water nanofluid

Accepted Manuscript Comparative study on thermal performance of twisted tape and wire coil inserts in turbulent flow using CuO/water nanofluid M.T. Naik, Syed Sha Fahad, L. Syam Sundar, Manoj K. Singh PII: DOI: Reference:

S0894-1777(14)00094-6 http://dx.doi.org/10.1016/j.expthermflusci.2014.04.006 ETF 8197

To appear in:

Experimental Thermal and Fluid Science

Received Date: Revised Date: Accepted Date:

13 November 2013 28 March 2014 2 April 2014

Please cite this article as: M.T. Naik, S.S. Fahad, L. Syam Sundar, M.K. Singh, Comparative study on thermal performance of twisted tape and wire coil inserts in turbulent flow using CuO/water nanofluid, Experimental Thermal and Fluid Science (2014), doi: http://dx.doi.org/10.1016/j.expthermflusci.2014.04.006

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Comparative study on thermal performance of twisted tape and wire coil inserts in turbulent flow using CuO/water nanofluid

M.T. Naik1,*, Syed Sha Fahad2, L. Syam Sundar3,*, Manoj K. Singh3 1

Centre for Energy Studies, JNTU College of Engineering, Kukatpally, Hyderabad, India

2

Muffakhamjah College of Engineering and Technology, Banjara Hills, Hyderabad. India

3

Centre for Mechanical Technology and Automation (TEMA-UA), Department of Mechanical

Engineering, University of Aveiro, 3810-193 Aveiro, Portugal *Authors: [email protected] (M.T. Naik), [email protected] (L.S. Sundar)

Abstract Heat transfer and friction factor analysis of CuO/water nanofluid flowing through a tube under turbulent flow conditions and with twisted tape (TT) and wire coil (WC) inserts were presented in this paper. The experimental investigations were performed in the Reynolds number range from 4000 to 20000, volume concentrations of 0.1% and 0.3%, twisted tape inserts of

and wire coil inserts of

. The experimental

results indicated that under same operating conditions and flow rates, heat transfer coefficient, friction factor and thermal performance factor associated with nanofluid in a tube with wire coil inserts are higher than those with the twisted tape inserts. The Nusselt number enhancement for 0.3% nanofluid in a tube without inserts is 17.62%, 0.3% nanofluid in a tube with TT-2 is 31.88% and 0.3% nanofluid in a tube with WC-2 is 44.45% at a Reynolds number of 20000 compared to water. Whereas, the friction factor enhancement for 0.3% nanofluid in a tube without inserts is 1.149-times, 0.3% nanofluid in a tube with TT-2 is 1.179-times and 0.3% nanofluid in a tube with WC-2 is 1.198-times at a Reynolds number of 20000 compared to water. The thermal performance factor of 0.3% nanofluid in tube with twisted tape and wire coil inserts are 1.24 and 1.36 compared against water data respectively. Keywords: CuO nanofluid, twisted tape, wire coil, heat transfer, friction factor.

1. Introduction Conventional single phase fluids such as water, engine oil, ethylene glycol and propylene glycol and transformer oil etc., plays an important role in thermal management of industries such as process industries, chemical plants, thermal power plants, but they have poor thermal characteristics, in particular thermal conductivity. To reach the industrial requirements heat transfer intensification of the fluids is very essential. Since solid materials posses higher thermal conductivities compared to conventional fluids. Many studies have been carried out on thermal properties of suspension of solid particles in conventional heat transfer fluids. Ahuja [1] and Liu et al. [2] have studied experimentally on heat transfer intensification of fluids by dispersing of millimeter or micrometer sized particles. They observed heat transfer enhancement but suffered from sedimentation of the particles in the fluids. Nanotechnology provides to manufacture solid particles down to millimetre or micrometer size to nanometer meter size. Fluids containing dispersion of nanometer sized particles are called nanofluids (Choi [3]). The heat transfer performance of these fluids is superior to suspended millimeter or micrometer particles in fluid and also other potential benefits such as large relative surface area, higher heat conduction, excellent stability and minimal clogging. Many researchers have explained thermal conductivity enhancement of nanofluids with influence of particle concentrations and temperatures [4-6]. The applicability of nanofluids in industrial sector can be analyzed by its convective heat transfer coefficient. Pak and Cho [7] experimentally studied the convective heat transfer of Al2O3/water and TiO2/water nanofluid in a tube under turbulent flow conditions and obtained approximately 75% heat transfer enhancement with 2.78% volume concentration of Al2O3 nanofluid. Sundar et al. [8] estimated the convective heat transfer of Fe3O4/water nanofluid and obtained Nusselt number and friction factor enhancements of 30.96% and 10.01% at 0.6% volume concentration compared to water at similar operating conditions. Ding et al. [9] observed

2

350% heat transfer enhancement with carbon nanotubes (CNT’s)/water flowing in a horizontal tube at 0.5% weight concentration at Reynolds number is 800. Chandraprabu [10] experimentally investigated the convective heat transfer of CuO/water nanofluid in the condensing unit of an air conditioner with particle concentrations of 1%, 2%, 3%, and 4 vol. % and they found heat transfer rate of CuO nanofluid improved up to 35%. Asirvatham [11] experimentally investigated the heat transfer of CuO/water nanofluid with low volume fraction of 0.003% in a copper tube with mass flow rate range from 0.0113 kg/s to 0.0139 kg/s and in the inlet temperatures of 10oC and 17oC and found that 8% heat transfer enhancement. Suresh et al. [12] experimentally investigated convective heat transfer of CuO/water nanofluids with particle concentrations of 0.1%, 0.2% and 0.3% and found that Nusselt number enhancements of 6%, 9.9% and 12.6%, respectively, compared to water. Hashemi and Akhavan-Behabadi [13] prepared nanofluids with different particle weight concentrations of 0.5%, 1% and 2% and observed highest heat transfer enhancement with helically coiled tube instead of straight tube. Kannadasan et al. [14] experiments were conducted in the turbulent flow for 0.1% and 0.2% CuO nanofluid in horizontal and vertical arrangements and observed the higher heat transfer rates for helically coiled heat exchanger. Suresh et al. [15] have conducted experiments by using helically dimpled tube with CuO/water nanofluid of 0.1%, 0.2% and 0.3% volume concentrations and the Nusselt number with dimpled tube and nanofluids under turbulent flow is about 19%, 27% and 39% higher than the Nusselt number obtained with plain tube and water. Most of the researchers have obtained enhancement in heat transfer with the use nanofluids flowing in a tube. Another passive technique to enhance the convective heat transfer of fluid flowing in a tube by inserting inserts. Generally used inserts such as twisted tape, wire coil, longitudinal strip and helical screw tape inserts etc. The rate of heat transfer enhancement is depending on flow conditions and geometry of the insert. Single phase fluids flowing in a tube with twisted

