Ragini Acharya

Combustion and emissions performance of low sulfur, ultra low sulfur and biodiesel blends in a DI diesel engine

Experiments were conducted with a commercially available six-cylinder, 4-valves per cylinder, turbocharged, direct injection (Dl) diesel engine. The engine was operated with low sulfur diesel fuel, ultra low sulfur diesel fuel and two other blends, low sulfur diesel fuel with 20 wt.% biodiesel and ultra low sulfur diesel fuel with 20 wt.% biodiesel, to investigate the effect of the base fuels and their blends on combustion and emissions. Combustion analysis, particulate matter emissions and exhaust gas composition (CO, NO x and total hydrocarbons) were determined at eight steady-state operating conditions according to the AVL 8-Mode test protocol. Combustion analysis showed at high load conditions a retarded start of injection, an earlier start of combustion and a lower premixed burn peak with ultra low sulfur diesel fuel. Mode averaged NO x emissions decreased with ultra low sulfur diesel fuel and biodiesel blends compared to low sulfur diesel fuel. A 20% PM reduction was observed with ultra low sulfur (15 PPM) diesel fuel compared to low sulfur (325 PPM) diesel fuel.

Effect of Pressure and Propellant Composition on Graphite Rocket Nozzle Erosion Rate

The objective of this work is to study the nozzle erosion rates at a broad range of pressures from 7 to 55 MPa with two baseline propellants: one is a nonmetallized propellant and the other is a metallized propellant, called propellants S and M, respectively. A comprehensive model for graphite nozzle erosion minimization and a numerical code has been advanced to predict the nozzle throat recession rates at high pressures. Four different kinetic schemes for heterogeneous graphite oxidation reactions were compared. The recession rate was found to increase almost linearly with pressure. The magnitudes of recession rates depend on the chemical kinetic scheme and the propellant composition. Contrary to popular belief, at lower pressures (P < 14 MPa), the heterogeneous kinetic rates showed a pronounced effect on the erosion rates, though at higher pressures, the nozzle throat erosion is mainly diffusion controlled. This observation stresses the importance of more accurate and definitive kinetic parameters for graphite oxidation reactions, especially at lower pressures. It was also observed that, besides H 2 O, the OH species affects the nozzle recession rate greatly. For the metallized propellant, the concentrations of major oxidizing species such as H 2 O, OH, and CO 2 are substantially reduced in comparison with the nonmetallized propellant, resulting in significant reduction of the erosion rates. A comparison of experimental data and predicted results from the graphite nozzle erosion minimization code shows excellent agreement especially for the nonmetallized propellant. To substantially reduce the throat recession rates at high pressures, it is suggested that the boundary-layer control at the throat region could be an effective method for future nozzle design considerations.

Implementation of Approximate Riemann Solver to Two-Phase Flows in Mortar Systems

This work presents a mathematical model for the two-phase flows in the mortar systems and demonstrates the application of approximate Riemann solver on such model. The mathematical model for the two-phase gas-dynamical processes in the mortar tube consists of a system of first-order nonlinear coupled partial differential equations with inhomogeneous terms. The model poses an initial value problem with discontinuous initial and boundary conditions that arise due to the design complexity and nonuniformity of granular propellant distribution in the mortar tube. The governing equations in this model possess characteristics of the Riemann problem. Therefore, a high-resolution Godunov-type shock-capturing approach was used to address the formation of flow structure such as shock waves, contact discontinuities, and rarefaction waves. A linearized approximate Riemann solver based on the Roe-Pike method was modified for the two-phase flows to compute fully nonlinear wave interactions and to directly provide upwinding properties in the scheme. An entropy fix based on Harten-Heyman method was used with van Leer flux limiter for total variation diminishing. The three-dimensional effects were simulated by incorporating an unsplit multidimensional wave propagation method, which accounted for discontinuities traveling in both normal and oblique coordinate directions. A mesh generation algorithm was developed to account for the projectile motion and coupled with the approximate Riemann solver The numerical method was verified by using exact solutions of three test problems. The specific system considered in this work is a 120 mm mortar system, which contains an ignition cartridge that discharges hot gas-phase products and unburned granular propellants into the mortar tube through multiple vent-holes on its surface. The model for the mortar system was coupled with the solution of the transient gas-dynamic behavior in the ignition cartridge. The numerical results were validated with experimental data. Based on the close comparison between the calculated results and test data, it was found that the approximate Riemann solver is a suitable method for studying the two-phase combustion processes in mortar systems.

Numerical Simulation of Graphite Nozzle Erosion with Parametric Analysis

Nozzle throat erosion is a major problem for solid rocket motors since it causes the degradation in the propulsive performance of solid rocket motors. The AP/HTPB composite propellants used in the rocket motors generate high concentrations of oxidizing species such as H2O, OH, and CO2 in the combustion products at temperatures ranging from 2,700 to 3,200 K for non-metalized propellants. Earlier, the authors utilized a comprehensive numerical program called graphite nozzle erosion minimization code for prediction of graphite nozzle throat erosion rates as a function of pressure and propellant composition. From these studies, it was established that various parameters affect the nozzle thermochemical erosion rate including oxidizing species concentrations, flame temperature, and operating pressure. In addition, the thermal properties of graphite could also affect the nozzle throat erosion rate since these are directly related to the surface temperature at the nozzle throat. In order to assess the relative importance of these parameters in terms of their impact on the nozzle throat erosion rate, a parametric analysis was performed in this study. Each of these parameters was systematically varied while keeping all the remaining parameters constant. Based upon this research, it is concluded that flame temperature can affect the thermochemical erosion rate most, followed by chamber pressure and major oxidizing species concentrations. The mechanisms associated with the influence of these parameters are explored and described. A comparison of predicted results with the available experimental data shows match within 20%. The parametric analysis performed in this research provides an in-depth understanding of the thermochemical erosion process and the controlling steps in the nozzle erosion phenomena.

