The influence of nanofluid PH on natural convection

The vast majority of experimental studies of nanofluids under natural convection have shown that the heat transfer rate decreases in contrast to observations of increased heat transfer rate for forced convection and boiling heat transfer. This surprising result has not been fully understood and the purpose of this study is to shed light on the physics behind the decrease of heat transfer in Al 2 O 3 – deionised (DI) H 2 O nanofluids under natural convection. A classical Rayleigh-Benard configuration has been employed, where the test medium is heated from the bottom and cooled from the top of an optically accessible chamber, while the sidewalls are insulated. Al 2 O 3 – H 2 O nanofluids with nanoparticle concentration within the range of 0.03 to 0.12 vol. % are used and tested under turbulent natural convection, Rayleigh number Ra ~ 10 9 , until steady state conditions are reached. For the synthesis of the nanofluid, pure DI water and high purity nanopowder, supplied by two different vendors, are involved with and without adopting the electrostatic stabilization method. The temperature measurements at different locations around the chamber allow the quantification of the natural convection heat transfer coefficient and the corresponding Nusselt and Rayleigh numbers. All the measured quantities are compared with those for DI water that serves as a benchmark in this study. It is found that the presence of nanoparticles systematically decreases the heat transfer performance of the base fluid under natural convection. An explanation for the reported degradation can be attributed to the buoyant and gravitational forces acting in the system that appear to be inadequate to ensure or maintain good nanofluid mixing. The results also show that as the nanoparticle concentration increases, the temperature of the heating plate increases, suggesting the presence of an additional thermal barrier imposed at the hot plate of the chamber. This can be attributed to the formation of a stationary thin layer structure of nanoparticles and liquid close to the heating plate that is qualitatively observed to increase in thickness as the nanoparticle concentration increases. The addition of a small amount of acetic acid to control the pH value of the nanofluid reduces the thickness of the thin layer structure close to the hot plate, leading to reduction of the rate of heat transfer decrease . A similar behaviour is observed when a different nanopowder that forms an acidic suspension is used. This behaviour is credited to the significantly increased nanofluid stability attained through the electrostatic stabilization method. Such a method takes advantage of the repulsive forces imposed due to the electric double layers that surround individual nanoparticles. The understanding of the influence of the nanofluid pH on the stability of nanosuspensions and its impact on heat transfer rate can lead to future guidelines for the effective use of nanofluids

Sedimentation in nanofluids during a natural convection experiment

This study presents an experimental investigation of the thermophysical behavior of γ-Al2O3–deionized (DI) H2O nanofluid under natural convection in the classical Rayleigh–Benard configuration, which consists of a cubic cell with conductive bottom and top plates, insulated sidewalls and optical access. The presence of nanoparticles either in stationary liquids or in flows affects the physical properties of the host fluids as well as the mechanisms and rate of heat and mass transfer. In the present work, measurements of heat transfer performance and thermophysical properties of Al2O3–H2O nanofluids, with nanoparticle concentration within the range of 0.01–0.12 vol.%, are compared to those for pure DI water that serves as a benchmark. The natural convective chamber induces thermal instability in the vertical direction in the test medium by heating the medium from below and cooling it from above. Fixed heat flux at the bottom hot plate and constant temperature at the top cold plate are the imposed boundary conditions. The Al2O3–H2O nanofluid is tested under different boundary conditions and various nanoparticle concentrations until steady state conditions are reached. It is found that while the Rayleigh number, Ra, increases with increasing nanoparticle concentration, the convective heat transfer coefficient and Nusselt number, Nu, decrease. This finding implies that the addition of Al2O3 nanoparticles deteriorates the heat transfer performance due to natural convection of the base fluid, mainly due to poor nanofluid stability. Also, as the nanoparticle concentration increases the temperature at the heating plate increases, suggesting fouling at the bottom surface; a stationary thin layer structure of nanoparticles and liquid seems to be formed close to the heating plate that is qualitatively observed to increase in thickness as the nanoparticle concentration increases. This layer structure imposes additional thermal insulation in the system and thus appears to be responsible in a big extend for the reported heat transfer degradation. Also, for relatively high nanoparticle concentrations of 0.06 and 0.12 vol.%, as the heating flux increases the rate of heat transfer deterioration increases. Specifically in the case of maximum nanoparticle concentration, 0.12 vol.%, when the turbulence intensity increases, by increasing the applied heat flux, the Nusselt number remains constant in comparison with lower nanoparticle concentrations. This behavior can be attributed mainly to the physical properties of the Al2O3 nanopowder used in this study and the resulting interactions between the heating plate and the nanoparticles

