Maurizio Santini

Three-dimensional X-ray microcomputed tomography of carbonates and biofilm on operated cathode in single chamber microbial fuel cell

Power output limitation is one of the main concerns that need to be addressed for full-scale applications of the microbial fuel cell technology. Fouling and biofilm growth on the cathode of single chamber microbial fuel cells (SCMFC) affects their performance in long-term operation with wastewater. In this study, the authors report the power output and cathode polarization curves of a membraneless SCMFC, fed with raw primary wastewater and sodium acetate for over 6 months. At the end of the experiment, the whole cathode surface is analyzed through X-ray microcomputed tomography (microCT), scanning electron microscopy, and energy-dispersive X-ray spectroscopy (EDX) to characterize the fouling layer and the biofilm. EDX shows the distribution of Ca, Na, K, P, S, and other elements on the two faces of the cathode. Na-carbonates and Ca-carbonates are predominant on the air (outer) side and the water (inner) side, respectively. The three-dimensional reconstruction by X-ray microCT shows biofilm spots unevenly distributed above the Ca-carbonate layer on the inner (water) side of the cathode. These results indicate that carbonates layer, rather than biofilm, might lower the oxygen reduction reaction rate at the cathode during long-term SCMFC operation.

X-ray computed microtomography for drop shape analysis and contact angle measurement

The interaction between an atomized fluid and a solid surface has a great importance in many fields, both in adiabatic conditions and when heat transfer is involved. To investigate the behavior of many drops in contact with a surface, the first step is to study a single one of them and in that, surface wettability is key parameter. Wettability analyses are usually performed by contact angle measurement, in most cases using the sessile drop or captive bubble techniques. Such techniques require optical acquisition of a side view of the drop or bubble, with a series of drawbacks when conventional optics are used, in particular for not uniform, not planar or rough base surfaces. X-ray micro-computed tomography is therefore used to acquire a 3D scan of a drop gently deposited on a surface, with the aim to reconstruct the drop surface and to perform contact angle measurements on true cross-sections of the drop-surface couple. Comparison with contact angle measurements performed on conventional images is performed. The results evidence that the proposed technique is very promising for surface characterization and to get more accurate and detailed information about wettability characteristics.

A novel technique for investigation of complete and partial anisotropic wetting on structured surface by X-ray microtomography

An experimental study about the anisotropic wetting behavior of a surface patterned with parallel grooves is presented as an application example of a novel technique for investigation of complete and partial anisotropic wetting on structured surface by X-ray microtomography. Shape of glycerin droplets on such surface is investigated by X-ray micro computed tomography (microCT) acting as a non-intrusive, full volume 3D microscope with micrometric spatial resolution. The reconstructed drop volumes enable to estimate the exact volumes of the drops, their base contours, and 3D static contact angles, based on true cross-sections of the drop-surface couple. Droplet base contours are compared to approximate geometrical contour shapes proposed in the literature. Contact angles along slices parallel and perpendicular to the grooves direction are compared with each other. The effect of the sessile drop volume on the wetting behavior is discussed. The proposed technique, which is applicable for any structured surface, enables the direct measure of Wenzel ratio based on the microCT scan in the wetted region usually inapproachable by any others. Comparisons with simplified models are presented and congruence of results with respect to the minimum resolution needed is evaluated and commented.

Micro computed tomography and CFD simulation of drop deposition on gas diffusion layers

Fuel cells are electrochemical power generation system which may achieve high energy efficiencies with environmentally friendly emissions. Among the different types, Proton Exchange Membrane fuel cells (PEMFC) seem at present one of the most promising choices. A very important component of a PEMFC is the gas diffusion layer (GDL), which has the primary role of managing water in the cell, allowing reactant gases transport to the catalyst layer while keeping the membrane correctly hydrated and preventing electrode flooding. Therefore, GDLs have to be porous and very hydrophobic. Carbon clothes or carbon papers coated with a hydrophobizing agent – typically a fluoropolymer – are used. Given the complex chemistry and morphology of the GDLs, wettability analyses on them present some critical issues when using the conventional contact angle measurement techniques. In this paper, the deposition of a drop on a GDL (produced using polytetrafluoroethylene-co-perfluoroalcoxy vinyl ether as the fluorinated polymer) was investigated by means of micro computed tomography (microCT) and numerical simulation. The microCT facility operational at the University of Bergamo was used to acquire a 3D tomography of a water drop deposed on a sample GDL. The reconstructed drop dataset allows thorough understanding of the real drop shape, of its contact area and contact line. The GDL dataset was used to create a realistic mesh for the numerical simulation of the drop deposition, which was performed using the OpenFOAM® interFOAM solver.

