Search

The measurement of biological fluid uptake into a scaffold sensor has been modeled by measuring the response of induced high-frequency temperature pulses. For this, a heat transport equation is used, developed from Extended Thermodynamics, also equivalent to Cattaneo's equation, as well as an effective thermal conductivity. The effective thermal conductivity is experimentally validated against data measurements of a carbon nanotube porous nanocomposite, embedded with silica nanoparticles. This nanocomposite serves also as the case study for the scaffold sensor. The uptake of the biological fluid in this scaffold sensor is equivalent to a change in the effective thermal conductivity, monitored by an increase of the interstitial volume fraction. By imposing a high-frequency temperature oscillation, the temperature response at the other end of the porous medium is calculated. Depending on the ratio of the relaxation time and the thermal diffusion time, the temperature response can be of oscillatory nature or of an exponential growth to an asymptotic limit. It is observed that an observed phase lag in the temperature response indicates a change in the effective thermal conductivity and thus is a criterion denoting the amount of uptake.

In this work, condensation flows of pure ethanol in a horizontal water-cooled circular cross-section heat exchanger were investigated for a mass velocity ranging between 30 and 100 kg m−2 s−1. The inner channel of condensation has inner and outer diameters of 4 mm and 10 mm, respectively, with a heat exchange length of 1020 mm. An adiabatic section consisted of a transparent circular cross-section channel, of 4 mm inner diameter and of 130 mm visible length, was placed downstream of the condenser in order to visualise the condensation flows by shadowgraph technique. Two main flow regimes were identified: the wavy-stratified regime and the wavy-annular regime. A flow regime map was developed and compared with flow regime maps available in the literature. Besides, the local vapour quality and internal heat transfer coefficient were determined and compared with prediction models of the literature. Finally, the total pressure drop in the condenser was determined.

Carbon nanotubes and silica nanoparticles are allowed to self-assemble into a nanocomposite by first forming an aqueous suspension, then depositing one drop after the other and finally letting them evaporate. Two types of composites are prepared. One by forming alternate layers and the other by forming several layers of a pre-mixed suspension. The thickness, thermal and electrical conductivity of the composites are measured versus the number of depositions. The pre-mixed composites showed an increase in the values in both the parallel and perpendicular directions of both the electrical and thermal conductivities, making them suitable for electrodes or battery-like applications. The values of the electrical and thermal conductivities in the perpendicular direction for the first composite decrease and increase, respectively, while for the parallel direction the values are significantly constant. As such, they would be useful as electrical insulators for optimal cooling. Thickness measurements showed that the pre-mixed composite is the denser one, due to a better alignment of the carbon nanotubes.

Evaporation can cause instability due to cooling effects on the density and surface tension. This causes, respectively, Rayleigh and Marangoni instabilities. When these instabilities grow sufficiently, self-sustained convection occurs. This convection causes changes into the evaporation rate and heat transfer rate. This also could change the heat transfer via the evaporation rate and can be important for industrial applications. It is the purpose of this paper to investigate the relation that exists between the overall evaporation rate and a set of parameters: temperature, gas flow and liquid thickness. Three-dimensional numerical simulations have been performed for this purpose and the results have been validated by means of an experimental setup, on which the numerical geometry has been based, characterized by a liquid evaporating through an opening in a cover sheet under a shear gas flow. It is shown that the temperature and the gas flow increase the evaporation rate. More interestingly, a maximum is observable for the evaporation rate as function of the liquid thickness. The explanation for these phenomena are drawn from the 3D numerical simulations. It appears that the maximum evaporation rate as a function of the liquid thickness depends on the confinement of the convection cells by the cover sheet, being assisted by the gas flow.

Early detection of ice formation on surfaces is a crucial requirement in many applications such as wind turbines, aeroplane’s wings, and radar domes. This paper presents the concept of a heat-pulse ice detection system. Its principle consists of recording the temperature evolution and assessing the relaxation time after the surfaces are subject to a heat excitation generated by an electrical pulse. The sensor comprises thin graphene films embedded in pre-preg layers of aeronautical grade as well as thermocouples to record the temperatures profiles at given locations. The criterion of ice detection exploits the high thermal capacity of ice compared to the air to capture the formation of icy spots. Numerical simulations and comparison with experimental investigations have been carried out to validate the principle and criteria of the ice detection system.

