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Research Papers

# Comparative Study on Heat Transfer Enhancement of Low Volume Concentration of Al2O3–Water and Carbon Nanotube–Water Nanofluids in Laminar Regime Using Helical Screw Tape InsertsOPEN ACCESS

[+] Author and Article Information
Sandesh S. Chougule

Discipline of Mechanical Engineering,
Indian Institute of Technology Indore,
e-mail: sandesh_chougule@yahoo.com

S. K. Sahu

Assistant Professor
ASME Member
Discipline of Mechanical Engineering,
Indian Institute of Technology Indore,
e-mail: santosh.sahu04@gmail.com

1Corresponding author.

Manuscript received February 5, 2014; final manuscript received June 23, 2014; published online July 15, 2014. Assoc. Editor: Calvin Li.

J. Nanotechnol. Eng. Med 4(4), 040904 (Jul 15, 2014) (9 pages) Paper No: NANO-14-1009; doi: 10.1115/1.4027913 History: Received February 05, 2014; Revised June 23, 2014

## Abstract

An experimental study has been carried out to evaluate the heat transfer and friction factor characteristics of helical screw inserts in Al2O3–water and carbon nanotubes (CNT)–water nanofluids through a straight pipe in laminar flow regime with constant heat flux boundary condition. Tests have been performed by using 0.15% volume concentration Al2O3–water and CNT–water nanofluid with helical tape inserts of twist ratio (TR) = 1.5, 2.5, and 3. The helical screw tape inserts with CNT–water nanofluid exhibits higher thermal performance compared to Al2O3–water nanofluid. The maximum enhancement in heat transfer was obtained for CNT–water nanofluid with helical tape inserts of TR = 1.5. The increase in pressure drop of Al2O3–water nanofluid with helical screw tape inserts is found to be higher compared to CNT–water nanofluid with helical screw tape inserts at lower value of TR. For both the nanofluids (CNT–water and Al2O3–water), the thermal performance factor was found to be greater than unity for all TRs.

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## Introduction

Conventional single phase fluids such as: water engine oil and ethylene glycol usually possess poor thermal properties. The conductivity of these fluids can be improved by dispersing nanosized particles with higher thermal conductivity. These fluids containing the nanometer sized particles with proper stability are termed as nanofluids [1]. Nanofluids were found to possess substantially higher thermal conductivities compared to their base fluids. Several heat transfer enhancement techniques have been used in various thermal devices such as: heat exchanger, air-conditioning system, and refrigeration system to improve the thermal performance. It may be noted that the size of the thermal device can be reduced by improving the heat transfer rate of the system. The heat transfer enhancement techniques are broadly divided into two categories such as: active method which needs an external power source and the passive method which does not need any external power source. The passive technique method mainly involves the use of twisted tapes, wire coil inserts, and helical screw tapes in the ducts leading to the increase in the heat transfer rate [2-8]. In such a case, the heat transfer rate increases because of various reasons, namely, decrease in hydraulic diameter, the increase in the length of the flow path due to helical configuration of twisted tape, the increase in the shear stress at wall and tube leading to improvement in fluid mixing. With the use of twisted tapes and helical screw tapes with nanofluids, the heat transfer rate can be enhanced due to enhanced thermal conductivity of nanofluids, improvement in fluid mixing, and increased heat transfer length.

Several studies have been carried out to investigate the heat transfer and friction factor characteristics of helical screw tapes and twisted tapes in pipe flow using nanofluids [9-18]. The heat transfer coefficient and friction factor for transition flow in a tube with twisted tape inserts and Al2O3nanofluid has been investigated experimentally by Sharma et al. [9]. The heat transfer coefficient of nanofluid flowing through a tube with 0.1% volume concentration was found to be 23.7% higher compared with water at 9000 Reynolds number. For 0.1% volume concentration of nanofluid, the maximum friction factor with twisted tape was found to be 1.21 times of the water flowing in a plain tube. The heat transfer and friction characteristics of Al2O3–water nanofluid in a circular tube with twisted tapes of various TRs have been reported by Sundar and Sharma [10]. The authors carried out experiments for varied range of volume concentration (0.02%, 0.1%, and 0.5%) and Reynolds number (10,000–22,000). For 0.5% volume concentration of Al2O3–water nanofluid, the enhancement in heat transfer coefficient and friction factor was found to be 33.51% and 1.096 times, respectively. In addition, the authors [11] studied the turbulent convective heat transfer and friction factor behavior of Al2O3–nanofluid in a circular tube with longitudinal strip inserts. Tests were carried out for varied range of volume concentration (0–0.5%), Reynolds number (22,000 > Re > 3000) and aspect ratio (0–18). The heat transfer coefficient found to increase with nanofluid volume concentration and decrease with aspect ratio. The heat transfer, friction factor, and thermal performance characteristics of CuO–water nanofluid in a circular tube fitted with modified twisted tape with alternate axis were studied experimentally by Wongcharee and Smith [12]. For 0.7% volume concentration of CuO–water and Reynolds number 1990, the twisted tape with alternate axis exhibits maximum increase in thermal performance factor. The heat transfer and friction factor characteristics of wire coil inserts with various pitch ratios in Al2O3–water nanofluid through a circular tube was experimentally investigated by Chandrasekar et al. [13,14]. The use of nanofluids increases the heat transfer rate with negligible increase in friction factor in the plain tube and the tube fitted with wire coil inserts. The comparative study on thermal performance of helical screw tape inserts with various TRs (1.78, 2.44, and 3) by using Al2O3–water and CuO–water nanofluids was reported by Suresh et al. [15,16]. Thermal performance factor of helical screw tape inserts using CuO–water nanofluid was found to be higher compared to the value by using Al2O3–water. The heat transfer and friction factor characteristics of Fe3O4–water nanofluid through a uniformly heated horizontal circular tube with and without twisted tape inserts for turbulent flow condition were studied experimentally by Sundar et al. [17]. For 0.6% volume concentration of Fe3O4–water nanofluid, the increase in the heat transfer and friction factor of twisted tape insert was found to be 51.88% and 1.231, respectively. The heat transfer and friction factor characteristics of water/propylene glycol based CuO nanofluids through a horizontal circular tube fitted with and without helical inserts for transition flow regime was reported by Naik and Lingala [18]. Tests were carried out for varied range of TRs (0–9) and Reynolds number (2500–10,000). For 0.5% volume concentration of CuO nanofluids, the Nusselt number was found to be 28% higher compared to the plain tube.

