International Journal of Heat and Mass Transfer 85 (2015) 1034–1040

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International Journal of Heat and Mass Transfer

journal homepage: www.elsevier .com/locate / i jhmt

Thermal performance analysis of Al2O3/R-134a nanorefrigerant

http://dx.doi.org/10.1016/j.ijheatmasstransfer.2015.02.0380017-9310/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +966 13 860 4628; fax: +966 13 860 7312.E-mail addresses: saidur@kfupm.edu.sa, saidur912@yahoo.com (R. Saidur).

I.M. Mahbubul a, A. Saadah a, R. Saidur b,⇑, M.A. Khairul a, A. Kamyar c

a Department of Mechanical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysiab Centre of Research Excellence in Renewable Energy, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran, 31261, Kingdom of Saudi Arabiac School of Mechanical and Mining Engineering, The University of Queensland, QLD 4072, Australia

a r t i c l e i n f o

Article history:Received 13 August 2014Received in revised form 12 February 2015Accepted 12 February 2015Available online 10 March 2015

Keywords:NanofluidThermal conductivityViscosityDensitySpecific heatCoefficient of performance

a b s t r a c t

Nowadays, nanofluids are being considered as an efficient heat transfer fluid in various thermalapplications. Refrigerant-based nanofluids, termed as ‘‘nanorefrigerants’’, have the potential to improvethe heat transfer performances of refrigeration and air-conditioning systems. This study analyzed thethermophysical properties and their effects on the coefficient of performance (COP) resulted by additionof 5 vol.% Al2O3 nanoparticles into R-134a refrigerant at temperatures of 283–308 K. The analysis hasbeen done for a uniform mass flux through a horizontal smooth tube using established correlations.The results indicate that the thermal conductivity, dynamic viscosity, and density of Al2O3/R-134ananorefrigerant increased about 28.58%, 13.68%, and 11%, respectively compared to the base refrigerant(R-134a) for the same temperature. On the other hand, specific heat of nanorefrigerant is slightly lowerthan that of R-134a. Moreover, Al2O3/R-134a nanorefrigerant shows the highest COP of 15%, 3.2%, and2.6% for thermal conductivity, density, and specific heat, respectively compared to R-134a refrigerant.Therefore, application of nanoparticles in refrigeration and air-conditioning systems is promising toimprove the performances of the systems.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction thermophysical properties of the refrigerant. Consequently, energy

A nanorefrigerant is one kind of nanofluid for which the basefluid is a refrigerant. Like other nanofluids, it is a mixture ofrefrigerant and solid particles. Nanorefrigerant is being consideredas a potential to enhance the thermal performance of refrigerationand air-conditioning systems because of the higher thermal con-ductivity of nanoparticles. Three main benefits have been reportedwhen using nanorefrigerants in a refrigerator [1]; Firstly, use ofnanoparticles can improve the solubility between the lubricantand the refrigerant [2]. Secondly, nanoparticles can enhance thethermal conductivity as well as heat transfer characteristics of arefrigerant [3,4]. Finally, reduction of the friction coefficient andwear rate is observed in nanorefrigerants compared to regularrefrigerants [5]. It is hoped that, the addition of nanoparticles intoconventional refrigerants will improve the heat transfer perfor-mance of refrigeration systems [6,7]. Almost all vapor compressionrefrigeration systems use lubricating oil making it possible to usenanoparticles in refrigeration systems in the form of a nanoparti-cles/oil suspension [8]. The existence of nanoparticles in oilsuspension influences the overall performance of the refrigerationsystem since nanoparticles significantly enhance the

consumption will be decreased along with reduction in emissionsthat lead to global warming and greenhouse-gas effects.

