Experimental investigation of diameter effect on heat transfer performance and pressure drop of TiO2–water nanofluid

doi:10.1016/j.expthermflusci.2012.08.014

Experimental Thermal and Fluid Science

Volume 44, January 2013, Pages 520-533

https://doi.org/10.1016/j.expthermflusci.2012.08.014 Get rights and content

Abstract

In this paper, an experimental study performed to investigate the convection heat transfer characteristics in fully developed turbulent flow of TiO₂–water nanofluid. The effect of mean diameter of nanoparticles on the convective heat transfer and pressure drop studied at nanoparticle volume concentration from 0.01 to 0.02 by volume. The experimental apparatus is a horizontal double tube counter-flow heat exchanger. The nanoparticles of TiO₂ with diameters of 10, 20, 30 and 50 nm dispersed in distilled water as base fluid. The results indicated higher Nusselt number for all nanofluids compared to the base fluid. It is seen that the Nusselt number does not increase by decreasing the diameter of nanoparticles generally. In this study both Nusselt number and pressure drop were considered in definition of thermal performance factor. The results show that nanofluid with 20 nm particle size diameter has the highest thermal performance factor in the range of Reynolds number and volume concentrations were studied.

Highlights

► It is an experimental study on Nu and pressure drop in turbulent flow of TiO₂–water. ► It was focused on diameter effect of nanoparticle on Nu and pressure drop. ► This study done for 10–50 nm size diameter and 0.01–0.02 volume concentration. ► Both Nu and pressure drop were considered for finding the best thermal performance. ► The 20 nm particle size diameter has highest thermal performance than other diameter.

Introduction

Heat transfer loads have rapid growth at various equipments used in industry, transportation, electronic and microelectronic, defense weaponry, etc. Conventional fluids such as oils and water are used widely in industries in order to heat transfer. In general, these fluids have poor thermal properties that restricted the heat transfer performance compared to those of most solids. Many techniques could be used to enhance heat transfer rate that results in reduction in the size of the heat transfer equipments. In recent years, many researchers developed new classes of fluids to enhance heat transfer rate by suspending small particles of solids in the ordinary fluids. Different types of nanoparticles, such as metallic, ceramics, ceramic oxides, ceramic nitrides, semi conductive material and carbon nanotubes (CNTs) can be used as solid. In the primitive studies in many years ago, uses of particles in size of millimetre or even micrometre in the fluid, results high thermal enhancement. But some problems such as poor stability of the suspension, clogging and high pressure drop creates. A decade ago, with the rapid development of nanotechnology, particles in order of micrometre (commonly between 1 nm and 100 nm) were replaced by nanometre-size particles. Choi [1] called this type of fluid by nanofluid. By using the nanofluid, compared with suspensions contains particles in size of millimetre or micrometre, heat transfer area decreases because of an enhancement in the heat transfer rate. Many experimental studies have been done by researchers. They reported that nanofluids have shown special advantages, such as better stability, greater thermal conductivity, and lower pressure drop. Although all of these benefits might does not occur at the moment. Some of these studies are expressed as follows.

Pak and Cho [2] studied on the heat transfer performance and pressure drop of γ-Al₂O₃ (13 nm) and TiO₂ (27 nm) nanoparticles suspended in water in turbulent flow through a horizontal circular tube. They observed that the heat transfer rate increases by increase in Reynolds number and nanoparticle volume fraction up to 3%, and it decreases for volume fraction of 3%.

Wen and Ding [3] studied on the convective heat transfer of water–Al₂O₃ nanofluid flowing through a copper tube in the laminar flow regime. Using the nanofluid showed considerable enhancement of convective heat transfer. The enhancement was particularly significant in the entrance region. Yang et al. [4] investigated the convective heat transfer coefficients of several nanofluids under laminar flow in a horizontal tube heat exchanger. The nanoparticles used in this research were graphitic in nature, with different aspect ratios. The graphite nanoparticles increased the convective heat transfer and static thermal conductivities significantly at low weight fraction loadings. He et al. [5] reported that addition of TiO₂ nanoparticles into the water or decreasing agglomerate size, enhances the thermal conductivity. The convective heat transfer coefficient increases with nanoparticle concentration in both the laminar and turbulent flow regimes through a vertical pipe at Reynolds <6500. The convective heat transfer coefficient does not change by average agglomerate size. The nanofluid pressure drop is very close to that of the water.

