Publications
281 results found
Downing GH, Hardalupas Y, 2024, Spectral characteristics of particle preferential concentration in turbulence computed by Eulerian and Lagrangian methods, International Journal of Multiphase Flow, Vol: 174, ISSN: 0301-9322
This study conducts a comprehensive analysis of the Fourier spectra characteristics of particle preferential concentration in turbulent flows obtained by Eulerian and Lagrangian modelling approaches for different Stokes numbers. Particle preferential concentration is characterized by clusters and voids in the spatial particle distributions formed in turbulent flows and can significantly influence processes such as droplet coalescence, evaporation, and gas-particle reactions. The research primarily focuses on comparing the performance of two Eulerian models—one with and one without second-order velocity moments—and one Lagrangian model, which are used to predict particle dispersion. Key findings include the impact of artificial diffusion in Eulerian methods, the superiority of methods incorporating second-order velocity moments contributions at higher Stokes numbers, and the sensitivity of the Lagrangian method to Poisson statistics. Notably, the study reveals minimal variation in mean velocity spectra across different Stokes numbers, as opposed to the marked variations in particle concentration, momentum, and energy spectra. Grid resolution emerges as a crucial factor in enhancing spectral energy predictions in Eulerian methods. The research underscores the nuanced distinctions between Eulerian and Lagrangian methods in modelling preferential concentration, providing a detailed spectral comparison of particle concentration, velocity, momentum, and energy, highlighting the importance of method selection based on specific modelling needs.
Samaras G, Bikos D, Cann P, et al., 2024, A multiscale finite element analysis model for predicting the effect of micro-aeration on the fragmentation of chocolate during the first bite, European Journal of Mechanics A: Solids, Vol: 104, ISSN: 0997-7538
The emerging need to reduce the calorific value of foods, while simultaneously improving the consumerperception drives the quest for developing new food structures that satisfy both criteria. Aiming to shedlight on the influence that micro-aeration has on the breakdown of chocolate during the early stages of theoral processing, this paper summarises the development of multi scale, in silico Finite Element (FE) modelsfor the first bite. A micro-mechanical analysis was first employed to predict the impact on the mechanicalproperties of chocolate at two microaeration levels, i.e. π = 10vol% and π = 15vol%. The estimated elastic,plastic and fracture properties from the micromechanical model were subsequently fed into a macroscopicsimulation of the first bite. Both micromechanical and the macromechanical models for the 10vol% and 15vol%porosity chocolate are compared to experimental data for validation purposes. The micromechanical modelsare compared to data from literature on mechanical testing of the same two chocolate materials whereas thefirst bite macromechanical model was compared to in vitro experimental data obtained in this study using a3D printed molar teeth test rig mounted to a mechanical tester. Finally, the particle size distribution of thefragmented chocolate during the first bite was estimated from the in silico model and compared to in vivoliterature data on the same chocolate materials and in vitro experimental data from this work. All comparisonsbetween the in silico models and the in vitro/in vivo data led to good agreement. Our modelling methodologyprovides a cost-efficient tool for the investigation of new food structures that reduce the calorific value whileenhancing the taste perception.
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.
Wang T, Hardalupas I, 2023, Combined optical connectivity and optical flow velocimetry measurement of interfacial velocity of a liquid jet in gas crossflow, International Journal of Multiphase Flow, Vol: 168, Pages: 1-18, ISSN: 0301-9322
Liquid jet in crossflow (LJIC) is a process in which a high-speed gas crossflow deforms and shears acontinuous liquid flow into tiny droplets. This study quantifies the liquid surface motion of LJIC duringthe primary breakup process, which has not been quantified due to the optical limitation close to thenozzle exit. The interfacial velocity of a breaking liquid jet indicates the local interaction of the gas andliquid flows and determines the initial velocity of the stripped droplets. The local interfacial liquidvelocities of LJIC have only been estimated from theoretical and computational studies, which have notbeen evaluated from measurements. Optical Connectivity (OC) introduces a laser beam through anatomiser nozzle and relies on total internal reflection at the liquid interface to propagate the laser lightinside the continuous liquid to record the instantaneous features of the interface of the continuous liquidduring the primary atomisation at the near nozzle region through imaging of the emitted fluorescentintensity from the liquid flow. The current study combines Optical Connectivity with Optical FlowVelocimetry (OFV) to quantify the time-dependent, local interfacial velocity of the liquid interfacestructures of the LJIC for gas Weber numbers between 14.9 - 112.6 and liquid-to-gas momentum ratiosbetween 2.1 - 36.4. The combined OC-OFV measurements of the spatial distribution of the mean andfluctuating values of the different components of the liquid interfacial velocity of LJIC demonstratehow the gaseous shear and liquid jet geometry interact to influence the atomisation process.
