Mass transport at gas-evolving electrodes

Direct numerical simulations are utilised to investigate mass-transfer processes at gas-evolving electrodes that experience successive formation and detachment of bubbles. The gas–liquid interface is modelled employing an immersed boundary method. We simulate the growth phase of the bubbles followed by their departure from the electrode surface in order to study the mixing induced by these processes. We find that the growth of the bubbles switches from a diffusion-limited mode at low to moderate fractional bubble coverages of the electrode to a reaction-limited growth dynamics at high coverages. Furthermore, our results indicate that the net transport within the system is governed by the effective buoyancy driving induced by the rising bubbles and that mechanisms commonly subsumed under the term ‘microconvection’ do not significantly affect the mass transport. Consequently, the resulting gas transport for different bubble sizes, current densities and electrode coverages can be collapsed onto one single curve and only depends on an effective Grashof number. The same holds for the mixing of the electrolyte when additionally taking the effect of surface blockage by attached bubbles into account. For the gas transport to the bubble, we find that the relevant Sherwood numbers also collapse onto a single curve when accounting for the driving force of bubble growth, incorporated in an effective Jakob number. Finally, linking the hydrogen transfer rates at the electrode and the bubble interface, an approximate correlation for the gas-evolution efficiency has been established. Taken together, these findings enable us to deduce parametrisations for all response parameters of the systems.</p

Coherence of temperature and velocity superstructures in turbulent Rayleigh-B\'enard flow

We investigate the interplay between large-scale patterns, so-called superstructures, in the fluctuation fields of temperature $\theta$ and vertical velocity $w$ in turbulent Rayleigh-B\'{e}nard convection at large aspect ratios. Earlier studies suggested that velocity superstructures were smaller than their thermal counterparts in the center of the domain. However, a scale-by-scale analysis of the correlation between the two fields employing the linear coherence spectrum reveals that superstructures of the same size exist in both fields, which are almost perfectly correlated. The issue is further clarified by the observation that in contrast to the temperature, and unlike assumed previously, superstructures in the vertical velocity field do not result in a peak in the power spectrum of $w$. The origin of this difference is traced back to the production terms of the $\theta$- and $w$-variance. These results are confirmed for a range of Rayleigh numbers $Ra = 10^5$--$10^9$, the superstructure size is seen to increase monotonically with $Ra$. Furthermore, the scale distribution of particularly the temperature fluctuations is pronouncedly bimodal. In addition to the large-scale peak caused by the superstructures, there exists a strong small-scale peak. This `inner peak' is most intense at a distance of $\delta_\theta$ from the wall and associated with structures of size $\approx 10 \delta_\theta$, where $\delta_\theta$ is the thermal boundary layer thickness. Finally, based on the vertical coherence relative to a reference height of $\delta_\theta$, a self-similar structure is identified in the velocity field (vertical and horizontal components) but not in the temperature.Comment: 17 pages, 10 figure

Electrolyte design for the manipulation of gas bubble detachment during hydrogen evolution reaction

During electrochemical gas evolution reactions, the continuous and vigorous formation of gas bubbles hugely impacts the efficiency of the underlying electrochemical processes. In particular, enhancing the detachment of gas bubbles from the electrode surface has emerged as an effective strategy to improve reaction efficiency. In this study, we demonstrate that the detachment of H2 gas bubbles can be controlled by the electrolyte composition, which can be optimized. We employ a well-defined Pt microelectrode and utilize electrochemical oscillation analysis to elucidate the features of H2 gas bubble detachment. Our investigation explores how the behaviour of H2 gas bubbles responds to variations in electrolyte composition and concentration. The coalescence efficiency of electrochemically generated microbubbles, a critical factor determining the mode of H2 gas bubble detachment (random detachment vs. single H2 gas bubble detachment), is profoundly influenced by the electrolyte composition. Specifically, coalescence efficiency follows the Hofmeister series concerning anions and coalescence is consistently inhibited in the presence of alkali metal cations. Furthermore, we establish a comprehensive model that accounts for both thermal and solutal Marangoni effects, allowing us to rationalize the trend of detachment size and period of single H2 gas bubbles under various conditions.</p

