Injectant Mole Fraction Research Articles

Influences of Angles of Attack and Sideslip on the Flow Field of a Cantilevered Ramp Injector in Supersonic Flows

The cantilevered ramp injector has been employed as an effective fuel injection strategy in the shock-induced combustion ramjet engine, and its flow field characteristics have attracted increasing attention. Three-dimensional ReynoldsAveraged Navier-Stokes (RANS) equations coupled with the RNG k- turbulence model have been employed to investigate the flow field of a cantilevered ramp injector in a freestream with a Mach number of 6.0. The effects of the angles of attack and sideslip on the flow field of a cantilevered ramp injector have been studied, and the predicted axial distribution of the maximum injectant mole fraction shows good agreement with the available experimental data in public literature. The obtained results show that the case with an angle of attack 3 � can promote the mixing process between the fuel and air most efficiently in a typical cantilevered ramp injector. However, it cannot prevent the premature ignition of the premixed combustible flow. The mixing process cannot be promoted when the normalized axial distance is larger than 7.55 in cases with different angles of sideslip, and larger angles of sideslip can reduce the effect of premature ignition for the premixed combustible flow.

Molecular weight and injector configuration effects on the transverse injection flow field properties in supersonic flows

The transverse injection flow field in the high speed conditions is more complex than that in the low speed, and more information should be explored to improve the overall performance of the airbreathing hypersonic propulsion system. The three-dimensional Reynolds-averaged Navier–Stokes (RANS) equations and the two equation SST k–ω has been employed to explore the influences of the molecular weight (hydrogen and nitrogen) and injector configuration (circular, square, diamond and equilateral triangular) on the mean flow field properties in the transverse injection strategy, and the wide range of the jet-to-crossflow pressure ratio (4.86, 10.29, 17.72 and 25.15) has been considered as well. The obtained results show that the low jet-to-crossflow pressure ratio can promote the mixing process between the injectant and the supersonic crossflow irrespective of the injectant species and the injector configuration, and the large molecular weight of the injectant can promote the mixing process as well when the jet-to-crossflow pressure ratio is fixed. The case with the equilateral triangular injector owns the highest mixing efficiency in the range considered in the current study, and it may be induced by the remarkable influence of the counter-rotating vortex pair. The injectant mole fraction decreases more rapidly with the increase of the streamwise distance for the case with the low jet-to-crossflow pressure ratio, and the jet plume spreads in the spanwise direction more rapidly for the case with the diamond injector irrespective of the injectant species.

Performance evaluation and parametric analysis on cantilevered ramp injector in supersonic flows

The cantilevered ramp injector is one of the most promising candidates for the mixing enhancement between the fuel and the supersonic air, and its parametric analysis has drawn an increasing attention of researchers. The flow field characteristics and the drag force of the cantilevered ramp injector in the supersonic flow with the freestream Mach number 2.0 have been investigated numerically, and the predicted injectant mole fraction and static pressure profiles have been compared with the available experimental data in the open literature. At the same time, the grid independency analysis has been performed by using the coarse, the moderate and the refined grid scales, and the influence of the turbulence model on the flow field of the cantilevered ramp injector has been carried on as well. Further, the effects of the swept angle, the ramp angle and the length of the step on the performance of the cantilevered ramp injector have been discussed subsequently. The obtained results show that the grid scale has only a slight impact on the flow field of the cantilevered ramp injector except in the region near the fuel injector, and the predicted results show reasonable agreement with the experimental data. Additionally, the turbulence model makes a slight difference to the numerical results, and the results obtained by the RNG k−ε and SST k−ω turbulence models are almost the same. The swept angle and the ramp angle have the same impact on the performance of the cantilevered ramp injector, and the kidney-shaped plume is formed with shorter distance with the increase of the swept and ramp angles. At the same time, the shape of the injectant mole fraction contour at X/H=6 goes through a transition from a peach-shaped plume to a kidney-shaped plume, and the cantilevered ramp injector with larger swept and ramp angles has the higher mixing efficiency and the larger drag force. The length of the step has only a slight impact on the drag force performance of the cantilevered ramp injector. However, it makes a difference to the flow field in the vicinity of the fuel injector, and the subsonic region becomes narrower with the increase of the length of the step.

