针对粗略测量启动思维 自制低成本宽带射频探头

在要求不高的测量中,可以利用自制探头进行测量.实验证明,探测50Ω电路,自制的探头在400KHz～1GMHz宽带范围内十分有用,且对电路引入的干扰也不大.     

Solidification of particle-reinforced metal-matrix composites

The solidification behavior of ceramic particle-reinforced metal-matrix composites (MMCs) is different from that of the bare matrix, not only because of the presence of the ceramic particles, but also due to their redistribution in the melt that results in nonhomogeneous thermophysical properties. The MMCs comprised of 10-to 15-μm SiC particles of varying volume fractions, dispersed uniformly in a modified aluminum A356 alloy by the melt stirring technique, were solidified unidirectionally in a thermocouple-instrumented cylindrical steel mold. The cooling rates were continually monitored by measuring temperatures at different depths in the melt, and the solidified MMCs were sectioned into disks and chemically analyzed for SiC volume fraction. The results point out that the cooling rate increased with increasing volume fraction of SiC particles. A small increase in the bulk SiC volume fraction of the cast MMC was observed due to particle settling during solidification. A one-dimensional enthalpy model of MMC solidification was formulated, wherein particle settling occurring in the solidifying matrix was coupled to the enthalpy equation by means of the Richardson-Zaki hindered settling correlation. A comparative study of simulations with experiments suggested that the thermal response of SiC particles used in this study was similar to that of single crystals, and their presence increased the effective thermal conductivity of the composite.     

A taguchi experimental design study of twin-wire electric arc sprayed aluminum coatings

An experimental study was conducted on the twin- wire electric arc spraying of aluminum coatings. This aluminum wire system is being used to fabricate heater tubes that emulate nuclear fuel tubes for use in thermal-hydraulic experiments. Experiments were conducted using a Taguchi fractional factorial design parametric study. Operating parameters were varied around the typical process parameters in a systematic design of experiments to display the range of processing conditions and their effect on the resultant coating. The coatings were characterized by hardness tests, optical metallography, and image analysis. This article discusses coating hardness, roughness, deposition efficiency, and microstructure. The study attempts to correlate the features of the coatings with the changes in operating parameters. A numerical model of the process is presented, including gas, droplet, and coating dynamics.     

Detection of initial damage in Norway spruce canopies using hyperspectral airborne data

Current broadband sensors are not capable of separating the initial stages of forest damage. The current investigation evaluates the potential of hyperspectral data for detecting the initial stages of forest damage at the canopy level in the Norway spruce (Picea abies (L.) Karst) forests of Czech Republic. Hyperspectral canopy reflectance imagery and foliar samples were acquired contemporaneously for 23 study sites in August 1998. The sites were selected along an air pollution gradient to represent the full range of damage conditions in even-aged spruce forests. The changes in canopy and foliar reflectance, chemistry and pigments associated with forest damage were established. The potential of a large number of spectral indices to identify initial forest damage was determined. Canopy hyperspectral data were able to separate healthy from initially damaged canopies, and therefore provided an improved capability for assessment of forest physiology as compared to broadband systems. The 673-724 nm region exhibited maximum sensitivity to initial damage. The nine spectral indices having the highest potential as indicators of the initial damage included: three simple band ratios, two derivative indices, two modelled red-edge parameters and two normalized bands. The sensitivity of these indices to damage was explained primarily by their relationship to foliar structural chemical compounds, which differed significantly by damage class.     

An Analysis of Recirculatory Flow in Gas‐Stirred Ladles

The paper is concerned with the fluid mechanics characterizing the bulk flow region in a gas‐stirred ladle. A theoretical framework, previously derived for the analysis of the gas‐liquid plume region, is extended to include the bulk flow phenomena. In this study, the liquid recirculation and mixing have been quantified in terms of simple dimensionless parameters related to the ladle geometry and gas flow rate. A new correlation for mixing time, consistent with the proper form of the Froude similitude criterion, is presented. The work has implications for the fundamental as well as applied aspects of ladle processing.     

A frontal solution program for finite element analysis

The program given here assembles and solves symmetric positive–definite equations as met in finite element applications. The technique is more involved than the standard band–matrix algorithms, but it is more efficient in the important case when two-dimensional or three-dimensional elements have other than corner nodes. Artifices are included to improve efficiency when there are many right hand sides, as in automated design. The organization of the program is described with reference to diagrams, full notation, specimen input data and supplementary comments on the ASA FORTRAN print-out.     

Bubble formation at nozzles in pig iron

An experimental study was undertaken to determine how several variables affect the size of gas bubbles formed at nozzles in liquid pig iron. The frequency of bubble formation was measured by an acoustic device, which could detect the vibrations produced by the bubble release. Accurate knowledge of the gas flow rate then enabled the calculation of bubble volumes. The use of large baths (60 Kg), melted by induction heating, permitted a wide range of experimental parameters: gas flow rate (0.5 to 1000 cc/s), outside nozzle diameter (0.64 to 5.1 cm), inside diameter (0.16 to 0.64 cm), chamber volume (23 to 2200 cc), nozzle depth (7.6 to 20 cm), surface tension (700 to 1500 dynes/cm) and nozzle orientation (up, down and sideways). The resulting bubble volumes were between 0.5 and 100 cc. The bubbles were found to form at the outer diameter of the nozzles due to the nonwettability of the nozzles. Furthermore, the bubbles were of a uniform size at low flow rates, but increased in volume with the flow rate, so that a constant frequency was established. In addition, the bubble volume was strongly dependent on the chamber volume upstream from the nozzle. This is known as a “capacitance” effect and is due to compressibility of the gas. “Doublets” or “double bubbles” at small chamber volumes and bubble “pairs” at large chamber volumes were also observed. These phenomena result in smaller bubbles, which make precise predictions of bubble size difficult. The results are compared with those obtained by other investigators in aqueous and metallic systems.     

Heat transfer and lance clogging during submerged powder injection

Nitrogen and silica particles of 30, 130, and 450 μm average diameters were injected at solid-to-gas loadings up to 280 kg/m³ into liquid lead at 400°C through a steel lance equipped with four thermo-couples. The lance was positioned adjacent to a transparent wall in the lead retort so that the flow patterns could be photographed. It was found that 130 and 450 μm particle injection produced bubling in the lead and clogging at high loadings, while the 30 μm particles produced jetting with no clogging. Analysis of the thermocouple responses permitted the determination of the heat transfer coefficients at the inner and outer lance surfaces. The inner surface heat transfer coefficient increased with loading, whereas the one at the outer surface was independent of loading. A two-phase, unsteady-state, one-dimensional model was developed for momentum and heat transfer in the lance permitting the calculation of gas and particle velocities, volume fractions, and temperatures as well as the lance temperatures. Using the experimentally determined heat transfer coefficients, it is shown that the gas and particles are heated only 20 to 40 K in the lance. Nevertheless, this is a large heat demand which chills the lance so that clogging will occur in the bubbling regime.