Development of a numerical model for the analysis of transient phenomena in HREs

In the rocket motors background the hybrid rocket engines (HREs) exhibit several advantages over the other two configurations, the liquid rocket engines (LREs) and the solid rocket motors (SRMs). There are also some disadvantages such as the low fuel regression rate and the combustion instability. The purpose of this work is to develop a numerical method capable of observing the combustion instability in the HREs with the aid of the previous studies developed about it. In the experimental tests on the HREs, it was found that the chamber pressure increases from its mean value and it oscillates into a nonlinear limit cycle, the so-called “DC shift“. Furthermore the chamber pressure oscillates with both low frequency and acoustic frequency, but the instability takes place in the low frequency regime. Unlike LREs and SRMs where the presence of combustion instability may lead to catastrophic events, in the HREs there are not this risks, nevertheless a better knowledge of this phenomenon is needed. A well known theory was developed to study the combustion instability in the LREs and in the SRMs; unfortunately that theory can not be used in the HREs. Thus, in this thesis, it was developed a numerical method based on three principal subsystems coupled. The first one is a computational fluid dynamic code in order to provide the gas dynamic properties inside the combustion chamber and the nozzle of the hybrid rocket system. Since the chemical reactions take place in the combustion chamber, a chemical code it has to be used; for this purpose it was employed the NASA CEA code []. Because of the complexity of the CEA code and in order to reduce the computational cost it was developed a numerical code. It takes as inputs the internal energy, density and molar fractions of the species inside the combustion chamber and it provides the temperature and the pressure at the chemical equilibrium as results. Once this code was validated comparing the results with the CEA code, it was coupled together with the CFD code and a further equation was added to the fluid dynamic system so as to define the quantities of oxidizer and fuel in the mainstream. It was studied the chemical composition in a specific nozzle configuration before using this code to analyse the flow in the combustion chamber and the results were compared with the results achieved in []. Once the code has shown the same results of [], it was able to study the mainstream flow in the combustion chamber of the hybrid rocket system in its steady-state condition.