Caratterizzazione parametrica delle prestazioni ed instabilita di induttori cavitanti

Nel campo della propulsione chimica ad uso spaziale, le turbomacchine rivestono un ruolo di fondamentale importanza. Tutti i lanciatori esistenti, infatti, presentano almeno uno stadio con motore a propellente liquido ed alle turbopompe è affidato il compito di innalzare la pressione del fluido prima dell’ingresso in camera di combustione, in modo da aumentare l’efficienza della reazione, e, contemporaneamente, diminuire il valore della pressione all’interno del serbatoio con una conseguente diminuzione del peso di quest’ultimo. Gli obiettivi fondamentali della ricerca nel campo della propulsione sono, da un lato, diminuire il peso dei componenti e, dall’altro, avere la possibilità di lavorare con densità di potenze sempre più elevate. Questo consente di aumentare il rapporto di carico utile del lanciatore, tuttavia necessita di uno studio approfondito di fenomeni quali la cavitazione e le instabilità fluidodinamiche e rotodinamiche. Generalmente, a causa dei molteplici effetti deleteri che ha la cavitazione, si cerca di limitare l’insorgere di questo fenomeno introducendo a monte della pompa principale un ulteriore elemento denominato induttore. Un induttore è una pompa assiale con un numero limitato di pale (solitamente 3 o 4) che ha il compito di fornire una prima compressione al fluido di lavoro, evitando, o quantomeno ritardando, lo sviluppo della cavitazione sul componente principale. Nello svolgere questo lavoro, l’induttore è spesso costretto a lavorare in condizioni nelle quali è inevitabile lo sviluppo della cavitazione, che può portare al manifestarsi di instabilità fluidodinamiche e rotodinamiche che, oltre a peggiorare l’efficienza dell’intero lanciatore, rischiano di compromettere l’integrità strutturale del sistema. Lo studio teorico di questi fenomeni risulta quanto mai difficoltoso e spesso inaffidabile, data l’enorme complessità fisica del sistema. È perciò assolutamente necessaria una caratterizzazione sperimentale della turbomacchina, che può essere molto onerosa a causa della difficoltà di lavorare con componenti criogenici o comunque instabili e pericolosi. La possibilità di comprendere come alcuni fattori fondamentali, quali la portata, il numero di giri del motore e la temperatura, influenzino sia le prestazioni sia l’insorgere delle instabilità, può portare ad un notevole risparmio economico e di tempo, rendendo possibile studiare il comportamento della turbomacchina in condizioni di similitudine geometrica e fluidodinamica. Il seguente lavoro si concentra sulla caratterizzazione di due induttori sperimentali, il DAPAMITO3 ed il DAPAMITO4, rispettivamente a tre e quattro pale, disegnati grazie ad un modello di ordine ridotto realizzato dal Prof. Luca d’Agostino e dal suo team di lavoro. È stato inoltre caratterizzato un ulteriore induttore commerciale a tre pale. L’intero lavoro è stato svolto presso il laboratorio di cavitazione di ALTA S.p.A., con la supervisione del Prof. Luca d’Agostino e degli ingegneri Lucio Torre ed Angelo Pasini. Dopo una breve introduzione riguardante gli aspetti generali della propulsione chimica e della cavitazione (Capitolo 1), verranno analizzate più in dettaglio le turbopompe, introducendo i parametri che caratterizzano lo studio delle prestazioni degli induttori (Capitolo 2). Il Capitolo 3 offre una descrizione del laboratorio di cavitazione e del circuito di prova, con particolare attenzione alle modifiche apportate all’impianto per una migliore interfaccia con gli induttori caratterizzati ed un’analisi più approfondita di fenomeni il cui studio non era previsto nella configurazione originale. Nel Capitolo 4 si descriveranno gli induttori oggetto dello studio. Il Capitolo 5 è dedicato alle curve di prestazione non cavitante ed alle loro caratteristiche principali, evidenziando quali sono i fattori che ne influenzano la forma e l’andamento. Il Capitolo 6 riguarda invece le curve di prestazione cavitante; verrà descritta accuratamente la loro forma e si delineeranno le modalità di sviluppo della cavitazione al diminuire della pressione in ingresso agli induttori. Successivamente si analizzerà l’influenza del coefficiente di flusso e della temperatura, confrontando i risultati sperimentali con i metodi attualmente più diffusi di scalatura termica delle prestazioni. Lo studio delle instabilità fluidodinamiche è riportato al Capitolo 7, che si apre con una breve introduzione dei principali strumenti, perlopiù di origine statistica, utilizzati per l’analisi dei segnali rilevati dai trasduttori. I vari tipi di instabilità verranno trattati singolarmente, esaminando le circostanze che portano ad un loro sviluppo ed i fattori che ne influenzano la forma e l’ampiezza, con riferimenti agli articoli presenti in letteratura che riportano l’insorgere di fenomeni simili. Infine il Capitolo 8 riporta le conclusioni del lavoro e termina con un breve accenno agli argomenti oggetto di futuro lavoro presso il laboratorio di cavitazione di ALTA S.p.A

