International Communications in Heat and Mass Transfer 61 (2015) 42–48

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International Communications in Heat and Mass Transfer

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Measurements and correlations of frictional pressure drop of TiO2/R123flow boiling inside a horizontal smooth tube☆

Omer A. Alawi a, Nor Azwadi Che Sidik a,⁎, A.Sh. Kherbeet b

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

☆ Communicated by W.J. Minkowycz.⁎ Corresponding author.

E-mail address: azwadi@fkm.utm.my (N.A.C. Sidik).

http://dx.doi.org/10.1016/j.icheatmasstransfer.2014.12.000735-1933/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Available online 27 December 2014

Keywords:NanorefrigerantDynamic viscosityPressure dropVolume concentrationTemperatureVapor quality

Nanorefrigerant is one kind of nanofluids. It is the mixture of nanoparticles with refrigerants. It has betterheat transfer performance than traditional refrigerants. Recently, some researches have been done aboutnanorefrigerants. Most of them are related to thermal conductivity of these fluids. Viscosity also deserves asmuch consideration as thermal conductivity. Pumping power and pressure drop depend on viscosity. In thispaper, the volumetric and temperature effects over viscosity of TiO2/R123 nanorefrigerants have been studied.Numerical conditions include temperature from 300 to 325 K, nanoparticle concentrations from 0.5% up to 2%,mass fluxes from 150 to 200 kg m−2 s−1, inlet vapor qualities from 0.2 to 0.7 and diameter of tube from 6 to10mm. The effect of pressure dropwith the increase of viscosity has also been investigated. Based on the analysisit is found that viscosity of nanorefrigerant increased accordingly with the increase of nanoparticle volumeconcentrations and decreaseswith the increment of temperature. Furthermore, pressure drop augmented signif-icantly with the intensification of volume concentrations and vapor quality. Therefore, low volume concentra-tions of nanorefrigerant are suggested for better performance of a refrigeration system.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

In recent years, refrigerant-based nanofluids formed by suspendingnanoparticles in pure refrigerants have been used as a new kindof working fluid to improve the performance of refrigeration systems[2,21,22]. The presence of nanoparticles may have effects on the pres-sure drop characteristics of refrigerant flow boiling inside tubes, andthen have impacts on the overall performance of the heat exchangersof refrigeration systems. Therefore, the pressure drop characteristics ofrefrigerant-based nanofluids must be known for the design and optimi-zation of the heat exchangers in refrigeration systems using refrigerant-based nanofluids.

The researches on the pressure drop characteristics of nanofluids canbe divided into two categories. One is to investigate the single-phasepressure drop characteristics of nanofluids, and the other is to investi-gate the phase-change pressure drop characteristics of nanofluids.

For the single-phase pressure drop characteristics of nanofluids,experimental studies [4,9,12] and simulation study [13] have been re-ported in literatures. Experiments on the single-phase pressure dropof CuO/H2O nanofluid in micro-channel heat sink showed that thepresence of nanoparticle causes a slight increase in pressure drop [4].Experiments on the single-phase pressure drop of TiO2/H2O nanofluid

6

flowing upward through a vertical pipe showed that the pressuredrop of nanofluid is a little larger than that of the host fluid at a givenReynolds number [9]. Experiments on the single-phase pressure dropof Al2O3/H2O nanofluid in micro-channel showed that the pressuredrop of nanofluid is larger than that of the host fluid, and increaseswith the increase of nanoparticle concentration at the same Reynoldsnumber [12]. Li and Kleinstreuer [13] simulated the fully developedpressure gradient of CuO/H2O nanofluid flow inside micro-channels.The simulation results show that: (i) comparing to the host fluid ata given Reynolds number, the pressure gradient enhancements areless than 2% and 5% at nanoparticle volume fractions of 1% and 4%,respectively; (ii) comparing to the host fluid at a given mean velocity,the pressure gradient enhancements are less than 5% and 15% at nano-particle volume fractions of 1% and 4%, respectively. All these researchesshow that the single-phase pressure drop of nanofluid is larger than thatof the host fluid, and the enhancement of the pressure drop is related tothe nanoparticle concentration.

