Publications
8 results found
Iqbal M, Kouloulias K, Sergis A, et al., 2023, Critical analysis of thermal conductivity enhancement of alumina-water nanofluids, Journal of Thermal Analysis and Calorimetry: an international forum for thermal studies, Vol: 148, Pages: 9361-9389, ISSN: 1388-6150
Nanofluids are colloidal suspensions constituted of nanoparticles and typical heat transfer fluids which have shown potential in yielding enhanced heat transport for many applications. Significant attention has been paid to their thermal conductivity enhancement which has been alleged, in some cases, to exceed theoretical limits classifying the enhancement as “anomalous”. The present study aims to quantitatively investigate the nature of the enhancements reported in the literature and classify their alignment with theoretical predictions. To do so, a rigorous and objective mathematical analysis method has been employed. The novelty and value of the present work lies in the deeper characterisation and understanding of the anomalous observations reported. The present analytical study focuses on (spherical) Al2O3–water nanofluids. It was discovered that studies involving low nanoparticle concentrations (ϕ ≤ 0.2 vol%) and the use of electrostatic stabilisation (through pH control) as opposed to steric stabilisation (using surfactants) as suspension stability control methods are likely to report anomalous effects. An exceptional case was observed for d < 15 nm, where to achieve anomalous enhancement, surfactants and pH controllers should not be used to prevent significant interfacial resistance. The shared characteristics of these anomalous observations indicate that nanofluid preparation effects are linked to the underlying physical mechanisms of heat transfer involved and those should be further investigated. The failure of studies attempting to replicate anomalous thermal conductivity enhancement in the literature could hence be understood, as these did not satisfy the conditions required to lead to an anomalous enhancement. The role of measurement errors was also considered.
Kouloulias K, Sergis A, Hardalupas I, et al., 2019, Visualisation of subcooled pool boiling in nanofluids, Fusion Engineering and Design, Vol: 146, Pages: 153-156, ISSN: 0920-3796
High-performance cooling is of vital importance for the cutting-edge technology of today, from micro-electronic devices to nuclear reactors. Boiling heat transfer is expected to play a critical role for the safe and efficient operation of components exposed to high heat flux in future nuclear fusion reactors. Recent advances in nanotechnology have allowed the development of a new category of coolants, termed nanofluids, which exhibit superior thermophysical characteristics over traditional heat transfer fluids. Qualitative experimental results of Al2O3-H2O nanofluids under subcooled pool boiling conditions are reported and compared to deionised water that served as a benchmark in the current work. A visual evaluation of the impact of nanoparticles on bubble dynamics and nucleation site activity at the heated surface of a bare NiCr wire is performed with the use of a Guppy F-080 FireWire camera. It was observed that the presence of nanoparticles significantly modifies the nucleation site density, bubble size at departure and frequency of bubble generation from the surface of the heating wire. Intense nanoparticle deposition on the heating wire surface was identified as a key mechanism for the observed differences via scanning electron microscopy. The deposited nanolayer reported to alter the surface texture of the wire. The outcome of this work is a step forward towards the evaluation of the applicability of nanofluids in cooling applications via boiling heat transfer.
Kouloulias K, Sergis A, Hardalupas I, 2019, Assessing the flow characteristics of nanofluids during turbulent natural convection, Journal of Thermal Analysis and Calorimetry: an international forum for thermal studies, Vol: 135, Pages: 3181-3189, ISSN: 1388-6150
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.
Kouloulias K, Sergis A, Hardalupas Y, et al., 2017, Measurement of flow velocity during turbulent natural convection innanofluids, Fusion Engineering and Design, Vol: 123, Pages: 72-76, ISSN: 1873-7196
Increased cooling performance is eagerly required for many cutting edge engineering and industrial technologies. Nanofluids have attracted considerable interest due to their potential to enhance the thermal performance of conventional heat transfer fluids. However, heat transfer in nanofluids is a controversial research theme, since there is yet no conclusive answer to explain the underlying heat transfer mechanisms. This study investigates the physics behind the heat transfer behavior of Al2O3–H2O nanofluids under natural convection. A high spatial resolution flow velocimetry method – Particle Image Velocimetry – is employed in dilute nanofluids inside a Rayleigh-Benard configuration with appropriate optical access. The resulting mean velocity and flow structures of pure water and nanofluids are reported and their overall heat transfer performances are compared for Rayleigh numbers, Ra, of the order of 109. This paper aims to identify the contribution of the suspended nanoparticles on the heat and mass transfer mechanisms in low flow velocity applications, as those occurring during natural convection. The outcome of this work is a first step towards the evaluation of the applicability of nanofluids in applications where more complex heat transfer modes, namely boiling and Critical Heat Flux, are involved that are of great importance for the cooling of Fusion reactors.
Kouloulias K, Sergis A, Hardalupas I, 2016, Sedimentation in nanofluids during a natural convection experiment, International Journal of Heat and Mass Transfer, Vol: 101, Pages: 1193-1203, ISSN: 0017-9310
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 th
Kouloulias K, Sergis A, Hardalupas I, et al., 2016, Measurement of flow velocity during natural convection in nanofluids, 29th Symposium on Fusion Technology (SOFT)
Kouloulias K, Sergis A, Hardalupas Y, 2016, The influence of nanofluid PH on natural convection, 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics (HEFAT 2016), Publisher: EDAS
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 Al2O3– deionised (DI) H2O 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. Al2O3– H2O 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 ~ 109, 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
Sarwar J, Georgakis G, Kouloulias K, et al., 2015, Experimental and numerical investigation of the aperture size effect on the efficient solar energy harvesting for solar thermochemical applications, ENERGY CONVERSION AND MANAGEMENT, Vol: 92, Pages: 331-341, ISSN: 0196-8904
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