It is evident that various experimental studies have been reported that predict the heat transfer and friction characteristics of nanofluids in plain tube. In addition, the heat transfer and friction factor characteristics of twisted and wire coiled inserts with various nanofluids through tubes have been reported by various researchers. It is reported that CNTs have the high thermal conductivity, high aspect ratio, low specific gravity, and large specific surface area (SSA) [19,20]. Although various nanofluids have been used to evaluate the heat transfer characteristics of helical screw inserts, the CNT–water nanofluid has not been used to analyze the heat transfer and friction factor characteristics of helical screw inserts in CNT–water nanofluids through a horizontal pipe in laminar flow regime. In view of this an attempt has been made to study the heat transfer and friction characteristics of helical inserts in Al2O3–water and CNT–water nanofluids through a straight pipe in laminar flow regime with constant heat flux boundary condition. The heat transfer, friction factor, and thermal performance factor of two different nanofluids such as Al2O3–water and CNT–water with helical inserts is evaluated through experimental investigation.

## Nanofluid Preparation

Multiwalled CNTs and alumina were received from M/S Nanoshel LLC. Table 1 lists the physical properties of Al2O3 and CNT nanoparticles. The SEM (scanning electron microscope) image of MWCNT and Al2O3, captured by using FESEM (Carl Zeiss Microscopy, Germany) is shown in Figs. 1(a) and 1(b). The MWCNT is written as CNT in the remaining text for the convenience. Here, the CNT–water nanofluid with 0.15% volume concentration was prepared by functionalization acid treatment method. According to this method, a simple acid treatment provides better stability to the CNT–water suspension. The acid treatment method essentially introduces hydroxyl group at the surface of CNT leading to conversion of the CNT surface from hydrophobic to hydrophilic. Initially, CNTs were initially immersed in a mixture of H2SO4/HNO3 (3:1) at room temperature to break up the winding state [21-29]. Later on, CNTs were treated in an ultrasound bath (USBT- 9.0L Ultrasonic Cleanser, 200 W, Rico Scientific Industries, India) for 2 h and upheld for 15 h. Further, this solution was neutralized by using ammonium hydroxide and filtered with a 0.22 mm cellulose acetate membrane. The CNTs were washed several times using de-ionized water until the pH was adjusted to 5.5. When a surface suffers oxidation, chemical elements are adsorbed, forming the functional groups. These groups are either positively or negatively charged. In this procedure, hydroxyl and carboxylic groups are inserted on the surface of the nanotubes. Because of the equal charge of these groups CNTs repel from each other leading to the dispersed form of the solution [21-29]. On the contrary, Al2O3–water nanofluid is prepared by simply dispersing specified amounts of nanoparticles in the de-ionized water without addition of any surfactant. The ultrasonic vibrator was used in order to make the nanofluid more stable and more dispersed in water. Sonication was done for 1 h continuously in order to obtain a more stable and evenly dispersed nanoparticle suspension. For 0.15% (by volume) concentration CNT–water nanofluid and Al2O3–water nanofluid, the average size of the particles are found to be 63.5 nm and 121 nm, respectively.

Here, the hydrophilic nature of the nanofluid was tested by the static wettability test. It may be noted that static wetting occurs when a small liquid droplet is deposited on a smooth, homogeneous stationary copper surface. The degree of static wetting is usually defined by the equilibrium contact angle (θe). For the higher values of contact angle (θe > 90 deg), the wetting is considered to be poor. While, for a lower contact angle (θe < 90 deg), the liquid wets the solid surface. With the decrease in the contact angle, the degree of wetting increases and the complete wetting is achieved for zero value of contact angle (θe = 0). In the present study, tests have been carried out to measure the contact angle of various fluids such as water, CNT–water nanofluid, and Al2O3–water nanofluid with a plain copper solid surface by using the contact angle meter (Model- HO-IAD-CAM-01A, Holmarc Opto-Mechatronics Pvt. Ltd.) and the results are shown in Figs. 2(a)2(c). The contact angle was found to be 57.62 deg, 48.45 deg, and 44.82 deg for water, Al2O3–water nanofluid, and CNT–water nanofluid, respectively. This shows that addition of nanoparticle increases the surface wettability.

The volume concentration of nanoparticles is evaluated as below:Display Formula

(1)$φ=Volume of nanoparticleVolume of nanoparticle+Volume of base fluid×100$
Display Formula
(2)$φ=[WsρsWsρs+Wbfρbf]×100$

A thermal properties analyzer (KD-2 Pro, Decagon Devices) with an accuracy of ±5.0% and viscosity meter (LVDVII + PRO, Brookfield Digital Viscometer) with an accuracy of ±1.0% were used to measure the thermal conductivity and viscosity of the nanocoolant at various sample temperatures.

## Experimental Method

###### Test Facility.

Figure 3 depicts the schematic view of the test facility to study the flow and convective heat transfer characteristics of the Al2O3 and CNT nanofluid in a tube with helical tape. The test facility includes the test section, power supply unit, nanofluid supply system, cooling section, and instrumentation scheme for measuring temperature. The test section is made of a 1000 mm long copper tube of 10.5 mm ID and 12.5 mm OD. One end of the copper tube is fitted to a 1000 mm long PVC tube of 10.5 mm inner diameter (ID) and 12.5 mm outer diameter (OD). The long PVC tube, termed as calming section, is kept long enough to ensure the fully developed flow condition at the entrance of the test section. The inlet and outlet of the test section are connected to the calming section and heat exchanger, respectively. The copper tube is heated uniformly by wrapping two heating elements made of nichrome heating wire (20 gauge, 53.5 Ω, 1 kW). The nichrome heating wire has ceramic bead insulation that prevents the direct contact of nichrome wire with the test section. The terminals of the nichrome heating wires are connected to an auto transformer and the power supply to the test section is varied by varying the voltage. The entire test section is insulated by using glass wool insulation in order to minimize heat loss from the test section to the surroundings. Six calibrated RTD PT100 type temperature sensors with an accuracy of ±0.1 °C are placed in the thermo wells mounted on the test section at distances 0.15 m, 0.30 m, 0.45 m, 0.60 m, 0.75 m, and 0.90 m from the inlet of the test section to measure the outside wall temperature. Two calibrated RTD PT100 type temperature sensors are located at the inlet (T1) and outlet (T8) of the test section to measure the inlet and outlet temperatures of the working fluid. The inside wall temperature of the test section was evaluated by calculating the tube wall temperature drop from 1-D radial heat conduction equation. The temperature at various locations was recorded by using a digital acquisition system (DAS) (Model 34970, Agilent Technologies). A peristaltic pump (RH-P1201, Ravel Hiteks Pvt. Ltd.) with a maximum capacity of 2.5 l/min is used to feed nanofluid from the coolant storage tank to the test section. The coolant flow rate of the peristaltic pump is controlled by varying the rotational speed. The peristaltic pump was calibrated by using a measuring glass jar. This was achieved by collecting the volume of water in the jar for a given time interval and comparing with the volume flow rate calculated from the rotational speed of peristaltic pump. The flow rate of the nanofluid is controlled by the bypass valve arrangement, and the remaining fluid is sent back to the storage tank. The nanofluid, heated in the test section, is allowed to cool by passing through an air cooler. A differential pressure transducer (D9824-6-005BD, Omicron) with an accuracy of ±0.25% mounted across the test section is used to measure the pressure drop across the test section. In addition, a U-tube manometer with Carbon Tetrachloride (CCl4) as manometer liquid is used to measure the pressure drop during the experimental investigation. The helical screw tape inserts with various TRs (Fig. 4) were fabricated by winding uniformly a copper strip of 3.5 mm width over a 2.5 mm copper rod and painted with insulating gel. The TR is defined as the ratio of length of one twist to the diameter of the twist. Three helical screw tape inserts with TR of 1.5, 2.5, and 3 are used in the present study.

