Publications
36 results found
Carnie J-T, Hardalupas I, Sergis A, 2024, Decarbonising building heating and cooling: designing a novel, inter-seasonal latent heat storage system, Journal of Renewable and Sustainable Energy Reviews, Vol: 189, ISSN: 1879-0690
The global heating and cooling demands have increased to mitigate the effects of the rise in extreme weather events due to climate change. This has led to an increase in global greenhouse gas emissions due to the use of fossil fuels to meet these demands. The current study evaluates how an alternative low-carbon heating and cooling system may provide thermal comfort in residential buildings, specifically in regions that have a humid temperature oceanic climate- for example, the United Kingdom. To meet the net-zero emissions targets set in the United Kingdom by 2050, greenhouse gas emissions generated from heating in residential buildings must fall by 95%. The leading decarbonisation strategy proposed by their government requires the electrification of the heating system through the installation of heat pumps. Consequently, the average electricity consumption per household is expected to increase. This will impose considerable pressure on electricity networks to source additional (ideally renewable) capacity, which will ultimately be costly. To circumvent this issue, the current study proposes a novel alternative method of providing nearly zero-carbon space and water heating, that can operate almost independently of the electricity grid. This requires the use of solar energy as the thermal energy source, and a solid-liquid phase change material as an inter-seasonal energy storage medium. A design optimisation study was thereafter carried forward to showcase the capability of such a system for a semi-detached house in London, United Kingdom.
Sergis A, Hardalupas Y, Flinders K, et al., 2023, A quantitative study on the thermal performance of self-modified heat transfer surfaces in high heat flux flow systems, International Journal of Heat and Mass Transfer, Vol: 215, ISSN: 0017-9310
The current work uses a novel fundamental heat transfer experiment to understand the morphological and thermal performance effects of nanoparticle deposition processes on heating surfaces under high heat fluxes. This is a unique fundamental study of nanosuspension induced nanoparticle coated boiling surfaces under realistic fusion relevant conditions. Al2O3–H2O nanosuspensions have been used under forced convection and boiling. The experiments were performed on a test bed able to simulate realistic fusion reactor heat flux. Nanosuspensions are found to deteriorate the cooling performance due to the formation of a complex self-assembled porous nanoparticle layer on the heating surfaces. This negative effect on thermal performance is irrespective of operation in nanoparticulate latent or pure coolants modes. For heat transfer in nanosuspensions, the increase of nanoparticle concentration reduced the observed negative thermal performance effects. Improvement of thermal performance beyond the break-even point, as witnessed for some conditions in the current work, could be potentially achieved by increasing the concentration of nanoparticles in the coolant. When the nanosuspension is removed and the heat transfer surfaces with the nanolayer deposit are washed and operated with pure liquids, it was discovered that the deposited layers survived and still affected (negatively) their heat transfer performance. The deposited layers are porous and are expected to extend the critical heat flux of surfaces in relevant industrial processes. The deposition process and the final thermal properties could be affected by several controlled parameters providing design opportunities for new or retrofitted applications that were otherwise inaccessible or unfeasible.
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, Vol: 148, Pages: 9361-9389, ISSN: 1388-6150
<jats:title>Abstract</jats:title><jats:p>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) Al<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>–water nanofluids. It was discovered that studies involving low nanoparticle concentrations (<jats:inline-formula><jats:alternatives><jats:tex-math>$$\phi $$</jats:tex-math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>ϕ</mml:mi> </mml:math></jats:alternatives></jats:inline-formula> ≤ 0<jats:italic>.</jats:italic>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 <jats:italic>d</jats:italic> < 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 underl
Yang D, Sergis A, Hardalupas I, 2023, Dynamics of forced flow boiling ebullition cycles at natural and artificial cavities, 11th International Conference on Multiphase Flow (ICMF 2023)
The bubble diameter evolution and growth time for forced flow boiling events at two natural and one artificial cavity are quantified. One of the natural cavities had depth of 18μm and a circular diameter of 12 μm and the second was smaller than the resolution of the confocal microscope (i.e. <0.25μm). The artificial cavity had diameter 200 μm and depth 975 μm. Even though the ebullition characteristics at the natural sites are similar to reports in the literature, the bubble evolution at the artificial cavity displays significant longer timescale and larger bubble sizes at lift-off. At the artificial cavity, the ebullition cycles are repeatable leading to lower fluctuations of ensemble averaged quantities than for natural cavities. The effect of cavity size on bubble characteristics is quantified. Finally, the results demonstrated that natural nucleation sites with size less than 0.25 μm can initiate boiling with superheat levels much lower than current expectations.
