Engineering Science and Technology, an International Journal xxx (2015) 1—15

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Heat transfer enhancement of TiO2/water nanofluid in a heat exchanger tube equipped with overlapped dual twisted-tapes

S. Eiamsa-ard a' *, K. Kiatkittipong b, W. Jedsadaratanachai c

a Department of Mechanical Engineering, Faculty of Engineering, Mahanakorn University of Technology, Bangkok 10530, Thailand b Department of Chemical Engineering, Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok 10520, Thailand c Department of Mechanical Engineering, Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok 10520, Thailand

HKSI-I

ARTICLE INFO

ABSTRACT

Article history: Received 29 October 2014 Received in revised form 14 January 2015 Accepted 14 January 2015 Available online xxx

Keywords:

Heat transfer enhancement Heat exchanger tube Overlapped dual twisted tapes TiO2/water nanofluid

Titanium dioxide (TiO2) in water as nanofluid was employed for heat transfer enhancement together with overlapped dual twisted tapes (O-DTs). The study encompassed Reynolds numbers from 5400 to 15,200, O-DTs with overlapped twist ratios (yo/y) of 1.5, 2.0 and 2.5 and nanofluids with TiO2 volume concentrations (f) of 0.07%, 0.14% and 0.21%. The experimental and numerical results indicated that O-DTs with smaller overlapped twisted ratio delivered a stronger swirl intensity and higher turbulent kinetic energy (TKE). The use of O-DTs at the smallest overlapped twist ratio of 1.5 enhanced heat transfer rates up to 89%, friction factor by 5.43 times and thermal performance up to 1.13 times as compared to those of plain tube. In addition, heat transfer increased as TiO2 volume concentration of nanofluid increased, owing to the increases of contact surface and thermal conductivity. The simultaneous use of the O-DTs having twist ratios 1.5 with the nanofluid with TiO2 volume concentration of 0.21% resulted in heat transfer enhancement around 9.9-11.2% and thermal performance improvement up to 4.5% as compared to the use of O-DTs alone. The empirical correlations of heat transfer rate (Nu), friction factor (f and thermal performance (h) in a constant wall heat flux tube equipped O-DTs at different overlapped twist ratios (yo/y) and volume concentrations of TiO2 nanoparticles (f) are also reported for heat transfer applications.

Copyright © 2015, Karabuk University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

Heat transfer processes are widely used in numerous areas including heat exchanger, cooling processes, heating and chemical processes. The poor heat transfer properties of common fluids (such as water, mineral oil and ethylene glycol) compared to most solids is a primary obstacle to effectiveness of heat processes. However, clogging in the process may be found in the tube when the fluid with large particles is employed. With awareness of this problem, nanometer particle in the fluid (as called "nanofluid") is an attractive solution which provides not only the enhanced thermal conductivity but also long term stability and low pressure drop [1]. Titanium dioxide (TiO2) is one of promising materials for heat transfer enhancement purpose due to its excellent chemical and

Abbreviations: O-DTs, overlapped dual twisted tapes.

* Corresponding author. E-mail address: smith@mut.ac.th (S. Eiamsa-ard). Peer review under responsibility of Karabuk University.

physical stability. In addition, TiO2 particles are cheap and commercially available. TiO2 nanoparticles suspended in conventional fluids were extensively utilized in various forms of heat exchangers, including circular tube [2,3], a double tube [4-6] and a shell and tube [7].

Another alternative method to enhance heat transfer is to insert twisted tape into a core tube. This approach induces secondary recirculation to the axial flow, leading to an increase in tangential and radial turbulent fluctuation and thus reducing a thickness of the boundary layer. Using nanofluid together with twisted taped for heat transfer enhancement was reported in numerous research works such as twisted tape inserts with Al2O3/water nanofluid [8,9], helical twist tape inserts with Al2O3/water nanofluid [1], twisted tape with alternate axis inserts with CuO/water nanofluid [10], twisted tape inserts with CuO/water nanofluid in corrugated tube [11], dual twisted tape inserts with CuO/water nanofluid in micro-fin tube [12], helical screw tape inserts with Al2O3/water nanofluids [13], helical screw tape inserts using CuO/water nanofluids [14], and propeller inserts with TiO2/water nanofluid [15].

