Experimental Analysis of the Thermal Performance Enhancement of a Vertical Helical Coil Heat Exchanger Using Copper Oxide-Graphene (80-20%) Hybrid Nanofluid

Abstract

The thermal performance enhancement of a vertical helical coil heat exchanger using distilled water-based copper oxide-graphene hybrid nanofluid has been analyzed experimentally. Accordingly, the focus of this study is the preparation of CuO-Gp (80-20%) hybrid nanoparticles-based suspensions with various mass fractions (0% ≤ wt ≤ 1%). The volume flow rate is ranged from 0.5 L·min⁻¹ to 1.5 L·min⁻¹ to keep the laminar flow regime (768 ≤ Re ≤ 1843) and the supplied hot fluid’s temperature was chosen to equal 50 °C. To ensure the dispersion and avoid agglomeration an ultrasound sonicator is used and the thermal conductivity is evaluated via KD2 Pro Thermal Properties Analyzer. It has been found that the increment in nanoparticles mass fraction enhances considerably the thermal conductivity and the thermal energy exchange rate. In fact, an enhancement of 23.65% in the heat transfer coefficient is obtained with wt = 0.2%, while it is as high as 79.68% for wt = 1%. Moreover, increasing Reynolds number results in a considerable augmentation of the heat transfer coefficient.

1. Introduction

In the last decades, the development of energy-efficient systems has caught the attention of intensive research focusing on heat transfer enhancement and size miniaturization of thermal devices, such as heat exchangers, which reduces their operating costs. In this regard, helical coil heat exchangers are more desirable than traditional straight tube heat exchangers owing to their compactness and higher heat transfer coefficients [1]. In fact, the induced centrifugal force with that type of heat exchanger intensifies the fluid motion promoting a much better heat transfer rate [2,3]. In addition, for further improvement of the thermal performance and energy-saving potential of these devices, a promising alternative consists of using nanosized particles in order to enhance thermophysical properties of the common base fluids [4,5,6,7,8,9,10,11,12,13,14,15,16,17]. However, nanoparticles’ concentration should be carefully selected to avoid excessive power of pumping caused by nanofluids’ viscosity increase [18,19]. Kumar et al. [20] performed an experimental study of the thermal energy exchange characteristics in a shell and helical coil heat exchanger utilizing Al₂O₃/water nanofluid under laminar flow conditions. They achieved a heat exchange rate enhancement of 7%, 16.9%, and 24.2% for a volume percentage of 0.1%, 0.4%, and 0.8%, consecutively. In another works of Kumar et al. [21], it was revealed that a gradual pressure decrease of 8%, 12% and 20% was recorded for nanoparticles concentration of 0.1%, 0.4%, and 0.8%, consecutively, and they have concluded that Al₂O₃/water nanofluids can be used as an efficient coolant in a helically coiled tube with a concentration up to 0.4%. Furthermore, compared to parallel flow, the counter flow configuration leads to an improvement of 4% to 8% in the thermal performance with nanoparticles volume percentage of 0.4%. Zan Wu et al. [22] studied the thermal and hydraulic effectiveness of aqueous MWCNT nanofluid in a helically coiled heat exchanger. The obtained viscosity increased significantly more than the thermal conductivity. Moreover, there is no noticeable improvement in the thermal energy exchange for a given pumping power and constant flow speed. Behabadi et al. [23] examined experimentally thermal energy exchange enhancement adopting the combination of multi-walled carbon nanotube nanofluids with helical coil tubes. Compared to the straight tube type, a considerable enhancement of the thermal energy transport was achieved. Leong et al. [24] made a comparison between 25°, and 50° helical baffles, and segmental type STHEs in terms of heat exchange and entropy generation. They indicated that the maximum thermal energy exchange rate is achieved using 25° helical baffles heat exchanger whereas the minimum generated entropy occurred with the 50° arrangement. Srinivas et al. [25] studied the effectiveness of a helical coil heat exchanger operated with different nanofluids. The increment in nanoparticles concentration was accompanied by an important augmentation of thermal transport. In comparison with the pure water, an amelioration of 26.8%, 30.37%, and 32.7% in thermal efficiency was recorded utilizing TiO₂, Al₂O₃, and CuO/water-based nanofluids, respectively. Rakhsha et al. [26] conducted both numerical and experimental studies of the turbulent forced convection flow inside helically coiled tubes operating with CuO-water nanofluids. It was revealed that the coefficient of heat exchange and the pressure decline were raised with the increase in curvature ratio and Reynolds number value. Larger increases were recorded with experimental results that exhibit 16% of rising in pressure reduction and 17% of enhancement in the heat exchange coefficient. Tohidi et al. [27] investigated numerically the combination of nanofluids and chaotic advection in coiled heat exchangers. Al₂O₃ and CuO nanoparticles were used to prepare nanofluids with concentrations up to 3%. Higher thermal energy exchange was achieved with chaotic coils and pure water when compared to the normal coils with nanofluids. However, small nanoparticles’ loading within the chaotic coils exhibits a considerable improvement in Nusselt number. Comparison between vertically and horizontally helical coiled heat exchangers, in terms of pressure loss and thermal energy exchange, was carried out by Kannadasan et al. [28]. The difference in heat transfer enhancement between vertical and horizontal arrangements was negligeable. Moreover, regardless of these arrangements, a better improvement in Nusselt number is achieved with higher nanoparticles loading and turbulent flow regime. Numerous researches have confirmed the advantages of using nanofluids in heat exchangers [29,30,31]. Some recent papers dealing with the effect of using nanofluids on the performances of helical coil heat exchanger can be found in the literature. Sundar and Shaik [32] studied the heat transfer and exergy production of a helical coil heat exchanger working with water ethylene glycol mixture diamond nanofluid. It was found that compared to the pure fluid the use of nanofluid leads to enhancements of the heat transfer rate and exergy efficiency by 36.05% and 36.48%, respectively. Based on numerical simulations and artificial neural network modeling, Fuxi et al. [33] investigated the effect of varying the coil pitch on the performances of a 3D helically coiled shell and tube heat exchanger working with a hybrid nanofluid. The authors mentioned that the combined variations of coil pitch and nanoparticles volume fraction can be used to optimize the heat transfer. Similar conclusions on the enhancement of the heat transfer in coiled heat exchangers by using nanofluids were presented in the recently published works of Singh et al. [34], Hasan et al. [35], and Tuncer et al. [36].

