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Many studies have been devoted to single drop impacts onto liquid films and pools, while just a few are available about double drop or drop train impacts, despite the fact that the latter are more realistic situations. Thus, computational fluid dynamics with a volume-of-fluid approach was used here to simulate the impact of multiple drops into deep pools. The aim was to verify if multiple drop impacts significantly differ from single drops ones, and if the models available in the literature for the crater depth in the case of single impacts are reliable also for the multiple drop cases. After validation against experimental data for single and double drop impacts, simulations for four to 30 drops, with a diameter of 2.30 mm, impact velocities 1.0, 1.4, 1.8, and 2.2 m/s, and random initial positions in the domain were performed. The results showed that the time evolution of the crater depth for multiple impacts is similar to the single drop case during the inertial phase, while the following behavior is very different. Consequently, the available models for the maximum crater depth during single drop impacts can still predict the upper and lower bounds of the values of the crater depth during multiple drop impacts within 5% deviation.

In this paper an experimental analysis of two different indirect evaporative cooling (IEC) systems is carried out: in the first one plates heat exchanger are finished with a standard epoxy coating (STD) while in the second one a novel hydrophilic lacquer is adopted (HPHI). Firstly, static contact angles, water retention and transient drop-surface interaction of both materials are evaluated. Secondly, IEC systems performance is measured in different operating conditions and, in particular, varying the water flowrate and nozzle position (top and side). Results highlight that contact angles of HPHI coating are always lower than the ones of STD treatment: according to these findings, in case of water distribution from the top of the IEC system, wet bulb effectiveness of HPHI device is higher than that of STD unit (up to 10%). Instead, in case of water flow supplied from the side of the system, no significant differences have been observed.

In the last decade, wind has experienced a strong expansion reaching 591 GW (2018) of installed capacity worldwide. The higher penetration of variable renewable energy sources (wind and solar) has led to a growing demand for reliable forecast methods, to properly integrate these sources in the electric grid, decreasing the cost of electricity production and power curtailments. The present work proposes diverse wind power predictive approaches based on a physical model, artificial neural networks and an hybridization of the two. The time series used is composed of two-years hourly measurements of a wind farm in Italy, consisting of 24 wind turbines with a nominal power of 0.66 MW. To ensure an adequate reliability and robustness of the results obtained from the performance evaluation, it was chosen to use eight different error metrics and to evaluate the accuracy considering two different predictive situations (yearly and daily), using the persistence model as benchmark. The evaluations of predictive performances, regarding both the analyses, confirmed the superiority of data-driven approaches in the daily wind power prediction. More in detail, the hybrid model managed to reduce the MAE, the NRMSE and the SS values, compared to persistence, by 50%, 47.82% and 47.68%, respectively.

Summary One of the most critical issues in the oil and gas industry is the dewatering of the pipelines used for natural gas transportation, and foam injection seems to be a prominent solution. This work has two goals: The main one concerns the development of an optical tool to measure the liquid holdup in foamy flows and perform the flow regime characterization, whereas the secondary goal is to quantify the effectiveness of surfactant injection in reducing the liquid loading. In this paper, we present the results of an experimental campaign aimed at the characterization of gas-liquid-foam flows in a horizontal pipe. Initially, liquid loading measurements for gas and liquid superficial velocities, ranging from 0.41 to 2.30 m/s and from 0.03 to 0.06 m/s, respectively, were performed by means of a specifically developed optical method. For each liquid superficial velocity, the minimum liquid holdup was found to lie in the proximity of the boundary between plug and stratified flow regime, with a superficial gas velocity between 0.44 and 0.90 m/s. Hence, the plug flow region corresponds to the best operating condition to perform the pipeline dewatering procedure. Moreover, the drift-flux model usually adopted for ordinary two-phase gas-liquid flows seems to fit well with the measured values of void fraction.

During the last decades, a rising interest was earned by two-phase oil-water and three-phase oil-water-gas flows, in particular for long-distance transportation of transitional and heavy oils. A technique to reduce the pressure drop is water injection into the oil stream, aimed at creating the so-called core annular flow (CAF). In the presence also of a gaseous phase, which is very common at the exit of an oil well, bubbles perturb and deform the oil core and water annulus. A technique is proposed here to measure the gas bubble velocity by image processing of frames from videos of the flow and by an analysis in a combined space-time representation. The main advantage of this procedure is that it does not require cross-correlation between different signals. Within certain ranges of the governing parameters such slug flow can be analysed as an equivalent gas-liquid flow, considering oil and water as a single liquid. Under this assumption the measured velocity can be used to estimate the in situ gas volume fraction. The results are in very good agreement with the predictions of the drift-flux model and the Armand correlation.

