Gel Propellants Research Articles

Investigation of the Evaporation Processes of Gel Propellant Droplets

Due to their high-performance and improved safety, gel propellants can be used in various boost motors and large launchers. The evaporation and combustion characteristics of gel propellants are the foundation for the gel-engine design. Especially, it is basal and important to study the gel droplet evaporation process. In this paper, the gel droplet evaporation model is developed to simulate the gel droplet evaporation process at first. Then the experiments to record the gel droplet evaporation process are conducted. During the droplet evaporation process, the decreased velocity of the droplet diameter increases gradually, whereas that of the droplet mass decreases gradually. The mass of both the liquid fuel and the gellant decreases gradually, however, the gellant mass concentration increases gradually and at the evaporation later stage the gellant mass is larger than that of the liquid fuel. The typical evaporation process characteristics captured by experiments are in reasonable agreement with the gel droplets evaporation mechanism. Especially, the “micro-burst” phenomenon of the gelled propellant may appears in rocket engines.

Wall friction effects and viscosity reduction of gel propellants in conical extrusion

Influence of wall friction on mean apparent viscosity reduction of gel propellants in conical radial flow extrusion is investigated analytically and numerically. A parametric study has been conducted to evaluate the effects of injector geometry, rheological constants, wall friction factor and volumetric flow rate on fluid viscosity profile. Gel propellants were modeled by the non-Newtonian power law constitutive relation. The influence of wall friction on the mean apparent viscosity, on wall slip and on pressure gradient is found to be substantial. Central results are supported by asymptotic analysis for small cone angles and low wall friction. Contact is made with lubrication type flow pattern.

Rheological Matching of Gel Propellants

A SIGNIFICANT effort has been undertaken by several research groups worldwide over the past decades to study various aspects of gel propellants. The principal objective of these basic studies is to realize a flexible energy-management propulsion system,which assures insensitivemunitions compliance. In addition, their increased energy density, whenmetal particles are introduced in the gel matrix, in comparison to neat-liquid propellants presents a significant potential advantage. The non-Newtonian complex rheological character of these propellants and the resulting systemlevel implications make their use in operational rocket engines very challenging. Nevertheless, their potentially full pulse-widthmodulation capabilities, combined with the ability for divert and attitude control system application, make them attractive for future rocket propulsion systems in both tactical and space applications [1– 4]. Key aspects of gel-propulsion technology, propellant preparation, rheology, atomization, and combustion are presented in an extensive review byNatan and Rahimi [5]. A general rheological classification of gel propellants and simulants has been proposed in a recent study on the shear rheology of gel propellants [6]. Rheologicalmatching of gel propellants is an intentional change in one or more relevant properties of the fluids, within specified ranges of temperatures and shear rates, to obtain a desired rheological behavior. Matching is achieved by using suitable techniques, depending on gel composition and preparation procedure. The reasons for rheological matching can vary as follows. 1) Gel-propellant simulant matching. The use of water-based simulants in lieu of toxic and corrosive materials enables the conduction of system development tests without the difficulties inherent in the handling of such materials, allows budget savings, and enhances safety. For example, hydrazine-based gel fuels are attractive in propulsion systems because of their performance characteristics, although safety precautions make the handling of such materials a difficult task. The use of a hydrazine-gel simulant allows adequate handling and testing of the feeding system without being exposed to safety hazards.Water-based gel simulants have been used for sprayflowvisualization [7], thixotropymodeling [8], atomization studies [9,10], elongational behavior [11], and propellant tank expulsion performance [4]. 2) Gel-propellant matching. Fuel and oxidizer can be matched for gel-feeding system design purposes. This matching provides the designer with an additional degree of freedom. In addition, certain fuel and oxidizer gels can bematched to complywith special requirements, such as a reduced temperature sensitivity of their rheological properties within a wide range of temperatures. Shear viscosity of gel-propellant simulants has been found to be significantly affected by shear rate and gel composition (gellant type and mass fraction) [6]. On the other hand, the effects of thixotropy and temperature field were found to be insignificant in comparison to the shear-thinning and composition effects. Rheological matching does not imply that gels of two different materials and gelling agents would have absolutely identical rheological parameters under all circumstances. However, these properties can be brought close enough, in relevant ranges of ambient conditions, such as flow rates, temperature, and external mechanical loads, to satisfy certain requirements. The gellant type and content provide a degree of freedom for the determination of the rheological properties of a gel propellant. The scope of the present study is to demonstrate the rheological matching of fuel, oxidizer, and simulant gels using various gellants, separately or combined. An investigation of the effect of temperature on the rheological parameters of water-based, gel-propellant simulants, formulated by various gellant combinations at different ratios among them, is also presented.

