Computational fluid dynamics modelling of PEM fuel cells to investigate transport limitations.

Modern technological advancements in our lifestyle have caused a significant increase in the consumption of energy. With this growing demand, people are more concerned about the rational use of existing limited energy and searching for alternative forms of environmentally friendly energy sources to reduce polluting emissions. Proton Exchange Membrane (PEM) fuel cell has shown and demonstrated that potential to be a suitable alternative power source because of its simplicity of design, load following capabilities, efficiency, feasibility and quick start-up. Although having these splendid advantages, cost and durability of PEM fuel cells are one of the major challenges that needed to be overcome. Three-dimensional single-phase and multi-phase isothermal PEM fuel cell models have been developed to investigate the transport limitations of fresh reactants and its effect on cell performance. The governing equations (continuity, momentum and species transport) with appropriate source terms were solved using computational fluid dynamics (CFD) technique. A user defined function (UDF) code was developed considering source terms for porous zones, effective diffusivity models for species transport inside cells and electrochemical reactions at catalyst layers to predict cell voltage at an average current density. The average current density and net water transfer coefficient, used to calculate the source terms, were calculated using auxiliary equations and linked to the solver through UDFs. Parametric studies were performed to determine the optimal operating conditions and geometrical design of PEM fuel cell. The simulation results show that gas diffusion layer permeability has no effect on cell performance for a value lower than 10-11 m2. GDL porosity is one of the major design parameters which have significant influence on limiting current density, hence on cell performance. Land area width of PEM fuel cell shows influence on cell performance. Low membrane thickness provides higher cell performance and approximately 50% reduction in membrane thickness results approximately 100% improvement in cell performance at high current density of 1.0 Acm-2. Bruggeman correlation was used in most of previous modelling work for explaining the diffusion of species though porous GDL and CL, but this thesis considered other types of effective diffusion models and investigated the effect of diffusion models on cell performance at high current densities. Tomadakis and Sotirchos (1993) anisotropic model produces cell voltage much closer to the experimental values. Therefore, anisotropic diffusion model should be utilized in PEM fuel cell modelling to minimize modelling uncertainties. A two-phase flow, steady-state, three-dimensional PEM fuel cell model considering the phase change effect of water has been developed in the final phase of the thesis. Flooding inside the cell was captured at high current density using the model for a condensation value of 10.0 s-1. Finally, parametric studies were performed based on isotropic and anisotropic GDL permeability cases. Modelling results suggest that isotropic permeability cases have strong influence on cell performance compared to anisotropic cases at high current density

Numerical modelling of proppant transport in hydraulic fractures.

The distribution of proppant injected in hydraulic fractures significantly affects the fracture conductivity and well performance. The proppant transport in thin fracturing fluid used during hydraulic fracturing in the unconventional reservoirs is considerably different from fracturing fluids in the conventional reservoir due to the very low viscosity and quick deposition of the proppants. This paper presents the development of a three-dimensional Computational Fluid Dynamics (CFD) modelling technique for the prediction of proppant-fluid multiphase flow in hydraulic fractures. The proposed model also simulates the fluid leak-off behaviour from the fracture wall. The Euler-Granular and CFD-Discrete Element Method (CFD-DEM) multiphase modelling approach has been applied, and the equations defining the fluid-proppant and inter-proppant interaction have been solved using the finite volume technique. The proppant transport in hydraulic fractures has been studied comprehensively, and the computational modelling results of proppant distribution and other flow properties are in good agreement with the published experimental study. The parametric study is performed to investigate the effect of variation in proppant size, fluid viscosity and fracture width on the proppant transport. Smaller proppants can be injected early, followed by larger proppants to maintain high propping efficiency. This study has enhanced the understanding of the complex flow phenomenon between proppant and fracturing fluid and can play a vital role in hydraulic fracturing design

Effect of fracture roughness on the hydrodynamics of proppant transport in hydraulic fractures.

