Abstract

In this study, heat transfer performance of nanofluids (Al2O3/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 Al2O3/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 Al2O3/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 Al2O3 and SiO2 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 Al2O3 and SiO2 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 Al2O3/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 Al2O3 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 Al2O3/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 Al2O3/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 Al2O3/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 Al2O3/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 Al2O3 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 Al2O3/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 Al2O3/water nanofluid. Zamzamian et al. [17] have experimentally investigated the forced convective heat transfer coefficient in Al2O3/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 Al2O3/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 Al2O3/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 Al2O3/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.

Fig. 1
(a) Layout of experimental setup. (b)
                        Photography of experimental setup.
Fig. 1
(a) Layout of experimental setup. (b)
                        Photography of experimental setup.
Close modal
Table 1

Specification of tube in tube condenser

DescriptionType/value
Type of the heat exchangerDouble-pipe heat exchanger
Type of flowParallel flow
Inner fluidRefrigerant (R410A)
Outer fluidAl2O3/water and CuO/water nanofluid
Outer tube outside diameter (Do)20 mm
Outer tube inside diameter (Di)18 mm
Inner tube outside diameter (do)10 mm
Inner tube inside diameter (di)8 mm
Length of the tube (L)8.1 m
Capacity of air-conditioning system2.48 kW
RefrigerantR407C
Refrigerant condensing temperature50 °C
Inlet temperature of the nanofluid20 °C
Inlet pressure of the nanofluid1 atm
Nanofluid flowrate150 to 300 lph
DescriptionType/value
Type of the heat exchangerDouble-pipe heat exchanger
Type of flowParallel flow
Inner fluidRefrigerant (R410A)
Outer fluidAl2O3/water and CuO/water nanofluid
Outer tube outside diameter (Do)20 mm
Outer tube inside diameter (Di)18 mm
Inner tube outside diameter (do)10 mm
Inner tube inside diameter (di)8 mm
Length of the tube (L)8.1 m
Capacity of air-conditioning system2.48 kW
RefrigerantR407C
Refrigerant condensing temperature50 °C
Inlet temperature of the nanofluid20 °C
Inlet pressure of the nanofluid1 atm
Nanofluid flowrate150 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 Al2O3/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

Nanofluid preparation is the important step for starting the experimental examination. Double step method was used to get ready the nanofluid. In this method, Initially Nanoparticles (30 nm sized Al2O3and 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:
Massofthenanoparticles=1×10-3×ν×ρs

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.

Table 2

Physical properties of Al2O3 and CuO nanoparticles

PropertyAl2O3CuO
Size (nm)3030
Density (kgm−3)36006350
Thermal conductivity (Wm−1 K−1)3669
Specific heat (kJkg−1 K−1)0.7650.5356
Viscosity (kgm−1 s−1)NilNil
PropertyAl2O3CuO
Size (nm)3030
Density (kgm−3)36006350
Thermal conductivity (Wm−1 K−1)3669
Specific heat (kJkg−1 K−1)0.7650.5356
Viscosity (kgm−1 s−1)NilNil

Data Analysis

The heat transfer performance of nanofluid through tube was defined in terms of convective heat transfer coefficient. Convective heat transfer coefficient and Nusselt number for the nanofluid are obtained through the following equation:
hnf(exp)=CpnfρnfUA(Tb2-Tb1)πdoL(Tw-Tb)
(1)
Nunf(exp)=hnf(exp).Dhknf
(2)
The experimental results obtained from the investigation were compared with the theoretical results using Seider–Tate equation for test rig verification. In this equation, nanofluid convective heat transfer enhancement is due to the increase in thermal conductivity
Nunf(th)=1.86(Renf.PrnfDhL)1/3(μnfμwnf)0.14
(3)
Renf and Prnf are defined as follows:
Renf=ρnf.U.Dhμnf
(4)
Prnf=Cpnf.μnfknf
(5)
The physical properties used for nanofluids were calculated from water and nanoparticles properties at average bulk temperature using the following correlation [18]:
μnf=μw(1+2.5ν)
(6)
ρnf=νρs+(1-ν)ρw
(7)
ρnfCpnf=(1-ν)ρwCpw+νρsCps
(8)
Lee and Choi [2] correlation was used for determining the nanofluids effective thermal conductivity
knf=[ks+(n-1)kbf-(n-1)ν(kbf-ks)ks+(n-1)kbf+ν(kbf-ks)]kbf
(9)

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.

