The effects of nanolubricants on boiling and two phase flow phenomena: A review

International Communications in Heat and Mass Transfer 75 (2016) 197–205

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International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt

The effects of nanolubricants on boiling and two phase flow phenomena: A review☆ Omer A. Alawi a, Nor Azwadi Che Sidik a,⁎, A.Sh. Kherbeet b a b

Department of Thermofluids, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor Bahru, Malaysia Department of Mechanical Engineering, KBU International College, 47800 Petaling Jaya, Selangor, Malaysia

a r t i c l e

i n f o

Available online 13 April 2016 Keywords: Nanorefrigerants Pool boiling Flow boiling Condensation

a b s t r a c t The study of nanorefrigerant boiling and two-phase flow phenomena is still very much in its infancy. This research field poses many opportunities to study new frontiers but also gives great challenges. This study presents a comprehensive review of nucleate pool boiling, flow boiling, condensation and two-phase flow of nanorefrigerants to summarize the current status of research in this newly developing interdisciplinary field and to identify the future research needs as well. This review has been realized that the physical properties have significant effects on the nanorefrigerant boiling and two-phase flow characteristics but the lack of the accurate knowledge of these physical properties has greatly limited the study in this interdisciplinary field. Therefore, effort should be made to contribute to the physical property database of nanofluids as a first priority. Secondly, systematic accurate experiments and flow regime observations on boiling and two-phase flow phenomena under a wide range of test conditions and nanofluid types should be emphasized to understand the fundamentals. Finally, physical mechanisms and prediction methods for boiling heat transfer and two phase flow characteristics should be targeted and applied research should also be focused on in the future. © 2016 Elsevier Ltd. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . Studies on boiling and two-phase flow phenomena of nanorefrigerants 2.1. Studies on nucleate pool boiling without and with lubricating oil 2.1.1. Studies on nucleate pool boiling without lubricant oil . 2.1.2. Studies on nucleate pool boiling with lubricants . . . 2.2. Studies on flow boiling of nanorefrigerants . . . . . . . . . . 2.3. Studies on condensation of nanorefrigerants . . . . . . . . . 3. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Boiling and two phase flow phenomena have been used in a variety of industrial applications and processes, such as refrigeration, airconditioning and heat pumping systems, energy conversion systems, heat exchange systems, chemical thermal processes, cooling of highpower electronics components, cooling of nuclear reactors, microfabricated fluidic systems, thermal processes of aerospace station and ☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (N.A.C. Sidik).

http://dx.doi.org/10.1016/j.icheatmasstransfer.2016.04.001 0735-1933/© 2016 Elsevier Ltd. All rights reserved.

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197 198 198 198 201 203 203 204 204

bioengineering reactors [8]. Boiling and two phase flow heat transfer enhancement may improve energy efficiency and achieve significant energy consumption reduction. One of the methods is to use nanofluids to enhance boiling and two-phase flow heat transfer [9,47]. As a new research frontier, nanofluid two-phase flow and thermal physics have the potential to improve heat transfer and energy efficiency in thermal systems for many applications, such as microelectronics, power electronics, nuclear engineering, heat pipes, refrigeration, and air-conditioning and heat pump systems [19,20,27,28,31]. Furthermore, a new concept described the application of nanoparticles as additives into refrigerants in which the obtained suspension is called “nanorefrigerant.” Refrigerant is a material used in heat transfer

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cycles which undergoes a phase change most of the time due to excessive heat transfer rate experienced during the process. Despite most of the fluids finding themselves a place in refrigeration cycles, only fluorocarbons or chlorofluorocarbons are considered as refrigerant according to general opinion. Two methods (single-step method and double-step method) are used for synthesis of nanorefrigerants. Double-step method is generally used to prepare nanorefrigerants. In this method, the nanomaterials are synthesized as dry powders by thermal decomposition and photochemical methods, transition metal salt reduction, ligand reduction and displacement from organometallics, metal vapor synthesis and electrochemical synthesis methods [6]. After production, the nanosized powder is put into the oil to form nanoparticle/oil mixture. Then, this mixture is dispersed by using different types of dispersion techniques such as ultrasonic agitation, magnetic force agitation, homogenizing, and high-shear mixing [50]. In single-step method, vapor nanophase powders are condensed into a liquid having low vapor pressure and dissolved in liquid at the same time. The nanoparticles are produced by applying a physical vapor deposition method or liquid chemical method [43]. To identify the research status of boiling and two phase flow, Cheng et al. [9] presented a review to understand the two-phase flow and boiling heat transfer characteristics of nanofluids and to identify particular areas requiring further study. Most of the available studies deal with water-based nanofluids. Researchers have given much more attention to the thermal conductivity of nanofluids rather than their heat transfer characteristics. Most of the available heat transfer studies are related to single phase flow heat transfer and some are related to nucleate pool boiling. However, the study of flow boiling and two-phase flow of nanofluids is very limited in the literature. This review paper focuses on the latest advances in the augmentation of heat transfer by using nanorefrigerants. The fundamental and applied research of boiling and two phase flow phenomena with nanorefrigerants with and without lubricants is reviewed. Tables 1–4 provide a summary of the review performed in this paper. 2. Studies on boiling and two-phase flow phenomena of nanorefrigerants Tables 1–4 present the summaries on the studies of nucleate pool boiling without lubricants, nucleate pool boiling with lubricant, flow boiling and condensation of nanorefrigerants, respectively. The following studies on several related topics such as nucleate pool boiling heat transfer without and with nanolubricants, flow boiling and condensation