Bikos D, Samaras G, Charalambides M, et al., 2023, A micromechanical based finite element model approach to accurately predict the effective thermal properties of micro-aerated chocolate, Innovative Food Science and Emerging Technologies, Vol: 83, ISSN: 1466-8564
Micro-aeration is a method to modify the sensorial attributes of chocolate but also affects the material properties of chocolate, which in turn, determine its material response during manufacturing and oral processes. This study aims to define the effect of micro-aeration on the thermal properties of chocolate by considering the changes of chocolate microstructure due to micro-aeration. Micro-aeration was found to alter the chocolate microstructure creating a layer of a third phase at the porous interfaces, which is argued to consist of cocoa butter of higher melting properties. A multiscale Finite Element Model is developed, which was confirmed by macroscale heat transfer measurements, to parametrically simulate the structural changes of micro-porous chocolates at the microscale level and estimate their effective properties, such as thermal conductivity and specific heat capacity. The developed multiscale computational model simulates the porous chocolate as a two-phase (chocolate- pores) or three-phase material (chocolate-cocoa butter layer- pores). The investigation identified a new, complex transient thermal mechanism that controls the behaviour of micro-aerated chocolate during melting and solidification. The results showed a maximum 13% reduction of keff and 15% increase of Cpeff with 15% micro-aeration resulting to a slower transient heat transfer through the micro-aerated chocolate. The reason is that the micro-aerated chocolate can store a larger amount of thermal energy than its solid counterpart. This effect slows down the transient heat transfer rate in the chocolate and modifies melting/solidification rate and impacts sensorial attributes during oral processing and cooling during manufacturing.
Iqbal M, Sergis A, Hardalupas Y, 2022, Stability of Nanofluids, Fundamentals and Transport Properties of Nanofluids, Publisher: The Royal Society of Chemistry, Pages: 41-70, ISBN: 9781839164194
<jats:p>Stability is key to sustaining the colloidal properties of nanofluids and by extension the beneficial thermophysical properties they exhibit for practical applications. Nanofluid suspensions are typically prepared through ultrasonic dispersion of nanoparticles, either using low-power ultrasonic baths or high-power ultrasonic probes. It has been observed that high-power probes, although achieving nanoparticle size reduction in a short time, can also cause considerable aggregation of particles and hence reduction in colloidal stability with excessive application. This effect is not observed in low-power ultrasonic dispersion applications. This discrepancy and its sources are explored and explained in the current chapter, through consideration of particle breakup mechanisms (fragmentation versus erosion) and the fusion of particles due to high-velocity interparticle collisions. Stability is known to be linked to solution pH; for example a pH value far from the isoelectric point yields a surface charge in the dispersed phase, which enhances stability through coulombic repulsion. Ultrasonication has been observed to affect the pH of nanofluid solutions. High-power devices are unable to affect pH change in dilute alumina–water nanofluids (ϕ &lt; 0.01 vol%), whereas low-power devices can. This is hypothesised to be due to the dominant breakup mechanism, i.e., erosion in low-power baths versus fragmentation in high-power probes. Hence, to improve nanofluid stability, it is recommended to use low-power sonication where possible, and source nanoparticles in aqueous form. If a high-power ultrasonic probe must be used, the duration and amplitude should be reduced to avoid the induction of significant stability reduction.</jats:p>
Iqbal M, Sergis A, Hardalupas Y, 2022, Stability of nanofluids, Fundamentals and Transport Properties of Nanofluids, Editors: Murshed, Publisher: Royal Society of Chemistry, Pages: 41-70, ISBN: 978-1-83916-646-4
Stability is key to sustaining the colloidal properties of nanofluids and by extension the beneficial thermophysical properties they exhibit for practical applications. Nanofluid suspensions are typically prepared through ultrasonic dispersion of nanoparticles, either using low-power ultrasonic baths or high-power ultrasonic probes. It has been observed that high-power probes, although achieving nanoparticle size reduction in a short time, can also cause considerable aggregation of particles and hence reduction in colloidal stability with excessive application. This effect is not observed in low-power ultrasonic dispersion applications. This discrepancy and its sources are explored and explained in the current chapter, through consideration of particle breakup mechanisms (fragmentation versus erosion) and the fusion of particles due to high-velocity interparticle collisions. Stability is known to be linked to solution pH; for example a pH value far from the isoelectric point yields a surface charge in the dispersed phase, which enhances stability through coulombic repulsion. Ultrasonication has been observed to affect the pH of nanofluid solutions. High-power devices are unable to affect pH change in dilute alumina–water nanofluids (ϕ < 0.01 vol%), whereas low-power devices can. This is hypothesised to be due to the dominant breakup mechanism, i.e., erosion in low-power baths versus fragmentation in high-power probes. Hence, to improve nanofluid stability, it is recommended to use low-power sonication where possible, and source nanoparticles in aqueous form. If a high-power ultrasonic probe must be used, the duration and amplitude should be reduced to avoid the induction of significant stability reduction.
