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Fluids and Surfaces
Full text linkFluids is pleased to present a Special Issue named “Fluids and Surfaces”, a curated collection of ten research articles focused on capillary phenomena and the interaction between fluids and surfaces [...
Modeling of Conventional Heat Pipes with Capillary Wicks: A Review
Full text linkConventional heat pipes (CHPs) with capillary wicks are fundamental in various engineering applications due to their exceptional heat transfer efficiency and minimal temperature gradients. Despite the recent advancements in heat pipe modeling, existing reviews predominantly emphasize loop or pulsating heat pipes, neglecting the extensive application and design challenges associated with CHPs. This review aims to address this lack by providing a comprehensive analysis of existing modeling techniques for CHPs, with a specific focus on their methodological innovations, validation strategies, and limitations, in order to outline a structured classification of models and provide useful suggestions for future research. The main findings of this work reveal a predominance of numerical lumped parameter models, which balance simplicity and computational efficiency, but often oversimplify complex phenomena. In fact, although numerical 2D and 3D models could offer greater accuracy at higher computational costs, they often share similar limitations with lumped parameter models. Additionally, some crucial aspects, including gravitational effects, real gas behavior in vapor modeling, activation effects, and operating limits, remain underexplored. Therefore, future research should address these gaps, to enhance the applicability of CHPs across different fields and operating conditions. In particular, an integrated approach is recommended, combining physics-based models with data-driven techniques, and supported by a robust and systematic experimental validation strategy, to ensure the reliability and generality of the developed models. Such modeling efforts are expected to guide the development of more effective and reliable heat pipe designs
Axisymmetric Drop Shape Analysis using a low-cost home-made setup
Full text linkSurface 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
Wettability analysis of desiccant beads for HVAC systems
Full text linkVapour 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
Crater Depth after the Impact of Multiple Drops into Deep Pools
Full text linkMany 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
A novel CFD model for design and performance prediction of recuperators for Indirect Evaporative Cooling
Full text linkIndirect evaporative cooling (IEC) appears to be a highly promising technology for incorporating and/or substituting traditional air conditioning systems, as it can guarantee good cooling performance with a reduced environmental impact. In this study, a Computational Fluid Dynamics (CFD) model for design and performance prediction of recuperators for IEC systems with dry primary and secondary channels was developed. The model was validated against experimental data for a cross-flow recuperator, obtaining a maximum difference between numerical and experimental results of 4.9% for the secondary air outlet temperature, 5.3% for the primary air outlet temperature, and 8.1% for the dry-bulb effectiveness. After validation, the model was used to find a new plate geometry which guarantees a 12.5-15.9% improvement in the dry-bulb effectiveness, without an excessive increase in the pressure losses along the channels
Liquid holdup measurement for gas-liquid stratified flows by means of resistive probes and image processing
Full text linkFlow patterns exert a fundamental influence on the behaviour of multiphase flows, and they must be brought into play when dealing with their modelling. This is usually done by means of summarizing quantities as the phase holdups and the interfacial area concentration. Many techniques have been designed during the years to measure them, among which the use of probes relying on electrical resistance is one of the simplest and less expensive. While having these points of strength, resistive probes are intrusive devices. This work is therefore devoted to a comparison between liquid height (and derived quantities) measurements - for stratified and stratified-wavy air-water flows - performed using a conventional resistive probe and by means of an image-based technique. Validation of the latter was performed using computer-generated flow images. Then, an experimental campaign was carried out for flows with liquid superficial velocities in the range 0.03 ÷ 0.06 m/s and gas superficial velocities in the range 0.77 ÷ 2.31 m/s. Results showed that the two methods give answers within very few percent of difference, which is more than satisfactory in this field. The results are also in good agreement with some of the most credited literature models and correlations
Wind power 24-h ahead forecast by an artificial neural network and an hybrid model: Comparison of the predictive performance
No full textIn 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
High energy rapid modular ensemble of satellites payload thermal analysis using openfoam
Full text linkIn a space mission design, the goal of the thermal control subsystem is to ensure that all components of the satellite stay within their operating temperature ranges. The problem becomes more critical if the mission involves nanosatellites with an astrophysics payload requiring a dedicated thermal design to keep it at low temperature, as in the case of the High Energy Rapid Modular Ensemble of Satellites mission. Therefore, a thermal analysis is performed using ESATAN, one of the most-used software packages among those suggested by the European Cooperation for Space Standardization. This gives a good overview of the temperature fields, but many geometrical simplifications must be introduced in the model and all thermal interfaces must be checked. In order to assess the effect of the latter, the ESATAN results are cross checked with those obtained using another software, OpenFOAM, in which the geometry is created directly from the computer-aided-design model of the satellite, resulting in a very accurate geometrical representation. By first comparing the view factor calculations against analytical test cases, some issues emerged. These are thoroughly analyzed and discussed, and it is finally shown that they do not affect the temperature field. The high level of detail of OpenFOAM highlighted interesting possible improvements in the thermal configuration of the satellite
Thermal performance against gravity of an AlSi10 AM heat pipe with a diamond lattice structure
Full text linkMetal Additive Manufacturing has gained momentum as a viable production technique for specialized heat transfer devices, in particular for industrial sectors characterized by small production numbers associated with high-performance requirements. Within the space industry, ESA is leading and funding several applied research programs to explore the possibility to employ AM for building electronic boxes with embedded heat pipes, in order to reduce manufacturing post-processing steps and contact thermal resistances. In the context of tender 1-10238, the project "Heat Pipe Solutions for High Power Systems"(HPS2) has developed and tested lattice-based heat pipes, that are intended to be integrated in electronic modules. In this paper, the heat transfer performances as function of input power and tilt angle against gravity of a 150 mm long heat pipe with a 20 mm evaporator section and a 40 mm condenser section are presented and compared with the results of the models of performance limits, based on the measured properties of the lattice
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