Preferential flow

In an od-wet rock, water resides in the larger pores, oil exists in the smaller pores or as a film on flow channel surfaces. Injected water preferentially flows through the larger pores and only slowly invades the smaller flow channels resulting in a higher produced water oil ratio and a lower oil production rate than in the water-wet case. 

Many appHcations use screws with constant pitch to feed material from a slotted opening. The configuration shown in Figure 9a shows a constant pitch and constant diameter causing a preferential flow channel to form at the back (over the first flight) of the screw. This type of flow destroys the mass flow pattern and potentially allows some or all of the problems discussed about fiinnel flow. 

Fig. 9. Preferential flow channel caused by (a) a constant pitch screw feeder and (b) a belt feeder.

An improperly designed interface to the belt can cause soHds compaction, abrasive wear of the belt, and excessive power requited to move the belt. The preferential flow channel shown in Figure 9b withdraws material from one end of the outlet. Depending on the gate opening, this could be at the back... 

A potential problem for rotary valve usage is that they tend to pull material preferentially from the upside of the valve, which can affect the mass flow pattern. Another problem is that once soHd drops from the vane, the air or gas that replaces it is often pumped back up into the bin. In addition, air can leak around the valve rotor. Such air flows can decrease the soflds flow rates and/or cause flooding problems. A vertical section shown in Figure 13 can alleviate the preferential flow problem because the flow channel expands in this area, usually opening up to the full outlet. To rectify the countercurrent air flow problem, a vent line helps to take the air away to a dust collector or at least back into the top of the bin. 

Fig. 13. (a) Preferential flow through rotary valve and (b) correction from installation of a vertical spool piece. 

Eor pesticides to leach to groundwater, it may be necessary for preferential flow through macropores to dominate the sorption processes that control pesticide leaching to groundwater. Several studies have demonstrated that large continuous macropores exist in soil and provide pathways for rapid movement of water solutes. Increased permeabiUty, percolation, and solute transport can result from increased porosity, especially in no-tiUage systems where pore stmcture is stiU intact at the soil surface (70). Plant roots are important in creation and stabilization of soil macropores (71). 

Preferential flow through root-mediated soil pores has been demonstrated for chloride, nitrate, and other ions that are not sorbed onto soil organic matter and clays. However, pesticide sorption onto soil affects both mobiUty of the pesticide as well as its residual life in the soil. Pesticide sorption onto root organic matter or organic linings of worm burrows may also slow transport of pesticides relative to water (72), thus countering the effects of increased permeabihty caused by roots. 

While phosphorus export from agricultural systems is usually dominated by surface runoff, important exceptions occur in sandy, acid organic, or peaty soils that have low phosphorus adsorption capacities and in soils where the preferential flow of water can occur rapidly through macropores (Sharpley et al., 1998 Sims et al., 1998). Soils that allow substantial subsurface exports of dissolved phosphorus are common on parts of the Atlantic coastal plain and Honda, and are thus important to consider in the management of coastal eutrophication in these regions. 

In practise some water will always be transported through the soil even if the local water content is less than the field capacity owing to preferential flow through cracks and large pores in the soil structure. This can be modelled simply by defining a bypass parameter to account for the fraction of water that can pass through each element. Modify the model to include bypass and see how this influences the solute profiles (see Corwin et al. (1991) for further details). 

Important studies of field-scale, water infiltration patterns are provided by Flury et al. (1994). Figures 9.1 and 9.2 show typical infiltration patterns of water, in various soils, for different degrees of initial saturation. A wide range of infiltration patterns is exhibited by these soils, from strong preferential flow to relatively stable, uniform flow (e.g.. Fig. 9.1, lower plot). 

Fig. 9.2 Patterns of vertical water flow in a porous medium, as a function of the initial water content. Shown here are two patterns of water infiltration, in two different dry and wet soils, following infiltration of 40 mm of water containing Brilliant Blue FCF as a dye applied to the ground surface (Flury et al., 1994). Reproduced by permission of American Geophysical Union. Flury M, Fluhler H, Jury W, Leuenberger J (1994) Susceptibility of soils to preferential flow of water A field study. Water Resour Res 30 1945-1954, doi 10.1029/94WR00871. Copyright 1994 American Geophysical Union...

Fig. 10.2 Pattern of contaminant advance (water containing dye) in a partially saturated soil. Note the high degree of spatial variability in the pattern, both laterally and with depth. Reprinted from Ghodrati M, Jury WA (1992) A field study of the effects of soil structure and irrigation method on preferential flow of pesticides in unsaturated soil. J Contam Hydrol 11 101-125 Copyright 1992 with permission of Elsevier...

Fig. 12.11 Examples of pesticide concentration vs. depth, indicating preferential flow of each pesticide in individual field plots. Symbols C continuous I intermittent P ponding S spiinkhng TG technical grade EC emulsifiable concentrate WP wettable powder U undisturbed D disturbed (Ghodrati and Jury 1990)...

Ghodrati M, Jury WA (1990) A field study using dyes to characterize preferential flow of water. Soil Sci Soc Am J 54 1558-1563... 

Greco, R. (2002). Preferential flow in macroporous swelling soil with internal catchment Model development and applications. ] Hydrol. 269,150-168. 

By independence we mean that the fluid loses its memory as it passes from vessel to vessel. Thus, a faster-moving fluid element in one vessel does not remember this fact in the next vessel and doesn t preferentially flow faster (or slower) there. Laminar flow often does not satisfy this requirement of independence however, complete (or lateral) mixing of fluid between units satisfies this condition. 

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