Flowfield characteristics of a transverse jet into supersonic flow with pseudo-shock wave

We performed an experimental investigation of the flowfield of a transverse jet into supersonic flow with a pseudo-shock wave (PSW). In this study, we injected compressed air as the injectant, simulating hydrocarbon fuel. A back pressure control valve generated PSW into Mach 2.5 supersonic flow and controlled its position. The positions of PSW were set at nondimensional distance from the injector by the duct height (x/H) of −1.0, −2.5, and −4.0. Particle image velocimetry (PIV) gave us the velocity of the flowfield. Mie scattering of oil mist only with the jet was used to measure the spread of the injectant. Furthermore, gas sampling measurements at the exit of the test section were carried out to determine the injectant mole fraction distributions. Gas sampling data qualitatively matched the intensity of Mie scattering. PIV measurements indicated that far-upstream PSW decelerated the flow speed of the main stream and developed the boundary layer on the wall of the test section. The flow speed deceleration at the corner of the test section was remarkable. The PSW produced nonuniformity in the main stream and reduced the momentum flux of the main stream in front of the injector. The blowing ratio, defined as the square root of the momentum flux ratio, of the jet and the main stream considering the effect of the boundary layer thickness was shown to be a useful parameter to explain the jet behavior.

Extended Quantitative Fluorescence Imaging for Multicomponent and Staged Injection into Supersonic Crossflows

The fluorescence ratio technique for processing planar laser-induced fluorescence data was extended for quantitative imaging of the injectant mole-fraction and density in multicomponent and multiinjection configurations in a nonreacting supersonic mixing flowfield. The method was experimentally validated in a mixing flowfield formed by single or staged sonic transverse injection into a Mach 2.0 supersonic air stream. Gaseous helium and air were used as injectants. Injectant mole-fraction distributions obtained by the present method were confirmed by comparison with gas-sampling data. Density distributions were validated with the oblique shock relation. The new method worked well for the multistaged injection configuration. Phase-locked measurement for the pulsed secondary injection was also tested. Results demonstrated the applicability of the new method to the pulse injection as well.

Quantitative Imaging of Injectant Mole Fraction and Density in a Supersonic Mixing

The fluorescence ratio technique for processing planar laser-induced fluorescence data was generalized for quantitative imaging of the injectant mole fraction and extended to quantify the density distributions in a nonreacting supersonic mixing flowfield. The original fluorescence ratio approach was first developed by Hartfield et al. (Hartfield, R. J., Jr., Abbitt, J. D., III, and McDaniel, J. C., Injectant Mole Fraction Imaging in Compressible Mixing Flow Using Planar Laser-Induced Iodine Fluorescence, Optics Letters, Vol. 1, No. 16, Aug. 1989, pp. 850-852.) for tests in a special closed-loop wind tunnel to eliminate the effects of thermodynamic property variations on planar laser-induced fluorescence signals in compressible flowfields. This approach provided us a quantitative means of planar mole-fraction measurement; however, it implicitly assumed that the tracer molecules were seeded at the same fraction in both the main and the secondary flows. In the present study, we generalized the Hartfield et al. method by considering differences in the tracer-seeding rates for obtaining planar images of mole fraction and density. Experimental validation of the new method was carried out in a mixing flowfield formed by sonic transverse injection into a Mach 1.9 supersonic airstream. The injectant mole-fraction distribution obtained from planar laser-induced fluorescence data processed by our new approach showed better agreement with the gas-sampling data than one based on the Hartfield et al. method. The density distribution was verified by comparison with the theoretical density ratio across the oblique shock wave.