http://dx.doi.org/10.1016/jjestch.2015.01.008

2215-0986/Copyright © 2015, Karabuk University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Nomenclature W tape width, m

y pitch length of twisted tape, m

A heat transfer surface area, m2 yo overlapped pitch length of tape, m

cp specific heat of fluid, J kg-1 K-1 yo/y overlapped twist ratio

D inside diameter of test tube, m

f friction factor = DP/((L/D)(pU2/2)) Greek symbols

h heat transfer coefficient, W m-2 K-1 p fluid density, kg m~3

k thermal conductivity of fluid, W m-1 K-1 5 tape thickness, mm

L length of test section, m m fluid dynamic viscosity, kg s_1 m_1

m mass flow rate, kg s-1 h thermal performance factor

Nu Nusselt number = hD/k f concentration of nanofluid, % by volume

P pressure of flow in test tube, Pa

DP pressure drop, Pa Subscripts

Pr Prandtl number = mCP/k b bulk

Q heat transfer rate, W conv convection

Re Reynolds number = pUD/m i inlet

t thickness of the test tube, m f nanofluid

T temperature, °C np nanoparticle

T mean temperature, °C o outlet

U average velocity, m s-1 p plain tube or particle

V voltage, V t twisted tape

V volume flow rate, m3 s-1 w wall or water

Heat transfer enhancement by inserting twisted tape is always escorted by the increase of flow resistance. The proper modification of twisted tape is a key approach to achieve a reasonable tradeoff between the increases of heat transfer rate and flow resistance. The twisted tape modification was made in order to improve the convective heat transfer and reduce flow resistance in heat exchangers. The modified twisted tape inserts include regularly spaced twisted-tape elements [16—18], regularly spaced short-length twisted tape [19], spacer at the trailing edge of twisted tapes [20], and loose-fit twisted tapes [21] were proposed. These modified twisted tapes enhanced heat transfer significantly in laminar flow regime. In the case of turbulent flow, the modification of geometrical twisted tapes are widely used due to the following advantages: (i) strengthening temperature uniformity in the core flow; (ii) increasing fluid disturbance in the core flow; (iii) reducing surface area of heat transfer component in the core flow; (iv) decreasing fluid disturbance in the boundary flow [22]. To increase fluid disturbance around the tape edges, the tapes were modified in different forms including delta-winglet twisted tape [23], squarecut twisted tape [24], V-cut twisted tape [25], trapezoidal-cut twisted tape [26], centre-cleared twisted tape [27,28], serrated twisted tape [29], edgefold-twisted-tape [30], peripherally-cut twisted tape with an alternate axis [31], perforated twisted-tapes [32], and notch twisted tape [33]. To increase the core flow disturbance, the tapes were modified in different forms such as non-uniform twisted tape [34], alternate clockwise and counterclockwise twisted-tape [35,36], and twisted tapes consisting of centre wings and alternate-axes [37].

The modified twisted tapes were also presented in the form of multiple twisted tapes which used as the multiple swirl generators for enhancing the heat transfer rate in a circular tube. Chang et al. [38] reported that the use of twin twisted tapes resulted in heat transfer improvement 98—180% over that of the plain tube. Eiamsa-ard et al. [39] carried out the experiment to study the thermohy-draulic characteristics in a round tube heat exchanger equipped with twin counter/co twisted tapes (counter/co-swirl tape) at four different twist ratios (y/W = 2.5, 3.0, 3.5 and 4.0) in turbulent regions under a constant wall heat flux conditions. The experimental results demonstrated that the heat transfer rate of the tube with the

counter twisted tapes was higher than those of the tube with co twisted tapes up around 12.5—44.5%. The use of counter twisted tapes and co twisted tapes with smallest y/W of 2.5 resulted in the highest thermal enhancement factors of 1.39 and 1.1, respectively. In addition, Eiamsa-ard et al. [40] reported the effect of the regularly-spaced dual twisted tape inserts at three different space ratios (s/D = 0.75, 1.5 and 2.25) on the pressure drop and heat transfer behaviours. It was found the heat transfer rate and pressure drop were decreased with increasing space ratio (s/D). Heat transfer rates of the tubes with regularly-spaced dual twisted tapes (s/D) of 0.75, 1.5 and 2.25 were respectively up to 140%, 137% and 133% over that of the plain tube. Zhang et al. [22] studied the heat transfer enhancement by triple and quadruple twisted tapes which induced multi-longitudinal vortices in a tube. Their results showed the tubes triple twisted tapes with the clearance ratio of a/D = 0.25, 0.3 and 0.35 possessed heat transfer rates higher than that of the plain tube around 162%, 164% and 171%, respectively, and higher than those of the tubes with quadruple twisted tapes at the same a/ D around 180%, 182% and 189%, respectively. The friction factors of the tube fitted with triple twisted tapes with a/D = 0.25, 0.3 and 0.35, were, respectively, around 5.33—6.27, 5.84—6.76 and 5.99—7.02 times of that of the plain tube and around 4.06—4.74, 4.36—5.06 and 4.45—5.19 times of those of the tubes fitted with quadruple twisted tapes at the same a/D. Hong et al. [41] carried out the numerical simulation to study turbulent heat transfer and flow characteristics in converging-diverging tubes (CDs) and converging-diverging tubes with twin counter-swirling twisted tapes (CDTs) inserts. They found that the increases in the heat transfer rate and friction factor for CDTs were higher than those of the CDs around 6.3—35.7% and 1.75—5.3 times, respectively.

