This thesis focuses on the development of fluorocarbon-based additives for high regression rate polymeric fuels, based on Hydroxyl-Terminated PolyButadiene (HTPB). Fluorocarbon polymers offer appealing features when considering their reaction with metal powders. In particular, fluoropolymer decomposition during combustion may release oxidizing species that can react with the metal fuel. This characteristic is particularly appealing when considering the possible insurgence of metal ignition/combustion in oxidizer-lean conditions (i.e. at/close the solid fuel surface). The first part of this work deals with nano-sized aluminum (nAl) loaded HTPB fuels. The approach concerns the evaluation of different manufacturing procedures on both mechanical properties and ballistics. In addition, a detailed investigation of the regressing surface of these fuels is performed, showing the insurgence of aggregation/agglomeration phenomena that can be related to the observed poor performance gain with respect to the baseline formulation (non-loaded HTPB) burned under similar operating conditions. This approach allows the evaluation of the nAl combustion behavior as well as the manufacturing techniques effects on the ballistic response of the following fluorocarbon-loaded formulations, whose production resorted to a combination of these techniques. Purposely developed manufacturing procedures are used in order to realize the inclusion of a compatibilized fluoropolymer (MCF-64) and PolyTetraFluoroEthylene (PTFE). The fluoropolymer was dispersed in the HTPB-based binder used in the manufacturing separately from the metal powders (an approach used for both MCF-64 and PTFE), or, in the case of the PTFE, it can be used in combustion for the production of composite powders produced by mechanical activation processes. In particular, both micron- and nano-sized aluminum are used in combination with 45 wt% PTFE for the production of the ActAl-15-T(45) and ActALEX-T(45) powders. The blends composed by a combination of the HTPB with MCF-64 and PTFE constitute two different solid fuels families, that is, HM35- and HT35- series respectively. The two new classes of fuels are comprehensively characterized, tested in gaseous oxygen and independently developed changing aluminum powder percentages and properties. The fluorinated formulation with PTFE and 10 wt% of nAl shows a fuel mass flow rate enhancement with respect to the baseline of 48 % at oxidizer mass flux (G_{Ox}) of 250 kg/(m^2s). Performance are improved also with respect to HTPB + 10 wt% nAl, pointing out the effectiveness of the adopted strategy. Among the HM35- series, appreciable performance gains are obtained by the activated uAl-loaded formulation with respect to the pure HTPB, while slight enhancements are observed for the other fuels. On the other hand, all the aluminized formulations of the HT35- series exhibit significant performance increases. For instance, the H-ActALEX-T(45)_54.5 shows percent regression rate (r_f) enhancement of 60 % at G_{Ox}=400 kg/(m^2s). %in the investigated oxidizer mass flux range, HT35-Al01f_30 shows an average r_f enhancement of 53 %, that rises to 59.8 % for the H-ActALEX-T(45)_54.5. The high density of the fuels leads to further stress this performance enhancement. Complex aggregation/agglomeration phenomena are observed and discussed thanks to the performed boundary-layer and regressing surface visualizations of the fluoropolymers-based formulations. These phenomena can be eventually connected to the significant performance gain of the investigated PTFE-based solid fuels. In particular, peculiar observations were made on the ballistics of the HTPB formulation loaded with 70 wt% of ActALEX-T(45), showing a low G_{Ox} sensitivity and an average r_f percent increase of 190 % over the whole investigated range, that yields an average fuel mass burning rate increase over the HTPB baseline of 416 %. The attractive performance of fluorocarbon-based additives produces significant r_f enhancement also when tested in N2O. In this case, the H-ActALEX-T(45)_70 exhibits an average fuel mass burning rate increase of 383 %.
