The 0.5 wt.% multiwalled carbon nanotubes/water nanofluids (MWNFs) were produced by using a two-step synthetic method with different types and concentrations of stabilizers. The static position method, centrifugal sedimentation method, zeta potential measurements, and rheological experiments were used to assess the stability of the MWNFs and to determine the optimal type and fixed MWCNTs-stabilizer concentration of stabilizer. Finally, MWNFs with different concentrations of MWCNTs were produced using the optimal type and fixed concentration ratio of stabilizer, and their stability, thermal conductivity, and pH were measured to assess the feasibility of using them in heat transfer applications. MWNFs containing SDS and SDBS with MWCNTs-stabilizer concentration ratio were 5 : 2 and 5 : 4, respectively, showed excellent stability when they were evaluated by static position, centrifugal sedimentation, zeta potential, and rheological experiments at the same time. The thermal conductivity of the MWNFs indicated that the most suitable dispersing MWNF contained SDBS. MWNFs with MWCNTs concentrations of 0.25, 0.5, and 1.0 wt.% were fabricated using an aqueous SDBS solution. In addition, the thermal conductivity of the MWNFs was found to have increased, and the thermal conductivity values were greater than that of water at 25°C by 3.20%, 8.46%, and 12.49%.
Research on the use of nanofluids in heat transfer applications has been highly productive. Relevant studies pertained to nanofluids having high thermal conductivity, heat transfer coefficient or capacity, viscosity, pumping power, and pressure loss or pipeline friction factor [
The preparation of nanofluids containing different materials, dispersing methods, and stabilizers/pH control is summarized in Table
Summary of nanofluids preparation and evaluation.
Authors | Nanofluid materials | Dispersing equipment | Stabilizers/pH control | Stability and stability evaluation method |
---|---|---|---|---|
Duangthongsuk and Wongwises [ |
TiO2/water | Ultrasonic vibrator for 3-4 h | CTAB/— | Agglomeration was observed by TEM |
Lotfi et al. [ |
MWNTs (MWCNTs)/water | Ultrasonic bath for 60 min and magnetic stirrer for 3 hr | COOH functional groups/— | — |
Zhu et al. [ |
Al2O3/water | Ultrasonic bath for at least 1 h | SDBS 0.1 wt.%/pH = 8.0-9.0 | Zeta potential, static position method, and spectrometer |
Phuoc et al. [ |
MWCNTs/water | Ultrasonic processor for 10 min and magnetic stirrer for 20 min (repeated two times) | Chitosan/— | (i) Stable for months |
Kumaresan and Velraj [ |
MWCNTs/water-ethylene glycol (EG) mixture | Magnetic stirrer for 30 min, followed by ultrasonication for 90 min | SDBS 0.1 vol.%/— | (i) Stable for more than 3 months |
Teng et al. [ |
Carbon/water | Magnetic stirrer for 1 hr, homogenizer for 30 min, and ultrasonic liquid processor for 30 min (repeated five times) | Water-soluble chitosan/— | Static position method, spectrometer, and zeta potential |
Ding et al. [ |
MWCNTs/water | Ultrasonic bath for 24 hr, high shear homogenizer for 30 min | Gum Arabic (GA)/pH = 6 | (i) Very stable for months |
Peng et al. [ |
Cu/R113 (refrigerant-based) | Ultrasonic processor for 1 h | SDS, CTAB, Span-80/— | (i) Stable for more than 24 hr |
Yousefi et al. [ |
MWCNTs/water | Ultrasonic disruptor for 30 min | Triton X-100/pH = 3.5, 6.5, and 9.5 | (i) Stable for more than 10 days |
Raveshi et al. [ |
Al2O3/water-EG mixture | Ultrasonic bath for 4 hr and magnetic stirrer for 5 h | SDBS/— | (i) Stable for more than 3 days |
Li et al. [ |
CuO/water | Ultrasonic cleanser for at least 1 hr | TX-10, CTAB, and SDBS/pH = 9.5 | Static position method and spectrometer |
Nieh et al. [ |