Although, heat transfer enhancement by nanofluid and twisted tape has been extensively studied and reported as literature review shown above, the explanation for the effect of twisted tape architecture together with nanofluid and also nanofluid concentration on the thermal performance is still limited and scarcely reported. This is the driving force for this research work. The finite volume method is used as a tool to study swirling flow and heat transfer characteristics by overlapped twisted tapes (with different overlapped twist ratio and twist lengths) together with TiO2—water

nanofluids (with different concentrations). Finally, the experimental data are used to develop the empirical correlations for Nusselt number, friction factor and thermal performance factor for a wide range of heat transfer applications.

S3400 15.0kV x30.0k SE

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2. Experimental setup

2.1. Overlapped dual twisted tapes

Fig. 1 demonstrated the geometries of the overlapped dual twisted tapes (O-DTs). The tapes were made of a thin aluminium sheets with thickness of 0.8 mm and tape width of 8 mm (W). Each twisted tapes was fabricated by twisting a straight tape, about its longitudinal axis, while being held under tension. Twisted tapes were twisted at 4 different twist lengths (180°/twist length): 36, 48 and 60 mm. The overlapped dual twisted tapes (O-DTs) were formed by coupling each of three twisted tapes at twist length of 24 mm (y) with each of other three tapes at different twist lengths (y0 = 36, 48 and 60 mm). Therefore, three pairs of the overlapped dual twisted tapes (O-DTs) with yo/y = 1.5, 2.0 and 2.5 were obtained.

2.2. Preparing of TiO2/water nanofluid

The TiO2 nanoparticles with an average diameter of 15 nm were purchased from Nanostructured and Amorphous Material, Inc, USA. Fig. 2 shows the scanning electron microscopy (SEM) photograph of

Fig. 2. SEM image of TiO2 nanoparticles.

the TiO2 particles. Prior to testing, TiO2 nanoparticles were dispersed in de-ionized water (the base fluid) at three different concentrations of f = 0.07%, 0.14% and 0.21% by volume. Then, the mixture was sonicated continuously for 180 min in an ultrasonic bath under ultrasonic pulses of 100 W at 36 ± 3 kHz for uniform dispersion of particles. Moreover, the sonication treatment significantly improved the stability of suspension. It took longer than

Fig. 1. O-DTs at various overlapped twist ratios: (a) yo/y = 2.5, (b) yo/y = 2.0, (c) yo/y = 1.5 and (d) front view (unit: mm).

180 min after preparation for TiO2 nanoparticles to start precipitated. Thus, TiO2 nanoparticles were still well dispersed in water before feeding into the tube.

2.3. Apparatus and procedure

The experimental facility of the present study is shown in Fig. 3. The facility mainly consisted of: (1) a heat exchanger tube, (2) cooling water tank and overhead fluid tank, (3) a set of thermocouple, (4) data logger, (5) manometer, (6) a centrifugal pump, (7) rotameter, (8) multi-meter and (9) variac transformer. The tube was made from copper having a diameter (D) of 19 mm, a length (L) of 1000 mm and thickness (t) of 1.5 mm. The length of a calm section was set at 1200 mm. In the heat transfer experiment, the tube was heated by an electrical heater wire attached around circular tube to

provide a constant wall heat flux boundary condition. The electrical output power was controlled by a variac transformer. The outer surface of the tested tube was well insulated to minimize convec-tive heat loss to surroundings. Prior to testing, the inlet and outlet temperatures of the bulk fluid were measured and recoded at certain points using a data logger in conjunction with RTDs calibrated within ±0.2 °C deviation by thermostat. Fifteen local temperatures on the upper, lower and side walls of the test tube were measured using type T thermocouples.

