Measurement of thermal conductivity,viscosity and density of ionic liquid[EMIM][DEP]-based nanofluids☆

Hua Xie ,Zongchang Zhao ,*,Jianhua Zhao ,Hongtao Gao

1 School of Chemical Engineering,Dalian University of Technology,Dalian 116024,China

2 Institute of Marine Engineering and Thermal Science,Dalian Maritime University,Dalian 116026,China

1.Introduction

As the economy developed rapidly,energy and environmental issues have become the hot topics.Searching for alternative energy sources,clean working fluids and optimizing current energy technologies are the key ways to solve these issues.Absorption heat pump(AHP)or absorption heat transformer(AHT)and absorption refrigeration are important energy-saving and cost-reducing devices which can recover various kinds of low temperature exhaust heat and reuse it for heating process or air-conditions.The conventional working pairs of AHP and absorption refrigeration are mainly water-ammonia and lithium bromide-water[1].However they have great disadvantages respectively such as toxicity,high operating pressure,corrosion and crystallization[2].Therefore it is necessary to explore new working pairs to overcome these shortcomings in industrial application.

Ionic liquids(ILs)are new kinds of solvent composed of organic cations and organic or inorganic anions.They are in the liquid state at room temperature.Studies have shown that the ILs possess considerable actual and potential applications in electrochemical process[3],solar collecting process[4],extraction separation[5],carbon dioxide capture[6],organic synthesis[7]and catalysts[8],etc.Imidazolium ionic liquids are important parts in the family of ionic liquids and have high thermal stability,negligible vapor pressure,wide liquid state temperature range,good solubility to water and alcohols as well as weaker corrosion tendency to ion steel equipments than lithium bromide-water[3].These excellent properties together with excellent thermodynamic properties make some aqueous solution of imidazolium ILs such as 1-butyl-3-methylimidazolium bromide,[BMIM]Br-water,1-ethyl-3-methylimidazolium dimethylphosphate,[EMIM][DMP]-water and 1-ethyl-3-methylimidazolium diethylphosphate,[EMIM][DEP]-water become the potential candidate of current working pairs LiBr-H2O used in AHP and absorption refrigeration[3,9-15].As the new working fluids of absorption cycles,the aqueous solution of ionic liquids should have not only excellent thermodynamic properties but also excellent heat and mass transfer properties.However the aqueous solution of ionic liquid mentioned above has lower thermal conductivity compared with that of LiBr-H2O,so enhancing the thermal conductivity of aqueous solution of ionic liquid mentioned above is very important for the application of these new working pairs.

IoNanofluids(INF),which are prepared by dispersing nanoparticles(tubes or rods)into ILs,are a specific type of nanofluids.The term IoNanofluids was first proposed by Nieto de Castro and co-workers in 2009[16].Nieto de Castroet al.[17,18]reported that INF(Imidazolium and Pyrrolidinium Liquids with MWCNTs)had higher thermal conductivity and heatcapacity than the base ILs.Wanget al.[19]studied the effect of five parameters(temperature,dispersion condition,particle size,surface state,and viscosity of base liquid)on thermalconductivity of INF(1-butyl-3-methylimidazolium hexa fluorophosphate with gold nanoparticles).Ferreiraet al.[20]determined the transport and thermal properties of quaternary phosphonium ionic liquids as well as IoNanofluids(phosphonium ILs with MWCNTs).Pauletal.[21]reported that the thermal conductivity was enhanced by 5%-6%and the specific heat capacity was enhanced by 23%.In addition,the heat transfer coefficient was increased by 20%for the INF composed of imidazolium ILs and aluminium oxide nano-particles(Al2O3).Liuet al.[22]measured the thermal conductivity,viscosity,specific heat and density of the graphene-dispersed nanofluids based on the ionic liquid 1-hexyl-3-methylimidazolium tetra fluoroborate([HMIM][BF4]).

All the previous studies were mainly about the thermophysical property of INF which shows better properties than the base ILs and makes itself a good alternative of heat transfer fluids.However,few studies consider the thermophysical property of the aqueous solution of IL-based nanofluids(SNF)which may be new important working pairs used in AHP orabsorption refrigeration in the future.In this article,INF will be prepared with ionic liquid,[EMIM][DEP]and MWCNTs.SNF consist of the aqueous solution of[EMIM][DEP](1)+H2O(2)and MWCNTs.The thermal conductivity,viscosity and density of INF along with SNF will be measured experimentally.In addition,the effects of the mass fraction of MWCNTs,the temperature and the mole fraction of water will also be evaluated.Meanwhile,the thermal conductivities and viscosities will be calculated using the models in literatures to prove the feasibility of the models applied to the new systems mentioned above.

