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- 저자 : Suresh Kumar Govindarajan (검색결과 4 건)
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Effect of random fracture aperture on the transport of colloids in a coupled fracture-matrix system
Nikhil Bagalkot,Govindarajan Suresh Kumar 한국지질과학협의회 2017 Geosciences Journal Vol.21 No.1
A variable aperture model, including the random variation of fracture aperture as against the conventional parallel plate model, has been developed to adequately examine the transport of colloids/suspended particles in a single coupled fracturematrix system. Rather than relying on a complex geostatistical method for an accurate representation of fracture aperture, which requires an enormous field data and resource for its validation, a simple statistical method (linear congruential generator) is implemented in the present article. The random variation of fracture aperture is an honest representation of the unpredictable geometry/ morphology of fracture aperture in comparison with widely applied the conventional parallel plate model or the simple mathematical functions based on fractal theory (self-affine structures). A considerable number of parameters are involved in investigating the extent of penetration of colloids into the rock matrix, which creates complexity and ambiguity in the analysis. To overcome this problem, a single parameter “Maximum Penetration Factor” has been introduced for simple and reliable assessment of diffusion of colloids within the rock matrix. Additionally, a non-dimensional parameter ‘Matrix Mitigation Factor’ has been introduced in the present study, which can provide a means of evaluating the diffusion of suspended particles within the rock matrix when it comes to real time applications like microbial enhanced recovery (MEOR) and chemical enhanced recovery (CEOR) in the petroleum industry (nanoparticles and nanofluids). A semi-implicit finite difference model has been adopted for solving the coupled partial differential equations in the present numerical study. Finally, Neumann and Robinson boundary conditions as a function of time have been applied at the fracture inlet to better represent the field scenario as against the conventional constant source condition (Dirichlet). The model results indicate that there is a difference in concentration between the parallel plate model and random fracture model when it comes to colloidal concentration in the fracture and rock matrix. The variance in concentration is due to the inclusion of variation of the aperture in the variable aperture model, which is absent in the parallel plate model. Additionally, the results suggest that the variable source boundary condition has a significant influence on the transport of colloids in fracture-matrix system. Finally, from the evaluation of the extent of diffusion of colloids into rock matrix, it was concluded that that variable aperture model is associated with more mitigation of colloids compared to the parallel plate model, especially in the case of random fracture.
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Nikhil Bagalkot,Govindarajan Suresh Kumar 한국지질과학협의회 2016 Geosciences Journal Vol.20 No.5
A variable aperture model, instead of a conventional parallel plate model, is utilized to study the transport of radionuclides in a single coupled fracture-matrix system. A fully implicit finite difference model has been developed, which incorporates fracture aperture width variation in the numerical study of two species radionuclide transport. Two distinct geometric profiles namely, sinusoidal and logarithmic have been used to capture the variation of aperture width. The dependence of advection, hydrodynamic dispersion, linear sorption, and matrix diffusion on aperture width is considered in the analysis of radionuclides transport. Two species (parent and daughter) radioactive decay chain is also incorporated. There is a greater retardation of radionuclides in fracture for the variable aperture model than the parallel plate model. Sensitivity analysis on fracture surface sorption coefficient, longitudinal dispersivity, matrix porosity, and matrix diffusion coefficient shows that the conventional parallel plate model overestimate the radionuclide concentration in the fracture when compared to the variable aperture model.
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Berlin Mohanadhas,Suresh Kumar Govindarajan 대한환경공학회 2018 Environmental Engineering Research Vol.23 No.4
The uranium ore residues from the legacies of past uranium mining and milling activities that resulted from the less stringent environmental standards along with the uranium residues from the existing nuclear power plants continue to be a cause of concern as the final uranium residues are not made safe from radiological and general safety point of view. The deposition of uranium in ponds increases the risk of groundwater getting contaminated as these residues essentially leach through the upper unsaturated geological formation. In this context, a numerical model has been developed in order to forecast the 238U and its progenies concentration in an unsaturated soil. The developed numerical model is implemented in a hypothetical uranium tailing pond consisting of sandy soil and silty soil types. The numerical results show that the 238U and its progenies are migrating up to the depth of 90 m and 800 m after 10 y in silty and sandy soil, respectively. Essentially, silt may reduce the risk of contamination in the groundwater for longer time span and at the deeper depths. In general, a coupled effect of sorption and hydro-geological parameters (soil type, moisture context and hydraulic conductivity) decides the resultant uranium transport in subsurface environment.
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Rakesh Thekke Veettil,Govindarajan Suresh Kumar 한국지질과학협의회 2016 Geosciences Journal Vol.20 No.5
The conventional dual-porosity model has been modified by considering the heat exchange term at the fracture-matrix interface in the governing equation for thermal transport within the low permeable rock-matrix as against its conventional consideration within the high permeable fracture. A finite volume numerical model has been developed in order to analyze the influence of the source/sink term which defines the heat transfer at the fracturematrix interface. The comparison of the spatial distribution for temperature within the fracture and within the reservoir matrix for two different models, (1) conventional model in which the source/ sink heat transfer term included in the equation for thermal transport within the fracture; (2) proposed model in which the source/ sink heat transfer term included in the equation for thermal transport within the rock-matrix, have been performed. In addition, the sensitivity of the reservoir matrix thermal conductivities, both horizontal and vertical, on thermal energy extraction from the reservoir matrix has also been analyzed using the proposed model. Numerical results suggest that the estimation of temperature distribution in the fracture and rock-matrix and thus quantifying the heat extraction from the reservoir matrix is underestimated in a fracture-matrix system by using the conventional thermal transport model. It has been also observed that the temperature distribution obtained in the fracture and the rock-matrix by considering the heat transfer term in the thermal transport equation within the fracture shows significant variation from the temperature distribution obtained by considering the heat transfer term in the equation for thermal transport within the rock-matrix.
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