Shock Waves最新文献

This study aims to validate the new developments in our in-house spectroscopic code (KAT-LIF) to perform NO-LIF simulations for detonation conditions, as well as evaluating the capabilities of the NO-LIF diagnostic for characterizing H(_2)-air detonations. This objective was achieved in several steps. First, our in-house spectroscopic tool, KAT-LIF, was updated to perform NO-LIF simulations by notably developing a database of NO(A-X) transitions, currently unavailable in conventional spectroscopic databases, as well as collecting and implementing species-specific line broadening, line shifting, and quenching parameters for NO-LIF. Second, the validation of KAT-LIF was performed by comparing the simulation results with pre-existing simulation tools (LIFSim and LIFBASE) and experimental NO-LIF measurements in a laminar CH(_4)-air flame and H(_2)-air detonation. The validation results present satisfactory agreement of KAT-LIF and other simulation tools (LIFBASE, LIFSim) with experimental results for several conditions. For example, less than 20% discrepancy between the simulated and experimental NO-LIF profiles is observed for stoichiometric H(_2)-air detonation, initially at 20 kPa and 293 K. Third, qualitative and quantitative capabilities of the NO-LIF technique for detonation characterization are discussed, which include: shock detection, induction zone length measurements, and quantitative number density measurements.

The aim of the present paper was to report on an experimental study of the characterization of a blast wave initiated by a solid explosive and its interaction with a rigid obstacle in the form of a hemicylinder. Pressure transducers located along the path of the blast wave and high-speed imaging allow (1) the measurement of the overpressure at different locations and (2) the characterization of the blast wave inception, propagation, and reflection off the hemicylinder. The scaling effect has been investigated by performing experiments in two different facilities, where one is at twice the scale of the other.

In the present work, operation characteristics of a disk-type rotating detonation engine (DRDE) with a constant chamber area were experimentally studied for various total mass flow rates and a wide variety of equivalence ratios of hydrogen–air mixtures. From the direct visualizations, the rotating detonation wave was found to propagate near the outer wall of the combustion chamber, regardless of the wave mode. For the present test conditions, single- and double-wave modes are observed, depending on the equivalence ratio of the mixture. The pressure gain was evaluated based on a one-dimensional flow model together with the chamber static pressure measured with the capillary tube average pressure technique. Although the present DRDE configuration provided a negative pressure gain for all the test conditions, it was found that the single-wave mode was superior to the double-wave mode in terms of the pressure gain.

The deformation and breakup characteristics of liquid rocket propellant 2 (RP-2) droplets are experimentally investigated in a shock tube. The RP-2 droplets are subjected to a weak shock wave, a strong shock, and a detonation wave to deduce the impacts of high-speed and supersonic reacting flows on droplet deformation and breakup. High-speed shadowgraph and schlieren imaging techniques are employed to characterize droplet morphologies, deformation rates, and displacement of the droplet centroid. The results reveal that the transition from a shock wave to a detonation suppresses the deformation of the droplet and augments small-scale breakup. A shift in dominant breakup mechanisms is linked to a significant increase in the Weber number due to an increase in flow velocities and temperatures when transitioning to the detonation case. The experimental data are combined with a droplet stability analysis to predict the “child” (or fragments of the initial “parent” droplet) droplet sizes of each test condition. The child droplet size is shown to decrease as the flow regime transitions toward a detonation. An analytical mass stripping model was also used to determine that the total mass stripped from the parent droplet increased when approaching supersonic reacting conditions. The child droplet sizes and mass stripping rate will ultimately influence evaporation timescales and ignition in supersonic reacting flows, which is important for the development of detonation-based propulsion and power systems.

Pre-ignition is an undesired combustion event known to restrict kinetic modeling validation. Previous methane oxidation studies reported premature ignition as part of ignition delay time measurements in shock tubes. In this context, the effect on the pre-ignition propensity and auto-ignition behavior of stoichiometric methane mixtures at different dilution levels of (hbox {N}_2), Ar, He, and (hbox {CO}_2) was studied at 10 bar and 25 bar and temperatures between 1080 K and 1350 K. In addition to conventional sidewall pressure and endwall light emission measurements, a high-speed imaging setup was utilized to visualize the ignition process. Relevant physicochemical parameters to describe and predict the pre-ignition phenomenon were used. The results suggest that dilution levels up to (80%) of bath gas are not successful in mitigating early ignition occurrence and its effects at moderate pressures. Replacing (hbox {N}_2) by He was found to suppress early ignition at 10 bar, attributed to an enhanced dissipation of temperature inhomogeneities in the test gas section. The present findings demonstrate that (hbox {CO}_2) has potential for pre-ignition heat release mitigation, while Ar was confirmed to promote premature ignition. To the best of our knowledge, we present the first detailed study on pre-ignition mitigation for methane mixtures in shock tubes, where further insights into its ignition non-idealities are given.