3

tape inserts have been analyzed by Smithberg and Landis [16], Lopina and Bergles [17], Manglik and Bergles [18]. Sarma et al. [19,20] also reported heat transfer enhancements of single phase fluids flowing in a tube with twisted tape inserts. In the similar way single phase fluid in a tube with wire coil inserts have been analyzed by Uttarwar and Rao [21], Vicente et al. [22,23], Wang et al. [24], Garcia et al. [25] and Akhavan-Behabadi et al. [26]. Most of the researchers have obtained enhancement in heat transfer for single phase fluid flowing in a tube with inserts. This indicates that, the similar passive technique may also be helpful to further heat transfer enhancement of nanofluids flowing in a tube. Nanofluid flowing in a tube with twisted tape inserts has been analyzed by Sundar and Sharma [27]. They conducted experiments with Al2O3/water nanofluid flowing in a tube and with twisted tape inserts and obtained 33.51% enhancement with twist ratio of

at

0.5% volume concentration and proposed Nusselt number and friction factor correlations. In the another study of Sundar and Sharma [28], they used Al2O3/water nanofluid flowing in a tube with longitudinal strip inserts and obtained 55.73% heat transfer enhancement at longitudinal strip insert of AR = 1 for 0.5% volume concentration. Wongcharee and Eiamsaard [29] observed Nusselt number increase of 12.8 and 7.2-times with CuO/water nanofluid in a tube with modified twisted tape and alternative twisted tape inserts under laminar flow. Wongcharee and Eiamsa-ard [30] also observed 1.57-times thermal performance factor for 0.7% of CuO/water nanofluid in a corrugated tube with twisted tape inserts. Eiamsa-ard and Wongcharee [31] studied the combined effects of nanofluids, dual twisted tapes and micro fin tube on the heat transfer rate, friction factor and thermal performance factor characteristics for 0.3% and 1.0% by volume in the Reynolds number between 5650 and 17000. Suresh et al. [32, 33] found maximum enhancement of 166.8% for Al2O3/water nanofluid and 179.8% for CuO/water nanofluid at helical screw tape ratio,

under the same flow conditions

by using 0.1% volume concentration. 4

Nanofluid in a tube with wire coil inserts has been analyzed by Chandrasekar et al. [34]. They considered 0.1% of Al2O3/water nanofluid in a tube under fully developed laminar flow with wire coil inserts and observed 21.5% heat transfer enhancement with wire coiled insert pitch of 3 and also developed Nusselt number correlation. Naik and Sundar [35] developed Nusselt number and friction factor correlations for propylene glycol and water based CuO nanofluid flowing in a tube with helical coil inserts. Saeedinia et al. [36] have prepared heat transfer experiments with CuO/base oil nanofluid in a tube with wire coil inserts. They considered particle concentrations from 0.07%-0.3% and five wire coil pitches of 25-35 mm and wire diameters of 0.9-1.5 mm and obtained 45% heat transfer enhancement with 63% penalty in pressure drop at high wire diameter coil inserts. Kahani et al. [37] investigated heat transfer behavior of Al2O3/water and TiO2/water nanofluid flowing in a tube with helical coil inserts in the volume concentrations from 0.25%-1.0% in the Reynolds number of 500 to 4500. Sundar and Singh [38] have provided the available correlations for nanofluid flowing in a tube with different kind of inserts in their review paper. The authors individually obtained further heat transfer enhancement for nanofluids flowing in a tube with inserts without any penalty in pumping power. Performance analysis between various inserted inserts in a tube for nanofluid flow is very essential. Till now, there is no such analysis is available in the literature. In this regard, the present work focuses on the estimation of convective heat transfer and friction factor of CuO/water nanofluid flowing in a tube with twisted tape and wire coil inserts under turbulent flow conditions. Thermal performance between twisted tape and wire coil inserts in a tube together with CuO/water nanofluid has been analyzed. Nusselt number and friction factor correlations were proposed based on the experimental data.

5

2. Preparation of CuO nanofluid In this study, nanofluids were prepared by dispersing CuO nanoparticles in distilled water. The physical properties of CuO nanoparticles and the equations for the estimation of thermal conductivity, density, specific heat and viscosity of distilled water were shown in Table 1. The nanofluids of 0.1% and 0.3% volume concentrations were prepared. Fig. 1a shows the transmission electron microscopy image of CuO nanoparticles dispersed in water and reveals that nanoparticles are in spherical shape. Sample nanofluid and bulk nanofluid preparations were represented in Fig. 1b and Fig. 1c. There are several procedures to prepare stable nanofluids, which include using dispersant, a stabilizer, a surface activator and an ultrasonic vibrator. Among these methods, surface modification method attracts more attention because of its unique, less cost and technological advantages [40]. Nanofluids could be stable a maximum of 30 min without using suitable surfactants. A surfactant can improve the stability of nanofluid dramatically. In this study, Cetyltrimethyl Ammonium Bromide (C-TAB) was used as surfactant for CuO nanoparticles. The required quantity of nanoparticles for given volume concentration was estimated from Eq. (1). The amount of surfactant is nearly equal to 1/10th of weight of nanoparticles for particular concentration was mixed with distilled water and stirred by high speed stirrer. The sonication was done continuously by ultrasonic processor (Hielscher, Germany) for at least 60 min to obtain a stable nanofluid. No settlement of nanoparticles was observed after 45 days. (1)

Volume concentration,

Where kg/m3,

is the percentage of volume concentration, = 100 g and

= 6300 kg/m3,

= 998.5

is the weight of the nanoparticles. The thermophysical

properties of prepared nanofluids were estimated from the properties of water (Table 1) and

6

properties of CuO nanoparticles at bulk temperature of fluid using the below equations for density, specific heat and viscosity [41-43]. (2) Where

and and

are the particle volume concentration and viscosity and the subscripts refer to particle, base fluid and nanofluid. The above equation is valid for

a very low particle volume concentration

. (3) (4)

The theoretical model to predict the thermal conductivity of solid liquid mixture, Maxwell [44] model can be described as follows: (5)