Al2O3 and TiO2/water-EG mixture | Magnetic stirrer for 1.5 hr, homogenizer for 30 min, and ultrasonic liquid processor for 30 min (repeated five times) | Water-soluble chitosan/— | Spectrometer |
Li et al. [ |
Cu/water | Ultrasonic bath for at least 15 min | SDBS 0.02 wt.%/pH = 8.5–9.5 | Zeta potential |
Hwang et al. [ |
Multiwalled carbon nanotubes (MWCNTs)/water | Ultrasonic disruptor for 2 hr | SDS/— | Visual observation |
Ho et al. [ |
Al2O3/water | Ultrasonic bath for at least 2 h | —/pH = 3 | visual observation |
Kathiravan et al. [ |
Cu/water | Ultrasonic bath for about 10 h | SDS/— | Agglomeration was observed by TEM |
Yousefi et al. [ |
Al2O3/water | Ultrasonic disruptor for 30 min | Triton X-100/— | Stable for three days |
Byrne et al. [ |
CuO/water | High intensity ultrasonic processor for 7-8 hr | CTAB/— | (i) Stable for more than 7 days |
Wang et al. [ |
Single-walled carbon nanotubes (SWNTs)/heavy water (D2O) | Ultrasonicated for 24 h | Triton X-100/— | Systematic small angle neutron scattering (SANS) |
Dong et al. [ |
CuPc-U (unsulfonated and hydrophobic) and CuPc-S (surface sulfonated and hydrophilic)/(water-NaNO3 mixture) | Ultrasonic bath for 30 min | Triton X-100/— | DLS measurement |
The static position method is the most commonly used method to evaluate the stability of nanofluids. The static position method leaves nanofluids in containers standing for a particular period, and the distance or color difference in sedimentation between nanofluids was observed by the naked eye [
The zeta potential (
Nanofluids may show solid-liquid separation in the flow state because of the density difference between the nanoparticles and the base liquid, thereby affecting the stability of the nanofluids. Therefore, a dynamic state can be applied to evaluate the stability of nanofluids, which in turn indicates the suitability of nanofluids for use in heat exchange applications in the long term. The preparation and evaluation of nanofluids is summarized in Table
Multiwalled carbon nanotubes (MWCNTs)/water nanofluids (MWNFs) with MWCNTs concentration of 0.5 wt.% and different stabilizers and stabilizer concentrations were produced using a two-step synthetic method. The static position method, centrifugal sedimentation method,
MWCNTs (Cheap Tubes Inc., USA) were used as nanoadditives to prepare the MWNFs. The external diameter, internal diameter, length, true density, specific surface area, and purity were approximately 20–30 nm, 5–10 nm, 10–30
FESEM images of MWCNTs.
The MWNFs were produced using a two-step synthetic method. First, an aqueous solution of the stabilizer was prepared by adding 1.6 wt.% of water-soluble chitosan (CH, (C6H11O4N)
Next, a base liquid with the required weight of MWCNTs was prepared by adding MWCNTs in several installments, which effectively reduced agglomeration by reducing the probability of MWCNTs between each other combining. A stirrer/hot plate (PC420D, Corning, USA) operating at 600 rpm with ultrasonic bath (400 W, D400H, TOHAMA, Taiwan) was used to disperse MWCNTs uniformly before more MWCNTs were added to the base liquid. After the concentration of MWCNTs was added to reach 0.5 wt.% and after using a homogenizer (YOM300D, Yotec, Taiwan) operating at 6000 rpm for 30 min, a stirrer/hot plate (PC420D, Corning, USA) operating at 600 rpm for 1.5 h, and an ultrasonic liquid processor (700 W, Q700, Qsonica, USA) operating for 30 min to ensure stable dispersion and suspension of MWCNTs in the base liquid, an intermittent oscillation process was used for the ultrasonic liquid processor (amplitude ratio: 25%; on/off duty: 10/10 s). These dispersal devices were used three times to effectively prevent the temperature in the dispersed equipment and nanofluid from increasing, thus achieving excellent dispersion.
