In this research project, multiphase behaviour in zero gap alkaline water electrolyzers is investigated, focusing on the influence of various parameters including current density, electrode-wall distance and electrode height. This involves a cell containing a vertical electrode pair separated by a membrane submerged in an electrolyte that has a liquid-air interface near the top of the electrodes. The study aims to identify and understand turbulent phenomena through different analysis methods. Additionally, the outcomes derived from the conducted experiments serve as validation data for concurrent modeling research.
The analysis begins by examining vortex formation on the cathodic side, where hydrogen bubbles originate, using Particle Image Velocimetry (PIV). The results indicate the presence of bubble vortices, especially near the top of the channel. At electrode-wall distances of 1.0 cm or more, the bubbles that fail to reach the liquid-air interface tend to circulate extensively within the channel without exhibiting any noticeable discrepancies, such as small vortices. It should be noted that some low-velocity vortices, typically on the order of millimeters per second, can be observed within the bulk of the channel. As a general rule, shorter electrode-wall distances lead to increased vortex formation throughout the entire channel. Additionally, higher current densities induce a greater amount of these vortices or chaotic and irregular motion of the bubbles in the bulk region. In modern configurations, elastic elements, such as materials of thin wires with high porosity and low solid volume fraction, are strategically incorporated into the electrolyzer design to enhance mechanical resilience and accommodate variations in pressure and temperature. These elements serve to ensure the stability and reliable performance of the electrolyzer system under diverse operational conditions. Much of the chaotic behaviour of the bubbles seems to be counteracted when the electrode-wall distance is large (1.5 cm) while an elastic element is placed in between the cathode and opposing wall. When the whole channel is filled by the elastic element (0.6 cm electrode-wall distance), this results in more small-velocity vortices than without the element.
Another examined phenomenon is the presence of a bubble mist. This mist arises when the bubbles that originate at the electrode do not escape the channel at the liquid-air interface, but recirculate towards the opposing wall and downwards from there. Light intensity coming from the bubble clouds is used to gain and calibrate results. Increasing the current density and decreasing the electrode-wall distance lead to deeper bubble mist plumes. Surprisingly, elevated electrolyte height directly influences mist depth, with larger electrolyte heights showing the highest mist depths, even when dividing these by the exposed electrode height. For these measurements, diamond shaped apertures on the electrodes are used. For another series of experiments where higher current densities are examined, slotted apertures are used and the effect of lowering the electrode-wall distance on the bubble mist depth is diminished. Whether this is due to the type of electrodes or the different measurement circumstances (for example lighting) can not be concluded from the results. This emphasizes the need for consistent experimental conditions.
Thirdly, the continuous release and upward movement of gas bubbles originating at the cathode, i.e. the bubble plumes, are investigated. The effect of changing the current density is investigated along different heights of the electrodes. The analysis performed employs three different strategies to assess turbulent behaviour. This includes the PIV method used initially to depict vortices, the light intensity coming from the bubble clouds and visual analysis to find discrepancies like bubble bursts and big bubble trajectories. The PIV method shows capabilities in tracking the plume width and depicting turbulent behaviour. The light intensity analysis however proves challenging due to disparities in lighting. Improved lighting and higher-resolution cameras could yield more reliable results, enabling further investigation into correlations between parameters and turbulence, such as bubble bursts.

This work provides a foundation for designing aerodynamic mudflaps used in heavy-duty vehicles, which will help these vehicles become greener. First, a simulated moving ground consisting of a leading edge, roller setup and a novel tangential blowing system was devised as an alternative to the expensive conveyor belts necessary for pertinent experiments. The crucial design parameters, such as rounded, smaller support walls and uniform pressure inputs, were determined using two initial blowing slot designs. Next, a boundary layer analysis and tests on an isolated wheel using planar PIV demonstrated its ability to drastically reduce the effects of the boundary layer and accurately emulate wheel flow. Finally, mudflaps of different solidity and solidity locations were tested using stereoscopic PIV measurements on a simplified setup. Drag estimates showed that more solidity led to higher drag and wake skewing, and having lesser solidity on the top gave 15% lower drag than the bottom.

