G.B. Deiters

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Master thesis (3)

3 records found

Flow measurements in a bubble-induced natural recirculation alkaline water electrolysis cell

Master thesis (2024) - J.R. de Vries (author), J.W. Haverkort (mentor), G.B. Deiters (mentor)

The greenhouse gases emitted from the excessive use of fossil fuels severely impact the global climate, and many effects are already being observed. Therefore, transitioning to a climate-neutral society is crucial, and hydrogen will play an essential role in this transition.

A promising technology for the production of green hydrogen is alkaline water electrolysis. However, for it to be competitive with conventional fossil fuel-based production, this technology needs to be optimised to increase efficiency, reduce costs and facilitate scalability. One approach to achieving this is using a flow through the electrolysis cell. This flow provides better temperature control, ensures a faster removal of gas bubbles and supplies new reactants, increasing efficiency and allowing larger and thinner cells to be built. This can be realised without the need for additional energy by using the buoyancy of the produced gas bubbles to induce the flow. This is called natural recirculation.

This research investigates how the flow rate and velocity of the recirculating electrolyte depend on the geometry and operating conditions of the cell. Several models have previously been made to predict this. However, they have different methods and results, and a general method of modelling natural recirculation has not yet been accepted. An experimental setup consisting of an alkaline electrolysis cell that uses natural recirculation has been designed and built to assess the validity of these models.

First, the recirculation flow rate was measured for multiple electrode-wall distances and current densities. These experiments showed that increased current density leads to higher recirculation flow rates. The relationship can be approximated with a power function with an exponent ranging from 0.3 to 0.6. Larger electrode-wall distances initially also lead to higher recirculation flow rates until a distance of 5.4 mm. Beyond this distance, the effects of backflow cause a slight decrease in the recirculation flow rate.

The pressure drop in the downcomer was measured as a function of flow rate. This function could then be inserted into the models, and the predicted recirculation flow rate was compared to the experimental data. Although the 1D models did not account for backflow and did not accurately estimate the pressure drop in the riser, they showed better results than the 2D models. The 2D models attempted to include backflow but overestimated its effect as it is challenging to predict its behaviour. Currently, the laminar 1D model most accurately predicts the recirculation flow rate.

The pressure drop in the downcomer was controlled using a valve, and the recirculation flow rate was measured to investigate the effects of backflow. Even though further research is required to understand the effects of backflow fully, this experiment did show that backflow reduces the recirculation flow rate.

Finally, the bubble plume and backflow were filmed using a high-speed camera, and PIV was used to analyse the behaviour of the backflow and determine the plume width.

To summarise, this research studied the relationship between the recirculation flow rate and the electrode-wall distance and current density. The experimental results were then used to assess the validity of different models.

The influence of flow shear on the bubble size and surface coverage in alkaline water electrolysis

Master thesis

Master thesis (2024) - K.M. Pardoel (author), J.W. Haverkort (mentor), G.B. Deiters (mentor), J.I. Postma (mentor), L. Botto (graduation committee member), B. Bera (graduation committee member)