Stabilizers affect the thermal properties and related physical and chemical properties of MWNFs [
The thermal conductivity, density, pH, electrical conductivity, and viscosity of the samples were measured using a thermal properties analyzer (KD2 Pro, Decagon Devices, USA; accuracy: ±5.0%), a pH meter (PH510, Eutech, Singapore; accuracy: ±1.0%), a liquid density meter (DA130, KEM, Japan; accuracy: ±1.0%), an electric conductivity meter (HTC-202U, HOTEC, Taiwan; accuracy: ±1.0%), and a resonant viscosity meter (Viscolite VL700HP T15-3, Hydramotion, England; accuracy: ±1.0%) in an isothermal unit (P-20, YSC, Taiwan), in which the sample was maintained at 25°C within an accuracy of ±0.5°C. To reduce measurement deviations, each parameter was measured six times. The four most concentrated measurements were then averaged and considered as the experimental value of the sample. Because the thermal properties analyzer showed high deviations, the thermal conductivity of each sample was measured 10 times for each experimental parameter, and the six most concentrated measurements were averaged as the experimental value of the sample.
The specific heat of the sample was measured using a differential scanning calorimeter (DSC, Q20, TA, USA) and a vapor compression refrigeration system (RCS40, TA, USA) in a high-purity nitrogen (5N) atmosphere. The temperature accuracy and calorimetric accuracy of the DSC were ±0.1°C and ±1%, respectively. The weight of samples was measured using precision electronic balance (0.01 mg/42 g, GR-202, A&D, Japan) and the weight of each sample was controlled at 12 ± 1 mg. The specific heat test method is a standard reference approach, and the standard reference was pure water [
The static position method is the most commonly used method to evaluate the stability of nanofluids. However, this method cannot evaluate the stability of nanofluids in heat exchange applications. Therefore, in this study, stability of nanofluids was determined using static and dynamic tests. First, we selected MWNFs with higher stability by using the static position method. The selected samples were then evaluated through centrifugal sedimentation, zeta potential measurement, and rheological tests to obtain the optimal stabilizer, and stabilizer concentration, and to evaluate the stability of MWNFs.
In the static position method, all samples were left standing for 30 days, and the difference in stability between samples was observed by the naked eye. However, when the difference was small, naked eye observation was difficult. Therefore, in the static position method, an ultraviolet-visible-near infrared (UV/VIS/NIR) spectrometer (V670, Jasco, Japan) was used at a wavelength of 800 nm to measure the absorbance of each sample initially and after 1 day, 7 days, and 30 days to determine the stability of each sample. Two milliliters of each sample was filled in a standard disposable cuvette (3.0 mL, Kartell, Italy). For measuring the absorbance changes in the sample, the central location of the optical path of the spectrometer was positioned 0.5 cm below the liquid level of the sample.
The samples selected using the static position method were centrifuged using a digital centrifuge (EBA 20, Hettich, Germany) at a speed of 5000 rpm for 30 min. The amount of samples centrifuged was 10 mL, and 0.7 mL of the sample was drawn using a quantitative pipette (DV-1000, HTL, Poland) from 1.0 cm below the level of the sample, and then to fill in a semi-micro disposable cuvette (7591-15, PlastiBrand, Germany). The UV/VIS/NIR spectrometer was used at a wavelength of 800 nm to measure the absorbance of samples and compare the stability of samples.
The zeta potential of the selected samples was measured using a particle size/zeta potential analyzer (SZ-100, Horiba, Japan). Because of the opacity of the MWNFs,
The rheological properties of the selected samples were determined by using a rheometer (DV3TLVCP, Brookfield, USA) with an accuracy of ±1.0%, and the sample temperature was controlled at 25°C by using an isothermal unit (HW401L, HILES, Taiwan). Structural damage and solid-liquid separation in nanofluids can be evaluated by considering thixotropic or rheopectic properties of the nanofluids. In Figure
Schematic of the rheological properties of fluids: (a) thixotropic properties versus shear rate, (b) thixotropic properties versus time, (c) rheopectic properties versus shear rate, and (d) rheopectic properties versus time.