Application of cone-beam micro-CT on high-speed Diesel flows and quantitative cavitation measurements

X-ray computed tomography (CT) is well-known and widely used in the medical sector for diagnosis of various illnesses. The technique is based on the absorption (i.e. attenuation) of the ionising electromagnetic radiation by the object. The amount of energy to be absorbed depends on the density and its thickness; the transmitted radiation through the object is then compared to the incident radiation that leads to a reconstruction of attenuation coefficients versus spatial position in the object. Thus, the resulting three-dimensional slices of the object are used (a) to identify internal geometric features of objects, and (b) to distinguish between media of different densities, i.e. liquid and air/vapour. In this study, the geometry extraction capability has been applied on time-averaged cavitation pocket shapes, as well as, the capability of density differentiation measurements on Diesel fuel flows. Results appear promising and pose a challenge in providing quantitative measurements of cavitation vapour fraction inside an injection hole.

Turbulence and Cavitation Suppression by Quaternary Ammonium Salt Additives

We identify the physical mechanism through which newly developed quaternary ammonium salt (QAS) deposit control additives (DCAs) affect the rheological properties of cavitating turbulent flows, resulting in an increase in the volumetric efficiency of clean injectors fuelled with diesel or biodiesel fuels. Quaternary ammonium surfactants with appropriate counterions can be very effective in reducing the turbulent drag in aqueous solutions, however, less is known about the effect of such surfactants in oil-based solvents or in cavitating flow conditions. Small-angle neutron scattering (SANS) investigations show that in traditional DCA fuel compositions only reverse spherical micelles form, whereas reverse cylindrical micelles are detected by blending the fuel with the QAS additive. Moreover, experiments utilising X-ray micro computed tomography (micro-CT) in nozzle replicas, quantify that in cavitation regions the liquid fraction is increased in the presence of the QAS additive. Furthermore, high-flux X-ray phase contrast imaging (XPCI) measurements identify a flow stabilization effect in the region of vortex cavitation by the QAS additive. The effect of the formation of cylindrical micelles is reproduced with computational fluid dynamics (CFD) simulations by including viscoelastic characteristics for the flow. It is demonstrated that viscoelasticity can reduce turbulence and suppress cavitation, and subsequently increase the injector’s volumetric efficiency.

Characterisation of water-oil drainage process through a reservoir rock sample by digital core analysis

Oil-water relative permeability and capillary pressure of a water-wet digital core are computed using the finite-volume/volume of fluid method and mimicking traditional laboratory steady-state experiments carried out on a water saturated sample. Commercial software is used for immiscible multiphase fluid flow simulations, carried out on a low-cost multicore workstation, and the porescale data are upscaled to derive relative permeability and capillary pressure curves as function of water saturation. The digital sample is obtained by high resolution X-ray computed tomography scanning.

Novel experimental technique for 3D investigation of high-speed cavitating diesel fuel flows by X-ray micro computed tomography

An experimental technique for the estimation of the temporal-averaged vapour volume fraction within high-speed cavitating flow orifices is presented. The scientific instrument is designed to employ X-ray micro computed tomography (microCT) as a quantitative 3D measuring technique applied to custom designed, large-scale, orifice-type flow channels made from Polyether-ether-ketone (PEEK). The attenuation of the ionising electromagnetic radiation by the fluid under examination depends on its local density; the transmitted radiation through the cavitation volume is compared to the incident radiation, and combination of radiographies from sufficient number of angles leads to the reconstruction of attenuation coefficients versus the spatial position. This results to a 3D volume fraction distribution measurement of the developing multiphase flow. The experimental results obtained are compared against the high speed shadowgraph visualisation images obtained in an optically transparent nozzle with identical injection geometry; comparison between the temporal mean image and the microCT reconstruction shows excellent agreement. At the same time, the real 3D internal channel geometry (possibly eroded) has been measured and compared to the nominal manufacturing CAD drawing of the test nozzle.

Characterization of highly hydrophobic textiles by means of X-ray microtomography, wettability analysis and drop impact

Highly hydrophobic surfaces have been intensively investigated in the last years because their properties may lead to very promising technological spillovers encompassing both everyday use and high-tech fields. Focusing on textiles, hydrophobic fabrics are of major interest for applications ranging from clothes to architecture to environment protection and energy conversion. Gas diffusion media - made by a gas diffusion layer (GDL) and a microporous layer (MPL) - for fuel cells are a good benchmark to develop techniques aimed at characterizing the wetting performances of engineered textiles. An experimental investigation was carried out about carbon-based, PTFE-treated GDLs with and without MPLs. Two samples (woven and woven-non-woven) were analysed before and after coating with a MPL. Their three-dimensional structure was reconstructed and analysed by computer-aided X-ray microtomography (CT). Static and dynamic wettability analyses were then carried out using a modified axisymmetric drop shape analysis technique. All the surfaces exhibited very high hydrophobicity, three of them near to a super-hydrophobic behavior. Water drop impacts were performed, evidencing different bouncing, sticking and fragmentation outcomes for which critical values of the Weber number were identified. Finally, a CT scan of a drop on a GDL was performed, confirming the Cassie-Baxter wetting state on such surface.