1,721,006 research outputs found
Positive Displacement Pumps: Pressure Modelling for Performance Analysis
No full textLa distribuzione della pressione all'interno di pompe volumetriche rotative varia periodicamente con la posizione del rotore; le conseguenti forze di pressione variabili rappresentano le principali fonti di vibrazioni e rumore. Questa tesi presenta diverse metodologie di modellazione per la simulazione dell'evoluzione di pressione in funzione della posizione del rotore in condizioni di lavoro, per questo tipo di pompe. Da un lato, viene sviluppato un approccio a parametri concentrati, applicato ad una pompa ad ingranaggi esterni per applicazioni automobilistiche; d’altro lato, viene studiato un approccio empirico, adottato nel caso di una famiglia di pompe a palette a cilindrata variabile.
Nel primo approccio, il volume fluido della pompa è suddiviso in volumi di controllo e viene risolto un sistema di equazioni di continuità. La metodologia di modellazione è generale e può essere applicata a qualsiasi pompa ad ingranaggi a denti diritti con profilo ad evolvente. I vari coefficienti di efflusso in regime laminare o turbolento sono stimati mediante opportune simulazioni fluidodinamiche CFD. Il modello è stato soddisfacentemente validato mediante il confronto con risultati sperimentali, in termini di efficienza volumetrica e di ripple di pressione in mandata, in differenti condizioni operative. Il modello validato ha poi permesso di svolgere un’indagine di sensibilità, utile per l’ottimizzazione di parametri progettuali e funzionali della pompa: profondità e larghezza della ”Epsilon Groove”, eccentricità delle ruote nel corpo pompa, caratteristiche e temperatura dell’olio. . I risultati di questa analisi saranno utili nell’ottimizzazione dell’efficienza e del comportamento dinamico.
Venendo all'approccio empirico, la modellazione della distribuzione di pressione non si basa in questo caso su leggi fisiche, ma sulla parametrizzazione di dati sperimentali: vengono individuate regole di correlazione tra le caratteristiche dell’evoluzione di pressione e i parametri geometrici e funzionali, relativamente ad una specifica famiglia di pompe di diverse dimensioni. Come applicazione del modello empirico, è stata condotta un'analisi del regime di usura e lubrificazione nel contatto tra le palette e l'anello statorico; tale analisi è stata effettuata mediante un metodo originale basato sul modello di usura di Archard, che associa il coefficiente di usura alle forze di contatto ottenute mediante il modello empirico, al tasso di usura ed alla durezza del materiale, misurati sperimentalmente; i risultati ottenuti vengono confrontati con i risultati forniti da un modello di lubrificazione elasto-idrodinamica al fine di valutare lo spessore del meato ed il regime di lubrificazione. Questo metodo è applicato ad un caso industriale al fine di dare indicazioni per la scelta appropriata dei materiali.
Come risultato generale, entrambi gli approcci di modellizzazione producono risultati affidabili, utili per l'analisi delle eccitazioni vibro-acustiche, per l'ottimizzazione strutturale, nonché per la valutazione del regime di lubrificazione ed usura. Tuttavia si rileva che, da un lato, l'approccio a parametri concentrati permette di simulare il comportamento di pompe di diversa tipologia e di valutare l’effetto di una vasta gamma di modifiche, da prendere in considerare per l’ottimizzazione del progetto. D'altro lato, i modelli empirici sono meno flessibili rispetto ai modelli a parametri concentrati, in quanto sono applicabili solo a una famiglia di pompe; tuttavia, essi rappresentano strumenti efficaci per la R&S in ambito industriale, in quanto sono caratterizzati da tempi di sviluppo più rapidi e da un più facile impiego.The pressure distribution inside rotary positive displacement pumps periodically varies with rotor position; the consequent variable pressure forces represent the main sources of casing vibration and noise emission. This dissertation presents different modelling methodologies for the simulation of pressure evolution in this kind of pumps, as a function of rotor position in working conditions: a lumped parameter approach is developed and applied to an external gear pump for automotive applications, while an empirical approach is studied and adopted in the case of a family of variable displacement vane pumps.