Finally, a different test method was used to determine the stability and to select the optimal type and concentration of stabilizer. A fixed MWCNT-stabilizer concentration ratio (MWCNT : stabilizer, w/w) was determined for the different stabilizers. MWNFs with different concentrations of MWCNTs were produced with the optimal type and fixed concentration ratio of stabilizers, and their stability, thermal conductivity, and pH were tested to confirm the usefulness of the selected stabilizer for heat transfer applications.
The UV/VIS/NIR spectrometer was used with the static positioning and centrifugal sedimentation methods to determine the difference between the absorbance of MWNFs initially (
The rheometer was used to measure the viscosity in the acceleration and deceleration processes at different shear rates. The results were used to determine the shear rate dependence of the thixotropic or rheopectic properties, and the shear rate dependence in (
The rheometer was used to measure the difference between viscosities at the fixed shear rate before and after the test period. The results were applied to (
The experimental results obtained with water were used as baseline values (
Table
Characteristics of water and aqueous solutions of the stabilizers at 25°C.
Property | Water | Aqueous solution of stabilizers of 1.6 wt.% | |||||
---|---|---|---|---|---|---|---|
CH | AG | TX100 | SDS | SDBS | CTAB | ||
Specific heat (kJ/kg °C) | 4.182 | 4.383 | 3.992 | 4.009 | 3.640 | 4.000 | 4.283 |
Thermal conductivity (W/m °C) | 0.604 | 0.604 | 0.585 | 0.576 | 0.577 | 0.590 | 0.612 |
pH | 8.68 | 4.46 | 6.98 | 7.17 | 7.88 | 7.92 | 6.95 |
Electrical conductivity ( |
92.8 | 2200 | 3020 | 105.8 | 1680 | 1970 | 1047 |
Viscosity (mPa s) | 0.8 | 1.3 | 5.9 | 1.1 | 1.1 | 1.1 | 1.2 |
Density (kg/m3) | 996.83 | 999.30 | 1003.95 | 998.03 | 998.78 | 998.95 | 996.73 |
Stabilizers added to water reduced the pH of water, with the addition of CH leading to the maximum reduction in the pH of water (by 48.62%). Adding CH causes substantial decrease in pH because the water-soluble chitosan is the chitosan (insoluble in water) treated by organic acids. Therefore, adding CH to the water leads to a substantial decline in pH of the aqueous solution.
Adding stabilizers to water increased the electrical conductivity of water, and adding AG led to the maximum enhancement of the electrical conductivity of water (by 3154.31%). Except TX100 which is a nonionic stabilizer, the other stabilizers are anionic (SDS, SDBS, and AG) or cationic (CH and CTAB) stabilizers. The electrical conductivity of the water increases with the number of ions in the water. Therefore, adding an anionic or cationic stabilizer in water substantially increases the electrical conductivity of the water.
Stabilizers are added to water to increase the viscosity of water. In this study, AG addition showed the maximum enhancement of the viscosity of water (by 637.50%). AG is capable of absorbing 200–300 times its own weight in water to form a gum of high viscosity. Therefore, adding AG in the water will greatly increase the viscosity of water. Furthermore, CH is generated by the hydrolysis of chitosan to form carbohydrate with low degree of polymerization (
The effect of stabilizers on water density is unclear. This phenomenon is mainly due to minor differences of density between the stabilizer and water, and moreover, the added amount of stabilizers is small. However, AG addition led to the maximum enhancement of the density of water (by 0.71%).
High viscosity results in a pressure drop in the pipeline in the heat exchange system; therefore, AG is not suitable for producing MWNFs that can be used as heat exchange working fluids. Furthermore, adding CH to water significantly lowers the pH of water; therefore, when CH is used to produce MWNFs that can be used as heat exchange working fluids, the material of the heat exchange system must be chosen carefully to avoid corrosion problems.