Comparing to the researches on the single-phase pressure dropcharacteristics of nanofluids, there are much fewer researches on thephase-change pressure drop characteristics of nanofluids. A literaturesurvey shows that the phase change pressure drop of nanofluid is men-tioned only by the paper of Bartelt et al. [1]. In the paper, the authorsfound that the presence of nanoparticle has an insignificant effecton the pressure drop of refrigerant/nanolubricant mixture (R-134a/POE/CuO nanofluid) flow boiling inside a horizontal tube, but no exper-imental data of the pressure drop were provided. The reason for such

Nomenclature

d diameter of nanoparticles (nm)FPD nanoparticle impact factorm mass flow rate (kg s−1)ΔP pressure drop (Pa)T temperature (K)f friction factor of Müller-Steinhagenh heat transfer coefficient (W/m2 K)L tube length (m)d diameter of tube (m)G mass flux (kg m−2 s−1)P pressure (Pa)q heat flux (Wm−2)x vapor qualityg gravitational acceleration (m/s2)k thermal conductivity (W/m K)X parameter of Lockhart and Martinelli

Greek symbolsμ dynamic viscosity (pa.s)σ surface tension (Nm−1)ω mass fraction of nanoparticlesρ density (kg m−3)φ volume fraction of nanoparticles

Subscriptsfrict frictionalL liquidr refrigeranti insiden nanoparticlesv vapor

Table 1Properties of TiO2 nanoparticle.

Property Unit Value

Purities (%) ≥99.5Average particle diameter (nm) ~20Molecular mass (g/mol) 78.87Density (kg/m3) 4260

Table 2Properties of R123 refrigerant.

Property Unit Value

Chemical formula – CHCl2CF3Molecular mass (g/mol) 153Normal boiling point (°C) 27.8Freezing point (°C) 107Critical temperature (°C) 184Critical pressure (MPa) 3.67Density (kg/m3) 1458.8Thermal conductivity (W/m.K) 0.075862Dynamic viscosity (mPa.s) 0.40805Specific heat (J/kg.K) 1022

43O.A. Alawi et al. / International Communications in Heat and Mass Transfer 61 (2015) 42–48

insignificant effect might be that the presence of lubricant oil can signif-icantly enhance the pressure drop of refrigerant flow boiling inside atube [10,24] and conceal the effect of nanoparticle on the pressuredrop. A definite conclusion of the nanoparticle effect on the pressuredrop characteristics of nanofluidmay not be obtained by the only reporton the pressure drop characteristics of refrigerant/nanolubricant mix-ture [1], and more experiments on the effect of nanoparticle on thephase-change pressure drop of nanofluid are needed.

The aimof this paper is to study the viscosity of TiO2/R123 refrigerant-based nanofluid for different volume concentrations at different tem-peratures. Pressure drop characteristics by using nanofluids in a simplesystem also have been investigated. The reasons for choosing TiO2 nano-particles have been chosen because: (i) TiO2 has been considered as asafe material for human being and animals, (ii) TiO2 nanoparticles areproduced in large industrial scale, (iii) TiO2 nanoparticles have beenused in different applications of nanotechnology including heat transferproperties, and (iv) metal oxides such as TiO2 are chemically more sta-ble than their metallic counterparts ([5]). The reasons for choosing re-frigerant R123 are as follows: it is a low-pressure fluid, and this airconditioner refrigerant is measured moderately halogenated as it con-tains methane or ethane in mixture with chlorine and fluorine.

2. Methodology

Viscosity of nanofluid rises tremendously with the increase of nano-particle volume concentrations. Therefore, in this paper a moderateconcentration of nanoparticles up to 2 vol.% of viscosity of TiO2/R123has been investigated. The viscosity of pure R123 refrigerant has been

taken from [14]. The properties of TiO2 nanoparticles and R123 refriger-ant have been presented in Tables 1 and 2, respectively. Viscosity ofnanofluids is an important transport property like thermal conductivity.Publications about the viscosity of nanofluids are still sparse comparedwith thermal conductivity literature. Most of the formulas have beendeveloped to express viscosity as a function of volume fraction of nano-particles. However temperature is an important influencing factor onviscosity as well, and as a result some correlations have been createdto be used to investigate the temperature effect on viscosity. There aresome theoretical formulae (model or correlations) available in literatureto calculate the viscosity of nanofluids (in general these formulas are forparticle suspension viscosity).