Statistics of turbulence in the energy-containing range of Taylor-Couette compared to canonical wall-bounded flows

Considering structure functions of the streamwise velocity component in a framework akin to the extended self-similarity hypothesis (ESS), de Silva \textit{et al.} (\textit{J. Fluid Mech.}, vol. 823,2017, pp. 498-510) observed that remarkably the \textit{large-scale} (energy-containing range) statistics in canonical wall bounded flows exhibit universal behaviour. In the present study, we extend this universality, which was seen to encompass also flows at moderate Reynolds number, to Taylor-Couette flow. In doing so, we find that also the transversal structure function of the spanwise velocity component exhibits the same universal behaviour across all flow types considered. We further demonstrate that these observations are consistent with predictions developed based on an attached-eddy hypothesis. These considerations also yield a possible explanation for the efficacy of the ESS framework by showing that it relaxes the self-similarity assumption for the attached eddy contributions. By taking the effect of streamwise alignment into account, the attached eddy model predicts different behaviour for structure functions in the streamwise and in the spanwise directions and that this effect cancels in the ESS-framework --- both consistent with the data. Moreover, it is demonstrated here that also the additive constants, which were previously believed to be flow dependent, are indeed universal at least in turbulent boundary layers and pipe flow where high-Reynolds number data are currently available.Comment: accepted in J. Fluid Mec

Cement as an Inorganic Binder for the Production of Formaldehyde-Free Bonded Plywood

For the bonding of veneers into plywood, formaldehyde-containing amino resins are mainly used. The advantages of these adhesives are good properties (e.g. fast curing), high availability, acceptable price and a mature technology. The formaldehyde emission (FE) is the major disadvantage of these adhesives. The limit of FE for wood-based panels has been reduced more and more in recent years. With the decision of the European Union from 1 January 2016, formaldehyde was obligatorily classified as carcinogenic and mutagenic. The result of this decision is a further reduction of FE limits for manufacturing and use of woodbased panels. Without expensive adhesive systems or coatings, plywood and its production will not comply with these limits in the future. Prospectively, the use of formaldehyde-containing adhesives will become uneconomical. For this reason, studies for bonding wood veneers with formaldehyde-free Portland cement were carried out. Ordinary Portland cement (OPC) is widely available, inexpensive and is in use in various industries including the wood-based panel industry (e.g. Cement-bonded particle board = CBPB). A new product called “cementbonded plywood (CBPly)” combines the advantages of organically-bonded plywood (e.g. great strength, low density) and CBPB (e.g. low FE, high fire resistance). In contrast to well-known cement-bonded composites based on wood particles (CBWC) like CBPB, cement-bonded wood-wool boards or wood-fibre-reinforced cement boards, the amount of wood is much higher in CBPly, since the cement is only located between the layers of wood (more specific information). The veneers ensure a high tensile strength of the material. This provides better bending properties compared to other particle-based CBCs. In addition to the increased strength, the handling and machinability is improved due to the lower density and lower cement content. Results from cone calorimeter tests indicate that if cement is also applied to the top veneers, the CBPly can be classified as Euroclass B (EN 13501-1). Due to the advantageous properties of CBPly, markets can be developed that are difficult or inaccessible for organically-bonded plywood and CBPB. Especially the use in applications with high demands on fire resistance and low emissions are regarded

Rising and settling 2-D cylinders with centre-of-mass offset

Rotational effects are commonly neglected when considering the dynamics of freely rising or settling isotropic particles. Here, we demonstrate that particle rotations play an important role for rising as well as for settling cylinders in situations when mass eccentricity, and thereby a new pendulum time scale, is introduced to the system. We employ two-dimensional simulations to study the motion of a single cylinder in a quiescent unbounded incompressible Newtonian fluid. This allows us to vary the Galileo number, density ratio, relative moment of inertia (MOI) and centre-of-mass (COM) offset systematically and beyond what is feasible experimentally. For certain buoyant density ratios, the particle dynamics exhibits a resonance mode, during which the coupling via the Magnus lift force causes a positive feedback between translational and rotational motions. This mode results in vastly different trajectories with significantly larger rotational and translational amplitudes and an increase of the drag coefficient easily exceeding a factor two. We propose a simple model that captures how the occurrence of the COM offset induced resonance regime varies, depending on the other input parameters, specifically the density ratio, the Galileo number and the relative MOI. Remarkably, depending on the input parameters, resonance can be observed for COM offsets as small as a few per cent of the particle diameter, showing that the particle dynamics can be highly sensitive to this parameter.</p