###### Experimental Test Procedure.

Tests were carried out to evaluate the convective heat transfer and friction factor characteristics of water and nanofluids through the tube with helical tape inserts. The storage tank is filled with the working fluid and the circulation pump is turned on to initiate the flow of coolant through the test section. The peristaltic pump is used to measure the coolant mass flow rate. The flow rate is adjusted to obtain the required Reynolds number (840–2280) to carry out the experimental investigation. After adjusting the required flow rate, the electric power is supplied to the heating element to heat the outer surface of tube. During heating, the DAS and a voltmeter monitor the surface temperature and the electric power, respectively. After attaining the steady state condition, the power supply to the heating element and the temperature at various locations of the test tube is noted by using voltmeter and DAS, respectively. Initially, tests were conducted with water and the results obtained from the experimental study were validated with the results of Shah [30]. Later on, two different working fluids, namely, CNT–water and Al2O3–water were used to study the friction factor and heat transfer characteristics in the horizontal tube. Subsequently, the screw tape inserts were inserted in the tube to study its effect on heat transfer and pressure drop characteristics. In order to evaluate the friction factor characteristics during the flow, the electric power supply to the heating coils was switched off and the pressure drop was measured by using a differential pressure transducer (D9824-6-005BD, Omicron). The tube wall temperatures, inlet and outlet temperatures of the working fluid, mass flow rate, electric power input are recorded during each test run. An error analysis is made to evaluate various errors associated in the experimental study following the procedure suggested by Coleman and Steel [31] and ANSI/ASME standard [32]. The uncertainties associated with various parameters, namely, flow rate, wall and fluid temperatures, voltage, current, and pressure drop were evaluated to estimate the combined uncertainty in Reynolds number, Nusselt number, and friction factor on the basis of 95% confidence level. The maximum uncertainty in various parameters, namely, Reynolds number, friction factor, and Nusselt number were found to be ±1%, ±3%, and ±2%, respectively.

## Results and Discussion

Experiments were carried out by using 0.15% volume concentration Al2O3–water and CNT–water nanofluid with helical tape inserts of TR = 1.5, 2.5, and 3. Reynolds number is varied in the range of 840–2280. The test data were used to calculate the Nusselt number, friction factor, and thermal performance factor for various Peclet numbers in Laminar flow regime with and without helical screw tape inserts. These are detailed below.

###### Thermal Conductivity and Viscosity of Measurement.

Figure 5(a) demonstrates the experimentally measured value of thermal conductivity for CNT–water and Al2O3–water nanofluids. The thermal conductivity of nanofluids increases with the fluid temperature and the viscosity is found to decrease with temperature. At higher fluid temperature, the Brownian motion of nanoparticles intensifies, consequently the micro convection increases resulting in an enhancement of the thermal conductivity of nanofluids. The enhancement in the thermal conductivity with temperature for CNT–water and Al2O3–water nanofluid is shown in Fig. 5(a). Earlier, several authors [9,33] reported similar observations during their experimental investigation. The absolute viscosities of nanofluid at different temperatures are estimated by using a viscosity meter (LVDV-II + PRO, Brookfield Digital Viscometer). Repeated tests are conducted with water and nanofluid to test the reliability of the values. The values of viscosity are found to be in excellent agreement with the values available in literature. Figure 5(b) shows the test data for viscosity of CNT–water and Al2O3–water nanofluid. It can be observed from these figures that the absolute viscosity of nanofluid increases with particle volume concentration and are in close agreement with the values reported by earlier researchers [9,34].

###### Heat Transfer Study.

The total heat supplied to the test section and the heat absorbed by the working fluid is estimated from Eqs. (3) and (4), respectively.Display Formula

(3)$Q1=VI (energy supplied)$
Display Formula
(4)$Q2=m·CP(Tout-Tin) (energy absorbed)$
Display Formula
(5)$(Q1-Q2)×100%/Q1<3%$

The heat balance between the heat input (Q1) and heat transfer rate of the water (Q2) is less than 3%, shown by Eq. (5).

Here, the Newton's law of cooling Eq. (6) is used for estimation of experimental heat transfer coefficient and the experimental Nusselt number is evaluated by using Eq. (7)Display Formula

(6)$h=Q2A(T¯w-T¯f)$

Here, $T¯w$ is the average temperature of the wall and $T¯f$ is the average bulk temperature of fluid.Display Formula

(7)$A=πDL, T¯w=T1+T2+T3+T4+T5+T66, T¯f=Tin+Tout2Nuexp=hDK$

where D, h, and K represents the diameter of the test section, average heat transfer coefficient, and thermal conductivity of the working fluid, respectively.

The Nusselt number of single phase fluid reported by Shah [30] is expressed below:Display Formula

(8)$Nu=1.953(RePrDx)1/3 for RePrDx≥33.33Nu=4.364+0.0722(RePrDx) for RePrDx≤33.33$

In this investigation, the tests have been carried out by using pure water in order to validate the test facility. The Nusselt number obtained from the present experimental study (Eq. (7)) is compared with the values obtained from Shah equation [30] and is shown in Fig. 6. The deviation between the present prediction obtained from Eq. (7) and the values obtained from Eq. (8) is found to be less than ± 7%. This indicates that the present test facility is in good condition and can be used to evaluate the heat transfer characteristics of both Al2O3–water and CNT–water nanofluid.

Figure 7 depicts the variation of Nusselt number with Peclet number for various working fluids and TRs. It is observed that Nusselt number increases with Peclet number. Tests have been carried out by using 0.15% volume concentration of Al2O3–water and CNT–water nanofluid. For given operating condition, the Nusselt number of both the nanofluids (Al2O3–water, CNT–water) was found to be higher compared to pure water. This may be due to the fact that the nanoparticles increase the thermal conductivity. In addition, the collision among nanoparticles and between the nanoparticles and tube wall increases the energy exchange rate leading to an increase in the heat transfer rate. Previous researchers have reported similar observations for Al2O3–water nanofluid in their experimental investigation [15]. Recently, Rathnakumar et al. [35] reported the experimental investigation of CNT–water nanofluids in circular tube with helical screw tape inserts in laminar flow regime. Their results exhibited lower heat transfer performance compared to present experimental study. This difference may be due to the use of surface modification method of nanoparticles during the synthesis of CNT–water nanofluids.