Among these theories, [7] is the pioneer and some other researchers'derived relations basically from this equation. The assumptions madefor this theory is linearly viscous fluid having dilute, suspended, andspherical particles for a low particle volume concentrations (φ b 0.02).The model is stated as:

μeff ¼ μ f 1þ 2:5ϕð Þ ð1Þ

Brinkman [3] modified the Einstein's model to a more generalizedform, using volume concentration, and viscosity of the nanofluid andbase fluid, as shown in Eq. (2).

μeff

μ f¼ 1

1−ϕð Þ2:5 ð2Þ

Furthermore, Wang et al. [23] proposed a model for calculating theviscosity of nanofluids which is defined as:

μeff

μ f¼ 1þ 7:3ϕþ 123ϕ2

� �ð3Þ

Gherasim et al. [8] introduced a correlation to predict the viscosity ofnanofluids with spherical nanoparticles, which is defined as Eq. (4).

μeff

μ f¼ 0:904e14:8ϕ ð4Þ

Pak and Cho [19] also measured the viscosity of two metallic oxideparticles, Al2O3–water and TiO2–water nanofluids with mean diameterof 13 and 27 nm, respectively. The viscosities of the dispersed fluids

Fig. 1. Test condition–Uniform mass flow in a horizontal smooth tube.

Volume Fraction (%)

Rel

ativ

e Vi

scos

ity

0 0.5 1 1.5 2 2.50.8

1

1.2

1.4

1.6

1.8

2

2.2Brinkman ModelCurrent ModelGherasim ModelPak and Cho ModelWang et al. Model

Fig. 3. Comparison between viscosity results with other model.

44 O.A. Alawi et al. / International Communications in Heat and Mass Transfer 61 (2015) 42–48

with Al2O3 and TiO2 at a 10% volume concentrationwere approximately200 and 3 times greater than that of water, as shown in Eq. (5).

μeff ¼ μ f 1þ 39:11ϕþ 533:9ϕ2� �

ð5Þ

The pressure drop can be calculated from friction factor, density,mass flux, vapor quality, tube length and diameter. A correlation pro-posed by Peng et al. [20] has been used to investigate the effect of nano-particles on frictional pressure drop of nanorefrigerant flow boilinginside a horizontal smooth tube, and the formula is:

ΔPr;n;frict ¼ FPD:ΔPr;frict ð6Þ

Where, FPD is the nanoparticle impact factor and is the frictional pres-sure drop of pure refrigerant. The nanoparticle impact factor is importantto correct the frictional pressure drop of pure refrigerant due to nanopar-ticle suspension. The nanoparticle impact factor can be determined by,

FPD ¼ exp φ� 2:19� 107� dpDi

þ 37:26� ρp

ρl;r−0:63� G−217:73� x� 1−xð Þ

" #( )

ð7Þ

Where, dp is the nanoparticle average diameter; is the tube internaldiameter; is the density of nanoparticle; is the liquid-phase density ofpure refrigerant; is the mass flux and is the vapor quality. A correlationproposed by Müller-Steinhagen and Heck [15] was used to determinethe frictional pressure drop of pure refrigerant [18]. The model is

Volume Fraction (%)

Vis

cosi

tyIn

crem

ent(

%)

0 0.5 1 1.5 2 2.5

5

10

15

20

25

30

TiO2/EG ( Chen et al., 2007)TiO2/R123 (Present Study)TiO2/Water (Duangthongsuk and Wongwises, 2009)

Fig. 2. Viscosity increases with the increase of particle volume fractions.

proposed for two-phase flow with the acceptable vapor quality in therange of 0 ≤ x ≤ 1.

