Low-pressure Cavity Research Articles

Keeping it "straight": how to do spinal tumor ablation with vertebral augmentation

This technical review provides a comprehensive overview of spinal tumor ablation and vertebral augmentation. These percutaneous minimally invasive procedures offer significant survival and palliative pain relief benefits for patients with pathological vertebral fractures. Vertebral augmentation, which includes vertebroplasty and kyphoplasty, involves injecting cement into fractured vertebral bodies to restore height. While vertebroplasty involves the direct injection of cement into a fractured vertebral body, kyphoplasty involves using a balloon to create a low-pressure cavity to allow for cement injection to restore the vertebral body height. Over the years, this technique has evolved into a straightforward process, though it presents certain technical challenges discussed in this article.

Optimization Determination Method for the Explicit Equation of Scraper Motion Quantity in an Elliptical Rotor Scraper Pump

Addressing the inherent drawbacks of conventional positive displacement pumps, such as complex structure, poor sealing, low volumetric efficiency, and high noise, an innovative design of an elliptical rotor scraper pump (ERSP) was proposed. By segregating the pump chamber into high-pressure and low-pressure cavities, the scraper minimizes operational noise and significantly improves volumetric efficiency. To analyze the motion state of the ERSP, a mathematical model was established, determining the coordinated movement between the scraper and rotor using different optimization methods. The equations of coordinated action were derived and validated with relevant software through constraints applied to three algorithms and polynomial fitting. The flow field model of the ERSP was defined based on the established coordinated movement equation, and computational fluid dynamics (CFD) simulations were conducted to analyze pressure and velocity fields within the pump. A prototype of the ERSP was fabricated and tested, confirming its feasibility and advantages in enhancing fluid pressure and flow speed. This study provides valuable insights into the dynamic characteristics and structural optimization of fluid rotor pumps, contributing to anticipating and resolving potential faults and promoting the development of fluid power machinery.

Analysis of circumferential gas force on star-wheel in a single screw expander

Service life of a star-wheel directly affects operation stability of a single screw expander. In addition, since star-wheel tooth of a single screw expander is prone to wear, circumferential force of the star-wheel rotor is investigated in this paper. Geometric gap models between the meshing pair with a single line envelope profile at different rotation angles are established and meshed section pressure fields and velocity fields of a referent tooth flank are simulated. Circumferential gas torque calculation model of a referent tooth and multi-tooth are established, and torque influencing factors are analyzed. Results indicate that the pressure difference between the expansion cavity and low-pressure cavity as well as gap geometry shape have a significant influence on the pressure field and velocity field. Both gas forces that act on the meshed section and the unmeshed section of the tooth have significant effects on the circumferential torque. Due to multi-tooth coupling, total gas torque periodically changes with the star-wheel rotation angle. Moreover, the negative values indicate that the gas torque on the left side of the star-wheel is greater than that on the right side. Unbalanced gas torque can cause star-wheel teeth deviation towards the side with less force. This consequently results in wear increase.

Numerical Simulation of Leakage and Diffusion Process of LNG Storage Tanks

To investigate the evolution process of LNG (Liquefied Natural Gas) liquid pool and gas cloud diffusion, the Realizable k-ε model and Eluerian model were used to numerically simulate the liquid phase leakage and diffusion process of LNG storage tanks. The experimental results showed that some LNG flashed and vaporized rapidly to form a combustible cloud during the continuous leakage. The diffusion of the explosive cloud was divided into heavy gas accumulation, entrainment heat transfer, and light gas drift. The vapor cloud gradually separated into two parts from the whole “fan leaf shape”. One part was a heavy gas cloud; the other part was a light gas cloud that spread with the wind in the downwind direction. The change of leakage aperture had a greater impact on the whole spill and dispersion process of the storage tank. The increasing leakage aperture would lead to 10.3 times increase in liquid pool area, 78.5% increase in downwind dispersion of methane concentration at 0.5 LFL, 22.6% increase in crosswind dispersion of methane concentration at 0.5 LFL, and 249% increase in flammable vapor cloud volume. Within the variation range of the leakage aperture, the trend of the gas cloud diffusion remained consistent, but the time for the liquid pool to keep stable and the gas cloud to enter the next diffusion stage was delayed. The low-pressure cavity area within 200 m of the leeward surface of the storage tank would accumulate heavy gas for a long time, forming a local high concentration area, which should be an area of focus for alert prediction.