Characterisation of hydrophone sensitivity with temperature using a broadband laser-generated ultrasound source

In this work, we present a novel method for characterising the relative variation in hydrophone sensitivity with temperature, addressing a key aspect of measurements in the field of ultrasound metrology. Our study focused on a selection of miniature ultrasonic hydrophones commonly used in medical applications. The method is based on using water as a temperature-sensitive laser-generated ultrasound (LGUS) source for calibration, allowing for flexible characterisation across a wide temperature range. The measurements were performed using both the LGUS method and the established self-reciprocity method. Our results demonstrate good agreement within 5% between the two methods, validating the effectiveness of the LGUS approach. We found that the sensitivity of the tested hydrophones exhibited low temperature dependence less than −0.2% per ∘C within the studied temperature range from 17 ∘C up to 50 ∘C. The presented LGUS method offers greater flexibility than current approaches as it allows for characterisation of membrane hydrophones with small element sizes and non-electrical transducers. By combining the relative sensitivity variation obtained through the LGUS method with the standard calibration at room temperature, absolute values of hydrophone sensitivity can be determined. The expanded uncertainty of our measurements, which was evaluated at temperature intervals of 8 ∘C, was determined to be on average 10%. Our work provides valuable insights into the temperature dependence of hydrophone sensitivity and lays the foundation for further investigations in this area. The LGUS method holds promise for future enhancements, such as increased bandwidth of the LGUS source and frequency domain analysis, to explore the frequency dependency of sensitivity variation with temperature

Development and Testing of a System for Controlled Ultrasound Hyperthermia Treatment With a Phantom Device

Hyperthermia is the process of raising tissue temperatures in the range 40 degrees C-45 degrees C for a prolonged time (up to hours). Unlike in ablation therapy, raising the temperature to such levels does not cause necrosis of the tissue but has been postulated to sensitize the tissue for radiotherapy. The ability to maintain a certain temperature in a target region is key to a hyperthermia delivery system. The aim of this work was to design and characterize a heat delivery system for ultrasound hyperthermia able to generate a uniform power deposition pattern in the target region with a closed-loop control, which would maintain the defined temperature over a defined period. The hyperthermia delivery system presented herein is a flexible design with the ability to strictly control the induced temperature rise with a feedback loop. The system can be reproduced elsewhere with relative ease and is adaptable for various tumor sizes/locations and for other temperature elevation applications, such as ablation therapy. The system was fully characterized and tested on a newly designed custom-built phantom with controlled acoustic and thermal properties and containing embedded thermocouples. Additionally, a layer of thermochromic material was fixed above the thermocouples, and the recorded temperature increase was compared to the red, green, and blue (RGB) color change in the material. The transducer characterization allowed for input voltage to output power curves to be generated, thus allowing for the comparison of power deposition to temperature increase in the phantom. Additionally, the transducer characterization generated a field map of the symmetric field. The system was capable of increasing the temperature of the target area by 6 degrees C above body temperature and maintains the temperature to within +/- 0.5 degrees C over a defined period. The increase in temperature correlated with the RGB image analysis of the thermochromic material. The results of this work have the potential to contribute toward increasing confidence in the delivery of hyperthermia treatment to superficial tumors. The developed system could potentially be used for phantom or small animal proof-of-principle studies. The developed phantom test device may be used for testing other hyperthermia systems

