Gel Propellants Research Articles

Hypergolic fuel impacting a gelled oxidizer wall: Droplet dynamics, heat release, ignition, and flame analysis

The combination of high-concentration solutions of hydrogen peroxide, known as High Test Peroxide (HTP), with the green fuel composed of tetramethylethylenediamine, dimethylaminoethanol, and methanol (TMEDA/DMEA/MeOH, 1:1:1 vol%), catalyzed by 1 wt% of copper chloride dihydrate, has significant potential for space hypergolic propulsion applications in terms of performance and safe operation. Besides the well-known and mature propulsion systems adopting liquid and solid propellants, gel propellants also present interesting characteristics that can be better explored to enable the creation of alternative propulsive systems. The present work employed HTP at 98 wt% with 6 wt% of fumed silica as a gelling agent to create gelled HTP (GHTP). Droplets of the green fuel impinged on a layer of GHTP placed over a glass slide, acting as a solid oxidizer, mimicked an element of a new hybrid gel/liquid hypergolic propulsion system. Data from infrared and visible light cameras enabled a detailed analysis of the dynamics of a reactive droplet impinging on a gelled oxidizer wall, as well as the heat release, ignition, and flame spread of the hypergolic GHTP/green fuel pair under open-air conditions. The heat release was observed to be distributed across concentric annular areas for both the fuel surrogate and the fuel with catalyst, before and after ignition, demonstrating a strong correlation with non-reactive droplet fluid dynamics, which was corroborated by spread diameter analysis processed from visible image data. Vapor and Ignition Delay Times (VDT and IDT) were found to be as low as 4 and 17 ms, respectively, for the highest droplet impact velocity and catalyst concentration. The reaction rate in the gas phase, considering the flame spread area, showed a dependency of both impact velocity and catalyst concentration, with the latter exhibiting a more pronounced and clear effect. The surface temperature ranges where first vaporization and ignition occurred were 60 °C ∼ 70 °C and 120 ∼ 170 °C, respectively, which is close to the boiling point of methanol and the auto-ignition temperature of TMEDA. This finding, along with the chemical mechanisms in the gas phase related to the presence of a catalyst in the fuel, may be important for hypergolic ignition. The relatively low ignition temperatures and the short ignition times represent additional advantages of the present hypergolic combination for propulsion applications.

A high-precision and near-zero residue laser-controlled gel propellant

Micro-nano satellites, with their advantages of small size, low power consumption, short development cycles, and capability for formation flying and networking, play a significant role in scientific research, national defense, and commercial applications. However, currently developed micro-nano satellites globally possess very limited maneuverability and lack effective attitude and orbit propulsion control. Solid propellants can offer some advantages such as high energy density and low cost, and they are extensively used in micro-and nano satellite propulsion systems. However, most of these propellants exhibit poor controllability and produce massive amounts of residual by-products. Laser-controlled gel propellants have been confirmed to be the most effective solution to address these issues in this study. The present study exhibits the preparation of ADN-based laser-controlled gel propellants using the freeze–thaw method and their controllable combustion characteristics under laser irradiation. The results show that the ignition delay time of gel propellants decreases from 93.0 ms to 34.6 ms, the extinction delay time is stable at around 2 ms, and the average burning rate increases from 0.6 mm/s to 1.5 mm/s under continuous laser irradiation with the power density ranging from 0.6 to 1.3 W/mm2. The average utilization rate of gel propellant is as high as 99.2 %. Upon testing, the residual solid matter was identified as unreacted propellant that had not been irradiated by the laser. Laser-controlled gel propellants represent a new type of energetic material that can be modulated to control its ignition, extinction, and burning rate. These capabilities overcome the challenges associated with the inability of traditional chemical propellants to extinguish and restart, thereby fulfilling the propulsion system requirements of micro-nano satellites.