In addition, the CNT–water nanofluid exhibits enormous enhancement in heat transfer compared to the Al2O3–water nanofluid for the similar conditions. This may be due to various reasons. CNTs possess higher thermal conductivity, higher aspect ratio, lower specific gravity, larger SSA, and lower thermal resistance compared to Al2O3 [19,20]. The CNT nanoparticles have porous structure and create capillary action leading to enhancement in heat transfer. It may be noted that the higher thermal conductivity and large SSA of CNT nanoparticles play an important role in the heat transfer enhancement. In addition, the average size of the CNT and Al2O3 nanoparticles used in the present investigation is found to be 10 nm and100 nm, respectively. Therefore, the enhancement in heat transfer for CNT–water nanofluid is found to be higher compared to Al2O3–water nanofluid.

Tests have been carried out in order to evaluate the heat transfer enhancement of pure water and nanofluids with helical screw tape insert. Three different TRs (TR = 1.5, 2.5, and 3) are considered in the present investigation and the results are depicted in Figs. 8 and 9. The helical screw tape insert exhibits higher Nusselt number in both nanofluids and pure water. The enhancement in Nusselt number was found to increase with the decrease in the TR for both pure water and nanofluids. The thermal performance of CNT–water nanofluid is found to be higher compared to Al2O–water nanofluid and pure water both in the plain tube and the plain tube with helical screw tape inserts. The Nusselt number of various working fluids such as CNT–water, Al2O3–water, and pure water for the plain tube with helical inserts is higher compared to the plain tube for a given value of Peclet number. This may be due to the fact that the plain tube fitted with helical inserts decreases the hydraulic diameter and increases the fluid velocity leading to the increase in the Reynolds number. Therefore, the Nusselt number increases with inserts. In addition, the turbulent intensity of the fluid increases at the tube wall leading to excellent mixing. In such a case, the redevelopment of thermal and hydraulic boundary layer takes place leading to the enhancement in the heat transfer rate. The average enhancement in Nusselt number of Al2O3–water and CNT–water in the plain tube were found to be 10.72% and 20.27%, respectively, compared to the water as the working fluid. The enhancement in Nusselt number decreases with the increase in Peclet number (Fig. 8). In the case of Al2O3–water nanofluid, the increase in the average Nusselt number was found to 230.14%, 175.07%, and 135.79%, for various TRs 1.5, 2.5, and 3, respectively. On the contrary, the enhancement in the Nusselt Number for CNT–water nanofluid was found to be 272.61%, 213.58%, and 166.45% for the TRs 1.5, 2.5, and 3, respectively. It may be noted that the enhancement in the convective heat transfer is due to the addition of nanoparticles in water and the use of helical screw tape inserts. For given Peclet number, the enhancement of Nusselt number of pure water with helical screw tape insert is larger compared to the enhancement of Nusselt number by using Al2O3–water and CNT–water nanofluids in the plain tube. It may be noted that the mechanism of heat transfer rate is different in both the cases. The heat transfer enhancement in the case of nanofluids is due to various reasons such as: improved thermal conductivity of nanofluids compared to pure water, Brownian motion of nanoparticles and particle migration. While, in the case of helical screw tape inserts, the random movement of the fluid particles increases the energy exchange rate leading to the enhancement in heat transfer. The higher turbulence intensity close to the tube wall results in excellent fluid mixing. This leads to redevelopment of thermal and hydraulic boundary layers and the enhancement in the heat transfer rate.

The present experimental results are used to derive the correlation among various parameters, namely, Nusselt number, Reynolds number, Prandtl number, and TR by using least square method of regression analysis (Eqs. (8) and (9)). The correlations are valid for laminar flow (Re < 2280), various TRs (1.5–3), and 0.15% volume concentration of both the nanofluids (Al2O3–water and CNT–water). The Nusselt number values predicted by the correlations are in reasonable agreement with the experimental results.

For Al2O3–water nanofluidDisplay Formula

(9)$Nu=0.508(RePr)0.529(T.R.)-0.438$

For CNT–water nanofluidDisplay Formula

(10)$Nu=0.631(RePr)0.517(T.R.)-0.430$

###### Pressure Drop Study.

The pressure drop (ΔP) measured across the test section is used to determine the friction factor (f) using the following relation:Display Formula

(11)$fexp=ΔP(LD)(ρu22)$

where u, ρ, D, and L represent the fluid velocity, fluid density, the diameter of the test section, and length of the test section, respectively. The friction factor values evaluated from the present experimental study are compared with the theoretical values obtained using the Hagen–Poiseuille equation given byDisplay Formula

(12)$f=64Re$

In this study, the friction factor of water is obtained by using Eq. (11) and shown in Fig. 9. Present test data are found to be in good agreement with the values obtained by using Hagen–Poiseuille correlation (Eq. (12)). The deviation between the present test data and theoretical values are found to be within 5%.

The variation of friction factor of various working fluids (CNT–water, Al2O3–water, and pure water) with Peclet number for the plain tube and the plain tube with helical screw tape inserts is shown in Fig. 10. The increase in the pressure drop of pure water due to the dispersion of nanoparticles (CNT and Al2O3) is minimal. However, the pressure drop increases significantly with the use of helical inserts in the plain tube for both nanofluids (Al2O3–water and CNT–water). The friction factor found to increase with the decrease in the TR. Compared to the plain tube with water, the maximum friction factor of CNT–water nanofluid with inserts was found to be 19.99, 13.24, and 11.80 times higher for TRs of 1.5, 2.5, and 3, respectively. The maximum friction factor of Al2O3–water nanofluid with inserts was found to be 20.43, 13.79, and 12.02 times the friction factor of plain tube with water for various TRs 1.5, 2.5, and 3, respectively. For a given TR (TR = 1.5), the Al2O3–water nanofluid exhibits higher pressure drop compared to CNT-nanofluid. This may be due to the fact that the larger size of Al2O3 particle (100 nm) compared to CNT nanoparticle (10 nm). Because of the larger size of the Al2O3 particle, the nanofluid (Al2O3–water) exhibits higher viscosity and reduces the free flow area in the presence of inserts. Therefore, the pressure drop in the case of Al2O3–water nanofluid is higher compared to the CNT–water nanofluid.

