Numerical simulation of thin liquid films over a solid non-wettable substrate assuming lubrication approximation.

When a continuous film flows over a non-wettable surface, it may break up with the consequent formation of a dry-patch. The actual shape of the resulting water layer is of great interest in several engineering applications, such as in-flight icing simulations, finned dehumidifier behavior modeling, coating process and chemical absorption/distillation through structured packing. A 2D numerical solver for the prediction of film flow is presented. The lubrication approximation is assumed, allowing for the description of liquid film flowing down an inclinate plate, driven by both gravity and shear. The effects of contact line and surface wettability are introduced combining the precursor film model and the disjoining pressure terms. The capillary pressure, which is usually modeled through the small slope approximation, is here defined imposing the membranal equilibrium of the gas-liquid free-surface, in order to investigate higher values of the imposed equilibrium contact angle between liquid and solid substrate. The in-house solver is first validated with both experimental and numerical results, available in literature. Numerical simulations are then performed with the aim of studying the beahvior of liquids in absorption/distillation process through structured packing. The lubrication theory is finally extended to the most general case of a 3D curved substrate, allowing to investigate problems involving complex geometries and configurations. Thus, a liquid film flowing down a packing layer, which is a wrinked surface composing the packed column used in absorption process, is simulated. Such a problem has been investigated in literature by means of a fully 3D approach only, but the huge computational costs do not allow to investigate several configurations, resulting in a lack of knowledge of the hydrodinamics driving the liquid flowing through the packing layers. The full modeling of the capillary pressure and the extension to general curved substrates clearly put a new effort to the well known lubrication theory and allow to simulate phenomena, that were not covered by such a theory.When a continuous film flows over a non-wettable surface, it may break up with the consequent formation of a dry-patch. The actual shape of the resulting water layer is of great interest in several engineering applications, such as in-flight icing simulations, finned dehumidifier behavior modeling, coating process and chemical absorption/distillation through structured packing. A 2D numerical solver for the prediction of film flow is presented. The lubrication approximation is assumed, allowing for the description of liquid film flowing down an inclinate plate, driven by both gravity and shear. The effects of contact line and surface wettability are introduced combining the precursor film model and the disjoining pressure terms. The capillary pressure, which is usually modeled through the small slope approximation, is here defined imposing the membranal equilibrium of the gas-liquid free-surface, in order to investigate higher values of the imposed equilibrium contact angle between liquid and solid substrate. The in-house solver is first validated with both experimental and numerical results, available in literature. Numerical simulations are then performed with the aim of studying the beahvior of liquids in absorption/distillation process through structured packing. The lubrication theory is finally extended to the most general case of a 3D curved substrate, allowing to investigate problems involving complex geometries and configurations. Thus, a liquid film flowing down a packing layer, which is a wrinked surface composing the packed column used in absorption process, is simulated. Such a problem has been investigated in literature by means of a fully 3D approach only, but the huge computational costs do not allow to investigate several configurations, resulting in a lack of knowledge of the hydrodinamics driving the liquid flowing through the packing layers. The full modeling of the capillary pressure and the extension to general curved substrates clearly put a new effort to the well known lubrication theory and allow to simulate phenomena, that were not covered by such a theory

Performance assessment of electro-osmotic flow of rectangular microchannels with smoothed corners

Microchannel heat sinks are a viable alternative to traditional thermal management systems when high fluxes over small surfaces are involved. To avoid high pressure drops especially when liquids are concerned, electro-osmotic flow, a phenomenon which is relevant at the microscales only, can be employed profitably. Joule heating, which occurs every time an electrical current is circulated through a conductor with finite electrical resistance, may hamper the application of electro-osmotic flows significantly; its effects must therefore be investigated, as should the influence of the entry length on the overall transport phenomena which occur in the microchannel, especially so since channels with uniform temperature at the walls tend to be somewhat short, to mitigate heat generation due to Joule heating. In this paper the transport phenomena occurring within a microchannel of rectangular cross-section with uniform wall temperature through which an electro-osmotic flow occurs is studied, while considering the flow fully developed hydrodynamically but thermally developing (Graetz problem). The corners are then smoothed progressively and the effect of this change in the shape of the cross-section over the non-dimensional dissipated power or temperature difference between wall and fluid is investigated using the performance evaluation criteria introduced by Webb. Correlations are suggested for the Poiseuille and Nusselt numbers for all configurations as are criteria to obtain the maximum allowable channel length, i.e. the length of the channel over which the walls start to cool the fluid, owing to Joule heating, in terms of the hydraulic diameter