Assessing the flow characteristics of nanofluids during turbulent natural convection

High-performance cooling is of vital importance for the cutting-edge technology of today, from nanoelectronic mechanical systems to nuclear reactors. Advances in nanotechnology have allowed the development of a new category of coolants, termed nanofluids that have the potential to enhance the thermal performance of conventional heat transfer fluids. At the present time, nanofluids are a controversial research theme, since there is yet no conclusive answer to explain the underlying physical mechanisms of heat transfer. The current study investigates experimentally the heat and mass transfer behaviour of dilute Al2O3–H2O nanofluids under turbulent natural convection—Rayleigh number of the order of 109—in a cubic Rayleigh–Bénard cell with optical access. Traditional heat transfer measurements were combined with a velocimetry method to obtain a deeper understanding of the impact of nanoparticles on the heat transfer performance of the base fluid. Particle image velocimetry was employed to quantify the resulting mean velocity field and flow structures in dilute nanofluids under natural convection, at three parallel planes inside the cubic cell. All the results were compared with that for the base fluid, i.e. deionised water. It was observed that the presence of a minute amount of Al2O3 nanoparticles in deionised water, φv = 0.00026 vol.%, considerably modifies the mass transfer behaviour of the fluid in the bulk region of turbulent Rayleigh–Bénard convection. Simultaneously, the general heat transport, as expressed by the Nusselt number, remained unaffected within the experimental uncertainty

Sedimentation in nanofluids during a natural convection experiment

AbstractThis study presents an experimental investigation of the thermophysical behavior of γ-Al2O3–deionized (DI) H2O nanofluid under natural convection in the classical Rayleigh–Benard configuration, which consists of a cubic cell with conductive bottom and top plates, insulated sidewalls and optical access. The presence of nanoparticles either in stationary liquids or in flows affects the physical properties of the host fluids as well as the mechanisms and rate of heat and mass transfer. In the present work, measurements of heat transfer performance and thermophysical properties of Al2O3–H2O nanofluids, with nanoparticle concentration within the range of 0.01–0.12vol.%, are compared to those for pure DI water that serves as a benchmark. The natural convective chamber induces thermal instability in the vertical direction in the test medium by heating the medium from below and cooling it from above. Fixed heat flux at the bottom hot plate and constant temperature at the top cold plate are the imposed boundary conditions. The Al2O3–H2O nanofluid is tested under different boundary conditions and various nanoparticle concentrations until steady state conditions are reached. It is found that while the Rayleigh number, Ra, increases with increasing nanoparticle concentration, the convective heat transfer coefficient and Nusselt number, Nu, decrease. This finding implies that the addition of Al2O3 nanoparticles deteriorates the heat transfer performance due to natural convection of the base fluid, mainly due to poor nanofluid stability. Also, as the nanoparticle concentration increases the temperature at the heating plate increases, suggesting fouling at the bottom surface; a stationary thin layer structure of nanoparticles and liquid seems to be formed close to the heating plate that is qualitatively observed to increase in thickness as the nanoparticle concentration increases. This layer structure imposes additional thermal insulation in the system and thus appears to be responsible in a big extend for the reported heat transfer degradation. Also, for relatively high nanoparticle concentrations of 0.06 and 0.12vol.%, as the heating flux increases the rate of heat transfer deterioration increases. Specifically in the case of maximum nanoparticle concentration, 0.12vol.%, when the turbulence intensity increases, by increasing the applied heat flux, the Nusselt number remains constant in comparison with lower nanoparticle concentrations. This behavior can be attributed mainly to the physical properties of the Al2O3 nanopowder used in this study and the resulting interactions between the heating plate and the nanoparticles

