1,721,218 research outputs found
Thermodynamic optimization of a solar adsorption cooling system.
No full textA dynamic model for the simulation of a solar based adsorption chiller system for domestic air conditioning is
presented. Solar irradiation is captured via an array of Parabolic Trough solar Collectors (PTCs), and stored via a
properly sized hot water tank. A commercial silica gel adsorption machine drives the HVAC system. Care is given
to the estimate of the available solar energy, and to the evaluation of the collector efficiency and its dependences
on geometrical and surface properties, as well as on the direct solar radiation and ambient temperature. System
performance at different locations is discussed, showing that in temperate climates (latitude ranging from 38° to
46°) it can offer a 4-6 kW cooling flow under most conditions, with 3-4 PTC panels. A tank size of around 1000
litres is enough for all of the investigated locations, and little advantage can be achieved by larger ones. The option
of the combined use as summer chiller and winter heat pump, and direct heating via PTC hot water is discussed
Numerical Modeling of Anti-Icing Operations Over Super-Hydrophobic Surfaces
No full textHydrophobic and super-hydrophobic coatings are currently considered promising tools to enhance the performances of thermal devices for in-flight icing protection. Icing protection is mostly obtained via heating of the critical surfaces: the simulation is quite complex, including the need for CFD solution of the flow around the wing, including the tracking of the cloud supercooled droplet, the simulation of the evolution of the interaction between droplets and airfoil (including coalescence into rivulets and continuous film, evaporation due to the de-icing heat flux, freezing on impact or later...). However, current commercial numerical prediction tools for in-flight icing simulation are based on the Messinger model [1], although extended to 3-D arbitrary surfaces [2], coupled with Lagrangian or Eulerian [3] droplet flow field analysis. Messinger's model assumes that the runback water layer is a continuous film driven by the shear stress and provides reliable and accurate results in several applications. Unfortunately, it intrinsically neglects any wettability effect and thus cannot assess hydrophobic coating performances. The main difficulty in the simulation of such coatings is that it involves an inherently multi-scale problem: wettability operates at a small scale, at most of the order of the single impinging droplet, but the local impinging mass flow, shear stresses, and heat transfer convective coefficients require state-of-the-art CFD computation around a whole wing or a whole aircraft. Since it is not practical to manage the large-scale computations with a grid fine enough to resolve the single droplet evolution (e.g., via VOF approach), a kind of intermediate bridge is required to derive average integral corrections from the small scale and transfer them to a coarser grid for the standard CFD solution of the large scale thermal and flow field. Here, a hierarchical approach is followed: first, a high-fidelity, small-scale model defines statistical distributions of relevant properties (rebounding droplet fractions, average runback water velocities, wet area fractions, coalesced droplet diameter distribution, including heat transfer and phase changes) as a function of the local fluid dynamic conditions; as a second step, statistical correlations are extracted by this small-scale computation campaign, providing integral corrections to the larger-scale CFD simulation. The high-fidelity simulation tool, an individual-based droplet phenomenological model, was described and validated in [4], and here is improved with regard to the phase change modeling (for both the ice beads freezing and the droplet evaporation due to de-icing system heat flux). The present work is then focused on the statistical models derived by the high- fidelity results, with special attention to the distribution of the small scale droplets (i.e., below the radius where coalescences become the main growth effect), following the approach described in [5] under different conditions. Such small droplets are of special interest for the antiicing devices, which ideally should operate mostly under such regime, allowing for quick evaporation and minimization of runback water rivulets. The statistical model is coupled with a CFD simulation of the heat and fluid flow around an airfoil under icing condition and a lagrangian droplet tracing code, allowing for thoughtful final validation versus literature experimental data provided in [6]. Finally, a parametrical analysis is conducted commenting on the usefulness of hydrophobic coatings of different properties for anti-icing operations under various environmental conditions. It demonstrates that it may offer a valuable aid in obtaining dry clean surfaces with no or little runback water with a relatively low energy consumption
Numerical simulation of dropwise condensation over hydrophobic surfaces using vapor-diffusion model
No full textDropwise condensation of humid air over hydrophilic and hydrophobic surfaces is numerically investigated using a phenomenological, Lagrangian model. Mass flux through droplets free surface is predicted via a vapor-diffusion model. Validation with literature experimental data is successfully conducted at different air humidities and air velocities. The accuracy of the implemented condensation model is compared with a standard analogy between convective heat and mass transfer, showing that the latter is not able to predict heat transfer performances in the investigated air velocity range
