Gel Propellants Research Articles

Rheological Characterization of Metalized and Non‐Metalized Ethanol Gel Propellants

AbstractThe gellation of metalized and non‐metalized ethanol with a methylcellulose gelling agent and its effect on the rheological properties (flow and dynamic study) of these gels is reported herein. The rheological study shows that increasing the shear rate reduces the apparent viscosity for a given yield stress (for a shear rate range of 1 to 12 s−1) for both shear rate ranges (1 to 12 and 1 to 1000 s−1) covered in present experiment. The gellant and metal particle concentrations significantly influence the gel apparent viscosity. Distinct changes in thixotropic behavior were observed, while decreasing the concentration of MC gellant and Al metal particles in the ethanol gels. The dynamic study showed that all of the linear viscoelastic regions (LVE) of the gel samples were independent of strain percentage (1 to 10). The G′ values depended on the frequency and exceeded the G′′ values, which indicated a gel‐like highly structured material. The tanδ values showed that all of the ethanol gels were elastic and weak physical gels with a high degree of cross‐linking.

Rheological Characterization of Hydrogen Peroxide Gel Propellant

An experimental investigation on the rheological behavior of gelled hydrogen peroxide at different ambient temperature (283.15, 293.15 and 303.15 K) was carried out in this study. The gel propellant was rheologically characterized using a rheometer, in the shear rate ranges of 1 to <TEX>$20s^{-1}$</TEX>, and 1 to <TEX>$1000s^{-1}$</TEX>. Hydrogen peroxide gel was found to be thixotropic in nature. The apparent viscosity value with some yield stress (in-case of shear rate 1 to <TEX>$20s^{-1}$</TEX>) drastically fell with the shear rate. In the case of the shear rate range of 1 to <TEX>$20s^{-1}$</TEX>, the apparent viscosity and yield stress of gel were significantly reduced at higher ambient temperatures. In the case of the shear rate range of 1 to <TEX>$1000s^{-1}$</TEX>, no significant effect of varying the ambient temperature on the gel apparent viscosity was observed. The up and down shear rate curves for hydrogen peroxide gel formed a hysteresis loop that showed no significant change with variation in temperature for both the 1 to <TEX>$20s^{-1}$</TEX> and the 1 to <TEX>$1000s^{-1}$</TEX> shear rate ranges. No significant change in the thixotropic index of gel was observed for different ambient temperatures, for both low and high shear rates. The gel in the 1 to <TEX>$20s^{-1}$</TEX> shear rate range did not lead to a complete breakdown of gel structure, in comparison to that in the 1 to <TEX>$1000s^{-1}$</TEX> shear rate range.

Thermal Diffusion Controlled Ignition of Hypergolic Gel Propellants

The present study proposes a simple theoretical model for the hypergolic ignition of gel propellants. The model assumes that the ignition process is governed by the time it takes for the heat wave produced by the reaction to diffuse through the whole droplet, which is described by the classical conduction time constant. The proposed model is based on a prior work by F. A. Williams and is further developed. The results obtained exhibit realistic ignition times in the range of tenths of milliseconds that are in close agreement to those obtained in various experimental works.

Al2O3나노입자가 젤(Gel) 추진제의 곡관 유동특성에 미치는 연구

Curved duct channel flow characteristics for non-Newtonian gel fluid is investigated. A simulant gel propellant mixed by Water, Carbopol 941 and NaOH solution has been chosen to analyze the gel propellant flow behavior. Rheological data have been measured prior to the flow analysis where water-gel propellant and water-gel propellant with nano particles are both used. The critical Dean number examined by the numerical simulation in the U-shape duct flow reveals that although water-gel-nano propellants have higher apparent viscosity, the critical Dean number do show no notable difference for both the two gel propellant. It is found that the power-law index may be a dominant parameter in determining the critical Dean number and that the gel with particles addition may be more vulnerable to Dean instability.

