This work focuses on the ballistic response to quasi-steady and forced transient operating conditions of solid fuels for hybrid propulsion. In the implemented hybrid configuration, the fuel is solid (HTPB, hydroxyl-terminated polybutadiene) while the oxidizer is gaseous (GOX, gaseous oxygen). Experimental tests were performed by a 2D-radial micro-burner enabling combustion under controlled operating conditions. A time-resolved data reduction technique was used for regression rate measurement. Quasi-steady ballistics was investigated under chamber pressure of 1.0 and 1.9 MPa, mass flow rates of 3 and 6 g/s and sample central port initial diameters of 4 and 6 mm. Under the investigated conditions HTPB exhibited a regression rate dependent from oxidizer mass flux but not influenced by the chamber pressure. The ballistic response of HTPB under forced transient conditions was investigated using the time-resolved data reduction technique in the quasi-steady part of the combustion run, and by a modified TOT in the transient leg. A mobile average algorithm was used to provide a smoothing of the data sampled in the transient leg, lessening the scattering induced by the high frequency sampling. Three different operating profiles were analyzed. In the first the mass flow rate is throttled down from 6 to 3 g/s. Tests showed a non-monotonic decrease of the regression rate during the throttling down transient. An initial, marked undershoot characterizes the forced transient leg. After this phenomenon, the regression rate exhibits faint oscillations. This is probably related to the fast variation of the reacting surface temperature. The following slow return of $r_f$ to values associated to a constant mass flow rate is probably influenced by the thermal lag of the condensed phase and possibly by surface reactions time, since the forced transient duration is approximately 4 ms, while the two characteristic times of heat diffusion are in a time scale between 10^(-2) and 10^(-3) s . The second profile was the extinction of the sample, accompanied by a combustion chamber depressurization. It was observed that the depressurization effect creates an additional mass flow that affects the ballistic response. All of the performed tests at this operating profile showed a similar behavior. During the closure of the feeding line, a marked undershoot effect is observed, followed by an overshoot caused by the flow induced by depressurization. Finally, re-ignition tests of the samples after extinction was tested. For this forced transient, only qualitative analysis was performed. The achieved results show that the vaporized fuel, generated by the pre-heated grain, reacts with the oxidizer flow in a very short time and the complete re-establishment of the grain combustion requires approximately 8 ms. This time interval is comparable to characteristic time that governs the reacting surface temperature variations.
Questo lavoro si concentra sulla risposta balistica di combustibili solidi per la propulsione ibrida, in condizioni quasi-stazionarie e di transitorio forzato. Nella configurazione ibrida implementata, il combustibile è solido (HTPB, hydroxyl-terminated polybutadiene) mentre l'ossidante è gassoso (GOX, ossigeno gassoso). Le prove sperimentali sono state eseguite con un micro bruciatore radiale a due dimensioni, in grado di mantenere controllate le condizioni operative. Una tecnica di riduzione dei dati risolta nel tempo è stata utilizzata per lo studio del rateo di regressione. La balistica quasi-stazionaria è stata investigata a pressioni di 1.0 e 1.9 MPa, portata massica di ossidante di 3 e 6 g/s e diametro iniziale del porto centrale di 4 e 6 mm. Nelle condizioni considerate l'HTPB ha mostrato un rateo di regressione dipendente dal flusso di massa dell'ossidante ma non da pc. La risposta balistica dell'HTPB in condizioni di transitorio forzato è stata studiata usando una tecnica di riduzione dati risolta nel tempo nel tratto quasi-stazionario, e una tecnica TOT nel tratto transitorio, modificata tramite un algoritmo di media mobile in grado di prevenire eventuali misurazioni spurie, causate dal campionamento ad alta frequenza. Sono stati studiati tre differenti profili. Nel primo la portata massica viene ridotta da 6 a 3 g/s. I test hanno mostrato una riduzione non monotona del rateo di regressione durante la riduzione della portata di ossidante. La prima parte del transitorio è caratterizzata da una marcata sottoelongazione, seguita da leggere oscillazioni del valore del rateo di regressione. Questo fenomeno è probabilmente collegato alla rapida variazione della temperatura della