High-pressure Spot Research Articles

Enhanced reactivity by energy trapping in shocked materials: reactive metamaterials for controllable output

Through the use of carefully designed numerical experiments on an explosive system, that use predictive models for subcomponents and multi-material simulation, we demonstrate enhanced reactivity by energy trapping in regions of the reactive flow that were previously shocked. Particles and inclusions are placed in designed patterns in an explosive matrix. New capabilities in additive manufacture make it possible to consider novel designs, that we refer to as ‘reactive metamaterials’. For a fixed amount of energy delivered by a shock impactor, an explosive that normally would not detonate, will detonate when particles are included. Enhanced reactivity correlates precisely with a change in the partition of energy from kinetic to internal, via reflective processes and flow stagnation in high pressure systems. We analyse cases associated with high shock impedance tantalum particles, and void inclusions, individually and placed in a test array. High impedance reflectors trap energy in regions of pre-shocked material. Whereas void shock collapse causes depressurisation of the material along with rapid material flow, and high pressure spot formation related to the jet impact blast. We analyse how these limiting cases of high impedance particle arrays and void arrays partition specific kinetic and internal energy, during the shock impact transient on the system of matrix explosive and embedded particle/voids. Both generate specific flow fields, pressure and temperature cycles in the matrix material over interval times, determined by the particle/void size and placement. Design variations of the configurations presented here can be tested by both experiment and simulation, and can be searched for optimal designs, aided by modern machine learning search methods.

Single Contaminated Drops Falling through Stagnant Liquid at Low Reynolds Numbers

Numerical simulations of contaminated spherical drops falling through a stagnant liquid at low Reynolds numbers are carried out using the finite difference method. The numerical results are used to describe the behavior of the surfactant concentrations and to understand the surfactant effects on the fluid motions in detail. The predicted interfacial surfactant concentration, Γ, is almost zero for angles, θ, below a certain value (the stagnant-cap angle, θcap), whereas it steeply increases and reaches a large value for θ>θcap (the stagnant-cap region). The increase in the initial surfactant concentration, C0, in the drop enhances the adsorption from the drop to the interface, which results in the increase in Γ and the decrease in θcap. Peaks appear in the predicted Marangoni stresses around θcap, which causes similar peaks in the pressure distribution. The high-pressure spots prevent the fluid motion along the interface, which results in the formation of the stagnant-cap region and the attenuation of the tangential velocity in the continuous phase. The surfactant flux from the bulk to the interface decreases C in the vicinity of the interface for θ<θcap and the weak diffusion cannot compensate for the reduction in C by adsorption, which results in C at the interface smaller than C0. The pattern of the low C region is determined by the advection and does not smear out because of a small diffusive flux.

Large-Eddy Simulations of the Unsteady Behavior of a Hypersonic Intake at Mach 5

Three-dimensional high-fidelity numerical simulations of a Mach 5 hypersonic ramjet intake are performed combining a high-order and time-accurate large-eddy-simulation model with a sharp-interface immersed boundary method. The analysis provides a detailed characterization of the unsteady behavior of the intake. A systematic investigation is carried out varying the blockage of the intake channel, and, in particular, three blockage levels are investigated as far as their effect on the flow. The analysis shows that two main unsteady phenomena occur, the former at high frequency (little buzz) and the latter at low frequency (big buzz). When the intake channel is entirely opened, only the little buzz is present, whereas in partially blocked conditions, the big buzz prevails. This paper will show how the big buzz is correlated to the convection of high-density, high-pressure spots from the entrance of the intake channel, which creates the oscillatory unsteady behavior.