Thermophysical properties are the performance parametersthat need to be analyzed in order to select the most suitable optionfor the energy conversion systems. Thermal conductivity is affect-ed by temperature and density. High thermal conductivity of therefrigerant is crucial in order to gain the maximum output fromthe system [9]. Addition of nanoparticles with high thermal con-ductivity and increasing their concentration can enhance the ther-mal conductivity of a nanorefrigerant [10,11]. Viscosity is anotherproperty that affects the pumping power and pressure drop para-meters [12]. It is known that pressure drop plays a significant rolewhen designing and optimizing refrigeration systems [13].Mahbubul et al. [14] studied the viscosity of R123-TiO2 nanorefrig-erant for different nanoparticle volume concentrations usingBrinkman’s model [15], and concluded that pressure drop increasessignificantly with the increase of viscosity. Moreover, rheologicalbehavior of Al2O3/R141b nanorefrigerant was studied and the mix-ture was found to behave in a non-Newtonian way [16]. As like vis-cosity, density of a fluid also has influences on the pressure dropand pumping power capacity. A solid substance has a higher den-sity in comparison to a liquid; therefore, the density of a nanofluidis found to be higher by increasing the concentration of nanoparti-cles within a fluid. Mahbubul et al. [17] measured the density of

Nomenclature

A heat transfer surface area (m2)bd bubble departure diameter (m)Bo boiling numberCo convection numberCp specific heat capacity (J/kg K)COP coefficient of performanceD tube diameter (m)E enhancement factorg gravitational acceleration (m/s2)G mass flux (kg/m2 s)h heat transfer coefficient (W/m2 K)hfg latent heat (kJ/kg)HTC heat transfer coefficient (W/m2 K)k thermal conductivity (W/m K)K orifice constantl tube length (m)L temperature lift (K)_m mass flow rate (kg/s)

Nu Nusselt numberP pressure (Pa)Pr Prandtl numberq heat flux (W/m2)Qout heat output (W)r radius of the tube (m)rp radius of the nanoparticles (m)Re Reynolds numberS suppression factort thickness of interfacial layer (m)T temperature (K)V volumetric flow rate (m3/s)

Wnet total work (W)x mass qualityXtt Martinelli parameter

Greek symbols/ particle volume concentration (%)q density (kg/m3)

l dynamic viscosity (N s/m2)r surface tension (N/m)

Subscriptscond condenserDB pool boilingdown downstreamevap evaporatorin inputl interfacial layer/nanolayerNpl no pressure lossesnr nanorefrigerantp nanoparticler refrigerants saturationSA single phaseup upstreamv vapor

PrefixD gradient

Table 1Properties of Al2O3 nanoparticles [26].

Properties Value

Radius 15 nmMolecular mass 101.00 kg/kmolDensity 3880 kg/m3

Thermal conductivity 40* W/m KSpecific heat 729 J/kg K

* Source: Wang and Mujumdar [27].

I.M. Mahbubul et al. / International Journal of Heat and Mass Transfer 85 (2015) 1034–1040 1035

Al2O3/R141b nanorefrigerant and found that density increases lin-early with increasing the volume concentration and decreases withincreasing the temperature. Specific heat is a measure of energystorage capability of the working fluid. Fluids with large specificheat require significant amounts of energy input to sensiblyincrease or decrease their temperature. Specific heat is proportion-al to the change of internal energy of a system, thus when the tem-perature of the system increases, the fluctuation of molecules willbe intensified and a higher heat capacity will be induced, as moreenergy levels can be filled up. This will be the reason of higher heattransfer rates. However, there is no literature available on thespecific heat capacity of nanorefrigerants.

High COP and environmental friendliness are considered as themajor selection criteria of a refrigerant. There are some studiesavailable about the pool boiling [18], flow boiling [19], convectiveheat transfer [20,21], pressure drop [22,23], migration characteris-tics [24,25], and energy performance [1,6] of the nanorefrigerants.To the best of authors’ knowledge, there are no studies availablediscussing the effect of thermophysical properties on the COP ofa system using nanorefrigerants. The objective of this study is toinvestigate the effect of temperature on thermal conductivity, vis-cosity, density, and specific heat of Al2O3 nanoparticles suspendedin R-134a refrigerant. Moreover, the effects of changed thermo-physical properties of nanorefrigerant on the COP are investigatedand compared with that of R-134a refrigerant.