During the experiments, the inlet bulk fluids at 26 °C were transferred by a 1.0 hp centrifugal pump through the fluid setting tank, rotameter and then the heat transfer test tube. The bulk fluids were heated by an adjustable electrical heater wrapping along the test tube. The temperature, volumetric flow rate and pressure drop data of the bulk fluids were recorded at steady state condition. The

Fig. 3. Layout of the experimental facility.

Reynolds number of the inlet fluid was varied from 5400 to 15,200. The properties of fluids for the flow and heat transfer evaluations were based on the average temperature of tube wall and inlet—outlet fluid temperatures. It should be remarked that the pressure drop of the heat transfer test tube was measured with manometer under an isothermal condition, without turning on heating unit. In the experiments, the overlapped dual twisted tapes (O-DTs) were inserted into the test section at different overlapped twist ratios of yo/y = 1.5, 2.0 and 2.5, and at different volume concentrations of TiO2 nanoparticles of f = 0.07%, 0.14% and 0.21% by volume (Table 1).

3. Mathematical model and numerical method

The available finite difference procedures were employed to solve the governing partial differential equations for swirling flows and boundary layer. Some simplifying assumptions were applied for conventional flow momentum and energy equations to model the heat transfer process in a constant heat flux tube with O-DTs. The major assumptions are; (1) the flow is steady and incompressible, (2) the flow through the O-DTs is turbulent, (3) natural convection and thermal radiation are neglected and (4) the thermo-physical properties of the fluid are temperature independence. Based on above approximations, the governing differential equations used to describe the fluid flow and heat transfer in a circular tube equipped with O-DTs were established. The continuity, momentum and energy equations for the three dimensional models were employed. For steady flow, the time-averaged incompressible Navier—Stokes equations in the Cartesian tensor notation can be written in the following form:

Continuity equation:

vx (rui) = 0

Momentum equation:

9 {pUjUj) = _9p+ _9_ dxj dxi dxj

dui dUj dxj dxi 3 d j dx

+dj - pu0uj

Energy equation:

U (pE + p)]

[keff dj '

. p u2 h-£ + — p 2

Table 1

Details of test tube and experimental conditions.

(a) Inner diameter of tube, D 19 mm

(b) Outer diameter of tube, Do 22 mm

(c) Wall thickness of tube, 1.5 mm

(d) Length of tube, L 1000 mm

(e) Material Copper

(f) Wall condition Constant heat flux

(g) Inlet temperature, Ti 26 °C

(h) Reynolds number, Re 5400-15,200

(i) Type of base fluid Water

(j) Type of nanoparticle TiO2

(k) Average diameter of nanoparticle, dp 15 nm

(l) Nanofluid concentrations 0.07, 0.14 and 0.21%

by volume

(m) Thermal conductivity coefficient of 13.7 W/m K

nanoparticles, kp

(n) Nanoparticle density, Pp 4170 kg/m3

(o) Density of nanofluid, Pnf See Eq. (4) kg/m3

(p) Specific heat of nanofluid, cnf See Eq. (5) J/kg K

(q) Thermal conductivity coefficient of nanofluid, knf See Eq. (6) W/m K

(r) Viscosity of nanofluid, mnf See Eq. (7) Pa s

In the present numerical solution, the time-independent incompressible Navier—Stokes equations and the turbulence model were discretized using the finite volume technique. QUICK (Quadratic upstream interpolation for convective kinetics differencing scheme) and central differencing numerical schemes were applied for convective and diffusive terms, respectively. The pressure—velocity coupling algorithm SIMPLE (Semi Implicit Method for Pressure-Linked Equations) was selected for evaluating the pressure field. Impermeable boundary condition was implemented over the tube wall. The turbulence intensity was kept constant at 10% at the inlet. The computation was performed until the difference between normalized residual of the algebraic equation and the prescribed value fell below a convergence criterion (10-6).

