Shell Perturbations of an Acoustic Thermometer Determined from Speed of Sound in Gas Mixtures

With the goal of achieving a better understanding of gas–shell coupling perturbations in the acoustic resonators used at INRiM for the determination of the Boltzmann constant, we measured the variation of their acoustic and microwave resonances induced by changing the composition of a binary He–Ar mixture which filled the cavity at constant temperature and pressure. As a consequence of the progressive dilution of a sample of initially pure He with Ar, the radial acoustic modes of the resonator spanned decreasing frequency intervals, partially overlapping, for several modes. In addition to the expected breathing mode of the shell, the results evidenced the presence of several other shell resonances at lower and higher frequencies, confirming that the elastic response of the assembled resonator significantly differs from that of a simple spherical shell. Experimental results are reported for two resonators which differ in design, dimensions, and constructing material. In spite of their being preliminary and susceptible of significant improvement, these results favor the interpretation of acoustic thermometry measurements with pure gases.     

Developing a Microhumidity Generator for Methane-Ethane Gas Mixtures

Methods are examined for drying natural gases and checking the degree of dryness by means of an experimental equipment. The working conditions are defined for a generator giving trace concentrations in a methane-ethane gas mixture.     

Measurements of Speed of Sound in Lean and Rich Natural Gas Mixtures at Pressures up to 37 MPa Using a Specialized Rupture Tube

Measurements of the speed of sound in 42 different compositions of lean, medium, and rich natural-gas mixtures using a specialized high-pressure rupture tube have been conducted. The rupture tube is made of stainless steel (internal diameter = 38.1 mm and length = 42 m), and is instrumented with 13 high-frequency-response dynamic pressure transducers (Endevco) mounted very close to the rupture end and along the length of the tube to capture the pressure-time traces of the decompression wave. Tests were conducted for initial pressures ranging from 10 MPa to 37 MPa and a temperature range from ?25°C to+68°C. Gas mixture compositions were controlled by mixing conventional natural-gas mixtures from an adjacent gas pipeline with richer components of alkanes. Temperature control is achieved by a heat tracer along the tube with a set point at the desired gas temperature of the particular test. Uncertainty analysis indicated that the uncertainty in the experimentally determined speed of sound in the undisturbed gas mixture at the initial pressure and temperature is on the order of 0.306 %. The measured speeds of sound were compared to predictions by five equations of state, namely; the Benedict–Webb–Rubin–Starling (BWRS), AGA-8, Peng–Robinson (PR), Redlich–Kwong–Soave (RK–Soave), and Groupe Européen de Recherches Gaziéres (GERG-2004) equations.     

Measuring Speed of Sound and Thermal Diffusivity in the Diamond-Anvil Cell

Transient grating spectroscopy in the diamond-anvil high-pressure cell permits, in favorable cases, determination of the equation of state and thermal transport properties of fluids at high temperatures and pressures. Measurements of the speed of sound and thermodynamic properties of aqueous Na₂SO₄to 3.4 GPa at 300°C and the thermal diffusivity of oxygen are reported as examples of the application of this technique.     

Photoacoustic Excitation of the Interior Surface of an Acoustic Resonator for Speed of Sound Measurement

A remote optical technique is proposed as a valid alternative to conventional electroacoustic excitation of sound inside an acoustic resonator. Efficient photoacoustic conversion of an amplitude-modulated light beam, impinging on the interior solid surface of the resonator wall, into a stationary acoustic field is achieved. The reported results were obtained in a cylindrical stainless steel resonator with a volume of about 572 cm³; the radiation source is the 514 nm line of an Ar⁺ laser. The laser beam can be focused on different points of the resonator internal surface. The signal frequency dependence is well interpreted in terms of gas-microphone detection theory. The precision obtained in the measurement of resonance frequency f _N and halfwidth g _N of the cavity modes is on the order of 10^-6 of f _N and is likely to be further improved by minor refinements of the experimental apparatus.     