Experimental investigation of vapour condensation on single cylindrical fin has been carried out under various gravity conditions: 1g0, 1.8g0 and 0.05g0. For the first time, a balance of forces acting on the condensate flow is analyzed. The fin surface is divided in seven areas, each one being characterized by the main force acting on the liquid motion: gravitational vs surface-tension pressure gradient. The magnitude of the areas does not depend on the gravity level. This confirms the correctness of the fin surface segmentation, as used by various authors, to simplify the condensation modelling. It has been shown that the most intensive vapour condensation takes place in the areas, where the surface-tension pressure gradient is higher than the gravity force. Analysis of the heat load shows that each region affected by the surface-tension pressure gradient provides 10–15% of the total heat load, a contribution that is not negligible and should be taken into account. In case of low gravity, the most intensive condensation takes place at the corner of the fin tip, where the curvature gradient is maximum, and on its cylindrical part. The heat loads in these regions are comparable.

Carbon nanotubes are allowed to self-assemble by depositing a droplet of a water dispersion thereof and letting it evaporate on a polycarbonate substrate. The effect of the number of droplets, evaporated on the same deposition spot, on the self-assembly density is assessed to be more than proportional for the first five depositions. The obtained nanoporous nanostructures are further tested for their electrical resistance and wettability. Two concentrations are used. It is found that a higher concentration and more importantly a higher number of droplet depositions causes the electrical resistance to decrease up to four orders of magnitude and the static contact angle to decrease more than three times. The contact angle hysteresis also increases due to an increasing advancing contact angle and a decreasing receding one. This is explained by the degree of coverage of the substrate by the carbon nanotubes as is also shown by scanning electron microscope images. A better coverage is suggested to cause more pinning for an advancing droplet and a higher capillary force for a receding droplet.

A new concept of vapour condenser is proposed to solve the drainage problem and to provide an effective condensate flow under weightlessness conditions. The concept combines the capability of a curvilinear surface to drive liquid, and a combination of a porous media and a pump, as an active condensate retraction system. The porous media collects and stores the condensed fluid and acts as a barrier for the vapour phase. Hydrodynamic validation of the liquid flow was done against retraction overflow under terrestrial conditions. It was figured out that the time available for adjusting the retraction system to avoid film disturbances decreased non-linearly with an increase in retraction overflow. The behaviour of the proposed condenser with the retraction system was tested under weightlessness condition during the ESA parabolic flights campaign as a part of concept validation. Three porous media with different values of the pore size and material were tested. It was found that the combination of the pump and porous media helped to improve the condensate flow. It was observed that the stabilisation effect was influencing the condensate flow not only during weightlessness condition but also under acceleration disturbances during the campaign., SCOPUS: ar.j, info:eu-repo/semantics/published

The present work attempts to model the case of combined gravitational and capillary motion of condensate for an axisymmetric fin under steady and transient fin operation conditions. The focus here is to examine the structure of the mathematical problem and to develop suitable numerical techniques rather than yield information on the macroscopic condensate flow rate and fin efficiency. The problem is formulated starting from general conditions and is simplified step by step by introducing corresponding assumptions. The particular fin shape of a paraboloid from revolution is chosen and the equations are properly non-dimensionalized. A vast reduction of the number of problem parameters is achieved in this way. The cases of isothermal fin, steady state operation and dynamic operation are treated separately using specialized numerical solution techniques developed for each case in order to improve computational efficiency and accuracy. Typical results of fin temperature and condensate film thickness are presented and discussed.

