An experimental investigation on the effects of surfactants on the thermal performance of hybrid nanofluids in helical coil heat exchangers

ICHMT-03520; No of Pages 6 International Communications in Heat and Mass Transfer xxx (2016) xxx–xxx

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International Communications in Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ichmt

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F. Hormozi, B. ZareNezhad ⁎, H.R. Allahyar Faculty of Chemical, Petroleum and Gas Engineering, Semnan University, Semnan, Iran

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a r t i c l e

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a b s t r a c t

In the present study, the effects of surfactants on the thermal performance of the hybrid nanofluid (Alumina–Silver) at constant wall temperature and laminar flow have been experimentally studied in a helical coil heat exchanger. Different surfactants such as anionic Sodium Dodecyl Sulfate (SDS) and nonionic Poly Vinyl Pyrrolidone (PVP) in the concentration of range of 0.1–0.4 wt.% are employed. It is found that the thermal performance can be maximized by using the 0.2 vol.% hybrid nanofluid and 0.1 wt.% SDS anionic surfactant in the helical coil. The maximum thermal performance in the presence of hybrid Alumina–Silver nanofluid and SDS anionic surfactant is 16% higher than that of the pure distilled water. The presented results can have potential application in process intensification and optimum design of heat exchangers. © 2016 Published by Elsevier Ltd.

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Keywords: Thermal performance Hybrid nanofluid Anionic surfactant Nonionic surfactant Helical coil Heat exchanger

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Effect of curved pipes on the thermal performance enhancement of fluids and nanofluids has been investigated for design of new heat exchangers in recent years. Most results have shown that the fluid thermal performance is improved because of creating more contact surface and centrifugal force [1–4]. For the first time in 1995, Choi [5] discussed the effect of nanofluid on thermal conductivity. After that, a lot of researches who have investigated the thermo-physical properties of nanofluid, reported the thermal conductivity enhancement in comparison with the base fluid [6–11]. Suresh et al. [11,12] have investigated the thermal performance of alumina and copper oxide nanofluid using the twisted tape inserts in tube under constant heat flux and laminar flow conditions. They have concluded that under constant thermal conditions in the twisted tape, copper oxide nanofluid shows better performance as compared to the alumina nanofluid. In addition, copper oxide nanofluid imposes higher pressure drop as compared to the alumina nanofluid and the use of twisted tape increases this pressure drop to a greater extent. Hybrid nanofluid is a new class of nanofluid which is made of two or more particles in combination with different percentages. The topic has attracted many researchers' attention in recent years. Most of them have found out that using nanocomposites in the base fluid results in improving thermal performance [12–16]. Lots of researches have also investigated the effects of surfactants on the stability of the nanoparticles [17–22].

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1. Introduction

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An experimental investigation on the effects of surfactants on the thermal performance of hybrid nanofluids in helical coil heat exchangers☆

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☆ Communicated by W.J. Minkowycz. ⁎ Corresponding author. E-mail address: [email protected] (B. ZareNezhad).

Since there are limited information on the effect of surfactants on thermal performance of hybrid nanofluids in the helical coil heat exchangers, the variations of Nusselt number, pressure drop and thermal performance in the presence of different compositions of SDS anionic and PVP nonionic surfactants are experimentally investigated in this work.

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2. Experimental

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2.1. Set-up and instruments

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According to Fig. 1, testing devices include stainless steel tank and temperature and pressure control systems. The tank is fully insulated with rock wool in order to avoid heat loss and a 2 kW heater is immersed in the tank to supply the required heat. In order to measure the inlet and outlet pressure, a very sensitive pressure transmitter has been used (SENSYS, 0.5BCIA PSCH). Two accurate thermocouples of T-type have been used for accurate measurement of the inlet and outlet temperatures. In addition, six thermocouples of K- type have been installed at various locations to measure the surface temperatures of the helical coil. Flow rate is estimated by an ultrasonic system with the accuracy of about 0.05 l per minutes. The used coil is made of copper, which its physical features are shown in Table 1. The device is first calibrated with pure water and then the main testing is started with hybrid nanofluid at different concentrations. As soon as the temperature reaches to a saturation state (constant temperature of 95 °C), we started to collect data including inlet and outlet temperature, inlet and outlet pressure as well as the surface temperature. An experiment has been repeated at least two times to ensure that the date is accurate. The employed hybrid nanofluid contains Alumina(97.5%)–Silver(2.5%)