Surface science and engineering gained an increasing importance in recent years, due to the potential benefit they can offer in many applications. An important parameter in this field is the surface wettability, that is in general evaluated by the contact angle. One of the most accurate techniques to measure this quantity is the axisymmetric drop shape analysis, based on the fitting of the theoretical Laplace-Young profile to the contour of experimental drops. In this paper the performance of a simple, low-cost setup – that can be built “at home” – to apply this technique is assessed, also including a detailed analysis about the influence of the most important parameters to set in the procedure. The latter aspect was evaluated by using computer-generated drop profiles and pictures, to have “realistic” images, but for drops of known properties. The experimental setup was built using a desk, a table lamp, a medical syringe, a support for the samples and a “bridge” camera. Measurements were performed on smooth and rough textile surfaces and the results were compared with previous measurements taken with a professional setup. From the comparison it can be affirmed that the performance of the home-made setup is very satisfactory.

In the present study, Volume of Fluid (VOF) multiphase flow model by ANSYS Fluent 16.2 is used to characterize core-annular flow patterns of highly-viscous oil-water flows through a Venturi Flow Meter (VFM) and a Nozzle Flow Meter (NFM) in a 40 mm i.d. horizontal pipe. Numerical simulations are compared with experiments, for oil superficial velocities in the range 0.25–0.75 m s−1 and water superficial velocities in the range 0.44–1.32 m s−1. Eight cases were considered for numerical runs. Two turbulence models were considered, namely realizable k − e and Shear Stress Transport ( ( SST ) k − ω . Two phase pressure drop, instantaneous radial velocity and holdup profile, cross-sectional time-averaged holdup and slip ratio were extracted from CFD simulations. Flow patterns were also compared to actual images taken by a Olympus E-M10 mirrorless camera. In terms of concentrated pressure drop and mean water holdup, the results of CFD simulations are consistent with the experimental data. Prediction of the total mass flow rate, computed by homogeneous model, for both the VFM and NFM also gave very satisfactory results.

Vapour adsorption plays a fundamental role in the operation and performance of innovative heat and mass transfer devices for latent heat recovery in HVAC systems. Materials to be used for such devices should present a high affinity with water vapour; at the same time pores should not be flooded in case the surface finds itself in contact with liquid water (e.g., due to moisture formation) and the surface should minimize the wetted parts. From the latter point of view hydrophobic microporous surfaces would be the most suitable; on the contrary, commonly used materials have hydrophilic and/or macroporous surfaces and their behaviour in presence of moisture is still not fully understood. Therefore, this paper is aimed at studying (mostly experimentally, with the support of numerical simulations) wettability and interaction with water, together with adsorption isotherms, of three commercially available desiccant beads (Silica Gel, Silica Gel impregnated with LiCl, Activated Alumina). Results evidence that the behaviour is significantly different between the three materials, even if their static and dynamic wettability is quite similar.

The progressive depletion of on-shore and light-oil reserves is forcing an increased use of transitional and heavy oils, which implies new challenges both during the extraction and the transportation. Focusing on the latter, a technique to reduce the pressure drop is water injection in the oil stream to create the so-called core annular flow (CAF), a flow regime with an oil core enveloped in a water annulus wetting the pipe wall, so that the apparent viscosity of the mixture is considerably reduced. Behaviour of CAF in ducts with non-uniform sections is still under research. This work is devoted to a CFD investigation about the pressure drop, pressure gradients, velocity profiles and in situ volume fractions in a duct including a Venturi flow meter. Unsteady RANS simulations were carried out using the Volume-Of-Fluid interFoam solver of OpenFOAM. Numerical results were experimentally validated for oil superficial velocities in the range 0.25-0.75 m/s and water superficial velocities in the range 0.44-1.10 m/s and comparisons between different approaches and sensitivity analyses were performed. Satisfactory agreement was found for the pressure drop and pressure gradients, and also for the in situ volume fraction with respect to the predictions of the Arney correlation.