A thermodynamic model for transient heat conduction in ceramic-polymer nanocomposites is proposed. The model takes into account particle’s size, particle’s volume fraction, and interface characteristics with emphasis on the effect of agglomeration of particles on the effective thermal conductivity of the nanocomposite. The originality of the present work is its basement on extended irreversible thermodynamics, combining nano- and continuum-scales without invoking molecular dynamics. The model is compared to experimental data using the examples of SiO 2 , AlN and MgO nanoparticles embedded in epoxy resin. The analysis is limited to spherical nanoparticles. The dependence of the degree of agglomeration with respect of the volume fraction of particles is also discussed and a power-law relation is established through a kinetic mechanism and experiments performed in our laboratory. This relation is used in our theoretical model, resulting into a good agreement with experiments. It is shown that the effective thermal conductivity may either increase or decrease with the degree of agglomeration.

The aim of this work is to understand the changes in the observed phenomena during particle-laden drop impact. The impact of millimeter-size drops was investigated onto hydrophilic (glass) and hydrophobic (polycarbonate) substrates. The drops were dispersions of water and spherical and nearly iso-dense hydrophobic particles with diameters of 200 and 500 μm. The impact was studied by side and bottom view images in the range 150 ≤ We ≤ 750 and 7100 ≤ Re ≤ 16400. The particles suppressed the appearance of singular jetting and drop partial rebound but promoted splashing, receding breakup, and rupture. The drops with 200 μm particles spread in two phases: fast and slow, caused by inertial and capillary forces, respectively. Also, the increase in volume fraction of 200 μm particle led to a linear decrease in the maximum spreading factor caused by the inertia force on both hydrophilic and hydrophobic substrates. The explanation of this reduction was argued to be the result of energy dissipation through frictional losses between particles and the substrate.

This chapter discusses the four main types of microorganisms that are used for the production of biodiesel: bacteria, yeast, fungi, and microalgae. Oleaginous microorganisms, that is, yeast, filamentous fungi, and microalgal species, are very interesting organisms that are able to accumulate lipids at levels greater than 20" of their cellular dry weight. Microorganisms have long been used in industrial processes, starting with alcohol fermentation using Saccharomyces cerevisiae. Microalgae are rich in some products, such as pigments and chlorophyll, which make them particularly attractive for other applications, like cosmetics, pharmaceuticals, and food. Microalgae can be classified into four main classes: diatoms, green algae, blue-green algae, and golden algae. Single-cell oils are lipids that are extracted from unicellular microorganisms. The oil extraction and purification of single cell oils still represent major costs in the production process. It is therefore important to have efficient downstream processing.

Liquid layers evaporating under the influence of a gas shear flow presents a non-uniform distribution of the evaporation rate all along the interface. Being the evaporation an endothermic process, a thermal gradient along the interface is generated and thermo-capillary flows are induced. Hence, two opposite mechanisms regulate the movement of the interface: the shear stress of the gas that entrains the interface in the direction of the flow and the thermo-capillary stress that forces the interface to move against the flow direction. The composition of these mechanisms at the interface generates an unstable thermal patterning. The dynamic evolution of the patterning and the relative evaporation rate are strongly influenced by the flow rate of inert gas, the layer thickness and the liquid thermo-physical properties. The goal of the present work is to study numerically how the evaporation process is influenced by the above-mentioned mechanisms. The focus will be on the evolution of the thermal patterning at the interface and the assessment of the main factors influencing the computed evaporation rate.

In this work a supercritical technology is proposed to improve the classical emulsification/diffusion technology for lipid nanoparticles (LNs) production. The process is based on the emulsion diffusion method improved by the addition of a continuous supercritical fluid processing step to eliminate the organic solvent from the nanosuspension obtained. Different emulsion/diffusion formulations for stearic acid nanoparticles production were tested and, then, processed by supercritical continuous extraction at 80 bar and 45 °C (liquid/gas ratio of 0.1) in a packed column, obtaining an efficient benzyl alcohol elimination. Solvent residues less than 100 ppm were measured. Stearic acid nanoparticles were not extracted or damaged by the supercritical processing step, did not stick on the packing elements and showed mean diameters of 30–50 nm; a value of one order of magnitude smaller than the ones obtained by the conventional emulsion/diffusion technology with a recovery efficiency of 100%. Indeed, the fast and complete elimination of the benzyl alcohol around the nanoparticles reduced the aggregation phenomena responsible of larger lipid particle sizes obtainable by traditional process.