2. Experimental method

The properties of Al2O3 nanoparticles and R-134a refrigerant areshown in Table 1 and Table 2, respectively. The analysis was car-ried out considering 5 vol.% of Al2O3 nanoparticles in R-134a refrig-erant with the temperature range of 283–308 K. Thermophysical

properties of Al2O3/R-134a nanorefrigerant were calculated usingMicrosoft Excel 2010 based on the established correlations fromliterature.

2.1. Thermal conductivity

Thermal conductivity of Al2O3/R-134a nanorefrigerant waspredicted using the Sitprasert et al. [29] correlation. This modelconsiders the effects of nanoparticle volume concentration,nanoparticle size, and temperature-dependent interfacial layer.

knr ¼ðkp � klÞ/kl½2b3

1 � b3 þ 1� þ ðkp þ 2klÞb31½/b3ðkl � krÞ þ kr �

b31ðkp þ 2klÞ � ðkp � klÞ/½b3

1 þ b3 � 1�ð1Þ

where,

b ¼ 1þ trp

ð1aÞ

b1 ¼ 1þ t2rp

ð1bÞ

Table 2Properties of R-134a refrigerant [28].

Temperature(K)

Pressure(Mpa)

Liquid Density(kg/m3)

Vapor density(kg/m3)

Liquid Specific heat(kJ/kg k)

Liquid Thermal conductivity(W/m k)

Liquid Viscosity(mPa s)

Surface Tension(N/m)

283 0.41461 1261.0 20.226 1.3704 0.087618 0.23487 0.010138288 0.48837 1243.4 23.758 1.3869 0.085444 0.22066 0.009441293 0.57171 1225.3 27.780 1.4049 0.083284 0.20737 0.008756298 0.66538 1206.7 32.350 1.4246 0.081134 0.19489 0.008081303 0.77020 1187.5 37.535 1.4465 0.078992 0.18313 0.007417308 0.88698 1167.5 43.416 1.4709 0.076853 0.17200 0.006766

1036 I.M. Mahbubul et al. / International Journal of Heat and Mass Transfer 85 (2015) 1034–1040

The thickness and the thermal conductivity of the interfaciallayer are calculated from Eqs. (1c) and (1d),

t ¼ 0:01ðT � 73Þr0:35p ð1cÞ

kl ¼ Ctrp

kr ð1dÞ

where C ¼ 30 is a constant for Al2O3 nanoparticles.COP is equal to heat output divided by total work input.

COP ¼ Qout

Wnet;inð2Þ

Eq. (2a) shows the basic equation used to calculate the heattransfer coefficient (HTC).

Q out ¼ hADT ð2aÞ

where, Qout is the heat output, h is the HTC, A is the heat transferarea, and DT is the temperature difference.

Eq. (2b) introduces the relationship between force convectiveboiling heat transfer of pure refrigerant and the output heat, takenfrom Wen et al. [30]

h ¼ EhDB þ ShSA ð2bÞ

In this equation, E is the enhancement factor, S is the suppres-sion factor, hDB is the pool boiling HTC obtained from the correla-tion by Dittus and Boelter [31]. hSA is the single phase heattransfer suggested by Stephan and Abdelsalam [32]. Eqs. (2c) –(2f) express all four parameters.

E ¼ C1BoC2 XC3tt ð2cÞ

hDB ¼ 0:023kD

Re0:8Pr0:4 ð2dÞ

The Reynolds number was calculated using, Re ¼ GDl and Prandtl

number by Pr ¼ CPlk . Here, mass flux,G, and tube diameter, D, were

assumed to be 150 kg/m2 sand 6 mm, respectively.

S ¼ C4CoC5 ð2eÞ

hSA ¼ 207k

bdqðbdÞkTs

� �0:674 qvq

� �0:581

Pr0:533 ð2fÞ

Referring to Eq. (2c), boiling number can be found from,Bo ¼ q

hfg G, while Xtt is the Martinelli parameter defined by

Xtt ¼ ð1�xÞx

h i0:9 qvq

� �0:5 llv

� �0:1. In Eqs. (2e) and (2f), Convection num-

ber, Co ¼ ð1�xÞx

h i0:8 qvq

� �0:5and Bubble departure diameter,

Table 3The constants in Eqs. (2c) and (2e) [30].