This study aims to relate the size, number density, and surface coverage of bubbles adhering to a vertical gas-evolving electrode to the current density and shear rate of alkaline electrolyte flow.
A force balance that includes the known forces on the bubbles is formulated. This force balance is made to predict the release radius of the adhered bubbles. Because the electric and Marangoni forces are still an open topic of research, they are approximated using a constant. Also, because the influence of the drag force on bubbles in alkaline water electrolysis is not exactly known, it is approximated using the Stokes drag force with a multiplicative fit factor. An equation for this force balance model that approximates the release radius of the bubbles is formulated. To know the exact value of the shear rate on the electrode, this study uses laminar, fully developed flow. Equations are formulated to relate the fully developed laminar flow to the shear rate.
The bubbles adhered to the electrode are visualized using a high-speed camera. The camera films through a PMMA rod that goes through the first electrode, allowing visualization of the bubble-covered surface of the second electrode. A long stainless steel rectangular duct develops the Laminar flow before it arrives at the electrode.
The experiment is to relate the bubble size, bubble number density, and surface coverage to the shear rate due to forced flow and the current density. Increasing the shear rate has the biggest effect on the largest bubbles adhered to the surface. The maximal radius reduces, while, for the range in shear rate used in this study, the median stays almost constant. The bigger bubbles will be replaced by smaller bubbles. This will reduce the surface coverage and increase the bubble number density. Increasing the current density, for the range in current density used in this study, means an increase in bubble interactions. This means an increase in bubble coalescence, collisions between bubbles, large bubbles being pushed off the surface by smaller ones behind them, and the increased shear rate from natural convection caused by moving bubbles. These increased bubble interactions will result in a reduction in bubble sizes, surface coverage and an increase in bubble number density.
The force balance model is related to the maximal radius of the bubbles. The constant of the electric and Marangoni forces is negative at the anode and positive at the cathode, which is expected because the forces should be attractive at the anode and repulsive at the cathode. The multiplicative fit factor of the drag force due to forced flow indicates a higher force than the Stokes drag force because the flow is not perfectly homogeneous and will flow through other bubbles, which can accelerate the local flow velocities. Also, bubbles will collide at a higher speed with increased forced flow. The multiplicative fit factor of the drag force due to natural convection indicates a higher force than the Stokes drag force because of the increased bubble interactions with increasing current density. Both the higher multiplicative fit factors are observed at the anode and cathode.

Velocity profile measurements in a zero-gap alkaline water electrolyser

Master thesis (2024) - A. Navarro Muñoz (author), J.W. Haverkort (mentor), Simone Dussi (mentor), G.B. Deiters (mentor), N. Kodur Venkatesh (mentor), Aviral Rajora (mentor), N. Valle Marchante (graduation committee member), G.E. Elsinga (graduation committee member)

The global climate crisis and rising greenhouse gas emissions highlight the urgent need to shift from fossil fuels to sustainable energy solutions. Hydrogen is increasingly recognized as a crucial tool for decarbonization, with significant potential to drive the transition to clean energy. This study examines hydrogen production via water electrolysis, focusing on forced-flow alkaline water electrolysers (AWE). It investigates how factors such as current density (j), height above the electrode (z), and superficial liquid velocity (w_l) influence velocity profiles, turbulence, and bubble dynamics within the electrolyser.

The results demonstrate that variations in current density j and height z strongly affect the velocity profile w(x), with higher values promoting buoyancy-driven flows and accelerating the liquid, especially near the electrode where gas flux is highest. These effects are more pronounced on the hydrogen (H2) side, where increased gas production generates larger velocities and broader plume dispersion. In contrast, higher superficial liquid velocity w_l shifts the flow regime toward forced convection, attenuating bubble plume effects and reducing the contrast between plume and bulk flow.

Turbulence intensity I(x) also depends on j, z, and w_l, increasing with higher j or z due to intensified bubble interactions and buoyancy effects, particularly near the hydrogen electrode. However, increased w_l reduces turbulence in buoyancy-dominated regions by structuring the plume. The model aligns well with experimental data, especially when fitting velocity profiles to a step-function regime for natural convection, achieving scaling exponents consistent with theoretical predictions.

This study identifies the Richardson number (Ri) as a valuable dimensionless measure of convection type, with a transition from natural to mixed convection observed around Ri = 1. Ri unifies the effects of j, z, and w_l on velocity distribution, offering enhanced design flexibility for electrolysers. Bubble-induced turbulence and wakes caused by large bubbles are the primary drivers of dispersion within the electrolyser, accounting for over 98% of total observed dispersion. This finding underscores the dominant role of turbulent dispersion over hydrodynamic or shear-induced effects in this forced-flow system.

Detached bubbles significantly enhance liquid and gas flow through buoyancy-driven effects and turbulence. Optimizing parameters such as j, z, w_l, and Ri can improve electrolyser performance, paving the way for more efficient hydrogen production technologies. These advancements are critical for accelerating the transition to a low-carbon future.