Numerical Simulation on Head-On Binary Collision of Gel Propellant Droplets

Binary collision of droplets is a fundamental form of droplet interaction in the spraying flow field. In order to reveal the central collision mechanism of two gel droplets with equal diameters, an axi-symmetric form of the Navier-Stokes equations are firstly solved and the method of VOF (volume of fluid) is utilized to track the evolution of the gas-liquid free interface. Then, the numerical computation model is validated with Qian’s experimental results on Newtonian liquids. Phenomena of rebound, coalescence and reflexive separation of droplets after collision are investigated, and structures of the complicated flow fields during the collision process are also analyzed in detail. Results show that the maximum shear rate will appear at the point where the flow is redirected and accelerated. Rebound of droplets is determined by the Weber number and viscosity of the fluid together. It can be concluded that the gel droplets are easier to rebound in comparison with the base fluid droplets. The results also show that the alternant appearance along with the deformation of droplets in the radial and axial direction is the main characteristic of the droplet coalescence process, and the deformation amplitude attenuates gradually. Moreover, the reflexive separation process of droplets can be divided into three distinctive stages including the radial expansion, the recovery of the spherical shape, and the axial extension and reflexive separation. The variation trend of the kinetic energy is opposite to that of the surface energy. The maximum deformation of droplets appears in the radial expansion stage; in the case of a low Weber number, the minimum central thickness of a droplet appears later than its maximum deformation, however, this result is on the contrary in the case of a high Weber number.

Prospects of UDMH Organic Gel Droplet Evaporation and Combustion Model

The organic gel propellants have the advantages of both solid and liquid propellant, but the catching fire and burning process of organic gel droplet appears obvious vapor ejection and flame disturbance. This unsteady combustion phe- nomenon of organic gel droplet is different from the quasi-steady combustion of conventional liquid droplet and produces great influence on the spray combustion flow field of gel liquid rocket engine and its optimization design. Based on the un- steady evaporation characteristics inside UDMH (unsymmetrical dimethylhydrazine) organic gel droplet and the micro molecule structure of organic gel propellant, three kinds of liquid phase evaporation model, the unsteady multicomponent evaporation model, the interface tracking multiphase flow evaporation model and the discrete element multiphase flow evaporation model, are proposed. The unsteady multicomponent evaporation model regards the gel droplet as a binary multi- conponent droplet, in which the gellant inside the gel droplet is been equivalent to a species with low volatility and high boiling temperature. In order to depict the shape and location variety of vapor bubble inside the gel droplet more precisely, the interface tracking multiphase flow evaporation model regards the gel droplet and the vapor bubble inside it as a single fluid, and uses the Level Set function tracking the interface location of vapor bubble. Based on the micro molecule structure of organic gel propellant, the discrete element multiphase flow evaporation model for the gel droplet regards the gellant and vapor bubble as the aggregation of micro-particles with different properties respectively, and uses the Lagrange method tracking the movement and aggregation of each kind micro-particle. The multicomponent evaporation model has been im- plemented in our foregoing work, so a summary of the model is presented. And next the main difficulties and the technique preparation about the last two multiphase flow models have been presented and analyzed in detail. In view of mixing and combustion process of gas phase surrounding the UDMH organic gel droplet, we propose that the finite rate chemical reac- tion model and the flamelet model should be used for the stagnation ambient and convective ambient respectively. The diffi- culties and numerical methods for handling the stiff chemical source term in finite rate chemical reaction model have been presented. And next the consistent flamelet model which takes consideration of species diffusion coefficient and non-unity Lewis number is presented. It would be more precise for the temperature and species interpolation of the gel droplet laminar non-premixed combustion flow field. Keywords organic gel; droplet; evaporation model; finite rate chemistry model; flamelet model; stiff source term

Numerical Simulation of Combustion for Freely Falling Gelled Fuel Droplets under Normal Gravity Conditions

Understanding the evaporation and combustion mechanisms of single droplets of gel propellant is the first stage to predict the burning characteristics in the combustion chamber. This paper, taking into account convection heat for freely falling gelled fuel droplets under normal gravity conditions, as well unsteady mass diffusion and thermal diffusion inside droplet, a theoretical model was developed to understand mass and heat transport mechanisms, and bubble growth within the gel droplet during processes of droplet combustion. The results show that at the first stage, shrinkage of the radius obeys the d2-law; steep temperature gradient and fuel mass concentration gradient appear within droplet, especially region near droplet surface. At the second stage, liquid fuel near the gellant layer within droplet starts to boiling, gellant layer formation resist the vaporizing fuel gas flow to extent; the vapor region appears between gellant layer and vaporizing surface within the droplet, and the droplet expands, swells, the layer thickness decreases until it ruptures.

Linear stability analysis of a slightly viscoelastic liquid jet

Gel propellant is one of the non-Newtonian viscoelastic liquids. The study of breakup of a gel propellant jet emanating from an injector has always been a fundamental task for a propulsion system with gel propellants. In this paper, a linear instability analysis method has been used to investigate the breakup of a slightly viscoelastic liquid jet. The disturbance wave growth rate of the axisymmetric mode has been determined by solving the dispersion equation of a slightly viscoelastic liquid jet, which was obtained by combining the linear instability theory and the linear viscoelastic model. Moreover, the maximum growth rate and corresponding dominant wave number have been observed, according to which maximum growth rate curves and dominant wave number curves have been plotted; this theory has been verified in the experiments of other authors. The time constant ratio, zero shear viscosity, liquid jet velocity, surface tension, liquid jet radius, liquid density and the gas density have been tested for their influence on the instability of the viscoelastic liquid jet. The results show that the effect of gas density is the greatest on the breakup of the viscoelastic jet, the jet velocity is affected second and the zero shear viscosity is least affected. However, the effects of other parameters are limited.