Exact solution to the inverse Womersley problem for pulsatile flows in cylindrical vessels, with application to magnetic particle targeting

An exact solution to the inverse Womersley problem was derived for the fully-developed, laminar pulsatile flow of a viscous Newtonian fluid, within a circular cylindrical vessel with rigid walls. In particular, given an arbitrary, time-periodic flow rate, the axisymmetric velocity profile was obtained by means of two neat and computable maps relating the corresponding Fourier coefficients. The study of such an inverse problem is motivated by the fact that flow rate is the main physical quantity which can be actually measured in many practical situations. The hypothesis of a fully-developed flow was deliberately introduced, in order to obtain an analytical solution (otherwise hardly achievable). Despite the intrinsic simplifications associated with the adopted position (which restrict the applicability of our results to 3D finite-length complex domains, and non-Newtonian fluids), the obtained solution provides a benchmark – and at the same time an approximation – for the inverse problem of pulsatile flows, it may serve as a debugging tool for more ambitious numerical approaches based on realistic data, and can also be used as an improved source of boundary data. As expected, the main advantage of our analytical solutions (compared to fully numerical approaches) resides in computational efficiency; this was quantitatively assessed through numerical tests. Moreover, the proposed solution was applied in the context of magnetic particle targeting, to highlight some peculiar effects on particle trajectories and capture efficiency due to pulsatility. Such a transport problem is increasingly drawing the attention of an interdisciplinary community, ranging from physicians to biomedical engineers, physicists and roboticists, thanks to its potential for targeted therapy, up to remote guidance of intravascular devices. More in general, the obtained benchmark solution holds potential for effectively exploitation in an interdisciplinary context

In vitro characterisation of ultrasound-induced heating effects in the mother and fetus:A clinical perspective

Introduction: The quantification of heating effects during exposure to ultrasound is usually based on laboratory experiments in water and is assessed using extrapolated parameters such as the thermal index. In our study, we have measured the temperature increase directly in a simulator of the maternal–fetal environment, the ‘ISUOG Phantom’, using clinically relevant ultrasound scanners, transducers and exposure conditions. Methods: The study was carried out using an instrumented phantom designed to represent the pregnant maternal abdomen and which enabled temperature recordings at positions in tissue mimics which represented the skin surface, sub-surface, amniotic fluid and fetal bone interface. We tested four different transducers on a commercial diagnostic scanner. The effects of scan duration, presence of a circulating fluid, pre-set and power were recorded. Results: The highest temperature increase was always at the transducer–skin interface, where temperature increases between 1.4°C and 9.5°C were observed; lower temperature rises, between 0.1°C and 1.0°C, were observed deeper in tissue and at the bone interface. Doppler modes generated the highest temperature increases. Most of the heating occurred in the first 3 minutes of exposure, with the presence of a circulating fluid having a limited effect. The power setting affected the maximum temperature increase proportionally, with peak temperature increasing from 4.3°C to 6.7°C when power was increased from 63% to 100%. Conclusions: Although this phantom provides a crude mimic of the in vivo conditions, the overall results showed good repeatability and agreement with previously published experiments. All studies showed that the temperature rises observed fell within the recommendations of international regulatory bodies. However, it is important that the operator should be aware of factors affecting the temperature increase

An innovative platform for treatment of vascular obstructions: system design and preliminary results

The quest for miniaturization of interventional devices and the integration between external surgical platforms and internal therapeutic tools are continuously fostering research in the biomedical field. Operating in the cardiovascular system poses dramatic challenges, but it also represent the elective application for highly targeted therapeutic devices. In this paper we present a robotic platform for treating vascular obstructions. It integrates a system for locomotion and navigation based on magnetic dragging and ultrasound tracking, a therapeutic module which involves mechanical attack to the obstruction by means of high intensity focused ultrasound, and a collection/retrieval module exploiting magnetic nano-particles to bind the obstruction debris and to drag them to a safe region for removal. Here we illustrate the system overview and the technical and theoretical instruments for developing the overall platform; preliminary results, together with future planned works, are reported in order to demonstrate the feasibility of the proposed approach