Modelling thermal fluxes at the soil surface

This paper investigates the impact that various representations of thermal fluxes at the soil surface have on the estimation of seasonal variations of temperature and stored thermal energy in the soil close to the surface. Three theoretical formulations representing; turbulent, nonturbulent and vegetation-covered soil surface conditions are considered. The influence of shading from nearby objects (e.g. vegetation) has also been investigated. Numerical predictions of soil temperature and stored thermal energy are compared with experimental results from a large scale field test (performed by others). The results of both 1D and 2D simulations are shown capable of representing specific aspects of field behaviour. Various sources of meteorological data have been used to define surface boundary conditions. In particular, simulations were performed using; i) data measured in-situ, ii) data obtained from The British Atmospheric Data Centre, and iii) data generated using analytical expressions found in the literature. It is found that the correct representation of the heat transfer processes occurring at the soil surface is of critical importance. In particular, it is shown that the use of publicly available sources of data, or mathematical/analytical expressions for meteorological data, may be adequate when in-situ measurements are not available

Analytical Solutions for Contaminant Diffusion in Double-layered Porous Media

Analytical solutions for conservative solute diffusion in one-dimensional double-layered porous media are presented in this paper. These solutions are applicable to various combinations of fixed solute concentration and zero-flux boundary conditions (BC) applied at each end of a finite one-dimensional domain and can consider arbitrary initial solute concentration distributions throughout the media. Several analytical solutions based on several initial and BCs are presented based on typical contaminant transport problems found in geoenvironmental engineering including (1) leachate diffusion in a compacted clay liner (CCL) and an underlying stratum; (2) contaminant removal from soil layers; and (3) contaminant diffusion in a capping layer and underlying contaminated sediments. The analytical solutions are verified against numerical solutions from a finite-element method based model. Problems related to leachate transport in a CCL and an underlying stratum of a landfill and contaminant transport through a capping layer over contaminated sediments are then investigated, and the suitable definition of the average degree of diffusion is considered

Factors influencing collection performance of near surface interseasonal ground energy collection and storage systems

The influence of surface boundary conditions, varying climatic conditions and engineering material parameters on the collection performance of near surface interseasonal ground energy collection and storage systems are investigated. In particular, the performance of a proposed design of an interseasonal heat storage system which has also been investigated by others as part of a full scale demonstration project is considered. A numerical model is developed and validated against field data. It is then applied to undertake a series of simulations with varying system parameters. It is found that (i) higher values of thermal conductivity of the storage layer result in increased storage of thermal energy and lower peak temperatures, (ii) system heat losses are strongly influenced by the performance of insulation layers, (iii) warmer climatic conditions provide more thermal energy available to be stored; however, changes in the amplitude of seasonal air temperature variations have an effect on the rate of collection of thermal energy and (iv) the use of correct surface boundary conditions is critical in modelling the dynamics of these systems

Design charts for contaminant transport through slurry trench cutoff walls

Slurry trench cutoff walls with low-permeability backfill material, such as soil-bentonite and slag-cement-bentonite, are used widely for containment of subsurface pollution. In the design of slurry walls the potential service life for a given thickness or the wall thickness for a target service life are typically determined via analyses of one-dimensional contaminant transport. The difficulty of selecting appropriate inlet and outlet boundary conditions and the mathematical complexity of analytical solutions hinder engineers in undertaking a contaminant transport analysis–based design. Design charts for nondimensionalized effluent flux are presented by developing and using an analytical solution. The methodologies of using these charts in design are demonstrated

Application of enzymatic and bacterial biodelignification systems for enhanced breakdown of model lignocellulosic wastes

This paper explores the extent to which enzymatic and bacterial biodelignification systems can breakdown lignocellulose in model wastes to potentially enhance biogas generation. Two representative lignocellulosic wastes (newspaper and softwood) commonly found largely undegraded in old landfills were used. A fungal peroxidase (lignin peroxidase) enzyme and a recently isolated lignin-degrading bacterial strain (Agrobacterium sp.) were used. Tests were conducted in stirred bioreactors with methanogens from sewage sludge added to produce biogas from breakdown products. Addition of lignin peroxidase resulted in ~20% enhancement in cumulative methane produced in newspaper reactors. It had a negative effect on wood. Agrobacterium sp. strain enhanced biodegradation of both wood (~20% higher release of soluble organic carbon and enhanced breakdown) and newspaper (~2-fold biogas yield). The findings of this paper have important implications for enhanced breakdown in old landfills that are rich in these wastes, and anaerobic operations utilising lignocellulosic wastes for higher degradation efficiencies and biogas production

Impact of single and multiple specimen suction control oedometer testing on the measurement of the soil–water characteristic curve

