Supersonic Air Stream Research Articles

Analysis of intermittency and probe data in a supersonic flow with injection

Probe measurements were performed in the flow field produced by injection of helium or air into a supersonic airstream. The injectant was seeded with water and Rayleigh scattering was used to image the injectant plume. The region of the flow containing injectant–air mixture is seen to be highly unsteady, leading to the intermittent presence of injectant in certain regions. The intermittency is inferred. It is shown that bias errors can occur when the probe data is analyzed by techniques which assume steady flow. A technique for relatively bias-free analysis utilizing the intermittency measurements is presented and the bias errors are estimated. The gas-sampling probe is shown to measure the mass-weighted-mean mass fraction of helium, which is significantly less than the simple mean. A new measure of mixing efficiency obtained from the combined probe and intermittency measurements is discussed.

アーク直接注入による超音速流中の着火

As ignition method in supersonic air streams, direct injection of dc arc discharges has been proposed and empirically examined. Since the electrodes are buried parallel to the streams in the same level of one of the chamber walls and the discharged arc is pushed into the main air flows by carrier gases, chemically active species are effectively produced and exposed in the combustion chamber. The experiments conducted so far showed 1) this direct arc injection method (DAIM) can ignite methane fuel in the flow of M=2.3 and total temperature T0 as low as 300K and 2) methane air stoichiometric premixed gas is most effective for ignition as the carrier gas used to produce active species and pushed into the main flow.

Tangential Injection from Overlaid Slots into a Supersonic Stream

The results of an experimental study of tangential supersonic slot injection into a supersonic airstream with tangential subsonic injection through an additional slot above the main slot is presented. Such a  ow is of interest for at least two propulsion-related applications. First, one can simulate a near-separation boundary-layer proŽ le by subsonic injection through the upper slot, which is then to be re-energized by supersonic injection through the lower slot. Second, this  ow is also of interest for application to fuel/ oxidizer/pilot injection from multiple overlaid slots. The experiments were performed in an intermittent, vacuum wind tunnel at a freestream Mach number of 2.85. The supersonic injectant had a Mach number of 2.00, and the subsonic injection was at Mach numbers of 0.26 and 0.72. The results are presented in the form of spark schlieren photographs, interferograms, and wall-static pressure measurements. Density proŽ les at several axial locations determined from the interferograms are presented, as well as streamwise and spanwise static pressure distributions. The major conclusions drawn from the tandem injection results, are Ž rst, that a near-separation boundary-layer proŽ le can be simulated in this way. Next, tandemly injected subsonic and supersonic  ow can be divided into separate components that closely resemble the respective individual injections into an undisturbed freestream. Also, the effects of the subsonic injection were completely mixed out at a downstream location of six slot heights. Therefore, adverse-pressuregradient-inducing devices should be positioned at least six slot heights downstream of the supersonic injection station if the effects of supersonic injection into a near-separation boundary layer for the purpose of re-energizing it are to be studied.

定断面流路内超音速二重せん断層流れの不安定性 ２次元流での超音速混合促進法を考える

The instabilities of confined, supersonic, double shear layer flow are examined in order to develop a mixing enhancement scheme in two-dimensional flow configuration. It is shown that flow confinement is essential to produce the skew-symmetric mode of instability wave which is responsible for rapid mixing in the subsequent nonlinear evolution stage. The linear stability analysis provides a design tool with which suitable flow configuration can be selected. Direct numerical simulation demonstrates that hydrogen gas slowly issued in between supersonic air streams menders, accelerates to supersonic speed and mixes with the air quickly without shock generation, thus leading to explosive combustion. The underlying new physics are explored.

Measured lengths of supersonic hydrogen-air jet flames—Compared to subsonic flame lengths—and analysis

Measurements of flame length are reported that help to quantify the overall fuel-air mixing process that occurs within a jet-like flame in the supersonic regime. A nonpremixed, turbulent, hydrogen-air jet flame is stabilized along the axis of a Mach 2.2 coflowing air stream. Supersonic flame lengths are compared to measured lengths of subsonic hydrogen-air flames with coflow that were stabilized on the axis of a subsonic wind tunnel for a range of fuel/air velocity and density ratios. The supersonic flames are found to be significantly shorter than (i.e., typically half as long as) corresponding subsonic flames, providing that both the velocity and density ratios are matched for the two cases. This difference implies that for axisymmetric jet geometries with combustion, mixing rates are larger in the supersonic case. Compressibility effects are not believed to be significant since typical convective Mach numbers are less than 0.45. One possible reason why the supersonic flames are relatively short is that the radial velocity, which controls entrainment, can be altered by compression/expansion waves that are inherent to the jet geometry. Both subsonic and supersonic flame lengths increase as the normalized fuel mass flux increases, which is predicted to occur by a scaling analysis. The density of the supersonic airstream was decreased in order to simulate an increase in altitude, and the supersonic flames became longer, as predicted by the analysis. Increasing the air stagnation temperature to 600K shortens the supersonic flames, which is not explained by the mixing-limited scaling analysis and may be due to finite-rate chemistry or partially premixed combustion, both of which are temperature dependent processes.