are respectively reviewed. For nucleate pool boiling, heat flux or superheated temperature degree is normally used as reference. For flow boiling heat transfer and two phase pressure drop, vapor quality, or total mass flux should be used as reference as in the available study. 2.1. Studies on nucleate pool boiling without and with lubricating oil Some experimental studies on nucleate pool boiling of nanorefrigerants have been presented. The experimental results on nucleate pool boiling are inconsistent with some studies showing a decrease or no change in nucleate boiling heat transfer with the addition of nanoparticles while some show an increase. An overall review on nucleate pool boiling heat transfer including lubricant effect is conducted. Table 1 presents a summary of studies on nucleate pool boiling of nanorefrigerants without lubricants while Table 2 presents a summary of studies on nucleate pool boiling of nanorefrigerants with lubricants or refrigerants with nanolubricants [8]. 2.1.1. Studies on nucleate pool boiling without lubricant oil Park and Jung [29,30] studied the effect of carbon nanotubes (CNTs) on nucleate boiling heat transfer of three halocarbon refrigerants (R123, R134a and R22). In these studies, 1 vol.% of CNTs was added to the refrigerants and they found that CNTs increased nucleate boiling heat transfer coefficients for both refrigerants. Fig. 1 shows their heat transfer coefficients of R134a with and without CNTs. In particular, enhancements up to 36.6% were observed at low heat fluxes. With increasing heat flux, however, the enhancement diminished due to more vigorous bubble generation according to their visual observations. In addition, no deposit of the particles on their heat transfer surface was observed in their study. Trisaksri and Wongwises [45] studied nucleate pool boiling heat transfer of TiO2-R141b nanofluid on a cylindrical copper tube surface at different nanoparticle concentrations of 0.01 vol.%, 0.03 vol.% and 0.05 vol.% and pressures of 200, 300, 400 and 500 kPa. Fig. 2 showed their measured nucleate pool boiling heat transfer of TiO2-R141b nanofluid versus superheated degrees at 300 kPa. Their results indicated that the suspended TiO2 nanoparticles deteriorated the nucleate boiling heat transfer of refrigerant R141b. However, almost no effect results from adding extremely small amounts of nanoparticles. This was in consistency with the results of Yang and Liu [49] at an extremely low concentration of 0.09 vol.%. However, the boiling heat transfer coefficient decreased with increasing particle volume concentrations, especially at high heat flux. This was contradictory to the results of Yang and Liu

Table 1 Summary of studies on pool boiling of nanorefrigerants without lubricants [8]. Literature

Nanofluid/nanoparticle

Test section

Conclusions

Park and Jung [29] Park and Jung [30]

CNTs-R123 and CNTs-R134a, 1 vol.% of CNTs CNTs-R22, 1 vol.% of CNTs

Trisaksri and Wongwises [45] Ding et al. [17]

TiO2-R141b, 0.01%, 0.03% and 0.05 vol.% Cu-R113/B68EP

Plain stainless steel horizontal circular tube of 19 mm O.D. and length of 152 mm. Plain stainless steel horizontal circular tube of 19 mm O.D. and length of 152 mm. Plain copper horizontal cylindrical heater of 28.5 mm O.D. and length of 90 mm. Boiling vessel of inside diameter of 30 mm and a height of 165 mm

Yang and Liu [49]

Au-R141b, 0.09%, 0.4% and 1%

Plain copper tube with an O.D. of 18 mm and length of 100 mm

Peng et al. [33]

CueR113, 0.1 wt.%, 0.5 wt.% and 1 wt.%

Horizontal copper flat surface with a diameter of 20 mm

Peng et al. [34,35]

Cu-R113, Al-R113, Al2O3-R113, CuO-R113, CuO-R141b, CuO-n-pentane

Boiling vessel of inside diameter of 50 mm and a height of 95 mm

CNTs increased boiling heat transfer coefficients, the enhancement was up to 36.6%. CNTs increased boiling heat transfer coefficients, the enhancement was up to 36.6%. TiO2 deteriorated the nucleate boiling heat transfer of refrigerant R141b. No effect from adding small amounts of nanoparticles. The nanoparticles migrated mass of nanolubricant increased with increasing the mass of nanoparticles and the mass of refrigerant. The migration ratio of nanoparticles decreased with increasing the nanoparticle concentration. Au particles increased boiling heat transfer coefficients. At concentration of 1%, the heat transfer coefficient was more than twice higher than those without nanoparticles. Surfactants enhanced the nucleate pool boiling heat transfer of Cu-R113 in most cases but deteriorated the heat transfer at high surfactant concentrations. The migration ratio of nanoparticles increased with decreasing the nanoparticle density, nanoparticle size, and dynamic viscosity of refrigerant, mass fraction of lubricating oil and heat flux, it increased with increasing the liquid-phase density of refrigerant or initial liquid level height.

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Table 2 Summary of studies on pool boiling of nanolubricant/refrigerant mixture or nanorefrigerants with lubricants [8]. Literature

Nanofluid/nanoparticle

Test section

Conclusions

Kedzierski and Gong [25]

CuO-R134a/polyolester

–

Kedzierski and Gong [26] Ding et al. [17]

CuO-R134a/polyolester (RL68H) mixture. Cu-R113/B68EP,

Horizontal copper flat surface

Kedzierski [21]