Development of an Apparatus for Testing of High-Temperature Static Seals

A test apparatus is described that evaluates the performance of static aviation seals under an environment similar to the lubrication system in gas turbine engines. The seals can be tested at operating temperatures as high as 400 °C in a static seal rig (SSR). Incorporating an air–oil mist generating system, the test facility exposes the seals to degraded oil, which can deteriorate the performance of seals through chemical attack. The test rig provides representative operating conditions to test four piston and four face seals at the same time. Throughout the aging process of seals, the rig carries out pressure decay leak tests at regular intervals and monitors temperature and pressure at several locations in the test chamber. The capabilities of the rig are demonstrated by presenting the results of durability testing with five types of seals. The results obtained with both successful and failed seals indicate that regular leak testing along with real-time measurement of temperature at the outlet of the low-pressure cavity can provide adequate information as to the operating status of the seals at different stages of testing and operating conditions.

Development of a mathematical model of molecular-selective gas transfer in a hybrid membrane-adsorption oxygen concentrator

Most modern oxygen concentrators use sorption air separation technology. The development of a hybrid membrane-sorption separation technology can improve the energy efficiency of the process. The paper considers a mathematical model of molecular-selective gas transfer in a hybrid oxygen concentrator consisting of adsorption and membrane units combined into a circulation loop. The considered hybrid model has based on a model of equilibrium isothermal sorption with linear isotherms without longitudinal mixing for numerical calculation of the adsorption unit of the system and a model of ideal displacement in the high-pressure cavity with perpendicular outflow in the low-pressure cavity to calculate the system membrane unit. In the course of the study, the high adequacy of the model was obtained with an experiment in the range of oxygen concentrations up to 90%, and a comparison was made with previously obtained results. A new model has been proposed that can take into account the equalization and countercurrent blowing stages.

Advanced numerical investigation of the heat flux in an array of microbolometers

The investigation of the thermal properties of an array of microbolometers has been carried out by mean of two independent numerical analysis, respectively the Direct-Simulation Monte Carlo (DSMC) and the classic diffusive approach of the Fourier’s equation. In particular, the thermal dissipation of a hot membrane placed in a low-pressure cavity has been studied for different values of the temperature of the hot body and for different values of the pressure of the environment. The results for the heat flux derived from the two approaches have then been compared and discussed.

Partial discharge characteristics of cavities with different appearances and positions in solid insulation

A cavity is a typical defect in a solid insulator which may cause the appearance of partial discharge (PD) activity in power equipment [1–4]. It is mainly caused by imperfections in the manufacturing process as well as by the creep during equipment operation [5]. In epoxy resin insulators, the creep brings irreversible deformations that increase with time at elevated temperatures and external stress [6,7]. Once a cavity has been formed, the PD can lead to insulation failure. Many studies on PDs in void defects, a type of gas-filled cavity, have been performed by pasting together three epoxy laminates and demonstrated factors that affect their intensity, including void size, shape and interface roughness [8–12]. However, such a test void is quite different in its structure and composition from what we further consider in this article, i.e., cavities appearing during actual insulator operation. Moreover, the pressure in a real cavity is usually very low, because of air leakage and polymerization shrinkage, whereas the test void typically remains under atmospheric pressure. Considering the above mentioned differences, we prepared experimental model insulators, containing various low-pressure cavities. The insulators were manufactured by means of automatic pressure gelation (APG) technique by changing the pressure and temperature during the resin pouring process, which allowed for simulating a more realistic operating environment.

