Buoyancy-driven heat transfer enhancement in a two-dimensional enclosure utilizing nanofluids

Heat transfer characteristics of nanofluids: a review

Review of convective heat transfer enhancement with nanofluids

Nanofluids are potential heat transfer fluids with enhanced thermophysical properties and heat transfer performance can be applied in many devices for better performances (i.e. energy, heat transfer and other performances). In this paper, a comprehensive literature on the applications and challenges of nanofluids have been compiled and reviewed. Latest up to date literatures on the applications and challenges in terms of PhD and Master thesis, journal articles, conference proceedings, reports and web materials have been reviewed and reported. Recent researches have indicated that substitution of conventional coolants by nanofluids appears promising. Specific application of nanofluids in engine cooling, solar water heating, cooling of electronics, cooling of transformer oil, improving diesel generator efficiency, cooling of heat exchanging devices, improving heat transfer efficiency of chillers, domestic refrigerator-freezers, cooling in machining, in nuclear reactor and defense and space have been reviewed and presented. Authors also critically analyzed some of the applications and identified research gaps for further research. Moreover, challenges and future directions of applications of nanofluids have been reviewed and presented in this paper. Based on results available in the literatures, it has been found nanofluids have a much higher and strongly temperature-dependent thermal conductivity at very low particle concentrations than conventional fluids. This can be considered as one of the key parameters for enhanced performances for many of the applications of nanofluids. Because of its superior thermal performances, latest up to date literatures on this property have been summarized and presented in this paper as well. However, few barriers and challenges that have been identified in this review must be addressed carefully before it can be fully implemented in the industrial applications.

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A review on applications and challenges of nanofluids

Experimental investigation into convective heat transfer of nanofluids at the entrance region under laminar flow conditions

Various suspensions containing Al2O3 nanoparticles with specific surface areas in a range of 5–124 m2 g−1 have been prepared and their thermal conductivities have been investigated using a transient hot-wire method. Nanoparticle suspensions, containing a small amount of Al2O3, have substantially higher thermal conductivity than the base fluid, with the enhancement increasing with the volume fraction of Al2O3. The enhanced thermal conductivity increases with an increase in the difference between the pH value of aqueous suspension and the isoelectric point of Al2O3 particle. For the suspensions using the same base fluid, the thermal conductivity enhancements are highly dependent on specific surface area (SSA) of nanoparticle, with an optimal SSA for the highest thermal conductivity. For the suspensions containing the same nanoparticles, the enhanced thermal conductivity ratio is reduced with the increasing thermal conductivity of the base fluid. The crystalline phase of the nanoparticles appears to have no ...

Thermal conductivity enhancement of suspensions containing nanosized alumina particles

Dependence of the thermal conductivity of nanoparticle-fluid mixture on the base fluid

Thermal analysis on nanosized TiO2 prepared by hydrolysis

Among various heat transfer enhancement techniques, an innovative way of improving the thermal conductivities of working media is to suspend ultrafine metallic or nonmetallic solid powders in traditional fluids since the thermal conductivities of most solid materials are higher than those of liquids. Theoretical and experimental studies have reported that suspensions containing solid particles have significantly larger thermal conductivities than the base fluids without solid particles [1, 2]. Unfortunately, suspended particles of micrometer, or even nanometer, dimensions may cause some severe problems such as abrasion and clogging in small passages. Furthermore, for all the large particle suspensions, stabilities are very poor since the coarse grained solid particles settle out eventually. Extensive research conducted on nanosized powder preparation and processing technology in recent years may offer a solution to the problem. Nanoparticle suspensions are expected to exhibit excellent properties. The large surface area and enhanced heat capacity of nanocrystalline materials [3–5] could improve the heat transfer capacities of nanoparticle suspensions. Also, Brownian movement of nanoparticles in a suspension increases stability [6]. Furthermore, nanoparticles can act as a lubricating medium because of their small sizes [7]. Adding a small amount of metal or metal oxide nanosized particles to a fluid has increased the thermal conductivity of the suspension over that of the base fluid [8–10]. The enhancement is highly dependent on the surface area of the particles and the thermal conductivity of base fluid. A nanosized metal particle suspension used as a new coolant in a microchannel heat exchanger offered significant benefits [11]. In this study, three types of α-SiC powder were dispersed in water and ethylene glycol (EG), respectively. The properties of SiC powders are summarized in Table I. SiC particles were deagglomerated by intensive ultrasonication after being mixed with base fluid and then the suspensions were homogenized by magnetic force agitation. The pH values of all the aqueous suspensions were adjusted to 10.0 such that the suspensions were stable. The thermal conductivities of the suspensions were measured by the hot-wire method [12] at a temperature of 4 ◦C. The thermal conductivities (λ) of the suspensions with SiC powers dispersed in water and EG as a function of solid particle volume fraction are shown in Fig. 1a and b, respectively. Fig. 2a and b show the ratio of the thermal conductivity enhancement (δλ = λ − λ0)