2.Experimental

2.1.Materials

Ionic liquid[EMIM][DEP]used in this work was synthesized and purified in a laboratory following the procedure described by Nieet al.[23].N-methylimidazole(AR)was purchased from Beshine of Chemical Science and Technology(Beijing)Co.,Ltd,and triethyl phosphate(AR)was purchased from Sinopharm Chemical Reagent Co.,Ltd.All reagents were checked using nuclear magnetic resonance spectrometer(1HNMR)and used without any further purification.An appropriate amount ofN-methylimidazole was put into a flask and then equimolar amount of triethyl phosphate was also added gradually.The reaction lasted and refluxed 10 h at 423.15 K.After the resultant was cooled down,the mixture was extracted with ether and then the product was treated with rotary evaporator to remove impurities.The structure of the[EMIM][DEP]was characterized with1HNMR.The purity of the product was 98%and the water content was 0.555%measured by Karl-Fisher titration.

The raw MWCNTs were purchased from Shenzhen Nanotech Port Co.,Ltd,with the following specification:diameter in(20-40)nm;length in(1-2)μm;purity＞97%;ash ＜3%;and special surface area of(70-150)m2·g-1.The raw MWCNTs were modified through procedure proposed by Esumiet al.[24],which is as follows:1 g of CNTs was added to 40 ml of concentrated nitric acid(≈68%)and refluxed for 8 h at 393.15 K.After washing with distilled water until a pH value around 7,the CNTs were dried at 353.15 K.The treated MWCNTs were added to IL and its aqueous solutions then mixed by a magnetic stirrer for 10 min and sonication[17]for 5 min at least for a homogeneous dispersion.The prepared samples were:[EMIM][DEP]+MWCNTs with mass fractions of MWCNTs of Φ=0.2%(0.1%by volume),Φ =0.5%(0.3%by volume)and Φ =1.0%(0.5%by volume)respectively;[EMIM][DEP](1)(x1=0.6 andx1=0.4 by mole fraction)+H2O(2)+MWCNTs with mass fractions of MWCNTs of Φ =0.2%(0.1%by volume),Φ =0.5%(0.3%by volume)and Φ=1.0%(0.5%by volume)respectively.Fig.1 shows the TEM picture of treated MWCNTs.Figs.2 and 3 show the steady nanofluids with 1.0%CNTs by mass fraction without any surfactants and the stable state can last for 20 h at least.

2.2.Experimental procedure

Fig.1.TEM picture of treated MWCNTs.

Fig.2.The picture of MWCNTs+[EMIM][DEP]+H2O at room temperature.

The thermal conductivity of the samples was measured at the temperature from 298.15 K to 353.15 K using a thermal conductivity measuring instrument(TC 3010L)purchased from Xi'an Xiatech Co.,Ltd.The principle of the TC 3010L is the transient hot-wire method.The specification of TC 3010L are as follows:measuring range of 0.001-5.0 W·m-1·K-1;pressure range of 0-10 MPa;accuracy of±2%;resolution ratio of 0.0005 W·m-1·K-1;repeatability of±2%.The amount of each sample was around 30 ml and temperature was controlled by an external water thermostat.The measurement at each temperature was repeated for three times and the average values were used in this article.

The viscosity of the samples was measured at the temperature from 298.15 K to 323.15 K using a capillary viscometer immerged in a water thermostat with a stirrer.The temperature of the water bath was controlled to±0.1 K.The diameter of the capillary pipe is 1.5 mm in this measurement.The capillary viscometer was calibrated by using diethylene glycol.The constant of the capillary viscometer is 0.3548 mm2·s-2.The measurement at each temperature was repeated for three times and the average values were considered as the finaldata.

Fig.3.The picture of MWCNTs+[EMIM][DEP]at room temperature.

The density of the samples was measured at the temperature from 298.15 K to 323.15 K using a liquid specific gravity balance immerged in a water thermostat which is also used in viscosity measurement.The liquid specific gravity balance was calibrated by using deionized water.The uncertainty of the density measurements was less than±0.0001 g·cm-3.

3.Results and Discussions

3.1.Thermal conductivity of[EMIM][DEP](1)+H2O(2)

The thermal conductivity of deionized water was measured with TC 3010L and the results were compared with the data in literature[25].The average relative deviation was 0.758%,which proved the feasibility of the measuring method used.