3. Experimental setup and procedure The schematic representation of experimental setup was shown in Fig. 2. The test section contains copper tube with 1750 mm long, 14 mm inner diameter (ID) and 16 mm outer diameter (OD). The other parts involved in the experimental setup are chiller, collecting tank, storage tank, variable pump and by-pass valve arrangement. Constant heat flux boundary condition was maintained by winding the nichrome heater with a gauge of 20 mm, resistance of 53.3Ω/m and a maximum capacity of 1000 W on outer surface of the copper tube. The test section was placed in a straight square duct in order to maintain horizontal position. The gap between the test tube and square duct was filled with rock wool insulation to reduce the heat loss to atmosphere. Seven PT-100 resistance temperature detector (RTD) sensors are provided; in which two were used to record the inlet and outlet temperatures, five were brazed on the outer surface of the test tube at distances of 187.5, 375, 750, 1125 and 1312 7

mm from the inlet of the test section to measure the wall temperatures of the tube. The resolution of all the thermocouples was ±0.1oC and they are calibrated before fixing at the specified locations. The aspect ratio,

of the test section was sufficiently large for

the flow to be hydrodynamically developed. The working fluid is circulated through the test section with an aid of pump; the suction side is connected to a storage tank. In order to measure the mass flow rate of working fluid a flow meter was used and it is connected between the pump and the test section. The storage tank is made of stainless steel of 30 liters capacity. The liquid which is heated in the test section is allowed to cool by passing it through a chiller. The liquid then flows to the storage tank by gravity. The provision of chiller helped in achieving steady state condition faster. The photographic representation of twisted tape (TT) and wire coil (WC) inserts were shown in Fig. 3. The twisted tape inserts were made in the laboratory from 1mm thick and 13 mm width of aluminum strip and the dimensions of twisted tape were shown in Table 2. A gap of 1 mm is provided between inner diameter of the tube and the width of the twisted tape for smooth insertion of inserts into a test section. The two ends of the aluminum strip were inserted into lathe; one end at the headstock and the other end at the tail stock, by rotating the head stock manually, the helix lengths of 70 mm and 140 mm were achieved. The twist ratios of twisted tape inserts such as TT-2 (

and TT-1 (

were obtained. The

twisted tapes are snug fit into the test tube and the tube fin effect is neglected. The convective heat transfer between twisted tape material and the adjacent fluid was neglected. The mass flow rate of nanofluid flowing through a tube with twisted tape inserts were estimated based on the inner diameter of the tube. The hydraulic diameter of tube with twisted tape inserts was considered as inner diameter of the tube, because the twisted tape has very negligible thickness i.e. 1 mm.

8

The wire coil inserts were made by winding uniformly aluminium wire of 2 mm diameter over copper core rod of 8 mm diameter with a length of 1750 mm. The aluminium wire is tightly wound on the copper core rod. The convective heat transfer between wire coil material and the adjacent fluid was neglected. The mass flow rate of nanofluid flowing in a tube with wire coil inserts were calculated based on the hydraulic diameter (Eq. (6)). The dimensions of the wire coil inserts WC-2 in Table 3. In this table,

is wire coil pitch,

and WC-1 is wire diameter,

were listed and

are inside and

hydraulic diameters of test tube, respectively. The hydraulic diameter is defined as given below [36]: (6) Where

is a wire coil diameter. The friction factor of nanofluid in a tube with twisted tape

and wire coil inserts were estimated based on the pressure drop across the test tube. The pressure drop was measured by placing the U-tube manometer between two ends of the test tube. For this purpose, two 4 mm holes were drilled at two ends of the test tube and U-tube manometer was connected with flexible tube. The manometer fluid was used as carbon tetrachloride and its equivalent height is recorded at different mass flow rates. Initial experiments were conducted with base fluid, after that different volume concentration of CuO nanofluid was considered one after the other. The flow rate of base fluid and nanofluid was measured with high precision flow meter supplied by Chambal Magnects, Ltd, India with an accuracy of ±0.1 liters/sec. The tube is cleaned with pure water between the experiments conducted with different concentrations of nanofluids. After reaching the steady state the inlet, outlet and wall temperatures are notes and the properties of the working is evaluated at bulk temperature of fluid. Similar procedure and data is collected for nanofluids flowing in a tube with twisted and wire coil inserts. The convective heat transfer coefficient was estimated based on the Newton’s law of cooling. 9

4. Data reduction 4.1. Experimental Nusselt number The amount of heat supplied to the test section and heat gained by the working fluid was estimated from Eq. (7) and Eq. (8) and found a maximum difference of ±2.5%. This indicates that negligible amount of heat loss takes place from test section to atmosphere. Experimental heat transfer coefficient and Nusselt number was estimated based on the given expressions: (Energy supplied)

(7)

(Energy absorbed)

(8) (9)

Where

,

, (10)

The correlations for estimation of Nusselt number for single phase fluid are given below: (i)

Gnielinski [45] correlation for turbulent flow (11)

, (ii)

,

Notter-Rouse [46] equation for turbulent flow (12)

4.2. Experimental friction factor The experimental friction was estimated by considering the pressure drop across the test section and the equation is given below: 10

(13)

The friction factor correlations for single phase fluid were given below: (i)

Blasius [47] equation for turbulent region (14)

(ii)

P etukov [48] equation for turbulent region (15)

The available correlations for Nusselt number and friction factor of nanofluids flowing in a tube with twisted tape and wire coil inserts were presented in Tables 4 & 5.

5. Results and discussion 5.1. Nusselt number 5.1.1. Nanofluid in a plain tube Heat transfer and friction factor experiments were initially conducted with water as working fluid. The experimental heat transfer coefficient and experimental Nusselt number was estimated from Eq. (9) and Eq. (10). Thermal conductivity of water and nanofluid was used for the estimation of experimental Nusselt number. The estimated Nusselt number from Eq. (10) was shown in Fig. 4 in comparison with the data obtained from Eq. (11) of Gnielinski [45] and Eq. (12) of Notter-Rouse [46]. The difference between experimental and theoretical Nusselt number for water was obtained a maximum of ±3%. Nanofluids of different concentrations were introduced into test section one by one for the estimation of heat transfer 11

coefficient. The Eq. (9) is used to analyze the experimental heat transfer coefficient of CuO nanofluid in a tube. Thermal conductivity of nanofluid estimated from Eq. (5); which is used to estimate the experimental Nusselt number of CuO nanofluid (Eq. (10)). The estimated Nusselt number for CuO nanofluid from Eq. (10) was represented in Fig. 5 along with base fluid data. Under similar operating parameters, the Nusselt number of nanofluid is higher than that of the base fluid (water), as the presence of nanoparticles directly results in an increase of thermal conductivity. Besides, the heat transfer improvement is also associated by the collision among nanoparticles and also that between the nanoparticles and tube wall, leading to an increase in the energy exchange rate. Nusselt number increases with increasing Reynolds number due the intensification of the nanofluid mixing fluctuation. In concentration range studied, Nusselt number slightly increases with the increase of nanoparticle concentration. In general, the increase of nanoparticle concentration in base fluid results the increases of thermal conductivity and collision of nanoparticles which are favourite factors for heat transfer enhancement and an increase of fluid viscosity which diminishes the fluid movement and thus heat transfer rate. The obtained result implies that for the present range, the effect of the increase in thermal conductivity and the collision of nanoparticles are more prominent than the increase of the fluid viscosity. According to the experimental results the Nusselt number of CuO nanofluid increases with increase of particle concentration. At 0.1% volume concentration, the enhancement in Nusselt number is 11.87% and 14.47% in the Reynolds number of 4000 and 20000. In the similar way, at 0.3% volume concentration, the enhancement in Nusselt number is 15.70% and 17.62% in the Reynolds number of 4000 and 20000 respectively. The enhancement is more in high Reynolds number compare to low Reynolds number, because of the effective mixing of fluid in fully developed flow condition.