Coaxial air blast atomization of a particulate gel suspension jet

The objective of this study is to experimentally and theoretically explore the impact of particle volume fractions and nozzle thread structure on the coaxial air blast atomization of particulate gel suspension jets through the high-speed flow visualization technology. The four breakup modes of particulate gel suspension jets are confirmed, namely oscillation mode, membrane-type breakup, fiber-type breakup, and superpulsating submode. However, the fiber-type breakup of jets disappears as the particle concentration reaches 40%, primarily attributed to the numerous particles sharply promoting the instability on the free jet surface. The experimental statistical results demonstrate that an increase in particle concentration and the adoption of the threaded nozzle both result in an expansion of the jet spray angle. The breakup length exhibits a reduction trend with the increase of particle concentration. However, it remains nearly the same value as that in the case of volume fraction 20% when the particle concentration continues to increase, contributed by the surge of viscous dissipation counteracting the enhanced instability on the jet interface caused by a large number of particles. The breakup length of a pure gel jet is predicted through the linear instability analysis, which is further modified by the viscosity model of particle suspensions to derive the breakup lengths of particulate gel suspension jets with different concentrations. The theoretical predictions align well with the statistical results in the experiment. The research is poised to have potential implications for numerous industrial processes and engineering applications including gel propellants.

Numerical simulations of secondary atomization characteristics of shear-thinning kerosene gels

Owing to their unique non-Newtonian characteristics, gel propellants have become a new type of fuel with promising applications in the aerospace industry. However, the complex rheological properties and high viscosity of gels pose significant challenges for flow control in gel engines, particularly in terms of fuel atomization and combustion efficiency. Focusing primarily on kerosene gel, this study investigates the secondary atomization characteristics of shear-thinning droplets, where the volume of fluid method is employed to capture interface dynamics on adaptive grids, and the viscosity of the gel is described by a power-law model. The accuracy of the numerical model is validated by comparing it with the experimental observations of the breakup process of 1% silica kerosene gel droplets. Numerical simulations are conducted to analyze the droplet breakup processes and formation mechanisms under bag, multimode, and shear breakup. Compared to kerosene, the apparent viscosity distribution inside the kerosene gel droplets is non-uniform, and the inhibition of viscous force delays the breakup time and reduces the droplet deformation, thereby affecting the transition of the breakup mode. By varying the Weber number, We, the rheological parameters, and the density ratio, the quantitative effects of these factors on droplet deformation, centroid velocity, average viscosity, the breakup time, etc. are revealed. The results indicate that the breakup becomes more severe with increasing We number, resulting in larger deformation and a greater number of smaller droplets; the rheological parameters significantly affect the droplet breakup by altering the fluid viscosity; moreover, under the same flow condition, the higher the density ratio, the more difficult it is for the droplet to break up; however, on dimensionless scales, the effect of density ratio appears weak. Additionally, a breakup regime diagram is constructed in the We–Oh (Ohnesorge) number space for low Weber numbers. This study validates the numerical method for simulating the secondary atomization of kerosene gel propellants, providing a thorough analysis of three breakup mechanisms and the impact of key parameters on kerosene gel droplets. These insights offer a theoretical foundation for the optimization of gel propellants, aiding in the development of efficient and stable propulsion systems.

Enhancement effect of Mo electrodes on the ignition characteristics of HAN-based gel propellant fuel and its combustion mechanisms

Hydroxylammonium nitrate (HAN) serves as a monopropellant fuel, renowned for its low toxicity and high-performance in space propulsion systems. HAN-based gel propellant may offer additional advantages, including long-term stability and excellent throttling capabilities. To better understand the ignition performance and combustion characteristics of this gel propellant, we prepared HAN-based gel propellant, and monitored its decomposition products using thermogravimetry (TG), Fourier Transform Infrared Spectroscopy (FTIR) and mass spectrometry (MS). Electrode materials of Mo, Cu, Ti and SUS 304 were fabricated to investigated their impact on ignition performance. It was found that the Mo electrode exhibited the shortest ignition delay time (ti) compared to Cu, Ti, and SUS 304. Meanwhile, the role of surface roughness of Mo electrode on the ignition delaytime (ti) and the ignition energy (Qi) were also studied. Our results revealed that, as the voltage increased from 100 V to 200 V, the fine-surfaced Mo electrode demonstrated a lower sensitivity to changes in ti compared to the rough-surfaced electrode. Additionally, the Qi of the rough-surfaced electrode decreased from 43.5 J to 16.7 J, while the Qi of the fine-surfaced electrode decreased from 52.5 J to 17.6 J. Combining a thermocouple, the combustion wave structure of the gel propellant was divided into pre-heating zone, condensed phase, electro chemical reaction zone, microexplosion-gases mixing zone and flame zone. Finally, the combustion model of HAN-based gel propellant was inferred to explain its combustion mechanism.