Validation of the Wilcox k-omega Model for Flows Characteristic to Hypersonic Airbreathing Propulsion

Numerical results obtained with the WARP code solving the 1988 Wilcox k‐ω two-equation model are compared with experimental data of flowfields characteristic of hypersonic airbreathing propulsion. The problems chosen for the comparison include a shock/boundary-layer interaction case, a reacting and inert planar mixing case, and two ramp injector mixing cases. In addition, a comparison is performed with empirical correlations on the basis of skin friction for flow over a flat plate and shear layer growth for a free shear layer. The agreement between the numerical and experimental results varies between being reasonable and excellent, with a discrepancy generally not exceeding 20%. It is found that the grid-induced error can be reduced to an acceptable level for most problems with a reasonable mesh size. However, the free shear layer and the shock/boundary-layer interaction cases require a considerably finer mesh. The Wilcox dilatational dissipation correction is seen to be beneficial in predicting the growth of a free shear layer at a high convective Mach number, but its use is considered either questionable or detrimental for the other cases. A proper choice of the turbulent Schmidt number is observed to be crucial in predicting the injectant mole fraction contours for one of the ramp injector cases, with the best agreement obtained with Schmidt number Sct fixed to 0.25. For the inert planar mixing case, overall better agreement is obtained when setting both the turbulent Prandtl and Schmidt number to 0.5.

Computer-controlled multiparameter flowfield measurements using planar laser-induced iodine fluorescence

A measurement technique for the mapping of complex compressible flowfields using planar laser-induced iodine fluorescence (PLIIF) is presented. Time-averaged values of all relevant flowfield properties are found : pressure, temperature, velocity, and, for mixing flowfields, injectant mole fraction. Several modifications have been made to a previous version of the PLIIF technique that allow multiple planes of data, needed for studying highly three-dimensional flowfields, to be collected and processed in greatly reduced time and with improved measurement accuracies. The technique is demonstrated with measurements in the flowfield of a Mach 2, rearward-facing step. The technique presented represents a unique tool that allows complex, compressible, three-dimensional flowfields to be mapped out completely and nonintrusively.

Complete three-dimensional multiparameter mapping of a supersonic ramp fuel injector flowfield

Planar laser-induced iodine fluorescence is used to map out the nonreacting mixing flowfield of an unswept ramp fuel injector using air injected at Mach 2.0 into a Mach 2.9 freestream. A fully automated test setup is used to measure time-averaged pressure, temperature, velocity, and injectant mole fraction on 21 crossflow planes and 7 axial planes. The measurement uncertainties are 5-8% for temperature, 4-10% for pressure, 10-20 m/s for velocity, and 2-3% for injectant mole fraction depending on the thermodynamic conditions. The measurements allow any desired gasdynamic quantity to be determined on a three-dimensional grid that spans the entire wind-tunnel test section. The experimental data set is comparable to the completeness of results normally available only from a computational fluid dynamics simulation. Results showing detailed flow features on specific planes, as well as overall quantities, such as global conservation checks, mixing performance, and flowfield losses, are presented. Mass, momentum, and energy flux, determined at the crossflow plane locations of the data set, show about a 2% standard deviation. The results are compared to a simulation using a three-dimensional Navier-Stokes solver. Agreement is reasonable with the exception of measurements in regions very close to walls, where the intensity of scattered light is high or where optical access is limited. The ability to generate extensive data sets, such as the one presented here, demonstrates that the planar laser-induced iodine fluorescence technique can be used 1) to generate detailed test cases for the validation of computational fluid dynamics codes and 2) as an alternative to computational fluid dynamics for performing design studies and performance evaluation in complex compressible flows.

