Charbel N. Raffoul

Bio: Charbel N. Raffoul is an academic researcher. The author has contributed to research in topics: Reynolds stress & Vortex shedding. The author has an hindex of 3, co-authored 3 publications receiving 19 citations.

An Experimental and Numerical Study of the Isothermal Flowfield Behind a Bluff Body Flameholder

Abstract: An experimental and numerical investigation was conducted to study the turbulent velocities and stresses behind a two-dimensional bluff body. Simultaneous three-component laser-Doppler velocimeter (LDV) measurements were made in the isothermal incompressible turbulent flowfield downstream of a bluff body placed at midstream in a rectangular test section. Mean velocities and Reynolds stresses were measured at various axial positions. Spanwise velocity measurements indicated that the flow is three dimensional in the recirculation zone of the bluff body. Confidence in the accuracy of the data was gained by calculating the mass fluxes at each axial station. These were found to agree with each other to within ±3 percent. A parallel Computational Fluid Dynamics (CFD) study was initiated to gage the predictive accuracy of currently available CFD techniques. Three solutions were computed: a two-dimensional steady-state solution using the standard k-∈ model, a two-dimensional time-accurate solution using the standard k-∈ model, and a two-dimensional time-accurate solution using a Renormalized-Group (RNG) k-∈ turbulence model. The steady-state solution matched poorly with the data, severely underpredicting the Reynolds stresses in the recirculation zone. The time-accurate solutions captured the unsteady vortex shedding from the base of the bluff body, providing a source for the higher Reynolds stresses. The RNG k-∈ solution provided the best match to the data.

An Experimental and Numerical Study of the Isothermal Flowfield Behind a Bluff Body Flameholder

Abstract: An experimental and numerical investigation was conducted to study the turbulent velocities and stresses behind a 2-D bluff body. Simultaneous three-component laser Doppler velocimeter (LDV) measurements were made in the isothermal incompressible turbulent flowfield downstream of a bluff body placed at midstream in a rectangular test section. Mean velocities and Reynolds stresses were measured at various axial positions. Spanwise velocity measurements indicated that the flow is three dimensional in the recirculation zone of the bluff body. Confidence in the accuracy of the data was gained by calculating the mass fluxes at each axial station. These were found to agree with each other to within ±3%. A parallel Computational Fluid Dynamics (CFD) study was initiated to gauge the predictive accuracy of currently available CFD techniques. Three solutions were computed: a 2-D steady-state solution using the standard k-e model, a 2-D time-accurate solution using the standard k-e model, and a 2-D time-accurate solution using a Renormalized-Group (RNG) k-e turbulence model. The steady-state solution matched poorly with the data, severely underpredicting the Reynolds stresses in the recirculation zone. The time-accurate solutions captured the unsteady vortex shedding from the base of the bluff body, providing a source for the higher Reynolds stresses. The RNG k-e solution provided the best match to the data.Copyright © 1995 by ASME

Three-Component Velocity Measurements Downstream of a Bluff-Body Flameholder.

Abstract: : In order to enhance fuel and air mixing for more efficient combustion, it has been suggested to use bodies that generate vortical and turbulent structures in their wake. The injected fuel from the surface of in-stream mounted fuel injectors will interact with the turbulent flow and thoroughly mix with the freestream air. Simple 2-d and 3-d bluff bodies have been the subject of experimental investigations for a number of years. However, the opportunity has never before existed for the full exploration of the flowfield downstream of such bodies. The introduction of laser Doppler velocimetry (LDV) must be credited for more accurate measurements in the area of fluid mechanics in general, and in particular in the recirculating and complex flows such as the case of this study. In order to establish a baseline frame of reference for future work employing bluff bodies with vortex generators, the flow characteristics around a 2-d bluff body were experimentally investigated in this effort. Three-component velocity measurements using LDV were made in the highly turbulent flow past a 2-d bluff body. The contribution of this study is unique as all three velocity components were simultaneously measured at each point without introducing disturbances to the flow. Vertical data profiles were obtained at 11 axial stations in the far-field and at 8 axial stations in the near-field. Spanwise profiles of the flowfield immediately downstream of the bluff body were also determined. The three orthogonal mean velocities, the six Reynolds stresses and all nine turbulent triple products were estimated. The production, convection, and diffusion of turbulent kinetic energy were computed directly from the experimental data using numerical differencing.

