Cavitation and Bubble Dynamics: Preface

Influence of Bubbles on the Energy Conversion Efficiency of Electrochemical Reactors

Bubbles are known to influence energy and mass transfer in gas evolving electrodes. However, we lack a detailed understanding on the intricate dependencies between bubble evolution processes and electrochemical phenomena. This review discusses our current knowledge on the effects of bubbles on electrochemical systems with the aim to identify opportunities and motivate future research in this area. We first provide a base background on the physics of bubble evolution as it relates to electrochemical processes. Then we outline how bubbles affect energy efficiency of electrode processes, detailing the bubble-induced impacts on activation, ohmic and concentration overpotentials. Lastly, we describe different strategies to mitigate losses and how to exploit bubbles to enhance electrochemical reactions.

Gas bubbles are almost a routine occurrence encountered by researchers working in the field of microfluidics. The spontaneous and unexpected nature of gas bubbles represents a major challenge for experimentalists and a stumbling block for the translation of microfluidic concepts to commercial products. This is a startling example of successful scientific results in the field overshadowing the practical hurdles of day-to-day usage. We however believe such hurdles can be overcome with a sound understanding of the underlying conditions that lead to bubble formation. In this tutorial, we focus on the two main conditions that result in bubble nucleation: surface nuclei and gas supersaturation in liquids. Key theoretical concepts such as Henry's law, Laplace pressure, the role of surface properties, nanobubbles and surfactants are presented along with a view of practical implementations that serve as preventive and curative measures. These considerations include not only microfluidic chip design and bubble traps but also often-overlooked conditions that regulate bubble formation, such as gas saturation under pressure or temperature gradients. Scenarios involving electrolysis, laser and acoustic cavitation or T-junction/co-flow geometries are also explored to provide the reader with a broader understanding on the topic. Interestingly, despite their often-disruptive nature, gas bubbles have also been cleverly utilized for certain practical applications, which we briefly review. We hope this tutorial will provide a reference guide in helping to deal with a familiar foe, the “bubble”.

Nip the bubble in the bud: a guide to avoid gas nucleation in microfluidics

Renewables challenge the management of energy supply and demand due to their intermittency. A promising solution is the direct conversion of the excess electrical energy into valuable chemicals in electrochemical reactors that are inexpensive, scalable, and compatible with irregular availability of electrical power. Membrane-less electrolyzers, deployed on a microfluidic platform, were recently shown to hold great promise for efficient electrolysis and cost-effective operation. The elimination of the membrane increases the reactor lifetime, reduces fabrication costs, and enables the deployment of liquid electrolytes with ionic conductivities that surpass those allowed by solid membranes. Here, we demonstrate a membrane-less architecture that enables unprecedented throughput by 3D printing a device that combines components such as the flow plates and the fluidic ports in a monolithic part, while at the same time, providing tight tolerances and smooth surfaces for precise flow conditioning. We show that inertial fluidic forces are effective even in millifluidic regimes and, therefore, are utilized to control the two-phase flows inside the device and prevent cross-contamination of the products. Simulations provide insight on governing fluid dynamics of coalescing bubbles and their rapid jumps away from the electrodes and help identify three key mechanisms for their fast and intriguing return towards the electrodes. Experiments and simulations are used to demonstrate the efficiency of the inertial separation mechanism in millichannels and at higher flow rates than in microchannels. We analyze the performance of the present device for two reactions: water splitting and the chlor-alkali process, and find product purities of more than 99% and Faradaic efficiencies of more than 90%. The present membrane-less reactor – containing more efficient catalysts – provides close to 40 times higher throughput than its microfluidic counterpart and paves the way for realization of cost-effective and scalable electrochemical stacks that meet the performance and price targets of the renewable energy sector.

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A versatile and membrane-less electrochemical reactor for the electrolysis of water and brine

Bubble nucleation in hydraulic systems

1 Stand der Technik Kavitation, ein weitreichendes Phänomen in hydraulischen Maschinen, entsteht infolge hydrodynamischer Druckabsenkung. Wenn der statische Druck in der Maschine lokal den Dampfdruck oder die Zugfestigkeit der Flüssigkeit erreicht, wachsen Keime zu Kavitationsblasen. Der anschließende Blasenkollaps führt zur Aussendung von Druckwellen, die schließlich zur Anregung von Bauteiloberflächen und damit zur Schallemission führen. Darüber hinaus folgt aus dem unmittelbar an Oberflächen stattfindenden Blasenkollaps im Einzelnen oder in Kollektiven erosiver Materialverschleiß. Bei dauerhaftem Einsatz hydraulischer Maschinen in einem sich durch starke Kavitation auszeichnenden Betriebsbereich kann es zu lokalen Bauteilschädigungen und einer deutlichen Reduzierung der Lebensdauer kommen. In vielen technischen Anwendungen wird der Einsatzbereich hydraulischer Maschinen nicht nur von der zur Verfügung stehenden Leistung begrenzt, sondern auch durch das aggressive Verhalten der Kavitation. Eine frühzeitige Erkennung kritischer Kavitationszustände während des Betriebs kann die Industrie in die Lage versetzen, die Lebensdauer hydraulischer Maschinen zu erhöhen, indem aktiv, z. B. durch Reduzierung der Drehzahl, in den Betrieb der Maschine eingegriffen wird. Darüber hinaus trägt eine möglichst detaillierte Kenntnis der durch die Kavitation verursachten Werkstoffbelastung bzgl. Ort und Intensität dazu bei, Bauteile strömungstechnisch zu optimieren und damit Standzeiten sowie Wartungsintervalle kostenorientiert abzustimmen.

T. F. Groß

Papers

Bubble nucleation in hydraulic systems

Zeitlich und örtlich hochaufgelöste Messung von Oberflächenbelastungen infolge Kavitation