Convection heat transfer of functionalized MWNT in aqueous fluids in laminar and turbulent flow at the entrance region

doi:10.1016/j.icheatmasstransfer.2010.03.003

International Communications in Heat and Mass Transfer

Volume 37, Issue 6, July 2010, Pages 717-723

https://doi.org/10.1016/j.icheatmasstransfer.2010.03.003 Get rights and content

Abstract

In this research, the convective heat transfer coefficients of water-based FMWNT nanofluid have been measured under both laminar and turbulent regimes flowing through a uniformly heated horizontal tube in entrance region. For the first time, we have compared effective parameters to measure the convective heat transfer coefficients for functionalized MWNT suspensions such as Re, mass fraction and temperature, altogether in entrance region. The experimental results indicate that the convective heat transfer coefficient of these nanofluids increases by up to 33–40% at a concentration of 0.25 wt.% compared with that of pure water in laminar and turbulent flows respectively and 20 °C.

Introduction

Carbon nanotubes have attracted much attention recently because of their extraordinary thermal, electrical, and mechanical properties [1]. It has long been recognized that the suspensions of solid particles in liquids provide useful advantages in industrial fluid systems, including heat transfer fluid, magnetic fluid, and lubricant fluid [1], [2], [3], [4], [5]. Since the working fluids have the limitation of heat transfer performance, solid particles were dispersed in the working fluids to improve their thermal properties or heat transfer characteristics. However, those previous practical (micro and millimeter) tended to quickly settle down. The nanofluid does not simply refer to a liquid–solid mixture. Some special requirements are necessary, such as even suspension, stable suspension, durable suspension, low agglomeration of particles, and no chemical change of the fluid. There are two major methods for producing nanofluids; (i) the one-step direct method represents the direct formation of the nanoparticles inside the base fluids, and (ii) the two-step method represents the formation of nanoparticles and subsequent dispersion of the nanoparticles in the base fluids. In general, these are effective methods used for preparation of suspensions [6], [7], [8], [9]: (1) to change the pH value of suspensions; (2) to use surface activators and/or dispersants; and (3) to use ultrasonic vibration. While increases in effective thermal conductivity as well as changes in density, specific heat, and viscosity are important indications of improved heat transfer behavior of nanofluids, the net benefit of nanofluids as heat transfer fluids is determined through the heat transfer coefficient. If nanofluids can improve the heat transfer coefficient of thermal energy systems, they can facilitate the reduction in size of such systems and lead to increased energy and fuel efficiencies. To this end, it is essential to directly measure the heat transfer performance of nanofluids under flow conditions typical of specific applications. To date, there has been limited research reported in this area.

In this study, nanofluid heat transfer research has been divided by fluid conditions of laminar flow and turbulent flow. The number of studies in these areas is small, with the smallest number of studies having been reported in the turbulent flow that is more useful in industry. Also we want to compare all parameters to measure convective heat transfer coefficients altogether in entrance region. According to literatures and our experiments, the effect of some factors such as mass fraction, temperature, and various velocities was selected and the effects of these parameters on the convective heat transfer coefficients in the water-based fluids are discussed.

Section snippets

Material and preparation of nanofluid

Distilled water and multi-walled carbon nanotubes were used to produce nanofluids. The carbon nanotubes were prepared in our groups (R.I.P.I.-MWNT). We synthesized the nanotubes by catalytic decomposition of 20% methane in hydrogen over Co–Mo/MgO catalysts at 1000 °C [10]. Fig. 1 shows respectively the SEM image of the sample. It can be seen that the nanotubes are entangled and some are in the format of agglomerates. For better dispersion, MWNTs were polarized by chemical treatment. One gram of

Physical properties

The heat capacity of nanofluids is measured and the value is similar to that of water. Measured values of the density do not agree well with the values calculated from mixing theory. The dynamic viscosity of water-based FMWNT nanofluids is increased by about 4% for a 0.25 wt.% at 33 °C. Also, Table 1 shows that the thermal conductivity of water-based FMWNT nanofluids is enhanced by 1.20% for a 0.25 wt.% at 33 °C compared with that of pure water. All the experimental data are summarized in Table 1.

Conclusions

The convection heat transfer performance of the functionalized MWNT fluids were studied in laminar and turbulent flows through a horizontal tube in entrance region. The experimental results show that the nanoparticles and temperature increase the heat transfer coefficient of the fluid system, but the increase is much more in low temperature at turbulent regime. The heat transfer coefficient increased about 25% compared to the pure water. Increasing the nanoparticles' concentration does not show

Acknowledgements

The authors would like to place on record, their appreciation for the support rendered by Research and Technology Directorate/National Iranian Oil Company, on the research leading to the present article and also would like acknowledge the Research Institute of Petroleum Industry (RIPI), and the Specialized Nanotechnology Research Center for granting this work.

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Convection heat transfer of functionalized MWNT in aqueous fluids in laminar and turbulent flow at the entrance region☆

Abstract

Introduction

Section snippets

Material and preparation of nanofluid

Physical properties

Conclusions

Acknowledgements

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