Enhanced thermal properties of nanodiamond nanofluids

Accepted Manuscript Title: Enhanced thermal properties of nanodiamond nanofluids Author: L. Syam Sundar Manoj K. Singh Antonio C.M. Sousa PII: DOI: Reference:

S0009-2614(15)00879-9 http://dx.doi.org/doi:10.1016/j.cplett.2015.11.028 CPLETT 33432

To appear in: Received date: Revised date: Accepted date:

9-9-2015 4-11-2015 18-11-2015

Please cite this article as: L.S. Sundar, M.K. Singh, A.C.M. Sousa, Enhanced thermal properties of nanodiamond nanofluids, Chem. Phys. Lett. (2015), http://dx.doi.org/10.1016/j.cplett.2015.11.028 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

High lights  The ultra-dispersed diamond nanopowders were treated strong acid treatment in order to make single nanodiamond particles.

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 The stable nanodiamond nanofluids were prepared by dispersing in ethylene glycol/water mixtures.

 At 1.0% vol., thermal conductivity enhancements are 17.8%, 14.2% and 11.4% for

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ND/20:80, ND/40:60 and ND/60:40 nanofluids. ND/20:80, ND/40:60 and ND/60:40 nanofluids.

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 At 1.0% vol., viscosity enhancements are 2.74-times, 1.73-times and 1.92-times for

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nanofluids in laminar–turbulent flow.

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 Theoretical approach was used to understand the heat transfer benefits of

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Graphical Abstract

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Enhanced thermal properties of nanodiamond nanofluids

L. Syam Sundar*, Manoj K. Singh*, Antonio C.M. Sousa

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*Authors: [email protected] (L.Syam Sundar), [email protected] (M.K.Singh)

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Department of Mechanical Engineering, University of Aveiro, 3810-193 Aveiro, Portugal.

Abstract

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Nanodiamond (ND) particles dispersed in ethylene glycol/water mixtures have been reported for their thermal properties and potential heat transfer applications. Commercially available

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ultra-dispersed diamond soot was treated with sulfuric acid–nitric acids to form single ND particles–characterized by various techniques–then prepared ND nanofluids and then measured thermal conductivity and viscosity by experimentally. The enhanced thermal

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conductivity for 1.0% of ND/20:80, ND/40:60 and ND/60:40 nanofluids is 17.8%, 14.2% and

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11.4%; enhanced viscosity is 2.74-times, 1.73-times and 1.92-times at temperature of 60oC; respectively. The heat transfer benefits of ND nanofluids in laminar to turbulent flow have

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been analyzed theoretically by using thermal properties.

Keywords:

diamond

nanoparticles;

nanofluids,

thermal

conductivity;

viscosity;

enhancement.

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1. Introduction Convective heat transfer fluids such as water (W), mineral oil (MO), ethylene glycol (EG) and propylene glycol (PG) plays an important role in a wide range of applications, among them industrial heating and cooling. The efficiency of fluid thermal systems would be

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enhanced substantially, if higher thermal conductivities of working fluids can be achieved. Maxwell [1] improved the thermal conductivity of base fluids by suspending small solid particles into them. Suspending micrometer-sized particles to fluids, presents several

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drawbacks such as increased pumping power, clogging of narrow channels, corrosion and erosion in the pipe and agglomeration of the particles. This particular problem of

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agglomeration of micrometer-sized particles can be overcome by using nanometer-sized particles; the Choi [2] and his team pioneered the use of nanoparticles dispersed in base fluids

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to prepare the so-called nanofluids, with consequent enhancement of the thermal conductivity as compared to that of the base fluids. They prepared nanofluids by dispersing carbon nanotubes in engine oil and obtained thermal conductivity enhancement of 160% compared

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to base fluid. Eastman et al. [3] prepared CuO/EG nanofluids and observed thermal conductivity enhancement of 40% at 0.3% volume concentration. Sundar and Sharma [4] prepared Al2O3/water and CuO/water nanofluids and observed thermal conductivity

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enhancement of 6.52% and 24.6% at 0.8% volume concentration. Gavali et al. [5] observed

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thermal conductivity enhancement of 200% for Fe3O4/water nanofluids at 5.0% particle concentration in the base fluid. Lee et al. [6] prepared Al2O3/water and CuO/EG nanofluids

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and obtained maximum thermal conductivity enhancement of 20% at 4.0% particle concentration of CuO/EG nanofluid. Pang et al. [7] prepared nanofluids by using Al2O3, SiO2 nanoparticles dispersed in methanol and obtained thermal conductivity enhancements of 10.74% and 14.29% at 0.5% volume concentration, respectively. Hwang et al. [8] prepared different kind of nanofluids such as MWCNT/water, CuO/water, SiO2/water and CuO/EG and found thermal conductivity enhancement of MWCNT/water nanofluids increased up to 11.3% at 0.01% volume concentrations. Lee et al. [9] prepared SiC/water nanofluid and obtained thermal conductivity enhancement of 7.2% and viscosity enhancement of 68% at 3.0% volume concentration and at a temperature of 28oC. Xie et al. [10] also prepared SiC nanofluids by dispersing them in water and EG fluids and observed thermal conductivity enhancement of 15.8% for SiC/water nanofluid at a volume fraction of 4.2%. Zhang et al. [11] prepared Au/toluene, Al2O3/water, TiO2/water, CuO/water and CNT/water nanofluids and measured thermal conductivity by using the transient short-hot-wire technique. Yu et al. [12] prepared ZnO/EG nanofluids and measured thermal conductivity and viscosity in the 4 Page 4 of 44

temperatures ranging from10 to 60oC and observed thermal conductivity enhancement of 26.5% at 5.0% volume concentration. Murshed et al. [13] prepared TiO2/EG and Al2O3/EG nanofluids and observed thermal conductivity enhancement of 18% and 45% at 5% volume fraction, respectively. In most of the earlier works, the nanoparticles of Al2O3, CuO, SiC,

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MWCNT, Fe3O4, ZnO, and TiO2, among others, are used for the preparation of nanofluids and almost all the nanofluids reveals that the enhancement of thermal conductivity, but the order of enhancement is different. The order of enhancement in thermal conductivity depends

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on the nanoparticles, concentrations and temperatures.

Nanodiamond (ND) is also one of the most outstanding and promising material

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properties of bulk diamond, where the material is in nanoscale. Some of the advantages with nanodiamond particles are higher thermal conductivity – hardness – electrical resistivity,

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biocompatibility, chemical stability and others. Among the entire nanomaterials, ND offering the higher thermal conductivity ranges between 900 and 2300 W/m K at room temperatures. So, these higher values attracting the research towards on ND-based nanofluids for heat

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transfer applications. In general, ND can be produced using the detonation technique and the resultant product (detonation nanodiamond) contains a mixture of diamond particles of 4-5 nm in diameter with other carbon allotropes and impurities. Early research work in this area

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was performed by Dalmatov [14] for the estimation of thermal conductivity of fluids with

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ultra-dispersed diamond (UDD) powders of average size of 0.1–2

, but faced difficulties

with agglomeration. The carbon impurities present in the UDD is one of the major reasons for

