Crossflow Momentum Flux Ratio Research Articles

The breakup and instability structures of liquid jets in subsonic crossflows

The breakup of water and Jet A fuel jets injected transverse into elevated temperature (up to 425 °C), combustion pre-heated, subsonic crossflows was investigated by internally fluorescing the liquid jet column. The fluorescent dye seeded test liquids were illuminated by a pulsed laser light that was transmitted within the injector using a quartz rod light-guide. High-resolution images isolating the intact liquid column from light scattered by the surrounding dense spray facilitated precise measurement of the liquid column breakup position and the size and distribution of windward column instability waves. The injector geometry consisted of a 1 mm circular orifice with an orifice length of 20 mm with a tapered inlet. Liquid jet to gaseous crossflow momentum flux ratios were varied from 6 to 266 and the gas Weber number was varied from 10.7 to 228. Empirical correlations were derived for the liquid column breakup coordinates and breakup time in addition to the average and local maximum instability wavelength and amplitude on the liquid column windward surface. The reported correlations provided key insight into atomization physics and aim to support the quantitative validation of high-fidelity numerical simulations of the liquid jet in subsonic crossflow.

Droplet evaporation of water and fuel jets in low and high temperature subsonic crossflows

Water and Jet A fuel were injected into low and high temperature crossflows of 65 °C and 425 °C, respectively, at a fixed crossflow pressure of 100 kPa, a liquid jet to gaseous crossflow momentum flux ratio of 7.5, and a crossflow Mach number of 0.175. Liquid jet injection temperature was also varied between 20 °C and 90 °C. The orifice had a diameter of 1 mm and a length of 20 mm. Visualization of the spray primary breakup processes was achieved using high-speed imaging. Droplet characteristics were measured at cross-sectional planes 300 mm, 400 mm, and 500 mm downstream of the orifice using a 2-component phase Doppler particle anemometry system. Results discussed included the variation of liquid volume flux, droplet size, and droplet velocities with increasing distance downstream of the injector. Analysis of the results revealed distinct changes in local and global droplet characteristics for both test liquids when injected into the high temperature crossflow compared to the low temperature crossflow. These changes, including increases in measured droplet size, were more significant at the bottom of the spray plume, especially for Jet A, due to the preferential heat-up and evaporation of droplets based on size and liquid thermal properties. For Jet A droplets smaller than 5 μm evaporated completely, leaving behind larger droplets that resisted evaporation. Increasing jet injection temperature was observed to reduce droplet size at a fixed crossflow velocity by reducing liquid surface tension which enhanced atomization. Higher liquid jet injection temperatures also resulted in increased droplet evaporation that initiated closer to the orifice exit.

Numerical study of a sonic jet in a supersonic crossflow over a flat plate

A sonic circular injector discharging into a Mach 1.6 freestream over a flat plate with a jet to crossflow momentum flux ratio of 1.73 is investigated numerically using a three-dimensional Reynolds-averaged Navier–Stokes equation. Menter’s shear stress transport k–ω turbulence model is employed to understand the complex flow features associated with jet–freestream interaction. The validation of the numerical solution is achieved by comparing the velocity and flat plate surface pressure measurements from the experiments, and the numerical solution shows good agreement with the experimental data. The present work emphasizes the flow field studies that include the identification of shocks, recirculation zones, and vortex structures. The Omega (Ω) vortex visualization method is employed for identifying the vortex structures. Comparison with high Mach number freestream conditions (M∞ = 2 and 4) shows that the vortex structures remain the same, irrespective of the freestream Mach number. A close analysis of the jet near-field shows several new vortex structures, including a secondary surface trailing vortex. The formation of each of these vortices lacks clarity to date. Considering the complex three-dimensional nature of the flow field, an attempt has been made to trace the formation of the vortex structures associated with a jet in supersonic crossflow.

Investigation of the mixing characteristics in a transverse hydrogen injection combustor with an inlet compression ramp

A shock-enhanced mixing in a transverse hydrogen injection combustor with an inlet compression ramp is carried out by using Large-eddy simulation (LES). Effects of a shock train induced by the inlet compression ramp on the mixing process have been investigated at three jet to cross-flow momentum flux ratios, J. The counter-rotating vortex pairs (CVP), promoting the mixing process of the fuel jet plume and mainstream air, is significantly affected by the reflected shock. The vorticity analysis is constructed to further understand the turbulent mixing mechanism. The shock-induced baroclinic torque is found to play an important role on the generation of the vorticity in the near field of the fuel jet, and the place where the reflected shock interacts with the jet plume. In addition, the probability density function (PDF) of mixture fraction is also investigated.

Characteristics and mixing enhancement of a self-throttling system in a supersonic flow with transverse injections

A three-dimensional self-throttling system is proposed in a scramjet combustor with transverse fuel jet, and investigated by Reynolds-averaged Navier-Stokes (RANS) simulations with the k-ω SST turbulence model. Numerical validation has been carried out against experiment and LES results. The effects of the jet-to-cross-flow momentum flux ratio and the throttling angle on mixing performance, fuel jet penetration depth and total pressure losses are all addressed. Through the proposed throttling system, the higher pressure upstream of the transverse fuel injection can drive part of the low momentum mainstream air into the downstream lower pressure region. The flow structures and the interactions between the shock waves and boundary layer are significantly changed to improve the mixing performance. The enhancement of mixing efficiency in the self-throttling system is closely related to the magnitude of the jet to crossflow momentum flux ratio, and a smaller throttling angle is found to further improve the mixing. On the other hand, the self-throttling system has a good performance in reducing the total pressure losses.