12

5.1.2. Nanofluid in a plain tube with twisted tape inserts Further heat transfer and friction factor experiments were conducted with nanofluids flowing in a tube with different ratios of twisted tape inserts. The experimental Nusselt number for nanofluid in a tube with twisted tape inserts are estimated from Eq. (10) and the data was represented in Fig. 6. Compared to the same concentration of 0.3% nanofluid in a tube with TT-1, the Nusselt number enhancement is 7% and 8.7% in the Reynolds number of 4000 and 20000, respectively. In the similar fashion, compared to same concentration of 0.3% nanofluid in a tube with TT-2, the Nusselt number enhancement is 10.57% and 12.21% in the Reynolds number of 4000 and 20000, respectively. Compared to the plain tube, the tubes with twisted tapes exhibit higher Nusselt number, because the tape inserts generate swirl flow offering a longer flowing path of fluid flow through the tube and also better fluid mixing, resulting in a thinner thermal boundary layer along the tube wall and thus superior convective heat transfer. Compared to water in a tube and 0.3% nanofluid in a tube with TT-1, the Nusselt number enhancement is 23.85% and 27.89% in the Reynolds number of 4000 and 20000, respectively. In the same way, compared to water in a tube and 0.3% nanofluid in a tube with TT-2, the Nusselt number enhancement is 27.94% and 31.88% in the Reynolds number of 4000 and 20000, respectively. The same trend has been observed by Sundar et al. [50] by using Fe3O4/water nanofluid in a tube with twisted tape inserts and Wongcharee and Eiamsa-ard [29] by using CuO/water nanofluid in a tube with twisted tape inserts. The experimental Nusselt number of CuO/water nanofluid in a tube with twisted tape inserts was shown in Fig. 7 in comparison with the data of Sundar et al. [50] for Fe3O4/water nanofluid in a tube with twisted tape inserts.

5.1.3. Nanofluid in a plain tube with wire coil inserts

13

Heat transfer and friction factor experiments were conducted with nanofluid in a tube together with wire coil inserts. The Eq. (10) is used to estimate the experimental Nusselt number and the data was represented in Fig. 8. From the figure, it was observed that the contribution of wire coil insert to the enhancement of Nusselt number is larger than the enhancement produced by CuO/water nanofluids. The convective heat transfer enhancement of nanofluids may be because of several factors such as improved effective thermal conductivity of the nanofluid over the base fluid (water), Brownian motion of nanoparticles, and particle migration as reported in literature [28,29]. The reason for heat transfer enhancement is due to the wire coil inserts which increase the irregular and random movement of the particles and the energy exchange rates in the nanofluid. The higher turbulence intensity of the fluid close to the tube wall and the wire coil insert is responsible promote thorough mixing of nanofluid and an efficient redevelopment of the thermal or hydrodynamic boundary layer which consequently results in the improvement of convective heat transfer. Compared to the same concentration of 0.3% nanofluid in a tube with WC-1, the Nusselt number enhancement is 11.47% and 16.26% in the Reynolds number of 4000 and 20000, respectively. In the similar manner, compared to same concentration of 0.3% nanofluid in a tube with WC-2, the Nusselt number enhancement is 14.33% and 22.81% in the Reynolds number of 4000 and 20000, respectively. Compared to water in a tube and 0.3% nanofluid in a tube with WC-1, the Nusselt number enhancement is 28.99% and 37.07% in the Reynolds number of 4000 and 20000, respectively. Same manner, compared to water in a tube and 0.3% nanofluid in a tube with WC-2, the Nusselt number enhancement is 32.28% and 44.45% in the Reynolds number of 4000 and 20000, respectively. Compared to wire coil and twisted tape inserts under same flow and volume concentrations, the heat transfer augmentation is caused with wire coil inserts because of secondary flow. Therefore, the Nusselt number effect of nanofluids in wire coil inserted tubes is more noticeable.

14

5.2. Friction factor 5.2.1. Nanofluid in a plain tube Friction factor experiments for water were conducted initially and the values are estimated from Eq. (16). Fig. 9 representing the experimental friction factor of water is in comparison with the data obtained from Eq. (17) Blasius [47] and Eq. (18) of Petukov [48] and found to be a maximum of ±2.5% deviation. Experimental friction factor of different volume concentrations of CuO nanofluid was estimated from Eq. (16) and the data was indicted in Fig. 10. The friction factor of CuO nanofluid increases with increase of Reynolds number and particle concentration. The viscosity of CuO nanofluid is also one of the major parameters for friction factor enhancement. Based on the Eq. (2), the viscosity enhancement for 0.1% and 0.3% volume concentrations of nanofluid are 2.5% and 7.5% respectively over the water. The friction factor of the nanofluid depends on the flow rate and viscosity of nanofluids. The friction factor enhancement for 0.1% volume concentration of CuO nanofluid is about 1.10times and 1.11-times at a Reynolds number of 4000 and 20000, respectively compared to water under same flow conditions. Similarly, the friction factor enhancement for 0.3% volume concentration of CuO nanofluid is about 1.10-times and 1.15-times at a Reynolds number of 4000 and 20000, respectively compared to water under the same flow conditions.

5.2.2. Nanofluid in a plain tube with twisted tape inserts Experimental friction factor of different volume concentrations of CuO nanofluid in a tube with different twisted tape inserts were calculated based on Eq. (16) and the data was 15

presented in Fig. 11. It observed that friction factor increases with increase of Reynolds number, volume concentration and decreases with decrease of twist ratio. It is clear that the use of twisted tape inserts results in a very high friction factor than that of plain tube. The friction factor of 0.3% nanofluid flowing in a tube with TT-2 enhances 1.139-times at a Reynolds number of 4000 and 1.026-times at a Reynolds number of 20000 compared to same concentration fluid without twisted tape insert. Compared to water flowing in a tube, the enhancement in heat transfer coefficient is about 1.26-times and 1.179-times under the same Reynolds number.