Energetical, environmental and economical feasibility analysis of a wind powered desalination plant in Sardinia
No full textNumerical analysis of surface coatings performances for in-flight icing device performance enhancement
No full textRecent developments in manufacturing techniques offer a wide range of high-performance coatings designed to improve heat and mass transfer processes. Superhydrophobic surfaces may enhance the performances of anti and de-icing devices for in-flight ice applications. Numerical modeling of the effect of such devices requires the analysis of the behavior of an ensemble of individual droplets, rather than assuming a continuous film as in standard icing codes. Here an individual-based method (IBM) is adopted. Droplets are assumed as the smaller element of the simulations, and phenomenological sub-models describe their behavior through impact on the surface, possible rebound, growth via coalescence or phase change (either freezing or evaporation), transition from still droplet to moving ones, the transition from running droplets to rivulets. The model is validated versus literature experimental data, showing the capability of the approach. It appears that hydrophobicity is not yet enough to build a passive anti-icing system but can significantly enhance the performances of typical thermal devices
La pubblicità televisiva: aspetti linguistici e retorici degli spot a episodi
No full textIn this article we analyse from the rhetoric and linguistic point of view some spots belonging to the television advertising campaigns of a famous Italian coffee brand (Lavazza) and of a multinational mobile phone company (Vodafone). As regards the rhetoric analysis, we try to describe some characteristics of the protagonists of the spots (i.e. of the testimonials) that constitute their ethos, i.e. their credibility and personal appeal towards the audience. We consider also the relationships between the testimonials and the emotions and the psychology (i.e. the pathos) of the audience. At a more detailed level, we pay attention to the figures of speech which are typically used in the spots (mainly word games such as paronomasia, calembour and the like). As regards the linguistic analysis, we focus attention on the way of speaking of the testimonials and of the secondary characters and make some sociolinguistic considerations about the language varieties used (mainly the variety of Italian spoken in Rome, but sometimes together with other Italian varieties and even foreign languages). At a more detailed level, we describe some phonetic, phonological, grammatical and lexical phenomena which characterize the way of speaking of testimonials and of minor characters
A selection criterion of solutions to a system of eikonal equations
No full textIn this article we consider a system of eikonal equations with a Dirichlet boundary condition. We propose a variational method to select the class of solutions which minimize the discontinuity set of the gradient
Numerical simulation of film instability over a corrugated sheet
Full text linkThe evolution of a liquid layer flowing down a corrugated sheet driven by gravity, which is the characteristic configuration of structured packing, is numerically analysed via the solution of the governing lubrication equation, which reduces the 3D physical problem to a 2D mathematical problem. Disjoining pressure is used to model contact line dynamics and surface wettability, while full implementation of capillary pressure allows to investigate contact angles up to 60°. The effect of corrugation is introduced via the definition, in the governing lubrication equation, of non-uniform gravity acceleration. Furthermore, the additional capillary force, arising from variations in the solid surface curvature, is also implemented. Different corrugation geometries and flow conditions, the latter being defined by the reference film Capillary number, are investigated, looking for configurations that allow enhancement of the liquid-gas interface area at low liquid flow rate. Such an analysis provides a novel approach in the design of structured packing, widely used in carbon capture via chemical absorption
Numerical modelization of contact angle hysteresis of falling droplet under enhanced lubrication approximation
Full text linkMoving contact lines are involved in several engineering applications: in in-flight icing phenomenon, the eventual transition from droplet to rivulet or continuous film regime is crucial for the prediction of ice accretion over the aircraft surface; absorption process through structured packing is also characterized by a thin film flowing over the corrugated sheets. Disjoining pressure together with the assumption of a thin precursor film is largely used in numerical simulations of thin films and moving droplets in order to model the dynamics of moving contact lines and the surface wettability properties, in terms of imposed static contact angle. The disjoining pressure model was largely validated in case of falling films with the well known Voinov-Hoffman-Tanner law. On the other side, the capability of the disjoining pressure to model the contact angle hysteresis, which is a crucial parameter for predicting moving droplets behavior, has not been discussed yet. Here, numerical simulations of both falling films and moving droplets under lubrication approximation are conducted and the disjoining pressure model is used to predict the contact line dynamics. After verification of the full curvature implementation for a 1D falling film, the effective contact angle hysteresis is estimated for a moving droplet under different flow conditions and the transition from droplet to rivulet regime detected
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