Supersonic Air Stream Research Articles

Numerical investigation on staged sonic jet interaction mechanism in a supersonic cross flow

The staged injection scheme has drawn an increasing attention for the airbreathing hypersonic propulsion system, and the fuel injection angle has a large impact on the mixing improvement between the fuel and the supersonic cross flow. The Reynolds-averaged Navier–Stokes equations associated with the SST k-ω turbulence model have been employed to investigate the interaction mechanism in the staged sonic injection flow field, and the influences of the injection angle, the injection angle arrangement, and the distance between the injectors on the flow field characteristics have been analyzed comprehensively. At the same time, three grid scales have been used to perform the grid independency analysis, and the predicted results have been compared with the experimental data in the open literature for the single transverse injection scheme. The obtained results show that the penetration height for the cases with the distance between the injectors being 1 mm is the highest in the range considered in the current study, and this may be due to the strongest shock wave/shock wave interaction between the injectors. At the same time, due to the blockage of the fuel injection, the penetration height increases as the supersonic air stream flows downstream, and the influence of the wave system generated by the first and third injectors cannot propagate downstream and upstream, respectively. The multi-port injection scheme can provide better fuel penetration performance than the single one when the flow flux keeps constant, and the multi-port injection scheme with a certain angle can provide a higher total pressure recovery efficiency than the staged transverse injection scheme. Further, the staged transverse injection flow field can provide a better recirculation zone for the mixing between the fuel jet and the boundary layer, and the separation length increases with the increase of the distance between the injectors.

Richtmyer-Meshkov Instability Induced Mixing Enhancement in the Scramjet Combustor with a Central Strut

Experimental and numerical study of Richtmyer-Meshkov instability (RMI) induced mixing enhancement has been conducted in a liquid-fueled scramjet engine with a central strut. To generate the RMI in the scramjet engine, transverse high temperature jets are employed downstream the strut injector. Compared to the transverse ordinary temperature jet, the jet penetration into the supersonic airstream of high temperature jet increases by 60%. The numerical results indicate that the RMI phenomenon markedly enhances the mixing efficiency (up to 43%), which is necessary to initiate the chemical reactions. Ground experiments were carried out in the combustor, which verify the numerical method from the perspective of wall pressures of the combustor. In particular, the experiment results indicate that the RMI can benefit flame-holding due to the mixing enhancement.

Modeling of Fuel Sloshing and its Physical Effects on Flutter

The sloshing effects of an internal fluid on the flutter envelope of an aeroelastic system have received little attention in the open literature. This issue is nevertheless relevant for many aircraft, especially high-performance fighter jets carrying stores. This paper addresses some aspects of this problem as well as related modeling and analysis issues. These include the importance or insignificance of accounting for the hydroelastic effect when modeling an internal fluid and its container as well as accounting for that container when modeling the aerodynamics of the overall aeroelastic system. The paper also reports on the findings of four independent sets of flutter analyses performed for a wing–store test configuration and various fuel fill levels in the subsonic, transonic, and early supersonic regimes. Two of these sets of numerica l experiments relied on a computational-fluid-dynamics-based computational technology, and two of them on the doublet-lattice method or a supersonic lifting-surface theory, where applicable. The geometry of the chosen test configuration is that of the AGARD Wing 445.6 with a blunt-nosed store. The obtained computational results show that, at least for the considered wing–store configuration, ignoring the aforementioned hydroelastic effect tends to overestimate the added-mass effect and underestimate the critical pressure and flutter speed. They also reveal that, whereas the aerodynamics of the store may be neglected in the subsonic regime, they cannot be ignored in supersonic air streams. Finally, the performed computational study suggests that, in general, the critical pressure and flutter speed decrease with an increasing fuel fill level.

スクラムジェットモデル燃焼器中における<i>n</i>–ドデカン熱分解成分の燃焼挙動

Experiments on the effects of thermally cracked components of n-dodecane on the ignition and the combustion performance of a scram jet model combustor at Mach 2 were performed. Gaseous components of the thermally cracked n-dodecane at various temperature and 1MPa were measured by a gas chromatograph. Surrogate mixtures including hydrogen, methane, ethane and ethylene were injected into the supersonic air stream in the model combustor, and ignition and combustion behaviors were investigated at total temperatures of 1800 to 2400K. Pressure measurements on the combustor wall and optical observations of combustion using a high speed video camera were conducted. Results indicated that three combustion modes of intensive, transient and weak combustions were observed. Ignition was observed in the boundary layer positioned at the downstream of the cavity, and the flame propagated upward to the cavity. The ignition performance of ethylene was superior to methane and ethane, and their performances were elucidated from the reactivities predicted by a 0-dimensional calculation of chemical reactions for stoichiometric mixtures. An increase in the methane concentration reduced the ignition performance, and an increase in ethylene enhanced the ignition and combustion.