C1 C2 C3 C4 C5

53.64 0.314 �0.839 0.927 0.319

bd ¼ 0:0146a 2rgðq�qg Þ

h i0:5with a ¼ 35�. Ts in Eq. (2f) is the saturation

temperature corresponding to the test section pressure for the flowboiling. The constants C1 to C5 in Eqs. (2c) and (2e), were obtainedby an iteration process to minimize the errors between the theore-tical calculated HTC and experimental results [30]. The constantvalues are shown in Table 3.

Substituting the above equations, the final relationship betweenCOP and thermal conductivity for refrigerant and nanorefrigerantare expressed in forms of Eqs. (3) and (4), respectively:

COP¼E 0:023 kr

D Re0:8Pr0:4h i

þS 207 k0:326rðbdÞ

qðbdÞTs

h i0:67 qvq

� �0:581Pr0:533

� � ADT

Wnet;in

ð3Þ

COPnr ¼E 0:023 knr

D Re0:8Pr0:4h i

þS 207 k0:326nrðbdÞ

qðbdÞTs

h i0:67 qvq

� �0:581Pr0:533

� � ADT

Wnet;in

ð4Þ

2.2. Viscosity

The viscosity of nanorefrigerant was calculated using Brinkmanmodel [15] in the following form:

lnr ¼ lr1

ð1� /Þ2:5ð5Þ

where, lnr and lr are the effective viscosity of nanorefrigerant andpure refrigerant, respectively. / is the particle volume fractionwhich is 0.05 (5%) in our case.

Klein et al. [33] analyzed the impact of pressure drop on therefrigeration performance using liquid-suction heat exchanger.They proposed a dimensionless correlation that indicates COP interms of pressure drop as follow:

COPCOPnpl

¼ 1� ð2:37� 0:0471Lþ 3:01� 10�4L2Þ � DPPevap

� �ð6Þ

where, L is the temperature lift which equals to Tcond � Tevap andPevap is the evaporator pressure.

Pressure drop of refrigerant in the compressor was calculatedusing Hagen–Poiseuille equation [34]. In this analysis, refrigerantwas assumed nearly incompressible. Hagen–Poiseuille model thatis used to calculate the pressure drop for a fluid flow through acylindrical tube, is expressed as follows:

DP ¼ 8llVpr4 ð6aÞ

Replacing pressure drop in Eq. (6) by Eq. (6a) gives a new rela-tionship between COP and viscosity:

COPCOPnpl

¼ 1� ð2:37� 0:0471Lþ 3:01� 10�4L2Þ 8llVpPevapr4

� �ð7Þ

Fig. 1. Variation of thermal conductivity as a function of temperature.

I.M. Mahbubul et al. / International Journal of Heat and Mass Transfer 85 (2015) 1034–1040 1037

2.3. Density

The density of nanofluid was calculated using Pak and Cho [35]correlation shown in Eq. (8):

qnr ¼ /qp þ ð1� /Þqr ð8Þ

Performance of the system is dependent on the mass flow rateof the refrigerant. This was suggested by Bukac et al. [36] accordingto the following relation:

COP ¼_mDhE

Wnet;inð9Þ

where, DhE is the change of enthalpy in the evaporator.The single-phase orifice equation [37] was used to calculate the

mass flow rate through a short tube:

_m ¼ K Affiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2g qðPup � PdownÞ

qð9aÞ

To obtain the relationship between COP and density, the massflow rate through a short tube in Eq. (9a) has been substituted intoEq. (9). The final equation is indicated as follows:

COP ¼ DhE K Affiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2g qðPup � PdownÞ

pWnet;in

ð10Þ

2.4. Specific heat

For a given particle volume fraction, specific heat of a nanore-frigerant can be calculated using the correlation suggested byPak and Cho [35].

Cp;nr ¼ /Cp;p þ ð1� /ÞCp;r ð11Þ

where, Cp;p is the specific heat of Al2O3 nanoparticles and Cp;r is thespecific heat of R-134a refrigerant.

