Ningfei Wang

Theoretical Modeling and Numerical Study for Thrust- Oscillation Characteristics in Solid Rocket Motors

Todiscover thrust-oscillation characteristics in solid rocketmotors, analyticalmodeling andnumerical simulation are carried out by an experimentalmotor in the vonKarman Institute for FluidDynamics. The numericalmethod by means of amesh sensitivity analysis is proposed for validation. Velocity profiles, oscillation frequencies, and pressure amplitudes were obtained by numerical simulations and then compared with the experimental data. Various cases with different inlet temperatures are proposed to investigate the influences of parameters on the oscillation characteristics. The results indicate that it is not a necessary condition for vortex-shedding frequency to approach a certain acoustic frequency when periodic oscillations are generated. Oscillations are more severe if the vortexsheddingphenomenon coupleswithhigh-order acousticmodes.Velocitymagnitude in the combustion chamber is the main factor that influences the vortex-shedding frequency; meanwhile, the pressure amplitude is mostly determined by themeanMach number. Theoretical modeling in conjunction with numerical calculations proves that the ratio of dimensionless thrust amplitude to pressure amplitude is predominantly determined by the throat-to-port-area ratio J, and it varies inversely as J. An integrated formula is presented to describe the relationship between thrust amplitude and pressure amplitude.

Diffusion Combustion of Liquid Heptane in a Small Tube with and without Heat Recirculating

In order to understand diffusion flame characteristics in a small tube, combustion of liquid n-heptane and air was experimentally and numerically studied. A tube of ID 4 mm and OD 6 mm made of quartz was used as the burner. Liquid n-heptane was delivered into a capillary from a syringe pump. Stable flames were established inside the burner with and without heat recirculating. Additionally, numerical simulations were conducted, and effects of equivalence ratio and external heat loss coefficient on diffusion flame were studied. Results show that, for a diffusion flame of liquid n-heptane in a small tube, as fuel flow rate increases, the flammable limits increase. The diffusion flame position moves downstream with increasing air flow, eventually stabilizing at the bottom of the outer tube until extinction. When the flame passes in the tube, the peak temperature would occur on the wall. If there is heat recirculating, the wall temperature of the inner tube is higher than the boiling point of liquid n-heptane. It is conducive to the pre-evaporation of liquid n-heptane. In contrast, if there is no heat recirculating, liquid fuel will be accumulated in the tube. The heat loss coefficient has a great influence on flammable limits of the tube burner without heat recirculating.

Study of breakdown in an ablative pulsed plasma thruster

Breakdown in ablative pulsed plasma thrusters (APPTs) must be studied in order to design new types of APPTs and measure particular parameters. In this paper, we studied a parallel-plate ablative pulsed plasma thruster that used a coaxial semiconductor spark plug. By operating the APPT about 500 times with various capacitor voltages and electrode gaps, we measured and analyzed the voltage of the spark plug, the voltage between the electrodes, and the discharge current. These experiments revealed a time delay (∼1–10 μs) between spark plug ignition and capacitor discharge, which may affect the performance of high-pulsing-rate (>10 kHz) and double-discharge APPTs, and the measurements of some of the APPT parameters. The delay time decreased as the capacitor voltage increased, and it increased with an increasing electrode gap and increasing number of ignitions. We explain our results through a simple theoretical analysis.

Numerical Analysis on Oscillation Characteristics in a Tailpipe Nozzle Solid Rocket Motor

DOI: 10.2514/1.48867 Based on vortex–acoustic coupling theory, large-eddy simulation with wall-adapting local eddy-viscosity model and finite element method are carried out to study the internal flowfield and acoustic field, respectively, in a tailpipe nozzle solid rocket motor with transition-section grain configuration. The numerical method by means of a mesh sensitivityanalysisisproposedforvalidation.Theinstantaneous flowfieldcharacteristicsinthecombustionchamber and tailpipe are analyzed. The excited low frequencies are close to that observed in experiment. The phenomenon in which acoustic signals, superimposed on the vortex-shedding motions, couple with an internal flowfield is proven to be one of the main reasons contributing to oscillation in the motor. According to fast Fourier transform, low frequenciespredominateinthecombustionchamber;however,highfrequenciespredominateinthetailpipe.Dozens of cases with different geometrical configurations are presented to investigate parameters that have impact on the low-frequency oscillation characteristics. The results indicate that the oscillation characteristics are mainly influenced by upstream mean velocity, transition-section angle, distance between vortex source and impingement points, tailpipe radius, and convergence angle of the nozzle.

