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A hybrid Reynolds-averaged Navier-Stokes/large-eddy simulation investigation of a strut-based scramjet is performed in the present paper. Both nonreacting and reacting cases in which the same sonic fuel jet mixes/reacts with two different supersonic airstreams (T=340 K vs T=850 K) are comparatively studied. The vorticity and its influencing factors are analyzed to understand the transport behaviors in the scramjet. Multiple mechanisms contribute to vorticity simultaneously but unequally in different regions. A large part of the flowfield is full of the supersonic airstream with small vorticity. In the mixing region, the averaged vorticity magnitude decreases gradually along the chamber length, and vortex stretch is the leading mechanism of vorticity transport within this region. The baroclinic torque effect is strengthened dramatically by combustion, but is weakened by the incoming flow with higher stagnation enthalpy. Although the near-wall region is small, the averaged vorticity magnitude in this region is extremely large due to the confinement and viscosity effects. Diffusion is the leading mechanism in the near-wall region, but dilatation can dominate vorticity transport due to shock-wave/boundary-layer interaction and pressure rise resulting from combustion. [ABSTRACT FROM AUTHOR]

This paper presents a numerical investigation of mixing in supersonic flows using the hybrid Reynolds-averaged Navier-Stokes/large-eddy simulation. Three configurations are comparatively studied to investigate the effects of confinement and heat release on the mixing: a nonreacting case without confinement, a nonreacting case with confinement, and a reacting case with confinement. First, the mixing region area is quantified and its development is characterized. Subsequently, the deformation and rotation of fluid are analyzed to illustrate the impacts of shocks and combustion on the mixing region development. The transport of vorticity is further discussed to explore the driving mechanism of mixing in different cases. The results show that the impingement of oblique shocks on the mixing region reduces its area due to compression but enhances the subsequent mixing by promoting vorticity via the dilatation and baroclinic torque effects. This leads to a wavelike pattern for the development of the mixing region. Heat release expands the mixing region and decreases vorticity in the upstream, where high heat-release rates are observed. Downstream, combustion products mix with air, and only weak heat release takes place. In that region, the baroclinic torque effect becomes dominant and results in higher vorticity as compared to the nonreacting case. [ABSTRACT FROM AUTHOR]

This paper examines the dynamics of the flame leading edge in a laminar premixed CH 4 /air flame stabilized on a bluff body in a channel. Harmonic fluctuations and step velocity change are used to simulate the flame response to acoustic oscillations, which are of primary importance in the study of thermo-acoustic instabilities. We use a fully resolved unsteady two-dimensional code with detailed chemistry and species transport, with coupled heat transfer to the bluff body. Calculations were conducted with different equivalence ratios, body materials, and steady state inlet velocity with step or harmonic perturbations. Results reveal that the flame leading edge dynamics displays a peak response around St = 0.5 suggesting that the leading edge motion is mainly due to the advection of appropriate ignition conditions as a result of the excitement of the wake recirculating flow. There is considerable augmentation of the flame wrinkles generated by the flame leading edge motion as result of the flow–flame interaction. Additionally, we show that a flame that anchors on average further upstream leads to stronger damping of the shear layer vortices and thus weaker vortex-flame interaction and heat release fluctuations. Hence, we identify two different mechanisms by which the flame leading edge location and oscillation amplitude impact heat release fluctuations. The study suggests a stronger dependence of the overall flame wrinkling and heat release fluctuations on the flame leading edge dynamics than recognized previously and the potential role it plays in combustion dynamics. [ABSTRACT FROM AUTHOR]

A tube-in-tube concept for separation of bodies in space was investigated theoretically and experimentally. The separation system is based on generation of high pressure gas by combustion of solid propellant and restricting the expansion of the gas only by ejecting the two bodies in opposite directions, in such a fashion that maximizes generated impulse. An interior ballistics model was developed in order to investigate the potential benefits of the separation system for a large range of space body masses and for different design parameters such as geometry and propellant. The model takes into account solid propellant combustion, heat losses, and gas phase chemical reactions. The model shows that for large bodies (above 100 kg) and typical separation velocities of 5 m/s, the proposed separation mechanism may be characterized by a specific impulse of 25,000 s, two order of magnitude larger than that of conventional solid rockets. It means that the proposed separation system requires only 1% of the propellant mass that would be needed for a conventional rocket for the same mission. Since many existing launch vehicles obtain such separation velocities by using conventional solid rocket motors (retro-rockets), the implementation of the new separation system design can reduce dramatically the mass of the separation system and increase safety. A dedicated experimental setup was built in order to demonstrate the concept and validate the model. The experimental results revealed specific impulse values of up to 27,000 s and showed good correspondence with the model. [ABSTRACT FROM AUTHOR]

In this paper we investigate the interaction between the flame structure, the flow field and the coupled heat transfer with the flame holder of a laminar lean premixed CH 4 /air flame stabilized on a heat conducting bluff body in a channel. The study is conducted with a 2-D direct numerical simulation with detailed chemistry and species transport and with no artificial flame anchoring boundary conditions. Capturing the multiple time scales, length scales and flame-wall thermal interaction was done using a low Mach number operator-split projection algorithm, coupled with a block-structured adaptive mesh refinement and an immersed boundary method for the solid body. The flame structure displays profiles of the main species and atomic ratios similar to previously published experimental measurements on an annular bluff body configuration for both laminar and turbulent flow, demonstrating generality of the resolved flame leading edge structure for flames that stabilize on a sudden expansion. The flame structure near the bluff body and further downstream shows dependence on the thermal properties of the bluff body. We analyze the influence of flow strain and heat losses on the flame, and show that the flame stretch increases sharply at the flame leading edge, and this high stretch rate, together with heat losses, dictate the flame anchoring location. By analyzing the impact of the flame on the flow field we reveal that the strong dependence of vorticity dilatation on the flame location leads to high impact of the flame anchoring location on the flow and flame stretch downstream. This study sheds light on the impact of heat losses to the flame holder on the flame–flow feedback mechanism in lean premixed flames. [ABSTRACT FROM AUTHOR]