The thermal conductivities of[EMIM][DEP](1)+H2O(2)solutions with different mole fractionsx1were measured from about 298.15 K to 353.15 K in this work.The thermal conductivities(kf)of IL[EMIM][DEP]and its aqueous solutions are shown in Table 1 and Fig.4.The results indicate that the thermal conductivities are in well linear dependence of temperature and the thermal conductivities of solutions withx1=0.4-1.0 decrease slightly as temperature increases.However,thermal conductivity increases apparently with increasing temperature for solution withx1=0.2.The reason is that the thermal conductivity of ionic liquid[EMIM][DEP]and that of water tends to follow an opposite direction as temperature increases,for the former the thermal conductivity will decrease as temperature increases,while opposite for the later one.The thermal conductivity of the solutions[EMIM][DEP](1)+H2O(2)are between those of water and[EMIM][DEP]and are affected by IL concentration.Whenx1is 0.2,the binary solution[EMIM][DEP](1)+H2O(2)contains more water and its thermal conductivity are mainly dominated by water.So it has the same variation tendency with temperature as that of water.Fig.4 also exhibits a well linear relationship between the thermal conductivitykfand the temperatureT.They were fitted with a linear equation:

wherek0andk1are the fitting parameters whose values and uncertainties are given in Table 2.

Table 1Thermal conductivities(k f)of[EMIM][DEP](1)+H2O(2)solutions at various temperatures and p0=101.325 kPa

Fig.4.Thermal conductivities of[EMIM][DEP](1)+H2O(2)solutions.(x1 is the IL mole fraction.■x1=1.0;▲ x1=0.8;★x1=0.6;▼x1=0.4;◆ x1=0.2;● x1=0.Solid lines are fitted ones with parameters in Table 3.)

3.2.Thermal conductivity of IL-based nanofluids and the aqueous solutions of IL based nanofluids

The effective thermal conductivity(keff)of the INF and SNF was measured from 298.15 K to 353.15 K in our work.As shown in Fig.5,the effective thermal conductivity with 1.0 wt.%MWCNTs varies almost linearly with temperature.The values and uncertainties of parametersin Eq.(1)are given respectively in Table 3.Fig.5 also shows that the effective thermal conductivity of the INF is decreased with increasing temperature,while that of SNF(x1=0.6,0.4)is opposite.Since the vibration and the rotation of the molecular chain in the equilibrium position are the main factors[23],the thermal conductivity of organic liquid decreases with increasing temperature.The enhanced Brownian motion of 0.01 mass fraction of MWCNTs cannot change the decreasing trend of the INF.While the enhanced molecular thermal motion,increased molecular collision and enhanced Brownian motion are the main factors[26]of the increasing effective thermal conductivity of SNF(x1=0.6,0.4)with temperature.This trend is favorable for heat transfer in falling film absorption process in AHP or AHT wherein absorption temperature is higher.

Table 2Fitting parameters,k0 and k1,in Eq.(1)for the thermal conductivity of the[EMIM][DEP](1)+H2O(2)

Fig.5.Thermal conductivities ofINF and SNF with a mass fraction of0.01 of MWCNTs.(x1 is the IL mole fraction.■x1=1.0;▲x1=0.6;●x1=0.4.Solid lines are fitted lines by Eq.(1)with parameters in Table 4.)

Table 3Fitting parameters,k0 and k1,of Eq.(1)for the thermal conductivity of INF and SNF with a mass fraction of 0.01 of MWCNTs

Fig.6.Variation of the enhancement ratio of thermal conductivities of the INF&SNF with a mass fraction of 0.01 of MWCNTs with temperature(x1 is the IL mole fraction).

Fig.6 shows the ratio of thermal conductivity of the INF&SNF with a mass fraction of 0.01 of MWCNTs to those of base liquids,namely thermal conductivity enhancement ratio.Compared with the pure IL,the enhancement ratio of INF ranges from 8.1%to 9.7%as the temperature varies from 298.15 K to 353.15 K.However,the ratio of SNF(x1=0.6,0.4)varies from(5.02%,5.1%)to(8.2%,7.9%)respectively,and the enhancement ratio approximately increases with increasing test temperature.Fig.7 shows that the enhancement ratio of both the INF and SNF increases with increasing mass fraction of MWCNTs at 300 K.Also,the enhancement ratio of INF is the highest.However,it is interesting to see that the enhanced ratio of SNF(x1=0.4)is higher than that of SNF(x1=0.6).To our consideration,the thermal conductivity enhancement ratio is not a simple linear function of concentration of IL.The structure of the interphase of particle/ fluid is the major mechanism responsible for the unexpected enhancement[17].As MWCNTs apparently increases the thermal conductivity of base liquids,the MWCNT-dispersed[EMIM][DEP](1)+H2O(2)may be better than its basedliquid to be used as working pairs in AHP and AHT.