R134a-CuO/RL68H

A smaller enhancement was observed for the R134a/nanolubricant (99/1) mixture, which had a heat flux that was on average 19% larger than that of the R134a/polyolester (99/1) mixture. The nanoparticles caused heat transfer enhancement relative to the heat transfer of pure R134a/polyolester. The migrated mass increased with increasing the original mass of nanoparticles and the mass of refrigerant. The migration ratio of nanoparticles decreased with increasing the volume fraction of nanoparticles. R134a/nanolubricant mixtures with 1 vol.% CuO nanoparticles had a larger heat flux than R134a/CuO/RL68H blend with 2 vol.% CuO nanoparticles. The nucleate pool boiling heat transfer coefficient of R113/oil mixture with diamond nanoparticles was larger than that of R113/oil mixture CNTs increased nucleate boiling heat transfer and had a higher heat transfer coefficient than the R113–oil mixture without CNTs and the enhancement can reach 61%. The migration ratio of nanoparticles increased with decreasing the nanoparticle density, nanoparticle size, and dynamic viscosity of refrigerant, mass fraction of lubricating oil and heat flux, it increased with increasing the liquid-phase density of refrigerant or initial liquid level height. Nucleate pool boiling heat transfer coefficient of R113/oil mixture with Cu nanoparticles increased by a maximum of 23.85 with decreasing nanoparticle size from 80 nm to 20 nm. A semi-empirical model was developed to predict the enhancement of refrigerant/lubricant pool boiling. R134a nanorefrigerant containing mineral oil and 0.1% mass fraction TiO2 nanoparticles led to a savings in energy consumption of 26.1%.

Peng et al. [38]

Diamond-R113/VG68, nanoparticles 0–15 wt.% Peng et al. [36] CNTs-R113/VG68 0–30 wt.% Peng et al. [34,35] Cu-R113, Al-R113, Al2O3-R113, CuO-R113, CuO-R141b, CuO-n-pentane Peng et al. [37] Cu-R113/VG68, 0–30% Kedzierski [22]

Al2O3-R134a/polyolester (RL68H) mixture. R134a-TiO2/POE

Saidur et al. [41] and Alawi et al. [2] Kedzierski [23]

R134a- Al2O3/RL68H

Kedzierski [24]

R134a-diamond/RL68H

Tang et al. [44]

R141b-/δ-Al2O3/SDBS

Diao et al. [16]

Cu-R141b/SDBS

Boiling vessel of inside diameter of 30 mm and a height of 165 mm 150 × 200 mm2 quartz windows Horizontal copper flat surface with a diameter of 20 mm Horizontal copper flat surface with a diameter of 20 mm Boiling vessel of inside diameter of 50 mm and a height of 95 mm.

Horizontal copper flat surface with a diameter of 20 mm Horizontal copper flat surface /

Copper, horizontal, flat, and rectangular-finned surface (the overall height and tip-width of a fin were 0.76 mm and 0.36 mm, respectively) Copper, horizontal, flat, and rectangular-finned surface (height and tip-width of a fin were 0.76 mm and 0.36 mm) Horizontal flat square copper surface Flat surface under atmospheric pressure

The experimental findings revealed that the application of nanoparticles in the R134a/polyolester lubricant (RL68H) mixture enhances boiling performance up to 113%.

Nanolubricant mass fraction of 0.5% gave a 98% enhancement in boiling heat transfer as compared to R134a/polyolester while the enhancement was 19% at 2% nanolubricant mass fraction. R141b/δ-Al2O3 with SDBS improved the pool boiling heat transfer compared with pure R141b. The enhancement ratio of the Cu-R141b–SDBS nanorefrigerant was different from that of the R141b–SDBS solution compared with pure R141b.

[49] for the same host refrigerant R141b with different nanoparticles and concentrations. They have also found that at higher particle concentrations, the effect of pressure on boiling heat transfer coefficients was less than that at lower concentrations. Fig. 3 showed the variation of boiling heat transfer with pressure for the boiling of 0.05 vol.% particle concentration. At a given heat flux, smaller differences in heat transfer coefficients are found among the various pressures. No further analysis of the mechanisms on why the heat transfer was deteriorated was given in their study. It should be realized that the nanoparticle types, sizes and concentrations may significantly affect the results. Especially the nanoparticle concentrations used in their study are extremely low, which

might not be the reason for the heat transfer deterioration but the heat transfer trend with the nanoparticle concentrations was contradictory to that of Yang and Liu [49]. Ding et al. [17] studied migration characteristics of nanoparticles in the Cu-R113 nanofluid pool boiling process experimentally and numerically. Their experimental results showed that the migrated mass of nanoparticles in the pool boiling process of both nanorefrigerants increased with increasing the original mass of nanoparticles and the mass of refrigerant, and the migration ratio decreased with increasing volume fraction of nanoparticles. The migrated mass of nanoparticles and migration ratio in the nanorefrigerant were larger than those in

Table 3 Summary of studies on flow boiling of nanorefrigerants [8]. Literature

Nanofluid/nanoparticle

Test section/material

Conclusions

Park et al. [32]

Al2O3-R134a, Cu-R134a, CNTs-R134a and SiO2-R134a, 0.5 vol.% R134a/POE/CuO/0.5 wt.%, 1 wt.% and 2 wt.%

Single horizontal circular tube/copper Single horizontal circular tube/copper

CuO-R113/0.1 wt.%, 0.2 wt.% and 0.5 wt.% CuO-R113/0.1 wt.%, 0.2 wt.% and 0.5 wt.% SiO2-R134a and SiO2-R134a/polyolester mixture/0.05 vol.%, 0.08% and 0.5 vol.% R134a/POE/CuO/0.5 wt.%, 1 wt.% and 2 wt.%. Cu-R141b, Al-R141b, Al2O3-R141b, and CuO-R141b

Single horizontal circular tube/copper Single horizontal circular tube/copper Single horizontal circular tube/copper

A noticeable decrease in heat transfer coefficient for a mixture of silica/R134a was observed. For a 0.5 wt.% nanolubricant with R134a, no apparent effect on flow boiling heat transfer was observed while for a 1 wt.% and 2 wt.% nanolubricants with R134a, significant enhancement of heat transfer was observed. Heat transfer enhancement was observed with a maximum enhancement of 29.7%

Bartelt et al. [5]

Peng et al. [39] Peng et al. [40] Henderson et al. [18] Sun and Yang [42]

Horizontal pipe

The two phase frictional pressure drops increased with a maximum increase of 20.8%. Heat transfer coefficient decreased for SiO2-R134a (up to 55%). Nanoparticles had an insignificant effect on the flow pressure drop of R134a/POE/CuO Cu-R141b nanorefrigerant had the biggest average heat transfer coefficient, followed by Al-R141b. Al2O3-R141b had a lower heat transfer coefficient than Al-R141b and CuO-R141b had the minimum heat transfer coefficient.