Aerodynamic breakup of gel suspension droplets loaded with aluminum particles

The persistent and frequent space explorations are affected by the performance of propellants, among which the closely related gel propellants have been widely used due to their unique characteristics. Currently, research efforts primarily concentrate on the metallization and atomized combustion of gel propellants, emphasizing the combustion performance of metallized gel propellants. However, little attention has been given to the secondary breakup process that occurs just before combustion. By utilizing high-speed flow visualization technology, this study examines the key aerodynamic breakup process of gel suspension droplets loaded with micro aluminum particles, experimentally and theoretically investigating the droplet dynamics induced by particle volume fraction. The experimental confirmation of the three breakup behaviors exhibited by gel suspension droplets, namely oscillation mode, bag-stamen breakup, and hemline breakup, is presented along with the construction of a phase diagram. The impact of heterogeneous effects resulting from an increase in particle volume fraction on the critical breakup Weber number is revealed, with a concentration of 9 % as the turning point. Within the range of moderate Weber numbers, the tail flick deformation of suspension droplets in airflow is observed and reported for the first time. At different particle concentrations, there keeps a linear correlation between the dimensionless tail length and Weber number. Spherical suspension droplets will deform into thin disks in the airflow, with a nearly constant deformation rate regardless of changes in particle concentration. Additionally, the relationships between characteristic times and droplet parameters are discussed through statistical analysis of the results. As Ohnesorge number and particle volume fraction increase, the characteristic times both show a monotonically increasing trend. Moreover, a model of unstable growth rate associated with the particle volume fraction is established through linear instability analysis, accurately predicting the variation in critical Weber number for gel suspension droplet breakup. The congruence between our theoretical framework and experimental findings not only enhances comprehension of the impact of micro aluminum particles on the secondary breakup of gel suspension droplets but also provides valuable guidance for the aerospace applications of metallized gel propellants.

Simulation study on the breakup process of kerosene Gel Jet in transverse airflow

The gel propellant not only inherits the characteristics of liquid propellants, such as high specific impulse, multiple ignition capabilities, and adjustable thrust, but also offers an advantage type of solid propellants, including reduced risk of leakage, long-term storage feasibility, and ease of maintenance. In this study, numerical simulations were conducted to investigate the atomization and mixing process of kerosene gel jets in high-speed crossflows. The study reveals that the jet breakup process primarily involves two forms of breakup: columnar breakup dominated by Rayleigh Taylor’s unstable wave and surface breakup dominated by Kelvin Helmholtz’s unstable wave. These two forms coexist, complement each other, and interact with each other. The numerical simulation of the kerosene gel breakup process provides favorable support for studying its combustion characteristics in the rotating detonation engine.

Combustion Enhancement of Gel Propellant Containing High Concentration Aluminum Particles Based on Carbon Synergistic Effect.

The addition of aluminum particles to gel propellants can improve combustion performance. However, the agglomeration of aluminum during the combustion process can result in a series of negative effects. In this paper, the aluminum agglomeration inhibition method of gel propellant based on carbon synergistic effect is proposed. Carbon particles exhibit excellent combustion properties, and the gaseous product CO2 generated during combustion can mitigate the agglomeration of aluminum. The research demonstrates that incorporating carbon particles into aluminum-containing gel effectively reduces the incomplete combustion of aluminum particles and increases the volumetric calorific value of the gel. When the mass fraction of carbon is 5 wt%, the volume calorific value of the gel reaches the highest. Meanwhile, the rheological experiments show that the addition of carbon particles can improve the shear-thinning properties of the gel, which is beneficial to the atomization and combustion processes of the gel.

SHORT OVERVIEW AND HIGHLIGHTS OF THE HISTORY OF THE GERMAN GEL PROPULSION RESEARCH AND TECHNOLOGY DEVELOPMENT ACTIVITIES

Gel mono- and bipropellants (i.e., gelled fuels and/or gelled oxidizers) are interesting candidates for advanced low-hazard rocket or ramjet propulsion systems and gas dynamic systems. Gel propellants combine major advantages of solid propulsion systems and liquid propulsion systems, such as easy and simple storage and handling characteristics (solid systems) and excellent throttleability (liquid systems). This article gives a short overview of the themes and the obtained results about the work conducted in Germany within the last two and a half decades, whereas a focus is set on the progress achieved in the last years.