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the agglomeration of the particles. For the large scale and uniform dispersion in fluids, the detonation soot is purified using liquid oxidants such as HNO3, a mixture of H2SO4 and HNO3, K2Cr2O7 in H2SO4, KOH/KNO3 to remove non-diamond carbon [15, 16]. Mixture of H2SO4 and HNO3 is one of the leading approaches to purify the detonated nanodiamond [1720]. Jee and Lee [21] used strong acid (98% H2SO4 and 70% HNO3 at a ratio of 3:1) for the purification of detonated nanodiamond and observed the formation of carboxyl groups on the surface of the ND particles based on the FTIR analysis. Xie et al. [22] used acid mixtures of perchloric acid, nitric acid and hydrochloric acid (based on the procedure of Jang and Xu [23]) to remove the carbon impurities present in the UDD and then prepared ND nanofluids by dispersing them in 55% ethylene glycol and 45% water mixtures; they observed thermal conductivity enhancement of 18% for a 2.0% volume concentration. Ma et al. [24] prepared nanofluids by dispersing diamond nanoparticles in high performance liquid chromatography (HPLC) grade water and observed thermal conductivity enhancement from 0.581 to 1.003 W/m K at 0.01% volume concentration. Ghazvini et al. [25] prepared ND/20:50 water/engine 5 Page 5 of 44

oil nanofluid and observed thermal conductivity enhancement of 25% at 1.0% weight concentration. Branson et al. [26] prepared ND/ethylene glycol and ND/mineral oil nanofluids and observed thermal conductivity enhancements up to 12% and 11% at 0.88% and 1.9% volume concentrations, respectively. Yu et al. [27] prepared ND/ethylene glycol nanofluid and observed thermal conductivity enhancement of 17.23% at 1.0% volume

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concentration at a temperature of 30oC. Tyler et al. [28] measured viscosity of ND/midel oil nanofluid and observed viscosity enhancement of 80% at 3.0% weight concentration.

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Yeganeh et al. [29] prepared ND/water nanofluid and obtained thermal conductivity enhancement up to 7.2% and 9.8% for 3.0% vol. at temperatures of 30 to 50oC, respectively.

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Taha-Tijerina et al. [30] prepared ND/mineral oil nanofluids and observed thermal conductivity enhancement up to 70% at 0.1% weight concentration. In the earlier work, ND

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nanofluids were prepared by using water, ethylene glycol, mineral oil and engine oil, among others, as base fluids and thermal conductivity enhancements were observed. Ethylene glycol/water mixture fluids are commonly used heat transfer fluid in heating

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industrial and residential buildings in the cold region countries such as Alaska, Europe and Russia, because of long winter conditions [31]. The freezing point of water can be increased by adding different weight ratios of ethylene glycol [32]. Namburu et al. [33], first time

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prepared nanofluids by dispersing CuO nanoparticles in 60% ethylene glycol and 40% water

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as a base fluid and obtained viscosity enhancement of 4.5-times and 3.1-times at particle loadings of 6.12% in the temperature range from -35 to 50oC. Later, Vajjha and Das [34]

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prepared nanofluids by dispersing Al2O3, CuO and ZnO nanoparticles in 60% ethylene glycol and 40% water and measured thermal conductivity up to 10% volume concentration and observed CuO nanofluids offering higher thermal conductivity enhancements compared to Al2O3 and ZnO nanofluids. Sundar et al. [35, 36] prepared nanofluids by dispersing Fe3O4 nanoparticles in 20:80, 40:60 and 60:40% EG/W mixtures and measured both thermal conductivity and viscosity at different particle concentrations and temperatures. Earlier work was mostly related with Al2O3, CuO, ZnO and Fe3O4 nanoparticles dispersed in ethylene glycol and water mixtures; therefore, there is no literature on the thermal transport properties for ND particles dispersed in different weight ratios of ethylene glycol and water mixtures. The present work is primarily focused on the preparation of nanodiamond (ND) nanofluids, by dispersing the nanoparticles in ethylene glycol and water mixtures. The ND particles were produced by acid treatment of commercially available ultra-dispersed diamond nanopowders. The treated ND particles were dispersed in 20:80, 40:60 and 60:40% ethylene glycol/water mixtures (weight ratio) for the preparation of nanofluids. The thermal transport 6 Page 6 of 44

properties such as thermal conductivity and viscosity were measured experimentally, whereas density and specific heat were determined indirectly and the results were compared against the literature values. A theoretical approach is used to understand the heat transfer benefits of these nanofluids flow in laminar flow conditions based on the Prasher et al. [42] analysis and

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turbulent flow conditions based on the Mouromtseff number [43].

2. Materials and methods

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2.1. Materials

The chemicals such as sulfuric acid (H2SO4, 99%), nitric acid (HNO3, 98%), and ethylene

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glycol (EG, 99%) were purchased from Sigma-Aldrich chemicals, USA and used without

2.2. Preparation of nanodiamond nanofluids

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purification.

The nanodiamond-soot (ND-soot) was purchased from International Technology Centre,

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USA (http://www.itc-inc.org/). Their specifications stated purity > 98%, primary average particle size 4-5 nm; cubic shape; grey color; specific surface area of 300-400 m2/g and the ND particles were produced by explosion detonation technique. The purchased diamond

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phase detonation soot contains graphitic carbon and incombustible impurities, metals and

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metal oxides. The metal impurities originate from the igniter used to initiate detonation (lead, silver, copper) and the steel walls (iron and other metals). The impurities can be inside the

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nanodiamond aggregates or attached to their outer surface and the nanodiamond aggregates should be disintegrated in order to remove the trapped impurities. The purchased ultradispersed diamond nanoparticles were treated with strong sulfuric acid and nitric acid solution [21, 37] to remove the impurities. The 10 g of ultra-dispersed diamond nanoparticles were dispersed in 300 ml of 3:1 mixture of concentrated sulfuric acid (H2SO4)(18.4 M)/nitric acid (HNO3)(16 M) and stirred magnetically for 72 h, after that acid treated ultra-dispersed diamond nanoparticles were dried in an oven at 80oC up to 12 h. The strong acid treatment results in the removal of carbon impurities and the formation of carboxyl groups on the surface of nanodiamond particles. Fig. 1 represents the schematic diagram of the synthesis of the above-mentioned surface-functionalized nanodiamonds. Depending on the surface area of the nanodiamond particles, there are many surface functional groups attached to the surface, but one surface functional group on the nanodiamond surface is shown in the figure, for clarity purpose. The acid treated nanodiamond (ND-treated) particles were characterized by X-ray diffraction using a Siemens D-500 diffractometer equipped with a copper anode and a 7 Page 7 of 44

nickel filters ( = 1.5404 Å) and the characterization of surface functionalization groups of ND was analyzed by FTIR (Bruker Equinox V70, wave number: 400-4000 cm-1) in KBr pellets. The Raman analysis is prominent analysis for carbon materials, since the ND is similar carbon material. In order to observe the G-band and D-band, the purchased ND-soot

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and ND-treated were performed for Raman analysis (Jobin-Yvon LabRam; 514 nm argon ion laser). The transmission electron microscopy (TEM: JEOL 2200F TEM/STEM; 200 kV) was also used for ND-soot and ND-treated. The average particle size distribution and zeta

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potential was measured with Zetasizer-2000 (Malvern, UK, 658 nm HeeNe laser).