superficie reagente. Il successivo lento ritorno del rateo di regressione a valori associati a una portata massica costante è probabilmente influenzato dal ritardo termico nella fase condensata e dal tempo caratteristico delle reazioni superficiali, poiche la durata del transitorio forzato (circa 4 ms) è comparabile con i tempi caratteristici relativi a queste due dinamiche (tra 10^(-2) e 10^(-3) s). Il secondo profilo riguarda il completo spegnimento del campione, associato a una depressurizzazione della camera di combustione. Le prove effettuate mostrano che la depressurizzazione crea un flusso di massa aggiuntivo che influenza la risposta balistica. Tutti i test eseguiti in questa condizione operativa hanno mostrato un comportamento simile. Durante la chiusura dell'alimentazione di ossidante, si osserva una marcata sottoelongazione nell'andamento del rateo di regressione, seguita da una sovraelongazione causata dal flusso indotto dalla depressurizzazione, e a sua volta seguito dal completo spegnimento del campione. In conclusione, sono state esguite prove di riaccensione di campioni precedentemente portati allo spegnimento per interruzione ddella portata massica. I risultati ottenuti mostrano che il combustibile vaporizzato, generato dal grano pre-riscaldato, reagisce con il flusso ossidante in un tempo molto ridotto, e la re-instaurazione completa della combustione avviene in circa 8 ms. Questo intervallo di tempo è comparabile il tempo caratteristico collegato alle variazioni di temperatura della superficie reagente.
Quasi-steady and forced transients of HTPB-based fuels in a micro burner
STOCCO, MARCO
2014/2015
Abstract
This work focuses on the ballistic response to quasi-steady and forced transient operating conditions of solid fuels for hybrid propulsion. In the implemented hybrid configuration, the fuel is solid (HTPB, hydroxyl-terminated polybutadiene) while the oxidizer is gaseous (GOX, gaseous oxygen). Experimental tests were performed by a 2D-radial micro-burner enabling combustion under controlled operating conditions. A time-resolved data reduction technique was used for regression rate measurement. Quasi-steady ballistics was investigated under chamber pressure of 1.0 and 1.9 MPa, mass flow rates of 3 and 6 g/s and sample central port initial diameters of 4 and 6 mm. Under the investigated conditions HTPB exhibited a regression rate dependent from oxidizer mass flux but not influenced by the chamber pressure. The ballistic response of HTPB under forced transient conditions was investigated using the time-resolved data reduction technique in the quasi-steady part of the combustion run, and by a modified TOT in the transient leg. A mobile average algorithm was used to provide a smoothing of the data sampled in the transient leg, lessening the scattering induced by the high frequency sampling. Three different operating profiles were analyzed. In the first the mass flow rate is throttled down from 6 to 3 g/s. Tests showed a non-monotonic decrease of the regression rate during the throttling down transient. An initial, marked undershoot characterizes the forced transient leg. After this phenomenon, the regression rate exhibits faint oscillations. This is probably related to the fast variation of the reacting surface temperature. The following slow return of $r_f$ to values associated to a constant mass flow rate is probably influenced by the thermal lag of the condensed phase and possibly by surface reactions time, since the forced transient duration is approximately 4 ms, while the two characteristic times of heat diffusion are in a time scale between 10^(-2) and 10^(-3) s . The second profile was the extinction of the sample, accompanied by a combustion chamber depressurization. It was observed that the depressurization effect creates an additional mass flow that affects the ballistic response. All of the performed tests at this operating profile showed a similar behavior. During the closure of the feeding line, a marked undershoot effect is observed, followed by an overshoot caused by the flow induced by depressurization. Finally, re-ignition tests of the samples after extinction was tested. For this forced transient, only qualitative analysis was performed. The achieved results show that the vaporized fuel, generated by the pre-heated grain, reacts with the oxidizer flow in a very short time and the complete re-establishment of the grain combustion requires approximately 8 ms. This time interval is comparable to characteristic time that governs the reacting surface temperature variations.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/110302