Gas-filled Cavity Research Articles

Investigation of ionization chamber perturbation factors using proton beam and Fano cavity test for the Monte Carlo simulation code PHITS.

The reference dose for clinical proton beam therapy is based on ionization chamber dosimetry. However, data on uncertainties in proton dosimetry are lacking, and multifaceted studies are required. Monte Carlo simulations are useful tools for calculating ionization chamber dosimetry in radiation fields and are sensitive to the transport algorithm parameters when particles are transported in a heterogeneous region. We aimed to evaluate the proton transport algorithm of the Particle and Heavy Ion Transport Code System (PHITS) using the Fano test. The response of the ionization chamber and beam quality correction factors were calculated using the same parameters as those in the Fano test and compared with those of other Monte Carlo codes for verification. The geometry of the Fano test consisted of a cylindrical gas-filled cavity sandwiched between two cylindrical walls. was calculated as the ratio of the absorbed dose in water to the dose in the cavity in the chamber. We compared the calculated using PHITS with that of a previous study, which was calculated using other Monte Carlo codes (Geant4, FULKA, and PENH) under similar conditions. The flight mesh, a parameter for charged particle transport, passed the Fano test within 0.15%. This was shown to be sufficiently accurate compared with that observed in previous studies. The calculated using PHITS were 1.116 ± 0.002 and 1.124 ± 0.003 for NACP-02 and PTW-30013, respectively, and the were 0.981 ± 0.008 and 1.027 ± 0.008, respectively, at 150MeV. Our results indicate that PHITS can calculate the and with high precision.

Pore development in viscoelastic foods during drying.

In this paper, we present a numerical model that can describe the pore formation/cavitation in viscoelastic food materials during drying. The food material has been idealized as a spherical object, with a core/shell structure and a central gas-filled cavity. The shell represents a skin as present in fruits/vegetables, having a higher elastic modulus than the tissue, which we approximate as a hydrogel. The gas-filled pore is in equilibrium with the core hydrogel material, and it represents pores in food tissues as present in intercellular junctions. The presence of a rigid skin is a known prerequisite for cavitation (inflation of the pore) during drying. For modeling, we follow the framework of Suo and coworkers, describing the inhomogeneous large deformation of soft materials like hydrogels - where stresses couple back to moisture transport. In this paper, we have extended such models with energy transport and viscoelasticity, as foods are viscoelastic materials, which are commonly heated during their drying. To approach the realistic properties of food materials we have made viscoelastic relaxation times a function of Tg/T, the ratio of (moisture dependent) glass transition temperature and actual product temperature. We clearly show that pore inflation only occurs if the skin gets into a glassy state, as has been observed during the (spray) drying of droplets of soft materials like foods.

Optimizing the thermal performance of the window with horizontal transparent insulation slats: A comprehensive analysis and comparative evaluation

Using slats made of transparent insulation material placed within the insulated glass unit (IGU) cavity can effectively reduce convective heat exchange in the gas cavity and improve windows' thermal resistance. A denser slat arrangement in the cavity of the Window with horizontal Transparent Insulation Slats (WTIS) contributes to better convective suppression within the gas-filled cavity. Still, it also raises concerns regarding excessive heat conduction along the slats. Therefore, further investigation is necessary to explore the optimal structure. In this study, we evaluated the thermal performance of WTIS using a validated numerical model and WINDOW7.8, considering various slat spacing, slat thickness, slat width, and cavity thickness. The air-filling and argon-filling cases were discussed. Furthermore, we experimented with testing the thermal performance of WTIS under different temperature conditions and validated the software calculation results. The results demonstrate that reducing the slat thickness and increasing the slat width is beneficial to improve the thermal resistance. The results identified an optimal slat spacing value of WTIS, which depends on both the cavity thickness and slat thickness and increases with their increase. The thermal resistance of optimized argon-filled WTIS is 39.47%-74.07% higher than that of argon-filled IGU.

Combined natural-convection and radiation in presence of internal heat generation source in absorbing-emitting-scattering medium

Numerical investigation of combined laminar natural convection and volumetric radiation with internal heat generation is presented in this paper and computations are performed for a grey gas-filled square cavity whose horizontal walls are adiabatic and vertical walls are differentially heated. The convection is treated under Boussinesq approximation by an approach based on finite-volumes and the volumetric radiation by the discrete ordinates method. Flow and heat transfer characteristics through isotherms, streamlines and average Nusselt numbers have been presented for an external Rayleigh number 106, internal Rayleigh number 0 to 4x1012, optical thickness 0 to 10 and Albedo 0 to 1. Representative results illustrating the effects of the optical thickness and the internal heat generation on the flow and the temperature distribution within the cavity are presented. The results reveal that the fluid flow and heat transfer are influenced significantly by the volumetric radiation and the internal heat generation. By comparing the solutions in pure convection, the results in combined convection-volumetric radiation show that when the medium is participating, the effect of internal source presence is very important.