The stable nanofluids were prepared by dispersing ND-treated particles in different

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base fluids. The base fluids used in this study are 20:80, 40:60 and 60:40 ratio (weight) of ethylene glycol/water mixtures. The physical properties of base fluids and nanodiamond

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(ND) particles were shown in Table 1. The ND nanofluids were prepared in the volume concentrations range from 0.2% to 1.0%. Eq. (1) is used to estimate the quantity of NDtreated particles required for 20:80, 40:60 and 60:40 EG/W-based nanofluids for known

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volume concentrations. The estimated values from Eq. (1) are shown in Table 2.

Where,

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Volume concentration,

is the percentage of volume concentration,

is the density of ND = 3100 kg/m3,

is the weight of ND and

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is the density of base fluid,

(1)

is the weight of base fluid.

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From Eq. (1) for 0.2, 0.6, 0.8 and 1.0% volume concentrations of 40 ml of 20:80% EG/W base fluid and amounts of 0.241, 0.726, 0.971 and 1.216 g were dispersed, respectively. For the preparation of 0.2% volume concentration, the required quantity of 0.241 g were directly dispersed in the base fluid (20:80%) and sonicated in ultrasonic bath for 2 h. No, surfactant is used for the preparation of nanofluids, because the acid treated-ND particles can have the carboxyl groups. Those will help for uniform dispersion in the base fluids. The same procedure is used for the preparation of other concentrations of 20:80% EG/W nanofluids and also prepared 40:60% EG/W and 60:40% EG/W nanofluids. The thermal properties such as thermal conductivity and viscosity were estimated both experimentally at different temperatures and concentrations. The other thermal properties such as density and specific heat were estimated with well-known mixture rule equations. These properties are very important to analyze the fluid as a benefit of heat transfer fluid. The KD 2 pro instrument (Decagon Devices, USA) is used to estimate the thermal conductivity of different nanofluids in the temperature range from 20 to 60oC. The 8 Page 8 of 44

sensor needle of KS-1 (length: 60 mm; diameter: 1.3 mm) is used for the thermal conductivity measurements with an accuracy of ±1%. The instrument is working under the transient line heat source method. The method used in the ASTM and IEEE thermal conductivity/resistivity measurement standards (IEEE 442 and ASTM 5334) is generally

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called the transient line heat source or transient heated needle method. If heat at a constant rate, q is applied to an infinitely long and infinitely small “line” source, the temperature

Where C is a constant,

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response of the source over time can be described by the equation:

(2)

is the change in tempeature, t is time and q is the heat source, k is

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the thermal conductivity.

The AND vibro-viscometer (A&D company, Japan) was used to measure the viscosity

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of different nanofluids in the temperature range from 0 to 60oC. The SV-10 vibro viscometer measures viscosity by detecting the driving electric current necessary to resonate the two sensor plates at constant frequency of 30 Hz and amplitude of less than 1 mm. It measures the

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viscosity in the range 0.3 to 10,000 mPa.s with an accuracy of ±1%. The nanofluids were directly poured into the fluid filling cup and placed on the table. The height of the table is

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adjusted so that the vibrating gold coated plates are filled into the fluid and only then the measurement is made. The direct viscosity value is displayed on the screen. Each sample is

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measured 5-times and the average value is recorded as final value. The other properties such as density and specific heat of nanofluids were estimated using the phase rule and the

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equations are given below:

Where

is the density,

(3) (4)

is the specific heat and the suffixes nf, bf and p stands for

nanofluid, base fluid and particle, respectively.

3. Results and discussion 3.1. Characterization

Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy provide valuable insights into phase composition and surface terminations of nanodiamonds. FTIR can detect functional groups and adsorbed molecules on the surface and it can also detect changes in the surface chemistry of functionalized nanodiamond. The signal-to-noise ratio (S/N = 45,000:1) of an FTIR spectrometer is the measure of performance for the time that is needed to 9 Page 9 of 44

accumulate a good IR spectrum. In the IR spectroscopy analysis, multiple scans of 1024 with resolution 4 cm–1 are accumulated for obtaining the final spectrum. The acquisition time is depends on the sample and sampling technique, but it always depends on the performance of the IR instrument insturment. Fig. 2a represents the IR spectra of ND-soot and Fig. 2b

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represents the IR spectra of ND-treated nanoparticles. The commercially purchased ND-soot inherently contains some functional groups of H, OH, COOH, in particular, which reflects in the IR spectra (black) of ND-soot, but those are not dominating peaks. The strong acid

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treatment with ND-soot, there may be possibility of formation of carboxyl groups on the surface of the ND particles were observed from the IR spectra (red). The peak near 1630 cm–1

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represents the O–H bending of physically adsorbed water and the hydrogen bonding; whereas the peak near 1755 cm–1 represents the C=O stretch of carboxylic groups. It represents the

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proof of covalent attachment of carboxylic groups on the surface of nanodiamond particle. The shoulder near ~1800 cm–1 may associate with the carboxylic anhydrite. The other bond near 3710 cm–1 represents the O–H stretching from the surface of COOH group. The broad

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band near 3000–3600 cm–1 represents the hydrogen bonded OH of physisorbed water on the surface. The peaks near 2300-2400 cm–1 are due to carbon dioxide from an ambient air. The

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broad band near 3000 cm–1 can be caused by the CH stretching on the ND surface. The bands diamond powders [38].

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near 700-1450 cm–1, mainly at 1275 cm–1 have been associated to either like groups on the The dry powder of ND-soot and ND-treated were placed on the quartz glass surface

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and the Raman spectra was recorded by using spot focus of the laser beam. In the final results the background spectra of quartz galss surface was subtracted. Fig. 3 represents the Raman spectrum of ND-soot and ND-treated. The diamond cores in the ND-soot seem to be completely covered by graphitic shells, and this is confirmed by the Raman spectrum (black line), which is dominated by the G-band of graphitic carbon at 1,590 cm–1 and D-band at 1410 cm–1 and has no diamond peak. In the acid treatment process (ND-treated), the graphitic layer is removed and the Raman spectrum (red line) shifted to 1328 cm–1 and also observed the surface functional groups and adsorbed molecules band at 1640 cm–1 and 1760 cm–1. The peak near 1328 cm–1 indicates the formation of pure diamond particles (sp3). The transmission electron microscopy (TEM) analysis was performed on the ND-soot and ND-treated. Two TEM samples were prepared by drying a droplet of aqueous dispersion of ND-soot and ND-treated on 300 mesh size copper grids coated carbon film. The diamond core (sp3) was covered with carbon impurities layer, which was cleared observed form the 10 Page 10 of 44

Fig. 4a. When the acid treatment, carbon impurities layers is removed and clear diamond particles were observed (Fig. 4b). The traditional mineral acids treatment on ND-soot eliminates unnecessary impurities including metal ions, carbon soot, etc. More importantly, this process converts the formation

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of carboxylic groups by oxidation on the surface of ND particles and also the process contributes to increase the stability of ND in the base fluids. The X-ray pattern of ND-treated was shown in Fig. 5a and the selected area X-ray diffraction (SEAD) from TEM analysis of

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ND-treated was shown in Fig. 5b. From the pattern it confirmed by the crystalline planes (111), (220) and (311) refection at 2 values are 43.74o, 75.36o and 92.34o, respectively. The

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lattice parameter obtained from these peaks was 3.56 Å on average, which agrees well with the literature value [21]. The Scherrer equation is used to calculate crystallite size from XRD

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data and the equation is given below:

(5)

Where D is the average crystallite size, β is the line broadening in radians, θ is the Bragg

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angle and λ is the X-ray wavelength (1.5404 Å). From the Eq. (1), the crystalline size of ND-

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3.2. Size distribution and stability

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treated was observed as ~5 nm with a lattice stain of 0.0258.