Improving the rotordynamic stability of short labyrinth seals using positive preswirl

Introducing a negative preswirl at the upstream of annular gas seals has been considered as an effective way to improve the system stability. This paper demonstrates a stability enhancement approach for a short labyrinth seal using positive preswirls. The static and dynamic characteristics of the labyrinth seal with various blade numbers (5, 10, 15), inlet preswirl ratios (–0.3, –0.15, 0, 0.15, 0.3) were studied. Results show that the inlet preswirl ratio has a dramatic effect on the circumferential location of the high-pressure spot for each seal cavity, particularly for the first cavity. The inlet preswirl ratio has opposite effects on the system stability due to the difference of high-pressure spot locations between the first cavity and the others. An increasing positive inlet preswirl could improve the system stability for the labyrinth seal with fewer blades (e.g. 5 blades). Its characteristics is mainly dominated by the first seal cavity. For the labyrinth seal with 10 blades, the system characteristics shows slight dependency on the inlet preswirl ratio. For the labyrinth seal with more blades (e.g. 15 blades), the negative inlet preswirl still increases the system stability, which agrees with the conventional conclusion. The paper provides a deeper understanding on the stability improvement of the labyrinth seal.

Experimental study of perforated-wall rotating detonation combustors

Perforated walls are potentially applied in rotating detonation combustors (RDCs) to stabilize combustion and perform transpiration cooling. This study involves an experimental investigation on the rotating detonation in perforated-wall combustors for the first time. Five types of walls with different hole sizes and perforated area ratios that range from 0 to 3.5% are examined to analyze acoustics and combustion characteristics, and performance of the RDC. The stable and unstable rotating detonation are both observed in the experiments, and the unstable phenomena mainly correspond to the counter two-wave rotating detonation that co-exists with the acoustic modes of the combustor. The acoustic modes are effectively suppressed by the perforated wall with area ratios over 1.75%, and the stability of rotating detonation significantly improves. The perforated walls significantly weaken the measured detonation pressure peaks and mitigate the impact of rotating detonation on the H2 plenum, while they do not evidently reduce the specific impulse. It is proposed that the acoustic modes are excited by local high-pressure spots generated by the collision of two detonation waves, and they induce the fluctuating pressure peaks and wave velocity by affecting the H2 injection. The perforated holes dissipate high-pressure spots, and thereby suppress the acoustic modes.

Liquid jet directionality and droplet behavior during emulsification of two liquids due to acoustic cavitation.

The present study numerically investigates liquid-jet characteristics of acoustic cavitation during emulsification in water/gallium/air and water/silicone oil/air systems. It is found that a high-speed liquid jet is generated when acoustic cavitation occurs near a minute droplet of one liquid in another. The velocity of liquid jet significantly depends on the ultrasonic pressure monotonically increasing as the pressure amplitude increases. Also, the initial distance between cavitation bubble and liquid droplet affects the jet velocity significantly. The results revealed that the velocity takes maximum values when the initial distance between the droplet and cavitation bubble is moderate. Surprisingly, the liquid jet direction was found to depend on the droplet properties. Specifically, the direction of liquid jet is toward the droplet in the case of water/gallium/air system, and vice versa the jet is directed from the droplet in the case of water/silicone oil/air system. The jet directionality can be explained by location of the high-pressure spot generated during the bubble contraction.

The Zeldovich spontaneous reaction wave propagation concept in the fast/modest heating limits