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Table 4 Summary of studies on condensation of nanorefrigerants [8]. Literature

Nanofluid/nanoparticle Research contents

Wang et al. [46]

Ni-R410A/Ze-GLES68 lubrication oil/1 wt.%

M.A. Akhavan-Behabadi R600a, R600a/oil and et al. [1] R600a/oil/CuO

Conclusions

The effect of nanoparticles on Analysis of nanoparticle behavior between lubrication oil and refrigerant and their role condensation in an air-conditioner on heat transfer enhancement and flow regimes for forced convection condensation in a tube. The maximum heat transfer augmentation was observed for nanorefrigerant with 1.5% Nanorefrigerant flow during mass fraction; 83% higher heat transfer rate compared to pure-refrigerant fluid flow at condensation inside a the same experimental conditions. horizontal smooth tube

the nanorefrigerant–oil mixture. They proposed a numerical model, which can qualitatively well predict the migrated mass of nanoparticles. Yang and Liu [49] conducted an experimental investigation on nucleate pool boiling heat transfer performance of refrigerant R-141b with and without nano-sized Au particles on a horizontal plain tube. For R-141b with 0.09 vol.% nanoparticles, there is no significant effect on pool boiling heat transfer performance for such a low concentration of nanoparticles. At particle concentration of 1.0 vol.%, the heat transfer coefficient was more than twice higher than those without nanoparticles. The addition of nanoparticles significantly increased R-141b boiling heat transfer behavior. Their results agreed with those by Wen and Ding [48] but were in contradiction to those of Das et al. [15] and Bang and Chang [4] who observed decrease of boiling heat transfer coefficients due to the presence of nanoparticles. Furthermore, they repeated their measurements of heat transfer coefficients four times with the intervals of every 5 days. It showed that the measured boiling heat transfer coefficients decreased for each test and finally close to those without nanoparticles. They attributed trapped particles on the surface and reduced the number of activation nucleation sites. The SPM investigation showed that the test tube surface roughness decreased from 0.317 mm before boiling test to 0.162 mm after the test. Further investigation by TEM and dynamic light scattering particle analyzer showed that the nanoparticles aggregated from the size of 3 nm before the test to 110 nm after the test. From their study, it can be concluded that the nanosized Au particles are able to significantly increase pool boiling heat transfer of refrigerant R-141b on a plain tube surface. However, tube surface roughness and particle size changed after boiling test. Both of these effects degrade the boiling heat transfer coefficients. Peng et al. [33] investigated the effect of Cu-R113 nanofluid surfactants on a horizontal flat test surface on nucleate pool boiling heat transfer. Three types of surfactants including sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB) and sorbitan monooleate (Span-80) were used in their experiments. The surfactant concentrations were from 0 to 5000 ppm, and nanoparticle concentrations were 0.1 wt.%, 0.5 wt.% and 1.0 wt.%. It can be seen that Cu nanoparticles

can enhance the nucleate boiling heat transfer of R113. The heat transfer coefficient increased with increasing the nanoparticle concentration. This was consistent with the results of Yang and Liu [49]. Furthermore, for different surfactants, the maximum heat transfer enhancement occurred at a different concentration. For R113 with SDS, the maximum enhancement occurred at a concentration of 2000 ppm, for R113 with CTAB, the maximum enhancement at a concentration of 500 ppm and for R133 with Span-80, the maximum enhancement occurred at a concentration of 1000 ppm. Their experimental results indicated that surfactants enhance the nucleate pool boiling heat transfer of nanorefrigerant on most conditions, but deteriorated the nucleate pool boiling heat transfer at high surfactant concentrations. The ratios of nucleate pool boiling heat transfer coefficient of nanorefrigerant with surfactant to that without surfactant were in the ranges of 1.12– 1.67, 0.94–1.39, and 0.85–1.29 for SDS, CTAB and Span-80, respectively. At a fixed surfactant concentration, the SER increased with the decrease of the nanoparticle concentration. They proposed a nucleate pool boiling heat transfer correlation for nanorefrigerant with surfactant. From the aforementioned studies on nucleate pool boiling of nanorefrigerant, it can be seen that the nanoparticle type, the concentration, the surface roughness and the material and the host fluid type have effects on the nucleate pool boiling heat transfer. Nucleate boiling heat transfer enhancement, deterioration or no effect with the presence of nanoparticles has been observed. It should also be realized that surfactants are often used to stabilize the nanofluids and they have a significant effect on the boiling heat transfer behaviors but most studies do not mention the effect except for the study of Peng et al. [33–35,37]. Cheng et al. [10] presented a comprehensive review on the studies of boiling phenomena of aqueous surfactants, which may be a good reference for the relevant research. It should be realized that the combined effects of a nanoparticle and a surfactant on the boiling heat transfer are unclear so far. This made it difficult to explain the experimental results and explore the heat transfer mechanisms. However, on the other hand, without a surfactant, it may be difficult to maintain the suspension of the nanoparticles stable in a fluid. A number of researchers did not mention if surfactants were used in the nanofluids in their studies. If a commercial nanofluid was used,

Fig. 1. Boiling heat transfer coefficients of R134a with 1 vol.% CNTs [29].