It is important to study the particle size distribution (PSD) of ND-soot and ND-treated,

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because the un-wanted carbon material (impurities) was removed from the ND-soot with strong acid treatment. The samples were prepared by dispersing 0.002 g of ND-soot and NDtreated in 2 ml of 20:80% EG/W. The PSD was measured with dynamic light scattering technique using the ZetaSizer NanoZS. The measurements were performed 3-times at a temperature of 25oC and the best results were presented. Figs. 6(a) and 6(b) show the particle size distribution of ND-soot and ND-treated. From the results, it can be observed that, the average particle-size of ND-soot is 268.3 nm with polydispersity index (PdI) of 0.338, whereas the average particle-size distribution of ND-treated is 19.27 nm with polydispersity index (PdI) of 0.374. Expected results were obtained from the PSD analysis, because the NDsoot is detonated-soot, which contains graphitic carbon and incombustible impurities, in addition to metals and metal oxides. The metal impurities originate from the igniter used to initiate the detonation (lead, silver, copper) and the steel walls (iron and other metals). In the ND-soot, the particles agglomerate with each other and it is difficult to achieve uniform dispersion in the base fluids. Not only carbon impurities, the other materials involved in the 11 Page 11 of 44

detonation process causes the increase of particle size, when they dispersed in base fluid. The materials such as lead, silver, copper and iron are surrounded by the core (sp2or sp3) diamond nanoparticles. The carbon impurities surrounded by the core diamond particles were cleared observed from the high-resolution TEM analysis, which is explained in characterization

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section. The acid treatment results the removal of carbon impurities, and finally achieved the reduction in particle size.

The colloidal stability of ND nanofluids was very important, while they dispersed in

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the base fluids. In the present case, the nanofluids were prepared without adding any surfactant, because surfactant suppresses the thermal properties of nanofluids. For obtaining

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the accurate values with the influence of nanoparticles in the base fluids, nanofluids should prepare without adding any surfactants. It is difficult to prepare the stable nanofluids without

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adding any surfactant, because of its density difference between base fluid and nanoparticles. Particle surface charge (negative or positive) is one of the important parameter for obtaining the stable nanofluids. The two advantages with strong acid treatment on ND-soot are (i)

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removal of carbon impurities (ii) formation of carboxyl groups on the surface. The stability of these ND-treated nanoparticles of 0.2% volume concentration of 20:80%, 40:60% and 60:40% EG/W was measured in terms of zeta potential. The zeta potential value is more than

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±30 mV, it represents the nanofluids are stable [39]. The zeta potential value of 0.2% volume

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concentration of ND-soot and ND-treated dispersed in 20:80% EG/W was measured as -15 mV and -33 mV, respectively. The surface charge on the ND-soot particles is much less than

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the measured value (-30 mV) because of low zeta potential value which results the agglomeration of particle in the base fluid. This means that, the nanofluids prepared with NDsoot were not stable; while, the zeta potential of ND-treated (-33 mV) is good enough to obtain colloidal stability. The surface charge (carboxyl groups) on the ND-treated particles is high which causes the higher zeta potential value. For both cases, the negative zeta potential value was achieved due to the dissociated acidic groups on the ND surface. The commercially purchased ND-soot surfaces are covered with many functional groups such as H, OH, COOH, CO (ketone), NH2, among many others. These functional groups create the negative zeta potential value, when ND-soot dispersed in the base fluid (20:80% EG/W). Similarly with the acidic treatment on the ND-soot, the carboxyl groups are dominated and suppress the other functional groups (H, OH, CO and NH2), which result the negative zeta potential value, when ND-treated dispersed in the base fluid. From the FTIR and zeta potential analysis, it is confirmed that the rich carboxyl groups are formed on the ND surface.

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The zeta potential of 0.2% volume concentration of ND-treated dispersed in the base fluids of 40:60% and 60:40% EG/W and observed as -36 mV and -38 mV, respectively. For the same particle volume concentration ( = 0.2%), the zeta potential value increases in the following order of base fluids 60:40%, 40:60% and 20:80% EG/W. Initially for the base fluid

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20:80% EG/W, the zeta potential is -33 mV, for 40:60% EG/W, is -36 mV and for 60:40% EG/W, is -38 mV. So, by adding ethylene glycol to the mixture, with the consequent decrease in percentage of water it occurs an increase in zeta potential value. This effect is caused by

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the increase in viscosity of the base fluid with the addition of ethylene glycol to the water [32]. This analysis reveals that, the zeta potential value is not constant for all the base fluids;

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it also depends on the effect of base fluid. It means that the zeta potential is indirectly depends on the viscosity of the base fluids. When the viscosity of the base fluid is more its

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zeta potential value is also more.

3.3. Thermal conductivity of nanodiamond nanofluids

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The optical image of different volume concentrations of ND nanofluids (base fluid: 20:80% EG/W) was shown in Fig. 6c. The other ND nanofluids (base fluid: 40:60% and 60:40% EG/W) were also prepared and by seeing with naked eye, all the nanofluids were appear like

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same. For understanding purpose, we just have shown one particular image of ND nanofluids.

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The bench mark studies were performed with the base fluids such as 20:80, 40:60 and 60:40 EG/W and its thermal conductivities were compared with ASHRAE [32] hand book data. A

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maximum of ±2.5% deviation was observed between them. DiGuilio and Teja [44] approach have been used to estimate the accuracy of the obtained thermal conductivity values. A systematic errors and limit the accuracy of the absolute measurements to a few percent that, for our particular setups, has been estimated as 2.5%–3%. Experimental thermal conductivity of different concentrations of ND/20:80 nanofluids were shown in Fig. 7a with effects of temperatures. The error bars (Fig. 7a) have been calculated by adding a 3% systematic error to the standard deviations. It is noticed that thermal conductivity of nanofluids increases with increase of volume concentrations and temperatures. The thermal conductivity of 0.2% vol. of nanofluid was enhanced up to 2.1% and 7.28% at a temperature of 20oC and 60oC, respectively. Similarly, the thermal conductivity of 1.0% vol. of nanofluid was enhanced up to 7.96% and 17.83% at a temperature of 20oC and 60oC, respectively. The linear-behavior of thermal conductivity enhancement was observed with increase of particle loadings in the base fluid. The similar trend in linear enhancement in thermal conductivity has been observed by Murshed et al. [13] for TiO2/EG; Xie et al. [10] for SiC/water nanofluid and Branson et al. 13 Page 13 of 44

[26] prepared ND/ethylene glycol and ND/mineral oil nanofluids. The enhancement is caused due to Brownian motion and micro-convection of the nanoparticles in the base fluid. Therefore, adding ND particles to the base fluid results in the formation of highly ordered arrangement of molecule around each particles and the stirring action caused by Brownian

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motion of the particles. At low particle concentrations the enhancement is lower, whereas high particle concentrations the enhancement is higher. For higher particle concentrations, one can expect the particle agglomeration in the base fluid. Some reports [40] showing,

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thermal conductivity enhancement with particle agglomeration. These agglomerations in the base fluids can break into primary nanodiamond particles with help of ultra sonication.

