Di Martino, Giuseppe Daniele
(2018)
Experiments and simulations of hybrid rocket internal flows and material behaviour.
[Tesi di dottorato]
Abstract
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, for both the cases of classical polymeric and liquefying paraffin-based fuels. A research effort is therefore of major importance in order to cover the lacking aspects and obtain quantitatively accurate results.
Another challenge for the hybrid rocket technology development is the optimization of the design of thermal insulations. The inner surface of the exhaust nozzle, through which the flow is accelerated to supersonic conditions producing the required thrust, is the most critical in this sense, as it is subjected to the highest shear stress and heat fluxes in a chemically aggressive environment. These severe conditions usually lead to removal of surface material due to heterogeneous reactions between oxidizing species in the hot gas and the solid wall. Because of the material erosion, there is an enlargement of the nozzle throat section and a consequent decrease of rocket thrust, with detrimental effects over the motor operation. Thus, the requirement that dimensional stability of the nozzle throat should be maintained makes the selection of suitable rocket nozzle materials extremely hard. In recent years, Ultra-High-Temperature Ceramics (UHTC) and Ultra-High-Temperature Ceramic Matrix Composites are the subject of considerable interest as innovative materials for rocket application, but still need to be properly characterized. Experimental testing along with computational fluid dynamic (CFD) simulations are, thus, both needed to improve the design and the current performance prediction capabilities of such propulsion systems. In this framework, the University of Naples is involved in the European project C3HARME – Next Generation Ceramic Composites for Combustion Harsh Environment and Space, in collaboration with other research centres, universities and industries, which aims at the design, manufacturing and testing of new-class high-performance UHTCMC for near-zero erosion rocket nozzles.
In the present work, the above-mentioned challenges are dealt with taking a combined experimental/numerical approach to improve understanding of the interaction between the gaseous combusting flow typical of hybrid rocket engines and the surface of solid materials involved in their operation, with a special focus to the fuel grain present in the combustion chamber, with the aim of predicting its consumption mechanism, and the exhaust nozzle inner surface, with the aim of identifying and validating new-class UHTCMC materials with improved erosion and structural resistance to the severe conditions experienced in particular in the throat region.
In particular, 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 in both the cases of polymeric and liquefying fuels. 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 and identifying possible future improvements.
Then, the research activities for the characterization of new-class UHTCMC materials are presented and discussed. This part of the work was mainly focused on an extensive experimental campaign for the characterization of new-class UHTCMC materials. In particular, first preliminary tests on small samples exposed to the supersonic exhaust jet of a 200N-class hybrid rocket operated with gaseous oxygen burning cylindrical port High-Density PolyEthylene (HDPE) fuel grains have been carried out for a fast characterization and a preliminary screening of the best candidates for the final applications. After that UHTCMC nozzle throat inserts has been manufactured and experimentally tested to verify the erosion resistance and evaluate the effects on the rocket performance by comparison with those obtained in similar operating conditions employing a graphite nozzle. The experimental activities are supported by simplified low-computational-cost numerical simulations, whose main objectives has been the prediction of the complex flow field in the hybrid rocket combustion chamber and the thermo-fluid dynamic conditions on the material. Future research activities will be then focused to the further development of the numerical models with the extension of the treatment for the gaseous flow/solid surface interaction in order to get a deeper insight on the new materials behaviour.
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