Numerical Simulation of Gel Propellant Droplet Breakup in Airflow

The droplet breakup is of importance in variety of spay combustion field. In order to investigate gel propellant droplet breakup in a gaseous phase, the Navier-Stokes equations in their axi-symmetric form was solved using the finite volume technique, and the Volume of Fluid method (VOF) was employed for tracking the free liquid-gas interface during process of droplet breakup. The results were compared with available published experimental data for Newtonian droplet, and validated the numerical model. The results show that increasing Weber number (We) can accelerate drop breakup; under the low shear rate, increasing K leads to droplet more resistant to breakup, drop breakup is dominated by K value; under the high shear rate, reducing n value results in stronger the shear-thinning behavior, and a decrease in resistant to breakup, n value governs the droplet breakup.

Numerical Argumentation of any Factors for Effecting Flow Property of a Power-Law Gel Propellant in a Converging Injector

To evaluate the effect of a converging injector geometry, volumetric flow rate and gallant content on the pressure drop, the velocity and viscosity fields, the governing equations of the steady, incompressible, isothermal, laminar flow of a Power-Law, shear-thinning gel propellant in a converging injector were formulated, discretized and solved. A SIMPLEC numerical algorithm was applied for the solution of the flow field. The results indicate that the mean apparent viscosity decreases with increasing the volumetric flow rate and increasing the gallant content results in an increase in the viscosity. The results indicate also that the convergence angle 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, and its value is limited by the Newtonian viscosity η∞. The effect of the convergence angle on the mean apparent viscosity is more significant than the effect of the volumetric flow rate and the gallant content on the mean apparent viscosity. Additional decreasing the viscosity results in increasing the pressure drop with increasing convergence angle. It is important to injector design that the viscosity decreasing and the pressure drop increasing are took into account together.

Numerical Solution on Flow Property of a Shear-Thinning Gel Propellant in a 90&lt;sup&gt;0&lt;/sup&gt; Pipe Bend

To evaluate an influence of the various bend diameter ratio Rc/R and velocity on the flow property of gel propellant in a 900pipe bend, the 3D governing equations of the steady, incompressible, isothermal, laminar flow of a power-law, shear-thinning gel propellant in pipe bend were formulated, discretized and solved, a SIMPLEC numerical algorithm was applied for the solution of the flow field, which on a series of sharp 900curved pipelines with nine kinds of bend diameter ratio and the inner diameters of 8mm were used on condition of seven kinds of Reynolds numbers. The pressure and velocity distributions were obtained, the empirical equation of local resistance coefficient from numerical experiments was conducted, providing the interrelations between the best bend diameter ratio and flow velocity in engineering design. The results indicate that the pressure and velocity distributions were non-linear, and which become tremendous with increasing Reynolds numbers. The results suggest that the dot of maximum velocity occurs the wall outside of a pipe bend, and which is more near to the wall outside of a pipe bend along the flowing direction and increasing the velocity. The phenomena of particle sedimentation should be took into account to investigate the flowing behavior of gel propellant in curved pipes on condition of lower Reynolds numbers.

Numerical Solution and Experimental Validation on the Flow Property of Gel Propellants in Round Pipes

To describe detailedly the flowing process and field of gel propellants in the round pipe, the experiments of the gel flowing in the tubes were conducted and the governing equations of the steady, incompressible, isothermal, laminar flow of a power-law, shear-thinning gel propellants in 3D pipe were formulated, discretized and solved. The results indicate that pressure drop per unit length increases with decreasing the tube diameter as well as increasing the mass flow rate, and the liquidity of gel is more difficultly than water, and the wall-slip effect of the tube flowing for gel propellants must be considered. The results indicate also that the velocity increases as the flow moves downstream as well as with decreasing the tube diameter, and the apparent viscosity decreases as the flow moves downstream, and the apparent viscosity on the radial center is the maximum. For the same tube geometry, the apparent viscosity at the exit plane decreases with increasing the mass flow rate. For the same tube length and the same mass flow rate, increasing the tube diameter results in an increase of the apparent viscosity at the exit plane.

