Influence of particle properties on convective heat transfer of nanofluids

© 2017 An experimental study is performed in order to examine how particle properties such as size and thermal conductivity affect the convection heat transfer of nanofluids. For this purpose, we prepare and study self-synthesized water-based nanofluids with different kinds of particles: polystyrene, SiO 2 , Al 2 O 3 and micelles. Concentrations of the nanofluids vary in the range of 0.1–1.8 vol-% and particle sizes between 8 and 58 nm. Full-scale convective heat transfer experiments are carried out using an annular tube heat exchanger with the Reynolds numbers varying in the range of 1000–11000. The pressure losses are also taken into account in the analysis in order to assess the feasibility of the nanofluids for practical forced convection heat transfer applications. The fluids are thoroughly characterized: viscosities, thermal conductivities, densities, particle size distributions, shapes and zeta potentials are all determined experimentally. In many previous studies, anomalous enhancement in convective heat transfer is observed based on comparison of the Nusselt numbers with equal Reynolds numbers. Also in this work, the nanofluids exhibit Nusselt numbers higher than water when compared on this basis. However, this comparison neglects the impact of differences in the Prandtl numbers, and therefore the altered thermal properties of nanofluids are not properly taken into account. In this study, no difference in Nusselt numbers is observed when the Prandtl number is properly considered in the analysis. All nanofluids performed as the Gnielinski correlation predicts, and the widely reported anomalous convective heat transfer enhancement was not observed with any nanoparticle types. Instead, we show that the convection heat transfer behavior of nanofluids can be explained through the altered thermal properties alone. However, addition of any type of nanoparticles was observed to change the fluid properties in an unfavorable manner: the viscosity increases significantly, while only moderate enhancement in the thermal conductivity is obtained. The more viscous nanofluids reach lower Reynolds numbers than water with equal pumping powers resulting in lower heat transfer coefficients. However, the increase in viscosity, and therefore also the deterioration of the convective heat transfer, is less pronounced for the nanofluids with smaller particle size indicating that small particle size is preferable for convective heat transfer applications

Facile preparation of concentrated silver and copper heat transfer nanocolloids

Concentrated, yet stable silver- and copper-in-water nanocolloids are prepared using a novel method combining formation of a metal ammine complex and use of a strong NaBH4 reductant. Maximum solid contents of the stable silver and copper nanofluids are 2000 and 5000 ppm (reported as mass fractions), respectively. The metallic nanoparticles are reduced in micellar microreactors, favoring formation of small nanoparticles. Use of stable metal ammine complexes ([Ag(NH3)2]+ and [Cu(NH3)4(H2O)2]2+) as metal ion sources prevent the formation of sparingly-soluble metal salts and thus, aid the nanocolloid synthesis. Several different stabilizers and combinations of them are tested for nanofluid synthesis: anionic sodium dodecyl sulfate, polymeric polyvinylpyrrolidone, sodium citrate, nonionic sorbitan trioleate and polysorbate 20. The particle sizes and size distributions are studied using dynamic laser scattering and transmission electron microscopy. Stability of the nanofluids is assessed by zeta potential measurements, repetitive particle size measurements and visual observations. The average particle sizes of the silver and copper nanofluids with optimized surfactants are < 20 nm and ~40 nm, respectively, and the fluids with optimized stabilizer compositions are stable over the storing period of a month. Specific heat and thermal conductivities of the fluids are measured using differential scanning calorimetry and modified transient plane source technique (TCi Thermal conductivity analyzer), respectively. In addition, the nanofluid viscosities are measured in order to assess the usability of the nanofluids in convective heat transfer. The chemistry of stabilizers is found to have a significant impact on the viscosity of nanofluids. Commonly used polymeric polyvinylpyrrolidone stabilizer produces viscous fluids, whereas the viscosities of the fluids stabilized with small size surfactants are close to that of water.Papers presented to the 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Costa de Sol, Spain on 11-13 July 2016

Retrotransposon insertions can initiate colorectal cancer and are associated with poor survival

Genomic instability pathways in colorectal cancer (CRC) have been extensively studied, but the role of retrotransposition in colorectal carcinogenesis remains poorly understood. Although retrotransposons are usually repressed, they become active in several human cancers, in particular those of the gastrointestinal tract. Here we characterize retro-transposon insertions in 202 colorectal tumor whole genomes and investigate their associations with molecular and clinical characteristics. We find highly variable retrotransposon activity among tumors and identify recurrent insertions in 15 known cancer genes. In approximately 1% of the cases we identify insertions in APC, likely to be tumor-initiating events. Insertions are positively associated with the CpG island methylator phenotype and the genomic fraction of allelic imbalance. Clinically, high number of insertions is independently associated with poor disease-specific survival.Peer reviewe

Ionic cross-linked polyvinyl alcohol tunes vitrification and cold-crystallization of sugar alcohol for long-term thermal energy storage