5.2.3. Nanofluid in a plain tube with wire coil inserts Experimental friction factor of CuO nanofluid flowing in a tube together with wire coil inserts were calculated from Eq. (16) and the data was shown in Fig. 12. It indicates that, the friction factor increases with increase of Reynolds number, particle concentrations and decrease of wire coil pitch. It is also observed that, under same range of Reynolds numbers, the effect of decreasing the pitch of wire coil insert is more prominent in friction factor enhancement. Therefore, the highest friction factor is obtained for the wire coil with the decreased pitch (WC-2). The trend of change in friction factor is in coherence with that for the plain tube at low Reynolds number, but with rising Reynolds number, the friction factor of wire coil inserted tubes is more increased compared to that of plain tube. It is also found that the increase of friction factor for 0.3% volume concentration of CuO nanofluid at higher Reynolds number is more. In fact, with the increase of Reynolds number, the coiled wire induces a secondary flow, which in turn, promotes turbulence that leads in friction factor increase [30]. The friction factor of 0.3% nanofluid flowing in a tube with WC-2 is enhances 1.196-times at a Reynolds number of 4000 and 1.042-times at a Reynolds number of 20000 compared to same concentration fluid without twisted tape insert. Compared to water flowing

16

in a tube, the enhancement in heat transfer coefficient is about 1.324-times and 1.198-times under the same Reynolds number. Compared to heat transfer coefficient, the magnitude of nanofluid friction factor with wire coil inserts is negligible and this will not affect any penalty on the pumping of nanofluid into the test section. The percentage enhancement in Nusselt number and friction factor of different volume concentrations of CuO/water nanofluid in a tube with twisted and wire coil inserts were summarized in Table 6.

5.3. Correlation for Nusselt number and friction factor The experimental Nusselt number of water, nanofluid, nanofluid with twisted and nanofluid with wire coil inserts (135 data points) are fit into general equation with an average deviation of 5% and standard deviation of 6% and the equation is given below: (16) ,

, ,

In the similar way, the experimental friction factor of water, nanofluid, nanofluid with twisted and nanofluid with wire coil inserts (135 data points) are fit into general equation with an average deviation of 5.12% and standard deviation of 6.33% and the equation is given below: (17)

, The data obtained from Eq. (16) and Eq. (17) is shown in Fig. 13 and Fig. 14 along with the experimental data.

5.4. Thermal performance factor

17

The thermal performance of twisted tape and wire coil inserts in turbulent flow of CuO/water nanofluids is evaluated in terms of thermal performance factor for constant pumping power condition. The thermal performance factor (η) can be defined as the ratio of the heat transfer coefficient (or Nusselt number) ratio to the friction factor (or pressure drop) ratio at the same pumping power:

(18)

Where

is the index. The larger the values of the thermal performance factor, the more

suitable the enhancement heat transfer technique. The index

is experienced different

values in previous literatures. In the laminar flow condition, Usui et al. [51] and Suresh et al. [33] have considered

= 0.1666 and Hashemi and Akhavan-Behabadi [13] have taken

. In the turbulent flow condition, Wongcharee et al. [30] and Abbasian-Arani and Amani [52] have considered,

. In order to investigate the influence of both twisted

tape and wire coil inserts and nanofluid techniques on thermal performance of the equipments, the following equation was used in this study:

(19)

The variation of thermal performance factor with Reynolds number for 0.1% and 0.3% nanofluid through twisted tape and wire coil inserts is illustrated in Fig. 15 and Fig. 16. The thermal performance factor for all twisted and wire coil inserts are greater than unity. It means that using both of the heat transfer enhancement techniques studied in this investigation is a good choice in practical application. Also, WC-2 shows the best thermal performance among other twisted tape and wire coil tubes. For example, at the highest Reynolds number the thermal performance factor for nanofluids with TT-2 is 1.24-times and 18

for WC-2 is 1.36 respectively. So, under same particle loading and flow rates, the thermal performance factor for wire coil inserts are more compared to twisted tape inserts.

6. Conclusions The present work focuses on the estimation of heat transfer and friction factor CuO/water nanofluid flowing in a plain tube with twisted tape and wire coil inserts. With the use of nanoparticles in base fluid, the heat transfer coefficient is increases. A maximum of 17.62% enhancement is obtained at 0.3% nanofluid at a Reynolds number of 20000. The heat transfer coefficient is further enhances for nanofluid in a tube with twisted tape insert. The enhancement is also depends on the twisted tape twist ratio. Higher heat transfer rates are obtained with decrease in twist ratio. It is noticed that, with the use of TT-2, a maximum of 31.88% enhancement at 0.3% nanofluid in Reynolds number of 20000. The heat transfer coefficient is also further enhances for nanofluid in a tube with wire coil inserts. A maximum of 44.45% heat transfer enhancement for 0.3% nanofluid in a tube with WC-2 at a Reynolds number of 20000 is observed. Nanofluid (0.3%) in a tube with twisted tape and wire coil inserts causes higher friction factors in the order of 1.17-times and 1.19-times compared to water flowing in a tube at Re = 20000. Compared to heat transfer enhancement, the enhancement in friction factor is negligible. The thermal performance factor for nanofluids with wire coil inserts is more effective than twisted tape inserts under same particle loading and flow rate.

19

Nomenclature Area, Specific heat, Inner diameter of the tube, Friction factor Twist tape pitch, Heat transfer coefficient, Current, Thermal conductivity, Length of the tube, Mass flow rate, Nusselt number, Power, Prandtl number, Heat flow, Heat flux, Reynolds number, Temperature, o Thickness of the tape, Voltage, Velocity,

Greek symbols 20

Uncertainty Pressure drop Volume concentration of nanoparticles, % Dynamic viscosity, Density,

Subscripts Bulk temperature Experimental Inlet Outlet Regression Wall temperature

Appendix Uncertainties associated with various parameters such as Reynolds number, heat flux, heat transfer coefficient, Nusselt number and friction factor is estimated based on the procedure of Beckwith et al. [49]. Uncertainties of various instruments are shown in Table 7. (A1)

Reynolds number,

Heat

flux,

(A2)

21

Heat transfer coefficient,

(A3)

Nusselt number,

(A4)

Friction factor,

(A5)

Acknowledgment The authors would like to acknowledge the Portuguese Foundation of Science and Technology (FCT) and J N T University-Hyderabad for performing the research work and the author (L.S.S.) would like to thank FCT for his Post-Doctoral research grant (SFRH/BPD/79104/2011).

References [1]

A.S. Ahuja, Augmentation of heat transport in laminar flow of polystyrene suspension: I. Experimental and results. Journal of Applied Physics 46 (1975) 34083416.

[2]

K.V. Liu, S.U.S. Choi, K.E. Kasza, Measurements of pressure drop and heat transfer in turbulent pipe flows of particulate slurries. Report, Argonne National Laboratory ANL-88-15 (1988) 1–91.

22

[3]

S.U.S. Choi, Enhancing thermal conductivity of fluids with nanoparticles. In Proceedings of the 1995 ASME International Mechanical Engineering Congress and Exposition San Francisco, CA, USA, 1995.