Application of Cerimetric Methods for Determining the Metallic Aluminum Content in Ultrafine Aluminum Powders

AbstractUltrafine (nano‐sized) aluminum is a promising component for solid propellants, gel propellants, explosives, etc. The work is focused upon a problem of determining active (metallic) aluminum content in nanoAl powders as one of the characteristics of their reactivity. It is shown that the high reactivity, the presence of either the gases adsorbed or coating matter on the particle surface restricts the traditionally used permanganatometric and volumetric analytic methods. A new technique determining the active aluminum content is presented. This is the adaptation of known cerimetric method based on the analytical reaction Ce4++e−=Ce3+. The method can be applied for analysis of metallic aluminum in the probes of nanoAl (including ones with organic coating), as well as micron sized aluminum, and their condensed combustion products.

On Shear Rheology of Gel Propellants

AbstractSelected fuel, oxidizer and simulant gels were prepared and rheologically characterized using a rotational rheometer. For fuel gelation both organic and inorganic gellants were utilized, whereas oxidizers and simulants were gelled with addition of silica and polysaccharides, respectively. The generalized Herschel‐Bulkley constitutive model was found to most adequately represent the gels studied. Hydrazine‐based fuels, gelled with polysaccharides, were characterized as shear‐thinning pseudoplastic fluids with low shear yield stress, whereas inhibited red‐fuming nitric acid (IRFNA) and hydrogen peroxide oxidizers, gelled with silica, were characterized as yield thixotropic fluids with significant shear yield stress. Creep tests were conducted on two rheological types of gels with different gellant content and the results were fitted by Burgers‐Kelvin viscoelastic constitutive model. The effect of temperature on the rheological properties of gel propellant simulants was also investigated. A general rheological classification of gel propellants and simulants is proposed.

ATOMIZATION OF GEL PROPELLANTS THROUGH AN AIR-BLAST TRIPLET ATOMIZER

An effort was made to relate the rheological properties of gel propellant simulants with their atomization behavior. The gels were atomized using a triplet impinging air-blast atomizer. The experimental investigation demonstrates that pseudoplastic water gels exhibit spray patterns that resemble those of Newtonian liquids, but they are more difficult to atomize. The Sauter mean diameter decreases along the injector axis and increases with increasing radial coordinate and the angle between the gas jets plane and the horizontal plane. Wide-angle convergent injectors require lower upstream pressure to achieve the same atomization performance with straight injectors. Sauter mean diameter decreases down to a minimum constant value with the increase of the air-to-liquid mass flow rate ratio. Sauter mean diameter increases with increasing gellant content due to the respective increase of the shear viscosity. An expression relating the Sauter mean diameter to the injection conditions and the mean apparent viscosity at the injector exit is presented. Corresponding author. Faculty of Aerospace Engineering, Technion Israel Institute of Technology, Haifa 32000, Israel. e-mail: aerbeny@aerodyne.technion.ac.il

Rheological Behavior of Nitromethane Gelled with Nanoparticles

Gel propellants provide rocket propulsion systems of high specific impulse, low sensitivity, and low vulnerability in combination with the capability to control the thrust, that is, variation of thrust and thrust cutoff. The rheological characterization is essential for the development of adequate gel propellant formulations and, thus, for the design of an applicable propulsion system. The rheological behavior of a gel propellant consisting of nitromethane as fuel and nanoparticles of silicon dioxide as gelling agent was examined. The experiments were conducted under steady-state shear flow and oscillatory shear. The nitromethane/silicon dioxide gels exhibit non-Newtonian flow behavior over the whole concentration range examined. Additionally, a yield stress is determined for all of the gels. The viscoelastic properties are typically gellike in that the storage and the loss moduli are both independent of frequency.

Mechanical Properties of Gel Propellants with Nanoparticles

Gel propellants provide rocket propulsion systems of high specific impulse, low sensitivity, and low vulnerability in combination with the capability to control the thrust, that is, variation of thrust and thrust cutoff. The characterization of the mechanical properties is essential for the development of gel propellant formulations and for the design of an applicable propulsion system. In this contribution the mechanical properties of a gel propellant consisting of nitromethane as fuel and nanoparticles of silicon dioxide as gelling agent are examined. The experiments were conducted under steady state shear flow and oscillatory shear. The nitromethane/silicon dioxide gels exhibit non-Newtonian flow behavior over the whole concentration range examined. The viscoelastic properties are typically gel-like in that the storage and the loss moduli are both independent of frequency.