A new sustainable material for storing heat and releasing it on demand has been demonstrated for long-term latent heat storage (LLHS). The material consists of a high-latent-heat sugar alcohol phase change material (PCM) dispersed within ionic cross-linked matrices of polyvinyl alcohol (PVA). This material's unique property is the inhibition of undesired crystallization of the PCM during cooling due to the strong intermolecular interactions of the polymeric matrices, which leads to vitrification instead of crystallization. The release of latent heat can be controlled due to the PCM's stability below its cold-crystallization, which is triggered by reheating, as demonstrated by differential scanning calorimetry (DSC), optical microscopy (OM) and in situ X-ray diffraction (XRD). The addition of an ionic citrate cross-linker further tunes the vitrification and cold-crystallization properties of the PCM. Homogeneity and the presence of hydrogen bonding of the cold-crystalizing PCM (CC-PCM) were studied by scanningelectron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), Fourier transform infrared spectroscopy (FTIR) and XRD. Thermal stability was confirmed by thermogravimetric analysis (TGA) and 100 consecutive DSC heating-cooling cycles. The CC-PCM demonstrated high latent heat of fusion, up to 266 J g(-1), depending on the composition. As a super-adsorbent, PVA was able to swell and hold the liquid PCM resulting in form-stability and leakage-preventive properties above the melting temperature. Taken together, these results confirm that PVA matrices are promising for the thermal and structural stabilization of sugar alcohol PCMs, overcoming unexpected heat release and phase separation, and withstanding repeated melting-cooling cycles for LLHS.Peer reviewe

Thermal properties and convective heat transfer of phase changing paraffin nanofluids

Fluids containing micro-sized solid-liquid phase changing particles have been proposed to be promising candidates as future heat transfer fluids. In addition, smaller nano-sized particles have been claimed to enhance the heat transfer performance of fluids even if the phase change is not exploited. For the first time, we conduct full scale convection heat transfer measurements combining these two consepts. Three water-based paraffin mixture nanofluids with particle mass fractions of 5–10% are prepared and measured with an annular tube heat exchanger with Reynolds numbers varying in the range of 700–11000. In addition, the fluids are characterized: latent heats, specific heats, viscosities, thermal conductivities, densities and particle size distributions are all determined experimentally. In agreement with previous studies of solid-particle nanofluids and nanoemulsions, also the phase changing nanofluids are found to exhibit Nusselt numbers clearly higher (up to ∼60% in the turbulent regime) than water when compared on the basis of equal Reynolds numbers. However, the differences in Prandtl numbers are shown to explain these deviations in Nusselt numbers. Indeed, the well-known Gnielinski correlation is able to explain the results and thus, significant anomalies in the convection heat transfer caused by neither the melting of the phase change material nor the presence of the nanoparticles are observed. However, the nanofluids have systematically slightly higher Nusselt numbers than the correlation would predict, but the deviations are within the accuracy of the correlation (10%). When compared by using equal pumping powers, the nanofluids exhibit heat transfer performance poorer than that of water. The positive impact of the latent heat is outweighed by the negative effects of the increased viscosity and decreased specific heat

Abstracts from The 34(th) Annual National Neurotrauma Symposium June 26-29, 2016 Lexington, Kentucky.

Objectives: Glial fibrillary acidic protein (GFAP) and ubiquitin Cterminal hydrolase-L1 (UCH-L1) have been studied as potential biomarkers of mild traumatic brain injury (mTBI). We report the levels of GFAP and UCH-L1 in patients with acute orthopedic injuries without central nervous system involvement and relate them to the levels in patients with CT-negative mTBI. Methods: Serum UCH-L1 and GFAP were measured from 73 patients with acute orthopedic injury. The injury types were recorded and most of the patients underwent also a head MRI. The results were compared to those found in patients with CT-negative mTBI (n = 93). Results: The levels of GFAP were higher in patients with acute orthopedic trauma than in patients with CT-negative mTBI (p = 0.026). The levels of UCH-L1 were not significantly different between these two groups. Those patients with orthopedic trauma, who had levels of either or both of these biomarkers in the upper quartile, had significantly higher levels of both biomarkers than patients with CT-negative mTBI (p < 0.001). Conclusions: Levels of GFAP and UCH-L1 were not able to distinguish patients with CT negative mTBI from patients with orthopedic trauma. Patients with orthopedic trauma and high levels of UCH-L1 or GFAP values may be falsely diagnosed as having a concomitant mTBI. This casts a significant doubt on their diagnostic value in cases with mTBI. Keywords: GFAP, UCH-L1, orthopedic injur

Glial fibrillary acidic protein and ubiquitin C-terminal hydrolase-L1 are not specific biomarkers for mild CT-negative traumatic brain injury

Glial fibrillary acidic protein (GFAP) and ubiquitin C-terminal hydrolase-L1 (UCH-L1) have been studied as potential biomarkers of mild traumatic brain injury (mTBI). We report the levels of GFAP and UCH-L1 in patients with acute orthopedic injuries without central nervous system involvement, and relate them to the type of extracranial injury, head magnetic resonance imaging (MRI) findings, and levels of GFAP and UCH-L1 in patients with CT-negative mTBI. Serum UCH-L1 and GFAP were longitudinally measured from 73 patients with acute orthopedic injury on arrival and on days 1, 2, 3, 7 after admission, and on the follow-up visit 3-10 months after the injury. The injury types were recorded, and 71% patients underwent also head MRI. The results were compared with those found in patients with CT-negative mTBI (n = 93). The levels of GFAP were higher in patients with acute orthopedic trauma than in patients with CT-negative mTBI (p = 0.026) on arrival; however, no differences were found on the following days. The levels of UCH-L1 were not significantly different between these two groups at any measured point of time. Levels of GFAP and UCH-L1 were not able to distinguish patients with CT-negative mTBI from patients with orthopedic trauma. Patients with orthopedic trauma and high levels of UCH-L1 or GFAP values may be falsely diagnosed as having a concomitant mTBI, predisposing them to unwarranted diagnostics and unnecessary brain imaging. This casts a significant doubt on the diagnostic value of GFAP and UCH-L1 in cases with mTBI