Preparation and performance characterization of temperature‐sensitive JP‐3 kerosene gel propellant

AbstractTwo types of JP‐3 kerosene gels were prepared using AC−C10 (N‐acetyl‐L‐isoleucyl decylamine, type A) and modified hydrogenated castor oil (type B) as gelling agents, and the gel formation mechanism between the two gelling agents and JP‐3 kerosene was investigated. The rheological properties of the gels, including stability, yield stress, shear thinning, and thixotropic behavior, were tested and discussed. The viscosity curves were fitted with the HBE constitutive equation and compared with the power‐law model. Non‐time‐dependent, time‐dependent, and temperature‐dependent constitutive equations of the kerosene gels were constructed. The effects of time and temperature on viscosity were analyzed. The results showed that the type B JP‐3 kerosene gel was more physically stable and less influenced by centrifugal force. Both types of gels exhibited a decrease in viscosity with increasing shear rate, indicating shear thinning behavior. Additionally, both gels demonstrated some degree of thixotropy, although the recoagulation was found to be weak. With an increase in temperature, the viscosity of the type A gel initially decreased, then increased, and finally decreased again until it approached the viscosity of kerosene at 75 °C. On the other hand, the viscosity of the type B kerosene gel gradually decreased and approached the viscosity of kerosene at 75 °C. Both types of gels exhibited high temperature sensitivity.

Preparation and properties of high-energy-density aluminum/boron-containing gelled fuels

Energetic nanofluid fuel has caught the attention of the field of aerospace liquid propellant for its high energy density (HED), but it suffers from the inevitable solid–liquid phase separation problem. To resolve this problem, herein we synthesized the high-Al-/B-containing (up to 30% (mass)) HED gelled fuels, with low-molecular-mass organic gellant Z, which show high net heat of combustion (NHOC), density, storage stability, and thixotropic properties. The characterizations indicate that the application of energetic particles to the gelled fuels obviously destroys their fibrous network structures but can provide the new particle–gellant gelation microstructures, resulting in the comparable stability between 1.0% (mass) Z/JP-10 + 30% (mass) Al or B and pure JP-10 gelled fuel. Moreover, the gelled fuels with high-content Al or B exhibit high shear-thinning property, recovery capability, and mechanical strength, which are favorable for their storage and utilization. Importantly, the prepared 1.0% (mass) Z/JP-10 + 30% (mass) B (or 1.0% (mass) Z/JP-10 + 30% (mass) Al) shows the density and NHOC 1.27 times (1.30) and 1.43 times (1.21) higher than pure JP-10, respectively. This work provides a facile and valid approach to the manufacturing of HED gelled fuels with high content of energetic particles for gel propellants.

High-energy-density metallized gel propellant by hydrogen-bonded polymer-small molecules for enhanced stability and shear-thinning performance

Metallized gel propellants with high metal content hold significant potential for enhancing the energy properties of hydrocarbon fuels. However, achieving an optimal balance between storage stability and atomization characteristics poses a significant challenge. In this study, we successfully synthesized metallized gel propellants that exhibit exceptional stability and remarkable shear-thinning properties by utilizing cooperative hydrogen bonding between polymer octanoyl cellulose and the small Thixatrol ST molecule. Through meticulous mixture design and formulation optimization, the metallized gel propellants demonstrated noteworthy characteristics, including a high calorific value of 48.48 MJ/kg, an impressive dynamic stability index of 89.12%, a surface tension of 23.21 mN/m, and sustained stability for at least six months under resting conditions. The metallized gel propellant, containing 50 wt% boron particles, exhibited a density 1.54 times higher than pure kerosene, meeting the requirements for enhancing specific impulse in rocket and ramjet engines. Moreover, optimized samples with varying boron concentrations were evaluated for their rheological properties and atomization behavior. The results revealed that the inclusion of boron particles increased the viscosity of the metallized gels. Consequently, the sample with 50 wt% boron exhibited modified viscosity-flow behavior, resulting in the formation of larger gel droplets during atomization. Finally, the metallized gel propellant demonstrated successful ignition in ramjet engine experiments. We anticipate that the cooperative hydrogen bonding between the polymer and small molecule will pave the way for the development of high-content metallized gel propellants with exceptional stability and superior shear thinning performance.