Development of Laboratory-Scale Gel Propulsion Technology

Selected gel propellants and simulants were formulated, prepared, rheologically characterized, and tested in the first phase of a program to develop gel-propulsion technology infrastructure. Hydrazine-based fuels, gelled with polysaccharides, were characterized as shear-thinning pseudoplastic fluids with low-shear yield stress (τ y i e l d ), whereas inhibited red-fuming nitric acid (IRFNA) and hydrogen peroxide oxidizers, gelled with silica, were characterized as yield thixotropic fluids with significant τ y i e l d . Safe storage and handling procedures were established. A laboratory-scale experimental setup was used to hot fire successfully a small 100-N nominal thrust rocket engine with selected hypergolic neat-liquid and gelled bipropellant combinations. One-element pentad-type injectors were utilized in the tests to inject the propellants into the combustion chamber. Continuous tests of up to 25-s firing duration and multipulse operations of up to 20 cycles of 0.1-s on/0.5-s off were successfully conducted with gelled-hydrazine/IRFNA bipropellants. Neat-liquid and gelled mono-methyl hydrazine/IRFNA bipropellants were also tested. The combustion pressure ranged between 20 and 35 bars. Experimental characteristic velocity, c* e x p , was determined as a function of the oxidizer-to-fuel (O/F) mass flow rate ratio. Maximum c* efficiency of more than 95 and about 90% was obtained in continuous firings for the neat-liquid and gelled hydrazine/IRFNA, respectively. In both cases, the maximum c* e x p values were obtained at higher O/F ratios than those that yield maximum theoretical c*.

Potential Usage of Energetic Nano-sized Powders for Combustion and Rocket Propulsion

ABSTRACTNano-sized energetic metals and boron particles (with dimensions less than 100 nanometers) possess desirable combustion characteristics such as high heats of combustion and fast energy release rates. Because of their capability to enhance performance, various metals have been introduced in solid propellant formulations, gel propellants, and solid fuels. There are many advantages of incorporating nano-sized materials into fuels and propellants, such as: 1) shortened ignition delay; 2) shortened burn times, resulting in more complete combustion in volume-limited propulsion systems; 3) enhanced heat-transfer rates from higher specific surface area; 4) greater flexibility in designing new energetic fuel/propellants with desirable physical properties; 5) nano-particles can act as a gelling agent to replace inert or low-energy gellants; 6) nano-sized particles can also be dispersed into high-temperature zone for direct oxidation reaction and rapid energy release, and 7) enhanced propulsive performance with increased density impulse. In view of these advantages, numerous techniques have been developed for synthesizing nano-particles of different sizes and shapes. To reduce any possible hazards associated with the handling of nano-sized particles as well as unwanted particle oxidation, various passivation procedures have been developed. Some of these coating materials could enhance the ignition and combustion behavior, others could increase the compatibility of the particles with the surrounding material. Many researchers have been actively engaged in the characterization of the ignition and combustion behavior of nano-sized particles as well as the assessment of performance enhancement of propellants and fuels containing energetic nano-particles. For example, solid fuels could contain a significant percentage of nano-sized particles to increase the mass-burning rate in hybrid rocket motors, the regression rate of solid propellants can be increased by several times when nano-sized particles are incorporated into the formulation. Specifically, hybrid motor data showed that the addition of 13% energetic aluminum powders can increase the linear regression rate of solid HTPB-based fuel by 123% in comparison to the non-aluminized HTPB fuel at a moderate gaseous oxidizer mass flow rate. Strand burner studies of two identical solid propellant formulations (one with 18% regular aluminum powder and the other with 9% aluminum replaced by Alex® powder) showed that nano-sized particles can increase the linear burning rate of solid propellants by 100%. In addition to solid fuels and propellants, spray combustion of bipropellants has been conducted using gel propellants impregnated with nano-sized boron particles as the fuel in a rocket engine. High combustion efficiencies were obtained from burning nano-sized boron particles contained in a non-toxic liquid-fuel spray. Materials characterization such as chemical analyses to determine the active aluminum content, density measurements, and imaging using an electron microscope have been performed on both neat nano-sized particles and mixtures containing the energetic materials. In general, using energetic nano-sized particles as a new design parameter, propulsion performance of future propellants and fuels can be greatly enhanced.