NUMERICAL SIMULATION OF HEAT TRANSFER IN CHEMICALLY REACTING SHOCK WAVE-TURBULENT BOUNDARY LAYER INTERACTIONS

Abstract The flow field of a transverse jet in a supersonic airstream subjected to shock wave-turbulent boundary layer interactions is simulated numerically by adaptive mixed explicit-implicit generalized-Galerkin finite element methods. In this scheme, convection and diffusion implicitness parameters are introduced to resolve shock wave discontinuities and widely disparate time and length scales of turbulence and finite rate chemistry. These parameters are flow field dependent, calculated from local Mach, Reynolds, and Damkohler numbers for each element. Effects of turbulence are taken into account with a two-equation (k-ε) model with a compressibility correction. Various cases of mixing, slot widths, and total pressure ratios with and without chemical reactions are examined. Favorable comparisons with experimental measurements are demonstrated.

Development of an Experimentally Flexible Facility for Mixing-Combustion Interactions in Supersonic Flow

A new experimental facility has been designed, built and calibrated to support research topics involving supersonic mixing and combustion and will be used to investigate: 1) molecular level mixing; 2) mixing-combustion coupling effects for gaseous and liquid fuels; and 3) atomization, droplet break-up, mixing, and combustion of high-energy/high-density slurry fuels in a supersonic airstream. The facility simulates flight enthalpies corresponding to altitudes between 26 and 36 km and flight Mach numbers up to 4.755. The major features of this facility are the geometric flexibility of the test section configuration, of the fuel injection, and of the experimental conditions, enabling the investigation of a broad range of topics. Parallel and transverse injection, independently controlled in four regions, is available. The construction of the test section is modular and the entrance Mach numbers can be varied between 1.6 and 3.6. An experimental program to define the test flow at the entrance of the facility’s test section is presented with emphasis on the test section entrance ture distribution and calibration. Selected research topics currently being investigated are summarized to highlight the usefulness of the present facility.

Numerical simulation of mixing for turbulent slot injection

Flowfields are simulated that are induced by sonic jets injected into supersonic airstreams. Results on structure and mixing of such jets are presented. Several two-dimensional cases are treated using a low-Reynolds-number q-ω turbulence model. The influence of some turbulence modeling corrections and of artificial dissipation are investigated for flat plate test cases where nitrogen is injected transversely into the supersonic airstream. The results demonstrate the necessity to use modeling corrections and to reduce artificial viscosity in near-wall regions. The numerical code is also used to calculate a similar transverse helium injection into a ducted airstream. In a further investigation hydrogen is injected axially into the mainstream. In this case mixing is strongly influenced by turbulent diffusion. Pressure and species profiles are compared with experimental data.

Numerical simulation of flame-stabilization and combustion promotion by plasma jets in supersonic air streams

The mechanisms of flame stabilization and combustion promotion by plasma jets, observed in scramjet bombustor experiments with transverse fuel injection, were investigated numerically, based on time-dependent, two-dimensional complete Navier-Stokes equations, with a six-species, eight-step reaction model combined with el'dovich chain reactions. Although most designs for scramjet combustors rely on the combination of a rearward-facing step and a plasma torch, the present study is conducted on a simple configuration without the step to make clearer the fundamental aspect of the mechanisms. The following results were obtained: o 1. A plasma jet with adequate input power and feedstock flow rate establishes a high-temperature recirculation zone with an adequate scale, which works for flame stabilization as that produced by the step. The plasma jet is also useful for the combustion promotion of the fuel jet, through the intensification of eddy diffusivity at its injection point and at the following downstream region. 2. The effectiveness of plasma jets in flame stabilization and combustion promotion increases with input electric power and feedstock flow rate, and the plasma jets with oxygen feedstock are more effective than those with air or hydrogen feedstock, as observed generally in scramjet combustor experiments. 3. A decrease of the plasma jet injection slit width, for a fixed input power and a fixed mass flow rate of feedstock, increases the effectiveness of plasma jets in some cases. 4. When the plasma jet injection slit is located downstream of the fuel injection slit, the required input power and feedstock flow rate for a desirable combustion progress are usually smaller than those when it is located upstream, in accordance with a previous supersonic-combustion experiment conducted without the step.