Fig.7.Variation of the enhancement ratio of thermal conductivities of the INF&SNF with mass fractions of MWCNTs at 300 K(x1 is the IL mole fraction).

3.3.Empirical correlations of thermal conductivity of INF and SNF

The effective thermal conductivity of the INF and SNF is correlated using four conventional models in the literatures for comparison to experiments.The four models are shown in Table 4.Here,kpandkfare the thermal conductivity of the MWCNTs and the base liquids.Maxwell model[27]only considered the effect of volume fraction of nanoparticles.Hamilton-Crosser(H-C)model[28]improved and considered not only volume fraction but also the shape of nanoparticles.Herenis the shape parameter which is an empirical constant depending on the shape of the nanoparticles and on the ratio of the conductivity of the two phases.When the particles are spherical,nis equal to 3 and independent of both the ratio and the particle size.The H-C equation becomes the same as the Maxwell's equation in this case.In our worknis taken as 6 because MWCNTs are the cylindrical particles according to the literature.Unit-Cell(U-C)model[29]is proposed to take specific shape factorKinto account by Yamada and Ota.K=2[30]represents shape factor for cylindrical particles.lpanddpare the length and diameter of the cylindrical nanoparticles.In this work,lp/dpis equal to 50,andxis equal to 0.2 for the INF as well as the SNF.Interfacial model[31]proposed by Murshedet al.gives better predictions for the effective thermal conductivity of nanofluids since it considered the interfacial layer between particle and the fluid medium.Here ω is the ratio ofklftokf,which are the thermal conductivity of interfacial layer and base liquid respectively.ω ＞1 is taken as empirical value.Herehis equal to 2 nm for CNTs and to 1 nm for spherical particle according to the literature.Over all the models,φvis the volume fraction of nanoparticles.Fig.8 shows the prediction results of the four models and experimental data.As shown in the figures,the values of Maxwell model and H-C model are lower than experimental ones obviously since only thinking of the volume fraction and the shape.The results of the U-C model are much better than the former ones.But comparing to the interfacial layer model,the latter one is closer to the experimental data.The values of ω are 15,7 and 9 for figure a,b,and c respectively.As ω is an empirical parameter depending on the orderings of base fluid molecules in the interface and the surface chemistry of nanoparticles.The interfacial layer model is more flexible and suitable to predict the effective thermal conductivity of nanofluids at low mass fraction of MWCNTs and the results are more approximate to the experimental data.

Table 4Models for the effective thermal conductivity of nanofluids

Fig.8.Simulated and experimental effective thermal conductivity of the nanofluids with[EMIM][DEP]based and[EMIM][DEP](1)+H2O(2)based((a)[EMIM][DEP];(b)x1=0.6;(c)x1=0.4.)

3.4.Dynamic viscosity of INF and SNF

The viscosity of diethylene glycol was measured by capillary viscometer and compared with the data in literature[32].The average relative deviation was 0.82%,which proved the feasibility of the measuring method.

The viscosity of the samples was measured from 298.15 K to 323.15 K.The results were correlated with the equations in literatures.There are many classical models as follows.

Drew and Passman[33]proposed the equation as:

Brinkman[34]modified the equation as:

and Wang[35]suggested a model as:

All above models fail to predict the viscosity of nanofluids until the W.Duangthongsuk[36]proposed an equation which is as follows:

where ηeffand ηfare the viscosity of nanofluids and base liquids respectively,φvis the volume fraction of MWCNTs.Thea,b,andcare the parameters which can be fitted according to the values of viscosity of the samples.The values of ηfof[EMIM][DEP]and its aqueous solutions were quoted from Shuanghua Yan[37].The fitted parametersa,b,cand the correlation coefficient(R2)were listed in Table 5.Fig.9 exhibited that the fitting curves coincided well with the experimental data.As shown in Fig.10,the viscosity increased with increasing the volume fraction of the MWCNTs.This trend is the same as that of MWCNTs dispersed in[BMIM][PF6]in literature[38].Moreover,the viscosity was reduced when the amount of water was increased.Fig.11 shows the variation of the viscosity of the samples with the measured temperature.The results were correlated with the following equation[39].

Table 5Fitting parameters a,b,and c in Eq.(9)at 298.15 K

Fig.9.Variation of the viscosity ratio of the nanofluids compared with their base liquids with various mass fractions of MWCNTs at 298.15 K.