The results obtained from the present experimental study are used to derive the correlations among various parameters such as: friction factor, Reynolds number and Twist ratio by using least square method of regression analysis (Eqs. (13) and (14)). These correlations are valid for laminar flow (Re < 2280), different TRs (1.5–3) and 0.15% volume concentration of both the nanofluids (Al2O3–water and CNT–water). The friction factor values predicted by the correlations are in reasonable agreement with the experimental data.

For Al2O3–water nanofluidDisplay Formula

(13)$f=289.20(Re)-0.743(T.R.)-0.791$

For CNT–water nanofluidDisplay Formula

(14)$f=250.98(Re)-0.726(T.R.)-0.796$

###### Thermal Performance Factor Analysis.

Thermal performance factor ($η$) is the parameter used to evaluate the effect of enhancement in heat transfer rate and the increase in the pressure drop [4,6,7,10,13-16]. This is defined as the ratio between the Nusselt number ratio $(Nu/Nupt)$ and friction factor ratio $(f/fpt)0.166$ for the same pumping power condition [15,36]. Mathematically one can defineDisplay Formula

(16)$η=(Nu/Nupt)(f/fpt)0.166$

where $Nu$ and $f$ denotes the Nusselt number and friction factor with helical inserts, respectively. While, $Nupt$ and $fpt$ represents the plain tube Nusselt number and friction factor, respectively. The variation of thermal performance factor of various working fluids, such as: Al2O3–water and CNT–water nanofluid with Peclet number is shown in Fig. 11. The values of $η$ were found to be higher for the low TRs irrespective of the working fluid and Peclet number. This may be due to fact that the helical screw inserts with lower TR generates stronger turbulence/swirl in the flow. This indicates that for a given operating condition, the helical screw tape inserts with the lower TRs should be considered for energy saving. The thermal performance factor ($η$) decreases with the increase in Peclet number. This may be due to the increase in the pressure drop with the increase in Peclet number.

It may be noted that the value of $η$ need to be greater than unity for the net energy gain in the system. The values of $η$ of the CNT–water nanofluid is found to be higher compared to the Al2O3–water nanofluid, and pure water for all TRs (1.5, 2.5, and 3) and for various Peclet number (5000–13,300). The values of $η$ were found to be higher for low TRs irrespective of the working fluid and Peclet number. This may be due to the stronger turbulence/swirl flow generated by the helical screw tape insert in the case of insert with lower TR. For TRs of 1.5, 2.5, and 3, the thermal performance factor varies between 2.29–1.69 and 2.03–1.55, respectively, by using CNT–water nanofluid. On the contrary, the thermal performance factor of Al2O3–water nanofluid (by volume 0.15%) was found to vary between 2.14–1.65, 1.90–1.47, and 1.58–1.31 for the TRs of 1.5, 2.5, and 3, respectively.

## Conclusion

An experimental study has been carried out to investigate the heat transfer and friction factor characteristics of helical screw inserts of various TRs (TR = 1.5, 2.5, and 3) in Al2O3–water and CNT–water nanofluids through a straight pipe in laminar flow regime with constant heat flux boundary condition. Tests have been performed by using 0.15% volume concentration Al2O3–water and CNT–water nanofluid. The Peclet number is varied within the range of 5000–13,300 during the present investigation. The conclusions obtained from the present study are detailed below:

1. (1)The addition of nanoparticle to base fluid (water) reduces contact angle leading to an increase in the wettability of the surface.
2. (2)The use helical screw tape inserts in plain tube causes intensification in heat transfer with significant increase in pressure drop.
3. (3)The Nusselt number for the tube fitted with helical inserts is found to be higher compared to the plain tube for a given Peclet number. The tube fitted with helical tapes decreases the hydraulic diameter leading to the increase in the fluid flow and swirl generation. The enhancement in Nusselt number decreases with the increase in Peclet number and increases with the decrease in the TR.
4. (4)The experimental results reveal that CNT–water nanofluid gives enormous enhancement in heat transfer compared to Al2O3–water nanofluid. This is because the CNTs offer a higher thermal conductivity, higher aspect ratio, lower specific gravity, and larger SSA and lower thermal resistance compared Al2O3–water nanofluid.
5. (5)The value of friction factor obtained by using the helical screw inserts were higher compared to the plain tube. The value of friction factor increases with the decrease in TR. The increase in the friction factor by the addition of Al2O3 and CNT nanoparticle in base fluid is minimal because of the low volume concentration of nanoparticles.
6. (6)The experimental results show that helical screw tape inserts give better thermal performance for CNT–water nanofluid compared to Al2O3–water nanofluid. For a given operating condition, the thermal performance factor for CNT–water nanofluid is higher compared to Al2O3–water for all TRs.

Nomenclature
• $Cp$ =

specific heat (J/kg K)

• $D$ =

diameter of the test section diameter (m)

• $f$ =

friction factor

• $h$ =

convective heat transfer coefficient (W/m2 °C)

• $I$ =

current (A)

• $k$ =

thermal conductivity (W/m K) (water, nanofluid)

• $L$ =

test section length (m)

• $m·$ =

mass flow rate (kg/s)

• $n$ =

Shape factor

• $Nu$ =

Nusselt number, $hD/k$

• P =

pitch of the helical screw tape insert (m)

• $Pe$ =

Peclet number, $ρCpvD/k$

• $Q$ =

heat input (W)

• $Re$ =

Reynolds number, $ρvD/μ$

• $T$ =

temperature (°C)

• $u$ =

fluid velocity (m/s)

• $V$ =

voltage (V)

• $W$ =

weight of nanoparticle (kg)

Greek Symbols
• $Δp$ =

pressure drop (N/m2)

• $μ$ =

dynamic viscosity (kg/m2 s)

• $ρ$ =

density (kg/m3)

• $φ$ =

volume concentration (%)