Abstract: An innovative concept for a rocket type thruster that can be beneficial for spacecraft trajectory corrections and station keeping was investigated both experimentally and theoretically. It may also be useful for divert and attitude control systems (DACS). The thruster is based on a combustion chamber discharging a weight through an exhaust tube. Calculations with granular double-base propellant and a solid ejected weight reveal that a specific impulse based on the propellant mass of well above 400s can be obtained. An experimental thruster was built in order to demonstrate the new idea and validate the model. The thruster impulse was measured both directly with a load cell and indirectly by using a pressure transducer and high speed photography of the weight as it exits the tube, with both ways producing very similar total impulse measurement. The good correspondence between the computations and the measured data validates the model as a useful tool for studying and designing such a thruster. [Copyright &y& Elsevier]

This paper presents a fundamental study of ultra-lean flames stabilized behind a thin, highly conducting metallic rectangular bluff body acting as a flame holder. Using high fidelity numerical simulations, we reproduce a phenomenon observed experimentally, showing that in this configuration steady hydrogen–methane flames can exist at equivalence ratios below the flammability limit associated with planar unstrained flames with the same hydrogen–methane proportion. These ultra–lean hydrogen–enriched mixtures exhibit a distinct stabilization mechanism compared to pure methane flames: they stabilize in the form of inverted closed V or U flames farther away from the flame holder as the inflow reactants velocity is reduced, leading eventually to blow-off for sufficiently small velocities. Conversely, as the reactants flow rate is increased, the flames anchor closer to the flame holder, and surprisingly no blow-off is observed at high velocities. This response is shown to be linked to the presence of hydrogen in the fuel mixture and its large diffusivity, which results in locally richer mixtures in the strained, curved flame base. [ABSTRACT FROM AUTHOR]

The paper presents a Reynolds-averaged Navier-Stokes computational aeroelastic investigation of the limit cycle oscillations phenomenon of the F-16 aircraft wing, with correlation to flight tests. The study is the first step toward a new computational capability, currently under development, to enable the prediction of the limit cycle oscillations phenomenon for Israeli Air Force purposes. The study presents aeroelastic simulations of the full-span F-16 fighter computational model including a linear modal structural model and an aerodynamic wing model. The simulations are performed at flow conditions for which limit cycle oscillations were encountered in flight tests. The effects of dynamic pressure, structural damping, angle of attack, and turbulence modeling on the characteristics of the phenomenon are discussed. The relationship between flutter and limit cycle oscillations is investigated, and the origin of limit cycle oscillations is identified as nonlinear, self-sustained, periodic, shock-wave oscillations on the upper surface of the wing. These oscillations are suggested to suppress flutter into limit cycle oscillations. [ABSTRACT FROM AUTHOR]

In this paper, we study the impact of injecting diluent gas into the outer recirculation zone of a swirl stabilized premixed methane/air flame on the flame shape and combustion dynamics. Two modes of instability were observed, a low frequency mode at 8–12 Hz which corresponds to a flame flickering mode in the outer recirculation zone and a high frequency thermo-acoustic instability at 250–275 Hz. Furthermore, air, N 2 and CO 2 were injected into the outer recirculation zone, at mass flow rates of at 1% and 3% compared with the main flow of premixed methane/air. The diluent addition shifts the outer recirculation zone flickering mode of instability to higher equivalence ratios and completely damps the thermo-acoustic instability for the current experimental conditions. Changing the diluent composition from air to N 2 and to CO 2 improved the extent of instability mitigation displaying the influence of diluent gas on the chemical kinetic in the ORZ region. Furthermore, the impact of the different diluent compositions on mitigating the combustion instability scaled according to the extinction strain rate of the diluted mixture in the outer recirculation zone. [ABSTRACT FROM AUTHOR]

This paper investigates the impact of fuel distribution on flame stabilization and local combustion modes in a cavity-based scramjet. Two injection schemes with a global equivalence ratio of 0.4 are comparatively studied. Ethylene is injected only from upstream of the cavity in the first case, whereas in the second case 75% of the fuel is injected from upstream of the cavity and 25% of the fuel is injected from the cavity floor. The reactive flow in the scramjet is numerically investigated using a hybrid RANS/LES method. The agreement between calculation results and experimentally measured pressure profiles and CH* chemiluminescence images is good. The simulation results are further employed to gain a fundamental understanding of local combustion modes from different aspects: supersonic vs. subsonic, and premixed (fuel-rich vs. fuel-lean) vs. diffusion by using filter functions. In the upstream-only fueling case, it is found that the flame is stabilized by the cavity shear layer and most of the heat is released by supersonic-premixed combustion. Moreover, subsonic-premixed combustion and supersonic-diffusion combustion are significant, and the leading mode changes along the combustor. In the dual fueling case, a cavity assisted jet-wake stabilized flame is observed with higher combustion efficiency. Diffusion combustion becomes the dominant mode, with similar heat release in subsonic and supersonic regions. Nevertheless, the flashback of a premixed flame along the walls plays an important role, leading to diffusion combustion around the fuel jet. The results elucidate the link between the fuel injection schemes, flame stabilization locations, and local combustion modes. [ABSTRACT FROM AUTHOR]