Fig. 2. Nucleate pool boiling heat transfer of TiO2-R141b nanofluid at 300 kPa [45].

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Fig. 3. Variation of boiling heat transfer with pressure for the boiling of 0.05 vol.% particle concentration [45].

apparently the effect of the surfactant was ignored. This may be the case of most studies. Furthermore, quite different results may be due to the experimental systems used. Peng et al. [34,35] carried out an experimental study in order to identify the impact of the type and size of nanoparticles, type of the refrigerant, lubricating oil's mass fraction (RB68EP), heat flux, and initial liquid-level height on the displacement of nanoparticles during pool boiling. In the study, R113, R141b and n-pentane were used as refrigerants. Cu, Al, Al2O3, and CuO nanoparticles having different average diameters were utilized as nanoparticles. The findings revealed that nanoparticles with a smaller size show a higher migration ratio. At a given nanoparticle size, the displacement ratio of Al nanoparticles was higher than other nanoparticles. They concluded that by reducing the particle density, dynamic viscosity, oil's mass fraction, and heat flux, the migration ratio of nanoparticles throughout the pool boiling of nanorefrigerant improves. However, decreasing the refrigerant's liquid-phase density or initial liquid-level height worsens the migration ratio. 2.1.2. Studies on nucleate pool boiling with lubricants The influence of CuO nanoparticles on the boiling performance of R134a/polyolester mixtures on a roughened, horizontal, flat surface has been experimentally studied by Kedzierski and Gong [25]. A lubricant based nanofluid (nanolubricant) was made with a synthetic ester and 30 run diameter CuO particles stably suspended in the mixture to a 4% volume fraction. For the 0.5% nanolubricant mass fraction, the nanoparticles caused a heat transfer enhancement relative to the heat transfer of pure R134a/polyolester (99.5/0.5) of between 50% and 275%. A smaller enhancement was observed for the R134a/nanolubricant (99/1) mixture, which had a heat flux that was on average 19% larger than that of the R134a/polyolester (99/1) mixture. Further increase in the nanolubricant mass fraction to 2% resulted in a still smaller boiling heat transfer improvement of approximately 12% on average. Consequently, significant refrigerant/lubricant boiling heat transfer enhancements are possible with nanoparticles. Thermal conductivity measurements and a refrigerant\lubricant mixture pool-boiling model were used to suggest that increased thermal conductivity was responsible for only a small portion of the heat transfer enhancement due to nanoparticles. Further research with nanolubricants and refrigerants is required to establish a fundamental understanding of the mechanisms that control nanofluid heat transfer. Boiling characteristics of refrigerant/lubricant mixture are very important in refrigeration and air-conditioning and heat pump systems. Lubricants have a great effect on the nucleate pool boiling heat transfer characteristic. In general, with addition of a lubricant, the nucleate pool

201

Fig. 4. Nucleate pool boiling curve of R134a/RL68H mixture on a plain surface [26].

boiling heat transfer deteriorates as shown in Fig. 4 [26]. However, with nanolubricant or nanorefrigerants with lubricants, different results may be obtained. In this section, the review of boiling with nanorefrigerants and lubricants or refrigerant and nanolubricant mixture is presented. It should be pointed out that the use of lubricants makes the base fluid a binary mixture. Boiling of a mixture has specific behavior showing, in principle, a degradation of the heat transfer with respect to the deal trend. However, in general, the oil content is very small in refrigeration system. In this case, most studies deal with refrigerant/oil mixture boiling using the saturation temperature of refrigerant but with the physical properties of the mixture. Kedzierski and Gong [26] researched the effect of CuO nanoparticles on the boiling characteristics of R134a/CuO/RL68H, refrigerant/ nanolubricant mixture for 0.5%, 1%, and 2% mass fractions, on a roughened, horizontal flat surface. At 0.5% mass fraction of nanolubricant, CuO nanoparticles enhanced the heat transfer between 50 and 275% as compared to the pure R134a/polyolester. With the usage of R134a/ nanolubricant blend, boiling heat transfer enhanced 19% on average as compared to the R134a/polyolester blend, and for R134a/nanolubricant blend, enhancement in heat flux was 12% on average. As a conclusion, the enhanced thermal conductivity of the mixture of CuO and lubricant led to a 20% increase in boiling heat transfer. Secondary nucleation and particle mixing might additionally be considered as other influences on the enhancement of boiling heat transfer. Ding et al. [17] performed an investigation on the influence of the original mass of nanoparticles and the mass of refrigerants on the moving characteristics of CuO nanoparticles in the pool boiling of R113/CuO nanorefrigerant and R113/CuO/RB68EP nanorefrigerant– oil blend for 0.0912%, 0.183%, and 1.536% volume concentrations at ambient conditions, experimentally and numerically. The developed numerical model was able to predict the effect of the original mass of nanoparticles on the displacement mass of nanoparticles in the refrigerant and refrigerant–oil blend with10.5% and 7.7% average deviation from the experimental results, respectively. In addition, it was able to estimate the impact of the refrigerant mass on the mass displacement of nanoparticles in the refrigerant and refrigerant–oil blend with a 32.2% and 38.4% average deviation of experimental results, respectively. More recently, Peng et al. [34,35] investigated the influences of nanorefrigerant composition and heating condition on the migration of nanoparticles during pool boiling. The nanoparticles included Cu (average diameters of 20, 50 and 80 nm), Al and Al2O3 (average diameters of 20 nm), and CuO (average diameter of 40 nm). The refrigerants included R113, R141b and n-pentane. The mass fraction of lubricating oil RB68EP was from 0 to 10 wt.%. Their experimental results showed that the migration ratio of nanoparticles during the pool boiling of

