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Heat transfer and friction factor investigations of CuO nanofluid flow in a double pipe U-bend heat exchanger
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ScienceDirect Materials Today: Proceedings 18 (2019) 207–218
www.materialstoday.com/proceedings
ICAMME-2018
Heat transfer and friction factor investigations of CuO nanofluid flow in a double pipe U-bend heat exchanger V. Nageswara Rao1, B. Ravi Sankar2 1
Department of Mechanical Engineering, Kallam Haranadhareddy Institute of Technology, Guntur, India 2
Department of Mechanical Engineering, Bapatla Engineering College, Bapatla, India
Abstract Experimental investigation was conducted for the estimation of convective heat transfer and friction factor of CuO nanofluids flow in a double pipe U-bend heat exchanger under turbulent flow conditions. The CuO nanofluid was flow through the inner tube of U-bend heat exchanger at different mass flow rates (8, 10, 12 and 14 LPM) and at different volume concentrations (0.01%, 0.03% and 0.06%). The hot water was flow through the annulus tube at a fixed mass flow rate of 8 LPM. The results indicate that, the Nusselt number of nanofluids increases with increase of Reynolds number and particle volume concentrations. The Nusselt number enhancement is about 18.6% at 0.06% volume concentration when compared to base fluid with a pumping penalty of 1.09-times. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Engineering, ICAMME-2018. Keywords: Double-pipe heat exchanger, Heat transfer, Friction factor.
1. Introduction The double pipe heat exchanger is one of the most common heat exchangers that are used in commercial and industrial applications because of its small size, non-manufacturing difficulty and compactness. Mainly used fluids in the heat exchanger equipments are water, ethylene glycol, propylene glycol, engine oil and etc. The performance of the heat exchanger equipment may be enhanced with the use of high thermal conductivity fluids. Cho [1] and his team developed high thermal conductivity fluids called as nanofluids, which is prepared by dispersing nanometer sized solid metallic particles in the fluids. The use of nanofluids in double pipe heat exchangers and its convective heat transfer coefficient have been estimated by researchers and few of them are given below. Reddy and Rao [2] observed heat transfer and friction factor enhancement by 10.73% and 8.73% for 0.02% volume concentration of 40:60% ethylene glycol and water mixture based TiO2 nanofluid flow in a double pipe heat exchanger at a Reynolds number of 15000. Zamzamian et al. [3] prepared Al2O3/EG and CuO/EG nanofluids and studied heat transfer coefficient while they flow in a double pipe and plate heat exchangers and they observed heat transfer enhancement of 26% for 1.0% weight Al2O3 and 37% for 1.0% weight CuO, where as 38% and 49% in the plate heat exchanger. Sajadi and Kazemi [4] observed
∗ Corresponding author. Tel.: 0919494966416 ; E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Engineering, ICAMME-2018.
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Nomenclature Specific heat, ⁄ Inner diameter of the tube, Friction factor Heat transfer coefficient, ⁄ Thermal conductivity, ⁄ Length of the tube, Mass flow rate, ⁄ Nusselt number, ℎ ⁄ ⁄ Prandtl number, Heat flow, Reynolds number, 4 ⁄ Velocity, ⁄ Greek symbols Uncertainty ∆ Pressure drop Volume concentration of nanoparticles, % Dynamic viscosity, ⁄ Density, ⁄ Subscripts Bulk temperature Cold Experimental Hot Inlet Outlet Particle Regression Water heat transfer enhancement of 22% at 0.25% of TiO2-water nanofluid flow in a double pipe heat exchanger in the Reynolds number of 5000. El-Maghlany et al. [5] observed augmentation of effectiveness and the number of transfer units (NTU) of the double pipe heat exchanger using Cu/water nanofluids. Abbasian Arani et al. [6] investigated heat transfer of TiO2/water nanofluids flow in a double pipe counter flow heat exchanger in the volume fraction range from 0.002 and 0.02 and the Reynolds number between 8000 and 51000. Sarafraz and Hormozi [7] observed heat transfer enhancement of 67% at 1.0% volume concentration of Ag/50:50% ethylene glycol/water nanofluid flow in a double pipe heat exchanger and they synthesized silver nanoparticles using plant extraction method from green tea leaves and silver nitrate. Khedkar et al. [8] observed over all heat transfer enhancement of 16% while 3% Al2O3/water nanofluids flow in a concentric tube heat exchanger. Hemmat Esfe and Duangthongsuk and Wongwises [9] observed heat transfer enhancement of 6-11% at 0.2% volume concentration of TiO2/water nanofluid flow in a horizontal double tube counter flow heat exchanger under turbulent flow conditions. Sudarmadji et al. [10] prepared hot Al2O3/water nanofluid is flowing inside tube, while the cold water flows annulus tube and estimated heat transfer coefficient for 0.15%, 0.25% and 0.5% and observed Nusselt number increment of 40.5% compared to pure water under 0.5% volume concentration. Darzi et al. [11] estimated heat transfer and pressure drop of Al2O3/water nanofluid in a double pipe heat exchanger at different temperature range of working fluid. Aghayari et al. [12] observed heat transfer coefficient and Nusselt number of 19% and 24% for 0.3% volume fraction of Al2O3/water nanofluid flow in a double pipe heat exchanger in counter flow direction. Asirvatham et al [13] observed heat transfer enhancement of 28.7% and 69.3% for silver/water nanofluid flow in a double pipe heat exchanger in the volume concentrations of 0.3 % to 0.9 %, respectively.