Quantitative mathematical models describe planar, spontaneous, reaction wave propagation (Zeldovich, Combust. Flame, vol. 39, 1980, pp. 211–214) in a finite hot spot volume of reactive gas. The results describe the complete thermomechanical response of the gas to a one-step, high-activation-energy exothermic reaction initiated by a tiny initial temperature non-uniformity in a gas at rest with uniform pressure. Initially, the complete conservation equations, including all transport terms, are non-dimensionalized to identify parameters that quantify the impact of viscosity, conduction and diffusion. The results demonstrate unequivocally that transport terms are tiny relative to all other terms in the equations, given the relevant time and length scales. The asymptotic analyses, based on the reactive Euler equations, describe both induction and post-induction period models for a fast heat release rate (induction time scale short compared to the acoustic time of the spot), as well as a modest heat release rate (induction time scale equivalent to the acoustic time). Analytical results are obtained for the fast heating rate problem and emphasize the physics of near constant-volume heating during the induction period. Weak hot spot expansion is the source of fluid expelled from the original finite volume and is a ‘piston-effect’ source of acoustic mechanical disturbances beyond the spot. The post-induction period is characterized by the explosive appearance of an ephemeral, spatially uniform high-temperature, high-pressure spot embedded in a cold, low-pressure environment. In analogy with a shock tube the subsequent expansion process occurs on the acoustic time scale of the spot and will be the source of shocks propagating beyond the spot. The modest heating rate induction period is characterized by weakly compressible phenomena that can be described by a novel system of linear wave equations for the temperature, pressure and induced velocity perturbations driven by nonlinear chemical heating, which provides physical insights difficult to obtain from the more familiar ‘Clarke equation’. When the heating rate is modest, reaction terms in the post-induction period Euler equations exhibit a form of singular behaviour in the high-activation-energy limit, implying the need to use a nonlinear exponential scaling for time and space, developed originally to describe spatially uniform thermal explosions (Kassoy, Q. J. Mech. Appl. Maths, vol. 30, 1977, pp. 71–89). Here again the result will be the explosive appearance of an ephemeral spatially uniform high-temperature, high-pressure hot spot. These results demonstrate that an initially weak temperature non-uniformity in a finite hot spot can be the source of acoustic and shock wave mechanical disturbances in the gas beyond the spot that may be related to rocket engine instability and engine knock.

Numerical investigation of the unsteady flow behaviors in a counter-rotating axial compressor

Unsteady numerical simulations of a counter-rotating axial compressor are performed to investigate the unsteady effects of rotor–rotor interaction process and the tip flow unsteady behavior near the stall operating condition. The results show that potential effects from the rear rotor have a more pronounced influence on the front rotor. Because of the interaction between the two rotors and the unsteady flow behaviors, the strongest fluctuant regions are different for the two rotors. Additionally, the oscillation on the pressure side is generally much stronger than that on the suction side. Frequency analyses show that self-unsteadiness occurs with the frequency of 0.76 blade passing frequency due to the impingement of the tip leakage flow on the pressure side near leading edge of the adjacent blade. An interesting phenomenon, i.e., the emergence of a local high pressure spot, is observed in the tip region. As a whole, it is concluded that there are two key effects of the emergence of the local high pressure zone in the tip region: (a) more leading edge spillage of tip leakage flow and increasing the risk to trigger stall; and (b) a decrease in blockage associated with double-leakage of tip clearance flow within the influenced region of the local high pressure zone.

Non-diffusive ignition of a gaseous reactive mixture following time-resolved, spatially distributed energy deposition

Systematic asymptotic methods are applied to the compressible conservation and state equations for a reactive gas, including transport terms, to develop a rational thermomechanical formulation for the ignition of a chemical reaction following time-resolved, spatially distributed thermal energy addition from an external source into a finite volume of gas. A multi-parameter asymptotic analysis is developed for a wide range of energy deposition levels relative to the initial internal energy in the volume when the heating timescale is short compared to the characteristic acoustic timescale of the volume. Below a quantitatively defined threshold for energy addition, a nearly constant volume heating process occurs, with a small but finite internal gas expansion Mach number. Very little added thermal energy is converted to kinetic energy. The gas expelled from the boundary of the hot, high-pressure spot is the source of mechanical disturbances (acoustic and shock waves) that propagate away into the neighbouring unheated gas. When the energy addition reaches the threshold value, the heating process is fully compressible with a substantial internal gas expansion Mach number, the source of blast waves propagating into the unheated environmental gas. This case corresponds to an extremely large non-dimensional hot-spot temperature and pressure. If the former is sufficiently large, a high activation energy chemical reaction is initiated on the short heating timescale. This phenomenon is in contrast to that for more modest levels of energy addition, where a thermal explosion occurs only after the familiar extended ignition delay period for a classical high activation reaction. Transport effects, modulated by an asymptotically small Knudsen number, are shown to be negligible unless a local gradient in temperature, concentration or velocity is exceptionally large.