This study presents a method to measure the void fraction in presence of a stratified three-phase flow with an opaque fluid like foam. The commonly used resistive probes, which were successfully applied for air-water flows, fail in detecting the liquid/foam interface due to the variable conductivity of foam. To overcome this problem, a new optical method was developed. A probe consisting of a steel rod covered in red vinyl plastic with a black measuring scale having 1 mm resolution was introduced radially into the flow; the foam layer, being opaque, can be easily identified against the measuring scale in a side view of the flow. The behavior over time of the liquid-foam interface was thus recorded through a video camera. A couple of small LED lamps provided the lighting to record the scene. The videos were then processed to count the measuring scale marks below the foam layer in order to get the instantaneous values of liquid layer depth. Measurements were performed at different pipe sections. The results were compared to those obtained for air-water flows at the same superficial velocities, with the latter ranging from 0.76 to 2.30 m/s for air and 0.03 to 0.06 m/s for water respectively. A liquid loading reduction up to 41 % was detected at the lowest gas superficial velocity, i.e. 1 m/s, while when the gas superficial velocity increases the difference in the liquid holdup lowers and becomes negligible at 2.30 m/s, regardless the value of the liquid superficial velocity. Since no specific model exists for foamy flows, as a first attempt the Zuber and Findlay drift-flux model was finally adopted to correlate the data.

This work presents and analyses the results of an experimental activity aimed at the characterization of stratified air&ndash, water flow conditions, which have been poorly analyzed in previous studies although they are significant for industrial applications. Tests were performed in a 24 m long, 60 mm inner diameter PMMA pipe, the superficial velocities ranged between 0.03 m/s and 0.06 m/s for the water and between 0.41 m/s and 2.31 m/s for air. The pressure gradient along the pipeline was determined and compared to the one obtained implementing two-fluid models available in the literature. Fair agreement with the models was found only at high values of the superficial gas velocities, i.e., above 1.31 m/s. Moreover, the void fraction was measured through a resistive probe and compared with the values predicted by available models. Since none of them was able to satisfactorily predict the void fraction in the whole range of superficial velocities, a drift flux model was successfully implemented. Eventually, with both the measured pressure gradient and the void fraction, a two-fluid model was implemented in order to determine the interfacial shear stress and to compare the outcome with the literature, emphasizing the influence of the operating conditions on the prediction performance.

Dropwise cooling can be of interest in many fields where it is necessary to remove heat from surfaces at a high rate and heat transfer by gaseous convection is not enough. Regrettably, the prediction of the heat transfer rates is still a major issue, particularly on innovative and engineered surfaces. Existing models offer fairly reliable predictions for smooth or isotropically textured surfaces, but information is lacking for surfaces with directional grooves and edges. A correlation is presented here to predict the heat removal rate during evaporation of sessile and impinging drops on microfin metallic surfaces, whose coefficients were fitted on experimental data acquired on aluminium and brass surfaces with triangular microfins. Eight samples, differing in material and microfin dimensions, and drops from sessile to impinging with Weber number up to 40, were studied. The correlation is based on the drop Fourier and Eotvos numbers, on the surface effusivity and on another quantity – the dimensionless free drop height – that summarizes the drop shape. It was developed using surfaces at 70 °C and then validated also against experimental tests at 60 °C and 80 °C. The mean absolute percentage error in the prediction of the heat removal rate in all the investigated conditions is within 6%.

Oil–water two-phase flow experiments were conducted in horizontal ducts made of Plexiglas® to determine the in situ oil fraction (holdup) by means of the closing valves technique, using mineral oil (viscosity: 0.838 Pa s at 20 °C; density: 890 kg m−3) and tap water. The ducts present sudden contractions from 50 mm to 40 mm i.d. and from 50 mm to 30 mm i.d., with contraction ratios of 0.64 and 0.36, respectively. About 200–320 tests were performed by varying the flow rates of the phases. Flow patterns were investigated for both the up- and downstream pipe in order to assess whether relevant variations of the flow patterns across the sudden contraction take place. Data were then compared with predictions of a specific correlation for oil–water flow and some correlations for gas–water flow. A drift-flux model was also applied to determine the distribution parameter.

The Thermogravimetric Heat Pump (TGHP) is a non-conventional system, implementing a reverse cycle, the main difference of which from the usual vapor compression (Rankine) cycle is a quasi-isothermal compression of the working fluid by a high heat capacity carrier fluid. Previous studies showed that employing HFC134a or PF5050 as working fluids may be promising in terms of thermodynamic performance, though the corresponding required plant heights confine its application to tall buildings (from minimum height of 10–12 storeys to skyscrapers). Accordingly, an investigation has been carried out in the present study in order to determine a group of fluids which allow lower heights under the same input conditions. In order to investigate the performance of the system and the required plant height, operation of a 100 kW TGHP has been simulated for 17 different fluids. Accordingly, the corresponding COPs and required heights are determined and based on the achieved COPs, the optimum fluid for each range of building height is selected. The resulting plant heights range from 20 m to nearly 200 m and R245ca is shown to be the most promising fluid for the lowest plant height range. A parametric study is next carried out in order to study the effect of variations in the condensation temperature and the dimensionless plant height on the performance of the system. The obtained results demonstrate that an increase in the former from 313 K to 348 K, for almost all of the analyzed fluids, causes a reduction of around 50% in the COP. It is also shown that, almost independent of the employed fluid, the maximum values of COP are reached for a dimensionless plant height of around 1.8. Moreover, all the analyzed fluids show basically the same COP trend and, at the same operating conditions, the COP values for all fluids are within a 10% range of variation. This leads to the conclusion that the thermophysical properties of the employed fluid mainly influences the required height of the system, while the COP values remain in a relatively small range.