In the former approach, the fluid volume of the pump is divided in several control volumes and a system of continuity equations is solved. This modelling methodology is general and can be applied to any spur gear pump with involute profile. The flow discharge coefficients in laminar or turbulent regime are estimated by appropriate CFD simulations. The validation of the model has been satisfactorily carried out for comparison with experimental results in term of volumetric efficiency and outlet pressure ripple, in different operating conditions. Through the validated model, a sensitivity analysis has been carried out, useful for optimizing some design and operating parameters of the pump: depth and width of the "Epsilon Groove", gear eccentricity with respect to the pump casing, oil temperature and properties. The results of this analysis will be useful for the optimization of the efficiency and the dynamic behaviour.
On the other hand, the empirical approach for modelling pressure distribution is not based on physical laws, but on the parametrization of experimental data: correlation rules are established between the characteristics of the pressure evolution and the design and functional parameters of a specific family of pumps of different size. As a model application, the analysis of the wear and lubrication regime in the sliding contact between the vanes and the pressure ring is developed; this analysis is carried out through an original methodology based on Archard’s wear model, which associates the wear coefficient to the contact forces given by the empirical model, the wear rate and the material hardness, experimentally measured; the results are compared with the outcomes obtained from an EHD lubrication model in order to evaluate the film thickness and the lubrication regime. This method is applied to an industrial case in order to give indications for the proper selection of materials.
As a general result, both modelling approaches give reliable results, useful for the analysis of the vibro-acoustic excitations, the optimization of the structural design, as well as the assessment of wear and lubrication regime. It is worth noting that, on the one hand, the lumped parameter approach allows the simulation of the behaviour of pumps of different type and design and makes it possible to account for a wide range of modifications, to be considered for optimization purposes. On the other hand, empirical models are less flexible than lumped parameter models, as they are applicable only to a single family of pumps; however, they
represent efficient tools for R&D in industrial environment, due to faster development time and more friendly use
Assessment of the lubrication regime in variable displacement vane pumps
No full textThis paper proposes a combined numerical-experimental methodology for the analysis of the lubrication regime and wear that occur between vanes and pressure ring in variable displacement vane pumps. The knowledge of the lubrication regime is essential for the improvement of the performance of high pressure vane pumps by reducing wear, increasing the volumetric efficiency and decreasing maintenance costs. Tests using pressure rings of different materials were carried out in order to identify the best material in terms of wear and friction. The proposed methodology is based on Archard’s law and takes advantage of wear experimental measurements, an empirical model for the estimation of contact pressure forces and hardness standard tests. The results of the analysis state that low wear and reduced friction can be obtained if elasto-hydrodynamic lubrication between vanes and pressure ring is established. Results have been also verified by analytical elasto-hydrodynamic models
ON THE WEAR AND LUBRICATION REGIME IN VARIABLE DISPLACEMENT VANE PUMPS
No full textThis paper proposes an experimental methodology for the analysis of the lubrication regime and wear that occur between vanes and pressure ring in variable displacement vane pumps. The knowledge of the lubrication regime is essential for the improvement of the performance of high pressure vane pumps by reducing wear, increasing the volumetric efficiency and decreasing maintenance costs. Tests using pressure rings of different materials were carried out in order to identify the best material in terms of wear and friction. The proposed methodology is based on Archard’s law and takes advantage of wear experimental measurements, an empirical model for the estimation of contact pressure forces and hardness standard tests. The results of the analysis state that low wear and reduced friction can be obtained if elasto-hydrodynamic lubrication between vanes and pressure ring is established. Results have been also verified by an analytical elasto-hydrodynamic model
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Controllo nervoso inibitorio non adrenergico non colinergico delle vie aeree: caratterizzazione farmacologica nella trachea di cavia.
No full text
Caratterizzazione farmacologica delle risposte inibitorie non adrenergiche non colinergiche nella trachea di cavia
No full textNeurotrasmettitori nelle vie aeree: caratterizzazione farmacologica del sistema non adrenergico non colinergico inibitorio nella trachea di cavia
No full textIn order to characterize the role of nitric oxide (NO), vasoactive intestinal peptide (VIP), ATP and carbon monoxide (CO) in the neurally mediated EFS-induced relaxation of isolated guinea-pig trachea, we evaluated the relaxing responses before and after treatment with known blockers of either synthesis or action of each mediator. The role of NO and ATP was shown to be both significant and similar. Peptide mediators did not show a significant role. The role of CO was shown only incases of blockage of the other mediators. In conclusion, apart from the most well-known mediators, at least two other mediators, i.e. ATP, or a related purine, and CO, are involved in the NANC relaxation of guinea-pig trachea
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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