Thermal conductivity of suspension containing SiC particles

Background: Knee osteoarthritis (KOA) is one of the most common chronic musculoskeletal disorders worldwide, for which exosomes derived from stem cells may provide an effective treatment. Purpose: To assess the effect of exosomes derived from human urine–derived stem cells (hUSCs) overexpressing miR-140-5p (miR means microRNA) on KOA in an in vitro interleukin 1β (IL-1β)–induced osteoarthritis (OA) model and an in vivo rat KOA model. Study Design: Controlled laboratory study. Methods: Exosomes derived from hUSCs (hUSC-Exos) were isolated and validated. The hUSCs were transfected with miR-140s using lentivirus, and exosomes secreted from such cells (hUSC-140-Exos) were collected. The roles of hUSC-Exos and hUSC-140-Exos in protecting chondrocytes against IL-1β treatment were compared by analyzing the proliferation, migration, apoptosis, and secretion of extracellular matrix (ECM) in chondrocytes. After vascular endothelial growth factor A (VEGFA) was identified as a target of miR-140, the mechanism by which VEGFA can mediate the beneficial effect of miR-140 on OA was investigated using small interfering RNA transfection or chemical drugs. The expression of VEGFA in cartilage and synovial fluid from patients with KOA was measured and compared with that of healthy controls. Surgery for anterior cruciate ligament transection and destabilization of the medial meniscus were performed on the knee joints of Sprague-Dawley rats to establish an animal model of OA, and intra-articular (IA) injection of hUSC-Exos or hUSC-140-Exos was conducted at 4 to 8 weeks after the surgery. Cartilage regeneration and subchondral bone remodeling were evaluated through histological staining and micro–computed tomography analysis. Results: Proliferation and migration ability were enhanced and apoptosis was inhibited in chondrocytes treated with IL-1β via hUSC-Exos, with the side effect of decreased ECM secretion. hUSC-140-Exos not only retained the advantages of hUSC-Exos but also increased the secretion of ECM by targeting VEGFA, including collagen II and aggrecan. Increased expression of VEGFA during the progression of KOA was also confirmed in cartilage and synovial fluid samples obtained from patients with OA. In the rat OA model, IA injection of hUSC-140-Exos enhanced cartilage regeneration and subchondral bone remodeling. Conclusion: Our results demonstrated the superiority of hUSC-Exos overexpressing miR-140-5p for treating OA compared with the hUSC-Exos. The effect of hUSC-140-Exos for suppressing the progression of KOA is in part mediated by VEGFA. Clinical Relevance: Exosomes derived from stem cells may provide a promising treatment for KOA, and our study can advance the related basic research.

Exosomes Derived From Human Urine–Derived Stem Cells Overexpressing miR-140-5p Alleviate Knee Osteoarthritis Through Downregulation of VEGFA in a Rat Model

Yan Liu

Papers

Thermal conductivity enhancement of suspensions containing nanosized alumina particles

Dependence of the thermal conductivity of nanoparticle-fluid mixture on the base fluid

Thermal analysis on nanosized TiO2 prepared by hydrolysis

Thermal conductivity of suspension containing SiC particles

Exosomes Derived From Human Urine–Derived Stem Cells Overexpressing miR-140-5p Alleviate Knee Osteoarthritis Through Downregulation of VEGFA in a Rat Model