According to the above summary of previous research, it is worth noticing that most investigations are devoted to the use of mono nanofluids, and it seems to be a shortage of hybrid nanofluids application in helical coil heat exchangers. From this perspective, the present work exhibits an experimental analysis of the thermal performance enhancement of an economical and cost-effective vertical helical coil heat exchanger using copper oxide-graphene (80-20%) hybrid nanofluid.

2. Experimental Procedure

2.1. Hybrid Nanofluid Preparation

Obviously, the most delicate and important step of this investigation is the hybrid nanofluid preparation. Therefore, we have opted for the two-step method, which is extensively applied in nanofluids preparation. Accordingly, CuO and Gp nanoparticles were suspended and dispersed separately and combined (80% CuO + 20% Gp) at different mass fractions in distilled water. To ensure the dispersion and avoid the agglomeration an Ultrasonic Homogenizer Sonicator is used and the thermal conductivity of the CuO-Gp hybrid nanofluid is evaluated via KD2 Pro Thermal Properties Analyzer (Figure 1).

2.2. Characterization

The transmission electron microscopy (TEM) analysis was used to characterize the morphology of the hybrid mixture of CuO-Gp nanoparticles. Moreover, a powder X-ray diffractometer was applied to record the X-ray diffraction (XRD) pattern of the CuO-Gp nanoparticles.

2.3. Experimental Setup

Figure 2 shows the graphic illustration of the experimental setup. It consists of a vertical helically coiled heat exchanger having a coil total diameter of 100 mm, a tube length of 1 m, and inside and outside tube diameters of 4 mm and 6 mm, respectively. Type T Thermocouples are fixed at different axial positions of the helical coil through which the hot hybrid nanofluid flows whereas the cold water passes across the surrounding vessel.