Finite Element Simulation of Photoacoustic Pressure in a Resonant Photoacoustic Cell Using Lossy Boundary Conditions

The finite-element method (FEM) is used to simulate the photoacoustic signal in a cylindrical resonant photoacoustic cell. Simulations include loss effects near the cell walls that appear in the boundary conditions for the inhomogeneous Helmholtz equation governing the acoustic pressure. Reasonably good agreement is obtained between theoretical results and experimental data. However, it was anticipated that loss mechanisms other than viscous and thermal boundary losses occur and should be included. Nevertheless, the feasibility to use FEM together with the derived boundary conditions to simulate the photoacoustic signal was demonstrated and good agreement with experiments for the actual resonance frequency and the quality factor of the cell was obtained despite its complicated geometry.     

Concentration-Dependent Diffusion Coefficients of Binary Gas Mixtures Using a Loschmidt Cell with Holographic Interferometry

An improved experimental setup and data evaluation procedure are presented for a Loschmidt cell combined with interferometry to measure concentration-dependent binary diffusion coefficients. We overcome long-standing discrepancies about the concentration dependence found in the literature. The systematic analysis of the residuals from parameter estimation enabled the improvement of the experimental setup and the identification of relevant fitting parameters. In particular, we found that it is crucial to account for uncertainties (1) in the initial conditions, (2) in the thermal stability of the optical setup, and (3) in camera calibration. The improved experimental setup and data evaluation procedure are validated with diffusion measurements of the system helium–krypton. The concentration dependence of the diffusion coefficient is successfully determined from multiple experiments with gas mixtures of various initial compositions in the half-cells of the Loschmidt cell. The agreement with literature data and the excellent quality of fit allow for high confidence in the results. In Part II of this paper (Wolff et al., in Int J Thermophys, 2018,  https://doi.org/10.1007/s10765-018-2451-7), the improved measurement setup is combined with a refined diffusion model to determine concentration-dependent diffusion coefficients from single measurements of mixing pure gases.     

基于光声技术的火灾气体探测系统设计

近年来，针对标识性气体的探测成为火灾探测技术中发展最活跃的领域之一。将可检测极低浓度的某一气体的光声检测技术应用于极早期火灾气体产物的检测是一个新的尝试，将可能实现高灵敏度、高可靠性的火灾探测。但常规光声气体检测设备结构复杂、价格昂贵，必须恰当的重新设计才能应用到火灾探测系统中。分析了该技术在火灾探测中应用的关键问题,并提出了一种利用光声腔和光源间的“自由吸收路径”进行测量的光声气体探测系统，避免了对光源的窄带滤波要求，实现了在线式的气体检测。起始状态下，光声腔密封有纯CO气体，吸收光源中4.6 μm的辐射，产生一定强度的初始光声信号；当火灾气体产物流经吸收路径时，其中的CO气体吸收使到达光声腔的光辐射在4.6 μm波长上发生衰减，导致光声信号减弱，这个信号的变化量就反映了吸收路径中的CO气体浓度。     

First Sound in Mixtures of Trapped Bosons and Fermions

We study the collisional density oscillations of a mixture of bosonic and fermionic potassium atoms confined in a spherical trap at zero temperature. We find that the lowest-lying fluctuations of the whole system occupy either a non-overlapping spatial domain or overlap with opposite velocities. For moderate number of particles, we compare these results to previous calculations within a random-phase approximation.     

Second Sound Contribution to Temperature Gradient Evolution in Superfluid Mixtures

Starting from the full system of hydrodynamic equations for helium isotopic mixtures, the problem of temperature and concentration relaxation is solved. The limiting case of ultralow temperatures, when the contribution of thermal excitations can be neglected, is considered. A comparison with the experimental data is carried out.      

Probing Sound Speed of an Optically-Trapped Bose Gas with Periodically Modulated Interactions by Bragg Spectroscopy

A Bose–Einstein condensate (BEC) with periodically modulated interactions (PMI) has emerged as a novel kind of periodic superfluid, which has been recently experimentally created using optical Feshbach resonance. In this paper, we are motivated to investigate the superfluidity of a BEC with PMI trapped in an optical lattice (OL). In particular, we explore the effects of PMI on the sound speed and the dynamical structure factor of the model system. Our numerical results, combined with the analytical results in both the weak-potential limit and the tight-binding limit, have shown that the PMI can strongly modify the sound speed of a BEC. Moreover, we have shown that the effects of PMI on the sound speed can be experimentally probed via the dynamic structure factor, where the excitation strength toward the first Bogoliubov band exhibits a marked difference from the non-PMI one. Our predictions of the effects of PMI on the sound speed can be tested using the Bragg spectroscopy.     