Subscripts
• $bf$ =

basefluid

• $in$ =

inlet

• $nf$ =

nanofluid

• $out$ =

outlet

• $pt$ =

plain tube

• $s$ =

nanoparticles

• $w$ =

wall

Abbreviations
• $HI$ =

helical screw tape inserts

• $ID$ =

inner diameter

• $OD$ =

outer diameter

• $SSA$ =

specific surface area

• $TR$ =

twist ratio

## References

Choi, S. U. S., Singer, D. A., and Wang, H. P., 1995, Development and Application of Non-Newtonian Flows, Vol. 231, ASME, New York, pp. 99–105.
Sivashanmugam, P., and Suresh, S., 2007, “Experimental Studies on Heat Transfer and Friction Factor Characteristics of Turbulent Flow Through a Circular Tube Fitted With Helical Screw-Tape Inserts,” J. Chem. Eng. Process, 46, pp. 1292–1298.
Chang, S. W., Yu, K. W., and Lu, M. H., 2005, “Heat Transfers in Tubes Fitted With Single, Twin, and Triple Twisted Tapes,” Exp. Heat Transfer, 18, pp. 279–294.
Murugesan, P., Mayilsamy, K., and Suresh, S., 2012, “Heat Transfer in a Tube Fitted With Vertical and Horizontal Wing-Cut Twisted Tapes,” Exp. Heat Transfer, 25, pp. 30–47.
Bhuiya, M. M. K., Ahamed, J. U., Sarkar, M. A. R., Salam, B., Masjuki, H. H. M., Kalam, A., Saidur, R., and Sayem, A. S. M., 2012, “Heat Transfer and Pressure Drop Characteristics in Turbulent Flow Through a Tube,” Exp. Heat Transfer, 25, pp. 301–322.
Patil, S. V., and Babu, P. V. V., 2014, “Laminar Heat Transfer Augmentation Through a Square Duct and Circular Tube Fitted With Twisted Tapes,” Exp. Heat Transfer, 27, pp. 124–143.
Bhuiya, M. M. K., Ahamed, J. U., Sarkar, M. A. R., Salam, B., Sayem, A. S. M., and Rahman, A., 2014, “Performance of Turbulent Flow Heat Transfer Through a Tube With Perforated Strip Inserts,” Heat Transfer Eng., 35, pp. 43–52.
Chougule, S. S., and Sahu, S. K., 2014, “Comparative Study on Heat Transfer Enhancement of Low Volume Concentration of Al2O3–Water and CNT–Water Nanofluids in Transition Regime Using Helical Screw Tape Inserts,” Exp. Heat Transfer (accepted).
Sharma, K. V., Sundar, L. S., and Sarma, P. K., 2009, “Estimation of Heat Transfer Coefficient and Friction Factor in the Transition Flow With Low Volume Concentration of Al2O3 Nanofluid Flowing in a Circular Tube and With Twisted Tape Insert,” Int. Commun. Heat Mass Transfer, 36, pp. 503–507.
Sundar, L. S., and Sharma, K. V., 2010, “Heat Transfer Enhancements of Low Volume Concentration Al2O3 Nanofluid and With Longitudinal Strip Inserts in a Circular Tube,” Int. J. Heat Mass Transfer, 53, pp. 4280–4286.
Sundar, L. S., and Sharma, K. V., 2010, “Turbulent Heat Transfer and Friction Factor of Al2O3nanofluid in Circular Tube With Twisted Tape Inserts,” Int. Commun. Heat Mass Transfer, 53, pp. 1409–1416.
Wongcharee, K., and Smith, E., 2011, “Enhancement of Heat Transfer Using CuO/Water Nanofluid and Twisted Tape With Alternate Axis,” Int. Commun. Heat Mass Transfer, 38, pp. 742–748.
Chandrasekar, M., Suresh, S., and Bose, C. A., 2010, “Experimental Studies on Heat Transfer and Friction Factor Characteristics of Al2O3/Water Nanofluid in a Circular Pipe Under Laminar Flow With Wire Coil Inserts,” Exp. Therm. Fluid Sci., 34, pp. 122–130.
Chandrasekar, M., Suresh, S., and Bose, C. A., 2011, “Experimental Studies on Heat Transfer and Friction Factor Characteristics of Al2O3/Water Nanofluid in a Circular Pipe Under Transition Flow With Wire Coil Inserts,” Heat Transfer Eng., 32, pp. 495–496.
Suresh, S., Venkitaraj, K. P., and Selvakumar, P., 2011, “Comparative Study on Thermal Performance of Helical Screw Tape Inserts in Laminar Flow Using Al2O3/Water and CuO/Water Nanofluids,” Superlattices Microstruct., 49, pp. 608–622.
Suresh, S., Venkitaraj, K. P., Selvakumar, P., and Chandrasekar, M., 2012, “A Comparison of Thermal Characteristics of Al2O3/Water and CuO/Water Nanofluids in Transition Flow Through a Straight Circular Duct Fitted With Helical Screw Tape Inserts,” Exp. Therm. Fluid Sci., 39, pp. 37–44.
Sundar, L. S., Kumar, N. T. R., Naik, M. T., and Sharma, K. V., 2012, “Effect of Full Length Twisted Tape Inserts on Heat Transfer and Friction Factor Enhancement With Fe3O4 magnetic Nanofluid Inside a Plain Tube: An Experimental Study,” Int. J. Heat Mass Transfer, 55, pp. 2761–2768.
Naik, M. T., and Lingala, S. S., 2014, “Heat Transfer and Friction Factor With Water/Propylene Glycol-Based CuO Nanofluid in Circular Tube With Helical Inserts Under Transition Flow Regime,” Heat Transfer Eng., 35, pp. 53–62.
Saidur, R., Leong, K. Y., and Mohamma, H. A., 2011, “A Review on Applications and Challenges of Nanofluids,” Renewable Sustainable Energy Rev., 15, pp. 1646–1668.
Chougule, S. S., and Pise, A. T., 2012, “Studies of CNT Nanofluid in Two Phase System,” Int. J. Global Technol. Initiatives, 1, pp. F14–F20.
Chougule, S. S., and Sahu, S. K., “Thermal Performance of Automobile Radiator Using Carbon Nanotube-Water Nanofluid—Experimental Study,” ASME J. Therm. Sci. Eng. Appl., 6(4), p. 041009
Chougule, S. S., Sahu, S. K., and Pise, A. T., 2014, “Thermal Performance of Two Phase Thermosyphon Flat-Plate Solar Collectors by Using Nanofluid,” ASME J. Sol. Energy, 136(1), p. 014503.
Chougule, S. S., and Sahu, S. K., 2014, “Comparative Study of Cooling Performance of Automobile Radiator Using Al2O3/Water and CNT/Water Nanofluid,” ASME J. Nanotechnol. Eng. Med., 5(1), p. 011001.
Chougule, S. S., Sahu, S. K., and Pise, A. T., 2013, “Performance Enhancement of Two Phase Thermosyphon Flat-Plate Solar Collectors by Using Surfactant and Nanofluid,” Front. Heat Pipes, 4(1), pp. 1–6.
Chougule, S. S., and Sahu, S. K., 2014, “An Integrated Effect of PCM and Nanofluid Charged Heat Pipe for Electronics Cooling,” ASME 12th International Conference on Nanochannels, Microchannels, and Minichannels (ICNMM2014), Chicago, IL, Aug. 13–17.
Pise, A. T., and Chougule, S. S., 2011, “Experimental Investigation Heat Transfer Augmentation of Solar Heat Pipe Collector by Using Nanofluid,” 21st National and 10th ISHMT-ASME Heat and Mass Transfer Conference, Madras, India, pp. 27–30.
Chougule, S. S., and Sahu, S. K., 2013, “Experimental Investigation of Heat Transfer Augmentation in Automobile Radiator With CNT/Water Nanofluid,” 4th ASME-International Conference on Micro/Nanoscale Heat & Mass Transfer (MNHMT2013), Hong Kong, China, Dec. 11–14.
Chougule, S. S., and Sahu, S. K., 2013, “Comparison of Augmented Thermal Performance of CNT/Water and Al2O3/Water Nanofluids in Transition Flow Through a Straight Circular Duct Fitted With Helical Screw Tape Inserts,” 22nd National and 11th ISHMT-ASME Heat and Mass Transfer Conference, IIT, Kharagpur, India, Dec. 28–31.
Chougule, S. S., Pise, A. T., and Madane, P. A., 2012, “Performance of Nanofluid-Charged Solar Water Heater by Solar Tracking System,” IEEE-ICAESM-2012, Nagapattinam, India, Mar. 30–31, Vol. VI, pp. 247–254.
Shah, R. K., 1975, “Thermal Entry Length Solutions for the Circular Tube and Parallel Plates,” Third National Heat Mass Transfer Conference, Indian Institute of Technology, Bombay, India, Vol 1, pp. 11–75.
Coleman, H. W., and Steele, W. G., 1989, Experimental and Uncertainty Analysis for Engineers, Wiley, New York.
ANSI/ASME, 1985, “Measurement Uncertainty,” The American Society of Mechanical Engineers, Paper No. PTC 19.
Assael, M. J., Chen, C. F., Metaxa, I. N., and Wakeham, W. A., 2004, “Thermal Conductivity of Suspensions of Carbon Nanotubes in Water,” Int. J. Thermophys., 25, pp. 971–985.
Chen, L., Xie, H., Li, Y., and Yu, W., 2008, “Nanofluids Containing Carbon Nanotubes Treated by Mechanochemical Reaction,” Thermochim. Acta, 477, pp. 21–24.
Rathnakumar, P., Mayilsamy, K., Suresh, S., and Murugesan, P., 2013, “Laminar Heat Transfer and Friction Factor Characteristics of Carbon Nanotube/Water Nanofluids,” J. Nanosci. Nanotechnol., 13(1–8).
Usui, H., Sano, Y., Iwashita, K., and Isozaki, A., 1996, “Enhancement of Heat Transfer by a Combination of Internally Grooved Rough Tube and Twisted Tape,” Int. Chem. Eng., 26, pp. 97–104.
View article in PDF format.