2.4. Calculation of Thermophysical Properties

Assuming uniform patterns of hybrid nanoparticles dispersion within the distilled water, the main thermophysical properties of the nanofluid were calculated using standard models of two-phase fluids. Accordingly, for all mass fractions, the density of CuO-Gp (80-20%) nanofluids was determined via the correlation of Lee et al. [37] and Pak and Cho [38]:

$${\rho }_{nf}=\varphi {\rho }_p+\left(1-\varphi \right){\rho }_{bf}$$

(1)

For all mass fractions of CuO-Gp (80-20%) nanofluids, the specific heat is evaluated applying the relation of Pak and Cho [38]:

$$C_P{}_{nf}=\varphi C{p}_p+\left(1-\varphi \right)C{p}_{bf}$$

(2)

The relationship between the nanoparticles volume fraction and mass fraction is:

$$\varphi =\frac{wt\%}{wt\%+\left(\frac{{\rho }_f}{{\rho }_n}\right)\left(1-wt\%\right)}$$

(3)

For all mass fractions of CuO-Gp (80-20%) nanofluids, the thermal conductivity and the viscosity were measured experimentally.

For a given axial distance ‘x’ after inlet, the coefficient of heat transfer is expressed by:

$$h\left(x\right)=\frac{q_S}{T_S\left(x\right)-T_b\left(x\right)}$$

(4)

where q_S is the exerted heat flux; T_S(x) and T_b(x) are the temperature of the tube-wall and the fluid bulk temperature at a distance ‘x’ from the inlet, respectively.

For helical coil, the coefficient of heat transfer might be evaluated as follows:

$$h_i\left(coil\right)=h_i\left(straight\right)(1+3.5\frac{D}{D_c})$$

(5)

where D_c is the helix’s diameter while D is the inside tube diameter. The heat flux is computed in this manner:

$$q_S=\frac{\dot{m}{C}_{P_{nf}}\left(T_{b,o}-T_{b,i}\right)}{A}$$

(6)

where A, T_b,i, and T_b,o are the inner surface area, the bulk fluid inlet and outlet temperatures, respectively.

The average heat transfer coefficient is determined as follows:

$$\overline{h}=-\frac{\dot{m}{C}_P}{2\pi rL}ln\frac{T_S-T_o}{T_S-T_i}$$

(7)

3. Results and Discussion

The XRD diffractogram of the prepared mixture is given in Figure 3. The analysis of this diffractogram shows that the measured powder’s mixture is formed of two phases; the first phase is constituted of hexagonal graphite (it has the same crystalline structure as the graphene), with a space group P63/mmc (Reference pattern PDF2: 00-012-0212), while the second phase is formed of monoclinic crystals of copper oxide, with a space group C2/c (Reference pattern PDF2: 00-045-0937) [39]. It is important to remark the presence of the peaks relative to the reflection (002), at 2θ equal to 26.5° and 35.5°. These two characteristic peaks are assigned to graphite and copper oxide, respectively. The full width at half maximum (FWHM) of diffraction peaks are narrow, indicating the high crystallinity of the prepared mixture.

The mixture crystallite size was evaluated using the Scherrer formula:

$$Crystallite size \left(L\right)=\frac{K·\lambda ·cos\theta }{w}$$

(8)

K is the shape factor (K = 0.94) and λ = 1.54178 A. The unit of W and θ should be converted into radian (rad) before using them in the Scherrer formula. The calculated crystallite sizes using the different peak positions are shown in

Table 1. Calculated crystallite sizes using the different peak positions.

Peak Position, 2θ (o)	26.5	32.5	35.5	38.7	48.7	53.4	58.3	61.5	66.1	68.0	72.4	75.1
Crystallite size, L (nm)	20.2	28.2	24.1	18.4	20.0	17.5	14.8	16.8	9.4	15.8	24.4	16.7

. The calculated average crystallite size is equal to 18.9 nm.

TEM image of CuO-Graphene nanoparticles is described in Figure 4. It is clear from the indicated scale that the nanometric size is well achieved without any agglomeration with uniform nanoparticles dispersion.

The impact of varying hybrid nanoparticles mass fraction and temperature on the thermal conductivity is illustrated in Figure 5. As it can be seen, the thermal conductivity is augmented with the increase in either of these two parameters and of both simultaneously. This trend can be explained by the accompanied increase in the kinetic energy of hybrid nanoparticles molecules, which in turn leads to the increase of Brownian motion and promotes nanoparticles interactions. Moreover, by increasing of dispersed nanoparticles mass fraction, their clustering and random collisions increase resulting in an increase in the hybrid nanofluid thermal conductivity. The latter increase is more pronounced for high mass fractions. In fact, for the base fluid (wt% = 0), the maximum thermal conductivity enhancement is nearly 8.2% (from 0.61 at 25 °C to 0.66 at 60 °C) whereas for the highest mass fraction (wt% = 1), it is about 24% (from 0.76 at 25 °C to 0.94 at 60 °C). The enhancement of the thermal conductivity becomes more important at higher mass fraction, due to the increase in nanoparticles Brownian motions

