A THERMOMECHANICAL MODEL OF SOLIDIFICATION ON A MOLD MOVING WITH CONSTANT VELOCITY

A thermomechanical model of pure metal solidification on a moving mold plate is considered. The goal of the model is to obtain a formula for the contact pressure at the shell/mold interface as the mold moves into the molten liquid. From the contact pressure it is possible to infer the effects of the mold velocity and the mold microgeometry on the time and location of gap nucleation which results from irregular distortion of the shell as it grows from the melt. The mold, which moves at a constant velocity into the molten liquid, has a sinusoidal surface with a low aspect ratio: this means that its wavelength greatly exceeds its amplitude. The mold is of infinite area and is assumed to be perfectly conducting and thermomechanically rigid. We therefore neglect the complexities associated with the physics of edge constraints and/or free boundaries of the solidifying shell and the interacting distortions between deformable mold and shell materials along their interface. The ratio of the velocity of the solid/liquid interface to the mold velocity is identified as another dimensionless parameter in the analysis. In order to arrive at an analytical solution for the contact pressure along the shell/mold interface, we assume that this parameter is small. This makes the velocity ratio a convenient perturbation parameter for the analysis of thermomechanical distortion of the thin shell material as it grows from the melt. This necessarily limits the analysis to situations where the mold moves at faster rather than slower speeds. It is assumed that there is zero tangential shear stress between the fluid and the solidifying shell. As the molten liquid flows over the mold, it perfectly wets the surface. This precludes wetting effects due to surface tension. A hypoelastic constitutive law, which is a rate formulation of thermoelasticity, is assumed to govern deformation of the shell as it grows from the molten liquid. Latent heat liberated at the freezing front is extracted across a constant contact resistance at the shell/mold interface. Peculiar fluid motion at the tip is neglected. A solution for the contact pressure that is valid near the liquid surface (i.e., the meniscus) is derived from the main theoretical developments. Beyond the time of gap nucleation at the shell/mold interface, the model is no longer valid since it cannot account for gross distortion of the shell (i.e., distortions that greatly exceed the spatial perturbations considered in the model).     

Comparison of three techniques used to measure diffusive gas exchange from sheltered aquatic surfaces

Three approaches commonly used to quantify diffusive gas exchange across aquatic surfaces were compared in a densely treed, low-wind environment Diffusive surface fluxes of carbon dioxide (CO2) and methane (CH4) from a small boreal reservoir were estimated using (i) surface water concentrations, the thin boundary layer (TBL) equation, and gas transfer velocities (k) calculated using sulfur hexafluoride (SF6); (ii) surface water concentrations, the TBL equation, and k estimated from wind speed; and (iii) static floating chambers (FCs). Comparisons were made during three different approximately 10-day intervals (August 2000, June and September 2001). CO2 and CH4 fluxes estimated from SF6-derived k were on average 1-3 times greater than those determined from wind-estimated k Overall agreement between FC CO2 and CH4 flux estimates and those based on SF6 and wind speed derived kvalues was much weaker, with FC CO2 and CH4 flux estimates ranging from -9 to 23 times those based on SF6 and wind-estimated k values. Chamber deployment likely enhanced gas transfer through disturbance of the surface boundary layer, and results of this study suggest that caution must be exercised concerning the use of FCs on very still water surfaces. Furthermore, findings of this study contradict the common belief that use of wind speed to approximate k is inappropriate for small bodies of water characterized by low winds and surface obstructions.     

Flow behaviour of carrot puree: modelling the influence of time,temperature and potato flakes addition

Rheological properties of carrot puree were investigated in the 1–1000 s^?1 shear rate range with the objective of modelling the influence of time, temperature and addition of potato flakes on the flow behaviour. Carrot puree exhibited a shear‐thinning behaviour that was well described by the Ostwald‐de Waele (Power Law) model with a flow behaviour index of 0.34 (±0.02) at 20 °C. The time‐dependent behaviour was characterised by a second‐order Structural Kinetic Model. The decay of the structural parameter with time was found to be independent of shear rate. The Arrhenius model was used to explain the effect of temperature in the range from 4 to 60 °C. The dry matter was increased by adding potato flakes (0–5%). A power law model (for the concentration) and the Arrhenius relationship (for the temperature) were combined to simultaneously describe the effects of temperature and concentration. This study provides essential data for equipment and process design.     