The effects of nozzle geometry and equivalence ratio on a premixed reacting jet in vitiated cross-flow

The effects of nozzle geometry, jet equivalence ratio (ϕj), and momentum flux ratio (J) on the flow field, stability characteristics, and flame topology of a premixed ethylene–air jet injected transverse to a vitiated cross-flow are investigated experimentally. Cross-flow conditions of 900 K and 100 m/s were chosen to simulate the environment of a secondary combustor in a staged combustion system. The dependence of the flame liftoff height on J suggests that at these conditions the flame stabilization process is flame-propagation controlled rather than autoignition assisted. A circular nozzle and high aspect ratio slotted nozzle of identical exit area were investigated for jet to cross-flow momentum flux ratios ranging from 5 to 65 for jet equivalence ratios of up to ϕj = 5.0. High-speed particle image velocimetry was utilized to study the time-averaged flow field and OH* chemiluminescence was used to capture time-averaged and instantaneous features of the flame behavior. The nozzle geometry was determined to have a significant effect on RJICF flame stability, with substantially expanded blow-out limits for the slotted nozzle. Enhanced operability of the high aspect ratio slotted nozzle was shown to be attributable to the substantially larger and stronger recirculation zone on the leeward side of the jet when compared to the circular nozzle. This area is characterized by a more disperse region of elevated vorticity levels, resulting in the entrainment of more hot combustion products with a longer residence time in the recirculation zone, which in turn provides a stronger and more stable ignition source to the oncoming, unburned reactants. A correlation for the isothermal JICF trajectory was modified to account for gas expansion effects and found to satisfactorily capture the jet trajectory for both the non-reacting and reacting slotted nozzle. The jet trajectory was demonstrated to be independent of ϕj, whereas the jet flame penetration decreases as ϕj increases, indicating that where the flame situates itself is determined by the local mixture concentration rather than changes in the flow field. Nevertheless, ϕj was also found to have an effect on the mean flow field, with slightly higher magnitudes of reversed flow velocity and an increase in mean recirculation zone length observed as the fuel content of the jet is increased.

Study of sonic injection from circular injector into a supersonic cross-flow using large eddy simulation

Large-eddy simulations have been performed to investigate the flow coherent structures and mixing characteristics in a Scramjet combustor having a sonic hydrogen injection from circular injector into a supersonic cross-flow with three different jet to cross-flow momentum flux ratios. The numerical results show that the jet penetration is not only influenced by momentum flux ratio but also significantly influenced by reflected shocks due to the upper wall. A bending down location of jet penetration could be observed just near the place where reflected shock interacts with the jet shear layer. Mixing efficiency was used to the quantitative analysis of the mixing process. In addition, the bow shock formed in the near field heats the mixture of air and hydrogen and leads to nonlinear distribution of temperature filed in mixture fraction space, which has the same logarithmic form in mixture space under the three momentum ratios with the same inlet stream Mach number.

Inverse diffusion and partially premixed flames with elliptical/swirling- and cross-flows

The combustion performance of inverse diffusion and partially premixed flames was optimized by employing concentric elliptical ports with variable relative angular orientation, co-swirling, and cross-flow opposing jets of different inclination angles. The flexible and favorably active control of combustion was pronounced by changing the nozzle relative angular orientation. The optimum angular shift was found to be 20° where the flame length was shortened by 39% from the case of 0° and the NOx levels were reduced to about 0.51 g/kg of fuel for the inverse partially premixed flames. At such optimum angular shift, the minimum CO and NOx emissions respectively decreased to 17.1 and 1.52 g/kg of fuel for the inverse diffusion flames. The elliptical jets increased the size of the swirling flow recirculation zone by about 28%. Increasing the inner aspect ratio consistently increased the combustion efficiency, where the CO emissions reach minimum values of 12.3 and 6.22 g/kg of fuel respectively for the inverse diffusion and partially premixed flames. The correspondingly minimum NOx emissions were thus 0.89 and 0.51 g/kg of fuel at a cross-flow momentum flux ratio of 2.0. Splitting the opposing cross-flow air jets by increasing their inclination angle above 0° duplicated the stagnation zone vortical impact to enrich the average turbulent energy at the major axis tips. The inclination angle of 15° was the optimum at no swirl while increasing the outer swirl angle shifted the optimum inclination angle to 25°. The inner/outer co-swirl of 45°/60° thus provided a minimum CO emission of 11.1 g/kg of fuel for the inverse diffusion flame and 2.65 g/kg of fuel for the inverse partially premixed flame.

