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Journal ArticleDOI

Gas and vapour movements in the soil: I. The diffusion of vapours through porous solids

01 Jul 1940-The Journal of Agricultural Science (Cambridge University Press)-Vol. 30, Iss: 03, pp 437-462
TL;DR: In this paper, the dependence of the coefficient of diffusion, D, upon the porosity, S, of a granular solid is investigated experimentally, and it is shown that a curve connecting D/D0 and S can be drawn which is independent of the nature of the solid, its moisture content and, within limits, its texture.
Abstract: The dependence of the coefficient of diffusion, D, upon the porosity, S, of a granular solid is investigated experimentally. For steady state conditions, using carbon disulphide and acetone vapours, it is shown that a curve connecting D/D0 and S can be drawn which is independent of the nature of the solid, its moisture content and, within limits, its texture. For a limited range of values of S (0·0 < S < 0·7) a good approximation is D/D0 = 0·66S and over this range the diffusion coefficients are larger than those found by Buckingham for carbon dioxide.Investigation of the non-steady state shows that in soils the attainment of pressure equilibrium is retarded by adsorption, and it is suggested that Buckingham's low values for steady-state conditions can be attributed to premature observations of the diffusion rates; the steady state had probably not been attained when his measurements were made.
Citations
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Journal ArticleDOI
TL;DR: In this paper, a theory of moisture movement in porous materials under temperature gradients is developed which explains apparently discordant experimental information, including (a) the large value of the apparent vapor transfer, (b) effect of moisture content on net moisture transfer, and (c) the transfer of latent heat by distillation.
Abstract: A theory of moisture movement in porous, materials under temperature gradients is developed which explains apparently discordant experimental information, including (a) the large value of the apparent vapor transfer, (b) effect of moisture content on net moisture transfer, and (c) the transfer of latent heat by distillation. The previous simple theory of water vapor diffusion in porous media under temperature gradients neglected the interaction of vapor, liquid and solid phases, and the difference between average temperature gradient in the air-filled pores and in the soil as a whole. With these factors taken into account, an (admittedly approximate) analysis is developed which predicts orders of magnitude and general behavior in satisfactory agreement with the experimental facts. An important implication of the present approach is that experimental methods used to distinguish between liquid and vapor transfer have not done so, since what has been supposed to be vapor transfer has actually been series-parallel flow through liquid ‘islands’ located in a vapor continuum. Equations describing moisture and heat transfer in porous materials under combined moisture and temperature gradients are developed. Four moisture-dependent diffusivities arising in this connection are discussed briefly.

2,179 citations

Book ChapterDOI
TL;DR: This chapter collates the mathematical approaches to the aeration process and explains the concepts of modeling in a simplified manner, which have culminated in the modeling of the oxygen movements within the plant.
Abstract: Publisher Summary This chapter illustrates the developments in the field of aeration since 1960, which have culminated in the modeling of the oxygen movements within the plant. The chapter collates the mathematical approaches to the aeration process and explains the concepts of modeling in a simplified manner. It is noted that the environment exerts a considerable influence on the directional flow of the respiratory gases within the plant and the directional exchange with the atmosphere. Oxygen can enter the plant body in a variety of ways. In non-aquatic species, the stomata and lenticels provide paths of low resistance for the entry and exit of both oxygen and carbon dioxide. In submerged astomatal aquatics, surface permeabilities are sufficiently high to allow the necessary gas transference. Plants rooted in unsaturated soils are exposed to an oxygen-rich environment over the greater part of their shoot and root surfaces. Oxygen enters the plant in the combined state as water. As water, it is transported from root to shoot in the xylem where a proportion is finally released into the liquid phase within the chloroplasts during the photolysis stage of photosynthesis.

1,358 citations

Journal ArticleDOI
TL;DR: The concept of tortuosity is used to characterize the structure of porous media, to estimate their electrical and hydraulic conductivity, and to study the travel time and length for tracer dispersion as mentioned in this paper.
Abstract: The concept of tortuosity is used to characterize the structure of porous media, to estimate their electrical and hydraulic conductivity, and to study the travel time and length for tracer dispersion, but different types of tortuosity—geometric, hydraulic, electrical, and diffusive—have been used essentially interchangeably in the literature. Here, we critically review the tortuosity models developed empirically, analytically, and numerically for flow in both saturated and unsaturated porous media. We emphasize that the proposed tortuosity models are distinct and thus may not be used interchangeably. Given the variety of models that have been developed, and the sharp differences between some of them, no consensus has emerged unifying the models in a coherent way. Related treatments of tortuosity are found in the literature on porous catalysts. In such materials, nonlinear reactions ordinarily accompany transport, and the effective diffusivity within the pore space in the presence of the reactions is distinct from the one in their absence. Thus, because tortuosity may be defined as the ratio of the effective diffusivities in the bulk material and within the pore space, a careful treatment of tortuosity may need to distinguish between transport with and without reactions. This complication is ultimately relevant to soils as well, because bioremediation and biodegradation in soils are always accompanied by nonlinear reactions. Common models of tortuosity include both logarithmic functions and power laws. In many cases, the differences between the logarithmic and power-law phenomenologies are not great, but power laws can usually be reconciled with percolation concepts. Invoking percolation theory provides both insight into the origin of the power functions and a framework for understanding differences between tortuosity models.

560 citations

Journal ArticleDOI
TL;DR: In this paper, the porosity, the tortuosity and the constrictivity of macroporous media were analyzed in terms of the effective bulk diffusion coefficient and the diffusion coefficient in the absence of the porous medium.