A fiber optic sensor is developed in order to measure film thickness along a curved surface. The technique is non-invasive, which has large bandwidth and good spatial resolution (150 μm and 300 μm). A “finger” type surface is used on top of which liquid is poured down in a continuing manner. Film thickness is measured with the fiber optic probe on 2 different locations along the “finger” surface. Film thickness of 163 and 79 μm was measured near the top and in the middle of the fin surface.

The splat morphology after the impact of suspension drops on hydrophilic (glass) and hydrophobic (polycarbonate) substrates was investigated. The suspensions were mixtures of water and spherical hydrophobic particles with diameter of 200μm or 500μm. The impact was studied by side, bottom and angled view images. At Reynolds and Weber numbers in the range 150⩽We⩽750 and 7100⩽Re⩽16,400, the particles distributed in a monolayer on the hydrophilic substrates. It was found that the 200μm particles self-arranged as rings or disks on the hydrophilic substrates. On hydrophobic substrates, many particles were at the air-water interface and 200μm formed a crown-like structure. The current study for impact of particle-laden drops shows that the morphology of splats depends on the substrate wettability, the particle size and impact velocity. We developed correlations for the inner and outer diameter of the particle distribution on the hydrophilic substrates, and for the crown height on hydrophobic substrates. The proposed correlations capture the character of the particle distributions after drop impact that depends on particle volume fraction, the wettability of both particles and the substrate, and the dimensionless numbers such as Reynolds and Weber.

The dynamics of thermal ripples at the interface of a volatile pure liquid (C2H5OH) is studied experimentally and numerically. Liquid evaporates under a flow of inert gas (N2) circulating along the interface. The evaporation rate is varied by regulating both the gas flow rate and the gas pressure. Experiments in microgravity environment allowed to identify a transition to “interfacial turbulence,” along which some particular events such as nearly periodic and possible intermittent behaviors. Direct numerical simulations have been performed, and compare qualitatively well with experimental results, offering new insights into the physical mechanisms involved. Small-scale ripples appear to arise from a secondary instability of large-scale convection cells and their motion seems to follow the corresponding large-scale surface flow. The relative role of surface tension and buoyancy in triggering these flows is assessed by comparing experiments and simulations in both microgravity and ground conditions. Qualitative features compare satisfactorily well such as typical speed and orientation of the thermal ripples, as well as spiral flow in the bulk.

In recent years, the development of two-phase flow cooling systems in micro-electronic as well as surface coating/patterning technologies, often based on thin films and liquid filled micro-enclosures, has renewed the interest in the classical problem of the cavity flow driven by a shear stress imposed at the gas-liquid boundary. In this paper, we study, numerically, the influence of the cavity geometry and boundaries on the three dimensional velocity field driven by a shear flow of inert gas. This study has been performed in the frame of the ESA sponsored Space Program on heat and mass transfer CIMEX-1 and it has to be considered as a contribution in the preparation of the Space experiment. The reference values for the flow parameters as well as the geometrical features encountered in the present paper target the main features of the CIMEX-1 experiment, although the main conclusions can be considered of general validity.

For several years, interfacial instabilities arising in evaporating layers of volatile liquid when subjected to a shear flow of inert and non-absorptive gas have been studied for their intrinsic complexity due to the interaction between different phenomena such as heat and mass transfer through the layer interface, thermo-capillarity, natural convection, and shear-induced stresses. More recently, the possibility of generating ordered thermal patterns in an evaporating layer by controlling such kinds of instabilities has been considered as an intriguing technique to enhance heat and mass transfer in precisely defined spots at the interface of the layers. We studied numerically the topology of the thermal patterns as well as the heat and mass transfer characteristics that can be induced in an evaporating liquid layer by controlling the thickness of the layer while keeping constant the gas flow intensity. Calculations have been conducted by considering ethanol as the working fluid and nitrogen as the inert gas. The thickness of the layer was varied in order to have aspect ratios with respect to the characteristic length of the evaporating interface in the range of 0.02-1. The inspiring reason for performing the simulations reported in this paper is the preparation of the CIMEX-1 experiment that will be performed on-board the International Space Station in the next future. For that reason, all the calculations presented refer to the condition of the absence of gravity. © 2011 by Begell House, Inc.