Rarefaction and scale effects on heat transfer characteristics for enclosed rectangular cavities heated from below

The fluid-flow and heat transfer in a buoyancy-driven microcavity heated from below are numerically investigated. In spite of the fact that microcavities are widely used in microelectro- mechanical systems, now a day, more interest in the evacuated cavity on Eqn. solar collectors are very common to reduce heat loss from the system. This paper provides a useful information for engineers to estimate heat transfer in low pressure cavities. The finite volume technique was used to solve the governing equations along with temperature jump and slip flow boundary conditions.The simulations are carried out for various cavity aspect ratios (H/L) and different Rayleigh number for both macroand micro-fluids. The effect of Knudsen number in the rarefied flow regime (microfluidic) has also been investigated. It is shown that for both cases the effect of aspect ratios on heat transfer becomes significant at high Rayleigh numbers and when the aspect ratio is below 5. It was also found that increasing Knudsen number reduces the heat transfer. The interaction between Nusselt, Rayleigh, Knudsen numbers, and the aspect ratio was investigated using the design of experiments, results show that no interaction between these parameters. To help engineers to estimate heat transfer in low pressure cavities, widely used in solar energy applications, a correlation for convection heat transfer coefficient is introduced.

The granular silo as a continuum plastic flow: The hour-glass vs the clepsydra

The granular silo is one of the many interesting illustrations of the thixotropic property of granular matter: a rapid flow develops at the outlet, propagating upwards through a dense shear flow while material at the bottom corners of the container remains static. For large enough outlets, the discharge flow is continuous; however, by contrast with the clepsydra for which the flow velocity depends on the height of fluid left in the container, the discharge rate of granular silos is constant. Implementing a plastic rheology in a 2D Navier-Stokes solver (following the μ(I)-rheology or a constant friction), we simulate the continuum counterpart of the granular silo. Doing so, we obtain a constant flow rate during the discharge and recover the Beverloo scaling independently of the initial filling height of the silo. We show that lowering the value of the coefficient of friction leads to a transition toward a different behavior, similar to that of a viscous fluid, and where the filling height becomes active in the discharge process. The pressure field shows that large enough values of the coefficient of friction (≃0.3) allow for a low-pressure cavity to form above the outlet, and can thus explain the Beverloo scaling. In conclusion, the difference between the discharge of a hourglass and a clepsydra seems to reside in the existence or not of a plastic yield stress.

Increasing the stability of fuel injection pump process

A patented construction of plunger and barrel assembly is proposed. A ball valve is placed in its axial bore to eliminate the fuel flowing into low pressure cavity during the injection process. The design of plunger and barrel assembly and its operation concept are considered.

Dissociation of molecular iodine in RF discharge for oxygen-iodine lasers

The dissociation of molecular iodine in 40 MHz-RF discharge was studied experimentally. This generation of atomic iodine is aimed at use in oxygen-iodine lasers. The discharge was ignited in a mixture of I2 + buffer gas fast-flowing through the cylindrical chamber and the discharge products were injected into a supersonic flow of nitrogen. The atomic iodine number density was measured in a low-pressure cavity after mixing with nitrogen and the dissociation fraction was calculated related to the input I2 flow rate. The dissociation fraction of 46.2% was achieved at 0.22 mmol/s of I2 and 7 mmol/s of Ar and RF power of 500 W. Argon and helium were used as a buffer gas; discharge stability and dissociation efficiency were better with argon. At the I2 flow rate corresponding to the operation of a 1 kW chemical oxygen-iodine laser, the dissociation fraction was about 20%. The dissociation efficiency (the fraction of absorbed energy used for the dissociation) significantly decreased with increasing in the specific energy. At a reasonable I2 flow rate (0.32 mmol/s), the maximum achieved efficiency was 8.5% and the corresponding energy cost was 8.9 eV per dissociating of one I2 molecule. The input energy of more than 3 kJ per 1 mmol of I2 is needed for dissociating at least 50% of I2. The obtained dependencies on the gas flow rates infer a good chance for scaling-up of the tested RF discharge generator for the intended application.

