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

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

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

Suprathermal electrons at Saturn's bow shock

The leading explanation for the origin of galactic cosmic rays is particle acceleration at the shocks surrounding young supernova remnants (SNRs), although crucial aspects of the acceleration process are unclear. The similar collisionless plasma shocks frequently encountered by spacecraft in the solar wind are generally far weaker (lower Mach number) than these SNR shocks. However, the Cassini spacecraft has shown that the shock standing in the solar wind sunward of Saturn (Saturn's bow shock) can occasionally reach this high-Mach number astrophysical regime. In this regime Cassini has provided the first in situ evidence for electron acceleration under quasi-parallel upstream magnetic conditions. Here we present the full picture of suprathermal electrons at Saturn's bow shock revealed by Cassini. The downstream thermal electron distribution is resolved in all data taken by the low-energy electron detector (CAPS-ELS, <28 keV) during shock crossings, but the higher energy channels were at (or close to) background. The high-energy electron detector (MIMI-LEMMS, >18 keV) measured a suprathermal electron signature at 31 of 508 crossings, where typically only the lowest energy channels (<100 keV) were above background. We show that these results are consistent with theory in which the "injection" of thermal electrons into an acceleration process involves interaction with whistler waves at the shock front, and becomes possible for all upstream magnetic field orientations at high Mach numbers like those of the strong shocks around young SNRs. A future dedicated study will analyze the rare crossings with evidence for relativistic electrons (up to ~1 MeV).Comment: 22 pages, 5 figures. Accepted for publication in Ap

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

Assessing the accuracy of the heat flux measurement for the study of boiling phenomena

The present work quantifies numerically the systematic errors present in experimental infrared heat flux studies of boiling surfaces. A transient conduction model for multilayer structures is proposed to describe the periodic heat fluxes encountered on boiling surfaces. The results of the current work show that the systematic error behavior of the infrared method is not uniform but dependent on the frequency of the heat flux signal of the boiling surface; which is a novel finding. As the frequency of the heat flux signal increases, the errors in the measured phase of heat flux signals are expected to increase. The errors in the amplitude of heat flux signals sharply increase at low frequencies (1-10 Hz) and decrease as the frequency increases. The maximum errors in the phase and amplitude of heat flux signals are 9% and 23%, respectively in the frequency range of nucleate boiling (10-80 Hz). Based on the current analysis, it is concluded that the systematic errors found arise from assuming that thermal contact resistances of such systems are negligible. This is an assumption universally adopted 2 by the field. By considering and correcting for the thermal contact resistance in the measurement of heat fluxes, the maximum errors in the phase and the amplitude of heat flux signals can be reduced to 7% and 9%, respectively. The results are applied to experimental data ensembles from the published public domain. Finally, the current work provides general guidelines to improve systematic errors in the measurement of heat flux for the study of boiling using infrared thermography found in the literature