This work aims at investigating the ballistic properties of various HTPB-based hybrid fuel formulations, burning in gaseous oxygen. Experimental analyses were conducted on several compositions containing nano-sized energetic powders (two different kinds of aluminum, called ALEX^TM and VF-ALEX, nano-iron and carbon nano-tubes). The formulations loaded with aluminum powders were manufactured using different pre-dispersion techniques in order to analyze their effects on the fuel ballistic behavior. The experimental tests were conducted on central-perforated cylindrical samples setting oxidizer volumetric mass flow at 210 nlpm with the combustion chamber pressure ranging from 4 to 16 bar to analyze the pressure effects on the fuel regression rate. Cured HTPB was taken as baseline for relative ballistic grading. Combustion tests were analyzed with an optical time-resolved technique. The research performed shows that the formulation containing VF-ALEX, under all of the investigated operating conditions, has the best ballistic properties (at 10 bar, the average increase with respect to the baseline was of 18.3% over the range of Gox from 400 to 200 kg/m^2s). Considering the manufacture techniques used to prepare loaded samples, the powders pre-dispersion in ethyl acetate provides the best ballistics performance enhancement (for HTPB loaded with ALEX^TM the average increase of regression rate with respect to the standard formulation at 10 bar was of 46.5% over the range of Gox from 400 to 200 kg/m^2s). Moreover, from the experimental investigation, a peculiar pressure dependence for the regression rate have been pointed out. A preliminary hypothesis were made connected the observed pressure effects to grain fragmentation and fuel pyrolysis. The gathered data were then used to define a semi-empirical model based on a surface energy balance, able to evaluate the effects of the convective and radiative heat flux on the hybrid combustion process. The results achieved on HTPB and loaded formulations show a good agreement between experimental and simulation results in particular for a surface temperature of 820 K. Finally an automatic procedure to analyze the combustion videos were developed. This software allows to achieve data standardization, reproducibility and to speed up the post processing of data collected during the experimental analysis.
Il lavoro qui presentato ha lo scopo di investigare la balistica di formulazioni combustibili per endoreattori ibridi a base di HTPB reagenti in ossigeno gassoso. Durante l’indagine sono state testate formulazioni combustibili additivate con nano-polveri (due tipi di alluminio, chiamati ALEX^TM e VF-ALEX, nano-ferro e carbo-nanotubi). Le formulazioni additivate con alluminio sono state realizzate utilizzando diverse tecniche per incrementare la dispersione delle polveri al fine di testarne l’effetto sulla balistica. I test sono stati condotti su campioni cilindrici a porto centrale, imponendo un flusso volumetrico di ossidante pari a 210 nlpm e variando la pressione in camera tra i 4 e i 16 bar in modo da verificarne possibili effetti sulla velocità di regressione. La balistica delle formulazioni additivate è stata caratterizzata relativamente ad una formulazione di riferimento a base di solo HTPB. I test sono stati analizzati mediante tecnica ottica time-resolved. L’indagine svolta ha permesso di evidenziare come, in tutte le condizioni operative investigate, il combustibile contenente VF-ALEX presenti le migliori proprietà balistiche (a 10 bar, l’incremento medio della velocità di regressione rispetto alla baseline è stato del 18.3% su un range di Gox da 400 a 200 kg/m^2s). Per quanto riguarda le tecniche di dispersione, l’uso di acetato di etile ha permesso il raggiungimento delle migliori prestazioni balistiche (per la formulazione additivata con ALEX^TM a 10 bar, l’incremento medio della velocità di regressione rispetto alla formulazione trattata con tecnica standard di dispersione è stato del 46.5% su un range di Gox da 400 a 200 kg/m^2s). Durante l’analisi sperimentale, inoltre, è stata evidenziata una peculiare dipendenza della velocità di regressione dalla pressione. Una spiegazione preliminare del fenomeno è stata fornita collegando l’effetto osservato alla frammentazione del grano e alla pirolisi del combustibile. I dati raccolti nell’indagine sperimentale sono stati, poi, utilizzati per sviluppare un modello semi-empirico, basato su un bilancio energetico alla superficie, in grado di valutare gli effetti del flusso termico convettivo e radiativo sul processo di combustione. I risultati ottenuti mediante simulazione e sperimentazione sono, tra loro, in ottimo accordo soprattutto per temperature superficiali di 820 K. Infine è stata sviluppata una procedura automatica per l’analisi dei filmati di combustione. Tale software ha permesso un incremento della standardizzazione dei dati, della loro riproducibilità e una velocizzazione di tutta la fase di analisi.
Experimental investigation and modeling of solid fuel regression rate for hybrid propulsion
MANZONI, MIRIAM
2010/2011
Abstract
This work aims at investigating the ballistic properties of various HTPB-based hybrid fuel formulations, burning in gaseous oxygen. Experimental analyses were conducted on several compositions containing nano-sized energetic powders (two different kinds of aluminum, called ALEX^TM and VF-ALEX, nano-iron and carbon nano-tubes). The formulations loaded with aluminum powders were manufactured using different pre-dispersion techniques in order to analyze their effects on the fuel ballistic behavior. The experimental tests were conducted on central-perforated cylindrical samples setting oxidizer volumetric mass flow at 210 nlpm with the combustion chamber pressure ranging from 4 to 16 bar to analyze the pressure effects on the fuel regression rate. Cured HTPB was taken as baseline for relative ballistic grading. Combustion tests were analyzed with an optical time-resolved technique. The research performed shows that the formulation containing VF-ALEX, under all of the investigated operating conditions, has the best ballistic properties (at 10 bar, the average increase with respect to the baseline was of 18.3% over the range of Gox from 400 to 200 kg/m^2s). Considering the manufacture techniques used to prepare loaded samples, the powders pre-dispersion in ethyl acetate provides the best ballistics performance enhancement (for HTPB loaded with ALEX^TM the average increase of regression rate with respect to the standard formulation at 10 bar was of 46.5% over the range of Gox from 400 to 200 kg/m^2s). Moreover, from the experimental investigation, a peculiar pressure dependence for the regression rate have been pointed out. A preliminary hypothesis were made connected the observed pressure effects to grain fragmentation and fuel pyrolysis. The gathered data were then used to define a semi-empirical model based on a surface energy balance, able to evaluate the effects of the convective and radiative heat flux on the hybrid combustion process. The results achieved on HTPB and loaded formulations show a good agreement between experimental and simulation results in particular for a surface temperature of 820 K. Finally an automatic procedure to analyze the combustion videos were developed. This software allows to achieve data standardization, reproducibility and to speed up the post processing of data collected during the experimental analysis.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/49901