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The further heat transfer enhancement in double pipe heat exchangers is possible by using return bend. Prasad et al. [14] experimentally studied heat transfer and friction factor of Al2O3/water flow in a double pipe heat exchanger with return bend and observed 25% heat transfer enhancement at 0.03% volume concentration in the Reynolds number of 22000. Clarke and Finn [15] numerically investigated the heat transfer mechanism of secondary refrigerant flow in an air chiller U-bends and observed with return bend there is a 20% heat transfer enhancement for up to 20 pipe diameters. Other experimental investigations found that heat transfer may be enhanced immediately downstream of a U-bend [16,17]. Hong and Hrnjak [18] observed further heat transfer enhancement in pipes with return bend and they concluded with the effect of fluid mixing, hydrodynamic and thermal development of secondary flows at immediate downstream of the U-bend. Choi and Zhang [19] numerically investigated heat transfer of Al2O3/water nanofluid low in a pipe with return bend and observed Nusselt number increase with increase of Reynolds number and Prandtl number, and the increment of specific heat of nanofluid. The experimental heat transfer and friction factor of CuO nanofluids flow in a double pipe U-bend heat exchanger data is not available. In this regard, the present paper focused on the estimation of heat transfer and friction factor of CuO nanofluids flow in an inner tube of double pipe heat exchanger at different mass flow rates and at different particle volume concentrations. The CuO nanofluid is flow through the inner tube, whereas the hot water flows through the annulus tube. The obtained results were compared with available literature values. 2. Preparation of CuO nanofluids The CuO nanofluids were prepared by dispersing CuO nanoparticles (Aarshadhaatu Green Nanotechnologies India Pvt. Ltd: with an average size of 47.5 nm) in distilled water at different particle concentrations of 0.01%, 0.03% and 0.06% in bulk quantity of 15 litres. The uniform dispersion of nanofluids was achieved by dispersing MB 143Nonidet P-40 surfactant nearly equal to 1/10th of weight of nanoparticles for a particular concentration. The surfactant was initially added in 15 litres of water and stirred vigorously, after that required quantity of CuO nanoparticles were added and stirred for 4 hours. The particles required for known percentage of volume concentration was calculated from the Eq. (1). Volume concentration (%),
=
(1)
= 6300 kg/m3, = 998.5 kg/m3, = 100 g and Where is the percentage of volume concentration, is the weight of the nanoparticles. The thermo physical properties of prepared nanofluids were estimated from the properties of water (Table 1) and properties of CuO nanoparticles at bulk temperature of fluid using the below equations for density, specific heat and viscosity [20]. =
(1 + 2.5 )
(2)
Where ( ) and ( ) are the particle volume concentration and viscosity and the subscripts ( ), ( ) and ( ) refer to particle, base fluid and nanofluid. The above equation is valid for a very low particle volume concentration( ≤ 0.02%). =
+ (1 − )
(3)
=
+ (1 − )
(4)
The theoretical model to predict the thermal conductivity of solid-liquid mixture, Maxwell [21] model can be described as follows: =
( (
) )
(5)
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Table 1 Thermophysical properties of CuO nanoparticles and base fluid. Particle/Base fluid
Diameter (nm)
Purity (%)
(kg/m3)
Surface area to mass, (m2/g)
(J/kg K)
(W/m K)
CuO Distilled water*
<50 nm 99 6310 29 525 17.65 = 999.79 + 0.0683 − 0.01074 + 0.0008214 . − 2.30309 × 10 . = 4.2174 − 0.005618 − 0.001299 . − 0.0001153 + 4.14964 × 10 1 = 557.8248 + 19.4084 + 0.13604 − 3.11608 × 10 − 0.00094129 = 0.56502 + 0.002636 − 0.0001251 . − 1.51549 × 10 *All temperatures are in degrees Celsius.
.
Fig. 1 Preparation of CuO nanofluids (a) sample nanofluid (b) bulk nanofluid 3. Experimental set-up and procedure: The schematic representation of an experimental setup is shown in Fig. 2a and the test section details were shown in Fig. 2b. The test section consists of two concentric tube heat exchangers, data logger along with personal computer, cooling water tank and heating water tank, a set of thermocouples, flow meters (both hot and cold) and U-tube manometer. The test section is two double pipe heat exchangers (Fig. 2b) and the inner tube bent at a radius of 0.160 m at a length of 2.2 m. So, the effective length of the heat exchanger is 2.2 m, but the length of the inner tube is 5 m. The inner tube inner diameter (ID) is 0.019 m and outer diameter (OD) is 0.025 m, which is made of stainless steel material and the annulus tube inner diameter (ID) is 0.05 m and outer diameter (OD) is 0.056 m, which is made of cast iron material. The annulus tube is wound with asbestos rope insulation in order to minimize the heat loss takes from the test section to the atmosphere. The four resistance temperature detectors (RTD) were installed to measure the inlet and outlet temperatures of the hot fluid (water or nanofluid) and cold fluid. Thermocouple needles are connected to the data acquisition system and the thermocouple readings are recorded in the computer for further processing. The thermocouples are calibrated (± 0.1oC) before installed in the test section. The aspect ratio ( ⁄ = 263, l: length; d: diameter) of the test section is sufficiently high to guarantee hydro-dynamically developed flow.
