Alberto Bettella

Testing and CFD Simulation of Diaphragm Hybrid Rocket Motors

Combustion efficiency in standard hybrid rocket motors is usually low. This is due to the poor mixing of the core oxidizer flow with the gasi fied fuel from the walls. Motor scale increase usually worsens this issue. Large and volu metric inefficient post combustion chambers are common devices used to improve efficiency, while mixers are somewhat more complex, but more effective devices. The addition o f a mixer at grain aft-end was originally suggested by Marxman [1], but literature data on mixers, and in general on techniques to improve combustion efficiency is lacking. The resea rch activity herein presented aim at further investigate this issue: a mixer placed in f uel grain was selected as efficiency enhancing device. This work starts from experiments performed by Dr. Grosse [1] and more testing, both at lab scale and at increased scale h ave been carried out. Simple 1-hole diaphragms fixed at 25% of the grain length were used. Centre-hole diameter has been optimized. Moreover, CFD simulations have been carried out to further understand the enhanced mixing phenomena. N2O was selected as oxidizer and as fuel the same par affin Sasol Wax 0907 mixture used by Dr. Grosse was chosen. Lab scale experiments showed that combustion efficiency is raised from 76% (without diaphragm) up to 95% using diaphragms. Also good agreement has been found with CFD simulations. At increased scale (three times the thrust of lab scale), the referenc e combustion efficiency was 80%, and it was raised at 94% with the addition of a diaphragm. Als o combustion stability was enhanced. Another plus of the diaphragm on post-chambers and aft-end mixers is to enhance regression rate downstream it. At lab scale and wit h the smaller diaphragm regression rate was increased up to +90% (4.5 mm/s) compared to literature data without diaphragm. At increased scale the increase was of +65 % (4.2 mm/s). Further research is needed, but these experiments showed that diaphragms can be used to design compact and efficient single-port and paraffin based hybrid rocket motors.

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...

Investigation of Effect of Diaphragms on the Efficiency of Hybrid Rockets

This paper describes computational fluid dynamics applied to the analysis of a hybrid rocket motor with a diaphragm in the combustion chamber to enhance rocket performance. This work follows the last author’s experiments: An engine was tested with nitrous oxide and paraffin wax as propellants. Several of the tests have been used as references for numerical simulations. The following approximations have been made: steady-state conditions, eddy dissipation model with one-step reaction, gaseous injection of fuel and oxidizer, and no droplets entrainment (typical of a paraffin grain). First of all, a single geometry without a diaphragm has been analyzed with different turbulence models (k-ω, k-ω shear stress transport, k-e, k-e renormalization group). It has been shown that the k-ω model predicts a lower flame temperature and chamber pressure than the k-e model. Then, five geometries have been studied to compare two different types of diaphragms (one hole and four holes) in two positions (24 and 33% of the to...

Computational Fluid Dynamics Simulation of Regression Rate in Hybrid Rockets

Computational fluid dynamics has been applied to the simulation of hybrid rockets using O2 as the oxidizer and hydroxyl-terminated polybutadiene or high-density polyethylene as the fuel. Simulations have been carried out using predefined fuel and oxidizer mass flow rates, and calculating the fuel mass flow rate as a function of the wall heat flux. In this second case, the calculated regression rate has been compared to the average value derived from the reference experiments; no tuning coefficients have been introduced. Computational fluid dynamics results are compared with the corresponding experiments in the literature. In those tests where regression rate is calculated as a function of the wall heat flux, the fuel grain is divided into segments and its average value is plotted as a function of the average oxidizer mass flux in the combustion chamber. Computational fluid dynamics regression rate is underestimated by 30% for high-density polyethylene and 50% for hydroxyl-terminated polybutadiene if only ...

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...

Use of Hybrid Rocket Technology for Submarine Emergency Deballasting

One of the most effective measures for a rapid return of military submarines to the surface in case of life threatening, hazardous or potentially catastrophic failures is the emergency blowing of the forward main ballast tanks. This can be achieved with various systems. Gas generator based modular emergency blow systems show several advantages compared to designs, which use highly compressed air in terms of specific gas work, mass flow and gas storage density. Operational gas generators for emergency blowing use either a liquid hydrazine monopropellant or a solid propellant consisting of GAP and strontium nitrate for gas generation. An alternative hybrid propellant gas generator is proposed to avoid safety issues, which both operational gas generators types possess. Also the relative free selection of the blow gas composition motivates the use of a hybrid propellant combination. A mass model for a hybrid gas generator is developed using design elements of the hydrazine gas generator and applied to a number of different hybrid propellant combinations. The hybrid propellant mass is determined by use of the assumption that the volume reduction due to heat losses, water condensation and gas dissolution in the ballast tank can be compared for a blow gas that is generated from a solid or a hybrid propellant. The effectiveness of the modeled hybrid gas generator is assessed amongst others by blow system mass, system volume and required gas generator quantity. Comparing these preliminary results to the operational solid and liquid propellant gas generator shows that even a nonoptimized hybrid propellant gas generator system is very probably competitive in terms of mass and volume.

Numerical Modeling of Paraffin-Based Fuels Behavior

In recent years paraffin wax has been introduced as a fuel for hybrid rocket motors. Its fundamental characteristic is a higher regression rate compared to classical polymeric fuels due to entrainment of liquid droplets from the fuel surface. For this reason paraffin wax is the main fuel used by CISAS in its research experiments. In order to study paraffin-based fuels behavior a 1D transient code of the fuel grain has been implemented. The first part of the paper describes the equations used by the numerical model and its validation. The second part of the papers deals with the phenomenology of supercritical entrainment. It is shown that above the critical pressure fluids behave in a different way respect to liquids. Three hypotheses have been done in order to describe supercritical entrainment. Finally the regression rate predictions of the model are compared with the experimental results. It is shown that the slope of the regression rate curve is strongly related to the entrainment law exponents because the vaporization regression rate is small compared to the entrainment part. The closure problem of droplet entrainment is described, highlighting the need for a physical based link between surface temperature, droplets temperature and vaporization temperature.

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.