Table
Characteristics of MWNFs containing different types and concentrations of stabilizer at 25°C.
Property | Stabilizer | Stabilizer concentration (wt.%) | ||||
---|---|---|---|---|---|---|
0.05 | 0.1 | 0.2 | 0.4 | 0.8 | ||
Viscosity |
CH | 1.2 | 1 | 0.9 | 1 | 1.1 |
AG | 1.2 | 1.2 | 1.4 | 1.9 | 3.0 | |
TX100 | 0.9 | 0.9 | 0.9 | 0.9 | 1.0 | |
SDS | 0.9 | 0.9 | 0.9 | 0.9 | 1.0 | |
SDBS | 0.9 | 0.9 | 0.9 | 0.9 | 1.0 | |
CTAB | 0.9 | 0.9 | 0.9 | 0.9 | 0.9 | |
|
||||||
pH | CH | 6.51 | 4.52 | 4.32 | 4.24 | 4.22 |
AG | 7.87 | 7.83 | 7.76 | 7.75 | 7.69 | |
TX100 | 7.62 | 7.63 | 7.64 | 7.32 | 7.12 | |
SDS | 7.32 | 7.34 | 7.41 | 7.74 | 7.83 | |
SDBS | 7.62 | 7.71 | 7.72 | 7.72 | 7.91 | |
CATB | 7.32 | 7.42 | 7.44 | 7.48 | 7.52 | |
|
||||||
Electrical conductivity ( |
CH | 225 | 309 | 511 | 820 | 1410 |
AG | 390 | 539 | 803 | 1238 | 1970 | |
TX100 | 201 | 196 | 134 | 125 | 126 | |
SDS | 343 | 430 | 567 | 885 | 1130 | |
SDBS | 330 | 357 | 402 | 610 | 1120 | |
CATB | 196 | 216 | 263 | 375 | 608 |
Figure
Photograph of experimental results of the static position method after 30 days.
Absorbance difference ratio among MWNFs in the static position experiment for different types and concentrations of stabilizers after 1 day.
Absorbance difference ratio among MWNFs in the static position experiment for different types and concentrations of stabilizers after 7 days.
Absorbance difference ratio among MWNFs in the static position experiment for different types and concentrations of stabilizers after 30 days.
Figure
Absorbance difference ratio among the MWNFs for the centrifugal sedimentation method for different types and concentrations of stabilizers.
Figure
Zeta potential of the MWNFs for different types and concentrations of stabilizers.
Figures
Viscosity of MWNFs containing CH at different shear rates.
Viscosity of MWNFs containing SDS at different shear rates.
Viscosity of MWNFs containing SDBS at different shear rates.
Figures
Viscosity of MWNFs containing CH at a specific shear rate during the test period.
Viscosity of MWNFs containing SDS at a specific shear rate during the test period.
Viscosity of MWNFs containing SDBS at a specific shear rate during the test period.
Table
Results for the stability of MWNFs.