Wang T, Liu Y, Chen C, et al., 2023, Combined two-photon optical connectivity and planar laser induced fluorescence for instantaneous characterisation of liquid interface during primary atomisation, Experimental Thermal and Fluid Science, Vol: 147, Pages: 1-15, ISSN: 0894-1777
A technique that combines two-photon excitation (TPE), planar laser induced fluorescence (PLIF) and optical connectivity (OC) has been applied to capture the instantaneous geometry of a cross-section normal to the main direction of the core of a liquid jet exposed to a crossflow of air during atomisation. This paper demonstrates that a nanosecond pulse laser can excite two-photon fluorescence in an atomising Rhodamine B-dyed water jet. It shows that TPE-PLIF on its own has limitations to capture the liquid interface during air-crossflow atomisation. The combination of TPE-PLIF and TPE-OC excites TPE-fluorescence at a targeted cross-section normal to the liquid jet axis at a fixed distance from the liquid jet exit and eliminates the limitations of the individual techniques. The optimisation of the instrumentation of the combined techniques and the associated image processing are described and the quantification of the instantaneous cross-section structures, developing during atomisation, on the surface of the liquid jet is demonstrated.
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 (Ο ≤ 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.
Bättig RJ, Jasper T, Hardalupas Y, et al., 2023, Zero Emission Hydrogen Internal Combustion Engine for a 5 kW Mobile Power Generator: Conversion Strategy for Carburetted SI Engines
A carburetted, spark ignited gasoline fuelled engine of a 5 kW rated power generator was converted to run on hydrogen. As opposed to large parts of current research, the engine conversion's foremost goal was not to maximise efficiency and power output but rather to find a cost-effective and low-complexity conversion approach to introduce clean fuels to existing engines. To allow for the increased volumetric fuel flow, the riser of the original carburettor was enlarged. The hydrogen flow into the venturi was metered with the help of a pressure regulator from a widely available conversion kit. The effects of different hydrogen-fuel-feed pressures on engine performance, operational stability and emission levels were examined experimentally. It was found that the hydrogen-line pressure before startup has to be set precisely (±5 mbar) to allow for stable and emission free operation. While the rated power of the converted engine was lowered to 57% of the original's generator, efficiency was increased by up to 9 percentage points while lowering NOx emissions drastically. In fact, under stable hydrogen supply conditions, it was shown that an average concentration of only 19 ppm NOx is feasible without any exhaust gas aftertreatment. An analytic derivation of lambda values from emission data has been examined. With a fairly constant offset of 4.3% between the measured and calculated lambda value, the stoichiometrically derived formula for lambda proved to yield precise results with little calculation effort. As a main limiting factor for continuous low emission levels, pressure fluctuations in the hydrogen supply line were identified to strongly influence the fuel flow rate leading to inconsistent air-to-fuel ratios. Furthermore, large, abrupt reductions in engine load were identified to be critical regarding running stability and backfiring events.
Samaras G, Bikos D, Skamniotis C, et al., 2023, Experimental and computational models for simulating the oral breakdown of food due to the interaction with molar teeth during the first bite, Extreme Mechanics Letters, Vol: 62, Pages: 1-11, ISSN: 2352-4316
The first bite involves the structural breakdown of foods due to the interaction with teeth and is a crucial process in oral processing. Although in vitro experiments are useful in predicting the oral response of food, they do not facilitate a mechanistic understanding of the relationship between the intrinsic food mechanical properties and the food behaviour in the oral cavity. Computer simulations, on the other hand, allow for such links to be established, offering a promising design alternative that will reduce the need for time consuming and costly in vivo and in vitro trials. Developing virtual models of ductile fracture in soft materials, such as food, with random and non-predefined crack morphology imposes many challenges. One of the most important is to derive results that do not depend on numerical parameters, such as Finite Element (FE) mesh density, but only physical constants obtained through independent standard mechanical tests, such as fracture strain and/or critical energy release rate. We demonstrate here that this challenge can be overcome if a non-local damage approach is used within the FE framework. We develop a first bite FE modelling methodology that provides mesh independent results which are also in agreement with physical first bite experiments performed on chocolate. The model accounts for key features found in chocolate and a wide range of compliant media, such as rate dependent plasticity and pressure dependent fracture initiation strain. As a result, our computational methodology can prove valuable in studying food structure-function relationships that are essential in product development.
