Daniele Pavarin

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.

CFD Simulation of a Hybrid Rocket Motor with Liquid Injection

In this paper, a commercial CFD code is used to simulate different hybrid rocket motor configurations applying liquid N2O as the oxidizer and paraffin wax as the fuel. This work is the prosecution of a previous study performed to simulate hybrid rockets with diaphragms of different geometries placed inside the combustion chamber, where N2O was injected in gaseous phase, instead of using liquid. With respect to the previous study, liquid injection has been introduced, together with the droplets vaporization inside the combustion chamber and their full coupling with the eulerian gas phase, in terms of both heat exchange and momentum exchange. The main objective is the description of the proper numerical models to be applied in test cases in which liquid injection has to be represented. The most important differences with respect to the simulations where only gas is injected are also discussed. In order to validate the CFD output, experimental results coming from two different design scales are used: a laboratory scale and an increased scale. For each of these two scales, different rocket configurations and geometries have been studied. The different geometries studied include: a lab-scale rocket with a cylindrical grain and with a 4-hole diaphragm inserted at the 24% of the grain length, a 1-hole diaphragm lab-scale motor and an increased-scale hybrid rocket with a 1-hole diaphragm and without any diaphragm. For each test case, a comparison with the related experiment is presented and discussed. The simulations have been run in steady state conditions, with simplified chemical reactions, liquid oxidizer injection and no paraffin entrainment. The simulations show a good agreement with the experimental results of the different rocket configurations analyzed: the maximum error on efficiency is 7%. The CFD predicts (both in the case of gas and liquid injection) a higher efficiency for the rocket geometries provided with a diaphragm with respect to the same geometries without a mixing device and this is in accord with experiments. CFD results also show some peculiar phenomena about liquid injection.

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 ...

SPIREs: A finite-difference frequency-domain electromagnetic solver for inhomogeneous magnetized plasma cylinders

Abstract We present SPIREs (plaSma Padova Inhomogeneous Radial Electromagnetic solver), a Finite-Difference Frequency-Domain (FDFD) electromagnetic solver in one dimension for the rapid calculation of the electromagnetic fields and the deposited power of a large variety of cylindrical plasma problems. The two Maxwell wave equations have been discretized using a staggered Yee mesh along the radial direction of the cylinder, and Fourier transformed along the other two dimensions and in time. By means of this kind of discretization, we have found that mode-coupling of fast and slow branches can be fully resolved without singularity issues that flawed other well-established methods in the past. Fields are forced by an antenna placed at a given distance from the plasma. The plasma can be inhomogeneous, finite-temperature, collisional, magnetized and multi-species. Finite-temperature Maxwellian effects, comprising Landau and cyclotron damping, have been included by means of the plasma Z dispersion function. Finite Larmor radius effects have been neglected. Radial variations of the plasma parameters are taken into account, thus extending the range of applications to a large variety of inhomogeneous plasma systems. The method proved to be fast and reliable, with accuracy depending on the spatial grid size. Two physical examples are reported: fields in a forced vacuum waveguide with the antenna inside, and forced plasma oscillations in the helicon radiofrequency range.

A microwave interferometer for small and tenuous plasma density measurements

The non-intrusive density measurement of the thin plasma produced by a mini-helicon space thruster (HPH.com project) is a challenge, due to the broad density range (between 10(16) m(-3) and 10(19) m(-3)) and the small size of the plasma source (2 cm of diameter). A microwave interferometer has been developed for this purpose. Due to the small size of plasma, the probing beam wavelength must be small (λ = 4 mm), thus a very high sensitivity interferometer is required in order to observe the lower density values. A low noise digital phase detector with a phase noise of 0.02° has been used, corresponding to a density of 0.5 × 10(16) m(-3).

Off-line ionization tests using the surface and the plasma ion sources of the SPES project.

The development of new target ion source systems for the selective production of exotic species (SPES) facility is currently in progress at Legnaro National Laboratories. In this context, the study of ion sources and their performance in terms of ionization efficiency and transversal emittance is a crucial point in order to maximize the available yields, particularly for short-lived isotopes. In this work, preliminary off-line ionization efficiency and emittance measurements for the SPES surface and plasma ion sources are presented. The plasma source emittance measurements are supported by dedicated numerical calculations.

Ion Velocimetry Measurements and Particle-In-Cell Simulation of a Cylindrical Cusped Plasma Accelerator

The Stanford Cylindrical Cusped Field Thruster (CCFT) has been experimentally and numerically investigated with particular focus on the exit plane acceleration region near the top magnetic cusp. Time-averaged xenon ion laserinduced fluorescence measurements using the 5d[4]7/2 -6p[3]5/2 (λ = 834.72-nm air) Xe II transition have mapped the total ion velocity vectors in this region. The thruster is also simulated using the fully kinetic 3-D particle-in-cell code F3MPIC. The consistent experimental and numerical results give physical insight into the mechanisms of ion acceleration and the role of the magnetic field topology in determining ion trajectories and plume divergence. The electrons are strongly magnetized and follow the magnetic field structure, grouping near the cusps. A steep potential drop over a few millimeters near the exit plane follows the magnetic separatrix of the top cusp, and is consistent with measured ion velocity vectors. A characteristic conical region of high ion density, peak ion velocity, and visible emission is observed in the experimental and simulated plume, with an estimated divergence half-angle of 30°.

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.