## References

Choi, S. U. S., Singer, D. A., and Wang, H. P., 1995, Development and Application of Non-Newtonian Flows, Vol. 231, ASME, New York, pp. 99–105.
Sivashanmugam, P., and Suresh, S., 2007, “Experimental Studies on Heat Transfer and Friction Factor Characteristics of Turbulent Flow Through a Circular Tube Fitted With Helical Screw-Tape Inserts,” J. Chem. Eng. Process, 46, pp. 1292–1298.
Chang, S. W., Yu, K. W., and Lu, M. H., 2005, “Heat Transfers in Tubes Fitted With Single, Twin, and Triple Twisted Tapes,” Exp. Heat Transfer, 18, pp. 279–294.
Murugesan, P., Mayilsamy, K., and Suresh, S., 2012, “Heat Transfer in a Tube Fitted With Vertical and Horizontal Wing-Cut Twisted Tapes,” Exp. Heat Transfer, 25, pp. 30–47.
Bhuiya, M. M. K., Ahamed, J. U., Sarkar, M. A. R., Salam, B., Masjuki, H. H. M., Kalam, A., Saidur, R., and Sayem, A. S. M., 2012, “Heat Transfer and Pressure Drop Characteristics in Turbulent Flow Through a Tube,” Exp. Heat Transfer, 25, pp. 301–322.
Patil, S. V., and Babu, P. V. V., 2014, “Laminar Heat Transfer Augmentation Through a Square Duct and Circular Tube Fitted With Twisted Tapes,” Exp. Heat Transfer, 27, pp. 124–143.
Bhuiya, M. M. K., Ahamed, J. U., Sarkar, M. A. R., Salam, B., Sayem, A. S. M., and Rahman, A., 2014, “Performance of Turbulent Flow Heat Transfer Through a Tube With Perforated Strip Inserts,” Heat Transfer Eng., 35, pp. 43–52.
Chougule, S. S., and Sahu, S. K., 2014, “Comparative Study on Heat Transfer Enhancement of Low Volume Concentration of Al2O3–Water and CNT–Water Nanofluids in Transition Regime Using Helical Screw Tape Inserts,” Exp. Heat Transfer (accepted).
Sharma, K. V., Sundar, L. S., and Sarma, P. K., 2009, “Estimation of Heat Transfer Coefficient and Friction Factor in the Transition Flow With Low Volume Concentration of Al2O3 Nanofluid Flowing in a Circular Tube and With Twisted Tape Insert,” Int. Commun. Heat Mass Transfer, 36, pp. 503–507.
Sundar, L. S., and Sharma, K. V., 2010, “Heat Transfer Enhancements of Low Volume Concentration Al2O3 Nanofluid and With Longitudinal Strip Inserts in a Circular Tube,” Int. J. Heat Mass Transfer, 53, pp. 4280–4286.
Sundar, L. S., and Sharma, K. V., 2010, “Turbulent Heat Transfer and Friction Factor of Al2O3nanofluid in Circular Tube With Twisted Tape Inserts,” Int. Commun. Heat Mass Transfer, 53, pp. 1409–1416.
Wongcharee, K., and Smith, E., 2011, “Enhancement of Heat Transfer Using CuO/Water Nanofluid and Twisted Tape With Alternate Axis,” Int. Commun. Heat Mass Transfer, 38, pp. 742–748.
Chandrasekar, M., Suresh, S., and Bose, C. A., 2010, “Experimental Studies on Heat Transfer and Friction Factor Characteristics of Al2O3/Water Nanofluid in a Circular Pipe Under Laminar Flow With Wire Coil Inserts,” Exp. Therm. Fluid Sci., 34, pp. 122–130.
Chandrasekar, M., Suresh, S., and Bose, C. A., 2011, “Experimental Studies on Heat Transfer and Friction Factor Characteristics of Al2O3/Water Nanofluid in a Circular Pipe Under Transition Flow With Wire Coil Inserts,” Heat Transfer Eng., 32, pp. 495–496.
Suresh, S., Venkitaraj, K. P., and Selvakumar, P., 2011, “Comparative Study on Thermal Performance of Helical Screw Tape Inserts in Laminar Flow Using Al2O3/Water and CuO/Water Nanofluids,” Superlattices Microstruct., 49, pp. 608–622.
Suresh, S., Venkitaraj, K. P., Selvakumar, P., and Chandrasekar, M., 2012, “A Comparison of Thermal Characteristics of Al2O3/Water and CuO/Water Nanofluids in Transition Flow Through a Straight Circular Duct Fitted With Helical Screw Tape Inserts,” Exp. Therm. Fluid Sci., 39, pp. 37–44.
Sundar, L. S., Kumar, N. T. R., Naik, M. T., and Sharma, K. V., 2012, “Effect of Full Length Twisted Tape Inserts on Heat Transfer and Friction Factor Enhancement With Fe3O4 magnetic Nanofluid Inside a Plain Tube: An Experimental Study,” Int. J. Heat Mass Transfer, 55, pp. 2761–2768.
Naik, M. T., and Lingala, S. S., 2014, “Heat Transfer and Friction Factor With Water/Propylene Glycol-Based CuO Nanofluid in Circular Tube With Helical Inserts Under Transition Flow Regime,” Heat Transfer Eng., 35, pp. 53–62.
Saidur, R., Leong, K. Y., and Mohamma, H. A., 2011, “A Review on Applications and Challenges of Nanofluids,” Renewable Sustainable Energy Rev., 15, pp. 1646–1668.
Chougule, S. S., and Pise, A. T., 2012, “Studies of CNT Nanofluid in Two Phase System,” Int. J. Global Technol. Initiatives, 1, pp. F14–F20.
