The oscillatory combustion behavior of solid propellants and energetic materials is of great interest for purposes of achieving stable combustion in solid rockets. This project is aimed at developing a new technique for determining the pressure-coupled response function of propellants that is simpler and more reliable than the standard T-burner method. The combustion response to an unsteady radiant heat source (laser) is measured from the recoil of the propellant. A transfer function to relate the heat flux-coupled response function to the pressure coupled response function is being developed based on a mathematical model of the unsteady burning rate of the propellant.
The objectives of this study are (1) to develop and validate a one-dimensional, premixed gas phase reaction code to be coupled with an existing condensed phase code which will calculate burning rate, temperature profile, and species profiles; (2) to use this code to investigate the domain of validity of existing activation energy asymptotics analytic results; (3) to develop new analytic results that can predict variations in steady burning rate and flame standoff distance with pressure, initial temperature, and radiant heat flux; and (4) to apply these models to steady and oscillatory (quasi-steady) burning of RDX and HMX.
Arcjet thrusters are currently being used as primary propulsion systems for station-keeping on low-Earth orbit satellites. Experiments are conducted on a 1-kW arcjet thruster using nitrogen/hydrogen-based propellants to evaluate a nonequilibrium arcjet plasma model. The results are used to determine model boundary conditions and accuracy. Application of flush-mounted electrostatic probes for internal diagnostics of the nozzle allows direct measurements of plasma parameters in the anode boundary layer. Such measurements allow an understanding of the physics of arc attachment and anode heating, both critical to improving arcjet performance and lifetime.
Experimental research is being conducted in the very-high pressure combustion facility to gain an understanding of the high-pressure (300 MPa) surface and gas phase
chemistry responsible for solid particulate boron combustion as it pertains to use as a fuel additive in propulsion devices and explosives. Boron has the potential for twice the volumetric energy release than typical hydrocarbon fuels, but the combustion processes are slower than most high-speed applications will allow. It is the intent of this research to investigate several mechanisms to enhance the burning rate of boron so that a fuller potential of the energy release can be achieved within high-speed combustion and explosive devices.
As new energetic materials are developed for use as propellants in solid rockets, it will be necessary to consider their combustion stability at an early stage of propellant development. This will require a multidisciplinary approach including complex chemistry, combustion, and fluid dynamics. The overall objective of this project is to conduct a coordinated, multidisciplinary investigation to advance our knowledge of dynamic burning response of new combinations of energetic materials. The specific objective is to develop an understanding of the combustion behavior of new energetic materials as monopropellants and combinations of new and conventional energetic materials as composite propellants.
A nonequilibrium model of kilowatt-class, radiation-cooled arcjet thrusters is developed. Separate electron and heavy species (molecules, atoms, and ions) kinetic temperatures permit more accurate predictions of the plasma transport processes and thruster performance. Kinetic nonequilibrium results indicate that regions of high nonequilibrium, with elevated electron temperatures, exist because of inefficient collisional energy transfer from the electrons to the heavies. The model is generalized to include the effects of thermal and chemical nonequilibrium, mass diffusion, electrical sheath losses, and line and continuum radiation transfer.
New energetic solid propellants will contain metals such as aluminum, magnesium, and boron. Experiments in a high pressure shock tube measuring boron ignition delay and combustion (burn) time, as well as measurements by emission spectroscopy of the transient reactive species, in the range 0.5 to 5 MPa and temperatures ranging from 1800° to 3000°K, will impact chemical kinetic theories on reaction pathways for such two-phase mixtures.
In order to explain rocket motor combustion instability, one must know the propellant burning rate response to pressure transients. End burning solid propellant rocket motors, where the motor throat area is modulated, will give rise to pressure fluctuations during which instantaneous burning rate, gas species, and gas temperature can be measured. Laser diagnostics and ultrasonic techniques are being used.
The task of developing a multicomponent, multiphase hydrodynamic model that describes all the salient features of an underwater explosion of metal-loaded high explosives is addressed. The model represents a significant advancement over previous efforts in this area because it includes many nonideal phenomena not previously considered (e.g., combustion of metal-loaded high explosives and bubble-water interfacial transport processes). The primary purpose of this research is to develop a basic understanding of the fundamental relationship between the available energy stored in the explosive system prior to detonation and the subsequent redistribution of energy throughout the bubble and surrounding fluid.
We are investigating gas-phase chemistry near the deposition substrate in diamond-forming flames using spatially
resolved laser diagnostics. High-quality, polycrystalline diamond films are deposited using acetylene/hydrogen/oxygen flat flames stabilized against molybdenum substrates. Coherent anti-Stokes Raman scattering (CARS) is used to measure major species concentrations (H2, CO) and temperature profiles near the diamond-forming substrate. Laser-induced fluorescence (LIF) and/or degenerate four-wave mixing (DFWM) will be used to measure the minor species H, C2H2, and CH3, which are thought to play an important role in diamond-forming flame chemistry.
We are investigating the interaction of a vortex with a diffusion flame sheet. Vortices induced by using an acoustic speaker to drive a fuel jet cause the flame sheet to bulge outward. The induced stretch is sufficient to extinguish the flame locally. We will use advanced laser diagnostics to study the flame-vortex interaction. Temperature measurements are performed using coherent anti-Stokes Raman scattering. Concentrations of the radical species OH and CH and the pollutant species NO will be measured using laser-induced fluorescence. The experimental measurements are compared with computational fluid dynamics (CFD) calculations that include detailed chemical kinetics.
Degenerate four-wave mixing (DFWM) is a promising combustion diagnostic, but lack of accurate models for lineshapes and signal intensities has impeded the quantitative application of the technique. A combined theoretical and experimental approach is used for the development and evaluation of strategies for quantitative measurements in flames. DFWM signal levels and lineshapes are calculated theoretically by solving the time-dependent density matrix equations for the DFWM process by direct numerical integration. Experimentally, DFWM measurements of OH and NO will be performed over a wide range of flame pressures and stoichiometries for comparison with these theoretical calculations.
There are several possible advantages in co-firing natural gas and coal compared to combustion of coal alone including the potential to reduce emissions, increase carbon burnout, and reduce flame length. In this program, a specially designed reactor is used to study the effects of natural gas on single-particle coal combustion in order to quantify the benefits for co-firing. Measurement capabilities include particle temperatures with a two-color pyrometer, particle size and shape with a two-dimensional imaging system, gas phase emissions, and burned particle analysis. Results show SOx reductions and decreased ignition delays with co-firing.
In a collaborative effort with General Electric Aircraft Engines, we will investigate experimental configurations that closely resemble actual GE gas turbine combustor configurations and operating conditions. Advanced nonintrusive laser diagnostics including CARS and LIF will be used to probe the mixing and combustion processes. The purpose of this research program is to ``bridge the gap'' from more fundamental experimental and modeling studies of turbulent mixing and combustion to the combustor design process. The technical issues that will be addressed in research at UIUC include (1) fuel/air mixing, (2) flame structure and stabilization, and (3) pollutant formation.