Lo scopo di questa tesi è lo sviluppo di combustibili polimerici ad alta velocità di regressione additivati con fluorocarburi, tali combustibili sono basati sul polibutadiene a terminazione idrossilica (HTPB). I fluoropolimeri offrono proprietà attraenti se si considerano le loro reazioni con le polveri metalliche. In particolare, la decomposizione del fluoropolimero durante la combustione può rilasciare specie ossidanti che sono in grado di reagire col combustibile metallico. Questa caratteristica è particolarmente apprezzata in quanto offre la possibilità di ignizione/combustione del metallo in condizioni di scarsa presenza di ossidante (ovvero in prossimità della superficie del combustibile). La prima parte di questo lavoro si è concentrata sui combustibili a base di HTPB e nano-alluminio (nAl, 10 % in massa). L'approccio punta a valutare l'effetto delle differenti manifatture usate sulle proprietà meccaniche e balistiche. In aggiunta, la superficie di regressione del combustibile è stata indagata in dettaglio, mettendo in evidenza l'insorgenza di fenomeni di aggregazione/agglomerazione che possono essere correlati allo scarso incremento delle prestazioni rispetto alla formulazione di riferimento (HTPB non alluminizzato) bruciata in simili condizioni operative. Questo approccio permette, inoltre, la valutazione del comportamento del nAl in combustione, così come l'effetto delle tecniche di manifattura sulla risposta balistica delle successive formulazioni basate sui fluorocarburi, la cui produzione si basa sull'uso combinato di queste tecniche. Procedure di manifattura appositamente sviluppate sono state usate per realizzare l'inclusione di un fluoropolimero compatibilizzato (MCF-64) e del politetrafluoroetilene (PTFE). Il fluoropolimero è disperso nel legante di HTPB separatamente dalla polvere metallica, durante la manifattura (sia per quanto riguarda l'MCF-64, sia per quanto riguarda il PTFE). Alternativamente, il PTFE può essere usato per la produzione di polveri composite, attraverso un processo di attivazione meccanica. In particolare, combinazioni di PTFE (45 %) e polveri di alluminio sia micrometriche che nanometriche hanno portato alla produzione dell'ActAl-15-T(45) e dell'ActALEX-T(45). I composti formati dalla combinazione dell'HTPB con l'MCF-64 e il PTFE costituiscono due differenti famiglie di combustibili solidi, ovvero, la serie HM35- e la serie HT35- rispettivamente. Queste due nuove classi di combustibili sono state esaurientemente caratterizzate, testandole in ossigeno gassoso, e sono state sviluppate separatamente variando le percentuali e le proprietà degli additivi metallici. Le formulazioni fluorurate con PTFE e nAl 10 % mostrano un incremento della portata massica di combustibile, rispetto al riferimento, del 48 % ad una portata specifica di ossidante (G_{Ox}) di 250 kg/(m^2s). Inoltre, le prestazioni risultano aumentate anche rispetto all'HTPB con il 10 % nAl, evidenziando l'efficacia della strategia adottata. Nella serie HM35-, aumenti apprezzabili di prestazioni sono stati ottenuti da formulazioni contenenti micro-alluminio (rispetto al puro HTPB), mentre incrementi molto modesti si sono osservati per gli altri combustibili. Viceversa, tutte le formulazioni della serie HT35- esibiscono significativi aumenti di prestazioni. Per esempio, l'H-ActALEX-T(45)_54.5 mostra un incremento della velocità di regressione (r_f) del 60 % a G_{Ox}=400 kg/(m^2s). L'alta densità dei combustibili porta ad un ulteriore aumento delle prestazioni. Fenomeni complessi di aggregazione/agglomerazioni si sono osservati, e sono stati discussi, grazie alle visualizzazioni di strato limite e superficie di regressione effettuate sulle formulazioni fluorurate. Questi fenomeni possono essere infine collegati al significativo aumento di prestazioni che è stato osservato per i combustibili a base di PTFE. In particolare, le osservazioni peculiari sulla balistica della formulazione ottenuta introducendo il 70 % di ActALEX-T(45) nel legante di HTPB, hanno mostrato una bassa sensibilità al G_{Ox} di questo combustibile. Inoltre si è osservato un incremento medio percentuale di r_f del 190 % su tutto l'intervallo studiato, che comporta un guadagno medio in termini massici del 416 % rispetto al riferimento. Le promettenti prestazioni ottenute con gli additivi basati sui fluorocarburi portano ad aumenti significativi di r_f anche nei test effettuati in N2O. In questo caso, l'H-ActALEX-T(45)_70 mostra un aumento medio della portata massica di combustibile del 383 %.