Model and simulation of liquid rocket organic gel spray droplet evaporation

Gel propellant has the advantage of controllable flux as liquid propellant and long-term reservation as solid propellant, however, the evaporation and combustion problem of gel spray droplet bores with the gel propellant development and combustor design all the time, and hampers gel propellant practical engineering applications. In this paper, the gel single droplet combustion experiment system is designed and constructed, and then the evaporation and combustion mechanism is explored deeply based on the experimental phenomena of organic gel unsymmetrical dimethylhydrazine (UDMH) single droplet burning in nitrogen tetroxide. The organic gel spray droplet multi-component evaporation model is developed for three different evaporation phases of the gel layer, i.e. the forming, expanding and bursting of the gel layer based on the single droplet evaporation characteristics in experiment, and then the gel single droplet vaporization in high temperature gas phase is numerically simulated and compared with the result of conventional liquid droplet using the elementary model parameters and physic property parameters. The result shows that the gel content on the droplet surface increases slowly at the beginning of evaporation, however it would increase rapidly to a mass fraction of 95% and form the gel layer in a limited time after exceeding a critical evaporation time, which results in surface mass flux dropping to 0 and surface temperature reaching the UDMH boil point rapidly. After the gel layer forming, the droplet radius and surface UDMH vapor mass fraction exhibit oscillation as the swelling-bursting phenomena in experiment. The gel droplet surface temperature holds above the boil point and the mass flux of gel droplet inner boiling evaporation is stronger than the conventional liquid droplet surface steady evaporation which makes the life time of gel droplet much shorter.

Numerical Solution and Experimental Validation on the Flow Property of Gel Propellants in Round Pipes

To describe detailedly the flowing process and field of gel propellants in the round pipe, the experiments of the gel flowing in the tubes were conducted and the governing equations of the steady, incompressible, isothermal, laminar flow of a power-law, shear-thinning gel propellants in 3D pipe were formulated, discretized and solved. The results indicate that pressure drop per unit length increases with decreasing the tube diameter as well as increasing the mass flow rate, and the liquidity of gel is more difficultly than water, and the wall-slip effect of the tube flowing for gel propellants must be considered. The results indicate also that the velocity increases as the flow moves downstream as well as with decreasing the tube diameter, and the apparent viscosity decreases as the flow moves downstream, and the apparent viscosity on the radial center is the maximum. For the same tube geometry, the apparent viscosity at the exit plane decreases with increasing the mass flow rate. For the same tube length and the same mass flow rate, increasing the tube diameter results in an increase of the apparent viscosity at the exit plane.

Breakup of a power-law liquid sheet formed by an impinging jet injector

Gel propellant is promising for future aerospace application, but because it behaves as the non-Newtonian power-law liquid, it is difficult to atomize. Impinging jet injectors are often used for atomization of gelled propellant. To understand the atomization mechanism of gelled propellant, a linear instability analysis method was used to investigate the instability and breakup characteristics of the sheet formed by a gelled propellant impinging-jet injector. The maximum disturbance wave growth rate and dominant wave number were determined by solving the dispersion equation of a power-law liquid sheet. It was found that the maximum disturbances growth rate and the dominant wave number both increase as Weber number of the liquid sheet increases. Consistency coefficient and flow index were tested for their influence on the stability of the power-law liquid sheet. A modified model, to predict the breakup length and critical wavelength of the power-law liquid sheet, was adopted. To validate the power-law liquid sheet breakup model, experiments were performed with injectors of different configurations and a high speed camera was used to show detailed information of the liquid sheet breakup process. The rheology of the power-law fluid used in the present study was also investigated. Comparison between the theoretical and experimental results shows that the linear instability analysis method can be applied to predict breakup length and wavelength of the power-law liquid sheet.

Effects of initial droplet diameter and pressure on burning of ATF gel propellant droplets

The increasing demand for higher energy density fuels and the ever-increasing concern for their safety have propelled research in the field of gel propellants. For studying the fundamental parameters without the interference of neighbouring droplets, isolated droplet burning of organic ATF (aviation turbine fuel) gel propellants was chosen to investigate experimentally the effects of initial droplet diameter and chamber pressure on the burning under normal gravity conditions at room temperature. Under ambient pressure condition, an increase in the burning rate constant was observed with increase in initial droplet diameter. For a given range of diameters, the burning rate constant also continued to increase with pressure. Experiments were also carried out to study how these variations were affected by initial droplet diameter and chamber pressure changes, respectively. The intensity of microexplosions was observed to decrease with increase in chamber pressure. A balance between the heat loss by the droplet to the surroundings and the heat gain by the droplet has been put forward to explain the variations of burning rate constant with varying pressures as well as varying initial droplet diameters.