Bikos D, Samaras G, Charalambides M, et al., 2022, Experimental and numerical evaluation of the effect of micro-aeration on the thermal properties of chocolate, Food and Function, Vol: 13, Pages: 4993-5010, ISSN: 2042-6496
Thermal properties, such as thermal conductivity, specific heat capacity and latent heat, influence the melting and solidification of chocolate. The accurate prediction of these properties for micro-aerated chocolate products with varying levels of porosity ranging from 0% to 15% is beneficial for understanding and control of heat transfer mechanisms during chocolate manufacturing and food oral processing. The former process is important for the final quality of chocolate and the latter is associated with sensorial attributes, such as grittiness, melting time and flavour. This study proposes a novel multiscale Finite Element Model to accurately predict the temporal and spatial evolution of temperature across chocolate samples. The model is evaluated via heat transfer experiments at temperatures varying from 16 °C to 45 °C. Both experimental and numerical results suggest that the rate of heat transfer within the micro-aerated chocolate is reduced by 7% when the 15% micro-aerated chocolate is compared to its solid counterpart. More specifically, on average, the thermal conductivity decreased by 20% and specific heat capacity increased by 10% for 15% micro-aeration, suggesting that micro-pores act as thermal barriers to heat flow. The latter trend is unexpected for porous materials and thus the presence of a third phase at the pore’s interface is proposed which might store thermal energy leading to a delayed release to the chocolate system. The developed multiscale numerical model provides a design tool to create pore structures in chocolate with optimum melting or solidifying response.
Yang D, Sergis A, Hardalupas I, 2021, Bubble growth and departure from an artificial cavity during flow boiling, 7th Micro and Nano Flows Conference (MNF 2021)
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 ofthe 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.
Sergis A, Wade WG, Gallagher JE, et al., 2021, Mechanisms of Atomization from Rotary Dental Instruments and Its Mitigation., J Dent Res, Vol: 100, Pages: 261-267
Since the onset of coronavirus disease 2019, the potential risk of dental procedural generated spray emissions (including aerosols and splatters), for severe acute respiratory syndrome coronavirus 2 transmission, has challenged care providers and policy makers alike. New studies have described the production and dissemination of sprays during simulated dental procedures, but findings lack generalizability beyond their measurements setting. This study aims to describe the fundamental mechanisms associated with spray production from rotary dental instrumentation with particular focus on what are currently considered high-risk components-namely, the production of small droplets that may remain suspended in the room environment for extended periods and the dispersal of high-velocity droplets resulting in formites at distant surfaces. Procedural sprays were parametrically studied with variables including rotation speed, burr-to-tooth contact, and coolant premisting modified and visualized using high-speed imaging and broadband or monochromatic laser light-sheet illumination. Droplet velocities were estimated and probability density maps for all laser illuminated sprays generated. The impact of varying the coolant parameters on heating during instrumentation was considered. Complex structured sprays were produced by water-cooled rotary instruments, which, in the worst case of an air turbine, included droplet projection speeds in excess of 12 m/s and the formation of millions of small droplets that may remain suspended. Elimination of premisting (mixing of coolant water and air prior to burr contact) resulted in a significant reduction in small droplets, but radial atomization may still occur and is modified by burr-to-tooth contact. Spatial probability distribution mapping identified a threshold for rotation speeds for radial atomization between 80,000 and 100,000 rpm. In this operatory mode, cutting efficiency is reduced but sufficient coolant effectiveness
Sergis A, Hardalupas I, 2021, Rapid humidification device and process, WO/2021/032962
This is an international application published under the patent cooperation treaty (PCT) and the World Intellectual Property Organisation (WIPO).This invention relates to a system that may be used as a flow seeder or as a rapid humidifier. The invention also relates to a method of high capacity seeding or humidifying a fluid flow.