[4]

S.U.S. Choi, Z.G. Zhang, F.E. Lockwood, E.A. Grulke, Anomalous thermal conductivity enhancement in nanotube suspensions, Applied Physics Letters 79 (2001) 2252-2254.

[5]

S. Lee, S.U.S. Choi, S. Li, J.A. Eastman, Measuring thermal conductivity of fluids containing oxide Nanoparticles, Journal of Heat Transfer 121 (1999) 280-289.

[6]

H. Masuda, A. Ebata, K. Teramae, N. Hishinuma, Alteration of thermal conductivity and viscosity of liquid by dispersing ultra-fine particles (dispersion of Al2O3, SiO2 and TiO2 ultra-fine particles), Netsu Bussei 4 (1993) 227-233.

[7]

B.C. Pak, Y.I. Cho, Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles, Experimental Heat Transfer. 11 (1998) 151-170.

[8]

L.S. Sundar, M.T. Naik, K.V. Sharma, M.K. Singh, T.Ch. Siva Reddy, Experimental investigation of forced convection heat transfer and friction factor in a tube with Fe3O4 magnetic nanofluid, Experimental Thermal and Fluid Science 37 (2012) 65-71

[9]

Y. Ding, H. Alias, D. Wen, R.A. Williams, Heat transfer of aqueous suspensions of carbon nanotubes (CNT nanofluids), International Journal of Heat and Mass Transfer. 49 (2006) 240-250.

[10] V. Chandraprabu, G. Sankaranarayanan, S. Iniyan, S. Suresh, Performance of CuO/water nanofluid as outer fluid in the tube in tube condensing unit of air conditioner: Experimental Study, Journal of Nanofluids. 2 (2013) 213-220. [11] L.G. Asirvatham, N. Vishal, S. K. Gangatharan, D.M. Lal, Experimental study on forced convective heat transfer with low volume fraction of CuO/water nanofluid, Energies. 2 (2009) 97-119. 23

[12] S. Suresh, M. Chandrasekar, S. Chandra Sekhar, Experimental studies on heat transfer and friction factor characteristics of CuO/water nanofluid under turbulent flow in a helically dimpled tube, Experimental Thermal and Fluid Science 35 (2011) 542-549. [13] S.M. Hashemi, M.A. Akhavan-Behabadi An empirical study on heat transfer and pressure drop characteristics of CuO–base oil nanofluid flow in a horizontal helically coiled tube under constant heat flux, International Communications in Heat and Mass Transfer 39 (2012) 144-151 [14] N. Kannadasan, K. Ramanathan, S. Suresh, Comparison of heat transfer and pressure drop in horizontal and vertical helically coiled heat exchanger with CuO/water based nanofluids Experimental Thermal and Fluid Science. 42 (2012) 64-70. [15] S. Suresh, M. Chandrasekar, P. Selva kumar, Experimental studies on heat transfer and friction factor characteristics of CuO/water nanofluid under laminar flow in a helically dimpled tube, Heat and Mass Transfer. 48 (2012) 683-694. [16] E. Smithberg, F. Landis, Friction and forced convective heat transfer characteristics in tube with twisted-tape swirl generators. Journal of Heat Transfer 86 (1964) 39-49. [17] R.F. Lopina, A.E. Bergles, Heat transfer and pressure drop in tape-generated swirl flow of single phase water. Journal of Heat Transfer 91 (1969) 434-442. [18] R.M. Manglik, A.E. Bergles, Heat transfer and pressure drop correlations for twistedtape inserts in isothermal tubes: part II–transition and turbulent flows, Journal of Heat Transfer 115 (1993) 890-896. [19] P.K. Sarma, T. Subramanyam, P.S. Kishore, V. Dharma Rao, Sadik Kaka, Laminar convective heat transfer with twisted tape inserts in a tube, International Journal of Thermal Science. 42 (2003) 821-828.

24

[20] P.K. Sarma, T. Subramanyam, P.S. Kishore, V. Dharma Rao, Sadik Kakac, A new method to redict convective heat transfer in a tube with twisted tape inserts for turbulent flow, International Journal of Thermal Science 41 (2002) 955-960. [21] S.B. Uttarwar, M. Raja Rao, Augmentation of laminar flow heat transfer in tubes by means of wire coil inserts, Journal of Heat Transfer. 107 (1985) 930-935. [22] P.G. Vicente, A. Garcia, A. Viedma, Experimental study of mixed convection and pressure drop in helically dimpled tubes for laminar and transition flow. International Journal of Heat and Mass Transfer 45 (2002) 5091-5105. [23] P.G. Vicente, A. Garcia, A. Viedma, Mixed convection heat transfer and isothermal pressure drop in corrugated tubes for laminar and transition flow. International Communications of Heat and Mass Transfer 31 (2004) 651-662. [24] L. Wang, B. Sunden, Performance comparison of some tube inserts. International Communications of Heat and Mass Transfer 29 (2002) 45-56. [25] A. Garcia, J.P. Solano, P.G. Vicente, A. Viedm, Enhancement of laminar and transitional flow heat transfer in tubes by means of wire coil inserts, International Journal of Heat and Mass Transfer 50 (2007) 3176-3189. [26] M.A. Akhavan-Behabadi, R. Kumar, M.R. Salimpour, R. Azimi, Pressure drop and heat transfer augmentation due to coiled wire inserts during laminar flow of oil inside a horizontal tube, International Journal of Thermal Sciences. 49 (2010) 373-379. [27] L.S. Sundar, K.V. Sharma, Turbulent heat transfer and friction factor of Al2O3 nanofluid in circular tube with twisted tape inserts, International Journal Heat and Mass Transfer. 53 (2010) 1409-1416. [28] L.S. Sundar, K.V. Sharma, Heat transfer enhancements of low volume concentration Al2O3 nanofluid and with longitudinal strip inserts in a circular tube, International Journal of Heat and Mass Transfer. 53 (2010) 4280-4286.