Power assisted fuel cell

A hybrid fuel cell demonstrated pulse power capability at pulse power load simulations synonymous with electronics and communications equipment. The hybrid consisted of a 25.0 W Proton Exchange Membrane Fuel Cell (PEMFC) stack in parallel with a two-cell lead-acid battery. Performance of the hybrid PEMFC was superior to either the battery or fuel cell stack alone at the 18.0 W load. The hybrid delivered a flat discharge voltage profile of about 4.0 V over a 5 h radio continuous transmit mode of 18.0 W.     

Analysis of reactions during sintering of CuO-doped 3Y-TZP nano-powder composites

3Y-TZP (yttria-doped tetragonal zirconia) and CuO nano powders were prepared by co-precipitation and copper oxalate complexation–precipitation techniques, respectively. During sintering of powder compacts (8 mol% CuO-doped 3Y-TZP) of this two-phase system several solid-state reactions clearly influence densification behaviour. These reactions were analysed by several techniques like XPS, DSC/TGA and high-temperature XRD. A strong dissolution of CuO in the 3Y-TZP matrix occurs below 600 °C, resulting in significant enrichment of CuO in a 3Y-TZP grain-boundary layer with a thickness of several nanometres. This “transient” liquid phase strongly enhances densification. Around 860 °C a solid-state reaction between CuO and yttria as segregated to the 3Y-TZP grain boundaries occurs, forming Y₂Cu₂O₅. This solid-state reaction induces the formation of the thermodynamic stable monoclinic zirconia phase. The formation of this solid phase also retards densification. Using this knowledge of microstructural development during sintering it was possible to obtain a dense nano–nano composite with a grain size of only 120 nm after sintering at 960 °C.     

In vivo lipolysis induced by heparin injection or fasting: a proton MR spectroscopy analysis of human plasma

The impact of induced lipolysis on the composition of plasma lipids is analyzed by proton magnetic resonance spectroscopy (MRS) in humans. The variations of the methylene and methyl resonances from lipids in lipoproteins are studied under two sets of lipolytic conditions: acute endovascular lipolysis induced by an intravenous injection of heparin and subacute lipolysis induced by short fasting. During acute lipolysis, the degradation of the very low density lipoproteins structures is well correlated to the modifications observed in the areas of CH₂ and CH₃ MRS signals. The comparison of regular spectra, spectra with water signal suppression, and spectra recorded with a spin-echo sequence provides information on the behavior of the different parts of the lipoproteins, that is, the neutral core, little affected by heparine-induced lipoprotein lipase (LPL) activation, and the surface layer supplying substrates to LPL. During 48 h of fasting, only limited modifications occur on the MR spectra, and lipolysis cannot be documented in details.Address for correspondence: CRMBM-CNRS, Faculté de Médecine, 27, bd Jean Moulin, 13005 Marseille, France.     

THE RULES OF PROJECT MANAGEMENT

You can't control people. It's never too early to plan. Project management and control must be built in, not added on. These are just a few of the 18 rules one project manager (a father) passes along in a letter to a new project manager (his daughter) so that she can better prepare, plan, and manage her organization's software projects.     

Beobachtungen zur Oberflächenveränderung durch-Erosion und Kavitation

Observations of the effect on surfaces of “erosion and cavitation” With the cavitation of material due to liquids, the meterial is damaged by blowholes. In addition, observations on highgloss polished pure metals, tested in H. Schröter's and O. Walchner's flow apparatus with weir and counter-weir, occasionally showed damage caused by small particles of solids entrained by the liquid. In order to simulate mechanical damage by small particles, steel shot of different edge lengths was entrained with the liquid so as to bit the surface of the specimen. The results were lines of sliding in the case of Armco iron and mechanical twinning in the case of zine. An energy comparison is then carried out between the entrained solid particles and the blowhole. Solid particles with a maximum energy content theoretically equal to the minimum energy content of a blowhole are able to cause a plastic deformation of the surfaces of materials within minute zones, whilst blowholes with equal energy content obviously do not cause any mechanical damage. This indicates the need, with metallographic investigations into initial damage by flow cavitation, to ensure adequate filtering of the test medium so as to preclude the possibility of sand erosion.