The computational domain for the flow in tube fitted with O-DTs was resolved by regular Cartesian elements. The pattern was applicable for only 180° twist length due to the periodic flow. The numerical analysis was made for O-DTs at three overlapped twist ratios (yo/y = 1.5, 2.0 and 2.5). Grid independent solution was obtained by comparing the solution for different grid levels. The total numbers of elements used are approximately 769,472, 519,852 and 1,267,687 for yo/y = 1.5, 2.0 and 2.5, respectively. The higher numbers of elements employed for the tape with yo/y = 2.5, are due to the larger twist length in comparison with the other tapes. The computation was performed at Reynolds number (at the inlet) of 10,000. The inlet temperature was kept constant at 300 K and the tube wall condition was maintained under constant wall heat flux of 1000 W/m2.

4. Data reduction

4.1. Thermophysical properties of nanofluids

The thermophysical properties (density, specific heat, viscosity and thermal conductivity) of the nanofluid were calculated as a function of nanoparticle volume concentration (f) together with properties of base fluid and nanoparticles. The density of nanofluid was evaluated using the general formula for the mixture:

Pnf = C1 - f)Pwater + fPnp (4)

The specific heat of the nanofluid was evaluated from:

cp,nf -

fpnpcp,np + (1 — f) pwatercp,water Pnf

These equations were recommended for nanofluids through experimental validation by Pak and Cho [2] and Xuan and Roetzel

[42]. The thermal conductivity was calculated from Maxwell model

[43] as shown in Eq. (6) which was recommended for homogeneous and low volume concentration liquid—solid suspensions with randomly dispersed, uniformly sized and non-interacting average spherical particles [44].

knf _ knp + 2kwater + 2f {knp — kwater)

knp + 2kwater — f ( knp — kwater)

Viscosity of nanofluids was calculated via the general Einstein's formula [45].

mnf = mwater i1 + hf) (7)

Where h = 2.5, as recommended for hard spheres [44].

4.2. Calculation of the heat transfer

In the present work, the heat transfer rate of working fluids was calculated by using the difference between inlet and outlet working fluid temperatures as

Qfluid = Mcp(To - Ti)

The thermal equilibrium test showed that the heat supplied by electrical winding in the test section is 5—8% larger than the heat absorbed by the working fluid.

qiv - Qfluid Qiv

x100%<5—8%

At the steady-state rate, the heat transfer taken by the fluid is equal to the convection heat transfer from the test section which can be expressed as

Qfluid — Qconv where

Qconv — hA(Tw - Tb

where Qcconv is the convection heat transfer from the test section, A is the heating internal surface area, Tb is average fluid bulk temperature in the tube and ~w is average wall temperature lined between the inlet and the exit of the test tube.

Tw ^ ] Tw/15

where Tw is the local wall temperature and evaluated at the outer wall surface of the test tube. The average heat transfer coefficient (h) was determined by combining Eqs. (8) and (11) as

h — Mcp(To - T)/A(TW - Tb)

For the local heat transfer coefficient, the bulk-fluid and wall temperatures were selected from a specific local station. Nusselt number is calculated using the following equation;

Nu — hD/k

120 110 100 90 80 70 60 50 40

X Plain tube A Blasius

" o Dittus and Boelter V Petukhov ö .

- Ö -

- Ö Ö -

- S & 8 ä ^ g M & & & Ù

X " o 1 X O i , i 1 , 1 -

5000 7000 9000 11000 13000 15000 Re

Fig. 4. Validation test of the present plain tube.

2.6 2.4 2.2

3e" 2.0

Z --s" £ 1.8

1.6 1.4

. o O-DTs, yjy = 1.5

□ O-DTs, yjy = 2.0

V O-DTs, yjy = 2.5 o

ST o □

x Plain tube O □ V

o □ V

o □ V

O S □ v V « ■tt

- 8 V ■b

8 v •ù x

" 8 V •k x x

* * •fr x x x

x 1 1 1 i i 1 1 1 1

5000 7000 9000 11000 13000 15000

O-DTs, yjy = 1.5 O-DTs, yjy = 2.0 O-DTs, yjy = 2.5 ST

£ _1_

9000 11000 13000 15000 Re (b)

Fig. 5. Effect of O-DTs on heat transfer rate: (a) Nu and (b) Nut/Nup.

where D is the inner test tube and k is the thermal conductivity of the fluid (water/nanofluid).

4.3. Calculation of the friction factor

The pressure drop (DP) across the test section length (L) is calculated from the difference of the levels of manometer fluid. Then pressure drop data were subjected to the calculation of friction factor via the following equation;

f — (D/L)( 2 DP I pU2)

4.4. Calculation of the thermal performance factor

where U is average velocity that calculated by dividing the measured volumetric water/nanofluid flow rate by the inlet cross-section area (A).

