## Abstract

In this study, heat transfer performance of nanofluids
(Al_{2}O_{3}/water and CuO/water nanofluid) is experienced
by using the condensing unit of an air conditioner. Nanoparticles at 30 nm are
suspended at various volume concentrations (1%, 2%, 3%, and 4%) in the base
fluid are produced for this current work. The nanofluids, considered as a
cooling fluid, flow in the outer side of the tube of condenser, and general
working condition of the air conditioner is applied for the investigation.
Experimental results highlight the enhancement of heat transfer rate because of
the existence of nanoparticles in the fluid. Two nanofluids show better heat
transfer rate than does the base fluid. The Nusselt numbers for CuO/water and
Al_{2}O_{3}/water nanofluids are enhanced up to 39.48% and
33.86%, respectively. The findings show that CuO/water nanofluids exhibit better
heat transfer rate than Al_{2}O_{3}/water nanofluids.

## Introduction

Compact air conditioners are now necessary for many applications. To obtain the compactness, smaller sized heat exchanger and high heat transfer rate fluid are needed. But conventional air conditioners have heat exchangers of larger size along with low heat transfer capacity fluids. To achieve high heat transfer rate, the thermal behavior of the fluid must be modified. Metals and metal oxides have high heat transfer capacity, so suspension of nanometer-sized metal oxide particles improves the performance of base fluid. Induced nanoparticles improve thermal conductivity of base fluid, thus increasing the thermal performance of base fluid.

Initially the word nanofluid was used by Choi et al. [1] in 1995 at Argonne National Laboratory, Lemont, IL. Nanofluids have
greater thermal conductivity and high heat transfer performance than the base
fluids. The studies showed that tempted nanoparticles in the base fluid enhance the
thermal conductivity and increase in the volume fraction of nanoparticles in the
base fluid increases the thermal conductivity. Lee et al. [2] have assessed the thermal conductivity of fluids that contain
oxide nanoparticles and found that the thermal conductivity of ethylene glycol with
a suspension of 4.0% volume of 35 nm CuO particles augmented up to 20%. Additional
results have shown that there is a linear augmentation in thermal conductivity ratio
up to 5 vol. %. A series of investigation has shown that the increased volume
fraction intensifies the heat transfer coefficient and Nusselt number for the
nanofluid. Eastman et al. [3] have experienced
the augmentation in effective thermal conductivities of ethylene glycol-based
nanofluids containing copper nanoparticles and have concluded that due to scattering
of 0.3 vol. % of Cu nanoparticles in ethylene glycol, the thermal conductivity
enhanced upto 40%. Wang et al. [4] have
studied the effect particle volume fraction with 24 and 23 nm CuO particles in a
base fluid of water and found that the thermal conductivity augmentation upsurges
linearly with amplified particle volume concentration, specifically the thermal
conductivity ratio increased by 34% at 10% volume fraction. Pang et al. [5] measured the thermal conductivity of methanol
based nanofluids with Al_{2}O_{3} and SiO_{2} nanoparticles
and concluded that for increasing volume fraction thermal conductivity increases and
the enhancement of thermal conductivity observed to be 10.74% and 14.29% over the
base fluid for the concentration of 0.5 vol. % of Al_{2}O_{3} and
SiO_{2} nanoparticles, respectively. Additional results have shown that
the linear augmentation in thermal conductivity ratio upto 5 vol. %. Series of
investigation has exhibited that the increased volume fraction intensifies the heat
transfer coefficient and Nusselt number for the nanofluid. Li and Peterson [6] have experimentally examined the effect of
temperature and volume fraction variants on the effective thermal conductivity of
nanoparticle suspension and reported 52% increase in the thermal conductivity at 6
vol. % of CuO/water nanofluids. Naraki et al. [7] have studied the overall heat transfer coefficient of CuO/water
nanofluids in a car radiator and shown that with the increase in flow rate and
volume concentration of nanoparticles in base fluid, the overall heat transfer
coefficient improved in the range of 6% to 8% compared with conventional working
fluids such as water. Fotukian and Esfahany [8] have examined the convective heat transfer of
Al_{2}O_{3}/water nanofluid inside a circular tube for turbulent
flow experimentally and attained 48% enhancement in coefficient of heat transfer
increases and showed that there is no effect in keep on increasing the nanoparticle
concentration for heat transfer enhancement in turbulent regime and pressure drop of
nanofluid rises with intensification in nanoparticle concentration. Nguyen et al.
[9] have analyzed the heat transfer
enhancement using Al_{2}O_{3} nanofluid for an electronic liquid
cooling system under turbulent regime and for particular volume concentration of
6.8%, heat transfer coefficient augmented up to 40% when compared with that of the
base fluids. The experimental data showed that nanofluid with smaller size of
nanoparticles offers advanced heat transfer coefficient for the similar flow rate.
Wongcharee and Eiamsa-ard [10] have studied
the heat transfer enhancement by using CuO/water nanofluid in corrugated tube
equipped with twisted tap and settled that convective heat transfer, and thermal
performance factor incline to rise with increasing concentration of nanoparticles in
nanofluid.

