Carlo Saverio Iorio

A benchmark study on the thermal conductivity of nanofluids

This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)], was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

Interfacial turbulence in evaporating liquids: Theory and preliminary results of the ITEL-master 9 sounding rocket experiment

Abstract 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.

Thermal patterns in evaporating liquid

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.

Complex Drop Impact Morphology

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.

Influence of Lateral Boundaries on Evaporative-Driven Convection Patterns

The evolution of convective patterns arising in evaporating liquid layers subject to a flow of inert gas depends on dynamical, thermo-physical and geometrical parameters. To the first group it is possible to associate the average velocity of the inert gas current and the total pressure of the gas phase — inert gas and vapor — insisting on the evaporating layer. The volatile liquid is also of importance in the convective patterns selection especially for what concerns the values of the latent heat of evaporation and of the cinematic viscosity. In this paper, we will focus on the influence that the lateral boundaries of the liquid pool have in the pattern selection process from the numerical point of view. A particular emphasis will be given to the heat transfer characteristics and to their dependence from both the liquid layer depth and the size of the evaporating surface.Copyright

Heat transfer enhancement in evaporating mini-layers: effects of an inert gas flow

When a layer of volatile liquid is subject to a flow of inert gas, a non-uniform distribution of the evaporation rate is generated all along the interface. Being evaporation stronger at the inlet boundary of the layer, because of the maximal efficiency of the inert gas flow in removing vapor from the interface, a thermal gradient along the interface is generated. 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. Moreover, because of the overall cooling of the interface due to the evaporative process, a gradient normal to the interface is also created. It results in a potentially unstable situation that is 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 if and how the dynamic evolution of the liquid layer is driven by the above-mentioned mechanisms. The main results concern the evaluation of the influence of the thermal instability patterns, eventually generated by the concurrent action of non-uniform evaporation and thermo-capillary motion, on the heat transfer at the bottom liquid. The distribution of temperature and velocity in the gas and liquid bulk phase for different mass flow rate of inert gas has also been of interest.Copyright

Impact of particle-laden drops: Particle distribution on the substrate

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.

Two-Phase Flow Pattern and Pressure Drop in a Microchannel

An experimental investigation has been carried out on two-phase flow characteristics in a 10 μm circular gap. The inlet of the gap is formed by two circular surfaces with radius 10 mm. The pressure drop is measured between two points placed — respectively — at the inlet and at the outlet of the gap. The upper and lower plates of the test section are transparent. Two-phase flow patterns were determined by video recording. The single flows of liquid FC-72 and nitrogen gas, as well as two-phase flow are investigated. In two-phase flow experiments the gap is filled by liquid. Liquid is maintained continuously by surface tension in the gap and in the meniscus formed between two inlet circular surfaces. The pressure difference includes two components, surface tension component in the meniscus and viscous one. Instability appears at the entrance of the gas into the gap. At small flowrate the gas flows in the gap has the form of chains of bubbles. Part of investigation was done in microgravity during a Parabolic Flights. In order to better understand experimental results, some numerical simulations have been done both in two and three dimensions for the real geometrical configuration for one-phase flows only.Copyright

3D Focusing of Micro-Particles by Sheath Flow Warping

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.Copyright

3D Focusing of Microparticles by Acoustic Standing Waves in a Flow Through Channel

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.Copyright