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Fig. 2a Schematic representation of an experimental setup
Fig. 2b Test section details The pressure drop across the test section was measured using U-tube manometer by connecting the pressure taps located at the inlet and outlet of the inner tube and mercury is used as manometric fluid. At the entrance of the concentric tube heat exchanger, two rotameters were used to measure the flow rates of cold and hot fluids. The nanofluid was flow through the inner tube, while the hot fluid was flow through the annulus tube. In both the double pipe heat exchangers, the flow paths of the two working fluids were arranged in the counter-flow direction. The temperature of the hot water (annulus side) was maintained around 80oC and kept constant flow rate of 8 LPM (0.133 kg/s). The CuO nanofluid (tube side) with constant inlet temperature of 29oC was supplied through the inner tube at different mass flow rates of 8, 10, 12 and 14 LPM. The experiments were conducted at different particle concentrations of 0.01%, 0.03% and 0.06% and used one after another. Each nanofluid of 15 litres were prepared and used in the experimental analysis. The temperatures of cold and hot fluids were recorded at steady state. The test section was calibrated with water as the working fluid, before using the CuO nanofluid. For each and every nanofluids experiments the test section is cleaned with pure water. The thermo-physical properties of the nanofluid were calculated at mean temperature. The logarithmic mean temperature different method is used to calculate the inside heat transfer coefficient of the nanofluid. The uncertainty of experimental results was determined based on the temperature, flow rate and pressure drop. The weight (W) of nanoparticles was measured by a precise electronic balance with the accuracy of ± 0.001 g, the precision temperature data acquisitions (T) is ± 0.1oC, flow rate (V) was measured by a rotameter with the full scale accuracy of ± 5% and pressure drop (P) was measured by a pressure transducer with the accuracy of ± 2%. The procedure of Kline and McClintock [22] was used to determine the uncertainties associated with the estimation of
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Nusselt number, ∆ ⁄ = (∆ℎ⁄ℎ) + (∆ ⁄ ) and obtained as ± 1.5%. The same procedure is also used to determine the uncertainty associated for estimation of friction factor, ∆ ⁄ = (∆ ⁄ ) + (∆ ⁄ ) + (2∆ ⁄ ) and obtained as ± 1.3%. 4. Data deduction 4.1. Nusselt number Rate of heat flow (annulus-side fluid),
= =
Rate of heat flow (tube-side fluid),
× ×
Overall heat transfer coefficient (tube-side),
,
,
×
×
,
∆
∆
,
−
;∆
=
,
−
,
;∆
(7)
,
∆ ∆
= =
(6)
(8)
(9) ∆
=
,
=
Overall heat transfer coefficient (annulus-side), Where,
−
,
−
∆ ∆
∆
,
For double pipe heat exchangers without considering the fouling factor term the below equation is used: =
+
+
(10)
or is the overall heat transfer coefficients for annulus side and tube side, is the thermal Where conductivity of tube material and is the length of the heat exchanger. The annulus heat transfer coefficient (ℎ ) is calculated based on the Gnielinski [23] and the expression is given below: =
( .
.
= (1.58
(
) .
(11)
⁄
) − 3.82) , 2300 <
< 10 , 0.5 <
< 2000
The obtained Nusselt number value from Eq. (11) is used to calculate the annulus heat transfer coefficient and the expression is given below: ℎ = Where
×
(12)
is the hydraulic diameter and =
=
is the thermal conductivity of annulus fluid
−
Where, A is the flow area i.e. = ( − ) The ℎ value from Eq. (12) is substituted in Eq. (10) for obtaining the tube side heat transfer coefficient can be determined as follows: (ℎ or ℎ ). That is the only unknown value in the equation. The value of =
×
The Reynolds number is based on the flow rate at the inlet of the tube. =
(13) (14)
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The Prandtl number is calculated based on the specific heat, thermal conductivity, and viscosity of nanofluids at mean temperature of the fluid. =
(15)
4.2. Friction factor The friction factor is calculated based on the pressure difference in the mercury column and the expression is given below: =
∆
(16)
×
where ∆ =
−
;
=
4.3. Effectiveness – NTU method ×
=
Number of transfer units,
=
Heat capacity of tube side fluid,
[
where,
(
=
× =
Heat capacity of annulus side fluid, is the smaller of where [ Effectiveness, =
⟹
(∆ )
×
(17) (18)
×
(19)
and
)]
(
)]
(20)
=
5. Results and discussion 5.1. Nusselt number The hot-water was used as annuls side fluid and cold water was used as tube side fluid, the experiments were conducted and the annulus side heat transfer coefficient (ℎ ) was calculated based on the Eq. (11) of Gnielinski [23] and those values were substituted in Eq. (10) for the calculation of tube-side heat transfer coefficient (ℎ ). The Eq. (13) was used to estimate the experimental Nusselt number and the values were shown in Fig. 3 along with the values obtained Gnielinski [23] and Dittus – Boelter [24]. There is a maximum of ± 2.5% was obtained between the experimental and theoretical Nusselt number. Dittus – Boetler [24] equation for single phase fluid: . . = 0.023 (21) The test section is divided into two double pipe heat exchangers in which the inner tube is bending at a distance of 2.2 m and connected to double pipe heat exchanger. The thermocouples were inserted at the inlet and outlet of the inner tube in order to measure the inlet and outlet temperatures of the water/nanofluids. The temperature drop in the bend region was neglected because of the inner tube is a smooth tube. The test section is perfectly designed to maintain counter flow direction between nanofluid and hot fluid in both the double pipe heat exchangers. The logarithmic mean temperature distribution of nanofluids and hot fluid was used for heat transfer calculations. The viscosity value at bulk mean temperature of nanofluid was used for the estimation of Reynolds number ( = ). The thermal conductivity value at bulk mean temperature of the nanofluid is used for the estimation of 4 ⁄ ⁄ ) and the values were shown in Fig. 4 along with base fluid. It is observed that Nusselt number ( =ℎ Nusselt number of nanofluid increases with increase of particle concentrations and Reynolds numbers. At, 0.01% volume concentration, the Nusselt number enhancement is about 4.5% and 7.23% at Reynolds number of 16545 and 28954, respectively compared to water data. Similarly, at 0.06% volume concentration, the Nusselt number enhancement is about 9.77% and 18.6% at Reynolds number of 16545 and 28954, respectively compared to water data. The enhancement in heat transfer coefficient for nanofluid is caused due to the effective fluid mixing by
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providing the return bend for the test tube. The similar behavior of active method of heat transfer augmentation for Al2O3 nanofluid in a tube with return bend have been observed by Choi and Zhang [20] based on the numerical study.