Multiphase flow is involved in a wide range of applications, and among the flow patterns that a multiphase mixture may develop in its flow, the intermittent one is particularly complex both in behaviour and for analysis. Experimental analysis about the characteristics of the flow structures (plugs and slugs) is therefore still mandatory for a detailed description of the phenomenon. In this work an image-based technique for the determination of the plug/slug characteristics was applied to air-water, oil-air and three-phase oil-water-air flows in horizontal ducts with different diameters, with superficial velocities of the phases in the range 0.2–2.1 m/s. The technique is based on the acquisition of a video of the flow and the conversion of each frame (or part of it) into a Boolean signal, in which the non-zero part represents the structure of interest. Concatenation of such signals along the singleton dimension creates a space-time representation of the flow, from which information about the flow velocities, the structure lengths and frequencies and the void fraction can be extracted. Focus here is particularly on the performances of the technique when using high-speed videos. The results were also compared with the predictions of the drift-flux model.

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.

Water transport through the cathode and oxygen presence/absence in the anodic solution, are important for operation and performance of single chamber microbial fuel cells (SCMFCs). This study focused on water transport and biofilm formation on carbon papers with different characteristics: hydrophobicity, thickness, and presence of a micro porous layer (MPL). The results showed that higher hydrophobicity, thicker structures and the presence of the MPL decreased the water transport over time. The carbon papers with low hydrophobicity had complete penetration of biofilms and higher water transport, while those with high hydrophobicity and MPL had no biofilm penetration and lower water transport, indicating a clear correlation of biofilm penetration and water transport across carbon papers. Salt precipitation on carbon papers clogged the pores and resulted in lower water transport. Cyclic voltammograms using Au/Hg microelectrode inserted near the cathode indicated the absence of oxygen in the anodic solution due to the oxygen consumption by aerobic/anaerobic biofilms on cathodes. The results of water transport and oxygen presence illustrated that the biofilms can be used as a cost-effective membrane/separator for SCMFCs.