Sergis A, Hardalupas I, Barrett T, 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, Pages: 1-10, 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.
Kolokotronis D, Sahu S, Hardalupas I, et al., 2023, Bulk cavitation in model gasoline injectors and their correlation with the instantaneous liquid flow field, Fluids, Vol: 8, Pages: 1-24, ISSN: 2311-5521
It is well established that spray characteristics from automotive injectors depend on, among other factors, whether cavitation arises in the injector nozzle. Bulk cavitation, which refers to the cavitation development distant from walls and thus far from the streamline curvature associated with salient points on a wall, has not been thoroughly investigated experimentally in injector nozzles. Consequently, it is not clear what is causing this phenomenon. The research objective of this study was to visualize cavitation in three different injector models (designated as Type A, Type B, and Type C) and quantify the liquid flow field in relation to the bulk cavitation phenomenon. In all models, bulk cavitation was present. We expected this bulk cavitation to be associated with a swirling flow with its axis parallel to that of the nozzle. However, liquid velocity measurements obtained through particle image velocimetry (PIV) demonstrated the absence of a swirling flow structure in the mean flow field just upstream of the nozzle exit, at a plane normal to the hypothetical axis of the injector. Consequently, we applied proper orthogonal decomposition (POD) to analyze the instantaneous liquid velocity data records in order to capture the dominant coherent structures potentially related to cavitation. It was found that the most energetic mode of the liquid flow field corresponded to the expected instantaneous swirling flow structure when bulk cavitation was present in the flow.
Goswami A, Hardalupas I, 2023, Simultaneous impact of droplet pairs on solid surfaces, Journal of Fluid Mechanics, Vol: 961, Pages: 1-37, ISSN: 0022-1120
This study investigates the dynamics of the simultaneous impact of two droplets on a dry substrate. We develop a new micro-controlled droplet generator that releases two equally sized water droplets simultaneously on-demand, with no trailing droplets. The impact Weber number, based on impact droplet size and velocity, and the inter-droplet spacing relative to the impact droplet size are varied in the ranges of 54 to 155 and 1.32 to 2.25, respectively, leading to the strong interaction of the spreading lamellae that form a central uprising sheet, which eventually deposits or breaks into tiny droplets. We analyse the impact processes for both deposition and splashing of the uprising sheets. Simultaneous high-speed imaging from two orthogonal views of the droplet impacts quantifies the three-dimensional structure of the sheet morphology, including the temporal evolution of the rim-bounded ‘semilunar’ shape, surface waves, rim corrugations and finger formation, and deposition or splashing of the liquid sheet. The characteristics of the sheet surface waves and the rim instabilities are quantified. Novel scaling is developed for the maximum sheet height, sheet width and thickness, which considers the geometrical constraints and mass balance of the interacting lamellae to describe the temporal evolution of a ‘semilunar’ uprising sheet and is in good agreement with the measurements. The uprising sheet splashing generates larger droplets than those from splashing of single-droplet impacts, and it occurs due to the end-pinching of sheet fingers and at conditions that single-droplet impacts lead only to liquid deposition.
Goswami A, Hardalupas I, 2023, Central sheet formation from simultaneous impacts of two droplets on a dry substrate, 11th International Conference on Multiphase Flow (ICMF 2023)
This study investigates the temporal and spatial evolution of a central sheet formed due to the interaction of lamellas of simultaneous impacts of two droplets on a dry substrate. A droplet generator released two equal-sized droplets simultaneously on demand. The impact Weber number and the inter-droplet spacing were varied between 54 to 128 and 1.64D0 to 2.25D0(D0: droplet diameter) respectively to study both deposition and splashing of the central uprising sheets. The sheet temporal evolution reveals its uprising ‘semilunar’ shape, cusps and finger formation, and eventual deposition or splashing behavior. The maximum height of the ‘semilunar’ uprising sheet scales with the inertial force at the lamella-lamella impact location. Forcases leading to splashing, rim fingers eject secondary droplets via an end-pinching mechanism and the sizes of the ejected droplets are larger than those commonly observed for equivalent single droplet impacts.
