Dynamic Analysis of Unidirectional Pressure Infiltration of Porous Preforms by Pure Metals

Unidirectional pressure infiltration of porous preforms by molten metals is investigated numerically. A phenomenological model to describe fluid flow and transport phenomena during infiltration of fibrous preforms by a metal is formulated. The model describes the dynamics of the infiltration process, the temperature distribution, and solid fraction distribution. The numerical results are compared against classical asymptotic analyses and experimental results. This comparison shows that end effects may become important and render asymptotic results unreliable for realistic samples. Fiber volume fraction and initial temperature appear as the factors most strongly influencing infiltration. Metal superheating affects not only the length of the two-phase zone but also the solid fraction distribution in the two-phase zone. The effect of constant applied pressure, although significant on the infiltration velocity, is almost negligible on the two-phase zone length and on solid fraction distribution. When the initial preform temperature is below the metal melting point, and constant pressure is applied under adiabatic conditions, the flow ceases when sufficient solidification occurs to obstruct it. A comparison with literature experiments proves the model to be an efficient predictive tool in the analysis of infiltration processes for different preform/melt systems

Correction of Basic Equations for Deep Bed Filtration with Dispersion

Copyright © 2006 Elsevier B.V. All rights reserved.Deep bed filtration of particle suspensions in porous media occurs during water injection into oil reservoirs, drilling fluid invasion into reservoir productive zones, fines migration in oil fields, bacteria, virus or contaminant transport in groundwater, industrial filtering, etc. The basic features of the process are advective and dispersive particle transport and particle capture by the porous medium. Particle transport in porous media is determined by advective flow of carrier water and by hydrodynamic dispersion in micro-heterogeneous media. Thus, the particle flux is the sum of advective and dispersive fluxes. Transport of particles in porous media is described by an advection–diffusion equation and by a kinetic equation of particle capture. Conventional models for deep bed filtration take into account hydrodynamic particle dispersion in the mass balance equation but do not consider the effect of dispersive flux on retention kinetics. In the present study, a model for deep bed filtration with particle size exclusion taking into account particle hydrodynamic dispersion in both mass balance and retention kinetics equations is proposed. Analytical solutions are obtained for flows in infinite and semi-infinite reservoirs and in finite porous columns. The physical interpretation of the steady-state flow regimes described by the proposed and the traditional models favours the former.Altoe, J. E., Bedrikovetski, P.G., Siqueira, A. G., de Souza, A. L., Shecaira, F.http://www.elsevier.com/wps/find/journaldescription.cws_home/503345/description#descriptio

The rate of herbage disappearance during periods of occupation in rotational grazing systems.

The rate of herbage depletion during a period of occupation in a rotational grazing system is dependent on daily intake of animals which in turn is a function of the quality and the quantity of the forage available. When herbage is abundant, daily intake per head increases as nutritive value decreases up to a critical level; thereafter it decreases progressively. The rate of herbage disappearance will increase with increasing intake, but decrease as less herbage is consumed. Where forage quality remains constant, both intake and the rate of herbage disappearance will be constant. As soon as the quantity of herbage available becomes limiting, intake per head is restricted and the rate of herbage disappearance will diminish progressively. In sweet grassveld, nutritive value is likely to remain constant while herbage is abundant, causing herbage to disappear linearly with time. This could have useful application in grazing experimentation.Keywords: experimentation; forage quality; grassveld; grazing; grazing systems; herbage; intake; model; period of occupation; rotational grazin

Growth analysis of Themeda triandra and Sporobolus fimbriatus tillers in semi-arid grassveld.

Marked populations of Themeda triandra and Sporobolus fimbriatus tillers were sequentially harvested at fortnightly intervals both under irrigation and under unamended moisture conditions. Sampling covered a period from tiller initiation through to senescence after flowering. Tiller masses and green leaf areas were used to derive absolute growth rates, relative growth rates, net assimilation rates and leaf area ratios respectively. Growth rates of Themeda triandra were higher, or were maintained at high levels for longer than those of Sporobolus fimbriatus. Moisture stress resulted in considerably depressed growth rates. Photosynthetic efficiency of spring emerging tillers was potentially greater in midsummer, but this seasonal influence was masked by moisture stress on non-irrigated treatments.Language: EnglishKeywords: absolute growth rate; botany; eastern cape; false thornveld; flowering; grassveld; growth analysis; growth rate; irrigation; leaf area; leaf area ratio; leaves; moisture stress; net assimilation rate; Plant growth; plant growth indices; relative growth rate; semi-arid grassveld; south africa; sporobolus fimbriatus; themeda triandra; tiller mass; tillers; treatments; Unamended moisture condition

Growth and desiccation of Themeda triandra and Sporobolus fimbriatus in relation to diminishing moisture availability.