On the flow of a bubble pair rising in temperature-sensitive gel propellants

The gel propellant is a novel type of propellant created by introducing gelling agents to the liquid propellant. Similar to other types of propellants, the occurrence of air bubbles in the gels is common and unavoidable during the manufacturing and casting process. The presence of bubbles can adversely impact the stable combustion of the engine and may even pose safety hazards when they enter the combustion chamber. In order to gain a better understanding of the bubble interaction within the gel propellant, we have conducted a study on the motion of a pair of bubbles rising in a quiescent water-gel. In this study, the volume of fluid (VOF) method is adopted for modeling the multiphase feature of the flow. The shear and temperature dependent viscosity of the gel has been modeled using the temperature-coupled Carreau–Yasuda model. The effects of various factors, including the initial distance between the bubble pair, bubble diameters, and the temperature, on the motion characteristics of the bubble pair are studied. It is found that an increase in the initial distance leads to a reduction in the bubble interaction; for side-by-side bubble pairs, the bubbles exhibit slower ascent rates and reduced lateral displacement; for inline bubble pairs, the coalescence of the bubble pair becomes slower. Increasing the diameter of the dominant bubble accelerates the coalescence of the bubble pair and the ascent. Elevating the temperature results in a decrease in viscosity, thereby accelerating the coalescence and ascent of inline bubble pairs and producing viscosity reduction around the bubble pair.

Preparation, characterization, and thermal decomposition kinetics of high test peroxide gel

High test peroxide gel was prepared by low temperature freezing under the action of calcium ion compound with sodium alginate as a gelling agent. The High test peroxide gel was characterized by ultra depth of field microscope, Fourier infrared spectrometer, rheometer, synchronous thermal analyzer, and electron microscope scanner. The thermal decomposition behavior and reaction kinetics of the gel were investigated. The effective content of H2O2 in the gel before and after aging was also analyzed by KMnO4 titration. The results show that the content of sodium alginate and the concentration of calcium ion compound directly affect the gelling and rheological properties of high test peroxide gel, and the gel with low sodium alginate content had a more uniform tissue structure. The Eα and lg(A/s−1) of the high test peroxide gel decomposition reaction are 69.10 kJ mol−1 and 9.13, respectively. Na2O, CaO, CaCl2 and CaSO4 are the main components of the high test peroxide gel thermal decomposition condensate products. After ignition, a boron-based high test peroxide gel propellant can produce a steady combustion. After being stored at 263 K for 20 days, the effective content of H2O2 in the gel is 96.50%, and the decomposition rate is only 0.63%.

Self-similarity formation of a pendant drop of power-law fluids

To fully explore the breakup behavior of gel propellant in both the macroscopical and microscopic scales, the present study started from the self-similarity analysis for a macroscopical power-law liquid thread, and a transition from viscocapillary to viscocapillary-inertia breakup regime was identified by the experimental verification, using high-speed photography technology and image processing. Furthermore, the dissipative particle dynamics method was employed to predict the profile of microscopic power-law liquid thread when the diameter of thinning liquid filament approaches the micro-/nanoscale. Three different breakup regimes, capillary-, viscocapillary-inertia-, and thermal-fluctuation-dominated modes, were carefully reproduced and validated. Results showed that the self-similarity breakup process can be retarded by the inertial and viscous effects. In addition, there are some similarities existing between the macroscopical and micro-/nanoscale thinning processes.

甲基肼凝胶推进剂粘弹性能分析

甲基肼凝胶推进剂粘弹性能分析

Study of Thixotropic Characteristics of a Kerosene Gel Propellant by Bayesian Optimization