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, Cann P, et al., 2023, Destructive and non-destructive mechanical characterisation of chocolate with different levels of porosity under various modes of deformation, Journal of Materials Science, Vol: 58, Pages: 5104-5127, ISSN: 0022-2461
Chocolate exhibits a complex material response under the varying mechanical loads present during oral processing. Mechanical properties such as Young’s modulus and fracture stress are linked to sensorial attributes such as hardness. Apart from this link with hardness perception, these mechanical properties are important input parameters towards developing a computational model to simulate the first bite. This study aims to determine the mechanical properties of chocolate with different levels of micro-aeration, 0–15%, under varying modes of deformation. Therefore, destructive mechanical experiments under tension, compression, and flexure loading are conducted to calculate the Young’s modulus, yield, and fracture stress of chocolate. The values of Young’s modulus are also confirmed by independent ultrasonic mechanical experiments. The results showed that differences up to 35% were observed amongst the Young’s modulus of chocolate for different mechanical experiments. This maximum difference was found to drop with increasing porosity and a negligible difference in the Young’s modulus measurements amongst the different mechanical experiments is observed for the 15% micro-aerated chocolate. This phenomenon is caused by micro-pores obstructing the microscopic inelastic movement occurring from the early stages of the material’s deformation. This work provides a deeper understanding of the mechanical behaviour of chocolate under different loading scenarios, which are relevant to the multiaxial loading during mastication, and the role of micro-aeration on the mechanical response of chocolate. This will further assist the food industry’s understanding of the design of chocolate products with controlled and/or improved sensory perception.
Castanet G, Hardalupas Y, 2023, Two-photon fluorescence lifetime imaging applied to the measurement of the droplet temperature in sprays, Measurement of instantaneous fully 3D scalar dissipation rate in a turbulent swirling flow
<jats:p> <jats:bold>Two-photon fluorescence lifetime imaging applied to the measurement of the droplet temperature in sprays</jats:bold> </jats:p> <jats:p>Droplets temperature is a key parameter for the study of heat and mass transfers in many spray applications. Time correlated single photons counting (TCSPC) is applied to monitor the fluorescence decay and determine the droplet temperature in the mixing zone of two sprays which are injected with significantly different temperatures. For some well-chosen fluorescent dye, like rhodamine B (RhB), the fluorescence lifetime strongly varies with the temperature. Provided sufficiently different fluorescence lifetimes for the droplets of the two sprays, the fluorescence decay is expected to follow a multiple exponential decay. In this study, different approaches are tested for measuring the temperature of the two sprays as well as their mixing fraction based on the analysis of the fluorescence decay. Firstly, the measurement of the mixture fraction alone is tested by considering a configuration where one spray is seeded with eosin Y (EY) and the other with rhodamine 6G (Rh6G). Given the very different lifetimes of these dyes, which are not temperature dependent, the fluorescence decay is function of the volume fraction of liquid from each spray in these tests. A calibration is necessary to evaluate the mixing fraction. Both sprays are mounted on an automated platform allowing 3D scanning and motions which allows obtaining maps of the fluorescence decay. The out-of-field fluorescence, observed in dense sprays when fluorescence is induced by one-photon absorption, is suppressed by using a two-photon fluorescence excitation. This approach significantly improves the spatial resolution of the measurements. Finally, both the droplet temperature and the mixing fraction are measured simultaneously using a single dye, namely RhB, whose fluorescence lifetime is temperature depen
Qiu S, Karlis E, Taylor AMKP, et al., 2023, On the aerodynamic instability in a swirl combustor operating with lean premixed hydrogen-enriched methane blend, 12th Mediterranean Combustion Symposium (MCS 2023), Publisher: The Combustion Institute
It is important to be able to understand and control the transition to strong thermoacoustic oscillation associated with the swirl combustor of, e.g., gas turbines. We focus on the contribution of aerodynamic instability to the onset of thermoacoustic oscillation on a 33kW swirl stabilized combustor operating with lean premixed H2-enriched CH4 blend. Global CH* chemiluminescence and dynamic pressure shows that the combustion transits from a quiescent to a limit-cycle state as the H2 molar concentration increases from 20% to 40%. The global (CH4 and H2) equivalence ratio was kept at 0.55 and the thermal heat release was almost constant at 33kW. Linear stability analysis is applied to solve the linear perturbation mode of the non-reacting swirling jet, and shows the existence of a dominant, helical mode perturbation and a secondary, center mode perturbation. This is confirmed experimentally by using proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) to extract coherent flow structure from time-resolved PIV snapshots of both non-reactive and reactive flow. It isfound that the helical mode gives rise to the precessing vortex core (PVC) by inducing a sequence of downstream propagating vortical perturbations. The helical mode and its induced PVC motion is found to diminish as the system transits from quiescent state to a limit-cycle oscillation. In the transitioning case, a streak-structure perturbation, which resembles the center mode, becomes dominant. The center mode is distinguished from the helical mode in that it is localized near the centerline of the swirl and is marginally stable under non-reacting condition. It is inferred from the present research that the PVC motion is not the only coherent flow structure that the flame interacts with to trigger thermoacoustic oscillation; the center mode may play a role in the triggering event, which needs further investigation.
