11 Matching Results

Search Results

Advanced search parameters have been applied.

Investigation of H2 Concentration and Combustion Instability Effects on the Kinetics of Strained Syngas Flames

Description: The flame extinction limits of syngas (H{sub 2}-CO) flames were measured using a twin-flame-counter-flow burner. Plots of Extinction limits vs. global stretch rates were generated at different mixture compositions and an extrapolation method was used to calculate the flame extinction limit corresponding to an experimentally unattainable zero-stretch condition. The zero-stretch extinction limit of H{sub 2}-CO mixtures decreases (from rich to lean) with the increase in H{sub 2} concentration in the mixture. The average difference between the measured flame extinction limit and the Le Chatelier's calculation is around {approx} 7%. The measured OH{sup -} chemiluminescent data indicates that regardless of mixture compositions the OH radical concentration reduces (within the experimental uncertainties) to an extinction value prior to the flame extinction. Flame extinction limits of H{sub 2}-CO mixtures measured in a flat-flame burner configuration also show a similar relation. Additionally, the measured laminar flame velocity close to the extinction indicates that regardless of fuel composition the premixed flame of hydrogen fuel blends extinguishes when the mixture laminar flame velocity falls below a critical value. The critical laminar flame velocity at extinction for H{sub 2}-CO premixed flames (measured in the flat flame burner configuration) is found to be 3.77({+-}0.38) cm/s. An externally perturbed H{sub 2}-CO twin flame was not experimentally achievable for the mixture conditions used in the present investigation. A slightest perturbation in the flow-field distorts the H{sub 2}-CO twin-flame. The flame becomes highly unstable with the introduction of an externally excited flow oscillation.
Date: August 7, 2006
Creator: Choudhuri, Ahsan R.
Partner: UNT Libraries Government Documents Department

Engine Control Improvement through Application of Chaotic Time Series Analysis

Description: The objective of this program was to investigate cyclic variations in spark-ignition (SI) engines under lean fueling conditions and to develop options to reduce emissions of nitrogen oxides (NOx) and particulate matter (PM) in compression-ignition direct-injection (CIDI) engines at high exhaust gas recirculation (EGR) rates. The CIDI activity builds upon an earlier collaboration between ORNL and Ford examining combustion instabilities in SI engines. Under the original CRADA, the principal objective was to understand the fundamental causes of combustion instability in spark-ignition engines operating with lean fueling. The results of this earlier activity demonstrated that such combustion instabilities are dominated by the effects of residual gas remaining in each cylinder from one cycle to the next. A very simple, low-order model was developed that explained the observed combustion instability as a noisy nonlinear dynamical process. The model concept lead to development of a real-time control strategy that could be employed to significantly reduce cyclic variations in real engines using existing sensors and engine control systems. This collaboration led to the issuance of a joint patent for spark-ignition engine control. After a few years, the CRADA was modified to focus more on EGR and CIDI engines. The modified CRADA examined relationships between EGR, combustion, and emissions in CIDI engines. Information from CIDI engine experiments, data analysis, and modeling were employed to identify and characterize new combustion regimes where it is possible to simultaneously achieve significant reductions in NOx and PM emissions. These results were also used to develop an on-line combustion diagnostic (virtual sensor) to make cycle-resolved combustion quality assessments for active feedback control. Extensive experiments on engines at Ford and ORNL led to the development of the virtual sensor concept that may be able to detect simultaneous reductions in NOx and PM emissions under low temperature combustion (LTC) regimes. An invention ...
Date: July 15, 2003
Creator: Green, J.B., Jr. & Daw, C.S.
Partner: UNT Libraries Government Documents Department

Combustion instability in an acid-heptane rocket with a pressurized-gas propellant pumping system

