Martina Faenza

Numerical and Experimental Investigation of Unidirectional Vortex Injection in Hybrid Rocket Engines

A study on vortex injection in hybrid rocket engines with nitrous oxide and paraffin has been performed. The investigation followed two paths: first, the flowfield was simulated with a commercial computational fluid dynamics code; then, burn tests were performed on a laboratory-scale rocket. The computational fluid dynamics analysis had the dual purpose to help the design of the laboratory motor and to understand the physics underlying the vortex flow coupled with the combustion process compared with axial injection. Vortex injection produces a more diffuse flame in the combustion chamber and improves the mixing process of the reactants, both aspects concurring to increase the c* efficiency. A helical streamline develops downstream of the injection region, and the pitch is highly influenced by combustion, which straightens the flow due to the acceleration in the axial direction imposed by the temperature rise. Experimental tests with similar geometry have been performed. Measured performance shows an incr...

The "Vortex Reloaded" project: experimental investigation on fully tangential vortex injection in N2O - paraffin hybrid motors

This paper deals with an experimental and numerical project intended to study fully tangential vortex injection in a hybrid motor of 1kN class. Due to the knowledge of the CISAS Hybrid Team, the choice for oxidizer has been pressurized nitrous oxide, while paraffin wax has been used as fuel. This investigation follows a previous project where a mixed axial/vortex device has been tested: also if an increase in performance has been observed, a non-uniform grain consumption showed an issue of such device. The work starts with a mission scenario that gives the design drivers for the subsequent preliminary design, performed with an iterative, transient and 0D numerical code. A successive step is the detailed design of the combustion chamber and test bed. The aim of the study was to investigate three objectives: vortex injection and a comparison with axial; throttling behavior at fixed mass flows (reduction of 75% and 50% of oxidizer flow); combustion chamber performance changing its configurations, in particular using inside a mixer. Performance parameters taken into account were chamber pressure oscillations, combustion efficiency and regression rate. In the preliminary phase two different issues have been discovered and solved: the first regards a chamber pressure behavior variation, linked to a too long postchamber; the second is referred to pressure pikes in the ignition phase, solved using a longer prechamber and a different ignition configuration. It has been shown that vortex injection lowers the chamber pressure oscillations respect to axial case from more than 7% down to 4%. Moreover, regression rate has been increased of 41%, and the a coefficient of its law up to 67% from axial. This last value, indeed, shows a constant behavior throttling down the oxidizer mass flow. The increase is due to the higher wall heat flux in the grain surface, given by the higher velocity and thermal gradient of the fluid. Combustion efficiency has been increased with vortex injection given by the higher turbulence flow that enhances the mixing of the reactants. Axial case showed a value of 76% of this last parameter, while vortex case went up to 90%. Coupling of injection and mixer (a diaphragm-like device inside combustion chamber) increases this value up to 96%. A cost analysis has been performed for this project, showing that hybrid propulsion is a low cost technology.

Computational Fluid Dynamics Simulation of Hybrid Rockets of Different Scales

CFX software is used to simulate different hybrid rocket configurations, applying liquid N2O as the oxidizer and paraffin as the fuel. This work is the prosecution of a previous paper analyzing liquid injection in a lab-scale hybrid rocket. It is focused on the formulation of the most suitable simulation technique to represent another type of liquid injector, compared with the one described in the previous paper. It also aims at extending the computational fluid dynamics simulation approach to hybrid rockets of larger scales. To validate computational fluid dynamics output, experimental results coming from both a laboratory scale and an increased-scale engine have been used. The different geometries studied include an increased-scale engine with a cylindrical grain having no diaphragm, the same rocket with a one-hole diaphragm inside the fuel grain, and a lab-scale rocket with a one-hole diaphragm. Simulations are steady state, and combustion derives from a single-phase chemical reaction. Liquid injection...