The Reynolds number based on inner test tube diameter is given

Re - pUD/m

All of the thermo-physical properties (k, P, m, cp) used for the calculation of the dimensionless number (Nu and Pr) are all evaluated at the bulk fluid temperature (Tb) from Eq. (17).

In the present study, the concept of equal pumping power is applied as performance evaluation criteria. For constant pumping power

The relationship between friction and Reynolds number can be expressed as

Tb - (To + Ti)/2

Fig. 6. Contour plots of velocity vector, turbulent kinetic energy, temperature field, streamline and local Nusselt number of O-DTs at overlapped twist ratio, yo/y = 2.5.

The thermal performance factor (h) of the tube fitted with overlapped dual twisted tapes (O-DTs) under same pumping power criteria is given by

"= Up;

where hp and ht are the heat transfer coefficient for the plain tube and the tube with O-DTs inserts, respectively.

4.5. Uncertainties of measurements

In this investigation, the inlet Reynolds number of cold water was varied from 5400 to 15,200. Uncertainties of measurements were estimated based on ANSI/ASME [46]. The uncertainties in the axial velocity, volumetric flow rate, pressure, and temperature measurements are found to be within ±6%, ±4%, ±5% and ±0.5%, respectively. The accuracies of the measured quantities are 1.8 x 10~5 kg/s for the mass flow rate, 0.05 °C for temperature different (DT), and 0.001 m for DL. In addition, the uncertainties of

Temperature, K

Nusselt-number : 100 109 118 127 136 145 155 164 173 182 191 200

Fig. 7. Contour plots of velocity vector, turbulent kinetic energy, temperature field, streamline and local Nusselt number of O-DTs at overlapped twist ratio, yo/y = 2.0.

Temperature, K

Nuss elt-numb er : 100 109 118 127 136 145 155 164 173 182 191 200

Fig. 8. Contour plots of velocity vector, turbulent kinetic energy, temperature field, streamline and local Nusselt number of O-DTs at overlapped twist ratio, yo/y = 1.5.

non-dimensional parameters were within ±5% for Reynolds number, ±7% for Nusselt number and ±9% for friction factor.

5. Results and discussion

In this section, the effects of the overlapped dual twisted tapes (O-DTs) and TiO2/water nanofluids on the heat transfer, friction factor and thermal performance factor characteristics are reported.

The results of water (the base fluid) in the plain tube are also reported for comparison.

5.1. Validation experiments of plain tube

Firstly, the heat transfer and friction results of the present plain tube (the tube without tape inserts) are validated by comparing the present Nusselt numbers with those obtained from the standard correlations of Dittus—Boelter and present

Fig. 9. Effect of TiO2/water nanofluid concentration on heat transfer rate: (a) Nu and (b) Nut/Nup.

friction factors with those obtained from Petukhov correlation [47] and Blasius correlation [47].

Dittus—Boelter correlation : Nu = 0.023Re08Pr04 (21)

Petukhov correlation : f = (0.79 ln Re - 1.64)-2 (22)

The comparisons between Nusselt number (Nu) and friction factor (f) for the present plain tube with standard correlations are shown in Fig. 4. Apparently, the present data for the current plain tube agree well with those from the standard correlations within ±3.5% for Nusselt number and ±2% for friction factor. Thus, it can be concluded that the results from the present experimental facility are reliable. Thus, the facility was employed for further investigation with O-DTs insert and nanofluids. Note that the Nusselt number (Nup) and friction factor (fp) of water in the present plain tube were used to normalize Nu and f obtained from the tube with O-DTs insert and nanofluids.

5.2. Heat transfer rate

5.2.1 Effect of O-DTs inserts

The heat transfer result in Fig. 5 indicates that Nusselt number (Nu) consistently increased with increasing Reynolds number. At a given Reynolds number, the tube fitted with overlapped dual twisted tapes (O-DTs) had higher than the plain tube (Fig. 5a). The superior heat transfer of the tube with O-DTs can be attributed to the dual swirl flows formed by the O-DTs. It was also found that heat transfer increased as overlapped twist ratios (yo/y) decreased. As indicated by Nusselt number ratio (Nut/Nup) in Fig. 5b, using O-DTs withyo/y = 1.5,2.0 and 2.5 respectively resulted in heat transfer enhancement of 1.77—2.07,1.68—1.98 and 1.59—1.88 times of those of the plain tube. To assess the effect of twisted tape modification, the single twisted tape (ST) insert with the same twist length was also tested for comparison. Obviously, all tested O-DTs gave better heat transfer than ST. This can be explained by the fact that the dual swirl flows generated by O-DTs give better fluid mixing and more efficient thermal boundary layer disruption than the single swirl flow produced by the ST.