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http://dx.doi.org/10.1016/j.icheatmasstransfer.2016.09.022 0735-1933/© 2016 Published by Elsevier Ltd.

Please cite this article as: F. Hormozi, et al., An experimental investigation on the effects of surfactants on the thermal performance of hybrid nanofluids in helical coil heat exchangers, Int. Commun. Heat Mass Transf. (2016), http://dx.doi.org/10.1016/j.icheatmasstransfer.2016.09.022

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F. Hormozi et al. / International Communications in Heat and Mass Transfer xxx (2016) xxx–xxx

Cp d D h K L Nu Re T U

T1:13 T1:14 T1:15 T1:16 T1:17 T1:18 T1:19

Greek letters ΔP Axial pressure drop (Pa) η Thermal performance factor ρ Density (kg·m−3) μ Dynamic viscosity (Pa·s) φ Nanoparticle volume fraction (%)

T1:20 T1:21 T1:22 T1:23 T1:24 T1:25 T1:26 T1:27 T1:28 T1:29 T1:30

Subscripts C Coiled tube ex Experimental f Base fluid nf Nanofluid nfs Nanofluid with surfactant p Particle S Straight tube th Theoretical w Wall

Specific heat (J·kg−1·K−1) Inside diameter of tube (m) Diameter of coil (m) Heat transfer coefficient (W·m−2·K−1) Thermal conductivity (W·m−1·K−1) Length of tube (m) Average Nusselt number Reynolds number Temperature (K) Average velocity (m·s−1)

Tube

d

t

L

D

λ

N

t1:3

Helical coil

5

1

2600

65

15

10

t1:4

presented in Tables 2 and 3. The measurement accuracies of different 101 instruments are given in Table 4 as well. 102

The thermophysical characteristics of the nanofluid are calculated 104 according to the following equations: [23–25]. 105 ρnf ¼ ð1− φÞ  ρ f þ φ  ρP μf ð1− φÞ2:5

R O

μ nf ¼

P


knf kP þ 2k f − 2 k f − kP  φ
: ¼ kf kP þ 2kf þ k f − kP  φ

97 98 99 100

111 113

ð4Þ

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116

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The experimental convective heat transfer coefficient and the Nusselt number are determined according to the following equations: 117

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95 96

ð3Þ

Thermal conductivity is also calculated using the following equation [26]: 114

nanocomposite with an average diameter of 80 nm (with spherical shape). The mixed hybrid nanofluids at a constant concentration 0.2 vol.% and using different concentrations of Sodium Dodecyl Sulfate (SDS) anionic and nonionic Poly Vinyl Pyrrolidone (PVP) surfactants in the range of 0.1–0.4 wt.% are provided by intense mixing via an ultrasonic device. There are numerous ways to prepare nanoparticles, one of which is Sol–gel. One of the advantages of this method is to prepare nanocomposites with high purity. Initially a homogenous suspension including solvent and precursor is solved and then the homogenous solution is turned into Sol by hydrolysis. After provoking the particles in Sol by HCl and NaOH, they join together and form a wet gel. Then after separating the solution and drying it, the nanoparticles are formed. Fig. 2 shows the TEM (transmission electron microscopy) images of dispersed nanoparticles in distilled water. The specifications of the employed nanoparticles and surfactants used in this study are