Fluorocarbon based additives for high regression rate polymeric fuels
DI LORENZO, MARCO
2014/2015
Abstract
This thesis focuses on the development of fluorocarbon-based additives for high regression rate polymeric fuels, based on Hydroxyl-Terminated PolyButadiene (HTPB). Fluorocarbon polymers offer appealing features when considering their reaction with metal powders. In particular, fluoropolymer decomposition during combustion may release oxidizing species that can react with the metal fuel. This characteristic is particularly appealing when considering the possible insurgence of metal ignition/combustion in oxidizer-lean conditions (i.e. at/close the solid fuel surface). The first part of this work deals with nano-sized aluminum (nAl) loaded HTPB fuels. The approach concerns the evaluation of different manufacturing procedures on both mechanical properties and ballistics. In addition, a detailed investigation of the regressing surface of these fuels is performed, showing the insurgence of aggregation/agglomeration phenomena that can be related to the observed poor performance gain with respect to the baseline formulation (non-loaded HTPB) burned under similar operating conditions. This approach allows the evaluation of the nAl combustion behavior as well as the manufacturing techniques effects on the ballistic response of the following fluorocarbon-loaded formulations, whose production resorted to a combination of these techniques. Purposely developed manufacturing procedures are used in order to realize the inclusion of a compatibilized fluoropolymer (MCF-64) and PolyTetraFluoroEthylene (PTFE). The fluoropolymer was dispersed in the HTPB-based binder used in the manufacturing separately from the metal powders (an approach used for both MCF-64 and PTFE), or, in the case of the PTFE, it can be used in combustion for the production of composite powders produced by mechanical activation processes. In particular, both micron- and nano-sized aluminum are used in combination with 45 wt% PTFE for the production of the ActAl-15-T(45) and ActALEX-T(45) powders. The blends composed by a combination of the HTPB with MCF-64 and PTFE constitute two different solid fuels families, that is, HM35- and HT35- series respectively. The two new classes of fuels are comprehensively characterized, tested in gaseous oxygen and independently developed changing aluminum powder percentages and properties. The fluorinated formulation with PTFE and 10 wt% of nAl shows a fuel mass flow rate enhancement with respect to the baseline of 48 % at oxidizer mass flux (G_{Ox}) of 250 kg/(m^2s). Performance are improved also with respect to HTPB + 10 wt% nAl, pointing out the effectiveness of the adopted strategy. Among the HM35- series, appreciable performance gains are obtained by the activated uAl-loaded formulation with respect to the pure HTPB, while slight enhancements are observed for the other fuels. On the other hand, all the aluminized formulations of the HT35- series exhibit significant performance increases. For instance, the H-ActALEX-T(45)_54.5 shows percent regression rate (r_f) enhancement of 60 % at G_{Ox}=400 kg/(m^2s). %in the investigated oxidizer mass flux range, HT35-Al01f_30 shows an average r_f enhancement of 53 %, that rises to 59.8 % for the H-ActALEX-T(45)_54.5. The high density of the fuels leads to further stress this performance enhancement. Complex aggregation/agglomeration phenomena are observed and discussed thanks to the performed boundary-layer and regressing surface visualizations of the fluoropolymers-based formulations. These phenomena can be eventually connected to the significant performance gain of the investigated PTFE-based solid fuels. In particular, peculiar observations were made on the ballistics of the HTPB formulation loaded with 70 wt% of ActALEX-T(45), showing a low G_{Ox} sensitivity and an average r_f percent increase of 190 % over the whole investigated range, that yields an average fuel mass burning rate increase over the HTPB baseline of 416 %. The attractive performance of fluorocarbon-based additives produces significant r_f enhancement also when tested in N2O. In this case, the H-ActALEX-T(45)_70 exhibits an average fuel mass burning rate increase of 383 %.| File | Dimensione | Formato | |
|---|---|---|---|
|
2016_04_DiLorenzo.pdf
non accessibile
Descrizione: Testo della tesi
Dimensione
45.73 MB
Formato
Adobe PDF
|
45.73 MB | Adobe PDF | Visualizza/Apri |
I documenti in POLITesi sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
https://hdl.handle.net/10589/118762