Self Ignition of Hydrogen-Air Mixtures with Inclined Porthole Injection in Supersonic Flows

Experiments were performed in the T4 shock tunnel to investigate the self ignition of hydrogen in a supersonic air stream. Hydrogen was injected into the flow over an inclined flat plate for oncoming Mach numbers of 7.9 to 8.0. The nozzle-supply enthalpy was kept between 3.1 and 3.4 MJ/kg and two different pressure levels were used in the tests. Measurements of surface pressures were used to infer the location of ignition but only small pressure increases were obtained when combustion occurred. Therefore multiple tests at nominally the same condition were used so that statistical methods could be used to identify the ignition lengths. The ignition lengths of the hydrogen air mixture directly behind a strong leading edge shock indicate that Pergament's method is able to predict the ignition length to within 35% for the observed autoignition over the range of conditions tested.

Effect of the incident shock wave interacting with transversal jet flow on the mixing and combustion

The interaction between an incident shock wave and a transverse jet flow for mixing and combustion in a supersonic airstream was investigated experimentally and numerically. NO planar laser induced fluorescence (NO-PLIF) and particle imaging velocimetry (PIV) for non-reactive flows and three-dimensional reactive/non-reactive numerical simulations were conducted to examine the effect of the incident shock wave on the three-dimensional flow structure and mixing mechanism between the airstream and the injected gas downstream of the injection slot. Results of NO-PLIF measurement and numerical simulation showed that, in the case without the incident shock wave, injected gas is seldom seen in the recirculation zone just downstream of the injection slot, while the injected gas with higher concentration is almost uniformly distributed in the recirculation zone when the incident shock wave is introduced downstream of the injection slot. Moreover, it was shown by the numerical simulations that the profiles of the local equivalence ratio is in the combustible range due to the enhanced entrainment of the airstream when the incident shock wave is introduced downstream of the injection slot. A large-scale recirculation in the direction parallel to the wall is generated by the three-dimensional flow effects, which enhances the mixing and extends the residence time in the recirculation zone in the case with incident shock wave downstream of the injection slot, the recirculation flow being confirmed successfully by PIV measurements as well. The results of three-dimensional reactive numerical simulations were in good agreement with the experimental flame-holding characteristics at a lower total temperature, which showed that flame-holding can be attained only when the incident shock wave was introduced downstream of the injection slot, confirming that the formation of three-dimensional and large-scale recirculation flow downstream of the injection slot enlarges the recirculation zone and enhances the mixing to produce the conditions for robust flame-holding.

Effects of Injection and Main Flow Conditions on Supersonic Turbulent Mixing Structure

The effects of injection and main flow conditions on the turbulent structure produced by a gaseous sonic transverse injection into a supersonic airstream were investigated. The turbulent structure inherent in the mixing flowfield was characterized by the single-time two-point spatial correlations based on the concentration fluctuation and measured with acetone planar laser-induced fluorescence. Results revealed that the most intensively fluctuating region appeared on the 50%-averaged-concentration track, irrespective of the injection conditions. The highly correlated region, which indicates an organized large-scale structure, appeared as a tilting ellipse in the lower-compressibility cases and an ellipse almost parallel to the streamline in the higher-compressibility cases. These differences in structure result in the difference of the averaged jet width. The injection with the higher-compressibility condition showed more stable structures and slower development in the size due to suppression of the instability growth. The structure was likely to be stretched toward the averaged-velocity direction as the main flow Mach number increased. Increasing the velocity difference between the jet and main flow caused the large-scale eddies to appear more frequently.

English

Thermochemical exploration of mixing and combustion of parallel hydrogen injection into supersonic vitiated air stream in a divergent duct is presented. Three-dimensional Navier Stokes equations along with twoequation turbulence models and Eddy dissipation concept (EDC)-based combustion models are solved using commercial CFD software. Chemical reaction for H2-air system is modelled by two different simple chemical kinetic schemes namely; infinitely fast rate kinetics as well as the single-step finite rate kinetics. Grid convergence of the solution is demonstrated and a grid convergence index-based error estimate has been provided. Insight into the mixing and combustion of high-speed turbulent reacting flow is obtained through the analysis of various thermochemical variables. Very good comparisons are obtained for the exit profiles for various fluid dynamical and chemical variables for the mixing case. For reacting case, the comparison between the experimental and the numerical values are reasonable. Parametric studies were carried out to study the effect of different turbulence models and turbulent Schmidt numbers. It is seen that Wilcox k-w turbulence model performed better than the other two-equation turbulence models in its class. Strong dependence of flow behaviour on turbulent Schmidt number was observed. The results indicate that simple chemical kinetics is adequate to describe the H2-air reaction in the scramjet combustor. Defence Science Journal, 2010, 60(5), pp.465-475 , DOI:http://dx.doi.org/10.14429/dsj.60.57

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.