ΔPr; frict ¼ G 1−xð Þ1=3 þ bx3 ð8Þ

Where, G is the factor:

G ¼ aþ 2 b−að Þx ð9Þ

In Eq. (9), a and b are frictional pressure gradients for the entire flowliquid and the entire flow vapor in the tube which can be determinedfrom:

a ¼ f L2G2

Diρr;Lð10Þ

b ¼ f G2G2

DiρGð11Þ

Temperature, K

Vis

cosi

ty(1

0-4),

Ns/

m2

300 310 320 3303

3.2

3.4

3.6

3.8

4

4.2

4.4

4.6

Volume Fraction = 0.5%Volume Fraction = 1%Volume Fraction = 1.5%Volume Fraction = 2%

Fig. 4. Effect of temperature over viscosity of nanorefrigerants.

45O.A. Alawi et al. / International Communications in Heat and Mass Transfer 61 (2015) 42–48

Where, fL and fG are the friction factors which depend on Reynoldsnumber; is density of vapor refrigerant. The Reynolds number can becalculated from Eq. (12):

Re ¼ G Di

μrð12Þ

Where μr is the viscosity of pure refrigerant.The friction factor can be obtained either from Eqs. (13) or (14):

f ¼ 16Re

For Re b 2000 laminar flowð Þ ð13Þ

f ¼ 0:079

Re0:25For Re ≥ 2000 turbulent flowð Þ ð14Þ

Some constant parameters of nanorefrigerant flows inside a hori-zontal smooth tube can be observed in Fig. 1 that has been used forthis analysis.

Particle Volume Fraction, %

Pre

ssur

eD

rop,

kPa

0 0.5 1 1.5 2 2.55

10

15

20

25

30

35

40

45

x = 0.2x = 0.3x = 0.4x = 0.5x = 0.6x = 0.7

(a)

Particle Volum

Pre

ssur

eD

rop,

kPa

0 0.5 15

10

15

20

25

30

35

40

45

50

x = 0.2x = 0.3x = 0.4x = 0.5x = 0.6x = 0.7

(c)

Fig. 5. Frictional pressure drop versus particle volume concentration: (a) G = 150 kg m−2 s−1

constant mass flux.

3. Result and discussion

The increase of viscosity for TiO2/R123 nanorefrigerants in respectof volume concentrations has been plotted in Fig. 2. It shows that vis-cosity increases with the increase of volume fractions. The other twoexperimental works about the viscosity of nanofluid have been com-pared with this result. Duangthongsuk and Wongwises (2009) inves-tigated the viscosity of nanofluid for TiO2 with water. The authorsfound that viscosity of nanofluid increases with the increase of volumeconcentrations. The increment rate is linear but not fully straight lineor constant rate. It may have happened because of the experimentalsetup, mixture/stability of nanofluid and also particle size, shapeor agglomeration. Chen et al. [5] studied the viscosity of nanofluidfor TiO2 with Ethylene glycol and found that viscosity increases ac-cordingly with the intensification of volume fractions. However, intheir studies, the increment rate is very high and almost of lineartrend. The authors reported large agglomeration of nanoparticles onthe suspensions.

Fig. 3 shows relative viscosity, μeff/μf of nanorefrigerant which is de-fined as the ratio of the nanorefrigerant viscosity and pure refrigerant

Particle Volume Fraction (%)

Pre

ssur

eD

rop,

kPa

0 0.5 1 1.5 2 2.55

10

15

20

25

30

35

40

45

x = 0.2x = 0.3x = 0.4x = 0.5x = 0.6x = 0.7

(b)

e Fraction (%)1.5 2 2.5

constant mass flux, (b) G = 170 kg m−2 s−1 constant mass flux, (c) G = 200 kg m−2 s−1

46 O.A. Alawi et al. / International Communications in Heat and Mass Transfer 61 (2015) 42–48

viscosity, as a function of particle volume fractions. Investigating the rel-ative viscosity of nanorefrigerant by using different models shows thatit is proportional to the particle volume fraction. The result from thepresent study has an almost similar trend to other models. However, aslope constructed by using Brinkman's model is parted away fromothers as the particle concentration increases since this model is validonly for particle concentration less than 2 vol.%. To overcome this limi-tation, Brownian motion of nanoparticle correlations [19] has been in-cluded to determine the viscosity of nanorefrigerant at higher particleconcentrations up to 5 vol.% in this study. Even though [8,19,23] useddifferent consideration in proposing their equations, the results fromdifferent models are apparently similar. The reason for this is thattheir theoretical works on particle suspensions were developed basedon Einstein's model, which introduced that the viscosity of nanofluidswas strongly influenced by particle concentration.

