Transient heat transfer characteristics and process intensification with Al2O3-water and TiO2-water nanofluids: An experimental investigation
Graphical abstract
Introduction
Efficient cooling process is very crucial for effective thermal management and energy utilization in industries. Controlling the working temperature by removing dissipated heat from machinery and various components is required for their proper functioning and durability. Still, it is extremely challenging to control the constantly increasing heat load driven by technological advancement. Different approaches were applied to improve the cooling efficiency of heat transfer devices. The replacement of traditional coolants with a new category of coolants is one of them. Nanofluids, which are stable dispersion of highly conductive metallic or non-metallic nanoparticles in the conventional base fluid such as water, oil glycol, etc., have emerged out to be very promising due to their outstanding heat transfer capability as compared to corresponding base fluids [1,2].
A wide variety of research work has already been carried out on the heat transfer capability of nanofluids. It is found from the literature that the thermal conductivity (k) of nanofluids is higher than the corresponding base fluids. Choi et al. [3] was first showed that the addition of metallic nanoparticles increased the thermal conductivity of basefluids. Later, Eastman et al. [4] reported 40 % enhancement in thermal conductivity of Cu-ethylene glycol nanofluids at 0.3 % concentration. Murshed et al. [5] measured thermal conductivities of Al, Al2O3 and TiO2 nanofluids with deionized (DI) water, ethylene glycol and oil as basefluids at various volume fractions 1–5 % at temperatures 20ºC-60ºC using transient hot-wire method. They also confirmed enhancements in the thermal conductivity of nanofluids. Recently, Xia et al. [6] described the stability and thermal conductivity of Al2O3-DI water nano fluids using PVP and SDS (sodium dodecyl sulphate). They reported that PVP has better effect than SDS on the stability of nanofluid. They further added that thermal conductivity increases with nanoparticle loading and surfactant addition have a negative effect on thermal conductivity enhancement of nanofluids.
Very recently, Al-Waeli et al. [7] reported that surfactant addition has a negative effect on the thermal conductivity of nanofluids, although the stability of nanofluids increased at the same time.
Further studies shows wide variation in the k values of nanofluids depending upon various parameters such as preparation method [8,9], dispersion quality [[10], [11], [12], [13]], particle morphology [[14], [15], [16], [17]], nanoparticle and base fluid properties [18,19], concentration of nanofluids [14,[20], [21], [22]], temperature [23,24] and stability [[25], [26], [27], [28]].
The study of heat transfer performance in nanofluids has been conducted extensively. Widespread application of nanofluids shows heat transfer enhancements in various applications including automobile radiator cooling [29,30], heat exchanger [[31], [32], [33], [34], [35], [36], [37]],nuclear cooling [38,39], electronic cooling [[40], [41], [42]], quenching [43], spray cooling [[44], [45], [46]] etc. In most cases, the heat transfer enhancement is done by controlling the thermal conductivity and heat transfer coefficient of the material. Hence, the measurement of k is very crucial. Various techniques were applied to measure the thermal conductivity of nanofluids. However, to the best of our knowledge, an accurate measurement technique is not reported. Less research on transient heat transfer performance of nanofluids is available in the literature. Manikandan et al. [47] experimentally determined the transient heat transfer characteristics of MgO-Therminol nanofluids using oleic acid as the dispersant. They observed enhanced heat transfer performance in nanofluids. Suganthi et al. [48] examined the transient heat transfer performance of ZnO-propylene glycol nanofluids. They reported 4.24 % increase in heat transfer and 26 % increment in heat transfer coefficient at2% concentration. Zennifer et al. [49] used CuO-ethylene glycol nanofluids and tested their thermal conductivity and transient heat transfer performance. They described 14.1 % increase in thermal conductivity with 1% vol. fraction at 50ºC and 11.8 % enhancement in transient heat transfer performance of nanofluids.
The effect of using different surfactants on thermal conductivity and heat transfer performance of the same nanofluids is not present. Therefore, a study on nanofluids including thermal conductivity measurement and transient heat transfer performance is of paramount importance.
The main focus of the present study is to study the viability of nanofluids as a coolant in various heat transfer applications. Nanofluids were prepared at various concentrations of 0.125 %, 0.25 %, 0.50 % and 1.5 % m/V dispersing ultrafine Al2O3 and TiO2 nanoparticles in water through two-step method. Magnetic stirring followed by ultrasonication was applied to achieve homogeneous mixing and de-clustering of nanoparticles. A very small amount of PVA (Polyvinyl alcohol) and PVP (Polyvinylpyrrolidone) were used to ensure better dispersion stability of nanofluids. The thermal conductivity of nanofluids was calculated using sonic velocity measured by an innovative thermal conductivity apparatus. A novel dedicated experimental setup was developed to analyze the heat transfer performance of nanofluids under transient conditions. The effect of concentration, temperature, and surfactant on the thermal conductivity and heat transfer of nanofluids was analyzed and reported in the present manuscript.
Section snippets
Materials
Nanoparticles of Al2O3 (20−30 nm), TiO2 (20 nm) were purchased from Sisco Research Lab (Bangalore, India). All the chemicals are laboratory grade; hence further purifications were skipped. DI (deionized) water was used to prepare nanofluids. The properties of nanoparticles and base fluid are summarized and tabulated in Table 1.
Preparation of nanofluids
In general, nanofluids are prepared in two ways; single-step approach and two-step approach. In this study, the two-step approach is adopted for the preparation of
Characteristics of nanoparticles
The nanostructure characteristics of Al2O3 and TiO2 nanoparticles obtained from SEM and TEM characterization techniques are shown in Figs. 4(a), (b) and 5 (a), (b) respectively. It is observed from the figures that Al2O3 and TiO2 nanoparticles are in aggregated form (Fig. 4(a),(b)) and the nanoparticles are nearly spherical in shape (Fig. 5(a),(b)). The particle size obtained from the post-processing of micrographs. The particle size varies from 16.25 nm to 31.75 nm for Al2O3 nanoparticles and
Conclusion
An experimental investigation on thermal conductivity and transient heat transfer in water based Al2O3 and TiO2nanofluidsare presented in this study. Al2O3 and TiO2 nanoparticles of an average diameter of 25 nm and 20 nm respectively are dispersed in water to prepare nanofluids at different m/v% of 0.125 %, 0.25 %, 0.5 % and 1.5 % via two-step approach. A very small amount of PVA and PVP are added as stabilizing agents. The stability of nanofluids was analyzed via settling behavior and zeta
Author statement
The Corresponding Author on behalf of all the co-authors state that the careful revision of the manuscript has been made based on the valuable suggestions provided by the esteemed reviewers. The authors convey their gratitude to the editor and reviewers for their valuable time and suggestion. Each comment of reviewers has been carefully read and revised as and when necessary.
Declaration of Competing Interest
The authors declare that there are no conflicts of interest.
Acknowledgment
The authors cordially acknowledge the financial support provided by the Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy, Govt. of India (sanction no. 39/14/04/2017-BRNS/34301).
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