Kulkarni et al. [6] performed a study on the convective heat transfer and viscosity of nanofluids. The nanoparticles CuO, Al₂O₃ and SiO₂ were used each in mixture of ethylene glycol and water. The results indicated that the heat transfer coefficient of nanofluid increases by increasing volume concentration. Kim et al. [7] investigated the convective heat transfer of nanofluid through a straight circular tube in the laminar and turbulent flow regimes. The base fluid was water and the nanoparticles were separate alumina and amorphous carbonic nanoparticles. Thermal conductivity and convective heat transfer coefficient at 3 vol.% Al₂O₃–water nanofluid were 8% and 20%, respectively. For amorphous carbonic nanofluid, the thermal conductivity was similar to that of water, and the convective heat transfer coefficient increased only 8% in laminar flow. Sharma et al. [8] evaluated heat transfer coefficient and friction factor in a tube with twisted tape at different twist ratio of nanofluid flows with Al₂O₃ nanoparticles. The results showed that heat transfer coefficient of nanofluid with 0.1% volume concentration is 23.7% higher than that of water in a tube at Reynolds number of 9000. The maximum friction factor with twisted tape at 0.1% nanofluid volume concentration was 1.21 times that of water flowing in a plain tube. Anoop et al. [9] studied on the effect of Al₂O₃ particle size in water base nanofluid on the heat transfer characteristics in the developing region of tube flow. Selected particle sizes were 45 and 150 nm. The nanofluid contains 45 nm particles have higher heat transfer coefficient compared with 150 nm particles. Both nanofluids showed higher heat transfer characteristics compared to the base fluid.

Duangthongsuk and Wongwises [10] studied the heat transfer coefficient and friction factor of the TiO₂ (21 nm)–water nanofluid with 0.2–2 vol.% in a horizontal double tube counter-flow heat exchanger under turbulent flow conditions. The heat transfer coefficient of nanofluid with 1 vol.% was approximately 26% greater than that of base fluids, while for volume concentration of 2.0 vol.% was approximately 14% lower than that of base fluids. The pressure drop of nanofluid was slightly higher than the base fluid and increases with increasing the volume concentrations.

Teng et al. [11] studied on the effect of particle size, temperature, and weight fraction on the thermal conductivity ratio of Al₂O₃–water nanofluid up to 2.0 wt.% and different nominal diameter 20, 50, and 100 nm. The results showed a correlation between high thermal conductivity ratios and enhanced sensitivity, small nanoparticle size and higher temperature. Xie et al. [12] investigated the convective heat transfer enhancement of nanofluid in laminar flows inside a tube. Nanofluid containing nanoparticles of Al₂O₃, ZnO, TiO₂, and MgO separately in a base fluid contains 55 vol.% distilled water and 45 vol.% ethylene glycol. They reported that the nanofluid heat transfer rate highly depended on several parameters such as the nanoparticle volume fraction, average size of nanoparticles, and the flow conditions. All nanofluids have higher heat transfer coefficient than that of water, and up to 252% enhancement occurs at a Reynolds number of 1000 for MgO nanofluid.

Farajollahi et al. [13] reported the heat transfer characteristics of aqueous nanofluid contains γ-Al₂O₃ and TiO₂ nanoparticles separately under turbulent flow condition in a shell and tube heat exchanger. The results showed that by uses the nanofluid, significant enhancement of heat transfer characteristics obtained and different optimum nanoparticles concentrations exist for nanofluid. Some of other researchers studied on the effect of some parameters such as nanoparticles volume fraction and type of nanoparticles on the convective heat transfer and the friction factor or pressure drop of nanofluid in turbulent flow condition [14], [15], [16], [17].