For a better understanding of flow and heat transfer phenomena associated with the use of O-DTs, the finite volume method is used to present the contour plots of velocity vector, turbulent kinetic energy, temperature field, streamline and local Nusselt number distribution of the tubes fitted with O-DTs at Reynolds number of 10,000 as shown in Figs. 6—8. Contour plots of streamline through the O-DTs in a circular tube under a constant heat flux were predicted by RNG k—s model using the QUICK scheme. The streamline distribution shows that the O-DTs induced two overlapped swirling flows while the velocity vector plot shows that the O-DTs possessed direct interaction around the clearance between two tapes, promoting turbulence around the tapes. The O-DTs with smaller overlapped twisted ratio (yo/y) gave higher turbulent kinetic energy (TKE) than the tape with larger overlapped twisted ratio due to a stronger swirl intensity. Thus, the use of O-DTs with smaller yo/y resulted in thinner thermal boundary layer (temperature fields), higher Nusselt number and more uniform Nusselt number distribution, as comparison shown in Figs. 6—8.

5.2.2. Effect of TiO2/water nanofluid

Fig. 9a, b shows the effect of concentration of TiO2/water nanofluid on Nusselt number of the tube with O-DTs. For the studied range, Nusselt number increased with increasing TiO2 concentration and all TiO2/water nanofluids gave higher Nusselt number than water as the based fluid. The higher heat transfer by nanofluids arises from: (i) the ability of suspended nano-particles enhancing thermal conductivity; (ii) movement of nanoparticles delivering energy exchange. The higher volume concentration of nanoparticles would increase thermal conductivity and contact surface, thus increasing heat transfer rate. Nusselt number ratio result in Fig. 9b shows that the presence of TiO2 at f = 0.07%, f = 0.14% and f = 0.21% enhanced Nusselt number by 4.5—5.5%, 6.7—7.8% and 9.9—11.2%, respectively, compared to that of water.

5.3. Friction loss

5.3.1. Effect of O-DTs inserts

The friction factor (f) and friction factor ratio (ft/fp) of the O-DTs inserts are depicted in Fig. 10a, b. Compared to the plain tube, the use of O-DTs resulted in an increase in friction factor. Friction factor increased as yo/y decreased, attributed to an increase in contact surface area and the flow mixing effects. As shown in Fig. 10b, the maximum friction factor ratios (ft/fp) of O-DTs with yo/y = 1.5, 2.0 and 2.5 were found to be 5.43 times, 4.95 times and 4.55, respectively. In addition, at the same Reynolds number, O-DTs caused 26.5—45.5% higher friction factor than the typical single twisted tape (ST), since double swirl flows give stronger flow disturbance than a single swirl flow.

5.3.2. Effect of TiO2/water nanofluid

The effect of TiO2 nanoparticles in water on the friction factor (f) and friction factor ratio (ft/fp) is presented in Fig. 11a, b. At a given Reynolds number, nanofluids caused higher friction factor than water (the based fluid). When volume concentrations of TiO2 nanoparticles increased, friction factor increased as seen in Fig. 11a. For the present range, the use of nanofluid at TiO2 volume concentration of (f) 0.21% caused up to 7.9% and 4.8% higher friction factors than those of the ones with f = 0.07% and 0.14%, respectively. The use of nanofluid at TiO2 volume concentration of 0.07% led to higher friction factor than the based fluid around 10%. The increase in friction factor can be caused by collision of small particles during fluid flow.