Numerical Simulation of Hybrid Rockets Liquid Injection and Comparison with Experiments

In this paper, CFX is used to simulate different hybrid rocket configurations applying liquid N2O as the oxidizer and paraffin wax as the fuel. This work is intended as the prosecution of a previous study about hybrid rockets with diaphragms of different geometries inside the combustion chamber, where N2O was injected in a gaseous phase. In this work, liquid injection is introduced, together with droplets vaporization and their coupling with the Eulerian gas phase, in terms of both heat and momentum exchange. The main objective is the description of the numerical models to be applied when liquid is injected. To validate computational fluid dynamics output, experimental results coming from a laboratory scale engine have been used. The different geometries studied include an engine with a cylindrical grain having no diaphragm and the same rocket with a four-hole diaphragm at 24% of the grain length. The simulations are steady state, and combustion derives from a single-phase chemical reaction. Liquid inject...

Numerical Investigation of Hybrid Motors for the EU FP7 SPARTAN Program

The SPARTAN (SPAce exploration Research for Throttleable Advanced eNgine) project is targeted at the development of a soft-lander demonstrator for Mars landing, based on throttleable hybrid rocket engine propulsion technology. Within the project, CISAS “G. Colombo” is in charge of the development of a new advanced CFD code for the hybrid rocket motor unsteady simulation, of other CFD simulations conducted with commercial software to investigate the fluid dynamics of the rockets developed by NAMMO, and of ground fire tests to be conducted at lab-scale. Thus, for the CFD simulations, an Open Source code (OpenFOAM) and a commercial code (Ansys CFX) have been used. The flow field has been investigated in its different aspects, to understand the physics behind the processes of injection and combustion. The main approximations applied to this study when analyzing the flow with commercial codes are steady state analysis and fixed chemistry in the combustion chamber. Both vortex and axial injection have been simulated by means of the CFD: an axial injector has been used for the CISAS lab-scale rocket motor and a vortex injector for the test cases reproducing NAMMO’s configurations. Regarding vortex injection, the results highlight that it improves the turbulent mixing between the reactants and pushes the flame near the wall, enhancing the heat flux transmitted and thus the regression rate, thanks to its strong helical flow and centrifugal effects. Further efforts have been oriented to take into consideration regression rate dependency on the wall heat flux: besides injecting a fixed and pre-established fuel mass flow, the latter has been coupled to heat exchange. This study is in a preliminary phase, but the first results show that CFD regression rate as a function of the oxidizer mass flux follows the same trend as predicted by: , with a and n experimentally estimated by NAMMO. Concerning OpenFOAM numerical solver, it provides a transient solution of the flow field, but uses corresponding boundary conditions compared to commercial software, in order to allow a full comparison of the steady state results obtained. The study performed with OpenFOAM is a work in progress. The starting point has been a comparison with the results obtained using commercial CFD, to validate the open source solver and boundary conditions. This comparison has showed that OpenFOAM can predict the experimental results as well as commercial software does. At the moment, a feasibility study is being conducted in order to develop a custom and flexible boundary condition, able to calculate regression rate as a function of the wall heat flux independently on the specific oxidizer/fuel combination. Other studies are also necessary to limit the computational time required by this open source tool.

Optimization of a H2O2 Gas Generator for a Hybrid Rocket Engine

The EU FP7 SPARTAN project (SPAce exploration Research for Throttleable Advanced eNgine) aims at developing a highly throttleable propulsion system (10:1 thrust ratio), to perform soft and precise landing in planetary environments; the innovative technology relies on a hybrid engine burning HTPB and decomposed 87.5% H2O2 with vortex injection. In this framework the University of Padua (UPD) is responsible for developing a prompt and reliable CFD tool to support both design phases and internal ballistic analysis of the hybrid engine. To validate numerical results UPD has developed its own engine and ground testing are still ongoing to characterize hybrid performance in deep throttling. The issues related to handling and storing high concentration hydrogen peroxide have been overcome integrating the hybrid motor with a dedicated gas generator to reproduce 87.5% H2O2 in dissociated condition. The mixture produced inside the gas generator is then injected as oxidizer into the combustion chamber of the hybrid motor. This paper presents the main results of the optimization tests campaign performed on the integrated gas generator, to assess the injected hot mixture characteristics and ignition capability of the HTPB fuel.