Sajadi and Kazemi [18] investigated turbulent heat transfer characteristics of TiO₂–water nanofluid in a circular pipe for maximum nanoparticles volume concentration of 0.25%. The results indicated that addition of small amounts of nanoparticles to the base fluid considerably augmented heat transfer, while Nusselt number are approximately the same for all nanoparticles volume concentration. The pressure drop of nanofluid increased with increasing the volume concentration while are slightly higher compared to the base fluid. Ji et al. [19] were investigated the effect of Al₂O₃–water particle size on the heat transfer performance of an oscillating heat pipe. Four nanoparticles with average diameters of 50 nm, 80 nm, 2.2 μm, and 20 μm were used. The results showed that all the nanofluids significantly affect the heat transfer performance and it depends on the particle size. The best heat transfer performance observed for nanoparticles with diameter of 80 nm. Zamzamian et al. [20] studied on the effect of nanofluid of aluminium oxide and copper oxide were prepared in ethylene glycol on the forced convective heat transfer coefficient in turbulent flow within a double pipe and plate heat exchangers. They found up to 50% enhancement in convective heat transfer coefficient of the nanofluid compared to the base fluid. Moreover, the results indicated that with increasing nanoparticles concentration and nanofluid temperature, the convective heat transfer coefficient of nanofluid increases.

Recently Abbasian Arani and Amani [21] presented an experimental study on heat transfer and pressure drop of TiO₂–water in turbulent flow regime for 30 nm particle size diameter. They carried out their experiment investigation for Reynolds number range between 8000 and 51,000 and 0.002–0.02 volume concentrations. They concluded that by using the nanofluid at high Reynolds number (greater than 30,000) more power compared to low Reynolds number needed to compensate the pressure drop of nanofluid, while increments in the Nusselt number for all Reynolds numbers are approximately equal. Therefore using nanofluids at high Reynolds numbers compared with low Reynolds numbers, have lower benefits.

Wang and Mujumdar [22], in their review article, explained that many factors such as particle size, shape and distribution, pH value, and the particle–fluid interactions may have important effect on the heat transfer performance of the nanofluids. The purpose of this study is to disclose the thermal fluid flow transport phenomenon of TiO₂–water nanofluid by studying the pressure drop, and the convective heat transfer performance for various diameter and concentrations of TiO₂–water nanofluids. TiO₂–water nanofluid is used as the working fluid under the constant heat flux boundary.

From the above literature review it must be mentioned that considerable enhancement in heat transfer coefficients were reported in the turbulent regime but studies on the effect of particle sizes in this regime has not been investigated comprehensively. Hence, the present investigation concentrates on the heat transfer enhancement in the turbulent flow regime with varying particle sizes and concentrations. The nanofluid used in this study is TiO₂–water with average particle sizes of 10 nm, 20 nm, 30 nm and 50 nm. The particle concentrations used in the experiments were of 1 vol.%, 1.5 vol.% and 2 vol.%.

We focus on titanium dioxide as a nanoparticle that was not studied extensively in literature such as aluminium and copper. Also titanium dioxide has important characteristics as safe material for human and excellent chemical and physical stability [9], [23].

In addition based on our literature review, it can be seen that all of the previous works on nanofluid heat transfer focused on heat transfer characteristics or pressure drop separately. Hence, the another aim of the present experimental investigation is to study both the convective heat transfer and friction factor characteristics in the fully developed turbulent flow of TiO₂–water nanofluid in a Reynolds number range of 9000–55,000 with 1–2 vol.% concentration.