The cathode performances are one of the main limitations related with microbial fuel cell (MFC) that strongly limit the overall output. The low power produced is an important problem that has to be addressed for the scale up of the MFCs. Cathodic overpotential is particularly high at circumneutral pH when carbonaceous or metallic catalysts are used [1]. Overpotentials are much lower for enzymes but enzymatic biocathodes are expensive and not durable [2]. Single chamber membrane-less MFCs are preferred due to the lower cost and higher power output. In membrane-less MFCs, both electrodes (anode and cathode) are exposed directly to the electrolyte or carbon-rich solution. The presence of growing bacteria inside the chamber generates electron acceptor-rich and electron donor-rich zones, which serve as cathode and anode, respectively. In fact, bacteria consume rapidly the oxygen inside the chamber creating anaerobic conditions that are useful for electrogenic bacteria for colonizing the anode electrode and oxidize organics. The cathode used is an air-breathing cathode and it is porous and allows oxygen to penetrate through the anodic solution. A biofilm is formed also on the cathode surface consuming the oxygen and forming a natural and cheap membrane that separate the cathode from the solution and stop the oxygen leaking inside the chamber, thus ensuring anaerobic conditions. Strict anaerobes grow quickly within 1 week of operations as recently showed [3]. It has been showed previously that the biofilm formation on the cathode initially increased the cathodic reduction reaction [4]. In this work, we analyzed the cathode performances and the MFC overall performance along 6 months operations [5]. At the end of the experimentation, the cathode has been examined with X-Ray Micro Computed Tomography (microCT), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX) for characterizing the organic/inorganic layer on the cathode surface facing the solution [5]. Single chamber MFC with volume of 125 mL was used and the cell was fed with raw wastewater and sodium acetate. The results obtained showed that cathode reduction reaction increased in the first 15 days but then decreased continuously over time [5]. The initial increase might be due to the formation of an initial biofilm layer that probably enhanced the cathodic reduction reaction. Over time, precipitation of compounds and fouling might take place. It has been speculated that thick biofouling decreased the performances. After 6 months, part of the cathode was dried and analyzed with SEM/EDX and the other part was analyzed as it is using microCT [5]. SEM has the great potential of very high resolution but images have to be taken after treating the sample and consequently change the morphology of the biofilm after drying it. Moreover, SEM is not able to pass through objects and consequently only the surface can be analyzed. The coupling with EDX allowed to identify sodium (Na) on the external part of the cathode and calcium (Ca) on the internal face [5]. Na and Ca were present in form of carbonate. The production of OH- due to the cathodic reaction, caused the alkalinization of solution at the interface with the cathode making the precipitation of carbonate species on the cathode is very probable. MicroCT images allowed making a complete 3D scan of the entire cathode analyzed. Interestingly, the different components (cathode, carbonate and biofilm) were clearly distinguishable (Figure 1). In fact, a compact and continuous thick layer of carbonate was clearly visible between the cathode and the biofilm (Figure 1, white color). Those results obtained using microCT clearly showed that the thick, not porous carbonate layer and not the biofilm itself might increase the proton mass transport resistance from the solution to the catalytic sites and lowering dramatically the cathode output during long terms operation. Reference [1] C. Santoro, F. Soavi, A. Serov, C. Arbizzani, P. Atanassov. Biosensors and Bioelectronics. 2016, 78, 229-235. [2] C. Santoro, S. Babanova, B. Erable, A. Schuler, P. Atanassov. Bioelectrochemistry. 2016, 108, 1-7. [3] M. Grattieri, S. Babanova, C. Santoro, E. Guerrini, S.P.M. Trasatti, P. Cristiani, M. Bestetti, P. Atanassov. Electroanalysis, 2015, 27(2), 327-335. [4] P. Cristiani, M.L. Carvalho, E. Guerrini, M. Daghio, C. Santoro, B. Li. Bioelectrochemistry, 2013, 92, 6-13. [5] M. Santini, M. Guilizzoni, M. Lorenzi, P. Atanassov, E. Marsili, S. Fest-Santini, P. Cristiani, C. Santoro. Biointerphases. 2015, 10, 031009. Figure 1. Cross Section of the cathode analyzed using microCT. Figure 1

The transition from annular to wavy-stratified oil–water adiabatic flow within horizontal pipes is experimentally analyzed, and a semiempirical model is proposed. The transition is referred to as critical because it occurs suddenly, giving rise to a sharp and strong increase in the pressure drop due to the contact of the high-viscosity oil with the pipe wall. This could lead to a dangerous accident in pipelines. Experimental runs were performed on eight test sections of both Plexiglas® and Pyrex® pipes with internal diameters ranging from 21.5 to 50 mm, using tap water and oil with viscosity about 880 times higher than that of water. On the basis of pressure drop measurement and flow pattern visualization, the transition boundary between annular and wavy-stratified flow was analytically determined and compared with flow pattern maps.

The shape and contact angles of drops on curved surfaces is experimentally investigated. Image processing, spline fitting and numerical integration are used to extract the drop contour in a number of cross-sections. The three-dimensional surfaces which describe the surface-air and drop-air interfaces can be visualized and a simple procedure to determine the equilibrium contact angle starting from measurements on curved surfaces is proposed. Contact angles on flat surfaces serve as a reference term and a procedure to measure them is proposed. Such procedure is not as accurate as the axisymmetric drop shape analysis algorithms, but it has the advantage of requiring only a side view of the drop-surface couple and no further information. It can therefore be used also for fluids with unknown surface tension and there is no need to measure the drop volume. Examples of application of the proposed techniques for distilled water drops on gemstones confirm that they can be useful for drop shape analysis and contact angle measurement on three-dimensional sculptured surfaces.

A deep understanding of the behaviour of sprays is of great importance in several fields, both industrial (combustion, heat transfer, spray deposition and painting, etc.), agricultural and medical, and the study of the behaviour of single drops is a fundamental step towards the comprehension of the whole phenomenon. Focusing the attention on spray/surface heat transfer problems, the most interesting aspect is the interaction of sessile and impinging drops on solid surfaces. The description or the prediction of the drop shape parameters (height, contact angle and contact area) is a necessary pre-requisite for the thermal analysis, especially when pure evaporation is involved. This paper describes an experimental analysis on the shape and behaviour of sessile and impinging water drops on smooth and microfinned surfaces of high effusivity materials, in isothermal condition and during evaporation. Drop shape, contact angle and contact area are characterised as a function of the surface material, microfins geometry and Weber number for impinging drops. Results about the evaporation transients for sessile drops on aluminium, brass and carbon steel surfaces are presented.