Physics and Regimes of Supersonic Combustion

Understanding the physics of supersonic combustion is the key to design a performing engine for scramjet-powered vehicles. Despite studies on supersonic combustion dating back to the 1950s, there are still numerous uncertainties and misunderstandings on this topic. The following questions need to be answered: How does compressibility affect mixing, flame anchoring, and combustion efficiency? How long must a combustor be to ensure complete mixing and combustion while avoiding prohibitive performance losses? How can reacting turbulent and compressible flows be modeled? Experimental results in the past have shown that supersonic combustion of hydrogen and air is feasible and takes place in a reasonable distance, which is a necessary requirement in actual hypersonic vehicles powered by supersonic combustion ramjets. These results are explained based on a theoretical analysis of the physical mechanisms driving mixing and combustion in supersonic airstreams, where they are found to be different from those in the incompressible regime. In particular, the classic Kolmogorov scaling is shown to be no longer strictly valid, and the flame regime is predicted to be significantly affected by compressibility and different from that of subsonic flames. This analysis is also supported by the results of the numerical simulations presented, showing that by generating sufficiently intense turbulence, a supersonic combustion flame is short and can indeed anchor within a small distance from fuel injectors, with the flame typically burning in the so-called flamelets-in-eddies regime.

Scalar Spatial Correlations in a Supersonic Mixing Flowfield

Single-time two-point spatial correlations of injectant concentrations in the supersonic mixing flowfield produced by a sonic transverse injection of air into a Mach 2.0 supersonic airstream were investigated using acetone planar laser-induced fluorescence data. Side-view and end-view contour maps were obtained in several planes to characterize the turbulent structure and three-dimensionality of the mixing flowfield. The correlation maps indicated an organized large-scale structure in the upper region of the jet. The side-view correlation maps revealed that the shape of the large dominating structure was elliptic and that its major axis turned from backward-leaning to forward-leaning as the reference point of correlation moved downstream. The end-view correlation maps showed that the instantaneous jet plume appeared by turns either in the top or in the lower sides of the time-averaged injectant plume in each cross section.

Preliminary Investigation on Lithium Hydride as Fuel for Solid-Fueled Scramjet Engines

The aim of this work is to study the combustion of a LiH grain in a supersonic airstream, as a preliminary to investigate its potential as fuel in a solid-fueled scramjet. This hydride is interesting because it reacts exothermically with many substances and contains hydrogen, suggesting its use when a much higher density (compared with that of LH 2 ) would be beneficial. In fact, LiH releases H 2 and Li by thermal decomposition; at the same time, the Li produced behaves as a highly energetic fuel in the presence of a high-temperature airstream. Because of the scarce data present in literature, the thermochemical properties of LiH and its decomposition were calculated, estimated, or modeled, and issues associated with its combustion in a hot supersonic stream were investigated; a simplified physical model describing its vaporization and combustion was built that was successively simulated by means of a computational fluid dynamics code. Results show that LiH is an ideal candidate for solid-fueled-scramjet applications, behaving not only as a high-energy-density bifuel system, but also as a safe and compact hydrogen carrier. An intense flame zone is predicted to be present over the decomposing surface and downstream of the grain; the flame is stable and high temperatures (on the order of 2900 K) are obtainable. Moreover, specific impulse and thrust density predicted at a flight Mach = 7 are also interesting, being 10,000 and 200―300 m/s, respectively.

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.

Investigations on ethylene combustion in a scramjet combustor using resistance heaters

In this article, the ignition characteristics of the gaseous ethylene hydrocarbon fuel is investigated in a supersonic clean airstream experimental facility with resistance heaters. A generic cavity flame holder is used to create a recirculation region and promote the fuel/air mixing at the lower wall of the combustor. Three different injection concepts are considered in this research: (a) ethylene injection upstream of the cavity; (b) ethylene and hydrogen injection upstream of the cavity simultaneously; and (c) ethylene injection preceded by pilot hydrogen injection. The pilot injection is a supportive tool for holding the flame of the main normal ethylene fuel injection. Therefore, using pilot hydrogen injection and cavity configuration necessitates optimizing the combustor length to ensure complete combustion and full liberation of the chemical energy stored in the fuel before exiting the combustor. This study proved the possibility of igniting the ethylene and maintaining its flame in the supersonic airstreams.