Fig. 3 Experimental Nusselt number of water is compared with the values of Gnielinski [23] and Dittus – Boelter [24]
Fig. 4 Experimental Nusselt number of CuO nanofluid at different particle concentrations and Reynolds number 5.2. Friction factor The experimental friction factor was calculated based on the pressure different between the entrance and exit of the tube. The pressure drop in the bend region is neglected. The U-tube manometer was used to collect the pressure difference between the entrance and exit of the tube and mercury (Hg) is used as manometric fluid. The mercury column height is converted into water column height for further friction factor calculations. The experimental friction factor is calculated from the Eq. (16) and the obtained values were shown in Fig. 5 along with the data obtained from Eq. (22) of Blasius [25] and Eq. (23) of Petukov [26]. It is observed that there is a maximum of ± 2.5% deviation was achieved between experimental end theoretical friction factors.
V. Nageswara Rao and B. Ravi Sankar / Materials Today: Proceedings 18 (2019) 207–218
a)
Blasius [25] equation for friction factor of single phase fluid: . = 0.0791 3000 < < 10
b) Petukhov [26] equation for single phase fluid: = (0.790 − 1.64) 3000 < < 5 × 10
215
(22)
(23)
The Eq. (16) is used to calculate the friction factor of different volume concentrations of CuO nanofluid and the values were shown in Fig. 6. The friction factor of CuO nanofluid increases with increase of particle volume concentrations and Reynolds number. The viscosity of nanofluid is also one of the major influencing parameter for friction factor enhancement. The friction factor of nanofluid flow through tube is purely depends on the mass flow rate and viscosity. At particle concentration of 0.01%, the friction factor increase of 1.043-times and 1.026-times at Reynolds number of 16545 and 28954 respectively compared to water data. Similarly, at particle concentration of 0.06%, the friction factor increase of 1.079-times and 1.092-times at Reynolds number of 16545 and 28954 respectively compared to water data. The friction factor penalty is very less compared to heat transfer enhancement.
Fig. 5 Experimental friction factor of tube side water is compared with the values of Blasius [25] and Petukov [26]
Fig. 6 Experimental friction factor of different volume concentrations of nanofluid along with base fluid
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5.3. Effectiveness – NTU method The effectiveness ( ) and number of transfer units (NTU) of heat exchanger at different volume concentrations of nanofluids were estimated from Eq. (17) and Eq. (20) and the results were shown in Fig. 7 and Fig. 8. It is observed from the figure 7, the NTU enhancement of 0.06% nanofluid is about 1.0245-times and 1.01-times at Reynolds number from 16554 to 28970. Fig. 8 shows the variation of effectiveness of heat exchanger with water and different volume concentrations of nanofluids. It is observed from the figure, the effectiveness increase of 0.06% nanofluid is about 1.033-times and 1.37-times at Reynolds number from 16554 to 28970. Based on the dispersion of CuO nanoparticles in distilled water, the Nusselt number, effectiveness and NTU have been increased with minimum enhancement in friction factor.
Fig. 7 Number of transfer units (NTU) variation of water and nanofluids at different Reynolds numbers
Fig. 8 Effectiveness of water and nanofluids at different Reynolds numbers 6. Conclusions The present work mainly focused on the estimation of heat transfer and friction factor of double pipe heat exchanger with return bend. In this analysis two methods of heat transfer augmentations were considered active method and passive method. In order to achieve active method of heat transfer augmentation, bend is provided in the test tube at
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a distance of 2.5 m. In order to achieve passive method of heat transfer augmentation nanofluid (nanoparticles dispersed in base fluid). The following conclusions have been obtained.
The Nusselt number enhancement of CuO nanofluids increases with increase of volume concentrations and Reynolds number. This is caused due to influence of particle Brownian motion and micro-convection of the nanoparticles in the base fluid. Maximum Nusselt number enhancement of 18.6% was obtained for 0.06% volume concentration and at a Reynolds number of 28970 compared to water. The friction factor of nanofluids increases with increase of particle concentrations and Reynolds numbers. Maximum of 1.092-times was observed 0.06% particle loading at a Reynolds number of 28970 compared to water. For a particular Reynolds number and temperature, the pressure drop due to the increase of friction factor is relatively negligible, when compared to the benefits arising from heat transfer enhancement. The NTU enhancement of 0.06% nanofluid is about 1.0245-times and 1.01-times at Reynolds number from 16554 to 28970, the effectiveness increase of 0.06% nanofluid is about 1.033-times and 1.37-times at Reynolds number from 16554 to 28970.