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 (Ο &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 (Ο < 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.
Downing G, Hardalupas I, Archer J, et al., 2022, Computational and experimental study of aerosol dispersion in a ventilated room, Aerosol Science and Technology, Vol: 57, Pages: 50-62, ISSN: 0278-6826
For many respiratory diseases, a primary mode of transmission is inhalation via aerosols and droplets. The COVID-19 pandemic has accelerated studies of aerosol dispersion in indoor environments. Most studies of aerosol dispersion present computational fluid dynamics results, which rarely include detailed experimental verification, and many of the computations are complex, making them hard to scale to larger spaces. This study presents a comparison of computational simulations and measurements of aerosol dispersion within a typical ventilated classroom. Measurements were accomplished using a custom-built low-cost sensor network composed of 15 commercially available optical particle sizers, which provided size-resolved information about the number concentrations and temporal dynamics of 0.3–40 µm diameter particles. Measurement results are compared to the computed dispersal and loss rates from a steady-state Reynolds-Averaged Navier–Stokes k-epsilon model. The results show that a newly developed aerosol-transport-model can accurately simulate the dispersion of aerosols and faithfully predict measured aerosol concentrations at different locations and times. The computational model was developed with scalability in mind such that it may be adapted for larger spaces. The experiments highlight that the fraction of aerosol recycled in the ventilation system depends on the aerosol droplet size and cannot be predicted by the recycled-to-outside air ratio. Moreover, aerosol recirculation is not negligible, as some computational approaches assume. Both modeling and measurements show that, depending on the location within the room, the maximum aerosol concentration can be many times higher than the average concentration, increasing the risk of infection.
Goswami A, Hardalupas I, 2022, Simultaneous Impact of two droplets on a solid surface: central sheet evolution and splashing, 75th Annual Meeting of the American Physical Society (APS), Publisher: American Physical Society, ISSN: 0003-0503
Horwich M, Hardalupas I, 2022, Obstruction of spreading droplets on a flat surface by non-deformable objects, 11th International Conference on Multiphase Flow (ICMF 2023)
Droplet impact onto a solid surface in the vicinity of a sessile solid cylinder is investigated. Water droplets impacted an acrylic surface with Weber numbers in the range of 406 – 616. The radius of the cylinder (0.52, 0.28 and 0.23 mm, ratios to droplet radius of 0.12 – 0.39) and the distance between the droplet impact point and the cylinder (1.75 – 10.57 mm, ratios to dropletradius of 1.26 – 5.53) were varied. Two high speed cameras captured the droplet temporal evolution simultaneously from two views and image analysis extracted the maximum spreading radius, number of corrugations and behaviour of ligament length along the rim at the circumference of the lamella. We observe a dry ‘V-shaped’ area behind the cylinder and discuss the conditions under which the lamella is able to re-join around the rear of the cylinder.
Downing G, Hardalupas I, 2022, Eulerian computation of droplet preferential concentration in turbulence, 11th International Conference on Multiphase Flow (ICMF 2023)
An expanded Eulerian method has been developed for the simulation of droplet preferential concentration in turbulence. The expanded Eulerian approach includes velocity fluctuation terms, which extends the applicability of the method to partially responsive droplet sizes with high Stokes numbers. The new Eulerian method is more computationally efficient than currentLagrangian methods simulating preferential concentration.