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.

Power output limitation is one of the main concerns that need to be addressed for full-scale applica- 28 tions of the microbial fuel cell technology. Fouling and biofilm growth on the cathode of single 29 chamber microbial fuel cells (SCMFC) affects their performance in long-term operation with 30 wastewater. In this study, the authors report the power output and cathode polarization curves of a 31 membraneless SCMFC, fed with raw primary wastewater and sodium acetate for over 6 months. At 32 the end of the experiment, the whole cathode surface is analyzed through x-ray microcomputed to- 33 mography (microCT), scanning electron microscopy, and energy-dispersive x-ray spectroscopy 34 (EDX) to characterize the fouling layer and the biofilm. EDX shows the distribution of Ca, Na, K, 35 P, S, and other elements on the two faces of the cathode. Na-carbonates and Ca-carbonates are pre- 36 dominant on the air (outer) side and the water (inner) side, respectively. The three-dimensional 37 reconstruction by x-ray microCT shows biofilm spots unevenly distributed above the Ca-carbonate 38 layer on the inner (water) side of the cathode. These results indicate that carbonates layer, rather 39 than biofilm, might lower the oxygen reduction reaction rate at the cathode during long-term 40 SCMFC operation

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.

Gas diffusion medium (GDM) is a crucial carbon-based component for water management in polymer electrolyte membrane fuel cells (PEMFCs). GDM consists of a macro-porous substrate, which is usually a carbon cloth or carbon paper, coated with a thin micro-porous layer (MPL). Traditionally, PTFE is used as hydrophobic agent and binder. In this work, three different GDMs were prepared using fluorinated polymers (polytetrafluoroethylene-co-hexafluoro-propylene (FEP), polytetrafluoroethylene-co-perfluoroalcoxy vinyl ether (PFA) and a fluorinated polyurethane based on perfluoropolyether blocks (PFPE)) in order to replace conventional PTFE as hydrophobic agent, which was used as reference. Inks composition and rheological behaviour were fixed in order to apply the blade coating technique for MPL deposition. These polymers allowed to decrease temperature during thermal treatments with respect to the one necessary for treating PTFE-based MPLs. Moreover, superhydrophobic layers were obtained for all the samples and the final electrical performances of the whole fuel cell system were improved with respect to the ones reached with PTFE-based samples. In particular, FEP allowed to reduce significantly mass transfer resistances in the operating conditions of the fuel cell.

When considering the flow of a two-phase mixture, the complete characterization of its behavior requires not only the evaluation of its temperature and pressure evolutions, but also the description of the distribution of the phases (the so-called "flow structure"). This is of great importance in problems concerning the analysis of the solicitations undergone by the containing ducts, both from a mechanical point of view and even more important when heat transfer at fixed power is involved. In the present paper some methodologies are presented, which proved to be very useful in the description and quantitative visualization of the structure of a two-phase flow: for liquid gas mixtures, they are based on relatively simple experimental acquisitions of a single quantity and allow a detailed visualization of the flow structure without requiring visual observation of the same.

A non-conventional heat pump working by a difference in density between two branches of a hydraulic vertical loop has been described. This system called thermogravimetric heat pump, TGHP, operates with a non-conventional regenerative thermodynamic cycle which remarkably improves COP values. The lower density in the ‘downward branch’ is obtained by a liquid–vapour two-phase flow. Performances and main geometrical characteristic trends, such as plant height Z and two-phase column diameter D T–PD have been drawn, varying the minimum cycle temperature between 15 and 25 °C and the user temperature, T max , in the range 60–70 °C. The carrier fluid is demineralized water; according to the peculiar working fluid—PP 50, HFC 134a and HFC 338cca—different solutions can be obtained, such as for 10–12 storey buildings or for skyscrapers. Yet, the results obtained with HFC 338cca must be accepted with some cautions while waiting for a better characterisation of such fluid. Chemical compatibility, thermal stability, environmental impact have been also taken into account in the choice of the operating couple, carrier fluid—working fluid. While the thermodynamic conversion process is non-conventional, the TGHP can be assembled by standardised technology. The compressor of a conventional plant is here replaced by a feeding pump and COP values obtained through a regenerative TGHP are globally larger than those of a common heat pump.

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.

