Francesco Barato

Numerical Investigation of the Effect of a Diaphragm on the Performance of a Hybrid Rocket Motor

This paper describes the use of a CFD code (Ansys CFX 12) for the analysis of a hybrid rocket motor with a diaphragm placed in the combustion chamber in order to enhance rocket performance. This work follows the experimental campaign of Matthias Grosse who tested the motor using nitrous oxide and paraffin wax as propellants. Several of his tests have been used as a reference for the numerical simulations. Several approximations have been made: steady state conditions, eddy dissipation combustion model with one-step reaction, gaseous injection of fuel and oxidizer, no droplets entrainment (typical of a paraffin grain). First of all, a single geometry without diaphragm has been analyzed with different turbulence models (k-ω, k-ω SST, k-e, k-e RNG). It has been shown that the k-ω model predicts a lower flame temperature and chamber pressure than k-e. Five geometries have been studied in order to compare the use of two different types of diaphragm (1 hole and 4 holes) in two positions (24% and 33% of the total length) respect to a configuration without mixer. The effect of the diaphragm is an increase of the mixing of the chemical species participating in the combustion process. The use of the diaphragm showed a performance enhancement, as showed in the experimental campaign. There is a good agreement between CFD results and experimental data: the efficiency is overestimated by less than 5.5%. This work proves the capability of CFD codes to predict global hybrid motor performances and to be a useful tool in the study of mixing devices.

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 ...

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...

Scaling Parameters of Swirling Oxidizer Injection in Hybrid Rocket Motors

Hybrid rockets present some disadvantages, mainly low regression rate and combustion inefficiencies. A promising technology to solve both is swirling oxidizer injection, which enhances the wall hea...

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 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.