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110




⋅

Nuð expÞ ¼




hð expÞ  d knf

ð5Þ

119 120

ð6Þ 122

where (Tw − Tb)M is a logarithmic temperature difference. 3. Results and discussion

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3.1. Heat transfer rate in the presence of ionic surfactant

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The trend of variation of hybrid nanofluid Nusselt number at different concentrations of anionic surfactant (SDS) is shown in Fig. 3. Comparing with the case of distilled water, the Nusselt number increases as anionic surfactant concentration is increased. As shown, an increase in the Reynolds number leads to an increase in heat transfer

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ð1Þ 107 ð2Þ

ðρ  C P Þnf ¼ ð1− φÞ  ðρ  C P Þ f þ φ  ρ  C p P :

 ð expÞ ¼ m  cp  T b1 − T b2 h A  ðT w − T b ÞM

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2.2. Determination of experimental Nusselt number

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T1:2 T1:3 T1:4 T1:5 T1:6 T1:7 T1:8 T1:9 T1:10 T1:11 T1:12

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Nomenclature

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t1:1 t1:2

Table 1 Geometrical characteristics of the Helical Coil (mm).

T1:1

Fig. 1. The experimental set-up employed in the present study.

Please cite this article as: F. Hormozi, et al., An experimental investigation on the effects of surfactants on the thermal performance of hybrid nanofluids in helical coil heat exchangers, Int. Commun. Heat Mass Transf. (2016), http://dx.doi.org/10.1016/j.icheatmasstransfer.2016.09.022

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F. Hormozi et al. / International Communications in Heat and Mass Transfer xxx (2016) xxx–xxx

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Fig. 3. Variation of Nusselt number for hybrid nanofluid by using different concentrations of anionic surfactant (SDS).

t2:1 t2:2

Table 2 Thermophysical characterization of nanopowders.

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rate at given concentrations of anionic surfactant. In addition, improvement of the stability of hybrid nanofluid by using an anionic surfactant (at all concentrations) has a positive effect on the Nusselt number enhancement at different agitation intensities. The maximum nanoparticle stability is obtained at the SDS concentration of 0.1 wt.% where the maximum Nusselt number is obtained. The variations of the Nusselt number enhancement ratio (ratio of hybrid nanofluid Nusselt number with anionic surfactant to hybrid nanofluid Nusselt number without anionic surfactant) are presented in Fig. 4. It is shown that by adding anionic surfactant to hybrid nanofluid, the stability of nanoparticles in base fluid is improved such that the rate of heat transfer is increased. It should be noted that at anionic surfactant concentrations higher

Particle/base fluid

Average diameter (nm)

Purity (%)

Actual density (kg/m3)

CP (J/kg·K)

k (W/m·K)

t2:5 t2:6 t2:7

Al2O3 Ag nanocomposite

55 25 80

99 99 99

3690 9320 3830.75

780 233 766

30.5 429 41

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t2:3 t2:4

Table 3 Properties of the employed surfactants.

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t3:1 t3:2 t3:3

Surfactant

t3:4 t3:5

Sodium Dodecyl Sulfate (SDS) Poly Vinyl Pyrrolidone (PVP)

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Fig. 2. TEM image of hybrid nanoparticles.

The molecular formula

Molecular weight

Density (g/cm3)

NaC12H25SO4 (C6H9NO)n

288.372 N105

1.01 1.2

Fig. 4. Variation of Nusselt number enhancement ratio for hybrid nanofluid by using different concentrations of anionic surfactant (SDS).

