Abhijit Chaudhuri

The paper highlights the collaborative research between University of Hawaii and the Indian Institute of Technology, Madras, to move forward the design of a compact wave powered desalination unit. Detailed hydrodynamic optimization is undertaken to optimize the weight, volume and overall performance of the unit. A fully coupled hydrodynamics/filtration model, developed with WEC-Sim software enables quick estimation of desalinated water and brine discharge in frequently observed seas. Several design considerations, that arose during the design stage are presented in this paper. We conclude with estimation of performance-desalinated water and brine discharge-in frequently observed ocean conditions.

Concentration polarization (CP) is a critical issue during desalination by cross-flow reverse osmosis (CFRO) filtration system and it is known that shear rate on the membrane surface can help its mitigation. Roto-dynamic filtration systems provide an easy way to generate high shear rate on membrane surface, and hence reducing CP. However the studies related to the growth of CP layer in roto-dynamic CFRO filtration is very limited. In this paper we developed a computational frame-work where flow and solute transport were simulated using ANSYS-Fluent V14.5 but physics of reverse osmosis through the membrane were implemented using "user defined functions" (UDFs). The development of the CP layer in a roto-dynamic CFRO filtration system has been discussed for different operating conditions such as inlet location, rotation speed, gap between the disk and the membrane and feed pressure. The effects of these parameters on permeate flux are also discussed. It is noted that angular speed of rotor and feed pressure are very effective parameters than others to enhance the permeate discharge. The understanding about the CP layer growth and its influence on permeate flux from the present study will help in better and efficient design of roto-dynamic CFRO filtration systems.

This study focuses on the development of a computationally efficient algorithm for the offline identification of system parameters in nonlinear dynamical systems from noisy response measurements. The proposed methodology is built on the bootstrap particle filter available in the literature for dynamic state estimation. The model and the measurement equations are formulated in terms of the system parameters to be identified - treated as random variables, with all other parameters being considered as internal variables. Subsequently, the problem is transformed into a mathematical subspace spanned by a set of orthogonal basis functions obtained from polynomial chaos expansions of the unknown system parameters. The bootstrap filtering carried out in the transformed space enables identification of system parameters in a computationally efficient manner. The efficiency of the proposed algorithm is demonstrated through two numerical examples - a Duffing oscillator and a fluid structure interaction problem involving an oscillating airfoil in an unsteady flow.

Deep CO2 storage reservoirs show a wide range of heterogeneous structures across different scales. Here, we show how high-resolution heterogeneity in both permeability and capillary entry pressure control dissolution and local capillary trapping of supercritical CO2 (ScCO2). Prior works mainly considered homogeneous reservoirs or simplified heterogeneity with horizontal and fixed capillary transition zones. Most of those studies ignored simultaneous free-phase ScCO2 spreading and solutal fingering. To advance understanding of heterogeneity effects, we have performed two-dimensional numerical simulations for ScCO2 injection into the middle of a storage reservoir. We have computed metrics including the spatial plume moments to compare the dissolution trapping and spreading of ScCO2 during the injection and post-injection period for different variances and correlation lengths of log-normal permeability and capillary entry pressure fields. The results show that the second-order spatial moment of the ScCO2 saturation field in the horizontal direction decreases by 10% as the permeability variance increases from 0.1 to 3.0. But it decreases by 50% as the capillary entry pressure variance becomes 1.1. For higher capillary entry pressure variance, the ratio of dissolved CO2 mass to free-phase ScCO2 mass decreases, showing higher local capillary trapping. The vertical distance between the centers of mass of the ScCO2 and dissolved CO2 plumes (representing the downward flow of dissolved CO2 through solutal fingering) is greater for heterogeneous reservoirs.

Heterogeneous aperture distribution is very common in large faults or fracture connecting injection and production wells. Recently the effects of heterogeneity on the temperature drawdown and transmissivity alteration due to dissolution/precipitation and thermo-elastic deformation of silicate reservoir were studied. The previous results based on numerical modeling have indicated that energy production rate is affected by correlation length and standard deviation of initial heterogeneous aperture field. The formation of a preferential flow path or channel is one of the most important consequences of heterogeneity when aperture alteration due to thermo-chemical and thermo-mechanical effects is significant. Previous studies have shown significant differences between the aperture alteration patterns of initially uniform fractures in carbonate and silicate reservoirs. Retrograde solubility and fast reaction rate of calcite with water are responsible for this. This has motivated us to study the effect of heterogeneity in carbonate reservoir. We have shown that the heterogeneity has much stronger effect in carbonate reservoir than that in silicate reservoir. In some cases the nature of evolution is completely different from homogeneous case under same injection conditions.