The effect of fracture roughness is investigated on proppant transport in hydraulic fractures using Joint Roughness Coefficient and a three-dimensional multiphase modelling approach. The equations governing the proppant transport physics in the fracturing fluid is solved using the hybrid computational fluid dynamics model. The reported proppant transport models in the literature are limited to the assumption of a smooth fracture domain with no fluid leak-off or fluid flow from fracture to rock matrix interface. In this paper, a proppant transport model is proposed that accounts for the proppant distribution in rough fracture geometry with fluid leak-off effect to surrounding porous rock. The hydrodynamic and mechanical behaviour of proppant transport was found directly related to the fracture roughness and flow regime especially under the influence of low viscosity fracturing fluid typically used in shale gas reservoirs. For the proppant transport in smooth fractures, the fracture walls employ mechanical retardation effects and reduce the proppant horizontal velocity resulting in more significant proppant deposition. On the contrary, for the proppant transport in rough fractures, the inter-proppant and proppant wall interactions become dominant that adds turbulence to the flow. It results in mechanical interaction flow effects becoming dominant and consequently higher proppants suspended in the slurry and greater horizontal transport velocity. Furthermore, the mechanical interaction flow effects were found to be principally dependant on the proppant transport regime and become significant at higher proppant Reynolds number

Numerical study of the effect of effective diffusivity and permeability of the gas diffusion layer on fuel cell performance.

A three-dimensional, single-phase, isothermal, explicit electrochemistry polymer electrolyte membrane fuel cell model has been developed and the developed computational model has been used to compare various effective diffusivity models of the gas diffusion layer. The Bruggeman model has traditionally been used to represent the diffusion of species in the porous gas diffusion layer. In this study, the Bruggeman model has been compared against models based on particle porous media, multi-length scale particles and the percolation-type correlation. The effects of isotropic and anisotropic permeability on flow dynamics and fuel cell performance have also been investigated. This study shows that the modelling of the effective diffusivity has significant effects on the fuel cell performance prediction. The percolation-based anisotropic model provides better accuracy for the fuel cell performance prediction. The effects of permeability have been found to be negligible and the specification of any realistic value for permeability has been found to be sufficient for polymer electrolyte membrane fuel cell modelling

Proppant transport in dynamically propagating hydraulic fractures using CFD-XFEM approach.

Numerically modelling the fluid flow with proppant transport and fracture propagation together are one of the significant technical challenges in hydraulic fracturing of unconventional hydrocarbon reservoirs. The existing models either model the proppant transport physics in static predefined fracture geometry or account for the analytical models for defining the fracture propagation. Furthermore, the fluid leak-off effects are usually neglected in the hydrodynamics of proppant transport in the existing models. In the present paper, a dynamic and integrated numerical model is determined that uses computational fluid dynamics (CFD) technique to model the fluid flow with proppant transport and Extended finite element method (XFEM) to model the fracture propagation. The results of fracture propagation were validated with the real field results and analytical models, and the results of proppant transport are validated with the experimental results. The integrated model is then used to comprehensively investigate the hydrodynamical properties that directly affect the near-wellbore stress and proppant distribution inside the fracture. The model can accurately model the proppant physics and also propose a solution to a frequent challenge faced in the petroleum industry of fracture tip screen out. Thus, using the current model allows the petroleum engineers to design the hydraulic fracturing operation successfully, model simultaneously fracture propagation and fluid flow with proppant transport and gain confidence by tracking the distribution of proppants inside the fracture accurately

A new CFD approach for proppant transport in unconventional hydraulic fractures.

For hydraulic fracturing design in unconventional reservoirs, the existing proppant transport models ignore the fluid leak-off effect from the fracture side wall and the effect of fracture roughness. In this paper, a model is proposed using three-dimensional computational fluid dynamics approach with fluid leak-off rate defined along the fracture length and considering the effect of fracture roughness on proppant distribution. Based on the simulation results, it is recommended that neglecting the fracture roughness in the proppant transport model can result in over predicting the proppant bed length and underpredicting the proppant suspension layer by 10–15%. Furthermore, neglecting the fluid leak-off effect can result in under predicting the proppant bed height by 10–50% and over predicting the proppant suspension layer by 10–50%. This study has enhanced the understanding of the proppant-fracturing fluid interaction phenomenon by accounting detailed physics to optimise the hydraulic fracturing design

Computational fluid dynamics simulation of natural gas hydrate sloughing and pipewall shedding temperature profile: implications for CO2 transportation in subsea pipeline.