Substituting Prandtl numbers in Eqs. (3), (4) by expressions ofspecific heat, viscosity and thermal conductivity, the relationshipbetween COP and specific heat for the refrigerant and nanorefriger-ant can be obtained from Eqs. (12) and (13), respectively.

COP¼E 0:023

D k0:6r Re0:8ðCp;rlrÞ

0:4h i

þS 207bd

� �1kr

� �0:207qðbdÞ

Ts

h i0:674 qvq

� �0:581ðCp;rlrÞ

0:533� �

ADT

Wnet;in

ð12Þ

COPnr ¼E 0:023

D k0:6r Re0:8ðCp;nrlrÞ

0:4h i

þS 207bd

� �1kr

� �0:207qðbdÞ

Ts

h i0:674 qvq

� �0:581ðCp;nrlrÞ

0:533� �

ADT

Wnet;in

ð13Þ

Fig. 2. Effect of the thermal conductivity of Al2O3/R-134a nanorefrigerant on COP atdifferent temperatures.

3. Result and discussion

3.1. Thermal conductivity

Fig. 1 shows the variation of thermal conductivity of refrigerantand nanorefrigerant with temperature ranging from 283 to 308 K.It can be seen in Fig. 1 that, the thermal conductivity of Al2O3/R-134a nanorefrigerant was linearly increased with increasingtemperature, while for pure refrigerant, thermal conductivitymoderately decreased with increasing temperature. As the thermalconductivity of Al2O3 nanoparticle is higher than the base fluid(refrigerant) therefore, the thermal conductivity of the nanorefrig-erant was found to be higher than pure refrigerant [3]. Again, withthe rise of temperature, the Brownian motion of nanoparticles willintensify and the contribution of micro convection in heat trans-port will increase, which results in the augmentation of thermalconductivity [9]. Therefore, the thermal conductivity of nanorefrig-erant tends to increase with increasing temperature. For pure

refrigerant, thermal conductivity was decreased with increasingtemperature. This is due to the fact that when temperature increas-es, the liquid is evaporated, which causes the atoms to be positive-ly charged and vibrate with greater amplitude. This is why thethermal conductivity for any substance is lower at the vapor statecompared with the liquid state. The increments in the thermal con-ductivity of Al2O3/R-134a nanorefrigerant are from 8.12% to 28.58%for 283 K to 308 K, respectively.

Fig. 2 demonstrates the effect of thermal conductivity on COP ofthe refrigeration system at different temperatures for R-134arefrigerant and Al2O3/R-134a nanorefrigerant. As noted fromFig. 2, COP increases with the increase of temperature (calculatedusing Eqs. (3) and (4), respectively). A maximum rise of 15% inCOP is observed for the nanorefrigerant compared to that of therefrigerant due to its higher thermal conductivity. Since thermalconductivity is proportional to HTC, HTC of the nanorefrigerantwith higher thermal conductivity is larger than that of the fluidwith lower thermal conductivity at the same Nusselt number [3].It can be stated that addition of more particles contributes to the

Fig. 4. Effect of the viscosity of Al2O3/R-134a nanorefrigerant on COP at differenttemperatures.

1038 I.M. Mahbubul et al. / International Journal of Heat and Mass Transfer 85 (2015) 1034–1040

increased effective surface area for heat transfer. As a result, theinherently greater thermal conductivity of nanoparticles enhancesthe thermal conductivity of the nanorefrigerant [9]. This is also thereason for maximization of HTC and minimization of entropy gen-eration rate i.e. improvement in exergy efficiency. Since higherenergy levels are more easily accessed, the heat transfer rateshould be increased. As a result, the overall COP of the system willincrease [38].