414 citations

Journal ArticleDOI
01 Sep 1966-Ecology
TL;DR: Analysis of basal rates of metabolism, conductances, and body temperature suggests that the first two are determined in relation to environmental characteristics, and that body temperature, "goodness" of thermoregulation, midpoint of thermoneutrality, and lethal ambient temperature are dependent upon the ratio M/C.
Abstract: Temperature regulation and rate of metabolism were studied in five species of fossorial rodents: Geomys pinetis, Spalax leucodon, Tachyoryctes splendens, Heliophobius kapeti, and Heterocephalus glaber. The burrows of these rodents are in well—drained soils of poor water—holding capacity. Burrow atmospheres have an oxygen concentration of 15 to 20%, a carbon dioxide concentration of 0.5 to 2.0%, and limited temperature fluctuations; they are saturated with water vapor. Body temperature are somewhat low (35 to 37°C) in four species; temperature regulation is good in these species at ambient temperature down to 5 or 10°C. Heterocephalus, however, has a much lower body temperature (about 32°C) and the poorest capacity for thermoregulation of any known mammal. Fossorial rodents have low basal rates of metabolism, high conductances, and high ranges of thermoneutrality. Lethal ambient temperatures are inversely proportional to the normal levels of body temperature. Analysis of basal rates of metabolism (M), conductances (C), and body temperature suggests that the first two are determined in relation to environmental characteristics, and that body temperature, "goodness" of thermoregulation, midpoint of thermoneutrality, and lethal ambient temperature are dependent upon the ratio M/C. It is concluded that the lowest body temperature in homoiotherms compatible with good thermoregulation is about 35°C. A comparison of Heterocephalus and lizards illustrates the energetic similarities and differences between endotherms and ectotherms. The reduction of M and the increase of C in fossorial rodents are directly related to the mean burrow temperatures. These modifications reduce the probability of overheating in an environment where evaporative and convective cooling are greatly reduced. Tachyoryctes and Heliophobius use forced evaporative cooling for emergency thermoregulation, in spite of the saturated burrow atmosphere. Heterocephalus facilitates heat loss during heat stress by increasing peripheral circulation over its naked body; Geomys does this by increasing the circulation to its naked tail and, possibly, naked feet. Geomys may lose up to 30% of its heat production via its tail. As a result of these modifications, the lethal ambient temperature is proportional to the mean burrow temperature. Geomys responds to geographic variation in heat loading either by a decrease in body size or by an increase in tail length. The smaller size of nonpregnant, female gophers helps to maintain their heat production during pregnancy below the limits established by the environment for males. Gopher distribution appears to be limited to soils with high rates of gas exchange. Interaction among species of gophers has, to some extent, a physiological basis. It is suggested that the physiological characters concerned with energetics are readily modified, in an evolutionary sense, to conform to the requirements imposed by the environment.

412 citations

References
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Journal ArticleDOI
TL;DR: In this paper, it is shown that clays do not conform to the Kozeny theory in its simple form, but that it may be modified to give a satisfactory representation of the data available.
Abstract: It is shown that the permeability of a water-saturated sand or fine powder can be calculated with considerable accuracy, if the porosity and the specific surface are known. In particular, the Kozeny theory here discussed leads to a very useful relationship between permeability and porosity. It is shown that clays do not conform to the theory in its simple form, but that it may be modified to give a satisfactory representation of the data available. The physical grounds for this modified theory are discussed in some detail, and it is shown that, while it is open to criticism, it is at least in harmony with our present knowledge of clays.An important deduction which follows from the modified theory is that clays may have zero permeability at quite considerable porosities, e.g. at ∈ = 0·207 for a clay soil, and ∈ = 0·355 for a plastic clay.

430 citations

Journal ArticleDOI
TL;DR: The nature of the concentration gradients of carbon disulphide in soil suggests that movement of the vapour is largely a simple diffusion process, but that in coarse textured or loosely packed soils a gravitational flow may occur to a limited extent.
Abstract: SUMMARY 1 Experiments on the lines indicated in the preceding paper but with improved methods confirm that the depth-distribution of carbon disulphide in soil fumigated by injection is characterized by: a. A high concentration in the zone of injection and immediately below it. b. A rapid decrease in concentration as the surface is approached. c. A somewhat slower decrease with depth below the zone of injection. d. A time-concentration curve for each individual layer representing a sharp rise to a maximum concentration followed by a slower decline to a low level which is normally reached in about 24 hours. 2 Variations in the depth of injection raise or lower the zone of high concentrations without appreciably affecting that in the surface 0–3 in. layer. 3 Increased amounts of carbon disulphide per injection increase the persistence of the effective concentrations of fumigant, without causing any marked change in distribution or in the maximum concentrations attained. 4 Experimental data indicate that the loss of fumigant from treated soil occurs very largely via the soil surface. 5 Effective concentrations of carbon disulphide could not be produced in the surface layer by variation of the depth or amount of the injections nor by covering the surface with sacking. 6 The nature of the concentration gradients of carbon disulphide in soil suggests that movement of the vapour is largely a simple diffusion process, but that in coarse textured or loosely packed soils a gravitational flow (as of a viscous fluid) may occur to a limited extent. There is no evidence that such gravitational flow proceeds to any considerable extent. The concentration of carbon disulphide is uniformly low at all depths after about 24 hours.

6 citations