Design of a Solid Rocket Motor for Characterization of Submerged Nozzle Erosion

Current understanding of physical and chemical processes involved in the erosion of submerged nozzles by highly-aluminized solid propellants is limited. The ability to predict the surface erosion rate of a given carbon-cloth phenolic (CCP) nozzle material is very important for the future design or modification of large solid rocket boosters for space launch applications. Although current erosion codes provide engineering accuracy for nozzle throat erosion rates, calculated rates for the forward surfaces of the submerged nozzle can vary significantly from observed values. The overall objective of this research study under the NASA Constellation University Institutes Project (NASA-CUIP) is to improve the understanding of nozzle erosion and related phenomena. In this work, the design of subscale solid rocket motor was performed based upon engineering analysis of the interior ballistic process and a series of CFD simulations of the flow and heat transfer processes in the region of the submerged nozzle. This motor design allows for the use a realtime X-ray radiography with a high-resolution image intensifier system to obtain submerged nozzle erosion data. From the CFD simulations, the maximum accretion rate of liquid alumina droplets was found to have a level of ~10 kg/s-m 2 in the nose-cone region. Elevated accretion rates in the submerged section of the nozzle were calculated and attributed to the impact of larger particles with higher inertia. These large particles could not follow the combustion product stream to flow out of the nozzle. Development of thermal waves in both the liquid film and the CCP material was investigated. Results showed that their interface temperature can reach 3,000 K in about 1 s. Future test results from this newly designed rocket motor will be highly beneficial for model validation as well as attaining in-depth understanding of interactions between the liquid alumina and nozzle material.

Pyrolysis/Evaporation Study of Succinic Acid/Polyvinyl Acetate for Reducing Nozzle Throat Erosion

A nozzle boundary-layer control system is under consideration for application in high-pressure rockets to mitigate the erosion rates of a graphite nozzle. The current design contains multiple center-perforated solid grains of fuel-rich materials consisting of succinic acid and polyvinyl acetate. This combination of the fuel-rich grains was selected due to high carbon and hydrogen contents along with relatively low evaporation temperature for generating fuel-rich gases. The characterization of the pyrolysis behavior of fuel-rich grains is a requirement for any subsequent quantitative analysis pertaining to the effect of the nozzle boundary-layer control system on graphite rocket nozzle erosion rates. Two separate experiments were conducted: 1) to determine the regression rate of solid fuel-rich grains under controlled heat flux or temperature conditions, and 2) to characterize its chemical decomposition and/or evaporation behavior. An empirical correlation between heat flux and surface regression rate of fuel-rich grains was developed. From Fourier transform infrared spectroscopy measurements, the fuel-rich grains were found to melt and evaporate at temperatures up to 773 K. These results have been used in parallel study nozzle throat erosion processes using computational simulation.

Comprehensive Three Dimensional Mortar Interior Ballistics Model for 120mm Mortar System with Experimental Validation

A three-dimensional mortar interior ballistic (3D-MIB) model and code have been developed and stage-wise validated with multiple sets of experimental data in close collaboration between The Pennsylvania State Univ. (PSU) and Army Research and Development Engineering Center. This newly developed MIB model and numerical code realistically simulates the combustion and pressurization processes in various components of the 120mm mortar system. Due to the complexity of the overall interior ballistic processes in the mortar propulsion system, the overall problem has been solved in a modular fashion, i.e., simulating each component of the mortar propulsion system separately. The physical processes in the mortar system are two-phase and were simulated by considering both phases as an interpenetrating continuum. Mass and energy fluxes from the flash tube into the granular bed of M1020 ignition cartridge were determined from a semi-empirical technique. For the tail-boom section, a transient one-dimensional two-phase numerical code based on method of characteristics (MOC) was developed and validated by experimental test results. The mortar tube combustion processes were modeled and solved by using a twophase Roe-Pike method with van Leer flux limiter, a fourth-order Runge-Kutta scheme, and an adaptive mesh generator to account for the projectile motion. For each component, the predicted pressure-time traces showed significant pressure wave phenomena, which closely simulated the measured pressure-time traces. The experimental data for the flash tube and ignition cartridge were obtained at PSU whereas the pressure-time traces at the breech-end of the mortar tube were obtained from the tests conducted at Yuma Proving Ground (YPG) and by using an instrumented mortar simulator at Aberdeen Test Center (ATC). The 3DMIB code was also used to simulate the effect of flash tube vent-hole pattern on the pressurewave phenomenon in the ignition cartridge. A comparison of the pressure difference between primer-end and projectile-end locations of the original and modified ignition cartridges with each other showed that the early-phase pressure-wave phenomenon can be significantly reduced with the modified pattern on the flash tube. The flow property distributions predicted by the 3D-MIB for a zero charge increment case are explained in details in this work.