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ScienceDirect Materials Today: Proceedings 18 (2019) 207–218
www.materialstoday.com/proceedings
ICAMME-2018
Heat transfer and friction factor investigations of CuO nanofluid flow in a double pipe U-bend heat exchanger V. Nageswara Rao1, B. Ravi Sankar2 1
Department of Mechanical Engineering, Kallam Haranadhareddy Institute of Technology, Guntur, India 2
Department of Mechanical Engineering, Bapatla Engineering College, Bapatla, India
Abstract Experimental investigation was conducted for the estimation of convective heat transfer and friction factor of CuO nanofluids flow in a double pipe U-bend heat exchanger under turbulent flow conditions. The CuO nanofluid was flow through the inner tube of U-bend heat exchanger at different mass flow rates (8, 10, 12 and 14 LPM) and at different volume concentrations (0.01%, 0.03% and 0.06%). The hot water was flow through the annulus tube at a fixed mass flow rate of 8 LPM. The results indicate that, the Nusselt number of nanofluids increases with increase of Reynolds number and particle volume concentrations. The Nusselt number enhancement is about 18.6% at 0.06% volume concentration when compared to base fluid with a pumping penalty of 1.09-times. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Engineering, ICAMME-2018. Keywords: Double-pipe heat exchanger, Heat transfer, Friction factor.
1. Introduction The double pipe heat exchanger is one of the most common heat exchangers that are used in commercial and industrial applications because of its small size, non-manufacturing difficulty and compactness. Mainly used fluids in the heat exchanger equipments are water, ethylene glycol, propylene glycol, engine oil and etc. The performance of the heat exchanger equipment may be enhanced with the use of high thermal conductivity fluids. Cho [1] and his team developed high thermal conductivity fluids called as nanofluids, which is prepared by dispersing nanometer sized solid metallic particles in the fluids. The use of nanofluids in double pipe heat exchangers and its convective heat transfer coefficient have been estimated by researchers and few of them are given below. Reddy and Rao [2] observed heat transfer and friction factor enhancement by 10.73% and 8.73% for 0.02% volume concentration of 40:60% ethylene glycol and water mixture based TiO2 nanofluid flow in a double pipe heat exchanger at a Reynolds number of 15000. Zamzamian et al. [3] prepared Al2O3/EG and CuO/EG nanofluids and studied heat transfer coefficient while they flow in a double pipe and plate heat exchangers and they observed heat transfer enhancement of 26% for 1.0% weight Al2O3 and 37% for 1.0% weight CuO, where as 38% and 49% in the plate heat exchanger. Sajadi and Kazemi [4] observed
∗ Corresponding author. Tel.: 0919494966416 ; E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advances in Materials and Manufacturing Engineering, ICAMME-2018.
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Nomenclature Specific heat, ⁄ Inner diameter of the tube, Friction factor Heat transfer coefficient, ⁄ Thermal conductivity, ⁄ Length of the tube, Mass flow rate, ⁄ Nusselt number, ℎ ⁄ ⁄ Prandtl number, Heat flow, Reynolds number, 4 ⁄ Velocity, ⁄ Greek symbols Uncertainty ∆ Pressure drop Volume concentration of nanoparticles, % Dynamic viscosity, ⁄ Density, ⁄ Subscripts Bulk temperature Cold Experimental Hot Inlet Outlet Particle Regression Water heat transfer enhancement of 22% at 0.25% of TiO2-water nanofluid flow in a double pipe heat exchanger in the Reynolds number of 5000. El-Maghlany et al. [5] observed augmentation of effectiveness and the number of transfer units (NTU) of the double pipe heat exchanger using Cu/water nanofluids. Abbasian Arani et al. [6] investigated heat transfer of TiO2/water nanofluids flow in a double pipe counter flow heat exchanger in the volume fraction range from 0.002 and 0.02 and the Reynolds number between 8000 and 51000. Sarafraz and Hormozi [7] observed heat transfer enhancement of 67% at 1.0% volume concentration of Ag/50:50% ethylene glycol/water nanofluid flow in a double pipe heat exchanger and they synthesized silver nanoparticles using plant extraction method from green tea leaves and silver nitrate. Khedkar et al. [8] observed over all heat transfer enhancement of 16% while 3% Al2O3/water nanofluids flow in a concentric tube heat exchanger. Hemmat Esfe and Duangthongsuk and Wongwises [9] observed heat transfer enhancement of 6-11% at 0.2% volume concentration of TiO2/water nanofluid flow in a horizontal double tube counter flow heat exchanger under turbulent flow conditions. Sudarmadji et al. [10] prepared hot Al2O3/water nanofluid is flowing inside tube, while the cold water flows annulus tube and estimated heat transfer coefficient for 0.15%, 0.25% and 0.5% and observed Nusselt number increment of 40.5% compared to pure water under 0.5% volume concentration. Darzi et al. [11] estimated heat transfer and pressure drop of Al2O3/water nanofluid in a double pipe heat exchanger at different temperature range of working fluid. Aghayari et al. [12] observed heat transfer coefficient and Nusselt number of 19% and 24% for 0.3% volume fraction of Al2O3/water nanofluid flow in a double pipe heat exchanger in counter flow direction. Asirvatham et al [13] observed heat transfer enhancement of 28.7% and 69.3% for silver/water nanofluid flow in a double pipe heat exchanger in the volume concentrations of 0.3 % to 0.9 %, respectively.