The increasing importance of drag reduction in commercial vehicles, traditionally motivated by rising energy costs, and more recently accelerated through policymaking in a bid to reduce emissions, has resulted in the development of increasingly sophisticated add-on aerodynamic devices for the semi-trailer. Of these devices, the greatest uptake amongst trucking fleets has been for devices fitted to the underbody, primarily on account of allowing free movement of freight into and out of the trailer, hence incentivizing a better understanding of the flowfield in this region. The current research addresses the aerodynamics of trailer wheels in tandem, which form a key component of the semi-trailer underbody, by investigating the effects of wheel rotation, trailer side-skirts, and wheel cavity covers on the flowfield and drag. The configurations tested include a short side-skirt terminating before the front edge of the leading wheel, a long side-skirt covering both the wheels, wheel covers without openings (solid disks), and wheel covers with openings varying in coverage area and radial position.

Stereoscopic (2D-3C) PIV is applied to examine the flow topology in the near wake and on selected planes besides the wheels. The measured velocity data in the wake is further used to derive the pressure field (by solving the Poisson equation for pressure), which together are then used to calculate the drag using a control volume approach. The uncertainty in the derived pressure field and drag values is determined using linear uncertainty propagation.

The largest drag reduction from the baseline case, i.e. without a side-skirt or wheel covers, is seen for the long side-skirt, followed by the short side-skirt with wheel covers, short side-skirt only, and wheel covers only, in that order. The effectiveness of both the long side-skirt and wheel covers is seen to increase with wheel rotation, with an appreciable reduction in drag with the wheel covers fitted only seen for rotating wheels. Investigation of the flowfield in the wake shows significant differences between the stationary and rotating wheel, particularly within the region of the projected wheel profile, and indicates to an earlier separation of flow along the upper surface of the rotating wheel. The effect of the side-skirt on the velocity in the wake is seen primarily in a lower streamwise velocity deficit, narrower wake, and higher horizontal symmetry of the wake for the skirted configuration. Wheel covers show a comparatively limited effect on the velocity field in the wake, showing a marginally wider wake and a slightly higher velocity deficit for the uncovered wheel. On planes beside the wheels, the side-skirt and wheel covers show a greater influence than wheel rotation, considered to be a consequence of a static floor combined with a gap between the wheel and the ground. The non-skirted configurations here exhibit a larger region of separated flow and greater velocity deficit whereas the uncovered wheels show an outflow at the bottom of the wheel cavity and an inflow at the top. Finally, the wheel covers with openings show a behavior between that of a covered and an uncovered wheel, with the inflow/outflow depending significantly on the coverage percentage and radial position.

In industry, fired heaters are used to heat process fluids and to generate steam. The efficiency of these devices is increased with air preheating. Gnerally, the flow through the air preheaters is driven by a centrifugal fan, via forced draft. Through a widening transition duct (large angle diffuser), air flows from the fan towards the air preheater. In terms of fluid mechanics, the geometry of such a duct is often unfavorable.

This research is aimed at improving the flow in large angle diffusers. A test platform was built with which the flow in diffusers of various geometries can be studied.

As part of a larger project called “Healing Water” this thesis project investigated aspects of minimally invasive water-jet drilling as a technology to be used in micro-fracture surgery. Drilling prototypes were developed and tested to gain a better understanding of the potential and the behaviour of high-pressure water jets in conjunction with minimally invasive devices. Specifically, this thesis focussed on the possible negative effects of inner diameter and curvature on drilling success. This thesis found that minimally invasive water jet drilling (in perspex, simulating bone) is possible. However, unwanted movement due to thrust reaction is a point of concern. Dealing with the thrust reaction as well as investigating real-world practicalities and limitations of the surgical procedure should be the focus of further R&D.