3.2. Viscosity

Fig. 3 shows the comparison between viscosities of Al2O3/R-134a nanorefrigerant and R-134a refrigerant in the temperaturerange of 283 K to 308 K. Viscosity of refrigerant and nanorefrigerantdecrease linearly with the increase of temperature. The viscosity ofnanorefrigerant is observed to be slightly higher compared to thatof pure refrigerant. The plot outlines the considerable effect of tem-perature on the viscosity of nanorefrigerant. When temperature offluid increases, the inter-particle and intermolecular adhesionforces are decreased [39] and as a result the viscosity of fluid is alsoreduced. This is a natural phenomenon that in most cases, viscosityof a liquid is decreases with the increase of temperature. Whentemperature of any substance increases, the movement amongthe molecules is also intensified. For the higher movement ofthe molecules, the resistance to flow of a material (referred to asviscosity) is decreases. Besides this, a higher temperature ofnanorefrigerant intensifies the Brownian motion of nanoparticles,and as a result, the viscosity of nanorefrigerant is decreased [40].Moreover, the viscosity of nanorefrigerant was found to be 13.68%higher than that of the base fluid (R-134a refrigerant).

Fig. 4 shows the effect of viscosity on COP for both Al2O3/R-134ananorefrigerant and R-134a refrigerant at different temperatures.In both cases, COP was increased with increasing temperature. Itis essential to study the flow resistance of the nanorefrigerant inaddition to the heat transfer improvement characteristic in orderto be able to utilize the nanorefrigerant in a refrigerator systemin a feasible way. The R-134a refrigerant and Al2O3/R-134a nanore-frigerants with 5 vol.% concentration are considered for the calcu-lation of the pressure drop. The COP of refrigerant in terms ofviscosity is calculated using Eq. (7). Apparently, no substantialaddition to the pressure drop is caused for the nanorefrigerant inall successions of the enquiry, which exposes that nanorefrigerantwill not result in an additional penalty as a rise of pumping power

Fig. 3. Variation of viscosity as a function of temperature.

[41]. As a result, though Al2O3/R-134a nanorefrigerant has higherviscosity compared to R-134a refrigerant, it shows better COP ratiocompared to R-134a refrigerant.

3.3. Density

The change in density with respect to refrigerant’s temperaturehas been shown in Fig. 5 for R-134a and Al2O3/R-134a nanorefrig-erant. Eq. (8) was used to calculate the density of nanorefrigerant.The density of Al2O3/R-134a nanorefrigerant and R-134a refriger-ant are moderately decreased with the increase of temperature.It is found that, Al2O3/R-134a nanorefrigerant exhibitsapproximately 11% higher density compared to R-134a refrigerantat the same temperature. Density is defined as the mass divided byvolume. The atoms of the refrigerant are starting to vibrate withincreasing temperature. Hence, the volume of the refrigerantincreases and the density will linearly decrease with the increaseof temperature. Moreover, the density of solid particles is muchhigher than that of liquids or gases (in this case the density of

Fig. 5. Variation of density as a function of temperature.

Fig. 7. Variation of specific heat as a function of temperature.

I.M. Mahbubul et al. / International Journal of Heat and Mass Transfer 85 (2015) 1034–1040 1039

Al2O3 and R-134a are about 3880 and 1220 kg/m3, respectively)causing the mixture of solid–liquid suspension to show higherdensity compared to the base fluid. This is why the density ofAl2O3/R-134a nanorefrigerant is found to be higher than theR-134a refrigerant.

The effect of density on the COP at different temperatures hasbeen shown in Fig. 6. COP for both Al2O3/R-134a nanorefrigerantand R-134a refrigerant are noted to decrease with the increase oftemperature. The COP of nanorefrigerant is about 3.2% higher thanthat of R-134a refrigerant due to the high density of nanorefriger-ant. COP and mass flow rate are calculated by Eq. (9) and (9a). Fromthe equations, the COP is directly proportional to mass flow rate,and the mass flow rate of Al2O3/R-134a nanorefrigerant is higherthan the pure refrigerant due to its higher density. Therefore,higher COP is observed for the nanorefrigerant. In a centrifugalcompressor, pressure rise is related to the density of the refriger-ant. A high value of density results in the high pressure rise whichindirectly reduces the overall pressure drop. This, in turn, willimprove the system performance.

