 0.7 0.6 0.5

0.3

0.1 0.0 SiC

TiN

a-TiO

r-TiO

-Fe O

Type of nanofluid without dispersant

Fig. 5 e Influences of boiling on dispersion stabilities of nanofluids without dispersant.

1.1

without boiling after boiling

1.0

Ratio of varying absorbency

respectively, to observe the dispersion stability of suspension. The types of dispersant for improving the dispersion of each type of nanoparticle in ammonia-water are preliminarily investigated. A new parameter called ratio of varying absorbency is defined to investigate the dispersion stability of nanofluids compared with the conventional method of measuring the height of supernatant layer or just measuring the absorbency. Several conclusions can be drawn:

0.4

0.2

0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 SiC + SDBS

TiN+PAA

a-TiO +DIPA

r-TiO +PAA

-Fe O +DIPA

Type of nanofluid with dispersant

Fig. 6 e Influences of boiling on dispersion stabilities of nanofluids with their optimal dispersants.

Fig. 6 shows the comparative ratio of varying absorbency of nanofluid with their optimal dispersants after standing for 24 h between after and without boiling. It can be seen that when adding dispersant, the dispersion stability of nanofluids are weakened a little after boiling 2 h. For SiC and TiN nanofluids, the dispersion stabilities after weakened were close to the nanofluid without adding dispersant. While for a-TiO2, r-TiO2 and a-Fe2O3 nanofluids, the dispersion stabilities after weakened by boiling were still much better than that of nanofluids without dispersant. Therefore, for the nanoparticles which can not disperse in ammonia-water stably by themselves, adding dispersant, on the whole, can improve the dispersion situations which are recommended to be employed.

4.

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Conclusions

20 types of nanoparticles mixed pairwise orthogonally with 10 types of dispersant are added in ammonia-water,

1) The parameter of ratio of varying absorbency is more practical than the height of supernatant layer to investigate the dispersion stability of some nanofluids which just fade and some well-dispersed nanofluids which are little stratified after storage, and is more efficient to contrast the dispersion stability of different types of nanofluids than just measuring the absorbency. 2) The dispersive characteristics of the dry nanoparticles influence and even decide to a great extent the dispersion stability of suspension. The nanoparticles with better dispersive characteristics in dry condition can more easily disperse in ammonia-water directly or strengthened dispersions by dispersant than those with poor dispersive characteristics in dry condition. 3) The dispersion stability of nanoparticles employed is divided into three levels. The fist level contains a-TiO2, TiN, SiC nanoparticles which have nice dispersion stabilities in ammonia-water and also can be improved further by adding related dispersants. The second level includes HAP-01, r-TiO2, Al2O3, a-Fe2O3, ZnFe2O4, Cr2O3, MgO nanoparticles whose dispersion stabilities can be greatly improved by adding dispersant but can not disperse in ammonia-water directly. Others nanoparticles are divided into the third levels, the ratios of varying absorbency of those kinds of nanofluids after standing for 24 h are below 0.5. 4) Boiling makes the good dispersion nanofluids disperse better, and makes the poor dispersion nanofluids disperse more poorly when adding no dispersant. And when adding dispersant, boiling makes the dispersion of selected nanofluids weakened.

Acknowledgment The work of this paper is financially supported by the Science Foundation of China (51176029), the 12th Five Year National Science and Technology Support Key Project of China (2011BAJ03B00) and the Foundation of Graduate School of Southeast University (ybjj1124). The supports are gratefully acknowledged.

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