Mixed explicit/implicit time integration of coupled aeroelastic problems: Three‐field formulation, geometric conservation and distributed solution

AbstractA three‐field arbitrary Lagrangian‐Eulerian (ALE) finite element/voluem formulation for coupled transient aeroelastic problems is presented. The description includes a rigorous derivation of a geometric conservation law for flow problems with moving boundaries and unstructured deformable meshes. The solution of the coupled governing equations with a mixed explicit (fluid)/implicit (structure) staggered procedure is discussed with particular reference to accuracy, stability, distributed computing, I/O transfers, subcycling and parallel processing. A general and flexible framework for implementing partitioned solution procedures for coupled aeroelastic problems on heterogeneous and/or parallel computational platforms is described. This framework and the explicit/implicit partitioned procedures are demonstrated with the numerical investigation on an iPSC‐860 massively parallel processor of the instability of flat panels with infinite aspect ratio in supersonic airstreams.

Gasdynamics of the scattering of the detonation products of a cylindrical explosive charge with inert metal particles

A mathematical model of the scattering of the detonation products of a condensed explosive with inert metal particles when the wave travels along the axis of a cylindrical charge is proposed and numerical calculations are carried out. Detonation product scattering is simulated by a two-phase nonequilibrium axisymmetric jet flow in a supersonic external airstream. A technique for calculating the product gas-suspension behind the detonation wave is developed. The optimal values of the difference scheme parameters ensuring the required calculation accuracy are found. The gasdynamic properties of the process associated with the particles and the lateral spread are investigated.

Numerical investigation of chemical reaction-turbulence interaction in compressible shear layers

Calculations of mixing of hydrogen and vitiated supersonic airstream with and without chemical reactions are presented. The Favre-averaged compressible flow equations and convection-diffusion equations for the chemical species are solved by a finite volume method that incorporates an incremental pressure equation for the conservation of mass. The chemical reactions are described by 9 chemical species and 24 pairs of reaction-steps. The turbulence field is described by multiple-time-scale turbulence equations. The calculated ignition delay and species concentrations obtained using the multiple-time-scale turbulence equations are in closer agreement with the measured data than those obtained using a probability density function method and k− ε turbulence models. It is shown that the multiple-time-scale turbulence equations successfully predict the increased shear layer thickness caused by the chemical reaction-turbulence interaction.

Parametrically Excited Nonlinear Two-Degree-of-Freedom Systems with Repeated Natural Frequencies

The method of normal forms is used to study the nonlinear response of two-degree-of-freedom systems with repeated natural frequencies and cubic nonlinearity to a principal parametric excitation. The linear part of the system has a nonsemisimple one-to-one resonance. The character of the stability and various types of bifurcation including the formation of a homoclinic orbit are analyzed. The results are applied to the flutter of a simply supported panel in a supersonic airstream.

Asymptotic and Numerical Analyses of Ignition of a Fuel Jet in a Supersonic Airstream

ABSTRACT Ignition of a cold fuel injection into a supersonic hot airstream is analyzed both asymptotically and numerically. The present study extends the Linan and Crespo's analysis of ignition in a mixing layer of parallel streams to the analysis of ignition in a jet flow. The ignition distance is related explicitly to the thermodynamic parameters and the width of jet The asymptotic analysis not only correlates the ignition distance in a fuel jet with that in a mixing layer of parallel streams, but also reveals the difference between them. It is shown that ignition mechanism of a fuel jet in an infinite air stream can be divided into three regimes, ignition regime in a mixing layer of parallel streams, ignition regime determined by jet scale and chemical scale, and ignition regime controlled by chemical scale only, according to the width of jet The present analysis theoretically exhibits the linear dependence of ignition distance on the logarithmic function of jet width. A good agreement is established b...

Parametrically Excited Nonlinear Two-Degree-of-Freedom Systems with Repeated Natural Frequencies

The method of normal forms is used to study the nonlinear response of two-degree-of-freedom systems with repeated natural frequencies and cubic nonlinearity to a principal parametric excitation. The linear part of the system has a nonsemisimple one-to-one resonance. The character of the stability and various types of bifurcation including the formation of a homoclinic orbit are analyzed. The results are applied to the flutter of a simply supported panel in a supersonic airstream.