Numerical Study of NO<sub>x </sub>Emissions in RQL Gas Turbine Combustor

RNG (Renormalization Group) k-ε turbulent model was applied to the numerical simulation of turbulent mixing processes in the RQL gas turbine combustor, and SIMPLE algorithm was used to solve the finite difference equations. The calculated conclusions were used to analyze temperature distribution of the mixed flow field and near-wall region of the flow field, and then discuss the NOx emissions. The results show that the effect of the injector zone geometry and the jet to crossflow momentum flux ratios on the NOx emissions is obvious. The reasonable control of jet is beneficial to reduce the local high temperature area and is able to improve the distribution of the exit temperature. And then achieve the goal of reducing the environmental pollution.

Large-Eddy-Simulation Modeling for Aerothermal Predictions Behind a Jet In Crossflow

Numerical investigations of a hot square jet in crossflow issuing perpendicularly to a flat plate are presented. This current work is initiated by Airbus Industries to better predict the aircraft aerothermal environment. Advanced numerical methods are tested in this specific context. In particular, the numerical approaches of large eddy simulations and Reynolds-averaged Navier-Stokes simulations are assessed in an industrylike configuration. Specific attention is brought to the large eddy simulations of a jet in a crossflow momentum-flux ratio of 0.77 and a Reynolds number of 93,900 (based on crossflow velocity and jet diameter). Numerical issues such as grid resolution and boundary condition choices are addressed. Comparison of the Reynolds-averaged quantities obtained from large eddy simulations with Reynolds-averaged Navier-Stokes simulation data at several cross-stream locations underlines the potential of large eddy simulations for aerothermal predictions. Large-scale motions observed experimentally and numerically are reproduced by the unsteady large-eddy-simulation fields, and spectral analyses reveal the ability of the large eddy simulations to reproduce the coherent structure frequencies.

Tomographic flow structures of a round jet in a crossflow

Flow structures in the near wake of an elevated jet in a crossflow at a large jet‐to‐ crossflow momentum flux ratio are experimentally studied in a wind tunnel. A laser‐light sheet is employed to illuminate fine smoke tracers for flow visualization. A laser Doppler velocimeter is used to measure the velocity field. Flows on the vertical symmetry plane and three horizontal planes across jet‐wake, jet‐exit, and tube‐wake regions are investigated. Smoke patterns show qualitatively apparent differences in different regions of the flow field. The measured velocity fields present quantitative characteristics of the streamlines, vorticities, and topological features of the flow structures. The flow characteristics on the symmetry plane show two bifurcation lines separated by a source point in the jet and tube wakes, respectively. Upstream from the bifurcation lines, reverse flows are observed. While downstream from the bifurcation lines, flows go directly forward. The flow structures and vorticity distributions on the horizontal planes show non‐closed recirculation zones of different topological structures in different characteristic flow regions, i.e., jet‐wake, jet‐exit, and tube‐wake. Details of the flow structures and differences in the jet and tube wakes are presented and discussed.

Comparison of the Flame Characteristics of Turbulent Circular and Elliptic Jets in a Crossflow

This study was directed to understand the coupling effects of the noncircular geometry of the burner and a crossflow on the combustion of gas jets. This paper compares the characteristics of turbulent propane jet flames from circular (diameter=0.45 cm) and elliptic (major axis/minor axis=3) burners of equivalent exit area in a crossflow. The elliptic burner was oriented with its major axis or minor axis aligned with the crossflow. Experiments were conducted in a wind tunnel provided with optical and probe access and capable of wind speeds up to 12.5 m/s. The burners were fabricated with metal tubes. Instrumentation included a Pt-Pt/13% Rh thermocouple, a quartz-probe gas sampling system, chemiluminescent and nondispersive infrared analyzers, a video-recorder, and a computer data acquisition system. The measurements consisted of the upper and lower limits of jet velocity for a stable flame, flame configuration, and visible length. Flame structure data including temperature profiles and concentration profiles of CO2,O2, CO, and NO were obtained in a two-zone flame configuration (at jet to crossflow momentum flux ratio=0.11), where a planar recirculation exists in the wake of the burner tube followed by an axisymmetric tail. The relative emission indicators of CO and NO were estimated from the composition data. Results show that the upper and lower limits of the fuel jet velocity increase with the crossflow velocity for all burners, and the rate of increase is highest for the elliptic burner with its minor axis aligned with the crossflow. That burner configuration also produces the longest flame. The relative emission indicators show that the CO production is lower and NO production is higher with elliptic burners than with circular burners in crossflow.

Burner Wake Stabilized Gas Jet Flames in Cross-Flow

ABSTRACT An experimental study of the stability and structure of a propane gas jet flame in cross-flow at jet to cross-flow momentum flux ratio less than unity is presented. When this ratio is 0.17 the flame structure is characterized by three distinct zones (Type II flame). A planar recirculation vortex attached to the burner tube in its wake forms the first zone. A second axisymmetric flame follows the first zone downstream. The junction of the two zones is characterized by a third zone with an intense mixing of jet and cross-flow streams. When the jet to cross-flow momentum flux ratio is 0.02 the axisymmetric part of the flame is extinguished (Type I flame). Temperature and composition profiles at different regions of these flames indicate that much of the Type II flame is akin to a premixed flame and the Type I flame is diffusion controlled.