Numerical Study on Air-involved Cavity during Water Exit of Underwater Vehicle

In the underwater vehicle launching process, some air in the launch tube will be involved in the low-pressure cavity formed at the shoulder part of the vehicle. This cavity mixed with air will collapse during water exit, causing dramatic change of flow field, which has a certain impact on the load environment. Based on homogeneous multiphase flow model, numerical simulation of the collapse of the cavity filled with vapor and involved air has been conducted. The results show that the nose shape with stronger cavitation ability is much easier to make the air in the launching tube involved in the low-pressure cavity. When air is mixed with vapor in the cavity, the rebound phenomenon will occur in the cavity collapse process. And there will also be multiple peaks of surface pressure.

Jet-regulated cooling catastrophe

We present the first implementation of Active Galactic Nuclei (AGN) feedback in the form of momentum driven jets in an Adaptive Mesh Refinement (AMR) cosmological resimulation of a galaxy cluster. The jets are powered by gas accretion onto Super Massive Black Holes (SMBHs) which also grow by mergers. Throughout its formation, the cluster experiences different dynamical states: both a morphologically perturbed epoch at early times and a relaxed state at late times allowing us to study the different modes of BH growth and associated AGN jet feedback. BHs accrete gas efficiently at high redshift (z>2), significantly pre-heating proto-cluster halos. Gas-rich mergers at high redshift also fuel strong, episodic jet activity, which transports gas from the proto-cluster core to its outer regions. At later times, while the cluster relaxes, the supply of cold gas onto the BHs is reduced leading to lower jet activity. Although the cluster is still heated by this activity as sound waves propagate from the core to the virial radius, the jets inefficiently redistribute gas outwards and a small cooling flow develops, along with low-pressure cavities similar to those detected in X-ray observations. Overall, our jet implementation of AGN feedback quenches star formation quite efficiently, reducing the stellar content of the central cluster galaxy by a factor 3 compared to the no AGN case. It also dramatically alters the shape of the gas density profile, bringing it in close agreement with the beta model favoured by observations, producing quite an isothermal galaxy cluster for gigayears in the process. However, it still falls short in matching the lower than Universal baryon fractions which seem to be commonplace in observed galaxy clusters.

A Numerical Study of Labyrinth Seal Flutter

A numerical study of a labyrinth-type turbine seal flutter in a large turbofan engine is described. The flutter analysis was conducted using a coupled fluid-structure interaction code, which was originally developed for turbomachinery blade applications. The flow model is based on an unstructured, implicit Reynolds-averaged Navier–Stokes solver. The solver is coupled to a modal model for the structure obtained from a standard structural finite element code. During the aeroelasticity computations, the aerodynamic grid is moved at each time step to follow the structural motion, which is due to unsteady aerodynamic forces applied onto the structure by the fluid. Such an integrated time-domain approach allows the direct computation of aeroelastic time histories from which the aerodynamic damping, and hence, the flutter stability, can be determined. Two different configurations of a large-diameter aeroengine labyrinth seal were studied. The first configuration is the initial design with four fins, which exhibited flutter instability during testing. The second configuration is a modified design with three fins and a stiffened ring. The steady-state flow was computed for both configurations, and good agreement was reached with available reference data. An aeroelasticity analysis was conducted next for both configurations, and the model was able to predict the observed flutter behavior in both cases. A flutter mechanism is proposed, based on the matching of the structural frequencies to the frequencies of waves traveling in the fluid, in the interfin cavities and in the high- and low-pressure cavities.

