William M. Roquemore

Study on Trapped-Vortex Combustor-Effect of Injection on Flow Dynamics

Low-velocity x8f ows in the cavities of a combustor can aid in establishing stable x8f ames. However, unsteady x8f ows in and around cavities may destabilize these x8f ames. By proper cavity design it is possible to lock (trap) the vortices spatially and, thereby, stabilize the x8f ames. The spatially locked vortices restrict the entrainment of main air into the cavity. For obtaining good performance characteristics with a trapped-vortex combustor, a sufx8e cient amount of fuel and air must be injected directly into the cavity. This mass injection can alter the dynamic characteristics of the x8f ow inside and around the cavity. The present study employed a numerical simulation to investigate the vortex dynamics of a cavity into which x8f uid mass is directly injected through jets. A third-order-accurate, time-dependent, computational x8f uid dynamics with chemistry code was used for simulating the dynamic x8f ows associated with an axisymmetric, centerbody trapped-vortex combustor under nonreacting and reacting conditions. It was found that mass injection increases the optimum size (width-to-diameter ratio) of the cavity. Injection of small amounts of x8f uid into a nonoptimum cavity increases the unsteadiness of the x8f ow. Fluid injected into the optimumsize cavity is transported along the outer core of the vortex, providing more efx8e cient mixing and a longer residence time for the fuel/air mixture. It was also found that use of thinner afterbodies results in the cavity x8f ow being more dynamic. Calculations made with a global-chemistry model revealed that at higher annulus air velocities, combustion is limited to the cavity region. As in the case of cold x8f ows, the injection jets in reacting x8f ows are pushed outward from the center when the cavity size is small.

Simulation of dynamic methane jet diffusion flames using finite rate chemistry models

Detailed calculations for methane jet diffusion flames under laminar and transitional conditions are made using an axisymmetric, time-dependent computational fluid dynamics code and different chemical-kinetics models. Comparisons are made with experimental data for a steady-state flame and for two dynamic flames that are dominated by buoyancy-driven instabilities. The ability of the three chemistry models-namely, 1) the modified Peters mechanism without C 2 chemistry, 2) the modified Peters mechanism with C 2 chemistry, and 3) the Gas Research Institutes Version 1.2 mechanism-in predicting the structure of coaxial jet diffusion flames under different operating conditions is investigated. It is found that the modified Peters mechanisms with and without C 2 chemistry are sufficient for the simulation of jet diffusion flames for a wide range of fuel-jet velocities. Detailed images of the vortical structures associated with the low- and transitional-speed methane jet flames are obtained using the reactive-Mie-scattering technique. These images suggest that a counter-rotating vortex is established upstream of the buoyancy-induced toroidal vortex in the low-speed-flame case and that the shear-layer vortices that develop in the transitional-speed flame are dissipated as they are convected downstream. The time-dependent calculations made using the modified Peters chemistry model have captured these unique features of the buoyancy-influenced jet flames. Finally, the unsteady flame structures obtained at a given height are compared with the steady-state flame structures.

Double-pulse and single-pulse laser-induced breakdown spectroscopy for distinguishing between gaseous and particulate phase analytes

We explore the use of a combination of double-pulse and single-pulse laser-induced breakdown spectroscopy (LIBS) methodologies as a means of differentiating between solid-phase and gaseous-phase analytes (namely, carbon) in an aerosol stream. A range of spectral data was recorded for double-pulse and single-pulse configurations, including both ns and fs prepulse widths, while varying the gas-phase mass percentage of the carbon from about 10% to 90% for various fixed carbon concentrations. The carbon emission response, as measured by the peak-to-continuum ratio, was greater for the double-pulse configuration as compared with the single-pulse response and was also enhanced as the percentage of solid-phase carbon was increased. Using a combination of the double-pulse and single-pulse emission signals, a monotonically increasing response function was found to correlate with the percentage of gas-phase analyte. However, individual data points at the measured gas-phase percentages reveal considerable scatter from the predicted trend. Furthermore, the double-pulse to single-pulse ratio was only pronounced with the ns-ns configuration as compared with the fs-ns scheme. Overall, the LIBS methodology has been demonstrated as a potential means to discriminate between gas-phase and particulate-phase fractions of the same elemental species in an aerosol, although future optimization of the temporal parameters should be explored to improve the precision and accuracy of this approach.