Wang T, Hardalupas I, 2022, Measurement of Interfacial velocity along the surface of a liquid jet in gas crossflow during primary breakup, 11th International Conference on Multiphase Flow (ICMF 2023)
The interfacial velocity along the surface of a breaking liquid jet in gas crossflow is measured using combined Optical Connectivity (OC) and Optical Flow Velocimetry (OFV) techniques. The interfacial motion on the surface structures is reported for a range of gas Weber numbers and momentum ratios. The interfacial liquid jet geometry was decomposed into individualProper Orthogonal Decomposition (POD) modes, including the contributions of flapping and wave motions, which characterise the oscillating and windward wave behavior of the liquid jet. The liquid jet images were reconstructed by using selected POD modes, which identified only the corresponding surface structures and quantified their lengthscales. By tracking these surface structures in time, the velocity of the different surface structures was quantified. The results showed that surface structures withdifferent wavelengths moved with different velocities and led a new non-linear atomization model.
Mulla I, Hardalupas I, 2022, Measurement of instantaneous fully 3D scalar dissipation rate in a turbulent swirling flow, Experiments in Fluids: experimental methods and their applications to fluid flow, Vol: 63, ISSN: 0723-4864
This paper describes the measurement methodology for quantifying the instantaneous full 3D scalar dissipation rate (SDR or χ) in order to characterize the rate of mixing. Measurements are performed in a near field of a jet-in-swirling-coflow configuration. All three components of χ are measured using a dual-plane acetone planar laser-induced fluorescence technique. To minimize noise, a Wiener filtering approach is used. The out-of-plane SDR component (χ3) is validated by assuming isotropy between axial and azimuthal components of SDR. An optimum laser-sheet separation distance (Δs) is identified by comparing the SDR components on the basis of instantaneous, mean, and probability density function data. The in-plane resolution needs to match the Batchelor scale (λB) for the central difference scheme-based SDR deduction. However, the out-of-plane resolution, Δs, requirement is different owing to the use of two-point difference based SDR and systematic biases. The optimum Δs is found to be 2.5λB. Finally, measurement guidelines are provided to assess the accuracy of 3D SDR measurements.
Emerique C, Jasper T, Hardalupas I, et al., 2022, Converting a production 6-cylinder Heavy Duty Engine to Dual-fuel mode operation using supervisory calibration and manifold injection, 10th International Conference on Modeling and Diagnostics for Advanced Engine Systems (COMODIA 2022)
Carbon dioxide emissions from heavy-duty vehicles can be reduced by converting, or retrofitting, conventional diesel engines into “dual fuel” mode operation using hydrogen and diesel fuel. Conversion can involve small and relatively cheap changes to design, thereby allowing a diesel engine to be fuelled by either conventionally, using diesel fuel (directinjection), or in conjunction with hydrogen (indirect injection – "fumigation”) in the intake manifold. We evaluate the conversion of a production, 6-cylinder 12.7L truck engine to run in dual fuel mode. Commercially viable conversions would permit, in the short term, the no-regrets ‘pump priming’ of a hydrogen pricing, production, storage and distribution infrastructure which, in the medium term, could also benefit the development of hydrogen-powered inshore marine vessels and trucks powered by, for example, fuel cells. The aim of this work was to establish the limitations and advantages of pursuing the conversion of an in-production heavy duty engine to dual fuel operation with the least changes to the engine. The conversion used supervisory calibration, which consisted in maximizing the amount of hydrogen injection on top of diesel fuel, without any optimization of the other engine parameters. The strategy was, at a given operating point, to decrease the load seen by the diesel ECU while also increasing hydrogen injection until the torque at this operating point was recovered. As a result, the load ‘detected’ by the diesel ECU was lower when the engine was running on the dual fuelmode than when it was running on diesel fuel only. The displacement of diesel fuel by hydrogen directly resulted in a reduction in carbon dioxide emissions. Nevertheless, the variability of the hydrogen-air premixed mixture combustion was kept under control and, in particular, knock had to be avoided.At low loads, up to 70% of the energy required to drive the truck could be provided by
Liu Y, Hardalupas Y, Taylor AMKP, 2022, A detailed CO2(1B2) chemiluminescence chemical kinetics model for carbon monoxide and hydrocarbon oxidation, Fuel, Vol: 323, Pages: 1-10, ISSN: 0016-2361
