Mixed Viewpoints Research Articles

Non-Isothermal Modeling of Co-Rotating and Contra-Rotating Twin Screw Extruders

The quality of the mixing of different raw materials, i.e. the uniformity of the mixture, is a key issue that will determine the morphology and the specific product properties of the resulting compound [1,2]. Numerical simulation of flow in extruder components provides a new insight, both qualitative and quantitative, into those features. 3-D transient numerical simulations of twin screw extruder (TSE) configurations are presented. A special method, the mesh superposition technique (MST), has been introduced to provide a convenient way to model intermeshing TSEs without involving any remeshing complexity [3]. It has been validated in previous work for isothermal cases [4]. To account for the important non-isothermal effects, the method is compared against numerical and experimental results for additional, non-isothermal validation [5]. We present an analysis of different non-isothermal features that characterize the flow induced by a co-rotating as well as a contra-rotating configuration. Both cases are compared in terms of pressure profiles, temperature fields, resulting torque imposed on the screws and from a mixing point of view to illustrate a typical analysis of different TSEs and provide objective information to select the most appropriate configuration for specific process requirements.

Magnetoresistance Effect of Co/bcc-Cr, Ni/bcc-Cr Artificial Superlattices

Epitaxial Co/Cr and Ni/Cr superlattices were prepared by using MBE. The crystal structure, interlayer exchange coupling, and magnetoresistance are discussed. The crystal structure of the CO and Ni grown on the bcc-Cr(OO1) were distorted hcp(1120) and, fcc(110), respectively. In the CO/ Cr system, an oscillatory exchange coupling, similar to that in an Fe/Cr system was observed, and the MR ratio was 2% at most. On the other hand, antiferromagnetic coupling was not observed in the Ni/Cr system, and the MR ratio was less than 0.1%. These differences are discussed from the viewpoint of mixing (alloying) at interfaces.

Simulation of airlift bioreactors: Steady-state performance of continuous culture processes

A tanks-in-series model was applied for mathematical modeling of the steady-state performance of continuous cultures in an airlift bioreactor. In the present computational algorithm, non-linear algebraic equations for the material balances of micro-organism, substrate and dissolved oxygen around the hypothetical well-mixed stages with backflow in the riser and downcomer were solved simultaneously using the Newton-Raphson technique. The concentration profiles of micro-organism, substrate and dissolved oxygen in the airlift bioreactor for continuous cultures were obtained under the same specifications. The simulation results were discussed mainly from the viewpoint of mixing in the airlift bioreactor. The relationships between the micro-organism productivity and design parameters of the airlift bioreactor such as liquid recycle flow rate and downcomer configuration were examined. The effect of mixing on the washout was also discussed. Numerical examples indicate that the proposed simulation scheme is very flexible and useful for the design and operation studies of airlift bioreactors.

Circulation and mixing of Mediterranean water west of the Iberian Peninsula

The spreading of water of Mediterranean origin west of the Iberian Peninsula was studied with hydrographic data from several recent cruises and current measurements from the BORD-EST programme. The vertical breakdown of the “Mediterranean salt” content reveals the dominant contribution of the so-called lower core of the outflow (60%), and the significant fraction (22%) brought downward to levels below 1500 m by diffusion. Intense salinity maxima in the upper core (18%) are only encountered south of 38°N in the vein flowing northward along the continental slope, and at a few stations in the deep ocean. Apart from the coastally trapped vein, other preferred paths of the water mass are revealed by the horizontal distributions of salinity maximum and Mediterranean Water percentage. One is southward, west of the Gorringe Bank, and two northwestward ones lie around 40°N and west of the Galicia Bank. Year-long velocity measurements in the Tagus Basin show westward mean values of 7 × 10 −2 m s −1 at 1000 m associated with a very intense mesoscale variability. This variability is related to the pronounced dynamical signature of the outflow which favours instability in any branch having detached from the slope current. From a mixing point of view, the strong interleaving activity occurring near Cape St-Vincent is illustrated, but its contribution to the downstream salinity decrease in the coastally trapped vein is weak. Current and meddy detachment play the dominant role, with a scaling estimate of their associated lateral diffusivity of order 500 m 2 s −1. The statistical distribution of the density ratio parameter, which governs double-diffusion at the base of the Mediterranean Water, was found to be very tight around R π = 1.3 in the temperature range of 5°C< φ < 8°C. North of 40°N, the presence of a fraction of Labrador Sea Water in the underlying water is shown to decrease that parameter and should favour the formation of salt fingers.

DYNAMICAL CLASSIFICATION OF CLMATE BASED ON THE AIR-MASS MIXING AND TRANSFORMATION

It is well known that the coastal belt of the continent corresponds to the dividing line of the climate. This subject is discussed from the viewpoint of air-mass mixing and transformation, and the following lines are attacked; (1): Spatial distributions of the diurnal variability of daily mean pressure (Fig. 1 and 2) and the standard deviation of yearly change of the monthly mean pressure (Fig. 4) are examined with the results along the trough line of these distributions. The transportation of air-mass is smaller than that of other regions. Using these results, the fields of wind and precipitation is divided into two parts. (2): Spatial distributions of the interdiurnal variabilty of daily mean temperature (Fig.5) and the standard deviation of year to year change of monthly mean temperature (Fig.6) are studied. Trough line of the latter distribution corresponds to the frontal zone (Fig.7). or locates between the zones of high and low temperature. These results are explainable as the products of the air-mass mixing and transformation during its long journey. Thus, it is shown that the classification of climate by trough line of the standard deviation of monthly mean temperature is much useful from the view-point of air-mass mixing and transformation. (3): Spatial distribution of the standard deviation of year to year variation of monthly mean vapour pressure is also studied. (Fig.8) Trough lire of this distribution means the dividing line of precipitation variability.(Fig.9) (4): Using the obtained results, climate near Japan is classified from the view-point of air-mass mixing and transformation. Two examples are as follows: (a): Winter air temperature over Japan is controlled by the air-mass over Siberian continent although the wind velocity blowing over Japan is affected by the depth of Aleutian Low, although the amount of precipitation is completely under the influence of the Sea of Japan. (b): In summer, the air temperature over Japan is affected principally by the temperature off the coast of Sanriku although the wind velocity proportional to the strength of Ogasawara High. Precipitation over Japan is influenced by the intensity of Ogasawara High along the southern coast, and is affected by the climatic condition off the coast of Sanriku.