The Effect of the Spargers Design on the Wastewater Treatment of Gas-Liquid Dispersion Process in a Stirred Tank.

The hydrodynamic and power characteristics of curved Rushton impeller in an air-wastewater system were investigated using the CFD-PBM method. Studies were conducted primarily in a mixing vessel of diameter 0.39 m. The inference of operating conditions, sparger distribution, and numbers on bulk flow patterns, gas-filled cavity formation, and power consumption have been investigated in detail. It found that the gassed power consumption is closely related to cavity shape and flow patterns. In particular, the development of large cavities causes a significant reduction in power drawn, impeller pumping capacity, and gas dispersion capability. The sparger distribution and location have a strong influence on relative power drawn, power required to disperse gas, and stability of operation. Of the sparger configurations studied, the use of three sparger distributions is suggested, since relative power drawn, gas dispersion capability, and flow patterns in dispersing gas are all enhanced.

Reaction–diffusion simulations of hydrogen isotope trapping and release from cavities in tungsten, I: Single cavity

We present simulations of deuterium thermal desorption spectra (TDS) from a single nm-sized spherical cavity in tungsten. We study both D2 gas-filled cavities and cavities with trapping only at surface sites. The simulations are based on the diffusion theory and the kinetic model of deuterium interaction with metal surfaces. We show that the previously used approaches based on local thermodynamic equilibrium between D2 gas and the subsurface solute deuterium are inadequate for simulating TDS spectra. We demonstrate the influence of cavity radius, D2 gas pressure, the parameters of the surface and the bulk. The deuterium release rate from a gas-filled cavity at a fixed temperature (below the TDS peak) stays constant until the D2 gas is depleted, in contrast to exponential decay observed for conventional deuterium trapping sites. Using the simulations and a simplified analytic model we demonstrate that for cavities the dependence of the TDS peak position on the heating rate does not follow exactly the Kissinger equation.

Using the swimbladder as a respiratory organ and/or a buoyancy structure-Benefits and consequences.

A swimbladder is a special organ present in several orders of Actinopterygians. As a gas-filled cavity it contributes to a reduction in overall density, but on descend from the water surface its contribution as a buoyancy device is very limited because the swimbladder is compressed by increasing hydrostatic pressure. It serves, however, as a very efficient organ for aerial gas exchange. To avoid the loss of oxygen to hypoxic water at the gills many air-breathing fish show a reduced gill surface area. This, in turn, also reduces surface area available for other functions, so that breathing air is connected to a number of physiological adjustments with respect to ion homeostasis, acid-base regulation and nitrogen excretion. Using the swimbladder as a buoyancy structure resulted in the loss of its function as an air-breathing organ and required the development of a gas secreting mechanism. This was achieved via the Root effect and a countercurrent arrangement of the blood supply to the swimbladder. In addition, a detachable air space with separated blood supply was necessary to allow the resorption of gas from the swimbladder. Gas secretion as well as gas resorption are slow phenomena, so that rapid changes in depth cannot instantaneously be compensated by appropriate volume changes. As gas-filled cavities the respiratory swimbladder and the buoyancy device require surfactant. Due to high oxygen partial pressures inside the bladder air-exposed tissues need an effective reactive oxygen species defense system, which is particularly important for a swimbladder at depth.

Influences of low temperature on internal gas-filled cavity discharges in EPR cable termination and its feature factors analysis

Ethylene–propylene rubber (EPR) cable is the most commonly used type in high-speed railway. Several flashover accidents take place in cable termination in an extremely cold environment. In this study, EPR cable terminations were tested in a simulated low-temperature system with varied applied voltages. The results showed that the differences in elastic modulus and free space volume between EPR insulation and stress control tube (SCT) in a frigid environment would lead to structural defects between them. Damages on the EPR insulation occurred in the form of flashover point when the ambient temperature declined to −30°C and below, with the growth rate of discharge and resulting extent of damage across the EPR/SCT interface being non-linearly related to ambient temperature. Besides, the partial discharge (PD) initial voltage and PD extinction voltage under low-temperature conditions reduced by >40% compared with those at 20°C. The flashover probability of EPR terminations increased significantly, and the shape characteristics of PD patterns have also changed obviously at low temperatures. It can be verified that under the high-voltage and low-temperature conditions, the EPR/SCT interface is prone to organisational mismatch, which will lead to the PD phenomenon and seriously affect the stable operation of EPR cable terminations.