The CO2(1B2) - CO2(X1Σ+g) transition is a source of chemiluminescence from CO and hydrocarbon premixed flames and can be used as a diagnostic; however, its chemistry is not well known due to its broadband nature. Although several attempts have been made to model CO2(1B2) chemiluminescence, none performs well in hydrocarbon flames. We propose a new detailed kinetic model for CO2(1B2) chemiluminescence, based on shock tube experiments in the literature and on opposed flame data presented here. The mechanism consists of 26 reactions which describe the formation of the lower excited state molecule CO2(3B2) (R1), the inter-system crossing reaction between CO2(3B2) and CO2(1B2) (R2), CO2(1B2), the formatting reaction path in hydrocarbon flames (R3), CO2(1B2) radiative quenching (R4) and collisional quenching of CO2(3B2) and CO2(1B2) (R5-R26). The reaction rates constants of R1 and R3 within ± 60% and ± 32% uncertainty, respectively, were determined as follows:
Chen C, Li Y, Akagi F, et al., 2022, Diffuse back-illumination extinction imaging of soot formation from a liquid fuel film, 20th Int Symp on Applications of Laser and Imaging Techniques to Fluid Mechanics
The transient combustion of a liquid iso-octane film, isolated from the often co-existing combustion of liquid sprays, was investigated within the nominally quiescent ambient of a constant volume chamber using a custom-made liquid fuel film generation system. Soot formation throughout the combustion process, from ignition to extinction, was visualized using high-speed diffuse back-illumination extinction imaging technique, providing temporally resolved spatial distribution of soot optical thickness (πΎπΏ) in the chamber. The impact of ambient pressure and ambient oxygen content on soot formation was examined over a range of 2 – 5 bar (absolute) and 16 – 30% (in terms of molar fraction of oxygen), respectively. Regardless of the test conditions, the fuel film combustion entailed three stages, namely flameinitiation, steady burning and flame extinction. While the ambient oxygen content was kept constant, the flame gradually became turbulent-like and the flame flickering less distinct as ambient pressure was increased. The totalamount of soot generated within the chamber was found to first increase then decrease with the ambient pressure, due to the competing impacts of increasing pressure on promoting soot-formation reaction rate and enhancing mixing of fuel vapour with the entrained air. Increasing ambient oxygen content, on the other hand, consistently enhanced soot formation, which may be associated with its impact on boosting flame temperature and consequently liquid fuel evaporation rate. In addition, flame flickering remained distinct for ambient oxygen content above atmospheric level,while becoming substantially less observable for that below atmospheric level. Flickering frequency, for all test conditions with distinct flame flickering, had a value of approximately 10 Hz and gradually increased with timeduring the steady burning stage, suggesting the shrinkage of the fuel film diameter. Flickering of the flame resulted in fluctuations in the total
Wang T, Hardalupas I, 2022, Combined optical connectivity and optical flow velocimetry for measurement of the interfacial velocity of a liquid jet in gas cross-flow, 20th Int Symp on Applications of Laser and Imaging Techniques to Fluid Mechanics (LXLASER2022), Pages: 1-19
Liquid jet in crossflow (LJIC) is a process in which a high-speed gas crossflow deforms and shears a continuous liquid flow into tiny droplets. This study quantifies the liquid surface motion of LJIC during the primary breakup process, which has not been fully assessed due to limited optical access close to the nozzle exit. The interfacial velocity of a breaking liquid jet indicates the interaction of the gas and liquid flows and the initial velocity of the stripped droplets. However, the local interfacial liquid velocities have not been measured, since no measurement technique is available, and they have only been estimated from theoretical and computational studies. Optical Connectivity (OC) is a new optical technique, which introduces a laser beam through an atomiser nozzle and relies on total internal reflection at the liquid interface to propagate the laser light inside the continuous liquid. This allows the recording of the instantaneous features of the disintegrating continuous liquid and its interface during the primary atomisation at the near nozzle region through imaging of the emitted fluorescent intensity from the liquid flow. The current research reports time-dependent OC measurements of the temporal evolution of the liquid interface structures along a LJIC. The LJIC breakup behaviour is reported for different atomisation regimes, as determined by non-dimensional parameters. Optical Connectivity is combined with Optical Flow Velocimetry (OFV) to quantify the local interfacial liquid velocities of liquid interface structures of the LJIC for a range of gas Weber numbers between 14.9 - 112.6 and liquid-to-gas momentum ratios between 2.1 - 36.4. The combined OC-OFV measurements report the spatial distribution of interfacial velocities along the surface of LJIC and reveal the physics of the contribution of gaseous shear and liquidjet geometry on the atomisation process.
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.
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