Devices that simultaneously facilitate controlling suction and applying a net stress on soil specimen provide soil–water characteristic curves (SWCCs) in terms of both the water content and degree of saturation, and volumetric deformations at various applied suctions. Such tests determine the water content of soil specimens based on the measured water volume changes at various applied suctions. However, studies have shown disagreements between the water volume–based calculated water content and the actual water content of soil specimens determined by the oven-drying method. Testing multiple soil specimens at predetermined suctions and measuring water content by the oven-drying method can overcome this but are a time-consuming approach. In this study, the impact of testing single and multiple soil specimens on the subsequently determined suction-water content and suction-degree of saturation SWCCs for the wetting process were studied. Statically compacted specimens of a sandy clay were used for establishing SWCCs using a suction control oedometer. Differences were noted between the calculated and measured water content and degree of saturation for an applied suction range of 0 to 95 kPa. Differences were noted between the SWCC fitting parameters obtained from the test results of single and multiple soil specimens. Statistical analysis suggested the differences between the results from single and multiple soil specimens testing were not significant. Corrections applied to the water volume change measurements were found to minimize these differences

The case for examining fluid flow in municipal solid waste at the pore-scale - A review

In this paper, we discuss recent efforts from the last 20 years to describe transport in municipal solid waste (MSW). We first discuss emerging themes in the field to draw the reader’s attention to a series of significant challenges. We then examine contributions regarding the modelling of leachate flow to study transport via mechanistic and stochastic approaches, at a variety of scales. Since MSW is a multiphase, biogeochemically active porous medium, and with the aim of providing a picture of transport phenomena in a wider context, we then discuss a selection of studies on leachate flow incorporating some of the complex landfill processes (e.g. biodegradation and settlement). It is clear from the literature survey that our understanding of transport phenomena exhibited by landfilled waste is far from complete. Attempts to model transport have largely consisted of applying representative elementary-scale models (the smallest volume which can be considered representative of the entire waste mass). Due to our limited understanding of fluid flow through landfilled waste, and the influence of simultaneously occurring biogeomechanical processes within the waste mass, elementary-scale models have been unable to fully describe the flow behaviour of MSW. Pore-scale modelling and experimental studies have proven to be a promising approach to study fluid flow through complex porous media. Here, we suggest that pore-scale modelling and experimental work may provide valuable insights into transport phenomena exhibited by MSW, which could then be used to revise elementary-scale models for improved representation of field-scale problems

Analytical solutions for ground temperature profiles and stored energy using meteorological data

Analytical solutions to estimate temperature with depth and stored energy within a soil column based upon readily available meteorological data are presented in this paper, which are of particular relevance in the field of ground heat extraction and storage. The transient one dimensional heat diffusion equation is solved with second kind (Neumann) boundary conditions at the base and third kind (Robin) boundary conditions, based on a heat balance, at the soil surface. In order to describe the soil-atmosphere interactions, mathematical expressions describing the daily and annual variation of solar radiation and air temperature are proposed. The presented analytical solutions are verified against a numerical solution and applied to investigate a case-study problem based upon results of a field experiment. It is shown that the proposed analytical approach can offer a reasonable estimate of the thermal behaviour of the soil requiring no information from the soil other than its thermal properties. Comparisons of predicted and measured soil temperature profiles and stored energy transients demonstrate there is reasonable overall agreement. The research contributes a practical approach that can provide surface boundary data that are vital in the thermal analysis of many engineering problems. Applications include: inter-seasonal heat transfer, energy piles and other more established ground source heat utilization methods

An indicator-based problem reduction scheme for coupled reactive transport models

A number of effective models have been developed for simulating chemical transport in porous media; however, when a reactive chemical problem comprises multiple species within a substantial domain for a long period of time, the computational cost can become prohibitively expensive. This issue is addressed here by proposing a new numerical procedure to reduce the number of transport equations to be solved. This new problem reduction scheme (PRS) uses a predictor-corrector approach, which ‘predicts’ the transport of a set of non-indicator species using results from a set of indicator species before ‘correcting’ the non-indicator concentrations using a mass balance error measure. The full chemical transport model is described along with an experimental validation. The PRS scheme is then presented together with an investigation, based on a 16 species reactive advective-diffusion problem, which determines the range of applicability of different orders of PRS. The results of a further study are presented in which a set of PRS simulations are compared with those from full model predictions. The application of the scheme to the intermediate-sized problems considered in the present study showed reductions of up to 82 % in CPU time with good levels of accuracy maintained

Near-boundary error reduction with an optimised weighted Dirichlet-Neumann boundary condition for stochastic PDE-based Gaussian random field generators

Random field generation through the solution of stochastic partial differential equations is a computationally inexpensive method of introducing spatial variability into numerical analyses. This is particularly important in systems where material heterogeneity has influence over the response to certain stimuli. Whilst it is a convenient method, spurious values arise in the near boundary of the domain due to the non-exact nature of the specific boundary condition applied. This change in the correlation structure can amplify or dampen the system response in the near-boundary region depending on the chosen boundary condition, and can lead to inconsistencies in the overall behaviour of the system. In this study, a weighted Dirichlet–Neumann boundary condition is proposed as a way of controlling the resulting structure in the near-boundary region. The condition relies on a weighting parameter which scales the application to have a more dominant Dirichlet or Neumann component, giving a closer approximation to the true correlation structure of the Matérn autocorrelation function on which the formulation is based on. Two weighting coefficients are proposed and optimal values of the weighting parameter are provided. Through parametric investigation, the weighted Dirichlet–Neumann approach is shown to yield more consistent correlation structures than the common boundary conditions applied in the current literature. We also propose a relationship between the weighting parameter and the desired length-scale parameter of the field such that the optimal value can be chosen for a given problem