Water was withheld from individual plants of Themeda triandra and Sporobolus fimbriatus growing in pots in a greenhouse. Leaf extension growth, followed by successive degrees of tiller desiccation, were monitored on a daily basis and related to diminishing moisture availability. Leaf extension growth ceased after about 40% soil water depletion in both species on the sandy clay loam used in the trial. Thereafter, leaves and growing points senesced progressively with increasing evaporative demand, despite a relatively small drop in soil moisture content. The tentative conclusion is that active leaf growth in semi-arid grassveld takes place in short sporadic spells subsequent to each effective rainfall event. It seems, for example, that after a rain of 50 mm in midsummer, the duration of active leaf growth could be no more than a week.Language: EnglishKeywords: botany; desiccation; growth; leaf growth; Leaves; Moisture; moisture content; mortality; semi-arid grassveld; shoot apices; soil moisture; south africa; sporobolus fimbriatus; themeda triandr

Partitioning, storage and remobilization of 14C assimilated by Themeda triandra Forssk.

The second youngest fully-expanded leaf on the main tiller of vegetative Themeda triandra plants was exposed to 14CO2. Thereafter, quantitative analysis of partitioning, storage and remobilization after defoliation of 14C-labelled assimilate was conducted in serially-harvested plants over a 27-day period. In undefoliated plants, 50% of assimilated 14C was ultimately lost through respiration, while that remaining in the plant reached its final destination within two days of assimilation. A small, but significant, portion of assimilated 14C was invested into long-term storage in roots and stem bases. These reserves were remobilized after defoliation, and a portion of the remobilized 14C was incorporated into new growth. Partially-defoliated plants regrew more rapidly than totally-defoliated plants, but they both made equal demands on 14C reserves. Reserve depletion took place from both roots and stem bases.Keywords: 14c; assimilation; defoliation; growth; labile carbon reserves; leaves; partitioning; plant physiology; regrowth; reserves; respiration; roots; stem; storage; structural carbon; themeda triandra; tille

Short term effects of defoliation on herbage yield at three veld condition sites in the False Thornveld of the Eastern Province.

All combinations of three frequencies and three intensities of defoliation were applied at three veld condition sites in the false thornveld of the Eastern Province during a single growing season. Treatment effects were recorded during the season of application, and their carry over effects recorded during the subsequent season. Veld condition had greatest influence on herbage yield. Frequency and intensity of defoliation affected yield to a lesser degree, and this was manifested both during the season of application of treatments and in the subsequent season. The effect was greatest on veld in good condition. Mean treatment yields were higher during the second than the first season. This is largely attributed to the difference in rainfall between the two seasons.Keywords: defoliation; eastern cape; false thornveld; frequency of defoliation; herbage; herbage yield; rainfall; south africa; university of fort hare; veld condition; yiel

An evaluation of the economic viability of commercial pastoralism in the Smaldeel area of the eastern Cape.

Carrying capacity models and financial data from study group farmers were used to derive attainable net revenue per ha on veld in good, moderate and poor condition. From these data, minimum farm sizes required for a reasonable standard of living (R20 000 net personal income per annum) were estimated for each veld condition class. These minimum sizes were compared with actual farming unit sizes determined from a farm survey. Most farming units are large enough to provide a reasonable standard of living, on condition that the farmer has little debt and does not have to incur costs of land development. However, most farmers cannot afford to implement the development required for currently recommended conservation practices. If conservation is required by society, subsidization of conservation practices should be considered.Language: EnglishKeywords: Agriculture; Carrying capacity; Commercial pastoralism; conservation; development; Eastern Cape; economic evaluation; Economic viability; farm size; Farm sizes; farmers; farming; free density; models; Net income; pastoralism; revenue; smaldeel; Smaldeel area; south africa; survey; Tree density; Veld conditio