Measurement and Diagnosis of PD Characteristics of Industrial Cable Terminations in Extreme Cold Environment

In the alpine region, there are many flashover failures of ethylene-propylene rubber (EPR) cable terminations due to cavity discharges in China high-speed railways (CHRs). The cable termination with cavity defects is used as test samples and a low-temperature PD test system is set up to study the characteristics of partial discharge (PD) of cavity defects in cable termination in those regions. Influences of the different temperatures on internal discharge in EPR insulation at −40°C and 20 °C have been analyzed. When the applied temperature dropped to −40°C, the PD initiation voltage and extinction voltage decreased by 31% and 28%, respectively, and the discharge pulse appeared at both positive and negative half-cycles. The results showed that, at 20 °C, the discharge was mainly caused by the internal discharge of the gas-filled cavity. Apart from the internal discharge of the gas-filled cavity, a marked trace of surface discharge was also detected at −40°C. Besides, the characteristics of PD development at −40°C are closely related to the discharge phase distribution and the asymmetry of the spectrum. The research can further reveal the development and evolution of PD of the gas-filled cavity and provides a basis for judging the severity of termination at low temperature.

The effect of photonic crystal fibre structure on the performance of Mach-Zehnder interferometer fibre optic gas sensors

A compact refractive index (RI) sensor, based on the Mach-Zehnder Interferometer (MZI), has been developed and experimentally evaluated for the highly sensitive detection and quantification of gases (Helium, Methane, and Carbon Dioxide). The RI sensor utilizes a variety of fibre types: Single Mode Fibre (SMF), Photonic Crystal Fibre (PCF), and Hollow-Core Photonic Crystal Fibre (HC-PCF). In order to fabricate the MZI sensors, a short length of sensing fibre was positioned between a lead-in and a lead-out single mode fibre (SMF) with an air gap at each interface. Three types of sensors were fabricated using this configuration employing 4 mm stub of: (i) PCF, (ii) 10 µm HC-PCF, and (iii) 20 µm HC-PCF as the sensing elements. The performance of these sensors, for detecting and measuring the quantity of gas present, were compared. The transmission spectrum of MZI sensors are formed by interference between the cladding and core modes. These transmission signals correspond to the frequency components in the sensor’s Fast Fourier Transform (FFT) spectrum. The effect of gap distance on the number and amplitude distribution of the modes was examined in an effort to optimize the design elements. The resulting fiber sensors can measure the RI of a gas-filled cavity and they showed high-sensitivity to helium, methane, and carbon dioxide. The highest RI sensitivity of 3210 nm/RIU was demonstrated in the RI range of 1.0000347-to-1.000449 by a sensor with a 4 mm long sensing stub element of 10 µm HC-PCF. Cyclic tests with the group of gases demonstrated that the measurements are highly repeatable. The measurement response and recovery times for all sensors were determined, and it was concluded that the 20 µm HC-PCF sensor has the fastest response/recovery time and the PCF sensor has the slowest. This research illustrates that the sensors fabricated by the proposed method have potential for improving the ability to detect and quantify pure gases. Additionally, the sensors are highly sensitive to low percentages of CO 2 , making them suitable for greenhouse gas measurement.

Experimental investigation on the cavity evolution and dynamics with special emphasis on the development stage of ventilated partial cavitating flow

Abstract Ventilated cavitating flows involve complex features and dynamic characteristics due to the strong interactions between gas and liquid. In the present work, we especially focus on the experimental investigation of the development stage of ventilated partial cavitating flow in a closed-loop water tunnel at Harbin Institute of Technology. In the experiments, a conical axisymmetric test body made of aluminum alloy with a ventilation slot at the nose is placed at the middle of the test section. A high-speed camera is used to capture the flow patterns. Five pressure transducers are installed in the test model to measure the unsteady dynamic pressure. A multi-field synchronous measurement system for cavitating flow is established to address the relationship between cavity evolution and instantaneous pressure. The effect of gas ventilation rate on cavity evolution and dynamic pressure are investigated. Three unsteady flow patterns are observed namely foamy cavity (FC), continuous transparent and foamy cavity (CTFC), continuous transparent and asymmetric cavity (CTAC) respectively. In addition, three distinct cavity regimes are also observed based on the dynamic cavity topology, referred to as gas-filled cavity region (GFCR), re-entrant jet region (REJR) and wake region (WR). The gas entrainment mechanism determined by the re-entrant flow model of a partial ventilated cavity is analyzed. A phenomenon we observed is that the increase in ventilation rate can improve the dynamic stability inside the cavity and can help reduce the load on the test body. Moreover, we find that there are location gaps in space between the pressure peaks and the foamy cavity closure position due to the effect of re-entrant flow.

