Abdollah S. Nejad

Breakup Processes of Liquid Jets in Subsonic Crossflows

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

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.

Comparison of Physical and Aerodynamic Ramps as Fuel Injectors in Supersonic Flow

An experimental investigation was conducted to compare the supersonic mixing performance of a novel e ush-wall aerodynamic ramp injector with that of a physical ramp injector. The aerodynamic ramp injector consists of nine e ush-wall jets arranged to produce fuel‐ vortex interactions for mixing enhancement in a supersonic crosse ow. Test conditions included a Mach 2.0 crosse ow of air with a Reynolds number of 3.63 10 7 per meter and helium injection with jet-to-freestream momentum e ux ratios of 1.0 and 2.0. Conventional probing techniques, including species composition sampling, were employed to interrogate the e owe eld at several downstream locations. Results show that with increasing jet momentum the aeroramp exhibited a signie cant increase in fuel penetration, whereas the physical ramp showed no discernible change. The near-e eld mixing of the aeroramp was superior to that of the physical ramp; however, the physical ramp reaches a fully mixed condition at approximately half the distance of the aeroramp. As the jet momentum was increased, the far-e eld mixing performance of the aeroramp approached that of the physical ramp. In all cases the total pressure loss incurred with the aeroramp was less than that caused by the physical ramp. For both injectors the total pressure loss decreased with increasing jet momentum. It was concluded that, although physical ramps may provide better far-e eld mixing, properly designed e ush-wall injection can provide comparable mixing performance while avoiding the practical problems associated with an intrusive geometry in a scramjet combustor.

EXPANSION AND MIXING PROCESSES OF UNDEREXPANDED SUPERCRITICAL FUEL JETS INJECTED INTO SUPERHEATED CONDITIONS

Theexpansionandmixingprocessesofunderexpandedsupercriticalfueljetsinjectedintosuperheatedconditions were experimentally studied. Ethylene was used as the fuel, and nitrogen was the ambient gas. The near-e eld jet plume structure was characterized by the location and size of the Mach disk and the expansion angle. The Mach disk location of the supercritical ethylene jet matches that of an ideal-gas jet. The size of the Mach disk and the expansion angle, however, increase as the injection temperatureapproaches the critical value. Thefar-e eld mixing processes were characterized by measuring fuel mole fraction and temperature distributions using spontaneous Raman scattering. Fuel mole fraction distributions follow a Gaussian function, whereas temperature distributions exhibit a dee cit inside the jet plume because of the expansion and acceleration of the fuel jet. As the injection condition approached the critical point, the following observations were made: 1 ) the ethylene centerline mole fraction increased, 2 ) the jet width at the stoichiometric level increased, 3 ) the jet width at half the maximum concentration remained the same, and 4 ) the temperature dee cit became more signie cant. These results were attributed to the larger injected fuel mass e ow and fuel condensation when the jet injection conditions approach the critical point.

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

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.

INJECTION OF SUPERCRITICAL ETHYLENE IN NITROGEN

The injection of supercritical fuel into a quiescent gas environment was experimentally studied to elucidate the effects of thermophysical and transport properties near the critical point on jet appearance, shock structures, and choking. Ethylene and nitrogen were used to simulate interactions between fuel and air. Conditions near the thermodynamic critical point of ethylene are considered, with supercritical temperatures and pressures upstream of the injector and subcritical pressures downstream of the injector. Flow visualization showed an opaque region resulting from fuel condensation when fuel was injected at near room temperature. At higher injectant temperatures, the ethylene jet was found to have a shock structure similar to that of an underexpanded ideal-gas jet. Mass flow rates were found to be insensitive to the variation of back pressure, indicating that the jet flow is choked. Mass flow rates were normalized by those values calculated for ideal-gas jets under the same conditions. The normalized mass flow rate first increases as injection conditions approach the critical temperature, apparently because of the rapid increase in fluid density, and then decreases, possibly as a result of the coexistence of liquid and gas phases at the nozzle exit.

Two-dimensional temperature field measurements using a molecular filter based technique

Abstract Filtered Rayleigh Scattering (FRS) has been investigated to determine the feasibility of the technique to obtain instantaneous two-dimensional temperature measurements in reacting flows. The laser frequency, of an injection seeded Nd:YAG laser, is tuned to an absorption line of iodine which is contained in an optical cell. The iodine filter is placed in front of an intensified CCD camera recording the scattered light. Background scattering from solid surfaces and particles is strongly absorbed by the iodine, while much of the Doppler broadened Rayleigh scattering is transmitted by the filter. The gas temperature can then be deduced from the measured transmission of the molecular Rayleigh scattering. Two different premixed flames were investigated, a hydrogen-air flame created using a Hencken burner and a methane-air flame. The accuracy of the FRS measurements was investigated by comparing FRS-derived temperatures with calculated values and temperatures recorded with coherent anti-Stokes Raman spe...

The Study of a Transverse Jet in a Supersonic Cross-Flow Using Molecular Filter Based Diagnostics

Transverse injection of sonic jets with circular and elliptical cross sections into a M = 1.98 flow have been experimentally investigated using Planar Doppler Velocimetry (PDV). The PDV technique illuminates the flow with the second harmonic of an injection seeded Nd:YAG laser which is tuned, in frequency, to an absorption line of iodine. By placing a pressure broadened iodine molecular filter in front of the camera the instantaneous velocity can be calculated from the transmission ratio. A unique feature of the current set-up is realized by retro-reflecting the incident laser so that the velocity component measured is in the streamwise direction. Streamwise velocities and turbulence intensities were measured upstream and downstream of the jet injection locations. The measurements show the velocity and turbulence in characteristic regions of the flow including the separation shock, bow shock, and the mixing layer of the jet as it develops into two counter rotating vortices. The elliptical jet showed a more rapid spanwise spreading of the shear layer and greater turbulence intensity, although it had a reduced lateral penetration compared to the circular jet.