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Recently, nanofluid is a hot research topic among thermal engineers to enhance the heat transfer. However, there are deviations between research groups in the effective thermal conductivity measurements by the transient hot-wire method. Since there has been no report for the researchers to select a proper data range of temperature to estimate the thermal conductivities, the deviation could be partly attributed to the poor selection of the temperature data range. In this study, the impacts of the data range selection, the power supplier response delay, the thermal coefficient of resistors, the hot-wire type and test section size on the thermal conductivity measurement using the transient hot-wire method are analyzed.

The proper selection of the temperature data range to be used to estimate the thermal conductivities of fluids using transient hot-wire method is suggested by investigating the impact of it on the measurement by both computationally and experimentally. The thermal coefficient of resistor is found to affect the measurement seriously whereas the type of hot-wire does not. The test section size of the measuring device is compared with the thermal penetration depth.

This review traces the development of nanofluid pool boiling from its beginning (1984) to the present through a sampling of studies that have interested the authors and which have led to the latest findings at the University of Texas at Arlington (UTA). The studies of thermophysical properties of nanofluids are briefly covered. Several works in the last 7 years are highlighted to illustrate the modes of nanofluid pool boiling testing, the variability of nanofluid boiling heat transfer (BHT), and the postulations of causes of this behavior. Starting in 2006, the wettability increase in the nanoparticle coating, generated during the nanofluid pool boiling, is recognized as the source of critical heat flux (CHF) enhancement through its effect on the dynamics of hot spots and departing bubbles. The reasons for the observed contradictory BHT behavior are not yet fully clear, but recently at UTA, nanofluid boiling heat transfer has shown to be transient due to the dynamic nature of the formation of the nanoparticle coating. Also at UTA, the mechanism of nanoparticle deposition on the heated surface has been further confirmed. Thus, nanofluid boiling has led back to heat transfer enhancement through surface modification in nanoscale. These developments from 2006 are covered in more detail.

This paper aims to study the convective heat transfer behavior of aqueous suspensions of nanoparticles flowing through a horizontal tube heated under constant heat flux condition. Consideration is given to the effects of particle concentration and Reynolds number on heat transfer enhancement and the possibility of nanofluids as the working fluid in various heat exchangers. It is found that (i) significant enhancement of heat transfer performance due to suspension of nanoparticles in the circular tube flow is observed in comparison with pure water as the working fluid, (ii) enhancement is intensified with an increase in the Reynolds number and the nanoparticles concentration, and (iii) substantial amplification of heat transfer performance is not attributed purely to the enhancement of thermal conductivity due to suspension of nanoparticles.

With the development of nanotechnology, building a new technology area in a variety of fields and achieving the best performance has become possible. Several studies on the performance of a solar heating system have been conducted using various nanofluids because the efficiency of heat transfer of nanofluids is high. Various previous studies, including theoretical, numerical, and experimental methods, were conducted using nanofluids for flat-plate, evacuated tube, direct solar absorption, parabolic trough, and heat pipe solar collectors. The present work provides an overview of the recent research on the performance of evacuated tube solar collectors using various nanofluids. The experimental and numerical results reported by several researchers, such as the thermal conductivity, heat capacity, and heat transfer coefficient of nanofluids, are first reported. The studies on the evacuated tube solar collectors with nanofluids were then investigated and summarized.

Lately, absorption of carbon dioxide using nanofluids has gained more attention as this acidic gas creates global warming effect. The absorption test was conducted in a custom designed high-pressure vessel made up of stainless steel 316 L, where CO₂ and nanofluid are in direct contact at static state. The type of nanoparticles and influence of its concentration on absorption of carbon dioxide are analyzed. TiO₂ and Al₂O₃ nanofluids at 0.02–0.14wt.% concentrations are prepared by dispersing in DI water. The CO₂ absorption tests were carried out for the above-mentioned nanofluids at said concentrations with operating conditions being an initial pressure of 3 bar and initial temperature of 302K. The results show that relative absorption index (RAI) of CO₂ absorption has increased to a maximum and then decreased with increase in nanoparticle concentration. The aqueous-based TiO₂, Al₂O₃ nanofluids are found to be most effective at 0.1 and 0.14wt.%, respectively, with RAI showing 39.81% and 22.3% increase in CO₂ absorption as compared to basefluid, respectively. The absorption test has also been conducted for saline-based TiO₂ and Al₂O₃ nanofluids at 1, 2, 3 and 3.1wt.% of salt concentration. The stability of saline-based nanofluids was analyzed using turbidity meter. It was found that increase in salt concentration decreases the stability of nanofluids and also decreases the CO₂ absorption rate because of unstability of nanoparticles in salt solutions. Absorption decreased by 11.93% for TiO₂, and 5.68% for Al₂O₃, when salt concentration was increased from 1 to 3.1wt.%.