Mechanisms behind postspinal headache and brain stem compression following lumbar dural puncture – a physiological approach

The cause of postspinal headache and its specific characteristics are unknown, and whether lumbar dural puncture (LP) triggers brain-stem compression in patients with brain oedema is still controversial. Hydrostatic effects of distal opening of the dural sac towards the atmosphere are described and applied to the normal brain and the brain with disrupted BBB. Analogue analyses from previous results using an isolated skeletal muscle enclosed in a rigid shell were applied to the brain in an attempt to simulate and verify the haemodynamic effects of distal opening of the spinal canal. The theoretical considerations and the experimental results are compatible with the hypothesis that hydrostatic effects of distal opening of the fluid-filled spinal canal may obliterate the normal subdural venous collapse after a change from the horizontal to vertical position, which may be compatible with postural postspinal headache as occurring close to pain-sensitive meningeal regions. The hydrostatic forces may also initiate transcapillary filtration and aggravate oedema when permeability is increased, which may cause a narrower situation in the brain stem region, perhaps aggravated by venous stasis and a Cushing reflex-induced increase in blood pressure. An magnetic resonance imaging (MRI) picture illustrates how this scenario may separate the subdural space into an upper high- and a lower low-pressure cavity, pressing the brain downwards with sagging of the brain. A life-threatening positive feedback situation for brain-stem compression may develop. The present study strongly suggests that postspinal headache and brain-stem compression and other LP-related effects are predictable following LP, without involving CSF leakage, and can be explained by hydrostatic effects triggered by distal opening of the normally closed dural space to the atmosphere.

"Burst" technology with feedback-loop control for capacitive detection and electrostatic excitation of resonant silicon sensors

A method for excitation and detection of resonant silicon sensors based on discontinuous, burst excitation is presented. The solution eliminates the crosstalk between electrostatic excitation and capacitive detection by separating them in time. High excitation voltages can be combined with highly sensitive detection electronics. The method facilitates the use of large distances between the resonator and electrodes used for excitation and detection. The method was successfully tested with feedback-loop control on silicon resonant density and pressure sensors where the electrodes were positioned outside a glass. Continuous measurements of gas pressures and liquid densities were realized. The simplified fabrication process utilized reduces the risk of leakage from the ambient pressure to the low-pressure cavities in which the resonators are encapsulated since electrical feedthroughs are not needed. Excitation voltages alternating between 0 and 150 V could be applied to the resonators with measured electronics sensitivities of 0.4 fF. Signal-to-noise ratios (SNRs) as high as 100 (density sensor) and 360 (pressure sensor) were obtained. The electronic evaluation revealed that the burst duty cycle (i.e., the excitation time relative to the free oscillation time) had a strong influence on the output detection voltage. As few as two excitation periods with a burst cycle frequency of 115 Hz and a burst duty cycle of 1% was sufficient to select and lock the resonance frequency (28 042 Hz) for the tested pressure sensor. The same electrodes could be used for both excitation and detection. A novel solution is also presented that eliminates the charging effect of dielectric surfaces which otherwise can be a problem for capacitive detection.

Radiation Heat Transfer in Multitube, Alkali-Metal Thermal-to-Electric Converter

Vapor anode, multitube Alkali-Metal Thermal-to-Electric Converters (AMTECs) are being considered for a number of space missions, such as the NASA Pluto/Express (PX) and Europa missions, scheduled for the years 2004 and 2005, respectively. These static converters can achieve a high fraction of Carnot efficiency at relatively low operating temperatures. An optimized cell can potentially provide a conversion efficiency between 20 and 30 percent, when operated at a hot-side temperature of 1000--1200 K and a cold-side temperature of 550--650 K. A comprehensive modeling and testing program of vapor anode, multitube AMTEC cells has been underway for more than three years at the Air Force Research Laboratory`s Power and Thermal Group (AFRL/VSDVP), jointly with the University of New Mexico`s Institute for Space and Nuclear Power Studies. The objective of this program is to demonstrate the readiness of AMTECs for flight on future US Air Force space missions. A fast, integrated AMTEC Performance and Evaluation Analysis Model (APEAM) has been developed to support ongoing vacuum tests at AFRL and perform analyses and investigate potential design changes to improve the PX-cell performance. This model consists of three major components (Tournier and El-Genk 1998a, b): (a) a sodium vapor pressure loss model, which describes continuum,more » transition and free-molecule flow regimes in the low-pressure cavity of the cell; (b) an electrochemical and electrical circuit model; and (c) a radiation/conduction heat transfer model, for calculating parasitic heat losses. This Technical Note describes the methodology used to calculate the radiation view factors within the enclosure of the PX-cells, and the numerical procedure developed in this work to determine the radiation heat transport and temperatures within the cell cavity.« less