Study on the wall ablation of heated compound-materials into discharge plasmas based on a modified model

Velocity of ablation vapor near the surface of heated compound-materials strongly affects the kinetic layer parameters modeled and manifested in the Knudsen layer. This paper discussed overlooked physics and clarified inaccuracies in the expression of velocity at the outer boundary of the kinetic layer induced by discharge plasma. The changes of average molecular mass coupling with discharge current on mass and momentum conservation equations in plasma layer were considered when modifying the expression of this boundary velocity. Our assessment of these effects indicated that velocity of ablation vapor showed a downtrend as the ratio of average molecular mass at inner and outer boundaries of plasma layer increased, which plays a decisive role in reducing the ablation rate. Compared with single species fluid model, the modified model that applies to the pyrolysis of heated compound-materials showed 56% drop in Teflons ablation rate when plasmas were fully ionized.

Studies on Effect of Head Cavity on Resonance Damping Characteristics in Solid Rocket Motors

In order to discover the effect of head cavity on resonance damping characteristics in solid rocket motors, large-eddy simulations with Wall-Adapting Local Eddy-viscosity subgrid scale turbulent model are implemented to study the oscillation flow field induced by vortex shedding on the foundation of VKI (von Karman Institute) experimental motor. The numerical method by means of a mesh sensitivity analysis is proposed for validation. Pressure oscillation frequencies and amplitudes are obtained, and then compared with the experimental data. It is investigated that oscillation amplitudes reduce remarkably after adding a cavity at the head-end. The results indicate that cavity volume, location and configuration have cooperative effect on the oscillation amplitude. Rayleigh criterion is proved to be of guiding significance of resonance damping. The substance of altering grain configuration is a comprehensive process of adding and abstracting mass. The suppression effect is not caused by the complicated flow field at the head-end. Additionally, it is neglected whether altering grain configuration at the acoustic pressure node. It is concluded that large mass flux added at pressure antinode could attribute to significant amplitude; meanwhile, the damping effect of cavity is stronger if the distance between cavity and pressure antinode becomes shorter. At last, this method is adopted by an engineering solid rocket motor. Ground test reflects that the oscillations are suppressed by the head cavity.

Analysis of Teflon Pulsed Plasma Thrusters Using a Modified Slug Parallel Plate Model

The Pulsed Plasma Thruster (PPT) is a reliable, relatively simple and low cost electric propulsion thruster that has been used widely in space flight mission. Numerical simulation is an important part of the overall research and development process for PPTs. To provide physical insights and guidance toward design of consistently efficient PPTs, a modified slug model for parallel-plate PPTs is presented in this paper. This new model is formed on the basis of electromechanical model (i.e. slug model), which has been published by American Worchester Polytechnic Institute. Different from common slug model, this new model assumes that after the plasma slug is formed there is additional mass accumulated to the plasma as it accelerates down the discharge chamber and force within the PPT comes from an gas dynamics due to build up of pressure within the thruster chamber and a magnetic acceleration from the Lorentz force. So the modified slug model can calculate the ablated mass per discharge and can describe the electromagnetic acceleration and aerodynamic efficiency in the process of plasma flow. The late time ablation (LTA) is sublimation of propellant that takes place after the main discharge, due to the propellant, temperature being above its sublimation point, and LTA is considered as one of the main contributors for PPT’s low efficiency. Thus the new model also analyze the impact of LTA for the performance of PPT according to the velocity of decomposed mixture of neutral atoms, monomers and free-radical chains of molecules (“slurry”) after the main discharge. Finally, influences of variations of electric parameters and configuration parameters on thruster performance are studied by numerical simulation.

Experimental and Numerical Studies on Methane/Air Combustion in a Micro Swiss-Roll Combustor

To understand effects of an air groove on working characteristics of micro Swiss-roll combustors, combustion of premixed CH4/air is conducted in 2 micro Swiss-roll combustors, one with an air groove and the other without. Experimental results show that stable combustion of premixed CH4/air in 2 combustors can be achieved and the flame is kept in combustors center. An air groove can extend flammable limit of the combustor and make it work at a larger excess air coefficient and a lower methane flow rate. Furthermore, it increases the radial surface temperature gradient on outer wall of the combustor. Additionally, the combustor with an air groove is numerically simulated. Numerical results indicate that hot combustion products play a strong role in heating incoming mixtures. On one hand, it makes premixed flame surface inclined across inlet channel; on the other hand, it makes flame position vary with flow velocity, excess air coefficient and heat loss to the environment.

Characteristics of plasma properties in double discharge ablative pulsed plasma thrusters

Ablative pulsed plasma thrusters, the earliest electric space propulsion devices, create highly transient plasmas in short discharges that are expelled to create thrust. In recent years, the double-discharge ablative pulsed plasma thruster design has been proposed to improve the low-thrust efficiency. In this study, optical emission spectroscopy was applied to investigate the plasma properties in different regions and energy distributions. The electron temperature and electron density of the plasmas are derived and discussed. This study provides a physical mechanism for double-discharge pulsed plasma thrusters.

Thrust Control by Fluidic Injection in Solid Rocket Motors

Cold-flow tests, thermal tests, and numerical simulation were performed to study the thrust-adjustment characteristics, and the thrust-vector-control characteristics, of a fluidic nozzle throat com...