The rheological behavior of gel propellants is crucial for their practical applications, especially in the rocket engine and ramjet fields. The thixotropic characteristics of gel propellants are an important component of their rheological properties and have a notable impact on their flow and injection process. However, most gel propellants contain rich, dynamic cross-linked network structures, which impart complex non-Newtonian fluid properties, and it is difficult to establish a unified mathematical model. In view of this, this study addresses the thixotropy of a prepared RP-3 kerosene gel and determines the mathematical model and model parameters describing its thixotropy. Experiments show that the kerosene gel exhibits shear-thinning properties as well as thixotropy. To describe the microstructural changes in the gel, three thixotropic constitutive models are introduced to analyze the rheological data, and the constitutive equation parameters are optimized. The three models are all structural dynamic models, which can be used to describe microstructural changes within the material. In addition, the fitting of the constitutive equation is a multiparameter optimization problem, and an appropriate optimization method must be used for parameter fitting. Therefore, the Bayesian optimization method combined with Gaussian process regression and the upper confidence bound (UCB) acquisition function is used in the multiparameter fitting of the constitutive models. Both experiments and numerical results show that the thixotropic model, which introduces a pre-factor with shear strain and assumes that the breakdown of the gel structure is related to energy dissipation rather than the shear rate, has a better fitting effect and prediction ability with regard to the gel. Combined with transient experiments at different shear rates, the model parameters of the constitutive law can be determined quickly by applying the Bayesian optimization method.

Rheological and atomization behavior of glycyrrhizic acid based supramolecular gel propellant simulant

Gel propellant has attracted much more attention due to its advantages in reducing the leakage risk, adjustable thrust, being capable of multi-time ignition and easy handling. Conventional polymeric gellant or inorganic gellant based gel propellant suffered from several problems, such as high viscosity, inadequate atomization and insufficient combustion. As several pioneer reports, low-molecular weight gelators (LMWGs) based supramolecular gel fuels could exhibit good shear-thinning properties. The corresponding atomization property, however, is still unclear and needs further study. Herein, glycyrrhizic acid (GA) based hydrogel was employed as supramolecular gel propellant simulant, and its rheological and atomization behavior were well investigated. The 1.0%− 2.0% GA hydrogel exhibited a certain mechanical strength with storage modulus between 30 and 90 Pa. The shear viscosity of hydrogel decreased sharply under low shear rate (below 2 s−1), and then further decreased to 10−2-10−3 Pa.s under higher shear rate (2–1000 s−1). The resulting shear viscosity of 1.0% GA hydrogel was close to the magnitude of pure water, and an order of magnitude higher than pure water for 1.5% and 2.0% GA hydrogel systems. The atomization performance of GA hydrogel was investigated under both self-impinging doublet injector and centrifugal injector, it was found that the spray pattern, breakup length of liquid film and spray cone angle of 1.0% GA hydrogel were all similar as the pure water under two kinds of injectors, while the atomization performance degradation for 1.5% and 2.0% GA hydrogel. It was concluded that the shear viscosity plays an important role in determining the atomization properties for GA based hydrogel.

Spray characteristics of the nanoparticle-containing gel propellants by using an improved single-phase nozzle

Due to the high viscosity and complex rheological properties, the atomization problem continues to be one of the key problems of gel propulsion technology. By analyzing the rheological properties of the gel propellants containing nanoparticles, an improved single-phase nozzle was proposed in this manuscript, which added a cone-like structure inside the nozzle. The equivalent cross-sectional area of propellant passage decreases gradually from inlet to outlet, which ensures that the apparent viscosity of gel propellant decreases continuously until it flows out of the nozzle. A series of experiments were conducted to investigate the atomization characteristics of the improved nozzle. A traditional direct current (DC) nozzle was also used for comparison. The results showed that the improved nozzle proposed in this manuscript can effectively improve the atomization fineness and uniformity of the gel propellant containing nanoparticles by efficiently decreasing Sauter mean diameter (SMD) and also reducing the volume fraction of large droplets in the spray field. Meanwhile, the spray cone angle of the improved nozzle was obviously larger than that of the DC nozzle, which was favorable for mixing and evaporation in the thrust chamber.

Empirical Estimation of Ethanol-Methylcellulose based Green Gel Propellant Viscosity at Varying Temperature

The rheological behavior of the Ethanol based green gels for use in propulsion systems at temperature range of 100C – 500C is investigated by employing a novel empirical model developed. The study is conducted at various shear rates. Empirically predicted results were observed to be in good agreement with experimental data at higher shear rates for both pure and energized gel system (>100 s-1). Considering the results, the present empirical model is deemed suitable for those Non Newtonian shear thinning fluids which obey Power law.