Chandrasekar et al. [11] experimentally
studied the heat transfer and friction factor characteristics of
Al_{2}O_{3}/water nanofluid in a circular pipe under laminar
flow with wire coil inserts and showed that the Nusselt number is increased by
12.24% when nanofluid at low concentration of 0.1% is used and equated the pressure
drop of nanofluids with distilled water; there is no major rise in pressure drop for
the nanofluid. Pandey and Nema [12]
experimentally analyzed the heat transfer and friction factor of nanofluid as a
coolant in a corrugated plate heat exchanger and stated that the pumping power
enlarged with increase in nanoparticle concentration and the coefficient of heat
transfer increased 11% than water with increased concentration of nanoparticles.
Zeinali et al. [13] examined the convective
heat transfer of Al_{2}O_{3}/Water nanofluid in circular tube under
laminar regime and observed that heat transfer coefficient ratio increases upto
1.41. They specified that due to the presence of nanoparticles in the base fluid
augment the temperature gradient at the wall and the heat transfer enhanced. Hashemi
and Behabadi [14] have experimentally
examined the heat transfer and pressure drop characteristics of CuO/base oil
nanofluid by made a flow through a helically coiled horizontal tube under constant
heat flux and observed increase in the heat transfer up to 30.4% and increase in
pressure drop up to 20.3% and proved that increased nanoparticles not only increase
the heat transfer rate but also raise the pressure loss in fluid flow. Vajjha et al.
[15] have analyzed the heat transfer
performance of Al_{2}O_{3}/water and CuO/water nanofluids in the
flat tubes of radiator. The results showed that both fluids have high potential in
terms of heat transfer rate with increased concentration. Specifically, the
percentage of increase in the normal heat transfer coefficient over the base fluid
for a 10 vol. % of Al_{2}O_{3}/water nanofluid is 94% and that for a
6 vol. % of CuO nanofluid is 89%. The analysis also showed that for the same amount
of heat transfer, the pumping power requirement is 82% lower for
Al_{2}O_{3} nanofluid of 10% concentration and 77% lower for a
CuO nanofluid of 6% concentration when compared with the base fluid. Nassan et al.
[16] compared the heat transfer
characteristics for Al_{2}O_{3}/water and CuO/water nanofluids in
square cross-sectional duct and concluded that heat transfer coefficient for
CuO/water nanofluid is greater than that of Al_{2}O_{3}/water
nanofluid. Zamzamian et al. [17] have
experimentally investigated the forced convective heat transfer coefficient in
Al_{2}O_{3}/ethylene glycol and CuO/ethylene glycol nanofluids
in a double-pipe and plate heat exchangers under turbulent flow. The findings
indicated a significant augmentation in convective heat transfer coefficient of the
nanofluids compared with that of the base fluid. The results have shown that in
plate heat exchanger for 1 vol. % of CuO/ethylene glycol and
Al_{2}O_{3}/ethylene glycol nanofluids, convective heat transfer
coefficient increased up to 49% and 38%, respectively, and in double-pipe heat
exchanger, increased up to 37% and 26%, respectively.

From the previous study, it is cleared that scattering of nanoparticles significantly
augments the heat transfer rate; increase in volume fraction increases the heat
transfer; and series of increase in particle volume fraction results in pressure
loss and high pumping power requirement. The aim of the present study was to compare
the heat transfer performance of CuO/water and Al_{2}O_{3}/water
nanofluids in the condensing unit of air conditioner for various flow rates. The
experimental results of nanofluids are compared with water for various volume
fractions. For this study, 1%, 2%, 3%, and 4% of volume fractions are used.