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nanorefrigerant increased with decreasing the nanoparticle density, nanoparticle size, dynamic viscosity of refrigerant, mass fraction of lubricating oil or heat flux while increased with increasing the liquidphase density of refrigerant or initial liquid-level height. Similar to the nanorefrigerants without lubricants, they developed a new model reflecting the influences of nanorefrigerant composition and heating condition on the migration characteristics of nanoparticles. Kedzierski [21] studied the effect of CuO nanoparticles in CuO/RL68H mixtures; however, CuO nanoparticle concentration was 0.5 vol.%, 1 vol.%, and 2 vol.% in the nanolubricants. Results showed that R134a/ nanolubricant mixtures with 1 vol.% CuO nanoparticles have a larger heat flux than R134a/CuO/RL68H blend with 2 vol.% CuO nanoparticles for all nanolubricant volume fractions. Finally, it was realized that the enhancements in the boiling heat transfer were related to nanoparticle interactions. Peng et al. [38] performed an experimental study with the aim of determining the heat transfer characteristics of R113/VG68 oil with diamond nanoparticles during nucleate pool boiling. In the study, nanoparticle concentrations in the R113/diamond/VG68 oil mixture were 0.05 wt.%, 0.1 wt.%, 0.15 wt.%, 0.25 wt.%, 0.3 wt.%, 0.45 wt.%, 0.5 wt.%, and 0.75 wt.%. They observed that the usage of diamond nanoparticles in the R113/VG68 oil mixture increased the heat transfer up to 63.4%. This enhancement was mainly due to the improvements on the fluid properties and modifications on the heating surface, although relative ratios of these effects did not specify. The enhancing effect of diamond nanoparticles/oil blend was more than 20% higher than CuO nanoparticles/oil blend. Peng et al. [36] did experiments to investigate the nucleate pool boiling heat transfer of nanorefrigerants having carbon nanotubes (CNTs). In the study, they used four different shapes of CNTs. TEM images of the CNTs are given in Fig. 5. The R113/CNT/oil mixtures were prepared in mass fractions of 0.1 wt.%, 0.2 wt.%, 0.3 wt.%, 0.6 wt.%, and 1.0 wt.%. The results showed that R113/oil blend having CNTs increased heat transfer coefficient up to 61% as compared to the R113/oil blend however this was mainly due to the reduction in surface tension which resulted in increased bubble frequency and led to an enhanced heat transfer rate. According to the experimental results, CNTs with a higher length and smaller outer diameter increased the heat transfer coefficient. Kedzierski [22] examined the pool boiling behavior of R134a/Al2O3/ polyolester nanorefrigerant on a rough and horizontal flat surface. In the study, the polyolester lubricant had three different mass fractions (0.5%, 1%, and 2%) and 1.6% volume fraction of the Al2O3 nanoparticle in nanorefrigerant was considered. For all mass fractions of lubricant, the heat transfer improvement was observed with the usage of Al2O3 instead of the R134a/polyolester mixture. Also it is found that the heat transfer surface was performed better by aging as its surface modified in that period. According to this model, nanorefrigerants that have a higher volume fraction and smaller size are good options to enhance the heat transfer. Peng et al. [37] performed experiments to investigate the effects of nanoparticle size during nucleate pool boiling of refrigerant R113, ester oil VG68, and Cu nanoparticles having varying sizes. The nanoparticle concentrations were 0.1 wt.%, 0.2 wt.%, 0.3 wt.%, 0.6 wt.%, and 1.0 wt.%. The findings revealed that the use of nanoparticles with an average diameter of 20 nm results in a better heat transfer performance (up to 23.8%) as compared to the nanoparticles having diameters of 50 nm and 80 nm. The rise of nanoparticle proportions in the nanoparticles/ oil blend improved the heat transfer. Also, by regarding the effects of the nanoparticle type and size, a general correlation was presented to estimate the heat transfer coefficient in nucleate pool boiling. Saidur et al. [41] and Alawi et al. [2] presented the effects of nanolubricants on boiling and two-phase flow phenomena. The mineral lubricant with different nanoparticles was used to investigate the lubrication and heat transfer performance. Under these conditions, the power consumption was reduced by about 2.4%, and the coefficient of

Fig. 5. Heat transfer enhancement as a function of mass flux for (a) 1% nanolubricant and (b) 2% nanolubricant [5].

performance was increased by 4.4%. The refrigerator's performance was found to be 26.1% better with 0.1% mass fraction of TiO2 nanoparticles compared to a refrigerator's performance with the HFC134a and POE oil system. In another study, Kedzierski [23] used a rectangular finned surface with the aim of investigating the effects of Al2O3 nanoparticles on the pool boiling characteristics of the R134a/polyolester lubricant (RL68H) mixture. The test section was copper, horizontal, flat, and rectangularfinned surface (the overall height and tip-width of a fin were 0.76 mm and 0.36 mm, respectively). The volume fractions of Al2O3 particles in the nanolubricant were 1.0%, 2.3%, and 3.6%. The experimental findings revealed that the application of nanoparticles in the R134a/polyolester lubricant (RL68H) mixture enhances boiling performance up to 113% on a rectangular finned surface. Also, the author remarked that the enhanced surfaces require higher quantities of nanoparticles in order to reach the same heat transfer enhancement as compared to smooth surfaces. Kedzierski [24] studied the effects of diamond nanolubricant on the heat transfer of R134a pool boiling. R134a/diamond/RL68H mixtures were prepared at 0.5%, 1%, and 2% diamond/RL68H (2.6 vol.%/97.4 vol.%) mass fractions. The results showed that the use of 0.5% nanolubricant mass fraction leads to a 98% enhancement in boiling heat transfer as