Experimental and numerical study of swept ramp injection into a supersonic flowfield

Time-averaged measurements of pressure, temperature, velocity, and injectant mole fraction are presented using the planar laser-induced iodine fluorescence technique in the complex three-dimensional compressible flowfield around a swept ramp fuel injector. Within the range of thermodynamic conditions present in the test case studied, the technique's accuracy is estimated to be 4% for pressure, temperature, and velocity and 3% for injectant mole fraction. Comparisons with numerical simulations using the SPARK three-dimensional Navier-Stokes computer code with an algebraic turbulence model are made at the centerplane of the flowfield as well as on three crossflow planes downstream of the injector. Calculations and measurements are in good agreement throughout the flowfield, with deviations on the order of 5%; however, in specific regions, such as in the base of the ramp, deviations are larger. A weak asymmetry in the incoming flowfield appears to be amplified by boundary-layer separation occurring when the ramp-generated shock reflects off the tunnel walls. Ramp-generated vortices are weaker in the calculated results due to the effects of numerical viscosity in the vortex cores. This leads to less turning and mixing of the jet plume than observed in the experiments. The rate of decay of the maximum injectant mole fraction with streamwise distance is greater for the present ramp injection scheme than for previously measured transverse injection schemes. In this recirculation region at the base of the injector, laminar calculations show better agreement with the measurements than turbulent calculations.

Quantitative investigation of compressible mixing - Staged transverse injection into Mach 2 flow

Planar measurements of the injectant mole fraction distribution and the velocity field within a supersonic mixing flowfield have been made using laser-induced iodine fluorescence. The flowfield investigated in this work is staged transverse injection of air into a Mach 2 freestream. A complete three-dimensional survey of the injectant mole fraction distribution has been generated, and a single planar velocity measurement has been completed. The measurements reveal the dramatic effect of streamwise vortices on the mixing in the near field of the injectors, as well as the rapid mixing generated by staging two fuel injectors. Analysis of the downstream decay of the maximum injectant mole fraction in this and other supersonic mixing flowfields indicates that the relative rate of injectant mixing well downstream of the injectors is independent of injection geometry, freestream Mach number, and injectant molecular weight. Mixing within this region of the flowfield is dominated by small-scale turbulence within the injectant plume. The transition of the dominant mixing mechanism, from vortex-driven mixing in the near field to small-scale turbulent mixing in the far field, was found to occur in the region about 10 diameters downstream of the injectors.

Experimental investigation of a supersonic swept ramp injector using laser-induced iodine fluorescence

Planar measurements of injectant mole fraction and temperature have been conducted in a nonreacting supersonic combustor configured with underexpanded injection in the base of a swept ramp. The temperature measurements were conducted with a Mach 2 test section inlet in streamwise planes perpendicular to the test section wall on which the ramp was mounted. Injection concentration measurements, conducted in cross flow planes with both Mach 2 and Mach 2.9 free stream conditions, dramatically illustrate the domination of the mixing process by streamwise vorticity generated by the ramp. These measurements, conducted using a nonintrusive optical technique (laser-induced iodine fluorescence), provide an accurate and extensive experimental data base for the validation of computation fluid dynamic codes for the calculation of highly three-dimensional supersonic combustor flow fields.

Mole-fraction imaging of transverse injection in a ducted supersonicflow

Laser-induced iodine fluorescence has been used to generate two-dimensional images of the mixing characteristics of air injected transversely as underexpanded jets behind a rearward-facing step into a ducted Mach 2 freestream; the images thus obtained were processed digitally in order to yield planar-injectant mole fraction distributions. The resulting planar images represent a three-dimensional data base of the injectant mole fraction distribution throughout the flowfield which is then used to reconstruct images exhibiting mole-fraction distributions normal to the duct. These images furnish a direct representation of the evolution of supersonic mixing along the duct, and facilitate the development of one-dimensional mixing schedules on the basis of the three-dimensional data base.

Injectant mole-fraction imaging in compressible mixing flows using planar laser-induced iodine fluorescence

A technique is described for imaging the injectant mole-fraction distribution in nonreacting compressible mixing flow fields. Planar fluorescence from iodine, seeded into air, is induced by a broadband argon-ion laser and collected using an intensified charge-injection-device array camera. The technique eliminates the thermodynamic dependence of the iodine fluorescence in the compressible flow field by taking the ratio of two images collected with identical thermodynamic flow conditions but different iodine seeding conditions. The resulting images are, to our knowledge, the first quantitative planar measurements of mole-fraction distributions in a nonreacting compressible flow field and allow mixing to be studied directly.