3.4. Specific heat

The specific heat of both Al2O3/R-134a nanorefrigerant andR-134a refrigerant linearly increase with the rise of temperaturefrom 283 K to 308 K as shown in Fig. 7. The specific heat of nanore-frigerant was calculated from Eq. (11). For a particular volume frac-tion, analysis showed that the minimum specific heat belonged tothe nanorefrigerant. This decrease is due to the lower specific heatof added particles. Moreover, higher specific heat of the base fluidis the reason why the specific heat of the mixture exceeds that ofthe nanoparticles. Therefore, the specific heat of the solid–liquidmixture becomes lower than the specific heat value of the base flu-id. Specific heat capacity of R-134a refrigerant is 2.4% higher com-pared to Al2O3/R-134a nanorefrigerant. It is because an increase inheat capacity will increase internal energy of the system. Theincrement in temperature will cause the liquid to fluctuate aboutits equilibrium value to a higher extent, and then the heat capacityof the system will increase, as more energy levels will be filled up.Most researchers agree upon the fact that specific heat capacity ofnanofluids are lower than that of the base fluids [42].

Fig. 8 demonstrates the effect of specific heat on the COP of arefrigeration system considering at different temperatures. Thevalue of COP linearly rises with the increase of temperature for

Fig. 6. Effect of the density of Al2O3/R-134a nanorefrigerant on COP at differenttemperatures.

Fig. 8. Effect of the specific heat of Al2O3/R-134a nanorefrigerant on COP atdifferent temperatures.

both Al2O3/R-134a nanorefrigerant and R-134a refrigerant.Approximately, a 2.6% higher COP was noticed for nanorefrigerantcompared to R-134a refrigerant. This parameter is increasedtremendously with increasing the output temperature.Refrigeration systems operated with nanorefrigerant provide moreefficiency due to their higher output temperature. The specific rea-son for higher output temperature is the more amounts ofnanoparticles in the base fluid. As we know, specific heat is definedas, ‘‘the heat required to raise the temperature of a unit mass of asubstance by one unit of temperature.’’ It is clear from the defini-tion of specific heat that any substance with lower specific heatprovides more output temperature for equal heat flow.

4. Conclusions

In the present study, thermophysical properties of Al2O3/R-134ananorefrigerant and the effect of these properties on the COP withrespect to temperature have been studied. The outcomes of thisanalysis could be drawn as follows.

1040 I.M. Mahbubul et al. / International Journal of Heat and Mass Transfer 85 (2015) 1034–1040

Thermal conductivity of the Al2O3/R-134a nanorefrigerant wasfound to increase by the increase of temperature. Approximately,a maximum of 28.58% enhancement of thermal conductivity isfound for the nanorefrigerant compared to the base refrigerant.Analytical results revealed that the thermal conductivity ofnanorefrigerant could enhance the COP of the refrigeration systemup to 15% in comparison with the pure refrigerant.

Both viscosity and density were found to be augmented forAl2O3/R-134a nanorefrigerant compared to pure refrigerant.About 13.68% and 11% enhancements have been also found forviscosity and density, respectively. Unlike, thermal conductivity;dynamic viscosity and density were decreased by the increase oftemperature for Al2O3/R-134a nanorefrigerant. When consideringthe viscosity, it can be concluded that the COP can be enhancedby replacing the working fluid with nanorefrigerant. COP increasedroughly by 3.2% as a function of density, for the case ofnanorefrigerant.

Specific heat capacities of refrigerant and nanorefrigerant werealso increased with increasing temperature. However, specific heatof nanorefrigerant was found to be slightly lower than the purerefrigerant. Nevertheless, 2.6% higher COP was observed fornanorefrigerant due to its specific heat capacity.

We hope that this study will shed light on the effect of usingnanorefrigerants on the performance of the refrigeration systemsand encourage more future investigations in this area. More studyand experimental work are required to check the improvement ofthe performance and the efficiency of the refrigeration systemsusing nanorefrigerants.

Conflict of interest

None declared.

Acknowledgment

‘‘The authors are thankful to University of Malaya for financialsupport under the High Impact Research MoE Grant: UM.C/625/1/HIR/MoE/ENG/40 (D000040-16001) from the Ministry ofEducation Malaysia.’’

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