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compared to R134a/polyolester while the enhancement was 19% where the nanolubricant mass fraction was 2%. Tang et al. [44] researched the influences of nanoparticle and the surfactant concentrations on pool boiling of R141b/δ-Al2O3 for 0.001%, 0.01% and 0.1 vol.% volume fractions. R141b as pure refrigerant and R141b/δ-Al2O3 at three different volume fractions with sodium dodecyl benzene (SDBS) at three ratios as mole number of surfactant were used in the tests. The results indicated that R141b/δ-Al2O3 with SDBS improved the pool boiling heat transfer compared with pure R141b. However, adding surfactant corrupted the heat transfer at 0.001 vol.% of Al2O3 and the pool boiling heat transfer of the nanorefrigerant without SDBS corrupted at 0.1 vol.% of Al2O3 compared with R141b/δ-Al2O3 with SDBS because of accumulation of the nanoparticles. The pool boiling heat transfer and critical heat flux (CHF) of the Cu-R141b–SDBS nanorefrigerant and SDBS–R141b solution were experimentally investigated by Diao et al. [16] on a flat surface under atmospheric pressure. The nanorefrigerant concentrations were 0.008 vol.%, 0.015 vol.%, and 0.05 vol.%. Pure R141b was also boiled on the nanorefrigerant-deposited surface to isolate the effect of the fluid properties. Experimental results showed that the Cu-R141b–SDBS nanorefrigerant can enhance the pool boiling heat transfer. The enhancement ratio of the Cu-R141b–SDBS nanorefrigerant was different from that of the R141b–SDBS solution compared with pure R141b. The CHF of the nanorefrigerant on the bare surface was lower than that of pure R141b on the deposited surface. 2.2. Studies on flow boiling of nanorefrigerants Limited studies in the literature concerning flow boiling of nanorefrigerants so far showed contradictory experimental results. Table 3 presented a summary of the available studies of flow boiling of nanorefrigerants. Park et al. [32] investigated flow boiling of SiO2/R134a nanorefrigerant in a horizontal tube. Their experimental data showed a noticeable decrease in the heat transfer coefficient for the mixture of SiO2 nanoparticles and R134a in comparison to the base fluid R134a data. Bartelt et al. [5] examined the heat transfer effects of CuO nanoparticles on R134a/POE mixtures in horizontal flow boiling conditions. Although a 0.5% mass fraction of nanoparticles showed no serious effect on the heat transfer rate, a 1% mass fraction mixture showed 42–82% improvement while a nanoparticle fraction of 2% showed 50–101% (Fig. 5). The reasons behind this large improvement were not understood clearly; however, additional nucleation sites formed by nanoparticles were argued to be one of the main factors. It is also noted that the saturation temperatures, found to be higher with nanofluid mixtures, and pressure drop differences are insignificant when compared with the base fluid. Peng et al. [39] examined the influence of the concentration of CuO nanoparticles on the heat transfer performance of R113-based nanorefrigerant flow boiling in a horizontal roughness pipe experimentally for 0.1 wt.%, 0.2 wt.%, and 0.5 wt.% nanoparticle concentrations, and a correlation was developed to predict the heat transfer enhancement of the R113/CuO nanorefrigerant. It was seen that dispersing nanoparticles in R113 pure refrigerant enhanced the flow boiling heat transfer of nanofluid, and the maximum enhancement of the heat transfer coefficient was 29.7%. Reduction in boundary layer height and nanoparticle surface modification was offered as the possible reasons for the observed improvement ratio. The correlation was able to predict the heat transfer coefficient experimental results in the range of ±20%. Peng et al. [40] also measured the two phase frictional pressure drop of the CuO-R113 flow boiling and have also found that the pressure drops of the nanorefrigerant were larger than those of the pure R113 flow boiling with a maximum increase of 20.8%. They also proposed a correlation for predicting the two phase frictional pressure drops. Henderson et al. [18] studied the heat transfer performance of R134a and R134a/polyolester mixtures with nanoparticles during boiling flow