水素・空気超音速噴流における着火の数値解析

Ignition of a parallel hydrogen injection into a supersonic hot air stream and ignition of a cold premixed combustible hydrogen/air stream heated by a hot air jet were investigated by a high-resolution TVD LU-SGS numerical algorithm. The present two-dimensional simulation included detailed chemistry in terms of elementary reactions, as well as a multispecies molecular transport model and the Baldwin Lomax algebraic turbulent model. In the case of a cold hydrogen injection into a hot air stream, numerical simulation showed that, with a decrease of the width of fuel jet, ignition point moves upstream quickly. The present results also showed that ignition distance was linearly dependent on the logarithmic function of the width of the fuel jet. Furthermore, the phenomenon could be still observed even for a very low concentration of hydrogen in the fuel jet. In the case of a premixed hydrogen/air stream ignited by a hot air jet, with a decrease of the width of air jet, ignition point moved very slightly downstream. However, there existed an ignition limit for the width of air jet, that is, if the width of the air jet was less than this limit, ignition became impossible. Furthermore, this ignition limit depended strongly on the initial temperatures of air stream and premixed hydrogen/air stream.

Simulation of transverse gas injection in turbulent supersonic air flows

The present paper deals with the simulation of the fluid dynamic behavior of transverse gas injection in turbulent supersonic air streams. The Favre-averaged Navier-Stokes equations are solved for a multicomponent mixture of gases. A two-equation k-e turbulence model is employed that properly accounts for low Reynolds effects. The governing equations are solved by a finite volume approach with the k-e model equations fully coupled with those of the mean flow, and an implicit treatment of the source terms. Several test cases are considered and the results are compared with available experimental measurements.

Numerical simulation of supersonic mixing and combustion

Problems of hydrogen mixing and combustion in supersonic air streams are numerically investigated. An implicit lower-upper (LU) scheme is used for solving the full, averaged, two-dimensional Navier-Stokes and species transport equations. For turbulence closure, a two-equation low-Reynolds-number q-ω turbulence model is employed. While the chemical source terms are treated fully coupled with the fluid motion, the conservation equations for q and ω are integrated in a decoupled way. This is done by the same LU algorithm as for the main flow equations. The combustion process in chemical nonequilibrium is based on a 20-step, 9-species finite-rate chemistry model. Because of the short residence times of the gas in supersonic combustors, the kind of hydrogen supply is very important for combustion process. This paper investigates hydrogen injections axial to the main stream at subsonic, sonic, and supersonic injection velocities. In addition to an optimal mixing that is strongly influenced by the shear layer behind the injector, it is important that the main stream remains supersonic. This results in a diverging geometry that compensates for the increase in boundary-layer thickness and rise in temperature due to combustion. There is also a strong interaction with shock waves, which often induce subsonic regions. The turbulent shear flow of hydrogen and air enables the gases to interpenetrate and can improve mixing.

CARS TEMPERATURE MEASUREMENTS IN A SUPERSONIC RAMJET COMBUSTION CHAMBER

The temperature distribution in the flowfield of a two-dimensional hydrogen jet in a supersonic air stream has been experimentally investigated. Hydrogen was injected parallel to the freestream air through the base of a strut centered in the combustion chamber. This strut, formed like a symmetrical wedge, also served as a flameholder. The aim of this work was the measurement of temperatures in a turbulent supersonic hydrogen-fueled ramjet for better understanding of combustion phenomena and improvement of theoretical predictions. Coherent Anti-Stokes Raman Spectroscopy (CARS) was used as a diagnostic tool for temperature mapping in the combustion chamber. Temperature profiles were determined at three different axial locations by translating the CARS probe-volume vertically across the combustor. Experimental parameters were the total inlet temperature of mainstream-air and the mass flow of injected hydrogen. Schlieren photographs and video pictures of the 306-nm-OH-intensity have been used to get a qualitative understanding of the flowfield and to determine the locations of interest for CARS measurements. Due to the nonlinear dependence of CARS on temperature and the highly turbulent flowfield, time averaging would lead to serious measurement errors. Thus, single pulse CARS measurements have been recorded using nitrogen as a probe molecule.

メタン‐水素混合燃料を用いた超音速燃焼に関する研究

In order to attain combustion of methane fuel in supersonic airstreams of relatively low temperature, hydrogen fuel was added to the fuel as a ignition promotor. In experiments, rectangular combustor with backward step was adopted and the methane-hydrogen mixture fuel was injected into supersonic airstreams. The region where the self-ignition occurs was confirmed by high speed direct photographs, and self-ignition characteristics were investigated with parameters of equivalence ratio and fraction of hydrogen in the fuel. The injection form was refined with use of the obtained results to attain self-ignition with low fraction of hydrogen. The combustion of methane-hydrogen mixture fuel was compared with that of hydrogen fuel by using wall static pressure distributions and OH/CH emission images of the flame. As results, it is confirmed that the combustion of methane fuel in the airstreams is obtained by addition of hydrogen fuel, and it is considered that the self-ignition in refined injection form is obtained under same conditions as that of hydrogen fuel, independently of the fraction of hydrogen in total injected fuels. The static pressure distribution and the flame shape in methane-hydrogen mixture fuel case were similar to those in hydrogen fuel case.