THE STATUS OF GEL PROPELLANTS IN YEAR 2000

The present study is a review of the main aspects in gel propellants past and current research and development. The work deals with the following subjects: Historical background of gel propellants; Advantages of gel propellants in comparison to liquid and solid propellants: safety, performance, storage; Theoretical performance evaluation of different gel propellant formulations for various missions; Rheological characterization: definition of the gel type, rheometry, identification of the parameters that affect the rheological properties; Preparation of gel propellants, gellants, compatibility of the ingredients; Flow behavior: power law, shear thinning fluids in pipes and injectors, thixotropic effect; Atomization: investigation of non-Newtonian fluid atomization characteristics, feeding systems; Combustion behavior: non-metallized fuel droplet ignition and burning, metallized fuel combustion and secondary atomization, combustion of gel fuels in small motors. Key word: gel, gel propellants, gel fuels, combustion, atomization, rheology.

Area 1: Reaction Kinetics of Energetic Materials (Solid, Liquid, and Gel Propellants)

Area 1: Reaction Kinetics of Energetic Materials (Solid, Liquid, and Gel Propellants)

Numerical Solution of the Flow of Power-Law Gel Propellants in Converging Injectors

The governing equations of the steady, incompressible, isothermal, laminar flow of a power-law, shear-thinning gel propellant in a converging injector were formulated. The equations were transformed to a ψ–ω system, then discretized and solved. A numerical algorithm was developed for the solution of the flowfield. A parametric investigation was conducted to evaluate the effect of the injector geometry and the flow rate on the velocity and viscosity fields, for various gel propellants. In comparison to a straight injector, a convergent one can produce additional decrease in the mean apparent viscosity of the fluid. The mean apparent viscosity decreases significantly with increasing the convergence angle of the injector. In general, increasing the gel flow rate results in a decrease of the mean apparent viscosity of the fluid. Increasing the gellant content of gel propellants results in an increase of the fluid viscosity that decreases when the gel flows through the injector. Gels of different gellant content that flow through the same geometry injector exhibit similar relative reduction in viscosity. Numerische Simulation des Stömungsfeldes von Gel-Treibstoffen in konvergenten Injektoren Es wurden die bestimmenden Gleichungen des stationären, inkompressiblen, laminaren Strömungsfeldes in konvergenten Injektoren für einen Gel-Treibstoff formuliert, der sich nach einem Exponentialgesetz unter Scherkräften verdünnt. Die Gleichungen wurden in ein ψ–ω-System transformiert, diskretisiert und gelöst. Ein Algorithmus für die Lösung des Strömungsfeldes wurde entwickelt. Des weiteren fand eine parametrische Untersuchung statt, um die Effekte der Injektorgeometrie und der Flußrate auf die Geschwindigkeits- und Viskositätsfelder für verschiedene Gel-Teibstoffe zu ermitteln. Ein konvergenter Injektor kann, im Vergleich zu einem geraden Injektor, eine zusätzliche Abnahme bewirken. Die mittlere scheinbare Viskosität nimmt mit zunehmendem Konvergenzwinkel des Injektors signifikant ab. Im allgemeinen erzeugt eine Erhöhung der Gel-Flußrate eine Abnahme der mittleren scheinbaren Viskosität des Fluids. Eine Zunahme des Gehaltes an Gelierungsmittel im Gel-Treibstoff bewirkt einen Anstieg der Fluidviskosität, die sinkt, wenn das Gel durch den Injektor fließt. Gels mit verschiedenen Gehalten an Gelierungsmittel, die durch die gleichen Injektorgeometrien fließen, verzeichnen eine ähnliche relative Reduktion der Viskosität. Simulation numérique du champ d'écoulement de propergols gels dans des injecteurs convergents On a formulé les équations régissant le champ d'écoulement stationnaire, incompressible, laminaire dans des injecteurs convergents pour un propergol gel, qui se dilue selon une loi exponentielle sous l'influence de forces de cisaillement. Les équations ont été transformées, discrétisées et résolues dans un système ψ–ω. On a développé un algorithme pour résoudre le champ d'écoulement. Par ailleurs, une étude paramétrique a été effectuée pour déterminer les effets de la géométrie de l'injecteur et du débit de l'écoulement sur les champs de vitesse et de viscosité pour différents propergols gels. Par comparaison avec un injecteur droit, un injecteur convergent peut entraîner une diminution supplémentaire. La viscosité apparente moyenne diminue lorsque l'angle de convergence de l'injecteur augmente. En règle générale, une augmentation du débit du gel entraîne une augmentation de la viscosité apparente moyenne du fluide. Une augmentation de la teneur en agent gélifiant du propergol gel entraîne une augmentation de la viscosité du fluide qui diminue lorsque le gel s'écoule à travers l'injecteur. Des gels à différentes teneurs d'agent gélifiant, qui s'écoulent à travers la même géométrie d'injecteur, donnent une réduction relative semblable de la viscosité.