Fig. 4 shows the effect of temperature over viscosity ofnanorefrigerants. Normally viscosity of thermo fluid decreases ac-cordingly with the increase of temperature. In this study it has been

Local Vapor Quality (x)

Pre

ssur

eD

rop,

kPa

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.85

10

15

20

25

30

35

40

45

Volume Fraction = 0.5%Volume Fraction = 1%Volume Fraction = 1.5%Volume Fraction = 2%

(a) (

Local Vapo

Pre

ssur

eD

rop,

kPa

0.1 0.2 0.3 0.45

10

15

20

25

30

35

40

45

Volume FractioVolume FractioVolume FractioVolume Fractio

(c)

Fig. 6. Frictional pressure drop versus local vapor quality: (a) G = 150 kg m−2 s−1 constant mmass flux.

seen that viscosity of nanorefrigerant also decreased accordingly withthe intensification of temperature.

The same trend for decrease of viscosity with the increase of tem-perature was found by some other researchers (e.g. [11,16]). Highnanorefrigerant temperature intensifies the Brownian motion of nano-particles and reduces the viscosity of nanorefrigerant. The highestviscosity was observed at 300 K and 2 volume concentrations (%) ofparticles.

Figs. 5 and 6 show the pressure drop characteristics of TiO2/R123nanorefrigerant as a function of nanoparticle volume concentrationand vapor quality, respectively. The highest frictional pressure dropoccurred at vapor quality of 0.7 where the value was 40.05 kPa with2 vol.% of particle concentration at mass flux (G = 150 kg m−2 s−1).Pressure drop of pure refrigerant with the same vapor quality wasonly 17.03 kPa. The lowest pressure drop was found at 6.647 kPaat vapor quality of 0.2 with 0.5 vol.% of particle concentration. How-ever, pressure drop was 5.083 kPa for pure refrigerant with the samecondition. Even with only 0.5 vol.% of nanoparticle concentration,

Local Vapor Quality (x)

Pre

ssur

eD

rop,

kPa

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.85

10

15

20

25

30

35

40

45

Volume Fraction = 0.5%Volume Fraction = 1%Volume Fraction = 1.5%Volume Fraction = 2%

b)

r Quality (x)0.5 0.6 0.7 0.8

n = 0.5%n = 1%n = 1.5%n = 2%

ass flux, (b) G = 170 kg m−2 s−1 constant mass flux, (c) G = 200 kg m−2 s−1 constant

47O.A. Alawi et al. / International Communications in Heat and Mass Transfer 61 (2015) 42–48

the pressure drop enhancement was 42.5% relative to pure R123refrigerant.

Suspending nanoparticles into the refrigerant generally increasesthe pressure drop even though the mass flow rate of the refrigerantwas considered to be inconstant in this study.When the particle volumefraction is suspended more than 1.5 vol.%, the enhancements ofpressure drop for all vapor qualities were found to be increased rapidly.By increasing the nanoparticle concentration, more collision betweenthe nanoparticles and wall interaction could have occurred and higherpressure drop compared to pure refrigerant was observed for thenanorefrigerant.

