International Journal of Heat and Mass Transfer
A new frontier of nanofluid research – Application of nanofluids in heat pipes
Introduction
Since the 1990s, researchers began to apply nano-material technology to heat transfer field and have achieved many meaningful results on heat transfer enhancement. In 1995, Choi [1] firstly proposed the concept of “nanofluid”, which is a fluid with some kinds of nanometer-sized particles suspended into a base liquid. Some examples of applied nanoparticles are pure metals (Au, Ag, Cu, Fe), metal oxides (CuO, SiO2, Al2O3, TiO2, ZnO, Fe3O4), Carbides (SiC, TiC), Nitrides (AlN, SiN) and different types of carbon (diamond, graphite, single/multi wall carbon nanotubes). Traditional liquids, such as water, ethylene glycol and engine oil are some examples of base fluids. Under appropriate operating conditions, nanofluids will exhibit high thermal conductivity and stability and are increasingly being used in many heat transfer applications in industrial fields. In recent years, the studies on nanofluids mainly focused on its thermal conductivity, and on forced convection and boiling heat transfer mechanisms. Various mechanisms of the heat transfer enhancement have been proposed including the interface effect (liquid layering around the nanoparticle makes the atomic structure of the liquid layer more ordered than that of bulk liquid, due to higher thermal conductivity of the nanoparticle than liquid, the liquid layer at the interface would reasonably have a higher thermal conductivity than the bulk liquid), Brownian motion, ballistic transport of energy carriers (ballistic phonon transport through the nanoparticles, heat is carried by phonons, i.e., by propagating lattice vibrations), and thermophoresis (nanoparticles can diffuse under the effect of a temperature gradient) [2], [3], [4].
Also, some researchers contribute their efforts on summarizing the latest reports of nanofluids on various fundamental studies. Keblinski et al. [5] made an interesting review to discuss the thermophysical properties of nanofluids and future challenges. Wang and Maunder [6] summarized the recent researches on flow and heat transfer characteristics of nanofluids in forced and free convective flows. Weerapun and Somchai [7] summarized the published experimental and numerical investigations of forced convective heat transfer of nanofluids. Bahrami et al. [8] provided an overview on the effective thermal conductivity of nanofluids. Up to 200 literatures have been published so far [9]. Recently, the fundamental study of nanofluids in heat pipes has been significantly developed [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47].
Heat pipes are high-efficient heat transfer devices and have been widely applied in various thermal systems. Since heat pipes utilize the phase change of the working fluid to transport the heat, the selection of working fluid is of essential importance to promote the thermal performance of heat pipes. Owing to the heat transfer enhancement effect of nanofluids in the single phase and phase-change heat transfer, some researchers have applied various nanofluids in heat pipes as the working fluids to enhance their heat transfer performance.
As a new kind of heat transfer working fluid, the nanofluid is a new technology attempt to use the special properties of this functional fluid to enhance the phase-change heat transfer in heat pipes, and will have wide application prospect. The fundamental studies of nanofluids applied in heat pipes are still in its initial stage, most of which are experimental study and some experimental results cannot be unified yet. The research on application of nanofluids in heat pipes was firstly published in 2003 [10]. Over 30 relevant articles have been published since then as shown in Table 1 and Fig. 1, Fig. 2, Fig. 3, involving miniature micro-grooved heat pipe, mesh wick heat pipe, sintered metal wick heat pipe, oscillating heat pipe (OHP), closed two-phase thermosyphon. The applied nano-materials included metal, metal oxides, diamond, carbon nanotubes and several other materials. The current studies are mainly experimental, theoretical studies were very few [11], [12], [13]. The type, size of heat pipes and operating conditions of heat pipes, the kind of the base fluids, the material and size of nanoparticles all varied in very wide ranges among these experiments. Therefore, it is difficult to quantitatively make the comparison among different experimental data and then the most exiting research conclusions are qualitative. This paper summarizes and reviews the latest researches of heat pipes using nanofluids as working fluids in recent years. It also discusses the mechanism of heat transfer enhancement or degradation, the existing problems for various heat pipes utilizing nanofluids, and explores the possible application prospects.
Section snippets
Micro-grooved heat pipe
Chien et al. [10] firstly carried out an experimental study on the application of nanofluids in FHP. They studied a disk-shaped aluminum miniature micro-grooved heat pipe. The diameter and the thickness were 9 mm and 2 mm, respectively. A total number of 18 micro-grooves were evenly distributed on the aluminum base to provide the capillary force. The depth and the width of rectangular micro-grooves were 0.4 mm and 0.35 mm, respectively. The nanofluid consisted of gold nanoparticles with a diameter
The comparison of the existing experimental data for the heat resistance and the maximum heat-transfer capacity
In the existing literatures, both the mass concentration and volume fraction were used for indicating the nanoparticle concentration in the base fluid. In order to arrange the data using the same parameter of the nanoparticle concentration, the mass concentration is used to describe the nanoparticle concentration in the present review.
The volume fraction θ can be estimated by following correlation
Fig. 16 gives out comparison of the relative reduction of the total heat resistance
Mechanism of transfer, existing problems and future research prospects
The current studies on nanofluids applied in heat pipes can be divided into three categories. The first category is the heat pipes with micro-grooves, meshes and sintered metal porous materials which provide the capillary force. The basic heat transfer mode for this category belongs to the convective evaporation and convective condensation of the fluid film. The boiling heat transfer may occur at high heat fluxes in heat pipes with micro grooves, but it cannot occur in the mesh and sintered
Conclusions
This paper describes the research results of heat transfer characteristics of various types of heat pipes using nanofluids as working fluids. Results of the limited number of available references have shown that nanofluids have great application prospects in various heat pipes. For the majority of micro-grooved heat pipes, mesh wick heat pipes, oscillating heat pipes and most closed two-phase thermosyphon, adding nanoparticles to the working liquid can significantly enhance the heat transfer,
Acknowledgments
This work was supported by the National Natural Science Foundation of China under Grant No. 51076092.
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