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The further heat transfer enhancement in double pipe heat exchangers is possible by using return bend. Prasad et al. [14] experimentally studied heat transfer and friction factor of Al2O3/water flow in a double pipe heat exchanger with return bend and observed 25% heat transfer enhancement at 0.03% volume concentration in the Reynolds number of 22000. Clarke and Finn [15] numerically investigated the heat transfer mechanism of secondary refrigerant flow in an air chiller U-bends and observed with return bend there is a 20% heat transfer enhancement for up to 20 pipe diameters. Other experimental investigations found that heat transfer may be enhanced immediately downstream of a U-bend [16,17]. Hong and Hrnjak [18] observed further heat transfer enhancement in pipes with return bend and they concluded with the effect of fluid mixing, hydrodynamic and thermal development of secondary flows at immediate downstream of the U-bend. Choi and Zhang [19] numerically investigated heat transfer of Al2O3/water nanofluid low in a pipe with return bend and observed Nusselt number increase with increase of Reynolds number and Prandtl number, and the increment of specific heat of nanofluid. The experimental heat transfer and friction factor of CuO nanofluids flow in a double pipe U-bend heat exchanger data is not available. In this regard, the present paper focused on the estimation of heat transfer and friction factor of CuO nanofluids flow in an inner tube of double pipe heat exchanger at different mass flow rates and at different particle volume concentrations. The CuO nanofluid is flow through the inner tube, whereas the hot water flows through the annulus tube. The obtained results were compared with available literature values. 2. Preparation of CuO nanofluids The CuO nanofluids were prepared by dispersing CuO nanoparticles (Aarshadhaatu Green Nanotechnologies India Pvt. Ltd: with an average size of 47.5 nm) in distilled water at different particle concentrations of 0.01%, 0.03% and 0.06% in bulk quantity of 15 litres. The uniform dispersion of nanofluids was achieved by dispersing MB 143Nonidet P-40 surfactant nearly equal to 1/10th of weight of nanoparticles for a particular concentration. The surfactant was initially added in 15 litres of water and stirred vigorously, after that required quantity of CuO nanoparticles were added and stirred for 4 hours. The particles required for known percentage of volume concentration was calculated from the Eq. (1). Volume concentration (%),
=
(1)
= 6300 kg/m3, = 998.5 kg/m3, = 100 g and Where is the percentage of volume concentration, is the weight of the nanoparticles. The thermo physical properties of prepared nanofluids were estimated from the properties of water (Table 1) and properties of CuO nanoparticles at bulk temperature of fluid using the below equations for density, specific heat and viscosity [20]. =
(1 + 2.5 )
(2)
Where ( ) and ( ) are the particle volume concentration and viscosity and the subscripts ( ), ( ) and ( ) refer to particle, base fluid and nanofluid. The above equation is valid for a very low particle volume concentration( ≤ 0.02%). =
+ (1 − )
(3)
=
+ (1 − )
(4)
The theoretical model to predict the thermal conductivity of solid-liquid mixture, Maxwell [21] model can be described as follows: =
( (
) )
(5)
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Table 1 Thermophysical properties of CuO nanoparticles and base fluid. Particle/Base fluid
Diameter (nm)
Purity (%)
(kg/m3)
Surface area to mass, (m2/g)
(J/kg K)
(W/m K)
CuO Distilled water*
<50 nm 99 6310 29 525 17.65 = 999.79 + 0.0683 − 0.01074 + 0.0008214 . − 2.30309 × 10 . = 4.2174 − 0.005618 − 0.001299 . − 0.0001153 + 4.14964 × 10 1 = 557.8248 + 19.4084 + 0.13604 − 3.11608 × 10 − 0.00094129 = 0.56502 + 0.002636 − 0.0001251 . − 1.51549 × 10 *All temperatures are in degrees Celsius.
.
Fig. 1 Preparation of CuO nanofluids (a) sample nanofluid (b) bulk nanofluid 3. Experimental set-up and procedure: The schematic representation of an experimental setup is shown in Fig. 2a and the test section details were shown in Fig. 2b. The test section consists of two concentric tube heat exchangers, data logger along with personal computer, cooling water tank and heating water tank, a set of thermocouples, flow meters (both hot and cold) and U-tube manometer. The test section is two double pipe heat exchangers (Fig. 2b) and the inner tube bent at a radius of 0.160 m at a length of 2.2 m. So, the effective length of the heat exchanger is 2.2 m, but the length of the inner tube is 5 m. The inner tube inner diameter (ID) is 0.019 m and outer diameter (OD) is 0.025 m, which is made of stainless steel material and the annulus tube inner diameter (ID) is 0.05 m and outer diameter (OD) is 0.056 m, which is made of cast iron material. The annulus tube is wound with asbestos rope insulation in order to minimize the heat loss takes from the test section to the atmosphere. The four resistance temperature detectors (RTD) were installed to measure the inlet and outlet temperatures of the hot fluid (water or nanofluid) and cold fluid. Thermocouple needles are connected to the data acquisition system and the thermocouple readings are recorded in the computer for further processing. The thermocouples are calibrated (± 0.1oC) before installed in the test section. The aspect ratio ( ⁄ = 263, l: length; d: diameter) of the test section is sufficiently high to guarantee hydro-dynamically developed flow.