## Experimental Setup

For analyzing the heat transfer performance of nanofluids (CuO/water or
Al_{2}O_{3}/water) in condensing unit of air conditioner
(tube-in-tube condenser), we envisioned and constructed an experimental setup.
Figures 1(a) and 1(b) show the schematic diagram
and photography of the setup. Table 1 shows
the specifications of the tube-in-tube condenser.

Description | Type/value |
---|---|

Type of the heat exchanger | Double-pipe heat exchanger |

Type of flow | Parallel flow |

Inner fluid | Refrigerant (R410A) |

Outer fluid | Al_{2}O_{3}/water and CuO/water nanofluid |

Outer tube outside diameter (D_{o}) | 20 mm |

Outer tube inside diameter (D_{i}) | 18 mm |

Inner tube outside diameter (d_{o}) | 10 mm |

Inner tube inside diameter (d_{i}) | 8 mm |

Length of the tube (L) | 8.1 m |

Capacity of air-conditioning system | 2.48 kW |

Refrigerant | R407C |

Refrigerant condensing temperature | 50 °C |

Inlet temperature of the nanofluid | 20 °C |

Inlet pressure of the nanofluid | 1 atm |

Nanofluid flowrate | 150 to 300 lph |

Description | Type/value |
---|---|

Type of the heat exchanger | Double-pipe heat exchanger |

Type of flow | Parallel flow |

Inner fluid | Refrigerant (R410A) |

Outer fluid | Al_{2}O_{3}/water and CuO/water nanofluid |

Outer tube outside diameter (D_{o}) | 20 mm |

Outer tube inside diameter (D_{i}) | 18 mm |

Inner tube outside diameter (d_{o}) | 10 mm |

Inner tube inside diameter (d_{i}) | 8 mm |

Length of the tube (L) | 8.1 m |

Capacity of air-conditioning system | 2.48 kW |

Refrigerant | R407C |

Refrigerant condensing temperature | 50 °C |

Inlet temperature of the nanofluid | 20 °C |

Inlet pressure of the nanofluid | 1 atm |

Nanofluid flowrate | 150 to 300 lph |

The experimental arrangement includes two flow loops comprising temperature- and
flow-rate-measuring units, cooling sections and flow-controlling system. In one
loop, normal air-conditioning cycle is carried out and in the other loop, cooling
fluid flows through external tube of the condenser. Refrigerant as a hot fluid takes
the inner tube as the passage and cold fluid, which may be water or nanofluid, takes
the outer tube as the passage. The outer tube is thermally insulated. In the
air-conditioning system, R410A refrigerant is used as a hot fluid and water or
nanofluid (CuO/water or Al_{2}O_{3}/water) is used as a cold fluid.
One rotameter with a flow range of 0–400 l h^{−1} is used to measure the
flow rate of the cold fluid and four pt100-type resistance temperature detectors
(RTDs) are used to indicate the inlet and outlet temperatures of both hot and cold
fluids. The RTDs are welded in the wall to ensure the isothermal state at the
boundary. Chiller unit gives the initial cooling to cold fluid and continuously
eliminates the heat from the fluid. Centrifugal pump is used to circulate the cold
fluid along the tube for varying flow rates. By controlling the pumping power, we
can obtain various flow rates.

## Nanofluid Preparation

_{2}O

_{3}and CuO nanoparticles) were added in to the water at the calculated mass. Then to create a uniform and stable mixture, Ultra sonic waves were passed through the fluid. Nanofluids with four different volume concentrations are prepared for the investigation (1%, 2%, 3%, and 4% of volume fraction). Mass of the nanoparticles added to the water calculated through the following equation:

A prepared nanofluid was supervised after 24 h, no sedimentation of nanoparticles was found. Sedimentation of nanoparticles is not occurring for the turbulent flow regime. This is due to higher imposed shear which interrupts the possible deposition of nanoparticles. The physical properties of nanoparticles are shown in Table 2.