Fig. 7(a)–(c) shows the nanoparticle impact factor FPD changingwith the vapor quality at different mass fluxes. FPD are in the rangesof 1.186–2.924, 1.113–2.273 and 1.013–1.557 at mass fluxes of 150,170 and 200 kg m−2 s−1, respectively. The reasons for the occurrenceof this phenomenon may include the following: 1) the viscosity ofnanofluid increases with the increase of mass fraction of nanoparticles([6,9,16,17]), causing the enhancement of FPD with the increase ofmass fraction of nanoparticles; 2) the collision among the nanoparticlesas well as the friction between the nanoparticles and the inside tubewall increase with the increase of mass fraction of nanoparticles,

Local Vapor Quality (x)

F PD

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.81

1.5

2

2.5

3

3.5

Volume Fraction = 0.5%Volume Fraction = 1%Volume Fraction = 1.5%Volume Fraction = 2%

(a) (

Local V

F PD

0.1 0.2 0.30.8

1

1.2

1.4

1.6

1.8

2(c)

Fig. 7.Nanoparticle impact factor FPD of TiO2/R123 nanofluid versus local vapor quality at differ

causing the enhancement of FPD with the increase of mass fraction ofnanoparticles. Fig. 7(a)–(c) shows that the nanoparticle impact factorFPD at low and high vapor qualities (x b 0.5 and x N 0.7) is larger thanthat at intermediate vapor qualities (0.5 b x b 0.7). The reasons for thisphenomenon may include: 1) the presence of nanoparticles promotesthe transition of flow pattern to annular flow at low vapor qualitiesand delays the transition of flow pattern from annular to dryout flowat high vapor qualities, which leads to the obvious enhancement of thefrictional pressure drop; 2) theflowpattern is kept as annular flow at in-termediate vapor qualities, thus the influence of nanoparticles on flowpattern is weak, which leads to the inapparent enhancement of the fric-tional pressure drop. Fig. 7(a)–(c) also shows that the nanoparticle im-pact factor FPD decreases with the increase of mass flux at given vaporquality and mass fraction of nanoparticles. It is conjectured that the in-crease of mass flux promotes annular flow, concealing the influence ofnanoparticles on the frictional pressure drop.

The variation of the total pressure drop versus the local vaporqualities for Pin = 91.358 kPa and different internal diameters isshown in Fig. 8. It can be noticed from this figure that the pressuredrop decreases as the internal micro tube diameter increases. This isdue to the increase of liquid viscosity and the decrease of vapor density.

Local Vapor Quality (x)

F PD

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.81

1.5

2

2.5

3

Volume Fraction = 0.5%Volume Fraction = 1%Volume Fraction = 1.5%Volume Fraction = 2%

b)

apor Quality (x)0.4 0.5 0.6 0.7 0.8

Volume Fraction = 0.5%Volume Fraction = 1%Volume Fraction = 1.5%Volume Fraction = 2%

ent mass fluxes: (a) G= 150 kgm−2 s−, (b) G= 170 kgm−2 s−1, (c) G= 200 kgm−2 s−1.

Local Vapour Quality (x)

Pre

ssur

eD

rop,

kPa

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.85

10

15

20

25

30

35

40

Di = 6 mmDi = 7 mmDi = 8 mm

Fig. 8. Frictional pressure drop versus local vapor quality at different internal diameters.

48 O.A. Alawi et al. / International Communications in Heat and Mass Transfer 61 (2015) 42–48

4. Conclusions

In this study, an attempt has beenmade to investigate theviscosity ofnanorefrigerants for TiO2 nanoparticles with R123 refrigerant. Throughthis study, it is found that volume fractions and temperature have signif-icant effects over viscosity of nanofluids. Results indicate that viscosityincreases with the increase of the particle volume fractions. However,it decreases with the increase of temperature. Furthermore, viscosity isdirectly related to pressure drop characteristics. Pressure drop increaseswith the increase of volume concentrations and vapor quality.

The frictional pressure drop of refrigerant-based nanofluid flowboiling inside the horizontal smooth tube is larger than that of pure re-frigerant, and increases with the increase of the mass fraction of nano-particles. The nanoparticle impact factor FPD at low and high vaporqualities (x b 0.5 and x N 0.7) is larger than that at intermediate vaporqualities (0.5 b x b 0.7). The nanoparticle impact factor FPD decreaseswith the increase of mass flux at given vapor quality and mass fractionof nanoparticles. It can be noticed from the results that the pressuredrop decreases as the internal micro tube diameter increases.

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