Stabilizer | Parameters | Stabilizer concentration (wt.%) | ||||
---|---|---|---|---|---|---|
0.05 | 0.1 | 0.2 | 0.4 | 0.8 | ||
CH |
|
−80.27 | −60.71 | −42.85 | −33.44 |
|
|
−88.60 | −73.91 | −56.29 | −56.92 |
|
|
|
38.0 | 45.1 |
|
44.0 | 40.0 | |
|
4.26 |
|
3.44 | 1.96 | 8.10 | |
|
3.29 | 1.99 | 1.57 | 1.60 | 2.26 | |
|
236.96 | 103.32 | 60.74 | 63.43 | 130.75 | |
|
3.12 | 2.78 | 3.45 |
|
2.89 | |
|
||||||
SDS |
|
−78.34 | −59.09 | −11.04 |
|
−10.99 |
|
−89.46 | −83.59 |
|
−58.17 | −54.44 | |
|
−63.0 | −61.7 | −65.0 | −76.3 |
|
|
|
0.76 | 4.55 |
|
3.34 | 1.78 | |
|
1.45 | 1.74 | 1.10 | 1.51 | 2.04 | |
|
48.72 | 78.45 | 12.47 | 54.48 | 108.95 | |
|
1.03 | 6.70 |
|
0.54 | 0.77 | |
|
||||||
SDBS |
|
−73.60 | −25.98 | −23.41 |
|
−10.98 |
|
−84.32 | −63.15 | −56.62 | −57.71 |
|
|
|
−54.9 | −69.5 |
|
−78.9 | −71.0 | |
|
0.65 | 16.15 | 5.05 |
|
1.55 | |
|
2.02 | 2.30 | 2.26 | 1.07 | 4.49 | |
|
106.39 | 135.23 | 130.75 | 9.78 | 359.14 | |
|
0.64 | 2.09 | 0.78 |
|
3.55 | |
|
||||||
Water | DRT | 0.07 | ||||
|
0.98 | |||||
RSD (%) | 0.46 |
Viscosity (
Generally, the concentration of the added stabilizer should increase with the MWCNTs concentration, and it is possible to fix the MWCNTs-stabilizer concentration ratio at a specific value. The optimal ratios of the MWCNTs concentration to the stabilizer concentration for SDS and SDBS were 5 : 2 (0.5 wt.% MWCNTs : 0.2 wt.% SDS) and 5 : 4 (0.5 wt.% MWCNTs : 0.4 wt.% SDBS), respectively, which showed excellent stability when they were evaluated by static position, centrifugal sedimentation, zeta potential, and rheological experiments at the same time. The optimal concentration ratios of MWCNTs-SDS and MWCNTs-SDBS were used to prepare MWNFs with different MWCNTs concentrations (0.25, 0.5, and 1.0 wt.%), and these MWNFs were used to conduct subsequent experiments pertaining to static positioning, thermal conductivity, and pH to identify the optimal stabilizer and confirm that the use of the optimal concentration ratio leads to MWNFs with excellent and stable performance.
Figure
Absorbance difference ratios among the MWNFs containing SDS and SDBS in the static position experiment for different MWCNTs concentrations.
Figure
Thermal conductivity of the MWNFs containing SDS and SDBS at different MWCNTs concentrations at 25°C.
Figure
Values of pH of the MWNFs containing SDS and SDBS at different MWCNTs concentrations at 25°C.
In this study, we used a two-step synthetic process to prepare MWNFs by using different stabilizers. The characteristics of aqueous stabilizer solutions and MWNFs were examined using suitable instruments and test methods. The findings of this study are summarized as follows. The viscosity, electrical conductivity, and pH of MWNFs are considerably different when different stabilizers are used. The maximum increase ratios of the viscosity and electrical conductivity for MWNFs were observed for AG, and the maximum decrease ratio of the pH for MWNFs was determined for CH. Based on the static position experiment, MWNFs with CH, SDS, and SDBS demonstrate higher stability. The static and dynamic tests showed that MWNFs containing SDS and SDBS demonstrate optimal stability. The optimal MWCNT-stabilizer ratios for MWNFs containing SDS and SDBS were 5 : 2 and 5 : 4 by weight, respectively. The static position experiment confirmed that using a fixed optimal concentration ratio is feasible for preparing MWNFs with excellent stability. The thermal conductivity values of MWNFs containing different stabilizers indicated that the most suitable stabilizer for MWNFs was SDBS. The maximum thermal conductivity of MWNFs containing SDBS was higher than the thermal conductivity of water at 25°C by 12.49%. The pH of MWNFs containing SDS and SDBS was near the neutral range; therefore, using these MWNFs should not cause pipeline corrosion problems.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank the Ministry of Science and Technology of Taiwan for their financial support to this research under Contract no. NSC 101-2221-E-003-011-MY2.