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Fig. 2a Schematic representation of an experimental setup
Fig. 2b Test section details The pressure drop across the test section was measured using U-tube manometer by connecting the pressure taps located at the inlet and outlet of the inner tube and mercury is used as manometric fluid. At the entrance of the concentric tube heat exchanger, two rotameters were used to measure the flow rates of cold and hot fluids. The nanofluid was flow through the inner tube, while the hot fluid was flow through the annulus tube. In both the double pipe heat exchangers, the flow paths of the two working fluids were arranged in the counter-flow direction. The temperature of the hot water (annulus side) was maintained around 80oC and kept constant flow rate of 8 LPM (0.133 kg/s). The CuO nanofluid (tube side) with constant inlet temperature of 29oC was supplied through the inner tube at different mass flow rates of 8, 10, 12 and 14 LPM. The experiments were conducted at different particle concentrations of 0.01%, 0.03% and 0.06% and used one after another. Each nanofluid of 15 litres were prepared and used in the experimental analysis. The temperatures of cold and hot fluids were recorded at steady state. The test section was calibrated with water as the working fluid, before using the CuO nanofluid. For each and every nanofluids experiments the test section is cleaned with pure water. The thermo-physical properties of the nanofluid were calculated at mean temperature. The logarithmic mean temperature different method is used to calculate the inside heat transfer coefficient of the nanofluid. The uncertainty of experimental results was determined based on the temperature, flow rate and pressure drop. The weight (W) of nanoparticles was measured by a precise electronic balance with the accuracy of ± 0.001 g, the precision temperature data acquisitions (T) is ± 0.1oC, flow rate (V) was measured by a rotameter with the full scale accuracy of ± 5% and pressure drop (P) was measured by a pressure transducer with the accuracy of ± 2%. The procedure of Kline and McClintock [22] was used to determine the uncertainties associated with the estimation of
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Nusselt number, ∆ ⁄ = (∆ℎ⁄ℎ) + (∆ ⁄ ) and obtained as ± 1.5%. The same procedure is also used to determine the uncertainty associated for estimation of friction factor, ∆ ⁄ = (∆ ⁄ ) + (∆ ⁄ ) + (2∆ ⁄ ) and obtained as ± 1.3%. 4. Data deduction 4.1. Nusselt number Rate of heat flow (annulus-side fluid),
= =
Rate of heat flow (tube-side fluid),
× ×
Overall heat transfer coefficient (tube-side),
,
,
×
×
,
∆
∆
,
−
;∆
=
,
−
,
;∆
(7)
,
∆ ∆
= =
(6)
(8)
(9) ∆
=
,
=
Overall heat transfer coefficient (annulus-side), Where,
−
,
−
∆ ∆
∆
,
For double pipe heat exchangers without considering the fouling factor term the below equation is used: =
+
+
(10)
or is the overall heat transfer coefficients for annulus side and tube side, is the thermal Where conductivity of tube material and is the length of the heat exchanger. The annulus heat transfer coefficient (ℎ ) is calculated based on the Gnielinski [23] and the expression is given below: =
( .
.
= (1.58
(
) .
(11)
⁄
) − 3.82) , 2300 <
< 10 , 0.5 <
< 2000
The obtained Nusselt number value from Eq. (11) is used to calculate the annulus heat transfer coefficient and the expression is given below: ℎ = Where
×
(12)
is the hydraulic diameter and =
=
is the thermal conductivity of annulus fluid
−
Where, A is the flow area i.e. = ( − ) The ℎ value from Eq. (12) is substituted in Eq. (10) for obtaining the tube side heat transfer coefficient can be determined as follows: (ℎ or ℎ ). That is the only unknown value in the equation. The value of =
×
The Reynolds number is based on the flow rate at the inlet of the tube. =
(13) (14)
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The Prandtl number is calculated based on the specific heat, thermal conductivity, and viscosity of nanofluids at mean temperature of the fluid. =
(15)
4.2. Friction factor The friction factor is calculated based on the pressure difference in the mercury column and the expression is given below: =
∆
(16)
×
where ∆ =
−
;
=
4.3. Effectiveness – NTU method ×
=
Number of transfer units,
=
Heat capacity of tube side fluid,
[
where,
(
=
× =
Heat capacity of annulus side fluid, is the smaller of where [ Effectiveness, =
⟹
(∆ )
×
(17) (18)
×
(19)
and
)]
(
)]
(20)
=
5. Results and discussion 5.1. Nusselt number The hot-water was used as annuls side fluid and cold water was used as tube side fluid, the experiments were conducted and the annulus side heat transfer coefficient (ℎ ) was calculated based on the Eq. (11) of Gnielinski [23] and those values were substituted in Eq. (10) for the calculation of tube-side heat transfer coefficient (ℎ ). The Eq. (13) was used to estimate the experimental Nusselt number and the values were shown in Fig. 3 along with the values obtained Gnielinski [23] and Dittus – Boelter [24]. There is a maximum of ± 2.5% was obtained between the experimental and theoretical Nusselt number. Dittus – Boetler [24] equation for single phase fluid: . . = 0.023 (21) The test section is divided into two double pipe heat exchangers in which the inner tube is bending at a distance of 2.2 m and connected to double pipe heat exchanger. The thermocouples were inserted at the inlet and outlet of the inner tube in order to measure the inlet and outlet temperatures of the water/nanofluids. The temperature drop in the bend region was neglected because of the inner tube is a smooth tube. The test section is perfectly designed to maintain counter flow direction between nanofluid and hot fluid in both the double pipe heat exchangers. The logarithmic mean temperature distribution of nanofluids and hot fluid was used for heat transfer calculations. The viscosity value at bulk mean temperature of nanofluid was used for the estimation of Reynolds number ( = ). The thermal conductivity value at bulk mean temperature of the nanofluid is used for the estimation of 4 ⁄ ⁄ ) and the values were shown in Fig. 4 along with base fluid. It is observed that Nusselt number ( =ℎ Nusselt number of nanofluid increases with increase of particle concentrations and Reynolds numbers. At, 0.01% volume concentration, the Nusselt number enhancement is about 4.5% and 7.23% at Reynolds number of 16545 and 28954, respectively compared to water data. Similarly, at 0.06% volume concentration, the Nusselt number enhancement is about 9.77% and 18.6% at Reynolds number of 16545 and 28954, respectively compared to water data. The enhancement in heat transfer coefficient for nanofluid is caused due to the effective fluid mixing by
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providing the return bend for the test tube. The similar behavior of active method of heat transfer augmentation for Al2O3 nanofluid in a tube with return bend have been observed by Choi and Zhang [20] based on the numerical study.