Reduction of increment of Rayleigh-Taylor instability in specially designed multilayer inertial-confinement-fusion targets

The problem of hydrodynamic stability and mixing is very important for inertial-confinement-fusion (ICF) systems based upon high compression of fuel before ignition. The ablative drive of foils and compression of shells are unstable. The fundamental isobaric ${f}^{\ensuremath{-}}$ mode is the most destructive one. It conserves pressures in the Lagrangian particles. A way to remove this dangerous mode is presented, based on special distributions of mass among subshells in the multishell target. The obtained solution follows from a consideration of new, inverse-density polytropes that have negative values of the polytropic index $N$, $\ensuremath{\rho}(r)\ensuremath{\propto}(r\ensuremath{-}{r}_{V}{)}^{N}$, where ${r}_{V}$ is the radius of an inner, low-pressure cavity filled with a fuel. Polytropes describe inhomogeneous incompressible and compressible cases. Density of material $\ensuremath{\rho}$ does not vanish in these distributions, as in the case of usual polytropes with $N>0$ considered previously in geophysics and astrophysics. Conversely $\ensuremath{\rho}$ rises when we approach the boundary with vacuum. This property allows us to simulate multilayer distributions of $\ensuremath{\rho}$ that are typical for ICF targets. In these targets the high-density subshells surround the low pressure or vacuum cavity, while the outer subshells are made from low-density materials such as plastics, foams, and/or from composite materials. The proposed distributions are self-similar. Therefore their linear dynamics is scale invariant. New acoustic fundamental modes ${f}_{P}^{\ifmmode\pm\else\textpm\fi{}}$ are found and an interesting correspondence between acoustic and gravity modes is presented. (The ${f}^{\ifmmode\pm\else\textpm\fi{}}$ or ${f}_{G}^{\ifmmode\pm\else\textpm\fi{}}$ fundamental modes are the well-known gravity modes.)

Partial suppression of hydrodynamic mixing in profiled shells

The problems of stability and mixing are important in the physics of high-energy densities. Ablation-induced acceleration of foils and compression of liners entail loss of symmetry and the development of instability. The most destructive instability is the fundamental f − mode, which conserves the pressure in Lagrangian particles. A means has been proposed to eliminate this dangerous mode, based on special profiling of the mass distribution among the subshells. The presence of this mode has led to novel proposals for limiting the degree of instability and optimization of the shells by profiling in the important case of very large density ratios at the ablation front. The solution is based on a class of new polytropes with an inverted density profile and a negative polytrope index N. In this class the density ρ of the material does not decrease towards the boundary with the vacuum, as for ordinary polytropes with N>0, but rather increases. This permits modeling multilayer distributions of ρ typical of inertial confinement fusion systems in which the high-density subshells form an inner core surrounding a low-pressure cavity, and the outer layers are made from low-density materials (plastic, foam type materials, composites). It is emphasized that the distributions are self-similar, and hence both the linear and the turbulent dynamics are scale-invariant. The spectral problem of perturbations in an incompressible fluid has a hidden symmetry. Isospectral deformations of the density profile I{ρ 0(y)} are known that leave the spectrum unchanged. It is of interest to apply the transformation I to the invariant f ± modes, since they are not tied to any specific profile of ρ 0(y). This paper analyzes a new type of invariant mode obtained in this way.