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.

Breakup Processes of Liquid Jets in Subsonic Crossflows

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.

Spray Structures of Liquid Jets Atomized in Subsonic Crossflows

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.

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

An experimental investigation was conducted to study and characterize the effects of injection angle on the breakup processes of turbulent liquid jets in a subsonic crosse ow of air. With water as the test liquid, the injection angle, freestream Mach number, and injection velocity were varied over a wide range to provide an extensive databaseofexperimentalresults.Pulsedshadowgraphphotographywasemployedtoascertaincolumntrajectories, column fracturelocations,and near-e eld spray characteristics.Resultsindicatethat column breakup behavior can be divided into two distinct regimes: aerodynamic and nonaerodynamic. Liquid column fracture locations were found to be governed by length scales, which depend on the corresponding breakup regime. For aerodynamic breakup, the column length scale was derived from thetimescale fortheanalogous process oftheaerodynamic secondary breakup ofadroplet.Fornonaerodynamicbreakup,thecolumnlengthscalewasderivedfromthetimescale for the breakup of a turbulent liquid jet issuing into a quiescent gas. A breakup regime parameter was dee ned to determine, based on jet operating conditions, the prevalent breakup regime and, therefore, the appropriate column length scale. Liquid column trajectories were correlated with an effective jet-to-freestream momentum e ux ratio and transverse injection angle by applying a force balance and momentum analysis. Comparisons between experimental data and analytical predictions are presented and show excellent agreement in most cases.

Effects of Injection Angle on Atomization of Liquid Jets in Transverse Airflow

Flows behind bluff bodies are unsteady for all but very small Reynolds numbers and are characterized by the presence of vortex shedding. Accurate prediction of time-mean reattachment length and downstream mixing of turbulent flow behind bluff bodies continues to be a challenging task for the CFD community. In the present paper, turbulent flow behind a triangular bluff body has been computed using several turbulence models as well as the Large Eddy Simulation technique. Results from the simulations have been compared with experimental data at a Reynolds number of 42,000. Influence of factors such as unsteadiness, three-dimensionality and effective viscosity on flow reattachment and mixing are discussed in the paper.

Unsteady Simulations of Turbulent Flow Behind a Triangular Bluff Body

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.

S. A. Spring

Papers

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