English

Numerical modelling of turbulent-reacting flow field of supersonic combustion ramjet(scramjet) combustors are presented. The developed numerical procedure is based on the implicittreatment of chemical source terms by preconditioning and solved along with unstedy turbulentNavier-Stokes equations explicitly. Reaction is modelled using an eight-step hydrogen-airchemistry. Code is validated against a standard wall jet experimental data and is successfullyused to model the turbulent-reacting flow field resulting due to the combustion of hydrogeninjected from diamond-shaped strut and also in the wake region of wedge-shaped strut placedin the heated supersonic airstream. The analysis could demonstrate the effect of interaction ofoblique shock wave with a supersonic stream of hydrogen in its (fuel-air) mixing and reactionfor strut-based scramjet combustors.

フレームジェットによる超音速メタン空気流れの着火

Ignition tests of methane fuel under supersonic air streams were conducted with the methane/oxygen combustion jets as an ignition source. The ignition capability of the combustion jets differed with their equivalence ratio φ; ignition at Mach 2 was successful at 2.4kW calorific input power with φ=0.3 jets while 6.5kW power was needed at stoichiometry (φ=1.0). Numerical calculations revealed the relative effectiveness on shortening the ignition delay times for methane/air mixture between active radicals such as H, O, and OH and ordinary oxygen gas O2 abundantly contained in the lean combustion jets. These results overall suggest that ignition by the methane/oxygen combustion jets has an advantage in simplicity from the practical viewpoint over the plasma jet method which needs a heavy electrical power source.

Vertical and horizontal water redistribution in Norway spruce (Picea abies) roots in the Moravian Upland

Hydraulic redistribution (HR) by roots of large Norway spruce (Picea abies (L.) Karst.) trees was investigated by means of sap flow measurements made with the heat field deformation method. Irrigation was applied to a limited portion of the root system to steepen gradients of water potential in the soil and thus enhance rates of HR. On completion of the sap flow measurements, and to aid in their interpretation, the structure of the root system of seven of the investigated trees was exposed to a depth of 30 cm with a supersonic air-stream (air-spade). Before irrigation, vertical redistribution of water was observed in large coarse roots and some adjacent small lateral roots. Immediately after localized irrigation, horizontal redistribution of water from watered roots to dry roots via the stem base was demonstrated. The amount of horizontal distribution depended on the position of the receiving roots relative to the watered roots and the absorbing area of the watered root. No redistribution from watered roots via dry soil to roots of neighboring trees was detected. Responses of sap flow to localized irrigation were more pronounced in small lateral roots than in large branching roots where release and uptake of water are integrated. Sap flow measurements with multi-point sensors along radii in large lateral roots demonstrated water extraction from different soil horizons. We conclude that synchronous measurements of sap flow in both small and large lateral roots are needed to study water absorption and transport in tree root systems.

B211 超音速空気流れによる水の微粒化と冷却現象(融解・凝固)

B211 超音速空気流れによる水の微粒化と冷却現象(融解・凝固)

流れ方向渦の導入が圧縮性二重せん断層の発達に及ぼす影響

Effects of streamwise vortex inducement on growth of a compressible double shear layer are investigated experimentally. Air is injected with a subsonic speed into a co-flowing supersonic air stream in the parallel direction, forming a compressible double shear layer. The Mach numbers of the subsonic and supersonic air streams are 0.29 and 1.78, respectively. The convective Mach number is 0.62. Several types of sine-curved trailing edges are used to induce streamwise vortices into the shear layer. The amplitude, wavelength and phase of the sine curve are varied widely. It is shown that the growth of the streamwise vortices is determined by their spacing. For the symmetric sine-curved trailing edges, the streamwise vortex inducement has no effect on the growth rate of the shear layer. For the asymmetric sine-curved trailing edges, with the increase in the streamwise vortex intensity, the growth rate of the shear layer is increased. At the maximum, 90% enhancement in the mixing is achieved when compared with the straight trailing edge.

Oxidation Behavior of Silicon-Infiltrated Carbon/Carbon Composites in High-Enthalpy Convective Environment

Thermal response and oxidation behavior of commercial metal-silicon-infiltrated carbon/carbon composites (MICMATTM; Si-CC) were evaluated in a high-enthalpy convective environment using an arc jet facility (an arc wind tunnel). Composite specimens were put into a supersonic plasma air stream having a gas enthalpy of 12.7–18.8 MJ/kg for 50–600 s. Cold-wall heat fluxes measured by a Gardon-type calorimeter ranged from 1.0 to 1.8 MW/m2, and the maximum surface temperature reached 1300°–1660°C. After the arc jet testing, no surface recession was observed in the samples, and the mass loss rate of the composites was far less than that of graphite. The excellent oxidation resistance was caused by formation of a porous SiC layer at the surface of the composite. Oxidation behavior of the composites is discussed based on a simplified airflow blocking model of the porous SiC layer. The composites exhibited excellent oxidation resistance for short-term exposure in high-enthalpy airflow.