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.

Rotational stabilisation of the Rayleigh–Taylor instability at the inner surface of an imploding liquid shell

A number of applications utilise the energy focussing potential of imploding shells to dynamically compress matter or magnetic fields, including magnetised target fusion schemes in which a plasma is compressed by the collapse of a liquid metal surface. This paper examines the effect of fluid rotation on the Rayleigh–Taylor (RT) driven growth of perturbations at the inner surface of an imploding cylindrical liquid shell which compresses a gas-filled cavity. The shell was formed by rotating water such that it was in solid body rotation prior to the piston-driven implosion, which was propelled by a modest external gas pressure. The fast rise in pressure in the gas-filled cavity at the point of maximum convergence results in an RT unstable configuration where the cavity surface accelerates in the direction of the density gradient at the gas–liquid interface. The experimental arrangement allowed for visualisation of the cavity surface during the implosion using high-speed videography, while offering the possibility to provide geometrically similar implosions over a wide range of initial angular velocities such that the effect of rotation on the interface stability could be quantified. A model developed for the growth of perturbations on the inner surface of a rotating shell indicated that the RT instability may be suppressed by rotating the liquid shell at a sufficient angular velocity so that the net surface acceleration remains opposite to the interface density gradient throughout the implosion. Rotational stabilisation of high-mode-number perturbation growth was examined by collapsing nominally smooth cavities and demonstrating the suppression of small spray-like perturbations that otherwise appear on RT unstable cavity surfaces. Experiments observing the evolution of low-mode-number perturbations, prescribed using a mode-6 obstacle plate, showed that the RT-driven growth was suppressed by rotation, while geometric growth remained present along with significant nonlinear distortion of the perturbations near final convergence.

Evaluation of a potential medical diagnosis application of sonoluminescence

Sonoluminescence (SL) is a phenomenon in which light is produced via violent collapse of a gas-filled cavity under excitation by a varying pressure field. Although the precise mechanism of this light production is not yet agreed upon in the literature, various applications of this effect have been established and are continually being developed. One such example of an application of this phenomenon that has not been extensively studied is the production of SL in biological fluids as a method for medical diagnostics. Measurements performed by Chernov et al. [in Proceedings of 14th International Symposium on Nonlinear Acoustics (1996), pp. 219–223] revealed varied intensities of SL emissions in blood plasma samples from groups of patients diagnosed with different diseases. We present an experimental apparatus for the production of single bubble sonoluminescence (SBSL) and subsequent measurement of radial oscillations using optical scattering techniques. This system will allow for characterization of the effects of both the biological fluid content and viscoelasticity on the spectra of SBSL light emissions. Additionally, these experimental results can be used to inform future computational models of this behavior. [This is a Plan II SAWIAGOS project.]

A hydrodynamic analysis of self-similar radiative ablation flows

Self-similar solutions to the compressible Euler equations with nonlinear conduction are considered as particular instances of unsteady radiative deflagration – or ‘ablation’ – waves with the goal of characterizing the actual hydrodynamic properties that such flows may present. The chosen family of solutions, corresponding to the ablation of an initially quiescent perfectly cold and homogeneous semi-infinite slab of inviscid compressible gas under the action of increasing external pressures and radiation fluxes, is well suited to the description of the early ablation of a target by gas-filled cavity X-rays in experiments of high energy density physics. These solutions are presently computed by means of a highly accurate numerical method for the radiative conduction model of a fully ionized plasma under the approximation of a non-isothermal leading shock wave. The resulting set of solutions is unique for its high fidelity description of the flows down to their finest scales and its extensive exploration of external pressure and radiative flux ranges. Two different dimensionless formulations of the equations of motion are put forth, yielding two classifications of these solutions which are used for carrying out a quantitative hydrodynamic analysis of the corresponding flows. Based on the main flow characteristic lengths and on standard characteristic numbers (Mach, Péclet, stratification and Froude numbers), this analysis points out the compressibility and inhomogeneity of the present ablative waves. This compressibility is further analysed to be too high, whether in terms of flow speed or stratification, for the low Mach number approximation, often used in hydrodynamic stability analyses of ablation fronts in inertial confinement fusion (ICF), to be relevant for describing these waves, and more specifically those with fast expansions which are of interest in ICF. Temperature stratification is also shown to induce, through the nonlinear conductivity, supersonic upstream propagation of heat-flux waves, besides a modified propagation of quasi-isothermal acoustic waves, in the flow conduction regions. This description significantly departs from the commonly admitted depiction of a quasi-isothermal conduction region where wave propagation is exclusively ascribed to isothermal acoustics and temperature fluctuations are only diffused.