Increased Expression of VEGF and CD31 in Postradiation Rectal Tissue: Implications for Radiation Proctitis

Background. Inflammation mediators related to radiation proctitis are partially elucidated, and neovascularization is thought to play a key role. Objectives. To investigate the expression of vascular endothelial growth factor (VEGF) and CD31 as angiogenetic markers in postradiation rectal tissue. Methods. Rectal mucosa biopsies from 11 patients who underwent irradiation for prostate cancer were examined immunohistochemically for the expression of VEGF and CD31 at three time settings—before, at the completion of, and 6 months after radiotherapy. VEGF expressing vascular endothelial cells and CD31 expressing microvessels were counted separately in 10 high-power fields (HPFs). VEGF vascular index (VEGF-VI) and microvascular density (MVD) were calculated as the mean number of VEGF positive cells per vessel or the mean number of vessels per HPF, respectively. Histological features were also evaluated. Results. VEGF-VI was significantly higher at the completion of radiotherapy (0.17 ± 0.15 versus 0.41 ± 0.24, P = 0.001) declining 6 months after. MVD increased significantly only 6 months after radiotherapy (7.3 ± 3.2 versus 10.5 ± 3.1, P < 0.005). The histopathological examination revealed inflammatory changes at the completion of radiotherapy regressing in the majority of cases 6 months after. Conclusions. Our results showed that in postradiation rectal biopsy specimens neoangiogenesis seems to be inflammation-related and constitutes a significant postradiation component of the tissue injury

ESTRO consensus guideline for target volume delineation in the setting of postmastectomy radiation therapy after implant-based immediate reconstruction for early stage breast cancer

© 2019 Elsevier B.V. Immediate breast reconstruction (IBR) rates after mastectomy are increasing. Postmastectomy radiation therapy (PMRT) contouring guidelines for target volumes in the setting of IBR are lacking. Therefore, many patients who have had IBR receive PMRT to target volumes similar to conventional simulator-based whole breast irradiation. The aim of this paper is to describe delineation guidelines for PMRT after implant-based IBR based on a thorough understanding of the surgical procedures, disease stage, patterns of recurrence and radiation techniques. They are based on a consensus endorsed by a global multidisciplinary group of breast cancer experts

Low dose cranial irradiation-induced cerebrovascular damage is reversible in mice

BACKGROUND: High-dose radiation-induced blood-brain barrier breakdown contributes to acute radiation toxicity syndrome and delayed brain injury, but there are few data on the effects of low dose cranial irradiation. Our goal was to measure blood-brain barrier changes after low (0.1 Gy), moderate (2 Gy) and high (10 Gy) dose irradiation under in vivo and in vitro conditions. METHODOLOGY: Cranial irradiation was performed on 10-day-old and 10-week-old mice. Blood-brain barrier permeability for Evans blue, body weight and number of peripheral mononuclear and circulating endothelial progenitor cells were evaluated 1, 4 and 26 weeks postirradiation. Barrier properties of primary mouse brain endothelial cells co-cultured with glial cells were determined by measurement of resistance and permeability for marker molecules and staining for interendothelial junctions. Endothelial senescence was determined by senescence associated β-galactosidase staining. PRINCIPLE FINDINGS: Extravasation of Evans blue increased in cerebrum and cerebellum in adult mice 1 week and in infant mice 4 weeks postirradiation at all treatment doses. Head irradiation with 10 Gy decreased body weight. The number of circulating endothelial progenitor cells in blood was decreased 1 day after irradiation with 0.1 and 2 Gy. Increase in the permeability of cultured brain endothelial monolayers for fluorescein and albumin was time- and radiation dose dependent and accompanied by changes in junctional immunostaining for claudin-5, ZO-1 and β-catenin. The number of cultured brain endothelial and glial cells decreased from third day of postirradiation and senescence in endothelial cells increased at 2 and 10 Gy. CONCLUSION: Not only high but low and moderate doses of cranial irradiation increase permeability of cerebral vessels in mice, but this effect is reversible by 6 months. In-vitro experiments suggest that irradiation changes junctional morphology, decreases cell number and causes senescence in brain endothelial cells