Performance Enhancement of Electrocatalytic Hydrogen Evolution through Coalescence-Induced Bubble Dynamics

The evolution of electrogenerated gas bubbles during water electrolysis can significantly hamper the overall process efficiency. Promoting the departure of electrochemically generated bubbles during (water) electrolysis is therefore beneficial. For a single bubble, a departure from the electrode surface occurs when buoyancy wins over the downward-acting forces (e.g., contact, Marangoni, and electric forces). In this work, the dynamics of a pair of H2 bubbles produced during the hydrogen evolution reaction in 0.5 M H2SO4 using a dual platinum microelectrode system is systematically studied by varying the electrode distance and the cathodic potential. By combining high-speed imaging and electrochemical analysis, we demonstrate the importance of bubble-bubble interactions in the departure process. We show that bubble coalescence may lead to substantially earlier bubble departure as compared to buoyancy effects alone, resulting in considerably higher reaction rates at a constant potential. However, due to continued mass input and conservation of momentum, repeated coalescence events with bubbles close to the electrode may drive departed bubbles back to the surface beyond a critical current, which increases with the electrode spacing. The latter leads to the resumption of bubble growth near the electrode surface, followed by buoyancy-driven departure. While less favorable at small electrode spacing, this configuration proves to be very beneficial at larger separations, increasing the mean current up to 2.4 times compared to a single electrode under the conditions explored in this study.</p

Rising and settling 2D cylinders with centre-of-mass offset

Rotational effects are commonly neglected when considering the dynamics of freely rising or settling isotropic particles. Here, we demonstrate that particle rotations play an important role for rising as well as for settling cylinders in situations when mass eccentricity, and thereby a new pendulum timescale, is introduced to the system. We employ two-dimensional simulations to study the motion of a single cylinder in a quiescent unbounded incompressible Newtonian fluid. This allows us to vary the Galileo number, density ratio, relative moment of inertia, and Centre-Of-Mass offset (COM) systematically and beyond what is feasible experimentally. For certain buoyant density ratios, the particle dynamics exhibit a resonance mode, during which the coupling via the Magnus lift force causes a positive feedback between translational and rotational motions. This mode results in vastly different trajectories with significantly larger rotational and translational amplitudes and an increase of the drag coefficient easily exceeding a factor two. We propose a simple model that captures how the occurrence of the COM offset induced resonance regime varies, depending on the other input parameters, specifically the density ratio, the Galileo number, and the relative moment of inertia. Remarkably, depending on the input parameters, resonance can be observed for centre-of-mass offsets as small as a few percent of the particle diameter, showing that the particle dynamics can be highly sensitive to this parameter.Comment: 32 pages, 13 figure

Diffusive and convective dissolution of carbon dioxide in a vertical cylindrical cell

The dissolution and subsequent mass transfer of carbon dioxide gas into liquid barriers plays a vital role in many environmental and industrial applications. In this work, we study the downward dissolution and propagation dynamics of CO2 into a vertical water barrier confined to a narrow vertical glass cylinder, using both experiments and direct numerical simulations. Initially, the dissolution of CO2 results in the formation of a CO2-rich water layer, which is denser in comparison to pure water, at the top gas-liquid interface. Continued dissolution of CO2 into the water barrier results in the layer becoming gravitationally unstable, leading to the onset of buoyancy driven convection and, consequently, the shedding of a buoyant plume. By adding sodium fluorescein, a pH-sensitive fluorophore, we directly visualise the dissolution and propagation of the CO2 across the liquid barrier. Tracking the CO2 front propagation in time results in the discovery of two distinct transport regimes, a purely diffusive regime and an enhanced diffusive regime. Using direct numerical simulations, we are able to successfully explain the propagation dynamics of these two transport regimes in this laterally strongly confined geometry, namely by disentangling the contributions of diffusion and convection to the propagation of the CO2 front.Comment: Submitted to Physical Review Fluid