Hybrid rocket engines (HREs) are an attractive thermochemical propulsive solution, offering safety and good performance with a contained cost and environmental impact. The employment of liquefying fuels (i.e. paraffin waxes) is required to develop high thrust HREs, thanks to their fast regression rate. At the same time, they exhibit poor mechanical properties, preventing their application in operating systems. The mechanical properties of paraffin-based fuels can be enhanced exploiting a 3D printed gyroid structure inserted into the fuel grain. The resulting solids are named armored grains. The aim of this work was to study the mechanical behaviour of the armored grain by means of finite element analyses (FEAs), after having identified the ideal materials for its realization. An investigation on the gyroid reinforcing armor was carried out to understand the ideal element type and mesh density that properly capture the real behaviour of the structure. Triangular CTRIA3 elements were chosen, with a density of 115 elements per gyroid unit surface. In addition, the effects of the specimen size and shape were taken into account to exploit the results of a small test case to predict the mechanical properties of larger structures. Furthermore, a geometrical approximation of the gyroid was considered. This new structure, named pseudo-gyroid, allowed to shorten significantly the computational time of the analyses, at the expense of a negligible approximation error (<4.6% in the Young modulus evaluation). Comparing the numerical and experimental results, it was possible to highlight the effect of the printing procedure on the mechanical properties of the structures. After having fully characterized the armor, further analyses were carried out on complete armored grains. Small cases were analyzed to predict the behaviour of larger sized structures, exploiting the considerations done on the reinforcing frame. The numerical results proved the usefulness of the pseudo-gyroid geometry: in less than 25% of time it provided a very similar Young modulus and yield stress (1.1% and 5.4% difference, respectively) compared to the real gyroid. The simulations predicted the Young moduli of the experimental cases with a difference of less than 11.3%. This result proves that finite element analyses can be used as a tool to understand the flaws introduced by the printing procedure and to estimate the mechanical properties of large scale HREs by means of smaller structures.
Gli endoreattori a propellenti ibridi (HRE) sono una tecnologia propulsiva promettente: riescono infatti ad unire sicurezza e alte prestazioni ad un costo contenuto e ad un ridotto impatto ambientale. Per sviluppare motori ibridi che possano garantire alte spinte, è necessario utilizzare combustibili basso fondenti, come le cere paraffiniche. Questi combustibili, dotati di alti ratei di regressione, hanno tuttavia scarse proprietà meccaniche, rendendo impossibile un loro impiego in sistemi operativi. Queste proprietà possono essere migliorate sfruttando l'inserimento di una struttura cellulare (giroide) all'interno del grano combustibile. Gli oggetti così ottenuti vengono chiamati grani armati. Lo scopo di questa tesi è studiare il comportamento meccanico del grano armato grazie all'analisi agli elementi finiti (FEA), dopo aver identificato i materiali migliori per la sua realizzazione. Uno studio sulla struttura di rinforzo è stato svolto per comprendere quale fosse la tipologia di elemento ideale e l'adeguata densità della mesh per cogliere il comportamento meccanico della struttura reale. La scelta è ricaduta sugli elementi CTRIA3, con una densità di 115 elementi per ogni superficie unitaria del giroide. Inoltre, gli effetti della dimensione e della forma del campione sono stati considerati per sfruttare i risultati di una piccola mesh per anticipare le proprietà meccaniche di strutture più grandi. In aggiunta, è stata creata un'approssimazione geometrica del giroide. Questa nuova struttura, chiamata pseudo-giroide, ha permesso di accorciare notevolmente i tempi di calcolo delle analisi, introducendo un errore trascurabile (<4.6% nella stima del modulo di Young). Grazie al confronto tra i risultati numerici e sperimentali, è stato possibile evidenziare gli effetti della procedura di stampa sulle proprietà meccaniche delle strutture. Dopo aver caratterizzato approfonditamente l'armatura, ulteriori analisi sono state effettuate su grani armati completi di paraffina. Per predire il comportamento di strutture dotate di grandi dimensioni sono stati analizzati casi di dimensione ridotta, sfruttando le considerazioni effettuate per le strutture di rinforzo. I risultati numerici hanno dimostrato l'utilità dello pseudo-giroide: infatti, in un tempo inferiore al 25%, ha ottenuto valori del modulo di Young e dello sforzo di snervamento molto simili (rispettivamente con una differenza dell' 1.1% e 5.4%) a quelli ottenuti con la geometria non approssimata. Le simulazioni sono state in grado di anticipare i moduli di Young dei casi sperimentali con un errore massimo del 11.3%. Questo risultato prova come le analisi agli elementi finiti possano essere uno strumento utile tanto per capire i difetti introdotti dal processo di stampa quanto per ottenere stime delle proprietà meccaniche di motori ibridi di grande scala anche con l'analisi di piccole strutture.
Finite element analysis of paraffin-based fuel reinforced by a gyroid structure
Bruschi, Daniele
2019/2020
Abstract
Hybrid rocket engines (HREs) are an attractive thermochemical propulsive solution, offering safety and good performance with a contained cost and environmental impact. The employment of liquefying fuels (i.e. paraffin waxes) is required to develop high thrust HREs, thanks to their fast regression rate. At the same time, they exhibit poor mechanical properties, preventing their application in operating systems. The mechanical properties of paraffin-based fuels can be enhanced exploiting a 3D printed gyroid structure inserted into the fuel grain. The resulting solids are named armored grains. The aim of this work was to study the mechanical behaviour of the armored grain by means of finite element analyses (FEAs), after having identified the ideal materials for its realization. An investigation on the gyroid reinforcing armor was carried out to understand the ideal element type and mesh density that properly capture the real behaviour of the structure. Triangular CTRIA3 elements were chosen, with a density of 115 elements per gyroid unit surface. In addition, the effects of the specimen size and shape were taken into account to exploit the results of a small test case to predict the mechanical properties of larger structures. Furthermore, a geometrical approximation of the gyroid was considered. This new structure, named pseudo-gyroid, allowed to shorten significantly the computational time of the analyses, at the expense of a negligible approximation error (<4.6% in the Young modulus evaluation). Comparing the numerical and experimental results, it was possible to highlight the effect of the printing procedure on the mechanical properties of the structures. After having fully characterized the armor, further analyses were carried out on complete armored grains. Small cases were analyzed to predict the behaviour of larger sized structures, exploiting the considerations done on the reinforcing frame. The numerical results proved the usefulness of the pseudo-gyroid geometry: in less than 25% of time it provided a very similar Young modulus and yield stress (1.1% and 5.4% difference, respectively) compared to the real gyroid. The simulations predicted the Young moduli of the experimental cases with a difference of less than 11.3%. This result proves that finite element analyses can be used as a tool to understand the flaws introduced by the printing procedure and to estimate the mechanical properties of large scale HREs by means of smaller structures.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/175537