THE INFLUENCE OF NANOFLUID PH ON NATURAL CONVECTION

ABSTRACT The vast majority of experimental studies of nanofluids under natural convection have shown that the heat transfer rate decreases in contrast to observations of increased heat transfer rate for forced convection and boiling heat transfer. This surprising result has not been fully understood and the purpose of this study is to shed light on the physics behind the decrease of heat transfer in Al 2 O 3 -deionised (DI) H 2 O nanofluids under natural convection. A classical Rayleigh-Benard configuration has been employed, where the test medium is heated from the bottom and cooled from the top of an optically accessible chamber, while the sidewalls are insulated. Al 2 O 3 -H 2 O nanofluids with nanoparticle concentration within the range of 0.03 to 0.12 vol. % are used and tested under turbulent natural convection, Rayleigh number Ra ~ 10 9 , until steady state conditions are reached. For the synthesis of the nanofluid, pure DI water and high purity nanopowder, supplied by two different vendors, are involved with and without adopting the electrostatic stabilization method. The temperature measurements at different locations around the chamber allow the quantification of the natural convection heat transfer coefficient and the corresponding Nusselt and Rayleigh numbers. All the measured quantities are compared with those for DI water that serves as a benchmark in this study. It is found that the presence of nanoparticles systematically decreases the heat transfer performance of the base fluid under natural convection. An explanation for the reported degradation can be attributed to the buoyant and gravitational forces acting in the system that appear to be inadequate to ensure or maintain good nanofluid mixing. The results also show that as the nanoparticle concentration increases, the temperature of the heating plate increases, suggesting the presence of an additional thermal barrier imposed at the hot plate of the chamber. This can be attributed to the formation of a stationary thin layer structure of nanoparticles and liquid close to the heating plate that is qualitatively observed to increase in thickness as the nanoparticle concentration increases. The addition of a small amount of acetic acid to control the pH value of the nanofluid reduces the thickness of the thin layer structure close to the hot plate, leading to reduction of the rate of heat transfer decrease. A similar behaviour is observed when a different nanopowder that forms an acidic suspension is used. This behaviour is credited to the significantly increased nanofluid stability attained through the electrostatic stabilization method. Such a method takes advantage of the repulsive forces imposed due to the electric double layers that surround individual nanoparticles. The understanding of the influence of the nanofluid pH on the stability of nanosuspensions and its impact on heat transfer rate can lead to future guidelines for the effective use of nanofluids

Optimisation of nanofluid properties for reduced in situ nanoparticle agglomeration

For the formulation of stable and durable nanofluidsattention should be paid on the preparation method. Key parameters to consider include the characteristics of the nanoparticles, the properties of the base fluids,the presence of surface-active agentsand the pH value of the nanofluids. An optimisation study of nanofluid properties was conducted tominimise nanoparticle agglomeration, thus reducing sedimentation during turbulent Rayleigh-Bénard convection(RBC). Three types of Al2O3-H2O nanofluidswith different consistencies and characteristics were prepared, with andwithout employing the electrostatic stabilisation method, tested and comparedin terms of the natural convective heat transfer performance. Transmission electron microscopy (TEM) and dynamic light scattering (DLS) measurements were performedtoobtain a spherical view of the nature and the characteristics of the nanofluids.It is reported that the electrostatic stabilization method and the proper selection of nanoparticles can improve the nanofluid stability, by reducing the nanoparticle agglomerationand improve the natural convectiveheat transfer performance of nanofluids. By comparing data from the TEM and DLS nanoparticle analysis, it is concluded that the average size of nanoparticles in the synthesized nanofluids is questionablein practice. Asystematic approach for the formulation of nanofluids and presentation of datais needed, so that reliable, reproducible and comparable results can be obtained that could eliminate the existing discrepancy among the findings in the literature