Simulation of beam-induced plasma in gas-filled rf cavities

Processes occurring in a radio-frequency (rf) cavity, filled with high pressure gas and interacting with proton beams, have been studied via advanced numerical simulations. Simulations support the experimental program on the hydrogen gas-filled rf cavity in the Mucool Test Area (MTA) at Fermilab, and broader research on the design of muon cooling devices. SPACE, a 3D electromagnetic particle-in-cell (EM-PIC) code with atomic physics support, was used in simulation studies. Plasma dynamics in the rf cavity, including the process of neutral gas ionization by proton beams, plasma loading of the rf cavity, and atomic processes in plasma such as electron-ion and ion-ion recombination and electron attachment to dopant molecules, have been studied. Through comparison with experiments in the MTA, simulations quantified several uncertain values of plasma properties such as effective recombination rates and the attachment time of electrons to dopant molecules. Simulations have achieved very good agreement with experiments on plasma loading and related processes. The experimentally validated code SPACE is capable of predictive simulations of muon cooling devices.

Recent studies on nanosecond-timescale pressurized gas discharges

The results of recent experimental and numerical studies of nanosecond high-voltage discharges in pressurized gases are reviewed. The discharges were ignited in a diode filled by different gases within a wide range of pressures by an applied pulsed voltage or by a laser pulse in the gas-filled charged resonant microwave cavity. Fast-framing imaging of light emission, optical emission spectroscopy, x-ray foil spectrometry and coherent anti-Stokes Raman scattering were used to study temporal and spatial evolution of the discharge plasma density and temperature, energy distribution function of runaway electrons and dynamics of the electric field in the plasma channel. The results obtained allow a deeper understanding of discharge dynamical properties in the nanosecond timescale, which is important for various applications of these types of discharges in pressurized gases.

Scaled experiments of explosions in cavities

Consequences of an explosion inside an air-filled cavity under the earth's surface are partly duplicated in a laboratory experiment on spatial scales 1000 smaller. The experiment measures shock pressures coupled into a block of material by an explosion inside a gas-filled cavity therein. The explosion is generated by suddenly heating a thin foil that is located near the cavity center with a short laser pulse, which turns the foil into expanding plasma, most of whose energy drives a blast wave in the cavity gas. Variables in the experiment are the cavity radius and explosion energy. Measurements and GEODYN code simulations show that shock pressures measured in the block exhibit a weak dependence on scaled cavity radius up to ∼25 m/kt1/3, above which they decrease rapidly. Possible mechanisms giving rise to this behavior are described. The applicability of this work to validating codes used to simulate full-scale cavity explosions is discussed.

Acoustic streaming with allowance for heat transfer

Acoustic streaming in a gas-filled cavity under vibration is numerically investigated. The case of thermally insulated cavity walls is compared with the case of walls maintained at a constant temperature. A strong influence of heat transfer on the acoustic streaming pattern is revealed for weakly nonlinear processes and vibration frequencies below resonance. Cavities of different diameters are considered.

Experimental evidence of a bubble-merger regime for the Rayleigh-Taylor Instability at the ablation front

Under the Discovery Science program, the longer pulses and higher laser energies provided by the National Ignition Facility (NIF) have been harnessed to study, first time in indirect-drive, the highly nonlinear stage of the Rayleigh-Taylor Instability (RTI) at the ablation front. A planar plastic package with pre-imposed two-dimensional broadband modulations is accelerated for up to 12 ns by the x-ray drive of a gas-filled gold radiation cavity with a radiative temperature plateau at 175 eV. This extended tailored drive allows a distance traveled in excess of 1 mm for a 130 μm thick foil, a factor 3x larger than previously achieved on other laser facilities. As a consequence, we have measured the ablative RTI in transition from the weakly nonlinear stage up to the deep nonlinear stage for various initial conditions. A bubble merger regime has been observed and the ablative stabilization strength varied by changing the plastic dopant from iodine to germanium.