Kim M, Sergis A, Kim SJ, et al., 2020, Assessing the accuracy of the heat flux measurement for the study of boiling phenomena, International Journal of Heat and Mass Transfer, Vol: 148, ISSN: 0017-9310
The present work quantifies numerically the systematic errors present in experimentalinfrared heat flux studies of boiling surfaces. A transient conduction model for multilayerstructures is proposed to describe the periodic heat fluxes encountered on boiling surfaces. Theresults of the current work show that the systematic error behavior of the infrared method isnot uniform but dependent on the frequency of the heat flux signal of the boiling surface; whichis a novel finding. As the frequency of the heat flux signal increases, the errors in the measuredphase of heat flux signals are expected to increase. The errors in the amplitude of heat fluxsignals 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 currentanalysis, it is concluded that the systematic errors found arise from assuming that thermalcontact resistances of such systems are negligible. This is an assumption universally adopted 2by the field. By considering and correcting for the thermal contact resistance in themeasurement of heat fluxes, the maximum errors in the phase and the amplitude of heat fluxsignals can be reduced to 7% and 9%, respectively. The results are applied to experimental dataensembles from the published public domain. Finally, the current work provides generalguidelines to improve systematic errors in the measurement of heat flux for the study of boilingusing infrared thermography found in the literature.
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.
Sergis A, Hardalupas I, 2019, Analysis of thermophoretic effects in nanofluids, 1st International Conference on Nanofluids (ICNf) and the 2nd European Symposium on Nanofluids (ESNf), Publisher: Nanouptake COST Action
Nanofluids are binary mixtures of nanosized solid particles (<100nm) and liquids with volumetric concentrations usually less than 5% that have exhibited enhanced thermalcharacteristics. The underlying heat transfer mechanisms of their behaviour are unconfirmed. The overall impact of global domain-wide thermophoretic effects in nanofluids for different nanoparticle sizes is believed to be one of the driving mechanismsgiving rise to the observed phenomena. Thermophoresis is analysed with the use of a custom-made molecular dynamics simulation code that models the kinematic behaviour of a simplified nanofluid in a domain with a temperature gradient. Despite nanoparticles found to be significantly more mobile than fluid molecules, no net migration effects ofnanoparticles are discovered across the temperature gradient of the system for all nanoparticle sizes tested throughout the simulation. It has been concluded that even though local stochastic nanoparticle-level thermophoretic effects are important, global domain-wide directional thermophoretic forces have not been observed under these circumstances.
Kouloulias K, Sergis A, Hardalupas I, 2019, Optimisation of nanofluid properties for reduced in situ nanoparticle agglomeration, 1st International Conference on Nanofluids (ICNf2019) / 2nd European Symposium on Nanofluids (ESNf2019), Publisher: Nanouptake COST Action
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.