5.4. Thermal performance factor

5.4.1. Effect of O-DTs inserts

Thermal performance factor result associated by the use of O-DTs and TiO2/water nanofluids based on the same pumping power criteria is shown in Fig. 12. Evidently, thermal performance factor increased as yo/y of O-DTs decreased. This implies that by decreasing yo/y, the augmentation of the heat transfer was more pronounced than pressure drop penalty. The thermal performance factor of O-DTs tended to decrease with the increase of Reynolds

Fig. 10. Effect of O-DTs on friction loss: (a) f and (b) ff

Fig. 11. Effect of TiO2/water nanofluid concentration on friction loss: (a) f and (b) ft/fp.

number. The use of the O-DTs had an advantage in energy saving at a Reynolds number range of 5400 and 15,200 (practical range for most industry process), indicated from thermal performance factors above 1 (h > 1.0). At Reynolds number of 5400, the thermal performance factors up to 1.13,1.11 and 1.08 were achieved by the use of the tubes with O-DTs with yo/y = 1.5, 2.0 and 2.5, respectively. It is also noteworthy that the thermal performance factors of the tube with O-DTs inserts were 12—25.7% higher than those of the tube equipped with ST.

5.4.2. Effect of TiO2/water nanofluid

Fig. 13 depicts the effect of TiO2/water concentration on thermal performance factor. At Reynolds number of 5400, the maximum thermal performance factor of the tube with O-DTs at yo/y = 1.5, increased from 1.13 to 1.15 and 1.16, 1.18 when the based fluid (water) was replaced by the nanofluids with for TiO2 concentrations of 0.07%, 0.14% and 0.21%, respectively. Over the range investigated, the thermal performance factors of nanofluids with f = 0.07%, 0.14% and 0.21% were higher than that of the based fluid around 1.7%, 2.6% and 4.2%, respectively. Comparatively, the use

nanofluids with f = 0.21% resulted in higher thermal performance factor than that of those with f = 0.07 and 0.14% around 2.2—2.8% and 1.4—1.8%, respectively. It is noteworthy that thermal performance factor was higher at lower Reynolds number, this implies that O-DTs are more suitable for practical application at lower Reynolds number.

5.5. Empirical correlations

The experimental results of Nusselt number, friction factor and thermal performance factor were used to develop the empirical correlations by using least square regression analysis. The resultant correlations are shown in Eqs. (23)—(25). The predicted data from the empirical correlations of the Nupred, fpred and hpred are plotted against experimental data of the Nuexp, fexp and hexp in Figs. 14—16. As shown from these figures, the maximum deviations between the experimental data and correlations are within ±4%, ±3% and ±2%, respectively, for Nusselt number, friction factor and thermal performance factor.

11000 Re

Fig. 12. Effect of O-DTs on thermal performance factor.

Fig. 14. Comparison of Nusselt number between experimental present data and those calculated from the present empirical correlations.

Nu = 0.267Rea617Pra4(yo/yr0'213(1 + f)0'505 ( 23)

f = 2.057Re-0'234 (yo/y)-0'311 (1 + f)0'886 (24)

h = 5.538Re-0' 179(yo /y)-0' 109(1 + f)0'209 (25)

6. Conclusions

Heat transfer enhancement by overlapped dual twisted-tapes (O-DTs) and TiO2/water nanofluids was experimentally and numerically investigated. The study encompassed O-DTs with overlapped twist ratios (yo/y) of 1.5,2.0 and 2.5 and nanofluids with

TiO2 volume concentrations (f) of 0.07%, 0.14% and 0.21%. The obtained results indicated that O-DTs induced overlapped swirling-flows which played an important role in improving fluid mixing and heat transfer enhancement. Nusselt number, friction factor and thermal performance increased with decreasing overlapped twist ratio and increasing TiO2 volume concentration. The maximum thermal performance factor of 1.18 was obtained by the use of O-DTs with the smallest overlapped twist ratio (yo/y = 1.5) and nanofluid at the maximum TiO2 volume concentration of 0.21%. Thermal performances of the tubes with O-DTs at yo/y = 1.5,2.0 and 2.5 varied between 0.94 and 1.18, 0.92 and 1.16, and 0.89 and 1.12, respectively. For the present range, the use of TiO2/water nanofluids resulted in 1.7—4.5% higher thermal performance factor than the use of the base fluid (water). In addition, the empirical correlations of heat transfer rate (Nu), friction factor (f) and thermal

Fig. 13. Effect of TiO2/water nanofluid concentration on thermal performance factor.

Fig. 15. Comparison of friction factor between experimental present data and those calculated from the present empirical correlations.

Fig. 16. Comparison of thermal performance factor between experimental present data and those calculated from the present empirical correlations.

performance (h) in a constant wall heat flux tube equipped O-DTs at different overlapped twist ratios (yo/y) and volume concentrations of TiO2 nanoparticles (f) are also reported for heat transfer applications.

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