Numerical Modelling of Droplets and Beads Behavior over Super-Hydrophobic and Hydrophilic Coatings under in-Flight Icing Conditions

Featured Application: Use of hydrophobic coatings as passive inflight icing safety device. Current technology has produced a wide range of advanced micro-structured surfaces, designed for achieving the best wettability and adhesion performances for each specific application. In the context of in-flight icing simulations, this opens new challenges since the current most popular and successful ice accretion prediction tools neglect the details of the droplet behavior opting for a continuous film model. Here, a phenomenological model, following, in a Lagrangian approach, the evolution of the single droplets from the impinging to the onset of rivulets, is developed to simulate the performances of super-hydrophobic surfaces in icing application. Possible rebound and droplet spread on the impact, coalescence, single ice bead formation and droplet to rivulet transition are taken into account. The first validation shows how the models are able to predict the anti-icing capability of a super-hydrophobic surface coupled with a heating system

Temporal dynamics of hippocampal neurogenesis in chronic neurodegeneration

The study of neurogenesis during chronic neurodegeneration is crucial in order to understand the intrinsic repair mechanisms of the brain, and key to designing therapeutic strategies. In this study, using an experimental model of progressive chronic neurodegeneration, murine prion disease, we define the temporal dynamics of the generation, maturation and integration of new neurons in the hippocampal dentate gyrus, using dual pulse-chase, multicolour ?-retroviral tracing, transmission electron microscopy and patch-clamp. We found increased neurogenesis during the progression of prion disease, which partially counteracts the effects of chronic neurodegeneration, as evidenced by blocking neurogenesis with cytosine arabinoside, and helps to preserve the hippocampal function. Evidence obtained from human post-mortem samples, of both variant Creutzfeldt-Jakob disease and Alzheimer’s disease patients, also suggests increased neurogenic activity. These results open a new avenue into the exploration of the effects and regulation of neurogenesis during chronic neurodegeneration, and offer a new model to reproduce the changes observed in human neurodegenerative diseases

Microbiological and chemical monitoring of Marsala base wine obtained by spontaneous fermentation during large-scale production

The present work was undertaken to evaluate the effect of the natural winemaking on the microbial and chemical composition of Marsala base wine. To this purpose, a large-scale vinification process of Grillo grape cultivar was monitored from harvesting to the final product. Total yeasts (TY) showed a rapid increase after must pressing and reached values almost superimposable to those registered during the conventional winemakings. Lactic acid bacteria (LAB) were registered at the highest levels simultaneously to yeast growth at the beginning of the process. Saccharomyces cerevisiae was the species found at the highest concentrations in all samples analysed. Several strains (n= 16) was registered at high levels during the alcoholic fermentation and/or aging of wine; only two of them were detected on the grape surface. Lactobacillus plantarum was the LAB species most frequently isolated during the entire vinification process. Ethanol content was approximately 14% (v/v) at the end of vinification. The value of pH did not greatly vary during the process and the volatile acidity (VA) was detected at low concentrations during the entire transformation. The concentration of malic acid rapidly decreased during the AF; on the other hand, lactic acid showed an irregular trend during the entire process. trans-caffeil tartaric acid was the most abundant hydroxycinnamoyl tartaric acid and volatile organic compounds (VOC) were mainly represented by isoamylic alcohol and isobutanol

Valutazione delle prestazioni termo-idrauliche di microcanali a spigoli smussati

In questo eleborato viene presentato uno studio focalizzato sull’ottimizzazione della geometria dei microcanali di un dissipatore di calore, con lo scopo di fornire una serie di relazioni operative e quindi direttamente utilizzabili per la progettazione di tali dispositivi. Alla definizione delle tradizionali funzioni obiettivo, legate ai Performance Evaluation Criteria (PEC), è stata aggiunta un’analisi dal punto di vista del secondo principio della termodinamica, per valutare l’entropia generata da un flusso fluido in un canale. Normalizzando l’entropia generata si è passati all’utilizzo di un numero di generazione entropica adimensionale e quindi più adatto alla valutazione delle prestazioni. Dopo una prima fase di analisi dal punto di vista fisico, il modello è stato applicato a casi concreti, in cui funzione obiettivo e numero di generazione entropica sono stati espressi in dipendenza dell’incognita geometrica da valutare. Inoltre, è stato approfondito anche il caso in cui non siano trascurabili gli effetti di dissipazione viscosa, che possono effettivamente incidere in modo determinante sullo scambio termico, soprattutto alle microscale