Description: Report presenting results of experimental measurements of low-frequency combustion instability of a 300-pound-thrust acid-heptane rocket engine as compared with the trends predicted by an analysis of combustion instability in a rocket engine. Results regarding the chugging frequency, combustion time delay, effect of rocket combustion-chamber characteristic length, effect of throttling, effect of injection velocity, effect of oxidant-fuel ratio, variation of chugging frequency with amplitude of chamber pressure fluctuations, and evaluation of the analysis are provided.
Date: May 1953
Creator: Tischler, Adelbert O. & Bellman, Donald R.
Partner: UNT Libraries Government Documents Department

Les Software for the Design of Low Emission Combustion Systems for Vision 21 Plants

Description: In this project, an advanced computational software tool was developed for the design of low emission combustion systems required for Vision 21 clean energy plants. Vision 21 combustion systems, such as combustors for gas turbines, combustors for indirect fired cycles, furnaces and sequestrian-ready combustion systems, will require innovative low emission designs and low development costs if Vision 21 goals are to be realized. The simulation tool will greatly reduce the number of experimental tests; this is especially desirable for gas turbine combustor design since the cost of the high pressure testing is extremely costly. In addition, the software will stimulate new ideas, will provide the capability of assessing and adapting low-emission combustors to alternate fuels, and will greatly reduce the development time cycle of combustion systems. The revolutionary combustion simulation software is able to accurately simulate the highly transient nature of gaseous-fueled (e.g. natural gas, low BTU syngas, hydrogen, biogas etc.) turbulent combustion and assess innovative concepts needed for Vision 21 plants. In addition, the software is capable of analyzing liquid-fueled combustion systems since that capability was developed under a concurrent Air Force Small Business Innovative Research (SBIR) program. The complex physics of the reacting flow field are captured using 3D Large Eddy Simulation (LES) methods, in which large scale transient motion is resolved by time-accurate numerics, while the small scale motion is modeled using advanced subgrid turbulence and chemistry closures. In this way, LES combustion simulations can model many physical aspects that, until now, were impossible to predict with 3D steady-state Reynolds Averaged Navier-Stokes (RANS) analysis, i.e. very low NOx emissions, combustion instability (coupling of unsteady heat and acoustics), lean blowout, flashback, autoignition, etc. LES methods are becoming more and more practical by linking together tens to hundreds of PCs and performing parallel computations with fine grids (millions of ...
Date: January 1, 2005
Creator: Smith, Clifford E.; Cannon, Steven M.; Adumitroaie, Virgil; Black, David L. & Meredith, Karl V.
Partner: UNT Libraries Government Documents Department

On pulsating and cellular forms of hydrodynamic instability in liquid-propellant combustion

Description: An extended Landau/Levich model of liquid-propellant combustion, one that allows for a local dependence of the burning rate on the (gas) pressure at the liquid/gas interface, exhibits not only the classical hydrodynamic cellular instability attributed to Landau, but also a pulsating hydrodynamic instability associated with sufficiently negative pressure sensitivities. Exploiting the realistic limit of small values of the gas-to-liquid density ratio {rho}, analytical formulas for both neutral stability boundaries may be obtained by expanding all quantities in appropriate powers of {rho} in each of three distinguished wavenumber regimes. In particular, composite analytical expressions are derived for the neutral stability boundaries A{sub p}(k), where A{sub p} is the pressure sensitivity of the burning rate and k is the wavenumber of the disturbance. For the cellular boundary, the results demonstrate explicitly the stabilizing effect of gravity on long-wave disturbances, the stabilizing effect of viscosity and surface tension on short-wave perturbations, and the instability associated with intermediate wavenumbers for negative values of A{sub p}, which is characteristic of many hydroxylammonium nitrate-based liquid propellants over certain pressure ranges. In contrast, the pulsating hydrodynamic stability boundary is insensitive to gravitational and surface-tension effects, but is more sensitive to the effects of liquid viscosity since, for typical nonzero values of the latter, the pulsating boundary decreases to larger negative values of A{sub p} as k increases through O(1) values.
Date: November 1, 1997
Creator: Margolis, S. B.
Partner: UNT Libraries Government Documents Department

Pulsating hydrodynamic instability and thermal coupling in an extended Landau/Levich model of liquid-propellant combustion. 2. Viscous analysis