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conditions in a horizontal tube. The R134a/SiO2 nanorefrigerants of volume concentrations of 0.5% and 0.05% were tested to determine the influences of nanoparticles on boiling heat transfer. For both volume concentrations, the convective boiling heat transfer coefficient decreased as compared to pure R134a because of poor dispersion. Also, they conducted experiments with R134a/POE mixtures having CuO nanoparticle volume fractions of 0.02%, 0.04%, and 0.08%. It was observed that R134a/CuO/POE nanorefrigerant at a volume fraction of 0.02% presented a slight enhancement in the heat transfer performance. A CuO volume fraction of 0.04% and 0.08% caused an average heat transfer enhancement of 52% and 76%, respectively. Further investigation revealed that this increase in heat transfer was not only due to thermal property changes, but also surface modifications caused by CuO particles. The authors also mentioned the possible secondary nucleation sites because of the presence of nanoparticles. It should be realized that the test mass fluxes were typically low in the studies of Peng et al. [39,40] and thus their proposed correlation had great limitation not only because of the limited test conditions but also because of the complex phenomena of flow boiling of nanofluids which have not yet well been understood. Furthermore, flow regimes should be observed to understand heat transfer and two phase flow mechanisms due to the effect of nanoparticles. No such observation is available so far. Overall, compared to the studies of nucleate pool boiling of nanorefrigerants, studies of flow boiling and two-phase flow phenomena of nanorefrigerants were very limited. There were also contradictory results from the available studies. Considering the very complex phenomena involved in nanorefrigerant two phase flow and flow boiling processes, it was obvious that more fundamental experimental studies were needed to understand the basic aspects of flow boiling and two phase flow phenomena such as flow regimes and heat transfer mechanisms. Thus, it was not the right time to develop correlations based on limited data before the fundamental mechanisms were understood. Especially flow regimes should be observed in connection to the corresponding two phase heat transfer and pressure drop behaviors and mechanisms [9,11–14]. The prediction methods should also be based on the observed flow regimes and mechanisms. The lubricant effect should also be focused on as it exists in the practical refrigeration systems [3]. Sun. and Yang [42] studied Cu-R141b, Al-R141b, Al2O3-R141b, and CuO-R141b as nanorefrigerants for 0.1 wt.%, 0.2 wt.%, and 0.3 wt.% mass fractions in a computer-aided test section to research the influences of material type and vapor quality on the flow boiling heat transfer in a horizontal pipe. As the same mass fraction, Cu-R141b nanorefrigerant had the biggest average heat transfer coefficient, followed by Al-R141b. Al2O3-R141b had a lower heat transfer coefficient than Al-R141b and CuO-R141b had a minimum heat transfer coefficient.

2.3. Studies on condensation of nanorefrigerants Table 4 showed the only study which focused on the effect of nanoparticles on condensation of R410A by Wang et al. [46]. The Ni nanoparticles were mixed with Ze-GLES68 lubrication oil. The mass of Ni nanoparticle was 1%. The effect of nanoparticles on condensation in an air-conditioner was investigated. It has been found that fluid subcooling with nanoparticles was larger than that without nanoparticles by 1.8 °C. The nanoparticle behavior between the lubrication oil and refrigerant, the role on heat transfer enhancement and the flow regimes for forced convection condensation in a copper tube was discussed. So far, no study on condensation of nanofluids inside channels was available in the literature, but this would be an interesting topic in the future as it was applied the refrigeration systems. Similar to flow boiling research, systematic experiments are needed to achieve accurate experimental data. Flow regimes should also be observed. Furthermore, the relevant heat transfer mechanisms and models should be developed in the

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long run when enough accurate experimental data and observations have been achieved. M.A. Akhavan-Behabadi et al. [1] carried out an experimental study on heat transfer characteristics of a nanorefrigerant flow during condensation inside a horizontal smooth tube. Experiments were conducted for three different working fluid types including: (i) pure refrigerant (R600a); (ii) refrigerant/lubricant (R600a/oil); and (iii) nanorefrigerant: refrigerant/lubricant/nanoparticles (R600a/oil/CuO). Polyolester oil (POE) was utilized as the lubricant in the two latter cases. In addition, nanorefrigerants (R600a/oil/CuO) were prepared by dispersing CuO nanoparticles with different mass fractions of 0.5%, 1% and 1.5% in the baseline mixture (R600a/oil). The implemented experiments covered a wide range of variables including: (i) mass fluxes from 154.8 to 265.4 kg/m2/s; (ii) vapor qualities between 10% and 80%; (iii) heat flux from 17 to 20 kW/m2; and (iv) condensation pressure from 5.1 to 6.2 bar. It was shown that significant heat transfer enhancement was achieved by adding nanoparticles to the baseline mixture and pure refrigerant. The maximum heat transfer augmentation was observed for nanorefrigerant with 1.5% mass fraction and 83% higher heat transfer rate compared to pure-refrigerant fluid flow at the same experimental conditions. 3. Conclusions Nanorefrigerant boiling and two-phase flow study are still in its infancy. Many disagreements exist with numerous conflicting experimental results and trends. In general, nanorefrigerants were found to increase, decrease or have no effects on nucleate pool boiling and flow boiling but consistently to increase both with nanolubricants. 1. Physical properties such as fluid density and viscosity, surface tension and specific heat have a significant effect on nucleate pool boiling, convective flow boiling and condensation. To present the experimental results and to understand the physical mechanisms related to the two-phase and thermal phenomena, the nanorefrigerant physical properties should be systematically investigated to set up a consistent database of physical properties in addition to thermal conductivity. 2. Nucleate pool boiling heat transfer mechanisms should be further investigated. The inconsistencies between different studies should be clarified. Furthermore, the effect of nanoparticles size and type on heat transfer should be studied. The heat transfer mechanisms responsible for these trends should be identified and be able to explain why nucleate heat transfer may be enhanced or decreased. Data should also be segregated by fluids which deposit on the boiling surface and those that do not, in order to prove if the fluid alone can enhance performance. Lubricant effect should be further studied. 3. More experiments on nanorefrigerant two-phase flow, flow boiling and condensation should be conducted to evaluate the potential benefits of nanofluids. These should also include heat transfer performance, two-phase flow patterns and pressure drop in various types of channels. Especially, the two-phase flow and heat transfer characteristics should be related to the corresponding flow patterns via flow visualization. Lubricant effect should also be studied and the corresponding flow regimes should be observed. 4. Nanoparticle deposition or coating on the surface of the heat transfer is a big issue to be resolved. For example, if the paint is useful, then it can be applied more easily by using the coating process instead of the deposition of nanorefrigerant. If such a nanoparticle layer has adverse effects, then ways to prevent it are needed or the correct nanofluids should be found. 5. Predicting the effects of nanoparticle methods should be developed based on different flow systems and to consider the impact of lubricants. 6. There are many unresolved problems of boiling and two-phase of nanorefrigerants, therefore great efforts should be made.

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