than 0.1 wt.%, the stability of nanoparticles is gradually decreased and thus a lower Nusselt number enhancement ratio is obtained [17,21]. According to Figs. 5 and 6, nanoparticles carry positive charge in a neutral aqueous environment. When a low concentration Sodium Dodecyl Sulfate (0.1 wt.%) is added to the hybrid nanofluid, the sodium cations of material (hydrophilic group) approach the base fluid and its sulfate anions (hydrophobic groups) that are made from a long dodecyl carbon chain (C12H25) are attracted on the surface of nanoparticles. Therefore, the nanoparticle surface has taken negative charge. Anionic groups are pulled into the inner layer and lead to strengthen the negative charge on the surface of nanoparticles. Moreover, it causes an increase in the thickness of electric double layer (EDL) and thus an increase in the absolute amount of zeta potential of the surface of nanoparticles such that a repulsive force between nanoparticles is created and the stability of nanoparticles in the base fluid is increased. As concentration of Sodium Dodecyl Sulfate increases, the concentration of sodium cations increases (positive charge) such that the cationic group enters into the inner layer. Thus, the net negative charge of the

Table 4 Uncertainty of measurement instruments. Parameter Fluid flow rate Steam temperature sensors Wall temperature sensors Inlet/outlet temperature sensors Pressure transmitters

Instrument ®

Flownetix 100series™ PT-100 Ω thermo resistance K-type Omega Thermocouples PT-100 Ω thermo resistance Keller, Series 35X-Bullet type

uncertainty ±1% of reading 0.1 K 0.1 K 0.1 K ±1% of reading

Fig. 5. (a) Chemical structure of SDS surfactant (NaC 12 H 25 SO 4 ), (b) schematic representation of surfactant micelle encapsulation over a nanoparticle.

Please cite this article as: F. Hormozi, et al., An experimental investigation on the effects of surfactants on the thermal performance of hybrid nanofluids in helical coil heat exchangers, Int. Commun. Heat Mass Transf. (2016), http://dx.doi.org/10.1016/j.icheatmasstransfer.2016.09.022

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F. Hormozi et al. / International Communications in Heat and Mass Transfer xxx (2016) xxx–xxx

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Fig. 8. Variation of Nusselt number enhancement ratio for hybrid nanofluid by using different concentrations of nonionic surfactant (PVP).

3.2. Heat transfer rate in the presence of nonionic surfactant

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According to Fig. 7 an increase in the concentration of nonionic surfactant (PVP) for the case of hybrid nanofluid leads to an increase in the stability of nanoparticles such that the Nusselt number of hybrid nanofluid at a given Reynolds number is improved. According to Fig. 8, the maximum increase in the rate of heat transfer occurs at the PVP concentration of 0.4 wt.%. As shown in Fig. 9, PVP doesn't have the potential of ionization in the base fluid. However its hydrophilic group which includes oxygen atoms can make hydrogen bond with water. At a low PVP concentration (i.e. 0.1 wt.%) in hybrid nanofluid, the molecules of PVP cannot cover the whole surface of nanoparticles such that nonionic surfactant molecule on nanoparticles can be connected to other nanoparticles from its side chain and it probably makes some of the nanoparticles agglomerate and cluster. As shown in Fig. 8, the more increase in the concentration of PVP nonionic surfactant, the more

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Fig. 10. Variation of Nusselt number enhancement ratio for hybrid nanofluid by using different concentrations of SDS or PVP.

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Fig. 7. Variation of Nusselt number for hybrid nanofluid by using different concentrations of nonionic surfactant (PVP).

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C

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Fig. 9. a: Chemical structure of SDS surfactant (NaC12H25SO4). b: Chemical structure of PVP surfactant(C6H9NO)n.

complete coverage of surfactant molecules around nanoparticles. This leads to a better repulsive force and a better stability of the nanoparticles. With an increase in the concentration of PVP up to 0.4 wt.%, the surface of nanoparticles is significantly covered with surfactant. Thus with an increase in the thickness of the electric double layer, the net charge of surface of nanoparticles and consequently absolute value of the Zeta potential of surface of nanoparticles are increased. Increasing the thickness of the electrical double layer creates a repulsive force between the nanoparticles resulting in further dispersion and the stability of the nanoparticles in the base fluid and thus improving the heat transfer. The greatest increase in Nusselt number enhancement ratio is about 6.425% at the PVP concentration of 0.4 wt.% and Re of 5100 [22,32] as shown in Fig. 8. The Nusselt number enhancement ratio for hybrid nanofluid in the presence of SDS anionic and PVP nonionic surfactants are compared in Fig. 10 at different species concentrations. At lower Reynolds number,