Numerical Solution of the Flow of Power-Law Gel Propellants in Converging Injectors

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Flow of Gel Fuels in Tapered Injectors

The governing equations of the steady e ow of gel propellants and fuels in a tapered tube injector have been formulated assuming a power-law rheological model. A parametric investigation was conducted to evaluate the effect of the injector geometry and pressure gradient on the e ow rate and the mean apparent viscosity of the gel for various fuels and metal loadings. The theoretical results indicate that the e ow rate increases signie cantly with decreasing thepower-law index and with increasing the pressuredrop in the injector. Theapparent viscosity is not uniformineachcrosssection;itismaximalattheaxisandreachesminimumatthewall.Themeanapparentviscosity of the gel exhibits a signie cant decrease with increasing the convergence angle of the injector. This implies that to obtain betteratomizationofthegel,high convergenceanglesarerequired.AcomparisonbetweenRP-1/Algelswith various aluminum mass fractions was conducted. The results indicate the existence of an optimal metal loading.

Performance Evaluation and Experimental Studies on Metallised Gel Propellants

Metallised gel propellants offer higher specific impulse and volumetric loading, reduced vaporisation loss, spillage and slosh problems and easy storage in comparison to the conventional liquid propellants. Theoretical performance analysis of gel propellant containing Al in unsymmetrical dimethyl hydrazine-dinitrogen tetroxide (UDMH-N/sub 2/O/sub 4} system shows peak Isp (vacuum condition) of 316.7 s and 318.3 s at oxidiser/fuel (O/f) ratios of 1.5 and 1.0, respectively for 30 per cent and 40 per cent UDMH-Al gel propellants, under standard conditions. The effect of other parameters like area ratio and chamber pressure on performance has been brought out in view of mission oriented applications. Aluminium has been found to be a better choice over magnesium in metallised gel propellants. Experimental studies on UDMH gellation using propellant grade (15 micrometer)and pyrotechnic grade (1.5 micrometer)Al in 500g batch level show that gellant(methyl cellulose) concentration could be reduced by 50 percent using pyrotechnic grade Al. The pseudoplastic-thixotropic behaviour, flow rate through die holes, burst pressure tests and bulk density are studied. UDMH -25 to 30 per cent Al gels with both grades of Al are found to be stable, pseudoplastic (shear thinning) and thixotropic (time-dependent shear thinning), but their flow pattern through die holes differ in nature.

Theoretical effects of aluminum gel propellant secondary atomization on rocket engine performance

A one-dimensional model of a gel-fueled rocket combustion chamber has been developed. This model includes the processes of liquid hydrocarbon burnout, secondary atomization, aluminum ignition, and aluminum combustion. Also included is a model of radiative heat transfer from the solid combustion products to the chamber walls. Calculations indicate that only modest secondary atomization is required to significantly reduce propellant burnout distances, aluminum oxide residual size, and radiation heat losses. Radiation losses equal to approximately 2-13% of the energy released during combustion were estimated. A two-dimensional, two-phase nozzle code was employed to estimate radiation and nozzle twophase flow effects on overall engine performance. Radiation losses yielded a 1% decrease hi engine 7>p. Results also indicate that secondary atomization may have less effect on two-phase losses than it does on propellant burnout distance, and no effect if oxide particle coagulation and shear-induced droplet breakup govern oxide particle size. Engine 7.p was found to decrease from 337.4 to 293.7 s as gel aluminum mass loading varied from 0-70 wt%. Engine 7,p efficiencies, accounting for radiation and two-phase flow effects, on the order of 0.946 were calculated for a 60 wt% gel, assuming a fragmentation ratio of 5.