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One of the most critical issues for wind energy exploitation is the high variability of the resource, resulting in very difficult forecasting of the power that wind farms can grant. A vast literature has therefore been devoted to wind speed and wind power quantitative forecasting, using different techniques. The widely used statistical and learning models that are based on a continuation in the future of the series’ past behaviour offer a performance that may be much less satisfactory when the values suddenly change their trend. The application to wind speed data of two techniques usually employed for the technical analysis of financial series–namely support and resistances identification and candlestick charts–is investigated here, with the main aim to detect inversion points in the series. They are applied to wind speed data series for two locations in Spain and Italy. The proposed indicators confirm their usefulness in identifying peculiar behaviours in the system and conditions where the trend may be expected to change. This additional information offered to the forecasting algorithms may also be included in innovative approaches, e.g., based on machine learning.

Scientific payloads onboard CubeSats usually have complex geometries and occasionally narrower allowed temperature ranges with respect to the rest of the spacecraft. In these cases, the capability to correctly predict the thermal behaviour of the payload once in orbit is mandatory. To achieve this ability, a thermal balance test is required to correctly identify the thermal model of the payload. The test consists in the application of different external thermal boundary conditions together with the addition of heat dissipation to simulate the thermal load produced by active electronics during operation. Those experimental data are fundamental to validate the numerical thermal model and make its predictions reliable. This paper presents the configuration and procedures of the thermal balance test performed on the Demonstration Model of the payload to be embarked on each satellite of the HERMES constellation. The test data is compared with the results of a finite volume thermal model of the payload, proving the application of this method to be reliable for space thermal analyses. The obtained test results show the temperature jumps caused by the heat dissipation applied to active components. A weak correlation between the payload interface and internal equipment has been observed, thus proving that the payload is almost decoupled from the Service Module temperature variations. Based on test outcomes, some modifications in the payload design have been implemented, with the aim to lower the operative temperature on critical, temperature-sensitive equipment.

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.

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.

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.

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.

Drop impacts (onto dry or wet surfaces or into deep pools) are important in a wide range of applications, and, consequently, many studies, both experimental and numerical, are available in the literature. However, such works are focused either on statistical analyses of drop populations or on single drops. The literature is heavily lacking in information about the mutual interactions between a few drops during the impact. This work describes a computational fluid dynamics (CFD) study on the impact of two, three, and four synchronized drops into a deep pool. The two-phase finite-volume solver interFoam of the open source CFD package OpenFOAM&reg, was used. After validation with respect to high speed videos, to confirm the performance of the solver in this field, impact conditions and aspects that would have been difficult to obtain and to study in experiments were investigated: namely, the energy conversion during the crater evolution, the effect of varying drop interspace and surface tension, and multiple drop impacts. The results show the very significant effect of these aspects. This implies that an extension of the results of single-drop, distilled-water laboratory experiments to real applications may not be reliable.

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 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.

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.

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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