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nanoparticles and the zeta potential of surface of nanoparticles are decreased and the stability of nanoparticles is reduced. This decrease in stability results in an increase in agglomeration and settling rate of nanoparticles which in turn reduces the thermal performance. Therefore the stability or instability of nanoparticles has a considerable role in the enhancement of thermal performance of hybrid nanofluid. The maximum heat transfer rate is obtained at 0.1 wt.% concentration of SDS anionic surfactant at all agitation intensities and the maximum Nusselt number enhancement ratio is about 6.283% at the Reynolds number (Re) of 5100 as shown in Fig. 4 [32–34].

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Fig. 6. Type of colloidal stabilization (electrostatic stabilization).

Please cite this article as: F. Hormozi, et al., An experimental investigation on the effects of surfactants on the thermal performance of hybrid nanofluids in helical coil heat exchangers, Int. Commun. Heat Mass Transf. (2016), http://dx.doi.org/10.1016/j.icheatmasstransfer.2016.09.022

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3.3. Pressure drop in the helical coil

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According to Fig. 11 an increase in Reynolds number and the concentration of SDS or PVP surfactants at a fixed concentration of hybrid nanofluid leads to a slight increase in helical coil pressure drop. This can be attributed to the increase of nanofluid viscosity due to an increase in the surfactant concentrations. Also an excessive increase of surfactant concentrations for the case of SDS speeds up the agglomeration process and settling of nanoparticles in the coil such that a higher pressure drop is experienced. In Fig. 12, pressure drop enhancement ratio of hybrid nanofluid with respect to distilled pure water at different SDS or PVP concentrations are presented. The greatest pressure drop is related to the hybrid nanofluid with SDS anionic surfactant at the concentration of 0.4 wt.% at the Reynolds number 5100. Using helical coil instead of straight pipe produces a more increase in pressure drop of hybrid nanofluid. [17,40]. The pressure drop enhancement ratio for 0.1 wt.% SDS (=1.03) is lower than that of the 0.4 wt.% PVP in the hybrid nanofluid (=1.07) indicating a 60% decrease in frictional loss when the

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3.4. Thermal performance of hybrid nanofluid in the helical coil

Thermal performance ratio of the helical coil to the straight tube for 250 the hybrid nanofluid with surfactant is defined as follows [41]: 251 NuC;nfS NuS; f η¼ 0:1666 : ΔP C;nfS ΔP S; f

ð7Þ

According to Fig. 13, the thermal performance ratio for all cases is greater than one. The helical coil exhibits better heat transfer performance than a straight tube because of centrifugal force and secondary flow. Also by using the optimum concentrations of surfactants in the employed hybrid nanofluid, the suspension stability and the heat transfer rate are enhanced. According to Fig. 15 the maximum increase in the thermal performance ratio belongs to 0.1 wt.% concentration of SDS anionic surfactant which is equal to 1.1636. Thus the hybrid Alumina–Silver nanofluid (with concentration of 0.2 vol.%) with 0.1 wt.% SDS anionic surfactant show the best thermal performance in the employed helical coil heat exchanger.

Fig. 11. The hybrid nanofluid pressure drop in the presence of SDS and PVP surfactants.

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4. Conclusion

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In the present study, the effects of surfactants on the thermal performance of the hybrid nanofluid (prepared by using Alumina–Silver nanocomposite with 0.2 vol.% concentration) have been studied experimentally in a helical coil. The Sodium Dodecyl Sulfate (SDS) anionic surfactant and Poly Vinyl Pyrrolidone (PVP) nonionic surfactant in the concentration range of 0.1–0.4 wt.% are employed. The maximum heat

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hybrid nanofluid in the presence of 0.1 wt.% SDS in helical coil is 247 employed. 248

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Fig. 12. The hybrid nanofluid pressure drop enhancement ratio in the presence of SDS and PVP surfactants.