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Higher ultra sonication time reveals the breaking of agglomerations and causes better stability.

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Experimental thermal conductivity of different concentrations of ND/40:60 nanofluids were shown in Fig. 7b with effect of temperatures. The thermal conductivity of 0.2% vol. of nanofluid was enhanced up to 1.98% and 6.51% at a temperature of 20oC and 60oC,

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respectively. Similarly, the thermal conductivity of 1.0% vol. of nanofluid was enhanced up to 6.73% and 14.23% at a temperature of 20oC and 60oC, respectively. Experimental thermal conductivity of different concentrations of ND/60:40 nanofluids were shown in Fig. 7c with

d

effect temperatures. The thermal conductivity of 0.2% vol. of nanofluid was enhanced up to

te

1.49% and 4.8% at a temperature of 20oC and 60oC, respectively. Similarly, the thermal conductivity of 1.0% vol. of nanofluid was enhanced up to 5.14% and 11.44% at a

Ac ce p

temperature of 20oC and 60oC, respectively. The similar fashion of increased thermal conductivity with increase of volume concentrations and temperatures for 40:60 and 60:40 based nanofluids were observed. The thermal conductivity enhancement with effects of base fluids was clearly observed. At same particle concentration

and temperature (T

=60oC) the thermal conductivity enhancement is not constant for all the base fluids, it varies from one base fluid to other base fluid. In the measured volume concentrations and temperatures, ND/20:80 nanofluid showing better results compared to other ND/40:60 and ND/60:40 nanofluids. The present experimental thermal conductivity ratio

for all

nanofluids with volume concentrations at a temperature of 30oC was shown in Fig. 8 along with Yu et al. [27] for ND/EG; Xie et al. [22] for ND/55:45 EG/W; Yu et al. [12] for ZnO/EG nanofluids. The thermal conductivity ratio of all the nanofluids increases linearly with increasing particle concentrations. The thermal conductivity enhancement of ND/EG [27] 14 Page 14 of 44

nanofluid has higher value that that of the present study. As discussed earlier, the thermal conductivity enhancement is not constant for all the base fluids. The particle size, morphology and particle/fluid molecules interface can attribute the different thermal conductivity enhancements in various base fluids. The present experimental thermal conductivity ratio

ip t

for all

nanofluids with temperatures at

= 1.0% vol. was shown in Fig. 9 along with Vajjha and

Das [34] for 60:40/Al2O3, 60:40/ZnO, 60:40/CuO; Branson et al. [26] for ND/mineral oil; Xie

cr

et al. [22] for ND/55:45 EG/W nanofluids. In the present study for all the nanofluids, thermal conductivity ratio increases with increase of temperatures; it well matches with the trend

us

explained by Vajjha and Das [34]. But, the authors [22, 26] observed constant thermal conductivity enhancement with increase of temperatures. There may be the base fluid effect

an

for observing different thermal conductivity enhancement ratios for temperatures. Some authors proposed regression equation for thermal conductivity of nanofluids, by considering linear increase with volume concentrations. The equations are given below:

M

Branson et al. [26] for nanodiamond nanofluids

(6)

(ND/mineral oil nanofluid)

(7)

d

(ND/EG nanofluid)

te

Pak and Cho [40] for Al2O3 and TiO2 nanofluids (8)

Ac ce p

Timofeeva et al. [41] for TiO2 nanofluids

(9)

In the similar way [26, 40 and 41], the present experimental data of all the nanofluids fit into regression equation by assuming linear enhancement of thermal conductivity with particle concentrations and the data obtained from Eq. (10) was shown in Fig. 10. (10)

15 Page 15 of 44

3.4. Viscosity of ND nanofluids The bench mark studies were performed with the base fluids such as 20:80, 40:60 and 60:40 EG/W and its viscosities were compared with ASHRAE [32] hand book data. A maximum of ±2.8% deviation was observed between them. Experimental viscosity of different

ip t

concentrations of ND/20:80 nanofluids were shown in Fig. 11a with effects of temperatures. It is noticed that viscosity of nanofluids increases with increase of volume concentrations, but decreases with increase of temperatures. The viscosity of 0.2% vol. of nanofluid was

cr

enhanced up to 1.58-times and 1.06-times at a temperature of 0oC and 60oC, respectively. Similarly, the viscosity of 1.0% vol. of nanofluid was enhanced up to 2.37-times and 2.74-

us

times at a temperature of 20oC and 60oC, respectively. The viscosity enhancement of nanofluids shows non-linear behavior with increase of temperatures. The enhancement is

an

caused due to increasing particle-to-particle interaction for higher concentrations, which can alter the intra-molecular forces and consequently the viscosity.

Experimental viscosity of different concentrations of ND/40:60 nanofluids were

M

shown in Fig. 11b with effects of temperatures. It is noticed that viscosity of nanofluids increases with increase of volume concentrations, but decreases with increase of temperatures. The viscosity of 0.2% vol. of nanofluid was enhanced up to 1.43-times and

d

1.15-times at a temperature of 0oC and 60oC, respectively. Similarly, the viscosity of 1.0%

te

vol. of nanofluid was enhanced up to 2.52-times and 1.73-times at a temperature of 20oC and 60oC, respectively. Experimental viscosity of different concentrations of ND/60:40

Ac ce p

nanofluids were shown in Fig. 11c with effects of temperatures. It is noticed that viscosity of nanofluids increases with increase of volume concentrations, but decreases with increase of temperatures. The viscosity of 0.2% vol. of nanofluid was enhanced up to 1.48-times and 1.56-times at a temperature of 0oC and 60oC, respectively. Similarly, the viscosity of 1.0% vol. of nanofluid was enhanced up to 1.86-times and 1.92-times at a temperature of 20oC and 60oC, respectively. From the results, it is noticed that viscosity enhancement not only depends on the particle concentrations and temperatures it also depends on the base fluid. Among all the base fluids, the particles dispersed in 20:80% EG/W base fluid exhibiting more viscosity enhancement compared to other base fluids. This is possible due to the more internal resistance between the two fluid layers. The present experimental viscosity ratio

for ND/60:40 nanofluids with

temperatures was shown in Fig. 12 along with Namburu et al. [33] for CuO/60:40 nanofluids. The viscosity ratio for all nanofluids at particle concentrations remains constant in the 16 Page 16 of 44

measured temperatures. Namburu et al. [33] observed decrease of viscosity ratio for all nanofluids at particle concentrations with increase of temperatures. The viscosity ratio of nanofluids is fitted quadratic expression in the volume concentrations range from 0% to 1.0% at different temperatures and the expression is given

ip t

below. The values from Eq. (11) are shown in Fig. 13 along with experimental values. )

(11)

Where A, B and C are the constants and its values are presented in Table 3.

cr

Namburu et al. [33] proposed the viscosity correlation for the CuO/60:40 nanofluids, which is (12)

an

us

given as follows:

The present viscosity data was fitted exponentially in similar way to Namburu et al. [33] and

(13)

te

d

M

the expression is given below:

The experimental values of thermal conductivity and viscosity; theoretical values of density

Ac ce p

and specific heat for all the nanofluids were shown in Table 4. A systematic theoretical approach was used to understand the heat transfer

capabilities of ND/20:80, ND/40:60 and ND/60:40 nanofluids in fully developed laminar and turbulent flow conditions based on the thermal properties without conducting the heat transfer experiments. For that purpose, Prasher et al. [42] analysis is used for laminar flow and Mouromtseff number (Mo) is used for turbulent flow.

