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

Experimental and numerical heat transfer studies of nanofluids with an emphasis on nuclear fusion applications

A nanofluid is a mixture of a low concentration of solid particles (10-100nm in size at concentrations below 10%vol.) and a carrier fluid (usually conventional coolants). These novel fluids exhibit anomalous heat transfer phenomena which cannot be explained using classical thermodynamic models. The fluids can be designed to offer unsurpassed heat transfer rates for heat transfer related applications at low costs of manufacturing. This PhD thesis describes the efforts to test whether these fluids can be utilised for high heat flux applications (similar to those encountered in proposed future fusion reactors) and also to discover the mechanisms which give rise to the phenomenal heat transfer enhancements observed. A broad metadata statistical analysis was performed on published literature which provided qualitative results regarding the heat transfer enhancement to be expected from nanofluids, indicated trends connecting by part mixture properties and heat transfer enhancement values exhibited and provided probable explanations of the heat transfer mechanisms involved. This study was performed to tackle the novelty and scientific uncertainty issues encountered in the field. Optical laser diagnostics experiments were performed on a high heat flux device (HyperVapotron) in isothermal conditions. The study provided extensive information regarding the flow structures formed inside the device using conventional coolants and nanofluids. This helped to both, understand the conventional operation of the device as well as review probable suitable geometries for the utilisation of the device using nanofluids. Finally, a Molecular Dynamics Simulation code was composed to model heat conduction through a basic nanofluid. The code results suggested the formulation of a new type of complex heat transfer mechanism that might explain the augmentation of heat transfer encountered experimentally. A new low cost high throughput platform (HTCondor®) has been used to run the code in order to demonstrate the capabilities of the system for less financially able institutions.Open Acces

Anomalous heat transfer modes of nanofluids: a review based on statistical analysis

This paper contains the results of a concise statistical review analysis of a large amount of publications regarding the anomalous heat transfer modes of nanofluids. The application of nanofluids as coolants is a novel practise with no established physical foundations explaining the observed anomalous heat transfer. As a consequence, traditional methods of performing a literature review may not be adequate in presenting objectively the results representing the bulk of the available literature. The current literature review analysis aims to resolve the problems faced by researchers in the past by employing an unbiased statistical analysis to present and reveal the current trends and general belief of the scientific community regarding the anomalous heat transfer modes of nanofluids. The thermal performance analysis indicated that statistically there exists a variable enhancement for conduction, convection/mixed heat transfer, pool boiling heat transfer and critical heat flux modes. The most popular proposed mechanisms in the literature to explain heat transfer in nanofluids are revealed, as well as possible trends between nanofluid properties and thermal performance. The review also suggests future experimentation to provide more conclusive answers to the control mechanisms and influential parameters of heat transfer in nanofluids

Large-scale solar wind flow around Saturn's nonaxisymmetric magnetosphere

The interaction between the solar wind and a magnetosphere is fundamental to the dynamics of a planetary system. Here, we address fundamental questions on the large-scale magnetosheath flow around Saturn using a 3D magnetohydrodynamic (MHD) simulation. We find Saturn's polar-flattened magnetosphere to channel ~20% more flow over the poles than around the flanks at the terminator. Further, we decompose the MHD forces responsible for accelerating the magnetosheath plasma to find the plasma pressure gradient as the dominant driver. This is by virtue of a high-beta magnetosheath, and in turn, the high-MA bow shock. Together with long-term magnetosheath data by the Cassini spacecraft, we present evidence of how nonaxisymmetry substantially alters the conditions further downstream at the magnetopause, crucial for understanding solar wind-magnetosphere interactions such as reconnection and shear flow-driven instabilities. We anticipate our results to provide a more accurate insight into the global conditions upstream of Saturn and the outer planets.Comment: Accepted for publication in Journal of Geophysical Journal: Space Physic

A combined model of pressure variations in Titan's plasma environment

In order to analyze varying plasma conditions upstream of Titan, we have combined a physical model of Saturn's plasmadisk with a geometrical model of the oscillating current sheet. During modeled oscillation phases where Titan is furthest from the current sheet, the main sources of plasma pressure in the near-Titan space are the magnetic pressure and, for disturbed conditions, the hot plasma pressure. When Titan is at the center of the sheet, the main sources are the dynamic pressure associated with Saturn's cold, subcorotating plasma and the hot plasma pressure under disturbed conditions. Total pressure at Titan (dynamic plus thermal plus magnetic) typically increases by a factor of up to about three as the current sheet center is approached. The predicted incident plasma flow direction deviates from the orbital plane of Titan by ≲10°. These results suggest a correlation between the location of magnetic pressure maxima and the oscillation phase of the plasmasheet. Our model may be used to predict near-Titan conditions from ‘far-field’ in situ measurements

Isothermal velocity measurements in two HyperVapotron geometries using Particle Image Velocimetry (PIV)

AbstractHyperVapotron beam stopping elements are high heat flux devices able to transfer large amounts of heat (of the order of 10–20MW/m2) efficiently and reliably making them strong candidates as plasma facing components for future nuclear fusion reactors or other applications where high heat flux transfer is required. They employ the Vapotron effect, a two phase complex heat transfer mechanism. The physics of operation of the device are not well understood and are believed to be strongly linked to the evolution of the flow fields of coolant flowing inside the grooves that form part of the design. An experimental study of the spatial and temporal behaviour of the flow field under isothermal conditions has been carried out on two replicas of HyperVapotron geometries taken from the Mega Amp Spherical Tokamak (MAST) and the Joint European Torus (JET) experiments. The models were tested under three isothermal operating conditions to collect coolant flow data and assess how the design and operational conditions might affect the thermal performance of the devices for single phase heat transfer. It was discovered that the in-groove speeds of MAST are lower and the flow structures less stable but less sensitive to free stream speed perturbations compared to the JET geometry. The MAST geometry was found to suffer from hydrodynamic end effects. A wake formation was discovered at the top of the groove entrance for the JET geometry, while this is absent from the MAST geometry. The wake does not affect significantly the mean operation of the device but it may affect the coolant pumping load of the device. For the JET variant, there is evidence that the typical operation with free stream flow speed of 6m/s is advantageous

Bubble growth and departure from an artificial cavity during flow boiling

Wall nucleation research has mainly focused on natural surface nucleation sites whose geometry is unknown and the effects of nucleation cavity geometry and size on flow boiling are not clear. The current research studies the effect of a blind hole with diameter 200 μm and depth 1 mm on bubble nucleation in a channel with water flow boiling. The boundary conditions were constant heat flux of 18.8 kW/m2, wall superheat of 8.7°C, water inlet temperature of 93.8°C and uniform velocity profile of 0.21 m/s at the inlet of the channel with cross-section of 30 mm x 10 mm, leading to a Reynolds number of 10038. High-speed imaging of the bubble behavior allowed the measurement of the bubble temporal and spatial evolution and quantified the bubble growth period and waiting period between departure and new growth and associated fluctuations. The bubble growth period reaches up to 40 seconds with a corresponding waiting time of 0.9 ms. It is observed that a wave front is induced by the breakage of the bubble neck which propagates through the bubble, resulting in distortions that serve as initial trigger of bubble movement along the nucleation wall

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