Property | Al_{2}O_{3} | CuO |
---|---|---|

Size (nm) | 30 | 30 |

Density (kgm^{−3}) | 3600 | 6350 |

Thermal conductivity (Wm^{−1} K^{−1}) | 36 | 69 |

Specific heat (kJkg^{−1} K^{−1}) | 0.765 | 0.5356 |

Viscosity (kgm^{−1} s^{−1}) | Nil | Nil |

Property | Al_{2}O_{3} | CuO |
---|---|---|

Size (nm) | 30 | 30 |

Density (kgm^{−3}) | 3600 | 6350 |

Thermal conductivity (Wm^{−1} K^{−1}) | 36 | 69 |

Specific heat (kJkg^{−1} K^{−1}) | 0.765 | 0.5356 |

Viscosity (kgm^{−1} s^{−1}) | Nil | Nil |

## Data Analysis

_{nf}and Pr

_{nf}are defined as follows:

In this equation “n” is the solid particle shape factor, and n = 3 was used to calculate the nanofluid thermal conductivity for spherical particles. The rheological and physical properties of the nanofluid were calculated at the mean temperature. Then the Nusselt number and convective heat transfer coefficient at different concentrations were calculated.

Experimental uncertainties of Nusselt number, Reynolds number and heat transfer rate were calculated using the ANSI/ASME standard (1986) (Table 3). The maximum uncertainties of Nusselt number, Reynolds number and heat transfer rate were found to be ±4.42%, ±4.09%, and ±4.24% respectively, and the details uncertainty calculation is shown in the appendix. To maintain the precision of the readings, the average value of ten reading is taken for the purpose of calculations.

Investigators | Fluid | Geometry | Dimensions (in mm) | Observations |
---|---|---|---|---|

Nasiri et al. [20] | Al_{2}O_{3}/water and
TiO_{2}/water nanofluid | An annular duct | L = 2100 | For the concentrations 0.1% to 1.5%, Nusselt number of
Al_{2}O_{3}/water nanofluid increased up to
23.8% and that for TiO_{2}/water nanofluid increased up to
10.1%. |

D_{i} = 10 | ||||

D_{o} = 22 | ||||

Suresh et al. [19] | Al_{2}O_{3}/water and CuO/water
nanofluid | Straight circular duct fitted with helical screw tape inserts | L= 1000 | For same concentration of 0.1 vol. %, Nusselt number of
Al_{2}O_{3}/water nanofluid increased upto
166.84% and that for CuO/water nanofluid increased upto
179.82%. |

D_{i} = 10 | ||||

D_{o} = 12 (tape twist ratios 1.78, 2.44 & 3) | ||||

Present author | Al_{2}O_{3}/water and CuO/water
nanofluid | Tube in tube condenser of air conditioning system | D_{o} = 20 | Heat transfer coefficient of
Al_{2}O_{3}/water nanofluid increased upto 49.84%
and for CuO/water nanofluid heat transfer coefficient enhanced upto
58%. |

D_{i} = 18 | ||||

d_{o} = 10 | ||||

d_{i} = 8 |

Investigators | Fluid | Geometry | Dimensions (in mm) | Observations |
---|---|---|---|---|

Nasiri et al. [20] | Al_{2}O_{3}/water and
TiO_{2}/water nanofluid | An annular duct | L = 2100 | For the concentrations 0.1% to 1.5%, Nusselt number of
Al_{2}O_{3}/water nanofluid increased up to
23.8% and that for TiO_{2}/water nanofluid increased up to
10.1%. |

D_{i} = 10 | ||||

D_{o} = 22 | ||||

Suresh et al. [19] | Al_{2}O_{3}/water and CuO/water
nanofluid | Straight circular duct fitted with helical screw tape inserts | L= 1000 | For same concentration of 0.1 vol. %, Nusselt number of
Al_{2}O_{3}/water nanofluid increased upto
166.84% and that for CuO/water nanofluid increased upto
179.82%. |

D_{i} = 10 | ||||

D_{o} = 12 (tape twist ratios 1.78, 2.44 & 3) | ||||

Present author | Al_{2}O_{3}/water and CuO/water
nanofluid | Tube in tube condenser of air conditioning system | D_{o} = 20 | Heat transfer coefficient of
Al_{2}O_{3}/water nanofluid increased upto 49.84%
and for CuO/water nanofluid heat transfer coefficient enhanced upto
58%. |

D_{i} = 18 | ||||

d_{o} = 10 | ||||

d_{i} = 8 |

## Results and Discussions

At first, some examinations are carried out using water to find the reliability and correctness of the measurements. Then the outcomes are compared with the results calculated using Seider–Tate equation under turbulent flow regime for varying flow conditions. Figure 2 presents the comparison between the results of experimental and expected values of Nusselt number for distilled water and achieved the worthy settlement between tested records and Seider–Tate equation outcomes, which highlights the correctness and reliability of the experiments

In the existing study CuO/water and Al_{2}O_{3}/water nanofluid at
different concentration of nanoparticles (1%, 2%, 3%, and 4% of volume fraction) in
water have been used for investigating the performance of heat transfer rate of
nanofluids in condensing unit of air conditioner. The results are compared by
varying the flow rate between 2.5 and 5 lpm.