3.5. Heat transfer benefits of nanodiamond nanofluids in laminar flow The potential applications heat transfer applications of ND/20:80, ND/40:60 and ND/60:40 nanofluids in a fully developed laminar flow conditions (Reynolds number, Re < 2300) can be evaluated based on the Prasher et al. [42] analysis. (14) (15) 17 Page 17 of 44

Where

and

are thermal conductivity and viscosity enhancement coefficients and

volume concentration. Based on the Prasher et al. [42] analysis, the heat transfer benefits of . For ND/20:80-nanofluid at 1.0% vol. and

nanofluids are possible when the value T = 30oC from Eq. (10) the value of

, from Eq. (11) the value of

, then

of

, from Eq. (11) the value of

ip t

. For ND/40:60-nanofluid at 1.0% vol. and T = 30oC from Eq. (10) the value , then

. For ND/60:40-

nanofluid at 1.0% vol. and T = 30oC from Eq. (10) the value of , then

The ratio of

cr

value of

, from Eq. (11) the

for all nanofluids is more than 4

and temperature (30oC). Therefore, the

us

in the estimated volume concentration (

prepared ND/20:80, ND/40:60 and ND/60:40 nanofluids are not benefit as a heat transfer fluid in fully developed laminar flow. The viscosity enhancement is more dominating

an

compared to thermal conductivity enhancement. Under very low particle concentrations, there may be possibility of benefit of nanofluids as a heat transfer fluids. The detailed

M

experimental analysis is needed before deciding the nanofluids as a heat transfer fluids.

3.6. Heat transfer benefits of nanodiamond nanofluids in turbulent flow

d

The heat transfer benefits of nanodiamond nanofluids in turbulent flow conditions (Re >

te

2300) are evaluated by means of the Mouromtseff number (Mo) (Simons. [43]). It is a figure of merit (FOM) to evaluate and compare the heat transfer capability of an alternative thermal

Ac ce p

fluid. The density, specific heat, thermal conductivity and viscosity values for all nanofluids were used to calculate the Mouromtseff number. The ratio of

should be more

than one for a fixed geometry and specific velocity, then that fluid is heat transfer capable fluid.

(16) (17) (18)

Where

are thermal conductivity, density, specific heat and viscosity,

respectively and the suffix bf, nf are base fluid and nanofluid. The nanofluids, figure of merit (FOM) > 1, provide a larger heat transfer benifits at the same velocity for a particular system. Fig. 14 represents the FOM of ND/20:80, ND/40:60 and ND/60:40 nanofluids at different volume concentrations and at a temperature of 30oC. In the estimated volume concentration 18 Page 18 of 44

range from 0.2% to 1.0%, the FOM for all the nanofluids (ND/20:80; ND/40:60; ND/60:40) is less than one. The viscosity enhancement for all the nanofluids prevails over that for the other properties. Table 5 represents the figure of merit of all the nanofluids (ND/20:80; ND/40:60; ND/60:40) at different concentrations and temperatures. It is clearly noticed that, when the temperature of the nanofluids increases from 30 to 60oC, the FOM value is greater

ip t

than one. At room temperature (30oC) FOM value is less than one, whereas at increase in temperature FOM is greater than one. This means that at higher temperatures all the

cr

nanofluids are suitable for use as heat transfer fluids in turbulent flow conditions. However, Mouromtseff number does not incorporate any additional heat transfer mechanisms that have

us

been observed in nanofluids heat transfer studies and therefore experiments need to be

an

conducted before conclusions can be drawn of the fluid potential.

4. Conclusions

The carbon impurities present in the UDD-nanopowders are successfully removed by strong

M

acid treatment. The acid treated nanodiamond nanoparticles are characterized by XRD, FTIR and TEM. The ethylene glycol/water mixture nanodiamond nanofluids are prepared and its thermal conductivity and viscosity is estimated by experimentally. Maximum thermal

d

conductivity enhancement is observed by ND/20:80 nanofluids compared to ND/40:60 and

te

ND/60:40 nanofluids. The thermal conductivity of 1.0% vol. is enhanced by 17.83%, 14.23% and 11.44% for ND/20:80, ND/40:60 and ND/60:40 nanofluids at a temperature of 60oC,

Ac ce p

respectively.

The viscosity of 1.0% vol. is enhanced by 2.74-times, 1.73-times and 1.92-times for

ND/20:80, ND/40:60 and ND/60:40 nanofluids at a temperature of 60oC, respectively. At same particle concentrations and temperatures, viscosity enhancements are more compared to thermal conductivity enhancements. Development of new theoretical models is necessary for composite nanofluids. The effectiveness of nanofluid in laminar and turbulent flow conditions were studied based on the Prasher et al. [30] model and Mouromtseff number [31]. The heat transfer benefits of ND/20:80, ND/40:60 and ND/60:40 nanofluids for all particle concentrations at a temperature of 30oC were not effective in both laminar and turbulent flow. The reason behind is the domination of viscosity, when compared to other thermal properties such as thermal conductivity, density and specific heat properties. But, when the temperature of the nanofluids increases from 30 to 60oC, all the nanofluids are showing benefit as heat transfer fluids in turbulent flow conditions. Before concluding the applicability of these nanofluids in heat transfer equipment detailed experimental study is to be required. 19 Page 19 of 44

Acknowledgment We are very much thankful to the Foundation for Science and Technology (FCT) Portugal for the financial support. The author LSS also acknowledges FCT for his post-doctoral grant

ip t

SFRH/BPD/100003/2014.

References

cr

[1] J.C. Maxwell, A treatise on electricity and magnetism, Clarendon Press. UK. 1891.

[2] S.U.S. Choi, Enhancing thermal conductivity of fluids with nanoparticles.

us

Developments and Applications of Non-Newtonian Flows, eds. D. A. Siginer and H. P. Wang, ASME: New York, FED 231/MD 66 (1995) 99–105.

an

[3] J. A. Eastman, S. U. S. Choi, S. Li, W. Yu, L. J. Thompson, Anomalously increased effective thermal conductivities of ethylene glycol-based nanofluids containing copper nanoparticles, Applied Physics Letters 78 (2001) 718–720.

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[4] L.S. Sundar, K.V. Sharma, Thermal conductivity enhancement of nanoparticles in distilled water, International Journal of Nanoparticles 1 (1) (2008) 66–77. [5] A. Gavili, F. Zabihi, T.D. Isfahani, J. Sabbaghzadeh, The thermal conductivity of

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[6] S. Lee, S.U.S. Choi, S. Li, J. Eastman, Measuring thermal conductivity of fluids

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of Heat and Mass Transfer 55 (2012) 5597–5602.

[8] Y.J. Hwang, Y.C. Ahn, H.S. Shin, C.G. Lee, G.T. Kim, H.S. Park, J.K. Lee, Investigation on characteristics of thermal conductivity enhancement of nanofluids, Current Applied Physics 6 (2006) 1068–1071.