A new reflected shock tunnel capable of generating hypersonic environments at realistic flight enthalpies has been commissioned at Sandia. The tunnel uses an existing free-piston driver and shock tube coupled to a conical nozzle to accelerate the flow to approximately Mach 9. The facility design process is outlined and compared to other ground test facilities. A representative flight-enthalpy condition is designed using an in-house state-to-state solver and piston dynamics model and evaluated using quasi-1D modeling with the University of Queensland L1d code. This condition is demonstrated using canonical models and a calibration rake. A 25-cm core flow with 4.6-MJ/kg total enthalpy is achieved over an approximately 1-ms test time. The condition was refined using analysis and a heavier piston, leading to an increase in test time. A novel high-speed molecular tagging velocimetry method is applied using in situ nitric oxide to measure the freestream velocity of approximately 3016 m/s. Companion simulation data show good agreement in exit velocity, pitot pressure, and core flow size.

When departure from the ideal gas equation of state is considered, the Noble–Abel stiffened gas model is an appealing and versatile candidate due to its simple form. The linear interaction approximation formalism is extended to consider non-ideal gas effects introduced by this equation of state. Kovásznay decomposition and adequate definition of the energy of disturbances are provided in the context of this equation of state. Changes with respect to ideal gas are investigated on transfer functions, critical angle, and compression factor. Those differences yield concrete effects on the damping and transfer of fluctuations across shock waves. Those changes are further illustrated by considering the interaction of an entropy spot with a Mach 3 stationary shock wave.

Detonation propagation in a stratified layer of combustible gas over an inert gas was investigated experimentally. The layer formed in a 12.7-mm-wide channel by opening a sliding door that initially separated a nitrogen-diluted stoichiometric hydrogen–oxygen mixture from argon, or nitrogen. As the lighter combustible gas layer spreads axially down the channel, diffusion across the interface produces a composition gradient across the layer height. A steady detonation wave, generated by deflagration-to-detonation transition in the driver section before the door location, was transmitted into the combustible layer. The axial distance the layer spreads and the amount of mass diffusion across the layer were controlled by the flame ignition delay time after the door opens. Schlieren video and soot foils were used to measure the extent of detonation propagation through the layer. It was shown that detonation propagation through the layer is self-limiting due to over-mixing at the layer leading edge. Three-dimensional numerical simulations, including viscous and multicomponent mass diffusion effects, predicted the composition distribution within the layer. The cell size distribution, calculated based on the theoretical ZND induction zone length, corresponding to the simulation composition distribution showed that a cell size gradient-based failure criterion successfully predicted the extent of propagation in the layer.

Separation shock unsteadiness in an incident-shock-induced interaction with and without control is evaluated in a Mach 2.05 flow using a (14^{circ }) shock generator. An array of mechanical vortex generators (MVGs) in the form of rectangular vanes (MVG1), ramp vanes (MVG2), and a delta ramp (MVG3) is placed (14delta ) upstream of the interaction region ((delta =5.2,{textrm{mm}}) being the local boundary layer thickness at the interaction). Among all the devices tested, MVG1 shows a maximum reduction of the separation length (about 28% relative to the no-control case). The spectra at separation also show a shift in the dominant frequency from 220 Hz without control to 539 Hz with MVG1. Interestingly, the peak rms (root mean square) value is seen to occur with control at much larger intermittency values ((upgamma _{upsigma ,{textrm{max}}}=0.92) for MVG1) in contrast to the no-control case in which it occurs at (upgamma _{upsigma ,{textrm{max}}}=0.5) as reported so far. The auto-correlation at the separation and reattachment shock locations indicates the presence of relatively small-scale structures with control as compared to the case without control. Out of all the control cases tested, MVG1 exhibits better separation control with a relatively lower unsteadiness level.