The continuous flow assurance in subsea gas pipelines relies heavily on the assessment of temperature profile during hydrate sloughing and pipewall shedding caused by hydrates, with similar implications for carbon dioxide (CO2) transportation under hydrate-forming conditions. Hydrate sloughing is the peeling off of some hydrate deposits from the pipeline inner surface. Similarly, pipewall shedding by hydrates involves the direct interaction of hydrates with the pipeline inner surface, resulting in the detachment or removal of hydrate deposits from the pipewall. While sloughing occurs within the deposit of hydrates, pipewall shedding is related to direct interaction of the gas phase with the thin layer of hydrates on the pipewall. In this study, a computational fluid dynamics (CFD) simulation approach is employed using a validated CFD model from the literature for predicting hydrate deposition rates (Umuteme et al., 2022), by applying a subcooling temperature to the pipe wall at hydrates-forming condition. We have deduced the presence of hydrates based on the stable temperature profile of natural gas hydrates along the pipeline model. The study shows that the simulated temperature contours align well with the reported hydrate deposition profile in gas pipelines (Di Lorenzo et al., 2018). The conversion of the consumption rate of natural gas to hydrates was achieved using the equation proposed in the literature (Umuteme et al., 2022). Two shear stress regimes have been identified for hydrate sloughing and pipewall shedding in this study, with the latter resulting in higher shear stress on the pipewall. Presently, there is a growing concern regarding the potential leakage of CO2 in pipelines (Lu et al., 2020; Wang et al., 2022; Wareing et al., 2016), which may escalate due to pipewall corrosion caused by hydrates (Obanijesu, 2012). The findings in this research can provide further knowledge that can enhance the safe transportation of CO2 in pipelines under stable hydrate forming conditions

Analytical modelling of the hydraulic effect of hydrate deposition on transportability and plugging location in subsea gas pipelines.

Accurate prediction of the hydraulic effect of hydrate deposition and plug location is critical to the safety and operability of natural gas transport pipelines, especially for gas-dominant subsea pipelines where maintenance and intervention activities are more difficult. To achieve this, the present work improved an existing two-phase pressure drop relation due to friction, by incorporating the hydrates deposition rate into the equation. In addition, a model has been developed to predict the pipeline plugging time. The transient pressure drop predictions in the present study for all six cases at high and low velocities are within 4% mean relative error. Similar predictions by Di Lorenzo et al. are within 40% maximum relative error, while the mean relative error of the transient pressure drop predictions by Zhang et al. was 7.43%. In addition, the plugging flowtime model underpredicts the plugging time by a mean relative error of 9%

Water dynamics inside a cathode channel of a polymer electrolyte membrane fuel cell.

The present study focuses on the investigation of water dynamics inside a polymer electrolyte membrane fuel cell using two different modelling approaches: Eulerian two-phase mixture and volume of fluid interface tracking models. The Eulerian two-phase mixture model has provided overall information of species distribution inside a fuel cell and identified that the liquid water usually accumulates under the land area. The volume of fluid interface tracking model has then been implemented to investigate the emergence of water droplets from the gas diffusion layer into the cathode channel and the subsequent removal of water from the channel. Further, the effects of the location of water emergence in the cathode channel on the dynamic behavior of liquid water have been investigated. The present study shows that the water emerging into the channel near the side walls greatly reduces the surface water coverage of the channel. In order to control the water path into the channel near side walls, a further discussion has been provided that a gas diffusion layer design based on hydrophilic fibres distributed inside a hydrophobic fibre matrix could provide a precisely controlled water path through the gas diffusion layer

An improved computational fluid dynamics (CFD) model for predicting hydrate deposition rate and wall shear stress in offshore gas-dominated pipeline.

Gas hydrates in pipelines is still a flow assurance problem in the oil and gas industry, and requires a proactive hydrate plugging risk predicting model. As an active area of research, this work has developed a 3D 10m length by 0.0204m diameter horizontal pipe CFD model based on the eulerian-eulerian multiphase modelling framework to predict hydrate deposition rate in a gas-dominated pipeline. The proposed model simulates the conditions for hydrate formation with user defined functions (UDFs) for both energy and mass sources implemented in ANSYS Fluent, a commercial CFD software. The empirical hydrate deposition rates predicted by this model at varying subcooling temperatures and gas velocities are consistent with experimental results within ±10% uncertainty bound. At lower gas velocity of 4.7m/s, the model overpredicted the hydrate deposition rates of the experimental results in Aman et al. (2016) by 9–25.7%, whereas the analytical model of Di Lorenzo et al. (2018) underpredicted the same experimental results by a range of 27–33%. Consequently, the CFD model can enhance proactive hydrate plugging risk predictions earlier than the analytical model, especially at low gas productivity. Similarly, at a velocity of 8.8m/s and subcooling temperatures of 2.5K, 7.1K and 8.0K, the CFD model underpredicted the hydrate deposition rates of the regressed experimental results in Di Lorenzo et al. (2014a) by 14%, 6% and 4% respectively, and overpredicted the results by 1% at a subcooling temperature of 4.3K. From the CFD model results, we also suggest that hydrate sloughing shear stress is relatively constant, and the wall shedding shear stress by hydrate vary during deposition. Finally, the CFD model also predicted the phase change during hydrate formation, agglomeration, and deposition