Numerical Studies on Cavity-Inside-Cavity-Supported Flames in Ultra Compact Combustors

Cavities are incorporated in the designs of the future gas-turbine combustors for providing flame stability and, thereby, for improving the lean blowout characteristics. Recently, a Cavity-inside-cavity (CIC) design was proposed for the Air Force Research Laboratory’s ultra compact combustor (UCC). Numerical studies are performed in the present study to understand the dynamics of the CIC-supported flames. The complex CIC that was used in the actual hardware has been simplified for making it amenable to two-dimensional models. Calculations are performed for the modified CIC using a two-dimensional, unsteady, reacting flow code known as UNICORN. Direct numerical simulations and calculations using k-e turbulence model are performed. A fast, global-chemistry model is used for studying the flame dynamics inside and in the wake region of CIC. Calculations are performed for several CIC geometries generated through varying the width of the cavity. The design CIC is found oversized for the secondary (circumferential) airflow used in UCCs. A detailed chemistry model is also used for understanding the blowout characteristics of the CIC-supported flames.© 2008 ASME

Hot electron dominated rapid transverse ionization growth in liquid water.

Pump/probe optical-transmission measurements are used to monitor in space and time the ionization of a liquid column of water following impact of an 800-nm, 45-fs pump pulse. The pump pulse strikes the 53-μm-diameter column normal to its axis with intensities up to 2 × 10(15) W/cm2. After the initial photoinization and for probe delay times < 500 fs, the neutral water surrounding the beam is rapidly ionized in the transverse direction, presumably by hot electrons with initial velocities of 0.55 times the speed of light (relativistic kinetic energy of ~100 keV). Such velocities are unusual for condensed-matter excitation at the stated laser intensities.

Simulation of PAHs in Trapped-Vortex Combustor

Residence time and thermo-chemical environment are important factors in determining soot-formation characteristics of jet engine combustors. For understanding the chemical and physical structure of the soot formed in these combustors knowledge on flow dynamics and formation of polycyclic aromatics-hydrocarbons (PAHs) is required. A time-dependent, detailed-chemistry computational-fluid-dynamic (CFD) model is developed for the simulation of the reacting flows in a trapped-vortex combustor. The axisymmetric trapped-vortex combustor of Hsu et al. was modeled by replacing injection holes with injection slots. Ethylene-air mixtures were used as fuel. Several calculations were made by varying the equivalence ratio and velocity of the main flow. Unsteady simulations revealed that the shearlayer vortices established outside the cavity flow enhance mixing of benzene in the wake region of the afterbody. However, in all the cases considered here, majority of the PAH species are produced in the cavity region. While fuel-rich condition resulted lower amounts of PAHs in the cavity region, soot is produced more in this region.© 2004 ASME

Vortex-Flame Interactions in Opposing- Jet Premixed Flames

A time-dependent CFDC code incorporating detailed chemical kinetics for methane combustion was developed for the simulation of local and temporal quenching process that is observed during a vortex-flame interaction in a premixed flame. The chemistry model was validated by simulating a weakly strained axisymmetric counter-flow premixed flame and by comparing the results obtained with GRI Version 1.2 mechanism. Calculations were made for the vortex-flame interactions in the counterflow flame for different injection velocities. The resulting changes to the flame structure along the stagnation line during a vortex-flame interaction are studied.

NOx in Methane-Air Jet Diffusion Flames

Dynamic simulations for NOx formation in an unsteady laminar, methane jet flame are made using an axisymmetric, time-dependent CFDC code and a detailed-chemical-kinetics model. Due to the buoyancy-induced instability vortical structures developed outside the flame surface and caused the flame to wrinkle. These simulations indicate that the temperature of the flame increases locally when it is compressed by the outer vortex and decreases when it is stretched. These effects are similar to those observed in a hydrogen diffusion flame and are attributed to the local nonunity Lewis numbers. Previous studies on dynamic hydrogen flames further reveled that the concentration of NO increases significantly in the compressed flame regions where temperature increases. For understanding the flame stretching and compression effects on the production of NO in methane diffusion flames calculations are made using different NOx chemistry models. It is observed that the thermal NO increases in the compressed flame regions; however, the flame stretch effect seems to be weak on the total NO (thermal + prompt) production.