Numerical simulation of fragmentation of melt drop triggered by external pressure pulse in vapor explosions

The fragmentation of molten drops is the key process in the fuel–coolant interaction (FCIs) which may occur during the course of a severe accident in a light water reactor (LWR). However, the mechanisms of this complicated process cannot be clarified sufficiently by experimental studies due to the rapid reaction. In this paper, a multi-phase thermal hydraulic code with the volume of fluid method (VOF) is developed and the fragmentation process of melt drops triggered by external pressure pulse is numerically analyzed to investigate the mechanism of fragmentation in vapor explosions. The simulation results show that the fragmentation process can be divided into several stages, including vapor film collapse, melt drop-coolant direct contact, formation of high pressure spots, rapid growth of a filament around the molten metal drop, rapid fuel coolant interaction area expansion, breaking up of the filament, and mixing of fragments with water. The calculation results are similar to Ciccarelli and Frost’s (Nucl. Eng. Des., 146, 109–132) experiment data. The simulation results suggest that growth and breaking up of a filament are the essential mechanism of melt tin drop fragmentation. Penetration and evaporation of the water jets, which are assumed as fragmentation mechanism in Kim’s model (Nucl. Sci. Eng., 98, 16–28, 1988), are not observed. In the calculation case, when molten metal density is hypothetically smaller, the water penetration is observed. Besides, the effects of external pressure pulse and molten metal temperature on the growth of filament and the explosion bubble are discussed.

Rehabilitation of a bilateral maxillectomy patient with a free fibula osteocutaneous flap*

Rehabilitation of patients who have undergone bilateral maxillectomy is difficult because of extensive loss of bone and soft tissue. In this clinical report, prosthodontic rehabilitation of oral function in a bilateral maxillecitomy patient combined with a new fibular osteocutaneous flap, which was designed to have two oronasal slits for the retention of an obturator prosthesis, was described. A 58-year-old man with a maxillary alveolar carcinoma underwent bilateral maxillectomy. The defect was reconstructed using a vascularized fibular bone wrapped circumferentially with a peroneal flap, which was fixed with miniplates between the right malar prominence and cut edge of the left zygoma remaining two slits anterior and posterior to the graft. Two and half weeks after the surgery, a delayed surgical obturator was delivered and an obturator prosthesis was delivered 6 weeks after the surgery. This obturator prosthesis could be extended into the slits to engage the tissue undercuts, and was stable during use. Mastication, deglutition, articulation and the mid-facial profile of the patient were rehabilitated. After installation of the obturator prosthesis, relining of the prosthesis base was carried out alongside the healing process of the graft, and adjustment of occlusions and high-pressure spots was carried out. No clinical disorders were observed either in the grafted tissue or the obturator prosthesis with a 3-year prognosis. Newly designing a fibular osteocutaneous flap combined with tissue-borne obturator prosthesis is one successful approach to the restoration of oral function, and increases the patient's quality of life after bilateral maxillectomy.

Missing pieces of the puzzle or about some unresolved issues in solid state chemistry of alkali metal aluminohydrides

The paper discusses some unresolved issues in the solid state chemistry of alkali metal aluminohydrides (alanates) relevant to high-capacity hydrogen storage. Analysis of experimental data available in chemical and materials science literature suggests a one-step mechanism for the thermal decomposition of both pure and Ti-doped aluminohydrides. Most likely, the presence of a titanium hydride phase in the catalyst is responsible for the catalytic effect of Ti-additives. Furthermore, ball-milling promotes chemical and phase transformations of solid alanates by enhancing mass transfer in the material and creating high-pressure spots where pressure-driven chemical reactions can take place.