25

[29] K. Wongcharee, S. Eiamsa-ard, Heat transfer enhancement by using CuO/water nanofluid

in

corrugated

tube

equipped

with

twisted

tape,

International

Communications in Heat and Mass Transfer. 39 (2012) 251-257. [30] K. Wongcharee, S. Eiamsa-ard, Enhancement of heat transfer using CuO/water nanofluid and twisted tape with alternate axis, International Communications in Heat and Mass Transfer. 38 (2011) 742-748. [31] S. Eiamsa-ard, K. Wongcharee, Single-phase heat transfer of CuO/water nanofluids in micro-fin tube equipped with dual twisted-tapes, International Communications in Heat and Mass Transfer 39 (2012) 1453-1459 [32] S. Suresh, K.P. Venkitaraj, P. Selvakumar, M. Chandrasekar, 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, Experimental Thermal and Fluid Science. 39 (2012) 37-44. [33] S. Suresh, K.P. Venkitaraj, P. Selvakumar, Comparative study on thermal performance of helical screw tape inserts in laminar flow using Al2O3/water and CuO/water nanofluids, Superlattices and Microstructures. 49 (2011) 608-622. [34] M. Chandrasekar, S. Suresh, A. Chandra Bose, Experimental studies on heat transfer and friction factor characteristics of Al2O3/water nanofluid in a circular pipe under laminar flow with wire coil inserts, Experimental Thermal and Fluid Science. 34 (2010) 122-130. [35] M.T. Naik, L.S. Sundar, Heat transfer and friction factor with water/propylene glycol-based CuO nanofluid in circular tube with helical inserts under transition flow regime, Heat Transfer Engineering. 35 (2014) 53-62. [36] M. Saeedinia, M.A. Akhavan-Behabadi, M. Nasr, Experimental study on heat transfer and pressure drop of nanofluid flow in a horizontal coiled wire inserted tube

26

under constant heat flux, Experimental Thermal and Fluid Science. 36 (2012) 158168. [37] M. Kahani, S. Zeinali Heris, S.M. Mousavi Comparative study between metal oxide nanopowders on thermal characteristics of nanofluid flow through helical coils, Powder Technology 246 (2013) 82-92 [38] L.S. Sundar, M.K. Singh, Convective heat transfer and friction factor correlations of nanofluid in a tube and with inserts: A review, Renewable and Sustainable Energy Reviews. 20 (2013) 23-35. [39] C.O. Popiel, J. Wojtkowiak, Simple formulas for thermophysical properties of liquid water for heat transfer calculation (from 0°C to 150°C), Heat Transfer Engineering 19 (1998) 87. [40] M.R.B. Romdhane, T. Chartier, S. Baklouti, A new processing aid for dry-pressing: a copolymer actin gas dispersant and binder, Journal of the European Ceramic Society 27 (2007) 2687. [41] D.A. Drew, S.L. Passman, Theory of Multi Component Fluids, first ed. Springer, Germany, 1999. [42] A. Einstein, Investigation on Theory of Brownian Motion, first ed. Dover publications, USA, 1956. [43] H.C. Brinkman, The viscosity of concentrated suspensions and solutions, Chemical Physics 20 (1952) 571. [44] J.C. Maxwell, A treatise on electricity and magnetism, 2nd Edition, Oxford University Press, Cambridge, UK, 1904. [45] V. Gnielinski, New equations for heat and mass transfer in turbulent pipe and channel flow, International Chemical Engineering. 16 (1976) 359-368.

27

[46] R.H. Notter, M.W. Rouse, A solution to the Graetz problem – III. Fully developed region heat transfer rates, Chemical Engineering Science 27 (1972) 2073–2093. [47] H.Blasius, Grenzschichten in Flussigkeiten mit kleiner Reibung (German), Z. Math. Phys., 56 (1908) 1-37. [48] B.S. Petukhov, Heat transfer and friction in turbulent pipe flow with variable physical properties, J. P. Hartnett and T. F.Irvine, (eds), Advances in Heat Transfer, Academic Press, New York, (1970) 504-564. [49] T.G. Beckwith, R.D. Marangoni, L.H. Lienhard, L.H., Mechanical measurements, 5th Edition, Addison–Wesley publishing company, New York, (1990) 45-112. [50] L.S. Sundar, N.T.R. Kumar, M.T. Naik, K.V. Sharma, 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, International Journal of Heat and Mass Transfer 55 (2012) 2761-2768. [51] H. Usui, Y. Sano, K. Iwashita, A. Isozaki, Enhancement of heat transfer by a combination of internally grooved rough tube and twisted tape, International Chemical Engineering 26 (1996) 97. [52] A.A. Abbasian Arani, J. Amani, Experimental study on the effect of TiO2–water nanofluid on heat transfer and pressure drop, Experimental Thermal and Fluid Science 42 (2012) 107.

Figure captions Fig. 1 (a) TEM image of CuO nanoparticles (b) sample preparation (c) bulk CuO nanofluid preparation. Fig. 2 Schematic representation of experimental setup. Fig. 3 Images of inserts (a) twisted tape (b) wire coil.

28

Fig. 4 Experimental Nusselt number of water is compared with the data of Gnielinski [45] and Notter-Rouse [46]. Fig. 5 Experimental Nusselt number of nanofluid with effect of Reynolds number and particle concentration. Fig. 6 Experimental Nusselt number of nanofluid in a tube together with twisted tape inserts. Fig. 7 Experimental Nusselt number of nanofluid in a tube with TT-1

and TT-2

is in comparison with Sundar et al. [27] for Fe3O4 nanofluid in a tube with twisted tape insert. Fig. 8 Experimental Nusselt number of nanofluid in a tube together with wire coil inserts. Fig. 9 Experimental friction factor of water compared with Blasius [47] and Petukov [48]. Fig. 10 Experimental friction factor of CuO nanofluid with effect of Reynolds number and particle concentration. Fig. 11 Experimental friction factor of nanofluid in a tube together with twisted tape inserts. Fig. 12 Experimental friction factor of nanofluid in a tube together with wire coil inserts. Fig. 13 Experimental Nusselt number is in compared with values from Eq. (16). Fig. 14 Experimental friction factor is in compared with values from Eq. (17). Fig. 15 Variation of thermal performance factor with Reynolds number for 0.1% and 0.3% concentration of nanofluid flows inside twisted tape tubes. Fig. 16 Variation of thermal performance factor with Reynolds number for 0.1% and 0.3% concentration of nanofluid flows inside wire coil tubes.

Table captions Table 1 Thermophysical properties of CuO nanoparticles and base fluid. Table 2 Dimensions of twisted tape inserts. Table 3 Dimensions of wire coil inserts. 29

Table 4 Various proposed correlations for the estimation of Nusselt number for nanofluid. flowing in a tube with twisted and wire coil inserts. Table 5 Various proposed correlations for the estimation of friction factor for nanofluid. flowing in a tube with twisted and wire coil inserts. Table 6 Percentage of Nusselt number and friction factor enhancements. Table 7 Uncertainties of instruments and properties.

Table 1 Thermophysical properties of CuO nanoparticles and base fluid. Particle/Base fluid

(nm) CuO

Surface area to

Diameter Purity (%)

<50 nm

99

(kg/m3)

mass, (m2/g)

(J/kg K)

(W/m K)

6310

29

525

17.65

Distilled water*

*All temperatures are in degrees Celsius [39].