Description: A pulsating form of hydrodynamic instability has recently been shown to arise during liquid-propellant deflagration in those parameter regimes where the pressure-dependent burning rate is characterized by a negative pressure sensitivity. This type of instability can coexist with the classical cellular, or Landau, form of hydrodynamic instability, with the occurrence of either dependent on whether the pressure sensitivity is sufficiently large or small in magnitude. For the inviscid problem, it has been shown that when the burning rate is realistically allowed to depend on temperature as well as pressure, that sufficiently large values of the temperature sensitivity relative to the pressure sensitivity causes the pulsating form of hydrodynamic instability to become dominant. In that regime, steady, planar burning becomes intrinsically unstable to pulsating disturbances whose wavenumbers are sufficiently small. In the present work, this analysis is extended to the fully viscous case, where it is shown that although viscosity is stabilizing for intermediate and larger wavenumber perturbations, the intrinsic pulsating instability for small wavenumbers remains. Under these conditions, liquid-propellant combustion is predicted to be characterized by large unsteady cells along the liquid/gas interface.
Date: January 1, 2000
Creator: Margolis, Stephen B.
Partner: UNT Libraries Government Documents Department

Stability of quasi-steady deflagrations in confined porous energetic materials

Description: Previous analyses have shown that unconfined deflagrations propagating through both porous and nonporous energetic materials can exhibit a thermal/diffusive instability that corresponds to the onset of various oscillatory modes of combustion. For porous materials, two-phase-flow effects, associated with the motion of the gas products relative to the condensed material, play a significant role that can shift stability boundaries with respect to those associated with the nonporous problem. In the present work, additional significant effects are shown to be associated with confinement, which produces an overpressure in the burned-gas region that leads to reversal of the gas flow and hence partial permeation of the hot gases into the unburned porous material. This results in a superadiabatic effect that increases the combustion temperature and, consequently, the burning rate. Under the assumption of gas-phase quasi-steadiness, an asymptotic model is presented that facilitates a perturbation analysis of both the basic solution, corresponding to a steadily propagating planar combustion wave, and its stability. The neutral stability boundaries collapse to the previous results in the absence of confinement, but different trends arising from the presence of the gas-permeation layer are predicted for the confined problem. Whereas two-phase-flow effects are generally destabilizing in the unconfined geometry, the effects of increasing overpressure and hence combustion temperature associated with confinement are shown to be generally stabilizing with respect to thermal/diffusive instability, analogous to the effects of decreasing heat losses on combustion temperature and stability in single-phase deflagrations.
Date: March 1, 2000
Creator: Telengator, Alexander M.; Margolis, Stephen B. & Williams, Forman A.
Partner: UNT Libraries Government Documents Department

Structure and Stability of Deflagrations in Porous Energetic Materials

Description: Theoretical two-phase-flow analyses have recently been developed to describe the structure and stability of multi-phase deflagrations in porous energetic materials, in both confined and unconfined geometries. The results of these studies are reviewed, with an emphasis on the fundamental differences that emerge with respect to the two types of geometries. In particular, pressure gradients are usually negligible in unconfined systems, whereas the confined problem is generally characterized by a significant gas-phase pressure difference, or overpressure, between the burned and unburned regions. The latter leads to a strong convective influence on the burning rate arising from the pressure-driven permeation of hot gases into the solid/gas region and the consequent preheating of the unburned material. It is also shown how asymptotic models that are suitable for analyzing stability may be derived based on the largeness of an overall activation-energy parameter. From an analysis of such models, it is shown that the effects of porosity and two-phase flow are generally destabilizing, suggesting that degraded propellants, which exhibit greater porosity than their pristine counterparts, may be more readily subject to combustion instability and nonsteady deflagration.
Date: March 1, 1999
Creator: Margolis, stephen B. & Williams, Forman A.
Partner: UNT Libraries Government Documents Department

Combustion Research Program: chapter from Energy and Environment Division annual report 1977