Kouloulias K, Sergis A, Hardalupas Y, 2019, Assessing the flow characteristics of nanofluids during turbulent natural convection, JOURNAL OF THERMAL ANALYSIS AND CALORIMETRY, Vol: 135, Pages: 3181-3189, ISSN: 1388-6150
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Sergis A, Hardalupas I, 2019, Analysis of Thermophoretic Effects in Nanofluids, 1st International Conference on Nanofluids (ICNf2019) / 2nd European Symposium on Nanofluids (ESNf2019)
Sergis A, Hardalupas I, Barrett T, 2018, Isothermal analysis of nanofluid flow inside HyperVapotrons using particle image velocimetry, NANOUPTAKE COST ACTION (CA 15119), Publisher: COST
Sergis A, Hardalupas Y, Barrett TR, 2018, Flow characteristics in HyperVapotron elements operating with nanofluids, Fusion Engineering and Design, Vol: 128, Pages: 182-187, ISSN: 0920-3796
HyperVapotrons are highly robust and efficient heat exchangers able to transfer high heat fluxes of the order of 10–20 MW/m2. They employ the Vapotron effect, a complex two phase heat transfer mechanism, which is strongly linked to the hydrodynamic structures present in the coolant flow inside the devices. HyperVapotrons are currently tested in the Joined European Torus (JET) and the Mega Amp Spherical Tokamak (MAST) fusion experiments and are considered a strong candidate for the International Thermonuclear Experimental Reactor (ITER). The efficiency of heat transfer and the reliability of the components of a fusion power plant are important factors to ensure its longevity and economical sustainability. Optimisation of the heat transfer performance of these devices by the use of nanofluids is investigated in this paper. Nanofluids are advanced two phase coolants that exhibit heat transfer augmentation phenomena. A cold isothermal nanofluid flow is established inside two HyperVapotron models representing the geometries used at JET and MAST. A hybrid particle image velocimetry (PIV) method is employed to measure the flow and create a dense velocity vector map of a region of interest. The vector spatial resolution is 30 μm. The instantaneous and mean flow structures of a nanofluid are compared to those present during the use of a traditional coolant (water) in order to detect any departure from the hydrodynamic design operational regime of the device. It was discovered that the flow field of the JET model is considerably affected when using nanofluids, while the flow in the MAST geometry does not change significantly by the introduction of nanofluids. Evidence of a shear thinning mechanism is found inside the momentum boundary layer of the nanofluid flows and it might be important to calculating the pumping power losses of a functional nuclear fusion power plant cooling system ran with nanofluids instead of water.
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Sergis A, Hardalupas I, Barrett T, 2017, Isothermal analysis of nanofluid flow inside HyperVapotrons using particle image velocimetry, Experimental Thermal and Fluid Science, Vol: 93, Pages: 32-44, ISSN: 0894-1777
Nanofluids are advanced two-phase coolants that exhibit heat transfer augmentation phenomena. Extensive research has been performed since the year 2000 onwards to understand the physical mechanisms of heat transfer in nanofluids when employed inside traditional heat exchanging geometries. The focus of this paper is to understand if and how the geometry of heat exchangers might be potentially affecting the nanofluid coolant flow boundary conditions established and how this might be hence further affecting their thermal characteristics. HyperVapotrons are highly robust and efficient heat exchangers able to transfer high heat fluxes of the order of 10–20 MW/m2. They employ a complex two-phase heat transfer mechanism which is strongly linked to the hydrodynamic structures present in the coolant flow inside the devices. A cold isothermal nanofluid flow is established inside two HyperVapotron model replicas. A high spatial resolution (30 μm) visualisation of the nanofluid flow fields inside each replica is measured and compared to those present during the use of a traditional coolant (water). Significant geometry specific changes are evident with the use of dilute nanofluids which is something unexpected and novel. Evidence of a shear thinning mechanism is found inside the momentum boundary layer of the nanofluid flows that might prove beneficial to the coolant pumping power losses when using nanofluids instead of water and is expected to affect their thermal performance from a hydrodynamic point of view.
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.