Table 2 Dimensions of twisted tape inserts Tube set

(mm)

TT-0 (Plain tube)

14

TT-1

14

(mm) Plain tube (Without inserts) 140

10 30

TT-2

14

70

5

Table 3 Dimensions of wire coil inserts Tube set

(mm)

(mm)

(mm)

(mm) Plain tube (Without insert)

WC-0 (Plain tube)

14

--

--

WC-1

14

41.4

12.158

2

2.95

0.1428

WC-2

14

27.6

11.389

2

1.97

0.1428

31

Table 4 Various proposed correlations for the estimation of Nusselt number for nanofluid flowing in a tube with twisted and wire coil inserts Nanofluid/( Insert) Al2O3-water (Twisted tape) CuO-water (Twisted tape)

Expression

Range

Ref.

10000 < Re < 22000 0-0.5%, 4.50 < Pr < 5.5 0 < H/D < 83 830 < Re < 1990 0.3% - 0.7%, H/D = 3

Sundar and Sharma [27] Wongchar ee and Eiamsa-ard [29] Naik et al. [38]

CuO70:30% W/PG (Twisted tape) Fe3O4-water (Twisted tape) TiO2-water Al2O3-water (Wire coil)

1000 < Re < 10000 0-0.5%, 4.50 < Pr < 5.5 0 < H/D < 83 3000 < Re < 22000 0-0.6%, 3.19 < Pr < 6.5 0 < H/D < 15 500 < Re < 4500 0.25% and 0.1% 5.89 < Pr < 8.95 115.3 < He<1311.4

Sundar et al. [50]

CuO-Base oil (Wire coil)

20 < Re < 120 0-0.3% p/d = 1.79, 2.14 and 2.50 Re < 2300 =0.1%, 2 < p/d < 3

Saeedinia et al. [36]

Al2O3-water (Wire coil)

Kahani et al. [37]

Chandrase kar et al. [34]

32

Table 5 Various proposed correlations for the estimation of friction factor for nanofluid flowing in a tube with twisted and wire coil inserts Nanofluid/

Expression

Range

Ref.

(Insert)

CuO-water (Twisted tape)

10000 < Re < 22000 0-0.5%, 4.50 < Pr < 5.5 0 < H/D < 83 830 < Re < 1990 0.3% - 0.7%, H/D = 3

CuO-70:30% W/PG (Twisted tape)

1000 < Re < 10000 0-0.5%, 4.50 < Pr < 5.5 0 < H/D < 83

Fe3O4-water (Twisted tape)

3000 < Re < 22000 0-0.6%, 3.19 < Pr < 6.5 0 < H/D < 15 500 < Re < 4500 0.25% and 0.1% 5.89 < Pr < 8.95 115.3 < He<1311.4 20 < Re < 120 0-0.3% p/d = 1.79, 2.14 and 2.50

Al2O3-water (Twisted tape)

TiO2-water Al2O3-water (Wire coil) CuO-Base oil (Wire coil) Al2O3-water (Wire coil)

Re < 2300 =0.1%, 2 < p/d < 3

Sundar and Sharma [27]

Wongcharee and Eiamsaard [29] Naik et al. [38]

Sundar et al. [50] Kahani et al. [37]

Saeedinia et al. [36] Chandrasekar et al. [34]

33

Table 6 Percentage of Nusselt number and friction factor enhancements Nanofluid

0.3%

Insert

Nusselt number, Nu

Friction factor, f

Re = 4000

Re = 20000

Re = 4000

Re = 20000

15.70%

17.62%

1.10-times

1.15-times

TT-1

23.85%

27.89%

1.13-times

1.08-times

TT-2

27.94%

31.88%

1.12-times

1.17-times

WC-1

28.99%

37.07%

1.20-times

1.11-times

WC-2

32.28%

44.45%

1.32-times

1.19-times

TT-0 (Plain tube)

Table 7 Uncertainties of instruments and properties Least division in measuring instrument

Min. and Max. values measured in experiment

Uncertainty,

Instrument name

Instrument range

Thermocouple, oC

0-120oC

Wall

temperature,

0.1oC

45.66-72.96

0.13706

Thermocouple, oC

0-120oC

Bulk

temperature,

0.1oC

31.25-42.9

0.23310

Voltage, V

0-220 V

Voltage,

0.1 V

0-220

0.04545

Current, I

0-20 I

Current ,

0.01 I

0-20

0.05

Resistance, R

0-53.3 R

Resistance,

0.1 R

0-53.3

0.1876

U-tube manometer, cm

0-50 cm

Height of the CCl4

1 mm

2.0-38.3 cm

0.003

Totalizer, liters Properties

Measured variable

0-9999 Mass flow rate, 1 liters 1-15 liters liters. kg/sec Thermal conductivity, density, specific heat, viscosity

0.00001 0.1

34

Fig. 1 (a) TEM image of CuO nanoparticles (b) sample preparation (c) bulk CuO nanofluid preparation.

35

Fig. 2 Schematic representation of experimental setup.

36

Fig. 3 Images of inserts (a) twisted tape (b) wire coil.

37

Fig. 4 Experimental Nusselt number of water is compared with the data of Gnielinski [45] and Notter-Rouse [46].

38

Fig. 5 Experimental Nusselt number of nanofluid with effect of Reynolds number and particle concentration.

39

Fig. 6 Experimental Nusselt number of nanofluid in a tube together with twisted tape inserts.

40

Fig. 7 Experimental Nusselt number of nanofluid in a tube with TT-1

and TT-2

is in comparison with Sundar et al. [27] for Fe3O4 nanofluid in a tube with twisted tape insert.

41

Fig. 8 Experimental Nusselt number of nanofluid in a tube together with wire coil inserts.

42

Fig. 9 Experimental friction factor of water compared with Blasius [47] and Petukov [48].

43

Fig. 10 Experimental friction factor of CuO nanofluid with effect of Reynolds number and particle concentration.

44

Fig. 11 Experimental friction factor of nanofluid in a tube together with twisted tape inserts.

45

Fig. 12 Experimental friction factor of nanofluid in a tube together with wire coil inserts.

46

Fig. 13 Experimental Nusselt number is in compared with values from Eq. (16).

47

Fig. 14 Experimental friction factor is in compared with values from Eq. (17).

48

Fig. 15 Variation of thermal performance factor with Reynolds number for 0.1% and 0.3% concentration of nanofluid flows inside twisted tape tubes.

49

Fig. 16 Variation of thermal performance factor with Reynolds number for 0.1% and 0.3% concentration of nanofluid flows inside wire coil tubes.

50

Highlights  Heat transfer and friction factor characteristics of nanofluid flowing in a tube with inserts are studied experimentally.  Increasing nanofluid volume concentration enhances the heat transfer and friction factor.  Further heat transfer and friction factor enhancements for nanofluid in a tube with twisted tape and wire coil inserts.  Maximum increase in Nusselt number of 45% for 0.3% nanofluid in a tube with wire coil insert-2 under turbulent flow.  Two empirical correlations are developed to predict Nusselt number and friction factor.

51