This thesis addresses the critical issue of gas crossover in small gap alkaline electrolysers, significantly impacting the safety of hydrogen production systems. The research aims to optimize electrolysis cell design by investigating how bubble coverage, gap size, and electrolyte flow influence hydrogen crossover, particularly for integration with renewable energy sources. Computational simulations using Ansys Fluent revealed that higher bubble coverage and controlled electrolyte flow significantly reduce crossover rates, ensuring safe Hydrogen-to-Oxygen levels. The study highlights the delicate balance required between minimizing gap size and managing crossover risks. Additionally, the research validates numerical results with an analytical model, providing a robust predictive tool for practical applications. The findings offer valuable insights and practical recommendations for the design and operation of alkaline electrolysis systems, enhancing their safety for industrial applications. This work supports global efforts to reduce greenhouse gas emissions, facilitate the transition to renewable energy sources, and advance the hydrogen industry, which is pivotal in the evolving energy landscape.

Thermal convection is a phenomenon seen in almost all facets of life, ranging from planetary convection to ocean currents and convection inside the Earth. The physics of thermal convection complicates when a porous wall layer is present. Flow over urban canopies, forest canopies or flow in underground aquifers are classic examples where thermal convection occurs in the presence of turbulent flow over a medium that may be described as a porous wall layer. The present work focuses on simulating pressure-driven turbulent flow over a simplified, ordered porous medium consisting of a regular array of cubes. The work further couples it with natural convection arising due to unstable stratification, to provide insight into its momentum and heat transfer characteristics. Direct numerical simulations (DNS) have been performed with a finite-difference Navier-Stokes solver to validate the model for buoyancy-driven convection and the classical Rayleigh-B´enard convection. Further, we extended the solver with a volume penalization Immersed Boundary Method (IBM) to model the ordered porous medium, which was validated against reference data. The bulk Reynolds in the overlying free channel region is fixed at 5500, the Prandtl number at 0.1, with an adiabatic boundary condition on the surface of the cubes. The bulk Richardson number, to cover different flow scenarios, from pure shear to purely buoyancy-driven flows, is also varied. The flow statistics show three distinct regimes spatially, (1) the porous region, (2) the turbulent channel and (3)the interface regime, where heat transfer timescales vary drastically between the porous region and the turbulent flow region. Looking at the regime transition from the perspective of changing Rib, the change starts at about bulk Richardson number Rib ≈ 0.1, with the flow switching from mostly buoyancy dominated to shear-dominated convection. The possible regimes as a function of the governing parameters are described, with three regimes formed using critical Rayleigh number limits for convection in free media and in porous media. The qualitative analysis of the thermal structures formed reveals, indeed, that the theorized critical limits are indeed seen through simulations. Lastly, length and velocity scalings are also suggested separately for shear-driven and buoyancy-driven regimes for different regions of the flow domain.

Stratified turbulent flows abound in environmental and industrial settings. Examples are atmospheric boundary layer flows, the transport of nutrients and organisms and the mixing of heat and salinity in the oceans, fluid flow in heat exchangers, and the transport of reactants and products in chemical reactions. These examples and many others consider stratified wall-bounded turbulence, in which the creation of turbulence by mechanical processes contends with its dissipation due to buoyancy effects. These flows are said to be stably stratified as these are inherently stable flows and are averse to mixing. The buoyancy effects alter the structure of the flow, and consequently the dynamics of mass, heat, and momentum transport. As density fluctuations become more severe, the Oberbeck-Boussinesq approximation becomes inaccurate and the resulting dynamics are not correctly predicted. In the current work, we developed and validated a numerical solver for direct numerical simulations (DNS) of turbulent flows featuring strong property variations. More precisely, we solve the Navier-Stokes equations in the limit of vanishing Mach number (so-called low-Mach number limit), with the fluid density given by the ideal gas law, and the dynamic viscosity and thermal conductivity also expressed as functions of temperature.
Our numerical solver is used to study stably-stratified turbulent channel flow under non-Oberbeck-Boussinesq conditions. The simulations are carried out at friction Reynolds number of  180, Prandtl number of 0.71, and friction Richardson number of the O(10). These non-dimensional numbers are the governing parameters and are defined based on the prescribed pressure drop and properties of the fluid at the reference temperature. Stratification is achieved by imposing constant temperature boundary conditions, with a high upper-to-lower wall temperature ratio (larger than 2), resulting in strong density variations in the flow. We will vary the temperature ratios and adjust gravity to maintain a similar Richardson number between cases, thereby isolating the effects of strong property variations in the flow dynamics. We will analyze the dynamics of heat and momentum transport under strong stratification for these conditions, also in light of DNS data of the same system under the Oberbeck-Boussinesq regime.