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thermal conductivity is the main influencing factor in controlling the rate of heat transfer. With increasing Reynolds number, the effect of thermal conductivity on the heat transfer becomes less considerable. [35,36]. On the other hand, the importance of other mechanisms including Brownian movement of nanoparticles, migration of the nanoparticles and reduction of thickness of boundary layer becomes more tangible in the increase in heat transfer rate [37–39]. In addition, at higher rate of Reynolds number, the effects of dispersion and turbulent movement lead to increase in fluid fluctuations and heat transfer rate. Moreover, it can be concluded that by increasing the concentration of both surfactants (anionic and nonionic) in hybrid nanofluid, the stability is increased and consequently the thermal performance of nanofluid is improved. As shown, the greatest increase in stability and the heat transfer rate is related to 0.4 wt.% of PVP nonionic surfactant in hybrid nanofluid in the helical coil. The results show that hybrid nanofluid with 0.1 wt.% of SDS anionic surfactant nearly has the same thermal performance as 0.4 wt.% of PVP nonionic surfactant. However, by increasing the concentration of SDS anionic surfactant from 0.1 wt.% to 0.4 wt.%, the stability of nanoparticles and the rate of heat transfer are gradually decreased. It is interesting to note that the PVP nonionic surfactant shows an opposite behavior. By increasing the concentration of PVP nonionic surfactant from 0.1 wt.% to 0.4 wt.%, the stability of nanoparticles and the rate of heat transfer are gradually increased according to the aforementioned discussion. As shown in Fig. 10, the maximum Nusselt number enhancement ratio of 1.063 (6.3% increase in heat transfer rate) can be obtained by using SDS anionic surfactant at a concentration as low as 0.1 wt.%. This concentration is 75% lower than the required concentration of 0.4 wt.% of PVP nonionic surfactant for the same duty.

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Fig. 13. The variation of thermal performance of the helical coil with respect to the straight tube for the hybrid nanofluid in the presence of SDS or PVP.

Please cite this article as: F. Hormozi, et al., An experimental investigation on the effects of surfactants on the thermal performance of hybrid nanofluids in helical coil heat exchangers, Int. Commun. Heat Mass Transf. (2016), http://dx.doi.org/10.1016/j.icheatmasstransfer.2016.09.022

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transfer rate is obtained at 0.1 wt.% concentration of SDS anionic surfactant at all agitation intensities and the maximum Nusselt number enhancement ratio is about 6.283% at the Reynolds number of 5100. This concentration is 75% lower than the required concentration of 0.4 wt.% of PVP nonionic surfactant for the same duty. The pressure drop enhancement ratio for 0.1 wt.% SDS (= 1.03) is lower than that of the 0.4 wt.% PVP in the hybrid nanofluid (= 1.07) indicating a 60% decrease in frictional loss when the hybrid nanofluid in the presence of 0.1 wt.% SDS in helical coil is employed. The maximum increase in the thermal performance ratio (about 1.1636) can be achieved by using 0.1 wt.% concentration of SDS anionic surfactant. Thus the maximum thermal performance in the presence of hybrid Alumina– Silver nanofluid and SDS anionic surfactant is 16% higher than that of the pure distilled water. The proposed approach can be used as an energy saving technique for design of new compact heat exchangers and economizers.

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Please cite this article as: F. Hormozi, et al., An experimental investigation on the effects of surfactants on the thermal performance of hybrid nanofluids in helical coil heat exchangers, Int. Commun. Heat Mass Transf. (2016), http://dx.doi.org/10.1016/j.icheatmasstransfer.2016.09.022

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