Description: A combustion system typically involves a complex interaction of chemical and fluid mechanical phenomena. It is a fertile field for sophisticated research and development which draw on the academic disciplines of high temperature chemical kinetics and turbulent fluid mechanics. A number of the most recent experimental and theoretical research techniques, such as laser based instrumentation, molecular beam techniques, and powerful computational and numerical analysis techniques in fluid mechanics can be fully exploited in well planned programs of combustion research. The initiation of research on problems associated with coal combustion is discussed in the first two articles. The subsequent twelve articles summarize research projects covering a wide variety of combustion problems. Several are directly related to pollution problems; in particular there is a coordinated program aimed at developing clean burning internal combustion engines. Another important general area being studied (in three experimental and two theoretical projects) is the complex interaction of fluid mechanical turbulence with combustion heat release.
Date: January 1, 1978
Creator: Budnitz, R.J.
Partner: UNT Libraries Government Documents Department

Studies in nonlinear problems of energy. Progress report, October 1, 1993--September 30, 1994

Description: The authors concentrate on modeling, analysis and large scale scientific computation of combustion and flame propagation phenomena, with emphasis on the transition from laminar to turbulent combustion. In the transition process a flame passed through a stages exhibiting increasingly complex spatial and temporal patterns which serve as signatures identifying each stage. Often the transitions arise via bifurcation. The authors investigate nonlinear dynamics, bifurcation and pattern formation in the successive stage of transition. They describe the stability of combustion waves, and transitions to combustion waves exhibiting progressively higher degrees of spatio-temporal complexity. One aspect of this research program is the systematic derivation of appropriate, approximate models from the original models governing combustion. The approximate models are then analyzed. The authors are particularly interested in understanding the basic mechanisms affecting combustion, which is a prerequisite to effective control of the process. They are interested in determining the effects of varying various control parameters, such as Nusselt number, Lewis number, heat release, activation energy, Damkohler number, Reynolds number, Prandtl number, Peclet number, etc. The authors have also considered a number of problems in self-propagating high-temperature synthesis (SHS), in which combustion waves are employed to synthesize advanced materials. Efforts are directed toward understanding fundamental mechanisms. 167 refs.
Date: September 1, 1994
Creator: Matkowsky, B. J.
Partner: UNT Libraries Government Documents Department

[The physics of coal liquid slurry atomization]. Annual report 1992

Description: In order to understand the physics of atomization and to predict and improve the performance of atomizers, a survey on the effects of turbulence on atomization has been made. The influence of gas turbulence intensity on the disintegration of a liquid jet, while a constant mean velocity in both gas and liquid streams has been maintained, has been studied. A study has been made of the influence of changing dynamic surface tension on liquid surface wave characteristics and atomization. The dynamic surface tension of water was changed by adding Triton X-100 non-ionic surfactant into the liquid supplied to a two dimensional slot atomizer. Wave frequencies were measured using laser beam attenuation. Dynamic surface tension changes were found to influence liquid sheet disintegration with little effect on wave frequencies. A series of experiments have been conducted to determine the fundamental processes of injection and atomization of liquid propellants for rocket combustion chambers because of their direct influence on combustion instability. For coaxial injectors, liquid and gas flow rates have been progressively changed. Microphotography was used to obtain details of wave disturbances on liquid surfaces. Direct measurements were made of wavelength and frequency of wave propagation on liquid surfaces. Frequency was found to remain constant along the length of the liquid surface. Pulsations in the liquid jet caused drops to form clusters with the same frequency as that of jet surface waves. Measured frequencies were in the range of those measured in combustion instability experiments. Detailed measurements have been made in the sprays using the phase Doppler particle analyzer. Measurements of drop size, velocity and number density are related to the disintegration process. Increasing turbulence intensity in the gas stream is a very effective means of reducing drop size, increasing spray width, and therefore, improving combustion.
Date: June 1, 1994
Creator: Chigier, N. & Brown, W. J.
Partner: UNT Libraries Government Documents Department