Figures 3 and 4 show the heat transfer coefficients of
Al_{2}O_{3}/water and CuO/water nanofluids versus Reynolds number at
different concentrations. As shown in Fig. 3,
outcomes indicate that the heat transfer coefficient of
Al_{2}O_{3}/water nanofluid is much higher than that of the base
fluid. The observed results indicate that with increase in Reynolds number and
volume fraction of nanoparticles in base fluid, heat transfer coefficient increases
up to 49.84%.

Figure 4 shows heat transfer coefficient of CuO/water nanofluid against Reynolds number at varying concentration of nanoparticles. As shown, heat transfer coefficients of nanofluid are better than those of base fluid—water. The observations confirm that for intensification in volume concentration, the heat transfer coefficient increases remarkably (up to 58%).

## Al_{2}O_{3}/Water Nanofluid Versus CuO/Water Nanofluid

To compare performance of two working nanofluids in this work, we calculated Nusselt
number for concentrations 1% and 4% through the experimental data. As shown in Fig. 5, there is no significant difference
between the two nanofluids. On the basis of the calculated results, it is found that
CuO/water nanofluid has higher thermal conductivity than
Al_{2}O_{3}/water nanofluid. As expected, CuO/water nanofluid is
found to offer higher heat transfer capacity. Summary of previous experimental
studies on the comparison of Nusselt number of two nanofluids is shown in Table 3.

## Conclusion

The present work related the heat transfer performance of
Al_{2}O_{3}/water and CuO/water nanofluid as cooling fluid in air
conditioner's condensing unit, flow through an outer tube passage. The series of
experiments were done under the normal working cycle of air conditioner at turbulent
regime through a Tube in tube condenser. The following conclusions have arrived from
the experimental data:

- (1)
Heat transfer coefficient of Al

_{2}O_{3}/water nanofluid and CuO/water nanofluid are superior to the base fluid, water. And keep on increasing the volume fraction, both fluids show significant increment in the heat transfer coefficient. This can be due to increased thermal conductivity of nanofluid and other reasons such as presence of Brownian motion and diffusion of nanoparticles in base fluid. - (2)
Heat transfer coefficient of Al

_{2}O_{3}/water nanofluid increased up to 49.84% and for CuO/water nanofluid heat transfer coefficient enhanced upto 58%. - (3)
As shown in the results, the Nusselt number also increases, compared to the base fluid. Nusselt number for Al

_{2}O_{3}/water nanofluid increases upto 33.86% and for CuO/water nanofluid upgraded upto 39.48%. - (4)
Comparison of two working fluids shows that CuO/water nanofluid has a superior convective heat transfer coefficient compare to Al

_{2}O_{3}/water nanofluid. This is due to enhancement in the thermal conductivities of nanofluid for increased concentration of nanoparticles.

*A*=cross-sectional area of the nanofluid flow path (

*m*)^{2}- Cp =
specific heat (kJ kg

^{− 1}K^{− 1})*d*=diameter of the inner tube (m)

*D*=diameter of the outer tube (m)

- D
_{h}= hydraulic diameter (m)

- h
_{nf}(exp) = nanofluid experimentally average heat transfer coefficient (W m

^{−2}K^{−1})*k*=thermal conductivity (W m

^{−1}K^{−1})- L =
length of the tube (m)

- Nu (exp) =
nanofluid experimental average Nusselt number

- Nu (th) =
nanofluid theoretical Nusselt number calculated from Seider–Tate equation

- Pr =
Prandtl number

*Q*=heat transfer rate (W)

- Re =
Reynolds number

- T
_{b}= average bulk fluid temperature (°C)

- T
_{w}= average duct wall temperature (°C)

- U =
average fluid velocity (m s

^{−1})*Δ*T =temperature difference (°C)