Fig.10.Viscosities of the nanofluids with various mass fractions of MWCNTs at 298.15 K.

where ηeffis the viscosity of nanofluid,Tis the absolute temperature,andA&Bare the parameters depending on the volume fraction of nanoparticles.The correlations can be expressed as follows:

The values of fitting parameters were listed in Table 6.As shown in Fig.11,the viscosities of the INF and SNF obviously decrease with increasing measured temperature,and approach to that of the base fluids respectively with further increasing temperature,although the viscosity apparently increased with increasing volume fraction of the MWCNTs.This trend would be favorable to the heat transfer process in the AHP and AHT in which output temperature of absorber will be higher.Moreover INF can serve as heating fluids for heat exchanger and reactors in chemical engineering owing to high thermal stability,negligible vapor pressure and wide liquid state temperature range.

Fig.11.Viscosity of INF and SNF as a function of temperature.x1 is the IL mole fraction.

Table 6Fitting parameters Ai B i and R2 in Eqs.(11)and(12)

3.5.Density of INF and SNF

The liquid specific gravity balance was calibrated by measuring the density of the deionized water and the average relative deviation was 0.05%compared with the data in literature[40].

The density of the samples was measured from 298.15 to 323.15 K.The results are shown in Fig.12 and were correlated with the following equation[41],

where

Fig.12.Density of INF and SNF with various mass fractions of MWCNTs.(x1 is the IL mole fraction,solid lines are fitted ones with parameters given in Table 7.).

where ρ is the density,α and β are the parameters related with the volume fraction(φv)of MWCNTs andaiandbiare the regression parameters and given in Table 7.As shown in Fig.12,the fitting lines coincided well with the experimental data.The density of the samples decrease with increasing temperature and increase with the increasing mass fraction of MWCNTs at the same temperature as MWCNTs have higher inherent density and thermal conductivity than those of[EMIM][DEP]and its aqueous solution.So the dispersed MWCNTs in the aqueous solution of[EMIM][DEP]can enhance the density and thermal conductivity of the solution.

Table 7Fitting parameters a i b i,in Eqs.(14)and(15)

4.Conclusions

The INF and SNF with various mass fractions of modified MWCNTs are stable in at least 20 h even at 353.15 K.The thermal conductivity of[EMIM][DEP]and its aqueous solutions(x1=0.8,x1=0.6,x1=0.4)decreases slightly with increasing temperature and shows linear dependence of temperature.However,the thermal conductivity of the deionized water and aqueous solution of[EMIM][DEP](x1=0.2)increase slightly with increasing temperature.The thermal conductivity of the INF increases significantly with the increasing mass fraction of MWCNTs and decreases slightly with the increasing measured temperature.While the thermal conductivity of the SNF increases as both the mass fraction of MWCNTs and the temperature increase.The enhanced thermal conductivity and the variation trend are in favor of heat transfer in falling film absorption in AHP and AHT.The most suitable model for predicting the effective thermal conductivity of the INF and SNF is the interfacial model.The viscosity of the INF and SNF will increase with the increasing mass fraction of MWCNTs and decrease dramatically as temperature rises.At the same time the viscosity of the samples tends to be equal to those of their base liquids with temperature rising.The density of the INF and SNF increases with the increasing mass fraction of MWCNTs or decreases with the increasing temperature.Consequently,[EMIM][DEP]+H2O with dispersed MWCNTs can enhance heat transfer in falling film absorption in AHP or AHT.

Nomenclature

dpdiameter of the cylindrical nanoparticle,nm

hempirical constant(=2 for cylindrical particles)

Kshape parameter of U-C model

keffthermal conductivities of nanofluids(INF and SNF),W·m-1·K-1

kfthermal conductivities of[EMIM][DEP]and its aqueous solutions,W·m-1·K-1

klfthermal conductivity of interfacial layer,W·m-1·K-1

kpthermal conductivity of nanoparticle,W·m-1·K-1

lplength of the cylindrical nanoparticle,nm

nshape empirical constant of H-C model(=6 for cylindrical particles)

rpradius of the cylindrical nanoparticle,nmTtemperature,K

x1mole fraction of[EMIM][DEP]

ηeffviscosity of nanofluids(INF and SNF),mPa·s

ηfviscosity of[EMIM][DEP]and its aqueous solutions,mPa·s

η viscosity of fluids,mPa·s

ρ density of fluids,g·cm-3

φ mass fraction of nanoparticle

φvvolume fraction of nanoparticle

ω the ratio ofklf/kf

Subscripts

f fluid

v volume

p particle

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