The growing number of currently operational satellites has introduced an overpopulation issue of LEO and GEO orbits. This requires the development of novels efficient methodologies intended to favor the process of the end-of-life satellites complete decommissioning. This purpose is usually fulfilled trough the heat generated by aerodynamic friction during the uncontrolled re-entry phase of the satellites itself. The critical aspect of the demise process concerns the complete thermal degradation of some structural components such as bearings or reaction wheels. In this framework, the use of a thermitic charge is subject of growing interest, because of the huge heat amount released trough their combustion. In addition, thermites are ignitable also in the absence of environmental oxygen. This peculiarity is extremely convenient for their application in the design for demise field. This thesis work has the objective of evaluating the effectiveness of these energetic materials from the point of view of the heat transfer to a metal structure. Numerous formulations of loose powders were then analyzed: Al-Fe2O3, Al-Bi2O3, Al-MoO2 and Al-WO3. The Al-Fe2O3 formulation was tested as standard powders, as pure mechanically activated powder and as a blend of these two. The others were analyzed only as activated thermites. In addition to loose powders, compacted powders were also introduced in the form of pellets. These charges were obtained varying the compacting pressure between 25 bar and 100 bar. After an initial study of the main thermochemical characteristics of these formulations, including the possible production of toxic substances, gaseous amount production and the dependence of the relative ignition temperature on heating rate and external pressure, the final experimental campaign was carried out. It had the objective of quantifying the heat exchange efficiency of the previously selected compositions. In these tests the thermite charge was lodged inside a closed metallic geometry, and it was ignited by means of a blower able to generate a hot air stream with a maximum temperature equal to 900 °C. The results of these experiments have shown how loose powder mixtures guarantee greater efficiency compared to pellets, and that their combustion process is decidedly faster than that recorded for compacted powders. The maximum efficiency was thus recorded for the loose activated Al-Fe2O3 formulation, which has reached a peak equal to the 75.1 %. In parallel with the experimental activity, this thesis includes a numerical model built up trough the Ansys simulation software. This code is intended to obtain a numerical tool able to simulate the heat transfer process between the reacting charge and the metallic substrate. An accurate model would allow an accurate sizing of the thermite mass needed for a certain practical application. The main parameters of the model have been previously tuned and validated trough the comparison of its results with a set of experimental measurements obtained trough tests in which the structure has been heated up by the heat generated via Joule effect by a resistive coil. The final assessment of the code's accuracy has been carried out simulating 3 among the tests done during the on-ground test campaign of the thermites. Concerning the heat amount transferred to the structure, numerical results have stressed percentage errors at most equal to the 14.7 %. The errors were calculated with respect to the heat absorbed in the interval between the instant of ignition and the instant in which the metal's temperature peak was recorded. In addition, the thermal history of each structural component is replicated with errors that fall below the 30 %.
Il numero crescente di satelliti attualmenti operativi ha introdotto un problema di sovrappopolazione delle orbite LEO e GEO. Ciò richiede lo sviluppo di nuove efficienti metodologie destinate a favorire il processo di completo smaltimento dei satelliti a fine vita. Questo scopo viene solitamente assolto attraverso il calore generato dall'attrito aerodinamico durante la fase di rientro incontrollato dei satelliti stessi. L'aspetto critico di un rientro incontrollato riguarda la completa degradazione termica di alcuni componenti strutturali come cuscinetti o serbatoi in titanio. In questo contesto, l'uso di una carica termitica è oggetto di crescente interesse, in virtù dell'enorme quantità di calore rilasciata attraverso la loro combustione. Inoltre, le termiti sono in grado di reagire anche in assenza di ossigeno ambientale. Questa peculiarità è estremamente conveniente per la loro applicazione nel campo del design for demise. Questo lavoro di tesi ha l'obiettivo di valutare l'efficacia di questi materiali energetici dal punto di vista del trasferimento di calore ad una struttura metallica. Sono state quindi analizzate numerose formulazioni di polveri libere: Al-Fe2O3, Al-Bi2O3, Al-MoO2 e Al-WO3. La formulazione Al-Fe2O3 è stata testata come polvere standard, come polvere attivata meccanicamente e infine sotto forma di miscela di queste due. Le altre composizioni sono state analizzate solo come termiti attivate. Oltre alle polveri libere, sono state introdotte anche polveri compattate sotto forma di pellet. Tali cariche sono state ottenute variando la pressione di compattazione tra 25 bar e 100 bar. Dopo un primo studio delle principali caratteristiche termochimiche di queste formulazioni, tra cui la possibile produzione di sostanze tossiche, la produzione di quantità gassose e la dipendenza della temperatura di accensione relativa dalla velocità di riscaldamento e dalla pressione esterna, è stata condotta la campagna sperimentale finale. Essa era finalizzata a quantificare l'efficienza di scambio termico delle composizioni precedentemente selezionate. In queste prove la carica di termite è stata alloggiata all'interno di una geometria metallica chiusa, ed è stata innescata per mezzo di un