The limitation of low regression rate in hybrid rocket propulsion can be overcome through the use of liquefying fuels. These form a melt layer over the grain, which allows mechanical mass transfer of fuel into the oxidiser flow by an entrainment process. The most promising fuel formulations are n-alkanes, specifically paraffin-based fuels, which show a good trade-off between structural properties and regression rate enhancement through entrainment. In this work the behaviour of different paraffin formulations under the effect of an oxidiser flow was studied by numerical means, using a multiphase solver of OpenFOAM. The influence of the viscosity and of the surface tension on the onset of the entrainment and on the primary break-up time was recorded, resulting to be in accordance with the theoretical model for hybrid combustion. The evolution of the temperature in the combustion chamber was considered; its influence is of essence for the residence time of the fuel droplets, but also for the characterisation of the transport and thermophysical properties that are required for numerical simulations. A simplified case was setup using a multi-region solver of OpenFOAM, as a basis for a future simulations, including both fluid interaction and heat transfer. The simplified implementation, considering only conjugate heat transfer, allowed for a sensitivity analysis of the two-dimensional temperature profile when changing the thickness of the melt layer and the flame location. The heat fluxes at the interfaces were also calculated. The results showed an increasing surface temperature and a decreasing heat flux of the liquid layer with its thickness, and a decreasing trend for both as the flame location is shifted upwards.
La limitata velocità di regressione nella propulsione dei lanciatori ibridi può essere superata grazie all'uso di propellenti bassofondenti. Questi formano uno strato liquido al di sopra del carburante solido, permettendo il trasferimento meccanico di massa del propellente nel flusso dell'ossidante, grazie ad un processo di entrainment. In questo lavoro è stato analizzato con processi numerici il comportamento di differenti paraffine sotto l'effetto di un flusso di ossidante, avvalendosi di un solver multifase di OpenFOAM. Sono inoltre state registrate l'influenza della viscosità e della tensione superficiale sulle condizioni di formazione dell'entrainment e sul tempo di primary break-up, ritrovando un accordo con il modello teorico relativo alla combustione ibrida. È stata considerata infine la temperatura nella camera di combustione; la sua influenza è fondamentale per il residence time delle gocce di carburante, ma anche per la caratterizzazione delle proprietà di trasporto e termofisiche richieste per le simulazioni numeriche. L'implementazione semplificata ha permesso un'analisi di sensitività del profilo di temperatura bidimensionale al variare dello spessore dello strato liquido e della posizione della fiamma. Sono stati inoltre calcolati i flussi di calore alle interfacce. I risultati hanno mostrato un aumento della temperatura superficiale e una diminuzione del flusso di calore con lo spessore dello strato liquido, e una diminuzione di entrambi quando la fiamma è spostata più in alto.
Characterization of liquefying fuels for hybrid rocket propulsion
Caielli, Francesca
2019/2020
Abstract
The limitation of low regression rate in hybrid rocket propulsion can be overcome through the use of liquefying fuels. These form a melt layer over the grain, which allows mechanical mass transfer of fuel into the oxidiser flow by an entrainment process. The most promising fuel formulations are n-alkanes, specifically paraffin-based fuels, which show a good trade-off between structural properties and regression rate enhancement through entrainment. In this work the behaviour of different paraffin formulations under the effect of an oxidiser flow was studied by numerical means, using a multiphase solver of OpenFOAM. The influence of the viscosity and of the surface tension on the onset of the entrainment and on the primary break-up time was recorded, resulting to be in accordance with the theoretical model for hybrid combustion. The evolution of the temperature in the combustion chamber was considered; its influence is of essence for the residence time of the fuel droplets, but also for the characterisation of the transport and thermophysical properties that are required for numerical simulations. A simplified case was setup using a multi-region solver of OpenFOAM, as a basis for a future simulations, including both fluid interaction and heat transfer. The simplified implementation, considering only conjugate heat transfer, allowed for a sensitivity analysis of the two-dimensional temperature profile when changing the thickness of the melt layer and the flame location. The heat fluxes at the interfaces were also calculated. The results showed an increasing surface temperature and a decreasing heat flux of the liquid layer with its thickness, and a decreasing trend for both as the flame location is shifted upwards.| File | Dimensione | Formato | |
|---|---|---|---|
|
Master_thesis_CAIELLI.pdf
accessibile in internet per tutti
Descrizione: Tesi di laurea magistrale
Dimensione
4.43 MB
Formato
Adobe PDF
|
4.43 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/10589/170945