Two-phase flows of viscous oil and water through singularities such as sudden area contractions/expansions have been taken into limited consideration in the relevant scientific literature. Nevertheless, they play a role of primary importance in industrial systems, for instance, but not only, in the exploitation of oil wells and pipelines. The proposed work is based on the comparison of photographic images of the flow patterns taken from three points of view, i.e. upper, lower and frontal, thanks to a couple of mirrors ±45° inclined with respect to the horizontal plane. Oil-water flow regimes have been observed both upward and downward of five horizontal test sections with diameter ratios d/D = 40/50, 30/50, 30/40, respectively. The observed structures of the oil-water interface, especially for core-annular flows, has suggested also detecting flow patterns in a 30 mm straight pipe for sake of comparison. Actually, the shape of the oil-core interface appears significantly influenced by the sharp-edged area change as well as by the expected momentum variation.

Surface roughness, porosity and contact angles of different carbon paper materials (TORAY paper with PTFE from 0% to 60% of and SGL paper with 0% and 20% of PTFE) suitable as electrodes in microbial fuel cells were investigated. The changes of contact angle between dry and clean anode surfaces and the ones after exposure to wastewater were measured using different liquids (pure water and sodium acetate solutions). The results showed that bacterial attachment to the carbon papers caused a significant decrease in the contact angle, shifting the surface property from highly hydrophobic to slightly hydrophobic or even hydrophilic. The quantity of biofilm attached on the anode surface decreased with the increase in PTFE content. Positive correlation between dry biomass content and the amount of pores at the small scale (5–10 lm) was observed. The start up time of MFCs was shortened by using the carbon anodes without PTFE or with low PTFE content (

The investigation of multiphase interfaces and contact lines is a challenging research topic for comprehension of countless physical problems, both natural and imitated from nature, at the microscale (nearly 1 μm). In presence of a three-phase interaction (liquid–gas–solid, or two immiscible liquids and a solid), different forces influence the evolution of the system and lead it towards its mechanical equilibrium. Even for the seemingly very simple case of drops on solid surfaces, there are many attempts to investigate the involved effects, with the main aim to profitably replicate them for functional applications (biomimetics). Here we show that it is possible to visualize the real multiphase interface by X-ray micro-computed tomography (microCT), which is essentially a full volume 3D microscope, non-intrusive, with micrometric spatial resolution and suitable to operate even with matter opaque at visible wavelength. Consequently, microCT can also be applied to investigate complex wetting scenarios, to validate the theoretical models already available for such cases.

The influence of endogen and exogen variables on the performances of small-scale trigenerative plants is investigated by means of a simulation model where the prime mover and the other components are considered as “black boxes” characterized by their performance parameters. The operating policy of the plant is not imposed a priori, but it is on the contrary dynamically and automatically chosen by the model itself using linear optimization techniques. The “constitutive equations” which formalize the relationships between the plant components and the energetic and economic target functions are expressed using linear terms only. By parametrizing the characteristics of the involved machines and of the end users, such a model allows to evaluate the influence of each parameter on the performances of the whole system. The aim is to give useful indications to the manufacturers, designers and end users in the field of trigeneration. The analysis is performed at two different level of details: the first one with a simplified version of the model, for a preliminary evaluation of the influence of the most important parameters, the second one using the complete model, focusing the attention on the Italian situation in terms of tariffs for energy and external temperatures.

The rheology of microporous layer (MPL) inks for polymer electrolyte membrane fuel cell (PEM-FC) applications was investigated, and its effect on thickness and morphology of final layer, prepared via doctor-blade technique, was examined. The effect of the carbon black (CB) on slurry viscosity was studied in the range CB/water = 0.11/0.17 (g/g) as well as the effect of surfactant and PTFE (Teflon®) addition. Slurry viscosity is mainly determined by the CB/water and CB/surfactant ratios, while PTFE addition has minor influence. Viscosities in the range 0.05–0.06 Pa s (at shear rate 100 s−1) were found to be appropriate for coating deposition. Two substrates of different morphology, a carbon cloth (CC) and a woven non-woven (WNW), were coated with slurries of different composition but showing the same rheology. Notwithstanding the use of slurries with the same rheological behavior, different MPL coating thicknesses were obtained. This suggested that the substrate morphology has a non-negligible influence on the final MPL thickness. The MPL layer definitely increased the hydrophobicity, and in some samples the region of superhydrophobicity (contact angle >150°) was reached. In-plane electrical resistance of gas diffusion layers (GDLs), i.e. substrate coated with MPL, was measured and it was found that the two different substrates have no influence on it (about 0.8 Ω). FCs assembling different GDLs showed different electrochemical performances in terms of I–V curves: MPLs coated onto WNW gave somewhat higher power densities (0.40 W/cm2 at 0.5 V).