[9] S.W. Lee, S.D. Park, S. Kang, I.C. Bang, J.H. Kim, Investigation of viscosity and thermal conductivity of SiC nanofluids for heat transfer applications, International Journal of Heat and Mass Transfer 54 (2011) 433–438. [10] H. Xie, J. Wang, T. Xi, Y. Liu, Thermal conductivity of suspensions containing nanosized SiC particles, International Journal of Thermophysics 23 (2002) 571–580.

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[11] X. Zhang, H. Gu, M. Fujii, Effective thermal conductivity and thermal diffusivity of nanofluids containing spherical and cylindrical nanoparticles, Experimental Thermal Fluid Science. 31 (2007) 593–599. [12] W. Yu, H. Xie, L. Chen, Y. Li, Investigation of thermal conductivity and viscosity of

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ethylene glycol based ZnO nanofluid, Thermochimica Acta 491 (2009) 92–96. [13] S.M.S. Murshed, K.C. Leong, C. Yang, Investigations of thermal conductivity and viscosity of nanofluids, International Journal of Thermal Science 47 (5) (2008) 560–

cr

568.

[14] V.Y. Dolmatov, Detonation synthesis ultra dispersed diamonds: properties and

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applications, Russian Chemical Review 70 (2001) 607–626.

[15] O.A. Shenderova, D.M. Gruen, Ultrananocrystalline diamond: synthesis, properties,

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and applications, William Andrew, 2006.

[16] V.N. Mochalin, O. Shenderova, D. Ho, Y. Gogotsi, The properties and applications of nanodiamonds, Nature nanotechnology, 7 (2012) 11–23.

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[17] A. Krüger, Y. Liang, G. Jarre, J. Stegk, Surface functionalisation of detonation diamond suitable for biological applications. J Mater Chem 16 (2006) 2322–2328. 4038–4041.

d

[18] A.S. Barnard, Self-assembly in nanodiamond agglutinates. J Mater Chem 18 (2008)

te

[19] M. Ozawa, M. Inaguma, M. Takahashi, F. Kataoka, A. Krüger, E. Ōsawa, Preparation and behavior of brownish, clear nanodiamond colloids. Adv Mater 19 (2007) 1201–

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1206.

[20] Y. Liang, M. Ozawa, A. Krueger, A general procedure to functionalize agglomerating nanoparticles demonstrated on nanodiamond. ACS Nano 3 (2009) 2288–2296

[21] A.Y. Jee, M. Lee, Surface functionalization and physicochemical characterization of diamond nanoparticles, Current Applied Physics 9 (2009) 144–147

[22] H. Xie, W. Yu, Y. Li, Thermal performance enhancement in nanofluids containing diamond nanoparticles, J. Phys. D: Appl. Phys. 42 (2009) 095413.

[23] T. Jang, K. Xu, FTIR study of ultradispersed diamond powder synthesized by explosive detonation, Carbon 33 (1995) 1663-1671. [24] H.B. Ma, C. Wilson, B. Borgmeyer, K. Park, Q. Yu, S.U.S. Choi, M. Tirumala, Effect of nanofluid on the heat transport capability in an oscillating heat pipe, Applied Physics Letters 88 (2006) 143116–143119.

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[25] M. Ghazvini, M.A. Akhavan-Behabadi, E. Rasouli, M. Raisee, Heat transfer properties of nanodiamond–engine oil nanofluid in laminar flow, Heat Transfer Engineering, 33 (2011) 525–532. [26] B.T. Branson, P.S. Beauchamp, J.C. Beam, C.M. Lukehart, J.L. Davidson,

ip t

Nanodiamond nanofluids for enhanced thermal conductivity, Nano, 7 (2013) 3183– 3189.

[27] W. Yu, H. Xie, Y. Li, L. Chen, Q. Wang, Experimental investigation on the thermal

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transport properties of ethylene glycol based nanofluids containing low volume concentration diamond nanoparticles, Colloids and Surfaces A: Physicochem. Eng.

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Aspects 380 (2011) 1–5.

[28] T. Tyler, O. Shenderova, G. Cunningham, J. Walsh, J. Drobnik, G. McGuire, Thermal

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transport properties of diamond-based nanofluids and nanocomposites, Diamond & Related Materials 15 (2006) 2078–2081.

[29] M. Yeganeh, N. Shahtahmasebi, A. Kompany, E.K. Goharshadi, A. Youssefi, L.

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Šiller, Volume fraction and temperature variations of the effective thermal conductivity of nanodiamond fluids in deionized water, International Journal of Heat and Mass Transfer 53 (2010) 3186–3192.

d

[30] J. J. Taha-Tijerina, T.N. Narayanan, C.S. Tiwary, K. Lozano, M. Chipara, P. M. 4778–4785.

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Ajayan, Nanodiamond-Based Thermal Fluids, Appl. Mater. Interfaces, 6 (2014)

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[31] F.C. McQuiston, J.D. Parker, J.D. Spitler, Heating Ventilating and Air-Conditioning, John Wiley & Sons Inc., New York, 2000.

[32] ASHRAE Handbook 1985 Fundamentals, American Society of Heating, Refrigerating and Air-Conditioning Engineers Inc., Atlanta, 1985.

[33] P.K. Namburu, D.K. Das, K.M. Tanguturi, R.K. Vajjha, Numerical study of turbulent flow and heat transfer characteristics of nanofluids considering variable properties, International Journal of Thermal Science 48 (2009) 290–302.

[34] R.S. Vajjha, D.K. Das, Experimental determination of thermal conductivity of three nanofluids and development of new correlations, International Journal of Heat and Mass Transfer 52 (2009) 4675–4682. [35] L. Syam Sundar, Manoj K. Singh, Antonio C.M. Sousa, Thermal conductivity of ethylene glycol and water mixture based Fe3O4 nanofluid, International Communications in Heat and Mass Transfer 49 (2013) 17–24.

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[36] L.S. Sundar, E.V. Ramana, M.K. Singh, A.C.M. Sousa, Viscosity of low volume concentrations of magnetic Fe3O4 nanoparticles dispersed in ethylene glycol and water mixture, Chemical Physics Letters 554 (2012) 236–242. [37] S. Kumari, M.K. Singh, S.K. Singh, J.J.A. Gracio, D. Dash, Nanodiamonds activate platelets

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Touhara, Diffuse reflectance infrared Fourier-transform study of the direct thermal fluorination of diamond powder surfaces, J. Chem. Soc., Faraday Trans. 91 (1995)

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3209–3212.

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[41] E.V. Timofeeva, A.N. Gavrilov, J.M. McCloskey, Y.V. Tolmachev, Thermal conductivity and particle agglomeration in alumina nanofluids: experiment and theory, Physical Review 76 (2007) 061203. 16 pages.

d

[42] R. Prasher, D. Song, J. Wang, P. Phelan, Measurements of nanofluid viscosity and its

te

implications for thermal applications, App Physics Lett. 89 (2006) 133108. [43] R.E. Simons, Comparing heat transfer rates of liquid coolants using the Mouromtseff

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number, Electronics Cooling. 12, 2006.

[44] M. DiGuilio, A. S. Teja, The thermal conductivity of the molten NaNO3–KNO3 eutectic between 525 and 590 K R, Int. J. Thermophysics 13 (1992) 575–592.