Un'applicazione meccanica delle superfici di traslazione

Un biliardo matematico è un poligono semplice nel quale un punto che si muove al suo interno, impattando con i lati, modifica la propria traiettoria secondo leggi di Snell per l'ottica geometrica. Studiando il moto di un punto all'interno di un biliardo, un problema che sorge naturale è quello di chiedersi sotto quali condizioni un'orbita, cioè l'in- sieme di tutte le traiettorie descritte dal punto, risulti periodica. La ricerca di oribite periodiche all'interno di biliardi poligonali è un problema abbon- dantemente studiato dai matematici e non ancora completamente risolto. La complessità del problema dipende in parte dalla geometria che caratterizza il biliardo. Scopo di questa tesi sarà quello di ricercare, sotto opportune condizioni, or- bite periodiche all'interno di triangoli con particolare interesse verso quelli rettangolari e mostrarne un'applicazione a un problema di natura meccani- ca. Il primo capitolo sarà dedicato alle definizioni preliminari con attenzione particolare alla definizione di mappa biliardo a partire dalle ipotesi sulla riflessione delle traiettorie. Verrano poi successivamente analizzati esempi semplici di specifici triangoli. Nel secondo capitolo verrano introdotte Superfici di Traslazione: superfici ottenute per identificazione di lati di poligoni disgiunti per mezzo di tra- slazioni affini. Verrà di seguito mostrato come a partire dalla costruzione di una superficie di traslazione sarà possibile individuare orbite periodioche all'interno di qualsia triangolo rettangolo avente angoli razionali. Infine, il terzo capitolo sarà dedicato a mostrare l'equivalenza tra lo studio di una traiettoria di un punto all'interno di un biliardo a forma di triangolo rettangolo e i possibili urti che due punti massivi {P1, m1} e {P2, m2} posso- no avere tra di loro e tra gli estremi di un segmento unitario [0, 1] nel quale sono vincolati a muoversi

Numerical Simulation of Dropwise Condensation of Steam over Hybrid Surfaces via New Non-Dimensional Heat Transfer Model

Dropwise condensation (DWC) of steam over hybrid hydrophobic–hydrophilic surfaces is numerically investigated via a phenomenological, Lagrangian model. The full non-dimensionalization of the heat transfer model, needed to determine the droplet growth, allows for generalization of computational results. Hybrid surfaces characterized by recursive geometries are implemented via the introduction of proper boundary conditions. The numerical size distribution of both the large and the small droplet populations, crucial for development of simplified, statistically sound models, is compared with empirical and theoretical correlations. Then, the validation with experimental data involving DWC over an hybrid surface is successfully conducted and the heat flux is enhanced under different operating conditions via hybrid geometry optimization

Optimization of Dropwise Condensation of Steam over Hybrid Hydrophobic–Hydrophilic Surfaces via Enhanced Statistically Based Heat Transfer Modelization

Steam condensation over a hybrid hydrophobic–hydrophilic surface is modeled via simplified heat transfer modelization. Filmwise condensation is assumed over the hydrophilic region. The standard film model is improved, accounting for the liquid flow rate crossing the hydrophobic–hydrophilic boundaries. A threshold for flooding occurrence is also presented. Dropwise condensation is assumed over the hydrophobic region. Compared to the heat transfer models in the literature, based on the statistical drop size distribution, a novel correlation is used for the size distribution of small droplets. The correlations of both the liquid flow rate crossing the hydrophobic–hydrophilic boundary and the size distribution of small drops are derived via Lagrangian simulations, using an in-house code previously developed and validated by the authors. The heat transfer model is validated with experimental data in the literature involving a hybrid surface, composed by alternate vertical hydrophobic–hydrophilic stripes. Then, the optimization of the hybrid surface geometry is performed in terms of hydrophobic width and hydrophilic width, with the aim of enhancing the heat flux