A mini splitterless-split-flow thin fractionation (SPLITT) device has been developed to achieve fast separations of micrometer-sized species. In this device, inlet and outlet steps have replaced the splitters, which are common to conventional SPLITT channels. By elimination of the splitters, it becomes straightforward to reduce channel dimensions while maintaining the classic method of fabrication. Reduced dimension channels allow high axial velocity at relatively low flow rate. These high axial velocities generate an enhancement of inertial lift forces and hydrodynamic shear-induced diffusion. Experiments carried out with particulate and biological species in a mini step-SPLITT channel demonstrate that these hydrodynamic effects yield highly enriched fractions of smaller species from binary mixtures.

Evaporation of a pure liquid into a inert gas is studied theoretically and experimentally. In contrast with the case where the gas phase is made of pure vapor, the thermocapillary (Marangoni) effect strongly destabilizes the system, and results in intensive and often chaotic forms of interfacial convection. Theoretically, a generalized one-sided model is proposed, which allows the solution of the thermo-hydrodynamic equations in the liquid phase only, still taking into account relevant effects in the gas phase. The equivalent heat transfer coefficient (Biot number) to be incorporated in this one-sided model appears to be high, which results in an acceleration of transitions to polygonal chaotic patterns. Chaotic interfacial patterns driven by the Marangoni effect have indeed been observed during the ITEL-Maser 9 sounding rocket experiment flown in March 2002, in preparation of the CIMEX (Convection and Interfacial Mass Exchange) experiment foreseen for the International Space Station.

In the field of life sciences, the monitoring of biological samples has become a great concern to control the ecosystem evolution. However, their characterization is often time-consuming because the typical size of the organisms/particles of interest is several orders of magni- tude smaller than the size of the sample under observation. Optical visualization systems require, then, high magnifi- cations that severely limit the depth of focus and conse- quently decrease the sampling rate. To tackle this issue, the most straightforward technique consists in focusing the samples to fit the observation field of view by means of so- called ''sheath flows''. This expedient allows for increasing the overall flow rate, inversely related to the sampling time. In this article, a cost-effective 3D hydro-focusing device is presented. Several flow rates have been tested for both sample and sheath flows, and a thorough investigation of the shape of the focused streamlines conducted in order to validate the prototype design. The 3D position of the sampled micro-objects has been located by digital holo- graphic microscopy and their distribution in cross-sections downstream the injection nozzle compared to numerical simulations. A maximum constriction—ratio between the part of the cross-sections where particles are present with and without focusing sheath flow—of 4.4 % has been observed confirming the potentiality of the technique. Also, a successful match between experiment and numerical simulation has been noted.

In this paper, some of the preparatory experiments of the ESA sponsored space program CIMEX-1 are presented. A liquid layer of variable thickness is subject to a flow of inert gas. The non-uniform evaporation induced by the gas flow creates a temperature gradient parallel to the interface triggering in that way thermocapillary convection. The combined action of evaporation, thermocapillarity and gravity has been not completely clarified both theoretically and experimentally. The experiment presented in this work concerns a liquid layer of ethanol of 2.2 mm thickness in presence of a mass flow of Nitrogen whose intensity varies in the range of hundreds of milliliter per minute. The experiments were performed at an initial liquid temperature of 21°C. The patterns observed are strongly dependent on the flow rate of inert gas. A change in the instability patterns has been observed for a gas flow of about 1.7 l/min.