23 Page 23 of 44

Table 1 Physical property of nanodiamond nanoparticles and base fluids at T = 30oC Nanoparticles/

(kg/m3)

Base fluid

k (W/m K)

(mPa.sec)

Cp (J/kg K)

Nanodiamond

3100

1000

---

516

2

20:80% EG/W

1029.72

0.492

1.65

3815

3

40:60% EG/W

1059.68

0.404

2.96

3468

4

60:40% EG/W

1086.27

0.334

5.38

3084

cr

1

ip t

S. No.

us

Table 2 Quantity of nanodiamond particle required for the volume concentrations Quantity of nanodiamond (g)

, (%)

40:60%

60:40%

0.2

0.241

0.234

0.228

0.6

0.726

0.706

0.689

0.8

0.971

0.943

0.920

1.0

1.216

1.181

1.153

d

M

an

20:80%

te

Table 3 Constants A, B and C used in Eq. (11) T = 20oC

Ac ce p

Nanofluid A

T = 40oC

T = 60oC

B

C

A

B

C

A

B

C

1.439

0.0481

1

1.533

-0.024

0.927

1.24

0.6027

ND/40:60 1.0523 2.034

-0.658

0.984

0.811

0.1903

1.01

0.5725 0.153

ND/60:40 1.0608 1.595

-0.7299

1.0683 1.718

0.8549

1.078

1.942

ND/20:80 0.995

-1.147

24 Page 24 of 44

Table 4 Thermophysical properties of nanofluids at a temperature of 30oC.

0.6

0.8

1.0

0.503

0.5212

0.5346

0.5412

0.5523

(mPa.sec)

1.170

1.700

2.270

2.555

2.840

(kg/m3)

1029

1033.8

1042.1

1046.2

1050.4

Cp (J/kg K)

3815

3808.4

3795

3788

3782

k (W/m K)

0.412

0.4210

0.4348

2.440

3.030

3.985

cr

(kg/m )

1059

1063.7

1071.9

1076

1080.1

Cp (J/kg K)

3468

3462

3450.2

3444.3

3438.4

k (W/m K)

0.338

0.345

0.3534

0.3567

0.3612

(mPa.sec)

3.690

5.740

6.815

7.3525

7.890

(kg/m3)

1086

1090.2

1098.35

1102.3

1106.4

Cp (J/kg K)

3084

3068.59

3063.4

3058.3

k (W/m K)

(mPa.sec) 3

3078.8

0.4402

0.4467

4.4625

4.940

Ac ce p

te

d

40:60%

ip t

0.2

us

40:60%

0.0

an

fluid 20:80%

Volume concentrations (%)

Property

M

Base

25 Page 25 of 44

Table 5 The Mouromtseff number (Mo) and the Figure of Merit

of

concentrations of all the nanofluids at various temperatures

30 ----

0.2

1973.6

2327

3004

3221

0.861

0.6

1761.4

1991

2335

2482

0.769

0.8

1685.6

1878

2147

2344

0.736

1.0

1629

1797

2038

2218

0.0

1406.2

----

----

----

0.2

1291.8

1593.0

1869.5

0.6

1166.8

1399.7

1715.1

0.8

1118.7

1332.0

1648.2

1.0

1079.3

1282.5

0.0

995.27

----

0.2

821.71

1001.2

0.6

774.28

0.8

753.46

1.0

736.65

Temperature (oC) 40 50 60 --------1.31

1.40

0.86

1.02

1.08

0.82

0.93

1.02

0.711

0.78

0.89

0.96

----

----

-----

-----

an

us

cr

1.01

ip t

60 ----

1985.8

0.91

1.13

1.33

1.41

1874.8

0.82

1.00

1.22

1.33

1789.9

0.79

0.95

1.17

1.27

1603.8

1733.1

0.76

0.91

1.14

1.23

----

----

----

----

----

----

1173.6

1269.2

0.83

1.01

1.18

1.28

950.62

1151.5

1217.1

0.78

0.96

1.16

1.22

930.50

1141.9

1217.1

0.76

0.93

1.15

1.22

917.55

1137.2

1210.1

0.74

0.92

1.14

1.22

Ac ce p

60:40

0.0

30 2289.9

Temperature (oC) 40 50 -------

M

40:60

Figure of Merit (FOM)

(%)

d

20:80

Mouromtseff number (Mo)

te

Base fluid

26 Page 26 of 44

ip t cr

Ac ce p

te

d

M

an

us

Fig. 1 Schematic diagram of the synthetic route for functionalized nanodiamonds

27 Page 27 of 44

ip t cr us an M d te Ac ce p Fig. 2 FTIR-spectra (a) ND-soot (b) ND-treated 28 Page 28 of 44

ip t cr us an M d te

Ac ce p

Fig. 3 Raman spectra of ND-soot and ND-treated

29 Page 29 of 44

ip t cr us an M d te Ac ce p Fig. 4 high-resolution TEM results for (a) ND-soot (b) ND-treated

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ip t cr us an M d te

Ac ce p

Fig. 5 (a) XRD pattern of ND-treated and (b) the corresponding selected area X-ray diffraction (SEAD) from TEM analysis

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ip t cr us an M d te Ac ce p Fig. 6 Average particle size distribution in 20:80 EG/W base fluid from DLS measurements (a) UDD-nanopowders (b) acid treated nanodiamond (c) different volume concentrations of 20:80% EG/W nanofluid 32 Page 32 of 44

ip t cr us an M d te

Fig. 7a Experimental thermal conductivity of 20:80% EG/W-based ND nanofluid at various

Ac ce p

concentrations and temperatures

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ip t cr us an M d te

Fig. 7b Experimental thermal conductivity of 40:60% EG/W-based ND nanofluid at various

Ac ce p

concentrations and temperatures

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ip t cr us an M d te

Fig. 7c Experimental thermal conductivity of 60:40% EG/W-based ND nanofluid at various

Ac ce p

concentrations and temperatures

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ip t cr us an M d te

Ac ce p

Fig. 8 Thermal conductivity enhancement of nanofluids with effects of base fluids in comparison with literature data at different volume concentrations

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ip t cr us an M d te

Fig. 9 Comparison of thermal conductivity enhancement

of present data at

Ac ce p

different temperatures with the literature values

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ip t cr us an M d te

Fig. 10 Linear-fit of experimental thermal conductivity ratio (

at

= 1.0% for

Ac ce p

ND/20:80, ND/40:60 and ND/60:40 nanofluids.

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ip t cr us an M d te

Ac ce p

Fig. 11a Experimental viscosity of 20:80% EG/W-based ND nanofluid at various concentrations and temperatures

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ip t cr us an M d te

Ac ce p

Fig. 11b Experimental viscosity of 40:60% EG/W-based ND nanofluid at various concentrations and temperatures

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ip t cr us an M d te

Fig. 11c Experimental viscosity of 60:40% EG/W-based ND nanofluid at various

Ac ce p

concentrations and temperatures

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ip t cr us an M d te

of 60:40% EG/W-based ND nanofluid is compared with

Ac ce p

Fig. 12 Viscosity ratio

the data of Namburu et al. [33] for 60:40% EG/W-based CuO nanofluid

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ip t cr us an M d te

Ac ce p

Fig. 13 Quadratic-fit of the experimental viscosity for all the ND nanofluids at 1.0% volume concentration

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ip t cr us an M d te

Fig. 14 Figure of merit (FOM) of different particle loadings of all the ND nanofluids at a

Ac ce p

temperature of 30oC

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