soffiante in grado di generare un flusso di aria calda ad una temperatura massima pari a 900 *C. I risultati di questi esperimenti hanno mostrato come le miscele di polveri libere garantiscano una maggiore efficienza rispetto alle pastiglie, e che il loro processo di combustione sia decisamente più veloce di quello registrato per le polveri compattate. L'efficienza massima è stata quindi registrata per la formulazione composta da Al-Fe2O3 attivata, che ha raggiunto un picco pari al 75,1 %. Parallelamente all'attività sperimentale, questa tesi include un modello numerico realizzato attraverso il software di simulazione Ansys. Questo codice ha lo scopo di ottenere uno strumento numerico in grado di simulare il processo di trasferimento di calore tra la carica reagente e il substrato metallico. Un modello accurato consentirebbe un preciso dimensionamento della massa di termite necessaria per una certa applicazione pratica. I parametri principali del modello sono stati preventivamente modulati e validati attraverso il confronto dei suoi risultati con una serie di misure sperimentali, ottenute attraverso prove in cui la struttura è stata riscaldata per mezzo del calore generato per effetto Joule da un avvolgimento resistivo. La valutazione finale dell'accuratezza del codice è stata effettuata simulando 3 dei test svolti durante la campagna di sperimentazione delle termiti. Per quanto riguarda la quantità di calore ceduta alla struttura, i risultati numerici hanno evidenziato errori percentuali al massimo pari a 14,7 %. Gli errori sono stati calcolati rispetto al calore assorbito nell' intervallo compreso tra l'stante di accensione e l' istante in cui è stato registrato il picco di temperatura del substrato metallico. Inoltre, la storia termica di ogni componente strutturale viene replicata con errori che scendono al di sotto dei 30 %.
Investigation of thermite exothermic reactions for satellites demise applications
Finocchi, Piero
2021/2022
Abstract
The growing number of currently operational satellites has introduced an overpopulation issue of LEO and GEO orbits. This requires the development of novels efficient methodologies intended to favor the process of the end-of-life satellites complete decommissioning. This purpose is usually fulfilled trough the heat generated by aerodynamic friction during the uncontrolled re-entry phase of the satellites itself. The critical aspect of the demise process concerns the complete thermal degradation of some structural components such as bearings or reaction wheels. In this framework, the use of a thermitic charge is subject of growing interest, because of the huge heat amount released trough their combustion. In addition, thermites are ignitable also in the absence of environmental oxygen. This peculiarity is extremely convenient for their application in the design for demise field. This thesis work has the objective of evaluating the effectiveness of these energetic materials from the point of view of the heat transfer to a metal structure. Numerous formulations of loose powders were then analyzed: Al-Fe2O3, Al-Bi2O3, Al-MoO2 and Al-WO3. The Al-Fe2O3 formulation was tested as standard powders, as pure mechanically activated powder and as a blend of these two. The others were analyzed only as activated thermites. In addition to loose powders, compacted powders were also introduced in the form of pellets. These charges were obtained varying the compacting pressure between 25 bar and 100 bar. After an initial study of the main thermochemical characteristics of these formulations, including the possible production of toxic substances, gaseous amount production and the dependence of the relative ignition temperature on heating rate and external pressure, the final experimental campaign was carried out. It had the objective of quantifying the heat exchange efficiency of the previously selected compositions. In these tests the thermite charge was lodged inside a closed metallic geometry, and it was ignited by means of a blower able to generate a hot air stream with a maximum temperature equal to 900 °C. The results of these experiments have shown how loose powder mixtures guarantee greater efficiency compared to pellets, and that their combustion process is decidedly faster than that recorded for compacted powders. The maximum efficiency was thus recorded for the loose activated Al-Fe2O3 formulation, which has reached a peak equal to the 75.1 %. In parallel with the experimental activity, this thesis includes a numerical model built up trough the Ansys simulation software. This code is intended to obtain a numerical tool able to simulate the heat transfer process between the reacting charge and the metallic substrate. An accurate model would allow an accurate sizing of the thermite mass needed for a certain practical application. The main parameters of the model have been previously tuned and validated trough the comparison of its results with a set of experimental measurements obtained trough tests in which the structure has been heated up by the heat generated via Joule effect by a resistive coil. The final assessment of the code's accuracy has been carried out simulating 3 among the tests done during the on-ground test campaign of the thermites. Concerning the heat amount transferred to the structure, numerical results have stressed percentage errors at most equal to the 14.7 %. The errors were calculated with respect to the heat absorbed in the interval between the instant of ignition and the instant in which the metal's temperature peak was recorded. In addition, the thermal history of each structural component is replicated with errors that fall below the 30 %.| File | Dimensione | Formato | |
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Thesis.pdf
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Exexutive_summary.pdf
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https://hdl.handle.net/10589/192415