Chougule, S. S., and Sahu, S. K., “Thermal Performance of Automobile Radiator Using Carbon Nanotube-Water Nanofluid—Experimental Study,” ASME J. Therm. Sci. Eng. Appl., 6(4), p. 041009
Chougule, S. S., Sahu, S. K., and Pise, A. T., 2014, “Thermal Performance of Two Phase Thermosyphon Flat-Plate Solar Collectors by Using Nanofluid,” ASME J. Sol. Energy, 136(1), p. 014503.
Chougule, S. S., and Sahu, S. K., 2014, “Comparative Study of Cooling Performance of Automobile Radiator Using Al2O3/Water and CNT/Water Nanofluid,” ASME J. Nanotechnol. Eng. Med., 5(1), p. 011001.
Chougule, S. S., Sahu, S. K., and Pise, A. T., 2013, “Performance Enhancement of Two Phase Thermosyphon Flat-Plate Solar Collectors by Using Surfactant and Nanofluid,” Front. Heat Pipes, 4(1), pp. 1–6.
Chougule, S. S., and Sahu, S. K., 2014, “An Integrated Effect of PCM and Nanofluid Charged Heat Pipe for Electronics Cooling,” ASME 12th International Conference on Nanochannels, Microchannels, and Minichannels (ICNMM2014), Chicago, IL, Aug. 13–17.
Pise, A. T., and Chougule, S. S., 2011, “Experimental Investigation Heat Transfer Augmentation of Solar Heat Pipe Collector by Using Nanofluid,” 21st National and 10th ISHMT-ASME Heat and Mass Transfer Conference, Madras, India, pp. 27–30.
Chougule, S. S., and Sahu, S. K., 2013, “Experimental Investigation of Heat Transfer Augmentation in Automobile Radiator With CNT/Water Nanofluid,” 4th ASME-International Conference on Micro/Nanoscale Heat & Mass Transfer (MNHMT2013), Hong Kong, China, Dec. 11–14.
Chougule, S. S., and Sahu, S. K., 2013, “Comparison of Augmented Thermal Performance of CNT/Water and Al2O3/Water Nanofluids in Transition Flow Through a Straight Circular Duct Fitted With Helical Screw Tape Inserts,” 22nd National and 11th ISHMT-ASME Heat and Mass Transfer Conference, IIT, Kharagpur, India, Dec. 28–31.
Chougule, S. S., Pise, A. T., and Madane, P. A., 2012, “Performance of Nanofluid-Charged Solar Water Heater by Solar Tracking System,” IEEE-ICAESM-2012, Nagapattinam, India, Mar. 30–31, Vol. VI, pp. 247–254.
Shah, R. K., 1975, “Thermal Entry Length Solutions for the Circular Tube and Parallel Plates,” Third National Heat Mass Transfer Conference, Indian Institute of Technology, Bombay, India, Vol 1, pp. 11–75.
Coleman, H. W., and Steele, W. G., 1989, Experimental and Uncertainty Analysis for Engineers, Wiley, New York.
ANSI/ASME, 1985, “Measurement Uncertainty,” The American Society of Mechanical Engineers, Paper No. PTC 19.
Assael, M. J., Chen, C. F., Metaxa, I. N., and Wakeham, W. A., 2004, “Thermal Conductivity of Suspensions of Carbon Nanotubes in Water,” Int. J. Thermophys., 25, pp. 971–985.
Chen, L., Xie, H., Li, Y., and Yu, W., 2008, “Nanofluids Containing Carbon Nanotubes Treated by Mechanochemical Reaction,” Thermochim. Acta, 477, pp. 21–24.
Rathnakumar, P., Mayilsamy, K., Suresh, S., and Murugesan, P., 2013, “Laminar Heat Transfer and Friction Factor Characteristics of Carbon Nanotube/Water Nanofluids,” J. Nanosci. Nanotechnol., 13(1–8).
Usui, H., Sano, Y., Iwashita, K., and Isozaki, A., 1996, “Enhancement of Heat Transfer by a Combination of Internally Grooved Rough Tube and Twisted Tape,” Int. Chem. Eng., 26, pp. 97–104.

## Figures

Fig. 1

(a) SEM image at magnification of 20,000× of MWCNT particles and (b) SEM image at magnification of 20,000× of Al2O3 particles

Fig. 2

Equilibrium contact angle on a plain copper surface: droplet resting on a level copper surface (a) pure water, (b) Al2O3–water nanofluid, and (c) CNT–water nanofluid

Fig. 3

Schematic diagram of test facility

Fig. 4

(a) Geometrical configuration of helical screw tape inserts and (b) helical twisted tape inserts used in the present work and their geometries

Fig. 5

(a) Variation of thermal conductivity with temperature for nanofluids with 0.15% volume concentrations and (b) Variation of absolute viscosity with temperature for nanofluids with 0.15% volume concentrations

Fig. 6

Comparison of experimental Nusselt number with Shah equation

Fig. 7

Variation of Nusselt number with Peclet number nanocoolant

Fig. 8

Enhancement in Nusselt number at different Peclet number

Fig. 9

Variation of friction factor with Reynolds number

Fig. 10

Variation of friction factor with Peclet number

Fig. 11

Variation of thermal performance factor with Peclet number

## Tables

Table 1 Geometrical specifications and characteristics of nanoparticles

## Discussions

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