Numerical Modeling of Injection of Shear-Thinning Gel Propellants Through Plain-Orifice Atomizer

A series of axisymmetric Navier–Stokes simulations were performed to study the mean and unsteady characteristics of gel propellant orifice flows at conditions representative of rocket injectors. The rheology of the gel was simulated assuming a shear-thinning fluid behaving in accordance with the Carreau–Yasuda model. The effects of Reynolds number (flow velocity), orifice L=D, fluid rheology, and orifice inlet chamfering were studied in a series of 200 independent simulations. Unsteady conditions were observed in most cases, due to the high injection velocities typical of rocket injection conditions. The magnitude and frequency of pulsations were characterized in these cases. In general, steady flows were obtained for lower Reynolds numbers and smoother inlet (i.e., greater chamfering) conditions. Mean discharge coefficients were computed for all cases to support design studies and engineering analyses.

Rheological properties of Carbopol containing nanoparticles

In this study we explored the rheological characteristics of Carbopol C934 gel (polyacrylic acid) containing SUS304 spherical nanoparticles of 100 nm as a simulant of gel propellants containing metal fuels. In comparison with the pure Carbopol gel, the SUS nanoparticle filled Carbopol gel exhibited stronger shear thinning and higher yield stress. As the concentration of nanoparticles increased yield stress increased, but viscosity and storage modulus increased first and then decreased abruptly beyond the critical limit. Also as the concentration of nanoparticles increased there was a transition in material characteristics from the ductile type to the brittle type, which means that highly filled Carbopol gels lost the structure almost instantaneously as the imposed stress was larger than the yield stress, while Carbopol gels of low particle loading sustained the structure even after the imposed stress was larger than the yield stress. The cryogenic scanning electron microscopy analysis revealed that the network structure changed abruptly when the rheological properties changed abruptly. The change in gel structure is attributed to the nanoparticles that compete with Carbopol chains in forming networks. The abrupt change in gel structure with the addition of particles beyond the critical limit should be an exclusive phenomenon of nanoparticles.

Effects of gellant concentration on the burning and flame structure of organic gel propellant droplets

The increasing demand for higher energy density fuels and the ever-increasing concern for their safety have propelled research in the field of gel propellants. For studying the fundamental parameters without the interference of neighbouring droplets, an isolated droplet was chosen to investigate experimentally the combustion process of gel propellants under normal gravity conditions. Phase separation of the gel propellant components leading to bubble nucleation, vapor jetting and microexplosions were found to be the main phenomenon involved during the combustion period. Experiments were comprehensively carried out to study the effect of gellant concentration on the burning rate constant as well as flame structure. The burning rate constant was found to decrease with increase in the gellant concentration. Decrease in the calorific value of the increasing gellant concentrations was proposed as one of the reasons for this variation. The flame exhibited a triple flame structure for all the cases for both C 2 ∗ radicals as well luminous flames. The horizontal and vertical flame standoff distances were observed to decrease with gellant content. Decrease in the content of the base fuel was proposed as the reason for the same.

Linear Stability Analysis of a Non-Newtonian Liquid Sheet

Gel propellant is a non-Newtonian visco-elastic liquid. The study on breakup of a gel-propellant sheet emanated from an injector has always been a fundamental task for a propulsion system with gelled propellants. A linear instability analysis method has been used to investigate the breakup of a visco-elastic liquid sheet under the combined influence of sinuous and varicose modes of disturbances at the liquid-gas interfaces. The disturbance wave growth rate of two disturbance modes has been worked out by solving the dispersion equation of a visco-elastic liquid sheet, which is obtained by combining the linear instability theory and the linear visco-elastic model. The linear visco-elastic model can describe the thixotropic behavior of the visco-elastic fluid. Moreover, the maximum growth rate and corresponding dominant wave numbers have been observed. The surface deformation growth rate curves, maximum growth rate curves, and dominant wave numbers curves have been plotted. Also, the breakup time of the liquid sheet has been computed by plotting the surface deformation curves based on the surface deformation equation on the liquid-gas interfaces. The time constant ratio, zero shear viscosity, surface tension, liquid sheet velocity, liquid sheet thickness, gas-to-liquid density ratio, and liquid density have been tested for their influence on the instability of the visco-elastic liquid sheet. Furthermore, the instabilities of Newtonian fluid and visco-elastic fluid are compared with each other. The results show that the visco-elastic liquid sheet is more unstable than the Newtonian liquid sheet. Similarly, the sinuous mode disturbance is more unstable than the varicose mode.