Heat extraction from the geothermal reservoir is sensitive to reservoir properties, operating parameters and coupling among various processes. Due to complex reservoir structure, heat extraction performances and flow field behaviour in geothermal reservoirs are very different than the small scale laboratory model experiments. In recent past, significant progress in reservoir scale numerical modeling has been made for quantification of challenges in geothermal energy system development. In this paper, a comprehensive review of state of the art geothermal reservoir modeling for heat extraction is presented. Various numerical tools and approaches such as the finite difference method, finite element method, finite volume method, etc. to model the geothermal reservoir in the last four decades are discussed. The thrust of this paper is on a critical review of individual evaluation of coupling among all possible processes that are thermo, hydro, mechanical, and chemical processes on heat extraction performance. Future directions in developing better understanding on geothermal reservoir systems are proposed.

Flow of undersaturated water in limestone aquifer can cause continuous permeability growth due to dissolution. We have simulated the evolution of permeability field of a 3-D porous limestone aquifer subjected to geothermal temperature gradient and vertical through-flow. The upward flow through porous limestone results in dissolution since calcite is a retrograde soluble mineral. In addition to permeability growth by promoting more dissolution, through-flow also inhibits Rayleigh Benard convection. To understand the temporal evolution of permeability and flow fields, we have performed several simulations with various combinations of initial permeability and through-flow magnitude. Since our computational domain is different in size and boundary conditions from past studies related to buoyant convection in porous medium, we have carried out simulations without reactive alteration to distinguish the hydrothermal systems as stable or unstable. The permeability growth is insignificant in the central part of the reservoir as the temperature gradient vanishes due to forced convection. Permeability growth is more near the edges, where temperature gradients are significant due to conductive heat transfer from the boundaries. For small magnitudes of through-flow, convection rolls are formed near the corners. However, the growth is very localized and rolls never form when magnitude of through-flow is large.

Carbon mineral trapping is commonly believed to be insignificant relative to other trapping mechanisms, yet, the role of cm-scale intraformational baffles is rarely considered in reservoir scale assessments. The present study aims to reveal conditions governing carbon mineral trapping and its quantification for a range of rock compositions in intraformational baffles. A series of 2-D multiphase reactive transport simulations were set-up to model the geochemical processes occurring at the interface of a baffle and a reservoir rock for a period of 1000 years. The study shows that the presence of baffles with certain rock properties within the reservoir rock can enhance or reduce carbon mineral trapping capacity relative to homogeneous reservoir rocks depending on clay and carbonate mineral content in baffles. The analysis shows that the precipitation of secondary carbonates occurs at the interface of the two rock types. Furthermore, it was found that higher clay and lower carbonate content of the baffle contribute towards enhancing mineral trapping while the mineralogy of reservoir rock does not have an important influence. The results are further utilized to derive averaged mineral composition for the integration of fine scale processes in coarser scale simulations.

The present work describes the design and analysis of a compact and modular wave-powered desalination (WPD) system. The design of the unit is influenced by the United States Department of Energy (DOE) Waves to Water competition rules. The system consists of a flap-type wave surge converter that converts wave energy into hydraulic piston force through a double-acting cylinder working as a power take-off (PTO) device. The dimensions of the flap are determined through frequency domain analysis under different sea states and the required discharge of desalinated water. Further, the whole system, including the reverse osmosis (RO) module, is modeled using WEC-Sim (an open-source code) and Simscape/SimHydraulics toolbox in MATLAB. The analysis of the system under six different sea states shows encouraging results for freshwater production. Considering the sea states studied here, the designed system can generate more than 50 bar pressure of the seawater feed discharge and produce up to 100 L/h of freshwater with TDS less than 500 mg/L. It was also observed that the use of an accumulator reduces the fluctuation in feed flow and feed pressure at the inlet of the RO module, resulting in improved system efficiency.

The temperature of deep hydrothermal sedimentary reservoirs varies between 50 and 250 °C, while the temperature of injected CO2 can be much below the reservoir temperature. Most past studies did not consider temperature effect on the CO2 dissolution trapping in storage reservoirs. We have performed numerical simulations of coupled multiphase flow, solute, and heat transport for liquid/supercritical CO2 injection in a storage reservoir for different reservoir temperature, injection temperature, and reservoir permeability. During the injection of relatively cool CO2 in a warmer storage reservoir, brine density increases with CO2 concentration causing solutal fingering in the diffusive boundary layer and the creation of an isolated dissolved CO2 plume (gravity trunk) below the injection zone. We have found that in many cases, the formation of a gravity trunk creates a large convection roll beneath the injection point. For low and moderate reservoir temperatures (100 and 175 °C), solutal fingers are pulled towards the injection zone by the large gravity trunk convection roll. In contrast, for higher temperature reservoirs (>206 °C), brine density decreases with dissolved CO2 concentration. Hence, for a reservoir temperature of 250 °C, solutal fingers do not develop, but reverse fingering from a dissolved CO2 mound is visible. We conclude that dissolution trapping achieves greater effectiveness for a reservoir temperature of 175 °C than those at 100 and 250 °C. We have also observed that free-phase CO2 below the caprock spreads faster for higher reservoir temperature and higher permeability, but it is not sensitive to injection temperature.