Gallo, Giuseppe
(2021)
Experiments and Simulations of Hybrid Rocket Engines Burning Paraffin Fuels.
[Tesi di dottorato]
Collection description
In the last decade a significant and ever growing interest has been addressed towards hybrid rocket propulsion, which offers the best-of-both-worlds by leveraging the favourable aspect of both traditional solid and liquid systems. Among the numerous advantages which characterize hybrid rockets, the most attractive ones are the re-ignition and throttling capabilities combined with the possibility of embedding environmentally sustainable propellants and, of the utmost importance, their intrinsic safety and lower operational costs. Moreover, hybrid rockets yield a better specific impulse than solid propellant rockets and a higher density impulse than liquids, which make them a promising technology in a number of space missions.
The widely recognized potentialities of the hybrid rocket warrant the renewed research efforts that are being devoted to its development, but the state-of-the-art of this technology still presents a number of challenging issues to be solved.
A first fundamental task is the definition of suitable models for the prediction of the motor internal ballistics and performance. In particular, rocket performance is governed by the rate at which the fuel is gasified, i.e. by the fuel regression rate, as this latter determines the total mass flow rate and the overall oxidizer-to-fuel mixture ratio, which, for a given chamber pressure, control the motor thrust and the ideal specific impulse. For a given fuel, regression rate is basically limited by the heat flux input to the solid grain, which mainly depends on the thermo-fluid-dynamics in the combustion chamber. This latter is significantly influenced by several geometrical parameters, such as, for example, the oxidizer injection configuration or the grain port shape. Furthermore, the recent efforts aimed at overcoming the main drawback of the hybrid rockets, which is the low regression rate of conventional polymeric fuels, have been focused on the development of new paraffin-based fuels, characterized by a consumption mechanism presenting additional complex phenomena compared to that of conventional polymers. Their intrinsic characteristic is the onset of a thin liquid layer on the fuel grain surface, which may become unstable, leading to the lift-off and entrainment of fuel liquid droplets into the main gas stream, increasing the fuel mass transfer rate. This phenomenon is strongly susceptible to the fuel composition, its manufacturing process and the obtained thermo-mechanical properties as well as to the engine operating conditions, which makes the prediction of the regression rate and combustion chamber internal ballistics even harder than in the case of a pure polymer. In this framework, computational fluid dynamics of hybrid rocket internal ballistics is becoming a key tool for reducing the engine operation uncertainties and development cost, but its application still presents numerous challenges due to the complexity of modelling the phenomena involved in the fuel consumption mechanism and the interaction with the reacting flowfield. A research effort is therefore of major importance in order to cover the lacking aspects and obtain quantitatively accurate results.
In addition, the hybrid combustion process tends to produce somewhat rougher pressure versus time characteristics than either liquid or solid rocket engines. However, a well-designed hybrid will typically limit combustion roughness to approximately 2 to 3% of mean chamber pressure. In any combustion device, pressure fluctuations will tend to organize themselves around the natural acoustic frequencies of the combustion chamber or oxidizer feed system. While significant combustion pressure oscillations at chamber natural-mode acoustic frequencies have been observed in numerous hybrid motor tests, such oscillations have not proved to be an insurmountable design problem. When pressure oscillations have occurred in hybrid motors, they have been observed to grow to a limiting amplitude. Unbounded growth of pressure oscillations, such as may occur in solid and liquid rocket motors, has not been observed in hybrid motors. Hybrid motors burning classic fuels have exhibited two basic types of instabilities in static test environments: acoustic and non-acoustic instabilities. Additional triggering mechanisms can arise when involving liquefying fuels. The large amount of fuel entrained in the chamber introduces an additional time-lag required to “vaporize” the liquid droplets. This delay can excite acoustic modes and start the characteristic feed-back loop occurring during combustion instability. Despite recent advances in understanding causes and solutions for combustion instability in hybrid motors, development of a comprehensive, predictive theory of combustion stability remains one of the major challenges in hybrid technology development.
Therefore, the first main objective of the present work is the definition of proper computational thermo-fluid-dynamic models of the hybrid rocket internal ballistics, including a dedicated gas/surface interface treatment based on local mass, energy and mean mixture fraction balances as well as proper turbulence boundary conditions, which can properly model the physical fuel consumption mechanism. For the validation of the computational models, a number of experimental test cases obtained from static firing of laboratory scale rockets, have been performed at the Aerospace Propulsion Laboratory of University of Naples “Federico II” and successively numerically reconstructed. The comparison between the numerical results and the corresponding experimental data allowed validating the adopted model.
Then, a comparative experimental study on combustion instability for liquefying fuels is presented. The campaign is aimed to a better understanding of triggering mechanisms. An analytical model for the evaluation of the acoustic modes in a system with different cross sectional area and gas properties is described to investigate the exhibited experimental frequencies. Then, a suitable one-dimensional acoustic model for reacting flows with distributed mass flow rate adduction in the chamber is presented for the prediction and the analysis of combustion instability.
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