Section snippets

Sample preparation

The schematic of the experimental apparatus is shown in Fig. 1. This set up have three closed-loop cycles. The nanofluid cycle contains a collection tank, a pump with bypass line, heat transfer test section, and a water heat exchanger in order to cool nanofluid. The heat transfer section was made of two centric tubes. According to equation (L_e/D ≈ 4.4 × Re^1/6) [24] the length of tube in order to create fully developed turbulent flow at Reynolds number of 51,000 (near maximum Reynolds number)

Preparation of nanofluid

In order to prepare the nanofluid by dispersing the nanoparticles in a base fluid, special mixing and stabilization methods of the nanoparticles are required. In the present study three effective methods were used to stabilize the suspension against sedimentation of nanoparticles. These methods are: change the pH value of the nanofluid, addition of surfactants or surface activators, and use of ultrasonic vibration. In this work, distilled water was used as liquid medium. The desired volume

Density and specific heat capacitance

The effective density of the nanofluid is given by: $ρ_{nf} = φ ρ_{p} + (1 - φ) ρ_{f}$ The heat capacitance is defined as: $c_{p, nf} = \frac{φ (ρ c_{p})_{p} + (1 - φ) (ρ c_{p})_{f}}{ρ_{nf}}$

Thermal conductivity of nanofluid

It must be mentioned that during our studies, a lake of experimental results about thermophysical properties variation by the diameter of nanoparticles or temperature observed. According to our studies several theoretical correlations existed but only one empirical correlation about thermal conductivity and viscosity of TiO₂–water nanofluid considered the effects of

Validation

In order to validate and estimate the accuracy of the experimental results, values of Nusselt number and friction factors for distilled water are compared with existing correlation. Values of Nusselt numbers compared with values of Gnielinski equation [46] and Petukhov equation [47].

Gnielinski equation: $\begin{matrix} \bar{Nu} = \frac{(f / 2) (Re - 1000) Pr}{1 + 12.7 (f / 2)^{0.5} ({Pr}^{2 / 3} - 1)}, 2300 < Re < 5 \times 10^{6}, 0.5 < Pr < 2000 \\ f = (1.58 \ln (Re) - 3.82)^{- 2} \end{matrix}$ Petukhov equation: $\bar{Nu} = \frac{(f / 8) RePr}{1.07 + 12.7 (f / 8)^{0.5} ({Pr}^{2 / 3} - 1)}, 10^{4} < Re < 5 \times 10^{6}, 0.5 < Pr < 200$ $f = (1.82 \log (Re) - 1.64)^{- 2}$

Results and discussion

The experiments were carried out using TiO₂–water nanofluid, with particles of average diameter of 10, 20, 30 and 50 nm and the following ranges of governing parameters: the Reynolds number from 8000 to 55,000, the particle volume fraction from 0% to 2%. The results and discussion presented hereafter focus on the effects of particle volume concentration, Reynolds number and particle size diameter on the flow and heat transfer behaviour of the nanofluid in the fully developed turbulent regime.

Conclusion

It is seen that the Nusselt number does not increase by decreasing the diameter of nanoparticles generally. But pressure drop increases significantly at high Reynolds number. Based on the values of Reynolds number and the nanoparticle volume fraction, change the diameter of nanoparticles could affect the Nusselt number and pressure drop of nanofluid. The Nusselt number increases by enhancing the Reynolds number and nanoparticle volume fraction. By increasing the Reynolds number, the Δp_nf

Acknowledgements

The authors would like to thank the referees for their valuable comments. The authors are grateful to University of Kashan for supporting this work by Grant No. 55806. They would also like to thank the Iranian Nanotechnology Development Committee for their financial support.

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Experimental investigation of diameter effect on heat transfer performance and pressure drop of TiO2–water nanofluid

Abstract

Highlights

Introduction

Section snippets

Sample preparation

Preparation of nanofluid

Density and specific heat capacitance

Thermal conductivity of nanofluid

Validation

Results and discussion

Conclusion

Acknowledgements

Int. J. Heat Mass Transfer

Int. J. Heat Mass Transfer

Int. J. Heat Mass Transfer

Appl. Energy

Int. Commun. Heat Mass Transfer

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Appl. Therm. Eng.

Phys. Let. A

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Int. J. Heat Mass Transfer

Int. J. Heat Fluid Flow

Int. Commun. Heat Mass Transfer

Exp. Therm. Fluid Sci.

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Experimental investigation of diameter effect on heat transfer performance and pressure drop of TiO₂–water nanofluid