高速涡轮发电机负载特性试验

以气体轴承支撑的高速涡轮发电机为研究对象,研究电机电负载对轴系稳定性影响,对比分析电机在不带电负载和带电负载两种情况下轴系振动特性,借助时间三维谱图、频谱图、轴心轨迹等详细分析升速区域内振动特性,并且对比分析以上两种工况下能量输出情况。实验结果表明发电机带负载后不仅会使轴系振动加剧,而且会改变能量的输出比例。     

An Improved Algorithm of Measurements of the Volume of Natural Gas in Gas Supply Using Measuring Systems with a Calculator

An algorithm for the measurement and calculation of the mean daily and average monthly volume of natural gas in a measuring system with a constricting device and a calculator, which takes into account the need to combine in time the channels for measuring the excess and barometric pressures, the gas temperature and density under standard conditions is proposed, which eliminates the systematic error inherent in existing methods of making measurements.     

Diffusion of Methylene Blue in Phantoms of Agar Using a Photoacoustic Technique

In this work, the kinetics of diffusion of methylene blue in agar aqueous solution is studied using a photoacoustic technique. Two agar phantoms solutions in water with a relation of mass/volume of 0.01% and 0.05% were analyzed. The study was performed using a modified Rosencwaig photoacoustic cell that is enclosed by transparent windows, on both sides. The sample is deposited directly on top of the upper window. A red light beam, at a fixed modulation frequency, is sent through the lower window illuminating the sample and inducing the photoacoustic effect inside the closed chamber of the cell. At the beginning of the experiment, a droplet of 100μL of agar solution is deposited; afterwards, the signal stabilizes, and 10μL of methylene blue aqueous solution (0.0125 g · mL^?1) is added to the surface of the agar. During the first seconds of the experiment, the photoacoustic signal amplitude increases followed by a gradual and long decay. Results for modulation frequencies in the range from 10Hz to 80Hz for both agar concentrations are presented. A simple theoretical approach is presented to analyze the experimental data. It is demonstrated that the kinetics of the process can be parameterized as a function of the changes of an effective optical absorption coefficient. From these results, the characteristic time, in which the dye diffusion process stabilizes, is obtained. It is found that this time is larger for samples with a higher agar concentration. These differences provide important results for biomedical sciences in which agar gels are used as phantoms resembling some of the properties of living organs and tissues.     

一种适用于低沸点混合制冷剂粘度测量装置的研制

针对混合制冷剂等挥发性混合物粘度测量的需要,研制了一种新的适用于测量低沸点混合物粘度的旋转式毛细管粘度计。该新型粘度测量装置在压力容器内嵌入旋转式毛细管粘度计,将旋转法升液和压力容器承压结合起来,避免了传统密封型毛细管粘度计由于抽放气的升液方式而导致混合物成分的变化,可以实现在较高压力下循环测量挥发性混合物溶液的粘度。采用R22和R290对旋转式毛细管粘度计进行了标定,并用R410A对粘度计的测量精度和性能进行了评价,粘度测量值与文献值最大相对偏差为0.81%。     

High-Resolution Shaft Speed Measurements Using a Microcomputer

A Motorola 6800 microcomputer has been applied to the measurement of instantaneous angular velocity at one degree intervals on a pair of coupled shafts. Velocity is determined by accurately measuring the time intervals between pulses produced by shaft encoders. Variations in relative velocity can be detected when precision timing is available. Using an essentially software approach time can be measured in 30-?s increments. The number of instructions in the timing loop can be reduced by adding a small amount of hardware to permit parallel data acquisition. Time increments of 12?s were obtained using inputs from two shafts. The results of measurements on a pair of shafts coupled by noncircular gears is included as an example.