Sergis A, Hardalupas I, Barrett T, 2016, Flow Characteristics in HyperVapotron Elements Operating with Nanofluids, 26th Fusion Energy Conference
HyperVapotrons are highly robust and efficient heat exchangers able to transfer high heat fluxes of the order of 10-20MW/m2. They employ the Vapotron effect, a complex two phase heat transfer mechanism, which is strongly linked to the hydrodynamic structures present in the coolant flow inside the devices. HyperVapotrons are currently tested in the Joined European Torus (JET) and the Mega Amp Spherical Tokamak (MAST) fusion experiments and are considered a strong candidate for the International Thermonuclear Experimental Reactor (ITER). The efficiency of heat transfer and the reliability of the components of a fusion power plant are important factors to ensure its longevity and economical sustainability. Optimisation of the heat transfer performance of these devices by the use of nanofluids is investigated in this paper. Nanofluids are advanced two phase coolants that exhibit heat transfer augmentation phenomena. A cold isothermal nanofluid flow is established inside two HyperVapotron models representing the geometries used at JET and MAST. A hybrid particle image velocimetry method is then employed to map in high spatial resolution (30μm) the flow fields inside each replica. The instantaneous and mean flow structures of a nanofluid are compared to those present during the use of a traditional coolant (water) in order to detect any departure from the hydrodynamic design operational regime of the device. It was discovered that the flow field of the JET model is considerably affected when using nanofluids, while the flow in the MAST geometry does not change significantly by the introduction of nanofluids. Evidence of a shear thinning mechanism is found inside the momentum boundary layer of the nanofluid flows and it might be important to calculating the pumping power losses of a functional nuclear fusion power plant cooling system ran with nanofluids instead of water. This work is a continuation of a previous study on HyperVapotrons and nanofluids, as documented by
Kouloulias K, Sergis A, Hardalupas Y, 2016, Sedimentation in nanofluids during a natural convection experiment, INTERNATIONAL JOURNAL OF HEAT AND MASS TRANSFER, Vol: 101, Pages: 1193-1203, ISSN: 0017-9310
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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
Sergis A, Hardalupas Y, 2016, Nanofluids: potential as future coolants, CRC Concise Encyclopedia of Nanotechnology, Editors: Ildusovich Kharisov, Vasilievna Kharissova, Ortiz-Mendez, Publisher: CRC Press, ISBN: 9781466580343
Sergis A, Hardalupas Y, 2015, Revealing the complex conduction heat transfer mechanism of nanofluids, NANOSCALE RESEARCH LETTERS, Vol: 10, Pages: 1-9, ISSN: 1556-276X
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Sergis A, Resvanis K, Hardalupas Y, et al., 2015, Comparison of Measurements and Computations of Isothermal Flow Velocity inside HyperVapotrons, 28th Symposium on Fusion Technology (SOFT), Publisher: Elsevier, Pages: 353-356, ISSN: 1873-7196
HyperVapotrons are two-phase water-cooled heat exchangers able to receive high heat fluxes (HHF) by employing a cyclic phenomenon called the "Vapotron Effect". HyperVapotrons have been used routinely in HHF nuclear fusion applications. A detailed experimental investigation on the effect giving rise to the ability to sustain steady state heat fluxes in excess of 10MW/m2 has not yet been possible and hence the phenomenon is not yet well understood. The coolant flow structures that promote the effect have been a major point of interest, and many investigations based on Computational Fluid Dynamic (CFD) simulations have been performed in the past. The understanding of the physics of the coolant flow inside the device may hold the key to further optimisation of engineering designs. However, past computational investigations have not been experimentally evaluated. Isothermal flow velocity distribution measurements of the fluid flow in HyperVapotron optical models with high spatial resolution are performed in this paper. The same measurements are subsequently calculated via commercial CFD software. The isothermal CFD calculation is compared to the experimental velocity measurements to deduce the accuracy of the CFD investigations carried out. This unique comparison between computational and experimental results in HyperVapotrons will direct future efforts in analysing similar devices.
Sergis A, Hardalupas Y, Barrett TR, 2015, Isothermal velocity measurements in two HyperVapotron geometries using Particle Image Velocimetry (PIV), Experimental Thermal and Fluid Science, Vol: 61, Pages: 48-58, ISSN: 0894-1777
Sergis A, Hardalupas Y, 2014, Molecular Dynamic Simulations of a Simplified Nanofluid, Computational Methods in Science and Technology, Vol: 20, Pages: 113-127, ISSN: 1505-0602
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