Microbial fuel cell (MFC) is a promising technology that explores biological and electrochemical processes to generate electricity from variety of organic compounds (wastes and wastewater) [1]. Developed at the beginning of 20-th century, regarding the significant achievements, MFCs are still “lab stage” devices. One way to overcome the “lab stage” is the materials characterization and optimization. Besides the necessity of well studied and understood bacterial behavior in MFCs, the knowledge of how materials and design, as well as design parameters are influencing MFCs performance is a main task placed in front of the researchers in this area. In the traditional fuel cells, the structure-to-property modeling is recognized as an effective approach to identify the key parameters influencing the system behavior and to discover the correlations between these parameters and the final characteristics. The same approach must be introduced in MFCs in order to address the questions that the conventional fuel cells faced and overcame years ago. In this study, structure-to-property relationship of different carbonaceous materials suitable for anode and cathode electrodes development have been developed in details and related to the performance of these electrodes was explored in real MFCs. Commercially available carbon paper (Toray® paper) was studied as material for anodes preparation. Surface parameters, such as wettability, porosity and roughness have been determined and related to the bacterial attachment and biofilm formation, as well as MFCs start up time. On the other end “home-made” activated carbon was studied as a material for the design of cathode electrodes and optimized varying the applied pressure and the magnitude of the temperature treatment step. These two parameters showed significant influence on the cathodes surface chemistry and morphology and thus on the cathodes electrochemical behavior. In both cases surface-to-property relationship approach was applied to understand the similarities and differences in the studied electrodes and highlight the properties that are important for improved MFCs performance. Carbon paper (Toray® H-090) with different PTFE content (0, 20, 40 and 60%wt PTFE) was used as anode material. The results showed that the increase in PTFE content led to an increase in roughness in both macro (100-300 μm) and micro-scale (5-10 μm) along with an increase of the porosity at macro-scale underling the presence of higher number of large pores [2]. At the contrary, the higher PTFE content led to a lower number of small pores (5-10 μm) that are the one preferred by bacteria for bacterial attachment and biofilm formation [2]. The contact angles measured varied between 135° and 155°, showing high hydrophobicity independent from the PTFE amount. After immersion in wastewater for 2 weeks, the contact angle dropped dramatically to slightly hydrophobic or completely hydrophilic. This phenomenon was mainly due to biofilm attachment on the surface that enhanced the wettability of the materials. Variation of anodes weight over time was also monitored to correlate the materials surface properties to bacteria attachment and further biofilm formation. The materials lost their hydrophobicity proportionally to the PTFE content and due to that the Toray® carbon paper with low PTFE content (0 and 20%wt PTFE) had higher increase in weight (wet and dry) compared to the other materials tested (40 and 60%wt PTFE) (Fig. 1) [2]. The reason is the increased number of small pores, which enhanced biofilm formation. The start up trend followed the biofilm attachment with the faster start up achieved by the Toray® with no PTFE treatment (Fig. 1) [2]. All of these points out the importance of the hydrophilic/hydrophobic properties and surface morphology on biofilm formation and subsequently the start up period of MFCs [2]. Even more detailed study was carried out for the developed gas-diffusion cathodes based on activated carbon. In this case except the surface morphology parameters, the surface chemistry and charge transfer resistance were included in the surface-to-property study and correlated with the electrochemical performance of the electrodes. Significant dataset was collected and processed through Principal Component Analysis (PCA), which is a statistical tool for data analysis (Fig. 2). The change in surface chemistry (determined using x-ray photoelectron spectroscopy) due to PTFE variation was found to have significant influence on the power output. The highest current was observed for samples with largest amounts of carbon oxides and oxygenated tetrafluroethylene (cathodes treated at 200°C). At the contrary, the highest resistance as well as lowest performance was noticed for the sample having increased amount of fully fluorinated carbons. It was found out that the magnitude of the applied pressure also determines the resistance of the cathodes showing reverse proportionality. Based on this study we can conclude that a step forward in the MFCs development can be done only with detailed investigation of surface-to-property relationships. Further studies on materials towards deeper characterizations should be addressed and parameter considered until know as insignificant or completely ignored have to be examined. [1] C. Santoro, Y. Lei, B. Li, P. Cristiani. Biochemical Engineering J. 2012;62:8–16. [2] C. Santoro, M. Guilizzoni, J.P. Correa Baena, U. Pasaogullari, A. Casalegno, B. Li, S. Babanova, K. Artyushkova, P. Atanassov. Carbon, 2013. DOI : 10.1016/j.carbon.2013.09.071

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