Table 3

Summary of experimental studies on Nusselt number of nanofluids

InvestigatorsFluidGeometryDimensions (in mm)Observations
Nasiri et al. [20]Al2O3/water and TiO2/water nanofluidAn annular ductL = 2100For the concentrations 0.1% to 1.5%, Nusselt number of Al2O3/water nanofluid increased up to 23.8% and that for TiO2/water nanofluid increased up to 10.1%.
Di = 10
Do = 22
Suresh et al. [19]Al2O3/water and CuO/water nanofluidStraight circular duct fitted with helical screw tape insertsL= 1000For same concentration of 0.1 vol. %, Nusselt number of Al2O3/water nanofluid increased upto 166.84% and that for CuO/water nanofluid increased upto 179.82%.
Di = 10
Do = 12 (tape twist ratios 1.78, 2.44 & 3)
Present authorAl2O3/water and CuO/water nanofluidTube in tube condenser of air conditioning systemDo = 20Heat transfer coefficient of Al2O3/water nanofluid increased upto 49.84% and for CuO/water nanofluid heat transfer coefficient enhanced upto 58%.
Di = 18
do = 10
di = 8
InvestigatorsFluidGeometryDimensions (in mm)Observations
Nasiri et al. [20]Al2O3/water and TiO2/water nanofluidAn annular ductL = 2100For the concentrations 0.1% to 1.5%, Nusselt number of Al2O3/water nanofluid increased up to 23.8% and that for TiO2/water nanofluid increased up to 10.1%.
Di = 10
Do = 22
Suresh et al. [19]Al2O3/water and CuO/water nanofluidStraight circular duct fitted with helical screw tape insertsL= 1000For same concentration of 0.1 vol. %, Nusselt number of Al2O3/water nanofluid increased upto 166.84% and that for CuO/water nanofluid increased upto 179.82%.
Di = 10
Do = 12 (tape twist ratios 1.78, 2.44 & 3)
Present authorAl2O3/water and CuO/water nanofluidTube in tube condenser of air conditioning systemDo = 20Heat transfer coefficient of Al2O3/water nanofluid increased upto 49.84% and for CuO/water nanofluid heat transfer coefficient enhanced upto 58%.
Di = 18
do = 10
di = 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

Fig. 2
Nusselt number versus Reynolds number for pure distilled water
Fig. 2
Nusselt number versus Reynolds number for pure distilled water
Close modal

In the existing study CuO/water and Al2O3/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 Al2O3/water and CuO/water nanofluids versus Reynolds number at different concentrations. As shown in Fig. 3, outcomes indicate that the heat transfer coefficient of Al2O3/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%.

Fig. 3
Heat transfer coefficient of Al2O3/water nanofluid and
                        water versus Reynolds number
Fig. 3
Heat transfer coefficient of Al2O3/water nanofluid and
                        water versus Reynolds number
Close modal
Fig. 4
Heat transfer coefficient of CuO/water nanofluid and water versus Reynolds
                        number
Fig. 4
Heat transfer coefficient of CuO/water nanofluid and water versus Reynolds
                        number
Close modal

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%).

Al2O3/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 Al2O3/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.

Fig. 5
Comparison of Nusselt number of Al2O3/water nanofluid
                        and CuO/water nanofluid versus Reynolds number
Fig. 5
Comparison of Nusselt number of Al2O3/water nanofluid
                        and CuO/water nanofluid versus Reynolds number
Close modal

Conclusion

The present work related the heat transfer performance of Al2O3/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. (1)

    Heat transfer coefficient of Al2O3/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. (2)

    Heat transfer coefficient of Al2O3/water nanofluid increased up to 49.84% and for CuO/water nanofluid heat transfer coefficient enhanced upto 58%.

  3. (3)

    As shown in the results, the Nusselt number also increases, compared to the base fluid. Nusselt number for Al2O3/water nanofluid increases upto 33.86% and for CuO/water nanofluid upgraded upto 39.48%.

  4. (4)

    Comparison of two working fluids shows that CuO/water nanofluid has a superior convective heat transfer coefficient compare to Al2O3/water nanofluid. This is due to enhancement in the thermal conductivities of nanofluid for increased concentration of nanoparticles.

Nomenclature
A =

cross-sectional area of the nanofluid flow path (m2)

Cp =

specific heat (kJ kg− 1 K− 1)

d =

diameter of the inner tube (m)

D =

diameter of the outer tube (m)

Dh =

hydraulic diameter (m)

hnf (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

Tb =

average bulk fluid temperature (°C)

Tw =

average duct wall temperature (°C)

U =

average fluid velocity (m s−1)

ΔT =

temperature difference (°C)

Greek Symbols
μ =

viscosity (Ns/m2)

μwnf =

nanofluid viscosity at duct wall temperature (Ns/m2)

ρ =

density (kg m−3)

ν =

nanoparticles volume fraction (%)

Subscripts
b =

bulk

i =

inside

nf =

nanofluid

o =

outside

S =

solid nanoparticles

W =

water

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