Analysis of environmental changes has become a great issue nowadays with the increase of pollution and global warming. Surveys about growing populations of species and apparition of new parasites are more and more relevant to preserve our environment and populations needs. The ability to observe for instance the presence of bacteria such as Criptosporidium and Giardia Lamblia in freshwater contributes to prevent people from diseases even in developed countries. The main issue in the observation of such species is that the size of the biological sample should be large enough to be representative of its surrounding environment, meaning that the amount of the scanned sample is several orders superior to the size of the micro species of interest. A major limitation for optical instruments in performing this kind of analysis is that the magnification required should fit the size of the particles, severely constraining the field of view. As a result, the rate of analysis is often so slow to be compatible with the large samples to analyze. One way to increase the rate of analysis is to manipulate particles in order to focus them in the field of view of the visualization system. In this article we use acoustic standing waves to achieve this goal by directing particles in the observation region. A 1mm × 1mm square section glass channel, excited by a piezo transducer is used. The carrier fluid is water while several types of particles, both synthetic and biological are tested. The dynamic behaviour of particles under the acoustic field has been investigated and the 3D position of each particle is reported. In order to increase the depth of investigation, we used a Digital Holographic Microscope. This technology permit to scan a larger volume of fluid and also to calculate the 3D position of each detected particle flowing in the channel. A focusing streamline of as thin as 1/20 of the channel cross section has been successfully achieved.

Observation of biological samples is a common issue in many applications relative to marine environment and in the field of water treatments. In particular, the ability to detect the presence of bacteria such as Criptosporidium and Giardia Lamblia in freshwater contributes to prevent people from critical diseases even in developed countries. The main challenge in this field is to analyze a large enough volume of biological sample to make it representative of the selected environment, while characterizing the species of interest whose size is often many order of magnitude smaller. In order to obtain detailed information of the observed species, the magnification of the visualization systems — often optical microscopes — should fit the size of the objects, involving a restrained field of view. As a result; the rate of analysis is lowered and the characterization of the samples time-consuming. To tackle this issue, an increase of the flow rate is possible by focusing of particles in the observation field of view. Such technique allows for increasing the overall flow rate, inversely related to the sampling time. In this article a new 3D hydrofocusing device is presented. A square section glass capillary (400×400μm2) used as the nozzle is inserted into a square section glass channel (2×2mm2). The inlets are fixed on a custom device that enables the sheath flow to completely wrap the sample flow after the injection. The 3D position of the particles used as representative substitutes of the biological species has been measured thanks to digital holographic microscopy and their distribution in cross-sections 2mm and 30mm downstream the injection nozzle compared to numerical simulations. A successful match of the location has been observed. The carrier fluid used in the experiments was water seeded with 27–45μm diameter neutrally buoyant particles. Several flow rates have been tested for both samples and sheath flow to investigate the shape of the focused stream line and to validate the prototype design. A maximum constriction — ratio between the part of the cross-sections where particles are present with and without focusing sheath flow — of 47% has been observed confirming the potentiality of the technique.

Efficient mixing in micro-channels is one of the most debated topics in the design of microfluidic devices for the outmost importance it has in many industrial processes. Different mixing mechanisms could be easily found in literature but a usual accepted classification divides the different devices in three major categories: diffusion, distribution-based and convective devices. For all of them, the goal is to break the intrinsic laminar character of very small Reynolds number flows typical of microfluidic to enhance the mixing of the different interacting streams. In this study, we focused on the numerical modeling of the so-called injection mixer. In these devices, the mixing is driven by the dynamic of the injection streams. Attention is given to the inlet region where the different streams first mix together. Computations tackled with the case of a three inlet micro-channel — thickness: 400μm, width: 3mm, length: 30mm — where the injection rates are modulated according to prescribed wave form and normalized to have a constant flow rate at the outlet. Linear and sinusoidal waveforms have been tested. For each configuration, an assessment of the mixing efficiency based on standard deviation of the stream concentration is evaluated together with a typical mixing length and time. Results are then compared with the case where the injection rate is constant. Preliminary results shows that injection mixer with modulated injections can achieve mixing efficiency that are at least comparable with the diffusion based systems. The waveform that is more suitable depends on the characteristic injection period and on the phase shift between different injections. Pulse waveform determines high efficiencies, but requires higher injection rates and creates pressure fluctuations that could be disturbing in applications. Experiments have also been carried and results are compared with the numerical ones. The results obtained are useful to better understand the behavior of injection mixing devices and to provide some hint to reduce the mixing length by coupling a preliminary mixing due to modulation of the inlet streams with other mixing configurations.Copyright © 2011 by ASME