Fig. 3 Experimental Nusselt number of water is compared with the values of Gnielinski [23] and Dittus – Boelter [24]
Fig. 4 Experimental Nusselt number of CuO nanofluid at different particle concentrations and Reynolds number 5.2. Friction factor The experimental friction factor was calculated based on the pressure different between the entrance and exit of the tube. The pressure drop in the bend region is neglected. The U-tube manometer was used to collect the pressure difference between the entrance and exit of the tube and mercury (Hg) is used as manometric fluid. The mercury column height is converted into water column height for further friction factor calculations. The experimental friction factor is calculated from the Eq. (16) and the obtained values were shown in Fig. 5 along with the data obtained from Eq. (22) of Blasius [25] and Eq. (23) of Petukov [26]. It is observed that there is a maximum of ± 2.5% deviation was achieved between experimental end theoretical friction factors.
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a)
Blasius [25] equation for friction factor of single phase fluid: . = 0.0791 3000 < < 10
b) Petukhov [26] equation for single phase fluid: = (0.790 − 1.64) 3000 < < 5 × 10
215
(22)
(23)
The Eq. (16) is used to calculate the friction factor of different volume concentrations of CuO nanofluid and the values were shown in Fig. 6. The friction factor of CuO nanofluid increases with increase of particle volume concentrations and Reynolds number. The viscosity of nanofluid is also one of the major influencing parameter for friction factor enhancement. The friction factor of nanofluid flow through tube is purely depends on the mass flow rate and viscosity. At particle concentration of 0.01%, the friction factor increase of 1.043-times and 1.026-times at Reynolds number of 16545 and 28954 respectively compared to water data. Similarly, at particle concentration of 0.06%, the friction factor increase of 1.079-times and 1.092-times at Reynolds number of 16545 and 28954 respectively compared to water data. The friction factor penalty is very less compared to heat transfer enhancement.
Fig. 5 Experimental friction factor of tube side water is compared with the values of Blasius [25] and Petukov [26]
Fig. 6 Experimental friction factor of different volume concentrations of nanofluid along with base fluid
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5.3. Effectiveness – NTU method The effectiveness ( ) and number of transfer units (NTU) of heat exchanger at different volume concentrations of nanofluids were estimated from Eq. (17) and Eq. (20) and the results were shown in Fig. 7 and Fig. 8. It is observed from the figure 7, the NTU enhancement of 0.06% nanofluid is about 1.0245-times and 1.01-times at Reynolds number from 16554 to 28970. Fig. 8 shows the variation of effectiveness of heat exchanger with water and different volume concentrations of nanofluids. It is observed from the figure, the effectiveness increase of 0.06% nanofluid is about 1.033-times and 1.37-times at Reynolds number from 16554 to 28970. Based on the dispersion of CuO nanoparticles in distilled water, the Nusselt number, effectiveness and NTU have been increased with minimum enhancement in friction factor.
Fig. 7 Number of transfer units (NTU) variation of water and nanofluids at different Reynolds numbers
Fig. 8 Effectiveness of water and nanofluids at different Reynolds numbers 6. Conclusions The present work mainly focused on the estimation of heat transfer and friction factor of double pipe heat exchanger with return bend. In this analysis two methods of heat transfer augmentations were considered active method and passive method. In order to achieve active method of heat transfer augmentation, bend is provided in the test tube at
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a distance of 2.5 m. In order to achieve passive method of heat transfer augmentation nanofluid (nanoparticles dispersed in base fluid). The following conclusions have been obtained.
The Nusselt number enhancement of CuO nanofluids increases with increase of volume concentrations and Reynolds number. This is caused due to influence of particle Brownian motion and micro-convection of the nanoparticles in the base fluid. Maximum Nusselt number enhancement of 18.6% was obtained for 0.06% volume concentration and at a Reynolds number of 28970 compared to water. The friction factor of nanofluids increases with increase of particle concentrations and Reynolds numbers. Maximum of 1.092-times was observed 0.06% particle loading at a Reynolds number of 28970 compared to water. For a particular Reynolds number and temperature, the pressure drop due to the increase of friction factor is relatively negligible, when compared to the benefits arising from heat transfer enhancement. The NTU enhancement of 0.06% nanofluid is about 1.0245-times and 1.01-times at Reynolds number from 16554 to 28970, the effectiveness increase of 0.06% nanofluid is about 1.033-times and 1.37-times at Reynolds number from 16554 to 28970.
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