Figure 6 shows the variations of the nanofluid viscosity with temperature and hybrid nanoparticles mass fraction. Obviously, the increase in temperature leads to the decrease in viscosity while the increase of nanoparticles mass fraction enhances the nanofluid viscosity. This trend can be attributed to the decrease in intermolecular cohesion and nanoparticles adhesion forces due to the temperature rise. However, the increment of dispersed nanoparticles loading results in shear stress augmentation and, thus, the increase in viscosity which is not desirable because it enhances the pressure loss in the heat exchanger coils. For the base fluid (wt% = 0), the maximum decrease in viscosity is nearly 48% (from 0.89 at 25 °C to 0.46 at 60 °C), whereas for the highest mass fraction (wt% = 1), it is about 45% (from 1.81 at 25 °C to 1 at 60 °C). It to be mentioned that except at the beginning of the variations the increase slop of the viscosity is more important for higher mass fraction values especially for lower temperatures. This augmentation is due to the increase in molecular interaction between the nanoparticles and base liquid at higher mass fractions.

The variation of the local coefficient of heat transfer with the axial position of the helically coiled tube with various mass fractions (wt% = 0.2% to wt% = 1%), for Re = 1590, is shown in Figure 7. Regardless of the axial position, this coefficient is markedly increased with the increase in the CuO-Gp nanoparticles mass fraction. At the beginning, this coefficient varies from 1500 W·m⁻²·K⁻¹ to 2692 W·m⁻²·K⁻¹, while at the exit it ranges from 234 W·m⁻²·K⁻¹ to 420 W·m⁻²·K⁻¹. For Re = 1590, the percentage of heat transfer coefficient enhancement versus nanoparticles mass fraction at the outlet of the helical coil (xi/D = 230) is plotted in Figure 8. As can be seen, the enhancement is nearly proportional to the mass fraction. In fact, it is around 23% for a mass fraction wt% = 0.2% while it reaches it maximum of 79.6% for wt% = 1%.

Variation of the average coefficient of heat transfer, for Re = 1590, with hybrid nanoparticles’ mass fraction is illustrated in Figure 9. Similar to the local heat transfer coefficient, there is a gradual augmentation with the rise of CuO-Gp nanoparticles loading. The average heat transfer coefficient reaches its maximum of 1012.14 W/m²·°C for wt% = 1% and Re = 1590.

The variation of the local coefficient of heat transfer with the axial position of the helically coiled tube with various Reynolds number values for wt% = 1% is illustrated in Figure 10. Disregarding the axial position, the increase in Reynolds number ameliorates the heat transfer coefficient. Furthermore, this amelioration is more pronounced at the coil entrance region where it is augmented from 1505 W·m⁻²·K⁻¹ to 4115 W·m⁻²·K⁻¹, while at the exit it was found to vary from 196 W·m⁻²·K⁻¹ to 536 W·m⁻²·K⁻¹. Figure 11 depicted the impact of Reynolds number on the average heat transfer coefficient, for wt% = 1%. Accordingly, a continuous increase is revealed with the rising of Reynolds number to attain its maximum of 1173 W·m⁻²·K⁻¹ for Re = 1843.

4. Conclusions

In the current experimental study, the thermal performance enhancement of helical coil heat exchanger using distilled water based CuO-Gp (80-20%) hybrid nanofluid has been analyzed in laminar flow regime (768 ≤ Re ≤ 1843) at various nanoparticles mass fractions (0% ≤ wt% ≤ 1%). The main findings can be summarized as follows:

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Increase in suspended CuO-Gp nanoparticles mass fraction, their clustering, and random collisions increase resulting in an increase in the hybrid nanofluid thermal properties.

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The increase in temperature leads to a reduction in viscosity while the rise of nanoparticles mass fraction enhances the nanofluid viscosity.

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At the coil outlet and set against the pure distilled water, an enhancement of 23% in the heat transfer coefficient was achieved with nanoparticles mass fraction wt% = 0.2%, while it is as high as 79.6% for wt% = 1%.

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Increase in the Reynolds number is accompanied by a significant increase in heat transfer coefficient which reaches its maximum of 1173 W·m⁻²·K⁻¹ for wt% = 1% and Re = 1843.