Breakup Processes of Liquid Jets in Subsonic Crossflows

Abstract: The breakup processes of liquid jets injected into subsonic air crosse ows were experimentally studied. Test liquids, injector diameters, and air Mach numbers were varied to provide a wide range of jet operation conditions. Results indicate that for larger injection velocity conditions liquid jets penetrate relatively far into the crosse ows and exhibit surface breakup processes before the column breaks. Liquid column trajectories were correlated by liquid/air momentum e ux ratios based on a force analysis of a cylindrical liquid element subjected to an aerodynamic drag force. Drag coefe cients were inferred from the column trajectories and were found to exhibit a weak dependence on liquid viscosity. The heights of the column fracture points were correlated using the time required for an analogous droplet to complete an aerodynamic secondary breakup process. The success of the resulting correlation justie es the assumption that the aerodynamic forces acting on a droplet and those acting on a liquid column have similar effects. This result, combined with the trajectory correlation, leads to the conclusion that the liquid column always breaks at the same streamwise location, in agreement with the present experimental observation.

Spray Structures of Liquid Jets Atomized in Subsonic Crossflows

Abstract: The structures of spray plumes from 0.5-mm waterjets injected into a subsonic crosse ow were experimentally investigated using phase Doppler particle anemometry. Droplet size, axial velocity, and volume e ux were measured across the spray plume at several axial distances downstream of the injector exit. Results indicate that large droplets can be found in the central portion of the spray plume for cases with small liquid/air momentum e ux ratios and in which the momentum exchange between column waves and the airstream is signie cant. For cases with large-momentum e ux ratios, the droplet size distribution exhibits a concave-layered structure, with the peak on the centerline and large droplets at the top. Droplets were found to concentrate in a small area within the spray plume, which indicates that the liquid mass distribution is not uniform. The height of the maximum volume e ux locations, an indicator of the location of the highest concentration of droplets, was measured and correlated with momentum e ux ratios and axial distances. It was found that more droplets are distributed toward the upper portion of the spray plume for larger momentum e ux ratios. Spray penetration, spray width, penetration-to-width ratio, and spray cross-sectional area were also found to increase with the momentum e ux ratio.

Development Needs for Advanced Afterburner Designs

Abstract: The afterburner on a modern aircraft gas turbine engine provides significant thrust augmentation critical to the performance and mission of tactical aircraft. Higher exhaust temperatures and survivability requirements for advanced aircraft have resulted in new constraints on the augmentor design which dramatically changes the design architecture. Many of the design methods established for afterburners over the past 50 years are insufficient for the augmentor configurations being developed today. This paper will describe some of the fundamental technical challenges being faced by augmentor designers, and will outline critical needs in terms of fundamental combustion sciences and engineering that needs to be acquired to support new design methods for advanced augmentors.

The influence of temperature ratio on the dynamics of bluff body stabilized flames

Abstract: The dynamics of bluff body stabilized flames are investigated using an unsteady, Lagrangian simulation technique . This paper presents an analysis of results from a parametric study of temperature ratio across a flame stabilized on a bluff body confined in a duct . The simulation presented in this study is a two -dimensional description of a premixed inflow of reactants with flame sheets attached to a triangular wedge. The analysis concentrates on flow field dynamics related to the transition from the asym metric Von Karman shedding found in non -reacting bluff body flows to independent shear layer vorticity commonly observed in many reacting flows. Details of this transition are described in terms of the instantaneous vorticity field, as well as mean and uns teady components of velocity and temperature. It is shown that for relatively small temperature differences across the flamesheets, the Von Karman shedding mode remains dominant, although the strength of the shed vortices decreases with increasing temperat ure ratio. As the temperature ratio increases, the Von Karman shedding mode is completely suppressed, and this suppression is clearly evident through RMS temperature profiles as well as turbulent stresses near the bluff body.

Breakup processes of liquid jets in subsonic crossflows

Abstract: The breakup processes of liquid jets injected into subsonic air crosse ows were experimentally studied. Test liquids, injector diameters, and air Mach numbers were varied to provide a wide range of jet operation conditions. Results indicate that for larger injection velocity conditions liquid jets penetrate relatively far into the crosse ows and exhibit surface breakup processes before the column breaks. Liquid column trajectories were correlated by liquid/air momentum e ux ratios based on a force analysis of a cylindrical liquid element subjected to an aerodynamic drag force. Drag coefe cients were inferred from the column trajectories and were found to exhibit a weak dependence on liquid viscosity. The heights of the column fracture points were correlated using the time required for an analogous droplet to complete an aerodynamic secondary breakup process. The success of the resulting correlation justie es the assumption that the aerodynamic forces acting on a droplet and those acting on a liquid column have similar effects. This result, combined with the trajectory correlation, leads to the conclusion that the liquid column always breaks at the same streamwise location, in agreement with the present experimental observation.